Is Heat Index in VWS always Lower?

[sam2004gp]

I got a high temp of 90.3 today and my Heat Index number from VWS only got up to 87.7.

Now that I think about I don't think it has ever got over the Outside temp reading.

Is it normal for VWS's Heat Index to always be lower than the Outside temp?

[mmorris]

I got Hi Temp 85 and the Heat Ix was 83 must be a dew point Humidity thing. So your not alone.

[WeatherBeacon]

I got a high temp of 90.3 today and my Heat Index number from VWS only got up to 87.7.

Now that I think about I don't think it has ever got over the Outside temp reading.

Is it normal for VWS's Heat Index to always be lower than the Outside temp?

What's the humidity? If the humidity is low enough, then yes, the heat index can be lower than the actual temperature. Check out this link:

   http://www.wxforum.net/index.php?topic=3696.msg30620#msg30620

and this heat index calculator:

   http://www.crh.noaa.gov/jkl/?n=heat_index_calculator

For example, at 90.2^oF and a relative humidity of 30%, the heat index is 88^oF.

Kevin...


EDIT:  Heat index is an inexact science.

[Mark / Ohio]

Our daytime humidity has been real low here once the warm air moved back in.  Wait until about July 4th and see what happens. 

Also the old prediction of overnight temperatures by the lowest afternoon dewpoint as not been working for me the last several days neither.  We're kind of in strange ground for the general climate in this area for this time of year.

[Anthony]

Same for me here with a temp of 83 and heat index of 81.

[LFWX]

I often see Heat Index lower than Temperature when the Humidity is low

But now that it's humid I'm getting strange Heat Index numbers from VWS

Conditions:
Temperature = 60.9
Humidity = 94%
Wind Speed = 1 mph

THW Index values shown:
VWS = 61.4
WL = 61.4

Heat Index values shown:
VWS = 67.1
WL = 61.4
VP2 = 62

Most Heat Index calculators will not take temperatures lower than 70 or 80, of the one's I found that would - most matched the VP2 & WL values, but a few did matched the VWS value. It's causing some strange things on my site.

[wxtech]

My heat index is almost always higher than actual temp and wind chill is almost always lower than the actual temp.

Now data @ 8:45 AM
65.0   Temperature             vxv007
68.4   Feels Like (virtual temperature)      vxv027  uni027
65.0   Wind Chill                vxv019 uni022
69.6   Heat Index                vxv021 uni021 
59.7   Dew Point                vxv022 uni022
83%   Humidity                vxv005%
0.8   UV Index                vxv017   
137   Solar Radiation             vxv018  Watts/Sq Meter 
0.00   Rain Today                vxv121uni121
In Lexington, Ga. high morning humidity, with dew, usually overcast early mornings.

[Anthony]

I have a heat index calculator on my web page. To get to it, click on the grapgs & guages link on the main page and on that page click on the weather computer link on the left at the bottom.

Yea I know what you are going to say. I need to get all of my links organized.

[LFWX]

Yea I know what you are going to say. I need to get all of my links organized

Have you looked at the mess on my page?

68.4   Feels Like (virtual temperature)      vxv027  uni027

Isn't "virtual temperature" a number used in calculating other numbers, not a "feels like" temperature?

EDIT: Link to virtual temperature discussion
http://www.wxforum.net/index.php?topic=4022.0

[wxtech]

68.4   Feels Like (virtual temperature)      vxv027  uni027

Isn't "virtual temperature" a number used in calculating other numbers, not a "feels like" temperature?

From the VWS manual:
VIRTUAL TEMPERATURE Virtual temperature is a fictitious temperature that takes into account moisture in the air.
The formal definition of virtual temperature is the temperature that dry air would have if
its pressure and specific volume were equal to those of a given sample of moist air. Virtual
temperature allows meteorologists to use the equation of state for dry air even though
moisture is present.
Yes, I think you're right.  But I'm also correct in calling it a "feels like" temperature.  It's different from heat index.  It's different from wind chill.
So, what is the consensus about what temperatures should I provide to the public?

[WeatherBeacon]

Heat index and wind chill are attempts at describing how hot or cold it feels to the body by accounting for humidity and wind. Apparent temperature Virtual temperature does not do that. Rather, it is more of a standardized temperature that meteorologists use to simplify calculations. Thus, apparent temperature virtual temperature is not another "feels like" temperature like heat index and wind chill are.

An analogy would be standardizing atmospheric pressure to sea level; it is more convenient for meteorological calculations.

Regards,

Kevin...

EDIT:  Sorry, folks. My brain was thinking of virtual temperature when I replied.

[LFWX]

Apparent temperature does not do that. Rather, it is more of a standardized temperature that meteorologists use to simplify calculations. Thus, apparent temperature is not another "feels like" temperature like heat index and wind chill are.

Did you mean to say virtual temperature?

From the info cited below, VWS shouldn't currently be calculating a heat index higher than the real temperature.
Temp = 53.9
Humidity = 87%
Dew Point = 50.4
and yet VWS has the heat index at 59.2 (VP2 console shows 54)

from
http://climate.virginia.edu/apparent.htm
The apparent temperature is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. The apparent temperature equals the actual air temperature when the dew-point temperature is 57.2F (14C). At higher dew-points, the apparent temperature exceeds the actual temperature and measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities. When the dew-point is less than 57.2F, on the other hand, the apparent temperature is less than the actual air temperature and measures the reduced stress and increased comfort associated with lower humidities and greater evaporative skin cooling


Also see wikipedia
http://en.wikipedia.org/wiki/Apparent_temperature
and the NWS
http://www.weather.gov/forecasts/graphical/definitions/defineApparentT.html

[WeatherBeacon]

Apparent temperature does not do that. Rather, it is more of a standardized temperature that meteorologists use to simplify calculations. Thus, apparent temperature is not another "feels like" temperature like heat index and wind chill are.

