due to a stronger southern jet stream, which can increase the length of the normal wet season in Houston. Increased clouds during El-Niño winters are what keep the temperatures from warming up. During La Niña events, the jet stream shifts further north, resulting in a warmer and drier winter. The chance of damage from tropical storms and hurricanes also goes up during La Niña events, due to decreased Atlantic wind shear. Post-El Niño weather during the spring usually result in increased rainfall, as demonstrated during the Memorial Day 2015 and Tax Day/Memorial Day/early June 2016 flood events, where the Houston Metro area (and the rest of the state) experienced a climate similar to the South Asian summer monsoon (in this case, a stalled low-pressure system—usually from an upper-level low from the northeast Pacific Ocean, migrates east, where the northern polar jet stream form the usual omega blocks—originates over the Four Corners region or the Sierra Madre Occidental/Sierra Madre Oriental mountain ranges in northern Mexico). Environmental issues Houston has had a troubled past with pollution of many types. This is due to the fact that Houston has been the home for the oil and gas industry since its inception in the early 1900s, when there was little environmental regulation. Harris County, where the bulk of the city has been historically located, is home to 15 Environmental Protection Agency Superfund sites, more than any other area in Texas. The list contains numerous companies, streets and waterways that have been considered hazardous to humans in various ways. Houston's environmental problems stem from a long history of pollution. Houston may be considered the air-conditioning capital of the world due to its hot and humid metropolitan. Since the 1930s, air-conditioning was built into all commercial buildings like theaters, malls, and corporations. Even the Astrodome was air-conditioned, making baseball players and fans comfortable for decades. This led to the possibility of an energy crisis, because of the popularity of air-conditioning in Houston, energy costs are generally higher than normal. Air pollution Houston is well known for its oil and petrochemical industries, which are leading contributors to the city's economy. The industries located along the ship channel, coupled with a growing population, have caused a considerable increase in air pollution for the city each year. Houston has excessive ozone levels and is ranked among the most ozone-polluted cities in the United States. Ground-level ozone, or smog, is