Mountain Climate
The climate of
mountains is kaleidoscopic, composed of myriad individual segments continually
changing through space and time.
Great environmental
contrasts occur within short distances as a result of:
-
diverse topography: slope and aspect
-microclimates
-
locally generated climatic effects such as winds
-
highly variable nature of the energy and
moisture fluxes
o elevational
gradients
o spacing
of valleys and ridges
o slopes,
o variety
of surfaces
§
ice and water (lakes and streams), snow patches,
shade, vegetation type and % cover, microtopography (hummocks), bare rock and
soil,
§
The complexity of climatic patterns in mountains
becomes truly overwhelming. Nevertheless, predictable patterns and
characteristics are found within this heterogeneous system; for example,
temperatures normally decrease with elevation while cloudiness and
precipitation increases, it is usually windier in mountains, the air is thinner
and clearer, and the sun’s rays are more intense.
Mountains also have a
major impact on regional climate:
Influencing wind systems
surrounding
areas warmer or colder, wetter or drier than they would be if the mountains
were not there.
The exact effect of the mountains depends upon
their location, size, and orientation with respect to the moisture source and
the direction of the prevailing winds.
EXTERNAL CLIMATIC CONTROLS
Latitude
Dictates solar energy
received, thus temperatures and general circulation.
Altitude
Fundamental to
mountain climatology are the changes that occur in the atmosphere with
increasing altitude, especially:
-
the decrease in temperature, air density, water
vapor, carbon dioxide, and impurities.
Continentality
The relationship
between land and water has a strong influence on the climate of a region.
Generally, the more water‑dominated an area is, the more moderate its
climate. An extreme example is a small oceanic island, on which the climate is
essentially that of the surrounding sea. The other extreme is a central
location on a large land mass such as Eurasia, far removed from the sea. Water
heats and cools more slowly than land,
so the temperature ranges between day and night and between winter and
summer are smaller in marine areas than in continental areas.
The same principle
applies to alpine landscapes, but is intensified by the barrier effect of
mountains.
Barrier Effects to General Circulation
The Cascade Mountains
are a climatic divide that creates unlike conditions on their windward and
leeward sides. All mountains serve as barriers to a greater or lesser extent,
depending on their size, shape, orientation, and relative location.
Specifically, the barrier effect of mountains can be grouped under the
following subheadings: (1) damming, (2) deflection and funneling, (3) blocking
and disturbance of the upper air, (4) forced ascent, and (5) forced descent.
Damming
Damming of stable air occurs when the mountains are high enough to prevent the passage of an air mass across them. When this happens, a steep pressure‑gradient may develop between the windward and leeward sides of the range.
Deflection and
Funneling
When an air mass is dammed by a mountain range, the winds can be deflected around the mountains if topographic gaps exist.
Blocking and
Disturbance of the Upper Air
High‑pressure areas prevent the passage of storms. Large mountain ranges such as the Rockies, Southern Alps and Himalayas are very efficient at blocking storms, since they are often the foci of anti-cyclonic systems (because the mountains are a center of cold air), the storms must detour around the mountains. In addition to the effect of blocking, mountains cause other perturbations to upper‑air circulation and subsequent effects on clouds and precipitation
Mountains have additional influence
on the location and intensity of the jet streams, which have vastly important
effects on the kind of weather experienced at any particular place and time. The jet streams may
also split to flow around the mountains; they rejoin to the lee of the range,
where they often intensify and produce storms
Forced Ascent
When moist air blows
perpendicular to a mountain range, the air is forced to rise; as it does, it is
cooled. Eventually the dew point is reached, condensation occurs, clouds form,
and precipitation results. This increased cloudiness and precipitation on the
windward slope is known as the orographic effect.
Forced Descent
Atmospheric‑pressure
conditions determine whether the air, after passing over a mountain barrier,
will maintain its altitude or whether it will be forced to descend. If the air
is forced to descend, it will be heated by compression (adiabatic heating) and
will result in clear, dry conditions. This is a characteristic phenomenon in
the lee of mountains and is responsible for the famous foehn or chinook winds.
Although heavy precipitation may occur on the
windward side of mountains where the air is forced to rise, the leeward side
may receive considerably less precipitation because the air is no longer being
lifted (it is descending) and much of the moisture has already been removed.
