Denver & Thornton, Colorado Climate

Thornton Climatology and Records   
Denver Climatology and Records   

Denver has a semi-arid steppe climate, with four distinct seasons. While Denver is located on the Great Plains, the weather of the city and surrounding area is heavily influenced by the proximity of the Rocky Mountains to the west. The climate, while generally mild compared to the mountains to the west and the plains further east, can be very unpredictable. Measurable amounts of snow have fallen in Denver as late as Memorial Day and as early as Labor Day, and trace amounts have been recorded in every month of the year.

The average temperature in Denver is 50.1 °F (10.1 °C), and the average yearly precipitation is 15.81 inches (402 mm). The season's first snowfall generally occurs around October 19, and the last snowfall is about April 27, averaging 61.6 inches (156 cm) of seasonal accumulation. Although Denver's Convention and Visitor Bureau claims Denver receives over 300 sunny days a year,[15] the National Oceanic and Atmospheric Administration places that figure at slightly above 250 days of sunshine a year.

Denver's winters can vary from mild to cold, and although large amounts of snow can fall on the mountains just west of the city, the effects of orographic lift dry out the air passing over the Front Range, shadowing the city from precipitation for much of the season. Additionally, warm chinook winds occasionally occur as air passing over the mountains heats as it descends, quickly melting snow accumulations and making Denver's winters milder than areas without this effect. The coldest temperature ever recorded in Denver was recorded on January 9, 1875 at -29 °F (-34 °C), though the last time Denver recorded a temperature below -20 °F (-29 °C) was in 1990.

Spring brings with it significant changes as Denver can be affected by air masses on all sides. Arctic air from the north can occasionally combine with Pacific storm fronts bringing snow to the city. In fact, March is Denver's snowiest month, averaging 11.7 inches (29.7 cm) of snow. Additionally, warm air from the Gulf of Mexico can bring the first thunderstorms of the season, and continental warm air can bring summer-like warm and dry conditions.

Starting in mid-July, the monsoon brings tropical moisture into the city and with it come frequent short (and occasionally severe) late-afternoon thunderstorms. However, despite this tropical moisture, humidity levels during the day generally remain very low. The average high during the summer is 85 °F (29 °C) and the average low is 56 °F (13 °C).

In the autumn, the tropical monsoon flow dies down and as Arctic air begins to approach, it can combine with moisture from the Pacific Northwest to bring significant snowfall to the city – November is Denver's second snowiest month, and Denver's greatest recorded snowfall from a single storm, 45.7 inches (116 cm), fell in late autumn from December 1 to December 6, 1913.