Difference in Heat Index calculation between WD and VP

I’ve noted interesting differences in the Heat Index calculation between WD and the Vantage Pro Plus Console (August 2002 software version B).

For example, right now WD is showing a temperature of 72.0 degrees, 63% humidity, and a Heat Index of 75.8 degrees. Meanwhile, the VP console is showing the same temp and hum, of course, but a Heat Index of 72 degrees. This appears to be a bug in the VP console as the NWS page calculates the same HI as WD…

Tom

I think you can definitely call this a VP feature rather than a bug. Davis documentation specifies that they are using a heat index calculation from 1979.

http://www.davisnet.com/product_documents/weather/app_notes/apnote_7.pdf

The NWS uses an updated calculation from May 2002.

http://www.hpc.ncep.noaa.gov/heat_index.shtml

Davis implements the calculation as a look up table, I’m reluctant to post it here but you can get the file thitable.h if you email Davis support.

Yes, Davis confirmed this after I left my original message on the subject. Why they use it is another matter (they were moot on that point) since the NWS current formula seems to be a much better real-world representation and probably also requires less console memory since it can be calculated by a formula and does not require a lookup table…

Tom

For those who are interested, here is the Steadman table that Davis uses for HI:

                              TEMPERATURE(degF) 

HUM(%) 70 75 80 85 90 95 100 105 110 115 120 125 130 135
— — — — — — — — — — — — — — —
0 64 69 73 78 83 87 91 95 99 103 107 111 117 120
5 64 69 74 79 84 88 93 97 102 107 111 116 122 126
10 65 70 75 80 85 90 95 100 105 111 116 123 131
15 65 71 76 81 86 91 97 102 108 115 123 131
20 66 72 77 82 87 93 99 105 112 120 130 141
25 66 72 77 83 88 94 101 109 117 127 139
30 67 73 78 84 90 96 104 113 123 135 148
35 67 73 79 85 91 98 107 118 130 143
40 68 74 79 86 93 101 110 123 137 151
45 68 74 80 87 95 104 115 129 143
50 69 75 81 88 96 107 120 135 150
55 69 75 81 89 98 110 126 142
60 70 76 82 90 100 114 132 149
65 70 76 83 91 102 119 138
70 70 77 84 93 106 124 144
75 70 77 85 95 109 130 150
80 71 78 86 97 113 136
85 71 78 87 99 117 140
90 71 79 88 102 122 150
95 71 79 89 105 126
100 72 80 90 108 131

I agree it would be preferable to use the NWS formula (below), but I don’t think they could calculate it on the fly using a small processor like the one in the console. Probably generate an interpolation table like the Steadman one.

Heat Index Calculation

The computation used for the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23). The regression equation of Rothfusz is

                     HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T 
                  - .05481717*RH*RH +  .00122874*T*T*RH +  .00085282*T*RH*RH - .00000199*T*T*RH*RH 

where T is temperature in degrees F and RH is relative humidity in percent. HI is the heat index expressed as an apparent temperature in degrees F. If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI:

                        ADJUSTMENT = [(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17} 

where ABS and SQRT are the absolute value and square root functions, respectively. On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI:

                        ADJUSTMENT = [(RH-85)/10] * [(87-T)/5] 

The equation for HI above with the appropriate adjustment is used to compute a maximum heat index using the HPC forecast maximum temperature and the 00 UTC ensemble averaged dew point temperature at each forecast point location for each forecast projection day. Similarly, a minimum heat index is computed using the HPC forecast minimum temperature along with the 12 UTC ensemble averaged dew point temperature. The forecast daily mean heat index for the projection day is the average of these two values, the maximum heat index and the minimum heat index.

My VP is showing 90 degrees F and 84% Heat index 116 degrees
WD is showng 115

the humidity is down a little today, a few days ago the VP was showing 133 degrees HI and WD was showing aprox 125
ks

temp and humidity was not much different than today

I got the formula from a link on a previous message in this thread and put it in a spreadsheet and came up with exactly the same as WD shows, so if this is correct heat index is frequently between 120 - 135 degrees F here on gulf coast of Texas, and most of the days at least 105 - 120 F. Boy I knew it was hot as H–l but that sure seems a bit high.
ks