1. Unit 2: Synoptic Scale (Regional) Weather & Climate
Length: ~1000km (~600miles) to ~6000km (~3500miles)
~Length of Alabama to the length of the U.S.
Time: Hours to Days (up to 1 week)
So far we’ve been talking about more global scale weather & climate
Synoptic scale includes air masses, fronts, mid-latitude cyclones, large hurricanes, large scale precipitation & temperature patterns

2. Airmasses and fronts

3. Air masses
An airmass is a large (usually thousands of km across) volume of air that has horizontally uniform properties of temperature and moisture.
Air masses acquire their properties from spending days to weeks over the same part of the Earth.
“Polar” air masses are colder than “tropical” air masses
“Maritime” air masses are wetter than "continental" air mass
Other specific air mass types include "arctic", "equatorial", and “monsoon”

4. Bergeron classification of air masses
3 letters: e.g. mTk, cPw
1st letter for moisture properties: c - continental, m - maritime
2nd letter for thermal characteristics: T - tropical, P -polar, A - Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air formed by significant downward motion in the atmosphere)
3rd letter for stability: k/w - air colder/warmer than ground

5. Source regions
The areas where air masses form are called source regions.

6. Air Masses on Weather Maps

7. Surface weather analysis
is a special type of weather map summarizing the information from all weather stations.

8. Wind speed code 5 10 50

9. Present weather
Type
Intensity

10. Fronts A weather front is a boundary separating two air masses
Types: cold front, warm front, occluded front, stationary front, dry line, squall line

11. What is an atmospheric front?
A front is a transition zone between two air masses of different densities.
The density contrast results from:
Difference in temperature;
Difference in humidity.
The frontal zone (surface) is the
upward extension of the front.
Sometimes the frontal zones can be very sharp.
The intensity of the weather along the front depends on the contrast of the air mass properties.
The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass.

12. Types of Fronts
Cold front: cold, dry stable air is replacing warm, moist unstable air.
Warm front: warm, moist unstable air is replacing cold dry stable air.
Stationary front: boundary between the two air masses is not moving.
The symbols on a map are in the direction of the air mass motion.
Occluded front: when a cold front catches up with a warm front

13. Weather Map
Shown: surface-pressure systems, air masses, fronts, isobars, winds and air flow (large arrows)
Green-shaded area: precipitation

14. What are the Signs of a Passing Front?
Sharp temperature changes over a relatively short distance.
Changes in the air’s moisture content (as indicated by changes in the dew point).
Shifts in wind direction
Pressure and pressure changes.
Clouds and precipitation patterns.
The location of the front is not always very obvious! Even meteorologists sometimes disagree…

15. Cold Fronts
A cold front is a mass of cold air advancing towards warm air.
Typically associated with heavy precipitation, rain or snow, combined with rapid temperature drops.
Since friction decreases with height, winds move faster at higher altitude. Then the surface of cold front becomes more steeper through time, leading to a narrow belt of precipitation.
Moving speed 0-30mph

16. Cold Fronts
Cold front- a front in which cold air is replacing warm air at the surface.
Notice the difference in
Temperature
Dew point
Wind direction
Pressure
Associated with low pressure centers (low pressure troughs): follow the dashed line
The pressure is minimum as the front passes (first decreases as the front approaches and then increases behind the front)

17. Satellite and radar images of cold fronts (narrow belt of clouds/precipitation)

18. Cold Front: cloud and precipitation patterns
The warm, moist air ahead of the front is forced upward and condenses
Cirrus clouds well ahead of the front
Strong thunderstorms with heavy showers and gusty winds along and ahead of the front: squall lines
Broad area of cumulus clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front).

19. Warm Fronts Warm fronts are warm air moving towards cold air.
This overrunning process produces large amounts of warm, moist air over cooler, drier air.
Shallow stratus clouds dominate and bring light precipitation to affected regions. Stable regions above the warmer air create vertically limited clouds and light precipitation. Frontal fogs may occur as rain evaporates in the colder air near the surface.
Moving speed about 12 mph

20. Warm Fronts
Warm front - in which warm air replaces cooler air at the surface
Notice the difference in
Temperature
Dew point
Wind direction
Pressure
Notice the presence of precipitation well ahead of the front

21. Slope of warm Fronts
Friction decreases with height, so winds move faster at higher altitude
This causes the surface of the front to become less steep through time. Then clouds will be spread to a wider region.

22. Warm Fronts: cloud and precipitation patterns
Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of stratus clouds and light to moderate continuous rain.
Warm fronts are usually preceded by cirrus first, then altostratus or altocumulus, then stratus and possibly fog.
At the warm front, gradual transition.
Behind the warm front, skies are relatively clear.

23. Satellite and radar images of warm fronts (wide region of clouds/precipitation)

24. Stationary Fronts
Stationary fronts do not move. They do not advance. They are two unlike air masses side by side.
They may slowly migrate and warmer air is displaced above colder.
From Environment Canada

25. Stationary Front
Stationary front- a front which does not move or barely moves.
Stationary fronts behave like warm fronts, but are more quiescent.
Many times the winds on both sides of a stationary front are parallel to the front and have opposite direction.
Typically stationary fronts form when polar air masses are modified significantly so as to lose their character (e.g., cold fronts which stall).
Typically there is no strong precipitation associated with stationary fronts (why? – no big contrast in the air mass properties, no air uplifting and condensation).

26. Occluded Fronts
Occluded fronts occur when two fronts meet, the warm air mass between them is displaced aloft.
This typically occurs when a cold front meets a warm front as it circulates the low pressure center of a mid-latitude cyclone.
The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point.

27. Formation of Occluded Fronts

28. Occluded fronts. Cold occlusion
Cold fronts move faster than warm fronts. They can catch up and overtake their related warm front. When they do, an occluded front is formed.  
Cold occlusion: very cold air behind, not so cold air ahead of, the warm front
The upper warm front follows the surface occluded front

29. Warm Occlusion
Very cold air ahead of, not so cold air behind, the warm front
The cooler air from the cold front cannot lift the very cold air ahead, rides “piggyback”
The warm front aloft precedes the surface occluded front

30. Different types of occluded fronts
A cold-type occlusion usually occurs in the eastern half of the continent where a cold front associated with continental/Polar air meets a warm front with maritime/Polar air ahead.
A warm-type occlusion is typical of the western edges of continents where the cold front, associated with maritime/Polar air, migrates to an area that is occupied by continental/Polar air.

31. Drylines
Drylines are boundaries between lighter humid air and denser dry air.
Air masses with similar temperatures but strong humidity gradients will act as fronts.
They frequently occur throughout the Great Plains, and are a favored location for thunderstorm development.

32. Weakening/Strengthening of the Front
Frontolysis:
The front weakens and dissipates
Why?-the air masses start losing their identities.
The temperature (humidity) contrast across the front is decreasing.
Typical for slow moving fronts
Frontogenesis:
The front intensifies.
Why? – The temperature (humidity) contrast across the front is increasing.
Example: cP air mass moves over warm ocean water.

33. Summary
Definition of airmasses. Bergeron classification of air masses (3 letters)
Surface weather analysis: Station model, wind speed code, present weather
Fronts: 6 types.
What is a cold front? Steep, narrow, fast
What is a warm front? Less steep, wide, slow
What is an occluded front? Two types