. Compendium of meteorology. Meteorology. LONG-WAVE RADIATION 43 receives more heat than it emits. Thus it cannot exist as a cloud and must evaporate. This is probably the reason for the phenomenon observed in the tropics, namely, that the highest cirrus clouds are not found near the stratosphere, but at an altitude of about 14 km. Also the diurnal variation in the cirrus clouds {, dis- solution toward noon, re-formation toward evening), which has been occasionally observed in the subtrop- ical deserts, can probably be ascribed to the fact that the ground temperature is very high at noon [
. Compendium of meteorology. Meteorology. LONG-WAVE RADIATION 43 receives more heat than it emits. Thus it cannot exist as a cloud and must evaporate. This is probably the reason for the phenomenon observed in the tropics, namely, that the highest cirrus clouds are not found near the stratosphere, but at an altitude of about 14 km. Also the diurnal variation in the cirrus clouds {, dis- solution toward noon, re-formation toward evening), which has been occasionally observed in the subtrop- ical deserts, can probably be ascribed to the fact that the ground temperature is very high at noon [36]. A further consequence of the interaction betAveen absorption of radiation from below and emission upward is the fact that a cloud layer must develop its own in- ternal convection system. The absorption of heat in the lower portions will lead to an evaporation of the drop- lets, while the emission from the upper portions will lead to increased condensation and a descent of the heavier cloud. Thereby, the stratified cloud is resolved into individual convection cells, stratus is turned into strato- cumulus and altostratus into altocumulus. This process may take place fairly rapidly. A stratified cloud J^ km thick, at an altitude of about 5 km, is converted and "destabilized" from the isothermal state to one with a temperature gradient of per 100 m in approxi- mately twenty minutes, whereas a similar cloud at a height of 2 km requires three quarters of an hour to complete the same change [32]. Keeping this in mind, it seems scarcely credible that an ordinary cloud layer can exist unchanged in the atmosphere for any length of time without being dissolved. If, in spite of the fore- going discussion, thin, closed, and stable altostratus cloud layers are observed, it becomes clear from the radiation calculations to what degree they must be sus- tained by a process which constantly re-forms them by new condensation. This process may be vertical austausch or upgliding
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