Houston's climate is classified as humid subtropical (Cfa in Köppen climate classification system). Spring supercell thunderstorms sometimes bring tornadoes to the area. Prevailing winds are from the south and southeast during most of the year, bringing heat across the continent from the deserts of Mexico and moisture from the Gulf of Mexico.
During the summer months, it is common for the temperature to reach over 90 °F (32 °C), with an average of 99 days per year above 90 °F (32 °C). However, the humidity results in a heat index higher than the actual temperature. Summer mornings average over 90 percent relative humidity and approximately 60 percent in the afternoon. Winds are often light in the summer and offer little relief, except near the immediate coast. To cope with the heat, people use air conditioning in nearly every vehicle and building in the city; in 1980 Houston was described as the "most air-conditioned place on earth. Scattered afternoon thunderstorms are common in the summer. The hottest temperature ever recorded in Houston was 109 °F (43 °C) on September 4, 2000.
Winters in Houston are fairly temperate. The average high in January, the coldest month, is 62 °F (17 °C), while the average low is 39 °F (4 °C). Snowfall is generally rare. Recent snow events in Houston include a storm on December 24, 2004 when one inch (2.5 cm) fell and more recent snowfalls on December 10, 2008. However, more recently on December 4, 2009 an inch of snow fell in the city. This was the earliest snowfall ever recorded in Houston. In addition, it set another milestone marking the first time in recorded history that snowfall has occurred on two consecutive years, and marks the third accumulating snowfall occurring in the decade of 2000–2010. The coldest temperature ever recorded in Houston was 5 °F (−15 °C) on January 23, 1940. Houston receives a high amount of rainfall annually, averaging about 54 inches a year. These rains tend to cause floods over portions of the city.
Houston has excessive ozone levels and is ranked among the most ozone-polluted cities in the United States. Ground-level ozone, or smog, is Houston’s predominant air pollution problem, with the American Lung Association rating the metropolitan area's ozone level as the 6th worst in the United States in 2006. The industries located along the ship channel are a major cause of the city's air pollution.
