CHAPTER 7 ATMOSPHERIC MOTIONS CHAPTER 7 ATMOSPHERIC MOTIONS

 Atmospheric Pressure is the force per unit area of a column of air above you  In other words, pressure is the weight of the column of air above you - a measure of how hard this column of air is pushing down  More fundamentally - atmospheric pressure arises from gravity acting on a column of air

 Horizontal Pressure Variations ◦ It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air ◦ Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure ◦ At surface, horizontal difference in temperature = horizontal pressure in pressure = wind

1000 mb 500 mb level

1000 mb 500 mb 1000 mb The heated column expands The cooled column contracts original 500 mb level

1000 mb new 500 mb level in warm air new 500 mb level in cold air 1000 mb The 500 mb surface is displaced upward in the warmer column The level corresponding to 500 mb is displaced downward in the cooler column original 500 mb level The surface pressure remains the same since both columns still contain the same mass of air.

1000 mb new 500 mb level in warm air new 500 mb level in cold air 1000 mb The 500 mb surface is displaced upward in the warmer column The 500 mb surface is displaced downward in the cooler column original 500 mb level The surface pressure remains the same since both columns still contain the same mass of air. A pressure difference in the horizontal direction develops above the surface High Low

1003 mb997 mb original 500 mb level Air moves from high to low pressure in middle of column, causing surface pressure to change. High Low Warm air aloft = high pressure Cold air aloft = low pressure

1003 mb997 mb original 500 mb level Air moves from high to low pressure at the surface… High Low High Low Where would we have rising motion?

1003 mb997 mb original 500 mb level High Low High Low Air diverges around the surface high Air converges around the surface low

 Rising air above the surface low leads to clouds and storms ◦ Low pressure centers aka “cyclones”  Sinking air above the surface high leads to fair weather ◦ High pressure centers aka “anticyclones”

Cold air aloft means low pressure (heights), warm air aloft means high pressure (heights).

 Above the ground, we typically look at maps showing the height of a given pressure level  If there are no horizontal variations in pressure, the pressure at a constant height level, or the height at a constant pressure level, are the same thing

 When there are horizontal variations, we see high heights in warm air aloft; low heights in cold air aloft

Steeper slope means the contour lines are closer together!

Table 7.1, p. 181

 The cause of the wind!  Horizontal pressure gradients lead to winds  PGF always directed from high to low pressure  The stronger the pressure gradient, the stronger the wind ◦ Or, in other words, the closer the isobars are together, the stronger the wind will be

The length of the red arrows indicate the strength of the PGF

 We don’t actually see the wind blow straight across from high pressure to low pressure  There must be other force(s) at work…

 “Apparent” force due to rotation ◦ An outside (nonrotating) observer doesn’t experience it ◦ An observer on the rotating body (like the earth, or the turntable) does experience it

 Since we (and the atmosphere) are rotating with the earth, we are affected by this force  Coriolis force turns moving objects/air parcels to the right in the northern hemisphere, to the left in the southern hemisphere  The faster the motion, the stronger the Coriolis force  Coriolis force is zero at the equator, relatively strong at the poles

 Pressure gradient force (PGF) ◦ Always from high pressure to low pressure  Coriolis force ◦ Always toward the right (in the northern hemisphere)  When these two are in balance, it is called the geostrophic wind ◦ Geostrophic = “earth turning”  If you’re traveling with the geostrophic wind, low pressure is always on your left! ◦ “when the wind is at your back, lower pressure is to your left (NH)”

The geostrophic wind blows parallel to straight isobars

 But what if the isobars (or isoheights) aren’t straight? ◦ (They’re usually curved – troughs and ridges)  When there is curvature, an observer (or an air parcel) in the rotating frame of reference experiences a force directed outward – the centrifugal force – think of being in a car going around a curve  Magnitude of centrifugal force is related to the velocity and the radius of curvature ◦ Faster speeds = greater centrifugal force ◦ Tight curves = greater centrifugal force

 Involves the PGF, Coriolis, and Centrifugal forces – flow is parallel to curved isobars  This is a good estimate of the winds, except right near the ground

Stepped Art Fig. 8-29, p. 214

 There’s one more force that’s important for winds near the ground

 Near the surface, the wind is slowed by drag from the ground, trees, buildings, etc.  What happens to force balance of geostrophic wind when the wind slows down?

