MET 2204 METEOROLOGY Presentation 4: Wind 1Presented by Mohd Amirul for AMC

MET 2204METEOROLOGY

Presentation 4: Wind

1Presented by Mohd Amirul for AMC

Recapitulate• Lets recap what we have done last week:

– What is the Atmosphere?– Composition of Atmosphere– Layers of Atmosphere– Variation in Atmosphere– Air stability


Presentation Outline• Introduction• Convection• Gradient Wind• General Circulation• Friction• Jet Stream• Local and Small Winds• Wind Shear• Wind Pressure Systems and Weather• Conclusion

Part 1

Part 2


Learning Outcomes

• At the end of this session, student should be able to:– Explain the concept of winds around the earth

surface and upper winds.– Understand the relation of winds to pressure

patterns and movement of weather system.


Introduction

Presented by Mohd Amirul for AMC 5

What is the WIND??


Cont.


How??If the temperature increase, the molecule inside the box will move rapidly. Then there are more and rapid collision happen between the molecule and wall. Therefore the pressure will increase. From this pressure difference the air will moving and it will create the WIND.

Convection


• Convection : transmits heat by transporting groups of molecules from place to place.

• Convection occurs in fluids such as water and air, which move freely.


Cont.

• Two surface are heated unequally– Warm air

• less dense and lifting to the top

• Spreads and cool

– Cold air• more dense and drawn to the

ground


Cont.

• The horizontal air flow in convective current is WIND. (sometime called ad ADVECTION)


Convective current

Cont.

General Circulation


steep pressure gradient(fast winds)

shallow pressure gradient(slow winds)

pre

ssu

re

distance

p

nn

Pressure Gradient Force (PGF)Idea:

• Pressure difference- create the FORCE and to drive the wind. The force we call as PGF.

• PGF: The difference in atmospheric pressure per unit distance

• Wind move from HIGHER PRESSURE to LOWER PRESSURE by PGF.


Cont.

• On the Isobars:– When there is difference in pressure the air begins

to moving from higher pressure to lower pressure.


Usage of ISOBAR on determine wind speed, strength, and direction.

Cont.

• Closer the spacing of isobars, stronger the PGF. Therefore stronger and faster the WIND.

• Widely space isobar – create light/slow wind• Closely space isobar – create strong/fast wind

• Therefore general idea of wind speed can get from isobar.


Cont.

• Surface pressure– Low in warm region (equatorial)– High in cold region (poles)

• Because of PRESSURE DIFFERENCE- create PGF

• Therefore, PGF develop from poles to equatorial.


Cont.

• Imagine that earth didn’t rotate. Therefore PGF is the only force acting on WIND.– Circulation happen between two hemisphere

(Circulation Process):• Cold air SINK at poles• Because dense air flows from poles to equator,

wind will blow straight from poles to equator.• Because less dense air rising, warm air force upward at

the equator.• High level wind blow directly toward the poles.


Cont.

Convective Current

• However above circulation is distorted since the earth does ROTATE!!!!

• So the PGF is not only force acting on the WIND.


Cont.

Coriolis Force (Geostrophic Force)• Idea:

– Imagine the rotation of the compact disk when the song played. Try draw a line during the rotation. Then stop the disk.

– Now you can see the line you draw is deflected away. Not a straight LINE!!

– This is the concept of Coriolis Force.


• Coriolise Force (Geostrophic Force) is the force caused by rotation of the earth

• It will make a moving mass of air turn to the right at northern hemisphere

• It will make a moving mass of air turn to the left at southern hemisphere.

• The angle of deflection/turn directly proportional to wind speed.


Cont.


Cont.

• In this course we will concentrate on Northen Hemisphere where mass of air will deflect to the right.

• Coriolis Force varies from equator to poles:– Maximum at poles and minimum at equator.

• So it will effect wind direction everywhere except at the equator.


Cont.

Differences of Coriolis and PGF

Pressure Gradient Force• Force created by different

pressure• Wind blow perpendicular to

isobar

Coriolis Force• Apparent deflection result

from earth rotation• Wind blow parallel to isobar


• PGF drives wind perpendicular to isobars (higher pressure to lower pressure).

• GF (coriolis force) drives wind to the right until defelected to 90˚ and become perpendicular to isobars.

