I frequently talk to people that tell me - adamantly - that they need a four season tent. I generally ask them a few questions to determine why they need a four season, as opposed to a three season tent, and the answers are rarely the correct answers.
"I need a four season tent because it is going to be really cold where I am going, and four season tents are warmer." while this is true, sort of, it isn't the reason to buy a four season tent, and there are actually a number of reasons not to buy a four season tent.
The first and most important reason not to buy a four season tent is weight. My three person tent weighs right around 10 pounds. Your average three season, three person tent weighs around 5 pounds - a quick search showed me a 3 person tent that weighed 3 pounds 15 ounces! - so more than double the weight.
The second reason is ventilation. Three season tents are loaded with mesh - which is one of the reasons they are so light - but the real reason for that mesh is airflow. You keep airflow moving through the tent even with a rain fly on, and that means you sleep dry. Four season tents have done away with all that mesh - because they are really designed for mountaineering, and all that ventilation isn't as big of an issue.
Four season tents are a bit warmer than three season tents. But it is a by-product of the lack of ventilation. Think of your four season tent as a rain shell jacket. It's job isn't to keep you warm it's to protect you from the elements - wind, rain, snow. If you want to be warm under your rain shell you add a fleece jacket or other insulating layer. If you want to sleep warmer in your tent you don't add a warmer tent, you add a warmer sleeping bag (the insulation layer!)
So why does anyone buy a fore season tent? Why do I own a four season tent? Most of my gear was purchased planning the inside passage trip - and other trips -ahem- that may be in planning now. I like paddling in places with extreme weather. Places where it can be beautiful when you go to sleep, and at three in the morning you are awakened by freight train winds. That is where a four season tent is worth its hefty weight in gold. This is why you buy a four season tent.
By definition, a four season tent is a one that can withstand winds in excess of 70 miles per hour. (a well made three season tent can do this too) and remain structurally sound under a heavy snow load. Try and visualize this, you go to sleep at 12,000 feet, and tomorrow morning is your summit push. Before you go to sleep you step outside, and marvel at how clear the night sky is, you can see more stars than you have ever seen before. Tomorrow should be a great day. After you get in your bag and drift off to sleep though, the weather changes. A storm front rolls through bringing, heavy wet snow with it. You awake to a foot of new wet snow and perfect avalanche conditions. Time to retreat down the mountain. If you had been sleeping in a three season tent the poles wouldn't have supported the heavy snow, and you would have woken - if you were lucky - to the sound of the poles snapping and nylon pressed heavily against your face. Good luck trying to find the zipper to your bag, and tent.
For most people a good three season tent will suffice, unless like me, you like to paddle in places like Alaska, or Patagonia. My four season tent is my only tent, and in the summer in the south it isn't ventilated enough. But on a beach in high winds it doesn't move, and that is why I like it. I don't mind the weight because it is going into my kayak and I am not carrying it in a back pack.
A final word about three season tents, when you buy a tent its rain fly should cover almost all of the tent. It should come all the way down towards the ground, stopping just 6 inches or so from the ground. Half rain flys and rain flys that just cover the very top of your tent are useless. You shouldn't have to pitch a tarp over your tent to keep you dry.
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