What is the concept of humidity in weather forecast?

Relative humidity The percentage value of the actual water vapor density contained in the air and the saturated water vapor density at the same temperature is called the "relative humidity" of the air. The degree of dryness and humidity of the air is related to the degree to which the amount of water vapor contained in the air is close to saturation, but has no direct relationship with the absolute amount of water vapor contained in the air. For example, when the pressure of water vapor contained in the air is also equal to 1606.24Pa (12.79 mm of mercury), at noon on a hot summer day, the temperature is about 35°C, and people do not feel wet, so the water vapor saturation pressure is still far away from the object. The moisture in it can continue to evaporate. In the colder autumn, around 15℃, people will feel humid, because the water vapor pressure at this time has reached supersaturation, and the water not only cannot evaporate, but also condenses into water, so we put the actual content in the air into water. The percentage of water vapor density ρ1 to the saturated water vapor density ρ2 at the same temperature ρ1/ρ2 × 100% is called relative humidity. It can also be expressed as the ratio of water vapor pressure:

For example, the pressure of water vapor in the air is 1606.24Pa (12.79 mmHg). At 35°C, the saturated vapor pressure is 5938.52Pa (44.55 mmHg). column), the relative humidity of the air

At 15°C, the saturated vapor pressure is 1606.24Pa (12.79 mmHg), and the relative humidity is 100%.

There is no functional relationship between the two physical quantities of absolute humidity and relative humidity. For example, the higher the temperature, the faster water evaporates, so there is a corresponding increase in water vapor in the air. Therefore, during the day, the absolute humidity is often greater at noon than at night. During the year, the absolute humidity in summer is higher than in winter. However, since the saturated vapor pressure of the air also changes with temperature, it may be that the relative humidity at noon is lower than that at night, and the relative humidity in winter is higher than that in summer. Since the saturated water vapor pressure at a certain temperature can be found from the "Saturated Water Vapor Pressure at Different Temperatures" table, as long as the absolute humidity or relative humidity is known, the relative humidity or absolute humidity can be calculated.