Quick answer
Chinch bugs suck sap from grass in hot, dry weather, causing yellow-then-brown patches that look like drought. If watering doesn't help a sunny patch, check for chinch bugs.
How to Check
Look at the edge where brown meets green. Part the grass and watch the soil surface, or push a can with both ends removed into the soil, fill it with water, and see if small black-and-white insects float up.
Conditions That Favor Them
- Hot, dry summers
- Heavy thatch layers
- Full sun near pavement
What to Do
Treat only when you confirm them. Proper watering and reducing thatch help prevent repeat problems.