What is a Micro-Climate?
Factors changing Climate in Cities?
Results of Micro-Climates in Cities?
Urban Heat Island
Video: Urban Heat Island
Mr. T. Tonna
Climates change according to a number of
Longitudinal and Latitudinal Location
In Cities and Large Urban Areas other
factors come in to place.
Mr. T. Tonna 3
Urban Heat Island
Mr. T. Tonna 4
What is a Micro-Climate?
A Micro-climate is a local atmospheric zone
where the climate differs from the surrounding
The term may refer to areas as small as a few
square feet (e.g a garden bed) or as large as many
square miles (e.g a valley).
Microclimates can be found near bodies of water
that can cool the local atmosphere, or in heavily
Mr. T. Tonna 5
Mr. T. Tonna
Climatic Zones of San Diego
1. Coastal Zone
2. Inland Area
3. Further Inland Area
4. Mountainous Region
Each Area is influenced by different
conditions, one of which is the
impact of the local population on
Upland- Upland areas have a specific type of climate that
is notably different from the surrounding lower levels.
Temperature usually falls with height at a rate of
between 5 and 10 °C per 1000 m, depending on the
humidity of the air.
Coastal-The coastal climate is influenced by both the
land and sea between which the coast forms a boundary.
The thermal properties of water are such that the sea
maintains a relatively constant day to day temperature
compared with the land.
Mr. T. Tonna
Forests- Tropical rainforests cover only about 6% of the
earth's land surface, but it is believed they have a
significant effect on the transfer of water vapour to the
atmosphere. This is due to a process known as evapo-
transpiration from the leaves of the forest trees.
Urban- These are perhaps the most complex of all
microclimates. With over 75% of the British population
being classed as urban, it is no surprise that they are also
the most heavily studied by students of geography and
meteorology. Therefore, the rest of these notes focus on the
various elements that constitute an urban microclimate.
Mr. T. Tonna 8
What Changes a Cities
Human activity has a big influence on the
climate of an urban area.
Climate is the long term behaviour of the
atmosphere in a specific area, with
characteristics such as
temperature, pressure, wind, precipitation,
cloud cover and humidity etc.
Mr. T. Tonna 9
An urban area is an area with a high
density of human created structures in
comparison with the areas surrounding it.
Climate in Urban Areas are affected by
human factors such as pollution, the colour
of buildings, people themselves and
Mr. T. Tonna 10
Companies that supply electricity typically rely on fossil
fuel power plants to meet much of this demand, which in
turn leads to an increase in air pollutant and greenhouse gas
The primary pollutants from power plants include Sulphur
Dioxide, Nitrogen Oxides, Particulate Matter, Carbon
Monoxide and Mercury.
Increased use of fossil-fuel-powered plants also increases
emissions of greenhouse gases, such as Carbon
Dioxide, which contribute to Global Climate Control.
Mr. T. Tonna 11
Colour affects the amount of heat which is reflected by a
surface. This is referred to as the Albedo.
Certain colours are able to absorb heat energy thus
warming the temperature of the surrounding area.
All weather is a result of the uneven heating of the Earth
caused by different areas of the planet having different
Generally the Lighter the colour the higher the Albedo
and the Darker the colour the Lower the Albedo.
The higher the Albedo, the less heat there is in an area.
Polar Regions have the highest Albedo
Mr. T. Tonna 12
There are a number of ways in which people can affect
Pollution is the main contributor by man.
Traffic- In Large Urban Areas Transport is always an
issue and the build up of Traffic leads to more emissions.
Industry- Industries are necessary for everyday life but
the pollutants they ‘spew’ into the atmosphere lead to
increase in temperatures.
Central Heating/Cooling- Electricity is needed for both
heating and cooling of houses. The more an urban area
continue to grow and warm the more energy will be
Mr. T. Tonna 13
Results of an Urban Micro-
Pollutants are harmful to human health and
also contribute to a reduction in Air
Quality- Smog or Acid Rain.
Warmer temperatures in cities mean that in
the summer during heat waves, many cities
and their residents experience even greater
temperatures and heat stress. In 2003 this
was a major problem in
Europe, particularly France, and many
Mr. T. Tonna 14
The mean winter temperatures are on average 1-2 degrees
Celsius higher in urban areas, in comparison to rural areas.
The mean summer temperature may be on average 5
degrees Celsius higher than surrounding rural areas.
Locally as warm air rises over an urban area it draws in
heat from the surrounding area and creates an area
of localised low pressure.
Strong pressure gradients develop between the windward
and leeward side of buildings and can lead to severe
Mr. T. Tonna 15
The table below summarises some of the differences in
various weather elements in urban areas compared
with rural locations:
Mr. T. Tonna 16
Urban Heat Island
As a result of the Micro-Climate in Urban areas the
Term Urban Heat Island was coined.
