WRAL WeatherCenter Blog

Heat index hiding away!

Posted August 19, 2013
Updated August 20, 2013

It's been a common observation for us all that this has been a cooler and wetter than normal summer so far, with clouds, humidity and rain keeping our high temperatures well on the short side of long-term averages while we've run above normal for morning lows. Even though it's been rather soupy at times in terms of humidity levels, with dew points often ranging in the upper 60s to mid 70s, with occasional interruptions with the passage of a cold front, we've really lacked the hot temperatures that often go along with fairly humid days in the summer around here. One notable result of that has been a relative lack of time with heat index values soaring up above 100 degrees.

This is quite a change from the past several years, as shown in the graphic above that I generated using a nice tool at the State Climate Office web site. They have a heat index climatology tool where you can pick any series of recent years for most hourly weather reporting station in North Carolina. You just pick the location and select the range of years you'd like to cover (you can go back as far as 1972 for RDU), and the tool will provide your choice of either a table of graph showing the data.

In this case, what we see is a decided lack of such combined heat and humidity for this summer, with only 12 hours at 100 or above as of the 19th of August. Of course, the bars representing the past five years include the entire summer and early fall, so we may add a few more this year, but we're still well short of even the rather mild summer of 2009, and find this year's number dwarfed by those from the hot summers we've experienced the past three years, with 2010 featuring 175 hours with the heat index topping the century mark!

We'll start this week on a mild note with some of us holding in the 70s, but appear headed for a warmup that could take us to around 90 or a little above late in the week. For now, though, it looks as though the heat index even at that point will only reach the mid or upper 90s. By the way, speaking of holding in the 70, we just finished a stretch of four days in a row with highs at RDU below 80 degrees. That's not a real common occurrence in August, although looking back at past data, I did find that we've had one stretch as long as six straight days in August with highs there under 80, which happened the last six days of the month in August 2002.


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  • Sundays Child Aug 20, 2013


    It said 175 HOURS of heat index over 100 in 2010.

  • Mike Moss Aug 20, 2013

    Yikes - wrote that up too fast yesterday. It is indeed hours, not days (obvious given numbers like 175!). I'll see if I can't correct the text. Thanks for the catch...

  • wjimcooper Aug 20, 2013

    Global warming was a popular topic during the hot summers of the last few years ... the East Coast must have other factors of influence!!!

  • inigo_montoya Aug 19, 2013

    It's unclear whether the data is really talking about number of days. The graph's Y-axis says number of hours and the tabular data from the Heat Index Climatology page lists "Heat Index Counts". If the data is sampled on an hourly basis, then there can be multiple samples (or "counts") that meet the threshold in a given day. Therefore, I'm not sure we can say there were really 175 days that had heat indices over 100 in 2010. There may have been a smaller number of days where the heat index stayed over 100 for multiple hours.