206 THE WEATHER 



about 260 ft. How much would the weight of the air remaining 

 below the half-mile level be reduced (Arts. 135 and 208)? Compute 

 this decrease in pressure in tons for 1 sq. mi. of area. 



Ans. About 280,000. 



231. Why a Windy Day Seems Cool. In cool weather a 

 wind makes the day seem colder because it forces air through 

 the clothing and brings fresh supplies of cold air into contact 

 with the body to carry away its heat. The faster the body 

 loses its heat, the colder seems the day. 



232. The Lag of Temperature ; Afternoon and Night. 

 Since all heat comes from the sun the hottest hour of the day 

 might be expected at noon, and the coldest is often supposed 

 to be at midnight. But we have seen that the warmest hour 

 is usually in early afternoon and the coldest shortly before 

 sunrise. The reason is that, while the earth receives heat 

 from the sun by day, it is all the time losing heat by radiation 

 into space. Commencing ordinarily about sunrise a locality 

 receives more heat than it radiates. Therefore, heat ac- 

 cumulates and the temperature continues to rise. Usually 

 about 2 or 3 p.m. the loss of heat by radiation becomes 

 greater per minute than the receipt of heat from the sun. 

 At that instant the temperature of the locality begins to fall; 

 and it continues to fall throughout the night, until the in- 

 creasing amount of heat received from the rising sun exactly 

 balances that lost by radiation, when the temperature fall 

 ceases. The temperature therefore usually rises during the 

 forenoon and early afternoon, and falls through the re- 

 mainder of the afternoon and all of the night. 



There are exceptions to this rule: (1) The march of tem- 

 perature on cloudy days is often different because clouds 

 interfere with both sunshine and radiation. (2) In winter, 

 the temperature is often controlled by the wind which brings 

 air from warmer or colder regions, and may thus cause the 

 maximum or minimum temperature to occur at any hour of 

 day or night. 



