The new air world

by Willis L. Moore

Vessel masters, long accustomed to forecast the near approach of storms from the action of their “glass” (barometer), now have learned that the daily weather map shows them at a glance the height of not one but of many barometers scattered over a wide area and read at the same moment of time. They see that the direction and the force of the wind are the results of differences in air pressure; that the air flows from a region where the air pressure is great, that is to say, where the barometers are high, towards a region where the pressure is less, or where the barometers are low; and that the velocity of the wind will be in proportion to the difference in the pressure of the air. Coast-wise and lake shipping are therefore not only affected by the forecasts made by the Weather Bureau but by the forecast made by the masters themselves when they can get access to the daily weather map. Their own lives and the lives and property of others are in their keeping. But the great mass of intelligent people have no idea of the methods employed in the making of the weather map and of the many and widely diversified uses to which a study of its data would lead.

One first must learn of the simple manner in which the map is constructed; then, by a comparison of the map each day with the preceding chart, he soon will be able to detect the beginning of storms, trace them through their various migrations as they cross the continent and finally pass out to sea, bidding them bon voyage as they go in quest of a more eastern continent on which to bestow their blessings of rain and active, purified air; or, as it often may happen, shuddering for the fate of the mariner who is caught in their fierce vortical whirls, and for the land areas that may be laid waste by their gyrating force.

How the Weather Map Is Made. At 8 A.M. to-day Washington time, which, by the way, is about seven o’clock at Chicago, six at Denver, and five at San Francisco, the observers at some two hundred stations in the United States and contiguous territory were taking their observations and from carefully standardized instruments noting the conditions of the atmosphere. By 8:20 A.M. the barometers at each station have been reduced to sea level, that is to say, they have been made to read what they would if they were located at the level of the ocean. Thus differences in air pressure that are due to differences in elevation are eliminated, so that they may not obscure those due to storm conditions. Then, for purposes of brevity and accuracy, the observations are reduced to cipher form, and each filed at the local telegraph office. During the next thirty or forty minutes the observations, with the right of way over all lines, are speeding to their destinations, each station contributing its own report, and receiving in return such observations from other stations as it may require. The observations from all stations are received at such important centers as Washington, New York, Chicago, and other large cities having Weather Bureau stations, and from these centers daily weather maps are printed and issued at 11 A.M. each day.

Chart 3.—Winter Storm, December 15, 1893, 8 a.m.

Black lines connect places having equal barometric pressure; arrows point in direction wind is blowing; figures at end of arrows show wind velocity, when it is more than light.

○ clear; ◓ partly cloudy; ● cloudy; R rain; S snow.

HIGH indicates center of anti-cyclone, or high-pressure area; LOW indicates center of cyclone, or low-pressure area.

Large figures show average temperature in each quadrant of cyclone.

Now turn to [Chart 3]. Heavy black lines (isobars, meaning equal pressure) are drawn through places having the same barometric reading. The readings are omitted from the printed Chart. By drawing lines for each difference of one tenth of an inch, the high and the low-pressure areas (called Highs and Lows) are soon inclosed in their proper circles. These lines run in oval or circular form, indicating that storms operate in the form of great atmospheric eddies; that there are central places of attraction towards which the air is drawn if the disturbance be a low-pressure area, with its usual accompaniments of warm, moist, and often rainy weather, and from which the air is driven if it be a high-pressure area, with cool, settled weather.