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…

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.

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:

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

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.

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