MICROCLIMATES
A microclimate is a local atmospheric zone where the climate differs from the
surrounding area. The term may refer to areas as small as a few square feet (for example
a garden bed) or as large as many square miles (for example a valley). Microclimates
exist, for example, near bodies of water which may cool the local atmosphere, or in
heavily urban areas where brick, concrete, and asphalt absorb the sun's energy, heat up,
and reradiate that heat to the ambient air: the resulting urban heat island is a kind of
microclimate.
Another contributory factor to microclimate is the slope or aspect of an area. Southfacing slopes in the Northern Hemisphere and north-facing slopes in the Southern
Hemisphere are exposed to more direct sunlight than opposite slopes and are therefore
warmer for longer.
The area in a developed industrial park may vary greatly from a wooded park nearby, as
natural flora in parks absorb light and heat in leaves, that a building roof or parking lot
just radiates back into the air. Solar energy advocates argue that widespread use of solar
collection can mitigate overheating of urban environments by absorbing sunlight and
putting it to work instead of heating the local surface objects.
A microclimate can offer an opportunity as a small growing region for crops that cannot
thrive in the broader area; this concept is often used in permaculture practiced in northern
temperate climates. Microclimates can be used to the advantage of gardeners who
carefully choose and position their plants. Cities often raise the average temperature by
zoning, and a sheltered position can reduce the severity of winter. Roof gardening,
however, exposes plants to more extreme temperatures in both summer and winter
Microclimates can also refer to purpose made environments, such as those in a room or
other enclosure. Microclimates are commonly created and carefully maintained in
museum display and storage environments. This can be done using passive methods, such
as silica gel, or with active microclimate control devices.
URBAN MICROCLIMATES
Urban Ecosystem
The urban ecosystem is made up of two infrastructures: grey and green. The grey
infrastructure consists of paved areas, buildings, utilities, and other structures commonly
associated with urban areas while the green infrastructure consists of vegetation.
Before we can identify ways that a green infrastructure can positively influence the urban
ecosystem we must first understand how the grey infrastructure influences the small
climatic region near or on the ground, called the urban microclimate.
Urban Heat Island
There are multiple factors responsible for the urban heat island effect. The most
important factors are the presence of dark surfaces and the absence of vegetation.
The presence of dark surfaces such as parking lots and roads absorb solar radiation during
the day and reradiate it as heat during the evening. Albedo is a measurement of the
ability of a surface to reflect back radiation. Asphalt has an especially low albedo,
meaning it absorbs almost all of the solar energy striking it. Many communities are
beginning to increase their asphalt's albedo by incorporating lighter-coloured aggregates
into asphalt or by applying light-coloured chip seal coatings over traditional blacktopped
surfaces. After three to five years, asphalt's albedo increases when the asphalt begins to
"grey out". Brick, cement, and other urban masonry can also absorb, store, and reradiate
more heat than vegetation and soil.
The absence of vegetation, especially trees, reduces the possibility of shade and severely
limits evapotranspiration. Evapotranspiration is the evaporation of water directly from the
soil and from the foliage of plants (transpiration). Evapotranspiration cools the
surrounding environment because heat is used to convert liquid water to water vapour.
Perhaps you've experienced this when water is sprayed onto your arm on warm day. As
the water evaporates your arm feels cooler because the heat from your arm is going into
the water to break bonds so that water vapour is released.
The density, height, and orientation of buildings can contribute to the urban heat island
effect. As building density increases the urban heat island effect will increase. There are
probably a number of additional factors responsible for this increase in the external air
temperature. More buildings mean less vegetation and soil to provide cooling through
evapotranspiration. More buildings also mean more people and more activities that
increase anthropogenic heat (i.e. heat generating devices). The orientation of the streets
influences air flow through urban street canyons as well as building solar heat gains. For
mid-latitude urban areas, an East-West axis street orientation is most preferable.
Other factors contributing to the urban heat island effect includes anthropogenic sources,
such as heat generated from cars, machines, and heat generating devices such as home
furnaces. Pollution may provide a small amount of cooling by blocking incoming
radiation but this is negated by the pollutants ability to prevent heat from escaping the
urban environment at night. Even the distribution of tall buildings and other architectural
structures create urban street canyons which prevent winds from carrying away the
trapped warmed air. This trapped air exposes many urban inhabitants to greater
concentrations of pollutants for longer periods of time compared to rural inhabitants. The
urban heat island effect also contributes to air pollution by increasing the photochemical
reactions that produce smog. There appears to be a direct correlation between increasing
urban temperatures and increasing concentrations of smog. The Heat Island Group
(2003), reported that in Los Angeles the incidence of smog increases by 3% for every
degree Fahrenheit the temperature rises above 70 degrees F. As a result, it's smoggy on
all days when temperatures are above 95 degrees in Los Angeles.
The heat island effect results in billions of dollars spent each year on increased
electricity demand, smog levels, ozone concentration and human discomfort. It is
estimated that the national electricity costs to combat the added urban heat island effect to
be over $1 billion each year. The increased usage of air conditioners in the summer
results in increased burning of fossil fuels at many power plants to generate more
electricity. This results in the release of large amounts of carbon dioxide and other
pollutants like sulphur dioxides, carbon monoxide, nitrous oxides, and suspended
particulates. High concentrations of particulates may serve as ice nuclei and increase the
likelihood of rain and fog, especially when mixed with heat island generated temperature
gradients and rough terrain of urban structures. This rain event is not as beneficial as one
might think since it contains many of the gaseous pollutants mentioned earlier.
Increased permafrost soil temperatures, melting glaciers, and rising sea levels leave little
doubt that the global average temperature of the air at the Earth’s surface is increasing.
The current rate of warming is likely greater than any previous rate in the past 10,000
years. Although a large percentage of carbon dioxide in our atmosphere comes from
natural sources it is the more recent emissions of carbon dioxide from the burning of
fossil fuels to produce energy, heat, and to power vehicles that is responsible for this
recent warming trend.
To summarise urban areas create microclimates due too:
 temperature
 sunlight
 wind
 relative humidity
 cloud
 precipitation
 atmospheric composition
RURAL MICROCLIMATES
Rural areas experience the opposite in the effects of the microclimate effect. Rural
regions contain:
 more vegetation – more evapotranspiration
 less development and infrastructure (e.g. roads, buildings)
 less pollution due to industries
 less anthropogenic heat
 wind is allowed to easily disperse heat
Mountain Microclimates
Mountainous regions experience microclimates due to the following reasons:
 aspect
 altitude
 vegetation (e.g. canopy)
 topography – valley atmosphere
 availability of water
Climate can change as you ascend a mountain.