UNIT 7 THE CLIMATE PUZZLE: THE ATMOSPHERE

A. INTRODUCTION

Overview

The atmosphere of the Earth is a huge and very complex system that interacts with the land, the oceans, and the Sun to produce the weather and climate of our planet. In this unit we shall look at the structure and dynamics of the atmosphere and learn the essential causes of weather. The chemical composition of the atmosphere is an important determinant of Earth's overall climate through the action of what is known as the Greenhouse Effect. Consistent planet-wide circulation patterns determine the directions of prevailing winds, affect local climate, and determine a number of factors that have strongly influenced human history and habitat such as the development of trade routes and the occurrence of monsoons in Southern Asia.

B. COMPOSITION AND STRUCTURE OF THE ATMOSPHERE

A summer thunderstorm brings high winds, bending tree limbs and swirling leaves, great splattering raindrops, lightning strokes, and booming thunderclaps. John Muir, one of America's early environmentalists, described how he once climbed a thrashing tree in the midst of a thunderstorm to better appreciate the spectacle of nature's show. Fortunately his tree was not struck by lightning, and those of us who share his love of natural phenomena, but do not share his daring, are content to watch from a porch or through a window. Others may prefer to draw the blinds closed and concentrate on something else.

Whether our reaction is one of awe or fear, all will agree that the weather exerts a powerful influence over our lives. The type of local agriculture is largely determined by the climate. Hurricanes, tornadoes, floods, and droughts can take lives and wreak havoc on local or even national economies. By 1985 a protracted drought had contributed to mass starvation in Ethiopia and other parts of Africa. By 1993, these droughts had eased in many parts of Africa, but a climate that limits agriculture combined with overpopulation and domestic turmoil had brought incredible suffering to Somalia and Ethiopia. On a longer time scale, it was only 10,000 years ago that the great ice sheets waned on the North American continent, uncovering much of Canada and the northeastern United States. What causes these changes in climate? Can they be predicted?

Before we can tackle such questions, we need to learn something about the weather and climate machinery of Planet Earth. The atmosphere forms the most important part of that machine. In this unit we will look at the composition, structure, and operation of the atmosphere.

Only two gases, nitrogen (N₂) and oxygen (O₂), make up 99% by volume of our atmosphere. Nitrogen accounts for 78%, oxygen for 21%, and one more gas, argon, accounts for most of the remaining one percent. Water vapor may locally account for as much as 3%, or it might be nearly absent. Water vapor is a clear, colorless gas -- what we see as steam or fog is really a collection of tiny droplets of liquid water suspended in the air. Other gases are present in minute amounts: neon, helium, methane (CH₄), carbon monoxide (CO), sulfur dioxide (S0₂), and ozone (0₃). Even carbon dioxide (C0₂), necessary to photosynthesis in plants, accounts for only 0.03% of the atmosphere.

The atmosphere also carries tiny particles of matter about on its winds. Larger particles settle out or are washed out by precipitation fairly quickly, but very small grains can stay aloft for months or years, kept suspended by the constant jostling of air molecules. Among the particulates found in the atmosphere are soil blown into the air as dust, sea salt resulting from the evaporation of ocean spray, smoke from forest fires and the combustion of fossil fuels, and fire ash or silicate dust and sulfate salts blasted into the atmosphere during volcanic eruptions.

Pollutants are gases or particulates in the atmosphere we breathe that are undesirable from the standpoint of the health of plants and animals. Carbon monoxide, nitrogen oxides, and sulfur dioxide are all toxic even at fairly low concentrations. Carbon monoxide is created by incomplete combustion and is often a product of automobile exhausts, as are oxides of nitrogen. Sulfur dioxide is a common byproduct in the burning of coal and oil, and chemical reactions with water vapor can convert it into droplets of sulfuric acid, leading to the formation of acid rain.

Hydrocarbons such as gasoline fumes can react with nitrogen oxides to form complex organic molecules that remain suspended in the air, turning it a yellow-brown color and making breathing difficult and sometimes even hazardous. We call it smog, and catalytic converters have been installed in American cars to reduce the amount of unburned gasoline that exhausts from the tailpipe. In California you may see special fittings on gas pump nozzles that trap gasoline vapor from the gas tank before it can escape to the air.

Not all pollutants are man-made, however. Volcanoes are among the greatest polluters, putting large quantities of sulfur dioxide, hydrochloric acid, and particulates into the atmosphere.

Water vapor varies strongly in concentration with elevation and temperature. Water exists as a solid (ice), liquid (water), or gas (water vapor) depending upon the pressure and temperature of its environment. Most water vapor is confined to the warm dense region of the atmosphere within ten kilometers of the surface.

Temperature, pressure, and humidity are the three most important properties that describe the physical state of the atmosphere. It is important to recognize the difference between temperature and heat. Heat and temperature both relate to the motion of molecules. Molecules and atoms are always in motion. In solids they vibrate about their fixed positions, while in gases they move about in a chaotic manner, interacting with their neighboring molecules and frequently changing direction. At high temperatures, molecules move more rapidly than at low temperatures. In fact, temperature may be regarded as a measure of the average kinetic energy of atoms in a region.

Heat, on the other hand, may be defined as energy in transit. When a hot region comes into contact with a cooler region, heat energy flows from one to the other, cooling the hot region and heating the cooler region until the temperatures of the two become the same. When this occurs, the flow of heat ceases. The tendency for heat to flow from warmer to cooler regions is the principal driving force for the weather machine.

The source of heat and the object being heated do not necessarily have to be in direct contact. The Sun is extremely hot and emits energy in the form of light. That light can fall on the much colder surface of the Earth and warm it, pumping heat into the ground and raising its temperature.

The temperature of the atmosphere can be measured by thermometers of one kind or another carried into the high atmosphere by rockets or into the lower atmosphere by instrumented balloons called radiosondes. Instrument packages attached to the balloons record the temperature, pressure, and humidity, and radio the data back to the ground as the balloon rises.

Atmospheric pressure refers to the force exerted on a unit of area due to the weight of all the air contained in a column directly above it. At sea level, average air pressure amounts to 1.01 x 10⁵ nt/m² (14.7 lbs/in²). That column of air, extending from sea level to the top of the atmosphere, weighs the same (and exerts the same pressure) as a column of water 10.3 meters (34 ft) high or a column of mercury 760 millimeters (29.9 in) high. In fact, a column of mercury forms the basis of the traditional barometer used in measuring atmospheric pressure.

A glass tube, closed at one end, is filled with mercury and inverted with its open end in a dish full of mercury (see Figure 7-l). If the tube is about a meter long, at sea level the mercury will be found to stand at a height of about l60 mm in the tube above the surface of the mercury pool in the dish. Above the mercury inside the glass tube is nothing -- a vacuum.

Above the mercury outside the tube is the atmosphere, exerting its pressure on the mercury surface and forcing it up into the tube until the column of mercury in the tube exerts the same pressure (force per unit area) on the mercury in the dish as the column of air. Changes in air pressure are reflected in changes in the height of the mercury column, which may be measured easily and accurately.