• At this time, GF and PGF is BALANCE!!!


Geostrophic Wind

• Geostrophic Wind is the wind that blows when the GF is balance with PGF.

• Geostrophic wind directly proportional to the PGF.

• Geostrophic wind speed increase as latitude decrease. (Wind speed becomes faster close to the equator)


Cont.

Geostrophic winds:•go in a straight line•go parallel to the isobars•have speeds proportional to the pressure gradient force.

1. pressure difference starts wind 2. wind gets going a little, starts

being deflected by Coriolis force3. wind goes faster in response to

pressure difference, 4. gets deflected more by Coriolis

Force5. Eventually, the two balance

How Geostrophic wind occur??

• But within the 15 degrees latitude from the equator GF is ZERO.

• For the wind to be geostrophic it must occur:– Above the friction layer– At a latitude greater than 15 degrees– When the pressure situation is not changing

rapidly– With the isobars STRAIGHT and PARALLEL.


Cont.

Gradient Wind

• The gradient wind occurs when the isobars are curved. The force makes the wind follow a curved path parallel to the isobars.

• Gradient wind is due to a combination of 3 forces:– PGF– GF (Coriolis Force)– Cyclostrophic Force


• Cyclostrophic force acts towards the center of pressure system when the isobars are curved.


Cont.

Friction


• Idea:– Consider two boxes of wood contact with each

other. Try move each of those boxes in the opposite direction. It will slow the movement or hardly to move smoothly because of FRICTION FORCE.

• Friction act as the resistance when one body is moved in contact with another.


Cont.

VFr

• Same goes with wind.

• There will be a friction between wind and terrain. It will slower the wind.

• Friction Layer: Greatest near and right at the surface. Usually confined to lowest 1000m (3000 ft).

• The friction increase when:– Rougher the terrain– Stronger the wind speed


Cont.

• Friction acting OPPOSITE in wind direction

• Friction increase will result decrease in GF.


Cont.

• Therefore, the friction will result:

GF and PGF no longer in balance!!

• Friction on sea less than from land.


Cont.

PGF + GF + FrictionChronology


Just a Pressure Gradient

Start with just a pressure gradient

Then wind blows straight from High P to Low P

And accelerates as it goes

H L

View from top

Pressure 1017 1015 1013 1011 1009 1007 1005 return

Just a Pressure Gradient

The wind would blow at the same speed regardless of altitude. H L

Pressure 1013 1011 1009 1007 1005return

Now add Friction

Then wind still blows straight from High P to Low P, but it doesn’t get moving as fast as soon, especially near the ground

H L

View from top

Pressure 1017 1015 1013 1011 1009 1007 1005 back up

Friction and Pressure Gradient

Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.

H L

Pressure 1013 1011 1009 1007 1005

Pressure Gradient and Coriolis

Forget friction.The wind starts out straight, but as soon as it starts building up speed, the Coriolis force turns it a bit to the right.

H L

The wind can’t accelerate any more over here because it’s going parallel to the isobars

This is the Geostrophic Wind

Reality: Pressure Gradient, Coriolis Force, and Friction

Coriolis Force turns the wind some, friction slows the wind some, and the result is roughly a 30º angle between isobars and wind.

H L30º


Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere.

H L

Pressure 1013 1011 1009 1007 1005


Coriolis Force is turning the wind toward us in the right part of the picture. H L

Pressure 1013 1011 1009 1007 1005


Since the Coriolis Force depends on wind speed, its effect decreases toward the ground where the wind speed is slower.

H L

Pressure 1013 1011 1009 1007 1005

Coriolis

Friction

Jet Stream

• In the Troposphere, wind will be stronger (increase in speed) as the height increase.

• The speed/strength of the wind will be increase to the maximum near tropopause.

• This maximum wind will concentrate in the narrow band and we call as the JET STREAM.


• Jet stream: narrow band of strong wind at level near the tropopause.


Cont.

Local and Small Scale Wind(Minor Wind system)


• Before this we dealt with general circulation and major wind system.

• But now, how the local terrain (e.g. mountain, valley, sea) can affect local weather?


Cont.

