The cooling effect produced by a draught does not necessarily arise from the wind being cooler, for it may be actually warmer, but arises from the rapid evaporation it causes on the surface of the skin. Vapor of water forms a blanket about the earth and prevents it from scorching during the day and freezing during the night.
How to Forecast Weather with Only an Aneroid Barometer. No one except an expert observer should use the mercurial barometer. The aneroid will answer as well for the purpose of forecasting from a single instrument; it is cheaper and less complicated. First learn your elevation above sea level; then add to the observed reading of your instrument .10 for each one hundred feet elevation. Note the fall or rise and the direction of the wind and with the aid of the table on [page 76] highly satisfactory forecasts may be made by any intelligent person. Skill will come with practice. Write down your forecasts each day as you make them and the following day note in a blank space left for the purpose the success or failure of your effort. Thus will you profit by your mistakes.
As a rule winds from the east quadrants and falling barometer indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather. The rapidity of the storm’s approach and its severity are indicated by the rate and the amount in the fall of the barometer. This applies to the Mississippi Valley and eastward to the Atlantic Ocean. Conditions are different in the Rocky Mountains, on the plateau of the mountains, and on the eastern Rocky Mountain slope, where precipitation seldom begins until after the barometer begins to rise after a fall, and the winds have shifted to the northwest.
Keep in mind that storms are great atmospheric eddies drifting from the west, with the winds blowing cyclonically toward the center; that when your wind is northeast the center of the storm is southwest of you; that when it is east the center is west; when it is south the center is north; when it is southwest the center is northeast, and when it is west or northwest the center is east of you.
| Wind Direction | Barometer Reduced to Sea Level | Character of Weather Indicated |
|---|---|---|
| SW. to NW. | 30.10 to 30.20 and steady. | Fair, with slight temperature changes, for 1 to 2 days. |
| SW. to NW. | 30.10 to 30.20 and rising rapidly. | Fair, followed within 2 days by rain. |
| SW. to NW. | 30.20 and above and stationary. | Continued fair, with no decided temperature change. |
| SW. to NW. | 30.20 and above and falling slowly. | Slowly rising temperature and fair for 2 days. |
| S. to SE. | 30.10 to 30.20 and falling slowly. | Rain within 24 hours. |
| S. to SE. | 30.10 to 30.20 and falling rapidly. | Wind increasing in force, with rain within 12 to 24 hours. |
| S. to SW. | 30.00 or below and rising slowly. | Clearing within a few hours, and fair for several days. |
| S. to E. | 29.80 or below and falling rapidly. | Severe storm imminent, followed, within 24 hours, by clearing, and in winter by colder. |
| SE. to NE. | 30.10 to 30.20 and falling slowly. | Rain in 12 to 18 hours. |
| SE. to NE. | 30.10 to 30.20 and falling rapidly. | Increasing wind, and rain within 12 hours. |
| SE. to NE. | 30.00 or below and falling slowly. | Rain will continue 1 to 2 days. |
| SE. to NE. | 30.00 or below and falling rapidly. | Rain, with high wind, followed, within 36 hours, by clearing, and in winter by colder. |
| E. to NE. | 30.10 and above and falling slowly. | In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours. |
| E. to NE. | 30.10 and above and falling rapidly. | In summer, rain probable within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE. |
| E. to N. | 29.80 or below and falling rapidly. | Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave. |
| Going to W. | 29.80 or below and rising rapidly. | Clearing and colder. |
Difference between Weight and Pressure of the Air. Air at sea level and at 32° temperature weighs one and one third ounces per cubic foot. A room twenty by twenty by ten feet contains some 333 pounds of air. The pressure of the air is a quite different thing. It is the sum of the weights of all the cubic feet of air that are stacked up, one on top of the other, clear to the top of the atmosphere. This is why the higher one goes, the less the pressure of the air, because there are a less number of cubic feet above. And then each cubic foot weighs a slight fraction less than the one just beneath it because the air has expanded. The room afore-mentioned sustains a pressure of 5880 on its floor and a like pressure on its ceiling, and a half of this pressure on each of the sides of the room. The room does not collapse because the air exerts a like pressure on the outside of the room and the two pressures are equal—one inward and the other outward.
Fig. 7.