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Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries

In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry, while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass's characteristics, as well as modification.

Classification and notation

[edit]
Source regions of global air masses

The Bergeron classification is the most widely accepted form of air mass classification, though others have produced more refined versions of this scheme over different regions of the globe.[1][2] Air mass classification involves three letters. The first letter describes its moisture properties – "c" represents continental air masses (dry), and "m" represents maritime air masses (moist). Its source region follows: "T" stands for Tropical, "P" stands for Polar, "A" stands for Arctic or Antarctic, "M" stands for monsoon, "E" stands for Equatorial, and "S" stands for adiabatically drying and warming air formed by significant downward motion in the atmosphere. For instance, an air mass originating over the desert southwest of the United States in summer may be designated "cT". An air mass originating over northern Siberia in winter may be indicated as "cA".[3]

The stability of an air mass may be shown using a third letter, either "k" (air mass colder than the surface below it) or "w" (air mass warmer than the surface below it).[3] An example of this might be a polar air mass blowing over the Gulf Stream, denoted as "cPk". Occasionally, one may also encounter the use of an apostrophe or "degree tick" denoting that a given air mass having the same notation as another it is replacing is colder than the replaced air mass (usually for polar air masses). For example, a series of fronts over the Pacific might show an air mass denoted mPk followed by another denoted mPk'.[3]

Another convention utilizing these symbols is the indication of modification or transformation of one type to another. For instance, an Arctic air mass blowing out over the Gulf of Alaska may be shown as "cA-mPk". Yet another convention indicates the layering of air masses in certain situations. For instance, the overrunning of a polar air mass by an air mass from the Gulf of Mexico over the Central United States might be shown with the notation "mT/cP" (sometimes using a horizontal line as in fraction notation).[4]

Characteristics

[edit]

Tropical and equatorial air masses are hot as they develop over lower latitudes. Tropical air masses have lower pressure because hot air rises and cold air sinks. Those that develop over land (continental) are drier and hotter than those that develop over oceans, and travel poleward on the southern periphery of the subtropical ridge.[5] Maritime tropical air masses are sometimes referred to as trade air masses. Maritime tropical air masses that affect the United States originate in the Caribbean Sea, southern Gulf of Mexico, and tropical Atlantic east of Florida through the Bahamas.[6] Monsoon air masses are moist and unstable. Superior air masses are dry, and rarely reach the ground. They normally reside over maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below, forming what is known as a trade wind inversion over the maritime tropical air mass.

Continental Polar air masses (cP) are air masses that are cold and dry due to their continental source region. Continental polar air masses that affect North America form over interior Canada. Continental Tropical air masses (cT) are a type of tropical air produced by the subtropical ridge over large areas of land and typically originate from low-latitude deserts such as the Sahara Desert in northern Africa, which is the major source of these air masses. Other less important sources producing cT air masses are the Arabian Peninsula, the central arid/semi-arid part of Australia and deserts lying in the Southwestern United States. Continental tropical air masses are extremely hot and dry.[7] Arctic, Antarctic, and polar air masses are cold. The qualities of arctic air are developed over ice and snow-covered ground. Arctic air is deeply cold, colder than polar air masses. Arctic air can be shallow in the summer, and rapidly modify as it moves equatorward.[8] Polar air masses develop over higher latitudes over the land or ocean, are very stable, and generally shallower than arctic air. Polar air over the ocean (maritime) loses its stability as it gains moisture over warmer ocean waters.[9]

Movement and fronts

[edit]
Picture of cold front (left part of the image) moving over the Czech Republic
Main article: Weather front

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.[10]

Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts.[11] When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline.[12] This is most common over the open ocean.

