288 THE SEASONS CLIMATE AND HEALTH 



you give some reasons why Illinois and Iowa are the greatest 

 corn-producing states of the Union? 



310. Summary. There are two chief causes of our summer 

 and winter seasons in the United States: (1) The change in 

 the altitude of the sun and the consequent change in its heating 

 power; (2) the great difference in the length of day. Both of 

 these are, of course, caused by the tipping of the earth's axis 

 23J^ from the perpendicular to the plane of the earth's orbit, 

 and the earth's revolution about the sun. Since these facts 

 are studied in geography further study of them is omitted here. 



II. CLIMATE AND LIFE 



311. Meaning of "Climate" and "Weather." By WEATHER, 

 we mean the condition of the atmosphere at some particular 

 time and at some particular place. The weather at Chicago 

 frequently differs considerably from that at Buffalo or Albany 

 upon any particular date, but the climate of these cities is 

 quite similar. In speaking of the weather we refer to the 

 temperature, the percentage of sunshine or cloudiness, the 

 wind, and the precipitation upon a particular date. 



By CLIMATE, we mean the average atmospheric conditions 

 existing in a certain locality for a period of time, especially as 

 they affect the animal and plant life of the region and the health 

 and comfort of the inhabitants. When considering the climate 

 of any region in reference to health we note: 



1. The average temperature and the changes in temperature. 



2. The average relative humidity and the changes in humidity. 



3. The prevailing direction and strength of the winds and how they 

 vary. 



4. The average relative amounts of sunshine and cloudiness. 



5. The average rainfall for the year and the particular seasons at 

 which it is heaviest and lightest. 



6. The prevalence of fogs and of dust and smoke in the air. 



7. The altitude of the region and the consequent density of the air. 



All these factors taken together determine, in a large measure, 

 the plant and animal life of the region and the health and com- 

 fort of the inhabitants. 



