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From the first table of mean temperature, supplied originally by the Surgeon General's Office, that excellent writer on the Cli. matology of the United States, Lorin Blodget, has indicated the following isothermal lines for Florida :
The spring isothermal of 70°, enters a little south of St. Augustine, and inclining north as the more elevated portion of the State is reached, rises above the thirtieth degree of latitude, and again declining south towards the Gulf, follows the thirtieth parallel of latitude along the Gulf coast of Middle Florida. The spring iso. thermal of 75o crosses the peninsula below the 27th degree of latitude, and has a considerable northward curve.
The summer isothermals of 81° strike each of the peninsula coasts between the 27th and 28th degrees of latitude, and respectively follow the coast-lines. Those of 82° traverse the interior of the peninsula, the one on the east side of the centre pursuing its course into Georgia, while that on the west side runs through Middle and West Florida. These summer isothermal lines follow the length of the peninsula, and consequently in it have a northwest and southeast direction.
For autumn the isothermal of 70° enters on the 30th parallel of latitude, and curving a little south as the more elevated region is reached, rises again even north of that parallel near the Gulf, and follows the coast of Middle and West Florida. That of 75o crosses the peninsula between the 27th and 28th degrees of latitude, passing out on the Gulf side a little south of its opposite point of entrance. The lowest position of an isothermal of 72o for the same season is indicated as crossing the peninsula between the 28th and 29th degrees of latitude. This has also a little southerly curve as it crosses the middle and western slope of the peninsula.
For winter the isothermal of 55° enters the State a little north of the 30th parallel of latitude, and curving southward in the ele. vated region, again rises north ward near the Gulf coast, which it follows through Middle and West Florida.
That of 60° crosses the peninsula near the 29th degree of latitude. The lowest posi. tion of isothermal of 62° is indicated as crossing the peninsula a little south of the 28th parallel of latitude. The isothermal of 65° crosses the peninsula between the 26th and 27th degrees of latitude. A minimum isothermal of 50° is also indicated as begin. ning at New Smyrna, descending south west to the centre of the peninsula, and thence running west to Tampa. Between the points of entrance on the Atlantic and of exit on the Gulf of this mini..
mum isothermal, there is a difference of three-fourths of a degree of latitude. While this isothermal is only occasional and not constant for the season, its great southerly deflection can only be explained by the greater altitude and drier atmosphere of the centre and western slope of the peninsula. To the same causes is to be also attributed the southerly curves of the winter isothermals in crossing the central ridge of the peninsula, and their exit on the Gulf side a little south of their opposite points of entrance.
The isothermal of 70°, as the mean temperature for the year, crosses the peninsula about the 29th degree of latitude ; and that of 75° a little south of the 27th degree of latitude. The lowest position of an isothermal of 72° is indicated as crossing the peninsula about the 28th degree of latitude.
Exposed as Florida is on the east to the Atlantic and on the south and west to the Gulf of Mexico, and having a large area of level country on the north, her climate, owing to frequent changes in the direction of the wind, may be considered rather variable during winter and spring as regards transitions of temperature. During March, 1873, the minimum temperature at Punta Rassa was 38°, a degree of cold sufficient for light frost in interior elevated districts. At the same place in December, 1872, the minimum was 35°, which may be considered as fully representing a light frost in the interior. A temperature of freezing, 32° for March has occasionally occurred at Tampa, and light frosts for the same month are almost an annual occurrence.
The average minimum temperature at Tampa for winter for a period of twelve years is 34o.4; though the thermometer may some winters fall even below 30°. It was down to 30° in 1843, 1849, and 1852. In 1857 the thermometer fell to 26° at Tampa, 32° at Fort Myers, 29° at Fort Pierce on Indian River, and to 30° at Fort Dallas on the Miami.” It is highly probable that such a depression has been reached sev. eral times since. In 1835 the thermometer, it is said, fell at Fort King (near Beala) one degree north of Tampa, to 11°, or 21° below freezing. At the same time it is reported to have fallen 7° below zero in the latitude of St. Augustine, and that “all kinds of fruit trees were killed in the ground and extensive orange groves destroyed."
