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2. WEATHER Specialty Course AUXWEA
Chapter 2 – Pressure and Winds

3. Section 1 Surface Pressure and Winds

4. Overview Surface Pressure: Surface Winds:
Definition of pressure, common units and common instruments.
Station pressure and sea-level pressure.
Pressure on common surface charts—isobars and station model.
Pressure Gradient Force (PGF).
Surface Winds:
Definition of wind direction and comparison with currents.
Wind circulation and the balance between Coriolis and the PGF.
The effect of friction on winds.
Changes in direction—veering and backing winds.
Where is the Low?—the Buys-Ballot Law.
Local effects—sea breeze and land breeze

5. Surface Pressure Pressure is force per unit area.
Pressure exerts equal force in all directions.
Atmospheric pressure is due to the weight of the over-lying air.
Pressure decreases with altitude (rate depends on temperature).
Useful measurement is the pressure tendency (change in 3 hrs).
The change of pressure with distance is called the Pressure Gradient, and it results in a horizontal force, the Pressure Gradient Force (PGF), that drives the winds.
Pressure is measured with a barometer and is reported in different units for different purposes (next slide).

6. Barometers and Units of Pressure
Examples of Barometers.
Mercury and aneroid.
All barometers are subject to error and must be calibrated.
Station pressure is converted to sea-level pressure for reporting.
Units of Pressure.
lb/square inch psi.
inches of Mercury in Hg.
mm of Mercury mm Hg.
Millibars (mb)* mb.
Kilo-Pascals kPa.
* Also called hecto-Pascals (hPa)
Kilo-pascals are used in some countries, like Canada.
©2008, US Power Squadrons. Reprinted with permission

7. Pressure Gradient Force (PGF)
©1997, USA Today. Reprinted with permission
A change of pressure with distance results in a force (PGF) that nature wants to even out. This causes the winds.
The higher the PGF, the stronger the wind.
Wind is affected by Coriolis and curves, and a balance is established between the PGF and the Coriolis “force.”

8. Surface Charts
Simple example of a surface chart, typical of a TV broadcast.
Pressure centers are shown in block letters.
Solid lines of constant pressure are isobars.
Intervals of 4 mb
Closed, or nearly closed, contours define highs and lows.
The low over Minnesota is a cyclone (see Chapter 4). The lows over Texas and Florida are called thermal lows, as the don’t generally have fronts.
©2008, US Power Squadrons. Reprinted with permission

9. Circulation in Highs and Lows
Upper image is circulation around a high.
Lower image is circulation around a low.
The PGF starts the winds.
Coriolis turns the wind.
The PGF and Coriolis balance.
Friction adds a third force (next slide).
The balloons represent small volumes of air. The arrows represent the PGF (toward low pressure) and the CF (to the right of the wind direction).
©1997, USA Today. Reprinted with permission

10. Effect of Surface Friction
Friction slows the wind.
Slower wind means weaker Coriolis effect.
Since the PGF is unchanged, it is greater than Coriolis.
The amount of friction depends on the nature of the surface.
Rougher surface, more friction.
Smoother, less friction.
Water normally smoother than land.
Angles shown are average.
The vector sum of the Coriolis “force” and the friction force balance the pressure gradient force (see Appendix A in the SSG)
©2008, US Power Squadrons. Reprinted with permission

11. Winds—Some More Definitions
Wind direction is defined as the direction FROM which it is blowing, contrary to currents.
When wind gradually changes direction, we say that it:
Is veering if the direction becomes more clockwise with time.
Is backing if the direction becomes more counter-clockwise with time.
We will see in Chapter 4 how that helps us.
To put it another way, veering is when the wind vector changes direction in a cyclonic fashion (CW in the N hemisphere). Backing is when the wind vector rotates in an anti-cyclonic fashion.
©2008, US Power Squadrons. Reprinted with permission

12. The Buys (“Bice”)-Ballot Law
In the northern hemisphere.
Face the wind.
The center of the low pressure system is on your right, or slightly behind.
We use this, along with veering and backing to understand how storms are progressing.
In the southern hemisphere, the low is to the left.
©1997, USA Today. Reprinted with permission