By Definition: An urban heat island (UHI) is a
metropolitan area that is significantly warmer than its
surrounding rural areas due to human activities.
The temperature different is usually larger at night
than in the day and recorded when wind is minimal.
Mr. T. Tonna 17
The formation of a heat island is the result of the interaction of the following
the release (and reflection) of heat from industrial and domestic buildings;
the absorption by concrete, brick and tarmac of heat during the day, and its
release into the lower atmosphere at;
the reflection of solar radiation by glass buildings and windows. The central
business districts of some urban areas can therefore have quite high albedo rates
(proportion of light reflected);
the emission of hygroscopic pollutants from cars and heavy industry act as
condensation nuclei, leading to the formation of cloud and smog, which can
trap radiation. In some cases, a pollution dome can also build up;
recent research on London's heat island has shown that the pollution domes
can also filter incoming solar radiation, thereby reducing the build up of heat
during the day. At night, the dome may trap some of the heat from the day, so
these domes might be reducing the sharp differences between urban and rural
the relative absence of water in urban areas means that less energy is used for
evapo-transpiration and more is available to heat the lower atmosphere;
the absence of strong winds to both disperse the heat and bring in cooler air
from rural and suburban areas. Indeed, urban heat islands are often most clearly
defined on calm summer evenings, often under blocking anticyclones.
Mr. T. Tonna 18
Overall Changes in Weather
Precipitation- As noted previously, the greater presence of
condensation nuclei over urban areas can lead to cities being
wetter and having more rain days than surrounding rural areas.
Indeed, it was often said that Rochdale, the famous mill
town, had significantly smaller amounts of rain on Sundays
when the town's factories were closed.
However, other factors play a major role, especially the heat
islands. These can enhance convectional uplift, and the strong
thermals that are generated during the summer months may
serve to generate or intensify thunderstorms over or downwind
of urban areas. Storms cells passing over cities can be 'refuelled'
by contact with the warm surfaces and the addition of
hygroscopic particles. Both can lead to enhanced rainfall, but
this usually occurs downwind of the urban area.
Mr. T. Tonna 22
Smog- Smogs were common in many British cities in the late 19th and early 20th
centuries, when domestic fires, industrial furnaces and steam trains were all
emitting smoke and other hygroscopic pollutants by burning fossil fuels. The smogs
were particularly bad during the winter months and when temperature inversions
built up under high pressure, causing the pollutants to become trapped in the lower
atmosphere and for water vapour to condense around these particles.
One of the worst of these 'pea-soup fogs' was the London Smog of the winter of
1952/53. Approximately 4,000 people died during the smog itself, but it is estimated
that 12,000 people may have died due to its effects. As a result, the Clean Air Act of
1956 was introduced to reduce these emissions into the lower atmosphere. Taller
chimney stacks and the banning of heavy industry from urban areas were just two
of the measures introduced and, consequently, fewer smogs were recorded in the
UK during the 1960s and 1970s.
Research in the 1990s has shown, however, that another type of smog -
photochemical - is now occurring in some urban areas as a result of fumes from car
exhausts and the build up of other pollutants in the lower atmosphere which react
with incoming solar radiation. The presence of a brown-coloured haze over urban
areas is an indication of photochemical smog, and among its side effects are people
experiencing breathing difficulties and asthma attacks.
Mr. T. Tonna
Wind- Tall buildings can significantly disturb airflows
over urban areas, and even a building 100 metres or so
high can deflect and slow down the faster upper-
atmosphere winds. The net result is that urban
areas, in general, are less windy than surrounding rural
However, the 'office quarter' of larger conurbations can
be windier, with quite marked gusts. This is the result
of the increased surface roughness that the urban
skyline creates, leading to strong vortices and eddies.
In some cases, these faster, turbulent winds are
funnelled in between buildings, producing what is
known as a venturi effect, swirling up litter and making
walking along the pavements quite difficult.
Mr. T. Tonna 24
1. Define what you understand by a n‘Urban Heat Island’.
2. What are the three major factors affecting the
temperature of an Urban Area?
3. Draw a detailed diagram of the effects of the Urban Heat
4. How is the precipitation and wind of an Urban Area
affected by the affects of the Urban Micro-Climate?
Mr. T. Tonna 25
Temperatures tend to be higher in Urban Areas.
This is a result of number of human-induced factors
which tend to increase the amount of heat in an urban
Urban Micro-climates do not only refer to hotter
temperatures in cities- Often due to the presence of
low pressure systems and high rise buildings, cities
are often very windy places.
Urban Heat Islands refer to urban areas which have a
significantly higher average temperature than the
Mr. T. Tonna 26
Using examples from your own experiences
in Urban areas, either in Malta or
abroad, discuss why you think that the
climate within these areas is different to its’
suburbs and other rural areas in Malta or
(200 - 300words).
Mr. T. Tonna 27