Quote Originally Posted by branng View Post
Was riding around in the extreme heat today in Beijing (high of 37 deg C / 100 deg F), in direct sunlight, while wearing a 3/4 helmet, a mesh jacket and thick denim jeans. I had to pull over to cool off in the shade for a bit as I began to exhibit clear signs of heatstroke: dizziness, an elevated pulse and feeling faint. All despite keeping fairly well hydrated by drinking plenty from the bottle of water I had with me. Removed my jacket and thankfully managed to cool off just enough to be able to continue riding to my destination.
I recall reading some articles about how mesh jackets, although feeling cooler, actually work towards faster dehydration. Despite this, I also wear mesh in heat, but try to replenish water with at least one bottle (500 ml) every hour, or so, when riding in heat & sun. Dehydration on a bike is dangerous territory and it can creep up on a person.

Soaking one's garments (especially something like a t-shirt under a jacket) provides some comfort but, again, can actually accelerate dehydration by encouraging perspiration.

The key is always to ensure you remain hydrated, and let your body's cooling system do its job. If, however, your body temp rises and you start to feel symptoms of dehydration (headache, dizziness, blurred vision, etc), it is very important to stop riding and try to cool down in the shade, a stream, an air conditioned building, etc, for at least an hour, or so. I was once told, and use it as my own personal rule-of-thumb, that you should be drinking enough water that you need to take a serious whiz every hour, or so.

The experts on staying cool in heat are those who live in the desert (e.g., Saudi, etc), and you will always see them covered from head to foot in wraps of clothing. Although appearing counter-intuitive, it actually works. When I lived in the Middle East, and rode around on a bike, one would always wrap oneself with light-colored clothes to prevent any exposure to sun and especially to the immense drying effects of hot air passing across exposed skin. Even one patch of exposed skin works like a drain for your body's perspiration.