FACTORS The standard observations carried out at any meteorological station consist of measurements of : (I) Pressure at the time of observation, and the change in pressure during the past three hours if a barograph is available. (2) Temperature of dry bulb and wet bulb thermometers, so that both temperature and humid ity can be derived from the readings. (3) Wind direction and velocity, the latter being estimated on the Beaufort scale, or read from an instrument in some convenient unit such as metres per second, miles per hour, or feet per second. (4) Weather and state of sky; i.e., number of tenths of the sky clouded, and nature of the cloud; whether rain or snow, hail, etc., is falling or not, whether there is fog, or mist present. (5) Visibility, usually given a numerical value by noting the most distant object visible among a number of previously selected standard objects at known distances from the observing point. (6) Maximum day tempera ture noted in the evening, and minimum night temperature noted in the morning. (7) Amount of precipitation since previous time of observation. (8) Amount of bright sunshine during the day. (9) Motion of cloud.
All these observations are carried out at most meteorological stations reporting by telegraph, telephone or wireless to a central meteorological office. For details of the use of such data for fore casting, see the standard textbooks on the subject.
In addition to the above observations, some or all of the fol lowing observations are made at a restricted number of stations in practically all countries of the world: (io) Upper wind meas ured by means of pilot balloons. (II) Upper air temperatures and humidities observed by means of instruments suspended on wing struts of aeroplanes. (12) Upper air temperatures and humidities at different heights by means of self-recording instru ments carried by free balloons. (13) Amount of atmospheric pollution. (14) Electric potential gradient in the atmosphere. ( I5) Various magnetic factors. When observations are made on board ship, to these are added: (16) Temperature of the sea surface, and state of the sea.
Temperature.—The factor which is most obviously related to solar radiation is the temperature of the air. In fig. 1, (a) gives the curve for diurnal variation of temperature at Aberdeen for August. This curve is typical of the curves obtained over the whole world at levels not much above mean sea level. This curve is definitely only the average over the month, and any one indi vidual interval of 24 hours may give widely differing forms for the curve of temperature, since the phenomenon may be compli cated by changes in wind direction, bringing up supplies of air from widely varying altitudes.
The effect of height upon the form of the variation of tem perature is shown by curves (b) for Parc St. Maur, Paris, so metres above sea level, and (c) for Eiffel tower, 335 metres above sea level, both for the month of July. The low-level station has a mean diurnal variation of over 9° C, while the higher station has a mean diurnal variation of a little over 5° C. There is a further noteworthy feature, that the time of maximum is about 2 hours later at the top of the tower. It is probable that at still greater heights the diurnal variation of temperature rapidly diminishes. (Hergesell, Lindenberg Publications, vol. xiv.) The temperature of air over the sea is subject to no consider able diurnal variation except possibly over shallow water. Ob servations made on board the Challenger indicated a diurnal range of about I° C, and this value has been confirmed (Q.J.R. Met. Soc., 1927) by observations made in the Mediterranean sea by N. K. Johnson. The changes of temperature are so slight that it is not possible to say with certainty how far they are vitiated by the changes of wind direction.