Under the Köppen climate classification, Guadalajara has a humid subtropical climate (Cwa) that is quite close to a tropical climate, featuring dry, mild winters and warm, wet summers. Guadalajara's climate is influenced by its high altitude and the general seasonality of precipitation patterns in western North America. Although the temperature is warm year-round, Guadalajara has very strong seasonal variation in precipitation. The northward movement of the Inter-Tropical Convergence Zone brings a great deal of rain, whereas for the rest of the year, the climate is very arid. The extra moisture in the wet months moderates the temperatures, resulting in cooler days and nights during this period. The highest temperatures are usually reached in May averaging 33 °C (91 °F), but can reach up to 38°C just before the onset of the wet season. March tends to be the driest month and July the wettest, with an average of 249 millimetres (9.8 in) of rain, over a quarter of the annual average of about 921 millimetres (36.3 in). Winters are relatively warm despite the city's altitude, with January daytime temperatures reaching about 21 °C (70 °F) and nighttime temperatures about 4 °C (39 °F). Frost may occur during the coldest nights, but since temperatures rarely fall below 0 °C (32 °F),it is an uncommon phenomenon, but it is possible. Snow is extremely rare, but flurries have been known to occur about every 90 years
