Chicago's highest official temperature is . It was recorded on July 24, 1934, There was a reading of recorded at Midway Airport during that month. The lowest temperature of was recorded on January 20, 1985, at O'Hare Airport. Bad winter cold waves and summer heat waves can last for many days. There are also many mild winter and summer days. Thunderstorms are common in spring and summer. Sometimes they make tornadoes. They are more common in the suburban areas and not in the city. The heaviest snowfall record was in January 1999. It snowed 18.6 inches (47.2 centimeters).