What's a heat warning? 10News breaks down the weather terms

Posted at 12:51 PM, Jul 25, 2018
and last updated 2018-07-25 18:10:47-04

SAN DIEGO (KGTV) - While San Diego County sweats through a heat wave, the National Weather Service is monitoring conditions to determine what type of warning the public needs. 

An Excessive Heat Warning is in effect through Friday from the valleys to the deserts. The coastal communities are under a Heat Advisory for the same time period.

The warnings levels are created by NWS meteorologists who monitor the heat index, which is determined by temperatures significantly above normal and high humidity, including humidities that can pose a threat to human life. The heat index measures how hot it really feels when relative humidity is factored in with the actual air temperature. Exposure to full sunshine can increase heat index values by up to 15 degrees, experts say.

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The NWS says one of two criteria must be met to reach the level: 

  • The heat index much be at least 105 degrees for more than three hours for two consecutive days
  • The heat index must be greater than 115 degrees.


Heat watches are issued when conditions are favorable for an excessive heat event within two to three days, according to the NWS. The watch is issued when there’s a higher likelihood of a heat wave but its timing is uncertain.


The Heat Advisory is issued within 12 hours of the start of extremely dangerous heat. In this situation, the heat index temperatures is expected to be 100 degrees or higher for at least two days, and nighttime air temperatures will not drop below 75 degrees.


NWS meteorologists report this condition is created when there’s potential for a heat event within one week.

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