A dry line (also called a dew point line, or Marfa front, after Marfa, Texas)
/ref> In general, thunderstorms and other forms of severe weather occur on the moist side of the dryline.
Characteristics
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 Wiley Online Library, March 18, 2010]
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.
Its location is close to the location of the 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.
While dry lines are most common in the
Great Plains, northern India also witnesses a similar moisture boundary.
In northeast India, it occurs mainly before the onset of their summer
monsoon,
while northwest India experiences it during the monsoon season.
Dry punch
A
dry punch is
meteorology 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
.
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.
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
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
tornado 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,
and the
Aravalli range in India.
If winds are strong enough,
can develop.
are common east of the dry line in the moist sector, though they are taller with greater development along the dry line itself.
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.
See also
