WindsWindsProfessor Jeff GawrychProfessor Jeff Gawrych

De Anza CollegeDe Anza College

Principle forcesPrinciple forcesQ: What drives the weather in the atmosphere?Q: What drives the weather in the atmosphere?

The uneven heating of the earthThe uneven heating of the earthQ: What causes the weather to change?Q: What causes the weather to change?

Pressure differences and windsPressure differences and windsWe must analyse the predominant forces in the We must analyse the predominant forces in the

atmosphere, namely:atmosphere, namely:– pressure and pressure gradients, pressure and pressure gradients, – gravitygravity– rotation of the earthrotation of the earth– frictionfriction

Pressure differencesPressure differences Determines the direction and speed of winds Determines the direction and speed of winds

Can help explain general circulation of Can help explain general circulation of atmosphere (next lecture)atmosphere (next lecture)

These differences are brought on by These differences are brought on by Temperature and density changesTemperature and density changes

Gas law Gas law p=p=RTRTp-Pressure (Pa), p-Pressure (Pa), - density (g m - density (g m-3-3))R - Gas Constant (287 J KR - Gas Constant (287 J K-1-1 kg kg-1-1))T - Temperature (K)T - Temperature (K)Cold air is more dense and warm air is less dense, so cold air Cold air is more dense and warm air is less dense, so cold air sinks and warm air rises!!!sinks and warm air rises!!!

It’s the It’s the differencedifference in pressure that in pressure that causes a fluid causes a fluid

like air to move.like air to move.

Pressure Gradient ForcePressure Gradient Force Pressure gradient: Pressure gradient:

– Difference in pressure between different Difference in pressure between different locations.locations.

– Force blows from higher pressure to lower Force blows from higher pressure to lower pressurepressure

– Force is directed perpicular to isobaForce is directed perpicular to isoba

– Stronger presure gradients produce Stronger presure gradients produce stronger windsstronger winds

Pressure Gradient ForcePressure Gradient Force

The Rotation of the The Rotation of the EarthEarth

Rockets, migrating birds, and large scale Rockets, migrating birds, and large scale weather systems are all deflected due to the weather systems are all deflected due to the rotation of the Earth.rotation of the Earth.

The Earth’s rotation causesThe Earth’s rotation causes– Rotational movement that deflects the winds.Rotational movement that deflects the winds.

The The Coriolis ForceCoriolis Force is the name of this is the name of this rotational force that deflects motion.rotational force that deflects motion.

Coriolis ForceCoriolis Force Affects direction, not speed of object Maximum at the poles Zero at the equator Not the reason the toilets flush a certain way Not the reason the toilets flush a certain way

in each hemisphere.in each hemisphere.

Always acts to the right of the motion in Northern Hemisphere.Always acts to the left of the motion in the Southern Hemisphere.

3 main types of wind3 main types of wind 1) Geostrophic:1) Geostrophic:

– winds aloft, where frcition is minimalwinds aloft, where frcition is minimal 2) Surface winds2) Surface winds

– Frition cannot be ignoredFrition cannot be ignored 3)Local/regional winds3)Local/regional winds

– Slope induced winds, thermal Slope induced winds, thermal circulations, etc,monsoon, etc.circulations, etc,monsoon, etc.

Geostrophic FlowGeostrophic Flow Assumes there is no friction at all!Assumes there is no friction at all!

– Not 100% realistic, but neraly true at Not 100% realistic, but neraly true at high altitudeshigh altitudes

PGF and Coriolis Force in balance PGF and Coriolis Force in balance Flow is parallel to isobarsFlow is parallel to isobars The jet stream is nearly geostophicThe jet stream is nearly geostophic

GeostrphicGeostrphic FlowFlow

9

Cylonic Cylonic flowflow Anticylonic Anticylonic flowflow

Low and High pressure systems

SurfaceSurface FlowFlow

Aloft, winds are nearly geostrophicAloft, winds are nearly geostrophic

At surface, friction alters direction of windsAt surface, friction alters direction of winds

Why do low-pressure systems Why do low-pressure systems cause rain?cause rain?

Aloft: flow is counter clockwise in NH and flow is geostrophic

Surface: flow is ~Clockwise, but NOT geostrophic,

Instead flow is inward towards low causing convergence

Leads to clouds/rain

Why do high-pressure systems Why do high-pressure systems cause clear conditions?cause clear conditions?

Aloft: flow is clockwise in NH and geostrophic Surface: flow is ~clockwise but NOT geostrophic Instead flow is outward from high causing

divergence Leads to sinking motion (subsidence) and clear

skies Sinking air WARMS and DRIES OUT

Surface WindsSurface Winds Friction is now a major factor, and Friction is now a major factor, and

slows down the wind.slows down the wind.– Trees, buildings, monuntqains,air Trees, buildings, monuntqains,air

resistance, etc. all contribute to frictionresistance, etc. all contribute to friction Winds DO NOT blow parallel to Winds DO NOT blow parallel to

isobars, but isobars, but – Inward towards low pressureInward towards low pressure– Outward from high pressureOutward from high pressure

Slope WindsSlope Winds

Day: warmer air rises and ascends upslope. Often causes cloud formationDay: warmer air rises and ascends upslope. Often causes cloud formation

Night: Cooler air descends downslope. Night: Cooler air descends downslope.

Slope flow II: Slope flow II:

Slope flow significantly Slope flow significantly alters precipataion patternsalters precipataion patterns

Thermal Thermal CirculationsCirculations– Localized events Localized events

sea breezesea breeze land breezeland breeze country breezecountry breeze

– Surrounding areas Surrounding areas heat/cool at heat/cool at different rates.different rates.

– Surface winds blow Surface winds blow from the colder from the colder place towards the place towards the warmer placewarmer place

Monsoon circulationMonsoon circulation