What are the 5 major air masses that affect north america?

Stable, cold, dry air masses originating over n. canada and alaska eventually plunge southward to interior of u.s. as a shallow dome of high pressure - reach gulf of mexico and florida - freeze cropsproduce lake effect snows as they move over the great lakes. usually do not move west of rocky mountains - mountains confine cold air to the east. upslope precipitation is common east of the rockies as the cp air mass slides to the south. during the summer, cp air mass can bring relief to hot, humid regions

Independent variable is something that is changed in a experiment. 

No bc a lot of things r still theories n unproven