• Figure 7-l The Operation of a Mercury Barometer

  The pressure of a column of mercury 760 mm high is balanced by the pressure exerted by a similar column of air extending to the top of the atmosphere.

  

The aneroid barometer uses a different method for measuring atmospheric pressure. A sealed bellows made of metal expands when air pressure falls, and contracts when it rises. Measuring the change in size of the bellows allows the recording of pressure changes.

Atmospheric pressure falls off rapidly with height above the Earth's surface. As a very rough guide, the pressure decreases by a factor of two for every five to six kilometers of elevation gained. A common unit of pressure is the millibar, where the pressure at sea level is approximately 1,000 millibars, or one bar. At 5.5 kilometers (18,000 ft), the pressure is down to about 500 millibars, at 11 km (36,000 ft) it is about 250 millibars, and so on. At 50 km (30 mi), the pressure is less than 0.1% of its sea-level value.

Relative humidity is a measure of the amount of moisture in the air (as water vapor, not as droplets) compared to the carrying capacity of the air. If the relative humidity is 50%, then the air contains just half the water vapor that it can carry at that temperature. Cold air can carry much less water vapor than warm air, and so the relative humidity can vary considerably as the temperature changes. As the temperature drops toward the end of the day, the relative humidity can reach 100%, at which point liquid water drops begin to condense from the water vapor, causing dew to form. Morning fogs often form for the same reason. Winter air in our heated houses is often very dry because cold air from outside the house, already low in moisture content, is heated with a resultant lowering of the relative humidity, often to very low percentage values.

The interplay of temperature, pressure, and humidity causes many features of the weather system. A body of air that rises assumes lower pressures at height and expands, which cools the air. If the air is moist, its relative humidity will increase and can exceed 100%, at which point droplets of water will form, giving rise to clouds. Condensation of water droplets is aided by the presence of particulates in the air that act as seeds or nuclei about which water droplets or ice pellets can begin to form.

1. Layers of the Atmosphere

We have already mentioned that when air rises, it expands and cools, and this provides a partial explanation of why the air temperature is usually lower at higher elevations, such as in the mountains. At one time it was assumed that this trend would continue to the top of the atmosphere, but radiosonde balloons and sounding rockets showed that this was not the case. Figure 7-2 shows that the temperature indeed drops as we rise to 10 km (6 mi), but above that elevation it warms again, reaching a maximum of about 0°C (32°F) at a height of about 50 km (30 mi). Above this elevation it cools once again, then above 80 km (50 mi) it once again rises, this time reaching very high temperatures at a height of about 200 km (125 mi).

On the basis of these temperature swings, the atmosphere has been divided into layers, called the troposphere, stratosphere, mesosphere, and thermosphere.

The troposphere is the lowest layer, extending to an average height of about 10 km (6 mi). Its top ranges in height from 8 km (26,000 ft) at the poles to 18 km (59,000 ft) at the equator. It contains most of the moisture in the atmosphere, and so most of what we regard as weather -- clouds and storms -- is confined to it. The ground, as its base, is warmed by the Sun and serves as the source of heat for this layer. Much of the troposphere is in constant motion, giving rise to constantly shifting weather patterns because warm air lies below cooler air, which is an unstable situation. Warm air tends to rise and be replaced by denser, cooler air, which in its turn is heated by the warm surface of the Earth. This keeps the troposphere in constant motion and brings an endlessly changing pattern of weather.

Between 10 and 50 km, however, the temperature rises. This places warm air above cooler air, which is a stable arrangement. The result is a layer in which little vertical mixing occurs. That is, the layer is stratified, hence its name, the stratosphere. Normally, neither clouds nor storms reach into this region. The boundary between the troposphere and stratosphere is called the tropopause. Its height ranges from 8 km (26,000 ft) at the poles to 18 km (59,000 ft) at the equator.

• Figure 7-2 Temperature Structure of the Atmosphere

  Adapted from R. S. Quiroz, Bulletin of the American Meteorological Society, Vol 53, pp 122-133, 1972. Included with permission.

  

Why does the temperature increase in the stratosphere? In Figure 7-2, note that there is a layer of ozone extending throughout much of this region. Ozone is a molecule made up of three atoms of oxygen bound together (O₃) instead of the usual two (O₂). It happens to be a very strong absorber of ultraviolet rays from the Sun, and absorbs much of the energy of these rays. In the process the air is heated by the energy of the absorbed rays. In addition to producing the temperature hump at about 50 km, it also blocks the shorter, more harmful ultraviolet rays that reach Earth's surface. Ultraviolet rays are responsible for sunburn and can cause skin cancer. Were it not for the ozone layer, most present forms of life probably could not exist on the continents.

There has been much concern in recent years about the continued integrity of the ozone layer. Some concerns centered on widespread use of chlorofluorocarbons (CFC's) such as Freon™ as propellants in spray cans. Chlorofluorocarbons are very stable and so eventually diffuse into the stratosphere where sunlight can act on it, releasing chlorine which is capable of destroying ozone. CFC is also used as a refrigerant in air conditioners and refrigerators. Since 1977, use of CFC in spray cans has been placed under strict governmental regulation in the United States, but development of a satisfactory substitute for it as a refrigerant has been slower. However, since January 1, 1996 CFC imports and production for domestic use have been banned in the United States. CFC-free refrigerators and air conditioners are now sold in the United States. Unfortunately chlorofluorocarbon-12, the air-conditioning gas called Freon™, has suddenly become the U.S. Customs Service No 2 problem, behind illegal drugs. One illegal Florida importing scheme broken up early in 1996, in which production of the CFC was being done in India where these chemicals are still legal, involved illegal CFC-12 shipments worth $52 million.

Table 7-2: Summary of Key Greenhouse Gases Influenced by Human Activities I

1 Ozone has not been included in the table because of lack of precise data.2 ppmv = parts per million by volume; ppbv = parts per billion by volume; pptv = parts per trillion by volume.

3 The current (1990) concentrations have been estimated based upon an extrapolation of measurements reported for earlier years, assuming that the recent trends remained approximately constant.

4 For each gas in the table, except CO₂, the "lifetime" is defined here as the ratio of the atmospheric content to the total rate of removal. This time scale also characterizes the rate of adjustment of the atmospheric concentrations if the emission rates are changed abruptly. CO₂ is a special case since it has no real sinks, but is merely circulated between various reservoirs (atmosphere, ocean, biota). The "lifetime" of CO₂ given in the table is a rough indication of the time it would take for the CO₂ concentration to adjust to changes in the emissions (see section 1.2.1 for further details).

From Climate Change, The IPCC Scientific Assessment, 1991.

Other concerns for the ozone layer have surfaced. High-flying jet aircraft such as the Concord supersonic transport and some military aircraft operate within the ozone layer, releasing nitrogen oxides in their exhausts. Nitrogen oxide is capable of reacting with ozone and destroying it, and because the stratosphere is so stable, these exhaust gases would remain within the ozone layer for long periods of time, maximizing the chances for ill effects. Other human sources of nitrogen oxides are automobile exhausts and nitrogen fertilizers at ground level, and atmospheric blasts of nuclear bombs. The principal regulator of atmospheric 03, however, is nitric oxide released from bacterial action in the soil. We will discuss this process in Unit 14, The Balance of Nature. Lightning also contributes nitrogen oxides to the atmosphere.