13. Local Effects on Wind Many things affect the local winds.
Buildings and trees block the wind.
The street space between buildings can “funnel” the winds and make them stronger.
Important to boaters are the sea breeze and the land breeze.
Remember, hot air rises and land heats and cools faster than water!
As the air rises, it creates low pressure, forcing the surface air to rush in to equalize the pressure. This creates sinking air over the cooler area, resulting in a vertical circulation.
©2008, US Power Squadrons. Reprinted with permission

14. Measuring Winds Winds are measured by Anemometers.
Most have vanes that determine direction.
They have either propellers (top picture) or cups (bottom picture) to determine speed.
They are located away from local influences, such as trees and buildings and are normally at or above 10 meters.

15. Station Model—Winds and Pressure
The winds are represented by the feather.
Triangles are 50 knots, long barbs are 10 and the short 5 knots.
Wind is from the SW at 15 knots.
The pressure and the 3-hour pressure tendency.
The sea-level pressure is mb.
The pressure tendency is a continuous fall of 2.3 mb in 3 hours.
-23\
Remember, the temperature is 67 degrees F.

16. Section 2 Upper-level Pressure and Winds

17. Overview Definition of upper level charts (isoheights and isotherms).
Ridges and troughs.
Usefulness of the 500 mb chart.
Thermal advection.
Upper-level convergence and divergence.
Rossby waves and global circulation.
Jet streams.

18. Upper-Level Charts
Upper-level charts are routinely produced every twelve hours for the following pressure levels: mb, 850 mb, 700 mb, 500 mb, 300 mb, and 200 mb.
Contrary to surface charts, the contours are called isoheights, rather than isobars—each chart is at a constant pressure.
Two of these are of most interest to us:
The 500 mb chart helps forecast the development of the storm systems.
The 300 mb chart best shows the Jet Stream, which steers the storm systems.

19. What is on an Upper-Level Chart?
The solid lines on the chart (next chart) are isoheights.
The red dotted lines are isotherms.
The station plots show temperature, winds, heights, and often dew points as well.
Rather than Highs and Lows, we talk about Ridges and Troughs (or Trofs).
Ridges are where the isoheights are convex to the pole.
Troughs are where they are convex to the equator.
There are sometimes “closed” isoheights, called lows or highs.
Upper-level charts can give us clues about what will be happening at the surface.
Convex means that a surface bulges out in the middle. Concave is the opposite.

20. Example of a 500 mb Chart
The details of upper-level charts will be covered much more fully in Chapter 7. The basics are: solid lined are isoheights (lines of constant altitude), the red dashed lines are isotherms, the red numbers are temperature in degrees Celsius, the green numbers are dew points (see Chapter 3) in degrees Celsius, and the blue feathers are wind direction and speed. Think of an arrow (the wind) travelling in the direction opposite the “feathers”. Triangles are 50 knots, long lines are 10 knots and short lines are 5 knots.
Courtesy of NOAA

21. Upper Level Global Circulation
If we look at the upper-level circulation from above the north pole, we see a system of ridges and troughs called Rossby waves.
Also called Planetary waves.
Normally from three (rare) to five, they travel around the globe from west to east.
The solid White line represents the polar jet stream. The short wave is a region where a low pressure system (cyclone) is likely to form.
©1997, USA Today. Reprinted with permission

22. Temperature Advection
Upper-level temperature advection is the change of upper-level temperature with time due to the winds.
As the temperature increases, the density, and therefore the weight*, of the air decreases, lowering surface pressure.
The opposite is true of negative temperature advection.
As the surface pressure decreases, low pressure systems can form, or deepen (lower central pressure).
Look at the previous 500 mb chart. It shows very little temperature advection, since the isoheights are nearly parallel with the isotherms.
* Weight is a force, and force = mass times acceleration—in this case, gravity
Positive temperature advection means that the temperature in a specific location increases with time. Negative temperature advection means the temperature decreases.

