Lecture 8

• winds: flow away from the equator in extra-tropics and towards the equator in sub-tropics and polar regions
• wet: in the troposphere air cools as it rises. as air cools it can hold less water vapor. the excess water vapor precipitates as rain or snow so precipitation happens when air rises
• dry: descending air loses water when it rises and warms as it descends but can not precipitate
• seasons are due to the orientation of the earth relative to the sun. solar heating changes over the year
• solar radiation is responsible for winds. winds blow due to the difference in solar heating between the equator and poles. only partly successful at eroding the latitudinal temperature gradient. lead to three overall circulation cells
• pressure

  • air molecules hitting a surface. those molecules exert a force
  • more mass -> more molecules -> more force -> greater pressure
  • warmer -> molecules hit faster -> more force -> greater pressure
• pressure is measured in Pascals (Pa) which is force per meter squared
• atmospheric pressure is typically quoted in hPa (1 hPa = 100 pa) typical surface values are around 1000 hPa
• air moves from high to low pressure - pressure gradient force (PGF)
• • warm air is less dense and can rise
  • less molecules at the surface so lower pressure
  • so low pressure at the equator
  • rising air moves away from the Equator to the cooler sub-tropics
  • higher pressure in the sub-tropics as air piles up and sinks
  • return flow at the surface from high to low pressure (PGF)
• the earth is rotating affecting the wind directions. this produces the Coriolis 'force' because we think of the earth as fixes and measure things relative to the earth
• coriolis effect deflects motion to the right in the northern hemisphere and to the left in the southern hemisphere
• the coriolis effect behaves like a force and is proportional to the speed and directed at right angles to motion
• effect is largest at the poles and smallest near the equator
• effect is small - the motion needs to be acting for long enough and over a large enough distance that the rotation of the earth is important
• hadley cell:

  • sun overhead -> lots of solar heating and surface air rises water condenses as air cools -> clouds and rain
  • the surface convergent flow, if over the oceans brings moisture
  • air diverges aloft and floes away from the equator and is deflected
  • the air descends around 30 degrees away from the upflow and is dry so little rain or clouds especially over land
  • explains the pressure patterns - low over equator and high in subtropics
  • forces drive this flow
• polar circulation:

  • thermally driven like hadley circulation
  • the poles are very cold - they have little incoming radiation from the sun
  • cold air is dense and sinks (high pressure) replaces by relatively cold air in the polar cell
• the ferrel cell:

  • mid-latitudes have warm descending air on equatorial side and cold ascending air on the polar side
  • cell is weak
  • not thermally driven - also called indirect thermal circulation
• in the mid-latitudes hear transfer is through eddies (waves) which becomes weather systems. at the surface these are mid latitude cyclones and anticyclones. PGF and CF forces important for mid-latitude flow
• Earth’s climate varies- warm wet tropics, hot dry sub-tropics, mid-latitudes and cold poles
• General circulation or vertical global wind patterns caused by differential solar heating
• Pressure gradient force and Coriolis “force” cause horizontal and vertical wind motions – explain flow around low and high pressure systems
• Descending branch of Hadley Cell is dry and leads to dry sub-tropics
• Hadley and Polar cells are thermally driven
• In between is the Ferrel Cell- indirect circulation
• Pressure gradient force and Coriolis “force”