Why is the weather forecast humidity 100%?

"Relative humidity 100%" in weather forecast means that the percentage of water vapor pressure in the air and saturated water vapor pressure at the same temperature is 100%.

Relative humidity is expressed by RH, which refers to the ratio of absolute humidity to saturated absolute humidity in the air at the same temperature and pressure. This number is a percentage, that is, at the same temperature and pressure, the ratio of the mass of water vapor contained in humid air to the mass of water vapor contained in saturated air is expressed as a percentage. For example, the normal humidity of the computer room is 60%, which is the relative humidity.

The original hygrometer with relative humidity is an old hygrometer for medical use. At present, most hygrometers are made of special temperature-sensitive and humidity-sensitive materials, and some even have mechanical rotating devices to form automatic hygrometers.

Extended data

Effect of relative humidity on PM2.5:

Relative humidity plays an important role in monitoring and forecasting fine particles in high pollution or low pollution cities. When the relative humidity of air without rainfall is below 60%-80%, the secondary generation of particulate matter is strong, and the concentration of PM2.5 is proportional to the relative humidity.

When the air humidity is more than 80%, it is easy to form rainfall, which will have a scouring effect on the particulate matter in the air, and the concentration of particulate matter is inversely proportional to the relative humidity of the air.

With the increase of humidity, the proportion of water-soluble organic carbon in particulate matter and organic carbon increases, and the proportion of water-soluble organic carbon and elemental carbon increases significantly under the condition of high relative humidity. The increase of urban relative humidity in winter is directly related to the formation of severe smog.