What is relative humidity? Explain the effect of relative humidity on the presence of a white trail released from a flying plane in a clear sky ?

Explanation

The ratio of actual mass of vapour content in the air for a given volume and temperature to that required to make the air saturated with vapour at that temperature is called the relative humidity.
The white trail is basically the vapour released by airplane engine condensing in sky to form cloud.
ii. If the relative humidity of surrounding air is more then, it takes longer time for the trail to disappear.
iii. If the relative humidity of surrounding air is less then, either small sized white trail may get formed or it may not get formed at all.