What Makes the Wind Blow

1. What Makes the Wind Blow? ATS 351 Lecture 8 October 26, 2009

2. Atmospheric Pressure Hold Pressure constant: as temperature increases, the gas will become LESS dense Hold temperature constant: linear relationship so as pressure increases, so does density air at a higher pressure is more dense than air at a lower pressure Air above a region of surface HIGH is more dense than air above a surface LOWHold Pressure constant: as temperature increases, the gas will become LESS dense Hold temperature constant: linear relationship so as pressure increases, so does density air at a higher pressure is more dense than air at a lower pressure Air above a region of surface HIGH is more dense than air above a surface LOW

3. Atmospheric Pressure It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure At a given level, more molecules exist above warm air than cold air = higher pressure

4. Two air columns, each with identical mass, have the same surface air pressure. Because it takes a shorter column of cold air to exert the same pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force thatcauses the air to move from a region of higher pressure toward a region of lower pressure.Two air columns, each with identical mass, have the same surface air pressure. Because it takes a shorter column of cold air to exert the same pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force thatcauses the air to move from a region of higher pressure toward a region of lower pressure.

5. Measuring Atmospheric Pressure Barometer, barometric pressure Aneroid barometer Altimeter, barograph Commonly used units Millibar (mb) Pascal (100Pa=1mb) Hectopascal (1hPa=1mb) Inches of mercury (in. Hg) Aneroid barometer = figure on the right. Aneroid barometer = figure on the right.

6. Converting from millibars to inches of mercury Point out highest and lowest pressures ever recored Also, typical pressures associated with strong H pressure and deep L pressure.Converting from millibars to inches of mercury Point out highest and lowest pressures ever recored Also, typical pressures associated with strong H pressure and deep L pressure.

7. Pressure Readings Barometer reading at a particular location ? station pressure Must adjust pressure at higher altitudes to sea level ? sea level pressure Add 10mb of pressure for every 100m above sea level Adjusted pressure is called sea level pressure Figure: Point C has a pressure of 894mb. Since it is 1100m above the surface, add 110 mb to the 894mb = 1004 mb at sea levelAdjusted pressure is called sea level pressure Figure: Point C has a pressure of 894mb. Since it is 1100m above the surface, add 110 mb to the 894mb = 1004 mb at sea level

8. Surface and Upper-Level Charts Sea-level pressure chart: constant height Upper level or isobaric chart: constant pressure surface (i.e. 500mb) High heights correspond to higher than normal pressures at any given latitude and vice versa

9. Cold air aloft: low heights or low pressure Warm air aloft: high heights or high pressures Ridge: where isobars bulge northward Trough: where isobars bulge southward

10. In Northern Hemisphere: High pressure: anticyclone (winds blow clockwise and outward from center) Low pressure: mid-latitude cyclone (winds blow counter clockwise and inward towards center) Notice wind direction around L and H on surface map Point out location of trough and ridge on 500mb map Point out that surface map is ISOBARS (pressure lines) whereas the upper-level 500mb map is HEIGHTS at 500mb (average value of 500mb is 5600) Point out how winds are flowing on the upper-level chart = blow parallel to contour lines (more on this later), whereas surface winds cross isobars (more on this later too)Notice wind direction around L and H on surface map Point out location of trough and ridge on 500mb map Point out that surface map is ISOBARS (pressure lines) whereas the upper-level 500mb map is HEIGHTS at 500mb (average value of 500mb is 5600) Point out how winds are flowing on the upper-level chart = blow parallel to contour lines (more on this later), whereas surface winds cross isobars (more on this later too)

11. Newton�s 2nd Law of Motion 2nd Law: F = ma F: net force; m: mass of object; a: acceleration At a constant mass, the force acting on the object is directly related to the acceleration that is produced. The object accelerates in the direction of the net force acting on it Therefore, to identify which way the wind will blow, we must identify all the forces that affect the movement of air Object accelerates in the direction of the NET FORCE acting on it. This is important when we begin to talk about winds and which direction they are going to blowObject accelerates in the direction of the NET FORCE acting on it. This is important when we begin to talk about winds and which direction they are going to blow

12. Forces that Affect Wind Pressure gradient force (PGF) Coriolis force Centripetal force Friction

13. Pressure Gradient Force Pressure gradient = ?p/d ?p: difference in pressure d: distance PGF has direction & magnitude Direction: directed from high to low pressure at right angles to isobars Magnitude: directly related to pressure gradient Tight lines (strong PGF) ? stronger wind PGF is the force that causes the wind to blow MAYBE DO WATER LEVEL EXPERIMENT: Shows that the greater the pressure difference, the stronger the force and the faster the water moves = same happens with airMAYBE DO WATER LEVEL EXPERIMENT: Shows that the greater the pressure difference, the stronger the force and the faster the water moves = same happens with air

14. Point out direction and magnitude of PGF vectors Basic run-down of how to calculate PGF. Take one pressure (1024) minus the second (1016) and divide it by the distance using the scalePoint out direction and magnitude of PGF vectors Basic run-down of how to calculate PGF. Take one pressure (1024) minus the second (1016) and divide it by the distance using the scale

