Class Notes - Deserts
Introduction
Take a few minutes and review pressure, forces, and winds in the desert link given in the Chapter Heading.
Begin by reviewing an animation that illustrates the effect of inclination of the Earth's axis of rotation. Inclination denotes the change in the tilt of the Earth's spin axis relative to the plane in which it orbits the Sun (the ecliptic plane).
The atmosphere serves to distribute the energy of solar radiation received by the Earth. Most of the radiant energy is converted into atmospheric heat energy before it is radiated back into space. Winds redistribute this energy, dissipating more of it in the process than by all other forces combined. Both the short-term fluctuations of the atmospheric system (the weather) and the longer-term
fluctuations of average weather (the climate) are an important part of the history of the Earth.
The atmosphere is relatively transparent to the incoming visible rays of the sun. Much of this radiation is absorbed by the Earth's surface and re-emitted as invisible heat rays. Just as a hot pavement radiates heat as it is warmed by the sun, the Earth's surface radiates heat back to the atmosphere.
If incoming solar radiation is 100 units:
- 16 units are absorbed by dust, water and ozone;
- 3 units are absorbed by clouds;
- 6 units are reflected back by air;
- 20 units are reflected by clouds;
- 4 units are reflected by the surface.
- Therefore a total of 49 units are removed from the 100 units. The Earth receives the remaining 51 units. Of the 49 units, 30 are reflected back into space. The albedo is a measure of the "reflectivity" of the surface - 30 units in the example given above.
If the albedo changes the temperature will change; a decrease in albedo will result in a net heating and vice versa.
The Electromagnetic Spectrum
The radiation that makes up the 100 units consists of a range of wavelengths including the infrared (longer than red visible light), the visible (ranging from long wavelength red to short wavelength violet), and ultraviolet. Take a few minutes to review the characteristics of the Electromagnetic Spectrum..
Note that radio waves are very long and x-rays are very short. The distance from crest to crest is the wavelength. The maximum height (at the crest) is the amplitude. The time interval between successive crusts is the period of the wave. The frequency of the wave is 1/period. A wave moves through a distance of one wavelength in a time of one period. The speed of the wave is the wavelength times the frequency. Since the speed of light is a constant, an increase in wavelength will be accompanied by a decrease in the frequency.
The Ozone hole
Ozone (O3) is a very small component of the Earth's atmosphere, but it plays a very important role.
Ozone absorbs radiation in the ultraviolet part of the spectrum (wavelengths less than the blue part of the spectrum), thus shielding the surface from harsh radiation. Anything that would promote destruction of ozone would be threat to the Earth.
Try this simulation of ozone formation and destruction in the stratosphere. The first scene is a cartoon. The second explains the action of UV light in breaking upoxygen molecules and forming ozone. The third shows the role of chlorine in the destruction of that ozone.
Greenhouse Gases
Much of the energy absorbed by the Earth is re-emitted. Much of this outgoing radiation is in the infrared (heat - longer wavelengths than the red part of the spectrum) and most is absorbed by water, carbon dioxide, and clouds in the atmosphere. If the amount of an absorber such as carbon dioxide increases, more radiation is absorbed by the Earth's atmosphere. The atmosphere radiates heat back to the surface. This is sometimes called the greenhouse effect (as the glass in the greenhouse allows sunlight to enter and blocks the escape of infrared radiation). However, in a greenhouse the glass roof prevents dissipation of heat by convection. This is not happening on Earth.
Many scientists are concerned about the increasing emission of infrared absorbers into the atmosphere and the potential for global warming. As the amount of these absorbers increases, Earth temperature will increase.
In the absence of the greenhouse effect it is estimated that the temperature of the Earth's surface would be less than 0 degrees centigrade .... ice would be the stable form of H2O. If the surface of the Earth were covered with snow, its high reflectivity would result in an increased albedo and the surface would cool.
In the following animation climate changes from stable to metastable - oscillating. Eventually the Earth becomes a Snow Ball.
Perhaps volcanism added water and carbon dioxide to the atmosphere thus causing a warming effect.
Dan Schrag and Paul Hoffman have revived the theory that the Earth was frozen over about 1 billion years ago. Microscopic fauna could have survived in oceans with thin ice cover and local ice-free patches. But macroscopic life could not get going until after the return to a warmer climate. Theories such as this represent the "bleeding edge" of current research. Students should check back after some months or years to see how well they have stood the test of time!
Winds and Deserts
Air pressure is a measure of the density of air exerted on its surroundings. At sea level this is 14.7 pounds per square inch (or 1 atmosphere) which is about 1 bar. When air is heated, its density decreases and the air rises as it expands. Warm air, therefore, exerts a lower pressure than cold air. When air is cooled it contracts, increases in density and sinks. Cold air exerts a higher pressure than warm air.
Air flows from high pressure areas to low pressure areas in an attempt to equalize pressure. At the equator the pressure is lower and at the poles it is higher. Therefore, other things being equal, air should flow from the poles to the equator. If the Earth did not rotate, wind would flow from the North to the South. Because of the rotation of the Earth (from the West to the East), the winds are deflected to the right of their normal path. That is, if you are at the North Pole the winds would blow due South; however, the rotation of the earth causes the winds to be deflected to the right. If you are at the South Pole the winds are deflected to the left. This is called the Coriolis effect.
The wind circulation pattern on Earth is not that simple because of the imposition of large convection cells related to solar heating. The Earth receives maximum solar radiation at the equator. This warm air rises and cools, and because cool air can contain less water vapor, rainfall is concentrated at the equator. The dry, cooler air flows North and South from the equator. When it is between 20 to 30 degrees N and S it begins to sink and warm up. This warm, dry air begins to flow toward the equator. Many deserts are located in these regions. The warm, dry air can hold a considerable amount of moisture, thus preventing rainfall.