Modification

[edit]
Lake-effect snow bands near the Korean Peninsula
See also: Precipitation and Lake-effect snow

Air masses can be modified in a variety of ways. Surface flux from underlying vegetation, such as forest, acts to moisten the overlying air mass.[13] Heat from underlying warmer waters can significantly modify an air mass over distances as short as 35 kilometres (22 mi) to 40 kilometres (25 mi).[14] For example, southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized precipitation since large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C or 23 °F) between the water surface and the air above.[15] Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes.[16]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Air mass

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An air mass is a large body of air with generally uniform and , typically spanning thousands of kilometers and acquiring its characteristics from the surface over which it forms. These masses originate in source regions where the air stagnates for several days, allowing it to take on the and properties of that area, such as continents for dry air or oceans for moist air. Air masses are classified using a two-part naming system: the first letter indicates moisture content—continental (c) for dry air from land sources or maritime (m) for moist air from water sources—and the second denotes — (A) for very cold air from polar ice caps, polar (P) for cold air from high latitudes, or tropical (T) for warm air from subtropical regions. Common types affecting include continental polar (cP), which brings cold, dry conditions; maritime tropical (mT), responsible for warm, humid ; and continental tropical (cT), delivering hot, arid air. As air masses move away from their source regions, they interact at boundaries called fronts, driving much of the day-to-day variability through processes like , shifts, and development in mid-latitudes.

Definition and Formation

Definition

In , an air mass is defined as a large volume of air that exhibits relatively uniform horizontal characteristics of , , and stability. These properties arise from the air's prolonged residence over a source region, resulting in a cohesive body that can span horizontally across hundreds to thousands of kilometers—often covering several million square kilometers—and extend vertically throughout much of the , typically up to 10–15 kilometers in height depending on . The concept of air masses was pioneered by Norwegian meteorologists and his son Jacob Bjerknes in the early , forming a cornerstone of the frontal theory that revolutionized weather analysis and forecasting. Their work emphasized how these expansive air bodies interact at boundaries, providing a framework for understanding large-scale atmospheric dynamics. Air masses differ fundamentally from small-scale air parcels, which are conceptual, localized volumes used to study thermodynamic processes like and stability on a micro level, whereas air masses highlight synoptic-scale uniformity over vast regions. This large-scale focus is essential for analyzing weather patterns within the , the atmospheric layer from the surface to the where virtually all significant meteorological phenomena occur. Air masses contribute to weather systems by advecting their properties across regions, influencing local conditions through gradual modifications.

Formation Processes

Air masses form primarily through the process of stagnation over extensive, uniform source regions, where the air undergoes gradual modification via interaction with the underlying surface. This stagnation is facilitated by large-scale high-pressure systems, known as anticyclones, which promote calm winds and descending motion, or , in the mid-troposphere. Subsidence inhibits vertical mixing and cloud formation, allowing the air to settle and homogenize horizontally while exchanging and with the surface below. As a result, the air acquires relatively uniform and characteristics that define the air mass. The formation process requires a sufficient period of relative immobility for the air to equilibrate with the surface, typically spanning several days to weeks depending on the initial conditions and surface type. For warm air masses developing over heated surfaces, the e-folding time for boundary layer growth is on the order of 1-2 days, with full development often completing in about one week through convective mixing. In contrast, cold air masses over cooler surfaces form more slowly, taking approximately two weeks, as radiative cooling and surface heat loss dominate without strong convection. During this time, synoptic-scale divergence associated with anticyclones further enhances subsidence, compressing the lower atmosphere and promoting stability that aids in achieving horizontal uniformity. The nature of the surface plays a crucial role in imparting specific properties to the forming air mass. Over continental land surfaces, which are generally drier and have lower , the air tends to become drier and more thermally variable, leading to continental (c) characteristics. Conversely, over maritime ocean surfaces, abundant and higher thermal inertia result in more humid and stable air, yielding maritime (m) characteristics. These distinctions arise from the differential rates of and flux at the air-surface interface, with subsidence ensuring that modifications propagate throughout the air mass depth.

