How to Read Weather Maps

Teacher’s Story

Weather maps may be obtained by writing to the nearest Weather Station, or by writing to the Chief of the Weather Bureau, Dr. Willis L. Moore, Washington, D. C., stating that you wish to post the maps in a public place. A supply of maps for three successive days for use in these lessons may be obtained at 20 cents per hundred. Sometimes they are sent free, if it is stated that they are to be used for school purposes.

The words isobar and isotherm have been bogies which have frightened many a teacher from undertaking to teach about weather maps, and yet how simple are the meaning of these two words. Isobar is made up of two Greek words, isos meaning equal and baros meaning weight. Therefore, an isobar means equal weight, and on a map one of these continuous lines means that, wherever it passes, the atmosphere there has equal weight and the barometer stands at equal height. The isobar of 30 means that the mercury in the barometer stands 30 inches in height in all the regions where that line passes.

“Isotherm” comes from the two Greek words, isos meaning equal and therme meaning heat. Therefore, on the map the dotted lines show the region where the temperature is the same. If at the end of the dotted line you find 60 it means that, wherever that line passes, the thermometer stands at 60 degrees.

Many of the “highs” and “lows” enter the United States from the Pacific Ocean about the latitude of Washington State or southwest British Columbia. They follow one another alternately, crossing the continent in the general direction of west to east in a path which curves somewhat to the north, and they leave the United States in the latitude of Maine or New Brunswick. If they enter by way of lower California, they pass over to the Atlantic Ocean farther south. The time for the passage of a high or low across the continent averages about three and one-half days, sometimes a little more. These areas are usually more marked in winter, and wind storms are more marked and more regular.

A low area is called a cyclone and a high area an anti-cyclone. The destructive winds, popularly called cyclones, which occur in certain regions, should be called tornadoes instead, although in fact they are simply small and violent cyclones. But a cyclone, when used in a meteorological sense, extends over thousands of square miles and is not violent; while a tornado may be only a few rods in diameter and be very destructive. The little whirlwinds which lift the dust in the roads are rotary winds also, but merely the eddies of a gentle wind.

In a cyclone or “low,” and also in a tornado, the air blows from all sides spirally inward toward the center where there is a column of ascending air.

In an anti-cyclone or “high” the air blows outward in every direction in curved lines from a column of descending air.

In the above map, the curved lines are isobars; the line of crosses from A to B indicates the course of the storm; the arrows indicate the direction of the wind, note that it is moving counter-clockwise around the area of low pressure; the shaded area indicates the region where it is raining or snowing; note that this is the area where the warm, moist air from the Gulf and the Ocean meets the colder air of the North.

The weather conditions during the passage of a cyclone are briefly as follows: Small, changing wisps of cirrus clouds appear about 24 hours before rain; these gradually become larger and cover the whole sky, making a nimbus cloud. The wind changes from northeast to east or southeast to south. The barometer falls; the thermometer rises, that is, air pressure is less to the square inch, and the temperature of the atmosphere is warmer. Rain begins and falls for a time, varying from an hour to a day or more. After the rain there appear breaks in the great nimbus clouds and finally the blue sky conquers until there are only a few or no clouds. The wind changes to southwest and west; the barometer rises, the temperature falls. The rain ceases, the sun shines out brightly. The low has passed and the high is approaching to last about three days.

U. S. DEPARTMENT OF AGRICULTURE, WEATHER BUREAU

EXPLANATION OF WEATHER SIGNALS

INTERPRETATION OF DISPLAYS

No. 1, alone, indicates fair weather, stationary temperature.
No. 2, alone, indicates rain or snow, stationary temperature.
No. 3, alone, indicates local rain or snow, stationary temperature.
No. 1, with No. 4 above it, indicates fair weather, warmer.
No. 1, with No. 4 below it, indicates fair weather, colder.
No. 2, with No. 4 above it, indicates rain or snow, warmer.
No. 2, with No. 4 below it, indicates rain or snow, colder.
No. 3, with No. 4 above it, indicates local rain or snow, warmer.
No. 3, with No. 4 below it, indicates local rain or snow colder.

WILLIS L. MOORE,
Chief U. S. Weather Bureau.

EXPLANATION OF WHISTLE SIGNALS

A warning blast of fifteen to twenty seconds duration is sounded to attract attention. After this warning the longer blasts (of four to six seconds duration) refer to weather, and shorter blasts (of one to three seconds duration) refer to temperature; those for weather are sounded first.

Blasts Indicate.

One long Fair weather.

Two long Rain or snow.

Three long Local rain or snow.

One short Lower temperature.

Two short Higher temperature.

Three short Cold wave.

By repeating each combination a few times, with intervals of ten seconds, liability to error in reading the signals may be avoided.

STORM AND HURRICANE WARNINGS

Explanation of Storm and Hurricane Signals

Storm warning—A red flag with a black center indicates that a storm of marked violence is expected.

The pennants displayed with the flags indicate the direction of the wind; red, easterly (from northeast to south); white (westerly from southwest to north). The pennant above the flag indicates that the wind is expected to blow from the northerly quadrants; below from the southerly quadrants.

By night a red light indicates easterly winds and a white light below a red light westerly winds.

Hurricane warning—Two red flags with black centers displayed one above the other indicates the expected approach of a tropical hurricane or one of those extremely severe and dangerous storms which occasionally move across the Lakes and northern Atlantic coast.

No night hurricane warnings are displayed.