Factors influencing pack selection are the season which the pack will be used, the amount of gear a person typically brings (kitchen sink vs. light and fast), the length of the trip, and the environment in which the trip will take place (desert vs. high alpine). Winter trips will require ~ 1000-2000 cubic inches more than a trip of the same duration taken in summer time. Many top loading internal frame packs have a spindrift collar which allows the top of the pack to be extended for this very purpose.

Courtesy of Kelty

 

EXTERNAL VS. INTERNAL:

External

Externals are the load warriors. Packs designed to carry large loads for long distances over relatively well established trails without a lot of low overhanging branches or narrow passages.

 

• For people backpacking on established trails or less demanding cross country
  
• Proper loading is accomplished more easily
  
• More weight can be carried more comfortably while allowing the wearer to walk erect
  
• External frames allow for increased ventilation, thereby, allowing you to remain cooler
   

Internal

An internal frame pack is ideally suited for trips where balance is critical or where narrow spaces will be encountered. Internal frame packs' backbone rests directly on your back, this brings the packs' center of gravity and your center of gravity as close as possible.

 

• For people who carry loads while doing activities that require independent arm movement
  
• For anyone skiing, climbing, or hiking rough trails
  
• Internal frames have a lower center of gravity thereby allowing you to do these activities that require delicate, balanced movement.
  

Courtesy of Kelty