Cold air damming
Encyclopedia
Cold air damming, or CAD, is a meteorological
phenomenon that involves a high pressure system
interacting with local geographic features. A cold-air damming situation typically involves a high pressure system located poleward of a mountain range. This allows for the spin of the high pressure to bring cold air in from the poleward direction and funnel it down a narrow stretch of land in front of the mountains.
Cold air damming typically happens in the mid-latitudes of the northern hemisphere, as this region is typically the southernmost limit of the northern jet stream
. Cold air damming is not widely observed in the southern hemisphere because of the lack of an appropriate arrangement of a mountain range in relation to the anticyclonic spin of the high pressure system. Some of the most common instances of cold air damming take place on the coastal plain of east-central North America, between the Appalachian Mountains
and Atlantic Ocean
. A typical situation here involves a high pressure area located in eastern Canada
or over northern New England
that pulls cold air from the region around Hudson Bay
or northern Quebec
and pushes it down the eastern side of the mountains, affecting the states from New York
to Georgia
, and sometimes even as far south as Alabama. Analogous situations can occur alongside the Rocky Mountains
over the western portions of the Great Plains
and various other mountain ranges (such as the Cascades
) along the west coast of the United States.
The effects of cold air damming become more prominent (and also more complicated) when a storm system interacts with the spreading cold air. This interaction is most noticeable on the East Coast of the United States
, as the combination of cold air damming and a moist low pressure system
is the most important component of a major winter storm. Depending on the strength of the cold air damming, the precipitation types and amounts can vary greatly across the affected region.
The effects of cold air damming east of the Cascades in Washington are strengthened by the bowl or basin-like topography of Eastern Washington
. Cold Arctic air flowing south from British Columbia
through the Okanogan River
valley becomes fills the basin, blocked to the south by the Blue Mountains
. Cold air damming causes the cold air to bank up along the eastern Cascade slopes, especially into the lower passes, such as Snoqualmie Pass
and Stevens Pass
. Milder, Pacific-influenced air moving east over the Cascades is often forced aloft by the cold air in the passes, held in place by cold air damming east of the Cascades. As a result, the passes often receive more snow than higher areas in the Cascades, which supports skiing at Snoqualmie and Stevens passes.
or sleet
, depending on the exact depths of the warm and cold layers. A thinner cold layer will lead to conditions favoring freezing rain, whereas a thicker or stronger cold layer will lead to sleet.
occurs when the northern high pressure system is very well established and is lying near or in the path of the advancing storm system. The blocking high pressure will funnel cold air down towards the oncoming storm as well as slow the storm's progress. If the high pressure is strong enough, it can even cause the storm to stall for a short period of time. Blocking situations can involve the presence of a wedge, though this is uncommon because of the typical strength and depth of the cold air the blocking high has put in place.
Blocking situations serve to effect precipitation amounts, and, by stalling storm systems, can greatly increase the storm's total precipitation over affected areas.
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
phenomenon that involves a high pressure system
High pressure area
A high-pressure area is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward due to the higher density air near their center and friction with land...
interacting with local geographic features. A cold-air damming situation typically involves a high pressure system located poleward of a mountain range. This allows for the spin of the high pressure to bring cold air in from the poleward direction and funnel it down a narrow stretch of land in front of the mountains.
Location
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...
. Cold air damming is not widely observed in the southern hemisphere because of the lack of an appropriate arrangement of a mountain range in relation to the anticyclonic spin of the high pressure system. Some of the most common instances of cold air damming take place on the coastal plain of east-central North America, between the Appalachian Mountains
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
and Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. A typical situation here involves a high pressure area located in eastern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
or over northern New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
that pulls cold air from the region around Hudson Bay
Hudson Bay
Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...
or northern Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
and pushes it down the eastern side of the mountains, affecting the states from New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
to Georgia
Georgia (U.S. state)
Georgia is a state located in the southeastern United States. It was established in 1732, the last of the original Thirteen Colonies. The state is named after King George II of Great Britain. Georgia was the fourth state to ratify the United States Constitution, on January 2, 1788...
, and sometimes even as far south as Alabama. Analogous situations can occur alongside 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...
over the western portions of 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...
and various other mountain ranges (such as the Cascades
Cascade Range
The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...
) along the west coast of the United States.
Effects
When cold air damming occurs, it allows for cold air to surge toward the equator in the affected area. In calm, non-stormy situations, the cold air will advance unhindered until the high pressure area can no longer exert any influence because of a lack of size or its leaving the area.The effects of cold air damming become more prominent (and also more complicated) when a storm system interacts with the spreading cold air. This interaction is most noticeable on the East Coast of the United States
East Coast of the United States
The East Coast of the United States, also known as the Eastern Seaboard, refers to the easternmost coastal states in the United States, which touch the Atlantic Ocean and stretch up to Canada. The term includes the U.S...
