C h a p t t e r r E i i g h t t
A i i r r M a s s s s e s s , , F r r o n t t s s , , a n d M i i d d l l e L a t t i i t t u d e C y c l l o n e s s
Goal for this Chapter
We are going to learn answers to the following questions:
•
What are the different types of air masses?
•
How fronts are formed?
•
How can warm fronts cause freezing rain & sleet during winter
•
What are Lake-effect snow? How does it affect certain regions in Michigan?
•
What are characteristics of warm front, cold front and occluded front?
•
What is a polar front theory?
Air Masses
•
A large body of air that has similar horizontal temperature & moisture characteristics
•
Weather forecasting: Determination of air mass characteristics, predicting how and why they change, and in what direction the systems will move
•
Source Region: Regions where air masses originate are known as source regions
•
For a huge mass of air to develop uniform characteristics, its source region should be generally flat and of uniform composition, with light winds – ideal regions are ice- and snow-covered arctic plains in winter and subtropical oceans and desert regions in summer
•
•
Air masses are grouped into four categories based on their source region
–
Polar (P): Originate in polar latitudes
–
Tropical (T): Originate in warm tropical regions
–
Continental (c): Source region is land, air mass is dry
–
Water (m): source region is water
Air Masses of North Americal
Continental Polar (cP) & continental Arctic (cA): Bitterly cold weather enters US in the winter; originate over ice- and snow-covered regions of northern Canada and Alaska; the air in contact with the surface becomes very cold and stable with little moisture; portion of this air breaks away and moves southward as a shallow high pressure area
Average upper-level wind flow (heavy arrows) and surface position of anticyclones during two winter season
(minimum temperature)
•
Cities located in the east of the Applachian mountains usually do not experience temperatures as low as those on the west side (WHY??)- compressional heating increase air temp downwind side
• cP air in summer has properties much different than from its winter counterpart; air is moderately cool and brings relief from the oppressive heat at the central and eastern US
•
Cold, dry cP air moving over the Gulf of Mexico warms rapidly and gains moisture; its original characteristics are no longer discernable

Visible satellite image showing the modification of cP air as it moves over the warmer Gulf of Mexico and the Atlantic
Ocean
•
During winter, cP air originating over Asia and frozen polar regions is carried eastward and southward over the Pacific Ocean by the circulation around the Aleutian low; the ocean water modifies the cP air by adding warmth and moisture to it, gradually changing into a maritime polar air mass
•
When the mP air moves inland, it looses much of its moisture and eventually become a drier, stable air mass called Pacific air
•
Along the East Coast, mP air originates in the North Atlantic as cP air moves southward some distance off the Atlantic coast;
North Atlantic is very cold and air mass travels only a short distance over water, wintertime Atlantic mP air masses are much colder their Pacific counterparts
•
Atlantic mP air masses are much less common
A winter upper-air pattern that brings mP air into the west coast of North America; large arrow: the upper-level flow; small arrows: trajectory of the mP air at the surface
•
The wintertime source region for the Pacific maritime tropical air masses is the subtropical east Pacific Ocean;
•
Air masses are warm and moist by the time they arrive along the West Coast because air travels over many kms of water before it reaches
•
The mT air that influences much of the weather east of the Rockies originates over the Gulf of Mexico and
Caribbean Sea; in winter, cold polar air tends to dominate the continental weather scene, so mT air is usually confined to the Gulf and extreme southern states.
Maritime tropical air (heavy red arrows) moving into northern California on January 1, 1997 – IR satellite image
Weather condition during a hot spell on 17 April, 1976; Red: maximum temp.; blue: minimum reached; heavy arrow: average upper-level flow; L & H: average position of the upper-level trough & ridge mT –contd.
