Radiation in the Atmosphere (Cont.)

Cloud Effects

(2) Cloud effects – occur only when clouds are present. (a) Absorption of the radiant energy by the liquid water droplets and ice

crystals in clouds. A (b) Reflection of the radiant energy back

to space by the liquid water droplets and ice crystals. R

Surface of the Earth

Clear sky

In a clear sky more solar radiation reaches the surface and warmer temperatures occur

58°F

AR

46°F

In a cloudy sky less solar radiation reaches the surface and cooler temperatures occur

Cloudy sky

Cloud Effects (Cont.)

The amount of solar radiation affected by clouds depends on the thickness of the clouds and their composition.

Thick clouds tend to absorb and reflect more solar radiation than thinner clouds.

Clouds with more liquid water tend to absorb and reflect more solar radiation than clouds with a higher percentage of ice crystals.

Cloud Effects (Cont.)

In general the net effect of clouds is to reduce the temperature in the daytime because they prevent some solar radiation from reaching the ground.

Surface Effects

(3) Surface effects occur when the solar

radiation reaches the surface of the

Earth.

(a) Reflection of solar radiation is the

primary surface effect.

Surface Albedo

The surface albedo is the percentage of solar radiation that is reflected by the surface.

surface albedo =

reflected solar radiation x 100%

incoming solar radiation

Surface Albedo (Cont.)

Incoming solar radiation = 800 W/m2

Reflected solar radiation =

200 W/m2

Surface albedo = ((200 W/m2)/(800 W/m2)) x 100%

Surface albedo = 25%

Typical Surface Albedos

Fresh snowFresh snow 75 to 95%75 to 95%

SandSand 15 to 45%15 to 45%

Grassy field 10 to 30%

Dry, plowed field 5 to 20%

Forest 5 to 15%

Measuring Solar Radiation

A pyranometer is an instrument used to measure solar radiation

Glass dome to keep dust and other contaminants off sensor. The solar radiation passes through the glass unaffected.

Sensor measures the incoming solar radiation.

If you want to measure the reflected solar radiation, you simply set out a second pyranometer turned upside down.

What happens to the incoming solar radiation?

19% is absorbed by gases and clouds 6% is scattered back to space 20% is reflected by clouds back to space 4% is reflected by the surface to space-------- 49% of the solar radiation is not absorbed by the Earth’s surface 51% is absorbed by the Earth’s surface

Other Important Surface Energy Transfers

1. Emission of terrestrial radiation by the surface.2. Absorption of terrestrial radiation emitted by the atmosphere to the surface.3. Latent energy transfer from the surface during evaporation.4. Transfer of internal energy between the

atmosphere and the surface.

Emission of Terrestrial Radiation by the Earth’s Surface

Most of the terrestrial radiation

(approximately 86% on average) emitted by

the surface is absorbed by gases in the

atmosphere.

Terrestrial Radiation

Methane

Nitrous oxide

Ozone, oxygen

Carbon dioxide

Water vapor

Sum of all gases

Absorption of Terrestrial Radiation

Water vapor is an excellent absorber of terrestrial radiation in wavelengths less than 7x10-6 m.

Carbon dioxide is an excellent absorber of terrestrial radiation in wavelengths longer than 12x10-6 m.

Methane is an excellent absorber of terrestrial radiation in wavelengths near

7x10-6 m.

Atmospheric Window

The band of wavelengths between 7x10-6 mand 12x10-6 m is sometimes called the atmospheric window because few gases in the atmosphere absorb terrestrial radiationin these wavelengths very well.Thus, terrestrial radiation emitted in these wavelengths is likely to leave the Earth’s atmosphere when the sky is clear.

Greenhouse Effect

The absorption of terrestrial radiation by the gases in the Earth’s atmosphere is sometimes called the greenhouse effect.

Greenhouse and Radiation

Solar radiation passes easily through the glass used in the walls and roof of a greenhouse.

Glass is a good absorber of terrestrial radiation emitted inside the greenhouse.

