# Atmospheric Forces and Winds

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Atmospheric Forces and Winds
Chapter 6 Atmospheric Forces and Winds

Figure CO: Chapter 6, Atmospheric Forces and Wind
Courtesy of RMS, Inc

Figure UN01: Winds over France on Feb. 27-28, 2010
Data from Météo-France

Figure UN02: Flooding in La Faute, France

Basics about Wind Wind direction is the direction from which the wind is blowing A north wind blows from the north to the south It is reported according to compass directions Prevailing wind direction is the most frequent direction Wind speed Reported on U.S. weather maps in knots 1 knot = 1.15 miles/hour = 0.5 meter/second If wind > 15 knots and highly variable, the weather report will include the wind gust, the maximum speed

Figure 01: Wind directions in angles, compass headings.

Forces Have magnitude (or strength) and direction
Multiple forces can act on the same point The resultant force is the net force If two forces act in opposite directions, the net force will have the direction of the stronger force and a strength equal to the difference of the two forces If two forces act at an angle to each other, the resultant force is along a diagonal and away from where the two forces are applied

Figure 02: Force diagram.

Figure 03: Graphical addition of force vectors.

Forces and Movement A force applied to an object often results in movement An object’s velocity is the magnitude and direction of its motion The speed of the object, the distance traveled in a given amount of time, is the magnitude of the motion Acceleration is a change in an object’s velocity—magnitude, direction or both

Forces cause the wind to blow
Forces that act on air create horizontal wind A force acting through a distance does work Work is equivalent to energy Ultimately, the sun provides the energy that allows the winds to blow Radiation causes temperature imbalances, which lead to pressure imbalances and a force

Newton’s second law of motion
Says that the sum of the forces = mass x acceleration Or that acceleration = sum of forces/mass Helps scientist forecast changes in the wind direction and speed, or its acceleration Requires that we specify which forces are acting and how strong they are Is also called the Law of momentum Momentum of an object is its mass x its velocity

Gravity, the strongest force
Does not act horizontally, so does not influence the horizontal winds. Does influence vertical air motions Is directed downward toward the center of Earth Is a very strong force Keeps our atmosphere from escaping Equals the mass x 9.8 m/s2

The force that results from pressure differences over distances in a fluid A pressure gradient is a change in pressure over a distance PGF always directed from high to low pressure Is stronger when isobars more closely spaced Is stronger when the difference in pressure is greater over a particular distance Determines the way air moves only if no other forces are acting

Figure B01A: Fan blowing on paper

Figure B01B: Air over plane wing, with lift and drag

Figure 04: Pressure gradient force in highs and lows.

Is always directed from high to low pressure Is stronger where the density is less—higher in the troposphere When stronger, causes stronger winds Is always important in horizontal winds Is not generally in the same direction the wind blows, because other forces can act

Figure 05: Surface weather map
From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].

Isobaric Charts Plot the altitude of a given pressure surface
Units of altitude are called geopotential meters Also called a constant-pressure chart Common levels are 850, 700, 500, 250, and 200 mb Are useful for portraying horizontal pressure gradients above the ground The spacing between the lines of constant height is proportional to the PGF The winds in general blow parallel to the height contours, at right angles to the PGF

Figure 06: 500-mb isobaric chart
From Plymouth State University Weather Center, [http://vortex.plymouth.edu/make.html.].

Figure 07A: Isolines of constant height are proportional to the PGF

Figure 07B: Isolines of constant height are proportional to the PGF

Figure 07C: Isolines of constant height are proportional to the PGF

Centrifugal Force/Centripetal Acceleration
Centripetal acceleration is a change in direction even if the speed does not change From the point of view of an observer experiencing the centripetal acceleration, there is an apparent force called the centrifugal force The faster the speed and the tighter the curve, the larger is the centripetal acceleration The sign of the centripetal acceleration is positive for cyclones, negative for anticyclones, and always directed inward to the center

Figure 08: Centrifugal force schematic

The Coriolis Force Deflects the wind to the right in the NH
Deflects the wind to the left in the SH Is strongest at the poles Is zero at the equator Is stronger for stronger winds Is weaker for weaker winds Is zero for calm. It cannot start a wind

Figure 09A: Curving path of ocean buoy
Adapted from Joseph et al., Current Science, 92 (2007).

Figure 6.10: The centrifugal (CENTF) and Coriolis forces acting on an air parcel moving with respect to the rotating Earth Modified from A. Persson, Bull. Amer. Meteor. Soc., 79 [1998]: 1378.).

Figure 11A: Coriolis force at different latitudes.

Figure 11B: variation of Coriolis force with latitude and wind speed

Figure B02B: Carl-Gustaf Rossby
Courtesy of University of Chicago News Office

The Friction Force Acts in the direction opposite to the direction the wind is blowing Acts to slow down the wind Is most important at Earth’s surface Gets stronger when the winds are stronger Is not important above the boundary layer (the lowest 1 km in the atmosphere) The rougher the surface and the stronger the wind the greater is the friction force

Figure 12: Frictional force diagram

Why force-balances are important
Force-balances simplify Newton’s second law of motion by limiting the number of forces Force-balances describe winds that come close to describing the observed winds Even though the forces are balanced, the wind need not be calm The PGF is important in every force balance Only the PGF can set calm air into motion

Figure T01: Some Atmospheric Force-Balances

Hydrostatic Balance Gravity (downward) balances the Vertical Pressure Gradient Force (upward) Does not apply inside cumulus clouds, because buoyancy is important there Does apply generally in the atmosphere Limits vertical motions to be much weaker than horizontal winds

Figure 6.13: Air parcel in hydrostatic balance
Reproduced from Lester, P., Aviation Weather, Second edition. With permission of Jeppsen Sanderson, Inc. Not for Navigation Use. Copyright © 2010 Jeppesen Sanderson, Inc.

