Question

causes of variation in humidity

Answer 1

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.