Weather Systems

A low pressure system over Iceland (Photo: NASA)

Learning objectives

• Describe the differences between continental / maritime and polar / tropical air masses.
• Explain the difference between warm fronts, cold fronts and occluded front.
• Explain what causes weather systems to develop and decay.

Air masses

An air mass is a large body of air (1000’s km across) with roughly uniform properties.

• Continental air tends to be drier with more extreme temperatures
• Maritime air tends to be wetter with more moderate temperatures
• Tropical air is warmer than Polar air

Air Mass Types

Air masses are classified according to their source region

• Maritime Tropical (mT): Warm & moist
• Continental Tropical (cT): Hot & dry
• Maritime Polar (mP): Cool & moist
• Continental Polar (cP): Cold & dry
• Continental Arctic (cA): Frigid & dry

Test your knowledge (iClicker)

Which of the air masses listed below would be #1 and #2 respectively in the figure below?

• A: cP and mT
• B: mP and cT

Test your knowledge

Which air mass is most relevant to us in Vancouver during the winter?

• A: Maritime Polar
• B: Maritime Tropical
• C: Continental Arctic
• D: Continental Polar

Fronts

A front is the transition zone between two contrasting air masses. There are 3 important characteristics of fronts:

• Vertical ascent of warm air mass, producing clouds.
• Horizontal motion of fronts, sometimes at more than 50 km/h, resulting in rapid change in weather conditions.
• Discontinuity of temperature, humidity and wind direction.

Two key types of fronts: Cold and Warm

Cold Fronts

Cold, dense air “undercuts” warm air forcing it aloft along the front. Commonly associated with:

• Cumulonimbus clouds & heavy rain
• Strong wind & severe weather
• Followed by clear skies
• Typical slope 1:40 – 1:50

Cold Fronts

How would you know if a cold front occurred?

• Heavy rain followed by a temperature drop and cold clear high pressure air

Severe Weather

Rapid upward displacement of warm air >> intense storms

• High winds
• Heavy rain
• Hail
• Lightning
• Tornadoes possible

Warm Fronts

Warm air gradually rides up over colder air ahead of it. Commonly associated with:

• Stratus clouds and moderate rainfall or snow
• Less intense but more widespread
• Followed by warmer conditions
• Typical slope 1:100 - 1:150

Cloud Sequence of a Warm Front

Cirrus clouds are the first to appear

Cloud Sequence of a Warm Front

Followed by cirrostratus as the cloud deck lowers

Cloud Sequence of a Warm Front

Then a transition to altostratus as the cloud deck continues to lower

Cloud Sequence of a Warm Front

Finally nimbostratus clouds and precipitation once the frontal boundary is close enough

Movement of Fronts

Cold fronts and warm fronts are continuously chasing each other around the mid-latitudes

• Cold fronts travel faster than warm fronts, so they eventually catch up

Occluded Front

The classical and usual view of an occluded front is that it initiates when a cold front overtakes a warm front near a cyclone, such that the warm air is separated (occluded) from the cyclone center at the surface

Jet Streams

Ribbons of high speed winds (160-400 km h-1) encircling the globe aloft (10-15 km).

• Snake around the globe from W to E and steer weather disturbances (mid-latitude Lows)
• Generally aligned with air mass boundaries at the surface

Source: NASA

Jet Streams

Flow dynamics in jet streams promotes the formation of high and low pressure systems.

High and Low Pressure Systems

Convergence aloft

Divergence aloft

Link Between Jet Stream and Ground

To form cyclones, it is important to consider the form of divergence in the jet stream.

• Cyclones cannot persist if they do not have upper-level support.

Life Cycle of Cyclones – Early

Stage starts with a relatively stationary front separating polar (cold) and subtropical (warm) air masses.

• A slight dynamic low created by the divergent jet stream aloft pulls air in, so cold air penetrates into the warm air, and warm air moves towards the cold air.
• The center of the disturbance starts to rotate.

Life Cycle of Cyclones – Open

Two fronts (one cold, one warm) are evident.

• The denser cold air undercuts the warm air, and the warm air overrides the cold air ahead of it
• The uplift is enhanced by the release of latent heat, as ascending air rises above the lifting condensation level & cloud develops.

Life Cycle of Cyclones – Occlusion

The occluded stage begins when the cold front catches up with the warm front near the low pressure center.

• The cold air at ground level undercuts the warm air pushing it further aloft.
• The ‘warm sector’ is systematically eliminated, as more cold air comes in.
  
• This influx destroys the system.

Life Cycle of Cyclones - Dissolving

The dissolving stage is the end of the cyclone’s life.

• Warm air aloft is separated from its source and gradually mix with the surrounding cold air.

A dissolving Pacific cyclone as seen from Apollo 9

On a Weather Map

Forecasting

Environment Canada is responsible for weather forecasting and research.

• They have multiple office located in Vancouver and Victoria

• Specialize in prediction of coastal and mountain weather

Douglas Jung Building, Vancouver BC

35 forecasters work in the PSPC

Forecasting 101

1. Analysis: examine to determine what is happening

2. Diagnosis: why is it happening?

3. Prognosis: forecast the likely outcomes

Scales of Forecasting

• Global: Largest patterns over longest time-scales (e.g., climate change)

• Synoptic: Continental scale, multi-day weather patterns

• Mesoscale: Smaller spatial scale, daily to hourly patterns

• Microscale: Ecosystem-scale, short time-scale (hours to minutes)

Forecasting Challenges in British Columbia

• Mountains and complex terrain
• Sources of moisture
• Pacific data void
  • Lack of observational data upstream of BC

Extreme Weather: Heat Domes

Intense high pressure systems that set up over an area and persist for days or weeks.

• Subsidence causes intense adiabatic heating and suppresses cloud formation

Record breaking heat dome, June 2021

Extreme Weather: Heat Domes

Intense high pressure systems that set up over an area and persist for days or weeks.

• Subsidence causes intense adiabatic heating and suppresses cloud formation
• Exacerbates fire risk

Lytton BC, after the heat dome

Extreme Weather: Atmospheric Rivers

Satellite image of water vapor

Washout on the Coquihalla Highway

Take home points

• Air masses is a large body of air with roughly uniform humidity and temperature depending on their origin
• Fronts are the transition zone between two contrasting air masses
• We distinguish between cold fronts (cold air undercuts warm air) and warm fronts (where warm air rides up over colder air ahead of it)
• The life-cycle of cyclones is a dynamic process starting with initially two separate fronts (cold / warm) that will combine to a occlusion and eventually dissolve the energy.
• Weather forecasting is difficult and especially difficult in BC
• Climate change has changed the timing and intensity of many weather events
• There is increasing focus on forecasting of severe events including wildfires, heat (and cold) extremes as well as precipitation events such as Atmospheric Rivers