It is one of the many ironies of nature that, while ozone in the stratosphere is a highly beneficial shield blanketing the Earth, at ground level in our cities it is a toxic pollutant created by the action of sunlight on automotive and industrial emissions.

Above the stratosphere, temperature once again falls with increasing elevation, marking the mesosphere. The final rise in temperature occurs within the thermosphere, above 80 km (50 mi) . This heating is caused by the action of high-energy radiation from the Sun on the extremely thin air. The effect is that a negatively-charged electron is knocked out of free oxygen (O), molecular oxygen (O₂), or nitrogen oxide (NO), leaving them as positively-charged ions. Their temperature is raised in the process, and because there are so few molecules per cubic centimeter at this height, it takes very little energy to produce large increases in temperature .

Because the lower portion of the thermosphere is characterized by the presence of ions, it is also referred to as the ionosphere. We will have more to say about this region and the next, nearly empty, one above it -- the magnetosphere -- in Unit 14, Interactions Between Sun and Man.

C. HEAT FROM THE SUN AND THE GLOBAL ENERGY BALANCE

1. Radiant Energy

The Sun puts out a tremendous amount of energy, but because the Earth is small and far away, we intercept only a tiny fraction of the total. Nearly all of that energy travels to us as radiation. In this unit, we shall use the term radiation to mean radiant energy (radiant heat). In this context, it has nothing to do with radioactivity, but simply refers to the sending out (transmission) or receiving (absorption) of radiant energy.

Radiation travels as electromagnetic waves. Some of the kinds of electromagnetic waves are radio and television waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Electromagnetic waves can traverse even the empty space that separates Earth from the Sun, traveling at the speed of light (299,000 km or 186,000 mi per second). Radiant heat is represented mostly in the infrared and visible regions of the electromagnetic spectrum.

Different types of radiant energy are distinguished from one another by their wavelengths, or the distance between successive wave crests. Figure 7-3 shows the characteristic wavelengths of different kinds of radiant energy.

• Figure 7-3 The Spectrum of Electromagnetic Radiation

  

All forms of radiation behave in a manner similar to that of light. Light rays travel through space in straight lines, and large objects that are opaque to light cast shadows. Some substances, like glass or water, are transparent and transmit light through them. Opaque substances do not transmit light, but either absorb or reflect it. Coal absorbs visible light and appears dark, while snow reflects it and appears bright. The transmission, absorption, and reflection properties of a material depend upon its structure and upon the wavelengths of light that strike it. For instance, ordinary glass is transparent to visible light but is nearly opaque to ultraviolet light: that is, it absorbs ultraviolet light. This explains why it is difficult to get a sunburn through a window, since ultraviolet rays are responsible for the effects of sunburn.

Radiation from the Sun comes to us in a broad range of wavelengths, but most of the energy is contained in a band ranging from the near infrared through the near ultraviolet, including the entire visible spectrum. For this reason, the Sun's light appears white, a mixture of all the colors.

In addition to what may be reflected from a body or transmitted through it, radiant energy is produced by a body and emitted by it into its surroundings. As a rule, the wavelengths of radiant energy emitted by a body depend on its temperature, with hot bodies emitting more short-wavelength (bluer) radiation, and cooler bodies emitting longer-wavelength (redder) radiation. While radiant energy is usually emitted in a broad band of wavelengths, a simple rule relates the wavelength for peak emission to the temperature of the body. This rule is known as Wien's Law and can be given in the form:

Rule 1

This rule applies strictly only to what is called a "black body," which is an object with a surface that absorbs all radiation falling upon it and that emits all radiation appropriate to the temperature. Rough, dark surfaces satisfy this requirement better than light-colored or reflective surfaces. A lump of charcoal is a good approximation to a black body, but we shall apply Wien's Law to objects with other kinds of surfaces to get at lease a rough idea of the peak wavelengths that they emit.

Using this rule, if we know the temperature of a black body, we can determine the dominant or peak wavelength of the radiant energy. Let us take some examples. The surface of the Sun has a temperature of about 5,700°C. Using Rule 1, we find that the dominant wavelength of its light should be about 4.8 x 10^-7 meters. This falls within the visible spectrum. Note that this gives us only the peak wavelength -- the actual band of radiation emitted by the Sun's surface extends from infrared to X-rays.

Another example is yourself. Normal skin temperature is about 34°C (93°F) which, when put into Rule 1 and the arithmetic carried out, yields a peak wavelength of 9.4 x 10^-6 m, or nearly 10^-5 m. From Figure 7-3, you can see that this falls in the infrared part of the spectrum. Our eyes are not sensitive to infrared rays; and for that reason you do not appear to "glow in the dark," but in fact you do. An instrument sensitive to the middle infrared part of the spectrum can literally take a picture of you in the absence of visible light. Your "glow" is entirely within the infrared.

Another familiar example of infrared emission is an asphalt street or other dark surface that has been heated in the sunlight all day. For a while after the surface becomes shaded, you can feel, but not see, the radiant heat being emitted from it in the infrared part of the spectrum.

We have spent so long on the concept of radiant energy because it is critical to understanding how the Earth interacts with sunlight, and this is the basis for all weather and climate. To be sure you understand it, let's take one last example. So far we have used the temperature of an object to find the wavelength of its radiation. We may turn that around and use the known wavelength emitted to obtain its temperature. We will need to rewrite Wien's Law as stated in Rule 1 into a new form:

Rule 2

Consider a brightly glowing coal in a furnace. What is its temperature? Because it glows red, we can choose a wavelength from Figure 7-3 of around 7 x 10^-7 m. Placing this number in the denominator of Rule 2, we might assume that the temperature of the coal is approximately 3,800°C (6,900°F). In addition to the visible red light that it emits, you can feel the infrared radiant heat that accompanies the visible light. Remember that not just one wavelength, but a broad range about the peak wavelength is emitted. Because of this effect, and the fact that our eyes cannot see the emitted infrared radiation, our estimate of the coal's temperature is on the high side, since the true peak of the radiation is in the infrared rather than in the red.

2. Light and Heat From the Sun and the Greenhouse Effect

When you leave a car parked in sunlight with its windows rolled up, the temperature can rise inside the car far above the outside temperature. A greenhouse works the same way, and even though a greenhouse may need a heater to keep the temperature above freezing in the winter, it takes less heat than if this natural heating effect were not in operation. This process in fact is called the Greenhouse Effect, and its operation depends upon two processes. The first is the fact that the enclosed structure traps heated air and prevents it from escaping to the outside air. The second relies upon the fact that glass is transparent to visible light but is relatively opaque to infrared radiation.

