Author Topic: Heat Index and Wind Chill thresholds  (Read 2308 times)

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Offline Cienega32

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Heat Index and Wind Chill thresholds
« on: August 14, 2009, 07:00:02 PM »
Understanding that either can mean nothing at certain temps, what are the "official" parameters for Heat Index and Wind Chill?

Searching around, I find different numbers. NWS states wind chill should to be calculated at 50F or below yet I've heard other numbers as well. I read Heat Index should be done at above 68F.


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Offline saratogaWX

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Re: Heat Index and Wind Chill thresholds
« Reply #1 on: August 14, 2009, 08:34:11 PM »
Well, the NWS calculator for Heat Index has the caveat of
Quote
Valid entries are, air temperatures greater than 80 F ( 27 C ), dew point temperatures greater than 60 F ( 16 C ), and relative humidities higher than 40 percent.

So.. heat index isn't valid below 80F.

Similarly, the NWS Wind Chill Calculator has the notation of
Quote
Note: Windchill Temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the wind chill temperature by 10 to 18 degrees F.
so Wind Chill is only valid at or below 50F (with a wind >= 3mph).

Hope this helps...
Best regards,
Ken
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Offline offroadjosh

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Re: Heat Index and Wind Chill thresholds
« Reply #2 on: August 14, 2009, 11:22:07 PM »
Thanks Ken! =)


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Offline Cienega32

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Re: Heat Index and Wind Chill thresholds
« Reply #3 on: August 15, 2009, 01:22:09 AM »
Thanks Ken.

I forgot the >=3mph on the wind chill.

The 68 on the heat index I found on some Wiki entry somewhere. I've also read 70 elsewhere. I guess the NWS would have a better number than a Wiki entry.

On another note - wxgraphic uses numbers of >=70 and <=60  to decide which to show so it made me wonder while I was placing the "$feelslike" function and dealing with the 'empty window' of temps.

I've heard the "Feels like" term used in place of "wind chill/heat index" in this 'hobby' and now I'm wondering just how different the two may be.



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Offline mackbig

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Re: Heat Index and Wind Chill thresholds
« Reply #4 on: August 15, 2009, 07:51:44 AM »
I checked the Enviro Canada site to see what they listed as rules to windchill and humidex.  Humidex call is better different though... :-P

For humidex they dont display unless it is at least 25C (77F), with a dp >0c and air temp of >20c.  Usually not discussed unless over 30C. 

For windchill.
It will be shown in current conditions if
0 (32F) with a wind over 2km/h (1.24mph)

It will only be shown in a forecast if significant
-25C or > (-13F) (with wind over 10km/h 6.2mph)

Interesting that NWS mentions somewhere that 50F is the breakpoint for windchill.  Both the enviro Canada site and the NWS calculators will not even give you a windchill value above air temp of 41F (EC limit is 39..)

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Offline W3DRM

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Re: Heat Index and Wind Chill thresholds
« Reply #5 on: October 26, 2009, 12:52:25 AM »
Well, the NWS calculator for Heat Index has the caveat of
Quote
Valid entries are, air temperatures greater than 80 F ( 27 C ), dew point temperatures greater than 60 F ( 16 C ), and relative humidities higher than 40 percent.

So.. heat index isn't valid below 80F.

Similarly, the NWS Wind Chill Calculator has the notation of
Quote
Note: Windchill Temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the wind chill temperature by 10 to 18 degrees F.
so Wind Chill is only valid at or below 50F (with a wind >= 3mph).

Hope this helps...
Best regards,
Ken

SaratogaWx Ken ---

I've been looking at your ajaxVWSwxf.js code and think I may have found an error. This is the 3/20/09 version of that file which I believe is the most current version available.

A snippet of the code is shown below and it seems to say that Wind Chill starts at 40F but I can't find any code relating to a minimum of 3mph being in the equation too.

...
Quote
      //BEGIN WIND CHILL DATA
      //CURRENT WIND CHILL
      if (wflash[9] <= 40) { // NOAA sez Wind Chill starts at 40F
         var windchill = convertTemp(wflash[21]);
         set_ajax_obs("ajaxwindchill",windchill+ajaxUOM(uomTemp));
      
         //CURRENT WIND CHILL RATE
         var windchillrate = convertTempRate(wflash[49]);
         set_ajax_obs("ajaxwindchillrate",windchillrate+ajaxUOM(uomTemp)+ajaxUOM(uomPerHr));
      
         set_ajax_obs("ajaxwindchillarrow",
            ajax_genarrow(windchill*1.0, windchill-windchillrate*1.0, '',
              langTempRising+uomTemp+langTempPerHour,
              langTempFalling+uomTemp+langTempPerHour,1)
         );   
      } else {
         set_ajax_obs("ajaxwindchill",'---');
         set_ajax_obs("ajaxwindchillrate",'---');
         set_ajax_obs("ajaxwindchillarrow",'');
      }
      //END WIND CHILL DATA
   

As you indicated above, NOAA says "Windchill Temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph"[/i]. Thus, the equation being used in the AJAX code doesn't seem to be correct.

I'm certainly no programmer and may be missing something obvious so pardon me if I'm wrong.

I've just recently implemented your AjaxVWS WxFlash routines and have noticed that neither the Heat Index nor the Wind Chill values seem to show any data. That's how I found the apparent problem with the code.

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