@gjr80: I am sure that you are right. I did the fairly obvious check of
forcing both derived parameters to "hardware" in weewx.conf, upon which
both show up as "N/A" (and blank on the graph) on the web page. I have a
vague memory that there are potential issues with having weewx use LOOP2
packets (otherwise, why not default to this). Can you elucidate?
On Friday, November 6, 2020 at 4:53:58 PM UTC-5 gjr80 wrote:
> Just to clear up obtaining heatindex and windchill from Davis hardware.
> Irrespective of any other settings, the only way you will get hardware
> provided heatindex and windchill from a Davis station is if you have WeeWX
> use the LOOP2 packet data from the console/logger. This is because
> heatindex and windchill are not included in the ARCHIVE record and LOOP
> packet provided by the console/logger. To use LOOP2 data the user must make
> a change to weewx.conf, a default install will use LOOP packet and ARCHIVE
> record data from the console/logger and not LOOP2 data.
>
> Based upon what you have posted so far my guess is the WeeWX is not
> receiving heatindex/windchill data from the console/logger and hence WeeWX
> is calculating both.
>
> Gary
>
> On Saturday, 7 November 2020 at 07:44:41 UTC+10 Peter Fletcher wrote:
>
>> 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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