Winds
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Downloadable Detailed Module Plan
Objective
Students must grasp the fundamental concept of pressure to understand what makes the wind blow.  Atmospheric pressure is covered.  Surface and upper air pressure charts are introduced.  Newton’s laws of motion lead into discussion of pressure gradient and coriolis forces and the resultant geostrophic and gradient winds.  Once students have a feel for fundamental forces behind wind flow, they will learn about winds interacting with the environment on different scales of motion. The special topic on the Sea Breeze and the Santa Ana Winds follow the text based section of the course on winds.

 

Lesson

1.      Lecture.  The lecture and reading material for the lessons on winds is taken from the sections of Chapter 9 and 10 of the text as listed below.

Chapter 9 The Atmosphere in Motion: Air Pressure, Forces, and Winds

Chapter 10 Wind:  Small-Scale and Local Systems

2.      Lecture Aid.  The WW2010TM Meteorology Online Guide “Forces and Winds” is used extensively as a lecture aid to the relevant text material beginning with the fundamentals of pressure all the way through the discussion on boundary layer winds.

3.      Lecture Aid.  To give students a feel for the apparent deflection represented by the coriolis force, use the Blue Skies TM CD Activity “Atmospheric Forces: Coriolis Force” to take a flight from one of the earth’s poles to a favorite getaway city.  Then fly from the other pole to a city in its hemisphere and compare the two apparent flight paths.

4.      Lecture Aid.  Take some class time to explore actual pressure patterns and the associated wind flow at the surface.  The Blue Skies TM CD “Atmospheric Forces: Winds in the Two Hemispheres” activity is a good summary to return to surface wind patterns after the lecture on upper level flow.  With internet connectivity, use the CD to overlay wind flow and pressure plots.  Plot a map of current US wind vectors and isobars, and compare/contrast with a similar map of Australia.

5.      Classroom Activity.  Have students begin to pay attention daily to the school’s weather station measurements of pressure and winds.  Each class, check whether there are middle level clouds and if so, determine the direction of upper level winds and check it against upper air charts available on line: http://www.rap.ucar.edu/weather/upper.html   …or http://www.hpc.ncep.noaa.gov/modelvrf/eta_500_F48.gif

6.      Forecast Competition.  Review the WW2010TM Online Guide/Reading Maps/Surface Obs/winds as well as the Reading Maps/Upper Air Obs/300mb heights & winds to demonstrate to students how to read the maps and to take the upper level winds into account in preparing their forecasts.

7.      Extra Credit.  Offer students the opportunity to complete QTP Trainee Workbook Forecasting Weather Elements Module 1: Surface Winds.  This module covers basic wind principles and procedures, rules, techniques, and rules-of-thumb needed to aid in surface wind forecasting. Allow at least two weeks to complete the questions. It may be necessary to help students through (or exclude) sections relying on concepts not covered in class.

 

Additional Online Resources

Links provided by Brooks/Cole.

Barometric pressure
A USA Today page describing units of atmospheric pressure, barometers, and the difference between station pressure, sea-level pressure, and altimeter setting. 

Forces in the Atmosphere
The first of three excellent modules from Texas A&M University that focus on atmospheric forces, air pressure, and winds, this series of pages introduces Newton's First and Second Laws, and discusses the main forces at work in the atmosphere.

Air Pressure, Height, and Temperature
Combining information about the balance of forces in the atmosphere with the ideal gas law, this module helps explain how pressure, height, and temperature are related in the atmosphere.

Winds and Pressure
This module explains the ways in which air responds to differences in horizontal pressure gradient, the concept of geostrophic balance, and how to relate the large-scale wind pattern to maps of pressure or height.

Wind Turbine Technology 
From the School of Mechanical Engineering at Cranfield University, this site gives a technical overview of current wind turbine technology. (http://www.cranfield.ac.uk/soe/ppae/)?

Wind Energy Potential in the United States 
 This paper from the National Wind Technology Center assesses the potential for using wind as a source of energy in the United States.

Wind Energy as a Significant Source of Electricity 
From a scientist at the National Renewable Energy Laboratory, this paper discusses wind energy technology and its viability as a significant source of electrical power.

Monsoon Mechanisms
From the Earth Space Research Group, this page illustrates the three basic physical mechanisms that all monsoons share.

Accident Prevention Program
Wind Shear  From a site devoted to aviation, this FAA pamphlet talks about the meteorological conditions associated with wind shear and how this phenomenon can affect aircraft.

Key Terms

Station pressure, mean sea level, isobars, constant height chart, constant pressure (isobaric) chart, ridges, troughs, anticyclones, midlatitude cyclones, pressure gradient, coriolis force, geostrophic wind, gradient wind, frictional or planetary boundary layer, hydrostatic equilibrium, microscale, mesoscale, sysnoptic scale, global scale, prevailing winds

 

Key Concepts

1.      Air pressure decreases with height more rapidly in cold air than in warm air.  Cold air is dense meaning the air molecules are closer together, so that there are more air molecules in a given vertical distance in a column of cold air.  Therefore the pressure changes more rapidly with height in the cold air.

