Regional, local, and microclimates
Regional, local, and microclimates are distinct scales of climate that reflect the variations in weather patterns influenced by geographical and environmental factors. Regional climates span large areas, often hundreds or thousands of kilometers, and are shaped by significant influences such as proximity to oceans and large topographic features. For example, the Gulf Stream affects the mild climate of western Europe, while the Asian monsoon brings heavy rain to South Asia during summer months.
Local climates are more limited in scope, covering a few kilometers, and can be influenced by specific phenomena like sea breezes, urban heat islands, and topographical features such as mountains. The urban heat island effect, for instance, causes cities to be warmer than surrounding areas due to human activities and infrastructure.
Microclimates, on the other hand, occur on a very small scale, from a few meters to a few kilometers, and are often dictated by immediate environmental features, such as the presence of water bodies or the orientation of slopes. For example, south-facing slopes in the Northern Hemisphere tend to warm up more quickly and retain heat longer than their north-facing counterparts. Understanding these different climate scales is crucial, especially in the context of climate change, as the impacts of global warming will vary significantly across regions and localities.
Regional, local, and microclimates
Climate change is not uniform across the world. Regional, local, and microscale processes often determine how the climate of a particular location will change in response to increasing concentrations of GHGs or other factors.
Background
Climate is the long-term average of weather patterns. Due to the spherical shape of the Earth, the climate at any location is primarily determined by its latitude. However, the climates of certain regions can be significantly different from those at similar latitudes. Depending upon the spatial scale of interest, these climates are categorized as regional (covering hundreds or thousands of kilometers), local (covering tens of kilometers), and micro (covering few kilometers or even meters). Often, the climate in each of these cases is dominated by atmospheric processes operating only at those spatial scales.
Regional Climates
Regional climates refer to climates of significantly large geographical area covering hundreds to thousands of kilometers with distinctive features distinguishing them from neighboring regions. The uniqueness of regional climates is driven by factors involving their locations, including distance from oceans and the proximity of large topographic features. Additionally, they are often characterized by specific large-scale atmospheric and oceanic processes such as the Atlantic and Pacific hurricanes, and the Asian and African monsoons that dominate other weather processes operating at smaller scales.
The oceanic influence is strongly evident in various regional climates of the United States. The cold water of the vast Pacific Ocean keeps the climate mild year-round in the western United States However, the relatively warm Atlantic helps generate massive hurricanes leading to hot and humid summers with lots of rain in the Southeast. The Atlantic Ocean strongly influences the regional climate of western Europe. In particular, the Gulf Stream, carrying warm tropical waters northward, keeps the western coast of Europe, especially the British Isles, unseasonably mild. Southern European and northern African climate is mild as a result of proximity to the Mediterranean Sea.
The Asian monsoon and the Indian Ocean play an important role in determining the distinctive climate of South Asia. The typical dry northeasterly reverse direction in the summer, bringing months of heavy and continuous rain to India and Southeast Asia. The influence of the Indian Ocean and the monsoon decreases, leading to drier climates, in the northern part of the Indian subcontinent.
Continental effects are evident in the climates of the midwestern United States, central and eastern Europe, midlatitudes of central Asia, and subtropical Africa. These regions experience hot summers, cold winters, and dryness throughout the year. As a consequence of this extreme climate, semiarid grasslands such as the prairie, savanna, and steppes are one of the most common vegetation in these regions.
Local Climates
Local climates are climates of limited geographical areas covering only a few kilometers or tens of kilometers. Examples of local climates include land, sea, and lake breezes, effects, and urban heat islands.
Sea/lake/land breezes are forced by the difference in thermal properties between land and water. Due to its high specific heat capacity, water heats up slower than land. Hence, on a warm summer day, the air over land gets warmer than the air over water. Since warm air is lighter, it rises, and cooler offshore air is drawn in to replace the rising air over land. Thus a cool sea/lake breeze develops. At night, the land cools down quickly, radiating heat to the atmosphere, but the sea remains relatively warm. Under these conditions the sea/lake breeze may reverse to form a land breeze.
Topographic features such as mountains/valleys can significantly influence the local climate. When moist air hits a mountain, it rises and cools, leading to orographic clouds and precipitation. Consequently, the windward side of a mountain is much cooler and wetter than the leeward side. Another important local climate process is mountain/valley winds. As the morning sun heats up the south- and east-facing slopes, air above the slopes warms and starts to rise, causing an upslope (anabatic) breeze. If the air is humid, it may lead to clouds and rain on mountain ridges. At night, the slopes cool radiatively and the cold air sinks back into the valley as downslope (katabatic) breeze. If there is a source of moisture in the valley, it may lead to extensive fog.
Another example of local climate is urban heat islands. They form in large developed metropolitan areas due to multiple reasons: increased heat absorption by concrete and asphalt structures, reduced evaporative cooling, trapping of outgoing radiation by buildings and waste heat from cars, air-conditioning, and industries. Consequently, cities tend to be several degrees hotter than surrounding regions.
Microclimates
Microclimates are climates of very small areas with spatial scales ranging from a few meters to a few kilometers. The major factor behind the existence of a microclimate is the proximity to a heat sink or source. Microclimates exist near ponds, lakes, and wooded areas that generate a cooling effect or near factories and construction sites where the waste heat warms the ambient air.
Topographic aspect (direction of slope) has a profound effect on microclimate. South-facing slopes in the Northern Hemisphere and north-facing slopes in the Southern Hemisphere are exposed to more direct sunlight. Hence, they warm up quickly, reach higher temperatures, and stay warm for longer. Due to this effect, the vegetation cover in opposing slopes of the same hill may be dramatically different. A similar effect is seen in areas that are shaded by natural or structures such as fences or tall buildings.
The city of San Francisco is a textbook example of microclimates. The city lies between the Pacific Ocean and the San Francisco Bay and the topography is complex, with 44 named hills. Due to these natural and anthropogenic reasons the city contains about 30 well-defined microclimate regions. The temperature within the city can vary by 5-10° Celsius, with similar range of variability in precipitation, wind speed, and incoming solar radiation.
Context
It is well established that global warming is going to affect different regions differently. Local impacts will depend on how regional, local, and microscale processes respond to greenhouse gas forcing. Extracting regional climate information from global data sets or general circulation model simulations is very difficult. Simulations with regional climate models can provide this information but are computationally very expensive. Statistical methods can also be used to downscale information from global scales, but these are based on the assumption that the fundamental nature of interactions between different meteorological parameters remain the same across various climate regimes. Hence, regional climate change is one of the major challenges in climate change science.
Key Concepts
- aspect: the direction of slope, such as south-facing or north-facing
- Gulf Stream: the warm current originating in the Gulf of Mexico that travels along the U.S. east coast before turning east to cross the Atlantic Ocean
- monsoon: a seasonal shift in prevailing wind, often associated with significant rainfall
- orographic process: any process triggered by topographic features
- sea breeze: a sea-to-land wind developed as a result of land-water temperature gradients
- trade winds: easterly surface winds in the tropics
- urban heat island: an urban region that is significantly warmer than the surrounding rural areas
Bibliography
Intergovernmental Panel on Climate Change. Climate Change, 2001—The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Edited by J. T. Houghton et al. New York: Cambridge University Press, 2001.
‗‗‗‗‗‗‗. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.
Oke, T. R. Boundary Layer Climates. New York: Routledge, 1988.
"What Is the Difference Between Weather and Climate?" NOAA, 16 June 2024, oceanservice.noaa.gov/facts/weather‗climate.html. Accessed 21 Dec. 2024.