•
In the previous figure, upper-level flow is directing cP southward and mT air northward
•
Continental Tropical (cT) Air Masses: Source region for hot, dry cT in North America is found during the summer in northern Mexico & the adjacent and southwestern US
•
Summary: Characteristics of each air mass depend upon the air mass source region & the type of surface over which the air mass moves. The winds aloft determine the trajectories of these air masses cT air covered a large area of the central and western US during 29,30 th June, 1990; #: maximum temp (°F); H shows with the isobar shows the upper level position of the subtropical high; winds aloft are weak
•
Lake-effect Snow: Snowstorms that form on the downwind side of a lake
•
Snow storms are highly localized, extending from a few km to >50 km inland; one side of a city may get more than the other side due to this effect
•
Most numerous from November – January; greater the contrast in temperature, greater the potential for snow showers
•
Longer the stretch of water over which the air mass travels, greater the amount of warmth & moisture derived from the lake & greater the potential for heavy snow showers
•
In Late winter, the frequency and intensity of lake-effect snows taper off as the temp contrast between air and water diminishes and larger portions of the lakes freeze (shaded purple: experience heavy L-E effect)
Formation of a Lake-Effect Snow; cold, dry air crossing the lake gains moisture & warmth from the water; buoyant air rises and cloud forms leading to snowfall
•
Front: Transition zone between two air masses of different densities; density differences are caused by temperature differences, fronts separate air masses with contrasting temperatures
•
Frontal surface or frontal zone: Upward extension of a front
•
Stationary Fronts: Any front that stops moving is called stationary front; drawn as an alternating red and blue line; semicircle face toward colder air & triangles point toward warmer air

•
Surface winds tend to blow parallel to the front, but in opposite directions on either side of it; upper level winds blow parallel to a stationary front
A simplified weather map showing surface pressure systems, air masses, and front (Green-shaded: precipitation)
•
Cold Front: Between points B & C on the surface weather map where cold, dry, stable polar air is replacing warm, moist unstable subtropical air
•
Triangles along the front showing its direction of movement
•
Criteria to locate a front on a surface weather map:
–
Sharp temperature change over a relatively short distance
–
Changes in the moisture content (dew point temp)
–
Shifts in wind direction
–
Clouds and Precipitation patterns
–
Pressure and pressure changes
Surface weather associated with a cold front in the southeastern US; temperature (top), dew point (bottom), present weather, cloud cover, sea level pressure (/:rising;\), wind speed & direction
Cold Front – contd.
•
Large contrast in temp & dew point on either side
•
Winds shift from southwesterly ahead of the front to northwesterly behind
•
Each isobar kinks as crosses the front, forming an elongated area of low pressure – a trough – which accounts for the wind shift
•
At the front, a relatively narrow band of thunderstorms produces heavy showers with gusty winds; behind the front, the air cools quickly & precipitation ends
•
Slope of the front – Horizontal distance/vertical distance = 50 (typical for fast-moving front, with >25 knots)
•
There are exceptions to ‘typical’ cold fronts
.
•
Warm front is drawn along the solid red line running from points C to D
•
Average speed of warm front ~ 10 knots (~1/2 of cold front)
•
Horizontal/ vertical ~ 150-200
Start from P’ --- ~1,200 km away ahead of the surface front --- surface winds are light & variable --- front is moving towards P’ --- cirrus clouds gradually thicken; --- clouds become altocumulus (Ac) and altostratus (As); closer to the fronts, snowflakes start falling; sheet like covering of nimbostratus (Ns) clouds --- as we approach, snow changes to sleet, then become freezing rain and finally rain & drizzle as the temp climbs above freezing
Vertical view of clouds, precipitation, & Winds across the warm front along a line P-P’ – horizontal / vertical ~150-200
•
•
Occluded Front (occlusion): The frontal boundary formed when a cold front overtakes a warm front; in surface weather map, represented as purple line with alternating cold-front triangles &warm-front half circles; both symbols point in the direction toward which the front is moving
•
Cold occlusion: Air behind the front is colder than the air ahead of it; upper warm front follows the surface occluded front
•
Warm occlusion: Air behind the front is milder than the air ahead of it; in a warm occlusion, the upper-level cold front precedes the surface occluded front
•
Most violent weather usually occurs where the cold front is just overtaking the warm front, where the greatest contrast in temp occurs
Cold-occluded front; faster moving cold front (a) catches up the slower-moving warm front (b) and forces it to rise off the ground (c)
