Transcript Microclimate

MICROCLIMATE
GLOBAL VEGETATION
DESERT
RAINFOREST
DESERT
Global climatic regions relate to:
� Latitude
� Continental location (E or W)
Regional climates relate more to:
� Altitude
� Ocean currents, winds
� Distance from sea
Colder - higher altitude, polar, and more continental.
FOR INSTANCE
Rainforest is close to the equator
NW Europe
temperatures
Deserts are along the tropics and in
in January
the interior of major continents
Ice is at high latitudes
Warmer - Gulf
Stream takes warm
water polewards
Warmer - southerly,
lower altitude, oceanic,
Warmer - southerly, oceanic
On a smaller scale,
weather and climate is
affected by smaller scale
variations in:
� Topography (relief)
� Albedo
� Aspect
� Urban Areas
� Vegetation
� Moisture and humidity
� Pollution, human activity
MICROCLIMATE (2)
Farmers alter albedo
by plastic sheeting.
Bare earth gains more
radiant heat, but loses
more heat through
evaporation loss and
wind at night.
Prevention of frost at
night can be crucial to
early growth.
Low lying valleys and hollows collect cold and humid
air (Frost Hollow); hilltops are exposed to wind; southfacing slopes (in Europe) are warmer, with longer days,
effectively, than north-facing slopes (Aspect). East or
west aspect may affect rainfall or snow coverThis in
Built-up areas are turn may affect vegetation, humidity, evaporation rates
2/3°C warmer
than rural areas,
especially at
night. This is an
Urban Heat
Island.
The Greenhouse Effect due to human
pollution is not intentional...
Thermal imaging of Atlanta shows the
correlation of warmer temperatures and
the built up area. The centre is warmest,
outside the city is coolest. Bodies of
water help reduce the effect.
The location of the CBD and tarmac
roads may be clearly seen. The effect is
to warm major urban areas by 2-3°C
by day and night more than rural areas.
URBAN HEAT ISLAND
Thermal images of Atlanta show radiant energy being
absorbed during the day (above) and retained during the
day (below). The roads can be seen as tarmac absorbs
radiation most effectively.
ATLANTA’s heat island
URBAN HEAT ISLAND - REASONS
RUSH
HOUR
TRAFFIC
THROUGH
A HAZE OF
FUMES
DOMESTIC HEATING
MOTOR
EXHAUSTS
FACTORY &
OTHER
POLLUTION
Human heat sources (domestic heating,
cars, factories) all warm the air.
Pollution by exhausts, factories and
other dusts absorb radiation and
prevent heat loss during the night. Dark
surfaces have a low albedo. Dry
surfaces reduce latent heat loss by
evaporation
In humid conditions, this may result in
smog (a mixture of fog and smoke)
which was common in pre-war London
and still is in LA, Rome, Athens,
Mexico City etc where surrounding
hills prevent the escape of polluted air.
DARK AND
DRY TARMAC
SURFACES
SMOG RESULTS
FROM POLLUTION
Ice is common on exposed
dark surfaces, as they lose
heat rapidly overnight.
Black ice is a hazard on
roads.and pavements
Cities designed on
the grid system
channel any wind
along streets that
contiue for many
kms (wind canyons).
Other cities reduce
wind speed by
ground level friction.
Warmer cities
reduce snow
cover and frost
frequency,
advancing plant
growth.
URBAN CLIMATES
The albedo of various
surfaces in urban areas
tends to be different to
rural areas; tarmac is
dark; glass is lighter.
Reduced snow and ice
cover reduces albedo.
Increased pollution by traffic and
other combustion tends to reduce
sunshine, espcially in winter when
the sun is at a low angle, passing
through ,more atmosphere. Air
pollutants increase condensation
and cloud development and so
rainfall intensity and amount.
