Part 8

Buys Ballot’s law of winds is, that in the Northern Hemisphere if one stands with his back to the wind, the low barometric pressure will be invariably to the left hand; in the Southern Hemisphere the lowest pressure is always to the right. This law explains one of the characteristics of low pressure storms.

AREAS OF HIGH PRESSURE

In speaking of low pressure storms we called them storm centers, because nearly always they are of sufficient intensity to bear that name, but in high pressure areas we do not speak of them as storm centers.

The Buys Ballot’s law applies to anti-cyclonic as well as cyclonic storms, that is, when one’s back is to the wind, the lowest barometric pressure is at the left and the highest at the right. This is probably understood by saying that in the cyclonic storms, the winds blow inward, contrary to the hands of a watch, and in the anti-cyclonic they blow outward, that is, in the same direction to the direction of the hands of the watch.

In the United States, the cyclonic storms are not as frequent as low pressure storms, and it is safe to say that probably not more than one-third of the entire anti-cyclonic areas can be classed as storm areas.

WHY AIR RISES

Another very interesting experiment is to secure a long-stemmed glass bulb (see Fig. 32). Arrange this apparatus as illustrated, with the stem of the bulb immersed in the water. The glass bulb condenses the air. When you first put it into the water nothing happens, but as soon as you apply heat the air bubbles come out of the end of the tube. This means that the air in the tube has expanded and part of it has come out through the stem of the tube and the remainder is lighter. It is well to remember, when air is heated it expands and becomes lighter. This fact is extremely important to remember, because it has a great deal to do with the important instrument, the barometer, which is used to measure the pressure of the atmosphere and is an important element in the question of humidity, as you will learn later. By this time you no doubt have learned that:

1. Air has weight.

2. Heated air expands, becomes lighter, and exerts less pressure.

3. Cold air comes from the side to take the place of hot air that rises.

When the rays of the sun heat an area of the earth, the air over such a place expands and becomes lighter, naturally rising, and the result of this is that the winds are produced by cool air moving in to take the place of the heated air. This cool air moves in from all directions. When such a thing happens at any point on the earth’s surface, it is known as a storm center, an area of low pressure.

WHAT IS A CYCLONIC STORM?

Because of the rotation of the earth on its axis, a force arises which tends to deflect to the right all motions in the northern hemisphere, and to the left all motions in the southern hemisphere. The winds flowing toward the storm center are turned to the right or left and move in a spiral around the storm center. This system of whirling winds around a central region of low pressure produce what is termed a cyclonic storm. Storms have a tendency to move in an easterly or northeasterly direction, and at a rate of from five hundred to seven hundred miles a day. Cyclonic storms, although we look upon them as being very severe, are very often mild and not of an intensive character.

WHICH WAY DOES THE WIND BLOW AFTER A STORM?

From the descriptions and experiments preceding, which illustrate the development of storms, reference was made only to the winds blowing in toward the storm center. Naturally the question comes to your mind: What happens to them after the cold air has taken the place of the warm air? They change to other directions when the storm has passed away. It is because of this fact that we look for a change in weather conditions when the wind changes—a very important sign that you will be interested in later on.

It is well to mention here a thing that is going to be very important to us when we study the barometer, that is, the pressure of the atmosphere. Should the pressure of the air, which is normally at sea level 14.7 pounds to the square inch, change, that is, become lighter, it would not exert so much pressure on the column of mercury in the tube of the barometer and the mercury would drop in the tube. (See Fig. 7.) On the other hand, if the weight of the air was increased, that is, if it became heavier, it would force the mercury to rise in the tube. This should be quite clear to you, because it is the lightness and heaviness of the air that is going to interest us more particularly than any other part of the subject when we get into the study of the atmospheric changes, what causes them, and the indications that lead up to our conclusions. In order that this principle is absolutely clear to you, you should perform Experiment 4, or if you have not facilities for doing it, it is well to see it performed in any physics laboratory.

Immediately you ask yourself: If air has such a tremendous pressure as 14.7 pounds to the square inch, why is it that a weight of air amounting to thirty-five thousand pounds bearing down on the average individual does not cave the body in? Simply because air penetrates the body so easily that it exerts as much pressure on the inside as on the outside, and thereby equalizes itself. For instance, if you go down into a subway or a caisson (a water-tight box or chamber within which submarine construction is carried on under great air pressure to keep out the water), where the pressure is sometimes greater than it is outside, have you noticed the effect this pressure exerts on the ear drums? As it becomes greater, you may equalize it by swallowing, which allows the air to get back of the ear drums through the Eustachian tubes, which lead from the mouth to the inner ear.

MOISTURE

Water vapor is always present in the air.

EXPERIMENT NO. 10

Expose a piece of dry potash to the air. You will soon discover that the potash will dissolve. It has taken up water from the air.

EXPERIMENT NO. 11

Put a piece of ice in a pitcher of water and allow it to stand in a warm room. You will soon notice that little beads of perspiration collect on the outside of the pitcher. This moisture is air being condensed.

Water vapor is part of the atmosphere. Some of it is always present in the air. The amount of vapor that the air can hold depends upon the temperature. When the temperature is warm, the air will hold more water. For instance, at 100° F. a cubic foot of air will hold 19.79 grains of vapor; at 80° F., 10.95 grains; at 50° F., 4.09 grains, and 32° F., 2.17 grains. At 32° F. is the freezing point on the Fahrenheit scale.

Air containing as much water vapor as it can hold is saturated. If the air is suddenly cooled down, that is, if the temperature falls when the air is saturated, air molecules are contracted, and it must give up the water, which produces rain. The ocean and the Great Lakes are the source from which the air gets its water. It rises into the air in the form of vapor, that is, vapor rising from the surface of the water, and the wind distributes it over the land. Condensation turns it into clouds, and when it is over-saturated, or rather, when the temperature drops and the air is unable to retain any more water, then it forms into drops of water and falls as rain. When the clouds get into the air, below the freezing point of the water, the drops of water are changed into ice crystals or snow flakes.

When the ice crystals are just at the point of melting into water, due to the rise in temperature, the snowflakes lose their form and the result is sleet.

HOW CAN WE USE THESE FACTS?

So far we have described, in a general way, certain facts about the elements of the air, such as temperature, pressure, humidity, precipitation, evaporation, clouds, winds, etc., and these facts of the elements enter into a very interesting phase of weather observation which we will designate as prophesying without instruments or forecasting by physical science. When we come to the more interesting and scientific part of weather observation, we will drop the word “prophecy,” because the instruments that are used to measure these elements are going to indicate certain things to us that will lead you to more definite conclusions. Hence, the following observations are what have given an opportunity to the weather prophet or to those people who have been credited with some mysterious power to prophesy what the weather is going to be. They are not definite or conclusive, and they cannot always be depended upon, but they certainly are significant and interesting, and a description of weather would not be complete without a list in chronological order of a series of phenomena or physical signs of this character that have lead certain men to gain quite a reputation for prophesying what the weather is going to be.