Page:Weather Facts and Predictions.djvu/22

 bulb thermometer will in England sometimes amount to 18°, and frequently be from 9° to 12°. This sort of thing will occur between April and September. During the Winter months the difference will be restricted to narrower limits, say from 4° to 9°.

The Temperature of the Dew-point is sometimes as much as 30° below the temperature of the air: and between April and September especially, is frequently 20°. During the Winter months the difference is much smaller, but is often between 6° and 15°.

When in summer a hot day is not followed by a dew, rain may be looked for.

A profuse dew is a very sure sign of fine weather.

Two principal currents blow over the northern hemisphere of the earth. The Equatorial northwards to the Pole, and the Polar, southward from the Pole.

The Equatorial current is warm; the Polar current cold. [Because winds bring with them the temperatures of the regions which they have passed over.]

Winds coming from the sea do not cause such variations in temperature as those coming from a continent. [Because marine temperatures are more uniform than continental ones.]

Moist winds blowing from the Ocean are accompanied by a mild temperature in winter, and by a cool temperature in summer. [Because air loaded with vapour obstructs both solar and terrestrial radiation.]