Feels-like temperature is a general term for indices that try to describe environmental effects on the human body. It is not a temperature measured by a thermometer.
Different indices use different variables and have specific limits. Wind chill applies to cold and wind, while heat index mainly describes warm, humid air in the shade.
Cold and wind
Wind increases heat loss from exposed skin and can make cold feel more severe. The standard wind-chill relationship applies only in low temperature and sufficient wind.
It does not predict object temperature or an exact frostbite time for every person.
Heat and humidity
High humidity slows sweat evaporation and reduces natural cooling. Heat index approximates this load for warm, humid conditions in the shade.
Direct sun, calm air, physical work, age, hydration and health can substantially increase actual risk.
More complex indices
UTCI and WBGT include more variables such as radiation and wind, but require suitable inputs and careful interpretation.
No index replaces medical advice or official occupational guidance. It is an indication for more conservative decisions.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.