When temps in the valley are above 92 degrees, descending the trail becomes hazardous. But you're going up Skyline, you say? Well, if you have never done Skyline before, or haven't done it recently, there is a good chance you might 'bonk' halfway through (many people do, especially when it's warmer) and then (unlike in December) you'll be in a situation where you can neither ascend nor descend and will need to be rescued (if you're smart.) If you're not smart, you might try descending midday anyway, and end up dead from heat stroke (this has happened to several unfortunate individuals.)
We are still having relatively cool nights, however (by May or June that will no longer be the case) so if you are a C2C novice and insist on trying the trail now anyway, then leave no later than 3 a.m. If you do bonk, you will still be able to descend during cooler morning hours when temps are still quite moderate. This advice does not apply late May through September, when it's dangerously hot even by 8 a.m. (IMHO a novice or non-regular should never ever try Skyline during these months.) Carry at least 4 to 5 liters of fluid, but remember that in itself, water won't protect you from heat stroke--you can die of heat stroke even if well-hydrated. One of the people who died on the trail was found with a full water bottle.
Every time I post this I always get a few responses along the lines of "But I did it last July and was perfectly fine," etc. Obviously not everyone who does it in summer dies or the trail would be littered with skeletons. Well-acclimatized 'regulars' who do it every month or so are usually fine doing it even in July. I'm just saying that for everyone else, you're taking a significant risk, which, let's not forget, also puts rescue personnel at risk, since flying the helicopters close to the mountain is hazardous. So consider waiting until late fall to do your C2C--as the saying goes, the mountain will still be there.
