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Fronts

Fronts. A refresher. Definition. A front is a boundary between relatively uniform warm air and a zone in which temperatures cools rapidly. Four Main Types of Fronts. Warm Front. Stationary Fronts. Occluded Front (a hybrid). As a front passes there are changes in. Temperature Dew point

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Fronts

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  1. Fronts

  2. A refresher

  3. Definition A front is a boundary between relatively uniform warm air and a zone in which temperatures cools rapidly

  4. Four Main Types of Fronts

  5. Warm Front

  6. Stationary Fronts

  7. Occluded Front (a hybrid)

  8. As a front passes there are changes in • Temperature • Dew point • Wind direction • Pressure • cloudiness

  9. Fronts and PressureFronts are associated with troughs of low pressure

  10. Fronts are associated with bands of clouds

  11. Vertical Structure of Fronts

  12. Cold Front • Slope 1:50, moves fast (20-30 mph), convection on leading edge

  13. Warm Front • Smaller slope (1:200), slower (1—15 knots), more stratiform clouds

  14. Stationary Front similar structure to warm front, but without movement

  15. There is a typical progression of clouds as cold and warm fronts approach and pass by • Cirrus • Cirrostratus • Altostratus • Nimbostratus • Cumulus after cold front

  16. There is another type of front: the occluded front • But to understand this this front, you need to learn about the life cycle of fronts and cyclones.

  17. For much of the 20th century the dominant paradigm for cyclone/frontal evolution has been the Norwegian Cyclone Model (Bergen School) Bjernkes, 1919

  18. Concept of Evolution of CyclonesBjerknes and Solberg1922

  19. Stationary Polar Front Wave Forming on Polar Front

  20. Wave Amplifies Occlusion as Cold Front Catches Up to Warm Front

  21. Occlusion Lengthens and System Weakens

  22. Warm and Cold Occlusions

  23. In the real world, only the warm occlusion is observed

  24. During the 1930s-1950s we learned the relationship between cyclones and fronts and upper level flow • Upper troughs associated with surface lows. Usually lagging to the west. • Upper ridges asociated with surface highs. Usually lagging to the west.

  25. https://atmos.washington.edu/~ovens/wxloop.cgi?h500_slp+/-168//https://atmos.washington.edu/~ovens/wxloop.cgi?h500_slp+/-168//

  26. What is the energy source of midlatitude cyclones?

  27. The answer: warm air rising and cold air sinking

  28. Warm (less dense) air rising and cold (more dense) air sinking lowers the center of gravity of the atmosphere • Like dropping a weight. • Potential energy (energy inherent in being aloft) is converted to kinetic energy (energy of moving air)

  29. The conversion to kinetic energy is enhanced by having large differences of temperatures (large horizonal temperature gradients)

  30. No accident that cyclones grow in regions of large temperature gradients

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