IDS 102
Movement of Air
Let’s make a prediction: Show on this surface pressure map, the direction that air
will travel. (This is “Blue-light Special” question—see what you can do!)
Check with an instructor before proceeding!
1
To answer the question posed on the previous page, think of what air does when a tire is
punctured. The air travels from the high-pressure area, the tire, to the outside where the
pressure is lower until the pressure is equalized. So, air in the atmosphere moves toward a
low pressure areas, this is called a pressure gradient force.
On the surface pressure maps, where will the pressure gradient force be the highest?
(Think of water running over steep land versus flat land and how this would appear
on a topographic map.)
Mark on the surface pressure map on the previous page where you think the pressure
gradient was the highest.
Coriolis effect and the movement of the atmosphere:
Things would be a lot simpler if the Earth did not rotate on its axis! The air would simply
move to the low-pressure areas and things would be hunky dory. However, the Earth does
rotate and this movement creates something called the Coriolis effect. Many people have
heard that the water in a toilet spins in one direction in the Northern Hemisphere and the
opposite direction in the Southern Hemisphere. This is an urban myth, which does not
have anything to do with the Coriolis effect!
When everyone gets to this spot, we will watch a short video from a web site on the
Coriolis effect. Here is a link to the web site:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
Take some time to view the Quicktime video and discuss with your classmates what you
observe. Summarize your discussion here.
2
Returning to the weather idea--- air would move straight from the high pressure to the
low pressure area, but due to the spin of the Earth, the air appears to be deflected to the
right in the Northern Hemisphere and the left south of the equator. Sometimes you will
read something in which the Coriolis effect is called the Coriolis force. The problem with
calling it a force is that there is nothing really deflecting the air; the deflection is due to
the change in the position of the Earth over time. Things are easier to understand if we
pretend that the Coriolis effect is like a force, but we have to remind ourselves that the
apparent deflection of the air to the right in the Northern Hemisphere is not due to a
force! Not due to a force! Ok! Ok!
The amount the air that is deflected is dependent primarily on the speed of the wind and
the latitude. The Coriolis effect is zero at the equator and a maximum at the poles. If the
air is moving faster, the Coriolis effect is greater. Another factor is the distance the air
moves. The greater distance the air moves, the greater the Coriolis effect. (This is why
the direction water moves in a toilet is not dependent on the Coriolis effect because the
distance is too small!)
When you combine the effect of the movement of the air by the pressure gradient force
and the deflection due to the Coriolis effect, the air travels in a curved path called the
geostrophic wind. See the illustrations at the University of Illinois web site.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/geos.rxml
If the pressure contours are close together, we would expect the wind velocity to be
__________________. If the contours are far apart, the wind velocity should be
__________________. Go back to page one of this handout and look at the maps again.
Do you have any differences in ideas about the direction and speed of the wind?
One more major factor affects the movement of air in the atmosphere, friction. As the air
moves across the ground surface, there is friction between the air and the trees, buildings,
hills, and us. This frictional force causes the air to move toward a low-pressure region
and away from a high-pressure region. So, the actual movement of air is close to parallel
with the pressure contour lines that you drew, but there are slight deviations due to the
friction between the air and the Earth. This deviation causes the air to angle toward low
pressure regions and away from high pressure regions.
Again, see the following U of I web site for more information about the role of friction in
the atmosphere.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/fric.rxml
3
Imagine there are a high pressure and low-pressure areas on the same map. What would
the wind vectors look like around that high pressure? How about around the low
pressure?
H
L
Wind Symbols
On weather maps wind speed and direction may be shown as a vector or with a weather
map symbol. The weather symbol points in the direction that the wind is going to. The
barbs indicate the wind speed in knots.
A
B
Weather symbol A indicates that the wind is blowing
from the west to the east and is blowing at 15 knots (1
knot = 1.15 mph).
Weather symbol B indicates that the wind is blowing
from the southeast to the northwest at 5 knots
See the web site below for a more complete description of the wind barb:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/wnd.rxml
At the University of Washington, the Department of Atmospheric Sciences produces
weather predictions (called the MM5 model) in the form of weather maps for the next
several days. The maps are animated to show how the maps are likely to change in the
next few days. Click on this link to go to this site:
http://www.atmos.washington.edu/mm5rt/
Click on the main MM5 link and explore some of the maps and observe the weather maps
for the next few days.
See if you can find:
A high pressure region
A low pressure region
A place in which the wind is blowing hard
A place where there is little if any wind
Feel free to explore the maps and test your ideas about the wind direction and velocity by
looking at the wind vector maps.
