dipping to , while readings of or below occur on average every several years, the last occurrence being January 7, 2014; the average window for freezing temperatures is October 31 thru April 6, allowing a growing season of 207 days. Summer is typically hazy, hot, and humid with long periods of temperatures and drought conditions at times. Louisville averages 35 days a year with high temperatures at or above , and the average window for such temperatures on average fall on June 7 and September 10, respectively. The mean annual temperature is , with an average seasonal snowfall of and an average annual rainfall of . The first and last measurable (≥) snowfalls of the season on average fall on December 8 and March 12, respectively. The greatest amount of precipitation in 24 hours was on March 1, 1997, and the heaviest 24-hour snowfall total was , occurring only two days before the all-time record low (see below). The wettest seasons are spring and summer, although rainfall is fairly constant year round. During the winter, particularly in January and February, several days of snow can be expected. January is the coldest month, with a mean temperature of . July is the average hottest month with a mean of . The highest recorded temperature was , which last occurred on July 14, 1936, and the lowest recorded temperature was on January 19, 1994. The record high daily minimum was on August 19, 1936, while the record low daily maximum was on January 12, 1918; the only other sub- daily maximum occurred on January 5, 1884. In 2012, Louisville had the fourth-hottest summer on record, with the temperature rising up to in July and the June all-time monthly record-high temperature being broken on two consecutive days. As the city exemplifies the urban heat island effect, temperatures in commercial areas and in the industrialized areas along interstates are often higher than in the suburbs, often as much as . A 2012 study by the Urban Climate Lab at Georgia Tech shows that Louisville has the highest index of the heat island effect in America. According to Brian Stone of the Urban Climate Lab and author of The City and the Coming Climate, "The average increase in the temperature difference between urban and rural environments in the Louisville area has been 1.67 degrees Fahrenheit every decade between 1961 and 2010. That's nearly double the