Vapour Pressure and Humidity.—The vapour pressure remains substantially constant during the 24 hours, at land stations. At Kew observatory, England, there is no appreciable diurnal varia tion of vapour pressure during any month of the year, and the same holds at nearly all stations for which data are available. Since temperature normally varies considerably between day and night, it follows that relative humidity varies in the opposite sense to temperature.
Cloudiness.—No simple rule can be stated for the diurnal variation of cloudiness. The wind-direction is of considerable im portance in this respect, and different types of cloud vary in dif ferent ways. The diurnal variation of cloudiness at Kew observa tory has been investigated by Brunt (M.O. Professional Notes, Nos. I and T4), who showed that with all wind directions there is also Arbeif en Prelim Aeronaut. Obs., Lindenberg, Bd. XIV.
a well-marked tendency for the amount of cloud to diminish in the evening. Similar results were derived for a number of stations in the Rhine valley. At Batavia, 6° S. of the equator, cloudiness attains its maximum in the evening or early night, and its mini mum about 4 A.M. local time (Observations, Batavia, xxxviii.). At Helwan cloudiness attains its maximum during the afternoon, and its minimum during the evening.
Thunderstorms.—The observations made by the "Challenger" showed that the frequency of thunderstorms at sea had a pro nounced maximum between 2 A.M. and 4 A.M., while the occur rence of "lightning only" showed a maximum between 8 P.M. and 10 P.M. Over the land, heat thunderstorms occur most frequently during the afternoons, but certain types of thunderstorms, as sociated with the line of separation between warm and cold air currents, show no special preference for any hour of day or night. Electric Potential Gradient.—The typical variation of potential gradient has a minimum in the early morning, and a maximum in late afternoon, the maximum being about twice the minimum ; but at many stations there is a subsidiary maximum about 8 A.M. and a minimum in early afternoon.
Seasonal Variation of Meteorological Elements.—While it is possible to consider the diurnal variation of the meteorolog ical elements separately, it is not possible to treat the seasonal variations in the same way. As is shown later, the whole of the circulation of the atmosphere and the distribution of pressure over the surface of the earth, with which this circulation is closely associated, are both subject to changes on a large scale during the course of the year. A noteworthy point which emerges from
the discussion of the diurnal variations is the difference in the phenomena over land and sea, the former showing the greater variability. The same is true of seasonal variations. The land heats up more rapidly than the sea in the summer, and cools more rapidly than the sea in winter. Hence the marked distinc tion between continental and oceanic climates, the former being subject to extremes of heat and cold, and the latter being rela tively equable. The centres of large land masses are associated with excessive heat in summer and excessive cold in winter, the lowest temperatures in the northern hemisphere occurring in northeast Siberia. In this region the seasonal change of tempera ture exceeds 6o° C, while over the major part of the Eurasian continent and North America the seasonal variation exceeds 3o° C. In the tropics the seasonal variation of temperature becomes relatively small, with the result that the change of temperature with latitude is greater in winter than in summer. All phenomena which are in any way related to or dependent on the horizontal gradient of temperature are therefore accentuated in winter.
Along the seaboard of a continent, the' nature of the seasonal variations will depend upon the direction of the prevailing wind, since winds from the interior of the continent and winds from the ocean will carry with them the characteristics of their region of origin. Rainfall again depends largely upon the interaction of different currents of air and on the form of the land, and it is not possible to give any general rules as to the nature of the seasonal variations of rainfall over the whole globe, except such as will arise in the course of the following brief discussion of the general circulation of the atmosphere. The general question of seasonal variation of meteorological factors is closely bound up with climatology, and a large amount of information on this topic will be found in any standard text book on the latter; e.g., Ken drew's Climates of the Continents, contains numerous tables giv ing seasonal variations of temperature, pressure and rainfall.