Meteorology Week 5: Air Masses and Fronts

Air Mass: A large section of the troposphere with uniform properties in horizontal temperature and moisture.

Naming: Based on initial properties derived from their region of origin.
- Moisture Types:
- Continental: Dry air formed over land.
- Maritime: Moist air formed over large bodies of water.
Temperature Regions:
1. Arctic (cA): Cold air formed over the Arctic Region (from pole to permafrost line).
2. Polar (cP): Cool air formed over the Polar Region (from permafrost to mean temperature of 10°C).
3. Tropical (cT): Warm air formed over the Tropical Region (from 30° latitude to the equator).

Winter Air Masses: mP, mA, cA are predominant.
Summer Air Masses: dominated by majority of continental regions covered in melted snow lakes, mA, mP, mT; cT is rare due to geographical limitations.
Gulf/Ocean Influence: Air masses often influenced by warmer ocean currents, typically maritime tropical (mT) for Canada.

Moisture Content:
- Dry air typically results in minimal cloud formation.
- Maritime air often results in cloud formation, precipitation, and fog.
Cooling Processes:
1. Radiation
2. Advection
3. Expansion
Different areas of the same air mass can experience varying weather due to these processes.
Stability of Air:
- Stable Air: Typically results in layer clouds and poor visibility.
- Unstable Air: Typically results in cumulus clouds and good visibility.

Cold Air Mass:
- Instability and turbulence.
- Good visibility, often cumulus clouds, showers, hail, thunderstorms.
Warm Air Mass:
- Stability with smooth air and poor visibility.
- Stratiform clouds and fog, often drizzle.

Major Modification:
- As an air mass moves, its properties can change extensively, which may result in a name change. For example:
- Continental Arctic (cA) moving over oceans gains moisture and is renamed Maritime Arctic (mA).
Minor Modification:
- Localized variations that affect weather without causing a name change.
- Temperature variations from differential heating effects,
- Movement across lakes adds moisture,
- Coastal air rising over mountains causing precipitation on one side and dryness on the other.
- Heating from below induces instability; cooling induces stability.

Continental Arctic (cA):
- Forms over ice and snow-covered regions.
- Moisture content is low.
- As it moves south, heating occurs, leading to turbulence. If it acquires moisture it forms stratocumulus clouds with light snow.
- Often moves across prairies into the eastern continent, rarely affecting the Pacific Coast.
Maritime Arctic (mA):
- Forms over Siberia or Alaska, moistening as it travels across the North Pacific.
- Results in stratocumulus clouds; thunderstorms develop when lifted by coastal mountains.
- Loses most moisture before hitting the prairies.
Maritime Polar (mP):
- Travels across the ocean; modification leads to a more stable air mass.
- Orographic lift creates extensive cloud formation and considerable rain, drier upon reaching prairies.
Maritime Tropical (mT):
- Originates over tropical oceans; very warm and moist with instability upon front lifting.
- Rarely reaches north of the Great Lakes in winter.
- Can lead to significant weather events, including snow and freezing rain in winter and fog.

Front: A transition zone or boundary between air masses, defined by the characteristics of colder air masses against warmer ones.

Characterizes a dome of cold air over the polar region, involving cA, mA, and mP air masses,
A dome of warm air exists around the equatorial zone, shifting with seasonal changes.

Fronts are named based on the motion of the colder air mass:
- Cold Front: Cold air advancing.
- Warm Front: Cold air retreating.
- On weather maps, front symbols indicate surface position.

The frontal boundary is sloped due to surface friction, known as the frontal surface:
- Cold Front: Steep slope (1:50 – 1:100).
- Warm Front: Gentle slope (1:200).

Occurs when prevailing winds move air opposite along the polar front,
Resulting in low pressure that initiates rotation with a bulge forming as warm air pushes up against cold air.
Calculated movements lead to the undercutting of warm air by advancing cold air, creating an occluded front (also referred to as trowal in Canadian charts).

Warm Front:
- Characteristics: long, gentle frontal slope.
- Weather: Typically forms stratiform clouds and rain, warm moist air.
- Weather sequence: CI, CS, AS, NS (rain), ST/FG.
Cold Front:
- Characteristics: steep frontal slope.
- Weather: Typically results in cumuliform clouds, heavy precipitation, possible thunderstorms.
- Weather pattern: Sharp drop in temperature and pressure upon passing.
Stationary Front:
- Characteristics: neither advancing nor retreating; winds blow parallel.
- Weather: Similar to warm front but less intense.
Occluded Front (Trowal):
- Characteristics: when a cold front catches up to a warm front.
- Weather: Traps warm air aloft between cold air masses; characterized by significant temperature differentials causing varying weather patterns.
Upper Front:
- Characteristics: non-occluded, not in contact with the surface.
- Typical occurrence: in the lee of mountains as one air mass crosses over another.