The hail I'd say, especially combined with the wind. We just decided to do a quick walk from Nant Peris to Pen Y Pass along the road. Going up was okay (the wind was behind us), but coming down it started alternating from hail to sleet, and the hail doesn't half sting when it hits you in the face. Luckily for me my fella is really tall and fat, so I used him as a wind sheild
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There was some crazy young lad on a Felt roadie though, just going up and down the Llanberis pass. He past us 3 times. Very exciting thing to do on a Saturday afternoon I'm sure. Having said that, it is one of the thing I plan to do - cycle up the Llanberis Pass and bomb it back down. See if I can break my 38mph max speed, should do it with ease - I'll just pick a day when it isn't really windy and hailing!