Ozone exists in various parts of the atmosphere; the usual measurements look at the total amount of ozone from the top to the bottom of the atmosphere. This is so we don't have to fly up to various altitudes to take ozone measurements. All we have to do is measure the total absorption of UV from the sun and this will give us the amount of ozone along the path of the sunlight. The total column ozone value will not tell us the ozone concentration vs. altitude directly, but we can extrapolate it from the total column value and the known average variation with altitude.

Most of the ozone in the atmosphere resides in the mid to upper stratosphere, about 20-40 km above sea level, with the maximum around 25-30 km altitude. The amount of ozone in this region of the stratosphere is higher in the polar latitudes than the equatorial latitudes (see next screen), due to the variation of the sun's intensity (the steeper the sun's angle [near the Equator], the higher the sunlight intensity, and the greater the amount of O3 loss through UV absorption).

The global average "normal" amount of ozone is 300 Dobson Units (DU), somewhat higher around the poles and somewhat lower in tropics. Because sunlight intensity varies with the season, the winter hemisphere ozone has a higher total column amount than the summer (we will see later that this is the opposite of the ozone concentration variation in tropospheric smog---in that case, the sunlight helps form the ozone, not destroy it).