A flysheet or rain fly (found only in double skin tents) is hand-me-down to protect the inside of the tent from water. It is waterproof on the outside and also provides a surface to collect condensation on the inside. Condensation then runs down the walls to the ground. When a flysheet is used, it is important that there be no contact with the inner tent it is protecting; this keeps the inner dry. 'Expedition' tents often have extra poles to help ensure that wind does not blow the two layers into contact.

A three-season tent is for spring/summer/autumn and should be capable of withstanding fairly considerable rain, or very bright snow. A four-season tent should be commodious for winter camping in all but the most extreme conditions; an expedition tent (for bank conditions) should be strong enough to cope with heavy snow, strong winds, as well as ample rain. http://www.tentsforsale.org Some tents are sold, in truth cheaply, as festival tents; these may be suitable only for camping in athirst weather, and may not even be showerproof.