 When wind speed slows down, Coriolis force also is reduced  Therefore, PGF is stronger than Coriolis, and wind blows across isobars toward lower pressure  Wind blows in toward a surface low, and away from a surface high

Aloft – flow parallel to isobars or isoheights Near surface – flow in toward low, away from high Cyclonic flow (counterclockwise in NH) Anticyclonic flow (clockwise in NH)

Fig. 7.17, p. 189

 Like a tornado, or your water in your bathtub  In these situations, the balance is between the PGF and centrifugal forces (Coriolis is unimportant) ◦ This is called a cyclostrophic wind  The water flowing out of your bathtub doesn’t change directions in different hemispheres!

ForceDirectionMagnitude Important when… Pressure Gradient (PGF) From high to low pressure Stronger when pressure differences are greater Pressure varies horizontally Coriolis To the right of wind in NH, to the left of motion in SH – always at 90º angle to wind Increases from equator toward pole, increases with increasing wind speed Earth is rotating, system is large and lasts a long time Centrifugal Outward from center of curvature Increases with increasing speed, increases with sharper curve Flow/motion is curved Friction In the opposite direction of the wind Increases with increasing speed, increases for rough surfaces Near the earth’s surface (lowest 1000 m)

Name Forces involved Result Valid when… ExampleGeostrophic PGF & Coriolis Flow parallel to straight isobars Isobars are straight, no friction Upper-level zonal wind Gradient PGF, Coriolis, Centrifugal Flow parallel to curved isobars Isobars are curved, no friction, system is large Upper-level low pressure center Surface / Boundary layer PGF, Coriolis, Friction Flow toward low, away from high Isobars are straight, friction important Wind near surface Cyclostrophic PGF & Centrifugal Flow parallel to curved isobars (can be either direction around a low) Isobars are curved, system is small (Coriolis unimportant) Tornado, draining sink

Fig. 7.13, p. 185

 Mercurial (Fortin)  Aneroid ◦ Recording: Barograph  Electronic (Pressure Transducer)

◦ Wind vane ◦ Cup anemometer ◦ Aerovane (Wind Monitor by R.M. Young) ◦ Sonic ◦ Rawinsonde (lifted by Weather Balloon)  Wind soundings  Wind Profiler

 A small increase in wind speed can greatly increase the wind force on an object ◦ F ~ V 2 ◦ Turbulent whirls (eddies) pound against the car’s side as the air moves past obstructions, such as guard railings and posts ◦ Similar effect occurs where the wind moves over low hills paralleling a highway ◦ Weird Stuff: Wind erosion, desert pavements, sand ripples, snow ripples, snow dunes, snow rollers, snow fences, windbreak, shelter belt

Table 7.2, p. 196

Fig. 7.24, p. 196

Fig. 7.25, p. 197

Fig. 7.26, p. 197

Fig. 7.27, p. 198

Fig. 7.28, p. 198  Winds which are more likely to come from a general direction can have a large influence on climate.  Wind sculptured trees (even in BCS, my backyard)

 Wind also influences water  Waves forming by wind blowing over the surface of the water  In general, the greater the wind speed, the greater the amount of energy added, and the higher the waves will be ◦ Wind speed ◦ Length of time wind blows ◦ Fetch (distance of straight wind over water)

 As waves travel across the open ocean into areas of weak winds, their crests become lower and more rounded, forming swells  ex.html ex.html  html html  Seiches ◦ Sloshing back and forth of a semi-enclosed body of water (Great Lakes, bays)

Fig. 7.29, p. 199

 The windiest region in the US is the Central Plains  Other windy spots include Alaska, Hawaii, and Atlantic and Pacific coasts  Mountaintops and passes tend to be windy

Fig. 7.30, p. 201

Table 7.3, p. 201

 The estimated maximum speed at which wind can blow at sea level is 200 to 225mi/hr  Above this speed, friction with the earth’s surface creates such a drag on the wind that it cannot blow any faster  Wind speeds in excess of 225 mi/hr are possible on mountaintops, narrow valleys, and tornadoes.

 Few locations in the world that have in place anemometers capable of measuring wind speeds over 200 mi/hr  Many instruments are simply blown away by winds of this magnitude  National Hurricane Center – anemometer died at 164 mph.

Fig. 7.31, p. 203