Southern latitude and maritime influences are the primary regulators of temperature in the Keys. Although situated just north of the Tropics, the Keys bear the tropical climate classification (i.e. the average temperature in the coldest month, January, is 68.7). Because of the nearness of the Gulf Stream in the Straits of Florida, about 12 miles south and southeast, and the tempering effects of the Gulf of Mexico to the west and north, the Keys have a notably mild, tropical maritime climate in which the average temperatures during the winter are only about 16 degrees lower than in summer. Temperature variations throughout the year average only about 10 degrees between the daily high and low. Continental cold fronts approaching from the north in winter are strongly modified by the warm ocean water. Consequently, there is no known record of frost, ice, sleet, or snow in the Keys.

There is little variation in temperature norms and extremes from one end of the Keys to the other, but the slightly lower monthly temperatures at Tavernier in the upper Keys can be attributed to its closer proximity to, and therefore the effects of the mainland compared to Key West.