Did you mean to say virtual temperature?

 
                            

Oh for crying out loud! Yes, that's what I had in mind. I think I need to take a break.

Thanks for the correction, LFWX!

[LFWX]

Thanks for the correction, LFWX!

No problem, I figured as much.

Well, so far, I've run across three Heat Index calculators which give numbers matching the VP2 console displayed value. So, until I can create a script to calculate this, I'll just stick with showing the THW Index.

Three calculators showing same Heat Index as the VP2 (at least for values I've tried - a cold front just passed through so things have cooled down a bit):

http://www.hpc.ncep.noaa.gov/html/heatindex.shtml

http://www.easysurf.cc/cnver16.htm

http://avc.comm.nsdlib.org/java/heatindex/heatindex.html

By the way, is it possible to just have WeatherLink calculate this and then somehow bring that value into my VWS created web page? That would be great!

-Mike

Weatherbeacon - love your "Right Wing Radicals" avatar.

[Mark / Ohio]

...By the way, is it possible to just have WeatherLink calculate this and then somehow bring that value into my VWS created web page? That would be great!..

You might be able to run the .htx file through one program and pick it up and run it again through the second program.  You really would need to be running both programs at the same time using Virtual VP.  I think I was able to do that experimenting one time a few years ago.  But I also think I ran into some issues getting all the tags to render correctly without confusing one program or the other.  Definitely some trial and error involved in it but it might work for what you want.

[tomcj2]

I had never given much thought to different Indices, or  Factors until I read this thread and did more research.  I will probably remove all of them from my site because of the different and confusing formats.  If it is 89°F, with no wind, 98% relative humidity, and I am standing in bright sunshine, that is exactly what it feels like, Gordon Ramsey Hot.

Why create an imaginary number when the factual observations will do?

[mackbig]

If you can have the weatherlink htx create a js file that is uploaded to your site, you can then put javascript in your vws htx, and once that htm/php file is on your site, it will call that value.  I use something similar for my humidex calc, and pool temp.  In my case vws creates both files, but there is no reason it would not worth with 2 diff programs creating the files.

Andrew

...By the way, is it possible to just have WeatherLink calculate this and then somehow bring that value into my VWS created web page? That would be great!..

You might be able to run the .htx file through one program and pick it up and run it again through the second program.  You really would need to be running both programs at the same time using Virtual VP.  I think I was able to do that experimenting one time a few years ago.  But I also think I ran into some issues getting all the tags to render correctly without confusing one program or the other.  Definitely some trial and error involved in it but it might work for what you want.

[LFWX]

Mark - Yes, I'll be installing VVP, worked great when I used a trial version on an older, much slower computer. Also, I made the mistake of allowing VWS to read a txt file to create a php file once and it erased something in the script. My brother was helping me trouble shoot when he found the error. Good idea though!

Andrew - That sounds like a plan, I'll have my, more computer savvy, brother help me work it out.

Why create an imaginary number when the factual observations will do?

In the case of someone involved in some physical activity outside these can be informative and helpful, if done correctly and consistantly.

Thanks
Mike

[mackbig]

Mike,
Let me know if you want me to post any code, that you can use as a base.

Why create an imaginary number when the factual observations will do?

In the case of someone involved in some physical activity outside these can be informative and helpful, if done correctly and consistantly.

I agree.  as long as the value is known.  We use Humidex here. its widely known and accepted.  It's similar to Heat index (better in my opinion since it has more values for lower temps high humidity)...   The other values (virtual temp, thw, thws...) might be interesting for other weather stations owners and yourself but if your site visitor has never heard of them .... so a link to a definition might be good.

Andrew

Andrew - That sounds like a plan, I'll have my, more computer savvy, brother help me work it out.
Thanks
Mike

[LFWX]

so a link to a definition might be good.

That's in the works - a page showing how the VP2 collects the data and calculates various parameters (rain rate, Dew Point, etc) and then an explaination of things like Sea-Level Pressure, Pressure Trend, Dew Point, THW Index, etc.

Next week looks pretty busy, but I may get back to you on the code, if I need any help, thanks for the offer!

Mike

[dan43]

Same here... current temperature, 28.1°C, humidex is 26.9°C.  Relative humidity is 22%.

I don't think my humidex has ever been above the temperature.

http://www.portcoquitlamweather.ca/

[mackbig]

You must have really low humidity readings all the time then.   

26.9 is your heat index reading, not humidex, your humidex if vws calculated it, would show 28.1

heat index calculator  http://www.srh.noaa.gov/elp/wxcalc/heatindex.shtml
humidex calculator  http://www.mackweather.com/humidex.php

Most heat index charts I have seen dont go below 40% humidity, and 80F is their base temp at well.

Andrew

Same here... current temperature, 28.1°C, humidex is 26.9°C.  Relative humidity is 22%.

I don't think my humidex has ever been above the temperature.

http://www.portcoquitlamweather.ca/

[Anthony]

Currently (at time of post) Temp 70, Humidity 80, Heat Index 74.

[sam2004gp]

Still noticing, "unsettling numbers" on our hottest day of the year so far.