EAS 106 LAB WINTER STORMS: CYCLONES, ANTICYCLONES, FRONTS

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EAS 106 LAB WINTER STORMS CYCLONES,
ANTICYCLONES, FRONTS Outside the tropics, the
winds are organized into large systems called low
and high pressure areas, that produce much of our
weather. Highs and lows are typically about 1000
miles in diameter and travel from west to east at
about 25 miles per hour. Earth's rotation gives
them the form of giant wind spirals. Air spirals
counterclockwise in toward lows (cyclones) and
then is forced to rise. Therefore, lows bring
mostly cloudy, wet weather. Highs (anticyclones)
have the opposite circulation. Air spirals
clockwise and outward from the center. Then air
from above sinks to fill the void. Highs bring
mostly clear weather but if sunshine heats the
ground enough thunderstorms are possible. As
winds spiral into low pressure areas, large
regions of polar and of tropical air called air
masses are brought close together. Temperature
contrasts become concentrated in long narrow
zones or fronts, which are basically boundaries
that separate air masses. It is important to
identify the fronts and predict their motion
because much of the major weather changes and
stormy weather outside the tropics forms along
fronts. The next slide contains a schematic
3-D illustration of a low pressure area with
winds, air masses (cold or warm), fronts and
clouds. The slide after that shows how weather
data is plotted at each city weather station.
Then guides for drawing fronts and illustrative
examples are shown. Finally come the
exercises. WEB Sources NOAA Summaries of Major
Winter Storms http//www.hpc.ncep.noaa.gov/winter_
storm_summaries/winter_storm_summaries.shtml Weath
er Archive You make the maps http//vortex.plymo
uth.edu/u-make.html
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Cold Front Cold Air Advances
Warm Front Warm Air Advances
Model Extratropical Cyclone (Low) with Cold and
Warm Fronts
In the drawing of the low to the left, the clouds
NE of the warm front should be much wider and
continuous, as in the warm front drawing above.
http//rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
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The Weather Station Model Each weather map
contains weather information from a large number
of weather stations across the country. In order
to read a weather map we must first know how to
read the weather information at each station,
which is presented in a standard coded form
called the weather station model. The station
model on this slide is highly simplified. The
station model on the next slide is more complete.
T (F) Upper left number. Td (F) Lower left
number. Weather symbols (to left). P (0.1hPa)
Upper right number Wind direction is parallel to
the long shaft. It points from the feathers to
the small circle and is named for the direction
it comes from. Wind Speed (kt) given by feathers
on shaft. Full feather 10 kt. Double circle
calm
Since sea level pressure is always near 1000
hPa, even though it appears as 95.6 hPa (to save
space) it is really 995.6 hPa.
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More Complete Weather Station Model
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Fronts are designated by thick lines studded
with triangles or semicircles that protrude in
the direction that the front moves. For a
stationary front the triangles and semicircles
extend in opposite directions. For an occluded
front alternating triangles and semicircles
protrude in the direction of motion. When a
cold front passes by, T can drop more than 10? F
in an hour. In the typical cold front (below
left) at two times 12 hours apart, the cold air
(shaded) blows from the ___ and the warm air
blows from the __. Temperature changes tend to
be more gradual at warm fronts. In the typical
warm front (below right), the cold air is on the
___ side and blows from the ___, while the warm
air is on the ____ side and blows from the ___.
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Simple Model of Surface Low Pressure Area
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Indicators for Finding Fronts
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Indicators for Finding Fronts
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• The next 3 slides contain surface weather maps
  for the storm of 05-06 JAN 2005.
• 1200 UTC 05 JAN. On this chart, isotherms and
  fronts are analyzed.
• 0000 UTC 06 JAN. Two charts are overlain. The
  first chart includes fronts and isobars but no
  weather stations. The second chart includes the
  weather station information and retains the
  fronts.
• 1200 UTC 06 JAN. Two charts are overlain. The
  top (or first) chart includes isobars but no
  weather stations. To see the map with weather
  stations right click on the chart with isobars,
  click on Order and then click on send to back.
  Then, draw the fronts.

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Sample Weather Map with Isotherms (10C
intervals) and Fronts
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To see weather station data right click then
click on Order - Send to Back
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• The storm of 30 November to 02 December 2008
  produced rain, snow, and ice across a wide swath
  in the middle of the USA. The satellite image on
  the next slide (taken after the storm had passed
  and the weather had cleared) shows the ice and
  snow put down by the storm.
• The four slides after the satellite image contain
  surface weather charts for 12 UTC 30 NOV 2006 and
  00 UTC 01 DEC 2006. The first chart at each time
  contains the pressure analysis with isobars at
  pressure intervals of 4 hPa and the second chart
  includes weather information at a large range of
  weather stations. The fifth chart gives
  conditions at 850 hPa (about 1500 m above sea
  level). The key to the station model for the 850
  hPa chart is given on two slides after the
  surface charts.
• EXERCISES
• Draw Isotherms (5C intervals) and Fronts for
  the surface charts of 12 UTC 30 NOV 2006 and 00
  UTC 01 DEC 2006.
• Draw Isotherms (5C intervals) for the 850 hPa
  chart of 00 UTC. Find a station (if there is one)
  where air with T 0C on the 850 hPa chart is
  situated directly over air with T surface chart at the same time (0000 UTC).
  Briefly explain what type of precipitation you
  would expect at this station.
• Find a station at which you expect temperature
  to A rise and B fall, by 12 UTC on 01 DEC and
  give your reasons in terms of the fronts and how
  they might move based on their movement in the 12
  hours from 12 UTC on 30 NOV to 00 UTC on 01 DEC.

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Snow and Ice Cover 03 Dec 2006 MODIS
Cumulus Cloud Streets
Cumulus Clouds
Jet Stream Cirrus Clouds
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Decoded Values for Weather Variables on the 850
hPa Chart
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