But, this trick only works on a dry day. When sweat evaporates off the body, energy is also absorbed into the air as part of the reaction. This helps to cool down the body. On humid days (where the air is already full of moisture), less sweat will evaporate and it won't be as effective. Also, if you've got decaf coffee or tea, it's probably best to stick to these. As Louis pointed out in our Facebook group, caffeine dehydrates the body. We all know that's a bad move in any weather (especially if you're hungover), let alone during a heatwave.
