Polar and alpine environments - Fundamentals of the Physical Environment - page 597

km
10.0

PLANETARY WIND BELT

Glacier

MM 1

1.0

MM 2

CRYONIVAL, BELT
ALPINE BELT

RM 1

RM 2

0.1

MONTANE FOREST BELT

0.01

FOREST BELT

Topoclimate Boundary
Microclimate Boundary

Mountain Macroclimate
MM 1 Windward, moist
MM 2 Leeward, dry; föhn

Regional Macroclimate
RM 1 Windward, moist
RM 2 Leeward, dry

Figure 24.4 The scale and variety of mountain climates. Mountain topoclimate (shaded) separates myriad surface micro-
climates from the broader mountain macroclimate. The logarithmic scale refers to the thickness of each layer, not to the absolute
altitude.
Source: Modified from Barry (1992)

effects. These include forced uplift of conditionally stable
air, warm air downflow, mountain circulation systems and
cold air drainage in more generally stable conditions.
Lee wave disturbance occurs where waves are triggered in
an air stream by surplus energy and approximate the
wavelength of the barrier. Clouds form as air ascends
through condensation levels, either in the waves or in
isolated rotors , and - in stable air - evaporates on the
descending limb, leaving stratiform lee wave or rotor
clouds ( Plate 24.2 ).
Fall winds describe air currents descending leeward
slopes, distinguishable by their thermal character and
development according to diurnal, seasonal or synoptic
conditions. Warm-air downflow is generated under
particular lapse rate conditions which lead to warming on
descent known as the föhn (Alps) or chinook (Canadian
Rockies) effect. This occurs when stable air loses moisture
over the barrier and descends at a dry adiabatic lapse rate
with an absolute gain in temperature ( Figure 24.5 ). Most
currents adiabatically warm on descent but are less
powerful mechanically and thermally. The föhn or
chinook occurs seasonally in most mountain systems; the
native Canadian term means 'snow eater', underlining
their important environmental influences as warm,
desiccating winds. Even with the small hills of the
United Kingdom föhn winds can give seasonally high
temperatures to the lee of Wales and the Cairngorms.

Plate 24.2 Fast-moving lee wave clouds above mountains
fringing Frobisher Bay, Baffin Island, in the east Canadian
Arctic, seen from 11 km altitude.
Photo: Ken Addison

Contrasting cold-air drainage occurs as gravity flows
of denser air, such as the bora (Adriatic Sea) or oroshi
(Japan). In their simplest form they are currents of cold