The magnitude of the thickness of the ozone layer is usually called total amount of ozone or simply total ozone. It is the amount of ozone molecules contained in a vertical column with unit base extending from surface to the "top" of the atmosphere.

Traditionally, the total ozone is usually expressed in Dobson units (1 Dobson unit = 2.687 E16 molecule/m2) in memory of the British scientist G. M. B. Dobson, who in the twenties of last century developed a spectrophotometer to measure the total amount of ozone operationally, on the basis of spectrum of ultraviolet radiation. Along the year and in medium latitudes, the total amount of ozone may range between 200 D and 500 D. In the Polar Regions, and especially over the South Pole and during ozone hole events, the total amount of ozone may decrease to values below 100 D.

Ozone monitoring in Portugal is carried out by IM, which currently operates a Dobson spectrophotometer (Lisbon) e 3 Brewer automatic spectrophotometers (Lisbon, Funchal and Angra do Heroísmo). These stations are part of the GAW (Global Atmosphere Watch) network of the World Meteorological Organization (WMO), contributing to a better global observational cover and being a reference to the remote sensing systems borne by satellites (TOVS, EP-TOMS, GOME, SCHIAMACHY, etc.). The results obtained are periodically sent to various international data centres (WOUDC and NHMP-LAP), where they are made available to the scientific community.

Observations of the total amount of ozone carried out in Lisbon from 1967 to 1998 show a statistically significant trend of 3,3% for each ten years period. This trend is comparable to those found in other stations in Europe with the same kind of instruments and over the same time interval.

The total ozone predictions issued are based on the large-scale prediction model disseminated by the German Meteorological Service (DWD) in its capacity of regional specialised meteorological centre of WMO for UV index prediction.