New wind chill calculation

[steve]

I've noticed a slightly odd effect with the wind chill calculation that I'm now using for Fine Offset stations. It has a cutoff of 10C, at or above which it decides that wind chill is no longer relevant, and so sets the wind chill to the same as the ambient temperature.

Now, while this might be strictly correct in meteorological terms (the NOAA say wind chill is only defined at or below 50F (10C) - see towards the end of this page - http://www.weather.gov/os/windchill/index.shtml) it does make the wind chill graph look a bit odd when the temperature is around 10C/50F.

I'm considering taking the 10C cutoff out of the calculation. Any thoughts?

Steve

[beteljuice]

Stick with NOAA

But what windchill graph ? or are you thinking V2, in which case you should switch windchill / HeatIndex and have a 'feelslike'.

[steve]

Stick with NOAA

I usually refer to my Davis VP2 and see what that does, but the documentation is a bit confusing as it depends on the firmware. But I think they don't cut off until about 90F (actually, I think the calculation just works out that way).

But what windchill graph ?

The one in the latest 1.8.5 builds.

If you look here (which is where it was brought to my attention) - http://www.c.chew.dsl.pipex.com/trends.htm and look at the outside temperature graph, you'll see the effect as the temperature hovers around 10C, with a sudden change in the value.

Steve

[beteljuice]

Had a look at graph outputs from other places and that 'gap' is correct, it is an unfortunate combination of the windspeed and graph scale (both time and amplitude) that makes it stand out so much

[steve]

But don't you think that sudden 'step change' in the wind chill graph as the temperature goes from 9.9C to 10C (and back again) is a bit silly? It suggests that in high wind at 9.9C there is a considerable wind chill, and then suddenly at 10C there is none. Even if it is being exaggerated in this case.

And look at it this way: If I change it to have no cut-off by default, and provide an ini-file switch to turn it back on, I will get far fewer repeated queries in here about it than I surely will if I leave it as it is

Steve

[beteljuice]

It's the nearest approximation in a complicated formula, just one of those things I'm afraid - just like they say you can't have a 'cooling' wind when it's hot !