CHAPTER 8
Micro and local climates
The climate near the ground provides one of the most easily experienced aspects of
climate. Large-scale climate is largely an abstraction based on averaging the multitude of
different weather systems recorded at standard sites above a grass surface. The marked
changes in weather conditions which can be experienced over short distances on the
ground surface demonstrate just how varied our surface microclimate can be. The nature
of the ground surface is very important. Depending upon the nature of the ground surface
and its surroundings, we can experience much higher temperatures, akin to more tropical
latitudes, or we can find temperatures more appropriate to more poleward latitudes. It is
easy to involve students in discussion about outdoor areas in which they feel warm or
cold. It may be possible to summarize the types of environment which normally appear
cool and those which appear warm and then try to explain why.
A note should be made about the effect of the wind. As our bodies try to maintain a fixed
temperature, which is about 30ºC for bare hands, wind will remove heat from the exposed
surface more rapidly and make it feel cooler than when there is no wind. The windiness
of a site may have as much effect as surface characteristics on the perceived microclimate
of the site. Many high-rise buildings generate strong winds or eddies at the ground
surface which can produce a harsh microclimate especially in well-shadowed locations.
Microclimate provides a useful scale for practical investigations if basic resources, such
as thermometers, are available. Most areas are sufficiently diverse in their surface
properties for small differences in air or surface temperature, wind speed and humidity to
be measured. The results can usually be explained at least at the qualitative level.
The climate near the ground
•
Large temperature differences occur near the ground because we are close to the
main energy exchange zones.
•
Absorption of solar energy by the ground surface is a key factor in determining
temperature levels.
•
Surface cloud, wetness, vegetation, topography and aspect all affect insolation
absorption and long-wave emission.
The microclimate over a bare soil
•
Moisture content and pore space in a soil are important in influencing soil
temperature.
•
Heat transfer from the soil surface is by convection into the atmosphere and by
conduction into the soil. The rates of heat transfer are very different.
•
At night the soil surface is the main source of radiation emission, so will become
the coldest area under calm conditions. Temperatures will increase with height to
produce a temperature inversion.
•
Above the bare soil an exponential increase of wind speed takes place.
The microclimate above a vegetated surface
•
The presence of vegetation produces a more complex microclimatic environment.
It acts as a zone rather than a surface through which the energy transfers take
place.
•
The amount of energy which reaches the soil surface depends upon the height of
the vegetation, the density of its leaves and the angle of the sun.
•
By day, maximum temperatures will be found within the crop canopy. The
location of the highest temperature will depend upon the balance between the
reduction in sunlight and the decrease in wind speed.
•
The wind speed profile will show a decrease of speed towards the surface. Its
precise form will depend upon the nature of the crop and the prevailing wind
speed.
The microclimate of woodland
•
As vegetation depth increases, so the degree of microclimatic modification
increases.
•
Forest ground temperatures show a decline in daytime values, an increase in
night-time values and a reduced diurnal range.
•
Temperature profiles through the forest will depend upon the density of the
canopy as well as on the type of climate, the time of year and weather conditions.
•
With lower temperatures and moisture provided by transpiration, humidity is
normally higher in woodland.
Urban climates
•
The non-natural surface properties of an urban area result in a very different heat
budget, which gives rise to the urban heat island.
•
In general, cities are warmer than their surroundings. The effects are greatest in
climates with light wind and clear skies.
•
Other climatic elements change but the effects are less striking.
•
Pollution is still a problem in urban areas though it is now more usually caused
by gases rather than particles.
The microclimate of slopes
•
The presence of a slope affects the insolation received at the ground; slopes facing
equatorwards are likely to receive more than a horizontal surface, slopes facing
polewards are likely to receive less.
•
The effect of slopes is that the radiation balance varies locally. As a result,
temperature differences between favoured and unfavoured sites can be
considerable.
•
At night the significance of aspect is limited; gradient and friction become more
important in influencing cold air drainage.
•
Under clear skies and light winds at night, cold air can flow downslope and
accumulate in valley bottoms, where the lowest temperatures are often measured.
These are called frost hollows.
Valley breeze systems
•
If cold air drainage develops, breeze systems may ensue, with cold air flowing
down-valley and linking with further tributary valley flows into a mountain
breeze.
•
By day the flow may reverse, with warmer air flowing up-valley and upslope to
give valley breeze.
Sea breezes
•
Temperature gradients which develop between land and sea can initiate a sea
breeze system if the large-scale pressure gradient is not too strong. The effects are
strongest in tropical areas, where strong heating and light gradient winds are
frequent.
