DFW Heat Index & the Heat Advisory Criteria for North Texas

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Current Conditions Around N. TX

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It’s pretty much a given that summertime in North Texas will be hot. However, there is a fine line between “hot” and dangerously hot. In other words, “only” a few degrees can take an enormous toll on the human body if you happen to be outdoors for an extended period of time. This is especially true when the humidity is very high. The human body cools itself through perspiration; as sweat evaporates from the skin, the latent heat of evaporation causes the skin to cool (evaporative cooling). The rate of evaporation increases with lower atmospheric moisture content (humidity), and decreases with higher humidity.

When the humidity is oppressive, sweat cannot evaporate from the skin and the body temperature will begin to rise. Research has shown that critical thresholds of temperature and/or heat index warrant enhanced warnings for the public, particularly for those who are more vulnerable such as children, those with inadequate ventilation/cooling systems in their homes, and the elderly. Therefore, the National Weather Service has established criteria for heat advisories and excessive heat warnings. The criteria vary regionally based on local climatology and how acclimated the general public is to heat/humidity. The following are the criteria used by the NWS in Fort Worth (FWD) and apply to all of North Texas. Generally, these same criteria apply for most surrounding NWS offices, with only slight variations.

When is a Heat Advisory Issued?

A Heat Advisory may be issued in North Texas using the following criteria:

  1. Daytime heat index expected to meet or exceed 105ºF -or-
  2. Daytime air temperature expected to exceed 103ºF
  3. For at least two consecutive days
  4. Typically issued 12 to 24 hours in advance

When is an Excessive Heat Warning Issued?

An Excessive Heat Warning (very rare) may be issued in North Texas using the following criteria:

  1. Daytime heat index expected to meet or exceed 110ºF -or-
  2. Daytime air temperature expected to meet or exceed 105ºF
  3. For at least two consecutive days
  4. Typically issued 12 to 24 hours in advance

In the past, the set of criteria included overnight low temperatures which required temperatures to stay 78ºF or warmer for two consecutive nights (in addition to the daytime temperature criteria) before an advisory or a warning could be issued. However, the nighttime temperature requirement has been eliminated. For a heat advisory or an excessive heat warning to be issued, the criteria above must be predicted to occur for at least two consecutive days. If the criteria above are predicted to occur for just one day, or for just a couple of hours on a single afternoon, no advisories/warnings will be issued.

✏️ Example: If the temperature for tomorrow is predicted to be 104ºF (technically “heat advisory” criteria), but the forecast for the next day is 100ºF, then the National Weather Service will not issue any advisories because the two-consecutive days criterion will not be met. Of course, a future forecast update may suggest that the day-2 temperatures will be warmer than expected, thereby warranting an advisory at forecaster discretion.

heat Calculator: Check out our heat index calculator and charts.

High-resolution, detailed heat index chart that uses relative humidity instead of dew point temperature

✏️ Read More: What is the heat index and how is it calculated?

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