National Weather Service United States Department of Commerce

Heat Advisories

 The Heat Index has to remain at or above 100°F for a minimum of 2 hours. Heat advisories are issued by zone when any location within that zone is expected to reach criteria. For example: If you expected the heat index to reach 100°F in the city of Elmira, a heat advisory would be issued for that county. 

 

A heat advisory means that people can be affected by heat if precautions are not taken. A quick study in the Corning area from the 2005 summer showed that early in the summer, emergency room visits increased when the heat index surpassed 95°F. The issuance of a heat advisory is important to raise public awareness that these precautions need to be taken. Heat advisories are also used to trigger other actions and regulations such as no evictions, no turning off of power, changing outdoor work requirements, etc.

Excessive

Heat Warnings

Criteria for an Excessive Heat Warning is a heat index of 105° F or greater that will last for 2 hours or more. Heat Warnings are issued by zone when any location within that zone is expected to reach criteria. For example: If you expected the heat index to reach 105°F in the city of Elmira, an Excessive Heat Warning would be issued for that zone.

 

A heat warning means that some people can be seriously affected by heat if precautions are not taken. Studies in Canada, Europe, and the U.S. have indicated that mortality begins to increase exponentially as the heat increases or stays above a heat index of 104°F. Note: This threshold will be a rare event in our CWA. Even the warm summer of 2005, would not have produced a heat warning.

In addition to raising public awareness, the issuance of a heat warning will alert hospitals and officials to take certain actions to prepare and respond to an increase in emergency calls, and activate programs to check on elderly and the home-bound. In some cases cooling centers can be open or designated and donation programs activated for fans and air conditioners. As in the case of an advisory, certain regulations may change such as turning off people's electricity, evictions, and outside work requirements.

Excessive

Heat Watches

Issued when Heat Warning criteria is possible (50-79%) 1 to 2 days in advance.  

 

 

Weather and Geographic Factors that Increase the Impact of Heat

  • Direct sun on buildings or people
  • Wind (increases dehydration)
  • Overnight minimum heat index - houses and buildings that do not have air conditioning will not cool down if the overnight minimum heat index remains above 75 - 80° and the area goes into a second hot day.
  • Successive days of heat with high nighttime temperatures is really bad - fatalities will occur. Impacts will begin to increase exponentially. Urbanization - more concrete, less green - impacts temperatures particularly at night.
  • Early season heat - a heat index of 95° - 100° will impact people in May and June or with the first few occurrences, where as later in the summer (August) it may take a heat index of 100° - 105° for the same effect. People modify their behavior after being affected by it and are less likely to be impacted again.

Human Factors that increase the Impact of Heat

  • Alcoholic consumption
  • Certain medications
  • Certain medical conditions
  • Elderly or infants
  • Physical exertion Exposure to sun

NOAA's heat alert procedures are based mainly on Heat Index Values. The Heat Index, sometimes referred to as the apparent temperature is given in degrees Fahrenheit. The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

To find the Heat Index temperature, look at the Heat Index Chart below. As an example, if the air temperature is 96°F and the relative humidity is 65%, the heat index--how hot it feels--is 121°F. The National Weather Service will initiate alert procedures when the Heat Index is expected to exceed 105°-110°F (depending on local climate) for at least 2 consecutive days.

Heat Index Chart

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