Monday, Feb. 6, 2012
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Reckoner
from Radiohead before class this afternoon.
The Experiment #1 reports were collected today. It generally
takes a week, maybe a little more, to get them graded. Meanwhile
the Experiment #2 materials should be distributed on Friday. If
you haven't returned your Experiment #1 materials please do so as soon
as you can so that they can be cleaned up and handed out to the Expt.
#2 people.
A take-home Optional
Assignment was handed out in class. It is
due on or before the start of class next Monday Feb. 13 (have it done
before coming to class).
We're starting a new block of material today - surface and
upper-level weather maps. We began by learning how
weather data are
entered onto surface weather maps.
Much of our weather is produced by relatively large
(synoptic scale)
weather systems - systems that might cover several states or a
significant fraction of
the continental US. To be able to identify and characterize these
weather systems you must first collect weather data (temperature,
pressure, wind direction and speed, dew point, cloud cover, etc) from
stations across the country and plot the data on a map. The large
amount of data requires that the information be plotted in a clear and
compact way. The station model notation is what meterologists
use.
We
worked through this material one step at a time (refer to p. 36 in
the photocopied ClassNotes). Some of the figures below were
borrowed from a previous semester or were redrawn and may differ
somewhat from what was drawn in class.
The center circle is filled in to indicate the portion
of
the sky
covered with clouds (estimated to the nearest 1/8th of the sky) using
the code at the top of the figure (which you can quickly figure
out). 3/8ths of the sky is covered
with clouds in the example above.
Then symbols are
used
to
identify
the
actual types of high, middle, and low altitude clouds observed in the
sky. Later in the semester we will learn the names of the 10
basic cloud types. Six of them are sketched above and symbols for
them are shown.
A complete list of cloud symbols was on a handout distributed in class
(a copy can be found here
) You do not, of
course, need to remember all of the cloud symbols.
A straight line extending out from
the center circle
shows the wind direction. Meteorologists always give the
direction the wind is coming from.
In this example the winds are
blowing from the NW toward the SE at a speed of 5 knots. A
meteorologist would call
these northwesterly winds.
Small barbs at the end of the straight
line give the wind speed in knots. Each long barb is worth 10
knots, the short barb is 5 knots.
Knots are nautical miles per hour. One nautical mile per hour is
1.15 statute miles per hour. We won't worry about the distinction
in this class, we will just consider one knot to be the same as one
mile per hour.
Here are four more examples.
What is the wind direction and wind speed in each case. Click here
for
the answers.
The air temperature and the dew point temperature are probably the
easiest data to decode.
The air temperature in this example
was 64o
F
(this is
plotted above and to the left of the center circle). The dew
point
temperature was 39o F and is plotted below and to the left
of the center circle. The box at lower left reminds you that dew
points range from the mid 20s to the mid 40s during much of the year in
Tucson.
Dew
points rise into the upper 50s and 60s during the summer thunderstorm
season (dew points are in the 70s in many parts of the country in the
summer). Dew points are in the 20s, 10s, and may even drop below
0 during dry periods in Tucson.
And maybe the most interesting part.
A symbol representing the weather
that is currently
occurring is plotted to the left of the center circle (in between the
temperature and the dew point). Some of
the common weather
symbols are
shown. There are about 100 different
weather symbols that you can choose
from (click here
if you didn't get a copy of the handout distributed in class today).
There's no way I could expect you to remember all of those weather
symbols.
The pressure data is usually the most confusing and most difficult
data to decode.
The sea level pressure is shown above and to the right
of
the center
circle. Decoding this data is a little "trickier" because some
information is missing. We'll look at this in more detail
momentarily.
Pressure change data (how the pressure has changed during
the preceding
3 hours)
is shown to the right of the center circle. You must
remember to add a decimal point. Pressure changes are usually
pretty small.
Here's what you need to know about the pressure data.
Meteorologists hope to map out small horizontal pressure
changes on
surface weather maps (that produce wind and storms). Pressure
changes much more quickly when
moving in a vertical direction. The pressure measurements are all
corrected to sea level altitude to remove the effects of
altitude. If this were not done large differences in pressure at
different cities at different altitudes would completely hide the
smaller horizontal changes.
In the example above, a station
pressure value of 927.3 mb was measured in Tucson. Since Tucson
is about 750 meters above sea level, a 75 mb correction is added to the
station pressure (1 mb for every 10 meters of altitude). The sea
level pressure estimate for Tucson is 927.3 + 75 = 1002.3 mb.
This sea level pressure estimate is the number that gets plotted on the
surface weather map.
Do you need to remember all the
details above and be able to calculate the exact correction
needed? No. You
should
remember that a correction for altitude is needed.
And the correction needs to be added to the station pressure.
I.e. the sea-level pressure is higher than the station pressure.
The calculation above is shown in a picture below
Here are some examples of coding
and decoding the pressure data.
First of all we'll take some sea level pressure values and show
what needs to be done before the data is plotted on the surface weather
map.
To save room, the leading 9 or 10
on the sea level pressure
value and
the decimal
point are removed before plotting the data on the map. For
example the 10 and the . in
1002.3 mb would
be removed; 023
would be plotted on the weather map (to the upper right of the center
circle). Some additional examples are shown above.
When reading pressure values off a
map you must remember to
add a 9 or
10 and a decimal point. For example
118 could be either 911.8 or 1011.8 mb. You
pick the value that
falls between 950.0 mb and 1050.0 mb (so 1011.8 mb would be the correct
value, 911.8 mb would be too low). Or pick the value
that is closest to 1000 mb, a typical value for sea level pressure.
Another
important piece of information on a surface map is the time the
observations were collected. Time on a
surface map is converted to a universally agreed upon time zone called
Universal Time (or Greenwich Mean Time, or Zulu time).
That is the time at 0 degrees longitude, the Prime Meridian.
There is a 7 hour time
zone difference between Tucson and Universal Time (this
never changes because Tucson stays on Mountain
Standard Time year round). You must add 7
hours to the time in Tucson to obtain Universal Time.
Here are several examples of conversions between MST and UT (most of these
weren't done in class)
to convert from MST (Mountain Standard Time) to UT (Universal Time)
10:20 am MST:
add the 7
hour time zone correction ---> 10:20
+ 7:00 = 17:20 UT (5:20 pm in Greenwich)
2:45 pm MST (this example was
done in class):
first convert to the 24 hour clock
by adding 12 hours 2:45 pm MST + 12:00 = 14:45 MST
add the 7 hour time zone
correction ---> 14:45 + 7:00 = 21:45 UT (7:45 pm in England)
7:45 pm MST:
convert to the 24 hour clock by
adding 12 hours 7:45 pm MST + 12:00 = 19:45 MST
add the 7 hour time zone correction ---> 19:45 + 7:00 = 26:45 UT
since this is greater than 24:00 (past midnight) we'll subtract 24
hours 26:45 UT - 24:00 = 02:45 am the next day
to convert from UT to MST
18Z:
subtract the 7 hour time
zone
correction ---> 18:00 - 7:00 = 11:00 am MST
02Z:
if we subtract the 7 hour time
zone correction we will get a negative
number.
So we will first add 24:00 to 02:00 UT then subtract 7 hours
02:00
+
24:00
=
26:00
26:00 - 7:00 = 19:00 MST on the previous day
2 hours past midnight in Greenwich is 7 pm the previous day in
Tucson
Here is a
gallery
of
surface weather map images.
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 (something we will
be learning more about on Wednesday)
