Thermal low

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Vertical cross-section of a thermal low ThermalLow.png
Vertical cross-section of a thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. [1] Thermal lows occur near the Sonoran Desert, on the Mexican Plateau, in California's Great Central Valley, in the Sahara, in the Kalahari, over north-west Argentina, in South America, over the Kimberley region of north-west Australia, over the Iberian Peninsula, and over the Tibetan Plateau.

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On land, intense, rapid solar heating of the Earth's surface causes the heating of the lowest layers of the atmosphere, via re-radiated energy in the infrared spectrum. The hotter air is less dense than surrounding cooler air and rises, leading to the formation of a low-pressure area. Elevated areas can enhance the strength of the thermal low because they warm more quickly than the atmosphere which surrounds them at the same altitude. Over water, instability lows form during the winter when the air overlying the land is colder than the warmer water body.

Thermal lows can extend to 3,100 metres (10,200 ft) in height and tend to have weak circulations. Thermal lows over the western and southern portions of North America, northern Africa, and Southeast Asia are strong enough to lead to summer monsoon conditions. Thermal lows inland of the coastline lead to the development of sea breezes which, combined with rugged topography near the coast, can lead to poor air quality. Owing to the very high temperatures in the centre of heat lows, there are relatively few direct observations of thermal lows.

Formation

An isolated thunderstorm rolls through Wah Wah Valley, Utah. This type of monsoonal pattern is very common in the late summer of the southwest US. Rain in the Wah Wah Valley.jpg
An isolated thunderstorm rolls through Wah Wah Valley, Utah. This type of monsoonal pattern is very common in the late summer of the southwest US.

In deserts, the lack of ground and plant moisture, that would normally provide evaporative cooling, can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. That, combined with the rise of the hot air, results in a low pressure area called a thermal low. [1] Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, which creates an associated heat low over the terrain, and enhances any thermal lows which would have otherwise existed. [2] [3] During the cold season, (winter), warm water bodies such as the Great Lakes can induce an instability low. [4] Thermal lows which develop near sea level can build in height during the warm season, or summer, to the elevation of the 700 hPa pressure surface, [5] which lies near 3,100 metres (10,200 ft) above sea level. [6] Heat lows normally are stationary and have a weak cyclonic circulation. [7] As they are strongest at the surface and warm near their center, and weaker aloft where the air is more stable, the thermal low is considered warm core. [8] [9] The strongest versions of these features globally are over Arabia, the northern portion of the Indian subcontinent, Arizona, Mexican Plateau, [10] northwest Argentina, [11] southwestern Spain, [12] Australia, [13] and northern Africa. The formation of the heat low over northern Africa leads to a low-level westerly jet stream from June into October. [14]

Role in the monsoon regime

Onset dates and prevailing wind currents of the southwest summer monsoon India southwest summer monsoon onset map en.svg
Onset dates and prevailing wind currents of the southwest summer monsoon

Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. That differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty meters deep, due to the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a meter or so. Additionally, the specific heat capacity of liquid water is significantly higher than that of most materials that make up land. Together, those factors mean that the heat capacity of the layer involved in the seasonal cycle is much larger over the oceans than over land, meaning that the air over the land warms faster and reaches a higher temperature than the air over the ocean. The hot air over the land tends to rise, creating an area of low pressure. That creates a steady wind blowing toward the land, bringing the moist near-surface ocean air with it. [15] Similar rainfall is caused by the moist ocean air being lifted upwards by mountains, [16] surface heating, [17] convergence at the surface, [18] divergence aloft, or from storm-produced outflows at the surface. [19] However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation.

In winter, the land cools off quickly, but the ocean retains its heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. [15] Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, much stronger, and seasonal. [20]

Role in sea breeze formation

Schematic cross section through a sea breeze front. If the air inland is moist, cumulus often marks the front's location. SeaBreeze.svg
Schematic cross section through a sea breeze front. If the air inland is moist, cumulus often marks the front's location.

The sea is warmed by the sun to a greater depth than the land due to its greater specific heat. [21] The sea therefore has a greater capacity for absorbing heat than the land, so the surface of the sea warms up more slowly than the land's surface. As the temperature of the surface of the land rises, the land heats the air above it. The less dense warm air rises, which lowers the sea level pressure by about 0.2%. The cooler air above the sea, with higher sea level pressure, flows towards the land into the area of lower pressure, creating a cooler breeze near the coast. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If the environmental wind field is greater than 8 knots (15 km/h) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop. [22]

Along the California coast, the cooler water creates a surface marine layer that is much cooler than inland areas during the summer. At the same time, the intense heating inland generates a pronounced thermal trough aligned with the Central Valley, and typically linked to the broader thermal low across the North American deserts. As a consequence, a strong pressure gradient is created which draws cool marine air landward. As temperatures plummet, fog and stratus stream in and through the gaps of the Coast Ranges, and especially through the Golden Gate at San Francisco (see San Francisco fog). The same thermal trough is sometimes pushed toward the coast, especially in late fall, when higher pressure develops to the east due to cooling even further east. That situation often brings the warmest temperatures of the year to the normally cool coastline, because the sea breeze stops or is even replaced by a dangerously dry land breeze (see also Diablo wind and Santa Ana wind).

