It would have to be pretty bad weather to cancel an appointment: ice on the roads or something along that line. I doubt my T would cancel his schedule due to a potential thunderstorm (heck, he'd never have an appointment from March through May if that was the case). If I cancelled simply because of a potential storm, I suspect I'd be expected to pay for it. If a horrible storm with hail actually did crop up at the time I needed to get to an appointment and I decided not to risk the drive, he'd probably understand that not charge me.