Water Source 5: Fly Leakage
Fly coverage, as already mentioned, is very important. As for fly leakage, factory-sealed fly seams are quite common. However, untaped, double lap-felled seams are fine when made with cotton-wrapped thread that expands when wet to fill needle holes, and especially when all seams slope to prevent water pooling. Being less subject to wear and pressure than the tent floor, coatings on flies can be made much thinner, lighter, and with lower waterproofness ratings than for floors. However, wind-driven rain exerts a surprising pressure and exploits weaknesses in inadequate or damaged coatings.

Shop for: a well-made fly. Flies become watertight by virtue of quality coatings, seam sealing or good seam sewing, and rain-shedding designs that prevent pooling from occurring.

Water Source 6: The Entrance
You and your tentmates can become a major conduit of water into the tent if the entranceway is poorly designed. The entrance design must let you enter and exit without exposing the tent floor to falling rain.

Shop for: vertical doors. If both the fly and tent doors slope at a low angle (45 degrees) to the ground, rainwater will fall directly onto the tent floor whenever you unzip to come and go. Look instead for doors that are more nearly vertical.

Water Source 7: Poor Ventilation
Gear-soaking condensed water vapor can cover cooler surfaces such as the inside of the fly and the lower walls of the tent in wet weather. Another benefit of close-to-ground fly coverage is that it keeps the lower walls warmer, reducing condensation. The vapor comes from your breath, from body heat drying out damp clothes, and especially from cooking. The vapor doesn't easily exit the tent in wet weather because the outside air is also very humid. With little difference between inside and outside humidity, the moisture only diffuses slowly through the tent fabric to the outside.

Even if a tent is made entirely of mesh, the fly can prevent exit of warm, damp air rising by convection. Convection venting requires a high fly vent or a fly door that can be left open at the top under an"eyelid" (shown) even in bad weather.

Shop for: convection venting. Look for vents up high on the fly, or fly door tops that can be left open without letting rain in. These capture wind gusts and, in the absence of wind, allow warm, moist air from within the tent to escape by convection. Without these high fly vents, even an all-mesh tent will be a sweatbox.

Water Source 8: A Wet Pitch
During a rainstorm, the tent body can get very wet as you fumble to set things up and get the waterproof fly in place.

Shop for: continuous pole sleeves. Continuous pole sleeves allow you to insert the poles with the fly laid over the tent to keep it dry. Alternatively, look for a tent that has a fly that's designed as a free-standing unit. These provide a surprising amount of room in which to sort out wet gear and organize before hanging up the tent underneath as a dry "inner sanctum." However, these tents usually require a "tent footprint" (a groundsheet with pole sockets), which adds weight.

Water Source 9: Tight Confines
A tent that's too small for you and your gear forces you and your sleeping bag into contact with the condensation-wetted walls.

Shop for: room to roam. Extra space at the sides and ends of a tent allows you to stuff waterproof bags full of unused gear, which in turn keeps sleeping bags out of contact with wet walls. Also, you'll be able to sit up and move more freely without risk of pushing the tent against the condensation-wetted fly. Extra space means extra weight, so look for a compromise you can accept between tent size (which affects comfort as well as dryness) and the weight you can happily carry.

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