evaporation

what causes water to evaporate

every fluid (pure) has a definite vapor pressure at a fixed temperature , now if any how the partial presuure of the fluiid is less then the the vapor pressure of the fluid then the fluid starts evaporating to attain the partial pressure equal to the  vapor pressure of the fluid, and then the equllibrium is reached and evaporation stops. 

                            basically at the top of the fluid there is a random motion of the fluid molecule due to the difference between the adhasive and cohesive forces, some molecule say 'n' goes into the vapur phase and some strikes back say m such that n>m then after the above explained condition is achived then the constraint m=n.

conclusion

whenever there is the fluid in liquid state is  present then in that system partial pressure is exactly to the vapor pressure.

and whenever there is no fluid in liquid state is present then partial pressure is always less then the vapor pressure.

geographical context

let us consider two location having altitude h1 and h2 they both have the same temperature t0 in such case sice at high altitude there is less partial presure of water vapour since it is very heavy so amount of evaporation is high in the h1 region then in the h2 region.