2.      Warm air aloft is associated with high pressure aloft while cold air aloft is associated with low pressure.  The reverse pressure pattern evolves at the surface.  Because a warm column of air extends farther vertically than a cold column with the same surface pressure, there is still more air aloft to cause pressure at a level in the warm column that is at the same elevation as the top of the cold column.  Since there is very little air left above the cold column at this elevation aloft, the air pressure is lower.

3.      Air flows from high to low pressure, so as winds begin to blow horizontally aloft from the warm high pressure to the low cold pressure, air leaves the warm column decreasing its surface pressure and enters the cold column, increasing its surface pressure.

4.      The ideal gas says:  Pressure is proportional to the Temperature times the Density.  This means the pressure of a gas is proportional to its density, as long as its temperature is constant.  Therefore, if air temperature is constant, as pressure increases (decreases), density increases (decreases).  For a given temperature, air at higher pressure is more dense than air at lower pressure.

5.      The standard atmospheric pressure is 1013.25milibars (mb); 29.92 inches of mercury (in.Hg); or 76centimeters (cm).  Millibars are used on US Surface maps.

6.      Mercury barometers must be corrected for temperature (being a fluid, mercury will expand when heated or contract when cooled), variations in gravity because the earth’s not a perfect sphere, and error unique to each instrument (instrument error).

7.      Station elevation is important to surface pressure measurements.  Pressure varies much more with height than horizontally, though it’s the horizontal changes in pressure of interest on a surface map.  Pressure changes more from the ground to the top of the Empire State building (1.2km) than from New York to Miami (1600km).  So a small difference in station elevation can yield a large difference in station pressure.  In order to monitor horizontal pressure patterns, station pressures are adjusted to report what the pressure would be at 0 meters or mean sea level assuming a standard lapse rate.

8.      A sea level pressure chart is a constant height (sea level) chart.  Lines of constant pressure (isobars) are plotted based on reports of pressure adjusted to sea level elevation (0 meters).  On a constant pressure (isobaric) chart, the height at which that pressure occurs is plotted, so the contours are lines of constant height.  The isobaric chart is commonly used in weather analysis.

9.      High heights on a constant pressure chart correspond to higher pressure, low heights correspond to lower pressure.

10. A surface of constant pressure rises in warm, less-dense air and lowers in cold, more dense air.  When the heights of the pressure surface are plotted, the wavelike patterns of the constant pressure surface reflect the changes in air temperature. An elongated region of warm air aloft shows up as higher heights and a ridge and the colder air show as lower heights and a trough.  Lines of constant temperature (isotherms) therefore tend to parallel the height contours on a constant pressure chart.

11. An anticyclone is an area of high pressure, also called a high. A midlatitude cyclone is a center of low pressure, a depression, or “low” which occurs at mid latitudes.  It is not a tropical storm or hurricane which are also associated with low pressure centers.  In the Northern Hemisphere, wind flows inward counterclockwise around a low, outward and clockwise around a high.

12. Newton’s First Law of Motion is that an object at rest will remain at rest and an object in motion will remain in motion (at a constant speed and direction) as long as no force is exerted.  The second law tells us that force exerted is equal to mass times acceleration (speeding up, slowing down, changing direction).  This means that the wind will accelerate in the direction of the force acting on it, so to understand wind flow, we have to understand the forces that affect movement of air.

13. Four forces affect wind flow: pressure gradient force, coriolis force, centripetal force, friction.

14. A pressure gradient is the change in pressure over distance. When differences in horizontal air pressure exist, there is a net force, the pressure gradient force, directed from high to low pressure.  Therefore, if it were the only force present, the pressure gradient force would cause the wind to blow from high to low pressure, perpendicular to the isobars. If isobars are closer together, there is a rapid change in a relatively short distance – a steep or strong pressure gradient and high winds.  If the isobars are spread widely apart, there is a weak pressure gradient and gentle winds.

15. Coriolis force is an apparent force that is due to the rotation of the earth. All free moving objects on earth, such as ocean currents, aircraft, artillery projectiles, and air molecules seem to deflect from a straight line path because the earth rotates beneath them.  The coriolis force causes the wind to deflect to the right of its path in the Northern Hemisphere and to the left of its path in the Southern Hemisphere.

16. Both wind speed and latitude influence the Coriolis force. The stronger the wind, the greater the deflection.  There is no Coriolis force at the equator so it is zero at the equator and increases to maximum values at the poles.

17. Geostrophic wind results from a balance between the pressure gradient force and the Coriolis force.  As the pressure gradient force moves air from high to low pressure, the coriolis force deflects it to the right in the NH until the two forces are balanced when wind become parallel to the isobars.  So the geostrophic wind is an approximation to the real wind and flows at constant speed parallel to isobars with high pressure to right and low pressure to the left in the NH.  The tighter the pressure gradient (isobars close), the stronger the gradient wind, the stronger the resulting geostrophic wind.

18. Pressure gradient force moves air from high to low pressure, or inward toward the center of a low, or outward from the center of a high.  In the NH, the coriolis force deflects the wind to the right, so air moving in towards a low is steered counterclockwise around it (parallel to isobars with low pressure to left).  Wind flowing outward from a high is deflected right steering it clockwise around it (parallel with the isobars with the high pressure to the right). In the SH, because the coriolis force deflects air to the left, wind blows clockwise around lows and counterclockwise around highs. 

