What's a heat advisory? What's a heat warning?

What's the Difference Between a Heat Advisory and a Heat Warning?


It's been another hot summer across Northeast Kansas but the flooding rains of June produced another consequence...very high humidity levels that were felt across our area for much of July and to some extent into early August.  The combination of air temperatures and the high humidity levels produced very high heat index values (heat indices) and so we have been hearing more and more about heat advisories and heat warnings.  So, what's the difference?  What does the NWS call a heat advisory and how hot does it have to get to be called a heat warning?  We'll answer both of those questions.  But first let's take a look at what the heat index is.

 What's the heat index?

The heat index tells you how hot it feels at a given humidity. Moist air feels hotter than dry air because it makes sweating less efficient. On a hot, dry day, your sweat will evaporate quickly and cool your skin; under humid conditions, sweat evaporates more slowly and doesn't do as much. Just as the wind chill attempts to measure how cold it feels under certain wind conditions, the heat index tries to measure how hot it feels given the humidity.

The National Weather Service will forecast air temperatures and humidity levels to calculate the expected heat index.  When that heat index is expected to approach or exceed certain levels for any period of time we call it a heat advisory.  If its going to be hotter or that heat is going to last for a more prolonged period of time we call it a heat warning.  Let's take a look...


A HEAT ADVISORY is issued when the following conditions are observed or anticipated:

If the maximum Heat Index (HI) is expected to be around 105.

So...the National Weather Service in Topeka will issue a heat advisory if that heat index is observed to be or is expected to be around 105 degrees for any period of time during the day.  Now, if the NWS expects daytime heat indices around 105 to persist for 4 days or longer we will issue a HEAT WARNING.  The idea is that the longer the heat lasts, the worse the impacts can be.

Simple enough so far.  Now, this is where it can get tricky.  There are a couple of situations in which a heat warning can be issued. We just mentioned one.

       The First:  A HEAT WARNING can be issued when the following conditions are observed or anticipated and are expected to persist for at least 48 hours:

Maximum HI around 110 & Minimum HI around 75 or higher.  The minimum part has to do with overnight low temperatures that stay above 75 degrees.  We still look at that as a part of our decision making process.  So as an example if the HI is expected to reach around 110 on day 1, the nighttime low is expected to remain above 75 degrees that night and the HI on day 2 is expected to reach around 110 again, then expect to hear about a heat warning for your area.

      The Second:   A HEAT WARNING is also issued when the following conditions are observed or anticipated and are expected to persist for 4 consecutive days or more:

Maximum heat index (HI) is expected to be 105 or higher for 4 or more consecutive days.   Remember, even if the daytime heat indices do not reach 110, if they get to around 105 and persist for 4 days or more, we'll issue a heat warning for those conditions too!  The longer the heat lasts without a break, the greater the impact.


Regardless of what the heat index is, please remember in the Kansas summertime heat that children are extremely vulnerable when left in a hot car for any period of time.  www.crh.noaa.gov/news/display_cmsstory.php


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