A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often slightly but sometimes substantially. The term may refer to areas as small as a few square meters or smaller (for example a garden bed, underneath a rock, or a cave) or as large as many square kilometers. Because climate is statistical, which implies spatial and temporal variation of the mean values of the describing parameters, within a region there can occur and persist over time sets of statistically distinct conditions, that is, microclimates. Microclimates can be found in most places but are most pronounced in topographically dynamic zones such as mountainous areas, islands, and coastal areas.^[1]

Microclimates exist, for example, near bodies of water which may cool the local atmosphere, or in heavy urban areas where brick, concrete, and asphalt absorb the sun's energy, heat up, and re-radiate that heat to the ambient air: the resulting urban heat island (UHI) is a kind of microclimate that is additionally driven by relative paucity of vegetation.^[2]

Background

The terminology "micro-climate" first appeared in the 1950s in publications such as Climates in Miniature: A Study of Micro-Climate Environment (Thomas Bedford Franklin, 1955).^[3]

Examples of microclimates

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 foreign surface objects.^[4]

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.

In an urban area, tall buildings create their own microclimate, both by overshadowing large areas and by channeling 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.^[5] 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.

Usually, if the inland areas have a humid continental climate, the coastal areas stay much milder during winter months, in contrast to the hotter summers. This is the case in places such as British Columbia, where Vancouver has an oceanic wet winter with rare frosts, but inland areas that average several degrees warmer in summer have cold and snowy winters.

Sources and influences on microclimate

Two main parameters to define a microclimate within a certain area are temperature and humidity. A source of a drop in temperature and/or humidity can be attributed to different sources or influences. Often a microclimate is shaped by a conglomerate of different influences and is a subject of microscale meteorology.

Cold air pool

Examples of the cold air pool (CAP) effect are Gstettneralm Sinkhole in Austria (lowest recorded temperature −53 °C (−63 °F))^[6] and Peter Sinks in the US. The main criterion on the wind speed ${\displaystyle v}$ in order to create a warm air flow penetration into a CAP is the following:

${\displaystyle \mathrm {Fr} ={\frac {v}{Nh))\geq \mathrm {Fr} _{c},}$

where ${\displaystyle \mathrm {Fr} }$ is the Froude number, ${\displaystyle N}$ — the Brunt–Väisälä frequency, ${\displaystyle h}$ — depth of the valley, and ${\displaystyle \mathrm {Fr} _{c))$ — Froude number at the threshold wind speed.^[7]

Craters

The presence of permafrost close to the surface in a crater creates a unique microclimate environment.^[8]

Caves

Caves are important geologic formations that can house unique and delicate geologic/biological environments. The vast majority of caves found are made of calcium carbonates such as limestone. In these dissolution environments, many species of flora and fauna find home. The mixture of water content within the cave atmosphere, air pressure, geochemistry of the cave rock as well as the waste product from these species can combine to make unique microclimates within cave systems.^[9]

The speleogenetic effect is an observed and studied process of air circulation within cave environments brought on by convection. In phreatic conditions the cave surfaces are exposed to the enclosed air (as opposed to submerged and interacting with water from the water table in vadose conditions). This air circulates water particles that condense on cave walls and formations such as speleothems. This condensing water has been found to contribute to cave wall erosion and the formation of morphological features. Some examples of this can be found in the limestone walls of Grotta Giusti; a thermal cave near Monsummano, Lucca, Italy. Any process that leads to an increase or decrease in chemical/physical processes will subsequently impact the environment within that system. Air density within caves, which directly relates to the convection processes, is determined by the air temperature, humidity, and pressure. In enclosed cave environments, the introduction of bacteria, algae, plants, animals, or human interference can change any one of these factors therefore altering the microenvironment within the cave.^[10] There are over 750 caves worldwide that are available for people to visit. The constant human traffic through these cave environments can have a negative effect on the microclimates as well as on the geological and archeological findings. Factors that play into the deterioration of these environments include nearby deforestation, agriculture operations, water exploitation, mining, and tourist operations.^[11]

The speleogenetic effect of normal caves tends to show a slow circulation of air. In unique conditions where acids are present, the effects of erosion and changes to the microenvironment can be drastically enhanced. One example is the effect of the presence of hydrosulfuric acid(H₂S). When the oxidized hydrosulfuric acid chemically alters to sulfuric acid(H₂SO₄), this acid starts to react with the calcium carbonate rock at much higher rates. The water involved in this reaction tends to have a high pH of 3 which renders the water almost unlivable for many bacteria and algae. An example of this can be found in the Grotta Grande del Vento cave in Ancona, Italy.^[12]

