1. 1

2. 2
Wind
Atmospheric
Motion

3. 3
Q: what produces wind?
wind is produced by pressure difference in horizontal~10 mb/100 km
recall vertical variations of pressure
Q: what are two primary factors
affecting atmospheric pressure?
 Density
Temperature
~1000 mb/12 km

4. 4
Idea gas law
Pressure ~ density * Temperature
Q: So, from the ideal gas law, how can change the pressure of a gas?
by increasing/decreasing density
by increasing/decreasing temperature

5. What is this?
This is the model of a
person, but not real.

6. We are to use a model to understand how
an atmospheric circulation is produced.

7. 7
Wind is produced by pressure difference
How? play a model
Assume two
columns have
same density,
same
temperature,
…

8. Air pressure = total air weight per unit area
of the atmospheric column above z
P0
P1 ?
P1=total air weight
P1
P0
P0=total air weight
P0
P1

9. Air pressure = total air weight per unit area
of the atmospheric column above z
P0
P1
P4
P2
P3
P4
Mount Everest 8,848 m
P3
P2
P1
P0
Pressure decreases with increasing of height

10. 10
Wind is produced by pressure difference
How? play a model
Assume two columns have same density,
same temperature, …
Q: What happens
if the column 1 cools
and column 2 warms?

11. 11
Wind is produced by pressure difference
How? play a model
Q: At which location (A or B),
pressure will be higher?
Q: As a result, will air move
from the cold to warm
column or from the warm
to cold column?
A
B
Fig. 6-2, p. 143

12. 12
Wind is produced by pressure difference
How? play a model
air moves from the warm
column to cold column due to
pressure difference
Rule
surface pressure is high as
the air sinks above the surface,
and surface pressure is low as
the air rises above the surface
Fig. 6-2, p. 143

13. Rule
Cool surface  air sinking  high surface pressure
Warm surface air rising  low surface pressure
Wind is blowing from high pressure to low pressure
Where is
clear sky?
Cool
High P
Warm
Low P

14. Surface pressure chart
14
isobars (lines of constant pressure) are plotted every 4 mb
Q: how is a pressure chart created from observations around the country?
a problem is that not all
stations are at sea level
Q: how can we correct
this problem?

15. Surface pressure chart
Rule
in the lower part of
atmosphere, pressure
changes about 10 mb for
every 100 meters of
elevation changes
use this rule, we adjust all
pressure measurements to sea
level, producing a constant
elevation sea-level pressure
chart, commonly refer to
surface weather map
15

16. 16
Tropopause height varies with latitude
Tropopause height is proportional to mean tropospheric temperature
Q: what percent of air is below 200mb level?
1000
200
1000
100%
800
100%
80%
1000
200mb
Q: What would the pressure change
from EQ to NP at level of Z = 5km?
500 mb
5 km
200mb
pressure would decrease as you
move northward
700 mb
500 mb
700 mb

17. 17
Isobaric Charts 500mb level
Each contour line tells
us the elevation above
sea level at which
pressure is 500 mb
Q: what is difference
between isobar and
contour line?

18. 18
Isobaric Charts 500mb level
Wavelike patterns of surface chart

19. Isobaric Charts
19
Q: where is the ridge? Where is the trough?
Ridge
Warm air aloft is
associated with a ridge.
Cold air aloft is associated
with a trough.
Trough

20. Ridges and Troughs
N
E
Ridge
Warm air aloft
Cold air aloft
Trough

21. 21
Isobaric Charts
The table below gives the “approximate” altitudes of
common isobaric charts used in meteorology
Isobaric Surface (mb)
Approximate Elevation (m)
1000
120
850
1,460
700
3,000
500
5,600
300
9,180
200
11,800
100
16,200