Air pressure. Relationships between pressure, density, and temperature (confined vs. unconfined gases). Measuring air pressure. Isobars. The pressure gradient force. Wind. Convection cell diagram. Out of the high, into the low. Local winds (sea/land breezes, mountain/valley breezes, Chinook/Santa Ana winds).

1. Atmospheric and Oceanic
Circulation
Chapter 4--Part 1
Or: What goes around, comes around.

2. Air Pressure and Wind

3. Have you ever noticed
changes in air pressure?

4. What is air pressure?
Pressure is the force a gas exerts on some specified area of
a container--it is the result of molecular collisions between
the gas and the container

5. Air pressure changes with altitude, from
place to place—and even in the same place,
changes over time

6. Air pressure changes with altitude, from
place to place—and even in the same place,
changes over time

7. Pressure, Density, and
Temperature
 Pressure (P), density (D), and temperature
(T) are all interrelated
 Pressure is the force of molecular collisions per
unit area (lbs/in2
)
 Density is the weight of a material per unit volume
(g/m2
)
 Temperature is a measure of molecular motion
 Changes to one of these variables can cause
changes in the others
For example….

8. Changing Density Pt.I
• There are three ways to change
the density of a gas:
1.Change the size of the container
What happens to pressure?
What happens to temperature?

9. 2

10.  What happens when you change the
temperature of a confined gas?
 Let’s take our original container full of
molecules and heat it up!

11. - What’s happening to the pressure?
- Is density changing, or not?

12. A little simplification:
 For confined gases:
(if D↑ then P↑)
(if D↑ then T↑)
(if P↑ then T↑)
(if T↑ then P↑--but only if confined)
Note:
(changing T will NOT affect D, if confined)

13. Changing Density—Pt.III
3. Change its
temperature (if it is
uncontained)
- What will happen
to the density?
- How will pressure
be affected?

14. In the atmosphere, gases are
uncontained, like this…

15. A little more simplification:
 For unconfined gases (like in the
atmosphere):
(if T↑ then D↓)
(if D↓ then P↓)
(if D↓ then T↓)

16. Measuring
Atmospheric Pressure
 In 1643 Evangelista Torricelli (a student
of Galileo) invented the first barometer…
 Today, we use an aneroid barometer

17. Average Sea Level Air
Pressure
 29.92 in. (inches of mercury)
 14 lbs/in2
 1013.2 mb (millibars of mercury)
 101.32 kPa (kilopascals, where 1
kilopascal is equivalent to 10 millibars)
We will use millibars, as this is the most
commonly used unit of measurement

18. Isobars
 Lines on a map that connect points of
equal barometric pressure are called
isobars
 Isobars follow the same rules as other
iso- lines (don’t cross, form closed
shapes, etc.)

19. Isobaric Maps

20. The Pressure Gradient Force

21. Wind
 Wind—Air moving horizontally in
response to pressure differences
 The process is called advection

22. Convection Cell Diagram
 Draw the convection cell diagram and
label it, just like you see it on the board
 Practice drawing a simplified version to
help you remember “out of the high,
into the low” on exam day

23.  Air always moves from regions of
higher air pressure to regions of lower
air pressure
 In other words:
“Out of the High, Into the Low!”

24. Local Winds
Convection Cells in Motion
 Land and Sea Breezes
 Mountain and Valley Winds
 Katabatic Winds (a.k.a. Mistral)
 Chinook Winds (a.k.a. Santa Anas,
Diablo Winds, Foehn winds, etc.)

25. Wind Direction
 Wind direction is determined by where
the wind is coming from
 For example, an east wind is one that is
coming from the east
 A sea breeze is one that is coming from the
sea and moving toward the land

26. Sea Breeze

27. Land Breeze

28. Valley Breeze

29. Mountain Breeze

30. Chinook/Santa Ana Winds