The humidity forecast is mostly relative humidity, such as 100% here. 100% means that 100% of the water vapor can be contained in the air. If water vapor enters the air, it will condense into water.

The upper limit of how much water vapor can be contained in the air is affected by temperature. The higher the temperature, the faster the water vapor molecules move and the less likely they are to condense. The air can hold more water vapor without condensation. So water vapor of the same quality in the air is easy to condense in winter and not easy to condense in summer. This is why white fog can be blown out in winter but not in summer.
The relative humidity shows how far the current water vapor content is from this upper limit. When the relative humidity is low, it means that the air is unsaturated and can absorb more water. At this time, the water is more likely to evaporate into steam. When the relative humidity is close to 100%, the air is about to be saturated. At this time, no matter how much water there is, the air does not welcome it to turn into steam.
People's sweat can evaporate into water, and the process of evaporation absorbs heat and makes people feel cool. When the air is close to 100% humidity, evaporation is difficult to occur, and people will feel unusually muggy. So some muggy cities are on the riverside, not only because of the heat, but also because of the high humidity, people can not dissipate heat through the evaporation of sweat.

Related  products: Digital Thermometer Hygrometer