Why is it so humid?

Because the air around you is carrying a lot of water, usually picked up over warm ocean or gulf water a day or two ago. The number that measures it is the dew point, not the humidity percentage.

Humidity percentage is the wrong number

Relative humidity tells you how full the air is compared with how much water it could hold at its current temperature. Warm air can hold far more water than cool air, so the same air reads 90% at dawn and 45% by mid-afternoon without gaining or losing a drop. That's why a 90% morning can feel fine and a 50% afternoon can feel like a sauna.

Dew point is the temperature the air would have to cool to before its water starts condensing. It only changes when water is actually added or removed, so it tracks what your skin feels all day.

Why water in the air feels bad

Your body cools itself by sweating: the sweat evaporates and carries heat away. Evaporation slows down when the air already holds a lot of water. Once the dew point climbs into the mid-60s °F, sweat lingers on your skin instead of evaporating, so you feel sticky and hotter than the thermometer says. Above about 70°F your body has a real struggle shedding heat, which is why humid heat waves are more dangerous than dry ones.

Where humid air comes from

Air picks up water by passing over warm water. In the eastern and central US the usual source is the Gulf of Mexico; on the East Coast it can be the Atlantic and the Gulf Stream; in the desert Southwest it's the summer monsoon flow from the Gulf of California and the eastern Pacific. A humid day almost always means the wind has spent the last few days blowing over one of them toward you.

Air Story traces exactly that: it follows the air at your spot backward for three days and colors the path by how much water it carried, so you can see where your humidity was picked up.