Solar radiation easily passes through the glass in the greenhouse.

Solar radiation is absorbed and stored as internal energy inside greenhouse increasing the temperature

Terrestrial radiation is absorbed by the glass and the energy stays inside the greenhouse.

The walls and sides of the greenhouse keep the cooler outside air from mixing with the warmer air inside the greenhouse.

Temperature stays warm

The Earth’s Greenhouse Effect

Roughly half the solar radiation passes through the atmosphere and is absorbed by the Earth’s surface.

Approximately 86% of the terrestrial radiation emitted by the surface is absorbed by gases in the atmosphere and only 14% escapes to space.

Incoming solar radiation

Terrestrial radiation reaching space

Terrestrial radiation absorbed by the atmosphere.

Terrestrial radiation emitted by the atmosphere back to the surface of the Earth

The Earth’s greenhouse effect occurs when terrestrial radiation emitted by the surface is absorbed in the atmosphere and then is radiated back to the surface as terrestrial radiation emitted by the atmosphere.

Effect of Clouds on Terrestrial Radiation

The liquid water droplets and ice crystals in clouds are excellent absorbers of terrestrial radiation.

Even thin clouds are capable of absorbing a lot of terrestrial radiation that would otherwise be emitted out to space.

The Effect of Clouds on Terrestrial Radiation (Cont.)

Clouds greatly enhance the greenhouse effect.

Clouds absorb almost all terrestrial radiation emitted by the surface and then radiate the energy back to the surface.

The Effect of Clouds on Terrestrial Radiation (Cont.)

The effect of clouds on terrestrial radiation has the biggest effect on temperature at night, especially in the winter.

Clear Sky

10°F

More energy is lost to space on clear nights and the temperature decreases more.

Terrestrial radiation is absorbed and radiated back to the surface.

28°F

Much less energy is lost to space on cloudy nights and the temperature decrease less.

Cloudy Sky

Terrestrial Radiation Emitted by the Atmosphere

The Earth’s atmosphere also emits terrestrial radiation.

Most of the radiation is rapidly absorbed by surrounding gas molecules.

The lower atmosphere emits terrestrial radiation back to the surface as part of the greenhouse effect.

Terrestrial Radiation Emitted by the Atmosphere (Cont.)

Gas molecules and clouds emit terrestrial radiation upwards which is lost to space.

The emission of terrestrial radiation to space is the major way the Earth and its atmosphere lose energy.

Types of GOES Images

1. Visible images

2. Infrared images

3. Water vapor images

Visible Satellite Imagery

Visible satellite imagery is constructed from the solar radiation reflected by an object to the sensor on the satellite. Objects that reflects a lot of visible light (i.e. have a high albedo) appear bright. These objects include fresh snow on the ground and thick clouds like cumuli and cumulonimbi.

Snow

Cumuliform

Cloud

High

Albedo

Bare soil (low albedo)

High Albedo

SNOW

Infrared Satellite Imagery

Infrared satellite imagery is constructed from the terrestrial radiation in the longwave band 1. Cold objects, such as the tops of cumulonimbi or cirrus clouds, emit less radiation. Generally, cold objects, are depicted as bright regions on infrared images. Warm objects, such as the Earth’s surface or low clouds, emit more radiation and are generally depicted as darker areas on infrared satellite images.

Surface T = 20°C

Cloud top

T = -45°C

Cumulo-

nimbus

Cirrus

T = -50°C

Low clouds

High clouds

Tall Thunderstorm clouds

Snow

High Surface Temperatures

Water Vapor Imagery

Water vapor imagery is constructed from emitted radiation in the upper level water vapor band. Regions of the upper troposphere with higher concentrations of water vapor emit more infrared (longwave) radiation in this band.

Water Vapor Imagery (Cont.)

Generally, more humid areas in the upper troposphere appear as bright regions on water vapor images and drier areas appear as darker regions.

Looping of water vapor images provides an indication of the flow of air in the upper levels of the troposphere.