2 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 1 10° 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 advantage 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- 