Warm-occluded front; faster moving cold front in (a) overtakes the slower-moving warm front in (b); surface map of the situation (c)

•
Polar Front Theory: A model to explain the life cycle of an extratropical storm; stages of birth, growth, & decay of mid-latitude cyclones; how weather develops along the polar front
•
Useful to describe the structure & weather associated with a migratory storm system
•
Consider a segment of the polar front as a stationary front --- cold air to the north and warm air to the south flow parallel to the front, but in opposite direction
•
Flow in opposite direction sets-up a shear; under a set of conditions, wavelike kink forms on the front (b); the wave forms is known as Frontal Wave
•
In b) we see the newly formed wave with a cold front pushing southward and warm front moving northward
•
In c): steered by wind aloft, the system moves east or northeast and gradually becomes a fully developed open wave in 12-24 hours; central pressure is lower and more closely-spaced isobars develop
•
In d): as the waves moves eastward, central pressures continue to decrease, and the winds blow more vigorously; cold front moves faster and moves closer to warm front; eventually cold front overtakes the warm front & the system becomes occluded
•
In e): storm becomes intense, with clouds & precipitation covering a larger area
•
In f): the intense storm gradually dissipates, because cold air lies on both side of the occluded front
•
Entire life cycle of a wave cyclone can last from a few days to over a week
•
Family of cyclones: Succession of storms that are various stages of development along the polar front in winter
•
Cyclogenesis: Development or strengthening of a mid-latitude cyclone
•
Regions of cyclogenesis in US: Eastern slopes of the Rockies (leeward), Gulf of Mexico, Atlantic ocean east of the
Carolinas, and Great Basin
•
Some waves develop into huge storms whereas others simply dissipate in a day or so – Answer lies in the upperwind flow, in the region of the high-level westerlies
•
Convergence: Piling up air; causes air density to increase directly above the surface low
•
Divergence: Will remove air from the column directly above the high; surface pressure falls and the system weakens
•
If upper-level (or low-level) pressure systems were always located directly above those at the surface would die out soon after they form
•
In two regions, we have H & L in the surface air; in the upper air we have convergence & divergence; surface winds are converging about the center of the low while aloft, the winds are diverging; for the surface low to develop into a major storm system, upper-level divergence must be greater than the surface convergence of air; more air must be removed above the storm than is brought in at the surface --- storm system is intensifying or deepening;
•
If the reverse of above takes place, the storm system will dissipate in a process called ‘filling’
Convergence, divergence, and vertical motions associated with surface pressure systems; for the surface storm to intensify, upper trough of low pressure must be located to the left (or west) of the surface low
•
When the surface winds are diverging about the center of the high, directly above the anticyclone, they are converging; in order for the surface high to strengthen, upper-level convergence of air must exceed low-level divergence of air; when this occurs, surface pressure increases and the high pressure is ‘building’

•
For a surface storm to intensify, the upper-level trough of low pressure must be located behind (to the west) of the surface low; when the upper-level trough is in this position, the atmosphere is able to redistribute its mass, as regions of low-level convergence are compensated for by regions of upper-level divergence, and vice versa
•
Jet Streak: A region of high wind speed that moves through the axis of a jet stream
•
Jet stream removes air above the surface cyclone and supplying air to the surface anticyclone; sinking of cold air and rising of warm air provide energy for the developing cyclone as PE is converted into KE
•
Summary: For a storm to intensify, there must be an upper-level counterpart – a trough of low pressure that lies to the west of the surface low
•
Horizontal & vertical motions, cloud patterns and weather that typically occur with a developing open-wave midlatitude cyclone are shown in the next figure
Summary of clouds, weather, vertical motions and upper-air support associated with a developing wave cyclone
•
Source region of air mass, cP, cA, mT, mP & their characteristics (temperature, humidity/moisture content, seasons when they are active, etc)
•
Air mass responsible for the summer rainshowers, thunderstorms in FL, southwest, Sierra Nevada mountains in
CA, and other Gulf Coast regions
•
Cold front, warm front, occluded front – how they are formed, properties; how they are shown in weather maps
•
Lake-effect snow – when, where, why?
•
Stationary front – why does not move?; thunderstorms in winter will form along which front?
•
Energy for a developing wave cyclone;
•
Convergence, divergence, cyclogenesis
Summary – contd.
•
Building of a anticyclone
•
When will a storm intensify?