WIND and PRESSURE
Geography 101 Lab
Name______________________________________
Purpose: Understand the properties of air, both stationary and in motion, including:
atmospheric profile, trade wind inversion, isobars and wind direction, and local winds.
Profile: In Hawai’i, the atmospheric profile is measured two times each day at 1 am and 1 pm
above Hilo and Lihue Airports. Weather balloons carrying instrument packages (called
rawinsondes) rise to about 30 km measuring temperature, humidity, pressure, and wind speed
and direction. In the first exercise, we will use data from a summer balloon flight to examine
the subtropical conditions over the Islands that help explain many features of local weather and
climate. In particular, we want to identify the Trade Wind Inversion, which limits cloud height
and keeps high elevation areas sunny and dry.
Wind and Pressure: Air is set in motion by pressure differences. On a weather map, air
pressure is generally expressed in millibar (mb) units and shown as equal value lines called
isobars. When isobars are close together they create a high pressure gradient and strong
winds. Winds are lighter when isobars are far apart, like near the center of a High pressure
area. Due to the balance of forces acting on the air (pressure gradient, Coriolis, and friction),
wind blows almost parallel to the isobars, but slightly away from the higher pressure and
slightly toward the lower pressure, as shown in the diagram. As you will be drawing many,
many wind arrows in this lab, I have shown the most common errors you need to avoid. Also
because of the balance of forces, wind circles High and Low pressure centers in opposite
directions in the Northern and Southern Hemispheres, as shown below.
Correct
1 Reverse direction
2 Not toward Low
3 Parallel to isobar
4 Perpendicular
Finally, the wind over the Islands generally follows the open ocean pattern for this part of the
Pacific. Exceptions are caused by local topography, especially the large mountains on Maui
and the Big Island that deflect prevailing winds and create their own sea and land breezes.
Hawaii Pressure and Temperature Profiles
1. Plot a profile of temperature using the underlined rawinsonde data. Connect the plotted points
with a line. Label the line.
2. Plot a profile of relative humidity using the underlined rawinsonde data. Connect the plotted
points with a line. Label the line.
3. On the diagram, draw a mountain representing Mauna Kea (4200 meters high).
4. What is the air pressure near sea level (10 m)? _________ Mauna Kea summit? __________
5. About ½ of Earth’s atmosphere (500 mb pressure) is below about what height? ___________
6. What is the approximate temperature difference between sea level (10 m) and the summit of
Mauna Kea on this day? ____________________
7. The layer in which the temperature increases with height is called the trade wind inversion.
Locate and label the trade wind inversion on the graph.
8. How many degrees did the temperature increase through the inversion on this day? _______
9. On the diagram, draw a cloud between 600 meters and the base of the inversion.
10. Haleakala is 3055 meters high. Do you think you can look down upon the cloud tops from
the top of Haleakala on this day?
_____________________________
Draw arrows showing wind direction around the pressure systems
in each diagram and name the wind directions (N, NE, E, SE, S,
SW, W, or NW) for locations A, B, and C in each diagram:
A=
L
C=
A=
H
A=
L
B=
B=
H
C=
C=
B=
______________________NORTHERN HEMISPHERE______________________________
A=
B=
A=
L
H
C=
C=
B=
H
B=
L
A=
C=
_____________________SOUTHERN HEMISPHERE______________________________
If the Low pressure system below moves eastward from position L1 to position L2 and then to L3, how
will the winds change over the island? (Hint: it will help to draw wind arrows)
Wind Direction
Position L1 _________
Position L2 _________
Position L3 _________
L
1
>
L
2
>
L
3
Isobars and Wind Direction
11. Draw arrows (lots and lots of arrows over the entire map) to indicate the estimated wind
directions for this area of the Pacific.
12. What is the approximate sea level pressure in Hilo?
________________________
13. What is the general direction of the winds over the Hawaiian Islands? (Note: winds are
named by the direction they come FROM) ________________________________
14. This is a normal northeast trade wind pattern for Hawaii. Describe what you think the
weather is like in Hawaii on this day. In other words, describe a nice Hawaiian weather day.
Isobars and Wind Direction
15. Draw arrows (lots and lots of arrows over the entire map) to indicate the estimated wind
directions for this area of the Pacific.
16. What is the general direction of the winds over the Hawaiian Islands? ________________
Are these typical northeasterly trade winds? ____________
17. What is the sea level pressure in Hilo, Big Island? __________ In Lihue, Kauai? ________
18. Is the pressure gradient higher near Hilo or near Lihue? (Note: pressure gradient is greater
when isobars are closer together). ______________________
19. Over which island do you think the winds are strongest?
causes stronger winds)
________________________
(Note: higher pressure gradient
20. Which island is most likely to get rain, Big Island or Kauai? (Note: rain is usually
associated with low pressure centers) ________________________
Variation in wind direction – Wind Roses
Monthly percent frequencies of wind directions for Honolulu
February
August
N
6
1
NE
16
62
E
11
32
SE
4
3
S
12
0
SW
14
0
W
9
00
NW
13
2
CALM
15
0
21. For February, what is the total frequency of the 3 westerly components (SW + W + NW)?
___________ The 3 easterly components (NE + E + SE)? _____________
Which is larger, easterly or westerly ________ and by how much (in %) _________?
22. For August, what is the total frequency of the 3 westerly components (SW + W + NW)?
___________ The 3 easterly components (NE + E + SE)? _____________
Which is larger, easterly or westerly ________ and by how much (in %) _________?
23. Which month sees more frequent trade winds (E + NE)?
_________________?
24. Plot the frequencies given above in the two Honolulu wind roses on the next page, using one
diagram for each month. To plot the data, draw a bar for each frequency extending in the proper
direction. (For example, if the N frequency is 20%, make a bar extending directly upward from
the center to the 20% circle). Write the percentage of calm days as a number in the center circle.
After you finish plotting the Honolulu wind roses, look at the accompanying diagram for the
entire island chain, entitled “Local Winds.”
25. Compare the Lihue and Honolulu wind roses. What is the most common wind direction for
both _________________________?
Why do you think they are similar?
26. Compare the Kona Airport (leeward) and Hilo (windward) wind roses. What is the most
common wind direction for both____________________________?
Why do you think they are the same as each other and opposite from Honolulu and Lihue?