Heat waves are attributable to stagnant atmospheric conditions. When air or water is stagnant, it has little or no direction or flow. The atmospheric conditions are said to be stagnant after a few days with very low wind speeds. There is no specific timeframe for such conditions to be called a heat wave, but most consider five consecutive days or more to be the standard. The atmosphere can become stagnant at any time during the year, but when it occurs during the summer, a heat wave can be the result.
The most common form of heat wave occurs when a high-pressure system settles over a geographic region. Atmospheric pressure is the pressure exerted by the atmosphere's weight. High pressure systems generally move from northeast to southwest in the northern hemisphere, but occasionally can become stationary. The position of the jet stream contributes to this, with areas on the warmer side more susceptible to heat wave conditions. The jet stream is a narrow strip of wind that circulates the Earth. It generally flows from west to east across the North American continent.
Cities and other highly congested areas are more prone to heat waves than their rural counterparts. This is in part because the stagnant atmosphere traps pollutants, dust and other debris. The relative paucity of plants in these areas is a factor as well. In cities, asphalt streets and concrete buildings can absorb heat, later radiating it back into the atmosphere, contributing to the already hot conditions. Buildings can also block what little wind there is during these times.
In inner city areas, where tall structures and narrow alleys are prevalent, heat waves can go beyond a nuisance, becoming deadly. Power outages can result as electrical conduits warp and air conditioners run day and night. The poor, including seniors on fixed incomes can find themselves without access to well-ventilated rooms. According to the U.S. Department of Commerce, during the 1978 heat wave in Texas, many low-income seniors "found the cost too high, turned off their units, and ultimately succumbed to the stresses of heat." Heat waves are America's biggest weather-related killer.