Geography 310
Lecture 12
Weather Map Interpretation and Constant Pressure Charts
Needs: Lect_310_12.ppt, maps and explanations at Unisys web site
A. General aspects of a "constant pressure surface"
1. isobars measure what pressure at MSL
PP1 2. In upper air no set height, so plot changes in height of pressure level
PP2
a. 850mb (moisture), 700mb (severe weather), 500mb (waves), 200 or 300 mb
(jet)
3. c.p. surface much like topographic map ("hills & valleys")
a. plan view (diagram below)
b. cross section (diagram below)
4. Isoheights (contours)--lines of constant height
a. used to relate changes in pressure surfaces to energy exchange
b. high heights "upper-level" highs
c. low heights "upper-level" lows
5. Examples from the Unisys web site (http://weather.unisys.com)
a. constant pressure surface maps (500hPa, 300hPa, and other levels)
b. other types of weather maps
i.
Surface maps
ii.
Radar maps
iii.
Model output maps
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B. Additional aspects of constant pressure surfaces
1. surface pressure-upper level pressure height related but not always same
a. temperature-pressure-height relationship
i. high temps=less dense=low pr.=expand=height rise
ii. low temps=more dense=high pr.=contract=height fall
b. heights respond to "mean layer or core temperature" so some temperature
distribution can produce cases where surface pressure and upper level
pressure height patterns are REVERSED (diagrams below)
i. cold-core highs have small sfc-500mb "thickness" and so low 500 mb
heights (sfc feature of most mid-latitude anticyclones)
ii. warm-core lows decrease in intensity with height (tropical cyclones,
MLCs), have large sfc-500mb thickness, so high 500 mb heights
(for MLCs the pattern is asymmetrical with the highest heights in
the warm sector (usually on the east side of the system in the NH)
c. other temperature distribution produces cases where surface pressure and upper
level pressure height patterns are the same (diagrams below)
i. warm-core highs have high heights above (sub-tropical highs, vertically
packed contours on bottom of troposphere)--contains sinking air
and dynamic compression on surface
ii. cold-core lows have low heights above (sub-polar low, vertically
packed contours in middle or top of troposphere)--contains rising
air and dynamic expansion on surface
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C. Uses of constant pressure surfaces
1. c.p. surfaces in mid-lats. reveal patterns of the G.C. transfer of latent and sensible heat
poleward
a. relates to jet stream and MLC & MLanti-Cyclone formation
2. energy exchange processes are driven by the "slope" or gradient of the c.p. surface
(remember asymmetrical shape of MLCs)
a. barotropic atmosphere--little or no slope, no gradients to drive energy
processes, found in areas of little temperature change, reason why tropical
systems are different than mid-latitude systems (see side and map view
diagrams below)
b. baroclinic atmosphere--sloping surfaces, gradients of temperature and pressure
so energy release is possible, found in the mid-latitude areas of temperature
contrasts (see side and map view diagrams below)
c. plan views--barotropic have widely spaced contours and isotherms don't cross
them; baroclinic have "packed" contours and isotherms cross (signifying
possible energy exchange)
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D. Aspects of c.p. surfaces (primarily 500mb)
1. winds blow parallel to height contours (no friction)
PP3 2. baroclinic zones--winds can carry different temperatures from one area to another
PP4
(cold and warm air advection)
a. this can change c.p. pattern over time (move waves)
b. wind speed is related to temperature gradient (feedback loop)
3. jet stream (200 or 300mb) name given to strongest winds found in the area of greatest
thermal contrast (greatest baroclinicity)--this is near the sfc polar front boundary
4. sfc irregularities (mountains and land-ocean sfc temps.) can affect waves, areas of
baroclinicity, and the jet stream
a. Rockies, Andes Mnts.
b. Oceans
5. zonal index--one measure of N-S gradient (waviness)
a. usually calculated between two parallels (ex. 30E to 50E)
b. measure height differences along several meridians and average
c. zonal index is an indication of the general wind speed, with higher index values
related to greater slopes and faster winds (diagnostic value of global
"vortex")
d. 500mb flow experiences modulations in mid-latitudes from more to less zonal
flow, not currently "predictable"