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Microclimate Definition

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What is a microclimate?

The Meteorological Glossary defines a microclimate as: ‘the physical state of the atmosphere close to a very small area of the earth’s surface, often in relation to living matter such as crops or insects. In contrast to climate, microclimate generally pertains to a short period of time.’

Generally, we take a microclimate to be the climate of a small environment such as a town, forest or garden. The German botanist Gregor Kraus who lived between 1841 and 1915 first contemplated Microclimatology, the study of microclimates. After his death, the German meteorologist Rudolf Geiger developed and quantified the subject into a branch of climatology.

How can we create a microclimate?
The most common microclimate that man has created is the ‘urban heat island’. This is used to describe how a city is relatively warmer than the surrounding rural areas. We see this on infrared satellite images in the Summer – major cities can be spotted as darker areas compared to the rest of the country.

So, why are cities warmer than the countryside? Major towns and cities contain little of the natural environment. In the heart of the city it is rare to see any trees, and the commercial environment consists mainly of concrete. Concrete absorbs heat and re-radiates it slowly much like an electric storage heater. In rural areas the trees use heat in the process of transpiration, and heat is also used in evaporation from streams and rivers.

However, the urban heat island is not the only microclimatological property of our cities. High concentrations of pollution can cause fog to linger longer than in the countryside. In particularly stagnant conditions ‘smog’ can form, causing health problems for certain groups, which are at risk.

A city can profoundly modify the hydrology of an area, owing to the lack of vegetation. In rural areas the vegetation absorbs precipitation thus creating a time lag between the onset of the rain and the run-off becoming appreciable. In cities the peak run-off occurs much earlier, increasing the risk of localised flooding. Winds in the city can be lighter and more variable in direction, owing to the irregular landscape and rougher surface causing greater drag.

On a smaller scale we can create a microclimate in our own gardens. The Victorians used walls to provide protection from the wind and in some cases the sun. Fruit was often grown against a sun-facing brick wall, which would act like a storage heater, re-radiating heat for ripening. Other walls could protect water-shy plants from the prevailing rain-bearing winds.

Today, we often water our gardens during dry periods, Drought can also be offset by using a mulch after watering, to prevent evaporation. On the other hand, excessive rainfall can only be offset by ensuring good drainage. Plants at risk from frost should be planted towards the top of a slope, as cooler air tends to ‘drain’ towards the bottom.

We can even create a microclimate in our own homes. Modern houses often have cavity walls packed with insulation to prevent heat loss, double glazed windows to reduce draughts, and thick loft insulation to prevent heat loss from the roof. Some people also add a porch to further reduce draughts. The temperature of the air indoors is often manipulated by central heating. As the temperature rises the humidity decreases, so some people also use humidifiers or place a bowl of water in each room.

Natural microclimates
The treetops of a high, dense forest can form an almost unbroken surface, which acts in a similar way to the ground. During the day the tree tops absorb solar radiation, resulting in high temperatures at canopy level. The temperatures decrease downwards, owing to the shading effect of the trees. Thus, the forest floor is generally cooler than the canopy and the surrounding countryside. In the Summer this temperature difference can be as much as 5 degrees Celsius. At night forests retain their heat and are generally warmer than their surroundings.

The airflow inside a forest is greatly reduced and results in higher humidities. Wind speeds tend to be very light near the ground, as they are reduced by the canopy. The effect of rainfall is difficult to define since it is hard to measure precipitation within a forest. However, deforestation can cause rainfall to have devastating effects.

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 contributing factor to microclimate is the slope or aspect of an area. South-facing 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. Advocates of solar energy 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 forien surface objects.[citation needed]

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.

Tall buildings create their own microclimate, both by overshadowing large areas and by channelling strong winds to ground level. Wind effects around tall buildings are assessed as part of a microclimate study.

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.


Written by James Thomas

23/09/2010 at 11:09

Posted in 004/ Environment

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