Page 247 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 211
Low pressure
(a) Convergent lifting.
(c) Orographic lifting.
▲Figure 8.3 Four atmospheric lifting mechanisms.
Orographic Lifting
The physical presence of a mountain acts as a topographic barrier to migrating air masses. Orographic lifting (oro means “mountain”) occurs when air is forcibly lifted upslope as it is pushed against a mountain (Figure 8.3c). The lifting air cools adiabatically. Stable air forced up- ward in this manner may produce stratiform clouds, whereas unstable or conditionally unstable air usually forms a line of cumulus and cumulonimbus clouds. An orographic barrier enhances convectional activity and causes additional lifting during the passage of weather fronts and cyclonic systems, thereby extracting more moisture from passing air masses and resulting in oro- graphic precipitation.
Figure 8.6a illustrates the operation of orographic lifting under unstable conditions. On the windward
(b) Convectional lifting.
Warm air
Ploughed fields
cP
Cold air
mT Warm air
Mountain
(d) Frontal lifting, cold-front example.
slope of the mountain, air is lifted and cools, causing moisture to condense and form precipitation; on the leeward slope, the descending air mass heats by com- pression, and any remaining water in the air evapo- rates (Figure 8.6b). Thus, air beginning its ascent up a mountain can be warm and moist, but finishing its de- scent on the leeward slope, it becomes hot and dry. The term rain shadow is applied to this dry, leeward side of mountains.
The province of British Columbia provides an excellent example of this concept, as shown in Figure 8.7, page 214. The Coast Mountains and the Rocky Mountains orographically lift invading mP air masses from the North Pacific Ocean, squeezing pre- cipitation onto the windward sides of the mountains and allowing dry air to descend the leeward sides. The
Animation
Atmospheric Stability
Leeward
Windward