Heat indices combine temperature and relative humidity to indicate how hot the body feels.

CNN: Heat wave sweeps across U.S. Northeast

The Met Office's "feels like temperature" takes into account expected air temperature, relative humidity and the strength of the wind at around 5 feet (1.5m) - the typical height of a person's face.

BBC: Who, What, Why: What is wind chill factor?

So depending on the relative humidity and temperature, 100 or more water molecules diffuse out of the leaf for every molecule of carbon dioxide that diffuses in.

WSJ: Harrison H. Schmitt and William Happer: In Defense of Carbon Dioxide