1. Chapter 10 Weather Systems of Middle Latitudes
Weather Studies Introduction to Atmospheric Science American Meteorological Society
Chapter 10
Weather Systems of Middle Latitudes
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-In-Point
Extra-tropical cyclones are major weather makers in middle and high latitudes
In 1703, Daniel Defoe was the first to propose that storms generally track from west to east in middle latitudes
In 1743, Benjamin Franklin was the first American to discover that storms usually move in an easterly or northeasterly direction
Based on observations, he concluded that wind direction in a storm was not an indication of the storm’s direction of movement

3. Driving Question
What systems shape the weather of the middle latitudes?
Middle latitudes extend from Tropics of Cancer and Capricorn, poleward to the Arctic/Antarctic Circles
Weather is particularly dynamic because of the migration of cyclones and anticyclones embedded in the prevailing westerlies
This chapter examines:
Air masses, fronts, cyclones, and anticyclones
Local and regional circulation systems

4. Air Masses
An air mass is a huge expanse of air covering thousands of square kilometers, and is relatively uniform horizontally in temperature and water vapor concentration (humidity)
Abbreviations for air mass types:
Cold (polar or P) or warm (tropical or T)
Dry (continental or c) or humid (maritime or m)
Arctic (A) air
Air mass source regions have nearly homogeneous surface characteristics over a broad area with little topographic relief
The air mass stays over the source region for an extended period, and takes on the characteristics of the source region

5. North American Types and Source Regions

6. North American Air Masses

7. Air Masses Modification of Air Masses
Air masses eventually move out of their source region
As they move, their properties are modified by the surface they pass over
Air mass modification occurs from:
Exchange of heat or moisture, or both, with the surface over which the air mass travels
Radiational heating or cooling
Adiabatic heating or cooling associated with large-scale vertical motion

8. Air Masses Modification of Air Masses, continued
In winter, as a cP air mass travels southeastward from Canada into the lower 48-states, its temperature usually modifies rapidly
By the time it arrives in the southern states, temperatures will not usually drop much below freezing
The sun warms snow-free ground, and the warmer ground heats the bottom of the air mass. Heat is then distributed vertically.
A similar process of heating and destabilization occurs when a cP air mass crosses the East Coast and moves over the western Atlantic. Evaporation from the sea surface leads to extensive areas of low clouds and fog
cP traveling over snow-covered ground experiences less modification
Much of the incoming solar radiation is reflected rather than being absorbed

9. Air Masses Modification of Air Masses, continued
Tropical air masses modify less than polar masses
They are often warmer than the ground they travel over
The bottom of the air mass cools, and stabilizes
Convective currents are suppressed
If a tropical air mass moves over a warmer surface, the air mass can become even warmer
Air masses undergo significant modification through orographic uplifting (e.g., mP air mass sweeping inland from the Pacific Ocean)
Rising air cools adiabatically, condensation/deposition occur, and precipitation is triggered on the windward slopes
Descent on the leeward side leads to adiabatic warming and cloud dissipation
Air mass emerges considerably milder and drier (e.g., modified Pacific air)

10. Frontal Weather
A front is a narrow zone of transition between air masses that differ in density
Density differences are usually due to temperature contrasts, hence the names cold fronts and warm fronts
Density differences may also be caused by humidity contrasts
A front’s transition zone may be 100+ km and a line representing a front on a weather map is drawn along the warm edge of the zone
A front is also associated with a trough in the sea-level pressure pattern, a corresponding wind shift, and converging winds
When air masses meet at fronts, the colder, denser air forces the warmer, less dense air to rise
This induces adiabatic cooling and often cloud/precipitation development
The slope of the front influences the types of clouds that form
Cold and warm fronts have different slopes associated with them

11. Frontal Weather Stationary Front
A front that exhibits essentially no lateral motion
This often happens along the Front Range of the Rocky Mountains when a shallow pool of polar air surges south over the plains and the leading edge is too shallow to cross the mountains. Milder air remains in the Great Basin to the west of the Rockies.
May also develop when a preexisting front becomes parallel to the upper-level flow pattern or along a boundary in the surface temperature pattern
Typical front
Slopes from Earth’s surface towards denser air
Lies in a trough in the pressure pattern
Wind changes direction rather abruptly across the front
May have broad region of clouds and precipitation (e.g., overrunning)

12. Stationary Front

13. Frontal Weather Warm Front Warm air advances while cold air retreats
Overall characteristics very similar to a stationary front
As a warm front approaches:
Clouds thicken and become lower in altitude
Sequence is cirrus, cirrostratus, altostratus, nimbostratus, and stratus when the advancing warm air is relatively stable
Initial cirrus appearance may be more than 1000 km (620 mi) ahead of the front
Just ahead of the front, steady precipitation usually gives way to drizzle and sometimes frontal fog
If advancing warm air is unstable, more vigorous uplift can occur with thunderstorms embedded in the overrunning zone

