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.