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Weather Home UK World Sports Coast and Sea Climate Change Features Understanding Weather Sport and Leisure Health and Culture History, Literature & Religion Science and Nature Coast and Sea Weatherwise About BBC Weather Site FAQ About the BBC	Microclimates

		A microclimate describes the climate of a small environment such as a town, forest or garden.	Key Points A microclimate is 'the physical state of the atmosphere close to a very small area of the earth's surface. The most common microclimate created by man is the 'urban heat island'. A city can profoundly modify the hydrology of an area. We even create a microclimate in our own home. Also in this Series Urban Heat Islands Also in BBC Weather Weather and Behaviour Disclaimer The BBC is not responsible for the content of external websites.

	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. Concrete absorbs heat and re-radiates it slowly much like an electric storage heater. 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. Winds in the city can be lighter... 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. 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.

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Microclimates