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Discrete microclimate of Mountain Palm Canyon, Anza Borrego Desert State Park, Ca. @ C.Michael Hogan

Microclimate is the set of meteorological parameters that characterize a localized area. The scale of geography associated with a microclimate is on the order of one square meter or as large as the order of 100 square kilometers. The chief factors comprising microclimate are: surface temperature, relative humidity, wind speed, solar insolation and precipitation. These factors derive from the confluence of larger scale meteorology with localized topographic elements.


Examples of areas for which the concept of microclimate can be applied are:

caption The microclimate of Mt.Lassen differs sharply from
valley conditions below @ C.Michael Hogano

A very small localized feature such as an individual tidepool or boulder face In these latter cases, microclimate is construed as extremely localized phenomena; for example, in the design of sound barriers for urban freeways, the scale of interest may be chiefly over tens of meters, and microclimate and topographic data may be collected within an extremely limited area in order to analyze the proper mitigation design. ParametersThe parameters typically associated with microclimate include conventional variables such as air temperature, Atmospheric humidity, solar insulation flux, Wind speed, wind direction; however, there may even greater attention on parameters that are associated with localized effects such as: air thermal gradient, soil temperature, soil thermal gradient, soil moisture content, wind turbulence (vertical and horizontal), near surface wind shear. Air pollution is often considered an element of microclimate, since it may affect plant and animal metabolism and animal behavior. In many circumstances of human impacts the parameters of noise pollution and light pollution are considered meaningful parameters of microclimate comfort.

Factors affecting microclimate

A number of natural and man-made circumstances can affect microclimate. Natural environment factors include: topography, sun angle exposure, latitude, soil type, vegetative cover as well as the host of meteorological factors that comprise the wider area meteorology: cloud cover, regional precipitation, high altitude wind characteristics. Topography is often critical since it can lead to localized ponding of water and hence localized anomalies in relative humidity; moreover, the topography greatly influences air flow patterns and localized wind shear, providing areas of relative shelter or exposure to winds. Topography also influences surface runoff, thereby altering upper soil water retention and hence local temperature and humidity. Vegetative cover is an extremely important influence on microclimate, dramatically altering air flow patterns, local temperature and humidity. Vegetative cover has a profound influence on near surface wind speed and direction; in extreme cases of a dense forest canopy, a quiescent zone can be created beneath a forest or chaparral canopy; in other cases the structure of vegetation simply modifies the airflow with surface friction effects, altering directionality, speed and vertical profile of wind speed. The presence of a stand of trees will also alter the wind speed and direction in the surrounding area, thus shaping not only the microclimate within the stand, but also nearby.  Finally, the type and density of vegetation will alter the efflux rate of water vapor from stomata on leaf surfaces, but will also affect the soil surface evaporation rate, both processes affecting local humidity and temperature. Man made influences on microclimate include built structures and roads; conversion of natural habitat to agriculture; replacing native forests with plantations; introduction of exotic species as landscaping elements; mass grading to alter topography; and installation of power generation schemes such as windfarms, solar arrays, hydroelectric dams or other power plants that generate thermal pollution or air pollution plumes

Relation to pollution

Microclimates are extremely important in understanding the phenomena of air pollutant dispersal, noise pollution, thermal pollution and other environmental science factors. In some cases microclimates have a critical influence in altering air pollutant dispersal, affecting the path of sound rays, or the extent of a thermal plume in surface waters. Air pollutant dispersal is most dramatically exemplified by the situation of an urban street canyon, where vehicular air pollution is effectively trapped in longer residence times. Noise pollution effects can be shaped by both natural landform and urban geometries; for example, massive masonry surfaces are effective reflectors, often doubling localized sound levels. Furthermore, the design of acoustic barriers is often influenced by the presence of known prevailing wind shear and thermocline, since those microclimate parameters have a strong impact upon atmospheric refraction of sound rays and hence sound intensity at a given receptor site.

