THE LOW
Cyclone is simply the generic name for a large rotating air mass. It is a barometric depression or LOW and is characterized by a flow of air inward and around a moving centre. The air circulation is counter-clockwise in the northern hemisphere and clockwise in the south.
Perhaps if the earth stopped rotating and there was no planetary circulation, with the great west-moving trades and east-moving "westerlies," the arrows on the weather map would all point directly toward the centre of the LOW; but, as things are, there are some very good reasons why air can not move directly into a LOW, that is at right angles to the isobars.
Moreover, the weather map does not indicate the true flow of the air, for observations of the wind made at the ground tell only a part of the story of the balance which the flowing air must maintain under the action of various forces, such as gravitation, rotational deflection, centrifugal tendency, and the various expansion and compression forces.
The winds near the ground are modified both in velocity and direction by friction. The free flow is often interfered with by topography.
THE TRUE AIR FLOW
One must rise above the ground some distance to get the true air flow, or what is known as the gradient wind, the flow which balances the gradient, i.e. a flow along the isobars. The gradient velocity is found about 300 metres above the ground, and the gradient direction a little higher. The lower clouds as a rule indicate true wind values very well; and so, it is desirable in studying winds to use cloud directions and velocities rather than surface values. In cloud work a nephoscope is essential. The unaided eye, unless properly shielded, suffers from the glare of a sunlit sky; and moreover, there are no fixed points or references. A black mirror, with suitable sighting rods and measuring devices, enables an observer to follow the cloud, estimate its height and determine with accuracy the direction from which it is moving. There is an average difference of 30 degrees between the cloud direction and the surface wind; the upper direction being more to the right. At times the directions may be opposite.