16. Coriolis Force Apparent deflection due to rotation of the Earth Right in northern hemisphere and left in southern hemisphere Stronger wind = greater deflection No Coriolis effect at the equator greatest at poles. Only influence direction, not speed Only has significant impact over long distances Coriolis = 2?sin? M = mass ; Omega = earth�s angular spin rate; V = velocity of object; phi = latitudeM = mass ; Omega = earth�s angular spin rate; V = velocity of object; phi = latitude

17. Geostrophic Winds When the force of PGF and Coriolis are balanced Travel parallel to isobars at a constant speed An approximation since isobars are rarely straight in real atmosphere but close enough to understand winds aloft Spacing of isobars indicates speed Close = fast, spread out = slow The wind starts to blow at point 1 and the Coriolis begins to deflect it right (in the NH) while PGF stays the same. When the strength of the Coriolis eventually equals the PGF = GEOSTROPHIC WINDS that blow parallel to the isobars. Point out how the vectors of PGF and Coriolis are the same at step 5The wind starts to blow at point 1 and the Coriolis begins to deflect it right (in the NH) while PGF stays the same. When the strength of the Coriolis eventually equals the PGF = GEOSTROPHIC WINDS that blow parallel to the isobars. Point out how the vectors of PGF and Coriolis are the same at step 5

18. Gradient Winds & Centripetal Force Gradient wind parallel to curved isobars above the level of frictional influence (winds aloft) An object accelerates when it is changing speed and/or direction. Therefore, gradient wind blowing around a low pressure center is constantly accelerating Centripetal acceleration: directed at right angles to the wind, inward toward center of low Centripetal force: inward-directed force Results from an imbalance between the Coriolis force and the PGF Since gradient wind is constantly acceleration - centripetal acceleration is producedSince gradient wind is constantly acceleration - centripetal acceleration is produced

19. Cyclonic flow: PGF > CF Anticyclonic flow: PGF < CF The NET force associated with the low is greater towards the center (PGF) due to the inward centripetal force. Why is the PGF going in towards the center of the low?? Because PGF is always going from H to L pressure and therefore is directed TOWARDS the L. Therefore, the PGF in anticyclonic flow is outward because it is going from H to L. The Coriolis is pointed inwards because it is to the RIGHT of the flow which is clockwise.The NET force associated with the low is greater towards the center (PGF) due to the inward centripetal force. Why is the PGF going in towards the center of the low?? Because PGF is always going from H to L pressure and therefore is directed TOWARDS the L. Therefore, the PGF in anticyclonic flow is outward because it is going from H to L. The Coriolis is pointed inwards because it is to the RIGHT of the flow which is clockwise.

20. Zonal & Meridional Winds Zonal winds: oriented in the W-E direction (parallel to latitude) Moves clouds, storms, surface anticyclones rapidly from west to east Meridional winds: oriented in a N-S trajectory As part of storm systems, tends to move warm air northward and cold air southward

21. Surface and Upper-Level Winds Winds on Upper-level Charts Winds parallel to contour lines and flow west to east Heights increase from north to south Surface Winds Winds normally cross isobars and blow more slowly than winds aloft Friction: reduces the wind speed which in turn decreases the Coriolis effect Friction layer: surface to about 1000m (3300ft) Winds cross the isobars at about 30� into low pressure and out of high pressure The reason winds are slower and cross isobars at the surface is FRICTIONThe reason winds are slower and cross isobars at the surface is FRICTION

22. PGF at surface is balanced by the sum of friction and Coriolis force Surface winds into low and outward from high The effect of surface friction is to slow down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure. Less Coriolis means wind is not deflected as much to right��so therefore wind is going to slightly turned at an angle - ALPHA. And cross isobars. It will also slow down. This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high. Aloft, away from the influence of friction, the winds blow parallel to the lines, usually in a wavy west-to-east pattern. The effect of surface friction is to slow down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure. Less Coriolis means wind is not deflected as much to right��so therefore wind is going to slightly turned at an angle - ALPHA. And cross isobars. It will also slow down. This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high. Aloft, away from the influence of friction, the winds blow parallel to the lines, usually in a wavy west-to-east pattern.

23. Winds & Vertical Motions Since surface winds blow into the center of a low, they are converging and that air has to go somewhere ? slowly rises Vice versa for winds blowing outward from H A surface low has convergence at surface and divergence aloft and a surface high has the opposite. Surface lows tend to be associated with stormy weather, as the rising motion leads to precipitation, while high pressure systems are associated with warmer, drier weather due to the sinking motions

24. Winds and air motions associated with surface highs and lows in the Northern Hemisphere.Winds and air motions associated with surface highs and lows in the Northern Hemisphere.

25. Hydrostatic Balance There is always a strong PGF directed upward Gravity balances the upward PGF When they are equal, hydrostatic equilibrium exists Good approximation for atmosphere with slow vertical movements Is not valid for violent thunderstorms and tornadoes. Why is there a strong PGF directed upward??? Because there is higher pressure at the surface than aloft - so therefore wind will want to move from surface upwards.Why is there a strong PGF directed upward??? Because there is higher pressure at the surface than aloft - so therefore wind will want to move from surface upwards.