Why steam turbine surface condencer vaccum decrease in hot summer but increase in cold nights?

Steam Turbine Surface condenser is a closed vessel kept in vacuum as it is connected with the last stage of Turbine exhaust Steam generally in -ve pressure.Inside the vessel there are tube bunches,through this tube cold water comes from Cooling tower, steam gives it's latent heat to the circulating water coming from Cooling tower,thus steam get condensed & it's volume decreases1; times,so vacuum is created in the Condenser, at the same time circulating cold water takes heat from Steam & it's temperature increases,goes to cooling tower.

In hot summer circulating water ambient temperature is low as compared with Cold nights.As circulation water temperature become lower, condenser Vacuum will Increase.... 

thank you for your answer.... but i think overall efficiency of steam turbine remain same. if we calculate overall energy consumption..it will be equal in cooled or hot  season... ???