Role in air pollution

In hilly or mountainous areas near the coastline, thermally-forced sea breezes, combined with wind circulations up the sides of the mountains, can encourage the production of chemicals which can lead to the development of smog. Pollution has been tracked into the mid-levels of the troposphere in the form of ozone, which is concentrated over the circulation of the thermal low as well as adjacent oceanic areas. [23]

Related Research Articles

<span class="mw-page-title-main">Convection</span> Fluid flow that occurs due to heterogeneous fluid properties and body forces

Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity. When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow.

<span class="mw-page-title-main">Monsoon</span> Seasonal changes in atmospheric circulation and precipitation

A monsoon is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains.

<span class="mw-page-title-main">Surface weather analysis</span> Type of weather map

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

<span class="mw-page-title-main">Anticyclone</span> Weather phenomenon of high pressure, as opposed to a cyclone

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

<span class="mw-page-title-main">Atmospheric circulation</span> Process which distributes thermal energy about the Earths surface

Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant. The smaller-scale weather systems – mid-latitude depressions, or tropical convective cells – occur chaotically, and long-range weather predictions of those cannot be made beyond ten days in practice, or a month in theory.

<span class="mw-page-title-main">Sea breeze</span> Wind blowing from sea to land

A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass. By contrast, a land breeze or offshore breeze is any wind that blows from a landmass toward or onto a large body of water. The term offshore wind may refer to any wind over open water. Sea breezes and land breezes are both important factors in coastal regions' prevailing winds.

<span class="mw-page-title-main">High-pressure area</span> Region with higher atmospheric pressure

A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation.

<span class="mw-page-title-main">Low-pressure area</span> Area with air pressures lower than adjacent areas

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

<span class="mw-page-title-main">Thermocline</span> Distinct layer of temperature change in a body of water

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<span class="mw-page-title-main">Prevailing winds</span> Strongest direction of wind on a region of Earths surface

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<span class="mw-page-title-main">Sea surface temperature</span> Water temperature close to the oceans surface

Sea surface temperature is the temperature of ocean water close to the surface. The exact meaning of surface varies in the literature and in practice. It is usually between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Sea surface temperatures greatly modify air masses in the Earth's atmosphere within a short distance of the shore. The thermohaline circulation has a major impact on average sea surface temperature throughout most of the world's oceans.

<span class="mw-page-title-main">Pressure system</span> Relative peak or lull in the sea level pressure distribution

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<span class="mw-page-title-main">Outflow boundary</span> Mesoscale boundary separating outflow from the surrounding air

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

<span class="mw-page-title-main">Walker circulation</span> Theorerical model of air flow in the atmosphere

The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical directions. This circulation, which is roughly consistent with observations, is caused by differences in heat distribution between ocean and land. In addition to motions in the zonal and vertical direction the tropical atmosphere also has considerable motion in the meridional direction as part of, for example, the Hadley Circulation.

<span class="mw-page-title-main">Atmospheric convection</span> Atmospheric phenomenon

Atmospheric convection is the result of a parcel-environment instability in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day expands the height of the planetary boundary layer, leading to increased winds, cumulus cloud development, and decreased surface dew points. Convection involving moist air masses leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

<span class="mw-page-title-main">Wind</span> Natural movement of air or other gases relative to a planets surface

Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet. Within the tropics and subtropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can prevail.

<span class="mw-page-title-main">Indian Monsoon Current</span> Seasonally-varying ocean current regime found in the tropical regions of the northern Indian Ocean

The Indian Monsoon Current refers to the seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean. During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea. During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal. These variations are due to changes in the wind stress associated with the Indian monsoon. The seasonally reversing open ocean currents that pass south of India are referred to as the Winter Monsoon Current and the Summer Monsoon Current. The cold Somali Current, which is strongly linked to the Indian monsoon, is also discussed in this article.

<span class="mw-page-title-main">Cold-core low</span> Cyclone with an associated cold pool of air at high altitude

A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.

