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Wind Lecture2.doc

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Environmental Science
Tanzina Mohsin

Wind – Lecture 2: Global Wind Circulation Troposphere !! What is wind?  Movement of air in an “ordered” fashion What causes air to move? - Differences in air pressure, vertical or horizontal - Differences in pressure forms a pressure gradient force o Two isobars  lines of constant pressure – wind moves from high to low pressure which creates the pressure gradient force - Air tends to flow from high pressure to low pressure - Called a pressure gradient force - Differences in the two figures: o The first figure  isobars were nicely spread out  100Meters apart o Second figure  they are randomly spread apart, not equally spread apart  Different pressure gradient force  Closely spaced isobar = STRONG pressure gradient force  strong wind! Coriolis Force: “fictitious” force due to rotation of the earth - Arises due to the rotation of Earth o Very hard to graph because this force is applicable to any moving object on Earth - In Northern hemisphere, causes a deflection to the right of the motion - In Southern hemisphere, deflection to the left of the motion o E.g., rocket launch  eventually curves to the right or left depending on the hemisphere - Related to the distance from the equator – at the equator, Coriolis force = 0 - Acts at right angle to the wind affecting the direction - Formulated mathematically by Gaspard Gustave de Coriolis in 1835; described by George Hadley a century before - Thought exercise: starting at North Pole, you aim your place towards Toronto. What city are you more likely to arrive at? Toronto, Montreal, or Winnipeg? o ??? - Summary Facts: o PGF (pressure gradient force) is always directed from high pressure towards the lower pressure o Steep PG (closely spaced isobars) indicate strong PGF and high winds o When the wind starts to blow, the Coriolis force causes it to bend to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern Hemisphere Geostrophic wind: balance between PGF and Coriolis results in geostrophic wind - Geostrophic wind is typically 1km or so above the Earth’s surface  blowing freely above 1km from the surface - One characteristic is that the wind is blowing parallel to the isobars - Therefore, PGF and Coriolis force comes to a balance at approx. 1km above the surface o So what is happening between 0-1km?  Surface winds: Below 1km – the wind is influenced by friction • Friction disrupts the balance between PGF and Coriolis Force • Wind is not blowing parallel to the isobars, they are blowing at an angle, 3 things are acting together now • Characteristic  at a lower pressure system, wind is blowing anti-clockwise  at a high pressure system, wind is blowing at clockwise rotation in NORTHERN HEMISPHERE o OPPOSITE FOR SOUTHERN HEMISPHERE  At high – wind clockwise  At low – wind anti-clockwise • Divergence at low, convergence at high pressure system Global Circulation: - Three broad categories of atmospheric circulation: o Global  10,000’s of km o Synoptic  100’s to 1000’s of km o Small scales  less than 100 km - Observation of global circulation: there is more energy released in the polar regions than is received from the sun – the reverse is true for the equatorial region o How can we account for this?  atmospheric circulation – poleward transport by wind One Cell Theory: one large overturning atmospheric cell - Air rises at the equator, moves poleward and then sinks at the pole – one large Hadley cell o Named after George Hadley – British Lawyer and amateur meteorologist - Atmospheric hear transport: o Sensible heat  heat you can “feel” o Latent Heat  stored as water vapour, heat is absorbed when water evaporates, heat is released when water condenses “hidden heat” - Energy transportation: o Energy is transported as sensible and latent heat o Latent heat transport occurs when moist equatorial air moves poleward, cools, condenses, and releases latent heat o Is this theory reasonable???  it would be ok if the Earth did not rotate  however the tilt and land/water contrast also play a role  3 main causes that One Cell Theory cannot explain: • Rotation introduces the Coriolis Effect • Tilt causes seasonality • Continentality causes land/sea variation due to varying thermal inertia (e.g., Monsoons)  One cell theory with a rotating Earth: • Prevailing wind from the east • Clearly not observed  prevailing wind in the midaltitudes is from the west • Therefore, NEED ABETTER THEORY Three Cell Theory: - 3 cells  Hadley Cell, Ferrel Cell, and Polar Cell - Hadley cell near the equator  Air rises at the equator, moves poleward and sinks at 30deg North and 30deg South - Surface flow is equatorward and produces winds from the east - Ferrel cell exists roughly between 30deg (arises) and 60deg (sinks) in each hemisphere - thermally indirect - Surface winds travel north and under the influence of Coriolis force veers to the east - Named after William Ferrel – American Meteorologist and mathematician - Polar cell  rising at 60deg, and sinking at both North and South Poles o Polar easterlies produced Definitions: - Intertropical Convergence Zone (ITCZ): area of rising air at or near the equator, heavy precipitation (0-5deg North and South) - Trade Winds: winds that blow to the south west, surface component of Hadley Cell, among from northeast (0-30deg N and S) - Subtropical High: subsiding air at 30deg North and 30deg South – little precipitation or descending air, referred to as the horse latitudes
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