The "Dobson unit" (DU) is named after professor G.M.B. Dobson (1889 - 1976), who has from the 1920s onwards done research on the ozone layer. Around 1930 he built the first "Dobson spectrophotometer", with which reliable measurements of the ozone layer became possible.
The "Dobson unit" indicates how much ozone there is in the air above a certain point on Earth. A proper unit would thus be "Kilogram per square meter".
The unit introduced by Dobson arrises as follows. Suppose that all the ozone in the air would be in a (gas) layer just above the ground, at standard pressure (1013.25 hPa) and at standard temperature (0.0 Celsius). The amount of ozone is then indicated by the thickness of this layer, expressed in 0.01 millimeter. (This is why the ozone layer is sometimes referred to as being "thick" or "thin".) The Dobson Unit is sometimes also used for other trace gases in the atmosphere.
Averaged over the entire world the ozone column has a valie of about 300 DU. For the Netherlands this is an average of 280 DU in autumn and 380 DU in spring.
During spring on the southern hemisphere, September-November, the so-called "ozone hole" develops, with ozone values (well) below 200 DU.