La Message Board Archives

Re: Some rain tonight? and heat index 101

Posted by Bret B on June 21, 2014, 8:48 am, in reply to "Re: Some rain tonight?"

Well, that rain last night was only 0.02", but there's still a chance of more today. The NHC Eastern Pacific tropical discussion ( mentions a surface trough attached to that Invest 95E far to our SSW, and an upper-level ridge in the same area. These two features are adding to the instability (i.e. thunderstorms) in the flow from the Pacific onto the western Mexican coastline.

Kathy, I don't have an instrument that displays heat index directly (although you can buy them.) I use the NOAA NWS heat index online calculator ( Just enter the current temperature (F or C) and the current relative humidity, or temp and dew point, and click Calculate. I also have printed out their heat index table for rough estimates without going online (link is on the calculator page.) In my example from the previous post, my backyard temp was 96F along with a RH of 62%. The calculator gives a heat index of 118F. Pretty steamy! I think the highest HI I've ever noted here was around 125F. And of course it's about the same inside the house. It's no wonder Donna and I are so lazy this time of year...

Heat index nerds can follow the "How do we calculate heat index" link at the top of the online calculator page. They use a fairly complex algorithm. And the Wikipedia article has even more long complex formulas to choose from: (I had to propose a correction to fix an error in one of these formulas last year, so as always take Wikipedia articles with a grain of salt. Anyone can edit them!)

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