With another impeding heat wave bearing down on the Carolinas. I thought I’d help explain what the Heat Index really is. In it’s purely mathematical form the calculation is as follows.
Heat Index = 16.923 + ((1.85212 x 10-1) x T)
+ (5.37941 x RH)
– ((1.00254 x 10-1) x T x RH)
+ ((9.41695 x 10-3) x T2)
+ ((7.28898 x 10-3) x RH2)
+ ((3.45372 x 10-4) x T2 x RH)
– ((8.14971 x 10-4) x T x RH2)
+ ((1.02102 x 10-5) x T2 x RH2)
– ((3.8646 x 10-5) x T3)
+ ((2.91583 x 10-5) x RH3)
+ ((1.42721 x 10-6) x T3 x RH)
+ ((1.97483 x 10-7) x T x RH3)
– ((2.18429 x 10-8) x T3 x RH2)
+ ((8.43296 x 10-10) x T2 x RH3)
– ((4.81975 x 10-11) x T3 x RH3)
where T is the dry bulb temperature (°F) and RH is relative humidity (%)
Looks crazy doesn’t it? Well it is when you think about it the Heat Index is really a measure of how the human body reacts to the combination of heat and humidity. More to the point how efficiently or in this case inefficiently it cools itself. So to calculate that you need lots of data besides the temperature and relative humidity. You need to know things like the surface area of an average human, rate of heat loss through skin, evaporative cooling, sweating…etc. So you can see lots of biology is in this as well as meteorology. This calculation is about how your body cools itself and the more humidity the less evaporation of sweat from your skin. This inefficiency makes you feel hotter, thus the Heat Index.
Here’s an easier way to understand it by just using this simple chart.
So lets look at some of the climatology of the Heat Index in Charlotte. Last week South Carolina set a state record for the highest Heat Index in the 20 years of record keeping at 124°. This was in Mount Pleasant, SC.
For Charlotte our worst year of Heat Indices was last summer which actually wasn’t our hottest ever for air temperatures. We’ve never gone above 114°. 2010 was more about the high heat Index values though it was plenty hot out there.
Notice last year we heat a Heat Index of 100°-104° or more for 130 hours.
We hit 105°-109° for 18 hours
We had 1 hour of a Heat Index of 110°-114°
We are way behind last years pace but we still have half the summer to go. Here’s a look at every year since these types of records have been kept. (Via The NC state Climate Office)
Heat Index Climatology: Output