A microclimate (or micro-climate) refers to localized atmospheric conditions in the near-surface layer, which includes the air immediately above a surface as well as the shallow soil and water environments below it. A microclimate can range in size from a few meters to at most a few kilometers across. It is characterized by a set of persistent, measurable differences in the climate conditions from those in the adjacent surrounding areas. These differences may be subtle or pronounced when evaluated over a diurnal (day-night) or seasonal cycle."Microclimate." Britannica Academic, Encyclopædia Britannica, 17 Mar. 2022. academic-eb-com.lib-e2.lib.ttu.edu/levels/collegiate/article/microclimate/52497
Surfaces associated with microclimates include both natural and human made materials, on land and water.
Microclimates are directly caused by weather and climate processes. These processes can be shaped or influenced by contributing factors, such as topography, soil composition, vegetation structure and plant diversity as well as the mass of nearby objects, natural or human made.
Because microclimates are hyperlocal systems, scientific instruments and best practices accounting for the physics of heat transfer and the measurement of microclimate air temperatures are needed to produce accurate results.
Important applications of microclimate knowledge include, agriculture and microclimate engineering, climate change impact, reforestation planning, and energy management.
Scales of climate Climate is commonly described as a hierarchy of spatial scales which are nested and interrelated. Understanding the scale at which climate is measured and the relationships between the classifications, provides important context.Bonan GB. Earth's climates. In: Ecological Climatology: Concepts and Applications. Cambridge University Press; 2008:68–87
A Macroclimate is the climate of a large region, typically covering an area defined as hundred to thousands of kilometers. It is shaped by broad atmospheric patterns. Mesoclimates are nested within the macroclimate and are defined by the topography and features of an area. Mesoclimates range in size between tens to hundreds of kilometers. Microclimates are present at much smaller scales, mostly controlled by the properties of the area and energy exchanges.Roger G. Barry and Peter D. Blanken, Microclimate and Local Climate (Cambridge: Cambridge University Press, 2016)
As spatial scale decreases from macro to micro, the influence of local surfaces, shade, wind obstruction and moisture present become dominant and have significant influence on the related properties of weather.Gordon B. Bonan, Ecological Climatology: Concepts and Applications, 2nd ed. (Cambridge: Cambridge University Press, 2008)
Microclimates occur within the Planetary boundary layer, which is the lowest level of atmosphere directly influenced by dynamics on the Earth's surface.Bonan, Ecological Climatology: Concepts and Applications (2008)
Causes of microclimates Microclimates develop when temperature and humidity are altered by exchanges of heat, moisture and air between the Earth's surface and the atmosphere.Gordon B. Bonan, Ecological Climatology: Concepts and Applications, 2nd ed. (Cambridge: Cambridge University Press, 2008)Roger G. Barry and Peter D. Blanken, Microclimate and Local Climate (Cambridge: Cambridge University Press, 2016)
These localized atmospheric conditions arise from differences in sunlight, heat, wind, and moisture near the surface. Terrain, vegetation, soil, water, and human-made features help shape and modify these conditions.
Direct physical processes Fundamental physical processes which directly create microclimate variation are:
Solar radiation and shading Solar radiation is the primary energy source creating microclimate formation. Shading from this radiation alters the amount of energy reaching the surface, resulting in variances of temperature.
Wind (or obstruction of wind) Wind redistributes heat and moisture, which allows localized air masses to develop distinct temperature and humidity properties. When wind is obstructed the wind speed velocity is reduced, which preserves the local weather processes.Stoutjesdijk, Ph., and J. J. Barkman. Microclimate, Vegetation and Fauna, Koninklijke Nederlandse Natuurhistorische Vereniging, Stichting Uitgeverij, 2014 Heat transfer and thermal properties of surfaces Surfaces differ in their ability to absorb, store, and release heat. Materials such as soil, water, rock and human-made surfaces have distinct properties that influence the surrounding air temperature due to conduction, convection, radiation and moisture content. Moisture, evaporation, and humidity Moisture and evaporation are influenced by soil, vegetation and nearby water. Areas with higher available moisture tend to have lower air temperature because evaporation transfers heat from the surface to the atmosphere.
Contributing environmental factors that shape microclimates
In addition to the direct physical processes, there are characteristics in the environment which can influence the development and persistence of microclimates.
Topography Changes in terrain such as elevation changes or depressions can strongly influence the development of microclimates by altering sun exposure, wind patterns, and air pooling. Another contributing factor of 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 periods of time, giving the slope a warmer microclimate than the areas around the slope. The lowest area of a glen may sometimes frost sooner or harder than a nearby spot uphill, because cold air sinks, a drying breeze may not reach the lowest bottom, and humidity lingers and precipitates, then freezes.