The so-called rainshadow effect is an arid area on the leeward or down-wind
side of mountains
Fig. 4.4 The
influence of the Olympic Mountains on the wind field and precipitation. The
arrows are flow lines indicating wind direction. Distance between the flow lines indicates relative speed, the
closer they are to one another the faster the wind in that region. Notice that
the flow lines are evenly spaced over the Pacific Ocean. As they are deflected through the Strait of
Juan de Fuca, the wind speed increases.
Also notice that winds are funneled up the western valleys of the
Olympics, concentrating moist air and increasing precipitation at the Hoh Rain
Forest (3800 mm), while Sequim only receives 430 mm in the rain shadow.
MAJOR CLIMATIC ELEMENTS
Solar Radiation
The effect of the sun
becomes more exaggerated and distinct with elevation. The time lag, in terms of
energy flow, between stimulus and reaction is greatly compressed in mountains.
Looking at the effect of the sun in high mountains is like viewing its effects
at lower elevations through a powerful magnifying glass.
The alpine environment has perhaps the most
extreme and variable radiation climate on earth. The thin clean air allows very
high solar intensities, and the topographically complex landscape provides
surfaces with a range of different exposures and shadowing from nearby peaks.
Amount of Solar
Radiation
-
solar radiation is rapidly depleted through
lower atmosphere.
o Due
to the increased density of the atmosphere and the greater abundance of water
vapor, carbon dioxide, and particulate matter near the earth’s surface.
o the
amount of energy reaching the surface at sea level is only about half that at
the top of the atmosphere
o High
mountains protrude through the lower atmospheric blanket and thus have the
potential for receiving much higher levels of solar radiation, as well as
cosmic‑ray and ultraviolet radiation
o solar constant is approximately 1365 Wm-2
at the top of the atm, and is typically ~800 Wm-2 at the surface at
noon on a clear day at low latitude.
o instantaneous
values as high as 1529 Wm-2 in the Alps, 112% of the solar constant
have been measured The additional radiation comes from sunlight reflected from
cloud bottoms and snow on higher slopes.
Quality of Solar Radiation
The alpine environment
receives considerably more ultraviolet radiation (UV) than low elevations.
If only wavelengths
shorter than 320 mm. are considered, then alpine areas
receive 50% more UV during summer solstice than does sea level.
Later in the year,
when the sun is lower in the sky (and therefore passes through denser
atmosphere), alpine areas receive 120% more UV than areas at sea level.
Effect of Slopes on Solar Radiation
The closer to
perpendicularly the sun's rays strike a surface, the greater their intensity.
The longer the sun shines on a surface, the greater the heating that takes
Mountains
are composed of a wide range of surface types, snow, ice, water, grassy
pastureland, extensive forests, desert shrub, soils, and bare bedrock. This
extensive variety of surface characteristics affects the receipt of incoming
solar radiation 
Fig. 4.7. Direct
solar radiation (Cal. cm‑2 hr‑1) received on
different slopes during clear weather at 50˚ N. lat. Three slopes are
shown: north, south, and east-facing (west would be a mirror image of east),
for summer and winter solstice and equinox (vernal is a mirror image of
autumnal). The lefthand side of each diagram shows the distribution of solar
energy on a horizontal surface (0˚ gradient) and is therefore identical
for each set of 3 in the same column. The righthand side of each diagram
represents a vertical wall (90˚ gradient). The top of each diagram shows
sunrise and the bottom shows sunset. As can be seen, the north- and south‑facing
slopes experience a symmetrical distribution of energy, while the east and west
reveal an asymmetrical distribution. Thus, on the east‑facing slope during
summer solstice the sun begins shining on a vertical cliff at about 4:00 a.m
and highest intensity occurs At 8:00 a.m. By noon the cliff passes into shadow.
The opposite would hold true for a west‑facing wall: it would begin
receiving the direct rays of the sun immediately past noon.
Fig. 4.8. Topo- and micro-climatic influences of slope
and aspect on vegetation types. The northern hemisphere example is given where
more solar receipt on south-facing slopes warms temperatures to where forest is
replaced by grass. North-facing slopes are shaded and cooler with more soil
moisture retention and thicker forests. On a larger scale, forests move down
valleys following moisture and cooler temperatures created by cold air
drainage. (After Kruckeberg 1991)
Temperature
The decrease of
temperature with elevation is one of the most striking and fundamental features
of mountain climate.
