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Wind Chill Calculation

Discussion specific to Davis weather stations
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williston
Posts: 28
Joined: Tue 25 Nov 2008 11:25 pm
Weather Station: Davis VP2 w/Daytime FARS
Location: Massachusetts

Wind Chill Calculation

Post by williston »

I have noticed with the option checked to have Cumulus calculate the wind chill, the display seems to reflect the current value on the console (average value). When it is unchecked, it is a "dynamic" display, i.e. it changes as the wind speed changes. I may be interpreting the instruction wrong for the option but thought I would pass it along to you. I like seeing the value change as the wind speed varies, so I keep it unchecked.
Station: VP2 running latest beta release
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steve
Cumulus Author
Posts: 26702
Joined: Mon 02 Jun 2008 6:49 pm
Weather Station: None
Operating System: None
Location: Vienne, France
Contact:

Re: Wind Chill Calculation

Post by steve »

Yes, Cumulus uses the 10-minute average wind speed to calculate wind chill. I thought that was what Davis did - I usually use them as a guide as to what to do. Their application note on 'derived variables' says this:
The Vantage Pro and Vantage Pro2 console uses the "10-minute average wind speed" to
determine wind chill, which is updated once per minute. When 10-minute of wind speed data is
unavailable, it uses a running average until 10-minutes worth of data is collected. The
WeatherLink® software uses the 10-minute average wind speed also. If it is unavailable, it uses
the current wind speed (which updates every 2.5 to 3 seconds). All other products use the
current wind speed to determine wind chill.

The reason an average wind speed is employed in the Vantage Pro and Vantage Pro2 to
calculate wind chill is as follows: The human body has a high heat capacity, thus high wind
speeds have no effect on the body's thermal equilibrium. So, an average wind speed provides
a more accurate representation of the body's response than an instantaneous reading. Also,
"official" weather reports (from which wind chill is calculated) provide average wind speed, so
using an average wind speed more closely matches the results that are seen in weather reports.
That seemed very reasonable to me, so that's what Cumulus does. The Davis DLL (which I use to get the data from the station) must be supplying a wind chill based on the current wind speed rather than the 10-minute average. That's the value you will get with the Cumulus option unticked. It seems odd that the DLL apparently doesn't supply the value shown on the console.

Steve
williston
Posts: 28
Joined: Tue 25 Nov 2008 11:25 pm
Weather Station: Davis VP2 w/Daytime FARS
Location: Massachusetts

Re: Wind Chill Calculation

Post by williston »

Hi Steve,
What I'm seeing is the wind chill value on the Cumulus display constantly changing according to the wind speed. (Not sure if I'm explaining it well) This is with the box -unchecked-. When the box is checked/selected, it mimics what the Davis console says, so I agree it's working correctly. Is this how Cumulus is supposed to work with the box unchecked? (Constantly changing/dynamic wind chill indication)
Thanks!
User avatar
steve
Cumulus Author
Posts: 26702
Joined: Mon 02 Jun 2008 6:49 pm
Weather Station: None
Operating System: None
Location: Vienne, France
Contact:

Re: Wind Chill Calculation

Post by steve »

I understand what you're saying.

With the option checked, Cumulus calculates the wind chill itself and uses the average wind speed. So it comes out the same as the VP2 console, because that's also how the console does it.

With it unchecked, Cumulus uses whatever the station supplies, if it supplies one. In this case, the Davis DLL I use appears not to supply the wind chill as displayed on the console, despite the fact that Davis say this is the best one to use. For some reason the Davis DLL appears to supply a wind chill based on the current (last 2.5 seconds) wind speed. I suspect this is actually an error in the DLL, because it has some other odd quirks, and I won't be using it for Cumulus 2.

Steve
williston
Posts: 28
Joined: Tue 25 Nov 2008 11:25 pm
Weather Station: Davis VP2 w/Daytime FARS
Location: Massachusetts

Re: Wind Chill Calculation

Post by williston »

Understood. I actually like seeing the dynamic display, especially during the last week or so when it has been very windy and we were getting some high gusts. Cumulus displays and stores some wind chill values that the console misses because it averages them. Thanks Steve.
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