As an anticyclone which travels north of us approaches, the wind shifts into the northwest, becoming cool and dry. In the winter the sudden change of temperature which gives us a cold wave is due to a high coming from the west or northwest, closely following a low. When the highs move through the Gulf states, the effect on the weather of the country to the north is very different. This is quite marked in summer, when the highs and lows may move eastward rather slowly. When a low moves slowly across the northern part, with the high moving across the southern border, we may have a hot wave. For days there may be a surface wind blowing up across the Southern states. A succession of these highs moving south of us will give an almost unbroken period of hot weather. But when the summer highs travel north of us, and the lows to the south, we have unseasonably cool weather. (These statements apply to the part of the United States lying east of the Rocky Mountains.)

The daily weather maps show the position of the highs and lows; and since their probable motion is known, the weather can be forecast for any locality.

300. Thunderstorms. Thunderstorms, especially the severe ones, usually occur in the late afternoon of a hot, sultry, and oppressive day, a day when dew will form on the outside of a glass containing only moderately cold water. During the day there has usually been a light breeze from the south or southeast, but the air feels stagnant. The storm usually appears in the west and moves eastward, often against a light southeast breeze. The typical thunderstorms usually occur in the south or southeastern section of a low, where the surface wind is a warm, humid layer of air blowing from the south or southeast. This warm air is usually forced in under a large mass of cold air, developing an unstable condition, so that here and there the warm lower layer breaks through and starts a convection current.

There are many types of thunderstorms, but they all have the same cause. They are caused by the rising of warm, moist air. As this air rises it is cooled by expansion until the dew point is reached, when condensation begins. In a well-developed storm the uprush of air is so great and carried to such a height that there is a very rapid rate of condensation, and the rainfall may be very heavy. The storm usually extends up into the prevailing westerly of the higher altitudes, so that it is carried along by this eastward drift of the atmosphere.

The motion of the air currents in the typical thunderstorm is of special interest. Fig. 178* shows a section of one, with the direction of the wind indicated by arrows. The storm is moving to the right. In front of it the warm, moist air is moving toward and up into the storm. This current supplies the water vapor which is condensed. The descending currents are cold air from above,

FIG. 178. Ideal section of a typical thunderstorm. A, ascending air; D, descending air; C, storm collar; S, roll scud; D', wind gust; H, hail; T, thunderheads; R, primary rain; R', secondary rain

which takes the place of the rising currents. To be more exact, it is this heavy, colder air that forces up the warmer and lighter air in front. The sudden drop in temperature as the thunderstorm approaches, caused by this cold air from above, is always well marked. The upward rush of air in front and the descending air in the storm cause the whirl, or rotational motion, about a horizontal axis which is typical of the front part of our summer thunderstorms.

The heat liberated by the condensation of so much water vapor furnishes much of the energy of such a storm. This heat warms the uprushing currents, so that even at a high elevation the air is warmer than that on either side of it. This increases the speed at which it rushes up and also the speed of the downward-moving cold air. Sometimes this upward-rushing air carries with it drops of water to such an elevation that they become frozen. They then fall as hail. Rain cannot fall through air with a relative velocity greater than 8 meters per second (sect. 110); hence if the upward current of air has a velocity greater than that, the drops are all carried upward. (It is now known that the intense electrification produced in such a storm is due largely to the way in which the uprushing air breaks the drops of water into a spray. Electricity is generated by such a process.)

301. Tornadoes. Rotational motions of the air are formed on all sorts of scales. They vary in size from the small dust whirl, or whirlwind, so commonly seen, to large, cyclonic motions several thousand miles in diameter. The small dust whirl is usually started by a convection current which forms on a sunshiny day over a hot roadway or bare ground. The air which rushes in is deflected by obstacles, so that a rotary motion is given. The direction of this rotation is purely accidental; but in the larger storms the direction of the rotation about a vertical axis in the Northern Hemisphere is always counterclockwise.

