A further note, from Davis's documentation on their wind chill
calculations:
"The formula below was adopted by both Environment Canada and the U.S.
National Weather Service to ensure a uniform wind chill standard in North
America. The formula is supposed to more closely emulate the response of
the human body when exposed to conditions of wind and cold than the previous
formula did:
35.74 + 0.6215T - 35.75 * (V^0.16 ) + 0.4275T * (V^0.16 )
This relationship takes into account the fact that wind speeds are measured
"officially" at 10 meters (33 feet) above the ground, but the human is
typically only 5 to 6 feet (2 meters) above the ground..... This newer
version of the formula addresses the fact that the latest National Weather
Service (NWS) formula was not designed for use above 40°F. The result of
the straight NWS implementation was little or no chilling effect at mild
temperatures. This updated version provides for reasonable chilling effect
at mild temperatures based on the effects determined by Steadman (1979) (see
THSW Index section), but as with the new NWS formula, no upper limit where
chilling has no additional effect."
On heat index, they say:
" Heat Index uses temperature and relative humidity to determine how hot
the air actually “feels.” When humidity is low, the apparent temperature
will be lower than the air temperature, since perspiration evaporates
rapidly to cool the body. However, when humidity is high (i.e., the air is
saturated with water vapor) the apparent temperature “feels” higher than
the actual air temperature, because perspiration evaporates more slowly.
Formulas: Heat Index is based upon a lookup table presented by Steadman
(1979) and loosely derived from the methodology outlined by Steadman (1998).
Thus, air temperatures below 50°F follow this 1998 procedure. Air
temperatures above 68°F follow his procedure outlined in 1979 (since the US
NWS continues to use this). Davis has made a smooth transition between the
two methods between 50°F and 68°F.
On Friday, November 6, 2020 at 4:33:15 PM UTC-5 Peter Fletcher wrote:
> Responding to both of your messages: You are right that my comments would
> equally apply to the heat index being different to the actual temperature.
> A relatively quick scan of the relevant pages on the NWS site found the
> explicit statement that the wind chill was undefined for air temperatures
> above 50 ºF but I did not notice the slightly less strongly worded caveats
> about heat index calculations being unreliable below 80 ºF
>
> My console initially showed the same value (60) for Heat index and Wind
> Chill, with outside temperature 62.0, RH 60, and wind in the low teens. At
> the same time, the weewx display showed a wind chill of 62.0 and a Heat
> Index of 60.7. A couple of minutes later, the temperature had dropped to 61
> 7, the RH had risen to 61, the heat index had risen to 61 (all on the
> console), with no significant change in the wind. The simultaneous weewx
> value for heat index was 60.5. It looks, therefore as if weewx is not using
> the console's calculated values for *either* derived parameter..
>
> In terms of definitions, at least as per the US NWS, it is not that (wind
> chill == temperature) above 50 ºF, but that wind chill is *undefined*
> above that temperature.They are not quite as clear about heat index, simply
> saying that "...The Heat Index calculation may produce meaningless results
> for temperatures and dew points outside of the range depicted on the Heat
> Index Chart linked below {40<= RH <=100; 80<= ºF <=110)}
>
> All my 'derived value" options in weewx.conf are set up "prefer_hardware',
> so weewx should presumably be using those transmitted by the console. As
> far as I can see, it isn't - certainly not for Wind Chill, and probably not
> for heat index, either!
> On Friday, November 6, 2020 at 3:25:32 PM UTC-5 Greg Troxel wrote:
>
>>
>> Also, check the values in your skin against what is on your console.
>>
>
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