, as the combination of cold air damming and a moist low pressure system
Low pressure area
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low-pressure area is known as...
is the most important component of a major winter storm. Depending on the strength of the cold air damming, the precipitation types and amounts can vary greatly across the affected region.
The effects of cold air damming east of the Cascades in Washington are strengthened by the bowl or basin-like topography of Eastern Washington
Eastern Washington
Eastern Washington is the portion of the U.S. state of Washington east of the Cascade Range. The region contains the city of Spokane , the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Hanford Nuclear Reservation and the fertile farmlands of the Yakima Valley and the...
. Cold Arctic air flowing south from British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
through the Okanogan River
Okanogan River
The Okanogan River is a tributary of the Columbia River, approximately 115 mi long, in southern British Columbia and north central Washington...
valley becomes fills the basin, blocked to the south by the Blue Mountains
Blue Mountains (Oregon)
The Blue Mountains are a mountain range in the western United States, located largely in northeastern Oregon and stretching into southeastern Washington...
. Cold air damming causes the cold air to bank up along the eastern Cascade slopes, especially into the lower passes, such as Snoqualmie Pass
Snoqualmie Pass
Snoqualmie Pass is a mountain pass that carries Interstate 90 through the Cascade Range in the U.S. State of Washington. The elevation of the pass summit is , and is on the county line between Kittitas County and King County...
and Stevens Pass
Stevens Pass
Stevens Pass is a mountain pass through the Cascade Mountains located at the border of King County and Chelan County in Washington, United States....
. Milder, Pacific-influenced air moving east over the Cascades is often forced aloft by the cold air in the passes, held in place by cold air damming east of the Cascades. As a result, the passes often receive more snow than higher areas in the Cascades, which supports skiing at Snoqualmie and Stevens passes.
Situations
Cold air damming can produce a number of different atmospheric profiles, and therefore its effects can vary greatly. The majority of the effects are in terms of the precipitation type, though some effect on the precipitation amount are noticeable as well.The Wedge
The effect known as "the wedge" is the most widely known example of cold air damming. In this scenario, the southern storm system will bring warmer air with it above the surface (at around 1500 meters). This warmer air will ride over the cooler air at the surface, which is being held in place by the high pressure system to the north. This temperature profile will lead to the development of freezing rainFreezing rain
Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
or sleet
Sleet
Sleet refers to two distinct forms of precipitation:*Rain and snow mixed, snow that partially melts as it falls. *Ice pellets, one of three forms of precipitation in a US-style "wintry mix", the other two being snow and freezing rain. The latter is referred to as sleet mainly by the United States...
, depending on the exact depths of the warm and cold layers. A thinner cold layer will lead to conditions favoring freezing rain, whereas a thicker or stronger cold layer will lead to sleet.
Seeping
While not as well known as the wedge, seeping is another occurrence in a dammed situation. Seeping occurs when the southern storm system develops or begins to move northward before the high pressure serving as the cold air damming mechanism has time to fully establish the dam. In this instance, cold air will seep southward along the surface, causing areas of rain to turn towards freezing rain, sleet, and, in situations that are strong enough or long enough, the precipitation can turn entirely to snow.Blocking
BlockingBlock (meteorology)
Blocks in meteorology are large scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones...
occurs when the northern high pressure system is very well established and is lying near or in the path of the advancing storm system. The blocking high pressure will funnel cold air down towards the oncoming storm as well as slow the storm's progress. If the high pressure is strong enough, it can even cause the storm to stall for a short period of time. Blocking situations can involve the presence of a wedge, though this is uncommon because of the typical strength and depth of the cold air the blocking high has put in place.
Blocking situations serve to effect precipitation amounts, and, by stalling storm systems, can greatly increase the storm's total precipitation over affected areas.
Historical Examples
- Northeastern United States Blizzard of 1978Northeastern United States Blizzard of 1978The Northeastern United States blizzard of 1978 was a catastrophic and historic nor'easter that brought blizzard conditions to the New England region of the United States and the New York metropolitan area. The "Blizzard of '78" formed on February 5, 1978 and broke up on February 7, 1978...
- North American blizzard of 2003North American blizzard of 2003The Blizzard of 2003, also known as the Presidents' Day Storm II or simply PDII, was a historical and record-breaking snowstorm on the East Coast of the United States and Canada, which lasted from February 14 to February 19, 2003. It spread heavy snow across the major cities of the Northeastern US,...
- North American blizzard of 2006North American blizzard of 2006The Blizzard of 2006 was a nor'easter that began on the evening of February 11, 2006. It dumped heavy snow across the Northeast United States from Virginia to Maine through the early evening of February 12 and ended in Atlantic Canada on February 13...