Page:Handbook of Meteorology.djvu/53

Rh parts. In centigrade terms, the increment is 0.00037 for each degree, measured from the absolute zero. Small as seems the rate of expansion, the actual increment over a continent, even for a rise of temperature of a few degrees, must be measured in terms of cubic miles, and its aggregate weight in millions of tons.

Observations show that the mean pressure over the earth varies from season to season; at any given locality existing pressure varies from hour to hour. It also varies according to latitude; but the variation for latitude is not regular. The maps on pages 40 and 42 show the several regions of high pressure. They show also that the pressure in these regions in January is slightly lower than in July. The regions of summer high pressure are situated in latitude 30° to 35° north and south.

The southerly regions cover the ocean and, for the greater part, are far removed from human activities. The northerly regions are of great importance from the fact that they modify the climate, the one of North America, the other of Europe and Asia.

The North Pacific high covers ocean waters in July, and it tends to carry cool air to the adjacent coast. In January it covers the western part of Canada and at times pours an enormous volume of cold air over the greater part of the United States.

The North Atlantic high covers western Asia in January; in July it covers the ocean east of the United States. At times, the area of maximum pressure, 30.30 inches (1026 mb) or more, is close enough to the continental coast to retard the easterly flow of air and cause a pretty general stagnation of air over the eastern part of the United States. It is therefore a feature in the formation of hot spells over that region.

The area covered by the North Pacific has a January pressure of 30.20 inches and a July pressure of 30.30 inches (1026 mb). The Bermuda, or Atlantic high, is about 30.25 inches (1024 mb); over Siberia, however, the winter high is not far from 30.50 inches (1033 mb). It is thought that the two zones of high pressure in mid latitudes are due to the descent of the upper currents that constituted the updraught in equatorial regions. This is denied by many meteorologists, however.