14. Warm Front
Cirrus clouds

15. Frontal Weather Cold Front Colder air displaces warmer air
In North America in winter, the temperature contrast along a cold front is usually greater than across a warm or stationary front
In summer, temperatures on either side of the front may be essentially the same
Density contrasts arise because of humidity differences
The slope on a cold front is much steeper than the slope on a warm front
Uplift is confined to a narrow area at or near the cold front’s leading edge
If the warm air is unstable, thunderstorms may form and a squall line can develop
If the warm air is relatively stable, nimbostratus and altostratus may form

16. Cold Front

17. Advancing Back-Door Cold Front
A cold front generally trails south or southwestward from the center of an extra-tropical cyclone. Back-door cold fronts move south along the eastern side of the Appalachian Mountains.

18. Frontal Weather Occluded Fronts
Typically form late in a cyclone’s life cycle as it moves into colder air
Faster moving cold front catches up with the warm front
There are 3 types of occlusions, distinguished by the temperature contrast between the air behind the cold front and ahead of the warm front
Cold occlusion
Air behind cold front colder than cool air ahead of warm front
Like a cold front at the surface but, with less air mass contrast
Warm occlusion
Air behind cold front is not as cold as the air ahead of the warm front
Like a warm front at the surface
Neutral occlusion
Little difference between air masses
Marked by a trough, wind shift line, band of cloudiness & precipitation

19. Cold-Type Occlusion
Air behind advancing cold front colder than cool air ahead of warm front
More common in eastern North America, where the colder air follows behind the front on northwest winds

20. Warm-Type Occlusion
Air behind the advancing cold front is not as cold as the air ahead of the warm front
Occurs in northerly portions of western coasts, such as in Europe or the Pacific Northwest, where mP air is behind the cold front

21. Air Masses
Summary
Fronts are characterized based on the movement of the cold air mass
Clouds and precipitation may develop along fronts when there is a significant density contrast between air masses and there is an adequate supply of water vapor
Properties that define a front are differences in temperature and humidity, wind shift, convergence, and a pressure trough
Frontogensis: front forms or grows stronger
Frontolysis: front weakens

22. Extra-tropical Cyclones
The extra-tropical cyclone (also called a low-pressure system or low), is a major weather maker of middle and high latitudes. Surface winds blow counterclockwise and inward. Surface winds converge, air rises, expands, and cools, resulting in clouds and precipitation.

23. Extra-tropical Cyclones
The comma-shaped cloud pattern is characteristic of a well-developed extra-tropical cyclone.

24. Extra-tropical Cyclones
Life Cycle
Norwegian cyclone model: conceptual model originally developed around WWI still closely approximates our current understanding
(A) Incipient cyclone: Cyclogenesis (birth of a cyclone) usually takes place along the polar front directly under an area of strong horizontal divergence in the upper troposphere
Air pressure at the bottom of the air column falls, a horizontal air pressure gradient develops, and cyclonic circulation begins
Westerlies aloft steer and support the cyclone as it progresses through its life cycle
West of the low center, the polar front pushes southeast as a cold front. East of the low, the polar front advances north as a warm front.

25. Extra-tropical Cyclones
warm sector
Life Cycle
(B) Wave cyclone: In this stage, the central pressure continues to drop and winds strengthen due to an increased pressure gradient. The upper-level trough deepens while remaining west of the surface low center.
Warm sector becomes better defined
Fronts form a pronounced wave pattern and comma cloud is seen in satellite images
Extensive stratiform cloudiness appears north of the warm front
Cyclone moves eastward or northeastward at km per hr (25-35 mph)

26. Extra-tropical Cyclones
Life Cycle
(C) Beginning of occlusion
Faster moving cold front advances on the warm front
Warm sector area diminishes and occluded front begins to form
Upper level pattern shows closed circulation and is directly over the surface low (vertically stacked)
Dry slot separates the cold front cloud band from the comma cloud
Cyclone moves slower at approximately 30 km per hr (20 mph)

27. Extra-tropical Cyclones
triple point
Life Cycle
(D) Bent-back occlusion
Surface low may become detached from the westerly steering flow and the occluded front is drawn around the low center
Warm sector is detached from cyclone center
Triple point favors development of a secondary cyclone
Eventually the cyclone weakens (cyclolysis)