Classification and Types

Source Regions

Air masses originate in large, relatively uniform geographic areas known as source regions, where high-pressure systems allow air to stagnate for extended periods, typically a week or more, enabling it to acquire the and characteristics of the underlying surface. The regions include polar areas, which produce cold, dry air over expansive ice caps and snow-covered lands, and regions, which generate even colder, drier air masses due to their extreme low temperatures and minimal over frozen surfaces. Tropical source regions, often over warm waters, form air masses that are warm and moist, reflecting the high rates from subtropical seas. In contrast, continental interiors serve as sources for dry air masses with extreme variations, as vast landmasses like deserts and plains provide little but intense heating or cooling. Notable examples include the , a wintertime high-pressure system over that sources continental polar air, characterized by its dryness from prolonged contact with frozen , and subtropical oceans such as the , which supply maritime tropical air through sustained interaction with warm, evaporating waters. These regions' surface features—whether icy expanses, arid soils, or humid seas—directly imprint initial properties onto the air mass before it departs. Seasonal variations significantly influence source region activity; for instance, the Desert acts as a more intense source of dry continental air in summer due to extreme surface heating, while in winter, cooler conditions reduce its thermal contrast and output. Globally, these source regions align with major patterns, with polar and zones concentrated around 60°–90°N/S latitudes over ice-covered areas, and tropical regions prominent near the and extending to subtropical high-pressure belts at approximately 30°N/S, where descending air promotes stagnation over oceans and deserts alike. Air masses formed here are classified using notation like "cP" for continental polar or "mT" for maritime tropical, as detailed in standard meteorological schemes.

Notation and Classification Scheme

The standardized notation for air masses employs a two-letter code that categorizes them according to their source region's content and latitudinal characteristics. The first letter indicates the surface type over which the air mass forms: "c" for continental (typically dry, originating over land) or "m" for maritime (typically moist, originating over water). The second letter denotes the and associated : "A" for (extremely cold), "P" for polar (cold), or "T" for tropical (warm). This scheme, rooted in the source regions that impart uniform properties to the air mass, provides a foundational framework for identification on weather maps. Examples of this classification include cP (continental polar), which forms over cold landmasses like and is characterized as cold and dry; mT (maritime tropical), originating over warm ocean waters such as the and noted for its warm and moist qualities; cT (continental tropical), from hot desert regions like the , dry and hot; and mP (maritime polar), from cooler ocean areas like the North Pacific, cool and moist. Additional modifiers occasionally appear in specialized contexts, such as "AA" for antarctic air or "E" for equatorial, though the core c/m and A/P/T system remains predominant. The classification scheme evolved from the early 20th-century work of the Bergen School of Meteorology in . introduced the concept of polar fronts in 1919, emphasizing boundaries between distinct air masses, while Tor Bergeron refined the air mass categorization in the 1920s by incorporating source-based properties like temperature, humidity, and stability. This system gained widespread adoption in the mid-20th century and is now standard in operational by organizations such as the (NOAA). Despite its utility, the notation has limitations in capturing the full spectrum of air mass variations. It primarily addresses basic source and thermal origins but does not fully account for nuanced distinctions, such as superior air masses (dry, subsiding air often aloft from high-pressure subsidence) versus inferior air masses (more humid, near-surface air influenced by storm tracks), which depend on the air mass's position relative to mid-latitude cyclone paths. This crudeness can overlook regional modifications or complex vertical structures, prompting supplementary analyses in advanced forecasting.