Alexander Von Humboldt was so struck by the
effect of temperature on the elevational zonation of climate and vegetation in
the tropics that he proposed the terms tierra calienfe, tierra templada, and
tierra fria for the hot, temperate, and cold zones.
Vertical
Temperature‑Gradient
Change
of temperature with elevation is called the environmental or normal
lapse rate. 
Fig. 4.11. Mean annual temperature with altitude in the southern
Appalachian Mountains. Dots represent U.S. Weather Bureau First Order Stations
in Tennessee and North
Carolina. Temperatures were calculated for period 1921‑1950. (Adapted
from Dickson 1959, p. 353)
Mountain Mass
(Massenerhebung) Effect
Large mountain systems create their own
surrounding climate (Ekhart 1948). Similar to the continentality effect, the
greater the surface area or land mass at any given elevation, the greater
effect the mountain area will have on its own environment. Mountains serve as
elevated heat islands where solar radiation is absorbed and transformed into
long-wave heat energy, resulting in much higher temperatures than those found
at similar altitudes in the free air Generally, the larger the mountain mass,
the higher the elevation at which vegetation grows.
Fig. 4.12. Distribution of mean annual temperature (˚C) in a
transect across the Mexican Meseta from Mazatlan to Veracruz. The temperature
over the plateau at 3,000 m (10,000 ft.) is about 3˚C (5.4˚F) higher
than over the coastal stations, owing to greater heating of the elevated land
mass. (Adapted from Hastenrath 1968, p.123)
Temperature
Inversion
Inversions are the
exception to the general rule of decrease in temperature with elevation. During
a temperature inversion the lowest temperatures occur in the valley and
increase upward along the mountain slope. Eventually, however, the temperatures
will begin to decrease again, so that an intermediate zone, the thermal belt,
will experience higher night temperatures than either the valley bottom or the
upper slopes.
Fig. 4.14. Cross‑section of an enclosed basin, Gstettneralm, in the
Austrian Alps, showing a temperature inversion in early spring. Elevation of
valley bottom is 1,270 m (4,165 ft.). Note increase in temperature (˚C)
with elevation above valley floor, especially the rapid rise directly above the
pass. This results from the colder air flowing into a lower valley at this
point. (After Schmidt 1934, p. 347)
Cold air is denser and
therefore heavier than warm air. As slopes cool at night, the colder air begins
to slide down slope, flowing underneath and displacing the warm air in the
valley. Temperature inversions are best developed under calm, clear skies,
where there is no wind to mix and equalize the temperatures and the transparent
sky allows the surface heat to be rapidly radiated and lost to space.
Consequently, the surface becomes colder than the air above it, and the air
next to the ground flows downslope.
As might be expected, distinct vegetation patterns are associated with these extreme temperatures. Normally, valley bottoms are forested and trees become stunted on the higher slopes, eventually being replaced by shrubs and grasses still higher up, but the exact opposite occurs here. The valley floor is covered with grass, shrubs, and stunted trees, while the larger trees occur higher up. An inversion of vegetation matches that of temperature (Schmidt 1934).
Precipitation
The increase of
precipitation with elevation is well‑known-- orographic effect.
-
demonstrated in every country of the world,
-
even on small hills.
-
an isohyetal map with its lines of equal
precipitation will look similar to a topographic map composed of lines of equal
elevation (Fig. 4.22).
-
the data on which most precipitation maps are
based are scanty, so that considerable interpolation may be necessary,
particularly in the areas of higher relief
-
Precipitation does not always correspond to
landforms. In some cases, maximum precipitation may occur at the foot or in
advance of the mountain slopes
-
Wind direction, temperature, moisture content,
storm and cloud type, depth of the air mass and its relative stability,
orientation and aspect, and configuration of the landforms are all contributing
factors in determining location and amount of precip
-
Down wind from mountains, air descends and
warms, thus dries, creating rain-shadowsareas of lower precipitation
totals
The orographic effect
involves several distinct processes: (1) forced ascent, (2) blocking (or
retardation) of storms, (3) the triggering effect, (4) local convection, (5)
condensation and precipitation processes, and (6) runoff.
Blocking of storms
Storms
often linger for several days or weeks as they slowly move up and over the
mountains, producing a steady downpour
Clouds. Cloud cover is
generally more frequent and thicker over mountains than over the surrounding
lowlands.
