10 2 15 and 10 5 15 Dr Frauenfeld Chapter 6 Atmospheric and Oceanic Circulations I Wind Essentials a Wind is measured with an anemometer speed and direction b Named based on direction that the wind comes from i e Northerly wind comes from the North and moves South c What causes the wind to blow i Sea Level Pressure 1 Standard sea level pressure 1013 25 mb 2 Variability highest at the surface lowest at top of the troposphere horizontal variability is only 30 vertical variability is much greater but balanced due to gravity so it doesn t affect the wind a Strong high pressure system Anti cyclone 1040 mb b Deep low pressure system Cyclone 980 mb ii Air Pressure 1 What causes horizontal pressure variations a Temperature temperature is inversely related to pressure and density 10 2 15 and 10 5 15 Dr Frauenfeld i High temperature hot low pressure and low density particles move apart Thermal Low ii Low temperature cold high pressure and high density particles get closer together Thermal High b Air Motion Warm air rises cold air sinks i Raising air low pressure on surface decrease particles on surface ii Sinking air high pressure on surface increase particles on surface Dynamic Low Dynamic High II Driving Forces Within the Atmosphere a Gravity don t need to worry about because it is constant across Earth s surface b Pressure Gradient Force PGF i Causes the wind to blow from high to low pressure areas ii The PGF will always be at 90 angles to the isobars iii The further apart the isobars are the less strong the PGF so the weaker lighter the winds iv The closer together the isobars the stronger the PGF so the stronger the winds The stronger the PGF the higher the wind speed v High PGF air diverges and sinks vi Low PGF air converges and rises Flows from high to low 10 2 15 and 10 5 15 c Coriolis Force Dr Frauenfeld i Apparent deflection of moving objects ii Due to Earth s rotation West East spin iii Effects only the direction of wind movement not speed iv Deflection is to the right counter clockwise of the wind motion in the Northern Hemisphere and to the left clockwise of the wind motion in the Southern Hemisphere v Variability in the Coriolis Force latitude strong Coriolis force 1 Varies by latitude 0 at the equator and is maximized at the poles higher 2 Varies by wind speed The faster the wind the strong the Coriolis Force d Coriolis Force Pressure Gradient Force i When you combine the two forces wind flows parallel to the isobars Geostrophic ii winds only happens in the upper atmosphere Is the reason why low pressure systems flow counter clockwise in the Northern Hemisphere and why high pressure systems flow clockwise in the Northern Hemisphere e Friction Force Gradient forces i Occurs below 500 meters 1600 feet ii Decreases wind speed which disrupts the balance between the Coriolis and Pressure 1 Coriolis becomes weaker because the slower the wind the weaker the Coriolis force 10 2 15 and 10 5 15 Dr Frauenfeld a So the PGF beats out the Coriolis effect and causes the wind to cross the isobars and not be parallel to them iii Caused by objects on the surface trees buildings vegetation etc f Coriolis Pressure Gradient Friction i Friction slows the wind weakens the Coriolis force wind crosses the isobars 1 Wind flow converges into a low pressure system 2 Wind flow diverges out of a high pressure system g Two main winds i ii If no friction Geostrophic Winds Upper Atmosphere If friction Surface Wind Surface III Global Circulation If Earth didn t rotate and if it had an uniform surface a Known as Hadley Circulation which is how he thought it would work there is only 1 convection cell in each hemisphere partially correct 10 2 15 and 10 5 15 Dr Frauenfeld Low pressure at the equator due to high temps and rising air high pressure at the poles due to low temps and sinking air If we take into account rotation b There is rising from the equator but it doesn t make it to the poles and doesn t go straight back to the equator because it gets deflected c Global Pressure Zones i Polar High at North Pole high pressure ii Subpolar Low at 50 60 N low pressure iii Subtropical High at 30 N high pressure iv Equatorial Low ITCZ Equatorial Low low pressure v Subtropical High at 30 S high pressure vi Subpolar Low at 50 60 S low pressure vii Polar High at South Pole high pressure d Global Surface Wind Zones correspond to pressure zones above H L H L H L H Polar Easterlies Midlatitude Westerlies Northeast Trade Winds Southeast Trade Winds Midlatitude Westerlies Polar Easterlies 10 2 15 and 10 5 15 Dr Frauenfeld i Equatorial Low Pressure 1 10 N to 10 S 2 A lot of energy a Constant angle of incidence b Consistent day length always 12 hours Surface Air Heats Up Thermal Low 3 Air Converges and Rises 4 Also called the Intertropical Convergence Zone ITCZ Does migrate because of the change in declination max 25 N and min 20 S migrates more over land areas because they heat up more quickly and have a more extreme temperature gradient ii Trade Winds 1 25 N NE Trade Winds to 25 S SE Trade Winds 2 Great variability stronger over the ocean because land heats up more 3 The ITCZ is caused by the converging air from the NE and SE trade winds this process makes up the Hadley Cells a Air converges and rises travels poleward cools and then sinks b Two Hadley Cells one in the Northern Hemisphere and one in the Southern Hemisphere iii Subtropical Highs Occur where the Hadley Cells have sinking air 1 20 to 30 North and South 2 High pressure hot dry low humidity descending air weather clear warm calm 3 Migrates 5 10 in latitude with the seasons Stronger in July in Northern Hemisphere and strong in January in the Southern Hemisphere 4 Strongest over the ocean 5 Where the global deserts are located because as air sinks it warms up and dries out which leads to desert formation iv Westerlies In between the Subtropical Highs and Subpolar Lows 1 Diverging winds North and South of Hadley Cells 2 30 to 60 North and South v Sub Polar Low Pressure 1 60 N and 60 S 2 Cool moist air 3 Clouds 4 Seasonal variability Bigger in the Southern Hemisphere in July Bigger in the Northern Hemisphere in January nonexistent in July 5 Strongest over the Oceans vi Easterlies In between the Subpolar Lows and Polar Highs 1 Highly deflected stronger Coriolis 2 Cold and dry vii Polar High Pressure Northern Hemisphere 3 Cold dry air 1 90 N and 90 S 2 Persistent in the Southern Hemisphere because of Antarctica land Seasonal in the 10 2 15 and 10 5 15 Dr Frauenfeld e