Blackbody Radiation

The physics of atmospheric science is primarily concerned with how matter and energy get from one place to another. We will start with the transfer of energy through light, then we will move on to the random motion of molecules, and we will finish with the large scale motion of fluids.

The primary source of energy for our planet is the light from the Sun. Our Sun is heated by the fusion of hydrogen deep within it's core. The surface of the sun is a balmy 6000 K, and it glows reddish yellow at that temperature.

A hot object, like the sun, or a stove burner, glows because the molecules and atoms within the object are vibrating and moving rapidly. These atoms and molecules are made up of charged particles, and the movement of charged particles causes the emission of radiation. This is called blackbody radiation. 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.

Because the sun is very hot (approximately 6000 K or 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 or 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.

Not only does a hot object give off higher energy light, but it also gives off more of it. The power (amount of energy per second) per unit area given off by a hot object is given by

It is important to note that the infrared, visible, and ultraviolet light are only a small part of the electromagnetic spectrum. (For a review, see 411A, Module 3, Unit 7.)