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Many people think I am crazy for all sorts of reasons. One of the reasons is the subject of this entry - the [http://www.amazon.com/Under-Armour-HeatGear%C2%AE-Compression-Midnight/dp/B00885166G Under Armor Heat Gear long sleeved top]. Why would someone wear a long sleeved top (or any top at all) in hot weather? The key is how the body looses heat in exercise. In cool weather, convection can be a significant way of loosing heat, which is why we put on clothes to prevent this heat loss. In hot weather, the heat loss through convection is much less than through evaporation. The heat gear top works by spreading the sweat out over a wider area and letting it evaporate. Being a form fitting top, it is tight to the body, so the cooling effect is felt directly. (I classify Under Armor as 'form fitting' rather than 'graduated compression' as they provide no significant pressure. See [[Why compression clothes]] for more details.) If sweat drips form the body, it's ability to cool you is very limited. The heat gear top is very thin and spreads the sweat out very well. I am cooler when running in hot weather with the heat gear top than running topless. The top does keep you very slightly warmer for the first few minutes, until the body starts sweating, but the top is so thin this is not a problem.
The heat gear top is also odor resistant and has a high UV factor. Being white is reflects the heat of direct sun as well. I suspect that when out of the sun, a dark top would work better, and I may try this out in the near future.
Any downsides to the top? Not many - it is so good at keeping you cool that you can chill quickly when you stop running. It is a compression top, so it reveals your body shape quite clearly, which might be a factor depending on your body image. The top is not cheap, but at $35 it is not outrageous. With the odor resistance, I have one top that I wear repeatedly and wash once a week or so. This is gear I can wholeheartedly recommend.