It depends on which part of Mexico you are refering to. The pacific side has a difference climate to the caribbean. There's a sticky at the top with lots of information on weather and where to find weather history. I'll assume the caribbean coast as you'll probably not want to be over on the pacific coast in july since it's rainy season! Generally speaking, the weather is warm and can be humid but also can be unsettled as it's the hurricane season.

Darren

we went last year end July first week August weather was lovely rained at night only. Had a few thunder storms again only at night. It was just about right not to hot but more than hot enough. we had the best holiday ever and woill return one day. (got loads of other places to vist first)