-
In middle and high latitudes, stratiform clouds
are common,
-
Middle latitude summers, continental,
subtropical, and tropical areas typically have cumulus clouds associated with
convection
-
A
number of cloud forms are unique to mountain environments
o A
cap or crest cloud forms over the top of an isolated peak or ridge.
o Banner
clouds are cap cloud which extent downwind from the peak like a flag waving in
the wind.
o Lenticular
clouds are lens-shaped clouds formed in regular spaced bands parallel to the
mountain barrier on the lee side
o Fig. 4.41. Lee‑wave clouds forming over the
Front Range of the Colorado Rockies. View is toward the west, so wind is
southwesterly (from left to right).
(Robert Bumpas, National Center for Atmospheric Research)
Fog
Drip. Fog drip is most significant in areas adjacent
to oceans with relatively warm, moist air moving across the windward slopes.
-
the moisture yield from fog drip may exceed that
of mean rainfall.
-
a tree will
yield more moisture than a rock, and a needle-leaf tree is more efficient at
"combing" the moisture from the clouds than a broadleaf tree
-
A tall tree will yield more moisture than a
short one, and a tree with front‑line exposure will yield more than one
surrounded by other trees
-
tropical and subtropical mountains sustain so‑called
"cloud forests," which are largely controlled by the abundance of fog
drip (Cavelier and Goldstein 1989). Along the east coast of Mexico in the
Sierra Madre Oriental, luxuriant cloud forests occur between 1,300‑2,400 m (4,300‑7,900 ft
-
On the northeast slopes of Mauna Loa, Hawai’i,
at 1,500‑2,500 m (5,000‑8,200 ft.), above the zone of maximum
precipitation, fog drip is likewise a major ecological factor in the floristic
richness of the forests. During a twenty‑eight‑week study, fog drip
was found to provide 638 mm (25.3 in.) of moisture at an elevation of 1,500 m
(5,000 ft.); and at 2,500 m (8,200 ft.) it provided 293 mm (11.5 in.), which
was 65% of the direct rainfall
Rime
Deposits. Rime is formed at subfreezing temperatures when
supercooled cloud droplets are blown against solid obstacles, freezing on them
.
Winds
Mountains are among
the windiest places on earth.
They protrude into the
high atmosphere, where there is less friction to retard air movement.
Wind is clearly an
extreme environmental stress; in many cases it serves as the limiting factor to
life. What may be the two most extreme environments in mountains are caused by
the wind: late‑lying snowbanks, where the growing season is extremely
short, and windswept, dry ridges. Both of these environments become more common
and more extreme with elevation, until eventually the only plants are mosses
and lichens‑or perhaps nothing at all. Trees on a windswept ridge may be
“flagged” with the majority of branch growth on the protected lee-side. In the
extreme conditions within the krummholz (crooked wood) zone, trees take on a
prostate cushion form.
Local Wind
Systems in Mountains
Winds
that blow upslope and upvalley during the day and downslope and downvalley at
night are thermally induced winds. The driving force for these winds is
differential heating and cooling which produces air density differences between
slopes and valleys and between mountains and adjacent.
-During the day, slopes are warmed more
than the air at the same elevation in the center of the valley; the warm air,
being less dense, moves upward along the slopes. Similarly, mountain valleys
are warmed more than the air at the same elevation over adjacent lowlands, so
the air begins to move up the valley.
-At night, when the air cools and becomes
dense, it moves downslope and downvalley under the influence of gravity. This
is the flow responsible for the development of temperature inversions. Although
they are interconnected and part of the same system, a distinction is generally
made between slope winds, and larger mountain and valley wind systems.
- Slope Winds. Slope winds consist
of thin layers of air, usually less than 100 m (330 ft.) thick. In general, the
upslope movement of warm air during the day is termed anabatic flow, and
the downslope movement of cold air during the night is referred to as katabatic
flow, or a gravity or drainage wind.
Mountain
and Valley Winds. 
Fig. 4.33. Schematic
representation of slope winds (open arrows) and mountain and valley winds
(black arrows). (a) and (b) Day conditions. (c) and (d) Night conditions.