Land and water mass distribution Large bodies of water can influence microclimates by lowering the surrounding air temperature and increasing humidity.Gordon B. Bonan, Ecological Climatology: Concepts and Applications, 2nd ed. (Cambridge: Cambridge University Press, 2008) Soil properties The soil composition makeup affect microclimates through its ability to store heat and moisture. Soils made of components that retain moisture tend to moderate temperature fluctuations while soils made of components which dry easily produce higher daily temperature changes.Stoutjesdijk, Ph., and J. J. Barkman. Microclimate, Vegetation and Fauna, Koninklijke Nederlandse Natuurhistorische Vereniging, Stichting Uitgeverij, 2014 The type of soil found in an area can also affect microclimates. For example, soils heavy in clay, retain heat and moisture which moderates the near ground temperature and humidity. On the other hand, if soil has many air pockets, then the heat could be trapped underneath the topsoil, resulting in the increased possibility of frost at ground level. Vegetation structure and plant diversity Vegetation and its structure has a significant impact on microclimates by modifying solar radiation and shading, blocking wind flow and supporting moisture retention.Jucker, T., Hardwick, S. R., Both, S., Elias, D. M. O., Ewers, R. M., Milodowski, D. T., Swinfield, T., & Coomes, D. A. (2018). Canopy structure and topography jointly constrain the microclimate of human‐modified tropical landscapes. Global Change Biology, 24(11), 5243–5258. https://doi.org/10.1111/gcb.1441 As pointed out by meteorologist Rudolf Geiger,R. Geiger. The climate near the ground. Harvard University Press, 1957. not only does climate influence the living plant, but the opposite effect of the interaction of plants on their environment can also take place, which is ultimately known as plant climate. Plant diversity can further impact the microclimate by creating variety in the canopy structure and transpiration rates. This produces measurable differences in temperature and humidity.Rémy Beugnon, Nolwenn Le Guyader, Alexandru Milcu, Jonathan Lenoir, Jérémy Puissant, Xavier Morin, and Stephan Hättenschwiler, "Microclimate modulation: An overlooked mechanism influencing the impact of plant diversity on ecosystem functioning," Global Change Biology 30, no. 3 (2024): e17214, https://doi.org/10.1111/gcb.17214De Frenne, Pieter; Lenoir, Jonathan; Luoto, Miska; Scheffers, Brett R.; Zellweger, Florian; Aalto, Juha; Ashcroft, Michael B.; Christiansen, Ditte M.; Decocq, Guillaume; De Pauw, Karen; Govaert, Sanne; Greiser, Caroline; Gril, Eva; Hampe, Arndt; Jucker, Tommaso (2021). "Forest microclimates and climate change: Importance, drivers and future research agenda". Global Change Biology. 27 (11): 2279–2297. doi:10.1111/gcb.15569. ISSN 1354-1013. Human-made structures The physical features and properties of the built environment such as the building form, layout and surface materials can impact the surrounding temperature, humidity, and wind patterns by changing air flow, shading and heat storage.Roger G. Barry and Peter D. Blanken, Microclimate and Local Climate (Cambridge: Cambridge University Press, 2016), chap. 10 Urban areas often develop microclimates which are warmer than the surrounding region. This is the result of the use of building materials like concrete and asphalt, which absorb and store heat, while building mass reduces airflow and wind patterns. All of this has a considerable impact on the local microclimate.Roger G. Barry and Peter D. Blanken, Microclimate and Local Climate (Cambridge: Cambridge University Press, 2016) See also urban microclimates in the examples section.
How to measure microclimates The measurement of microclimates presents challenges not encountered with observing broader regional atmospheric conditions. Environmental conditions often vary considerably over short distances due to differences in shading, terrain, vegetation and surface materials. Measurements are also highly sensitive to placement, exposure and the design of the sensor itself. Temperature sensors record their own physical temperature which can be influenced by their materials, heat exchange, and solar radiation exposure. Instruments placed above the near surface layer may be subject to conditions which are different from the microclimate being observed. Due to these challenges, the accurate measurement of microclimates requires careful sensor placement, radiation shielding, and thoughtful analysis of observation data.Maclean, I. M. D., Mosedale, J. R., and Bennie, J. J. (2021). "On the measurement of microclimate." Methods in Ecology and Evolution 12 (8): 1397–1410. https://doi.org/10.1111/2041-210X.13627
Microscale meteorology is the scientific field which studies atmospheric processes responsible for the creation of microclimates. It focused on processes which occur in distances as small centimeters to a few kilometers, which are the same scales at which microclimates exist. This field supplies the methods and techniques by which accurate microclimate measurement can occur.Maclean, I. M. D., Mosedale, J. R., and Bennie, J. J. (2021). "On the measurement of microclimate." Methods in Ecology and Evolution 12 (8): 1397–1410. https://doi.org/10.1111/2041-210X.13627Geiger, Rudolf, Robert H. Aron, and Paul Todhunter. (2009). The Climate Near the Ground. 7th ed. Lanham, Maryland: Rowman & Littlefield.