Just as you wouldn't wear only a tee shirt to hike through falling snow or sweatpants in the tropics, it's important to choose the appropriate backpack for the weather in which you'll be hiking. If you're like most backpackers, you'll find yourself eventually owning different backpacks to use in different climates.
Almost as important as what you'll use the backpack for and being sure of a personal fit, is matching the backpack to the weather you'll be experiencing. Your personal comfort, and more importantly the condition of the contents in your backpack, can be adversely affected by the weather. It's no fun to hike through the rain looking forward to a dry campsite, only to find everything in your backpack soaking wet because you didn't choose the water-resistant model (not to mention the extra weight those wet items create).
Here in Florida it is almost a given that you'll be hot, wet or most likely both when you hike in our tropical climate. That kind of weather demands a backpack that is light, water-resistant and constructed in such a way that air (no matter how warm and wet it may be) can circulate between your back and the backpack. A wet backpack constantly rubbing against your back as you walk will only serve to make your hike miserable and possibly chafe your back to the point of being raw.
Colder, yet still wet with snow climates will mean you should be choosing a backpack that is a little more rugged, but still water-resistant, with at least a small amount of insulation to keep your contents dry and at least lukewarm. The insulation (as well as the more rugged material) will also help keep YOU warm...or at least your back.
If you are able to choose only one backpack, then make it an All-Weather backpack that can be used in a variety of climates. This kind of backpack tries to be everything to everyone and, depending on which kind you choose, it will either succeed or fail. In general, an All-Weather backpack will work well in most climates and conditions and is a good beginner backpack, but as mentioned at the beginning of this article, you'll eventually find yourself with different backpacks for different weather.
This article continues the series Choosing A Backpack, with general guidelines for beginner backpackers and others about how to pick the right backpack. The series articles also include:
Choosing A Backpack - Usage
Choosing A Backpack - Personal Fit