As someone who has successfully claimed for piste closure/lack of snow, I can see the logic of an insurance that pays for lack of sun on a beach holiday - ie £xx per day when it rains for more than y hours, or cloud cover is more than z% for more than y hours, etc. However, this type of insurance pays out after you have suffered the lack, not before, and doesn't generally cover all the costs incurred in getting there, accommodation, etc.