•
By night, the sea may become warmer than the land and a lighter, reverse
circulation may develop, called the land breeze.
•
Both sea and land breezes are accompanied by a reverse flow at higher levels to
provide a roughly elliptical convection cell.
CASE STUDY – Urban Areas and Photochemical Smog
We have seen in this chapter of the main text that the presence of an urban area at the
surface has the potential to change most aspects of the local “natural” climate. Within a
city there are buildings of various heights, shapes and materials that will modify the
airflow and will present different surfaces towards the sun; some will be in continuous
sunlight on clear days, others may be in semi-permanent shadow (Figure 1). In addition
the natural atmosphere of the area can be modified by the addition of pollution from a
variety of ground sources, such as industry and transport. On windy days this may not be
significant but under anticyclonic conditions with little air movement, the concentration
of pollution can become marked (Figures 2 and 3). Such pollutants can affect the
radiation budget of the city as well as causing health problems when concentrations get
above certain limits. The World Health Organization has listed air quality standards to
indicate the maximum duration of particular concentrations of pollutants that should not
be exceeded for health reasons.
The most obvious impact of the city is on temperatures. Virtually all cities of the world
have higher temperatures than their surroundings. Some of the largest increases are in
temperate latitudes of the developed world where space heating and industry generate
much waste heat. This tends to reduce the frequency of snow and ice at the ground
surface and so more of the incoming energy is used for heating the air rather than being
reflected or used in the latent heat of fusion as snow melts. Cities such as Montreal,
Chicago and Moscow demonstrate this effect. The higher temperatures can have human
impact as outlined in the Application Box in this chapter.
To demonstrate the nature of photochemical smog and its impact, the city of Adelaide
will be used. It is chosen because it is not a city that is renowned for this problem but
because it demonstrates that even cities which might appear unlikely to suffer, in fact, do
have some problems.
Photochemical smog is a mixture of pollutants that are formed when the primary
pollutants, nitrogen oxides and volatile organic compounds (VOC), react with sunlight to
produce a brown haze that is caused by very small liquid and solid particles of the
secondary pollutants scattering sunlight. These secondary pollutants include ozone
which is a dangerous substance when found at ground level (in contrast to its vital role in
the stratosphere). The other significant pollutant is called peroxyacetyl nitrate, more
commonly abbreviated to PAN, which forms in reactions between nitrogen dioxide and
hydrocarbons from the VOC.
Both of the primary pollutants are produced naturally in burning vegetation, through
microbial action in soils and from the evaporation of naturally-occurring compounds such
as terpenes; in South Australia, eucalypt forests can release significant amounts of these.
The main source of nitrogen oxides from human sources is by the combustion of fossil
fuels, mainly power stations and motor vehicles. VOC are produced from incomplete
combustion of fossil fuels, from the evaporation of solvents and fuels and from burning
plant matter. In Adelaide, it was estimated that two thirds of the nitrogen oxides come
from motor vehicles which also produce 44% of VOC emissions.
To get photochemical smog we therefore need sunlight and sources of pollution.
However if the city is in a windy location, the pollutants may well be dispersed and so
smog is not a problem. It is most likely to develop where air near the ground is trapped
by an atmospheric inversion of temperature (see Chapter 4) and there is limited air
movement. This is a common situation in sub-tropical latitudes where high pressure
dominates for part of the year and there is much sunshine.
The presence of a city leads to a reduction of air movement, so restricting ventilation and
increasing the potential for smog. There is very little we can do to alleviate the smog
potential caused by an inversion, so the main ways to reduce smog frequency is (1) to
plant open avenues in the city to develop ventilation of sea breezes or other natural
winds; or (2) to reduce the amount of pollutants into the atmosphere. The problem of
photochemical smog in Adelaide is less severe than the other cities listed in the caption of
Figure 3, nevertheless efforts are being made for the residents to keep motor vehicles
well serviced, to use more efficient engines and to turn off unnecessary electrical
appliances. In cities where the problem is more severe other measures have been taken.
In Athens during conditions favouring smog, motor vehicles are restricted with licence
plates ending with an even number being alternated with those whose plate ends with an
odd number. Drivers are encouraged to use public transport where this is available.
More affluent residents of the city can, of course, buy two cars to cover both days as long
as one has an odd-numbered plate and the other is even-numbered; it is not always easy
to legislate! Surprisingly even London occasionally experiences photochemical smog.
This is during heat waves in summer when prolonged sunlight reacts with the pollution
associated with motor transport.
