Houston’s climate is classified as humid subtropical , typical of the Southern United States. While not in Tornado Alley, like much of Northern Texas, spring supercell thunderstorms sometimes bring tornadoes to the area. Prevailing winds are from the south and southeast during most of the year, which bring heat and moisture from the nearby Gulf of Mexico and Galveston Bay.
During the summer, with temperatures reaching or exceeding 90 °F (32 °C) on an average of 106.5 days per year, including a majority of days from June to September; additionally, an average of 4.6 days per year reach or exceed 100 °F (37.8 °C). Houston’s characteristic subtropical humidity often results in a higher apparent temperature, and summer mornings average over 90% relative humidity. Air conditioning is ubiquitous in Houston; in 1981, annual spending on electricity for interior cooling exceeded $600 million (equivalent to $1.71 billion in 2020), and by the late 1990s, approximately 90% of Houston homes featured air conditioning systems. The record highest temperature recorded in Houston is 109 °F (43 °C) at Bush Intercontinental Airport, during September 4, 2000, and again on August 27, 2011.
Houston has mild winters, with occasional cold spells. In January, the normal mean temperature at George Bush Intercontinental Airport is 53 °F (12 °C), with an average of 13 days per year with a low at or below 32 °F (0 °C), occurring on average between December 3 and February 20, allowing for a growing season of 286 days. Twenty-first century snow events in Houston include a storm on December 24, 2004, which saw 1 inch (3 cm) of snow accumulate in parts of the metro area, and an event on December 7, 2017, which precipitated 0.7 inches (2 cm) of snowfall. Snowfalls of at least 1 inch (2.5 cm) on both December 10, 2008, and December 4, 2009, marked the first time measurable snowfall had occurred in two consecutive years in the city’s recorded history. Overall, Houston has seen measurable snowfall 38 times between 1895 and 2018. On February 14 and 15, 1895, Houston received 20 inches (51 cm) of snow, its largest snowfall from one storm on record. The coldest temperature officially recorded in Houston was 5 °F (−15 °C) on January 18, 1930. The last time Houston saw single digit temperatures was on December 23, 1989. The temperature dropped to 7 °F (−14 °C) at Bush Airport, marking the coldest temperature ever recorded there. 1.7 inches of snow fell at George Bush Intercontinental Airport the previous day.
Houston generally receives ample rainfall, averaging about 49.8 in (1,260 mm) annually based on records between 1981 and 2010. Many parts of the city have a high risk of localized flooding due to flat topography, ubiquitous low-permeability clay-silt prairie soils, and inadequate infrastructure. During the mid-2010s, Greater Houston experienced consecutive major flood events in 2015 (“Memorial Day”), 2016 (“Tax Day”), and 2017 (Hurricane Harvey). Overall, there have been more casualties and property loss from floods in Houston than in any other locality in the United States. The majority of rainfall occurs between April and October (the wet season of Southeast Texas), when the moisture from the Gulf of Mexico evaporates extensively over the city.
Houston has excessive ozone levels and is routinely 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 twelfth on the “Most Polluted Cities by Ozone” in 2017, after major cities such as Los Angeles, Phoenix, New York City, and Denver. The industries along the ship channel are a major cause of the city’s air pollution. The rankings are in terms of peak-based standards, focusing strictly on the worst days of the year; the average ozone levels in Houston are lower than what is seen in most other areas of the country, as dominant winds ensure clean, marine air from the Gulf. Excessive man-made emissions in the Houston area led to a persistent increase of atmospheric carbon dioxide over the city. Such an increase, often regarded as “CO2 urban dome,” is driven by a combination of strong emissions and stagnant atmospheric conditions. Moreover, Houston is the only metropolitan area with less than ten million citizens where such CO2 dome can be detected by satellites.