Let us examine this second effect more closely. Visible light from the Sun passes through glass readily and falls on objects inside the greenhouse, warming them and raising their temperature. These objects radiate part of the energy away from them at a wavelength that is determined by their temperature, according to Rule 1. Even if they become quite hot to the touch (around 70°C), their peak emission is at around 8.4 x 10-6 m, well within the infrared range. Because glass does not transmit infrared radiation (that is, it is opaque to it), radiant energy cannot escape from the greenhouse and is trapped within it.

The Greenhouse Effect, then, is partly dependent on an enclosing substance being transparent to incoming visible radiation and opaque to outgoing infrared radiant heat. It is fortunate for us that the Earth's atmosphere does not form a very effective greenhouse. If it were as effective as glass, the average temperature would rise to the point that living on Earth would become intolerable. As it turns out, oxygen and nitrogen are largely transparent to infrared, allowing the warm ground to radiate a significant amount of infrared energy into outer space. In addition, atmospheric circulation carries heat away from the surface quite effectively .

On the other hand, water vapor, carbon dioxide, and methane are much more opaque in the infrared, and their presence tends to produce an additional Greenhouse Effect that is dependent on the concentration of these gases. Clouds are also extremely effective blocks to outgoing infrared radiation. This explains why the coldest nights are those clear, crisp times when there are no clouds and the humidity is low. As we mentioned earlier, carbon dioxide accounts for only about 0.03% of the atmosphere, but its concentration has been increasing for some time due to the activities of mankind. This has raised concerns about possible long-term effects on climate. We shall return to this subject in the next unit.

From Climate Change, The IPCC Scientific Assessment, 1991

As it is, with all other factors held equal, the total Greenhouse Effect on Earth at the present time produces a warming of about 35°C (63°F) over what the temperature would be without any such effect at all. That much of a temperature difference, of course, is a critical one. Without it, Earth would be largely a frozen planet.

D. CLIMATE AND THE SEASONS

Seasons are caused by the tilt of Earth's rotational axis. Like a navigational gyroscope, the axis points in the same direction in space from year to year. Over long periods of time, it can precess and has other motions similar to those of a spinning top. These motions need not concern us now. Figure 7-4 shows the situation as Earth rotates about the Sun during the course of a year. In the northern hemisphere, in June the Sun stands very high in the sky at noon and its rays beat directly down, efficiently warming the ground. In December, on the other hand, the Sun's rays hit the northern hemisphere obliquely, and the same energy is spread out over a much larger area. Note from the diagram that the exact opposite is the case in the southern hemisphere, resulting in a reversal of the seasons.

At the equator, the Sun is never far from overhead at noon, somewhat farther north in June and farther south in December, but making about the same angle with the ground in both months. As a result, the temperature is generally very warm and much the same all year. At the poles, on the other hand, the sun never rises very high above the horizon, resulting in a very cold climate.

There is one further effect worth noting. Earth's orbit around the Sun is not perfectly circular. It is somewhat elliptical, with the result that the Earth is 147 million km from the Sun in January but 152 million km from the sun in July. This might seem to contradict the seasons in the northern hemisphere (but not in the southern hemisphere), except that the difference in energy received from the Sun due to this effect is small compared to the effect of the tilt of the rotation axis. Nevertheless, it introduces some asymmetry into the climate, tending to exaggerate seasonality in the southern hemisphere and reduce it in the northern hemisphere. As we shall see in the next unit, changes in these orbital parameters have been implicated in long-term climate changes.

• Figure 7-4 Seasons on Earth

  

E. HEAT TRANSPORT MECHANISMS IN THE ATMOSPHERE

We have already discussed one method for transporting heat from one place to another: radiation. This process is all-important in determining the net heat input from the Sun into the Earth climate system. The remaining heat transport processes -- atmospheric convection, and ocean currents -- serve to redistribute this energy throughout the world, determining local weather and climate in the process.

Convection is the process by which heat is carried by a moving mass. In Unit 4, Continental Tectonics and Earth's lnterior we discussed convection in the mantle as a means of transporting heat from the Earth's interior to the surface. It might be well to mention at this point that this quantity of heat coming up through the ground and into the atmosphere is minuscule compared with the energy flux from the Sun, and so is very unlikely to have any significant effect on climate.

There are many familiar examples of convection. A radiator heats cool air. The air expands, becomes less dense and rises, displacing cooler air which sinks and enters the radiator at its base to complete the convection cycle. A pot of water placed on a stove is heated from below. Warm water tends to expand and rise over the hottest part of the pot bottom, displacing cooler water that sinks near the edge of the pot.

• Figure 7-5 Examples of Horizontal Convection

  

Convection, then, is a cyclic motion of a freely-moving fluid in which a net flow of heat is carried from a warm to a cooler region by the fluid. Heat transport does not always have to be vertical in convection. Figure 7-5a shows a room with one cold wall and one warm wall. Its floor and ceiling are well insulated so that they will not affect the process. Air near the warm wall is heated and rises while air near the cold wall is cooled and sinks. Even though vertical motion always occurs in convection, the net result may be the horizontal transport of heat energy. In this case, heat is removed from the warm wall and carried to the cold wall. It is also important to realize that there is an actual transfer of heat. In the process the warm wall is cooled and the cool wall is warmed. Ultimately, thermal processes strive to reduce everything to the same temperature. They will continue to act only as long as there is a source of energy (such as solar radiation or a furnace) to maintain a temperature difference.

On Earth, solar radiation maintains temperature differentials in many ways, but the two most important are the temperature differences between the equator and the polar regions, and that between continental and oceanic areas. During the summer the latter results in the phenomenon of the sea breeze along the ocean shore. The ocean does not change temperature so readily as the surface of the land and remains at a nearly constant temperature during day and night. The surface layer of the land, however, becomes quite hot during sunny days and cools noticeably at night. During the day, then, air over the land is heated and rises, being replaced by cool air from over the ocean. This produces an onshore breeze -- the sea breeze (see Figure 7-5b). At night, the breeze dies as the land cools, or may even reverse direction if the land temperature falls below that of the sea, producing an offshore breeze.

On a larger scale, a similar situation is an important contributor to the monsoons that are so important to southern Asia and elsewhere. Seasonal temperature differences between continental landmasses and major oceans help to govern the annual alternation of rainy and dry seasons in these parts of the world. We shall return to this point in the next unit.

Moving air masses may transfer heat from one place to another in two different ways. One is by the movement of a warm or cold air mass, carrying its thermal energy with it. Meteorologists refer to this thermal energy as sensible heat. Heat transfer also may be accomplished using energy storage within the air mass due to the evaporation and condensation of water. In order to evaporate water, energy must be absorbed, and this energy input is called the latent heat of vaporization. That energy is effectively stored in the new physical state of the water (as gaseous water vapor). When you step out of a tub or swimming pool, your skin feels cool because of the water that is evaporating from it. Part of your body heat is being used to change the water from a liquid to a gaseous state. The latent heat is released when water vapor condenses back into the liquid state as rain or fog. The latent heat can then be transported if evaporation occurs in one place and the moist air mass then moves to another place before condensation occurs.

Heat transfer in the atmosphere is often a combination of both processes: the transfer of sensible heat and the transfer of latent heat. In the absence of external forces, the two taken together will act to transfer heat from a warm to a cooler region.

