Why Does the Wind Blow, and in What Patterns?

1. Why Does the Wind Blow,and in What Patterns? Observations (generally): Near the earth’s surface: winds blow across isobars toward the lower-pressure side of isobars at an angle rotated to the right of (clockwise from) the perpendicular [opposite in So. Hem.] faster where pressure gradient is larger(for similar underlying surface) faster over water than over land(for same pressure gradient)

3. Why Does the Wind Blow,and in What Patterns? Resulting patterns: • Around low pressure center: • Winds form inward, counter clockwise spiral pattern around low [inward, clockwise spiral in So. Hem.] • Air converging into low must go somewhere; can only go up • Pressure decreases with altitude, so rising air expands as it rises • Air that expands under lower pressure will cool • When temperature drops below dew point, water vapor begins to condense, forming cloud • Surface low pressure areas tend to be cloudy

4. Why Does the Wind Blow,and in What Patterns? Resulting patterns: • Around high pressure center: • Winds form outward, clockwise spiral pattern around low [outward, counter-clockwise spiral in So. Hem.] • Air must come from somewhere to replace air diverging out of high; can only come from above • Pressure decreases with altitude, so pressure on sinking air increases and so air compresses as it sinks • Air that compresses under higher pressure will warm • No cloud can form in warming air; any existing cloud will evaporate • Surface high pressure areas tend to be clear

5. Why Does the Wind Blow,and in What Patterns? Observations (generally): • Aloft (significantly far above the earth’s surface): • winds blow roughly parallel to isobars • with lower-pressure to the left of the wind’s direction of motion [opposite in So. Hem.] • faster where pressure gradient is larger

7. Why Does the Wind Blow,and in What Patterns? Resulting patterns: • Around lows and troughs aloft: • Wind blows counterclockwise [opposite in So. Hem.] • But (generally) no convergence into them • Around highs and ridges aloft: • Wind blows clockwise [opposite in So. Hem.] • But (generally) no divergence out of them • At midlatitudes, see narrow zone of fast winds (polar jet stream) • Winds blow from (roughly) west toward east in both hemispheres (speeds at least 60 knots, up to 200+ knots) • Jet stream location wobbles across midlatitudes • Wobbles migrate eastward (typically 20-25 knots) • Wobbles associated with troughs and ridges in pressure pattern

9. Why Does the Wind Blow,and in What Patterns? Explanations based on fundamental physical principle Conservation of momentum([Isaac] Newton’s 2nd Law): Rate at which sum of forces an object’s motion = acting on the object (velocity) changes (per unit mass of the object)

10. Why Does the Wind Blow,and in What Patterns? Forces acting on air: • Pressure-gradient force (PGF) • Arises from difference in pressure between places • Larger where pressure gradient is larger • Pushes on air toward lower pressure side • On isobar map: • larger where isobars are closer together (pressure gradient larger there) • pushes in direction perpendicular to isobars, toward lower-pressure side • zero at pressure maxima and minima (highs and lows) because no pressure gradient there

11. Why Does the Wind Blow,and in What Patterns? Forces acting on air (cont’d) • Coriolis force • Apparent force arising from rotation of the earth beneath objects as the move around • Appears to push on objects in direction perpendicular to their direction of motion • To the right of the direction of motion [opposite in So. Hem.] • So can’t change speed of object, only direction • Appears to push harder the faster the object moves • zero when object isn’t moving (so can’t push anything into motion!) • zero at equator

12. Why Does the Wind Blow,and in What Patterns? • Note: Combination of PGF and Coriolis force seems to explain pattern of winds aloft very well • When PGF and Coriolis are the only forces acting, winds tend to blow so that Coriolis force acting on it roughly balances PGF acting on it • Balance occurs when wind blows parallel to isobars, with lower pressure on left [opposite in So. Hem.], and faster where PGF is larger • Wind that blows this way is called geostrophic wind • Doesn’t explain surface winds well

13. Why Does the Wind Blow,and in What Patterns? Forces acting on air (cont’d) • Friction • Pushes in direction opposite wind direction • Therefore tries to slow wind down • Can’t create wind (can’t slow down air that’s not moving!) • Results from drag when air blows across underlying surface • Less drag (so less friction) over ocean than over land • Therefore not important aloft

14. Why Does the Wind Blow,and in What Patterns? Does combination of PGF, Coriolis, and friction explain observed pattern of winds at earth’s surface? (See handout, “Geostrophic Winds Aloft vs. Winds at Earth’s Surface in the Northern Hemisphere”)

15. Some Other Important Questions • What accounts for the pressure pattern aloft? • Why is pressure aloft generally lower at higher latitudes and higher at lower latitudes, with a large gradient in between, in the midlatitudes? • Why are there troughs and ridges in the pressure pattern aloft at midlatitudes? • Why do we see maxima and minima (highs and lows) at midlatitudes at the earth’s surface (and sea level)?