NOTES

6.A Aspects of the hydrologic cycle 6.B Alternative ways of stating the humidity 6.C Saturation deficit and crop growth air's 6.D Psychrometer measurements

Plate 6.1 Moisture in the atmosphere (especially in the upper troposphere) can be detected by means of satellite photographs such as these. They came from the geostationary satellite Meteosat over the equator near Africa, at 1155 GMT in February 1988. The upper image shows radiation in the infra-red band, i.e. indicating the temperatures of the radiating surfaces, with the cold tops of high clouds being whitest and the warm land of Africa black. Notice the band of cloud at the ITCZ over the Indian ocean at about 20°S. Also, there was intense convection over much of southern Africa, except on the west where skies are shown clear as a result of the anti-cyclone over the South Atlantic ocean. Heavy cloud obscures Brazil, far to the west.

The lower image was obtained at the same time for the same area, but shows the amount of water vapour in the atmosphere, in terms of radiation of other wavelengths. Whiteness here means moisture. There is a clear resemblance to the upper image, with dry (i.e. subsiding) air over the South Atlantic ocean, but there is a more clearly defined trough line.

6.E The weather-stress index (WSI)

6.F A thermal sensation scale

6.G The Standard Effective Temperature

6.H Evaporative coolers

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