In addition, many deserts form on the windward side of high mountains. When air is forced over the mountains it cools and precipitation falls - the mountain acts as an effective rain shadow. The warm, dry air that descends on the other side favors the formation of an arid environment.
Suggestion
Find a map showing the distribution of Earth's deserts. Try and account for the placement of the major deserts using the model described in these notes and in the text.
Lake Storage of Solar Energy
Electromagnetic radiation travels with the speed of light - 186,000 miles per sec. Since the sun is about 93,000,000 miles from the Earth, how long does it take for sunlight to reach the Earth's surface.
about 8 hours
about 8 minutes
about 500 hours
The amount of energy falling on the surface of the Earth depends on a number of factors; the angle between the sun's rays and the surface, the length of exposure, and the nature of the cloud cover. In the middle of the summer about 1,000 calories per square centimeter reach the upper atmosphere. (1 calorie is the amount of heat needed to raise the temperature of 1 gram of water 1 degree centigrade)
Absorption and scattering reduce the incident radiation by as much as 20% and cloud cover will reduce it further still.
When sunlight falls on a body of water the radiant energy is gradually absorbed through a surface layer many meters thick. About half of the energy, however, is absorbed in the top 1 meter.
In changing from a liquid to the gaseous state the more rapidly moving water molecules leave the surface of the water carrying energy with them. This lowers the temperature at the surface of the body of water. The amount of heat required to evaporate a gram of water is about 600 calories. The rate of evaporation depends upon the temperature and the vapor pressure. Wind promotes evaporation because it sweeps the vaporized water molecules away which allows more evaporation to occur.
A portion of the incoming energy is reflected back (back-radiation) into the atmosphere.
This is an overestimate of the temperature because some of the energy is lost at night. However, it does indicate that under the right conditions, a standing body of water can store energy.
If the cloud cover is heavy, the incident radiation is reduced but the evaporative transfer can remain the same. Thus, in some time periods the lake will be reduced in energy.
Daisyworld
Follow through the logic of the following discussion of Daisyworld.
Predictions of climate change depend on feedback loops. One type of a feedback loop is
called a vicious circle. The most famous example of a vicious circle is blood feuds: a
member of one tribe or family is killed by another tribe or family, so that injured party strikes back, often killing more than one. This in turn leads to a response, often more severe, which leads to more and more killing.
Similarly, some people worry that the increase in global temperature could begin a cycle of increases where the earth will get warmer and warmer until all the water on the planet boils off and the atmosphere becomes pure carbon-dioxide. This condition is called 'runaway greenhouse effect' and accurately describes what happened on Venus.
The cycle would proceed as follows: the carbon-dioxide that people have added to the atmosphere leads to increased temperatures, this temperature increase leads to more water being evaporated, which raises temperatures more and (since over 99% of the world's carbon is stored in the ocean) the evaporation leads to more carbon being released into the atmosphere.
Vicious circles can be broken when there is some type of negative feedback or force that brings a system to balance instead of continuing the change. For example, feuds can be broken when the level of grief overwhelms the survivors and stops the cycle.
Similarly, the Earth might be able to stop the vicious circle of climate warming by having a negative feedback. One source of negative feedback could come from the adaptation of life systems (or the biosphere). This possibility has been explored by James Lovelock who calls his theory the Gaia hypothesis. This hypothesis claims, for example, that if the level of CO2 in the atmosphere increases, then plant life will increase in order to absorb the CO2 and convert it back into oxygen through photosynthesis.
Imagine a world populated only by black and white daisies. As with life on earth, the
daisies are at the mercy of the climate and can only survive moderate temperatures.
However, just like the rainforests, even the modest little daisies can effect the
environment by absorbing varying amount of sunlight, altering the temperature.
On Daisyworld there are black daisys and white daisys. When the temperature is high white daisys flourish as they reflect heat (increased albedo) and the temperature is lowered. At lower temperatures black daisys flourish as they absorb heat (lowered albedo) and the temperature increases. Lovelock hypothesized that such a simple world could (under certain conditions) reach a steady state such that the temperature is stabilized at a value which allows both kinds of daisys to grow.
At temperatures lower than 5oC and higher than about 40oC daisys cannot survive.
What happens is based on the assumption that
daisies have a set temperature where they grow best. Think of this optimum temperature as the "goal". If the temperature is too low, then conditions will shift that raise the temperature and vice versa.
If the earth is colder than this optimum
black daisies will grow more than white daisies, since black objects absorb more of the sun's
energy than white objects do.
Having more black daisies causes the earth to absorb more energy since black objects absorb
more sunlight than white objects. This increase in energy absorption leads to higher
temperatures on earth. Unchecked, the temperature would increase beyond the optimum value. Now the feedback occurs -- and white daisies begin to increase in amount thus lowering the temperature.
Alternatively, if
the temperature were higher than the optimum daisy temperature white daisies will grow
better since when they reflect more of the sun's energy than the black daisies do. Having
more white daisies means the earth will absorb less energy (since white reflects better than
black) which will in turn lower the earth's temperature. This lower temperature again serves
to bring the earth's temperature closer to the temperature daisies prefer.
This feedback cycle
shows how living systems on earth can create feedback loops that regulate the earth's
temperature and cause it to gravitate towards temperatures which are best for their growth.
Can you think of other processes that involve both positive (ever increasing) and negative (decreasing) feedback loops? What about population?
Copyright by John C. Butler, July 29, 1995