CHANGE OF TEMPERATURE WITH HEIGHT DURING ANTI-CYCLONES. 511 



and, during the long winter nights, cools yet more by radiation through the clear 

 diathermanous air above — radiation which cools itself without affecting the temperature 

 of the air above it. 



This vertical distribution of temperature involves an abrupt change at the boundary 

 between the upper surface of the moist air and the base of the warm dry air. Such 

 change is indeed necessary, because — dry air being heavier than moist at the same 

 temperature — the former could not float above the latter unless it was at least so much 

 warmer as to compensate for its naturally greater density. At low temperatures the 

 required change is small ; at 15° perfectly dry need only be 0°"5 warmer than saturated 

 air to become as light ; at 30° a difference of 1° is required, at 48° it must be 2°, at 59° 

 it must be 3°, at 67° it must be 4°, at 76° it must be 5°, and at 80° it must be 6° 

 warmer to be as light as saturated air. Certain observations at Ben Nevis Observatory 

 confirm this. When a cloud layer extends just about the level of the hill top, and 

 there is clear sky above, we usually have the cloud rising and falling a little from time 

 to time, so that the Observatory is alternately in the top of the cloud and the base of 

 the dry air. Under these circumstances, the temperature is invariably lower in the 

 cloud than in the dry air above it, and this quite independent of whether the change 

 takes place by day or night. An observation of the rise of temperature experienced 

 immediately above a cloud layer was made by Professor Piazzi Smyth at the Peak of 

 Teneriffe in 1856 (see Journal Scottish Meteorological Society, vol. vii. p. 273). 

 His observations were all made in the daytime, and he supposed the rise to be largely 

 due to the sun's heat reflected from the upper surface of the cloud ; but the fact that 

 the same conditions are found at night indicates that other agencies than the direct 

 heat of the sun control these temperatures, namely, the heat due to descent and 

 consequent compression of the dry air, and the cooling of the cloudy air by evaporation 

 and radiation. For example, on the 8th February 1892 up till 11 p.m. there was fog 

 on the top of Ben Nevis, the temperature being 25° to 26°, and the air saturated ; 

 after 11 p.m. the fog sank, leaving the hill top just clear above it, the temperature for 

 the next few hours being 30° to 33°, with a wet bulb depression of from 3° to 5°, 

 equivalent to a relative humidity of 40 to 60 per cent. On the 9th February, at 

 8 p.m., the fog rose again and enveloped the summit, the temperature falling 8°"5 in 

 one hour, and the air becoming, of course, saturated. Many other instances, at all 

 hours of the day, could be given ; the average of twenty cases shows a rise of tempera- 

 ture of about 3° in two hours as the fog sinks, and an equal fall when it rises again — 

 the wet bulb remaining almost steady in both cases, indicating that the upper surface 

 of the cloud acts as an evaporating surface in regard to the dry air above it. No 

 similar change is observed when a cloud settles down on the hill from above. Another 

 fact bearing on the subject is, that in the cloud or fog the air is calm, while as soon as 

 it sinks and the hill top emerges into the dry air a strong breeze is usually felt, 

 showing that the dry air is flowing away over the cloud surface, forced, no doubt, by 

 the descending current of the anti-cyclone. 



