Mexico 's climate is defined by its topography. As a rule of thumb, with rising elevation - temperature and humidity fall. This means cool temperate conditions in central Mexico(the Sierra Mardre ranges) and hot humid conditions on the coastal lowlands.
Visitors to the north of Mexico will barely notice the rainy season(between July and September) but on the southern beaches it can get oppressively hot with just the occasional tropical cloudburst to cool things down.
Summer is also hurricane season on the Yucatán Peninsular, and as prices rise with the influx of European holidaymakers this time of year is best avoided. Winter is the ideal time to visit(between October and May).