Humidity levels change with temperature.

So 100% humidity is at hotter temperatures is different than 100% at lower temperatures. That’s simply because there is more space for water vapor at higher temperatures.

100% humidity at 55 degrees (f) will feel comfortable, but 100% humidity at 75 degrees (f) will feel hot and muggy.

This is due to dew-point, the point (temperature) at which water vapor condenses and become dew. Or what the temperature would have to be cooled to to be at 100% humidity.

Generally speaking a dew point of 55 degrees (f) and under is perceived as comfortable, which would also mean 100% humidity and any temperature 55 degrees (f) and under would also be comfortable (with in reason of course).

Outdoor athletes will often use dew point to judge if their activity will be comfortable, uncomfortable, or even down right dangerous because temperature and humidity on their own don’t tell the whole story until looked at together.