Have you ridden much in the rain?
Apart from fogging up and wetting your spectacles (wait wait lazer surgery - non issue now)
The road, however much rain rains upon it seems to stay dirty. Lots of dirt ends up on your back from the very clean tyre spinning and picking up lovely wet dirt to spray on you.
Are you going for a 'get wet' option ride sarvo?
Please post pictures of your arse when you get back!