Measurements at the longer four wavelengths, which penetrate into the lower atmosphere, are used to obtain estimates of total column ozone.

The new satellite measurements refer to what is called "column ozone," the amount of ozone over any given spot on the earth.

Inside the hole, column ozone has been depleted by as much as 50 percent in some winters since the mid-1980's.

Total column ozone reached the minimum reading of 98 Dobson units on September 29, compared to 90 Dobson units in 1999.

The minimum total column ozone occurred around September 29th, reaching 90, 98, and 100 ?

Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere.

Through the 1990s, total column ozone in September and October have continued to be 40-50% lower than pre-ozone-hole values.

This, they said, translates to about a 6 percent reduction in "column ozone" - the total amount of ozone over any given spot on Earth.

Total column ozone indicated a 60 per cent drop from early August measurements.

We can't really expect much of an improvement in the UV levels reaching the biosphere until we can detect an increase in total column ozone.