[Ref #Understanding Weather & Climate, Books a la Carte Edition (6th Edition) ISBN-13: 978-0321773227 #ISBN-10: 0321773225 CH 9 AIR MASSES and FRONTS # Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-18 to 12-22]

What’s Fronts?

Fronts – a narrow boundary regions between two types of air masses – transition zone at which there are great temperature differences with relatively short distances between two air masses

Generally speaking, the larger the difference in temperature between two air masses the more intense the resultant weather that the front brings.

There are four (4) most common types of front:

1. Warm front
2. Cold front
3. Stationary
4. Occluded front

 

	Warm Front	Cold Front	Stationary Front	Occluded Front
Symbol on WX chart	solid lines with semicircles and are often colored in red edges of the semicircles shows the direction of movement of the warm air.	Cold fronts on weather charts usually represented by solid lines with triangles and are often colored in blue. The tips of the triangles indicate the direction of movement of the cold air	Stationary fronts on weather charts usually represented by a series of symbol of cold front and warm front interchangeably connected and the triangle of  cold front is opposite direction to the semicircles of the warm front	Occluded fronts on weather charts usually represented by solid lines with interchangeable triangles and semicircles in same direction. The color of occluded front are often in purple
Formation	warm mass of air advances and replace the body of colder air	cold, dense and stable air advances and replaces a body of warmer air	two air masses are relatively equal	Occluded front occurs when a fast-moving cold front replacing the slow-moving warm front Two (2) types of  occluded fronts: Cold front occlusion (worse) Warm front occlusion (worser)
Moving speed	10-25 mph Up to 2 days for its passage	25-30 mph (up to 60 mph) Can spend only few hours for its passage	The weather is the mixture that can be observed in cold front and warm front	The weather is the mixture that can be observed in cold front and warm front
Prior the passage	Ci, Cs, As, CB Fog precipitation (rain, sleet, snow or drizzle) wind: from southeast to south-southeast dew point ↑ barometric pressure continues to fall ↓	Ci, TCU and CB Rain showers No significant change in wind direction High dew point barometric pressure fall ↓
During the passage	stratiform clouds Poor visibility Precipitation Drizzle (DZ) Variable (VRB) winds dew point and barometric pressure levels off →	TCU and CB continuous existing Poor visibility Heavy Rain (RA) VRB winds and gusty dew point and barometric pressure drop rapidly ↓
After the passage	Sc rain shows (SH RA) Wind: south-southeast to south-southwest (SSW) Dew point rises and then levels off ↑→ barometric pressure  slightly rises then decrease ↑↓ Temperature is warmer	Cu precipitation ↓ Wind direction : west-northwest (WNW) barometric pressure continues to rise ↑ Temperature is cooler

 

Warm front

• Warm fronts on weather chart usually denoted by solid lines with semicircles and are often colored in red. The edges of the semicircles shows the direction of movement of the warm air.
• Warm front occurs when a warm mass of air advances and replace the body of colder air.
• Warm fronts typically move at 10-25 mph, of which passage take up to two days

(6) Six indicators of warm front:

1. Clouds
2. Fog/Visibility
3. Precipitation
4. Wind
5. Dew point
6. Barometric pressure

• Prior to the passage of the warm front:
  • cirriform (Cs/Ci) /stratiform (As) clouds, along with fog. Cumlonimbus clouds (thunderstorms – CB/TS) are likely to develop during summer time;
  • precipitation is light to moderate from of rain, sleet, snow or drizzle;
  • wind from south-east to south-southeast
  • Dew point increase
  • barometric pressure continues to fall
• During the passage of the warm front:
  • visibility is generally poor due to precipitation
  • dew point and barometric pressure levels off
• After the passage of the warm front:
  • Stratocumulus clouds
  • precipitation is form of rain shows
  • Wind from south-southeast to south-southwest
  • dew point rises and then levels off
  • barometric pressure slight rise and then decrease again

 

Cold front

• Cold fronts on weather charts usually represented by solid lines with triangles and are often colored in blue. The tips of the triangles indicate the direction of movement of the cold air

Notes:

Cold front in the Northern Hemisphere is normally oriented in a northeast to southwest manner  – WHY ???

• Cold front occurs when a cold, dense and stable air advances and replaces a body of warmer air
• Cold fronts typically move faster at a rate of 25-30 mph, sometimes up to 60 mph. The passage can be as fast as just a few hours
• Prior to the passage of the cold front
  • Cirrifrom, towering cumulus and Cumulonimbus clouds could be observed
  • Precipitation in the form of rain showers
  • wind direction has no significant changes
  • high dew point
  • barometric pressure fall
• During the passage of the cold front
  • towering cumulus and Cumulonimbus clouds continuous existing
  • visibility is generally poor
  • dew point and barometric pressure drop rapidly
• After the passage of the cold front
  • Cumulus clouds
  • precipitation decrease
  • Wind direction changes to west-northwest
  • barometric pressure continues to rise

 

Fast-Moving Cold Front generally creates a steeper frontal surface and give a very narrow band of weather along the leading edge of the front. Behind a fast-moving cold front, the skies usually clear rapidly, and the front leaves behind gusty, turbulent winds and colder temperatures. 

If the warm air being overtaken by the cold front is relatively stable, overcast skies and rain may occur for some distance behind the front;

If the warm air is unstable, scattered thunderstorms and rain showers may form. A continuous line of thunderstorms, or squall line, may form along or ahead of the front.

 

Stationary front

• Stationary fronts on weather charts usually represented by a series of symbol of cold front and warm front interchangeably connected and the triangle of  cold front is opposite direction to the semicircles of the warm front
• Stationary front occurs when two air masses are relatively equal
• The influences from the stationary front can be last for days
• The weather associated with stationary front is mixture that can can be found in  both warm and cold front

 

Occluded front

• Occluded fronts on weather charts usually represented by solid lines with interchangeable triangles and semicircles. The color of occluded front are often in purple
• Occluded front occurs when a fast-moving cold front replacing the slow-moving warm front
• (2) Two types of occluded fronts:
  1. Cold front occlusion (worse)
     • The fast-moving cold front is colder than the air ahead of the slow-moving warm front
  2. Warm front occlusion (worser)
     • The air ahead of the slow-moving warm front is colder than the air of the cold front

Embedded thunderstorms are likely found in warm from occlusion

• Generally speaking, weather is more severe in warm front occlusion than the cold front occlusion

 

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