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Name: Ariel O.
Status: educator
Age:  50s
Location: N/A
Country: N/A
Date: 2000-2001

Is the heating of the atmosphere by Earth's surface a result of long wave radiation from Earth's surface, or is it a result of a direct contact with the warm Earth's surface? Which of these too effect is more important? Also, air is cooler as we go up in altitude as a result of adiabatic cooling. Is this the most important factor, and how much of this cooling a result of simply being far from Earth's surface? Ariel -

The earth's atmosphere is rather transparent to radiation. Atmospheric heating is largely the result of conduction.

There are two terms - standard lapse rate and adiabatic lapse rate. Standard lapse rate is likely what you are referring to and is measured by traveling up through the atmosphere. Adiabatic lapse rate requires a lifting force which causes a parcel of air to move up and reduce pressure causing a drop in temperature. If there is no lifting force, there is no adiabatic cooling.

Larry Krengel

Dear Ariel-

You have asked a fairly simple question...that has a very complex answer..! The "heating of the atmosphere" has several sources, and is part of the earth's radiation balance. There are several "heat exchanges" occurring simultaneously between the earth/atmosphere/space interface. I'll try to simplify the radiation balance equation as much as possible, and use percentages rather than the actual numerical values of the radiation.

First, we must assume that there is in fact a "balance"...i.e., that the heat gain and loss from both the earth and atmosphere equals out over time. Otherwise, the earth would be warming or cooling (which in fact may very well be happening), but we will assume that there is a zero balance from incoming and outgoing radiation. It would be much easier explained if I could use a diagram or graphic, to draw the respective radiation exchanges, but I'll try to use a table, similiar to a auditor or bookkeeper's journal, such that the income and outgo balance. I'll detail the heat gains at the earth, and the heat losses from the earth, and then the heat gains to the atmosphere, and the heat losses from the atmosphere.
Earth Heat Gains
short-wave radiation from the sun...............34.7%
long-wave radiation from the atmosphere.........65.3%

Earth Heat Losses
Long-wave radiation to the atmosphere...........75.5%
Long-wave radiation to space.................... 4.1%
Evaporation from oceans/lakes/land..............15.6%
Convection and conduction to atmosphere......... 4.8%

Atmospheric Heat Gains
Short-wave radiation from the sun...............11.9%
Heat to atmosphere from condensation............14.4%
Heat to atmosphere from convection/conduction... 4.4%
Long-wave radiation from earth..................69.4%

Atmospheric Heat Losses
Long-wave radiation Radiated to Space...........40.0%
Long-wave radiation radiated to earth...........60.0%

These values derived from graphs and charts from the textbook "Meterorology Today," (4th Ed.) by C. Donald Ahrens.

This is a greatly simplified summary of the radiation balance. Each of the above factors involve other effects, such as albedo, solar angle, selective absorption of radiation by atmospheric gases such as carbon dioxide, ozone, water vapor,, scattering and reflection of solar radiation, etc.

You can see some interesting observations from the above data though. One suprise may be that the earth receives almost twice as much heat from the atmosphere as it does from the sun. But the sun only shines on a given point on the earth half the time, while the atmosphere radiates continuously.

The earth receives about one-third of its heat directly from the sun, but less than 5 percent of the heat from the earth is radiated back to space or about the same amount that is involved in conduction and convection in the atmosphere.

The atmosphere receives about one-eighth of its heat directly from the sun in the form of short-wave radiation absorption, but receives almost 70 percent, or 6 times as much heat from the earth, in the form of long-wave radiation.

The atmosphere receives 3 times as much heat from the latent heat of condensation as it does from conduction/convection.

The atmosphere loses heat in only 2 ways-radiation to space, and radiation back to the earth.

As to your second question...the air is warmer close to the earth's surface, because that is where most of the heat is added. The heat is added to the upper levels by convection and long and short wave absorption, and by condensation of water vapor. Adiabatic processes are important in some types of cloud formation.

Dale Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

Earth's surface is primarily from long wave radiation. The very lowest part of the atmosphere, called the boundary layer (up to about 1.5 kilometers on a normal day) warms from the warmer earth surface (mostly by long wave radiation), with the warmer air being mixed upwards by convective air parcels (air warmer than the surrounding air) and mechanical turbulence (from wind). As the air rises (and expands, thereby decreasing in air pressure) it cools, resulting in the adiabatic lapse rate that you mentioned. Since temperature and pressure are roughly proportional for the same volume of air, the temperature decreases with height (during the day).

Long wave radiation is absorbed by the air, primarily water vapor, carbon dioxide, and methane in the air (so-called "greenhouse gases"). Since there is a greater concentration of these gases near the earth's surface, more is absorbed closest to the surface and less with increasing height. Therefore, for this reason also, during the day, the temperature decreases with height in the boundary layer.

So, adiabatic cooling and the distance from the earth's surface are both important. Long wave radiation is most effective in heating the air, with direct contact of the air with the ground being less important.

At night, long wave radiation continues to be lost from the Earth's surface (a lot during cloudless nights) and often causes the temperature at the surface to become less than at greater heights, causing a slight increase in temperature with height (called an inversion) and a decrease in the height of the boundary layer to as little as 300 meters.

David R. Cook
Atmospheric Section
Environmental Research Division
Argonne National Laboratory

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