Generally, extreme heat is defined by temperatures that hover 10 degrees or more above the average high temperature for the region, last for prolonged periods of time, and are accompanied by high humidity.

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 as a result of stagnant atmospheric conditions that trap pollutants in urban areas, thus adding unhealthy air to excessively hot temperatures. In addition, asphalt and concrete store heat longer, then gradually release it at night, which produces significantly higher nighttime temperatures in urban areas. This is known as the "urban heat island effect."

HEAT INDEX (APPARENT TEMPERATURE): A number in degrees Fahrenheit that indicates how hot it feels when relative humidity is added to the actual air temperature. Exposure to direct sunlight can increase the heat index by 15 degrees. See the National Weather Service Heat Index Chart for more information.

HEAT WAVE: Prolonged period of excessive heat, often combined with excessive humidity. The National Weather Service defines a heat wave as at least three consecutive days when the temperature reaches 90°F.

ULTRAVIOLET RADIATION: Ultraviolet or UV radiation, which is emitted by the sun, can damage the skin. UV radiation can lead to severe sunburn following an intense short-term overexposure, or serious skin cancers after long-term overexposure.

National Weather Service Terms

HEAT ADVISORY: Issued by the National Weather Service when the heat index is expected to reach at least 100°F but less than 105°F, or when nighttime lows are expected to remain above 80°F.

EXCESSIVE HEAT WATCH: Issued by the National Weather Service, 24-48 hours in advance, when it is possible the heat index will reach 105°F for at least three hours for at least two consecutive days, or when there is a predicted heat index of 115°F.

EXCESSIVE HEAT WARNING: Issued by the National Weather Service 24 hours in advance of a possible 105°F heat index for at least three hours for at least two consecutive days, or when there is a predicted heat index of 115°F.

OZONE HEALTH ADVISORY: Issued when ozone levels in outdoor air are predicted to be greater than 0.08 parts per million when averaged over an eight-hour period.

UV INDEX: Forecast of the amount of skin-damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (solar noon). The UV Index can range from 0 (at night time) to 15 or 16 (in the tropics at high elevations under clear skies), with a higher numeric value corresponding to a shorter time required for skin damage to occur. See the National Weather Service UV Index Chart for more information.