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NYC Hazards: Extreme Heat Basics


 
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What is Extreme Heat?

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-Related Terms and Information
  • HEAT INDEX: An estimate of how it feels when air temperature and humidity are combined. See the National Weather Service Heat Index Chart for more information.
  • HEAT WAVE: The National Weather Service defines a heat wave as at least three consecutive days with high temperatures of at least 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: For New York City, a Heat Advisory is issued when the heat index is forecast to reach 95°F to 99°F for at least 2 consecutive days, or 100°F to 104°F for any length of time.
  • EXCESSIVE HEAT: Issued by the National Weather Service when the heat index values are forecast to reach or exceed 105°F for at least two consecutive hours.
  • EXCESSIVE HEAT WATCH: Issued by the National Weather Service, 24 to 48 hours of an event when the heat index values are forecast to reach or exceed 105°F for at least two consecutive hours.
  • EXCESSIVE HEAT WARNING: Issued by the National Weather Service within 24 hours of an event when the heat index values are forecast to reach or exceed 105°F for at least two consecutive hours.
  • 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.
  • AIR QUALITY INDEX: Issued by the New York State Department of Environmental Conservation and relayed by the National Weather Service when the Air Quality Index is forecast to exceed 100.