Fronts

• Fronts are boundaries between air masses of different temperatures.
• Fronts are zones of transition, sometimes with a sharp frontal zone.
• Fronts are typical in midlatitudes weather, uncommon in tropical and polar regions.
• Front is a three-dimensional boundary zone formed between two converging air masses with different physical properties.
• The frontal concept was developed by Norwegian meteorologists during World War I.
• The term front was coined because these scientists considered the clash between unlike air masses to be analogous to a confrontation between opposing armies along a battlefront.
• Air masses retain their separate identities as one is displaced by the other.

Front Formation

• Frontogenesis is the process of front formation (war between two air masses).
• Frontolysis is the dissipation of a front (one of the air masses wins against the other).
• Frontogenesis involves convergence of two distinct air masses, while frontolysis involves overriding of one air mass by another.
• In the northern hemisphere, frontogenesis happens in an anticlockwise direction, and in the southern hemisphere, it happens in a clockwise direction due to the Coriolis effect.
• Mid-latitude cyclones, temperate cyclones, or extra-tropical cyclones occur due to frontogenesis.

Characteristics of Fronts

• Temperature contrast affects the thickness of the frontal zone inversely.
• A sudden change in temperature through a front causes a change in pressure.
• Wind shift occurs due to pressure gradient and Coriolis force.
• Wind shift is a change in wind direction of 45 degrees or more in less than 15 minutes with sustained wind speeds of 10 knots or more throughout the wind shift.
• Frontal activity is associated with cloudiness and precipitation due to ascent of warm air, which cools adiabatically, condenses, and causes rainfall.
• Intensity of precipitation depends on the slope of ascent and the amount of water vapor present in ascending air.

Classification of Fronts

• Based on the mechanism of frontogenesis and the associated weather, the fronts can be studied under the following types:
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• Stationary Front:
  • A front that forms when two air masses are unable to push against each other and the surface position of the front does not change
  • Wind motion on both sides of the front is parallel to the front
  • The name “stationary front” is because the warm or cold front stops moving
  • Once this boundary resumes its forward motion, it becomes a warm front or cold front
  • Weather along a stationary front includes the formation of cumulonimbus clouds and frontal precipitation caused by overrunning of warm air
  • Cyclones migrating along a stationary front can dump heavy amounts of precipitation, resulting in significant flooding along the front.
  • 
• Cold Front:
  • A cold front is formed when a cold air mass replaces a warm air mass by advancing into it or when the warm air mass retreats and the cold air mass advances.
  • Cold fronts move up to twice as quickly as warm fronts.
  • Frontolysis begins when the warm air mass is completely uplifted by the cold air mass.
  • The weather along a cold front depends on a narrow band of cloudiness and precipitation, and severe storms can occur.
  • During the summer months, thunderstorms are common in the warm sector of a cold front, and in some regions like the USA, tornadoes occur in the warm sector.
  • Cold fronts produce sharper changes in weather, and temperatures can drop more than 15 degrees within the first hour.
  • The approach of a cold front is marked by increased wind activity in the warm sector and the appearance of cirrus clouds, followed by lower, denser altocumulus clouds.
  • At the actual front, dark nimbus and cumulonimbus clouds cause heavy showers.
  • A cold front passes off rapidly, but the weather along it is violent.
  • 
• Warm Front:
  • Warm front is a sloping frontal surface along which active movement of warm air over cold air takes place.
  • Frontolysis (front dissipation) begins when the warm air mass makes way for the cold air mass on the ground.
  • Weather along a warm front is marked by gradual changes in temperature and wind direction.
  • The passage of warm front is marked by a rise in temperature, pressure and change in weather.
  • Clouds along a warm front are cirrus, stratus and nimbus. There are no cumulonimbus clouds as the gradient is gentle.
  • Cirrostratus clouds ahead of the warm front create a halo around sun and moon.
  • 
• Occluded Front:
  • Occlusion is a meteorological process where the cold front of a low-pressure system catches up to the warm front, forcing warm air between them to rise.
  • It occurs when a cold air mass overtakes a warm air mass and goes underneath it.
  • Frontolysis occurs when the cold air completely undertakes the warm air mass on the ground.
  • A long and backward swinging occluded front is formed which could be a warm front type or cold front type occlusion.
  • Weather along an occluded front is complex, a mixture of cold front type and warm front type weather. Such fronts are common in western Europe.
  • The formation of mid-latitude cyclones involves the formation of an occluded front.
  • Clouds along an occluded front are a combination of clouds formed at the cold front and warm front. Warm front clouds and cold front clouds are on opposite sides of the occlusion.
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Air Masses, Fronts, and Major Atmospheric Disturbances

• Atmospheric disturbances can be stormy or produce calm weather.
• They are smaller than the components of the general circulation and migratory.
• They have a relatively brief duration and produce characteristic and relatively predictable weather conditions.
• Midlatitude disturbances are associated with where polar and tropical air masses meet, including midlatitude cyclones and anticyclones.
• Tropical disturbances occur in low latitudes and include tropical cyclones and easterly waves.
• Localized severe weather, such as thunderstorms and tornadoes, can occur in many parts of the world.