Lecture 11

• an airmass is a large body of air that has uniform horizontal properties of:

  • temperature (warm or cold)
  • moisture (dry or moist)
• horizontal extent of an airmass = 1000's km
• airmasses extend vertically throughout the troposphere
• airmasses have the properties of their source regions
• airmasses move slowly from source region but are modified en-route
• boundary between two airmasses is called a front
• British Isles are usually affected by a series of fronts usually separating polar maritime and tropical maritime air
• front:

  • the transition zone between two different airmasses - regions of warm and cold air extends vertically through troposphere
  • temperature contrasts on wither side of front
  • air converges on either side of the front to produce strong winds
  • at a front, warm air rises over the cold air
  • as the air rises it cools and condenses to form clouds and produce rainfall
  • surface pressure lowers
• warm front:

  • warm moist air rises, condenses producing clouds and precipitation ahead of and at the surface of front boundary
  • clouds are thicker and lower nearer to the front boundary and extensive
  • gentle slope ~1:150-1:200, speed about 15 mph or 25 km/h
• cold front:

  • warm moist air condenses into high clouds ahead of the surface front boundary
  • uplift more rapid than for warm fronts, cumulonimbus form
  • thick clouds - narrow, short-lived band of precipitation (can be thunderstorms) near front boundary
  • steep slope 1:50-1:70, speed typically twice as fast as for warm fronts ~30 mph or 50 km/h
• occluded front:

  • cold front moves about twice as fast as warm front and catches up to the warm front
  • rising air along occluded front with cold air wither side
  • part of life cycle of mid-latitude cyclone
  • low pressure centre of mid latitude cyclone at tip of occluded front as rising air lowers mass of air colums
• mid-latitude cyclone:

  • area of low pressure in the mid-latitudes associated with warm, cold and occluded fronts
  • brings stormy weather
  • air converges at the surface
  • cyclonic motion - anticlockwise in NH
  • typically form between 30-60 degrees along the polar front - boundary between cold polar air and warm tropical air
  • above the polar front is the polar front jet stream - strong winds
• convergence and divergence


  cyclone formation

  • situation of converging surface air and divergence aloft
  • if upper-level divergence is stronger than surface convergence -> surface pressure drops -> surface low deepens
• UK storm centre

  Met Office website set up for winter 2015/16 to raise awareness and aid the communication of approaching severe weather through media and government agencies. storms are named when it has the potential to cause an amber 'be prepared' or red 'take action' wind (and possibly rain) warning