Declan, I cannot comment on American laws, but as far as I understand it here in the UK, accidental exposure (such as the effects of an unexpected gust of wind) is not an offence, but purposeful exposure is.

We have a kilt-wearing British police officer in our midst (Al Sporrano), and there's another over at "Tom's Café" (Stu). They would be able to corroborate the UK position.