Moisture is the presence of water vapor occurring in the atmosphere from evapotranspiration that occurs on both liquid and vegetable surfaces.
We measure air humidity with hygrometers / hygrographers.
Hygrograph used to record relative humidity
It is the amount of water vapor in the atmosphere at any given time.
It is the ratio between the absolute humidity of the air and its saturation point (it is the limit by which the atmosphere can increase the amount of water vapor. When it is reached, we say that the air is saturated; the higher the temperature, the greater the amount of water vapor the air may contain, and hence the higher the saturation point). It is measured as a percentage.
Fog is the most visible effect of air humidity.