Low Relative Humidity in the Atmosphere

Relative humidities below 3 percent are rare and last less than 24 hours. The limitation on the duration of extreme relative humidities exists because of the diurnal cycle. The lowest relative humidities are associated with high temperatures. When air temperature decreases after sunset, the relative humidity increases because there is not a comparable decrease of water vapor content. On a day with a minimum relative humidity of 3 percent, the maximum in the diurnal cycle should be about 8 percent. When ambient relative humidity is 3 percent, the relative humidity in a hot storage container may be only 1 percent. The maximum in the diurnal cycle in the container should then be about 7 percent. The most extensive areas of low relative humidity are in northern Africa and southwestern Asia. Typical afternoon minima are in the range 10 to 20 percent over a major portion of these deserts in spring and summer. In small areas the average afternoon minimum in the driest months is slightly less than 10 percent. In some parts of the world, very rapid changes of relative humidity occur in certain weather situations. At coastal stations with well-developed sea breezes, relative humidities may increase from 10 percent to 80 percent in one or two hours. In the lee of mountain ranges, abrupt decreases of a comparable magnitude can occur with chinook foehn winds.