Relation to Ecology

The study of microclimates is often geared toward understanding impacts upon habitat and biodiversity, since plants and animals are strongly affected by microclimate variation. Moreover,  Landform, water content of soil and other abiotic factors of a habitat may also be influenced by microclimate temperature, wind speed and wind direction. Temperature, precipitation and sunlight exposure are obvious microclimate factors influencing plant metabolism and animal behavior. Less obvious is the significant impact that wind speed has on plant gas exchange at the stomatal openings, and hence upon vegetative metabolism as a whole. Higher wind speeds decrease the thickness of the boundary layer (the stationary layer at the leaf surface)); this decreased thickness enhances gas diffusion at the stomata opening, where diffusion rates are governed by the concentration of carbon dioxide and water vapor within the boundary layer.

Impact upon humans

Microclimate has a significant effect upon agricultural production and upon human comfort. Agricultural yields and crop growing season is affected by microclimate variation in temperature, rainfall, solar insolation, humidity and wind velocity. The first three factors being the greatest. Humidity can have an impact by impeding water vapor diffusion away from the leaf surface. While moderate wind speeds may promote plant metabolic rates, it is equally clear that high winds can dramatically reduce yields by increasing wind soil erosion, reducing soil water content and mechanically breaking stalks and shredding grains. Impacts of microclimate upon human comfort can be considerable; for example, in the outdoor environment there is a optimum range of human comfort with respect to temperature, humidity and wind speed. In the case of sunlight exposure, which can vary by topographic impact upon the microclimate, there are also optima, which of course have considerable variations among individuals. Some descriptions of microclimate affecting humans also include the factors of noise pollution, light pollution and air pollution. In any case, one of the fields of modern architecture is the mathematical modeling and prediction of microclimate regimes in the vicinity of man-made structures. These methods are used, for example, in siting of outdoor functions such as swimming pools, patios, and tennis courts. Indoor environments have similar issues, except that mitigation design to optimize comfort can make use of forced air heating, air conditioning and ventilation as well as other forms of climate control such as radiant heating, fireplaces, lightwells and use of geothermal heat. Building orientation

Relationship to climate change

Changes in climate have occurred throughout the history of the Earth; however, presently there are concerns among many that the rate of climate change is exceeding historic norms, placing some ecosystems and human environments at risk for inability to adapt with sufficient swiftness. The notion of microclimate has two different relationships to climate change:

(1) Measuring regional or world trends in climate is often dependent on data acquisition in a microclimate setting; and

(2) The differential impacts of climate change upon proximate locales with differing microclimates.

Temperature is a fundamental parameter for assessing trends in climate change. The microclimate concept of urban heat island is well known to meteorologists, but the issue is whether increased intensity of a given urban heat island over the past several decades may bias the temporal trend of temperature in a given location. In order to elucidate this issue, researchers will need to produce a much more complete picture of each microclimate where sensors have been placed; furthermore in the analysis of climate change, future selection of sensor locations must include not only a clear understanding of microclimate stability, but also make use of locations which are least vulnerable to forecasted influences of man such as further urban development, grading or water diversions. In addition Petersen et al. note that in 1970 only twenty percent of the worldwide thermal sensor network for monitoring temperature trends were in rural areas (less subject to heat island bias), and that the number of rural stations shrank to seven percent by the year 1998. A broader concern of the present global thermal sensor network is that only ten to forty percent (percentages varying over the span of the last 120 years of record) of the Earth are included in the monitoring network, implying a much more complete representation of regional and microclimate sampling locations are needed. Similar microclimate sampling questions may be raised with regard to other meteorological variables such as precipitation and wind speed. The second relationship of microclimate to climate change is the phenomenon of differential impact of diverse microclimates within a given region. For example, concensus forecasts indicate a likelihood of future temperature to Northern Europe. While this effect is viewed as favorable for crop production in the region, it fails to account for microclimates within the region which may be warmer than average; in such cases the resulting temperature rise would be higher than the region norm, and may in fact not be so favorable to global warming. Conversely, the failure to account for cooler microclimate regimes in Southern Europe, may be contributing to an overstatement of adverse agricultural impacts from global warming in Southern Europe. To illustrate the European microclimate differences further, it is known that the ninety percent of Western Europeans living in urban or suburban microclimates live in a decidedly milder climate than those living in rural settings.


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