Winter and summer temperatures have generally been above average since 1980. The horizontal black lines show the long-term (entire period) averages: (a) 16.1☏, (b) 66.7☏, (c) 2.6 days, (d) 4.4 nights, (e) 3.6 inches, (f) 11.8 inches. Bars show averages over 5-year periods (last bar is a 6-year average). Ice-cover duration on Lake Mendota has exhibited a consistent downward trend since the late 19th century (Figure 4).įigure 2: Observed (a) winter (December–February) average temperature, (b) summer (June–August) average temperature, (c) annual number of very hot days (maximum temperature of 95☏ or higher), (d) annual number of warm nights (minimum temperature of 70☏ or higher), (e) total winter precipitation, and (f) total summer precipitation for Wisconsin from (a, b, e, f) 1895–2020 and (c, d) 1900–2020. For example, the annual average maximum ice coverage during 2003–2013 was less than 43%, compared to the 1962–2013 average of 52%. Ice coverage in the Great Lakes has been declining since the 1970s. The increase in winter temperatures has also reduced lake ice cover. ![]() The number of very cold days has been near or below average since 2000, reflecting a winter warming trend (Figure 3). ![]() The lack of summer warming is reflected in a below average occurrence of very hot days (Figure 2c) and no overall trend in warm nights (Figure 2d). In 2012, a “killer frost” closely followed an abnormally warm March, resulting in significant damage to fruit crops. Warmer spring temperatures present the additional threat of frost-freeze damage to early-budding fruit trees. Like much of the Midwest, this warming has been concentrated in the winter and spring, while summers have warmed less (Figures 2a and 2b). The hottest year was 2012, with a statewide average temperature of 47.4☏, which is 5☏ above the long-term average. The hottest 5-year period on record was 2000–2004. Since the beginning of the 20th century, temperatures in Wisconsin have risen more than 2☏, and temperatures in the 2000s have been warmer than in any other historical period (Figure 1). Less warming is expected under a lower emissions future (the coldest end-of-century projections being about 2☏ warmer than the historical average green shading) and more warming under a higher emissions future (the hottest end-of-century projections being about 12☏ warmer than the hottest year in the historical record red shading). Historically unprecedented warming is projected during this century. Observed temperatures are generally within the envelope of model simulations of the historical period (gray shading). Shading indicates the range of annual temperatures from the set of models. Temperatures in Wisconsin (orange line) have risen more than 2☏ since the beginning of the 20th century. Projected changes for 2006–2100 are from global climate models for two possible futures: one in which greenhouse gas emissions continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Annual average temperatures vary from 39☏ in the north to 50☏ in the south.įigure 1: Observed and projected changes (compared to the 1901–1960 average) in near-surface air temperature for Wisconsin. The state has borders along Lake Superior to the north and Lake Michigan to the east, and the proximity to the lakes provides a moderating effect on temperatures for locations along the shorelines. However, periodic intrusions of cooler air from Canada provide breaks from summer heat. The summer is characterized by frequent warm air masses, either hot and dry continental air masses from the arid west and southwest or warm and moist air from the south. The winter season is dominated by dry and cold air, with occasional intrusions of milder air from the west and south. The southern part of the state experiences cold winters and mild to hot summers, while the northern part of the state experiences frigid winters and generally cool summers with brief bouts of excessive heat. ![]() Wisconsin’s location in the interior of North America and the lack of mountains to the north and south expose the state to incursions of bitterly cold air masses from the Arctic in the winter and warm, humid air masses from the Gulf of Mexico in the summer, causing a large range of temperatures across the state.
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