Since we have some summer heat around today, I'd would like to explain why dewpoint is a better measure of how sticky it feels rather than using relative humidity.
First and foremost, the relative humidity is not an absolute value but rather a ratio. It is a measure of how much water vapor is currently present in the given volume of air, compared to how much water vapor the same volume of air is capable of holding at a given temperature. Air's capacity to hold water vapor increases with increasing temperature (i.e. warmer air can hold more vapor than colder air)...and that is why 55% humidity at a temperature of 95° feels much more oppressive than 55% humidity at a temperature of 45°.
Dewpoint temperature, on the other hand, is totally independent of actual air temperature. The definition of dewpoint is as follows:
The temperature to which a given air parcel must be cooled in order for saturation to occur.
This is much more representative of the actual quantity of water vapor contained within a given volume of air, and a dewpoint temperature of 65° feels every bit as oppressive when the air temperature is 95° than it does when the air temperature is 70°.
Here are some key dewpoint temperatures to guage how uncomfortable it feels.
50-55... Fairly Comfortable... barely feel the humidity
55-60... Slightly Humid
65-70... Very Uncomfortable