We can now see the operation of the familiar hydrologic cycle, diagrammed in Figure 7-6. Both latent heat and sensible heat play important roles. The Sun supplies energy to evaporate water from the ocean (latent heat) and also supplies the temperature differentials needed to move the moist air mass onto the continents (transfer of sensible heat and latent heat). As the warm, moist air rises it expands and cools until its relative humidity reaches 100%, at which time condensation occurs and precipitation becomes possible. On the average, both latent and sensible heat processes tend to transfer energy from the warm surface of the Earth to higher, cooler regions of the troposphere, but more than 50% of solar energy reaching the surface is transported back into the atmosphere via latent heat transfer. Horizontal winds, updrafts (rising air), downdrafts (descending air), clouds, and precipitation are all byproducts of this energy transfer. Taken together, the whole scheme is a heat engine that produces motion and weather effects by attempting to reduce, on average, the temperature differences introduced by the Sun.

• Figure 7-6 The Hydrologic Cycle

  

It takes a substantial amount of heat energy to raise the temperature of water. That is, water has a high heat capacity, which is just the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. Because of this, the world's oceans constitute an immense reservoir of heat energy that may be exchanged with the atmosphere at the sea surface. Warm and cold ocean currents can strongly influence climate locally, as you have seen in the units on oceanography. They are another example of the transfer of sensible heat, where this time the moving mass is water instead of air.

F. CIRCULATION OF THE ATMOSPHERE

1. Wind

What makes the wind blow? You already know part of the answer. Wind is simply motion of air masses in response to pressure differences, acting in such a way as to transport heat energy from warm regions such as the equator to cold regions such as the poles. This statement is fine as a general description, but it omits a number of complicating factors that must be taken into account if we are to be able to understand and predict winds. Many of these complications arise from the Coriolis Effect, which we found to be an important influence on oceanic circulation. We shall find it to be equally important in the case of the atmosphere.

HM 7-5 (Quicktime Video Animation): Atmospheric weather patterns over northern Africa. Atmospheric circulation over the northern Africa and Europe. (Animation from Met. Office, UK). To begin animation click on image to load, then click to start animation.

In the previous unit you learned that oceanic circulation is largely governed by the prevailing winds. Now we need to determine what governs the directions of these persistent winds. Let us begin by considering the case where we ignore effects of the Earth's rotation and then introduce the Coriolis Effect to see what changes it introduces.

You are probably familiar with the way in which high and low pressure areas move across daily weather maps. Recall that the atmospheric pressure is just the weight of the column of air directly above a unit area. The greater the mass of air contained within the column, the higher the atmospheric pressure measured at its base. The total mass of air above a given area is a function of several variables: temperature, humidity, and the extent to which air masses are converging toward that region or diverging from it.

When you blow up a balloon, it is clear that the pressure inside the balloon is greater than in the atmosphere outside the balloon. If the mouth of the balloon is allowed to open, the air inside will rush out in response to the pressure difference. In general, air tends to accelerate from a region of high pressure toward one of lower pressure. In addition, it is not the value of the pressure that is important, but how rapidly the pressure changes over a given distance .

The pressure gradient is defined to be the difference in pressure over a unit distance, where that distance is measured in the direction of maximum pressure change. The direction of the gradient is just that direction of maximum change. Figure 7-7a shows how this works. At the bottom of the diagram the pressure is high (1010 millibars) and at the top it is low (990 millibars). The horizontal lines are isobars: lines of equal pressure. The pressure gradient points from bottom to top, perpendicular to the isobars.

A volume of air in this diagram will experience a force due to the pressure gradient in the same direction as the gradient, that is, toward the top of the diagram. Newton's Second Law of Motion states that a mass acted on by a force will accelerate in the direction of the force, moving ever more rapidly with time. This is shown in the same diagram for a small volume of air at successive time intervals, starting at rest and accelerating in response to the pressure gradient force.

• Figure 7-7 Pressure Gradients, Winds, and the Coriolis Force

  a. The unlabeled arrows show the motion of a volume of air on a horizontal surface at successive time intervals in response to the pressure gradient force on a non-rotating Earth. b. In the northern hemisphere of a rotating Earth, the Coriolis force deflects the volume of air to the right until the wind blows parallel to the isobars.

  

Once we introduce a rotating Earth, however, the situation changes dramatically, even though the pressure gradient may remain unchanged. In Figure 7-7b, the volume of air begins from rest and initially is accelerated toward the top of the diagram by the pressure gradient force. As soon as it begins to move, however, it is deflected to the right by the Coriolis force. So long as the air moves toward lower pressure, it is accelerated to higher velocity. The Coriolis force increases with velocity, however, and bends the motion ever more to the right.

Eventually the air will be moving parallel to the isobars. At this point, the Coriolis force will exactly balance the pressure gradient force and the air will continue to move parallel to the isobars at a constant velocity. Such a wind is referred to as a geostrophic wind. A geostrophic wind is attained when an air mass has reached a sufficient velocity that the pressure gradient force is exactly balanced by the Coriolis force. Winds associated with high and low pressure zones come close to being geostrophic, especially at altitudes high above the ground where there is little to impede the air's motion. Near the ground, however, friction with the land surface slows the air and prevents it from becoming truly geostrophic. The result may more closely resemble not the rightmost set of arrows in Figure 7-7, but the one or two just preceding it to its left, in which the motion of the air still has a component moving it from high toward low pressure in addition to the motion to the right. Because of this frictional effect, there is a net flow of air away from high pressure regions and toward low pressure regions.

From this we may state a general rule: in the northern hemisphere, if the wind is blowing into your left ear, then your nose points toward the low pressure region. In the southern hemisphere, the same is true if the wind blows into your right ear, since the Coriolis force works in the opposite direction south of the equator.

If the isobars are curved, the same general rules hold. In Figure 7-8 we see how winds circulate around high and low pressure regions in each hemisphere. Note that the "ear and nose" rules continue to work regardless of where you happen to be standing.

A large circulating air mass associated with a low pressure region is referred to as a cyclone, and the direction of rotation of any cyclone is determined by the Coriolos Effect. The term cyclone also has a colloquial use, however, in referring specifically to tropical cyclones such as hurricanes.

• Figure 7-8 Wind Directions around High and Low Pressure Regions

  

Major storms are often associated with low pressure regions, and hurricanes (called typhoons or cyclones in some parts of the world) invariably have winds that circulate in a counterclockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. One reason that storms can have lifetimes of days or weeks before they finally dissipate is that the Coriolis Effect prevents the winds from moving air directly into the low pressure zone and wiping it out by the mass of air converging upon it. Storm systems are really long-persisting eddies in the atmosphere. H. S. Saffir and R. H. Simpson have defined a Hurricane Intensity Scale which relates scale numbers to the atmospheric pressure of the central region of the hurricane, wind speed, height of the storm surge, and potential damage to structures. This scale has proved very useful in studying hurricanes.

