The diurnal variation of the temperature of the surface of the sea is probably very slight, but the difficulty of obtaining a fair sample of the surface water of the sea makes any accurate esti mate of the true surface temperature impossible.
Lapse-rate of Temperature.—In overcast weather there is little or no variation of the lapse rate between day and night. In clear weather, however, for the lowest layers, there is a pronounced maximum in the middle of the afternoon, its value exceeding many times the dry adiabatic lapse rate. The lapse rate dimin ishes rapidly in the evening, giving place to a pronounced inver sion. Over the sea there is no appreciable diurnal variation of the lapse rate.
Pressure.—In fig. 2, (a), (b) and (c) give for July the diurnal variations of pressure (from data in Manual of Meteorology, vol. ii.) at Aberdeen, Batavia and in the Arctic, the times being Greenwich mean time in (a) and local times in (b) and (c).
It will be noted that within the tropics there is a very strongly marked pressure wave of 12 hours' duration, with maxima at 9 A.M. and 9 P.M., superposed on a 24 hourly wave. Some fur ther details of these diurnal variations are given later in the section Dynamical Aspects. Curve (d) in the same figure shows the diurnal variation for July on Ben Nevis, 1343 metres above mean sea level, a single ill-defined maximum at 2 P.M.
Wind.—The surface wind velocity shows a maximum about 2 P.M. and a minimum in the early morning. The direction shows a corresponding change, the wind backing as it diminishes. At the top of the Eiffel tower the diurnal variation of wind velocity is the reverse of that observed at the ground, the maximum velocity occurring during the early morning and the minimum during the afternoon. At intermediate heights the nature of the diurnal variation is to some extent dependent on the strength of the wind. Hellmann (Met. Zeit., Jan. 1915) set up three anemometers at heights of 2, 16 and 32 metres above the ground, in a flat meadow at Nauen. He found that when strong winds were blowing, all three anemometers yielded maxima during the day, and minimum during the night. With light winds the anemometer at 2 metres
showed a day maximum and a night minimum, but those at 16 metres and 32 metres showed two maxima, one about mid-day, and the other about midnight. At 16 metres the maxima were approximately equal in winter, but in summer the day maximum was slightly the greater. At 32 metres the night maximum was the greater in winter and summer. With light winds the height at which the night maximum becomes equal to the day maximum is thus about 16 metres in winter, and between 16 and 32 metres, in summer. With strong winds this height exceeds 32 metres.
Durward (M.O. Professional Notes, No. i 5) investigated the diurnal variation of winds at i,000 to 6,000 ft. by an examination of 1,736 pilot balloon ascents, made by the meteorological serv ice with the British Armies in France during March 1917–Sept. 1918. The winds at i,000 ft. showed a maximum about mid night, and a well marked minimum between 9 A.M. and Io A.M. in reasonably good agreement with the Eiffel tower observations. The variation at 2,000 ft. and 3,00o ft. was of the same general character, but when all wind directions were taken together no appreciable diurnal variation was found at 4,00o and 6,000 feet.
At sea, the diurnal variation of wind is much less marked. GAO (K. Ned. Met. Inst., No. 102) states that from May to October the maximum velocity of the south-east trade-winds of the Indian ocean occurs during the night-hours. No detailed ob servations at one place are available, and it is not at present possible to give any definite statement of the variation of winds over the sea.
Land and Sea Breezes.—At places on a coast there is usually a pronounced tendency for a wind to blow from the sea to the land in the morning, and from the land to the sea in the evening. In some places, this wind follows the sun in the course of the day. For example, at Aberdeen, the sea breeze sets in suddenly in the morning as a wind from sea to land at right-angles to the coast line, but during the day it veers until it blows parallel to the coast.