Fig. 34.—Sun Spot in Photosphere.
Changes of heat, moisture and wind are the cause of all the kinds of weather we have, and we have a good many kinds, be it hot or cold, dry or wet, calm or windy, clear or stormy, good, bad and indifferent.
To Forecast the Weather by a Barometer.—The best way to tell what the coming weather will be in the next few hours is by the rise and the fall of the pressure of the air, or barometric pressure, as it is called.
A simple barometer for showing the changes in the pressure of the air can be made of a glass tube about 3 feet long, ½ inch in diameter, and closed at one end as shown in [Fig. 35]. Fill the tube with mercury and, placing your finger over the mouth of the tube, turn it upside down and put the open end into a cup, or other vessel, which is half full of mercury; in placing it into the cup be careful that no air gets into the tube.
Only a small part of the mercury in the tube will run into the cup and this will leave a space in the top of the tube. Now fasten a yardstick, the purpose of which is to show the changes in the height of the mercury in the tube, with a string or wire to the tube, and your barometer will be complete, as shown in [Fig. 36].
Since the air presses on the mercury in the cup but not in the tube, the pressure of the air on the mercury in the cup just balances the weight of the mercury in the tube and, hence, any increase or decrease in the pressure of the air, which ordinarily is about 15 pounds to the square inch, is shown by the rising or the falling of the mercury in the tube.
The barometer in helping to forecast the weather shows that: (1) when the mercury rises in the tube, that is when it is high, the weather will be fair; (2) when the mercury falls in the tube, that is when it is low, bad weather may be looked for; (3) when the mercury suddenly falls in the tube a storm is coming, and (4) when the mercury continues at a high point the weather will remain fair.
Fig. 35.—Barometer Tube.