More on Hydrostatic Balance
The pressure gradient force is stronger when the air is less dense The density of air is less when the air Temperature is higher Pressure decreases upward less rapidly when the air has a higher temperature Hydrostatic balance helps explain the sea breeze and other thermal circulations

Pressure levels on weather maps
The atmosphere is very close to hydrostatic balance This means that the height of a particular pressure level is roughly equivalent to the pressure at a related height level An altimeter is a barometer with a height scale Upper-level weather maps are labeled in m Winds on a weather map are strong when the height contours are close together, weak where they are farther apart

Geostrophic Balance Is a balance between the horizontal pressure gradient force and the Coriolis force Ignores the friction force Has isobars that are straight lines Does not mean that the wind is calm Has a wind called the geostrophic wind Winds on weather maps above the surface are close to the geostrophic wind Blows with lower pressure (height) on the left (NH)

Figure 14: Geostrophic balance

The Geostrophic Wind Is a wind in geostrophic balance
Is parallel to the isobars In the NH has low pressure on the left In the SH has low pressure on the right In the NH the wind blows clockwise around high pressure centers and counterclockwise around low pressure centers In the SH CW flow around lows and CCW flow around highs

Figure 15: Geostrophic wind in highs and lows

Gradient balance is between the PGF, the Coriolis force and the centrifugal force Gradient balance allows curving wind patterns called the gradient wind The centrifugal force is always outward Around a low the centrifugal force opposes the PGF and the resulting flow is subgeostrophic Around a high the centrifugal force adds to the PGF and the resulting flow is supergeostrophic

Figure 16: As in Figure 6-15, except now we also include the centrifugal force, leading to gradient balance.

Initially there is an imbalance of forces Air parcels move toward lower pressure (PGF) As soon as there is a wind, the Coriolis force acts Parcels oscillate towards a balance between the PGF and the Coriolis force Adjustment takes minutes to hours Adjustment is temporary and incomplete

Figure 17: Wavy path of parcel adjusting to balance

Guldberg-Mohn Balance
Is a balance between the PGF, the Coriolis force, and friction Friction slows the wind and the Coriolis force weakens The winds blow across the isobars at an angle toward low pressure (away from high pressure) Between 15° and 30° over water Between 25° and 50° over land Friction damps oscillations during adjustment to balance

Figure 18: Guldberg-Mohn balance

Figure 19: A numerical simulation of how varying amounts of friction affect the adjustment to Guldberg–Mohn balance. Modified from Knox, J., and Borenstein, S., J. Geoscience Ed., 46 [1998]: 190–192.

Figure B03A: Chart of wind speeds and max wave heights

Figure 21: The isobars at the surface drawn over a satellite image of a cyclone
Image created by Prof. Joshua Durkee, Western Kentucky University, using GREarth software.

The Thermal Wind The thermal wind relates temperature and winds to each other The winds are more westerly as you go up wherever it’s colder toward the poles

Putting horizontal and vertical winds together
At the surface, the wind blows across the isobars into low-pressure areas At the center of the low-pressure area the air must rise Low-pressure areas are usually cloudy and wet At the surface, the wind blows across the isobars out of high-pressure areas At the center of the high-pressure area the air must sink High-pressure areas are usually clear and dry These patterns are the result of Guldberg-Mohn balance

Figure 22: Schematic of pressure levels when air is heated

Figure 23: Cross-section of winds at various pressure levels

Figure 24A: How surface wind patterns induce vertical wind motions
Figure 24B: How surface wind patterns induce vertical wind motions

Figure 24A: How surface wind patterns induce vertical wind motions

Figure 24B: How surface wind patterns induce vertical wind motions

The thermal circulation
The sea breeze is a thermal circulation A thermal circulation has both horizontal and vertical air motions The horizontal pressure gradient force is most important in a thermal circulation Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column

The sea breeze Is a daytime circulation
Depends on differential heating at the surface between land and water Has the warmer, rising air column over the land, which absorbs more incoming solar radiation Has the cooler, sinking air column over the water, which absorbs less radiation Air flows from warmer to cooler column aloft Air flows from cooler to warmer column at the surface

Figure 25: Sea breeze schematic

Figure 26A: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

Figure 26B: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

Figure 26C: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

Figure 26D: Satellite image of sea breeze
Courtesy of SSEC, University of Wisconsin-Madison

The sea breeze and the land breeze
As solar heating diminishes in the late afternoon, the sea breeze weakens At night, differential cooling occurs The cooler, sinking air column is over land, where radiational cooling is more rapid than over the water The warmer, rising air column is over the water The land breeze develops at night Air flows towards the land aloft Air flows towards the water at the surface

Figure 27: Schematic of land breeze

Scales of motion in the atmosphere
Describe the size and lifetime of wind patterns in the atmosphere Determine which forces are most important to forming the wind patterns Are largest when the lifetimes are longest Are smaller when the lifetime is shorter Have a variety of names and definitions

More on scales of motion
Microscale: <1 km in diameter PGF, centrifugal, friction forces are important Mesoscale: Between 1 and 1000 km in size PGF, centrifugal, friction, and Coriolis Force for largest sizes Synoptic scale: At least 1000 km in size Balance between PGF and Coriolis Force dominates Planetary scale: Roughly 10,000 km in size