Lecture 12

• anticyclones:

  • a large high pressure region
  • air spirals away from it at the surface and this creates descent
  • descending air gets warmer
  • assuming the total water content is conserved, this means the air gets less saturated, so any clouds (liquid water) will evaporate (to water vapour)
  • tend to be cloud-free and sunny
  • in the summer they generate hot weather
  • slow moving to they can persist for several days - can lead to heatwaves
  • well spaced isobars - weak pressure gradients - low winds - can accumulate air pollution
• anticyclone formation:

  • converging air aloft, and surface divergence
  • if upper-level convergence is stronger than surface divergence -> surface pressure rises -> surface high deepens
  • vertical motion - descent ~1cm/sec (1km/day)
• ITCZ


  intertropical convergence zone

  • easterly trade winds converge near the equator along the ITCZ
  • warm moist air rises all the way to the tropopause -> thunderstorms form
  • the air then drives the Hadley cell circulation with descending air about 30 degrees
• ITCZ migration

  follows the migration of the Sun's overhead position with a delay of around 1-2 months. as the ocean heats up more slowly than the land, the ITCZ moves further north and south over land areas than over water. in July and August the ITCZ lies north of the equator over Africa, Asia and Central America. in January and February lies further south into South America, Central Africa and Australia. responsible for wet (short and long rains) and dry seasons in the tropics
• monsoon = wind reversal

  • important seasonal feature
  • closely related to ITCZ position
  • it is a regional large-scale sea breeze circulation
  • in summer strong solar heating over continent leads to low-pressure as warm air rises; brings in moist oceanic air and heavy rainfall
  • wind reversal: sea to land (summer); land to sea (winter) season
  • occurs in India, W Africa, E Asia, Northern Australia, Central America
• tropical cyclones

  typically a few 100 km in size and up to 500 km (mid-latitude cyclones are bigger ~2000 km). typical central pressure of 950 hPa, but record low 870 hPa (mid-latitude cyclones ~970 hPa)
• tropical cyclones only form over oceanic regions where sea-surface temperatures are greater than 26.5 degrees C
• tropical cyclones do not form within 5 degrees of the equator due to the negligible Coriolis force there
• tropical cyclones form in regions where the vertical wind shear between the surface and upper troposphere is low (less than ~23 mph or 10 m/s)
• tropical cyclones form from tropical disturbances


  there are 2 main mechanisms:

  • easterly waves - originate over continents as air moves across mountains/deserts e.g. off west coast of Africa
  • the ITCZ - easterly trade winds converge