The Prescott daily news., 1914-05-02
HOW THEY FORECAST THE
 WEATHER

ther forecaster does not look out of the window and guess. He is not weatherwise like the old salts who scents the approach of storm with out knowing how. To the signs by which the ordinary citizen decides whether to take his umbrella with him or leave 1t at home, he pays no atten- tion. Give him his telegraph wires and he can make his predictions as ac- curately in windowless celler as on the top of New York's highest sky scraper.
 The private individual sees the clouds settle on a nearby mountain or the rain sweep over a field a mile from his house and knows that the storm will be on him in a few minutes With the aid of the telegraph the weather man sees the same storm when it is a thousand miles away, and not only this storm but every other one in the country. He knows in what direction and at what speed each is moving, and can thus calculate with reasonable accuracy the approximate time when it will reach any place in it's route.
 Twice a day, at in the morning and at 8 in the evening, reports are telegraphed to Washington from about 200 observers stationed in as many different localities in the United States and Canada. In these reports the observers do not volunteer their personal opinions about what it going to happen. They confne themselves to plain statement of the actual con 'ditions at that particular moment, the pressure, or weight of the atmosphere, the temperature, the direction and velocity of the wind, the amount of rain since the last report, and so forth From this information the Weather Map of the United States is made up, showing the conditions that prevail in every part of the country. Since there are two maps for each day it is ob vious that by comparing them the forecaster can keep track of the course and progress of both storms and periods of clear weather. From that the next step is to predict what sort of weather will prevail for a day or two in any given locality.
 This map is the basis of all scien- tifc weather forecasting. A glance at it will show that it is divided into Thigh pressure areas" and flow pres sure areas These are technical terms used to describe the regions in which the weight or pressure of the atmos- phere is great (high) or small Qow). At sea level the barometer, which is used to measure this weight, will reg
 ster 30 inches under normal condit-
 ions of the atmosphere. When 1t reg isters more than this, say 30.5 or 3l, the pressure is "high;" when 29.5 or less, "low" In this way the height of a column of mercury in barome- ter indicates the weight of the air just as, in thermometer, it indicates the temperature.
 Low pressure usually mean strong winds, rain and rising temperature; high pressures, cool, clear weather. For reason to be explained later, these "high" and "lows," as they are called, travel in a general direction from west to east. The forecaster notes their progress on the map, pre- ceives their speed and their route, and then predicts the time of their arrival at any specified point. If they tra- veled, like a ship steered by compass, an exact course to the east, and if they moved invariably at the same speed, then weather forecasting would be simple sum of arithmetic, like calcu Hating the time when a railroad train running 90 miles an hour will arrive at a station 300 miles away. But storms are not railroad trains. They travel in an easterly direction, but they do not travel due east. Their speed is liable to change, and they are affected by the presence of other storms, by mountain ranges, large bodies of water, and many other things which make weather prophecy ing the complicated science that it is. The skeleton of the science, however, is the progress of those ''high" and '1owss" eastward across the country.
 This progress is caused by the shape of the earth and the well-known fact that hot air rises The tropical sun