Dry line#Dry punch
{{Short description|Boundary between moist and dry air}}
{{Globalize|article|USA|2name=the United States|date=March 2018}}
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A dry line (also called a dew point line, or Marfa front, after Marfa, Texas){{cite web | date=2007-05-04|author = Scott Girhard | title = Lecture 3 - Thunderstorms | publisher = San Antonio College | url = http://www.accd.edu/sac/earthsci/sgirhard/1370.090/chap3.htm | access-date = 2008-03-15 |archive-url = https://web.archive.org/web/20070927000112/http://www.accd.edu/sac/earthsci/sgirhard/1370.090/chap3.htm |archive-date = 2007-09-27}} is a line across a continent that separates moist air and dry air. One of the most prominent examples of such a separation occurs in central North America, especially Texas, Oklahoma, and Kansas, where the moist air from the Gulf of Mexico meets dry air from the desert south-western states. The dry line is an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the High Plains states in the warm sector of an extratropical cyclone and stretches into the Canadian Prairies during the spring and early summer.{{citation needed|date=February 2014}} The dry line is also important for severe convective storms in other regions of the world, such as northern India and Southern Africa.Howard, E., and R. Washington, 2019: Drylines in Southern Africa: Rediscovering the Congo Air Boundary. J. Climate, 32, 8223–8242, https://doi.org/10.1175/JCLI-D-19-0437.1. In general, thunderstorms and other forms of severe weather occur on the moist side of the dryline.
Characteristics
File:Dry line satellite description.png
Near the surface, warm dry air is denser than warm moist air of lesser or similar temperature, and thus the warm dry air wedges under the moist air like a cold front.Carlson and Ludlam, "Conditions for the occurrence of severe local storms", orig. Tellus, Vol.20, No.2, pp.203-226 (May 1968), republished online at [http://onlinelibrary.wiley.com/doi/10.1111/j.2153-3490.1968.tb00364.x/abstract Wiley Online Library, March 18, 2010]{{cite web|author=Huaqing Cai |url=http://www.atmos.ucla.edu/~caihq/pic/fig23.html |title=Dryline cross section |access-date=2006-12-05 |date=2001-09-24 |publisher=University of California Los Angeles |url-status=dead |archive-url=https://web.archive.org/web/20080120180130/http://www.atmos.ucla.edu/~caihq/pic/fig23.html |archive-date=2008-01-20 }}
At higher altitudes, the warm moist air is less dense than the cooler, drier air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front. The dry line is most common in the spring.{{cite web|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=dryline&submit=Search|title=Dryline|author=Glossary of Meteorology|date=June 2000|publisher=American Meteorological Society|access-date=2010-04-25|url-status=dead|archive-url=https://web.archive.org/web/20110606085326/http://amsglossary.allenpress.com/glossary/search?p=1&query=dryline&submit=Search|archive-date=2011-06-06}} Its location is close to the location of the {{convert|55|F|C}} isodrosotherm, or line of equal dewpoint. The location of the dryline may not be marked with a surface pressure trough or shift of the wind direction. It bulges more to the east underneath the location of the highest winds within the jet stream.{{cite web|url=http://home.earthlink.net/~rhulecki/drylinetstorms/Dryline.html |title=Dryline Thunderstorms |author=Daniel Dix |date=June 2000 |publisher=The Weather Channel |access-date=2010-04-25 |url-status=dead |archive-url=https://web.archive.org/web/20070729204713/http://www.home.earthlink.net/~rhulecki/drylinetstorms/Dryline.html |archive-date=2007-07-29 }} While dry lines are most common in the Great Plains, northern India also witnesses a similar moisture boundary.{{cite book|year=1991|pages=449–481|title=Mid-Latitude Weather Systems|author=T. N. Carlson|publisher=HarperCollinsAcademic|isbn=978-0-04-551116-7}} In northeast India, it occurs mainly before the onset of their summer monsoon,{{cite journal|url=http://www3.interscience.wiley.com/journal/113521473/abstract?CRETRY=1&SRETRY=0|archive-url=https://archive.today/20130105131114/http://www3.interscience.wiley.com/journal/113521473/abstract?CRETRY=1&SRETRY=0|url-status=dead|archive-date=2013-01-05|title=The dry-line of Northern India and its role in cumulonimbus convection|author=K. J. Weston|journal=Quarterly Journal of the Royal Meteorological Society|volume=98|issue=417|pages=519–531|date=1972-03-07|access-date=2010-04-25|doi=10.1002/qj.49709841704|bibcode = 1972QJRMS..98..519W |url-access=subscription}} while northwest India experiences it during the monsoon season.
