In addition to profile measurements, there is a third way to measure ozone. It is the one you're most likely to have encountered in the media. It is called a Dobson Unit. Instead of measuring the amount of ozone at a particular height, the Dobson Unit is a measurement of the total amount of ozone in a column extending vertically from Earth's surface to the top of the atmosphere. This measurement of ozone is directly related to the amount of UV light reaching the surface. It is therefore a measure of UV exposure received at the surface: the less total ozone in the column, the more UV light penetrates, the faster you get sunburned. While the consequences of less ozone are rather straightforward, understanding the unit in which column ozone is measured will require a bit of explanation.

Let's start at the top of the atmosphere. Let's also say we've created a special net that collects ozone molecules but lets every other molecule that makes up the air (e.g. nitrogen, oxygen, argon, water vapor, etc.) pass right through. We pull our net straight down from the top of the atmosphere to the ground and stack up our ozone molecules nice and neat at the surface where the temperature is 0°C and the air pressure is standard sea level pressure, 1013.25 mb, also given by 760 mm or 29.92" of mercury (Hg). The Dobson Unit is a measure of the height of our stack of ozone molecules: 1 DU = 10-5m. A typical midlatitude air column contains about 300 DU of ozone, which is equivalent to a stack of ozone molecules about 3 millimeters high, about the thickness of two stacked pennies, as is shown in Figure 3.03.

For comparison, a column taken inside the Antarctic ozone hole (see Chapter 10) contains only about 100 DU of ozone, a stack of molecules about the height of a dime.