NATS 101
Lecture 23
Fronts
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
Homogeneous
COLD
-10oC
-5oCo
0C
5oC
Strong
Thermal
Contrast
Surface Wind
WARM
Frontal Motion
Homogeneous
Cold Front Animation
Frontal Types
Warm-warm air advances, cold air retreats
Homogeneous
COLD
-10oC
-5oCo
0C
5oC
Strong
Thermal
Contrast
Surface Wind
WARM
Frontal Motion
Homogeneous
Warm Front Animation
Frontal Types
Stationary-neither air mass advances significantly
Homogeneous
COLD
Strong
Thermal
Contrast
-10oC
-5oC
0oC
5oC
WARM
Homogeneous
Surface Wind
Frontal Motion
Frontal Types
Occluded-Looks like a hybrid between a cold
and warm front, with warmest air along front
Homogeneous
-10oC -5oC
0oC
COLD
Surface Wind
5oC
Strong
Thermal
Contrast
Frontal Motion
WARM
Strong
Thermal
Contrast
COLD
Occluded Front Animation
Homogeneous
2) Frontal Passage
3) Post-Cold Front
Pressure Trough
Temp: Rapid Cooling
Wind Shift
Press: Rapid Rising
Abrupt Temp Fall
Wind: W-NW
Rain, T-Showers
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
Ahrens Fig. 8.12
Cross-Section: Cold Front
Cumulus-type Clouds
High Clouds
Ahrens Fig. 8.13
Slope = 1 in 50
Cold Front Animation
Narrow band of rising warm air at cold front
Widespread sinking cold air behind cold front
Typical Cold Front Weather
2) Frontal Passage
Pressure Trough
Wind Shift
Steady Warming
Rain Ending
1) Pre-Warm Front
Temp: Warming
Press: Falling
Wind: E-SE
3) Post-Warm Front
Temp: Warm
Press: Steady/Rises
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
Ahrens Fig. 8.14
Dew Pt: Rising
Sky: Lowering Ceiling
Wx: Widespread Precip
Cross-Section: Warm Front
Low Clouds-Middle Clouds-High Clouds
Ahrens Fig. 8.15
Slope = 1 in 300
Warm Front Animation
Widespread region of rising warm air ahead of warm front
Leads to widespread region of precipitation ahead of front
Typical Warm Front Weather
Occluded Fronts
Cold-Warm Hybrid
Cold Occlusion
Occluded Front Animation
Warm Occlusion
Ahrens Fig. 8.17 and 8.18
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
L
Temp: Slow Warming
Press: Falling
Temp: Rapid Cooling
Wind: E-SE
Press: Rapid Rising
Dew Pt: Rising
Wind: W-NW
Sky: Lowering Ceiling
Dew Pt: Lowering
Wx: Precipitation, Low Vis.
Sky: Clearing
Wx: Improving
Temp: Warm
Press: Steady
Wind: S-SW
Dew Pt: High
Sky: Variable
Wx: Showers
Summary: Frontal Weather
L
Temp: Slow Warming
Press: Falling
Temp: Rapid Cooling
Wind: E-SE
Press: Rapid Rising
Dew Pt: Rising
Wind: W-NW
Sky: Lowering Ceiling
Dew Pt: Lowering
Wx: Precipitation, Low Vis.
Sky: Clearing
Wx: Improving
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