Wednesday Feb. 6, 2013
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I'm sorry you weren't able to see the video that went along
Woman Blues" sung by Valerie June.
40 sets of Experiment #2
materials were distributed in class today. About 55
people have signed up for the experiment so that wasn't enough
for everyone. I should have enough extra materials on
Friday for everyone. Expt. #2 reports are due Mon., Feb.
Quiz #1 is one week from today (Wed., Feb. 13). A
preliminary version of the Quiz #1
Study Guide is now available. Quiz #1 will cover
material on both the Quiz #1 Study Guide and the Practice Quiz Study Guide.
Reviews are scheduled for Mon. and Tue. afternoon next week
from 4 - 5 pm in Haury (Anthropology) 129.
Assignment was handed out in class today. It is
due next Monday, but you can always turn it in early.
On Monday we learned how weather data is plotted on a surface
map using the station model notation (and you should go back to
the end of the Monday notes because there were one or two topics
not covered in class). Today we'll start to see what
analysis of that data can start to tell you about the weather.
A bunch of weather data has been
plotted (using the station model notation) on a surface weather
map in the figure below (p. 38 in the ClassNotes).
Plotting the surface weather data on a map is just the
beginning. For example you really can't tell what is causing
the cloudy weather with rain (the dot symbols are rain) and
drizzle (the comma symbols) in the NE portion of the map above or
the rain shower along the Gulf Coast. Some additional
analysis is needed. A meteorologist would usually begin by
drawing some contour lines of pressure (isobars) to map out the
large scale pressure pattern. We will look first at contour
lines of temperature, they are a little easier to understand (the
plotted data is easier to decode and temperature varies across the
country in a more predictable way).
Isotherms, temperature contour lines, are usually drawn at 10o F intervals. They
do two things: (1) connect points on the map that all
have the same temperature, and (2) separate regions that are
warmer than a particular temperature from regions that are
colder. The 40o F
isotherm above passes through a city which is reporting a
temperature of exactly 40o
(Point A). Mostly it goes between pairs of cities: one with
a temperature warmer than 40o (41o at
Point B) and the other colder than 40o (38o
F at Point C).
generally decrease with increasing latitude: warmest temperatures
are usually in the south, colder temperatures in the north.
Now the same data with isobars drawn in. Again they separate
regions with pressure higher than a particular value from regions
with pressures lower than that value. The
isobars also enclose areas of high pressure and low
pressure. Isobars are generally drawn at 4 mb intervals
(starting with a base value of 1000 mb). Isobars
also connect points on the map with the same pressure.
The 1008 mb isobar (highlighted in yellow) passes through a city
at Point A where the
pressure is exactly 1008.0 mb. Most of the time the isobar
will pass between two cities. The 1008 mb isobar passes
between cities with pressures of 1009.7 mb at Point B and 1006.8 mb at Point C.
You would expect to find 1008 mb somewhere in between those two
cites, that is where the 1008 mb isobar goes.
The pressure pattern is not as predictable as the isotherm
map. Low pressure is found on the eastern half of this map
and high pressure in the west. The pattern could just as
easily have been reversed.
site (from the American Meteorological Society) first shows
surface weather observations by themselves (plotted using the
station model notation) and then an analysis of the surface data
like what we've just looked at. There are links below each
of the maps that will show you current surface weather data.
Here's a little practice (this figure wasn't shown in class).
Is this the 1000, 1002, 1004, 1006, or 1008 mb isobar? (you'll
find the answer at the end of today's notes)
Now we'll look at what you can learn about the weather once
you've drawn in some isobars and mapped out the pressure pattern.
We'll start with the large nearly circular centers of High and Low
pressure. Low pressure is drawn below. These figures
are more neatly drawn versions of what we did in class.
