So there’s one or two heat warnings and advisories up this morning …
… and a lot of talk about the Heat Index. Let’s take a closer look. First off while the temperatures are above normal, at least here in coastal GA/SC they won’t be threatening the historical record highs. The air temperatures are going to be high Friday/Saturday, but won’t set any records. The average for July 30th is 92, the records this time of year are over 100 (101-103), and forecast highs in downtown Savannah are 97 today and 96 Saturday. The problem is that humidity is way above normal. That is due to the pattern of air flow over the region is keeping moist Gulf and Atlantic air “trapped” over us (recall the low pressure system that NHC was looking at earlier this week, AL90, wandered over us, then off of North Florida before drifting back over us). And it’s the humidity that’s the problem.
The Heat Index has an interesting history, and there are several versions. The method currently in use by the US National Weather Service is not a complex equation, if you want to see it look here:
The TLDR is that the “heat index” is supposed to represent how hot if feels, given that the higher the humidity, it “feels” hotter because your body can’t cool itself as efficiently. The technical reason is because we cool ourselves by sweating (ewww), and the evaporation of that sweat cools us down, since it takes energy – heat – to convert water from the liquid to gas states. The evaporation rate depends temperature and humidity – the higher the humidity, the less moisture evaporates, and the less heat is transferred from your body to its surroundings. Drier air means that evaporation works better, so it “feels” cooler (although that can be misleading), thus all the jokes about a Dry Heat …
Either way, especially if you are not adapted to it, the heat can be dangerous (and in the ranges expected today, even if you are). By the way, the NWS has different criteria for when to issue heat advisories around the country, depending on normals. So a heat advisory in Vermont is issued at much “cooler” temperatures than in Savannah.
To sum up, technically speaking, it’s not that it’s so hot, it’s because it’s kinda hot and really humid … it would be uncomfortable at 97, but all that humidity today will make it feel like it’s well over 110, maybe as high as 120 in parts of town where the temperature and humidity gang up. So if you can avoid working outside this afternoon, don’t, and if you absolutely have to, drink lots of fluids, protect yourself, and be careful.
Update: at 2:30pm, in midtown Savannah the air temperature was 93, the humidity 70%, which adds up to a heat index of 120F :O