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16.4: Heat Transfer in the Atmosphere

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    Heat moves in the atmosphere the same way it moves through the solid Earth or another medium. What follows is a review of the way heat flows and is transferred, but applied to the atmosphere. Radiation is the transfer of energy between two objects by electromagnetic waves. Solar radiation heats the ground, then heat radiates from the ground into the lower atmosphere.

    In conduction, heat moves from areas of more heat to areas of less heat by direct contact. Warmer molecules vibrate rapidly and collide with other nearby molecules, transferring their energy. In the atmosphere, conduction is more effective at lower altitudes where air density is higher. Heat transfers upward to where the molecules are spread further apart or heat transfers laterally from a warmer to a cooler spot, where the molecules are moving less vigorously.

    Heat transfer by movement of heated materials is called convection. Heat that radiates from the ground warms the air, which rises and initiates convection cells in the atmosphere.

    The Sun heats the ground through radiation. The ground heats the air through conduction. Warm air rises through convection.
    Figure \(\PageIndex{1}\): The transfer of heat energy through Earth's atmosphere by radiation, conduction and convection. (NOAA) Image is in the public domain.

    Energy from the Sun

    The Earth constantly tries to maintain an energy balance with the atmosphere. Most of the energy that reaches the Earth’s surface comes from the Sun. About 44% of solar radiation is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths. When viewed together, all of the wavelengths of visible light appear white. But a prism or water droplets can break the white light into different wavelengths so that separate colors appear.

    Of the solar energy that reaches the outer atmosphere, UV wavelengths have the greatest energy. Only about 7% of solar radiation is in the UV wavelengths. The three types are:

    • UVC: the highest energy ultraviolet, does not reach the planet’s surface at all.
    • UVB: the second-highest energy, is also mostly stopped in the atmosphere.
    • UVA: the lowest energy, travels through the atmosphere to the ground.
    Three types of ultraviolet light radiation: UVA, UVB, UVC. UVA wavelengths 320-400 nm; reaches surface of Earth. UVB wavelengths 290-320 nm; mostly stopped in the atmosphere. UVC wavelengths 100-290 nm; blocked by ozone layer.
    Figure \(\PageIndex{2}\): Three types of ultraviolet light emitted from the Sun. Image by Trudi Radtke team is used under a CC BY 4.0 license.

    The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we feel as heat. Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases. Ozone completely removes UVC, most UVB and some UVA from incoming sunlight. Oxygen, carbon dioxide, and water vapor also filter out some wavelengths.

    Heat at Earth’s Surface

    About half of the solar radiation that strikes the top of the atmosphere is filtered out before it reaches the ground. This energy can be absorbed by atmospheric gases, reflected by clouds, or scattered. Scattering occurs when a light wave strikes a particle and bounces off in some other direction.

    About 30% of incoming solar radiation is reflected back into the atmosphere. Another 47% is absorbed by rocks, soil, and water and then radiated back into the air as heat. These infrared wavelengths can only be seen by infrared sensors.

    Incoming solar radiation: 47% absorbed by ocean and land, 6% reflected by surface, 17% reflected by clouds, 7% scattered to space, 23% absorbed by air, water vapor, ozone, clouds, dust. Infrared radiation emitted from the surface: 24% latent heat, 7% conduction, 10% longwave radiation, 6% absorbed by water vapor, ozone, clouds, carbon dioxide (20% radiated from clouds, 40% radiated from water vapor, ozone, carbon dioxide)
    Figure \(\PageIndex{3}\): Earth's heat budget. Of all of the solar radiation reaching Earth, 30% is reflected back to space and 70% is absorbed by the Earth (47%) and the atmosphere (23%). The heat absorbed by the land and oceans is exchanged with the atmosphere through conduction, radiation, and latent heat. The heat absorbed by the atmosphere is eventually radiated back into space. Image by Paul Webb licensed under CC BY 4.0.

    Because solar energy continually enters Earth’s atmosphere and ground surface, is the planet getting hotter? The answer is no (although the next section contains an exception) because energy from Earth escapes into space through the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then average global temperature stays the same. This means that the planet’s heat budget is in balance. What happens if more energy comes in than goes out? If more energy goes out than comes in?

