NATS 101
Lecture 23Fronts
Review
• Air Masses
Large regions with “uniform” temperature and moisture distributions and distinctive weather
• Classified by Source Region Continental (c) or Maritime (m)Polar (P) or Tropical (T)
• Source Regions
Big in area (>1600 km by 1600 km)
Dominated by light winds (long resident times)
Air Mass Characteristics
Ahrens Table 8.1
Air Mass Source Regions
Ahrens Fig 8.2
Weather Map with Air Masses, Fronts, Extratropical Cyclone
Ahrens Fig 8.11
Extratropical Cyclones and Fronts
• In mid-latitudes, significant weather is often associated with a particular type of storm:
Extratropical Cyclone
• Cyclone denotes the circulation around a low pressure center
• The energy for extratropical cyclones comes from horizontal temperature contrasts
Extratropical Cyclones and Fronts
• ET cyclones often form on a boundary between a warm and cold air mass, associated with the jet stream
• They tend to focus temperature contrasts along frontal zones, bands of very rapid horizontal temperature changes
Extratropical Cyclones and Fronts
• Strongest temperature gradients occur at warm edge of frontal zone, called a front
• There are four types of fronts Classified by their movement Each has its own symbol, color scheme
Cold, Warm, Stationary, Occluded
Frontal Types
Cold-cold air advances, warm air retreats
0oC5oC
-5oC-10oC
Surface Wind
Frontal Motion
COLD
WARM
Cold Front Animation
Strong Thermal Contrast
Homogeneous
Homogeneous
Frontal Types
Warm-warm air advances, cold air retreats
0oC5oC
-5oC-10oC
Surface Wind
Frontal Motion
COLD
WARM
Warm Front Animation
Strong Thermal Contrast
Homogeneous
Homogeneous
Frontal Types
Stationary-neither air mass advances significantly
Surface Wind
Frontal Motion
0oC5oC
-5oC
-10oC
WARM
COLDStrong Thermal Contrast
Homogeneous
Homogeneous
Frontal Types
Occluded-Looks like a hybrid between a cold and warm front, with warmest air along front
0oC
5oC
-5oC-10oCSurface Wind
Frontal Motion
COLD
WARMCOLD
Occluded Front Animation
Homogeneous
Homogeneous
Strong Thermal Contrast
Strong Thermal Contrast
3) Post-Cold Front
Temp: Rapid Cooling
Press: Rapid Rising
Wind: W-NW
Dew Pt: Lowering
Sky: Clearing
Wx: Drying
1) Pre-Cold Front
Temp: Warm
Press: Steady/Falls
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
2) Frontal Passage
Pressure Trough
Wind Shift
Abrupt Temp Fall
Rain, T-Showers
Ahrens Fig. 8.12
Cross-Section: Cold Front
Slope = 1 in 50
Narrow band of rising warm air at cold front
Widespread sinking cold air behind cold front
Cumulus-type Clouds High Clouds
Ahrens Fig. 8.13
Cold Front Animation
Typical Cold Front Weather
1) Pre-Warm Front
Temp: Warming
Press: Falling
Wind: E-SE
Dew Pt: Rising
Sky: Lowering Ceiling
Wx: Widespread Precip
3) Post-Warm Front
Temp: Warm
Press: Steady/Rises
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
2) Frontal Passage
Pressure Trough
Wind Shift
Steady Warming
Rain Ending
Ahrens Fig. 8.14
Cross-Section: Warm Front
Slope = 1 in 300
Widespread region of rising warm air ahead of warm front
Leads to widespread region of precipitation ahead of front
Low Clouds-Middle Clouds-High Clouds
Ahrens Fig. 8.15
Warm Front Animation
Typical Warm Front Weather
Occluded FrontsCold-Warm Hybrid
Ahrens Fig. 8.17 and 8.18
Cold OcclusionWarm Occlusion
Occluded Front Animation
Typical Occluded Front Weather
Summary Fronts
• ET cyclones tend to focus temperature contrasts along frontal zones
• Strongest temperature gradients occur at warm edge of frontal zone, called a front
• Fronts classified by movement, each has own symbol and characteristic weather
Cold, Warm, Stationary, Occluded
Summary: Frontal Weather
Temp: Warm
Press: Steady
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
Temp: Slow Warming
Press: Falling
Wind: E-SE
Dew Pt: Rising
Sky: Lowering Ceiling
Wx: Precipitation, Low Vis.
Temp: Rapid Cooling
Press: Rapid Rising
Wind: W-NW
Dew Pt: Lowering
Sky: Clearing
Wx: Improving
L
Summary: Frontal Weather
LTemp: Rapid Cooling
Press: Rapid Rising
Wind: W-NW
Dew Pt: Lowering
Sky: Clearing
Wx: Improving
Temp: Slow Warming
Press: Falling
Wind: E-SE
Dew Pt: Rising
Sky: Lowering Ceiling
Wx: Precipitation, Low Vis.
Temp: Warm
Press: Steady
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
Assignment for Next Lecture
• Topic - Extratropical Cyclone Formation • Reading - Ahrens pg 219-228• Problems - 8.14, 8.15, 8.17