The relatively small rotary storms which have so great a destructive effect in the United States are tornadoes. They have a very well-developed counterclockwise rotation about a vertical axis, and always occur in the southern or southeastern part of a cyclone and from two hundred to eight hundred miles from its center, under conditions which are favorable for severe thunderstorms. Apparently the cause is a very violent upward convection current. The air blowing in from the sides is deflected, either by the rotation of the earth or, more probably, by the general cyclonic motion in which it is an eddy, so that a counterclockwise whirling motion is produced. On account of the violent uprush of the air in the center, the whirling mass of air moves toward its center rapidly. It has been explained before that when air or any mass which has angular momentum has its radius of rotation decreased, there must be an increase in the speed. It behaves like water running out of an opening in the bottom of a washbowl (sect. 135). Certainly the wind in a tornado does attain enormous speeds. This speed has never been measured, but estimates indicate that it may be as high as five hundred miles per hour. On account of the enormous speed of rotation the centrifugal forces are very great, and atmospheric pressure on the outside of the tornado must be very much larger than that at the center. The pressure at the center of a severe tornado has never been measured, but it must be quite low.

The destructive effect is due not only to the high speed but also to the sudden drop in the barometric pressure, causing the air inclosed in buildings to expand outward with explosive violence. Windows and sometimes walls are forced outward.

The path of a tornado is from a few feet to a thousand yards wide and from a mile to several hundred miles long. Tornadoes occur almost exclusively in the part of the United States which lies east of the Rocky Mountains and are most frequent over level country. In hilly or mountainous country the topography encourages the flow of convection currents, and this is probably the reason that the excessively unstable conditions which sometimeş occur over a level country are avoided.

302. The averaging of temperature. It is found that the daily temperature of any one place, when averaged over twelve months, is nearly a constant quantity, varying only a few degrees from one year ear to another. It does not make any difference what are the limits of the period taken, whether from January to January or from June to June. For example, a very hot August can be averaged in with the preceding eleven months or with the succeeding ones. But because this average is approximately constant, many argue that the weather must change in such a way that this average will be a constant; for example, a hot summer must be followed by a cold winter. Does the student think this reasoning sound?

The temperature of the earth's surface is determined by the rate at which it receives heat from the sun and by the rate at which it loses heat into space. Since both these rates, when averaged over the surface of the earth, are practically constant, it follows that the temperature at any one time, averaged over the entire surface of the earth, must be a constant.* If at any place the temperature is unusually high, it is because it is receiving more than its share of the warm winds. Some other place must be getting the cold winds. Indeed, the facts show that when one locality has a hot season, another has a cold one. The average of temperature is struck at the time, not months later.

A study of the factors that cause changes in the weather shows that when it is unusually cool in one locality it should be warmer in some other. For example, a high traveling eastward should bring warmer weather to the regions north of its track and cooler weather to those lying to the south.

303. The averaging of rainfall. There is a relatively small amount of water stored in the form of vapor in the atmosphere. Estimates show that if all the vapor were condensed and fell as rain, it would give less than two inches of water. In other words, the storage capacity of the atmosphere is relatively small, and water must fall as rain at almost the same rate at which it evaporates. The greater part of the earth's surface is water, and the total evaporation from this is fairly uniform; hence the total daily amount of water falling as rain must be approximately constant. From this it follows that if any section of the country is having a rainfall above the average, there must be other localities which are not receiving their share. There can be no such thing as a world-wide drought nor a world-wide excess of rain.

PROBLEMS

1. The dew point was found to be 12° C. when the air temperature was 20°. Find, by the use of the tables in the text, the absolute and relative humidities; also the pressure of water vapor.

2. On a humid day in summer the dew point was 78.8° F., and the barometer 74 cm. What part of the atmospheric pressure was due to the water vapor? 3. The relative humidity on a day when the temperature was 30° C. was 63 per cent. Compute the mass per cubic meter of water in the air.

4. The dew point was found to be 68° F. on a day when the temperature was 86° F. Find both the absolute and relative humidities.

* Any appreciable change in the average temperature of the earth would mean a tremendous change in its thermal energy.