Climate of Florida

In July the trade winds south of the northward-moving subtropical ridge expand northwestward into Florida. On occasion, dust from the Sahara moving around the southern periphery of the ridge moves into the state, suppressing rainfall and changing the sky from a blue to a white appearance and leads to an increase in red sunsets. Its presence negatively impacts air quality across the Southeastern United States during the summer, by adding to the count of airborne particulates. This is in sharp contrast to the normally clean air over Florida and the southeastern USA, which on average is the cleanest air in the USA.[5] Over 50% of the African dust that reaches the United States affects Florida.[6] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year.[7] Dust events are possibly linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.[8]

Fall foliage in North Central Florida.

On average, Florida has the mildest winters in the Continental United States. Average lows range from 65 °F (18 °C) in Key West to near 41 °F (5 °C) degrees Fahrenheit at Tallahassee, while daytime highs range from 64 °F (18 °C) at Tallahassee to 77 °F (25 °C) at Miami.[9][10] Predominant tropical easterly winds across central and southern Florida keep temperatures warm during the winter. Occasional strong cold fronts move southward down the peninsula with freezing or near freezing temperatures on a few nights into inland areas of central Florida every few years. A few times each decade Miami might see a winter night fall below 45 °F (7 °C). El Niño winters tend to be cooler due to increased cloud cover, but tend to have fewer freezes.[11]

Four hardiness zones exist. USDA zone 11b with average extreme annual lows between 45 to 50 °F (7 to 10 °C) is found from Key West to Marathon and 11a, 40 to 45 °F (4 to 7 °C), in the remaining keys and Miami Beach. Zone 10b is found in coastal South Florida and much of the Everglades where annual extreme low temperatures range from 35 to 40 °F (2 to 4 °C). Zone 10a, 30 to 35 °F (−1 to 2 °C) is found in the rest of South Florida, and in certain coastal regions as far north as St.Petersburg on the west coast and roughly Vero Beach on the east coast mainland. Next is zone 9b, 25 to 30 °F (−4 to −1 °C), across interior central Florida, changing over to 9a, 20 to 25 °F (−7 to −4 °C), just north of Orlando. The coolest, zone 8b, is located in northwestern Florida from roughly Gainesville and northwest including Tallahassee. Low temperatures range from 15 to 20 °F (−9 to −7 °C). Formally, there exist very small isolated areas near the Georgia border classified as zone 8a with extreme low averages of around 14.9°F.[12][13]

Florida has experienced 12 major freezes. This includes four "impact" freezes, sufficiently severe to kill entire groves of citrus trees, resulting in a noticeable economic effect on citrus growers, prompting them to shift groves further southward. These impact freezes are indicated by asterisks in the following: Great Freeze of 1894-5*, February 13–14, 1899, February 2–6, 1917, December 12–13, 1934, January 1940, December 12–13, 1957, December 12–13, 1962*, January 18–20, 1977, January 12–14, 1981, December 24–25, 1983*, January 20–22, 1985, and December 22–26, 1989*.[14]

Despite being the mildest on average, the winter climate was a crucial contributing factor of the Challenger disaster on January 28, 1986 in which overnight temperatures at Titusville, adjacent to the Kennedy Space Center, had dropped as low as 24 °F (−4 °C)[15] and were still below freezing at 28.0 to 28.9 °F (−2.2 to −1.7 °C) on launch day. The severe cold had caused the O-Rings on the right-side SRB to crack as they only had a redline tolerance of 39 °F (4 °C).

Typical summer afternoon shower from the Everglades travelling eastward over Downtown Miami.

During the summer, minimal temperatures range from near 70 °F (21 °C) in northern Florida to near 80 °F (27 °C) in the Keys. High temperatures during the summer average in the lower 90s Fahrenheit statewide.[16] Heat index can easily reach 103-110 degrees. Relief from the heat during the summer comes in the form of afternoon and evening thunderstorm activity, late morning and afternoon sea breezes off the relatively cooler ocean, and during the passage of a tropical cyclone. The record high temperature for the state is 109 °F (43 °C) at Monticello in 1931.[17]

Statewide, the highest rainfall amounts occur during the summer months. In northern Florida, there is a weak winter secondary maximum while statewide the driest months of the year are during the spring.[16] During El Niño, Florida sees greater rainfall between November and March.[11] Due to the lack of the secondary maximum across the peninsula, a distinct dry season is seen in the averages from winter through spring. This dry season provokes brush fires annually as temperatures rise during the late spring, before they fade during early June as the rainy season gets underway.

Hurricane Easy's Rainfall across the Southeastern United States

The heaviest rainfall to occur in 24 hours was measured in Yankeetown during Hurricane Easy (1950), 38.70 inches (983 mm). This is also the highest known point storm total maximum related to any tropical cyclone which has impacted Florida,[20] and by itself would be the highest known rainfall total for any month from any location within Florida. This rainfall amount remained the national 24-hour rainfall record until Tropical Storm Claudette (1979).[21] Heavy rainfall events have fallen due to stalled fronts near the state as well, and occur during the March through May and October through November timeframe.

The wettest month recorded at a Florida climate station was during May 1891 when Gainesville, Florida received 30.90 inches (785 mm).[22] The wettest year on record for a Florida climate station was during 1879 when 127.24 inches (3,232 mm) fell at Pensacola, Florida.[23] The driest year for a climate station statewide was during 1974 when only 19.99 inches (508 mm) fell at Key West.[24]

One of the worst years for wildfires was in 1998, at the end of an El Nino cycle. 480 wildfires consumed 500,000 acres (2,000 km²) statewide.[25] In 2010, the National Weather Service issued more fire alerts in Florida than anywhere else in the nation.[26]

A statewide drought began in November 2005, one month after Hurricane Wilma's passage through the state, and persisted until 2009.[27] The previous significant drought occurred in 2000, which was the state's driest year on record.[28]

Snow in Jacksonville on December 23, 1989

Snowfall is rare in Florida. The earliest recorded occurrence of snow or sleet occurred in 1774 in the far northern portion of the state.[29] The latest occurrence of snow or sleet fell on January 3, 2018, as a winter storm brought snow flurries in Tallahassee and northern portions of the state.[30]

The state record for snowfall is 5 inches (13 cm), set in northern Florida during January 1800, though some debate exists about the accuracy.[31] The earliest in the season that frozen precipitation has fallen was during the Late November 2006 Nor'easter on November 21 across central Florida.[32] The latest in the season that snow or sleet has fallen was on April 8, 2007. Snow flurries have been reported in the air (not on the ground) as far south as Homestead, during a January 19, 1977 event.[33]

There are more tornadoes per square mile in Florida than any other state.[35][36] However, these tornadoes tend to be much weaker and short-lived than in other states like the Midwest or Great Plains. Strong tornadoes do occasionally form in Florida, usually in conjunction with a cold frontal passage in the winter or spring. A total of 42 people died in February 1998 from the deadliest such tornado outbreak in Central Florida, which occurred during the nighttime hours.[37]

While tornadoes in the Midwest are more severe, a higher rate of deaths are experienced in Florida, and Brevard County, specifically, due to higher population density and quantity of manufactured homes.[38]