The Feels Like Temperatures map show what the outdoor temperature will feel like for the current day. Feels Like Index is a factored mixture of the Wind Chill Factor
and the Heat Index. The combination of the heat index and the wind chill factor are denoted collectively by the single terms "apparent temperature" or "relative outdoor temperature" or simply "Feels Like".
is the apparent temperature
felt on exposed skin, which is a function of the air temperature and wind speed. The wind chill
temperature (often popularly called the wind chill factor) is always lower than the air temperature, except at higher temperatures where wind chill
is considered less important. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead.
The heat index (HI) is an index that combines air temperature and dew point in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity
, allowing an index to be made which corresponds a temperature and humidity
combination to a higher temperature in dry air.
The heat index is derived from work carried out by R. G. Steadman. Like the wind chill index
, the heat index contains assumptions about the human body mass and height, clothing, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.
In Canada, the similar humidex is used in place of the heat index.
At high temperatures, the level of relative humidity
needed to make the heat index higher than the actual temperature is lower than at cooler temperatures. For example, at 80°F (approximately 27°C), the heat index will agree with the actual temperature if the relative humidity is 45%, but at 110°F (roughly 43°C), any relative-humidity reading above 17 % will make the Heat Index higher than 110°F. Humidity is deemed not to raise the apparent temperature at all if the actual temperature is below approximately 68°F (20°C) — essentially the same temperature colder than which wind chill is thought to commence. Humidex and heat indexes are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun.