Winds and up-and-down currents

The existence of separate regions of high and low pressure is the cause of wind, or bodily movement of large portions of the atmosphere. Winds vary from the extensive trade winds caused by belts of high and low pressure sur­rounding the earth’s surface to the purely local gusts and ‘bumps’, caused by local differences of temperature and pressure. On the earth’s surface we are usually only concerned with the horizontal velocity of winds, but when flying the rising convection currents and the corresponding downward movements of the air are also important. The study of winds, of up-and-down convection currents, of cyclones and anti-cyclones, and the weather changes produced by them – all these form the fascinating science of meteorology, and the reader who is interested is referred to books on that subject.

In the lower regions of the atmosphere conditions are apt to be erratic; this is especially so within the first few hundred feet. It often happens that as we begin to climb the temperature rises instead of falling – called an inversion of temperature. This in itself upsets the stability of the air, and further distur­bances may be caused by the sun heating some parts of the earth’s surface more than others, causing thermal up-currents, and by the wind blowing over uneven ground, hangars, hills, and so on. On the windward side of a large building, or of a hill, the wind is deflected upwards, and on the leeward side it is apt to leave the contour altogether, forming large eddies which may result in a flow of air near the ground back towards the building or up the far side of the hill, that is to say in the opposite direction to that of the main wind. Even when the surface of the ground is comparatively flat, as on the average airfield, the wind is retarded near the ground by the roughness of the surface, and successive layers are held back by the layers below them – due to viscosity
– and so the wind velocity gradually increases from the ground upwards. This phenomenon is called wind gradient. When the wind velocity is high it is very appreciable, and since most of the effect takes place within a few metres of the ground it has to be reckoned with when landing.

Quite apart from this wind gradient very close to the ground, there is often also a wind gradient on a larger scale. Generally, it can be said that on the average day the wind velocity increases with height for many thousands of feet, and it also tends to veer, i. e. to change in a clockwise direction (from north towards east, etc.); at the same time it becomes more steady and there are fewer bumps.