Man's physical universe

xanabras

WEATHER-FORECASTING 267

ditions. Dry air is more dense than moist air; the average molecular

weight of dry air is 29, while the molecular weight of water vapor is 18.

The precipitation of moisture is typical of storms. In the northern

hemisphere the winds blow spirally outward in a clockwise direction

around anticyclone areas and spirally inward in a counterclockwise

direction around cyclone areas; the directions of wind circulation

are opposite in the southern hemisphere.

Fig. 87A. An atmospheric pressure record for the week, June 6-12, 1932, at

Stockton, California. The weather was fair all week.

Monday I Tufidau I

Wednciday IVednttday f/

Thunday / Friday /

Saturday I Sunday /

. . I iij^Bt « « « .OH'? « f. i loril; 4 6 I iofi « I I ,0^2 « » t I..II'? . « t loOit . 6 « lolT; 4 t i .o ifl ? . t i lolft 4 t'i

-lliiflllMi-t!lln^''W

Fig. 87B. An atmospheric pressure record for the week, January 11-17, 1932,

at Stockton, California. The weather was stormy during this week.

In general, a storm is approaching when the barometer indicates

falling air pressure. The more rapidly the air pressure is falling, the

more rapidly is the storm approaching and the more severe it is likely

to be.

The indications afforded by the wind and barometer are the best

guides now available for determining future weather conditions. In

most regions winds from the east and a falling barometric pressure

usually indicate foul weather, and winds shifting to the west indicate

clearing and fair weather.

The south winds usually bring warmth, the north winds bring cold,

the east winds in the middle latitudes indicate a storm approaching

from the westward; and the west winds show that the storm has

already passed eastward.

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