Physical and Thermodynamic Properties

Temperature and Moisture Characteristics

Air masses exhibit distinct vertical profiles, primarily characterized by their environmental lapse rates—the rate at which decreases with altitude. Polar air masses, originating from high-latitude source regions, often display shallow lapse rates in their lower layers, typically less than the average tropospheric value of 6.5°C per kilometer, frequently featuring inversions that indicate stability. In contrast, tropical air masses from low-latitude regions tend to have lapse rates closer to the dry adiabatic rate of approximately 9.8°C per kilometer, reflecting their potential for convective activity under certain conditions. These profiles contribute to the overall uniformity of within an air mass, a key defining feature. Moisture content varies significantly between air mass types, with maritime air masses acquiring higher levels of due to over surfaces, resulting in specific humidity values often exceeding 10 g/kg. For instance, maritime tropical air masses commonly exhibit specific humidities of 15–20 g/kg near the surface, supporting abundant formation and potential. Continental air masses, forming over land with limited , maintain lower levels, typically below 5 g/kg; continental polar air masses, for example, show values around 1–3 g/kg, leading to drier conditions. This contrast in absolute content underscores the role of source regions in determining an air mass's profile. Dew point temperature and relative further delineate moist versus dry air masses by quantifying saturation potential. In moist maritime air masses, dew points are relatively high (e.g., 15–20°C in tropical varieties), indicating substantial that can readily condense upon cooling, whereas dry continental air masses feature low dew points (often below 0°C in polar types), signifying minimal availability. Relative , the ratio of actual to saturation at the current temperature, tends to be higher in cooler moist air masses but can vary; it complements as a measure of how close the air is to saturation, with values near 100% signaling imminent in humid environments. The horizontal and vertical uniformity of an air mass is often evaluated using potential temperature, a conserved thermodynamic property for dry adiabatic processes. It is calculated as θ=T(1000p)R/Cp\theta = T \left( \frac{1000}{p} \right)^{R / C_p} where TT is the air temperature in , pp is the in hectopascals, RR is the specific for dry air (287 J kg⁻¹ K⁻¹), and CpC_p is the specific heat capacity at constant (1004 J kg⁻¹ K⁻¹), yielding R/Cp≈0.286R / C_p \approx 0.286. In an air mass's source region, potential temperature remains nearly constant with height, confirming the homogeneous conditions that define it, and deviations from this constancy signal modification or boundaries with adjacent air masses.

Stability and Other Properties

The stability of an air mass is assessed by its response to vertical displacements, influenced by and profiles. Warm, moist air masses, such as maritime tropical types, typically exhibit conditional , where dry parcels remain stable but saturated parcels become unstable upon lifting due to release during . In contrast, cold, dry air masses like continental polar exhibit absolute stability, resisting vertical motion even when saturated because the environmental is shallower than both dry and moist adiabats. (θ_e), which accounts for both sensible and , serves as a for assessing moist stability; an increase in θ_e with height indicates stability, while a decrease signals potential in lifted moist parcels. A key metric for evaluating air mass stability is the (LI), defined as the difference between the environmental temperature at 500 hPa (T_{500}) and the temperature of a surface parcel lifted adiabatically to that level (T_{parcel,500}):
LI=T500−Tparcel,500\text{LI} = T_{500} - T_{parcel,500}
Positive LI values (>0) denote stable conditions, while negative values (<0) indicate instability conducive to convection, with more negative values signaling greater potential for severe weather in unstable air masses. Beyond stability, air masses possess other notable properties tied to their thermodynamic state. Visibility is often reduced in moist air masses due to haze formation, as high relative humidity promotes hygroscopic growth of aerosols, scattering light and limiting visual range to below 10 km in maritime tropical flows. Continental air masses carry higher pollutant content, including elevated levels of particulate matter (PM_{2.5}) and trace gases from anthropogenic sources, with concentrations up to 20 times greater than in clean maritime air, affecting air quality during advection. Pressure tendencies in air masses reflect their large-scale dynamics; subsiding motion in source regions fosters rising pressure (positive tendency) under high-pressure ridges, while divergence at boundaries can lead to falling pressure (negative tendency) as air masses advance. Modern observations enhance characterization of air mass properties through satellite-derived aerosol loading, such as aerosol optical depth (AOD) from instruments like MODIS, which quantifies particulate burdens to distinguish polluted continental air (AOD > 0.5) from cleaner maritime types (AOD < 0.2), aiding in tracking transport and impacts on .

