Page:Weather Facts and Predictions.djvu/41

 After the storm is past a great fall occurs in the thermometer.

Thunderstorms coming up with an E. wind while the barometer is falling do not cool the air; it remains sultry, and another thunderstorm may be looked for as at least likely to occur in the neighbourhood. Not till the wind gets round towards the W., and the barometer begins to rise, will the temperature of the air fall.

If several thunderstorms come on in succession from the W., each storm has usually a more northerly drift than the one which preceded it.

Thunderstorms in spring lie at a low level and do not last long; they are usually followed by a period of cool weather.

The general direction of a thunderstorm is either from E. to W., or from N. to S.; not often is its direction oblique.

Thunder occurs commonly when the wind is S.; very rarely when it is N. In summer or early autumn, if after the wind has been S. for two or three days, the air becomes very sultry, and numerous clouds with white summits and blackish bases present themselves, thunder and rain are imminent; if two independent masses of such clouds are seen approaching on different sides, a storm is very near.

There is more lightning in summer and autumn than in winter and spring.

That form of lightning known as "silent"