Session 2: Radiation Basics

Part 2:
Radiation Basics

It is extremely important to understand that all objects radiate. Actually, it is more proper to say all objects radiate if their temperature is above absolute zero. All molecules, if they are moving at all (i.e. their temperature is above absolute zero), are emitting electromagnetic waves. The wavelength associated with each object depends on that object's temperature. Basically, the warmer the object, the faster the molecules vibrate, and the shorter the wavelengths of the emitted radiation. Radiation is important to meteorology because it is the form of energy that makes the air move, clouds form, and chemicals in the atmosphere react with one another.

Electromagnetic Spectrum

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Wavelengths are measured in units of micrometers, because of the small distance between respective wave crests. The sun emits radiation on many wavelengths, but its highest intensity of energy is emitted at wavelengths from 0.4 to 0.7 micrometers. Luckily, this is the band we know as visible radiation or light (if we saw at very different wavelengths, objects would not be as distinguishable because the reflecting radiation would be significantly less). Energy with wavelengths smaller than 0.4 micrometers is known as ultraviolet, or UV radiation. Energy with wavelengths larger than 0.7 micrometers is known as infrared, or IR, radiation. Humans can see neither UV nor IR radiation. However, the radiation we feel as heat is IR radiation. The only beings that can see UV and IR radiation are aliens or cool people on Star Trek . Sometimes, you might hear about military people using IR goggles to see in total darkness. The goggles do not actually "see" IR radiation, but interpret IR radiation and present it in a band of visible radiation that humans can see.

Because the sun is very hot (approximately 6000 Kelvin (K), which is about 10000°F), it emits radiation at very short wavelengths, most of it less than 2 micrometers. In contrast, the earth has an average temperature of around 25 degrees C (288 K, 77°F). Therefore, the earth emits radiation of much longer wavelengths, generally between 5 and 25 micrometers. Thus, solar radiation is known as shortwave radiation, and terrestrial radiation (that from the earth) is known as longwave radiation. These two categories are used widely when discussing heat balances (in next session). There are, of course, many other forms of radiation, some with wavelengths on the order of 100 or so meters. A list of common radiations and their respective wavelengths are presented below.

Type of Radiation	Approximate Wavelength
AM radio waves	100 meters
Television waves	1 meter
Microwaves	1 millimeter
Infrared waves	1 micrometer
Visible light	0.3 to 0.7 micrometers
Ultraviolet waves	0.1 micrometers
X rays	0.001 micrometers

Albedo

Radiation is not only emitted, absorbed, and transferred, but also reflected. The albedo of a surface is its reflectivity, or the ratio of radiation that is reflected to that which hits the surface. Snow, for example, can reflect over 90 percent of the light that hits it, so the albedo for snow would be 0.9. Still water on the other hand, absorbs around 90 percent of the light that reaches it. So the albedo for still water would be 0.1. A chart of typical albedos for various surfaces is given below. The sum of the albedos of the many surfaces on the earth are important in determining the earth's energy balance, which is discussed in the next section.

Typical Albedo of Selected Surfaces
Surface	Albedo (%)	Surface	Albedo (%)
Snow	79-95	Dark Soil	5-15
Ice	30-40	Grassy Field	10-30
Thick Clouds	60-90	Forest	5-15
Thin Clouds	30-50	Water	10 (avg.)
Wet Sand	20-30	Venus	78
Dry Sand	35-45	Mars	17
Concrete	17-27	Moon	7
Asphalt	5-10	Earth	34-42

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Part 2: Radiation Basics

Electromagnetic Spectrum

Albedo

On to Factors Influencing Energy Balance

Back to Energy and Heat Transfer

Part 2:
Radiation Basics