A heat wave, defined by the National Weather Service as three or more consecutive days where the temperature reaches at least 90 F, typically occurs when a ridge of high pressure air inflates over a large area. The likelihood of heat waves has increased due to global warming.
When a ridge of high pressure occurs, the atmosphere inflates at both its lower and upper levels. This inflation prevents other weather systems from moving into the area, and keeps the sky free of clouds. The effects of compression warm and dry the air that sinks down from the upper atmosphere, and the unclouded sunshine ensures that the mass of warm air maintains its heat level. A lack of winds in the upper atmosphere means that it can take a long time for the heat wave to dissipate.
A 2011 study from the National Oceanic and Atmospheric Administration attributes a large increase in the probability of heat waves to the effects of global warming. The severe heat wave that hit Texas in 2011 was made more likely by global warming according to the NOAA. While researchers in Oregon and Britain who contributed to the report attribute the heat wave to the natural weather pattern known as La Nina, they also discovered that global warming made that heat wave 20 times more likely.