Question

how does atmospheric pressure affect boiling and evaporation

Answer 1

Boiling point of a substance is defined as the temperature at which its vapour pressure become equal to the atmospheric pressure.Since vapour pressure of substance increase with temperature so atmospheric pressure is low then less temperature has to be supplied to the substance so that vapour pressure becomes equal to atmospheric pressure and therefore at less atmospheric pressure the boiling point of the substance will be low and the evaporation will be fast.
Similarly if atmospheric pressure is high then more temperature is required by the substance so that vapour pressure becomes equal to atmospheric pressure and therefore at high atmospheric pressure boiling point of the substance will be high and the evaporation will be slow.