Chapter 5
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Transcript of Chapter 5
Chapter 5Chapter 5
Winds and Global Circulation
IntroductionIntroduction
Reason for winds goes back to concepts of insolation and radiation
Differential (unequal) heating of Earth’s surface by latitude and at smaller scales leads to variations in pressure gradient between locations air motion
Like opening a bottle or can of beerEarth’s rotation plays role in direction of flow
How Pressure Leads to How Pressure Leads to Air MotionAir Motion
So What is Pressure?So What is Pressure?
Force per unit areaDad says, “Have you checked the air in
your tires lately?” – can measure because higher pressure in tire exerts force on gauge
In day-to-day life, why do we feel atmospheric pressure?
Air has mass and is being pulled downward by Earth’s gravity
Measurement of PressureMeasurement of Pressure
Various units – in tire example, use pounds per square inch; also kg/cm2, pascals (Pa), millibars (mb), cm or mm of Mercury, inches of Mercury
Standard sea level pressure = 101,320 Pa = 1013.2 mb = ~ 76 cm Hg = 760 mm Hg = 29.92 in Hg
Measure using barometer – mercury or aneroid
Mercury BarometerMercury Barometer
Very accurate! Hg in a glass tube Tube placed in dish of Hg Hg flows out of tube due to
gravity a vacuum at top Hg stops flowing when air
pressure pushing on Hg in dish = pull of gravity on Hg in tube
As pressure on Hg in dish increases, Hg in tube rises
Pressure Change with AltitudePressure Change with Altitude
What do you notice if you swim to the bottom of a pool?
Same concept with atmosphere – less of atmosphere above you as altitude increase, so less pressure
On weather maps, use a formula to adjust pressure readings to sea level, which enables pressure analyses (isobars)
WindWindAir motion or flow of air with respect to
surface, usually only horizontal movementsMeasured in meters per second or miles per
hour (1 m/s approximately = 2 mph)Speed determined using anemometer (most
common is cup anemometer)Direction determined with wind vane and
always given as direction from which wind is coming (wind blowing from NW to SE = northwesterly wind); also use degrees
Measuring Wind Speed Measuring Wind Speed and Directionand Direction
Wind vane and anemometer Compass directions/
degrees
Forces Affecting WindForces Affecting Wind
Many features affect wind (buildings, hills, valleys, etc.); think of swirling motion in a football/baseball stadium
Three main forces determine wind speed and direction on larger scales:
1. Friction 2. Pressure gradient force 3. Coriolis effect
FrictionFriction
“Drag” due to a surfaceCauses wind speeds to decrease and has
minor influence on wind directionIf looking at a profile of wind vs. height,
would see a half-U shape, with slower speeds near the surface and increasing values higher in atmosphere
Negligible at high levels
Pressure Gradient ForcePressure Gradient Force
Due to difference in pressure (a gradient) between locations
PGF “pushes” air from areas of higher to lower pressure
A stronger gradient (greater difference in pressure) between locations produces stronger winds
Therefore, affects both direction and speed If only PGF, then wind is perpendicular to isobars,
but of course this is too simple....
Pressure Gradient ForcePressure Gradient Force
Pressure Gradient ForcePressure Gradient Force
Coriolis EffectCoriolis Effect
Deflection of motion of an object (including wind) from its path
So mainly affects direction Due to rotation of Earth In Northern Hemisphere, deflection is to the right
of pressure gradient force, while in Southern Hemisphere, deflection is to the left of PGF
Most deflection at poles, least at equator
Coriolis EffectCoriolis Effect
Local WindsLocal Winds
Friction and PGF have greatest effect on small-scale winds; Coriolis negligible
Some local winds include:– Convective winds: heating causes pressure
gradient; at low levels, wind flows toward warm region (convergence), and at high levels, wind flows away from warm region (divergence)
– Mountain and valley breezes: during day, mountainsides heated causing air flow up valleys; opposite effect at night
Local Winds (cont’d)Local Winds (cont’d)– Land and sea breezes: due to specific heat
attributes (land heats and cools faster than water); during the day, land heats, and air flow from cooler water surface towards land (sea breeze); opposite at night, because water warmer than land surface (land breeze)
Cyclones and AnticyclonesCyclones and Anticyclones Cyclones
– Areas of low pressure– Air spirals inward and upward (convergence)– Due to Coriolis, air moves counterclockwise in NH and
clockwise in SH– Associated with cloudy weather and precip
Anticyclones– Areas of high pressure– Air spirals outward and downward (divergence)– Due to Coriolis, air moves clockwise in NH and
counterclockwise in SH– Associated with fair weather
Winds in Cyclones (L) and Winds in Cyclones (L) and Anticyclones (H)Anticyclones (H)
SUNWarmLow Pressure
ColdHigh Pressure
How is heat transported from the Equator to the Poles?
