The "Greenhouse Effect"
The actual flow of heat is complicated by the atmosphere, which has three strong effects:
- Clouds in the atmosphere reflect some of the sunlight before it reaches the ground, reducing the heating of the ground. This process, hard to estimate, has been monitored by measuring "earthshine", the faint glow from the dark part of the Moon when only a thin crescent is visible.
- 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.
- Air can flow, and thus carry its heat from one place to another. That is what produces our weather.
The 3rd process is discussed further below. It is a big subject--made even bigger by the influence of water vapor, which produces rain, hurricanes and other interesting phenomena--and two additional sections are also devoted to it, starting here, and also one (written later) stressing the energy exchange processes.
The second process (which keeps us warmer) is stronger than the first (which reduces warming), 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. It is produced at high altitudes by the action of sunlight on ordinary oxygen and its peak concentration is around 25 kilometers. It is also a greenhouse gas, 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 from the break-up of escaping refrigerant gases, of the types preferred until recently for use in 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.
Many additional details are available on the web--unfortunately, the ones that go into additional details are also usually more difficult. Some of them:
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.