You could try a Core Cooler, Tues did a review of it HERE
on HU, and because I believe every word she says apart from which way nuts and bolts undo, I bought one on the strength of her reviex. It's fantastic!! So good in fact that because it's 38°C here in my tiny office with the non-opening window I've actually got it on now.
My gran always used to say if you can keep your wrists, ankles and neck cool then the rest of you should feel cool, I think it's because you've got quite high blood flow with the veins/arteries very close to the surface, The Core Cooler works for your neck, but you could get yourself some of those wrist sweatbands tennis players love and keep them soaked in water which will also cool you down a lot. If you wear shorter boots you could put those round your ankles too.
Finally, a fine misting bottle, small enough for your pocket can be great too. I got mine from Boots.
Just open your visor and squirt a bit in across your brow*. It helps clear the sweat out of your eyes and again, makes you feel cooler than you probably are.
*probably best to do this when stopped.
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