Topic: Wind chill readings

[ioinn]

My software (VWS 14.) seems to have accurate readings, while Underground doesn't.  During this cold snap my software is showing -25 with temp of -13 and 5 mph wind. Underground displays -16 with same info.  I use rapid fire for updating and the wind chill doesn't change with temp and/or wind changes.  Does Underground use a time interval or average over time interval?  Anyone know?

[Zephirus]

I don't know what they are using.  But I have been looking today at how varied wind chill numbers are too.  I made a spreadsheet to look at various calculation methods.  My Davis station uses an average method of some time like 10 minutes.  In ~2001 NWS revised their WC calcs too so be aware of that.  In my area the wind almost pulses rather than blows so what you feel does not match too well to the WC stations report in my experience.  But hey, alot of people like their wind speed measured at 10 meters, and mine is at 3. 

[Anthony]

It's hard to compare one site to another when it comes to wind speed/wind chill. Wind speed can vary greatly over short distances. Plus surrounding objects can play a role in the speeds that are recorded. Temperatures can also vary over short distances. All this will effect wind chill readings.

[mackbig]

Was it just temporary glitch.  Took me a while to find windchill, does not show on personal WU page.  Need to look at the city, then the PWS's underneath. 

Your town, mine, and my station all show accurate windchill calcs right now.  The wind speed used is just the currect measure, or at least it was for the ones I looked at.

But you are correct, the rapidfire update does not seem to affect the wc displayed on the page.

Andrew

[blackjack52]

I don't know what they are using.  But I have been looking today at how varied wind chill numbers are too.  I made a spreadsheet to look at various calculation methods.  My Davis station uses an average method of some time like 10 minutes.  In ~2001 NWS revised their WC calcs too so be aware of that.  In my area the wind almost pulses rather than blows so what you feel does not match too well to the WC stations report in my experience.  But hey, alot of people like their wind speed measured at 10 meters, and mine is at 3. 

The newer versions of VWS use the new NWS WC calcs. You are given the option to use this under SETTINGS/ADV PARAM.

[ioinn]

Just to clarify, my question is why does my VWS display show a very different WC reading then WU is displaying?
Even checking a neighboring PWS site on WU with both mine and his showing identical temps and about same wind speeds, we had a 12 degree difference in WC readings on the WU display.  And neither were same as my VWS display which of course changes as wind speed was changing (temp was constant).
Thanks for everyones input so far.

[blackjack52]

I just watched 6 PWSs not change WC readings for 15min. 3 diff PWS manufacturers, all w/ very diff wind readings and most Rapid Fire.
That tells me it is computed at their end at an interval greater than 15min, or the reading is taken from our tags at more than a 15min update period.

[mackbig]

I thought I aswered this, but perhaps it was digressing a bit.  My answer would be no, they are not different, WU and VWS appear to use same calculator

My website shows -11.3c wind 12.6km, wc -17.7  (values provided by VWS)

The noaa calulator gives that as well.  EC calculator rounds up to -18.

I went on WU for your town (Wakefield, MI).  Its shows -10.6c wind 11km, and a wc of -16
The noaa calculator gives -16.3, EC calc shows -16

Not sure why your live VWS display and WU were not in agreement.  Are they close now that its not -13f anymore
I do notice that the rapidfire ones do not appear to be accurate, so it is possible that WU picks a time to use the wind speed from.

Andrew

Just to clarify, my question is why does my VWS display show a very different WC reading then WU is displaying?
Even checking a neighboring PWS site on WU with both mine and his showing identical temps and about same wind speeds, we had a 12 degree difference in WC readings on the WU display.  And neither were same as my VWS display which of course changes as wind speed was changing (temp was constant).
Thanks for everyones input so far.

[ioinn]

I just checked WU weather stations site and found that 3 stations according to WU WC display was warmer then the actual displayed temp with moderate winds (1-4 mph).  How can WC be warmer then the temp (note that temps had been increasing, but lower then WC)?My PWS is Sunday Lake.
Just toggled back to the PWS list and now mine is blank for WC?

[tinplate]

Note that the wind chill formula is only defined above 3 MPH wind, so for wind speed 3 and below, the temperature and wind chill are the same. The calculated wind chill in wunderground can sometimes be a little above the temp because of rounding. Also note that the formula takes into consideration the fact that the standard height of wind measurement is 10 meters, but the human face is around 5 ft high. The formula adjusts for this. So if your anemometer is lower than 10 meters or otherwise sheltered, your calculated wind chills will be higher than they really should be (had your wind speed been measured at 10 meters as the formula assumes).

[blackjack52]

How does the formula know your anemometer is below 30'

[tinplate]

It doesn't. That's why the formula will scale your wind down too much if your anemometer is low. It is assuming it's at 10 meters.

[Zephirus]

The WC formula(new and old) is simple and and does not truncate for "out of range" inputs such as wind speed < 4 MPH.  Most of the Java calculators I have seen have code that must fix inputs that are out of range.  If you use the "raw" equations for WC and you enter either temperature (above) or wind (below) defined values you will get WC numbers that don't make sense (e.g. WC > current AT).   For my area, I think WC would be better calculated as an instantaneous value that would be averaged at some defined interval.  There are standards for a reason however.  An example would be last night in our area AT was about 26F, and wind surged to about 6 MPH off and on at about 5 minute intervals.  So if you went out side for a few minuted when the wind was gusting,   you would think it didn't feel too good in your face.  But if you look at a WC calculation it would indicate no WC because of averaging below 4 MPH. 

[blackjack52]

It doesn't. That's why the formula will scale your wind down too much if your anemometer is low. It is assuming it's at 10 meters.

I think I follow,
It assumes  that if the wind is blowing at 5mph, and it should be at 30', then the wind should be only 2mph at 5', therefore, if the anemom is at 5' already, it will scale it down further, reporting less WC.

[blackjack52]

Would increasing the gain for wind spd and gust help someone compensate if they didn't have the anemometer sited correctly

[sam2004gp]

Yes it would, but is a highly subjective modification.  Everybody's "wind surrondings" and obstacles are different.

[blackjack52]

Thanks. I thought so.  I'm just trying to offer up some alternatives for those that might not be sited properly.  I would imagine, if your station is at 15', the gain would have to be 2, assuming no obstructions.

I'm at 22', and have gain of 1.1.

[sam2004gp]

Perhaps, but I don't assume that all anemometers are at 30.  I use a gain because the OS-WMR968 units are know for reporting low numbers consistently.
Plus I have never heard anywhere to alter your height in WU for the anemometers.  So I think what your reporting wind speed wise and temp. wise., that WU calculates it's own and since they have alot of stations to do, they only do it at an interval like 15 mins or so.