Plant microclimate

As pointed out by Rudolf Geiger in his book^[13] not only climate influences the living plant but the opposite effect of the interaction of plants on their environment can also take place, and is known as plant climate. This effect has important consequences for forests in the midst of a continent; indeed, if forests were not creating their own clouds and water cycle with their efficient evapotranspiration activity, there would be no forest far away from coasts,^[14] as statistically, without any other influence, rainfall occurrence would decrease from the coast towards inland. Planting trees to fight drought has also been proposed in the context of afforestation.^[15]

Dams

Artificial reservoirs as well as natural ones create microclimates and often influence the macroscopic climate as well.

Slopes

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.

Soil types

The type of soil found in an area can also affect microclimates. For example, soils heavy in clay can act like pavement, moderating the near ground temperature. 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.^[16]

Cities and regions known for microclimates

Americas

• Northern California above the Bay Area is also well known for microclimates with significant differences of temperatures.^{[citation needed]} The coastline typically has daytime temperatures of 17 and 19 °C (63 and 66 °F) during summer months along that coastline, but inland towns not far from the ocean such as Lakeport, can be as hot as 34 °C (93 °F) in an average summer day, in spite of being just around 40 miles (64 km) inland. Even as far north as the Klamath River valley around the 41st parallel north between Willow Creek and Eureka averages such temperatures, which is extremely hot for such northerly areas. At this parallel, the temperature at the coast is so cool that Willow Creek beats Eureka's all-time record temperature on average 79 times per year. This is in spite of the areas being less than 50 miles (80 km) from each other.
  • San Francisco is a city with various microclimates. Due to the city's varied topography and influence from the prevailing summer marine layer, weather conditions can vary by as much as 9 °F (5 °C) from block to block and a full 30 °F (17 °C) between the coastal fog belt and the heat island of downtown. The Noe Valley district for example, is typically warmer and sunnier than adjacent areas because the surrounding hills block some of the cool fog from the Pacific.
  • The region as a whole, known as the San Francisco Bay Area can have a wide range of extremes in temperature. In the basins and valleys adjoining the coast, climate is subject to wide variations within short distances as a result of the influence of topography on the circulation of marine air. The San Francisco Bay Area offers many varieties of climate within a few miles. In the Bay Area, for example, the average maximum temperature in July is about 64 °F (18 °C) at Half Moon Bay on the coast, 87 °F (31 °C) at Walnut Creek only 25 mi (40 km) inland, and 95 °F (35 °C) at Tracy, just 50 mi (80 km) inland.^[17]
• The Los Angeles and San Diego areas are also subject to phenomena typical of a microclimate.^[18][19] The temperatures can vary as much as 36 °F (20 °C)) between inland areas and the coast, with a temperature gradient of over one degree per mile (1.6 km) from the coast inland. Hills and mountains can also block coastal air masses. The San Fernando Valley is usually much warmer in summer than most of Los Angeles, because the Santa Monica Mountains usually block the cool ocean breezes and fog. Southern California has also a weather phenomenon called "June Gloom" or "May Grey", which sometimes gives overcast or foggy skies in the morning at the coast, but usually gives sunny skies by noon, during late spring and early summer.
• The Big Island of Hawaii is also an area known for microclimates,^[20] as Kailua-Kona and Hilo, Hawaii, experience rainfall of 18 in (460 mm) and 127 in (3,200 mm) per year, respectively, despite being just 60 mi (97 km) from each other.
• Calgary, Alberta, is also known for its microclimates.^{[citation needed]} Especially notable are the differences between the downtown and river valley/flood plain regions and the areas to the west and north. This is largely due to an elevation difference within the city's boundaries of over 1,000 ft (300 m), but can also be attributed somewhat, to the effects of the seasonal Chinooks.^[21]
• Halifax, Nova Scotia, also has numerous microclimates.^{[citation needed]} Coastal temperatures and weather conditions can differ considerably from areas located just 5–15 km (3.1–9.3 mi) inland. This is true in all seasons. Varying elevations are common throughout the city, and it is even possible to experience several microclimates while traveling on a single highway due to these changing elevations.
• Vancouver and its metro area also has many microclimates.^[22] North Vancouver and other regions situated on the mountain slopes get over 2,000 millimetres (79 in) of precipitation a year on average, while other regions to the south get around 1,000 millimetres (39 in), although they are less than 40 kilometres (25 mi) away. Temperatures in the Fraser Valley inland may be up to 10 °C (18 °F) warmer than the coast, while in winter they are several degrees colder.
• Chesapeake Bay is also known for its subtropical microclimate.^[23] It is most notable for its mild climatic effects on the area east and west of the lowlands of Maryland and Delmarva. Having over 64,000 square miles (170,000 km²) of water; (most of which is a mix of fresh and salt water) creates higher levels of humidity and heat in the spring and summer months. An example of this effect is the survival of subtropical palm trees and plants such as water hyacinths^[24] in the area.^[25][23]
• Chile Chico and Los Antiguos on the southern shores of General Carrera Lake have favourable conditions for agriculture despite being in inner Patagonia.^[26]
• New York City and its surrounding metro area feature an extensive urban heat island, and influence from the Atlantic ocean. These factors cause it to be the northernmost major city in the US that Köppen describes as humid subtropical, with the city being in the 7a/7b/8a USDA zones, compared to nearby cities south of it, which feature lower zones.

