Intro. to Atmospheric Sciences Plymouth State University

Intro. to Atmospheric Sciences Plymouth State University
PRESSURE, WIND & FORCES Dr. Sam Miller Intro. to Atmospheric Sciences Plymouth State University 1

Atmospheric Pressure

Atmospheric Pressure It is the pressure exerted because of the weight of the air above Pressure = Force/Area Varies much more in the vertical than the horizontal Horizontal variations Tens of millibars in thousands of kilometers Vertical variations Hundreds of millibars in ten kilometers Always decreases with height

Less mass over your head here… …than here.

Atmospheric Pressure Average sea-level pressure (SLP)
mb (a.k.a. hecto-Pascal; hPa) 29.92 in. Hg 14.7 lb/in2 “Normal” range of sea level pressure 960 to 1045 mb (hPa) 960 – center of strong hurricane 1045 – strong high in the winter Average height of 500 hPa is 5.5 KM ASL (5500 meters)

Measuring Pressure Barometers are used to measure pressure
Mercurial barometer Classic barometer Aneroid barometer More recent Digital barometer Commonly used at modern airports An altimeter is a type of barometer calibrated to show altitude, instead of pressure

Mercury Barometer

Aneroid Barometer

Station vs. Sea Level Pressure
Pressure always falls with height A station above sea level will always have lower pressure than a station at sea level Vertical changes in pressure overwhelm horizontal changes As meteorologists, we’re interested in identifying weather systems, not making topographic maps

Station vs. Sea Level Pressure
It’s the horizontal differences in pressure that are meteorologically significant Solution: Convert all station pressures to sea level pressures This is the pressure a given station would have if it were at sea level Makes it possible to “cancel out” vertical changes in pressure, and visualize only the horizontal changes.

Correcting to Sea Level pressures at elevation
Observed station pressures at elevation Correction applied Resulting sea-level pressure Sea-level pressures with isobars

Correcting to Sea Level
Observed station pressures at elevation LOOKS AS IF LOWEST PRESSURE IS HERE Correction applied Resulting sea-level pressure Sea-level pressures with isobars

Observed station pressures at elevation Correction applied Resulting sea-level pressure ADD ABOUT 1 MB FOR EACH TEN METERS OF ALTITUDE REDUCTION Sea-level pressures with isobars

Correcting to Sea Level pressures at elevation
Observed station pressures at elevation Correction applied Resulting sea-level pressure Sea-level pressures with isobars

Observed station pressures at elevation Correction applied LOWEST PRESSURE IS REALLY HERE Resulting sea-level pressure Sea-level pressures with isobars

Typical sea-level pressure chart

Uneven heating of the Earth’s surface causes horizontal temperature differences
Horizontal temperature differences result in horizontal pressure differences

Cold Dense Heavy Warm Thinner Lighter

Uneven heating of the Earth’s surface causes horizontal temperature differences
Horizontal temperature differences result in horizontal pressure differences Horizontal pressure differences push mass from high to low pressure

Wind Movement of mass (air) in response to pressure differences
If no other factors affected it, wind would always blow directly from H to L pressure

How Fast Does the Wind Blow?
If pressure difference is large wind blows faster If pressure difference is small wind blows slower

Pressure Gradient Gradient = Change of some quantity over a distance
Pressure Gradient (PG): Change in sea-level pressure per distance

Small PG Large PG

Light Wind Strong Wind

Measuring Wind Wind vane Anemometer Measures wind direction
Measures wind speed

Wind Direction Direction that it is COMING FROM

The wind speed and direction depend on the sum of the forces acting on the atmosphere
Pressure gradient is not the only force at work

Forces

Which forces affect the wind?
Pressure gradient force (PGF) Due to horizontal differences in pressure Coriolis force (CoF) Due to the Earth’s rotation Centrifugal force (CeF) Due to curved motions Friction force (FrF) Due to interaction with the surface

Pressure Gradient Force (PGF)
Force that “pushes” the air from regions of higher pressure to regions of lower pressure The “prime mover” Gravity in disguise Always directed away from H and toward L Perpendicular to isobars Strongest where pressure gradient is strongest Important in entire atmosphere

Coriolis Force (CoF) “Apparent” force due to the rotation and curvature of the Earth Deflects objects from a straight path Acts perpendicular to the wind direction To the right in the Northern Hemisphere To the left in the Southern Hemisphere Affects all free-moving objects Projectiles Aircraft Ocean currents Air molecules Important in entire atmosphere

