Meteo 3: Chapter 7Meteo 3: Chapter 7

Analyzing weather above Earth’s Analyzing weather above Earth’s surfacesurface

Read: pp. 251-277 (ignore Read: pp. 251-277 (ignore confluence) confluence)

Why Care about Upper-Air? Why Care about Upper-Air?

Talked about surface pressure and what Talked about surface pressure and what winds we expect to see at the surfacewinds we expect to see at the surface

Lots of atmosphere not at the surface that Lots of atmosphere not at the surface that affects weather conditions at the surface!affects weather conditions at the surface!

Need to have ways to observe and analyze Need to have ways to observe and analyze the atmosphere kilometers from the surfacethe atmosphere kilometers from the surface

Maps of weather above the surfaceMaps of weather above the surface

Upper-air weather conditions plotted on maps Upper-air weather conditions plotted on maps of constant pressureof constant pressure

Data obtained from radiosondes: heights, Data obtained from radiosondes: heights, temperatures, wind speeds and directions, temperatures, wind speeds and directions, humidityhumidity

Constant Pressure SurfacesConstant Pressure Surfaces

Simplifies mathSimplifies math All upper-air maps plotted All upper-air maps plotted

on constant pressure on constant pressure surfacessurfaces

Mandatory pressure Mandatory pressure levels:levels: pressures where pressures where radiosondes always take radiosondes always take observationsobservations

Pressure decreases faster Pressure decreases faster with height closer to with height closer to groundground

A 500 mb chart- Height contours, isohypses, A 500 mb chart- Height contours, isohypses, or heights = isopleths of equal heightor heights = isopleths of equal height

Density decreases with height, so a 150 mb Density decreases with height, so a 150 mb layer must be thicker higher in the atmospherelayer must be thicker higher in the atmosphere

A volume of air will become larger when heatedA volume of air will become larger when heated

Pressure decreases more rapidly with height in Pressure decreases more rapidly with height in cold air columns than warm air columnscold air columns than warm air columns

Warm columns of air taller than cold columnsWarm columns of air taller than cold columns

A 500 mb chart- Height contours, isohypses, A 500 mb chart- Height contours, isohypses, or heights = isopleths of equal heightor heights = isopleths of equal height

Temperatures near the SurfaceTemperatures near the Surface

Forming upper-level troughs/ridgesForming upper-level troughs/ridges

Cold air masses move south, warm air Cold air masses move south, warm air masses move north…forming upper-level masses move north…forming upper-level troughs/ridgestroughs/ridges

Relationship between heights and pressureRelationship between heights and pressure

On a constant pressure map, a minimum in height On a constant pressure map, a minimum in height corresponds to a low pressure center on a constant height corresponds to a low pressure center on a constant height surface on an altitude equal to that heightsurface on an altitude equal to that height– Treat a center of low heights on a constant pressure surface as if it Treat a center of low heights on a constant pressure surface as if it

were a center of low pressurewere a center of low pressure

On a constant pressure map, a maximum in height On a constant pressure map, a maximum in height corresponds to a high pressure center on a constant height corresponds to a high pressure center on a constant height surface at an altitude equal to that heightsurface at an altitude equal to that height– Treat a center of high heights on a constant pressure surface as if it Treat a center of high heights on a constant pressure surface as if it

were a center of high pressurewere a center of high pressure

Height gradients α pressure gradientsHeight gradients α pressure gradients

A contour map ofpressure on a constantheight surface looks thesame as a contour map ofheights on a constantpressure surface.

Therefore, the relationshipbetween the wind andpressure fields is the sameas the relationshipbetween the wind andheight field.

Upper-level winds: Geostrophic approximationUpper-level winds: Geostrophic approximation

Forces on parcel: pressure (height) gradient Forces on parcel: pressure (height) gradient force….Coriolis force increasing in magnitude until it force….Coriolis force increasing in magnitude until it becomes equal in magnitude but opposite in direction to becomes equal in magnitude but opposite in direction to PGF…wind blows parallel to height contours with lower PGF…wind blows parallel to height contours with lower heights to leftheights to left

PGF

COR

5400 m

5460 m

5520 m

A 500 mb chart- with upper-air wind A 500 mb chart- with upper-air wind observationsobservations

Geostrophic wind (wind aloft)Geostrophic wind (wind aloft)

Geostrophic wind: Geostrophic wind: Wind that results due to a Wind that results due to a balance between the height gradient and Coriolis balance between the height gradient and Coriolis forceforce– Good approximation to wind above the groundGood approximation to wind above the ground– There is some friction to throw this balance off, but its There is some friction to throw this balance off, but its

effects are minimal at high altitudeseffects are minimal at high altitudes– Near the ground, friction causes wind to cross isobars Near the ground, friction causes wind to cross isobars

toward lower pressure at ~30º angletoward lower pressure at ~30º angle

Upper-level wind speed & Jet StreamUpper-level wind speed & Jet Stream

Horizontal temperature gradients α height gradients AND Horizontal temperature gradients α height gradients AND the larger the height gradient, the faster the wind speedthe larger the height gradient, the faster the wind speed

Wind speeds increase with altitude up to about 250 mb Wind speeds increase with altitude up to about 250 mb because height gradient on constant pressure surface because height gradient on constant pressure surface increases with altitudeincreases with altitude

Jet StreamJet Stream

When warm and cold air When warm and cold air masses collide, the masses collide, the strongest winds occur just strongest winds occur just below the tropopause (the below the tropopause (the top of the troposphere, top of the troposphere, about 250 mb)about 250 mb)

This fast flowing river of air This fast flowing river of air over mid-latitudes = over mid-latitudes = mid-mid-latitude jet streamlatitude jet stream

Subtropical jet?Subtropical jet?

250mb Heights and Wind Obs250mb Heights and Wind Obs

More on jet streamsMore on jet streams

Embedded in mid-latitude “westerlies”Embedded in mid-latitude “westerlies” Only several hundred kilometers wide, thousands of km Only several hundred kilometers wide, thousands of km

longlong DiscontinuousDiscontinuous Sharp surface front underneath jet stream Sharp surface front underneath jet stream Moves south in winter, north in summerMoves south in winter, north in summer Stronger in winter than summerStronger in winter than summer Jet Streak:Jet Streak: Pocket of faster winds embedded in the jet Pocket of faster winds embedded in the jet

streamstream– Located in regions with enhanced height gradients at ~ 250 mbLocated in regions with enhanced height gradients at ~ 250 mb

Summer and winter jetsSummer and winter jets

low-amplitude pattern (no majorstorms); “zonal pattern”

high-amplitude pattern (major stormspossible); “meridional pattern”