28. Extra-tropical Cyclones
Entire cycle can occur over several days, or a much shorter period
Speed of development depends on upper air support
Weak divergence aloft will cause poorly defined systems
Sometimes cloudiness and precipitation occur with an upper-level or surface trough, which is not associated with a closed surface cyclonic circulation
When upper-level conditions are ideal, the entire life cycle can occur in less than 36 hours

29. Extra-tropical Cyclones
Cyclone Bomb
This is the term applied to a rapidly intensifying cyclone, and is defined as a central pressure drop of at least 24 mb in 24 hours
Few cyclones meet this criteria, and most that do occur in winter over a warm ocean surface current (e.g., Gulf Stream)
Conveyor Belt Model
This is an alternate, 3-D model to the steps discussed previously (Norwegian cyclone model)
Combines horizontal and vertical air motions
Depicts the circulation in a mature cyclone in terms of three broad interacting systems called conveyor belts, which transport air with certain properties from one location to another
Belts are (1) warm and humid, (2) cold, and (3) dry

30. Extra-tropical Cyclones
Conveyor Belt Model, continued
(1) Warm and humid conveyor belt originates in the cyclone’s warm sector
Ascends slightly as it flows northward in the warm sector at low levels and then ascends more rapidly over the warm front
Helps explain the broad region of clouds/precipitation north of the warm front
(2) Cold conveyor belt originates north of the warm front
Ascends as it progresses toward the west
Forms the comma cloud and produces precipitation
Turns clockwise at upper levels and follows westerly flow aloft
(3) Dry conveyor belt
This air originates high in the troposphere and low stratosphere upstream of the upper-level trough
One branch descends southward behind the cold front; the other forms the dry slot that separates the head & tail of the comma cloud

31. Conveyor Belt Model

32. Extra-tropical Cyclones Cyclone Weather
Figure below represents an intensifying cyclone in the Upper Midwest
Four sectors about the low center:
Strong cold air advection, stratiform clouds, and non-convective precipitation northwest of the low
Cold front south of low is accompanied by convective precipitation. Sinking air and mostly clear skies characterize the southwest sector behind the cold front.
The mildest air is in the southeast (warm) sector of the cyclone
An extensive overrunning zone is found to the northeast of the low center

33. Principal Cyclone Tracks
As a general rule, the cyclone center moves forward in the same direction and at about one-half the speed of the 500-mb winds
Principal storm tracks tend to converge toward the northeast
Storm tracks appear to originate just east of the Rocky Mountains, but actually form over the Pacific Ocean near Alaska
As a cyclone travels over the mountains, it often loses its identity, but reforms over the Great Plains
Nor’easters often intensify off the North Carolina coast and track toward the northeast along the East Coast
2 motions exist
Movement of the cyclone along the coast
Counterclockwise flow of winds around the storm center; winds in northeast sector of the cyclone blow from the northeast (gives the name “nor-easter”)
Some may become powerful systems; drawing copious amounts of water vapor from the ocean and producing large amounts of precipitation over a broad area
Generally, cyclones that form in the south yield more precipitation because they have access to greater amounts of mT air
Cyclogenesis is more frequent in the winter when the mean position of the polar front and jet stream shift southward

34. Principal Cyclone Tracks

35. Extra-tropical Cyclones
Cold Side/Warm Side
Storm track determines weather at points on the ground
Track A puts Chicago on the warm side with passage of the warm and cold fronts
Track B puts Chicago on the cold side with no frontal passage
Table summarizes the general sequence of weather conditions at Chicago

36. Extra-tropical Cyclones
Winter Storms
An extra-tropical cyclone that produces any combination of frozen or freezing precipitation
An associated hazard is a cold wave, which often follows a winter storm
Necessary ingredients include cold air (typically brought in by a sprawling cold high to the north), a moisture supply, and uplift mechanisms
A major storm requires warm and humid air brought northward
A storm moving to the northeast produces heaviest snow to the north and west of the low center
Blizzard: a severe storm characterized by high winds and reduced visibility due to falling or blowing snow

37. Colorado-track Winter Storm System

38. Extra-tropical Cyclones Cold and Warm Core Systems
An occluded cyclone is a cold-core system
Lowest temperatures occur in a column just above the surface low
Depth of low increases with altitude
Cyclonic circulation prevails throughout the troposphere and is most intense at high altitudes
The requirement that thickness (mean temperature) be lowest at the low center produces the classic isobar pattern

39. Extra-tropical Cyclones Cold- and Warm-Core Systems
A non-occluded cyclone is a warm-core system
Lowest temperatures are northwest of the cyclone’s center, and highest temperatures are to the southeast
Low aloft is displaced to cold side of the storm
The system tilts with altitude
Upper-level low lags behind surface low
Vertical cross-section of a low from northwest (cold) to southeast (warm)

