High humidity on summer days makes it much harder for us to keep cool. The increased moisture in the air doesn’t make the temperature hotter—a 90°F day is still 90°F—but it does make it harder for our bodies to release heat and cool us down. This is why humid days feel hotter: we have all this extra heat stored up.
Of course, you have your home’s air conditioning system to help keep you cool. But does the AC do anything about the humidity aside from blast out enough cool air to overcome the heat? Well … a little. But it’s rarely enough.