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Thermal low
From Wikipedia, the free encyclopedia
Thermal lows, or heat lows, are non-frontal low-pressure areas that
occur over the continents in the subtropics such as near the Sonoran
Desert, the Mexican plateau, California's Great Central Valley,
Sahara, South America over northwest Argentina, Australia, the
Iberian peninsula, and Tibetan plateau during the warm season as
the result of intense heating when compared to their surrounding
environments.[1] Over land, intense, rapid solar heating of the land
surface results in heating of the lowest layers of the atmosphere via
Vertical cross-section of a thermal low
reradiated energy in the infrared spectrum. The resulting hotter air
is less dense than surrounding cooler air. This, combined with the
rising of the hot air, results in the formation of a low pressure area. Elevated areas can enhance the strength of
the thermal low as they warm more quickly than the atmosphere which surrounds them at the same altitude.
Over the water, instability lows form during the winter when the air overlying the land is colder than the
warmer water body. Thermal lows tend to have weak circulations, and can extend to 3,100 metres (10,200 ft) in
height. 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. Development of thermal lows inland of the
coastline lead to the development of sea breezes. Sea breezes combined with rugged topography near the coast
can encourage poor air quality.
Contents
1 Formation
2 Role in the monsoon regime
3 Role in sea breeze formation
4 Role in air pollution
5 References
Formation
In deserts, 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. This,
combined with the rising 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
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Thermal low - Wikipedia, the free encyclopedia
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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
Monsoons are caused by the larger amplitude of the seasonal cycle of
land temperature compared to that of nearby oceans. This differential
warming happens because heat in the ocean is mixed vertically through a
"mixed layer" that may be fifty meters deep, through 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,
these factors mean that the heat capacity of the layer participating in
the seasonal cycle is much larger over the oceans than over land,
with the consequence 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. This
creates a steady wind blowing toward the land, bringing the moist
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.
near-surface air over the oceans 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 keeps the 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,
Onset dates and prevailing wind currents of
the southwest summer monsoon
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, stronger and seasonal.[20]
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Role in sea breeze formation
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 warm
air is less dense and so it rises. This rising air over the land lowers
the sea level pressure by about 0.2%. The cooler air above the sea,
now with higher sea level pressure, flows towards the land into the
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
Schematic cross section through a sea
breeze front. If the air inland is moist,
cumulus often marks the front's location.
a possible sea breeze, the sea breeze is not likely to develop.[22]
Along the California coast, the cooler water sets up a surface marine layer that is much cooler than inland areas
during the summer. At the same time, the intense heating inland creates a pronounced thermal trough aligned
with the Great 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 as higher pressure develops to the east due to cooling further
east. This setup often brings the warmest temperatures of the year to the normally cool coastline as the
seabreeze stops or is even replaced by a dangerously dry land breeze.
Role in air pollution
In areas where it is hilly or mountainous 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]
References
1. ^ a b Glossary of Meteorology (2009). Thermal Low. (http://amsglossary.allenpress.com/glossary/search?id=thermallow1) American Meteorological Society. Retrieved on 2009-03-02.
2. ^ National Weather Service Office in Tucson, Arizona (2008). What is a monsoon?
(http://www.wrh.noaa.gov/twc/monsoon/monsoon_whatis.php) National Weather Service Western Region Headquarters.
Retrieved on 2009-03-08.
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3. ^ Douglas G. Hahn and Syukuro Manabe (1975). The Role of Mountains in the South Asian Monsoon Circulation.
(http://adsabs.harvard.edu/abs/1975JAtS...32.1515H) Journal of 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. (http://ams.allenpress.com/perlserv/?request=getdocument&doi=10.1175%2F1520-0493(2000)128%3C3654%3ASGLSAT%3E2.0.CO%3B2) 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 southwestern north America. (http://www3.interscience.wiley.com/journal/113491306/abstract?CRETRY=1&SRETRY=0)
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.
(http://www.nrlmry.navy.mil/forecaster_handbooks/Arctic/Forecasters%20Handbook%20for%20the%20Arctic%20App
endix%20B.pdf) Retrieved on 2009-03-08.
7. ^ National Weather Service Forecast Office Columbia, South Carolina (2009). Weather Terms.
(http://www.erh.noaa.gov/er/cae/FireWx/weatherterms.shtml) National Weather Service Eastern Region Headquarters.
