The parameters of an extreme heat watch, warning, or advisory can
vary by location. Generally, temperatures that hover 10 degrees or
more above the average high temperature for the region, last for
prolonged periods of time, and are often accompanied by high
humidity, that the body cannot tolerate are defined as extreme heat.
A heat wave is a very dangerous situation.
People living in urban areas may be at greater risk from the
effects of a prolonged heat wave than people living in rural
regions. An increased health problem, especially for those with
respiratory difficulties, can occur when stagnant atmospheric
conditions trap pollutants in urban areas, thus adding unhealthy air
to excessively hot temperatures. In addition, asphalt and concrete
store heat longer and gradually releases heat at night, which
produces significantly higher nighttime temperatures in urban areas
known as the "urban heat island effect."