Atmospheric Composition
and
Contour Weather Maps
Val Bennington
AOS 101
September, 2008
Station Map Review
Atmospheric Composition
• How does temp change as we go up in the
atmosphere?
• Why?
• What makes up the atmosphere? Where do these
species reside?
Temperature in Atmosphere
What’s in the Atmosphere?
• Nitrogen (78%)
• Oxygen (21%)
• Argon (1%)
• Water vapor (0-4%)
• CO2 (380 ppm)
• Ozone (O3) – (4 ppb)
• Methane (CH4) – (1.7 ppm)
• CFCs
Questions
Where is the ozone layer?
Why does temp go up in the
stratosphere?
What level are we usually
talking about when we talk
about weather?
Where is most of the water
vapor?
Why is it hard to breathe at
high altitudes if it is still 21%
oxygen??
Contour Maps
What Do They Tell Us?
Why Contour?
• Contour maps organize all the available data so we can
make sense of it
• We have all that station map data!!!! Let’s use it. Let’s
be able to forecast from it!
• Once contoured, you can determine wind direction,
high and low pressure systems, locations of possible
precipitation, fronts, regions of strong winds and
changing temperatures --- ALL FROM A MAP!
BEFORE
AFTER
What They Tell Us
• Regions of High and
Low Pressure
• Fronts
• Temperature
• Wind Direction and
Speed
• How your weather is
going to change!
How to Think about Contouring
• Like topographic
map
• Lines of constant
height in this picture
• Walk along one of
these lines -- stay at
exactly the same
altitude the ENTIRE
time
What Else Do We Contour?
• Isopleth is a line on a map that connects all the points of a
given variable with the SAME SPECIFIED VALUE
• Isobar - line of constant pressure
• Isotherm - line of constant temperature
• Isotach - line of constant wind speed
• Isodrosotherm - a line of constant dewpoint
• Isohyet - a line of constant precipitation accumulation
• Isoneph - a line of constant cloudiness
• Isohaline - a line of constant salinity (saltiness in the ocean)
• Isoheight - a line of constant height
Isobars in Blue (lines of constant pressure)
Can you spot Hurricane Ike???
Which Way Do the Winds Go?
Geostrophy
Notes about Isobars
• Closeness of lines tells us how strong pressure gradient
force (PGF) is
• Very close lines = great pressure difference over small
distance
----> large PGF!
**Think of topographic map
** Steep terrain where lines close together
** Here: large PGF when lines close together = Big Force
Isotachs (wdsp) and Isoheights
Isotherms
Contouring Rules
• Contours never cross each other
• A contour line has values higher than the line value on
one side and values lower on the other side of it
• Isotherm of 10°F has temps below 10°F on one side
and temps above 10°F on the other
• Contours are only drawn where there is data!
• Evenly spaced increments are always used (say every 5
degrees and every 4 mb for pressure)
• Easiest to find a high or low value first and work from
there
Examples
• Temperature
observations
• Where do we draw
the 15°F
isotherm????
Examples
• Temperature
observations
• Where do we draw
the 15°F
isotherm????
Examples
• Temperature
observations
• Where should we
draw the 75ºF and
80ºF isotherms?
• Where should we
draw the 75F and
80F isotherms?
Examples
• Temperature observations
• Where should we draw the
75ºF and 80ºF isotherms?
• Where should we draw the
75F and 80F isotherms?
New Times
• In the olden days, all weather maps and contour
maps were done by hand!
• Now, computers draw contour maps and determine
wind direction and speed - as well as many other
things
• Contouring is very tedious - so you will only be
asked to do simple contouring