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