Movement and Interactions

Advection and Movement Patterns

refers to the horizontal transport of air masses by , which drives their movement across large distances after formation. In the , often propel continental polar air masses equatorward from high-latitude source regions like , carrying their cold, dry characteristics into mid-latitude areas. This process is fundamental to synoptic-scale patterns, as the winds redistribute and moisture globally. The trajectories of air masses are significantly influenced by large-scale atmospheric dynamics, including Rossby waves and the jet stream. Rossby waves, which are planetary-scale undulations in the upper-level winds, cause meandering paths that can steer polar air masses southward in troughs or block their progress in ridges, leading to prolonged weather episodes. The polar jet stream, a narrow band of strong westerly winds at around 9-12 km altitude, further guides these movements by confining air mass boundaries near the polar front, where temperature contrasts are sharp. During advection, air masses retain their core physical properties, such as temperature and humidity, until surface interactions modify them. Typical rates for air masses range from 10 to 30 km/h, allowing them to traverse continental distances—such as from the to the —in a matter of days and influencing over vast regions. These speeds correspond to the movement of associated fronts, which advance at similar velocities under the influence of gradients and upper-level . Observational methods for tracking air mass include the deployment of balloons equipped with radiosondes, which provide vertical profiles of , , and to delineate air mass boundaries in real time. models, such as those from the European Centre for Medium-Range Weather Forecasts (ECMWF), simulate trajectories by integrating fields and enable backward or forward tracking of air parcels over synoptic timescales. These tools are essential for forecasting the arrival of specific air masses and associated changes.

Formation of Fronts and Interactions

When contrasting air masses meet, they form boundaries known as weather fronts, where the denser air mass typically undercuts or overrides the lighter one, leading to dynamic interactions that drive significant weather changes. These interactions occur due to differences in temperature, density, and moisture between air masses, such as continental polar (cP) and maritime tropical (mT), resulting in the sharpening of thermal gradients at the interface. The primary types of fronts arise from the relative motion of these air masses. A forms when a colder, denser air mass, like cP, advances into a warmer one, such as mT, forcing the warm air aloft rapidly due to the cold air's wedging action. In contrast, a develops as a warmer, less dense air mass, like mT, advances over a colder one, such as cP, with the warm air gradually rising over the denser cold air. An occurs when a cold front overtakes a warm front, lifting the warm air mass completely off the surface as cooler air from both sides converges. Stationary fronts form when neither air mass dominates, resulting in a quasi-stationary boundary with persistent but slower-moving patterns. Frontogenesis, the process intensifying these boundaries, is driven by atmospheric convergence and deformation of the flow field. Convergence perpendicular to the thermal gradient concentrates air mass boundaries, increasing the horizontal , while deformation—stretching along an axis of dilatation and contracting perpendicular to it—further sharpens the front if the isotherms align within 45 degrees of the dilatation axis. These processes are quantified by the frontogenesis function, which measures the rate of change of the temperature gradient magnitude. Frontal surfaces exhibit characteristic slopes, typically 1:50 to 1:100 for cold fronts due to frictional slowing of the dense cold air near the surface, and shallower slopes for warm fronts. Interactions at fronts produce distinct weather phenomena tied to the lifting of moist air. Cold fronts often generate narrow bands of intense , including showers and thunderstorms, as the steep uplift promotes convective in the overridden warm air. Warm fronts lead to broader, stratiform from layered clouds like nimbostratus, resulting from the gradual ascent over hundreds of kilometers. Occluded and stationary fronts can sustain prolonged cloudy conditions and , though less intense than active cold fronts. The speed of a front, vfv_f, can be approximated from the speed vgv_g and the convergence angle α\alpha as vf≈vgsin⁡αv_f \approx v_g \sin \alpha, where α\alpha represents the angle between the geostrophic flow and the frontal orientation, reflecting the component driving frontal motion. This approximation highlights how synoptic-scale winds influence the propagation of air mass boundaries.

