When warm and cold air masses collide, the warm air is forced ...

[amal]

When warm and cold air masses collide, the warm air is forced upward. Why does this occur?
  What will be the ideal response?

Question 2

Why is the formation of air masses associated with high pressure systems rather than low pressure systems?
  What will be the ideal response?

[cam1229]

Answer to Question 1

ANSWER: When warm and cold air masses come together, the cold, dense air wedges under the warm air, forcing the warm air upward, much like a snow shovel forces snow upward as the shovel glides through the snow.

Answer to Question 2

ANSWER: In order for a huge mass of air to develop uniform characteristics, its source region should be generally flat and of uniform composition with light surface winds. The longer the air remains stagnant over its source region, or the longer the path over which the air moves, the more likely it will acquire properties of the surface below. Consequently, ideal source regions are usually those areas dominated by surface high pressure. They include the ice- and snow-covered arctic plains in winter and subtropical oceans in summer. The middle latitudes, where surface temperatures and moisture characteristics vary considerably, are not good source regions. Instead, this region is a transition zone where air masses with different physical properties move in, clash, and produce an exciting array of weather activity.

[amal]

Thanks for the timely response, appreciate it

[cpetit11]

Gracias!