(S-1) Sunlight and the Earth

The Sun is the brightest and most familiar object in the sky. Life on Earth would not be possible without it:

• The food we eat exists because of sunlight falling on green plants, and the fuel we burn comes either from such plants, or was accumulated by them (in the forms of coal, oil and natural gas) long ago.

• The Earth would probably not be fit for life. Life as we know it needs liquid water, and Earth is the only planet to have it: without the Sun, Earth would be an icy rock in space. Even now, Earth is probably the only place in our solar system fit for life: any water on Venus and Mercury would become steam, any on Mars or on more distant planets would freeze.

All hot substances radiate light, either the visible kind or beyond the rainbow spectrum, in the "infra red" (IR; "below red") and "ultra violet" (UV; "above violet") ranges. This glow [called "black body radiation" by physicists--the glow of a body with no color of its own] is the way a red-hot piece of iron or the filament in an electric light bulb produce light. The hotter the object, the brighter it shines, and the further away from red is its color. Conversely, the color of a hot object (if it is dense) tells us how hot it is. In the case of the Sun, the color of the photosphere suggests a temperature of 5780 degrees Kelvin (degrees Celsius measured from the absolute zero, about 5500° C.)

The Heating of the Earth

Because the ground is nowhere as hot as the Sun, its emission is also much weaker. However, at any location the ground sends out radiation in all directions in the half-sky that is visible, while receiving radiation only from the small solar disk, covering only a small circle in the sky, 0.5 degrees across. Because of this, the total energy any area receives should be equal to the total energy it returns back to space.

Think it over! If all of Earth's heat comes from the outside (neglecting internal heat), and if it maintains a steady temperature, no other way exists. Of course, only the average temperature is steady. Actually the ground is heated only in the daytime, but radiates back day and night, so nights, when energy only goes out and hardly any comes in, are cooler than days.

The "Greenhouse Effect"

• Clouds in the atmosphere reflect some of the sunlight before it reaches the ground, reducing the heating of the ground.

• The atmosphere absorbs the infra-red (IR) light radiated from the ground and thus delayes the escape of heat to outer space, keeping the ground warmer than it would otherwise be.

The second process is stronger, so the net effect is that like a blanket, the atmosphere helps keep Earth warmer than it would be otherwise. This is called the "greenhouse effect," because the same process operates in greenhouses used for growing vegetables in cold climates. A greenhouse is enclosed and roofed by glass panes, which let sunlight enter, but absorb the IR emitted back by the ground, and thus keep the greenhouse warm.

The chief absorbers of IR in the atmosphere are not nitrogen and oxygen, the main constituents of air, but a relatively minor percentage of "greenhouse gases" such as water vapor (H₂O), carbon dioxide (CO₂) and methane (CH₄), which are strong absorbers of IR.

Another molecule, responsible for an important effect even though only a very small amount of it is present, is ozone, a variant of the oxygen molecule--O₃ rather than the usual O₂ produced at high altitudes, with its peak around 25 kilometers. It is also a greenhouse effect, but more important, it absorbs the Sun's ultra-violet (UV) light, which on can cause skin burns and hurt eyes. The ozone found near the ground and forming part of the urban air pollution comes from a completely different process.

. High altitude ozone is destroyed by the presence of chlorine, and recently attention has been drawn to ozone removal by chlorine produced by escaping refrigerant gases, of the types preferred until recently for use air conditioners, refrigerators, aerosol cans and also some industrial applications. These gases are very, very stable, and can persist in the atmosphere for many years. Unfortunately, sooner or later their molecules wander into the stratosphere, where the ultra-violet sunlight is capable of breaking them up and releasing chlorine. Because of the damage from these gases to the ozone layer, their use is being phased out.

The greenhouse effect helps keep Earth at temperatures comfortable for life, but that is a finely balanced situation. In the last half century, the burning of fossil fuels--coal and oil-- has steadily increased the atmospheric content of CO₂. The average temperature of the Earth has also risen, and this rise is believed to be due to the added CO₂.

Further Exploring:

Site on the Earth's raditation balance, with several subfiles.
Site on the global solar heat budget.
Similar to the above, from a university course.

A hurricane viewed from space.

Rising air expands, and expansion of a gas cools it down, which is why mountaintops are cooler. Ultimately, a height is reached where not enough air remains on top to stop the IR radiation from escaping to space. The air then cools by radiation and stops rising, producing a relatively stable layer of the atmosphere known as the stratosphere.

Just below the boundary of the stratosphere ("tropopause"), air which has cooled is forced down again by warmer air rising from below. The result is a circulation of air, rising hot and returning cold, going around again and again, a motion known as convection. On a cold winter day such convection also occurs in homes: near poorly insulated windows the air cools and descends (as the flame of a candle will show--but careful with that fire!), while further inside the room it rises again. The region between the ground and the stratosphere where convection and weather take place is known as the troposphere.

Sunlight also evaporates water--from the oceans, from lakes and rivers and from green plants. Energy is invested in turning liquid water into vapor, and therefore humid air has more energy stored in it than dry air.

The capacity of air to hold water vapor depends strongly on temperature, and is smaller in cold air (just as less sugar can dissolve in cold water). As warm humid air rises, it expands and cools, and since it then cannot hold as much water as before, the excess is forced out: Initially into the tiny droplets of clouds, then if the cooling is more drastic, into raindrops.

The remaining air is drier and warmer--warmed by water vapor turning back to liquid and returning energy to its surroundings--and warmer air is better able to radiate its heat into space. That is how water, clouds and rain play a major role in the transport of solar heat from the ground back to space and help create the complex patterns of weather and climate.

Additional Exploring