evaporated again into the air. The water precipitated as rain comes therefore from these two distinct sources, each of which may preponderate according to the nature of the storm. Thus, thunderstorms derive their supply mainly from land surface evaporation, while cyclonic storms which are caused primarily by warm moisture bearing winds blowing from the tropics, are fed largely by evaporation from the ocean.

UNITED STATES.

The normal annual precipitation, as will be seen from an inspection of the isohyetals, exhibits a decided variation in amount for different parts of the eastern United States. This variation is a climatic feature traceable to factors, both general and local in character, the most important of which are latitude, proximity to oceans, and altitude.

The annual amount of rainfall tends to diminish as the latitude increases. Broadly stated, in the eastern United States, the normal annual precipitation ranges from about 60 inches along the gulf coast to about 30 inches along the Canadian line.

Rains caused by moisture-laden air from the Gulf of Mexico and the Atlantic Ocean exert a marked influence in swelling the annual precipitation in the southeastern parts of the United States. Conversely, as the distance from these bodies of water increases this influence becomes less. This is particularly noticeable in the great plains region immediately west of the Mississippi River, where the lack of moisture from the gulf region causes the annual rainfall to diminish rapidly in a westward direction. The isohyetals here run nearly north and south with a marked easterly inclination in the northerly latitudes caused by the influence of these latitudes tending to decrease rainfall.

Increase of altitude usually is accompanied by increased rainfall, but this is largely local in effect. Except for these local variations, which may be very appreciable in amount, the effect of topography on rainfall is popularly much overestimated. Mountain ranges, by forcing air currents to higher altitudes, induce precipitation and so cause the annual precipitation to run high on the sides exposed to prevailing winds. This very action is, however, responsible for greatly decreased rainfall on the leeward sides of such mountain ranges, although the elevation there may also be considerable. The great plains region west of the Mississippi River rises 5000 feet in about 750 miles to the foot of the Rocky Mountains. Yet its normal annual precipitation shows a decrease in going westward of over 25 inches. This shows that latitude and proximity to the ocean may be more potent factors than altitude.

It is convenient to consider storms in the eastern United States as being of three classes: cyclones, West Indian hurricanes, and thundershowers.

Cyclones

Cyclones, frequently called extratropical cyclones, to distinguish them from the tropical cyclones or West Indian hurricanes, are disturbances of varying intensity which pass over the United States with more or less regularity, in a general easterly direction, and are usually accompanied by rainfall over extensive areas. Their action will be understood from the following considerations.

When a mass of air covering a large area moves from south to north there must be, as explained on page 47, a tendency towards the

accumulation of pressure on the right hand or east side of the moving mass; or, what is the same thing, the production of an area of low pressure on the western side. Conversely, if air moves from north to south the pressure will be low to the east of the moving mass and high to the west of it.

On the daily weather map, isobars or lines of equal atmospheric pressure at the earth's surface are drawn in accordance with the daily simultaneous observation of the pressure at stations scattered over the whole country. The high and low pressure areas are usually inclosed by oval shaped isobars, and are commonly spoken of as the highs and lows of the weather maps. The lows are generally more definite than the highs. The lows and highs are usually from 500 to 1000 miles apart and drift across the country from west to east in a more or less regular and constant succession.

A low pressure area, as shown on a weather map, furnishes a most convenient and useful epitome or short-hand indication of the weather conditions existing and impending over an area extending for several hundred miles in every direction from its center. Around a low the wind circulation is in general counter-clockwise, that is, in a northerly direction to the east of, and in a southerly direction to the west of the low. This gyratory or cyclonic movement has given rise to the use of the term cyclone as applied to weather disturbances accompanying low pressure areas. The result of this general wind circulation is that east of a low the southerly winds bring a slowly rising temperature, while west of a low the northerly winds produce cold. As a low in its easterly motion drifts over a given station, there is often a sudden change in wind and temperature, producing the marked cold waves particularly noticeable in winter.

In the area to the east of a low the southerly winds bring moisture from the Gulf of Mexico and Atlantic Ocean into the cooler northern latitudes, leading to the formation of clouds and precipitation. Hence, in this area there is always a more or less general rainfall. The intensity of rainfall is dependent on a variety of causes, sometimes it seems somewhat proportional to the strength of the winds. The condensation of water vapor apparently takes place chiefly in the lower mile of the atmosphere, and is probably assisted by local vertical displacements of masses of air in a manner similar to the action in tropical hurricanes and thundershowers to be described later. The prediction of weather changes is based chiefly upon the geographical movement of low pressure areas as determined by observation of previous similar cases. The lows are much more frequent, definite, and energetic in winter than in summer, with the result that our alternations of warm threatening weather and cold clear weather are

correspondingly more frequent and marked in the winter; but notwithstanding this fact, our precipitation is, on the average, greater in summer because of the greater capacity of the air for carrying water vapor on account of the higher temperatures prevailing during that

season.

