Our body temperatures and chickens' isn't all that different from each other. If it's too hot for us, it's too hot for chickens.
Baby chicks are especially vulnerable because they aren't able to regulate their body temps with the insulating effect of feathers, having only fuzzy down at the moment. This is why it's crucial to always provide them with both cool space as well as warm space in which to move in and out as they feel the need to shed excess heat or warm up.
Direct sunlight is especially dangerous to baby chicks on very warm days since they very quickly absorb far more heat than their bodies can tolerate.