Geology

Geology

Underpinning Houston's land surface are unconsolidated clays, clay shales, and poorly cemented sands up to several miles deep. The region's geology developed from river deposits formed from the erosion of the Rocky Mountains. These sediments consist of a series of sands and clays deposited on decaying organic marine matter, that over time, transformed into oil and natural gas. Beneath the layers of sediment is a water-deposited layer of halite, a rock salt. The porous layers were compressed over time and forced upward. As it pushed upward, the salt dragged surrounding sediments into salt dome formations, often trapping oil and gas that seeped from the surrounding porous sands. The thick, rich, sometimes black, surface soil is suitable for rice farming in suburban outskirts where the city continues to grow.^[52][53]
The Houston area has over 150 active faults (estimated to be 300 active faults) with an aggregate length of up to 310 miles (500 km),^[54][55][56] including the Long Point–Eureka Heights fault system which runs through the center of the city. No significant historically recorded earthquakes have occurred in Houston, but researchers do not discount the possibility of such quakes having occurred in the deeper past, nor occurring in the future. Land in some areas southeast of Houston is sinking because water has been pumped out of the ground for many years. It may be associated with slip along the faults; however, the slippage is slow and not considered an earthquake, where stationary faults must slip suddenly enough to create seismic waves.^[57] These faults also tend to move at a smooth rate in what is termed "fault creep",^[49] which further reduces the risk of an earthquake.

Climate

Main article: Climate of Houston

Allen's Landing after Tropical Storm Allison, June 2001

Houston's climate is classified as humid subtropical (Cfa in the Köppen climate classification system), typical of the lower South. While not located in "Tornado Alley", like much of the rest of 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.^[58]
During the summer, temperatures commonly reach over 90 °F (32 °C), with an average of 106.5 days per year, including a majority from June to September, with a high of 90 °F (32 °C) or above and 4.6 days at or over 100 °F (38 °C).^[59] However, humidity usually yields a higher heat index. Summer mornings average over 90% relative humidity.^[60] Although sea breezes are present, they don't offer substantial relief, except in the southeastern areas of the city closer to the Gulf.^[61] To cope with the strong humidity and heat, people use air conditioning in nearly every vehicle and building. In 1980, Houston was described as the "most air-conditioned place on earth".^[62] Officially, the hottest temperature ever recorded in Houston is 109 °F (43 °C), which was reached both on September 4, 2000, and August 28, 2011.^[59]
Houston has mild winters in contrast to most areas of the United States. In January, the normal mean temperature at Intercontinental Airport is 53.1 °F (11.7 °C), while that station has an average of 13 days with a low at or below freezing. Snowfall is rare. Recent snow events in Houston include a storm on December 24, 2004 when 1.0 in (2.5 cm) of snow accumulated in parts of the metro area.^[63] Falls of at least 1.0 in (25 mm) 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. The coldest temperature officially recorded in Houston was 5 °F (−15 °C) on January 18, 1930.^[59] Houston has historically received an ample amount of rainfall, averaging about 49.8 in (1,260 mm) annually per 1981–2010 normals. Localized flooding often occurs, owing to the extremely flat topography and widespread typical clay-silt prairie soils, which do not drain quickly.
Houston has excessive ozone levels and is routinely ranked among the most ozone-polluted cities in the United States.^[64] Ground-level ozone, or smog, is Houston's predominant air pollution problem, with the American Lung Association rating the metropolitan area's ozone level sixth on the "Top 10 Most Ozone-Polluted Cities" in 2014.^[65] The industries located along the ship channel are a major cause of the city's air pollution.^[66] In 2006, Houston's air quality was comparable to that of Los Angeles.^[66]