PDF - Wallace Online

i CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 221places, 90 Fahr. being sometimes reached with us and notbeing often very much exceeded at Batavia.Causes of the uniform High Temperature near the EguatwIt is popularly supposed that the uniform high temperatureof the tropics is sufficiently explained by the greater altitude,and therefore greater heating-power of the midday sun ;buta little consideration will show that this alone by no meansaccounts for the phenomenon. The island of Java is situatedin from six and a half to eight and a half degrees of southlatitude, and in the month of June the sun's altitude at noonwill not be more than from 58 to 60. In the same monthat London, which is fifty -one and a half degrees of northlatitude, the sun's noonday altitude is 62. But besides thisdifference of altitude in favour of London there is a still moreimportant difference, for in Java the day is only about elevenand a half hours long in the month of June, while at Londonit is sixteen hours long, so that the total amount of sun-heatreceived by the earth must be then very much greater atLondon than at Batavia. Yet at the former place the meantemperature of the day and night is under 60 Fahr., whilein the latter place it is 80 Fahr., the daily maximum beingon the average in the one case about 68 and in the otherabout 89.Neither does the temperature at the same place dependupon the height of the sun at noon for at Batavia it is ;nearlyvertical during October and February, but these are far frombeing the hottest months, which are May, June, and September,while December, January, and February are thecoldest months, although then the sun attains nearly itsgreatest altitude. It is evident, therefore, that a difference of30 in the altitude of the sun at noon, at different times ofthe year, has no apparentinfluence in raising the temperatureof a place near the equator, and hence we conclude that otheragencies are at work which often completely neutralise theeffect which increased altitude must undoubtedly exert.There is another important difference between the temperateand tropical zones, in the direct heating effect of thesun's rays independently of altitude. In England the noondaysun in the month of June rarely inconveniences us or