Short-period Variations of the Wind

IF one examines the autographic records of wind velocity and direction obtained at any inland observatory on a summer's day, it will be found that both traces are markedly different during the day-time as compared with the night. An occasion on which this effect was very well developed is represented in the traces obtained at Porton on July 5-6, 1923. The anemometer vane is mounted at a height of 13.5 metres and has an excellent exposure. The direction chart shows that during the day-time the wind is characterised by very large variations in direction. These variations are, moreover, of a peculiar type. It is observed that the wind assumes a fairly steady and definite direction for an interval of the order of ten minutes. It then suddenly swings through an angle of perhaps 60° or even 90°. After maintaining this new direction for some minutes, the wind shifts again with equal suddenness. Regarded as short-period variations, these shifts are quite irregular in occurrence, although when considered over a period of some hours, the mean direction agrees with that demanded by the pressure gradient. In contrast with this behaviour we may compare the trace obtained during the night. This is the normal type of trace in which deviations from the mean direction are both small in amplitude and of short duration. The wind velocity record also shows a characteristic effect during the day-time: short intervals of nearly dead calm alternate discontinuously with intervals during which the velocity is nearly constant with a value of two or three metres per second. It may be noted that the durations of these lulls and gusts are about the same as those of the changes in wind direction. At night-time the velocity trace becomes normal like the direction trace.