April 24th-Air Masses and Fronts

Air Masses

  • Classify air masses:
    • Continental (c) vs. Maritime (m)
    • Polar (p) vs. Tropical (t)
    • Arctic (a) - Super cold, a subset of the polar region
  • Types:
    • CA - Continental Arctic
    • CP - Continental Polar
    • MP - Maritime Polar
    • MT - Maritime Tropical
    • CT - Continental Tropical (dry and hot, e.g., Southwestern US, Northern Mexico)
  • Air masses are latitude-dependent.
  • CA can transform into MT over time due to atmospheric circulation.
  • Air mass modification occurs as air masses move.

Fronts

  • Fronts are positions where air masses with different characteristics meet.
  • Characteristics:
    • Energy levels (temperature and humidity).
    • Temperature reflects the internal energy of an air parcel.
    • Humidity is crucial due to latent heat release, leading to atmospheric instability and weather (storms, precipitation, hurricanes).
  • Mid-latitude regions experience mixtures of polar and tropical air.
  • Boundaries are delineated by large temperature gradients.
  • Gradients in energy cause weather phenomena like wind, convection, and instability.
  • The jet stream originates from the intersection of cold and warm air masses.

Atmospheric Circulation

  • Hadley cell: Driven by radiation input at low latitudes.
    • Sinking around 30-35 degree latitude band.
  • Mid-latitude convergence zone: Around 40-60 degree latitude band.
  • Temperature gradient is largest in the mid-latitudes, not the subtropics (e.g., San Diego, Miami).
  • Example: Current weather map
    • Large temperature difference over a short distance indicates an air mass boundary.
    • Altitude affects temperature (e.g., cold in Wyoming, Montana due to mountains).

Jet Stream and Rossby Waves

  • Polar jet: Located at the gradient between warm and cold air at 8-12 km altitude.
  • Rossby waves: Undulating patterns in the jet stream, indicating the boundary between warm and cold air parcels.
  • Counterclockwise flow: Low pressure
  • Clockwise flow: High pressure
  • Rossby waves are undulations between warmer air to the south and colder air to the north.
  • L: Low-pressure regions with counterclockwise flow, associated with cold air.
  • H (Omega High/Blocking High): A stable high-pressure system that can lead to drought, especially in agricultural regions like Nebraska and Eastern Washington.
  • Subtropical jet is not discussed in detail.

Fronts: Intersection of Air Masses

  • Moving fronts:
    • Warm fronts
    • Cold fronts
  • Fronts are the intersections of air masses with different temperatures and humidities.

Cold Fronts vs. Warm Fronts

  • Cold Front:
    • Depicted in blue with triangles. Triangles indicate the direction of movement.
    • Cold air pushes warmer air.
  • Warm Front:
    • Depicted in red with semicircles. Semicircles indicate the direction of movement.
    • Warm air pushes cold air.
  • The only difference between a cold front and a warm front is the direction of travel.
  • Stationary Front:
    • Alternating red semicircles and blue triangles on opposite sides of the line.
    • No movement.

Interaction of Warm and Cold Fronts

  • Warm Fronts:
    • Warm air is less dense, so it rises over colder air.
    • As warm air rises, it cools, leading to condensation, cloud formation, and precipitation.
    • Sequence of events when a warm front approaches:
      • High clouds appear first.
      • Clouds thicken and lower.
      • Precipitation begins while it is still cold.
      • Temperature increases after the precipitation stops.
    • Surface temperature is the last thing to change with the onset of a warm front.
    • Conditionally unstable air leads to cumulonimbus clouds and heavy precipitation.
    • Stable conditions result in dreary high clouds and light precipitation.
  • Cold Fronts:
    • Cold air is denser and pushes under warm air, lifting it abruptly.
    • Steeper slope compared to warm fronts.
    • Cloud formation and precipitation occur quickly.
    • Weather progression: Nice → windy → cloudy → heavy precipitation → cold → clears up.
    • Clearing occurs once within the homogeneous cold air mass.

Front Characteristics

  • Warm Front: Gradual slope, with warm air rising over cold air over a long distance (e.g., 100 miles).
  • Cold Front: Steep slope, with cold air forcing warm air upwards rapidly over a short distance (e.g., 20-30 miles).

Occluded Fronts

  • Occlusion: When a cold front catches up to a warm front.
  • The fronts ultimately disappear as all warm air is pushed upwards, eliminating the temperature gradient.

Life Cycle of a Frontal System

  • Process:
    1. Stationary front: Cold air to the north, warm air to the south.
    2. Cold air pushes southward, forming a cold front, and warm air moves northward, forming a warm front.
    3. The faster-moving cold front catches up to the warm front, resulting in an occluded front.
    4. Warm air is pushed aloft, and the system dissipates.

Storm Formation and Tracking

  • Storm track: The gradient between warm and cold air, associated with the jet stream.
  • Location is seasonally dependent, dipping further south in the winter.
  • The storm track is influenced by temperature gradients and historical observations.
  • Isothermal charts (lines of constant temperature) help identify fronts and temperature gradients.