19. Winds that blow west to east, parallel to latitudes are called “zonal”.  Winds that blow more in a north south meandering trajectory are called “meridional”.

20. We generally find westerly winds aloft in both hemispheres.  The air is colder therefore the pressure aloft lower at the poles than at the equator.  As the pressure gradient force moves air from the high pressure at the equator to the low pressure at the poles, it is deflected to the right (east) in the NH and to the left in the SH which is also towards the east.

21. The surface wind is different from wind aloft relative to the pressure patterns.  Surface winds tend to cross the isobars and flow more slowly than a geostrophic wind aloft because frictional drag of the ground slows the wind down in the frictional layer, also know as the planetary boundary layer from the surface to about 1000m (3300ft).  As friction reduces the wind speed, the coriolis force is reduces, so that the coriolis force doesn’t balance the pressure gradient force and the resulting wind is not parallel to the isobars but crosses them..  Therefore, near the surface in the NH, the wind flows counterclockwise and into a low; clockwise and out of a high. The angle at which the wind crosses isobars depends on the roughness of the terrain.

22. Vertical air motion associated with surface pressure systems typically involves sinking air above a surface high and rising air above a low.  As air at the surface moves in toward the center of a low (converges), it must go somewhere, so it slowly rises and at about 6km above the ground, it begins to diverge (move outward) to compensate for the converging surface air.  If the divergence aloft is stronger than the surface convergence, it strengthens the surface low.  Conversely, air diverges around a high at the surface causing air to converge aloft and descend to the surface to replace the diverging air.  The rate at which air rises above a low or descends above a high is very small (about 1.5km per day) compared to horizontal wind speeds. 

23. The vertical pressure gradient (pressure decreasing with altitude) doesn’t cause air to rush off into space because the pressure gradient force upwards (from high pressure at the surface to decreasing pressure aloft) is balanced by the downward force of gravity.  This is known as hydrostatic equilibrium.

24. “Microscale” circulation, such as eddies of smoke from a smokestack are has a diameter of a few meters or less.  Mesoscale is from a few kilometers to hundreds of kilometers such as local coastal or mountain winds, thunderstorms, tornadoes.  Synoptic scale is hundreds to thousands of kilometers, the scale of weather maps with circulations around highs and lows.  The largest scale is planetary (global) and refers to wind patterns over the entire earth.

25. Gusts are irregular air motions where air fluctuates rapidly in speed and direction.  They are caused when wind blows over landscape obstructions and breaks into turbulent whirling eddies or “mechanical turbulence”.  The stronger the wind, the stronger the mechanical turbulence.  Surface heating, which reaches a maximum in the afternoon, gives rise to “thermal turbulence” magnifying the effects of mechanical turbulence.  The eddies carry surface air upward and pull faster upper level winds downward enhancing gust.  If the atmosphere is unstable, there is a greater exchange of faster moving air aloft with surface air and gusty winds at the surface are more likely.

26. Wind shear occurs where wind speed and direction change suddenly aloft.  The shearing force produces eddies along a mixing zone.  If the eddies form in clear air, it’s Clear Air Turbulence (CAT).

27. Eddies in clear air might have diameters up to several hundred meters.  If an aircraft flies into the zone of descending air within an eddie, it can drop suddenly as though there is no air to support the winds, thus the term “air pocket”.

28. Three factors that affect the height of a wave in water caused by wind: wind speed, how long the wind blows, and the distance of deep water over which the wind blows.   Sustained 50 knot wind blowing steadily for three days over 1600 miles can create 100 ft waves.  This can happen with stationary storms on the open ocean.  When waves move into a region of weaker winds they change shape and become swells. 

29. Wind direction is generally it is given as the direction from which it is blowing, ex. a north wind blows from the north.  However, “onshore”, “offshore”, “upslope”, and “downslope wind” refer to the direction it is blowing to. 

30. Prevailing wind is the name given to the wind direction most often observed in a given place during a given time.  In LA, the prevailing wind direction is on shore, carrying cool air with moisture causing fog along the coast.  When the wind shifts occasionally to offshore, the dry desert air warms the coastal areas. 

31. In the northeastern half of the US, the prevailing wind is northwest in winter, bringing in the cold arctic air.  In the summer, the prevailing wind there is southwest.  Windows of houses should face southwest to keep the winter wind out and allow summer wind in.

32. Thermal circulation is circulation brought on by changes in air temperature in which warmer air rises and cold air sinks.  Suppose air to the north is cooled, becoming denser.  There is a greater decrease in pressure with altitude than in the warmer air so that a low develops aloft in the north.  This causes a pressure gradient aloft and wind aloft blows from higher pressure in south to lower pressure to north.  The air leaving aloft, causes rising vertical motion in the south and drop in pressure at the surface, while it piles up to the north causing descending air an increase in surface pressure.  Now there’s a pressure gradient at the surface and the wind blows from the high in the north to the low in the south, completing the circulation pattern.