Forecasting the Weather 



287 



day. If the mountain were warmer 

 than the cloud mass there would be no 

 condensation, but some of the moisture 

 of the cloud would be evaporated and 

 float away in the prevailing winds. 



An unusual amount of moisture in 

 the air in summer produces a feeling of 

 closeness ; physical labor is more ener- 

 vating that when the air is drj r and crisp. 

 The change from sultry, oppressive 

 weather is nearly always brought about 

 by a series of thunderstorms, sometimes 

 lasting over two days. 



Summarizing the indications that may 

 be drawn from the temperature and 

 moisture of the air, it would appear that 

 an increase in the amount of moisture 

 in the air is a sign of a change from fair 

 to foul weather, both winter and sum- 

 mer. In the colder months an increase 

 in the temperature of the air above the 

 average for the season, coupled with an 

 increase in moisture, is a sign of rain or 

 snow within twenty-four to forty-eight 

 hours. In the summer an increase of 

 temperature alone is not always an indi- 

 cation of rain ; but these are not in- 

 fallible rules. The old proverb, "All 

 signs fail in dry weather," is as true 

 today as when first formulated. 



THE WIND AND ITS SUCCESSIVE 

 CHANGES 



The wind is less prophetic in character 

 than the clouds, since it is affected by 

 the form of the land over which it blows. 

 Thus it has a tendency to blow up a 

 valley in the daytime and in the contrary 

 direction at night, no matter in what 

 direction the valley may extend. Winds 

 also have a tendency to blow toward and 

 up the sides of a mountain slope in the 

 daytime and down the side of a moun- 

 tain at night, and this movement of the 

 air generally extends for some distance 

 out from the foot of the mountain on the 

 level slopes. There are also the well- 

 known land and sea breezes of all coun- 

 tries, where during the t went}'- four 

 hours of the day the temperature of the 



land becomes alternately warmer and 

 colder than that of the sea. These winds 

 (valley, mountain, land, and sea breezes) 

 are called diurnal winds. They are 

 caused by differences in temperature 

 that are not general, but confined to the 

 valley or mountain slope of a particular 

 locality. In order that these differences 

 of temperature may arise there must be 

 clear weather and unobstructed sun- 

 shine. It is easily seen, then, that all 

 such winds must be most active in fair 

 weather, and that when they cease, or 

 fail to appear at the usual time, the 

 atmosphere as a whole must have come 

 under an influence greater than that 

 which produced the diurnal winds. 



In the open country or other exposed 

 situation where the true direction of the 

 wind can be determined, it should be 

 noticed, first, what is the prevailing di- 

 rection of the wind in fair weather, and 

 what is the direction from which storms 

 usually come. The direction of the 

 wind during the twenty-four hours im- 

 mediately preceding the storm should 

 be especially noticed . To do this a short 

 journal or diary of the weather should 

 be kept. The direction of approach of 

 storms in the United States varies in 

 different localities. It is quite impor- 

 tant that each observer determine for 

 his immediate neighborhood the shift of 

 the wind with the approach of storms, 

 during the colder months at least. 



In the warm months the winds are 

 light and rather variable, and changes 

 in direction have not the same impor- 

 tance as in the colder months. The rain 

 of summer generally occurs in connec- 

 tion with thunderstorms; it will be found 

 that these are most frequent for a certain 

 direction and with the wind in a partic- 

 ular quarter. Beyond the fact that more 

 thunderstorms come from a westerly 

 quarter than from any other direction, 

 little can be said that will be of value 

 in forecasting their approach by the di- 

 rection of the surface winds only. The 

 coming of a thunderstorm can generally 



