MICROCLIMATE

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MICROCLIMATE
GLOBAL VEGETATION

• Global climatic regions relate to
• Latitude
• Continental location (E or W)
• Regional climates relate more to
• Altitude
• Ocean currents, winds
• Distance from sea

DESERT
RAINFOREST
DESERT
Colder - higher altitude, polar, and more
continental.
FOR INSTANCE Rainforest is close to the
equator Deserts are along the tropics and in the
interior of major continents Ice is at high
latitudes
NW Europe temperatures in January
Warmer - Gulf Stream takes warm water polewards
Warmer - southerly, lower altitude, oceanic,
Warmer - southerly, oceanic
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MICROCLIMATE (2)

• 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

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 south-facing 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 turn may affect
vegetation, humidity, evaporation rates
Built-up areas are 2/3C warmer than rural areas,
especially at night. This is an Urban Heat Island.
The Greenhouse Effect due to human pollution is
not intentional...
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URBAN HEAT ISLAND
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 2-3C by day and night more than rural
areas.
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.
ATLANTAs heat island
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URBAN HEAT ISLAND - REASONS
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.
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URBAN CLIMATES
Ice is common on exposed dark surfaces, as they
lose heat rapidly overnight. Black ice is a
hazard on roads.and pavements
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.
Warmer cities reduce snow cover and frost
frequency, advancing plant growth.
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.
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.
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FROST HOLLOW
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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)
Low-lying cloud in valleys seems like fog at
ground level
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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 Mistral in
S.France).
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ALTITUDE
Temperature decreases with height by 0.6C 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.

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ASPECT

• 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 1C higher in annual average
  temperature),
• Frosts are less frequent, less severe
• Maximum temperatures are higher.

Isolated snow patches are likely to remain in
spring on north facing slopes (in Britain) where
the sun takes longer to melt the snow.
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
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VEGETATION - WOODLAND

• Trees reduce temperature during the day, but
  retain heat during the night. Temperatures are
  thus more even (less extreme)
• Wind speed is reduced
• Evaporation are lower, especially in the day but
  also at night. Locally, air becomes saturated
  (and is not blown away) due to transpiration
  this reduces evaporation.
• Humidity levels remain high and constant due to
  transpiration and low evaporation rates. Mosses
  are common on the forest floor

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
British forest floors with moss and marsh at
ground level
Rainforest transpiration also increases cloud and
rainfall
To reduce windspeed in orchards (evaporation,
frost and blossom loss) windbreaks are planted.
They may reduce soil loss in arable fields.
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VEGETATION - OTHER PLANTS
Other plants (freshwater reeds, right) may also
reduce windspeed or water current, retain
sediment and allow other vegetation to colonise.

• 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

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.