Originally Posted by
Dan The Man
I would say the opposite for extreme heat. You can sweat a lot faster than you can absorb water, so if you are relying on only sweating to cool you down, you will dehydrate eventually. You will also need more electrolytes because you lose those too. It takes a fixed amount of energy to evaporate a given mass of water, whether it's sweat or just sprayed on. That means it drops your temperature either way. As long as you aren't spraying half of it onto the pavement, you will get the same cooling from spraying yourself down as you will from sweating buckets. Obviously your body is better at fine-regulating your temperature, but if it's 100 degrees outside, you are going to be dumping a lot of heat anyways.
In extreme dry heat, at every mile marker I would take a mouthful of water and spray a squirt on my front and thighs. Works out to about a liter every 15 miles or so.
While it is true that the energy to evaporate a given unit of water is the same independent of the source, it's not necessarily true that water sprayed on you is the same as water sweated out of you. Nor that water on the outside of you is the same as water inside you. Heat exhaustion and heat stroke are the result of dehydration or too little water inside your body for you to continue to function. To avoid them, you need water
inside you. Pouring water on yourself may cool you but it does nothing to address the dehydration.
We are also talking about touring. While touring, as opposed to recreational riding, you probably won't know where water sources are. A liter of water wasted on the outside of you may be a liter of water you need at mile 40, 50 or beyond. If you have excess water, by all means, pour it on yourself. Nancysv is absolutely correct unless you can be sure you have excess water, it's better to have it on the inside than the outside. In other words, drink.