4
IDS 102
Fronts and Weather Maps
Sometimes we hear on weather reports that a front is approaching and usually this is
associated with a warning of bad weather. To understand fronts we must understand
different types of air masses. The classification of air masses depends on where the air is
coming from. If the air is coming from the land it is continental. If the air comes from the
ocean or a large lake, it is termed maritime. We also class air based on whether the air is
coming from the poles or from the tropics. See the table below:
Continental (land)
Maritime (ocean or large
lake)
Polar
Cold, dry air
Cool, moist air
Tropical
Hot, dry air
Warm, moist air
A front is created when one of these types of air masses meets another type of air mass.
In general, there are four types of fronts: cold front, warm front, stationary front and an
occluded front.
A cold front happens when a mass of cooler, more-dense air meets warmer, less-dense
air. The warmer and usually more moisture-rich air is forced to rise over the on-coming
colder air. This rise of warm air leads to clouds and sometimes rain or snow showers. A
cold-front is shown by a line with a saw-tooth pattern on a weather map. The saw-teeth
point in the direction of the movement of the cold air.
When a cold front passes a spot on the Earth’s surface, clouds form, sometimes there is
precipitation (snow or rain or …), and the temperature of the air decreases.
5
a)
b)
c)
d)
e)
The drawing below is a side view of the cold front illustrated in the previous
page. Label the following in this view:
the cold air mass
the warm air mass
the direction of movement of the cold air mass
the direction of movement of the warm air mass
where the precipitation would occur
A warm front occurs when warm air collides with cooler air. The warm air is less dense
and rises over the cold air. Again, the moisture from the warmer air creates clouds and
precipitation as it rises. This type of weather system may generate temperature inversions
in which the cold air is trapped near the ground.
⎝
Label the cold air, the warm air, the precipitation, and direction of the two air
masses in the side view below:
6
Stationary fronts and occluded fronts are relatively common, but are beyond the scope of
this class.
A place you can observe current weather maps is at the following web site:
http://ww2010.atmos.uiuc.edu/(Gh)/wx/home.rxml
Click on Surface weather and select the type of map and/or image you want to look at.
See if you can find:
A cold front
A warm front
High pressure region
Low pressure region
If you want to learn more about weather symbols, go to the next section. This section is
not required—you will not be tested over the weather symbols.
7
Weather symbols
Earlier in this unit you learned about the wind barb and how we determine wind direction
and speed. The other parts of the weather symbol are described at the following
University of Illinois web site:
http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/sfcobs.rxml?hret=/indexlist.rxml
This symbol tells us the direction of the wind, wind speed, temperature, dew point, and
weather conditions (fog, clear, etc) for each location. Another unusual part of the symbol
is the manner in which air pressure is reported. If the air pressure value given is greater
than 500 mbar, the air pressure is below 1000 mbar. Add a “9” to the front and divide by
10. This will give you the air pressure in mbar. If the value is under 500, add a 10 to the
front and divide the resulting number by ten. Confusing?? Let’s try an example or two:
What is the air pressure if the value on the map is 210?
What is the air pressure if the value on the map is 820?
See an instructor to determine if you are on the right track!
Draw weather map symbols for the following conditions:
Clear skies
Air temperature= 23 degrees C
Dew Point= 20 degrees
Wind direction= 92 degrees
Wind speed= 10 knots
Atmospheric pressure= 1010 mb
Light Rain- total cloud cover
Air temperature= 5 degrees C
Dew Point= 5 degrees
Wind direction= 210 degrees
Wind speed= 18 knots
Atmospheric pressure= 990 mb
8
Once you get to this site, go to the link titled:
http://www.atmos.washington.edu/cgi-bin/latest.cgi?sfc_wa
The computer will load a series of weather maps for the past six hours. Use the controls
on the left side of the screen to select a single image. Select the most current map.
What is the weather symbol for the site closest to GRCC. What does the
symbol indicate the temperature is outside? What is the dew point? What
is the direction the wind is blowing from and what is the speed of the wind
in knots? What is the cloud cover?
Answer the same questions in the previous question for Aberdeen,
Washington.
If you go to the top of the screen you will find a drop down box that is titled “Select
Plot”. Select “WA with pressures”. A new set of maps will load. Now the corrected sealevel pressures are noted making the map more complicated.
What is the pressure as close to GRCC as you can find?
What is the pressure in Aberdeen?
If you have questions, please let us know!
9