Chapter 8

Air Masses, Fronts, and Mid-Latitude Cyclones

1. Air Masses (pg. 204)

· Temperature and humidity determined by source regions after which they are named (Table 8.1, Fig. 8.2).

· Continental polar (cP), e.g., Canada; V. cold, dry (Fig. 8.2).

· Continental arctic (cA), e.g., Alaska; V. cold, dry (Fig. 8.2).

· Maritime polar (mP), e.g., Gulf of Alaska; cool, moist; much of our winter weather (Fig. 8.2).

· Maritime tropical (mT), e.g., Gulf of Mexico, tropical Pacific ocean (“pineapple express”Fig 8.8); warm, moist (Fig. 8.2).

· Continental tropical (cT), e.g., northern Mexico during our monsoon season; warm, moist; much of our summer weather (Fig. 8.2).

2. Fronts (pg. 214)

· Marks the boundary between two air masses (Fig. 8.11, 8.12).

· Can be located by noting one or more of the following on a WX map:

· Rapid T change

· Sharp wind direction change

· Sharp P and P-tendency changes

· Rapid dew point temperature changes

· Boundaries in cloud and precipitation patterns

· Cold front – cold air mass advances into warmer air. Cold air is denser therefore hugs the ground and forces warm air upwards where it cools and moisture may condense into clouds leading to precipitation (Fig. 8.15).

· Normally found in axis of a trough (Fig. 8.13).

· Steep leading edge often gives “concentrated” WX patterns near the front itself.

· Often move at about 25 mph.

· We can often forecast WX associated with passage of a cold front (Table 8.2).

· Warm front – warm air mass advances into colder air. Warm air is less dense therefore rides up over the cold air mass and moisture may condense into clouds leading to precipitation (Fig. 8.18).

· Normally found in axis of a trough (Fig. 8.17).

· Shallow leading edge often gives widespread WX patterns far removed from the front itself.

· Often move at about 10 mph.

· We can often forecast WX associated with passage of a warm front (Table 8.3).

· Stationary front – one that does not move.

· Occluded front – cold front overtakes warm front (Fig. 8.20).

· WX intensity hard to predict at occluded front (Table 8.4).

· University of Illinois - Air Masses and Fronts web page.

3. Mid-Latitude Cyclones (pg. 222)

· Reasonable (but inexact) working model for the formation of fronts and cyclones (low pressure systems) comes from “polar front theory” (Fig. 8.23).

· Called “cyclogenesis”, it helps explain commonly observed family of cyclones (Fig. 8.24).

· Factors favoring the formation of cyclones include:

· Upper level divergence over a surface L (Fig. 8.28).

· Strong jet stream to sweep away air and intensify surface L (Fig. 8.29).

· New England sometimes experiences “nor’easters” (Fig. 4, pg. 226).