I guess the PM can give any assumption that he or she wishes to but it would be uncommon for them to assume something that is already their risk and also very difficult to measure. Weather happens every day but when would it be classed as bad weather? Much more sensible and measurable that the Contractor prices for items that are already their risk under the contract. Any assumptions that are given should be clear and measurable. They are for matters that otherwise would be difficult for the Contractor to price with a sensible allocation for cost/time.
If the PM really did state "make no allowance for any rain delays" then I guess that is the assumption to be priced. The Contractor would then have to prove with good records/diaries that indeed any rain did cause an issue.
If the PM really did state "make no allowance for any rain delays" then I guess that is the assumption to be priced. The Contractor would then have to prove with good records/diaries that indeed any rain did cause an issue.