8 , TROPICAL AGRICULTURE 



As is well known, the Tropics are included within a zone 

 about the center of the earth extending 23^ north and south 

 of the Equator. The northern and southern boundaries of the 

 Tropics coincide nearly with the isotherm 68 F. for the cold- 

 est month of the year. If, therefore, the Tropics are defined 

 not as a geographical zone 47 wide, but as the area bounded 

 by the isotherm just mentioned, it will be found that this area 

 is only about 30 wide at the west coast of Africa and of 

 America instead of the normal 47. As is already indicated, it 

 is in the subtropics or so-called temperate climates that the 

 highest temperatures and greatest range of temperature are 

 recorded. For example, temperatures of 110 to 120 F. during 

 the summer months are not of rare occurrence in certain parts 

 of the mainland of the United States. In Jacobabad, India, 

 a temperature of 127 F. has been recorded. This locality 

 is outside of the tropical zone. Moreover, the high tempera- 

 tures which occur in summer in mainland cities like Washing- 

 ton, Cincinnati, St. Louis, and Chicago, are frequently accom- 

 panied with a high relative humidity making the weather com- 

 bination as a whole much more trying and difficult to endure 

 than the times of highest temperature in strictly tropical cli- 

 mates. Temperatures in the Tropics are affected by elevation 

 in the same manner as in temperate climates. Everywhere the 

 mean temperature falls about 4 for every 1,000 feet of eleva- 

 tion. At the Equator the elevation at which frost occurs is 

 about 18,000 feet. On the Island of Hawaii at an altitude of 

 20 north the frost elevation is about 4,500 feet. An idea 

 of the range of temperature in certain well known tropical 

 cities may be gathered from the following data: In Cairo, 

 Egypt, the mean winter temperature is 56 and the mean sum- 

 mer temperature 83 F. Bogota, Colombia, lies at consider- 

 able elevation and possesses the advatage of perhaps the most 

 remarkably uniform temperature of any city in the world. Its 

 average daily temperature is 60 F. the year round. In 

 Colombo a rather uniform temperature alternation occurs, giy- 



