VIII 



A tmospheric Phenomena 



127 



tion proved that it really depends on the changes of 

 atmospheric temperature, and so is a result of the radiant 

 energy of the Sun. The Morning Minimum of pressure 

 about 4 A.M. results from the cooled dust-motes condensing 

 upon themselves most of the water-vapour contained in the 

 air, the vapour pressure is greatly reduced, and the total 

 observed reduction of atmospheric pressure is thus accounted 

 for. When the Sun appears, the dust-motes are warmed up, 

 the vapour returns to the atmosphere, and the temperature 

 of the air rapidly increasing, produces the Forenoon Maxi- 

 mum of pressure about 10 A.M. When the temperature of 



FIG. 24. Daily Range of Atmospheric Pressure in temperate 

 and tropical climates (after A. Buchan). 



a gas is raised it must either expand or press more strongly 

 on the vessel containing it ; and in the forenoon the heated 

 air is prevented from expanding for a time by the resistance 

 of the cooler layers of atmosphere above, against which it 

 presses with increasing force, and the barometer rises. 

 After 10 o'clock the continued heating enables the air to 

 overcome the resistance, and ascending currents set in, the 

 air rises and, meeting the west winds of the upper atmo- 

 sphere, is carried away to the eastward. The density of the 

 whole column of atmosphere is diminished by the removal 

 of the ascending air, and the Afternoon Minimum of 

 pressure occurs about 4 P.M. As the surface air cools 

 in the evening it grows denser and sinks, the upper air 

 welling over from the heated regions to the west, where it 

 is still only early afternoon, flows in above, cools and sinks, 

 so raising the pressure to produce the Evening Maximum 

 about 10 P.M. Thus the morning minimum and maximum 