    To say that the Earth’s heat budget is balanced ignores an important point. The amount of incoming solar energy is different at different latitudes. Where do you think the most solar energy ends up and why? Where does the least solar energy end up and why? The table below compares day length, Sun angle, solar radiation and albedo at the equatorial region and polar regions. The difference in solar energy received at different latitudes drives atmospheric circulation.

    Table \(\PageIndex{1}\): Comparison of day length, Sun angle, solar radiation and albedo at the equatorial region and polar regions.
      Day Length Sun Angle Solar Radiation Albedo
    Equatorial Region Nearly the same all year High High Low
    Polar Regions Night 6 months Low Low High
    Effect of the Earth's shape and atmosphere on incoming solar radiation. Compared to equatorial regions (b), incoming solar radiation of the polar regions (a) is less intense for two reasons: the solar radiation arrives at an oblique angle nearer the poles, so that the energy spreads over a larger surface area, lessening its intensity. The radiation travels a longer distance through the atmosphere, which absorbs, scatters and reflects the solar radiation.
    Figure \(\PageIndex{4}\): Effect of the Earth's shape and atmosphere on incoming solar radiation. Compared to equatorial regions (b), incoming solar radiation of the polar regions (a) is less intense for two reasons: the solar radiation arrives at an oblique angle (low Sun angle) nearer the poles, so that the energy spreads over a larger surface area, lessening its intensity. The radiation travels a longer distance through the atmosphere, which absorbs, scatters and reflects the solar radiation.

    The Greenhouse Effect

    The exception to Earth’s temperature being in balance is caused by greenhouse gases. But first, the role of greenhouse gases in the atmosphere must be explained. Greenhouse gases warm the atmosphere by trapping heat. Some of the heat radiation out from the ground is trapped by greenhouse gases in the troposphere. Like a blanket on a sleeping person, greenhouse gases act as insulation for the planet. The warming of the atmosphere because of insulation by greenhouse gases is called the greenhouse effect. Greenhouse gases are the component of the atmosphere that moderate Earth’s temperatures.

    Energy from the Sun arrives to the Earth as solar radiation. Some solar radiation is reflected by the Earth and the atmosphere. Most radiation is absorbed by the Earth's surface and warms it. Infrared (heat) radiation is emitted by the Earth's surface. Some of the infrared radiation passes through the atmosphere. Some is absorbed and re-emitted in all directions by greenhouse gas molecules. The effect of this is to warm the Earth's surface and the lower atmosphere.
    Figure \(\PageIndex{5}\): The greenhouse effect. This diagram shows the Earth's greenhouse effect. The Earth absorbs some of the energy it receives from the Sun and radiates the rest back toward space. However, certain gases in the atmosphere, greenhouse gases, absorb some of the energy radiated from the Earth and trap it in the atmosphere. These gases essentially act as a blanket, making the Earth’s surface warmer than it otherwise would be. While this greenhouse effect occurs naturally, making life as we know it possible, human activities in the past century have substantially increased the amount of greenhouse gases in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth’s temperature.

    Greenhouse gases include CO2, H2O, methane, O3, nitrous oxides (NO and NO2), and chlorofluorocarbons (CFCs). All are a normal part of the atmosphere except CFCs. The table shows how each greenhouse gas naturally enters the atmosphere.

    Table \(\PageIndex{2}\): Greenhouse gases and where they come from
    Greenhouse Gas Where It Comes From
    Carbon dioxide (CO2) Respiration, volcanic eruptions, decomposition of plant material; burning of fossil fuels
    Methane (CH4) Decomposition of plant material under some conditions; biochemical reactions in stomachs
    Nitrous oxide (NO and NO2) Produced by bacteria
    Ozone (O3) Atmospheric processes
    Chlorofluorocarbons (CFCs) Not naturally occurring; made by humans

    Different greenhouse gases have different abilities to trap heat. For example, one methane molecule traps 23 times as much heat as one CO2 molecule. One CFC-12 molecule (a type of CFC) traps 10,600 times as much heat as one CO2 molecule. Still, CO2 is a very important greenhouse gas because it is much more abundant in the atmosphere. Human activity has significantly raised the levels of many of greenhouse gases in the atmosphere. Methane levels are about 2.5 times higher as a result of human activity. Carbon dioxide has increased more than 35%. CFCs have only recently existed.

    What do you think happens as atmospheric greenhouse gas levels increase? More greenhouse gases trap more heat and warm the atmosphere. The increase or decrease of greenhouse gases in the atmosphere affect climate and weather the world over.

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