Air will start moving toward low
pressure (like a rock sitting on a hillside that starts to roll
downhill), then something called the Coriolis force will cause
the wind to start to spin (we'll learn more about the Coriolis
force later in the semester). In the northern hemisphere winds
spin in a counterclockwise (CCW) direction around surface low
pressure centers. The winds also spiral inward toward the
center of the low, this is called convergence. [winds spin
clockwise around low pressure centers in the southern hemisphere
but still spiral inward, don't worry about the southern
hemisphere until later in the semester]
When the converging air reaches the center of the low it starts to
rise. Rising air expands (because it is moving into lower
pressure surroundings at higher altitude), the expansion causes it
to cool. If the air is moist and it is cooled enough (to or
below the dew point temperature) clouds will form and may then
begin to rain or snow. Convergence
is 1 of 4 ways of causing air to rise (we'll learn what
the rest are soon, and, actually, you already know what one of
them is - warm air rises, that's called convection). You often see cloudy
skies and stormy weather associated with surface low pressure.
Everything is pretty much the exact opposite in the case of
surface high pressure.
Winds spin clockwise (counterclockwise in the southern
hemisphere) and spiral outward. The outward motion is called
Air sinks in the center of surface high pressure to
replace the diverging air. The sinking air is compressed and
warms. This keeps clouds from forming so clear skies are
normally found with high pressure.
Clear skies doesn't necessarily mean warm weather, strong surface
high pressure often forms when the air is very cold.
Here's a picture summarizing what we've learned so far.
It's a slightly different view of wind motions around surface
highs and low and wasn't
shown in class.
The pressure pattern will also tell you something about where
you might expect to find fast or slow winds. In this case
we look for regions where the isobars are either closely spaced
together or widely spaced. Portions of the two figures
that follow can be found on p. 40c in the ClassNotes.
A picture of a hill is shown
above at left. The maps at upper right is a topographic
map that depicts the hill (the numbers on the contour lines are
altitude). A center of high pressure on a weather map, the
figure at bottom left, could have exactly the same
appearance. The numbers on the contours lines (isobars)
are now pressure values in millibars.
Closely spaced contours on a topographic map indicate a steep
slope. More widely spaced contours mean the slope is more
gradual. If you trip walking on a hill, you
will roll rapidly down a steep hillside, more slowly down a
On a weather map, closely spaced contours (isobars) means
pressure is changing rapidly with distance. This is known
as a strong pressure gradient and produces fast winds (a 30 knot
wind blowing from the SE is shown in the orange shaded region
above). Widely spaced isobars indicate a weaker pressure
gradient and the winds would be slower (the 10 knot wind blowing
from the NW in the figure).
The winds around a high pressure
center are shown above using both the station model notation and
arrows. The winds are spinning clockwise and spiraling outward
slightly. Note the different wind speeds (25 knots and 10
knots plotted using the station model notation). Fast
winds where to contours are close together and slower winds
where they are further apart.
Winds spin counterclockwise and
spiral inward around low pressure centers. The fastest
winds are again found where the pressure gradient is strongest.
This figure is found at the bottom of p. 40 c in the
photocopied ClassNotes. You should be able to sketch in the
direction of the wind at each of the three points and determine
where the fastest and slowest winds would be found. (you'll find
the answer at the end of today's notes).
Here are the answers to the two questions found earlier in the
Pressures lower than 1002 mb are colored purple.
Pressures between 1002 and 1004 mb are blue. Pressures
between 1004 and 1006 mb are green and pressures greater than 1006
mb are red. The isobar appearing in the question is
highlighted yellow and is the 1004 mb isobar. The 1002 mb
and 1006 mb isobars have also been drawn in (because isobars are
drawn at 4 mb intervals starting at 1000 mb, 1002 mb and 1006 mb
isobars wouldn't normally be drawn on a map)
And here's the answer to the question about wind directions and
The winds are blowing from the NNW at Points 1 and 3. The
winds are blowing from the SSE at Point 2. The fastest winds
(30 knots) are found at Point 2 because that is where the isobars
are closest together (strongest pressure gradient). The
slowest winds (10 knots) are at Point 3. Notice also how the
wind direction can affect the temperature pattern. The winds
at Point 2 are coming from the south and are probably warmer than
the winds coming from the north at Points 1 & 3. We'll
be looking at how the pressure pattern (because it largely
determines the winds) can indirectly affect the temperature