robint
Contributor
I did Coba in August arriving late in the morning and it was hot but with two quart thermoses of cold water in my backpack - doable. 8am sounds much better for either, and at least CI doesn't seem like a mountain bike park.
Don - Coba is surrounded by jungle, so yes it is hot but there is at least some shade. The site at Chichen Itza has been cleared of all trees in the efforts to uncover the 10-20% of ruins that are now available. Very little trees means swealtering hot open areas. There are a few small gift shops around the huge site, so we ducked into those (air conditioning, restrooms, and cold beverages) a few times during our 3 hours there. It helped but if the place is crowded I would expect them to be packed with people.
Coba
Chichen Itza - a few clusters of trees here and there, by mid-day these were full of people trying to excape the heat
many of these were taken by the vendors so you couldn't even get in the shade!
robin