PRESSURE & WIND, GENERAL CIRCULATION, JET STREAMS

FORCES that move AIR: Winds Aloft: (top of troposphere) 1. Gravity 2. Pressure gradient 3. Coriolis effect Surface Winds: 1. Gravity 2. Pressure gradient 3. Coriolis effect 4. Friction

1. Gravity Earth exerts gravitational force on atmosphere. (This causes pressure and density to be greater closer to earth.) Acceleration due to gravity = 9.8 m/sec 2

surface Pressure Gradient a) Vertical (Remember hydrostatic equilibrium)

2. Pressure Gradient b) Horizontal (wind) PGF is perpendicular to isobars. Pressure Gradient Force (PGF)

high wind speed.low wind speed. Wind speed determined by steepness of gradient.

 Current weather map Current weather map

3. Coriolis Effect / Force Apparent deflection of moving object due to rotation of earth.

 Animation Animation  Animation Animation  Animation Animation  Animation Animation

Deflection… 1. Is to the right of the path of motion in the northern hemisphere and to the left of the path of motion in the southern hemisphere. 2. Increases with latitude: maximum at poles; zero at equator Plane of deflecting force is parallel to earth’s surface at poles; no component of deflection parallel to surface at equator.

Deflection Increases with wind speed. 4. Increases with mass of object.

CE is perpendicular to path of motion. If PG and CE were only forces on atmosphere, wind would blow parallel to isobars

4. Friction  Surface provides friction to atmospheric movement; “slows down” the air.

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

geostrophic balance “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 Southern Hemisphere H L H L Around and clockwise Around and counterclockwise Around and clockwise

 How do surface winds differ from these upper tropospheric winds?

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.

Southern hemisphere HIGH PG CE

Resulting wind direction: HIGH PG CE FRIC WIND Southern hemisphere Out and counterclockwise

S. hem, HIGH HIGH

Southern hemisphere LOW PG CE

Southern hemisphere LOW PG CE In and clockwise

S. hem, LOW LOW

Northern hem, HIGH HIGH Out and clockwise

Northern hem, LOW LOW In and counterclockwise

General Circulation

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

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

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) 35 o - 60 o N & S  not steady or persistent

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

Polar Easterlies 65 o - 80 o 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

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

Monsoonal Low Azores Bermuda High Pacific High

Upper Air Movement

HEAT COOL Isobaric surfaces City

HEATCOOL 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.

meters 2300 meters Constant Pressure Map (isobaric maps)

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

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

Constant Altitude Map  Shows pressure at a given altitude

m 550 mb 810mb

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

 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 (20°-90° N & S)  Easterly  in Tropics (15°N - 15°S)

 Upper Level Westerlies have ridges and troughs:  “Rossby Waves” (Longwaves)  Wavelength = 1000s km  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 Current 300 mb map

Subtropical Jet Stream  Between midlatitude tropopause and tropical tropopause

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

Enhanced warming in Arctic is affecting Rossby waves

Highs and Lows move horizontally  Highs move towards convergence aloft  Surface pressure rises in direction High is moving and falls in its wake  Rising barometer means air is being ADDED aloft and sinking air (clear skies) are coming