The state of Alabama is classified as humid subtropical (Cfa) under the Köppen climate classification. The state’s average annual temperature is 64 °F (18 °C). Temperatures tend to be warmer in the state’s southern portion with its proximity to the Gulf of Mexico, while its northern portions, especially in the Appalachian Mountains in the northeast, tend to be slightly cooler. Alabama generally has very hot summers and mild winters with copious precipitation throughout the year. The state receives an average of 56 inches (1,400 mm) of rainfall each year and experiences a lengthy growing season of up to 300 days in its southern portion.Hailstorms occur occasionally during the spring and summer here, but they are seldom destructive. Heavy fogs are rare, and they are confined chiefly to the coast. Thunderstorms also occur year-around. They are most common in the summer, but they are most commonly severe during the spring (i.e. March through May) and late autumn (i.e. November). That is when destructive winds and tornadoes occur frequently, especially in the northern and central parts of the state. Central and northern Alabama are squarely within Dixie Alley, the primary area in the U.S. outside the Southern Plains (i.e. the traditional Tornado Alley) with relatively high tornado risk. Alabama is ranked second in the U.S for the deadliest tornadoes. Hurricanes are quite common in the state, especially in the southern part. Major hurricanes occasionally strike the coast, such as Hurricane Frederic in September 1979 and Hurricane Ivan in September 2004; both storms resulted in significant to devastating damage in the Mobile area.
Summers in Alabama are among the hottest in the United States, with high temperatures averaging over 90 °F (32 °C) throughout the summer in many parts of the state. In the extreme south, summer’s heat is tempered slightly by winds from the Gulf of Mexico which often blow inland by up to 10–15 miles. In parts of northern Alabama, the elevation above sea level similarly tempers the summer heat. In summer, average afternoon high temperatures are especially high in Montgomery and Tuscaloosa (both averaging 92 °F (33 °C) in July and August). The average nighttime humidity is highest in southwest Alabama, which results in the highest average (high plus low temperature) for reporting stations in the state. While afternoon highs in Mobile are slightly cooler than those in Alabama’s inland cities, the warm nights produced by proximity to the Gulf of Mexico results in the average annual temperature for Mobile being the highest in the state. Cool morning temperatures help keep the northeast highlands among the more temperate locations in the state and lowest in the northeast among the highlands. Thus at Mobile the annual mean is 67 °F (19 °C), the mean for the summer 81 °F (27 °C), and for the winter 52 °F (11 °C); and at Valley Head, in DeKalb county, the annual mean is 59 °F (15 °C), the mean for the summer 75 °F (24 °C), and for the winter 41 °F (5 °C). At Montgomery, in the central region, the average annual temperature is 66 °F (19 °C), with a winter average of 49 °F (9 °C), and a summer average of 81 °F (27 °C). The average winter minimum for the entire state is 35 °F (2 °C), and there is an average of 35 days in each year in which the thermometer falls below the freezing-point. At extremely rare intervals the thermometer has fallen below 0 °F (−18 °C), as was the case in the extreme cold wave of 12–13 February 1899, when an absolute minimum of −17 °F (−27 °C) was registered at Valley Head. The highest temperature ever recorded was 112 °F (44 °C) in Centreville, on September 6, 1925 (https://www.ncdc.noaa.gov/extremes/scec/records). During the exceptional August 2007 heat wave, three instances of official 108 °F (42 °C) heat were recorded at Maxwell AFB in Montgomery, each tying the all-time record for the capital city. Winters are generally mild in Alabama, as they are throughout most of the southeastern United States, with average January low temperatures around 40 °F (4 °C) in Mobile, around 31 °F (−1 °C) in Huntsville, around 35 °F (2 °C) in Montgomery, and around 33 °F (1 °C) in Birmingham.