What is the correct mowing height for a lawn in summer?

FAQ
Question: 

What is the correct mowing height for a lawn in summer?

Answer: 

Kentucky bluegrass and other cool-season grasses thrive in the cool weather of spring and fall.  Hot, dry conditions in summer are stressful for cool-season grasses.  Kentucky bluegrass lawns should be mowed at a height of 3 to 3½ inches during the summer months.  Bluegrass lawns should be mowed at a height of 2½ to 3 inches in spring and fall.  The additional leaf area during summer shades and cools the crowns of the turfgrass plants.  The higher mowing height also provides more food-producing foliage and promotes deeper root growth. 

When mowing the lawn, never remove more than 1/3 of the total leaf area at any one time.  Removing more than 1/3 of the leaf area severely injures the turfgrass plants and reduces their ability to withstand additional environmental stresses.