Glossary

Dobson Unit: The unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 Celsius) and pressure (1013.25 millibars, or one atmosphere, or "atm"), the column would be about 0.3 centimeters thick. Thus, the total ozone would be 0.3 atm-cm. To make the units easier to work with, the "Dobson Unit" is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 Dobson Units (DU).
ozone: A molecule consisting of three oxygen atoms. Ozone strongly absorbs ultraviolet light with wavelengths in the range of around 290 to 300 nanometers (0.000000001 meters), which makes it useful in the stratosphere, because it protects life on earth from the damaging effects of this radiation. It is also a very reactive compound, which makes it harmful down here at the surface, because it can damage (i.e. react with) the lung tissue of those who breathe it.
ozone layer: That level of the atmosphere which encompasses a peak in ozone concentrations. As electromagnetic radiation comes in from the sun and hits the earth's atmosphere, certain wavelengths in the ultraviolet range (180-240 nanometers) are absorbed by and break apart oxygen molecules (which are made of two oxygen atoms). Some of the resulting unattached pairs of oxygen atoms then recombine into triplets to form ozone. A different range of wavelengths of ultraviolet (290-300 nm) are strongly absorbed by ozone, which breaks down as a result and reforms into molecular oxygen again. The higher up in the atmosphere you go, the thinner the air is, and thus the less oxygen there is to absorb the 180-240 nm ultraviolet to form ozone; this means that ozone amounts tend to decrease you go higher. The lower down in the atmosphere you go, the more oxygen the ultraviolet has to pass through to get there, and the greater the chances are that it has already been absorbed to create ozone somewhere higher up; this means that way down low, the ozone amounts tend to be lower. From roughly 12 to 30 km, the two tendencies balance out, and the highest ozone concentrations are found there, in what is called the "ozone layer".
Total ozone: The total amount of ozone in a column of air stretching from the earth's surface to space.

Glossary of Terms