PRESSURE & WIND, GENERAL CIRCULATION, JET STREAMS

Current weather map Current wind map

Animation

Deflection... Increases with latitude . Increases with wind speed. Increases with mass of object.

Minimal friction aloft > 3000 ft in troposphere “friction layer” : 0 – 3000 ft Winds aloft blow parallel to isobars: geostrophic wind

geostrophic balance Show air parcel moving in response to pressure gradient, then deflected, as in Fig 8.9 , until pressure gradient force and Cor effect are in opposite directions; eventually this balance is reached : “Geostrophic balance”; the resulting wind is geostrophic wind and it is parallel to isobars; (Note: Coriolis effect is to the right of MOTION not to the right of gradient; define geostrophic balance as” balance between pressure gradient and Coriolis forces acting on a parcel so that the forces are equal in magnitude but in opposite directions; wind produced is geostrophic.) “balance between pressure gradient and Coriolis forces acting on a parcel so that the forces are equal in magnitude but in opposite directions”

GEOSTROPHIC WIND Northern Hemisphere L H Around and clockwise Around and counterclockwise Southern Hemisphere L H Around and clockwise Around and counterclockwise

Current upper air map

Friction and Surface Winds Drag produced by surface. Frictional force is applied opposite to direction of air motion; causes wind to blow across the isobars.

At surface, friction reduces wind speed, which reduces Coriolis effect. Coriolis can not balance PGF so wind crosses isobars.

Current weather Current satellite IR

General Circulation REMINDER: Go Over wind speed

Global Wind Systems driven by Highs and Lows at surface Where are Highs and Lows?

Imagine the earth with no rotation HIGH There would be a single cell of convection in each hemisphere LOW

But the earth rotates Coriolis deflection causes air to be deflected from those simple convective pathways Creating 3 cells in each hemisphere and a surface High Pressure in subtropics

Let’s look at SURFACE Components of each cell

Hadley Cells Strong and persistent Warm air rising at Intertropical Convergence Zone (ITCZ) At top of troposphere, spreads poleward, sinks at Subtropical Highs Blows towards ITCZ at Surface, creating…

Trade Winds Between subtropical Highs and ITCZ NE in N. Hem SE in S. Hem

Ferrel cells Not as strong, persistent, well- defined

Westerlies (surface component of Ferrel cells) 35o - 60o N & S not steady or persistent

Polar Front Zone 60o - 65o N & S zone of conflict between differing air masses

Polar Easterlies 65o - 80o N & S more prevalent in Southern, variability in Northern

Distribution of land masses disturbs this idealized system of Highs, Lows, winds Why? Uneven heating of land and water creates temperature differences and therefore pressure differences over land vs water with seasonal changes

Canadian High Siberian High Icelandic Low Aleutian Low Azores Bermuda High Pacific High

Pacific High Azores Bermuda High Monsoonal Low

Upper Air Movement

500 625 Isobaric surfaces 750 875 1000 City A City B COOL HEAT

DECREASED DENSITY INCREASED DENSITY It takes a shorter column of cold air to exert the same surface pressure as a tall column of warm air. 500 625 500 625 750 750 875 875 1000 1000 HEAT COOL

Hot Cold 2300 meters 500 Constant Pressure Map (isobaric maps) 625 750 850 750 850 1000 1000 Hot Cold

Constant pressure map shows elevation of a certain pressure. Low heights and troughs represent cold air. High heights and ridges represent warm air.

5400 5520 5580 5640 5700

Currently: Current surface temperature map Current map of heights of 500 mb layer

Constant Altitude Map Shows pressure at a given altitude

500 625 550 mb 750 810mb 500 1000m 1000m 1000m 625 850 750 850 1000 1000 Hot Cold

On a constant altitude map: low pressures indicate Cold Air high pressures indicate Warm Air

High heights on a constant pressure surface map are equivalent to high pressures on a constant altitude map Low 500 mb heights are associated with low pressure at any given altitude; High 500 mb heights are associated with high pressure at any given altitude.

Therefore, high and low heights tell you where high and low pressures are (for a given altitude)

Upper Level Winds Westerly in mid- and high latitudes Easterly (20°-90° N & S) Easterly in Tropics (15°N - 15°S)

Upper Level Westerlies have ridges and troughs: “Rossby Waves” (Longwaves) Wavelength = 1000s km 3 - 6 loops around earth above 500 mb layer influence surface weather

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Converging height lines make wind speeds increase

On warm side, pressure drops less rapidly with altitude than on cold side; Note isobaric surfaces slope and slope increases with altitude

Therefore, wind speed increases with altitude JET STREAMS : zones of high wind speed (Narrow bands, speed increases toward center (up to 150 mph)) Embedded in upper level Westerlies below tropopause Jet streams are located above strong temperature contrasts at surface

Polar Jet Stream Subtropical Jet Stream

Polar Front Jet Stream Between midlatitude tropopause and polar tropopause

Polar Jet above Polar Front Zone : Where cold dense polar air meets warmer air from mid-latitudes

Can see polar jet on 300 mb maps Current 300 mb map

Subtropical Jet Stream Between midlatitude tropopause and tropical tropopause

Subtropical Jet due to Conservation of Angular Momentum: greatest wind speed at North edge of Hadley cell due to Conservation of Angular Momentum: (smaller radius of rotation, faster the spin)