Unfortunately as coastal regions which historically are within the path of hurricanes are populated, catastrophic damage becomes a predictable outcome. For example, during 1989 and 1992 two Category 4 hurricanes, Andrew and Hugo, caused about 32 billion dollars of damage to the southeastern coast of the United States. Fortunately, conscientious monitoring of hurricane activity, increased awareness of hurricane danger, stricter building codes and coordinated governmental response have significantly decreased fatalities related to these events.

2. Global Circulation Patterns

In our description of atmospheric circulation as a heat engine, we mentioned that heat is transported from the equator to the poles. You might expect that this would be accomplished by a single convection cell in which warm air at the equator rises, travels at height to the polar region, cools and sinks, and returns at low elevations to the equator. This simple kind of convection circuit is called a Hadley Cell. For a number of reasons, this picture does not work for the entire global circulation pattern. The thickness of the troposphere is very small compared to the distance from equator to pole, and this would make the convection very inefficient. The warm air would have to spend so much time in the high troposphere that it would cool and begin to sink. This in fact occurs at a latitude of about 30°N (approximately the latitude of New Orleans in the United States or Cairo in Egypt). There are other, more complex reasons involving the Coriolis Effect and the conservation of angular momentum of the Earth. The result is a breaking up of the circulation pattern into at least two zones between equator and pole (see Figure 7-9).

Subtropical latitudes are dominated by the Hadley Cell, with the southerly-flowing surface winds in the northern hemisphere deflected somewhat to the right (west) by the Coriolis Effect, which is weak at these low latitudes. Note that the northerly-flowing tropical surface winds in the southern hemisphere are deflected to the left, which is also to the west. The descending cooled air at the northern edge of the Hadley Cell generates persistent high-pressure regions at about 30°N latitude. North of these, the Coriolis Effect becomes stronger and more dominant in its effects, largely replacing the vertical circulation of the Hadley Cell with horizontal circulation about low-pressure regions. This serves to continue the process of moving heat from the warmer temperate zones to the colder polar regions. A weak vertical circulation pattern, suggested by the dashed loops in Figure 7-9, also appears to exist at mid- and high latitudes, but horizontal convection is the dominant process. A similar situation exists in the southern hemisphere.

The high-pressure regions at about 30°N indicate that the principal circulations in the zone just to the north of them should be from west to east, and in fact they are. The reason for this westerly flow is more complex, and once again has to do with conservation of angular momentum of the Earth. These persistent winds in the mid-latitudes are known as westerlies while those that blow from the northeast or southeast in the tropics are called trade winds. In the days of sailing ships, trade routes between Europe and North America would take advantage of the pattern, using a more southerly route for travel from Europe to America and a more northerly route for the return.

From the direction of the trade winds in Figure 7-9, perhaps you can see why Columbus, setting sail from Spain, made landfall not on the continental United States, which is at the same latitude as Spain, but farther south, in the West Indies.

Figure 7-9 shows that the trade winds in the northern and southern hemispheres converge near the equator in a low pressure region called the intertropical convergence zone. Here warm, moist air rises and cools, producing heavy rainfall characteristic of equatorial regions. Because the pressure tends to be uniformly low in the convergence zone, the pressure gradient is small and the result is a general lack of strong winds. Early sailors labeled this zone the doldrums. Other zones of generally light winds occur at the horse latitudes, in the high pressure regions separating the trade winds from the westerlies.

The general pattern described above is an average- for all seasons, and for the entire Earth. A more detailed description requires that we modify it to account for the seasons and for the effect of Earth's landmass distribution. Because the temperature of the ocean does not change very much from winter to summer, the bands of high and low pressure shown in Figure 7-9 tend to be persistent over the oceans, giving rise to features such as the Pacific High off the west coast of North America and the Bermuda High in the Atlantic Ocean.

• Figure 7-9 Persistent Patterns of Global Air Circulation, (a more detailed view), a simplified global model, and the seasonal position of the Intertropical Convergence Zone (after Moran, Morgan and Pauley, 1997)

  

  

Persistent downwelling masses of dry air associated with these high pressure zones are responsible for the existence of the Sonora Desert in Mexico and the Sahara Desert in northern Africa. In the southern hemisphere, the zone of persistent highs governs the existence of the Peru and Atacama Deserts seaward of the Andes Mountains, the Namib Desert of southwestern Africa, and the deserts of western Australia.

The situation is very different on the continents, however. They tend to be hot in the summer and cold in the winter. In summer, they warm the air above them, which rises and produces a local low pressure region. In winter, the air is cooled and sinks, resulting in a high pressure zone. The huge mass of the Asian continent displays this effect very strongly, giving rise to an annual phenomenon that affects the lives of over two billion people: the monsoons.

Figure 7-10, Origin of the Monsoons

During the northern hemisphere winter, a large high pressure zone forms over the central Asian continent, pushing the intertropical convergence zone south of the equator (see the heavy line in Figure 7-10.) Cool, dry winds blow out of central Asia from north to south bringing rainless winter monsoon winds to China and much of southeast Asia. In summer, a low pressure zone forms over southern Asia, and the intertropical convergence zone shifts far north, looping up over Nepal and China. The result is a flow of warm, moist air from the persistent high pressure zone over the Indian Ocean. The summer monsoons bring deluges of rain that make farming, and especially rice culture, possible in these regions. A failure or even delay of the summer monsoon can bring drought and famine to vast numbers of people.

The monsoon phenomenon is most active in the eastern hemisphere because it is caused by the asymmetrical arrangement of landmasses -- Asia north of the equator and the Indian Ocean south of the equator. The western hemisphere, however, has North and South America more symmetrically placed on either side of the equator, with the result that wet and dry seasons are less prominent.

G. FRONTS AND STORMS

We tend to take for granted the rapidly shifting patterns of weather in most parts of the world. Around the Rocky Mountain area, notorious for its rapid changes, there is an expression, "If you don't like the weather, just wait ten minutes." Most natural processes, as you have seen, operate on vastly longer time scales, with change coming slowly. Why is weather so changeable?

Weather as we know it, with its clouds and storms, is confined to the lowermost layer of the atmosphere, the troposphere. Look again at Figure 7-2. We have already noted that warm air underlies cold air in the troposphere, and that this potentially unstable situation accounts for the constantly shifting patterns of weather. Warm air tends to rise and be displaced by descending colder air. Global circulation tends to keep this process going at all times, and local instabilities can develop into storm systems, fed energy by the heat engine of the atmosphere and guided by the patterns of convection and the Coriolis Effect.

Many storm systems are generated where large masses of warm and cold air interact at fronts. We first will examine the structure of different kinds of fronts and then go on to relate them to the development of storms.

• Figure 7-11 Cold and Warm Fronts

  a. A cold front tends to bring in stormy weather due to moist warm air being forced up the steep face of the front. b. A warm front has a gentler shape and tends to produce widespread cloudiness and precipitation.

  

Figure 7-11 illustrates the differences between cold and warm fronts. The views in the lower part of the diagram show the structures of the fronts while the symbolic representation in the upper part shows how they are depicted on weather maps. In both cases the fronts are moving from left to right.

