Unit 2: Synoptic Scale (Regional) Weather & Climate

Synoptic scale:• 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

Airmasses and fronts

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”

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

Source regions The areas where air

masses form are called source regions.

Air Masses on Weather Maps

Surface weather analysis

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

Wind speed code

5

10

50

Present weather

Type

Intensity

Fronts A weather front is a boundary separating two air masses Types: cold front, warm front, occluded front, stationary

front, dry line, squall line

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.

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

Weather Map Shown: surface-pressure systems, air masses, fronts,

isobars, winds and air flow (large arrows) Green-shaded area: precipitation

What are the Signs of a Passing Front?

Signs• 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…

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

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)

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

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).

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

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

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.

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.

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

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

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).

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.

Formation ofOccluded

Fronts

Occluded fronts. 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

Cold occlusion

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

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.

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.

Weakening/Strengthening of the Front Frontogenesis:

• The front intensifies.• Why? – The temperature

(humidity) contrast across the front is increasing.

• Example: cP air mass moves over warm ocean water.

• 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

Summary

1. Definition of airmasses. Bergeron classification of air masses (3 letters)

2. Surface weather analysis: Station model, wind speed code, present weather

3. Fronts: 6 types. 4. What is a cold front? Steep, narrow, fast5. What is a warm front? Less steep, wide, slow6. What is an occluded front? Two types