Air Pressure, Winds, El Nino summary and notes




Air Pressure, Winds, El Nino summary and notes


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Air Pressure, Winds, El Nino summary and notes


The Atmosphere

Summary notes.

Part III: Air Pressure, Winds, El Nino



pressure is exerted by weight of air above – on a global scale atmosphere is in hydrostatic balance, i.e. pressure at every altitude is exactly that needed to balance the weight of air above (so that air doesn’t fall to ground or take off into space)

it takes a shorter column of cold, more dense air to exert the same surface pressure as a tall column of warm, less dense air

atmospheric pressure decreases more rapidly with elevation in a cold column of air

warm air aloft is associated with high pressure aloft and cold air aloft is associated with low pressure aloft

thermal circulation cell – high pressure aloft moves air from warm column of air to cold column of air aloft (PGF). This reduces surface pressure in warm column and increases it in cold column. This change in surface pressure generates a PGF at surface in opposite direction to PGF aloft. This horizontal movement of air is accompanied by vertical motion – sinking where air is coming into cold column aloft above surface high, and rising where air is coming into warm column at surface low.

high surface pressure is accompanied by sinking air – clear weather

low surface pressure is accompanied by rising air – cloudy weather

divergence at surface accompanied by convergence aloft

convergence at surface accompanied by divergence aloft

pressure charts are used for sea-level (i.e. 0 m), all surface data is converted to 0 m altitude.

lines of constant pressure are isobars (know typical surface pressure)

‘H’ = center of high pressure = anticyclone

‘L’ = center of low pressure = cyclone

elongated highs = ridges

elongated lows = troughs



wind is caused by horizontal differences in pressure – which are caused by unequal heating

if earth did not rotate and there was no friction, air would go directly from high -> low pressure

pressure gradient force (PGF):

Pressure gradient force is perpendicular to isobars – in a direction from high to low pressure.

spacing of isobars shows pressure gradient = difference in pressure/distance. The closer the isobars the greater the pressure gradient, and the greater the pressure gradient force, the stronger the wind.

PGF =(change in pressure/horizontal distance)

Coriolis force (CF):

apparent force due to rotation of earth (if we were fixed out in space we would not experience this force. It is ‘apparent’ because it exists only because we are on a rotating earth and so to us free-moving objects are deflected from straight-line travel.)

in NH objects (winds) are deflected to the right

in SH objects (winds) are deflected to the left

Coriolis Force –

is at right angles to direction of movement (direction of wind)

affects only direction, not speed of an object

the stronger the wind, the stronger the force

strongest at poles, 0 at equator

Geostrophic wind = wind generated aloft by balance between PGF and CF. – This is the wind ‘aloft’ or the ‘upper level’ wind (ie. above the influence of the surface)

flows parallel to isobars

stronger for closely-spaced isobars

stand with low pressure on your left, high on your right. In NH wind will be at your back (in SH wind will be in your face)

For a Low, flow is anticlockwise in NH, clockwise in SH

For a High, flow is clockwise in NH, anticlockwise in SH


Surface wind = wind at/close to earth’s surface where friction is important

surface wind is in same direction as upper level wind but angles across isobars because of friction

friction caused by trees, buildings, topography

PGF now balanced by CF plus friction

PGF directed from H to L

CF at right angles to wind

friction directly opposes wind

surface wind blows at an angle to isobars (in towards L, out away from H) – angle depends on roughness of terrain, 10° over smooth water, 35° near hills

because of friction, surface winds blow slower than geostrophic winds aloft

Vertical air motion

surface lows (cyclones) cause air to blow inwards = convergence at surface -> air must rise -> divergence aloft. Therefore surface lows are associated with rising air, clouds, precipitation.

surface highs (anticyclones) cause air to blow outwards at surface = divergence at surface -> convergence aloft -> subsidence of air. Therefore surface highs are associated with sinking (and warming) air and fair weather.


3 scales of winds –

       driven by differences in temperature between land and ocean, or between different latitudes

  • local (mesoscale) = land-sea breeze (wind blows from sea to land during warmest part of day because land heats up faster than water. Wind blows from land out to sea in evening as water retains its heat longer.)
  • regional (synoptic) = winds we see on a weather map = monsoon, cyclones, anticyclones
  • global circulation:

3-cell model (see chart)

Hadley cell: equator - 30° - rising air at equator

Ferrel cell: 30° - 60°

Polar cell: 60° - 90°

Equatorial low, Doldrums; 30° high, Horse latitudes, 60° sub-polar low, 90° polar high

Surface winds – Trade winds, Westerlies, Polar Easterlies

Causes precipitation to be high in equatorial region (low pressure, rising air), low precipitation in sub-tropics (30º latitude) due to high pressure and sinking air, more

precipitation in mid-latitudes (low pressure), dry at poles (high pressure, sinking air, cold air – contains little moisture)


Ocean Currents

drag exerted by winds blowing across ocean causes surface water to move -> force ocean currents. Leads to motion several 100m deep into water. Friction greater in water than air so ocean currents are slower than winds (typically kms/day rather than kms/hr)

Coriolis force deflects water to R in NH, L in SH relative to direction of wind.  Leads to ocean currents in gyres.  Circulation direction follows that of subtropical highs.

Ocean current have strong effect on climate –

     N. Atlantic Drift/Gulf Stream: keeps Britain and N. Europe warm

     Cold currents along W. side of continents. Warm currects along E. side of continents

     Ocean currents transfer heat from tropics to poles.

Ekman spiral (ocean flow is diverted progressively further to R. in NH as you go deeper into water) – leads to net motion of water at 90° to direction of surface wind – leads to cold upwelling where wind blows parallel to coast (eg along parts of CA coast).

thermocline = boundary separating warm surface water and cold deep water


El Nino

anomalous warming of sea surface in equatorial Pacific

occurs in conjunction with Southern Oscillation (an atmospheric see-saw between a high pressure center in SE Pacific at Easter Island and low pressure center over Indonesia)

Need to know - direction of winds and current, position of warm puddle of water, atm. pressure associated with El Nino, depth of thermocline associated with normal and El Nino conditions.


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