In a cold front, a cold air mass displaces a warm air mass, with the cold air forcing itself under the warm air in a blunt wedge that rapidly forces the warm air to greater height. The warm air cools as it rises and its moisture begins to condense into clouds. This process of condensation releases the latent heat of the water vapor, which warms the air and encourages it to continue rising. Powerful updrafts can result in the production of thunderstorms that are often associated with cold fronts impinging on warm, moist air.

The effect of the latent heat is a very important one in this case. It accounts for the fact that in temperate climates, thunderstorms tend to occur in the spring and summer and not in the winter. Moist summer air provides the extra energy of latent heat to fuel the violent updrafts of a thunderstorm. This effect is usually much reduced in the dryer winter air.

A warm front behaves differently from a cold front because its structure is different. The warm air usually slides up and over the resident cold air mass along a gently sloping plane (see Figure 7-11b). Cloudiness and precipitation often occur, but in a far more extended version than in the cold front model. As a warm front approaches, high clouds are often seen. The clouds thicken and lower, and precipitation in the form of rain or snow may follow, perhaps lasting for some time.

A third type of front may form when a fast-moving cold front approaches and overtakes a warm front. The result is an occluded front, in which the warm air mass is lifted up between the colliding cold masses and no longer contacts the ground. At the ground surface the temperature may not drop much as an occluded front passes, but cloudiness and precipitation in the high warm air mass may result nonetheless.

Cyclones develop around low pressure regions, and are especially abundant in the polar frontal system separating the cold polar air masses from the warmer mid-latitude westerlies (see Figure 7-9). These extra-tropical cyclones, as they are called, are not usually similar to hurricanes and typhoons, which are tropical cyclonic storms. In general they are massive circulating low-pressure weather systems that tend to move from west to east in the mid-latitudes. They develop as the result of wave-like instabilities in the polar frontal system.

More spectacular are tropical cyclones that develop over the open oceans in low latitudes. Called hurricanes or typhoons (or called cyclones in the South Pacific and Indian Oceans), their structure is similar to but more compact than extra-tropical cyclones and they derive their often devastating power from the warmth and moisture of the oceans. They are characterized by strong updrafts associated with a low-pressure region. Condensation and the release of latent heat intensifies the updraft, causing a convergence of air around its base. The Coriolis Effect twists the air currents into cyclonic motion, while the converging air streams speed up as they spiral in toward the central updraft, much as a twirling ice skater spins faster as she draws her arms in closer to her body.

Tropical cyclones can grow in strength while they are over warm ocean and become of hurricane strength with wind speeds in excess of 33 m/sec (73 mi/hr) around a central region called the eye of the storm. In this central low pressure zone, winds are more gentle and cloud cover is light and sometimes absent. The reason is that the principal updrafts occur in a band surrounding the center of the storm. Warm dry air from elevations over 10 km is sucked down into the eye of the storm, producing the deceptively clear air of the eye. When a hurricane passes directly overhead, people who have taken shelter will sometimes emerge, thinking the storm has passed, only to find the fury of the winds renewed once the eye has passed. Their ordeal is only half over but now, of course, the winds will come from the opposite direction. Hurricanes begin to weaken and dissipate once they are over land because the land surface exerts a greater frictional drag upon them and also because they are deprived of moisture input from the ocean.

Another feared type of storm occurs on a much smaller scale than hurricanes. Tornadoes occur almost always in conjunction with thunderstorms and can develop violent twisting winds in their funnel-shaped vortices with speeds of 100 m/sec (220 mi/hr). The funnels range in size from a hundred meters or so up to a kilometer across. So much energy packed into such a small package makes them capable of immense damage, albeit restricted to the small areas actually touched by the funnel.

Tornado funnels apparently form aloft and propagate downward, and seem to be associated with the strong downdrafts accompanying thunderstorms. Even so, the worldwide distribution of tornadoes shows that they form mostly in land areas where conditions are particularly favorable. They are found in the mid-latitudes in Europe, Japan, northeastern India, South Africa, coastal Australia, New Zealand, and a small area of central South America. But by far the largest number are found in the United States, mostly east of the Rocky Mountains, where they most commonly result from the interaction of warm, moist air sweeping north from the Gulf of Mexico beneath cool, dry air moving east at a greater height. Tornadoes are essentially unknown in the huge landmass of Asia north of the Himalayas.

The common summer thunderstorm is a much less imposing affair than a hurricane or tornado, but it has its own majesty and can be capable of wreaking damage as well. Thunderstorms develop when warm, humid air near the ground wells upward in a strong updraft, causing condensation and the familiar latent heat intensification of upwelling. Because there is so much water condensing and falling as rain (and sometimes as hail), strong downdrafts develop in parts of the storm, producing high winds in advance of the storm cell where the downdrafts encounter the ground and spread out.

The presence of both up- and downdrafts in thunderstorms can toss around even large airplanes, and because a thunderhead can extend to heights of over 12 km (39,000 ft), jetliners are usually routed around them. Infrequently it is necessary for a commercial airliner to land through the fringes of a thunderstorm, providing a memorable experience for its passengers. When conditions are too turbulent, flights are forced to circle aloft until the storm clears or are rerouted to other airports.

Small, local thunderstorms may last only an hour or less before they dissipate, but a series of storms may become organized ahead of an advancing cold front in what is called a squall line. These tend to be more dangerous and can spawn hail and tornadoes.

Large thunderstorm systems sometimes form over North America, typically 160 to 400 km (100 to 250 mi) across, and account for more than 50% of the summer precipitation in the farm belt. These super storms are called mesoscale convective clusters.

Lightning and thunder add considerable drama and some danger to the performance put on by a thunderstorm. The exact mechanism by which lightning is generated is still not known for certain, but the process appears to be related to the precipitation of ice and water droplets within the thunderclouds. In this process, negatively-charged electrons are stripped from atoms and transported downwards to the lower reaches of the cloud, leaving the upper portions positively charged. Lightning bolts occur as a means of reducing the separation of electric charges that build up, either within the cloud or between the ground and the cloud.

Lightning is simply a flow of a large electric current through the air, as Benjamin Franklin demonstrated with his famous (and dangerous) kite-flying experiment. These currents can reach 50,000 amperes or more in a single stroke, and can electrocute people. Prompt treatment with cardio-pulmonary resuscitation (CPR) techniques can sometimes revive a victim of lightning stroke.

The electric current spreads out once it hits ground, taking whatever routes good electric conductors may afford it. For this reason, safe shelter from lightning may be found in automobiles or houses, where metal walls or pipes will provide a route for the electric current that generally will detour it away from you. For this reason it is best to stay away from plumbing or television antenna leads during thunderstorms.

All in all, more people in the United States are killed by lightning than by the effects of hurricanes and tornadoes combined. If you are indoors and are not part of the electrical circuit, you have little to fear from lightning. On the other hand, persons outdoors are in exposed positions, and should attempt to find shelter in a nearby building or auto. Trees, however, are poor choices for shelter, since trees are frequent targets for lightning strokes. Golfers, boaters, and mountain climbers often find themselves in exposed situations during thunderstorms, and these enthusiasts are advised to consult weather forecasts before setting out.

