The balanced atmosphere

Earth's Heat Budget

In the atmosphere, heat is transfered by conduction, convection, and radiation.

Conduction:

transfer of heat from molecule to molecule within a substance.

flow: warmer --> colder regions

Convection:

transfer of heat by the mass movement of a fluid (liquid or gas).

Radiation:

radiant energy or radiation travels in the form of waves that release energy when they are absorbed by an object.

All things, no matter how big or small, emit radiation.

The wavelength (or frequency) that each object emits depends primarily on the object's temperature:

• sun's temperature - 6000 K (10,500 F) --> solar radiation also called shortwave radiation;
• earth's temperature - 288 K (15 C, 59 F) --> infrared radiation also called longwave radiation.

Gases in our atmosphere are selective absorbers --> that is, they are good absorbers of certain wavelengths but may not be a good absorber is all wavelengths.

Good absorbers are also good emitters at that particular wavelength.

Water vapor and carbon dioxide are strong selective absorbers of infrared radiation and poor absorbers of solar radiation. These molecules are also strong emitters at the same wavelength.

The following figures illustrate the main terms of the mean global energy budget.

We will follow a beam of solar energy and study its interaction with the atmosphere and the Earth's surface. Assume the beam contains 100 units of energy as it enters the atmosphere:

On a worldwide, long-term average, air molecules absorb 16% of the incoming sunlight and reflect another 6%. Clouds remove another 24%. 54 units remain at this point.

Complete solar portion of the Earth's energy budget. 4 of the 54 units are reflected back to space. Therefore, the 50 units left must be absorbed by the Earth's surface.

Upward longwave radiation from Earth's surface is largely absorbed by clouds and certain atmospheric gases. Some, however, escapes directly into space. In normalized units, the Earth's surface radiates 105 units.

The Earth's energy budget including all solar fluxes plus upward longwave radiation from the Earth's surface. Atmospheric gases (water vapor, carbon dioxide, etc.) and clouds absorb 99 of the 105 units emitted by the Earth's surface. Note that the net flux at the Earth's surface is negative (50-105 = -55) while the atmosphere shows a large surplus.

Greenhouse gases (water vapor, carbon dioxide, methane, tropospheric ozone, CFCs, and nitrous oxides) and clouds emit radiation upward (back to space) and downward (85 units) to warm the Earth's surface. This behavior is commonly referred as the Greenhouse Effect.

The Earth's energy budget including the sensible heat flux, transfered through the processes of conduction and convection, and the latent heat flux , transfered through the process of phase change (evaporation, etc.) In a balanced budget, the energy storage is neither increasing or decreasing, that is: Energy Input - Energy output = 0 The sum of the inputs equals the sum of the outputs; the budget balances, and the entire system is neither warming nor cooling.

The Alistair B. Fraser's Bad Greenhouse page. This page does not discuss that the "greenhouse effect" is bad but it is intended to replace bad science with good science.