Recording-2025-03-11T15_35_13.985Z

Chapter 9: MS and the Front

Overview of MS (Air Masses)

• Air Mass: A large body of air that has uniform temperature and humidity characteristics.

• Influenced by two main factors:

  • Temperature of the Atmosphere

  • Humidity Level (Osmosis)

• MS significantly impacts larger areas and are not static; they tend to move, affecting different regions.

Characteristics of Air Masses

• Movement: MSs are dynamic; they can migrate to different locations, altering the temperature and humidity of the new area.

• Boundary (Front): The front is the boundary between two differing MSs, often characterized by temperature and humidity differences.

  • Spatially Limited: Fronts occupy smaller areas compared to the MSs they separate.

  • Linked to Mid-Latitude Cyclones: Fronts are essential to understanding the development of mid-latitude cyclone systems, typically discussed in further detail in advanced classes.

Source Regions of Air Masses

• Source regions are areas where air masses form, impacting the atmosphere above them.

• Heating/Cooling: Long-term heating or cooling must occur in the source region for MS characteristics to be established (stable conditions needed).

• Geographical Influence: MSs can develop over land (continental - dry) or oceans (marine - humid), impacting humidity levels in the air masses.

Classification of Air Masses

• Moisture Content:

  • Continental (C): Dry air mass formed over land.

  • Maritime (M): Moist air mass formed over oceans.

• Temperature:

  • Tropical (T): Warm air mass from tropical regions.

  • Polar (P): Cool air mass from polar regions.

  • Arctic (A): Very cold air mass from Arctic regions.

  • Antarctic (AA): Extremely cold air mass from the Antarctic (mainly outside North American context).

• Combined Classification: MSs can be classified by a combination of lowercase (moisture) and uppercase (temperature) letters, e.g., cT (continental tropical), mP (maritime polar).

Movement and Interaction of Air Masses

• Air masses are not confined to their source regions. They migrate, leading to significant changes in temperature and humidity in the areas they move to.

• Impact on Regions: As one air mass replaces another (e.g., from A to B), significant weather changes (temperature/humidity) occur.

  • New air masses can moderate extreme conditions of the previous air mass encountered.

  • Over time, the moving air mass may assimilate some characteristics of the new region.

Example of Air Masses Movement

• Case Study: Minneapolis, St. Louis, and Birmingham

  • Day Zero Observations: Minneapolis at -5°C, St. Louis at 0°C, Birmingham at 5°C.

  • After 24 Hours: Cold front moves south, affecting temperatures significantly.

    • Minneapolis experiences a temperature drop approx. 20°C due to the cold air mass.

    • St. Louis and Birmingham show smaller temperature changes (1-2°C).

  • Further Movement: Cold front can move into warmer regions, drastically changing temperatures across affected areas (e.g., a significant temperature drop seen in St. Louis and Birmingham).

Conclusion and Next Steps

• Importance of understanding MS and fronts in predicting weather patterns.

• Further discussions on different types of air masses in subsequent sessions.

• Reminder for students to submit any pending assignments before the next class.