In the summer months we tend to focus on the temperatures to know how hot it's going to be outside. But how hot it really feels depends on the heat index.

When the human body heats up, it will cool itself by sweating. Heat is then removed from the body by the evaporation of that sweat.

However, when the relative humidity is higher, it reduces the evaporation rate. The result of a lower evaporation rate leads to slower cooling of the body, hence the sensation of overheating.

So, the more "humid" it is, the higher the heat index will be at a given temperature.

Now, the precise equation that is used to calculate the heat index by National Weather Service is long and complex, but it can be found by clicking here.

A simpler way to see the relation of temperature and humidity can be seen on this chart: