Turf grass does better in the high heat of summer if you allow it to grown out a bit. By allowing your lawn to grow out a little you are promoting deeper grass roots that can suck up moisture and shade the soil which minimizes evaporation. If you cut your lawn too short you are reducing grass’s ability to produce the energy needed for growth.
TIP: For mowing in the heat of summer, follow the one-third rule but let your grass grow roughly .5 of an inch higher than the recommended cutting height. Never remove more than one-third of your grass height at one time because this will lead to more stress and potential scalping of the lawn and/or disease.