What does "relative humidity 100%" mean in the weather forecast?

Relative Humidity The percentage of the actual water vapor density contained in the air to the saturated water vapor density at the same temperature is called the "relative humidity" of the air. The degree of dryness and wetness of the air is related to the degree that the water vapor contained in the air is close to saturation, but not directly related to the absolute amount of water vapor contained in the air. For example, when the pressure of water vapor contained in the air is equal to1606.24pa (12.79mm Hg), at noon in hot summer, the temperature is around 35℃, and people do not feel wet, so it is far from reaching the saturated pressure of water vapor, and the water in the object can continue to evaporate. In the cold autumn, people will feel wet around 15℃, because the water vapor pressure has reached supersaturation, and water can not only evaporate, but also condense into water, so we call the actual density of water vapor in the air ρ 1 and the percentage of saturated water vapor density ρ2 at the same temperature ρ1ρ 2×100. It can also be expressed by water vapor pressure ratio:

For example, the pressure of water vapor in air is 1606.24Pa( 12.79 mm Hg), the saturated vapor pressure at 35℃ is 5938.52Pa(44.55 mm Hg), and the relative humidity of air.

At 15℃, the saturated vapor pressure is1606.24pa (12.79mmhg) and the relative humidity is 100%.

There is no functional relationship between absolute humidity and relative humidity. For example, the higher the temperature, the faster the water evaporates, so the water vapor in the air also increases accordingly. Therefore, in a day, the absolute humidity at noon is often greater than at night. In a year, the absolute humidity in summer is higher than that in winter. However, because the water vapor saturation pressure of air also changes with temperature, it may be that the relative humidity at noon is smaller than that at night, and the relative humidity in winter is larger than that in summer. Because the saturated vapor pressure at a certain temperature can be found out from the table of "saturated vapor pressure at different temperatures", as long as the absolute humidity or relative humidity is known, the relative humidity or absolute humidity can be calculated.