Wet weather can severely impact the delivery schedule (and associated costs) of a project that includes works outside or that is dependent on outdoor work.
If you are the project manager on a project for delivery for a client, you should allow for wet weather in your schedule.
Most clients will usually only allow variations (time or costs) for wet weather delays if those wet weather delays meet a certain criteria. That criteria is usually based on rainfall being recorded in that area (either by official sources or from an onsite rain gauge) for more than a set number of days for that month.
For example, a contract may officially state that you should allow for 5 days of wet weather in each month of the project (or a variation of that for each month), and that variations will only be granted for wet weather delays if the recorded rainfall is more than that number of days. If the contract does not state this, you should get the official local government average rain days for that area and allow for that in your contract offer (and state the number of rain days allowed for each month).
By showing the number of wet weather days clearly in your schedule, you make it much easier to get extension of time or cost claims approved by your client. When wet weather does occur that influences your works schedule, you should make clear records of this, and communicate to the client that the weather may delay the schedule. If it does then affect the schedule, you should inform the client as soon as possible and also inform them that you will need to make an extension of time claim and possible cost claim as per the contract.
If you are the project manager on the client side, you should make the same allowances in your schedule, and you should ideally state the wet weather day allowances in your tender / offer request, and in the contract. A clearly written section on this in the contract makes the effects of rain clear to both parties.
You should allow for wet weather days, both in your proposals that you submit to clients (or to potential clients) and also if you are managing a project that requires works by construction companies (i.e. if your company is the client). Show this clearly in your schedule.