=Dry punch=
A dry punch is meteorological slang for a synoptic scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of warm, moist (buoyant) air at low levels often increases the potential for severe thunderstorms.
Daily progression in North America
The dry line typically advances eastward during the afternoon and retreats westward at night, mainly due to the increased mixing down to the surface of moist air aloft, rather than the air mass' surface density contrast. The movement of the dry line during daylight hours is quickest in areas where low level moisture is most shallow, as dryline movement slows in areas with deeper low-level moisture. Weaker winds aloft also slow its progression.{{cite web|url=http://www.srh.noaa.gov/topics/attach/html/ssd97-39.htm|date=1997-09-01|access-date=2010-04-25|publisher=National Weather Service Southern Region Headquarters|author=Todd Lindley|title=Effects of Texas Panhandle Topography on Dryline Movement}} However, a strong storm system can sweep the dry line eastward into the Mississippi Valley or Texas/Louisiana border, regardless of the time of day. Stronger dry line passages result in a sharp drop in dew point, clearing skies, and a wind shift from south or south-easterly to west or south-westerly. Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward during the evening and nighttime hours. Severe and sometimes tornadic thunderstorms often develop along the slope-reversal zone east of the surface dry line, especially when it begins moving eastward.
Associated weather
In the dry sector west of the dry line, clear skies are the rule due to the dryness of the air mass sweeping in from the Desert Southwest in North America,{{cite web|date=2006-10-11|access-date=2010-04-25|url=http://www.nssl.noaa.gov/primer/tornado/tor_climatology.html|title=Tornado Climatology|author=National Severe Storms Laboratory|publisher=National Oceanic and Atmospheric Administration|archive-url=https://web.archive.org/web/20120320044359/http://www.nssl.noaa.gov/primer/tornado/tor_climatology.html|archive-date=2012-03-20|url-status=dead|author-link=National Severe Storms Laboratory}} and the Aravalli range in India.{{cite journal|title=A Numerical Study of the Hydrometeorological Dryline in Northwest India During the Monsoon|author=Sen Chiao and Ana P. Barros|journal=Journal of the Meteorological Society of Japan|volume=85A|pages=337–361|issn=0026-1165|doi=10.2151/jmsj.85A.337|year=2007|bibcode=2007JMeSJ..85A.337C |doi-access=free}} If winds are strong enough, dust storms can develop. Cumulus clouds are common east of the dry line in the moist sector, though they are taller with greater development along the dry line itself.{{cite book|url=https://books.google.com/books?id=4VY39mJKmO0C&q=dry+line+weather+occurrence+outside+the+United+States&pg=PA70|author=Michael William Carr|title=International Marine's Weather Predicting Simplified: How to Read Weather Maps|year=1999|access-date=2010-04-25|page=70|publisher=McGraw-Hill Professional|isbn=978-0-07-012031-0}} The moist sector is normally capped with a lid of an elevated mixed drier layer which represents subsidence from aloft as the surface air cools and contracts at night. The same process promotes the development of a low level jet to the east of the dryline. During the daytime, if heating and/or convergence are sufficient, the cap can be broken, resulting in convective clouds.