Page 249 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 213
(b)
▲Figure 8.6 Orographic precipitation, unstable conditions assumed. (a) Prevailing winds force warm, moist air upward against a mountain range, producing adiabatic cooling,
eventual saturation and net condensation, cloud formation, and precipitation. On the leeward slope, as the “dried” air descends, compressional heating warms it and net evaporation dominates, creating the hot, relatively dry rain shadow. (b) The rain shadow produced by descending, warming air contrasts with the clouds of the windward side. Dust is stirred up by leeward downslope winds. (c) The windward slopes of the Coast Mountain range and leeward rain shadow conditions of the interior Dry Plateau are clearly visible despite the light dusting of snow in this true-colour MODiS image from the Terra satellite. [(b) robert Christopherson.
(c) MODiS rapid response Team, gSFC/NASA.]
wall of clouds. Precipitation usually is heavy, contain- ing large droplets, and can be accompanied by hail, lightning, and thunder.
The aftermath of a cold front’s passage usually brings northerly winds in the Northern Hemisphere and southerly winds in the Southern Hemisphere as anticy- clonic high pressure advances. Temperatures drop and
(a)
Windward
MAR
–4 °C Saturation
Altitude 4000 m 3000 2000 1000
DAR
Leeward
2 °C
6 °C
Condensation level
18 °C 28 °C
8 °C
16 °C Chinook winds
DAR
26 °C
Dry, hot
Rain shadow
Warm, moist Ocean
31 °C
in advance of the cold front lifts upward abruptly and experiences the same adiabatic rates of cooling and fac- tors of stability or instability that pertain to all lifting air parcels.
A day or two ahead of a cold front’s arrival, high cirrus clouds appear. Shifting winds, dropping temperature, and lowering barometric pressure mark the front’s advance due to lifting of the displaced warmer air along the front’s leading edge. At the line of most intense lifting, usually travelling just ahead of the front itself, air pressure drops to a local low. Clouds may build along the cold front into characteristic cumulonimbus form and may appear as an advancing
(c)