Dry line
Encyclopedia
A dry line 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
.
, and thus the warm dry air wedges under the humid air like a cold front. 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 55 °F (12.8 °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
. 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.
/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 temperature
s 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
thunderstorm
s often develop along the slope reversal zone east of the surface dry line, especially when it begins moving eastward.
sweeping off the Rocky mountains
in North America
, and the Aravalli range
in India. If winds are strong enough, dust storm
s can develop. Cumulus cloud
s 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 dry layer, which can extend well downwind and poleward of their source region, which are Earth's desert
s. 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.
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
(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
Severe weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
frequency in the Great Plains
Great Plains
The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...
of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. It typically lies north-south across the High Plains
High Plains (United States)
The High Plains are a subregion of the Great Plains mostly in the Western United States, but also partly in the Midwest states of Nebraska, Kansas, and South Dakota, generally encompassing the western part of the Great Plains before the region reaches the Rocky Mountains...
states in the warm sector of an extratropical cyclone
Extratropical cyclone
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
and stretches into the Canadian Prairies
Canadian Prairies
The Canadian Prairies is a region of Canada, specifically in western Canada, which may correspond to several different definitions, natural or political. Notably, the Prairie provinces or simply the Prairies comprise the provinces of Alberta, Saskatchewan, and Manitoba, as they are largely covered...
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
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
.
Characteristics
Near the surface, warm dry air is denser than warm moist air of lesser temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, and thus the warm dry air wedges under the humid air like a cold front. 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
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...
. The dry line is most common in the spring
Spring (season)
Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...
. Its location is close to the location of the 55 °F (12.8 °C) isodrosotherm, or line of equal dewpoint. The location of the dryline may not be marked with a surface pressure trough
Trough (meteorology)
A trough is an elongated region of relatively low atmospheric pressure, often associated with fronts.Unlike fronts, there is not a universal symbol for a trough on a weather chart. The weather charts in some countries or regions mark troughs by a line. In the United States, a trough may be marked...
or shift of the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
direction. It bulges more to the east underneath the location of the highest winds within the jet stream
Jet stream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...
. While dry lines are most common in the Great Plains
Great Plains
The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...
, northern India also witnesses a similar moisture boundary. In northeast India, it occurs mainly before the onset of their summer monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
, while northwest India experiences it during the monsoon season.
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. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley or TexasTexas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
/Louisiana
Louisiana
Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...
border, regardless of the time of day. Stronger dry line passages result in a sharp drop in dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
, clearing skies, and a wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
shift from south or south-easterly to west or south-westerly. Blowing dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
and rising temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s 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
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s 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 massAir mass
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or maritime...
sweeping off the Rocky mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, and the Aravalli range
Aravalli Range
The Aravalli Range literally meaning 'line of peaks', is a range of mountains in western India and eastern Pakistan running approximately 800 km from northeast to southwest across states of Rajasthan, Haryana, and Gujarat and Pakistani provinces of Punjab and Sindh...
in India. If winds are strong enough, dust storm
Dust storm
A dust / sand storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, causing soil to move from one place and deposition...
s can develop. Cumulus cloud
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...
s 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 dry layer, which can extend well downwind and poleward of their source region, which are Earth's desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
s. Clouds such as altocumulus
Altocumulus cloud
Altocumulus is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection...
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.