Dry line

Idealized depiction of where a dryline is located around an extratropical cyclone

A dry line (also called dew point line, or Marfa front)^[1] separates moist air from the Gulf of Mexico (to the east) and dry desert air from the south-western states (to the west). 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. The dry line is also important for severe convective storms in other regions of the world, such as northern India.

Characteristics

Near the surface, warm dry air is denser than warm moist air of lesser temperature, and thus the warm dry air wedges under the humid air like a cold front.^{[2][not in citation given (See discussion.)]} 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.^[3] Its location is close to the location of the 55 °F (13 °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.^[4] While dry lines are most common in the Great Plains, northern India also witnesses a similar moisture boundary.^[5] In northeast India, it occurs mainly before the onset of their summer monsoon,^[6] while northwest India experiences it during the monsoon season.^[7]

Daily progression

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.^[8] 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 off the Rocky mountains in North America,^[9] and the Aravalli range in India.^[7] If winds are strong enough, dust storms can develop.^[5] 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.^[10] The moist sector is normally capped with a lid of an elevated mixed dry layer, which can extend well downwind and poleward of their source region, which are Earth's deserts. Clouds such as altocumulus or altocumulus castellanus can develop at the vertical boundary between the low-level moisture and the dry mixed layer above the low-level moist air mass east of the dry line.^[5]

See also

• Surface weather analysis
• Weather front

References

1. ^ Scott Girhard (2007-05-04). "Lecture 3 - Thunderstorms". San Antonio College. Archived from the original on 2007-09-27. http://web.archive.org/web/20070927000112/http://www.accd.edu/sac/earthsci/sgirhard/1370.090/chap3.htm. Retrieved 2008-03-15. 
2. ^ Huaqing Cai (2001-09-24). "Dryline cross section". University of California Los Angeles. http://www.atmos.ucla.edu/~caihq/pic/fig23.html. Retrieved 2006-12-05. 
3. ^ Glossary of Meteorology (June 2000). "Dryline". American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?p=1&query=dryline&submit=Search. Retrieved 2010-04-25. 
4. ^ Daniel Dix (June 2000). "Dryline Thunderstorms". The Weather Channel. http://home.earthlink.net/~rhulecki/drylinetstorms/Dryline.html. Retrieved 2010-04-25. 
5. ^ ^{a b c} T. N. Carlson (1991). Mid-Latitude Weather Systems. HarperCollinsAcademic. pp. 449–481. ISBN 0045511160. 
6. ^ K. J. Weston (1972-03-07). "The dry-line of Northern India and its role in cumulonimbus convection". Quarterly Journal of the Royal Meteorological Society 98 (417): 519–531. Bibcode 1972QJRMS..98..519W. doi:10.1002/qj.49709841704. http://www3.interscience.wiley.com/journal/113521473/abstract?CRETRY=1&SRETRY=0. Retrieved 2010-04-25. 
7. ^ ^{a b} Sen Chiao and Ana P. Barros. "A Numerical Study of the Hydrometeorological Dryline in Northwest India During the Monsoon". Journal of the Meteorological Society of Japan 85A: 337–361. doi:10.2151/jmsj.85A.337. ISSN 0026-1165. http://www.jstage.jst.go.jp/article/jmsj/85A/0/85A_337/_article. 
8. ^ Todd Lindley (1997-09-01). "Effects of Texas Panhandle Topography on Dryline Movement". National Weather Service Southern Region Headquarters. http://www.srh.noaa.gov/topics/attach/html/ssd97-39.htm. Retrieved 2010-04-25. 
9. ^ National Severe Storms Laboratory (2006-10-11). "Tornado Climatology". National Oceanic and Atmospheric Administration. http://www.nssl.noaa.gov/primer/tornado/tor_climatology.html. Retrieved 2010-04-25. 
10. ^ Michael William Carr (1999). International Marine's Weather Predicting Simplified: How to Read Weather Maps. McGraw-Hill Professional. p. 70. ISBN 9780070120310. http://books.google.com/books?id=4VY39mJKmO0C&pg=PA70&lpg=PA70&dq=dry+line+weather+occurrence+outside+the+United+States#v=onepage&q=dry%20line%20weather%20occurrence%20outside%20the%20United%20States&f=false. Retrieved 2010-04-25.