(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.

How sunlight is created

The Sun has no sharply defined surface like that of the Earth, because it is too hot to be anything but gas. Rather, what appears to us as the surface is a layer in the Sun's atmosphere, the "photosphere" (sphere of light) which emits light ("radiates") because ot its high temperature.

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

Sunlight carries energy, which warms up the Earth and is the driving force behind all our weather and climate. As the ground is heated by sunlight, it begins to radiate, but being too cool to radiate even a dull red, its radiation is in the infra-red range. A hot pot or a hot laundry iron also radiates IR, and your hand can easily sense that radiation (as heat), if held close without touching.

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"

The actual flow of heat is complicated by the atmosphere, which like a blanket helps keep Earth warmer than it would be otherwise. It does so by absorbing the infra-red (IR) light radiated from the ground and thus delaying the escape of heat to outer space. 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. In the high stratosphere (see further below), solar ultra-violet (UV) light is absorbed by ozone (O3), a form of oxygen produced there (in rather small quantities) by the UV itself. The ozone found near the ground and forming part of the urban air pollution comes from a completely different process.

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.


By absorbing infra-red (as well as by its contact with the hot ground), air heats up. As hot air expands, each cubic meter (or cubic foot) of it weighs less than before heating. Where the heating is most pronounced, the warm air is more buoyant than the cooler air surrounding it, and tends to float upwards: soaring birds and glider pilots seek such "thermal currents and allow themselves to be carried upwards by them. This buoyancy is the basic process responsible for weather.

 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.

Next Stop: (S-2) Our View of the Sun

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Author and curator: David P. Stern
Last updated 20 August 1999