Transcript Lecture 5 (10/01) METR 1111

Lecture 5 (10/07)
METR 1111
Isolining and
Upper Air Maps
What are isolines?
• Isolines (also called isopleths)- lines
that connect equal values of a variable
• In meteorology, we frequently use:
- isobars (pressure)
- isotherms (temperature)
- isotachs (wind speed)
Isolines
• Isolines make it easy to see different
values on a map
• Allow us to interpolate values at a point
• Can pick out different features like
fronts, jetstreams, drylines, pressure
fields, etc.
• How can I draw an isoline?
Drawing Isolines
• When drawing isolines/isopleths, keep
in mind:
• Lines cannot cross.
• They must close themselves off or start
and end at the edge of the map
• You must have greater values on one
side and smaller values on the other
Upper Air
• “Upper air” can refer to anything not
on the surface
• Rawinsonde (weather balloons)
launched twice a day from stations
across country & world at 0 and 12 Z
• Measures temperature, pressure,
humidity, and wind speed and direction
Upper Air Maps
• UAM have many stations’ readings at a
certain constant pressure level
• Ex: On an 850mb map, all stations are
at a pressure of 850mb.
• Most common upper level maps:
850mb, 700mb, 500mb, 250mb,
200mb.
Temperature
• Temperature is in upper left corner
• Measured in Celsius
Dew Point/Dew Point Depression
• Dew point or dew point depression is in
lower left corner
• Measured in Celsius
• Dew point depression = difference
between the temperature and dew point
• Can look for map label or use intuition
• If you see a dew point greater than a
temperature, dew point depression is
plotted (here dew pt = -2° C)
Geopotential Height
• Upper right corner = geopotential
height
• Geopotential height -- the height above
sea level that the instrument is at when
it reads the specified pressure level.
• Often truncate last number or two but
measured in meters
• Need to decode geopotential height.
To try clear up confusion in class:
• The following pattern appears between isobars and
lines of constant geopotential height:
• If we draw lines of constant height on a constant
pressure surface it will look like lines of constant
pressure (isobars) drawn at a given height (as long
as data at about the same elevation above the earth
is used)
• Thus, an area of low heights means you have an
area of low pressure
• An area of high heights means you have an area of
high pressure
What does it all mean?
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Here’s an example if you’re interested:
Compare this pattern of constant height lines on a constant pressure surface:
http://weather.unisys.com/upper_air/previous/ua_250-1.html
With this pattern of constant pressure lines on a constant height surface:
http://weather.unisys.com/upper_air/previous/ua_36000ft-2.html
The height contours from the first should approximately line up with the
isobars from the second (except that they use fewer lines on one).
To see this best, look in regions where the lines are closest together (greatest
pressure gradient) and compare between the two maps
A viewing tip: Click on the first url to display it on your web browser and then
cut and paste the second url into the box and hit return—then hit the forward
and back button to switch between the two
Decoding Geopotential Heights
• Not all map-makers use the same truncation
rules
• These are the most common:
• 850 mb add 1000m
533=1,533m
• 500 mb multiply by 10 591=5,910m
• 250 mb multiply by 10 098=10,980m
& add 10,000m
Wind Barb
• Wind symbols are the same as with
surface plots
• Remember a triangle means 50 knots
• Ex: 1 triangle, 3 long flags and 1short
flag add up to 50+30+5=85 kts
• This wind is 90° (Easterly) at 50 kts
Announcements:
• Pick up graded hmwks & quizzes after
you turn in your quiz
• Read Ch 6 (Floods & Droughts)
• Complete homework 5 for next time
(available on web)