Summer temperatures are hot, although are moderated somewhat by the cool breezes from the Great Salt Lake and by the city's elevation. The lack of cold fronts in summer allows the temperatures to become consistently hot due to powerful, long-lasting high pressure . Occasional thunderstorms give almost the only relief in temperatures. In an average year, 5 days hotter than , 23 days greater than , and 56 days greater than can be expected. . However, such days also have very low humidity. The low humidity and the altitude create ideal conditions for radiational cooling, and hence, large swings in temperature. Summer nights are rather cool; the record low even in July is . July is the warmest month, with an average temperature of . Salt Lake City's record high minimum temperature is , set on July 18, 2016, and its record high temperature is , first set on July 26, 1960 and again on July 13, 2002 (although the temperature in 2002 was slightly higher). The last summer-like weather is typically experienced in September; the latest 90°F (32°C) temperature was recorded on September 30. Temperatures cool down rapidly in fall. The first major cold fronts typically arrive anytime from September to October. The first winter-like weather is usually experienced in November. Salt Lake City's average yearly temperature is , and the freeze-free period lasts an average of 167 days, from April 30 to October 15, although it has lasted anywhere from 124 to 236 days. Freezing temperatures have occurred as early as September 13 (in 1928), and as late as May 28 (in 1954). Both precipitation and humidity are highest from March through May and lowest from June through mid-September. Major sources of precipitation are winter snow storms originating in the Gulf of Alaska, late winter and spring rains from the Pineapple Express created in Hawaiian waters, and summer monsoons from the Gulf of California. The airport averages of precipitation per year, with bench areas receiving up to , mainly due to increased snowfall. May is the wettest month, averaging of precipitation. Average year-round humidity is 55%. The summer monsoon rising from Mexico and Arizona passes through the region beginning in mid-July and continuing into September, bringing intense but short-lived thunderstorm activity. Tornadoes have been known to occur during this time period, the most notable recent example being in 1999 when an F2 tornado struck downtown Salt Lake City, causing extensive damage and resulting in one death. Many of these thunderstorms consist of dry lightning, which occurs when humidity is too low to support rainfall and the rain evaporates before reaching the ground.