What set the atmosphere in What set the atmosphere in motion?motion?

Review of last lectureReview of last lecture Thickness of the atmosphere: less than 2% of EarthThickness of the atmosphere: less than 2% of Earth’’s s

thicknessthickness Definition of temperature and its unit. Definition of temperature and its unit. Four layers of the atmosphere, what separate them?Four layers of the atmosphere, what separate them? Definition of pressure and its unit. Definition of pressure and its unit. Definition of pressure gradient. Pressure gradient sets Definition of pressure gradient. Pressure gradient sets

the air in motion.the air in motion. Equation of state (Relationship between P, Equation of state (Relationship between P, ρρ, and T), and T) Vertical Pressure Distribution. How does pressure Vertical Pressure Distribution. How does pressure

change with height? What is the hydrostatic change with height? What is the hydrostatic equilibrium?equilibrium?

Horizontal Pressure Distribution. Most common Horizontal Pressure Distribution. Most common atmospheric circulation structureatmospheric circulation structure

The most common The most common atmospheric circulation atmospheric circulation

structurestructure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Horizontal Pressure Gradients and wind

The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure

Horizontal wind speeds are a function of the strength of the pressure gradient

Pressure and winds plotted on same chart

Notice the strong winds in Ohio – due to tight pressure gradient

Forces Affecting the Speed and Direction of the Wind

Horizontal pressure gradients responsible for wind Horizontal pressure gradients responsible for wind generationgeneration

Three factors affect wind speed and/or direction Three factors affect wind speed and/or direction ((velocityvelocity):):

1.1. Pressure Gradient Force  (PGF)Pressure Gradient Force  (PGF)

2.2. Coriolis Effect  (CE) Coriolis Effect  (CE)

3.3. Friction Force (FF)Friction Force (FF)

1. Pressure 1. Pressure GradientGradient

ForceForce(PGF)(PGF)

• pressure gradient: high pressure low pressure

• pressure differences exits due to unequal heating of Earth’s surface

• spacing between isobars indicates intensity of gradient

• flow is perpendicular to isobars

Video: Coriolis effectVideo: Coriolis effect http://www.youtube.com/watch?v=aeY9tY9vKgs

2. The 2. The CoriolisCoriolis Effect Effect

objects in the atmosphere are influenced by the Earthobjects in the atmosphere are influenced by the Earth’’s rotations rotation Rotation of Earth is counter-clockwise looking down from N. Pole.Rotation of Earth is counter-clockwise looking down from N. Pole.

results in an results in an ‘‘apparentapparent’’ deflection (relative to surface) deflection (relative to surface) deflection to the right in Northern Hemisphere (left in S. Hemisphere)deflection to the right in Northern Hemisphere (left in S. Hemisphere) Greatest at the poles, 0 at the equatorGreatest at the poles, 0 at the equator Increases with speed of moving objectIncreases with speed of moving object and distance and distance CE changes direction not speedCE changes direction not speed

Winds in the upper air:Geostrophic Balance

• Now the wind speed/direction is simply a balance between the PGF and CE. This is called GEOSTROPHIC BALANCE. • Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection• Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force

Friction is very small in the upper air:

Winds near the surfaceWinds near the surface

Friction slows down wind speed and reduces Coriolis deflectionFriction slows down wind speed and reduces Coriolis deflection

Friction is important for air within ~1.5 km of the surface (the so-Friction is important for air within ~1.5 km of the surface (the so-called called planetary boundary layer).planetary boundary layer). It varies with surface texture, It varies with surface texture, wind speed, time of day/year and atmospheric conditions. wind speed, time of day/year and atmospheric conditions. Friction above 1.5 km is often small (often called the Friction above 1.5 km is often small (often called the free free atmosphereatmosphere), except over regions with storms and gravity ), except over regions with storms and gravity waves.waves.

The third term (friction) must be considered:The third term (friction) must be considered:

Effect of frictional force

Upper airw/out Friction(geostrophic balance)

Near surfacew/ Friction

Counterclockwise in NH (opposite in SH)characterized by ascending/diverging air which cools to form clouds/precipitation

Clockwise airflow in NH (opposite in SH) Characterized by descending/converging air which warms creating clear skies

Upper airUpper air: isobars usually not : isobars usually not closed offclosed offTroughsTroughs (low pressure areas) (low pressure areas)RidgesRidges (high pressure areas) (high pressure areas)

Near surfaceNear surface: isobars usually : isobars usually closed off due to surface frictionclosed off due to surface frictionCyclonesCyclones (Low pressure areas)(Low pressure areas)AnticyclonesAnticyclones (High pressure (High pressure areas)areas)

Cyclones, Anticyclones, Troughs and Ridges

Summary

Know 3 Forces that affect wind speed /directionKnow 3 Forces that affect wind speed /direction Especially work on Coriolis force, as this is the hardest Especially work on Coriolis force, as this is the hardest

to understand. Which direction is air deflected to by to understand. Which direction is air deflected to by Coriolis force?Coriolis force?

What is the geostrophic balance? At which level is it What is the geostrophic balance? At which level is it valid? Difference between upper level and surface valid? Difference between upper level and surface windswinds

Does cyclones correspond to high or low surface Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter-pressure? Is the air moving clockwise or counter-clockwise around them? How about anticyclones?clockwise around them? How about anticyclones?