23. Cold and Warm Core Lows
In warm core lows, lower density at the core results in the depth of the low (center compared with edges) decreasing with height.
Cold core lows have increasing depth with height.
The dashed lines represent isoheights, or lines of constant pressure, viewed from the side. The variation is due to the fact that colder air is more dense and warmer air is less dense. In dense air, the pressure surfaces are closer together.
Warm core low
Cold core low

24. Convergence and Divergence
The surface feature to the right is a cyclone, which has fronts associated with it (see Chapter 4). The “dome” represents a high pressure area.
©1997, USA Today. Reprinted with permission
Convergence means the winds are coming closer together or slowing down. Divergence is the opposite.
Convergence increases pressure (weight of air above) and deepens highs. Divergence decreases pressure.

25. Summary (1 of 3)
Atmospheric pressure is due to the weight of the air above it.
Standard sea-level pressure is:
29.92 inches (760 mm) of mercury.
millibars (hecto-Pascals), or 14.7 psi.
Pressure is measured by barometers.
They need calibrating.
The measured pressure is “reduced” to sea level (i.e., corrected for altitude).
Pressure gradient force is pressure difference over distance.
Increasing PGF increases wind speed.
On surface charts:
Isobars are lines of constant pressure.
Highs and lows are centers of pressure features.
The word “reduced” does not mean the pressure decreases. It is simply an historical name for correcting to sea level.

26. Summary (2 of 3)
Winds are referred to by the direction FROM which they blow.
They circulate into lows and out of highs.
Counter clockwise (N hemisphere) circulation called cyclonic.
Clockwise circulation is called anti-cyclonic.
In the southern hemisphere, cyclonic is CW and anticyclonic is CCW.
They are measured by anemometers.
In the absence of friction, the PGF is balanced by Coriolis Effect and the winds blow along isobars.
Including friction causes winds to cross isobars.
Change in wind direction with time is called:
Veering if the direction changes in a clockwise manner.
Backing if the direction changes in a counter clockwise manner.

27. Summary (3 of 3) The Buys-Ballot law shows the direction of the Low.
Face the wind, the low is to your right or slightly behind.
(Sometimes defined as back to the wind, low is on the left).
There are many kinds of local winds. Two major ones are:
Sea Breeze (on-shore winds) due to land heating during the day.
Land Breeze (off-shore) due to land cooling faster than water at night.
On upper level charts, lines of constant altitude are isoheights.
Ridges are where isoheights are convex toward the pole.
Troughs (or trofs) are where they are convex toward the equator.
A series of trofs and ridges around the pole are called Rossby waves.
Upper level convergence raises surface pressure, while divergence lowers surface pressure.

28. Chapter 2 Questions QUESTION ANSWER
In millibars (mb), standard sea-level pressure is:
A north wind blows:
The most important correction to surface pressure measurements is due to:
The force caused by the difference in pressure over distance is called what?
What causes surface winds to cross isobars?
mb.
From the north.
Altitude above sea-level.
Slide 28: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
The pressure gradient force (PGF).
Friction.

29. Chapter 2 Questions QUESTION ANSWER
Winds that flow clockwise in the northern hemisphere are called what?
When the wind direction gradually changes direction in a clockwise fashion, it is said to be:
Isobars are lines of constant:
The Jet Stream shows up best on which upper-level chart?
Lines of constant temperature are called:
Anti-cyclonic.
Veering.
Pressure.
Slide 29: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
The 300 mb chart.
Isotherms.

30. Chapter 2 Questions QUESTION ANSWER Surface pressure is caused by:
Isobars on a surface chart are drawn every:
Higher pressure gradients cause:
The standard instrument for measuring wind direction and speed is called what?
Half if the total mass of the atmosphere is in the lowest:
The weight of the air above.
4 mb.
Stronger winds.
Slide 30: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
An anemometer.
18,000 feet.

31. End of Chapter 2 Are there any questions?
Chapter 3 covers moisture, latent heat, fog and stability.