In

About 120 cyclonic disturbances cross the country per year. passing across the continent, cyclonic centers show generally a tendency to avoid crossing areas of high elevation, such as mountain ranges and high plateaus, except in the cold season.

Rates of Travel of Cyclones and Precipitation Areas

Cyclones have a fairly definite rate of motion in passing across the continent. An examination of the charts published by the Weather Bureau, in Supplement No. 1 of the Monthly Weather Review, 1914, which illustrates types of storms and their movements, shows that the average hourly movement of a cyclone center varies from less than 20 miles in summer to over 40 miles in winter.

It might be supposed that the center of precipitation would have about the same velocity of translation as the center of the low pressure area, since it generally occurs in the south or southeast quarter of the latter in the eastern part of the United States. A study was made of 33 of the most important rainstorms in the eastern United States, for which data are available. Further reference to these, together with maps, will be found in a subsequent chapter. This study showed that the rate of travel of the area of precipitation was so erratic as to be practically indeterminable, although definitely greater in winter than in summer. About all that can be said is that the direction of movement is generally the same as that of the low. Several reasons suggest themselves for the erratic nature of rainfall travel: (a) the lows themselves are far from uniform in velocity of movement; (b) in the Upper Mississippi and Ohio River valleys the direction from which moisture bearing winds can approach a low is restricted; (c) rainfall gaging stations are too far apart to permit drawing accurate conclusions; (d) the rate of precipitation is far from uniform; (e) the distance from the center of the low to the center of precipitation varies greatly in the same and in different storms.

West Indian Hurricanes

West Indian hurricanes, or tropical cyclones, have many features in common with the extratropical cyclones just described, but differ from them in some important respects. The differences, however, are of consequence only when the tropical cyclone appears in its

typical form. When a tropical cyclone reaches a continental land area its prime characteristics of enormous wind velocities, calm central eye, and uniformly distributed cloud area, are rapidly transformed by the changed conditions and it takes on the characteristics of an extratropical cyclone. Thus it is only the Gulf and South Atlantic Coast regions in the United States which receive the tropical cyclone in its full force. The reasons for this will appear more clearly after this type of storm is described.

As its name implies the tropical cyclone originates in the tropics. Those which reach the United States are always formed north of the Equator, usually in from 8 to 12 degrees, north latitude, a region of calm or very light variable winds. The capacity of the air for water vapor is great and it is almost completely saturated, producing an ideal condition for vigorous convection currents. This is what actually takes place: The superheated moist air rises, some of the water vapor condenses at a relatively low altitude, releasing latent heat, thus giving new impetus to the rising current and consequently to the spirally inflowing air at the base of the convection flue. This of course results in a greater rate of condensation, release of latent heat, and increased velocity of convection current. Thus the embryo tropical cyclone grows and builds itself up, feeding on the energy stored in water vapor as latent heat. The winds soon attain great velocities, frequently more than 100 miles an hour, and as the velocity increases a very low pressure area is formed at the center of the whirl. Nimbus clouds, evenly distributed over the entire area of disturbance, except for the calm central eye, are formed by the rapid condensation, and torrential rains fall. The temperature and moisture of the atmosphere in all the quadrants is the same, the former being greater and the latter less in the calm central eye than in other parts of the area.

The necessary conditions for the development of a tropical cyclone, that is, great absolute humidity, a surface of little frictional resistance, and undisturbed terrestrial winds, are to be found only on a water surface near the equator, and, since in the northern hemisphere the trade winds come from the northeast, the most favorable conditions exist on the west side of an ocean. Furthermore, since tropical conditions extend farthest north in late summer, this is the most favorable time for the development of tropical cyclones in this hemisphere, and it is then that most of them occur. The direction of the spirally inflowing winds at the bottom of the convection flue is in a counterclockwise direction, due to the influence of the terrestrial wind system. In general, the cyclone moves in a northwesterly direction, slowly at first, and then with somewhat increased velocity. About latitude 30 the direction of movement of the tropical cyclone gradually changes