44 A MONTANE RAIN-FOREST. 



HUMIDITY. 



I am not able to give any figures or records to show satisfactorily 

 what differences there may be between the moisture conditions at 

 different altitudes on the Windward Slopes of the Blue Mountain 

 Range. The rainfall of 130 inches at Port Antonio is due to heavy 

 showers which are often confined to the vicinity of the coast. Localities 

 off the coast at 1,000 to 3,000 feet altitude receive less than that amount 

 of rain. The upper zone of heavy rainfall begins at about 4,000 feet, 

 and extends to the summit of Blue Mountain Peak. The fact that the 

 fall for New Haven Gap is 113 inches and that for Blue Mountain 

 Peak 168 inches indicates that the fall increases steadily with increase 

 of altitude. Even more important than the rainfall conditions is the 

 behavior of the cloud mass which is so characteristic of the windward 

 slope. Judging from my repeated visits to the windward side of the 

 range, from one to three times a week at all seasons of the year, I may 

 hazard the estimate that during February, July, and August these 

 slopes are enveloped in cloud for 30 per cent of the daylight hours, 

 and during the other months of the year for 70 per cent of them. The 

 nights are always clear, and it not infrequently happens in the winter 

 months that lower humidities occur at night than those prevalent dur- 

 ing the day. Several times I have watched the sunrise from Blue 

 Mountain Peak or from the Main Ridge in the vicinity of Sir John 

 Peak, and have noticed that it was only 5 to 15 minutes thereafter 

 when clouds began to form. An hour to two hours after sunrise there 

 would be a solid cloud blanket over the entire north slope. Detached 

 fragments from this cloud mass are being continually blown across the 

 main ridge and they melt quickly as they are borne down over the 

 sunny leeward slopes. I feel assured that on the windward slopes 

 above the lower limit of our area at 4,500 feet there are not any dif- 

 ferences of moisture conditions of a kind or amount capable of influ- 

 encing the vegetation. The cloud blanket gives a uniformity to the 

 conditions, which can scarcely be rendered pronouncedly different by 

 a rise of rainfall from as high an amount as 113 inches to 168 inches per 

 annum. 



The percentages of fog during the day at Cinchona are roughly 

 10 per cent for February, July, and August, and 30 per cent for the 

 other months, and it is to this difference rather than to its slightly 

 lower rainfall of 105 inches that we must look for the basis underlying 

 the principal habitat distinction which I have made that between 

 the leeward and windward sides of the range. The cloud blanket 

 seldom settles for any length of time over localities on the leeward 

 side below 4,500 feet, and the rapidity with which the rainfall dimin- 

 ishes below that altitude is shown in the 67 inches fall for Resource, 

 which is located southof Cinchona, at 3,700 feet (1,128 meters) elevation. 



