Scarifying gets rid of moss and thatch. Doing so in the summer means the soil dries out and becomes too warm. Grass tends to grow slower in dry hot weather. I find I mow more in April and May, and less in June, July and August. After scarifying, I normally add fertiliser and grass seed.
I don't see to much harm in scarifying during the summer. Id recommend looking at the weather forecast and choose a day before it's going to rain which should save you having to hand water it. Once scarified I would cut it in two directions.
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