Page:Weather Facts and Predictions.djvu/36

 not deposited in cloudy weather, because the clouds obstruct the escape of heat into space by radiation, nor in windy wheather because wind constantly changes the air in contact with the ground, and thus prevents its temperature from falling sufficiently low. When the temperature falls below freezing the dew becomes converted into hoar frost.

As dew is not formed during the prevalence of clouds or wind it serves as an indication of fine weather.

Hoar frosts on three successive mornings in early spring or autumn betoken rain, but in April or May are generally followed by dry weather.

More rain falls on land than at sea, especially in hilly or mountainous countries, and so the temperature will be raised by the latent heat thus given out. [For this reason the northern hemisphere, as containing more land, is warmer than the southern hemisphere.

The drainage of agricultural land has been proved to raise the mean annual temperature, and so such operations improve the produce of the crops (to the detriment of the wells).

Places having a considerable rainfall are characterised by a low mean pressure. More showers of rain happen between 2 and 3 p.m. than during any other hour. Rain is rare between midnight and 1 a.m.

In weather which is showery rather than steadily wet, when between the intervals of sunshine a cloud appears in the W., passes over the spectator, and as it passes pours