THE FACTORS WHICH DETERMINE CLIMATE 297 



about 2 o'clock in the afternoon, and that the lowest temperature is 

 usually about 5 or 6 o'clock in the morning. The records of the 

 Weather Bureau show that the relative humidity is usually highest 

 at about the hour of the lowest temperature and that the relative 

 humidity is lowest at about the hour of the highest temperature. 

 Review Art. 248 and explain why this should be so. Show that this 

 would necessarily be so unless there were a change in the absolute 

 humidity. Since weather changes alone produce changes in the 

 absolute humidity of the atmosphere, and since such weather changes 

 are likely to occur at any hour of the day, show that on the average 

 we should expect to find the morning relative humidity considerably 

 higher than the evening humidity. 



We frequently hear people speak of the dampness of evening air 

 and the danger of " catching cold" through being out in the evening. 

 Rarely do we hear any suggestion that morning air is damp, and we 

 never hear that there is danger of " catching cold" on account of 

 morning dampness. Judging from the average relative humidity, 

 evening and morning, as shown by these maps, and also from the fact 

 that the temperature is almost always lower in the morning than in 

 the evening, would you say that the theory that people " catch cold" 

 through exposure to the evening air because of its dampness or low 

 temperature is well founded? 



THE EXPLANATION. It is very possible, however, that there 

 is some truth in the supposition that we are more likely to suffer 

 from exposure in the evening than in the morning, not on 

 account of the dampness or low temperature of the evening 

 air, but on account of our own physical condition. In the 

 evening we are more or less exhausted tired out by the exer- 

 tion of the day's work; our vitality is at low ebb. During the 

 day's labor, waste materials accumulate in the system; our re- 

 sisting powers are correspondingly reduced. In the morning, 

 after the night's rest, our bodies are recuperated; our vitality 

 and resisting powers are at full flow. It is also often true 

 that we dress more warmly in the morning than in the evening. 

 We sometimes fail to put on extra wraps soon enough with the 

 fall of temperature in the evening. We doubtless do often 

 first realize that we are developing a cold during the evening 

 rather than the morning hours. But, if this is so, it is not be- 

 cause of damp air but is due to insufficient clothing and to our 



