Explanation of terms

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 wind chill temperature is always lower than the air temperature for values where the wind chill formula is valid. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead.

Wind chill temperature on schijf.org is calculated according the following JAG/TI formula:

    Twe = 13.12 + 0.6215 * Ta - 11.37 * (3.6 * V)0.16 + 0.3965 * Ta * (3.6 * V)0.16

where:

   Ta = air temperature in degrees Celsius
   V = average windspeed in meters per second
   Twe = wind chill temperature in degrees Celsius

Although it's nice to have a formula, what does this mean in reality? Well, fortunately for us there is some research available. In 2008 TNO did research into loss of finger dexterity in increasing wind chill conditions. Here's a short recap; 12 military men, aged 30 to 37, were dressed for winter and had to undergo winter like conditions. This resulted in two conclusions:

• Manual dexterity decreases in lower wind chill temperatures and longer exposures;
• Warm clothing does help keep the body temperature longer, but does almost nothing for manual dexterity. Fingers will get too cold and loose their dexterity once skin temperature drops below 14°C.

To keep your fingertips above the minimum 14°C limit one should keep the exposure time limited according the formula:

   Twe * (duration)0,48 = -113,07

Given this formula we can make the following table:

Temperature	Duration
-10	2,5 hours
-20	27 minutes
-30	16 minutes
-40	9 minutes
-50	5 minutes

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Dew point

The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point. When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.

The dew point temperature on schijf.org is calculated according the following formula:

    Tdp = (b * y) / (a - y);
    y = (a * Ta) / (b + Ta) + ln(Hrel / 100);

where:

   Ta = air temperature in degrees Celsius;
   Hrel = relative humidity;
   Tdp = dew point temperature