Vertical tectonics Orogeny

1
Vertical tectonics - Orogeny Effects of
vertical motions of crust, either as plateau
uplift or mountain building, may have major
effects on climate Most easily observed and
obvious are direct orographic effects of
mountain ranges on winds, and also water content
of air Evolution of grasslands of North
America and associated faunas have been
attributed to drying effect of rising Sierra
Nevada and American West More subtle, but of
greater global importance, are large-scale
plateau uplifts which act as physical barriers to
zonal flow of air around planet (and which also
affect radiation balance because of shorter path
through atmosphere) Finally, uplift and
mountain building at high latitudes may be
responsible for initiation of ice-sheet formation
2
Orografic effects in orogens

• Air changes temperature with pressure
• Rising dry air cools with expansion because of
  reduced pressure of thinner overlying atmosphere
• Sensible heat (temperature) is converted to
  potential (gravitational) energy (latent heat) as
  air rises this is returned as sensible heat when
  air descends

Most obvious regional climatologic effects are
precipitation and rain-shadow effects due to
cooling and moisture loss of rising air and
heating of descending air after it has passed
over crest of mountain range To understand why
mountain ranges induce precipitation on windward
side and arid conditions on leeward side it is
necessary to consider changes in air temperature
and pressure as it rises and falls, and
relationship between moisture content of air and
temperature
3
Few interesting facts about AIR (Bill Bryson,
A short history of nearly everything)

• every 1 km T drops 1.6ºC
• atoms jiggle and jounce and the more they do so
  the more heat the release - on hot summer day you
  feel excited atoms !
• high up there are fewer molecules, i.e. fewer
  collisions - less heat
• air is not weightless
• when barometer rises slightly over night, about
  half a ton has been quietly piled upon us
• you do not really feel the burden - air is made
  mostly of incompressible fluids (cannot be
  compressed and has relatively constant density
  throughout)
• air in motion different issue - hurricane, stiff
  breeze
• overall, 5,200 million million tonnes of air
  around us - if they move at 50-60 km/h roofs may
  lift off etc.
• typical weather front 750 million tonnes of
  cold air pinned beneath a billion tons of warmer
  air - thunderstorm has energy equivalent to 4
  days use of electricity in US

4
Rain shadow effect
Christchurch
West coast
Mountain range is barrier to atmospheric
circulation, especially if perpendicular to
circulation e.g. ANDES
Mountain range traps water vapour converting
latent to sensible heat Effect of air passing
over orographic obstruction is to cause
precipitation as air rises and to greatly
increase its evaporative potential as it descends
Rain shadow on lee side of a mountain range
is not merely lack of precipitation air has
extreme drying effect Moisture sources in lee
are not likely open water surfaces, but plants
Herbaceous plants can return soil moisture to air
almost as effectively as evaporation off a
lake Xerophytes (plants which live under arid
conditions) have developed special mechanisms
for conserving and storing water Plants
adapted to retention of water were not widespread
until later Cenozoic when orogenies in many
parts of world promoted rain shadows on large
scale Rain shadow effects can extend for
hundreds of kilometers downwind mountain
ranges and have major effect on regional climate
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Orografic effects in orogens Quantitative aspects
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Dry air
Relationship between altitude, pressure, and
temperature for rising and falling dry air
Red, diagonal lines are dry adiabats
Temperature of rising air decreases following
diagonals up and to left
Rate at which temperature declines with
elevation is termed lapse rate For dry air
lapse rate in troposphere is 10C km1
(troposphere is convecting lower half of
atmosphere and is 20 km thick at equator and 10
km thick at poles) If dry air rises or descends
without exchanging heat with its surroundings
(i.e. adiabatically), temperature changes follow
almost straight diagonal lines
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Moisture and air
Relationship between altitude, pressure,
tempera-ture, and absolute humidity of air at
saturation Numbers in diagram are absolute
humidity, expressed as grams of water vapour per
kilogram of air Figure shows mass of water,
which can be accommodated in saturated air at
different temperatures and elevations or
pressures Ability of air to hold moisture is
exponential function of temperature
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Amount of vapour in saturated air doubles
with every 10C temperature increase
Figure shows that most moisture in air is
trapped in convecting troposphere (80 of mass
of the atmosphere is in troposphere)
Meridional temperature gradient at Earths
surface explains why most moisture in atmosphere
is contained in troposphere at low latitudes
Transformation of water to vapour through
evaporation and back to water through
precipitation is significant part (60) of
atmospheric energy transport system
because of rapid increase in capacity of warm
air to hold water, increasingly large amounts of
energy are required to saturate warmer air with
vapour rapid increase in energy involved in
evaporation from sea surface to saturate
overlying air with vapour tends to limit
temperature of tropical seas to 30C
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Saturated air
Relationship between altitude, pressure, and
temperature for rising and falling
moisture-saturated air Diagonal lines are wet or
saturated adiabats Temperature of saturated air
decreases following curved green lines up and to
left
Rising saturated air which does not exchange
heat with surroundings follows path indicated by
green curved sloping lines (wet or saturated
adiabats) At very cold temperatures amount of
moisture in air negligible, curves similar to dry
adiabats and lapse rate also similar at 10C
km1 However, at warm temperatures lapse rate
very much less than that for dry air for
saturated air at 30C near sea level lapse rate
5C km1, i.e. at lower elevations and warmer
temperatures wet adiabats steep, i.e. wet air
does not cool fast
When saturated air rises, temperature decrease
reduces amount of water which can be carried as
vapour and excess water transformed back into
liquid phase as droplets (which may fall out as
precipitation) In transformation from vapour
to liquid, latent heat of vapourization released
as sensible heat, warming air
10
At point A temperature is 28C, and air
contains 16gH2O kg1AIR If saturated, air at
this temperature/ pressure could hold 24gH2O
kg1AIR and has relative humidity of 67
(relative humidity being ratio of observed
mois-ture content of air to moisture content at
saturation) As air rises it cools along dry
adiabat (path from A to B), and relative humidity
increases At elevation of 900 m (B),
tempera-ture is 19C, relative humidity is 100
From here up to crest of mountain range air
cools along wet adiabat following path from B to
C, with moisture falling out as precipitation
At 3 km crest of mountain range air temperature
is 9C and saturated air contains 10gH2O kg1AIR,
still saturated Descending lee side of mountain
range air warms along dry adiabat At sea level
(D) temperature has increased to 39C, relative
humidity has decreased to 21
Left Relationship between altitude, pressure,
temperature, absolute humidity (solid lines), dry
air (straight dashed lines), and saturated air
(curved dashed lines) Diagram can be used to
estimate orographic effects of air passing over
mountain
Right Path of air starting from sea level,
passing over 3-km mountain range, and descending
into basin below sea level