Coriolis Force (CoF) “Apparent” force due to the rotation and curvature of the Earth Deflects objects from a straight path Acts perpendicular to the wind direction To the right in the Northern Hemisphere To the left in the Southern Hemisphere Affects all free-moving objects Projectiles Aircraft Ocean currents Air molecules Important in entire atmosphere Gaspard-Gustave de Coriolis, early 19th Century French mathematician, scientist, and mechanical engineer

Strength of CoF depends on:
Rotation of the Earth Speed and direction Constant for our purposes Latitude CF zero at equator CF stronger at higher latitudes CF maximum at poles Speed of the object Faster objects deflected more Slower objects deflected less

Centrifugal Force (CeF)
Another “apparent” force directed radially outward from the center of a system’s rotation. Regardless of direction of rotation Important throughout the entire atmosphere Does not require “closed” circulation – only curvature Points outward from focal point (center) of rotation

Frictional Force (FrF)
Force that “slows down” the wind, due to interactions with the surface. Always directed opposite to the wind Only Important from the surface up to about 1 km Boundary layer – layer of the atmosphere where friction is important Depth of the boundary layer varies, depending on time of day, wind speed, and surface roughness

If forces are balanced, air flows at constant speed and direction
If they are not balanced air will keep accelerating (changing direction and or speed) The balance of forces is different close to the surface than it is in the upper atmosphere Wind behaves differently at the surface than aloft No friction aloft (above about 1 km)

Combining Forces

Wind Aloft Applies to “free troposphere” above the boundary layer
Above ~ 1 km Friction not important Straight motion: PGF and CoF Curved motion: PGF, CoF and CeF

Case 1: Geostrophic Wind
PGF balances CoF No CeF or FrF Wind direction parallel to straight isobars Wind speed is constant Remember Buys Ballot’s Law L isobars PGF wind CoF H

Buys Ballot’s Law “If you stand with your back to the wind in the Northern Hemisphere, lower pressure is on your left and higher pressure is on your right”

Buys Ballot’s Law “If you stand with your back to the wind in the Northern Hemisphere, lower pressure is on your left and higher pressure is on your right” C.H.D. Buys Ballot, 19th Century Dutch Chemist and Meteorologist Professor of mathematics and physics

Case 2: Gradient Wind PGF, CoF and CeF balance
Wind direction parallel to curved isobars Wind speed varies depending on direction of curvature

L Case 2a: Gradient Wind Cyclonic flow around low pressure area
CeF isobars CoF Cyclonic flow around low pressure area Counterclockwise in Northern Hemisphere Slower than geostrophic flow - CeF weakens PGF wind PGF L PGF wind CoF CeF

H Case 2b: Gradient Wind Anticyclonic flow around high pressure area
CeF isobars PGF Anticyclonic flow around high pressure area Clockwise in Northern Hemisphere Faster than geostrophic flow - CeF adds to PGF wind CoF H CoF wind PGF CeF

Boundary Layer (BL) Wind
Near the Earth’s surface Below ~ 1 km Friction must be considered Straight motion: PGF, CoF and FrF Curved motion: PGF, CoF, CeF and FrF

L H Case 3: Straight BL Wind PGF balanced by CoF & FrF
No CeF Wind direction crosses straight isobars toward low pressure Amount of crossing depends on type of surface Smooth: Not much Rough: May be straight across Wind speed is constant L isobars PGF wind CoF FrF H

Case 4: Curved Flow Surface high: Surface low: Anticyclones
Air flows clockwise out of high Low-level divergence Surface low: Cyclones Air flows counterclockwise into low Low-level convergence NORTHERN HEMISPHERE

Summary

Review

Definition of atmospheric pressure
- Normal range at sea level Types of pressure measuring instruments Station pressure vs. sea-level pressure About 1 mb for every 10 meters Horizontal variations in SLP are the important variable - Results in WIND – The movement of mass

Net wind is result of balance of up to four different forces
- PGF - CoF - CeF - FrF Geostrophic wind and Buys Ballot’s Law Gradient wind above the atmospheric boundary layer - Cyclones – CCW in Northern Hemisphere - Anticyclones – CW in Northern Hemisphere

Below about 1 km, friction is important
- Results in wind flow crossing isobars toward lower pressure - Divergence out of highs Convergence into lows

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