Wind chill

Wind chill is the apparent temperature felt on exposed skin due to wind.

The degree of this phenomenon depends on both air temperature and wind speed. The greatest wind chill reported in extremes, results from a combination of a low temperature and a high wind speed.
The wind chill temperature (often popularly called the wind chill factor) is always lower than the air temperature. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead.

Some weather stations provide a Wind Chill reading and, by default, that reading will be used by Cumulus.

If within Cumulus you apply corrections (Configuration menu, Calibration screen) to either temperature or wind speed, then any wind chill output by your station will not be compatible, and you should set Cumulus to Calculate Wind Chill.
On Configuration menu click Station and tick the option to Calculate Wind Chill.

There may be other reasons why you do not want to use the reading provided by your station and tick this option.
If your weather station does not provide a Wind Chill reading, Cumulus will calculate it (regardless of whether the option is ticked or not).

Wind Chill Temperature is officially defined for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph).

For lower wind speeds, Cumulus will report a wind chill temperature that will be the same as reported for temperature.
By default, for higher temperatures, Cumulus will report a wind chill temperature that will be the same as reported for temperature. Cumulus can make a wind chill calculation for temperatures above 10°C, so if you have asked Cumulus to calculate wind chill and don't wish it to comply with the ten degree limit, edit your cumulus.ini and add to the [Station] section the line:
```
UseWindChillCutoff=0
```

More information on Wind Chill here in Wikipedia. Cumulus uses the 'new North American' formula, as described on that page.