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.