MIDTERM 1: APPROXIMATE GRADES TOTAL POINTS = 45 AVERAGE = 33 HIGH SCORE = = A = B = C < 20.0 NP

Transcription

1 MIDTERM 1: TOTAL POINTS = 45 AVERAGE = 33 HIGH SCORE = 43 APPROXIMATE GRADES = A = B = C < 20.0 NP

2 Forces to consider: 1) Pressure Gradient Force 2) Coriolis Force 3) Centripetal Force 4) Friction Force Force is a VECTOR!! It has a magnitude AND a direction

3 Pressure Gradient Force () pressure gradient = change in pressure distance is directed from HIGH pressure to LOW pressure at right angles to isobars. isobars H L

4 Clicker Question Set Frequency to "BB" At which location is the pressure gradient force () largest? (A) A (B) B (C) Same at A and B A B isobars (4mb interval)

5 Clicker Question Set Frequency to "BB" At which location is the pressure gradient force () largest? (A) A (B) B (C) Same at A and B At "A", the distance between isobars is smaller, so pressure gradient is largest ~ P / distance A B isobars (4mb interval)

6 Clicker Question Set Frequency to "BB" N HIGH San Diego LOW isobars W S E What would be the wind direction? (A) to the north (B) to the east (C) to the west (D) to the south

7 Clicker Question Set Frequency to "BB" N HIGH San Diego LOW isobars W S E What would be the wind direction? (A) to the north (B) to the east (C) to the west (D) to the south

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10 MIT Coriolis Movie

11 Coriolis Force: An "apparent" force we add to compensate for viewing motion on a rotating reference frame

12 Coriolis Force important when length-scale is large ( s of km)

13 Coriolis Force (CF) - Deflect objects to right in Northern Hemisphere - Deflect objects to left in Southern Hemisphere - Zero at equator - Maximum at poles - Proportional to speed of object - Acts at right angles to direction of motion If interested: CF = 2 m Ω V sin(φ) m = mass of object Ω = rotation rate of Earth V = Velocity of object φ = latitude

14 Low Pressure High Pressure

15 Low Pressure Initially, no motion, so only force to start with is the High Pressure

16 Low Pressure Initially, no motion, so only force to start with is the At point 0 V 0 = 0 High Pressure

17 Low Pressure Air begins to move in direction of. V 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

18 Low Pressure Air begins to move in direction of. - remains the same, but... V 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

19 Low Pressure Air begins to move in direction of. - remains the same, but... - Now there is a Coriolis Force V 1 CF 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

20 Low Pressure V NET 1 CF 1 At point 1 V 1 > 0 Air begins to move in direction of. - remains the same, but... - Now there is a Coriolis Force - Net force is combo of CF and At point 0 V 0 = 0 High Pressure

21 Low Pressure V NET 1 CF 1 At point 1 V 1 > 0 Air begins to move in direction of. - remains the same, but... - Now there is a Coriolis Force - Net force is combo of CF and - Velocity increases and changes direction At point 0 V 0 = 0 High Pressure

22 Low Pressure V 2 Air moves in direction of NET force At point 2 V 2 > V 1 V NET 1 CF 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

23 Low Pressure V 2 At point 2 V 2 > V 1 Air moves in direction of NET force - Coriolis perpendicular and proportional to velocity CF 2 V NET 1 CF 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

24 Low Pressure V 2 NET At point 2 V 2 > V 1 CF 2 Air moves in direction of NET force - Coriolis perpendicular and proportional to velocity - NET force adjusts and bends more V NET 1 CF 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

25 Low Pressure V 3 V 2 NET At point 2 V 2 > V 1 CF 2 As NET force bends, so does the velocity V NET 1 CF 1 At point 1 V 1 > 0 At point 0 V 0 = 0 High Pressure

26 Low Pressure V 3 V 2 NET CF 3 At point 2 V 2 > V 1 V NET 1 CF 1 At point 1 V 1 > 0 CF 2 As NET force bends, so does the velocity - Coriolis Force keeps changing direction as velocity change At point 0 V 0 = 0 High Pressure

27 Low Pressure V 3 V 2 NET NET CF 3 At point 2 V 2 > V 1 V NET 1 CF 1 At point 1 V 1 > 0 CF 2 As NET force bends, so does the velocity - Coriolis Force keeps changing direction as velocity change - and NET force also adjusts At point 0 V 0 = 0 High Pressure

28 Low Pressure V 3 V V 2 NET NET CF 3 CF At point 2 V 2 > V 1 V NET 1 CF 1 At point 1 V 1 > 0 CF 2 Finally, Coriolis force is equal and opposite of the - Now, no net force - Velocity remains constant - Velocity is parallel to the isobars At point 0 V 0 = 0 High Pressure

29 GEOSTROPHIC WIND - Straight isobars; no friction - Balance between Pressure Gradient Force and Coriolis Force LOW isobars WIND Coriolis Force HIGH

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31 GRADIENT WIND isobars WIND LOW V 2 /R Coriolis Force HIGH = CF + V 2 /R (Flow around Low Pressure)

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34 NOW INCLUDE FRICTION - Friction and Coriolis act together to balance - Wind crosses isobars at an angle LOW isobars Friction WIND HIGH Coriolis Force

35 Generally, friction is stronger over land As a result, surface winds cross the isobars at a greater angle over land than over water. Over Land Over Water wind wind

36 NOW INCLUDE FRICTION - Friction and Coriolis act together to balance - Wind crosses isobars at an angle LOW isobars Friction WIND HIGH Coriolis Force Stronger the friction, the more the wind 'veers' across isobars - Ocean (less friction) angle is about degrees - Land (more friction) angle can be up to degrees

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39 Clicker Question Set Frequency to "BB" Would you expect more clouds near a surface low pressure or a surface high pressure? (A) surface high pressure (B) surface low pressure (C) equal chance for clouds

40 Northern Hemisphere Air motion around surface low and high pressures Surface LOW: - surface air converges towards low - air rises away from surface - generally cloudy skies, stormy Surface HIGH: - surface air diverges away from high - air sinks towards surface - generally clear skies, fair weather