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Summertime can be "cool" but is often "hot". Radiation and convection are efficient methods of removing heat. However, once the air temperature reaches 95°F, heat loss by radiation and convection ceases. It is at this point that heat loss by sweating becomes all-important. But sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation (sweat changing to water vapor). The downside of this method of cooling is that high relative humidity retards evaporation.
The Heat Index uses relative humidity and dry bulb temperature to produce the "apparent temperature" or the temperature the body "feels".
Find out more about the Heat Index JetStream - An Online School for Weather.