Europe

• Known for its wines, the Ticino region in Switzerland benefits from a microclimate in which palm trees and banana trees can grow.^{[citation needed]}
• Gran Canaria is called "Miniature Continent" for its rich variety of microclimates.^{[citation needed]}
• Tenerife is known for its wide variety of microclimates.^[27]
• Istanbul exhibits a multitude of distinct microclimates because of its hilly topography and maritime influences.^[28] Within the city, average summer mean temperatures range from 20–24 °C (68–75 °F) depending on proximity to the Black Sea, with more significant differences on certain days. Rainfall also varies widely owing to the rain shadow of the hills in Istanbul, from around 600 millimeters (24 in) on the southern fringe at Florya to 1,200 millimeters (47 in) on the northern fringe at Bahçeköy.^[29] Furthermore, while the city itself lies in USDA hardiness zones 9a to 9b, its inland suburbs lie in zone 8b with isolated pockets of zone 8a, restricting the cultivation of cold-hardy subtropical plants to the coasts.^[30]
• Leeds, located in Yorkshire, England, is known to have a number of microclimates because of the number of valleys surrounding the city centre.^{[citation needed]}
• The central west coast of Portugal, similarly to California, has huge differences in summer temperatures from the surrounding inland regions. In less than 60 km (37 mi), average daily summer temperatures can vary through as much as 10 degrees Celsius/18 degrees Fahrenheit, from 21 °C (70 °F) in Peniche or São Pedro de Moel to around 31 °C (88 °F) in Santarém or Tomar. This phenomenon is caused by local upwelling created by the northern Nortada winds.^[31]
• The coastal areas in the Andalusia region of Spain has a microclimate.^{[citation needed]} Further north along the coast, Cádiz has a summer average of 27 °C (81 °F) with warm nights, whereas nearby Jerez de la Frontera has summer highs of 33 °C (91 °F) with inland areas further north such as Seville being even hotter.
• Sorana, a commune in Italy's Pescia Valley with a microclimate considered ideal for growing the Sorana bean.^[32]
• The Nizza (Nice) district of Frankfurt-am-Main, Germany is a small area on the north bank of the River Main where wind shelter and sunlight reflected off the river produces a Mediterranean climate and supports one of the largest gardens of southern European plants north of the Alps.^[33]

Asia and Oceania

• Amman, Jordan, has extreme examples of microclimate, and almost every neighbourhood exhibits its own weather.^[34] It is known among locals that some boroughs such as the northern and western suburbs are among the coldest in the city, and can be experiencing frost or snow whilst other warmer districts such as the city centre can be at much warmer temperatures at the same time.
• Sydney, Australia, has a microclimate occurring prominently in the warmer months.^{[citation needed]} Inland, in Sydney's western suburbs, the climate is drier and significantly hotter with temperatures generally around 3–7 °C (5–13 °F) above Sydney CBD and Eastern Suburbs (the coast), as sea breezes do not penetrate further inland. In summer, the coast averages at 25.9 °C (78.6 °F), while inland varies between 28 and 30 °C (82 and 86 °F), depending on the suburb.^[35][36] In extreme occasions, the Coast would have a temperature of 24 °C (75 °F), while a suburb 20 km (12.43 mi)) inland bakes in 36 °C (97 °F) heat. However, winter lows in the West are around 3–5 °C (5–9 °F) cooler than the coastal suburbs, and may provide mild to moderate frost.^[37] Within the city and surrounds, rainfall varies, from around 682.5 mm (26.87 in) in the far-west to 1,213.8 mm (47.79 in) at Observatory Hill (the east or the coast).^[38]