Humidity tells you the moisture content of the atmosphere, or how much water vapor there is in the air. When the humidity is high it feels oppressive outside because sweat doesn't evaporate and provide cooling. When the humidity is low you feel cooler, but your skin dries out and you get dehydrated more easily because more moisture is being evaporated from your body. The same thing applies to crops in high and low humidity. A high water vapor content is necessary to produce clouds and precipitation. Relative humidity and dew point are the two main ways to describe humidity.
Relative humidity is given as a percentage. This percentage tells you how close the air is to being saturated. If the relative humidity is 100%, the air is saturated. If the relative humidity is 50%, the air contains half the water vapor required for it to be saturated. If the amount of water vapor in the air increases, the relative humidity increases, and if the amount of water vapor in the air decreases, the relative humidity decreases.
However, relative humidity is dependent on air temperature, too. If the water vapor content stays the same and the temperature drops, the relative humidity increases. If the water vapor content stays the same and the temperature rises, the relative humidity decreases. This is because colder air doesnâ€™t require as much moisture to become saturated as warmer air.
If you watch the weather in the summer, youâ€™ll notice that the relative humidity is actually higher in the morning than in the afternoon. This is because the cooler morning air is closer to saturation than the hot afternoon air, even with the same amount of water vapor. Surprisingly, there is no significant difference in daily average relative humidity between summer and winter. Since warm air is less dense than cold air, there is more room for water vapor in warm summer air as compared with cold winter air.
Dew point is a better indicator of humidity than relative humidity because it is not a percentage dependent on temperature. The dew point is the temperature to which the air would have to be cooled to become saturated. Below the dew point, water will condense out of the air onto surfaces. In the early morning, grass surfaces will be coated with water if the nighttime temperature has dropped below the dew point. When humidity is high, the dew point temperature is only a few degrees below, or equal to, air temperature. In dry places, like deserts of the southwest, air temperature can be 50 or 60 degrees above the dew point. Generally dew point is a more reliable indicator of humidity than relative humidity because dew point is not changed by a change in air temperature and doesnâ€™t fluctuate much throughout the day.