H. WEATHER FORECASTING

1. Observations and Methods of Forecasting

Few scientific activities are as visible to the layman as weather forecasting. Evening news programs feature "meteorologists" who stand before colorful weather maps and radar pictures and deliver detailed forecasts for the next 24 hours and more general forecasts for the next few days. Some of the television personalities on your screen are trained scientists who participate actively in preparing the forecast while others are simply delivering forecasts prepared by private meteorological firms or the National Weather Service. All, however, rely on data collected worldwide under the guidance of the World Meteorological Organization in a carefully organized and standardized program.

At hundreds of weather stations around the world, instrumented weather balloons called radiosondes are released twice each day at synchronized times -- at whatever local time corresponds to midnight and noon in Greenwich, England. These balloons carry standardized instruments that measure atmospheric pressure, temperature, and humidity as they rise through the troposphere and into the stratosphere. As the balloon rises to regions of lower pressure, the gas in the balloon expands. The balloon eventually bursts and the instrument package is lowered by parachute to the ground. In many cases, the motion of the balloon is tracked during its travels so that information on wind speed and direction is also obtained.

Today, weather maps are constructed from information compiled from the radiosondes, along with observations made from the ground and from satellites. Published maps showing whole continents usually display atmospheric pressure at sea level, temperature, winds, and frontal systems (Figure 7-12). These large-scale maps are called synoptic maps, because they give a synopsis or overall view of the weather in a large region at a particular time. Weather forecasters also make extensive use of similar maps showing conditions aloft, allowing them a more three-dimensional view of the atmosphere.

Two more recent techniques have added to the forecaster's tools -- satellites can scan the atmosphere from above it and determine profiles of temperature, humidity, and ozone concentration as well as images of cloud cover. Weather radar is now mostly used to determine local patterns of precipitation.

Even with this background of worldwide data, however, the weather forecasting task remains profoundly complex. Several prominent meteorologists have maintained that the weather forecasting problem is the most difficult that exists in nature except for that of understanding the human condition. The atmosphere is big, has many external and internal interactions, and is constantly changing on scales that range from a few meters to the span of the entire Earth. The physical laws governing atmospheric behavior are reasonably well understood, but the problems associated with applying them to all parts of the atmosphere simultaneously and throughout an extended period of time are among the most formidable found in all of science. It is not surprising that weather and climate researchers are among the best customers for manufacturers of the world's largest and fastest supercomputers.

• Figure 7-12 A Synoptic Weather Map

  U.S. Weather Service Map. From NCAR, National Weather Service and NOAA.

  

As it is practiced today, weather forecasting utilizes a number of different approaches. The traditional method relies on the synoptic maps and so is referred to as the synoptic approach. From the existing pattern of highs, lows, fronts, temperature and humidity data, and the relation between surface conditions and those at height, the meteorologist applies his or her experience and knowledge of prevailing and historical weather behavior to predict the motion and development of fronts and storms. Initially, this practice was as much art as science, as it often relied upon the forecaster's experience and intuition, and the best practitioners often found it difficult to teach their skill to others.

Another approach uses what climatologist Stephen H. Schneider.; calls a kind of meteorological uniformitarianism -- weather records are searched to find close matches to the present synoptic maps. The weather that resulted from the past situation can then be used as a guide to forecasting the weather at the present time. This method is called the analog approach. It simply assumes that similar conditions will give rise to similar weather. While this is often the case for a broad-brush view of the weather, differences in details of past and present conditions can sometimes lead to substantial differences in the resulting weather.

A somewhat different method is the statistical approach that uses statistical analysis to relate different kinds of data to valid predictions on the basis of past records. Present data are then used to make predictions using the statistical guidelines.

More recently, numerical weather models have been used to supplement the other three approaches. Here, a synoptic description of the state of the atmosphere in terms of pressure, temperature, humidity, wind velocity and other variables measured at several different elevations are fed into a computer which then calculates the evolution of the weather system as time goes on. In order to do this, the computer uses the physical laws that govern interactions between all the different parts of the atmosphere and between them and external influences such as the Sun and the oceans. We shall discuss numerical models and how they are constructed in some detail in the next unit.

At present, fairly reliable weather forecasts can be made covering a span of about two days, but only large-scale events such as temperature changes and the arrival of fronts can be predicted with accuracy. Smaller scale phenomena such as local thunderstorms and showers, hail, and downpours can only be predicted in a statistical sense. A forecast that predicts a 20% chance of precipitation for your area means that there is one chance in five that you will encounter measurable precipitation during the time covered by the forecast wherever you happen to be. Very transient and localized phenomena such as tornadoes can hardly be predicted at all, but once they have formed and are sighted, they are tracked and tornado warnings are sent out to people in their vicinity. Similarly, conventional weather radar can track thunderstorms and help to predict where the most severe activity is likely to occur during the next hour or so.

Noise in the data actually makes the first 24-hour forecast less reliable than that for the second 24-hour period. Useful but less accurate forecasts may be made out for five to seven days.

HM 7-13 (A-F) Near real-time satellite derived data which present a short term (24 hour) weather prediction (all images from NOAA)
A) Wind directions and speeds.	B) Variation in vorticity.
C) Surface precipitation.	D) Atmospheric pressure.
E) Air temperature.	F) Tropospheric pressure.

RECOMMENDED READING

The principles of meteorology are well presented in Batan (1979) and Moran et al., (1994). Fishman and Kalish present the ongoing revolution in meteorology, resulting from supercomputing and advanced analysis techniques, in a very interesting and engaging manner. Frazier (1982) presents an interesting article on deadly weather. Hartmann (1994), Sanderson (1990) and Musk (1988) give descriptions of weather systems and climatology in a clear and interesting fashion. Specifics and details regarding the atmospheric system are presented in Salby (1996). Fein and Stephens (1987) present a series of papers discussing the historical and meteorological aspects of the Monsoons. Baker (1990) discusses satellite observation of the earth, past, present and future. Smith (1997) successfully presents "geodesy made easy", a wonderfully accessible introduction to this important field of study.

• D. James Baker, Planet Earth, the view from space, Harvard University Press, 1990.
• Louis J. Battan, Fundamentals of Meteorology, Prentice-Hall, Inc., 1979.
• Jay Fein and Pamela Stephens, editors, Monsoons, Wiley and Sons, 1987.
• Jack Fishman and Robert Kalish, The Weather Revolution, Plenum Press, 1994.
• Kendrick Frazier, "Storms on a Deadly Scale," Mosaic, Nov.-Dec. 1982.
• Dennis Hartmann, Global Physical Climatology, Academic Press, 1994.
• Leslie Musk, Weather Systems, Cambridge University Press, 1988.
• Joseph Moran, Michael Morgan, and Patricia Pauley, Meteorology, Fourth Edition, Prentice Hall, 1994.
• Murry Salby, Fundamentals of Atmospheric Physics, Academic Press, 1996.
• M. Sanderson (editor), UNESCO Sourcebook in Climatology, UNESCO, 1990.
• James Smith, Introduction to Geodesy, Wiley-Interscience Publication, 1997.