There are pure measurement from the data sent directly from the various sensors — things like temp, humidity, wind speed, station pressure, etc. Then there are “derived” values which are calucluated from the pure measurements — things like Heat Index, Wind Chill, sea-level Pressure, Rain Rate, Delta T, etc.
To see the formulas for each derived value, please see our Derived Formulas page.
For example, here is how Heat Index is calculated:
I really appreciate the time you and many others take to provide the answers for the Top 50 Things ~ Did You Know. On the same line of thought would like to know if WeatherFlow is using the same formula as other weather related agencies do.
I ask because there have been several instances where the X station noted windchill / heat index / feels like etc. Yet the WF system did not show the same in the lower / higher ranges for cold vs hot weather.
ie. Air Port indicates the windchill is -10’C yet the WF station does not indicate a comparative value. In some instances I can understand because the Sky has not detected the same amount of wind speed. Yet other times the same wind speed is seen but the windchill is not represented in the windchill line of the App / Web Portal??
This is often seen at the lower wind speeds from my experience. 
Hi @Phoenix Good questions. When determining our derived formulas, our entire staff of meteorologists was tasked to arrive at the most common and scientifically sound calculations. In some cases these calculations may differ by region. Where possible, we have noted the source (ie. ‘Weather.gov’ in above example). “Feels like” is one of the most squishy as many organizations calculate differently.
When you see a suspect value, please let us know so we can immediately investigate to ensure the inputs and the math is performing as expected.
Here’s the Wind Chill formula:
So I had the right formulas, just did a boneheaded move and named them backwards.
–Sam
“Feels like heat chill”. That’s a new one! 
touché @WFmarketing, touché.
Best line ever . . . 
As always, thank you for your insight and endless support. Looking forward to next winter to offer more empirical data to fine tune the windchill (Feels Like). 
Hello all!
I’ve wrote something on this, last year:
I know my english skills are not perfect but I promise I’ve done my best… 
You have done well, Pierre.
Thanks @GaryFunk, you’re a gentleman… 
Awesome pawsome . . .
One question I’ve always had is what do you do for the “Feels Like” temperature between the ranges that Wind Chill and Heat Index are valid?
I know in practice that it will just bet set to the observed temperature, but it’s not as though wind speed/humidity have no affect at these ‘out of range’ temperatures (ie. between 50 and 80 F). Is it just that the effect is small enough (or not dangerous enough) that it is ignored?
What we need is a good formula that will use temperature, wind speed, humidity and dew point. To date, Tim Nicholas (@WFstaff) has yet to develop one. 
What about something like the ‘Australian Apparent Temperature’. It seems to take most of those factors into account.
actually I don’t care a lot about windchill and heat index because they just give you how much the human feels in particular temperature but it’s actually vary depending on person, so I can get better idea of how much the air is hot or cold by wind speed and dew point, because two of these parameter can give me a better idea even when the temperature under 80°F(26°C) or above 50°F(10°C), as the wind speed is 20/mph(32kph) or more and dew point less than 50°F(10°C) then you will feel much colder, and of the wind speed is calm or too weak with dew point more than 62°F(approx 17°C) then you will feel more hot. but the wind actually not work like that every time because this also depend on the direction and the time like on the day or night.
To be consistent I think this should read:
Wind Chill is calculated for temperatures at or below 50°F and wind speeds at or above 5mph.
Or do you really not calculate it if the speed is exactly 5mph?