40. Extra-tropical Cyclones Cold and Warm Core Systems
Warm-core cyclone (thermal low)
Stationary, have no fronts, and are generally associated with fair weather
From over a broad expanse of arid/semiarid land in response to intense solar heating of the ground
Hot surface heats the overlying air and lowers the density of the air column enough for a low to form
Usually very shallow
Anticyclone aloft overlies low

41. Anticyclones
In anticyclones, subsiding air and diverging surface winds favor formation of a uniform air mass, no fronts, and generally fair skies
Arctic and Polar Highs (cold-core anticylone)
Labeled either a polar high (cP air) or arctic high (A air) and are products of extreme radiational cooling, often over snow-covered land
Clockwise circulation weakens with altitude, and may reverse
Usually has a cold trough overlying it
These exert the highest pressure in winter
They are extremely stable, with an inversion in the lower km or so
Interact with the circulation of an extra-tropical cyclone by helping to maintain and strengthen the temperature contrast along the cyclone’s cold front

42. Anticyclones Warm High (warm-core anticyclone)
Forms south of the polar front and consists of extensive areas of subsiding warm and dry air
These strengthen with altitude
Examples are Bermuda-Azores high and systems that may develop over the interior of North America, especially in summer
The greater mass of air over the anticyclone center (related to a higher tropopause) is responsible for the high surface pressure
A cold-core anticyclone can become warm-core as it moves south and modifies

43. Anticyclones Anticyclone Weather
Fair weather system because surface winds blowing in a clockwise and outward direction (Northern Hemisphere) induce subsidence over a broad area
Arctic highs produce the lowest temperatures of winter
A stalling warm anticyclone can lead to drought and excessive summer heat
A weak horizontal air pressure gradient near the center leads to intense nighttime radiational cooling
Ahead of an anticyclone, there may be strong northwest winds ushering in polar or arctic air
May bring heavy lake-effect snows to the lee side of the lakes
In the summer, the most noticeable effect is not a lowering of temperatures, but a lowering of humidity
Highs may become entrenched east of the Rockies in summer, and form blocking highs
High temperatures and eventually drought result

44. Local and Regional Circulation Systems Land and Sea (or Lake) Breezes
Sea Breeze
Under exposure to the same intensity of solar radiation, the land surface warms more than the water surface
Highest pressure over water, and cool breeze sweeps inland
Shallow circulation has maximum strength in mid-afternoon
Uplift may lead to thunderstorms
Land Breeze
By late evening, winds blow offshore due to a reversal in the heat differential between land and water
Obtains maximum strength around sunrise but is weaker than a sea breeze

45. Local and Regional Circulation Systems Chinook Winds
A relatively warm and dry wind that develops when air descending the leeward slopes of a mountain range is adiabatically compressed
Strong winds cause stable air in the lower troposphere to ascend on the windward side
On the leeward side, stable air descends to the original altitude, and the larger scale of circulation causes further descent
Called Santa Ana winds in southern California
The figure is a schematic representation of the surface weather pattern that favors development

46. Local and Regional Circulation Systems Chinook Winds
Boulder, CO, situated in the foothills of the Rocky Mountains, experiences particularly strong and destructive downslope winds, sometimes gusting to 160 km per hr (100 mph)
On average, the community sustains about $1 million in property damage each year due to these winds

47. Local and Regional Circulation Systems Desert Winds
Hot surfaces (i.e., deserts) may develop superadiabatic lapse rates in the lowest levels of the atmosphere
These are highly unstable, and generate vigorous upwelling and gusty surface winds, but very few clouds form
A dust devil is a whirling mass of dust-laden air formed by localized hot spot
Air is heated, and rises rapidly
Cooler surface winds converge on the hot spot
Horizontal wind shear causes the column of rising hot air to spin about a nearly vertical axis
Dust and debris are picked up, making these visible to altitudes topping 900 m (3000 ft)
May cause damage, as some have winds as higher than 75 km per hr (45 mph)
Strong thunderstorm downdrafts may generate dust storms known as a haboob

48. Local and Regional Circulation Systems Mountain or Valley Breezes
Bare valley walls absorb solar radiation and heat the surrounding air
Cooler, denser air over the valley sinks and air adjacent to the valley walls blows upslope
Cumulus clouds may form near summit
Best developed between late morning and sunset
Mountain Breeze
Bare valley walls are chilled by radiational cooling and cool the surrounding air
Colder, denser air near the valley walls sinks and gusty breeze blows downslope
Fog or low stratus clouds may form in the valley