0o
30oN
60oN
30oS
60oS
90oN
90oN
Earth
Warm air rises at the equator producing Low pressure and flows towards the poles
L0o
30oN
60oN
30oS
60oS
90oN
90oN
Cold air sinks at 30o
N and S latitude Creating high pressure(Subtropical High pressure)
L0o
30oN
60oN
30oS
60oS
90oN
90oN
H
H
Northeasterly and southeasterly surface winds flow from the subtropical high pressure belts (30o N and S) to the low pressure belt (ITCZ) at the equator (calm winds: doldrums)
westerly surface winds flow from the subtropical high pressure belts towards higher latitudes
L0o
30oN
60oN
30oS
60oS
90oN
90oN
H
H
IG4e_05_19
westerly surface winds are forced to rise around 60o N and S latitude when they encounter cold polar easterly winds from the poles resulting in Subpolar Low pressure (SPL) belts
L0o
30oN
60oN
30oS
60oS
90oN
90oN
H
H
L
L
cold air sinks at the poles producing polar high (PH) pressure regions
L0o
30oN
60oN
30oS
60oS
90oN
90oN
H
H
L
L
H
H
Three-cell ModelThree-cell Model
2-D Glance at Surface2-D Glance at Surface
Semi-permanent pressure cells around globeCalled semi-permanent, because location and
intensity vary with season (Fig. 5.17, p. 164)Seasonal shifts lead to
– Movement of Intertropical Convergence Zone (ITCZ) – moves northward in July, southward in January – why?
– Monsoon: shift in wind direction from offshore flow in winter (dry season) to onshore flow in summer (wet season)
Semi-permanent Pressure CellsSemi-permanent Pressure Cells
Winds AloftWinds Aloft As you move farther from the Earth’s surface,
friction has less impact, so can focus mainly on PGF and Coriolis
Geostrophic wind: wind that moves parallel to isobars and at a right angle to the PGF (Fig. 5.22a)
Flow is initially in direction of PGF, but in Northern Hemisphere, Coriolis deflects motion to the right
At some point, PGF = Coriolis (sum = 0), so speed and direction of air flow no longer changes
See Fig. 5.22b for illustration of final two points
Winds AloftWinds Aloft
Other Characteristics of Other Characteristics of Winds AloftWinds Aloft
General pattern: – Weak easterly winds in
tropics (Equatorial Easterlies)
– High pressure at Tropics of Cancer and Capricorn, westerly winds to Arctic and Antarctic Circles (Westerlies)
– Spiraling motions from Circles to poles
Specific Features of Specific Features of Winds AloftWinds Aloft
Rossby waves– Undulations or waves in Westerlies which move cold air
toward equator and warm air toward poles– Primary mechanism for poleward heat transfer
Polar front– Sharp boundary between cold polar air and warm tropical air
Jet streams– Narrow zones (tube-like) of very fast wind speeds (center
has highest speeds)– Occur along strong pressure gradients– Two affecting US – polar jet and subtropical jet
•Smooth westward flow of upper air westerlies
•Develop at the polar front, and form convoluted waves eventually pinch off
•Primary mechanism for poleward heat transfer
•Pools of cool air create areas of low pressure
Rossby WavesRossby Waves
Ocean CirculationOcean Circulation
Why are we talking about the oceans during the same class at winds?
Atmospheric circulation drive the direction of surface ocean currents
Ocean currents also act as heat transfer mechanisms (help global energy balance)
General pattern is warm currents on eastern flank of continents and cold currents on western flank (Gulf Stream vs. California current)
Ocean CurrentsOcean Currents Upwelling is vertical movement in the oceans Important because it brings nutrients to upper
levels of ocean Occurs along western edges of continents – one of
strongest is along South America What does this have to do with weather and
climate? Teleconnection – relationship between circulations
in one region and weather/climate in another region; impacts vary across globe and even on same continent
Ocean CurrentsOcean Currents
El NinoEl Nino
El Niño-Warmer than normal waters in the Equatorial Pacific
La Niña-Cooler than normal waters in the Equatorial Pacific
Normal vs. El Nino and Associated Normal vs. El Nino and Associated Weather EffectsWeather Effects