Page:Weather Facts and Predictions.djvu/18

 differ much from the true mean of the day ascertained under circumstances of precision. This is especially true of 9 a.m. and p.m., 10 ditto, 3 ditto, and 4 ditto.

Where the daily range is small, that is to say in the tropics, and in temperate regions in winter, the maximum temperature occurs at about 1.30 p.m.; but in temperate regions in summer not until between 2.30 and 3.30 p.m.

In winter, and at night in dry calm clear weather, the air is warmer at some height above the ground than it is at the surface. [This explains why fog, which is vapour condensed by chilled air, is so frequently visible in low-lying places, whilst neighbouring eminences are clear.] In such cases the upper rooms are warmer than those nearer the ground—a consideration for invalids.

Houses most protected against severe weather are those on a gentle acclivity, a little above the plain or valley from which it rises, and which have a southern aspect, with trees on the rising ground in the rear.

Comparing Great Britain with places on the continent or in America having the same latitude, the comparative mildness of the former is due to the influence of the Gulf Stream.

It is an undoubted fact that the mean temperature of Great Britain is higher than it was some centuries ago. This is due to the drainage of land generally and to the reclamation of waste lands. Glaisher considers that the mean temperature of the year at Greenwich has risen 2° in the last 100 years and