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July 18, 2011

2

The Heat Index & Records for Charlotte

by wxbrad

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.

heat_index

 

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.

graph1560

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

Heat Index Counts for KCLT from 1972 through 2011

Year 100-104 105-109 110-114 ≥115 Total
1972 5 0 0 0 5
1973 11 0 0 0 11
1974 3 0 0 0 3
1975 12 0 0 0 12
1976 4 0 0 0 4
1977 98 5 0 0 103
1978 40 8 0 0 48
1979 34 0 0 0 34
1980 80 10 0 0 90
1981 31 1 0 0 32
1982 2 0 0 0 2
1983 52 25 4 0 81
1984 2 0 0 0 2
1985 10 0 0 0 10
1986 73 0 0 0 73
1987 64 0 0 0 64
1988 46 5 0 0 51
1989 13 0 0 0 13
1990 37 0 0 0 37
1991 76 2 0 0 78
1992 29 0 0 0 29
1993 100 4 0 0 104
1994 3 0 0 0 3
1995 44 0 0 0 44
1996 9 0 0 0 9
1997 0 0 0 0 0
1998 24 0 0 0 24
1999 56 31 5 0 92
2000 26 0 0 0 26
2001 26 3 0 0 29
2002 47 1 0 0 48
2003 11 0 0 0 11
2004 0 0 0 0 0
2005 40 6 0 0 46
2006 29 1 0 0 30
2007 44 3 1 0 48
2008 17 0 0 0 17
2009 10 0 0 0 10
2010 111 18 1 0 130
2011 30 10 0 0 40
Sum
Avg		 1349
34		 133
3		 11
0		 0
0		 1493
37

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