Globally photochemical smog is becoming an increasing problem as urban populations,
motor vehicle usage and energy consumption all increase. Health concerns are increasing
as the levels of pollution stay above critical levels for longer periods of time.
Figure 1. The new multi-storied development of Pudong in Shanghai. Built on old
agricultural plots, the skyscrapers present a barrier to the wind that will slow down its
movement at ground level and allow the concentration of pollutants. Shadowing will be
a noticeable feature despite its latitude of 30ºN. Source: Peter Smithson
Figure 2 Beijing viewed from one of its multistorey buildings. Despite being in midsummer, the amount of pollution in the atmosphere is considerable leading to a decline in
visibility. Coal consumption in power generation, industry and road traffic are the main
sources of pollution. Wind speeds in summer are not high and monsoonal rainfall only
amounts to about 550mm so opportunities for cleaning the atmosphere are not frequent.
This problem has been raised in connection with the Olympic Games based there in 2008.
Source: Peter Smithson
Figure 3 The city of Săo Paulo is renowned for its traffic density and jams. It has a vast
area of urbanization covering about 1500 square kilometres with an estimated population
of its metropolitan area of about 19 million. Being in the sub-tropics, like Beijing, it
experiences many days of bright sunshine. The sunlight can react with the pollution
created by traffic to produce a photochemical smog as shown above. This is a common
problem in many cities in subtropical latitudes where sunlight is abundant; Los Angeles,
Athens, Tokyo, Mexico and Tehran are some of the cities most affected. This photograph
also indicates the density of buildings in this central area as well as the dominance of
light coloured material which helps to increase surface albedo.
Source: Peter Smithson
Figure 4 This view of central Vancouver demonstrates the contrast in air quality with
Beijing. Being located on the western side of the North American continent with
prevailing onshore winds from the Pacific, the air is naturally very clean. Industrial
activity is relatively light so frequently the atmosphere is clean with good visibility of the
mountains beyond North Vancouver in the distance. Photochemical fog is rare.
Source: Peter Smithson
Essay and discussion questions
1 Why is the ground surface normally hottest by day but coolest at night?
2 To what extent are there different microclimates in woodland as compared with grass
cover?
3 In what ways do the microclimates of evergreen forest differ from those of deciduous
forest for the same macroclimate?
4 Why should cities be warmer than rural areas?
5 With greater urbanization, are city temperatures likely to become even hotter relative to
their rural surroundings?
6 Why do the lowest temperatures at night often occur in valley bottom locations?
7 In your local area, see if you can see any evidence of the microclimate differences that
you would expect between north- and south-facing slopes or the effect of cold air
drainage into valleys and hollows.
8 Under what conditions do sea breezes develop?
Further reading
Geiger, R., Aron, R.H. and Todhunter, P. (2003) Climate near the Ground, sixth edition,
Cambridge, Mass.: Harvard University Press. A recent revision of a classic work on
microclimate. It provides a vast resource of information and explanation about the
climate near the ground surface.
Landsberg, H.E. (1981) Urban Climate. New York: Academic Press.One of the few
books specifically on urban climate. Concentrates on description or urban climate with
emphasis on role of pollution. Little on energy budget components of cities.
Oke, T.R. (1987) Boundary Layer Climates, second edition, London: Methuen. An
intermediate to advanced-level book demonstrating the significance of the ground surface
in determining microclimate. Very clearly presented but still needs careful reading.
Simpson, J.E. (1994) Sea-breezes and local winds. Cambridge: CUP.Written by an
experienced researcher in the field, it is an impressive study of the controls and nature of
sea-breezes and local winds.
Website addresses
http://www.metoffice.gov.uk/education/secondary/students/microclimates.html
Part of the UK MetOffice site that provides basic information about the nature and
variety of microclimates near the ground. Follows a similar approach to that taken in this
book and even includes an identical diagram.
http://www.field-studies-council.org/urbaneco/urbaneco/introduction/microclimate.htm
Provides examples of a range of microclimates, primarily from the urban heat island
viewpoint and its wildlife applications. Useful links to other related sites too.
http://mediatheek.thinkquest.nl/~ll118/en/development/types.list.html
If you are interested in local winds and their names, this site has a good list of local winds
around the world with some detail about their nature and origin.
http://www.rmets.org/activities/schools/local_winds.php
The website of the Royal Meteorological Site, its educational section covers local winds
at the above URL where details of their origins are described. Links are provided to
other information on local winds around the world.
http://www.urban-climate.org/
The official website of an organization studying urban climates. Has a wide range of
information and useful links.