(After Defant 1951, p. 665, and Hindman 1973, p. 199)
Fig. 4.34. Valley fog in the Coast Range of northern
California beginning to dissipate as slope winds strengthen and the return flow
develops in the center of the valley. Top photo taken at 9:58 A.M.; bottom
photo taken at 10:07 A.m. (Edward E. Hindman, U. S. Navy)
-
glacier wind,
which arises as the air adjacent to the icy surface is cooled and moves
downslope due to gravity. The glacier wind has no diurnal period but blows
continuously, since the refrigeration source is always present.
-
Maloja wind,
named after the Maloja Pass in Switzerland between the Engadine and Bergell
valleys. This wind blows downvalley both day and night and results from the
mountain wind of one valley reaching over a low pass into another valley, where
it overcomes and reverses the normal upvalley windflow.
Mountain
Winds Caused by Barrier Effects
-
barrier effect
introduces turbulence to the winds, increasing and decreasing speeds, changes
directions, and modifies storms
-
Most commonly the
wind will fall down the lee-side of mountains under the influence of gravity.
These surface winds are sometimes collectively termed fall winds, but are known by a variety of local terms because
they have long been observed in many regions downwind from mountains, and have
associated with them distinct weather phenomenon.
-
Fig. 4.39.
Diagrammatic representation of classical development of a foehn (chinook) wind.
Temperatures at different locations are based on the assumption that air at the
base of mountain on windward side is 10˚C (50˚F). By the time the air
has undergone the various thermodynamic processes indicated in its journey
across the mountains it reaches the base on the leeward side at 18.1˚C
(64.6˚F). (Author)
-
-
When the winds leave
the surface in a hydraulic jump, they often travel through the atmosphere in a
wave motion, producing unique cloud-forms.
-
Foehn
Wind. Of all the transitory climatic phenomena of
mountains the foehn wind (pronounced
"fern" and sometimes spelled föhn) is the most
intriguing.
-
Many legends, folklore, and misconceptions have
arisen about this warm, dry wind that descends with great suddenness from
mountains.
-
In North America it is called the
"Chinook;" in the Argentine Andes the "zonda;" in New
Zealand the "Canterbury north‑wester;" in New Guinea the “warm
braw;” in Japan the “yamo oroshi;” in the Barison Mountains of Sumatra the
“bohorok;” the “halny wigtr” in Poland; the “autru” in Romania; other mountain
regions have their own local names for it (Brinkman 1971; Forrester 1982). The
“Santa Ana” of southern California forms in a similar fashion.
- The foehn produces distinctive weather: gusts of wind, high temperatures, low humidity, and very transparent and limpid air
- The primary
characteristics of the foehn are a rapid rise in temperature, gustiness, and an
extreme dryness that puts stress on plants and animals and creates a fire
hazard
Lee Waves. The behavior of
airflow over an obstacle depends largely on the vertical wind profile, the
stability structure, the shape of the obstacle and the surface roughness.
When
wind passes over an obstacle, its normal flow is disrupted and a train of waves
may be created that extends downwind for considerable distances (Figs. 4.26 and
4.39). The major mountain ranges produce large‑amplitude waves that
extend around the globe
The most distinctive visible features of lee waves are the lenticular (lens‑shaped) or lee‑wave clouds that form at the crests of waves.
Fig. 4.40. Lee waves resulting from air passing across a mountain barrier. Lee‑wave clouds often form at the ridge of the waves. Rotors may develop nearer the ground in the immediate lee of the mountain. (Adapted from Scorer 1967, p. 93)
Microclimates
Large differences in
temperature, moisture and wind can be found within a few meters, or even
centimeters. The thin atmosphere at high elevation means surfaces facing the
sun on a clear day can warm dramatically, but shaded surfaces remain cold.
The mosaic of microclimates determines the
local variability in ecosystem processes. The distribution of vegetation zones,
and even individual species may follow the distribution of microclimates. A
simple classification using solar receipt, wind exposure, depth of winter snow
cover and density and height of vegetation cover, can help to characterize alpine
microclimates.
On this basis the following general
microclimates can be differentiated:
Sunny, windward slope
– solar radiation and windspeeds high
Sunny, lee slope –
solar radiation high, windspeeds low
Shaded, windward slope
– solar radiation low, windspeeds high
Shaded, lee slope –
solar radiation and windspeeds low.
The resolution of most
weather station networks in mountains is far too coarse to capture the spatial
variability of climates in mountains.