Wednesday Feb. 10, 2016
Dave McGraw and Mandy Fer: "Slumbering
Rose" (5:25), "Seritony"
(4:52), "Cat
Creek" (5:52), "Grow"
(4:17), "Comin
Down" (5:12)
The first real quiz of the semester is Wednesday next week and the
Quiz #1 Study Guide is now available.
Time on a surface map is converted to a
universally agreed upon time zone called Universal Time (or Greenwich Mean Time, or Zulu time). We
quickly went through how to convert from Universal Time to
Mountain Standard Time. All of this was stuck onto the
end of the online notes from Monday's
class,
there's no need to repeat it here.
Surface weather map analyses
A bunch of weather data has been plotted (using the
station model notation) on the surface weather map in
the figure below (p. 38 in the ClassNotes).
A couple of stormy regions have been circled in
green.
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.
1st step in surface map
analysis: draw in some contour lines to reveal the large
scale pressure pattern
Pressure
contours = isobars
( note the word bar
is in millibar, barometer, and now isobar ,
they all have something to do with pressure)
Temperature contours = isotherms
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
Isotherms, temperature
contour lines, are usually drawn at 10o F intervals. They
do two things:
isotherms (1) connect points on the map with the same
temperature
(2)
separate regions warmer
than a particular temperature
from regions colder
than a particular temperature
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). The temperature pattern is also
somewhat more predictable than the pressure pattern: temperatures
generally decrease with increasing latitude: warmest temperatures
are usually in the south, colder temperatures in the north.
Here's another example starting with just a bunch of temperature
numbers (Question #13 on the current Optional Assignment)
Our "job" is to try to make some sense of this data. To
do that we'll draw in an isotherm or two. Colors can help
you do this.
There is one temperature below 40 it has
been colored blue, temperatures
between 40 and 50 are green and temperatures in the
50s are colored yellow. It
should be pretty clear where the isotherms should go.
The isotherms have been drawn in at right; not how the
isotherms separate the colored bands. Note how the 40 F
isotherm goes through the 40 on the map.
Isobars
These are a little harder to draw because you have to
be able to decode the pressure data
isobars (1) connect points on the map with equal pressure
(2) separate regions of high pressure from regions with lower pressure
and
identify and locate centers of high and low pressure
Here's the same weather map with isobars drawn in.
Isobars are generally drawn at 4 mb intervals (above and below a
starting value of 1000 mb).
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 isobars separate regions of high and low pressure.
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.
This
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
A single isobar is shown. Is it the 1000, 1002, 1004, 1006,
or 1008 mb isobar? (you'll find the answer at the end of today's
notes)
What can you begin to learn about the weather once you've
mapped out the pressure pattern?
1a. Surface centers of low pressure
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 (don't worry about the Coriolis force
at this point, we'll learn more about it 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, we won'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.
Convergence
causes air to rise (1 of 4 ways)
rising
air e-x-p-a-n-d-s
(it moves into lower pressure surroundings at
higher altitude)
The expansion causes the air to cool
If you cool moist air enough
(to or below its dew point temperature) clouds can form
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.
1b. Surface centers of high
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 divergence.
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.
Divergence causes air to sink
sinking air is compressed and warms
warming air keeps clouds from forming - clear
skies
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 that tries to combine all
the key features in as simple a sketch as possible.
2. Strong and weak
pressure gradients - fast or slow winds
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. I handed out a
replacement for p. 40c in the ClassNotes (don't throw p. 40c
away).
A picture of a hill is shown above at left. The map at
upper right is a topographic map that depicts the hill
(the numbers on the contour lines are altitudes). A center
of high pressure on a weather map, the figure at the
bottom, has the same overall appearance. The numbers on
the contours are different. These are contours (isobars)
of 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 roll a rock downhill on a
steep slope it will roll more quickly than if it is on a gradual
slope. A rock will always roll downhill, away from the
summit in this case toward the outer edge of the topographic
map. Air will always start to move toward low pressure
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).
This is as far as we got in class
today. The situation with low pressure is
comparable as shown below.
Winds spin counterclockwise and spiral inward around low
pressure centers. The fastest winds are again found where
the contour lines are close together and the pressure gradient
is strongest.
Contour spacing
closely
spaced isobars = strong pressure gradient
(big change in pressure with distance) - fast
winds
widely spaced isobars = weak
pressure gradient (small change in
pressure with distance) - slow winds
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). Once
you know which directions the winds are blowing you should be able
to say whether the air at each of the points would be warmer or
colder than normal.
There are a total of 4 processes that cause air to
rise. They are all sketched below.
1. Convergence, winds spiraling in toward centers of low
pressure was mentioned earlier in class today.
2. We'll be learning about fronts in class on Friday. Cold
and warm fronts are shown in cross section above. You'll
better understand what is going on here after class Friday and
next Monday.
3. Sunlight striking and being absorbed at the ground warms the
ground. Air in contact with the ground warms and becomes
buoyant. If it is warm enough (low enough density) it will
float upward on its own. This is called free
convection. This came up in class last Fri. (see the Fri., Feb. 5 notes)
4. Finally when winds encounter a mountain they must move upward
and over the mountain. You might expect to see clouds and
precipitation on the upwind side of the mountain because that is
where the air is rising and cooling. You'll sometimes find a
"rainshadow" (lack of rain) on the dry downslope side of a
mountain.
Here are answers to the two questions embedded in today's notes
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, the 1002 mb and 1006
mb isobars wouldn't normally be drawn on a map)
Winds from the NW at 20 knots at Point #1,
SE winds at 30 knots at Point #2, and NW winds at 10 knots at
Point #3.
The southerly winds in the middle of the picture at Point
#2 would probably be the warmest. You would find colder
air coming from the north at Points #2 and #3.