The first heat wave of the summer has come and gone, roasting much of the central and eastern United States during the final two weeks of June.
The hottest conditions in Wisconsin were felt between June 19 and 23, when temperatures reached the 90s, and 70-degree dew points caused heat indices to exceed the triple digits. The hottest temperature reported in the state was 98°F in Kenosha (Kenosha County) on both June 22 and 23, setting their new daily record for those days.
Over 365 observers and automated weather stations have been recording daily maximum temperatures around Wisconsin over the last 132 years.
Historically, the hottest day of the year in Wisconsin is July 19, with a statewide average high temperature of 82.2°F. This is nearly one month following the peak in the incoming solar radiation on the summer solstice.
Temperatures peak after the maximum solar radiation because it takes time for the ground and lower parts of the atmosphere to warm, like how water temperatures in lakes are warmest in August rather than July. This is called thermal or seasonal lag.
Triple-Digit Temperatures
One way of gauging the intensity of summer heat in the Badger State is to look at the number of times that the temperature has reached at least 100°F.
Nearly 74% of all the recording stations have reported high temperatures reaching the century mark at least once; many of the others had too short a record or were in northern sections of the state.
The earliest that a 100-degree day has occurred in a year was on May 31, 1934, in Prairie du Chien (Crawford County) with a high of 109°F!
The latest occurrence of 100°F in any year has been in September, with Richland Center (Richland County) reaching 104°F on September 10, 1908.
The highest temperature in state history was 114°F recorded in Wisconsin Dells (Columbia County) on July 13, 1936.
| Location | Hottest recorded temperatures (°F) | Number of triple-digit temperatures recorded | Longest stretch of 100°F |
|---|---|---|---|
| Eau Claire | 111 (July 14, 1936) | 75 | 11 days (July 6 – 16, 1936) |
| La Crosse | 108 (July 13, 1995) | 75 | 9 days (July 7 – 14, 1936) |
| Wausau | 107 (July 13, 1936) | 15 | 4 days (July 11 – 14, 1936) |
| Madison | 107 (July 14, 1936) | 36 | 5 days (July 10 – 14, 1936) |
| Milwaukee | 105 (July 24, 1934) | 34 | 4 days (July 8 – 11, 1936) |
| Green Bay | 104 (July 13, 1936) | 17 | 4 days (July 7 – 10, 1936) |
An interesting point of note is the larger number of hot days in La Crosse and Eau Claire compared to Madison, despite being north and west of the capital city.
Since these cities are farther west, they are often subjected to warm, southwesterly winds from the Central Plains beginning earlier in the season. Additionally, the sandy soils of the western regions can dry out and heat up faster in the spring compared to the clay or rocky soils of other regions of the state, giving them a warmer starting point by the time summer months arrive.
Heat Safety
In recent years, attention has also been paid to how elevated levels of humidity contribute not only to discomfort but also pose a health hazard.
Since 1996, heat has been the second most common cause of weather-related fatalities in Wisconsin, behind cold weather. The Department of Health Services reports that an average of eight heat-related deaths occur in the state every year.
The National Weather Service (NWS) calculates anticipated values of the heat index when issuing heat-related alerts to the public. These alerts are issued when heat index values approach levels that are dangerous for human health.
The heat index, also called the apparent temperature, is what the temperature “feels like” to the human body when the relative humidity is combined with the air temperature.
When the heat index is between 90°F and 105°F, heat cramps and heat exhaustion are possible with prolonged exposure and/or physical activity. Someone experiencing symptoms of heat exhaustion should rest in a cool area and rehydrate. If heat exhaustion symptoms aren’t treated, heat stroke can develop.
Heat stroke is possible with prolonged exposure and/or physical activity when the heat index is between 105°F and 130°F. Someone experiencing symptoms of heat stroke should seek medical attention as soon as possible.
Heat Index Records
Since the mid-1990s, the hottest heat index measured in the state was 124°F at the Kenosha Regional Airport in July 1999. Over a dozen cities reached triple-digit temperatures during that late-month heat wave.
The killer heat wave of July 1995 resulted in heat index readings of 123°F at Madison’s Dane County Regional Airport and 122°F at Milwaukee’s Mitchell International Airport. That event resulted in 71 directly heat-related deaths in the state.
On the opposite end, Rhinelander’s Oneida County Airport has the ”coolest” maximum heat index of any Wisconsin Automated Surface Observing System station, with 106°F, which was reached in June 2018.