Measuring Temperatures Aloft

Measurements on mountains, with kites and from manned balloons (since the Montgolfier brothers' ascent in 1783) show that not only does the pressure fall with increasing elevation, but the temperature does too.

The change of air temperature with height, i.e. the temperature profile is nowadays measured in three principal ways. The most

Figure 1.8 Variation with latitude of the annual-average mean-sea-level pressure. The curve shows the meridional profile (i.e. variation with latitude) of zonal means (i.e. averages of places at the same latitude but different longitudes).

common method since 1930 is by means of a radiosonde. This is a balloon-borne instrument package invented by Pavel Molchanov (18931941). It rises at about 6 m/s, transmitting a radio signal of the surrounding temperature, humidity and pressure as it goes. (The pressure and temperature values are used to indicate the altitude of the measurements.) Something like a thousand radiosondes are launched once or twice daily around the world. The ascending rubber balloon expands in the low pressures and is made brittle by the low temperatures, so that eventually it bursts at an altitude of perhaps 20 km. The package then descends by parachute. Some meteorological services encourage people to look for sondes which have come down and mail them back for a reward, so that they can be re-used.

A second source of information about temperatures has become available recently. Passengers on long-distance flights are periodically shown the current altitude of the plane and also the temperature outside, so that one could plot an instructive graph of temperature against height as the plane rises to cruising level, and descends from it.

In addition, meteorological satellites are nowadays used to determine the temperature profile. This is done by measurement of the amount of radiation of various wavelengths emitted by molecules of either carbon dioxide or oxygen at each level in the atmosphere, as explained in Chapter 2.

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