• Local winds: Winds that blow over short distances• Caused by unequal heating of the earth’s surface

within a small area• Form only when no winds are blowing from far

away

Cont.

Mountain and Valley Winds

• During daytime– Air contact with the mountain slope heated from

solar radiation.– This air will become warmer than air at the same

altitude which are not in contact to the ground (farther from the slope)

– The colder air (denser air) will going downward. It will force warmer air near the ground up to the mountain slope.


– This movement of the air will be called as VALLEY WIND (because air flowing up out of the valley).

• During night:– Air contact with the mountain slope cooled by

terrestrial radiation and becomes heavier (dense) than surrounding air.

– It will sinks along the slope– We call the wind as MOUNTAIN WIND (flow like

water down the mountain slope)


Cont.

• Mountain Wind stronger than Valley Wind (especially in winter)


Cont.

Katabatic Wind• Katabatic Wind: Any wind blowing down an incline

when inclination is influential in causing the wind.

• Therefore, mountain wind is the katabatic wind.

• Originated from cold (dense air) spills down slopping terrain displacing warmer air (less dense air ahead of it).


Land and Sea Breezes• Land surface warm and cool more rapidly than

water surface.

• During daytime:– Land will be warmer than sea– Wind will blow from cool (high pressure) to warm

land(low pressure)– Therefore, it will blow from sea (cooler) to land

(warmer).– We call this wind as SEA BREEZE.


• During night– Wind reverses– Wind will blow from cool land(high pressure) to

warmer water (lower pressure)– We call this wind as LAND BREEZE.

• Land and sea breezes develop only when the overall pressure gradient (change of pressure) weak.– Because wind with a stronger pressure gradient mix the air

so rapidly. Thus, local temperature and pressure gradient don’t develop along the shore line.


Cont.


Cont.

Land and Sea Breeze

Wind Shear


• Idea: Try rubbing 2 objects against each other. It will create the friction.– If 2 objects are SOLID, they will have no exchange

of mass.– If 2 objects are FLUID, they will creates eddy(whirl)

along common shallow mixing zone.

• This shallow layer induce (produce) eddies and mixing is a SHEAR ZONE.

*We will discuss more on wind shear and relate with the turbulence in Turbulence chapter.


Cont.


Cont.

Wind Shear

• Wind shear is defined as a change in wind speed and/or direction within a short distance. it can be in either a horizontal or vertical direction, or both. this change causes a shearing or tearing effect, which causes great concern for pilots and airline industries.


Cont.


Wind, Pressure System, and Weather.

• Rising air– Low pressure

• Bad weather• Conducive to cloudiness and precipitate

– High Pressure• Good weather• Because it breaking up the cloudiness and make it

scatter.


Cont.

• Stagnant air– When downward motion of air prevent the

convection and any upward motion.– Form a stratus (a large dark cloud), fog, haze and

smoke.

• Warm air– Pressure decrease more slowly.


Cont.

• Remember:

• Low Pressure- Bad Weather• High Pressure- Good Weather


Cont.

Conclusion• Wind is the moving air from higher pressure to lower

pressure.• Isobar provides general information of wind speed

and direction.• Wind speed and direction influenced by the pressure

gradient force, coriolis force and friction.• Two major wind systems are gradient wind and

geostrophic wind.• Jet stream: narrow band of strong wind at level near

the tropopause.


• The local terrain might affect the local wind.• Wind shear is defined as a change in wind

speed and/or direction within a short distance.

• Low pressure of air will create bad weather while good pressure of air will create good weather.


Cont.

Key Points• What is the Wind?• Concept of Covection• How the air move?• Use of Isobar• The force acting on the wind: PGF, GF, Friction• Major Wind: Geostrophic and Gradient Wind• Jet Stream• Local and Minor Surface Wind: mountain and Valley; Katabati;

Land and Sea.• Wind Shear


End of Presentation #45 Minutes for Q/A session


Quiz 21. What is the air? What is the atmosphere? Is it

same?2. Why tropopause is important in meteorology?3. What is the solar and terrestrial radiation?4. Differentiate heat and temperature.5. How density of air vary with altitude?6. Define humidity.

Duration: 20 minutes


Documents

MET 2204 METEOROLOGY Presentation 4: Wind 1Presented by Mohd Amirul for AMC