Anti-cyclones are associated with calm and cold in the centre, while on the borders the wind blows around the centre, spirally outwards in the direction of the hands of a clock. An anti-cyclone is usually accompanied by a blue sky, dry cold air, a hot sun, a hazy horizon, and little or no wind.
Wedge-shaped Isobars, unlike V’s, usually point north. They are areas of high pressure moving along between two cyclones, being really projecting parts of an anti-cyclone. The fine weather accompanying them is only temporary, because they are never stationary, and are generally followed by cyclonic disturbances. At the narrow end of the wedge thunderstorms or showers often occur, and at the wide end fog is common.
Cols or necks of relatively low barometric pressure occur between two anticyclonic areas. Like straight isobars they are intermediate systems. Over cols the weather is dull and gloomy; in summer they may be associated with thunderstorms.
Straight Isobars obviously do not enclose any area of high or low pressure. They form an intermediate condition, preceding the formation of a cyclone; and are usually associated with a blustering wind and hard sky.
Weather forecasting is necessarily somewhat difficult and uncertain. If one is dependent on observations at a single point the following rules are useful:—
- (a) If the barometer falls slowly and steadily bad weather will follow.
- (b) The barometer falls for rain with S.W., S.E., and W. winds.
- (c) When the barometer falls rapidly, heavy storms may be expected.
- (d) The barometer rises rapidly for unsettled weather.
- (e) The barometer rises gradually for fine, settled weather.
The Thermometer also is of great value as a weather indicator, especially if one knows what is the average temperature at the place of observation for each day of the year. Thus:—
- (a) A temperature continued for some time above or below the average, indicates a probable change.
- (b) Electric storms follow unusual warmth in summer.
- (c) A low thermometer and almost steady barometer are succeeded in winter by gales from N.N.W. or N.E.
The veering of the wind in England is also useful as an indicator. Thus:—
- (a) When the wind, in shifting, goes round in the same direction as the hands of a clock—i.e., from N. by E. to S., or from S. by W. to N.,—favourable changes of weather may be looked for.
- (b) When the wind backs—that is, veers round in the opposite direction—bad weather generally follows.