During winter water vapour is not evaporated quickly in the atmosphere as the tempt. is low. so the tempt. remains constant for a long time
  during summer the water vapour evaporates quickly as the tempt is high n during this process it takes heat from the atmosphere n so the tempt. of the surrounding air decreases little 
         So in this way specific of water plays a greater role in stabilising atmospheric tempt. during winter n summer seasons