Topic 6 Ch 13 Air Masses Fronts and Mid-Latitude Cyclones

Air Masses
  • Definition: Large bodies of air with uniform temperature and moisture conditions.
  • Development: Form over source regions where they adopt characteristics of the underlying land/water surfaces.
  • Fronts: Transition zones where air masses from different source regions meet.
Classification of Air Masses
  1. Maritime Polar (mP)
    • Characteristics: Cool, humid, unstable all year (average specific humidity, SH: 4.4 g/kg).
    • Winter Pattern:
      • Continental Arctic (CA): Very cold, very dry, stable (average SH: 0.1 g/kg).
      • Continental Polar (CP): Cold, dry, stable with high pressure (average SH: 1.4 g/kg).
  2. Maritime Tropical (mT)
    • Characteristics: Warm, humid, unstable (varying SH between 10 - 17 g/kg)
    • Subcategories of mT:
      • Stable to conditionally unstable: averages SH around 10 g/kg.
      • Very humid and very unstable: average SH can reach up to 17 g/kg.
  3. Continental Tropical (cT)
    • Characteristics: Hot, low relative humidity, stable aloft, unstable at surface (average SH: 13 g/kg).
Lake-Effect Snowfalls
  • Impacts: Depending on proximity to lakes, snowfall amounts can vary significantly. Regions may see snow accumulation ranging from less than 100 cm to over 300 cm near major lakes (e.g., Great Lakes).
Fronts
  • Definition: Boundaries between air of different properties, primarily temperature and moisture.
  • Effects of Fronts: Uplift of less dense air results in:
    • Clouds
    • Precipitation
    • Drop in surface pressure
Types of Fronts

  1. Cold Fronts: Cold air advances and replaces warm air.

    • Associated Weather:
      • Temperature and humidity drop.
      • Winds shift clockwise.
      • Rapid uplift of warm air leads to cumulus clouds, thunderstorms, and heavy precipitation.

  2. Warm Fronts: Warm air moves in, retreating cold air.

    • Associated Weather:
      • Temperature and humidity increase.
      • Winds shift clockwise.
      • Warm air rises slowly over cold air, leading to stratiform clouds and light precipitation.

  3. Stationary Fronts: No significant movement observed.

Cyclogenesis
  • Definition: Development of a mid-latitude cyclone along a polar front.
  • Process: A stationary front develops a "kink" due to upper atmospheric flow, resulting in a surface low.
Life Cycle of a Mid-Latitude Cyclone
  1. Initial Stage: Stationary front exists.
  2. Frontal Wave: A kink develops.
  3. Open-Wave Stage: Full development with distinct fronts.
  4. Mature Cyclone: Active cyclone with significant weather impacts.
  5. Occluded Stage: Cold front overtakes warm front.
  6. Dissolving Stage: System weakens and dissipates.
Migration of Mid-Latitude Cyclone
  • Direction: Typically moves from west to east due to the prevailing westerly winds in the mid-latitudes.