REPORT ON ATMOSPHERIC CIRCULATION. 19 



tion of the tension by a lowering of the temperature and change of state of a part of the 

 aqueous vapour. 



TJie Morning Maximum. — The diurnal heating of the atmosphere proceeds with 

 the ascent of the sun. As the water condensed on the surfaces of the dust particles 

 is evaporated, tension is increased by the simple change from the fluid to the gaseous 

 state ; and as the dust particles in the sun's rays rise in temperature above that of the 

 films of air in contact with them, the temperature of the atmosphere is thereby raised, 

 thus further increasing the tension. Under these conditions the barometer steadily 

 rises with the increasing tension to the morning maximum. It is to be particularly 

 noted that this rise of the barometer is not due to any accessions to the mass of air 

 overhead, but only to increasing temperature and change of part of the watery vapour 

 from the liquid to the gaseous state. Owing to the rapidity of the heating and increase 

 of tension of the atmosphere through its whole height by the sun's rays, but more par- 

 ticularly in the lowermost strata where the dust particles are more numerous and, as the 

 colours of sunset suggest, grosser than prevail in the upper regions of the atmosphere, 

 some time must elapse before the greater expansive force thus called into play is able 

 to counteract the vertical and lateral resistance it meets from the inertia and viscosity 

 of the air. The only effect of the conversion of latent to sensible heat in these con- 

 densations, and the converse after sunrise, is but a slight retardation of the phenomena. 

 The Afternoon Minimum. — When this resistance has been overcome, an ascending 

 current of the warm air sets in, and pressure gradually falls, as the mass of air 

 overhead is reduced by the ascending current flowing back as an upper current to 

 eastward, in other words, over the section of the atmosphere immediately to eastward, 

 the temperature of which has now fallen considerably lower than that of the region 

 from which the ascending current rises. 



'The Evening Maximum. — When the daily maximum temperature is past and 

 temperature has begun to fall, the air becomes gradually more condensed in the lower 

 strata, and, as a consequence, pressure at great heights is lowered, and, be it particularly 

 noted, lowered most as compared with the pressure at the same height over the region 

 from which the ascending current is rising. Hence it follows that owing to this relative 

 difference of pressure, the ascending current, which rises from the longitudes where at the 

 time the afternoon pressure is at the minimum, flows back to eastward, thus increasing 

 the pressure over those longitudes where temperature has now greatly fallen. This 

 atmospheric quasi-tidal movement occasions the evening maximum of pressure, which 

 occurs from 9 p.m. to midnight, according to latitude and geographical position. As 

 midnight and the early 'hours of morning advance, these contributions through the 

 upper currents become less and less, and finally cease altogether, and the effects of the 

 nocturnal radiation now going forward again introduce the morning minimum, as already 

 described. Thus the afternoon minimum is occasioned by the removal of part of the 

 mass of the atmosphere by the ascending current and its connected upper current, 



