Geography

Main article: Geography of Houston A simulated-color image of Houston

According to the United States Census Bureau, the city has a total area of 656.3 square miles (1,700 km2); this comprises 634.0 square miles (1,642 km2) of land and 22.3 square miles (58 km2) of water. Most of Houston is located on the gulf coastal plain, and its vegetation is classified as temperate grassland and forest. Much of the city was built on forested land, marshes, swamp, or prairie, which are all still visible in surrounding areas. Flatness of the local terrain, when combined with urban sprawl, has made flooding a recurring problem for the city. Downtown stands about 50 feet (15 m) above sea level, and the highest point in far northwest Houston is about 125 feet (38 m) in elevation. The city once relied on groundwater for its needs, but land subsidence forced the city to turn to ground-level water sources such as Lake Houston, Lake Conroe and Lake Livingston. The city owns surface water rights for 1.20 billion gallons of water a day in addition to 150 million gallons a day worth of groundwater.

Houston has four major bayous passing through the city. Buffalo Bayou runs through downtown and the Houston Ship Channel, and has three tributaries: White Oak Bayou, which runs through the Houston Heights community northwest of Downtown and then towards Downtown; Braes Bayou, which runs along the Texas Medical Center; and Sims Bayou, which runs through the south of Houston and downtown Houston. The ship channel continues past Galveston and then into the Gulf of Mexico.

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.

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), including the Long Point–Eureka Heights fault system which runs through the center of the city. There have been no significant historically recorded earthquakes 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. These faults also tend to move at a smooth rate in what is termed "fault creep", 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 Köppen climate classification system). While not necessarily part of "Tornado Alley" like much of the rest of Texas, spring supercell thunderstorms do sometimes bring tornadoes to the area. Prevailing winds are from the south and southeast during most of the year, bringing heat from the western deserts 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) and at least 3 days at or over 100 °F (38 °C) . However, the humidity causes 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 showers and thunderstorms are common in the summer. The hottest temperature ever recorded in Houston is 109 °F (43 °C), which was reached both on September 4, 2000 and August 28, 2011.

Winters in Houston are mild. The average high in January, the coldest month, is 63 °F (17 °C), while the average low is 43 °F (6 °C). Snowfall is very 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. 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 in two consecutive years, and was 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 50 inches (1,270 mm) a year. These rains tend to cause floods over portions of 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 as the 8th worst in the United States in 2011. The industries located along the ship channel are a major cause of the city's air pollution. In the past, Houston's air quality has been compared to that of Los Angeles.

Climate data for Houston (Intercontinental Airport), 1981–2010 normals, Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Record high °F (°C) 87 (31) 91 (33) 96 (36) 95 (35) 99 (37) 107 (42) 105 (41) 109 (43) 109 (43) 99 (37) 89 (32) 85 (29) 109 (43) Average high °F (°C) 62.9 (17.2) 66.3 (19.1) 73.0 (22.8) 79.6 (26.4) 86.3 (30.2) 91.4 (33) 93.7 (34.3) 94.5 (34.7) 89.7 (32.1) 82.0 (27.8) 72.5 (22.5) 64.3 (17.9) 79.7 (26.5) Average low °F (°C) 43.2 (6.2) 46.5 (8.1) 52.5 (11.4) 59.4 (15.2) 67.6 (19.8) 73.5 (23.1) 75.1 (23.9) 74.8 (23.8) 69.8 (21) 60.9 (16.1) 52.1 (11.2) 44.6 (7) 60.0 (15.6) Record low °F (°C) 5 (−15) 6 (−14) 21 (−6) 31 (−1) 44 (7) 52 (11) 62 (17) 54 (12) 45 (7) 29 (−2) 19 (−7) 7 (−14) 5 (−15) Precipitation inches (mm) 3.38 (85.9) 3.20 (81.3) 3.41 (86.6) 3.31 (84.1) 5.09 (129.3) 5.93 (150.6) 3.79 (96.3) 3.76 (95.5) 4.12 (104.6) 5.70 (144.8) 4.34 (110.2) 3.74 (95) 49.76 (1,263.9) Avg. precipitation days (≥ 0.01 in) 9.6 9.2 8.8 6.8 8.0 10.6 9.1 8.3 8.0 7.9 8.2 9.5 104.1 Mean monthly sunshine hours 142.6 155.4 192.2 210.0 248.0 282.0 294.5 269.7 237.0 229.4 168.0 148.8 2,577.6 Source: NOAA (extremes 1889–present), Hong Kong Observatory (sun only, 1961–1990) Cityscape Further information: Geographic areas of Houston and List of Houston neighborhoods

Houston was incorporated in 1837 under the ward system of representation. The ward designation is the progenitor of the eleven current-day geographically-oriented Houston City Council districts. Locations in Houston are generally classified as either being inside or outside the Interstate 610 Loop. The inside encompasses the central business district and many residential neighborhoods that predate World War II. More recently, high-density residential areas have been developed within the loop. The city's outlying areas, suburbs and enclaves are located outside of the loop. Beltway 8 encircles the city another 5 miles (8.0 km) farther out.

Though Houston is the largest city in the United States without formal zoning regulations, it has developed similarly to other Sun Belt cities because the city's land use regulations and legal covenants have played a similar role. Regulations include mandatory lot size for single-family houses and requirements that parking be available to tenants and customers. Such restrictions have had mixed results. Though some have blamed the city's low density, urban sprawl, and lack of pedestrian-friendliness on these policies, the city's land use has also been credited with having significant affordable housing, sparing Houston the worst effects of the 2008 real estate crisis. The city issued 42,697 building permits in 2008 and was ranked first in the list of healthiest housing markets for 2009.

Voters rejected efforts to have separate residential and commercial land-use districts in 1948, 1962, and 1993. Consequently, rather than a single central business district as the center of the city's employment, multiple districts have grown throughout the city in addition to downtown which include Uptown, Texas Medical Center, Midtown, Greenway Plaza, Memorial City, Energy Corridor, Westchase, and Greenspoint.

The western view of Downtown Houston skyline The southern view of Downtown Houston skyline The Uptown Houston skyline