<span class="mw-page-title-main">Monsoon of South Asia</span> Monsoon in Indian subcontinent

The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

  1. 1 2 Glossary of Meteorology (2009). Thermal Low. Archived 2008-05-22 at the Wayback Machine American Meteorological Society. Retrieved on 2009-03-02.
  2. ↑ National Weather Service Office in Tucson, Arizona (2008). What is a monsoon? National Weather Service Western Region Headquarters. Retrieved on 2009-03-08.
  3. ↑ Douglas G. Hahn and Syukuro Manabe (1975). The Role of Mountains in the South Asian Monsoon Circulation. Journal of the Atmospheric Sciences, vol. 32, Issue 8, pp. 1515-1541. Retrieved on 2009-03-08.
  4. ↑ Nathaniel S. Winstead and Pierre D. Mourad (2000). Shallow Great Lake–Scale Atmospheric Thermal Circulation Imaged by Synthetic Aperture Radar. Archived 2021-03-22 at the Wayback Machine Monthly Weather Review: pp. 3654–3663. Retrieved on 2009-03-09.
  5. ↑ David R. Rowson and Stephen J. Colucci (1992). Synoptic climatology of thermal low-pressure systems over south-western north America. International Journal of Climatology, Vol. 12 Issue 6, pp. 529-545. Retrieved on 2009-03-08.
  6. ↑ United States Navy (2008). Forecasters Handbook for the Arctic Appendix B: Mean Monthly Sea Level Pressure, Air Temperature, and 700-mb Height. Archived 2016-12-26 at the Wayback Machine Retrieved on 2009-03-08.
  7. ↑ National Weather Service Forecast Office Columbia, South Carolina (2009). Weather Terms. National Weather Service Eastern Region Headquarters. Retrieved on 2009-03-08.
  8. ↑ Glossary of Meteorology (2009). Warm Low. Archived 2007-08-17 at the Wayback Machine American Meteorological Society. Retrieved on 2009-04-04.
  9. ↑ Gongyuh Lin (2008). Synoptic Weather Systems. California State University, Northridge. Retrieved on 2009-04-04.
  10. ↑ Donna F. Tucker (1998). The Summer Plateau Low Pressure System of Mexico. Archived 2021-04-19 at the Wayback Machine Journal of Climate: pp. 1002–1015. Retrieved on 2009-03-09.
  11. ↑ Marcelo E. Seluchi, A. Celeste Saulo, Matilde Nicolini, and Prakki Satyamurty (2003). The Northwestern Argentinean Low: A Study of Two Typical Events. Archived 2021-04-19 at the Wayback Machine Monthly Weather Review: pp. 2361–2378. Retrieved on 2009-03-09.
  12. ↑ Roger Graham Barry and Richard J. Chorley (2003). Atmosphere, Weather, and Climate. Routledge, p. 199. ISBN   978-0-415-27171-4. Retrieved on 2009-03-08.
  13. ↑ Bureau of Meteorology. "Climate of Giles". Archived from the original on 2008-08-11. Retrieved 2008-05-03.
  14. ↑ B. Pu and K. H. Cook (2008). Dynamics of the Low-Level Westerly Jet Over West Africa. American Geophysical Union, Fall Meeting 2008, abstract #A13A-0229. Retrieved on 2009-03-08.
  15. 1 2 Dr. Louisa Watts (2009). What causes the west African monsoon? National Centre for Environmental Science. Retrieved on 2009-04-04.
  16. ↑ Dr. Michael Pidwirny (2008). CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes. Physical Geography. Retrieved on 2009-01-01.
  17. ↑ Bart van den Hurk and Eleanor Blyth (2008). Global maps of Local Land-Atmosphere coupling. Archived 2009-02-25 at the Wayback Machine KNMI. Retrieved on 2009-01-02.
  18. ↑ Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press, p. 66. ISBN   978-0-12-548035-2. Retrieved on 2009-01-02.
  19. ↑ Glossary of Meteorology. Gust Front. Archived 2011-05-05 at the Wayback Machine Retrieved on 2008-07-09.
  20. ↑ BBC Weather. The Asian Monsoon. Retrieved on 2008-05-22.
  21. ↑ Dr. Steve Ackerman (1995). Sea and Land Breezes. University of Wisconsin. Retrieved on 2006-10-24.
  22. ↑ JetStream: An Online School For Weather (2008). The Sea Breeze. Archived 2006-09-23 at the Wayback Machine National Weather Service Southern Region. Retrieved on 2006-10-24.
  23. ↑ A.C. Carvalho, A. Carvalho, I. Gelpi, M. Barreiro, C. Borrego, A.I. Miranda, V. Perez-Munuzuri (2006). Influence of topography and land use on pollutants dispersion in the Atlantic coast of Iberian Peninsula. [ permanent dead link ]Atmospheric Environment 40 (2006) 3969–3982. Retrieved on 2009-03-09.

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