THE ATMOSPHERE AS AN ENVIRONMENT 



the thermal, as a slow sinking of the atmosphere. The sinking speed may 

 be comparable with that of a fungus spore (Hirst, 1959), but the local 

 rising velocity may commonly be 100 or more times this velocity. Some 

 bird species soar in large thermals, as do dragonflies in smaller ones near 

 the ground. Other birds haunt thermals to prey on the insects carried 

 upwards (Scorer, 1954). 



The Stratosphere 



In this region, which extends upwards from the tropopause to the 

 limit of the atmosphere, the temperature lapse-rate, characteristic of the 

 troposphere, is zero or may even be reversed. The height of the tropo- 

 pause varies with season, latitude, and other factors. The bottom of the 

 stratosphere may be found at an altitude of about 10 km., though under 

 special conditions it may reach temporarily to much nearer ground-level. 



The dust of the stratosphere is believed to be meteoric and to have 

 entered the Earth's atmosphere from space. It is generally believed that 

 terrestrial dust, including organic spores, is almost, if not entirely, con- 

 fined to the troposphere — except for occasional incursions in air currents 

 dragged up into the stratosphere by volcanic eruptions (or hydrogen 

 bombs). However, recent studies of atmospheric circulation, discussed by 

 Machta (1959), may point to exchange of air between troposphere and 

 stratosphere — with rising air over the equator and descending air in middle 

 latitudes. 



Circulation of the Atmosphere 



Under the influence of pressure difl^erences resulting from solar 

 heating, and of friction between wind and the rotating earth, a general 

 pattern of atmospheric circulation is set up. The surface winds sho\Mi in 

 atlases are the ground-level part of a three-dimensional system that has 

 not yet been well explored. The pattern still being worked out shows a 

 complex circulation, with air over the Equator ascending and flowing 

 discontinuously to the poles, which are themselves regions of generally 

 subsiding air (Palmen, 195 1). Across the Equator there is relatively little 

 interchange between northern and southern hemispheres. 



AIR MASSES 



The fact that air may have the same temperature and humidit}' over a 

 wide geographical area has given rise to the concept of the discrete air 

 mass, with properties different from adjacent air masses and separated 

 from them by 'fronts'. When an air mass remains stationary for some 

 time, it acquires a temperature and humidity dependent on the surface 

 on which it rests. These characteristics will be retained for some time 



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