Relative humidity (%) measurement cannot be exist without air temperature (°C) measurement.

Relative humidity is what most of us know as humidity. It is defined as how much water is in air as a percentage of the maximum water that air can hold at a particular temperature before it condenses out. The amount of moisture that air can hold before it starts to rain is highly dependent on temperature and the amount of dust in the air which initializes water droplet formation. To put it simply, the hotter the air is, the bigger a sponge it becomes. Even a small air temperature change can change relative humidity readings by multiple percent. Air at 30 °C (94 F) can hold twice as much water in it before it starts to rain as air at 15°C (60 F). It also means the following: 90 % humid air at 15 °C, if heated to 30 °C, will only show 45 % humidity. Since this also works in reverse, we can see that when temperature on a warm and humid day drops due to a weather front coming through, the humidity at lower temperature is likely to reach 100 % and it will probably start to rain. This lower temperature, where humidity level reaches 100 % is called a dew point temperature and is reported in most weather forecasts.

Dew point is the temperature, at which moisture in the air reaches the point where is starts to condensate. It is the temperature to which we have to cool air for relative humidity to reach 100 % level. Without any other influences on the amount of water absorbed in air, this is the temperature at which dew may form and it may also start to rain. This effect can be clearly seen when driving on a cold rainy day. In a warm car the air can hold a certain amount of moisture, but if the glass is cooler than the dew point temperature of the air inside the car, windows will start to fog up.The solution is to warm the windows above dew point temperature by blowing hot air on them or to turn up the ventilation to reduce air humidity levels inside the car.

For more information on rain drop shape and size, please read the following article: Rain drop size and speed of a falling rain drop

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