Modification and Evolution

Mechanisms of Modification

As air masses advect away from their source regions, their , , and stability profiles undergo modification through interactions with new underlying surfaces and atmospheric processes, altering their initial uniform characteristics. Surface exchange plays a primary role in modification, where and fluxes between the air mass and the lead to heating or and moistening or drying. When a air mass moves over a warmer surface, such as a continental polar mass over unfrozen land, sensible transfer warms the lower layers, increasing the depth and potentially destabilizing the profile through enhanced . Conversely, a warm air mass over a cooler surface experiences from below, promoting stability and limiting vertical mixing to shallower depths. exchange follows similar patterns: maritime paths over oceans facilitate , raising specific humidity in the lower , while passage over arid land enhances drying via deficits. These fluxes are governed by bulk aerodynamic transfer coefficients, typically resulting in gradual adjustments that homogenize the . Vertical mixing further modifies air masses by entraining drier, potentially warmer or cooler air from the free atmosphere above the , leading to isobaric dilution of the original properties. This entrainment occurs at the top, where turbulent eddies incorporate overlying air, reducing extremes in and gradients. For instance, in a warming continental air mass, entrainment introduces drier free-tropospheric air, which counteracts surface moistening and maintains relative aloft. The process is most pronounced during daytime convective heating, when the grows to several kilometers, but from large-scale can oppose it, slowing the rate of dilution. Orographic effects contribute to modification when air masses are lifted over elevated , inducing adiabatic cooling and often . Ascent over mountains forces expansion and cooling at rates of approximately 9.8°C per kilometer for dry air, or less for moist ascent, which can saturate the air mass and trigger , thereby removing moisture and altering stability. On the leeward side, descending air undergoes adiabatic warming, drying further and creating rain shadows with modified, warmer, and drier profiles compared to the original mass. Radiative effects, particularly nocturnal cooling, also influence air masses by allowing longwave radiation loss from the surface and lower atmosphere under clear skies, cooling the near-surface layer by 2–3°C per day in stable conditions and enhancing inversions. This is amplified in dry, clear environments and contributes to diurnal variations in modification. The time scales of these modification processes vary with air mass type, speed, and surface contrast, ranging from rapid changes in 12–24 hours for intense surface fluxes to slower evolution over several days to weeks for deeper layers. Warm air masses often reach near-equilibrium with new surfaces in 3–5 days, while cold masses may require up to two weeks due to counteracting .

Examples of Modified Air Masses

One prominent example of air mass modification occurs when a continental polar (cP) air mass, originating over cold, dry land areas such as , advects eastward over the warmer . As the air mass traverses the ocean, it undergoes sensible and latent heating from the sea surface, leading to a increase and significant uptake through . This transformation typically results in the air mass evolving into a maritime polar (mP) type, which retains much of its cold character but becomes considerably more humid, often fostering conditions for stratiform clouds and light precipitation upon reaching coastal regions. A contrasting case involves maritime tropical (mT) air masses from the warm, moist moving northward over cooler land surfaces in the U.S. Midwest during winter or early spring. Upon encountering the colder continental surface, the lower layers of the mT air cool rapidly through conduction and radiation, reducing its temperature and promoting that often manifests as advection fog, low stratus clouds, and . This cooling diminishes the air mass's , shifting it toward a more stable profile that suppresses convective activity while enhancing persistent low-level cloudiness across the Mississippi Valley and . Observational evidence of such modifications is evident in the 1993 Superstorm (also known as the ), a rapid event over the from March 12–14. In this case, southerly flow ahead of an approaching low-pressure system warmed and moistened a low-level air mass over the warm Gulf waters, creating an exceptionally unstable environment. This modified mT-like air interacted with an incoming cold polar surge, fueling explosive deepening of the to 960 hPa and producing widespread , including convective outbreaks and heavy across eastern . In contemporary contexts, urban heat islands (UHIs) accelerate local modifications of passing air masses by injecting additional anthropogenic heat into the , particularly in major cities. For instance, UHIs can enhance warming of cooler polar air masses transiting urban areas, increasing low-level instability and altering moisture profiles faster than in rural surroundings, thereby influencing mesoscale weather patterns like initiation. This effect is projected to heighten the frequency of oppressive air masses in urbanized regions under scenarios.

References

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  44. https://www.eoas.ubc.ca/books/Practical_Meteorology/mse3/Ch12-Fronts.pdf
  45. https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/air-masses/modification
  46. http://www.nwcg.gov/publications/pms425-1/8-air-masses-and-fronts
  47. https://journals.ametsoc.org/view/journals/atsc/2/2/1520-0469_1945_002_0094_topcat_2_0_co_2.xml
  48. https://www.weather.gov/media/ilm/Overview_Kocin_Schumacher_Morales_Uccelini.pdf
  49. https://vtechworks.lib.vt.edu/bitstream/handle/10919/103468/Van_Tol_ZC_T_2021.pdf
  50. https://ams.confex.com/ams/pdfpapers/36644.pdf
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