Now, when it is remembered that the difference in the latitude
| See Blodget's Isothermal Charts.
of the two places does not amount to a degree, it is impossible to credit the statement and believe that there was between the two places a difference in temperature of 18o. Besides there is no authority for it outside of historical tradition. Assistant Surgeon R. F. Simpson, writing from Fort Dallas in April, 1857, says: " There was frost and ice on December 25 and 26, 1856, and January 20, 1857, with the thermometer at 30° at sunrise." Remembering that Fort Dallas is low down on the eastern coast of the peninsula, below the 26th degree of latitude, it becomes very ques. tionable whether there is any part of the peninsula universally exempt from frost, though still of not sufficient intensity to materially affect tropical plants. So far as I know, there are no records for the same year (1857) in the upper eastern part of the State; but at Mount Vernon Arsenal, near the Florida and Alabama line, in latitude 31°12' with 200 feet altitude, the minimum was 10°, or 22° below freezing. This difference of 20° of temperature between Fort Dallas and Mount Vernon Arsenal would only give about 3° difference of temperature for each degree of latitude. Now, remembering that the winter of 1856–57 was the coldest of which we have any record-although the observations at Tampa embrace twenty-seven years—we are compelled to discredit the excessive degree of cold for St. Augustine in 1835. Nevertheless these facts go to demonstrate pretty conclusively that during some winters a considerable degree of cold, as compared with the mean winter temperature, is experienced in every section of the State, though occurring at rather irregular and mostly long intervals of several years. Such depressions are never of such intensity south
. of the 29th degree of latitude as to jeopardize bearing sweet orange trees, though sometimes fatal to those of only a few years' growth, and such perennial tropical plants as the banana, pine-apple, etc. In these unusual depressions, however, the climate forms no exception, as the same thing does occur in the milder temperate latitudes of the Eastern Hemisphere.
The maximum temperature of summer generally ranges from 92° to 95°, rarely exceeding the latter except in the northern part of the State. This fact is so contrary to the impression generally entertained by the public outside of the State, that to many the statement appears at first incredible. It need not appear so strange, however, when it is remembered that in the north temperate zone, the days are longer and the nights shorter during summer as we advance from the lower to the higher latitudes, and that
consequently the rays of heat from the sun are longer concentrated on the earth's surface with a proportionately shorter night for cooling by radiation. The reverse being the case in the winter, it is thus that an equal distribution of heat for the year in the lower and higher latitudes of the same zone is insured, the winter deficiency being compensated by the summer excess.
The regular alternation of the land and sea breezes, the latter being the cooler by several degrees, greatly ameliorates also the summer heats of Florida, and markedly so all along her extensive coast. In corroboration of these statements I quote from Sur. geon-General Lawson, who says:
“The climate of Florida is remarkably equable and proverbially agreeable, being subject to fewer atmospheric variations, and its thermometric ranges much less than any other part of the United States, except a portion of the coast of California.
“For example, the winter at Fort Snelling, Minnesota Territory, is 48° colder than at Fort Brooke (Tampa), but the summer at Fort Brooke is only about 8° warmer. The mean annual temperature of Augusta, Georgia, is nearly 8°, and at Fort Gibson, Arkansas, upwards of 10° lower than at Tampa, yet in both those places the mean summer temperature is higher than at Tainpa. In the summer season, the mercury rises higher in every other part of the United States, and even in Canada, than it does along the coast of Florida. This is positively shown by meteorological statistics on my file in this bureau. At Portland, and on the coast of New England, the thermometer attains an average height in summer of 94° Fahrenheit, and in winter descends to—7°, having an annual range of 101o. At Fort Mifflin, near Philadelphia, the thermometer has an annual range of 87°, rising to 95° Fahrenheit in summer, and falling to go in winter. At Fort Moultrie, Charleston barbor, the average range is 70°, being in summer 90°, and in winter 20o. At Fort Pierce (or Capron), Indian River, Florida, the range is 59°, the annual range averaging for its highest point 94°, and the lowest 35o. At Tampa, the annual range is 58°,
, being 92° in summer, and 34° in winter; while at Key West the annual average range is only 35°, being for summer 89°, and 54° for winter. If we were to extend our comparison by including the stations in the interior of the United States, remote from the influences of the sea-breezes, the difference would be still more apparent. The point under consideration will, however, be illustrated sufficiently by comparing the temperature of Florida, as