Retrieved on 2009-03-08.
8. ^ Glossary of Meteorology (2009). Warm Low. (http://amsglossary.allenpress.com/glossary/search?id=warm-low1)
American Meteorological Society. Retrieved on 2009-04-04.
9. ^ Gongyuh Lin (2008). Synoptic Weather Systems. (http://www.csun.edu/~hcgeg004/maps.html) California State
University, Northridge. Retrieved on 2009-04-04.
10. ^ Donna F. Tucker (1998). The Summer Plateau Low Pressure System of Mexico. (http://ams.allenpress.com/perlserv/?
request=get-document&doi=10.1175%2F1520-0442(1999)012%3C1002%3ATSPLPS%3E2.0.CO%3B2&ct=1) 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. (http://ams.allenpress.com/perlserv/?request=getdocument&doi=10.1175%2F1520-0493(2003)131%3C2361%3ATNALAS%3E2.0.CO%3B2) Monthly Weather Review:
pp. 2361–2378. Retrieved on 2009-03-09.
12. ^ Roger Graham Barry and Richard J. Chorley (2003). Atmosphere, Weather, and Climate.
(http://books.google.com/books?
id=MUQOAAAAQAAJ&pg=PA199&lpg=PA199&dq=wind+associated+with+thermal+low&source=bl&ots=e3fEqPpMk&sig=XBh6jjCvVJ66B2IXpvy70RUe_s8&hl=en&ei=-O6zSZznK9BtwfWoo2xCQ&sa=X&oi=book_result&resnum=10&ct=result) Routledge, p. 199. ISBN 978-0-415-27171-4.
Retrieved on 2009-03-08.
13. ^ Bureau of Meteorology. "Climate of Giles" (http://www.bom.gov.au/weather/sa/giles/climate.shtml). Retrieved 200805-03.
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14. ^ B. Pu and K. H. Cook (2008). Dynamics of the Low-Level Westerly Jet Over West Africa.
(http://adsabs.harvard.edu/abs/2008AGUFM.A13A0229P) American Geophysical Union, Fall Meeting 2008, abstract
#A13A-0229. Retrieved on 2009-03-08.
15. ^ a b Dr. Louisa Watts (2009). What causes the west African monsoon? (http://www.ncas.ac.uk/index.php?
option=com_content&task=view&id=291&Itemid=9) National Centre for Environmental Science. Retrieved on 200904-04.
16. ^ Dr. Michael Pidwirny (2008). CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes.
(http://www.physicalgeography.net/fundamentals/8e.html) Physical Geography. Retrieved on 2009-01-01.
17. ^ Bart van den Hurk and Eleanor Blyth (2008). Global maps of Local Land-Atmosphere coupling.
(http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf) KNMI. Retrieved on 2009-01-02.
18. ^ Robert Penrose Pearce (2002). Meteorology at the Millennium. (http://books.google.com/books?
id=QECy_UBdyrcC&pg=PA66&lpg=PA66&dq=ways+to+moisten+the+atmosphere&source=web&ots=-0MYq5qyS6&
sig=gz5lOAPIc54v5qfO7nZ098KmVGE&hl=en&sa=X&oi=book_result&resnum=6&ct=result) Academic Press, p. 66.
ISBN 978-0-12-548035-2. Retrieved on 2009-01-02.
19. ^ Glossary of Meteorology. Gust Front. (http://amsglossary.allenpress.com/glossary/search?
p=1&query=gust+front&submit=Search) Retrieved on 2008-07-09.
20. ^ BBC Weather. The Asian Monsoon. (http://www.bbc.co.uk/weather/features/understanding/monsoon.shtml) Retrieved
on 2008-05-22.
21. ^ Dr. Steve Ackerman (1995). Sea and Land Breezes. (http://cimss.ssec.wisc.edu/wxwise/seabrz.html) University of
Wisconsin. Retrieved on 2006-10-24.
22. ^ JetStream: An Online School For Weather (2008). The Sea Breeze.
(http://www.srh.weather.gov/srh/jetstream/ocean/seabreezes.htm) 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.
(http://www.meteogalicia.es/galego/informacion/documentos/Carvalho_Atmos_Environ_2006.pdf) Atmospheric
Environment 40 (2006) 3969–3982. Retrieved on 2009-03-09.
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