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Weather fronts

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dedicated to Mr.shaukat awan

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Weather fronts

  1. 1. Presented to: Mr. Shaukat Ali Awan Fronts, Formation, Identification and Frontal Weather Department of Space Science ,University of the Punjab, Lahore
  2. 2. Contents  Fronts  Formation  Identification and Frontal Weather
  3. 3. Fronts  Fronts are boundary surfaces that separate air masses of different densities one of which is usually warmer and contains more moisture than the other.  The zones that Separate the vast sizes of air masses.  A front is the transition zone between two air masses of different densities.  Separate air masses with contrasting temperatures. Often, they separate air masse s with different humidity's as well called Fronts.  Fronts can form between any two contrasting air masses.
  4. 4. Air Masses  An air mass as the term implies is an immense body of air usually 1600 km whic h is characterized by homogeneous physical properties in particular temperature and moisture content at any given altitude.
  5. 5. Frontal Surface or Frontal Zone The upward extension of a front is referred to as a frontal surface or front al zone.
  6. 6. Formation of Fronts  Generally, the air mass located on one side of a front moves faster than the air mass on the other side. Thus, one air mass actively advances into the region occupied by another and collides with it.  As one air mass moves into the region occupied by another, minimal mixing occurs along the frontal surface.  Instead the air masses retain their identity as one is displaced upward over the other. No matter which air mass is advancing, it is always the warmer less dense air that is forced aloft.  Whereas the cooler, denser air acts as a wedge on which lifting occurs.  The term overrunning is applied to the process of warm air gliding up over a col d air mass.
  7. 7. Types of Fronts  Warm fronts  Cold fronts  Occluded fronts  Stationary fronts
  8. 8. 2D view of Fronts 3D view of Fronts
  9. 9. Warm Front  When the surface position of a fr ont moves so that warmer air in vades territory formerly occupie d by cooler air, it is called a warm front.  Warm fronts are usually as sociated with maritime tropi cal (mT) air “glides” over cooler air positioned over land.  The boundaries separating th ese air masses have very gradual slopes that average ab out 1:200  This rising of warm air over cold, called overrunning
  10. 10. Clouds Pattern in Warm Front
  11. 11. A temperature inversion called a frontal inversion exists in the region of the uppe r level front at the boundary where the warm air overrides the cold air. Frontal Inversion in Warm Front
  12. 12. Identification and Frontal Weather Weather Element Before Passage During Passage After Passage Temperature Cool or Cold Rising Warmer Winds East or southeast Variable South or Southwest Precipitation Light-to-moderate rain, snow, or freezing rain in winter. Heavy rain possible in summer. None or light rain None, occasionally showers in summer Clouds Cirrus, cirrostratus, stratus, nimbostratus Cumulonimbus when air is Conditionally unstable. None, stratus, or fog Clearing, cumulus, or cumulonimbus in summer Pressure Falling Falling or steady Falling then rising Humidity Moderate to high Rising High, particularly in summer
  13. 13. Cold Fronts  When cold air actively advances into a region occupied by warmer air the zo ne of discontinuity is called a cold front.  On the average, cold fronts are about twice as steep as warm fronts, having slopes of perhaps 1:100.  In addition, cold fronts advance at speeds up to 80 kilometers (50 miles) per hour, about 50 percent faster than warm fronts
  14. 14. Contd…
  15. 15. Contd…  As the temperature contrast across a front lessens, the front will often weaken and dissipate. Such a condition is known as frontolysis.  An increase in the temperature contrast across a front can cause it to streng then and regenerate into a more vigorous frontal system, a condition called frontogenesis.
  16. 16. Identification and Frontal Weather Weather Element Before Passage During Passage After Passage Temperature Warm Sharp drop Colder Winds South or southwest Variable and gusty West or northwest Precipitation None or showers Thunderstorms in summer, rain or snow in winter Clearing Clouds None, cumulus, or cumulonimbus Cumulonimbus None or cumulus in summer Pressure Falling then rising Rising Rising Humidity High, particularly in summer Dropping Low, particularly in winter
  17. 17. Occluded Fronts  If a cold front catches up to and overtakes a warm front, the frontal boundary create d between the two air masses is called an occluded front, or simply an occlusion. Formation of a Warm Front
  18. 18. Contd…
  19. 19. Types of Occlusion Cold-Type Occluded Fronts
  20. 20. Warm-Type Occluded Fronts
  21. 21. Occluded Front Helps to Generate a Mid-Latitude Cyclone
  22. 22. Stationary Fronts  Airflow on both sides of a front is neit her toward the cold air mass nor to ward the warm air mass  It is almost parallel to the line of the fro nt.  The surface position of the front does not move, or it moves sluggishly. This conditi on is called a stationary front.
  23. 23. Weather Typically Associated with a stationary Front  Some overrunning usually occurs along stationary fronts, gentle to moder ate precipitation is likely.  Stationary fronts may remain over an area for several days, in which case flooding is possible.  When stationary fronts begin to move, they become cold or warm fronts, depending on which air mass advances.
  24. 24. Thanks 

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