AmbientWeather.com: What Humidity Level does Fog, Mist, and Rain Occur?
Fog. Fog forms when the difference between air temperature and dew point is less than 2.5 °C (4.5 °F).
As an example:
Temperature = 70.0 degF
Dew Point = 65.5 degF.
Humidity = 83%
In this example, fog begins forming at 83%.
Mist. Mist forms when the relative humidity is greater than about 70%.
Rain. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. For example, it can rain at a ground humidity of 60%, but over time, the humidity will increase.
High Humidity as it Relates to a Weather Station:
100% humidity is not possible as the air is completely saturated and water droplets fall out. The maximum a weather station will measure is 99%.
High (and low) humidity are very difficult to measure. The accuracy is generally guaranteed to 95% and then degrades due to the non-linearity and high gain of the capacitive sensor vs. water vapor in the air.