winds professor jeff gawrych de anza college
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Winds Professor Jeff Gawrych De Anza College. Q: What drives the weather in the atmosphere? The uneven heating of the earth Q: What causes the weather to change? Pressure differences and winds We must analyse the predominant forces in the atmosphere, namely: - PowerPoint PPT PresentationTRANSCRIPT
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
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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