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Climatic Conditions of the Tortugas

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While no permanent records are available from the Tortugas, the writer assumes that the records from the office of the United States Weather Bureau at Key West, only 68 miles distant, will for this pur pose be applicable to the climate of the Tortugas. The records in this office have been kept since 1871 and are very reliable.

For botanical consideration the subject of rainfall is quite important. According to the Key West records the total average rainfall is 38.66 inches. The precipitation is quite varied and depends largely in sum mer on tropical squalls which suddenly appear on the horizon and sweep over the islands with a fierce rush of wind and driving torrents of rain. These perhaps last only 20 minutes to an hour. The maximum total precipitation for 24 hours, for the year 1914, for instance, was 4.80 inches, which occurred in November of that year. An idea of the varia tion may be gained by comparing this with the maximum precipitation for 24 hours in January of the same year, which was only 0.31 inch. It may be mentioned in this connection that the greatest precipitation in this region occurs during the months of September, October, and November. As stated above, the average annual rainfall for Key West (and presumably the approximate for Tortugas) is 38.66 inches. By comparing this with the total average precipitation for Miami, situated on the mainland of Florida, 125 miles north of Key West, which is 46.56 inches, it is seen that the average is slightly lower for the southern keys than for the main peninsula.

The average number of clear days for these southern keys is 151. During the summer months of May, June, and July the writer has lived in the Tortugas, when for periods of several weeks there was a continuous succession of clear, brilliantly sunny days with not even an occasional tropical storm or squall. According to the meteorological Summary for Miami, the total average of clear days is only 68 and partly cloudy 135. The comparison again, with the record for Key West, with its average of 151 days, taken together with the difference in total precipitation, shows the climate of the southernmost keys to be slightly drier than that of the mainland. This difference is perhaps due largely to the prevailing winds.

The prevailing winds are eastwardly, as reported by the Weather Bureau at Key West, and as also observed at Tortugas by Vaughan, in his paper on the influence of the prevailing wind in atoll formation, and also by the writer in various physiological experiments undertaken at the latter place. The velocity of these winds 18 very variable. The average velocity is 9.6 miles per hour. But in this region of hurricanes all winds over 75 miles per hour are rated as hurricane winds. These usually occur in the fall during the months in which the heaviest precipitation occurs, viz, September, October, and November, which months are called the "hurricane months." In the Gulf region, as recorded by the Key West office, the following severe hurricanes, of which the directions and velocities are here given, did a great deal of damage: On September 25, 1894, 87 miles per hour from the south west; October 19, 1896, 88 miles per hour from the southwest; October 11, 1909, 83 miles from the northeast; October 17, 1910, 100 miles per hour from the south. This last hurricane, known as the "great hurri cane of 1910," not only did vast damage on land but, as observed by the writer and others by means of a diving helmet and glass-bottomed boats, the coral and sponge fauna and the algal flora of the sea-bottom in the region suffered great changes and in some places on the bottom immense windrows of broken corals may still be seen which were mashed and heaped up by the force of giant wind-driven waves.

The temperature of the Tortugas is fairly constant. The variations in January and February, which are the coolest months, range from 60° F. as a minimum for these months to 75° F. as the maximum; in June, July, and August, which are perhaps the hottest months, the minimum is 77° F. and the maximum 88° F. The Key West records show the average annual temperature to be 76.8° F. and for the entire period over which the records extend—i. e., 1871 to 1913 inclusive—the maxi mum was 100° F. and the minimum 41° F. For Miami the mean annual maximum temperature is 80° F. and the minimum is 68° F.; that is, the annual mean temperature is slightly lower than that of the Tortugas and the southern keys.