I. Earth-Atmosphere System Energy Relationships

Here we'll examine what happens to the flow of radiant energy from the sun as it enters and travels through the Earth's atmosphere to the surface of the Earth

Heat Transfer :

• Always from warmer to cooler objects

• Forms:

1. conduction: the transfer of heat energy by molecular activity - molecule to molecule contact
2. convection: the transfer of heat energy by the movement of a mass or substance from one place to another
3. radiation: can be transfered through a vacuum

Forms of Atmospheric Radiant Energy:

Sun (shortwave): ultraviolet, visible, near IR;

Earth-Atmos (longwave): thermal IR

Basic Radiation Laws:

1. all objects emit radiant energy
2. hotter objects radiate more total energy per unit area than cooler objects
3. the hotter the radiating body, the shorter the wavelength of maximum radiation
4. objects that are good absorbers of radiation are good emitters as well

• the rate @ which solar energy falls on a surface located at the top of the Earth's atmosphere is a constant (solar constant)

• As insolation moves through the atmosphere three things can happen to it:

1. reflection - occurs at the interface between two different media (e.g. air & cloud) when some of the radiation striking the interface is thrown back

albedo - the ratio of reflected to incident radiation (an object w/albedo of 1 (or 100%) is a perfect reflector) - light colored objects have high albedos

dark objects have low albedos

2) scattering - dispersal of radiation in all directions

3) absorption - radiation is converted to heat and emits that heat (radiation) according to #2 (above)

• most absorption within the atmosphere is by oxygen, ozone, water vapor and various aersols (solid and liquid particles)

Direct Insolation - insolation that is transmitted directly through the atmosphere to the earth's surface

Diffuse Insolation - insolation that is scattered &/or reflected to the earth's surface

• the direct and diffuse insolation ("total solar")are either absorbed or reflected by the earth's surface

• (that portion which gets absorbed is converted to heat)

The (Atmospheric) Greenhouse Effect

• while the earth behaves like a black body - the atmosphere does not!!

• the gases that comprise the atmos. are "selective absorbers" - they absorb some wavelengths and are transparent to others.

• selective absorbers usually emit radiation at the same wavelength which they selectively absorb at.

important selective absorbers:

water vapor (H2O) **

carbon dioxide (CO2) **

nitrous oxide (N2O)

methane (CH4)

Ozone (O3)

these gases are poor absorbers of visible (shortwave) radiation, but good absorbers of infrared (longwave) radiation

• the greenhouse effect thus works in the following manner:

• direct and diffuse shortwave radiation from the sun is absorbed at the earth's surface

• the earth radiates longwave energy into the atmosphere where some of it is absorbed by the various greenhouse gases

• these gases gain kinetic energy and collide w/neighboring air molecules which increases the average KE of the air, which results in an increase of air temperature.

• these same greenhouse gases also emit longwave radiation - some of which is transmitted to the earth's surface where it is absorbed and thus heats the ground.

• the earth then reradiates longwave energy upward, where, once again, it is absorbed by the greenhouse gases and warms the lower atmosphere

• thus, the greenhouse gases act as an "insulating layer" keeping some of the earth's radiant energy from escaping to space; and keeping the lower atmosphere considerably warmer than it would otherwise be (~59 F higher than without the GH gases)