Light rain and even a sprinkle of snow won't delay or stop you taking your heli ride; what does cancel helicopter tours is pouring rain, thunder, lightening, heavy snow fall and hail stones.
If any of these weather conditions happen before your flight, then it may not get cancelled at all, but rather pushed back until the weather as subsided, if it does.
If cancellation occurs due to the weather, it likely because the helicopter company deems the poor visibility due to long spells of fog or mist won't ease off in a suitable time.
Therefore, they will contact you to advice on a new time and day; or you'll be given the chance to cancel all together - results in a full refund at no cost to you.