Fig. 43.
The Seven Fundamental Shapes of Isobars (after Abercrombie).
Cyclones or depressions are areas of low barometric pressure. A cyclonic system (Fig. 43) is formed by circles of concentric isobars. The differences between cyclones and anti-cyclones are as follows:—
| Cyclones. | Anti-cyclones. |
| Wind moves in the opposite direction to the hands of a watch. | Wind moves in same direction as the hands of a watch. |
| Barometer is lowest in the centre. | Barometer is highest in the centre. |
| Area comparatively small. | Area comparatively large. |
| Gradient from centre to circumference steep. | Gradient not steep. |
| Short duration. | Long duration. |
| Velocity of wind great. | Air comparatively quiet. |
| Weather bad; much rainfall. | Weather fine. |
| Cool in summer; warm in winter. | Hot in summer; cold and frosty in winter. |
Cyclones usually travel from west to east, and are always associated with bad weather. The essential point in determining the character of the weather, both in cyclones and anti-cyclones, is the barometric gradient. Thus, according to the gradient, a cyclone may mean mild wet weather, a gale, or a hurricane. The turning point of a cyclone, just before the barometer begins to rise again, is called the trough. Cyclones are usually oval in shape, except in the tropics, where they are smaller and circular. The ordinary course of events in a cyclone is shown in Fig. 44, reading it from left to right.
In Secondary Cyclones, bad weather is usually associated with a stationary barometer and no wind. They are incompletely circular looped concentric isobars, with the lowest pressure in the centre. They frequently follow primary cyclones.
V-shaped Depressions are angular areas, with the lowest pressure in the centre, frequently forming between adjoining anti-cyclones (Fig. 43). In the northern hemisphere the tip usually points south. They usually move with great rapidity from east to west, and are always associated with squalls or thunderstorms. Their movement is very uncertain, and their forecast therefore more difficult than that of cyclones and anti-cyclones.
Fig. 44.
Weather Sequence in a Cyclone (after Abercrombie).
The tracing indicates the line which a self-recording barometer would have marked. The arrows mark the shift of the wind, and the number of barbs denote the varying force of the wind.