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Adry line (also called a dew point line, orMarfa front, afterMarfa, Texas)[1] 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 centralNorth America, especially Texas, Oklahoma, and Kansas, where the moist air from theGulf of Mexico meets dry air from the desert south-western states. The dry line is an important factor insevere weather frequency in theGreat Plains ofNorth America. It typically lies north-south across theHigh Plains states in the warm sector of anextratropical cyclone and stretches into theCanadian Prairies during the spring and early summer.[citation needed] The dry line is also important for severe convective storms in other regions of the world, such as northernIndia andSouthern Africa.[2] In general, thunderstorms and other forms of severe weather occur on the moist side of the dryline.
Near the surface, warm dry air is denser than warm moist air of lesser or similartemperature, and thus the warm dry air wedges under the moist air like a cold front.[3][4]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 acold front. The dry line is most common in thespring.[5] Its location is close to the location of the 55 °F (13 °C) isodrosotherm, or line of equaldewpoint. The location of the dryline may not be marked with a surface pressuretrough or shift of thewind direction. It bulges more to the east underneath the location of the highest winds within thejet stream.[6] While dry lines are most common in theGreat Plains, northern India also witnesses a similar moisture boundary.[7] In northeast India, it occurs mainly before the onset of their summermonsoon,[8] while northwest India experiences it during the monsoon season.[9]
Adry punch ismeteorological slang for asynoptic scale ormesoscale 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 severethunderstorms.
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.[10] However, a strong storm system can sweep the dry line eastward into theMississippi Valley orTexas/Louisiana border, regardless of the time of day. Stronger dry line passages result in a sharp drop indew point, clearing skies, and awind shift from south or south-easterly to west or south-westerly. Blowingdust and risingtemperatures 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 sometimestornadicthunderstorms often develop along the slope-reversal zone east of the surface dry line, especially when it begins moving eastward.
In the dry sector west of the dry line, clear skies are the rule due to the dryness of theair mass sweeping in from theDesert Southwest inNorth America,[11] and theAravalli range in India.[9] If winds are strong enough,dust storms can develop.[7]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.[12] 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.[7]
