Part 7

Fig. 28 ]

There are really four simple cloud formations and three compound formations:

=1. The Cirrus Cloud.= (Fig. 24.)

The Cirrus cloud is always seen high in the sky and at a great elevation. Its formation is fibrous and it is particularly characterized for its many varieties of shapes. It also has a marked delicacy of substance and it is pure white.

=2. The Cumulus Cloud.= (Fig. 25.)

The Cumulus cloud is of moderately low elevation. It is a typical cloud of a summer day. It may be recognized by little heaps or bushes rising from a horizontal base. In summer-time we are all familiar with the cumulus clouds rising with the currents of air in huge masses. They form one of the most accurate indications of fair weather when you see them gradually dissolving. Sometimes these clouds become very large, and, while the texture is generally of a woolly white, naturally, when they assume such large sizes, they gradually change in color to a darkish tint.

=3. The Stratus Cloud.= (Fig. 26.)

This is the opposite of the Cirrus cloud, because it hangs the lowest of all, in gray masses or sheets, with a poorly-defined outline.

=4. The Nimbus Cloud.= (Fig. 27.)

Any cloud can be classed as a nimbus cloud from which rain or snow is falling.

Of the Compound Clouds we have:

Fig. 29 ]

1. The Cirro-Cumulus Cloud (Fig. 28), which has all the characteristics of both the Cirrus and the Cumulus. The most characteristic form of this cloud, and the one most commonly known, is when these clouds form small round masses, which appear to be cirrus bands broken up and curled up. This is what people call the “mackerel” sky.

2. The Cirro-Stratus Cloud (Fig. 29), which is known when the clouds arrange themselves in thin horizontal layers at a great elevation.

Fig. 30 ]

3. The Cumulo-Stratus (Fig. 30) is the cumulus and the stratus blended together. Their most remarkable form is in connection with approaching thunder storms, and are often called thunder heads. They rapidly change their outline and present a beautiful spectacle in the sky at times.

The Cirrus, Cirro-Cumulus and Cirro-Stratus are known as the upper clouds and the others are known as the lower.

ATMOSPHERIC DISTURBANCES

Disturbances of the atmosphere are classified as follows: Cyclonic, or low area storms, or anti-cyclonic, or high area storms.

Fig. 31 ]

The word “cyclone” to most people immediately means a terrific storm, whereas in weather observing the cyclonic storm is not really a cyclone or hurricane at all. It is a storm with an atmospheric pressure below average. Particularly important is the wind that blows about this area, which is always spirally inward, due to the rotation of the earth on its axis. This is probably why it is given the name of cyclonic storm, for it bears one of the important characteristics of a real cyclone. As the wind is deflected and moves into the storm center, it turns to the right and in the form of a whirlwind, spirally, moves around the storm center. (See Fig 31.) It is this whirling process that has given it the name, cyclonic storm.

As the air rises over the point of low storm area, or, in other words, the area of low pressure, and travels into the atmosphere, it is not permitted to rise to any great height, because it is always acted upon by the force of gravity and is being pulled back to earth again. We assume that because of this fact, this rising air which has been pulled back to the earth again piles up in certain places, causing the barometer to rise. Such a center as this is known as a high barometric center or the anti-cyclonic area. Here the circulation of the air is exactly opposite to that of the cyclonic area.

Fig. 32 ]

We are all more or less acquainted with these anti-cyclonic storms, because in winter these great masses of air rise up from the warm areas, pile up, and form high pressure areas over the mountains of Canada, and soon this high pressure works down upon us as blizzards and cold waves.

We have described quite minutely the movement of the wind about these points of high pressure and low pressure and have shown you the map and have illustrated the high pressure and low pressure areas, but there is still another feature that is of great importance to us, and that is the movement of the storms and the fact that storms have a progressive movement from west to east.

These storms move more rapidly in the United States than elsewhere, and are more rapid in their movement in winter than in summer. Their speed is almost one half again as great. The average velocity of the low area storm in the United States is about twenty-five miles an hour in June, July, August, and September, and from October on they continue to increase.

LOW PRESSURE

We can summarize low pressure storms generally in the following manner: They have a wind circulation inward and upward and are elliptical in form. Their velocity varies from six hundred to nine hundred miles per day, moving in the same general direction. They are characterized in their eastern quadrants by cloudy weather, southerly and easterly winds, precipitation, temperature oppressive in summer and abnormally high in winter, falling barometer, increasing humidity and followed by clear weather, rising barometer, decreasing humidity and falling temperature in the western quadrants.