Most events at London 2012 will go ahead as normal even if it pours down. However, some might be affected at times of particularly bad weather. In any case the terms and conditions on the London 2012 website are very clear.
Generally, there are no refunds available if a session is delayed during the day, or is "substantially completed" when the interruption occurs.
If a session is not substantially completed and is rescheduled to take place as a new session (i.e. separate from an existing, scheduled, session), the ticket holder may retain their original ticket for admission to the rescheduled session.
If a session is cancelled, then the person who bought the ticket will need to apply for a refund of the face value by sending back the ticket along with a refund application form.
http://www.tickets.lo...om/purchaseterms.html