Clouds above Tripoli aided by fires
A permanent haze hangs
over Mexico City
Denver’S ‘Brown cloud’
FROST HOLLOW
1
2
The Frost Hollow effect tends to
operate when the ground
surface cools, usually overnight
when cloud cover is limited. It
is common in mountains where
snow and ice cover maintain
cold surfaces for long periods,
reflecting insolation and may
cause cold winds down slope (eg
In hollows, humidity is often
high (rivers, estuaries, marsh
land) and towns (usually on
lower land) increase air
pollution. Both tend to make
fog or cloud more likely. Smog
(smoke - fog) occur with bad
pollution (as in Athens, LA,
Mexico City, pre-war London)
3
Low-lying cloud in valleys
seems like fog at ground level
4
Temperature decreases with
height by 0.6°C per 100m. This
can result in permanent
snowcaps on mountains above
forests where snow is seldom seen
(here, on Cotopaxi volcano in
Mexico near the equator, the
snowline is at 5000m).
In Britain, upland areas such as Snowdonia which range from
0-1000m above sea level, produce climates ranging from
temperate maritime to the almost Arctic.
� Lower temperatures cause greater soil saturation; higher
altitude also tends to increase precipitation (and making it more
likely to be snow rather than rain). At t higher altitudes, the
growing season is shorter, frosts are more frequent and harder
while winters are longer.
� Agriculture is strongly affected. Some arable crops are possible
at low level, on valley floors. Higher up, pasture becomes is
enclosed. Above this, open moorland is used for sheep to roam,
but is covered mainly with heather and other hardy plants.
Besides temperature, wind speed, evaporation rates, and
humidity are also affected.
� Elsewhere, sensitive crops (fruit orchards, right - or vineyards)
can only exist below the cold and windy upper slopes due to frost
frequency in early spring. The lowest points may also be
unsuitable due to the frost hollow effect.
ALTITUDE
�
A hill farm in Snowdonia
An orchard on a slope below
woodland in Devon
In the northern hemisphere, a
southerly aspect gives effectively a
higher angle of sun in the sky, and
longer days.
� In the southern hemisphere, a
northerly aspect is warmer
� The growing season is longer (by
about a month for each 1°C higher in
annual average temperature),
� Frosts are less frequent, less severe
� Maximum temperatures are higher.
�
ASPECT
The right hand slope (above) is
facing the sun, keeping it free of
snow for longer.
In some arid environments,
shade is important, reducing
temperature, humidity and
evaporation rates.
Shaded areas, especially
if north facing, remain
damper with reduced
temperatures,
evaporation and
humidity. This also
affects vegetation, soil
moisture which may, in
turn, affect frosts and
temperature variations
Isolated snow patches
are likely to remain in
spring on north facing
slopes (in Britain)
where the sun takes
longer to melt the snow.
Sensitive crops may nly be
possible on south-facing slopes in
Europe (eg vines below); the
opposing slope is pasture alone.
VINES
VEGETATION - WOODLAND
Trees reduce
temperature during
the day, but retain heat
during the night.
Temperatures are thus
more even (less
extreme)
�
Shade can be welcome in
the desert, but on the forest
floor, the lack of sunlight is
a serious deterrent to other
plants. Thick undergrowth
occurs only in clearings or
where old trees fall
Wind speed is
reduced
�
British forest floors
with moss and marsh
at ground level
Evaporation are
lower, especially in the
day but also at night.
Locally, air becomes
saturated (and is not
Rainforest transpiration also
blown away) due to
increases cloud and rainfall
transpiration; this
To reduce
reduces evaporation.
windspeed in
� Humidity levels
orchards
remain high and
(evaporation, frost
constant due to
and blossom loss)
transpiration and low windbreaks are
evaporation rates.
planted. They may
Mosses are common on reduce soil loss in
the forest floor
arable fields.
�
VEGETATION - OTHER PLANTS
Vegetation on sand (marram grass,
above) not only anchors the moving
sand with its roots but also:
� Reduces wind speed which
stabilises sand (ripples show wind)
� Increases humidity locally (cms)
� Keeps temperature more even
� Reduces frosts, evaporation.
� Dune systems grow as a result
Other plants
(freshwater
reeds, right)
may also reduce
windspeed or
water current,
retain sediment
and allow other
vegetation to
colonise.
Temperatures are more extreme
where vegetation is absent; ice
forms (left) on a bare rock surface
due to rapid radiation loss
overnight.
By reducing light
penetration to the forest
floor (right) , trees are
prevent the growth of
competing species.