NATS 101

Lecture 15Surface and Upper-Air Maps

Supplemental References for Today’s Lecture

Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

Summary

• Because horizontal pressure differences are the force that drives the wind

Station pressures are adjusted to one standard level…Mean Sea Level…to mitigate the impact of different elevations on pressure

Ahrens, Fig. 6.7

PGF

Correction for PhoenixElevation of PHX Airport is ~340 m

Station pressure at PHX was ~977 mb

So, SLP for PHX would be

SLP = 977 mb + (1 mb / 10 m) 340 m

SLP = 977 mb + 34 mb = 1011 mb

Surface Maps

• Pressure reduced to Mean Sea Level is plotted and analyzed for surface maps.Estimated from station pressures

• Actual surface observations for other weather elements (e.g. temperatures, dew points, winds, etc.) are plotted on surface maps.

NCEP/HPC Daily Weather Map

Isobaric Maps

• Weather maps at upper levels are analyzed on isobaric (constant pressure) surfaces.(Isobaric surfaces are used for mathematical reasons

that are too complex to explain in this course!)

• Isobaric maps provide the same information as constant height maps, such as:

Low heights on isobaric surfaces correspond to low pressures on constant height surfaces!

Cold temps on isobaric surfaces correspond to cold temperatures on constant height surfaces!

Isobaric Maps

Ahrens, Fig. 2, p141

504 mb504 mb

496 mb496 mb

PGF

Downhill(Constant height)

Some generalities:

1) High/Low heights on an isobar surface correspond to High/Low pressures on a constant height surface

2) Warm/Cold temps on an isobaric surface correspond to Warm/Cold temps on a constant height surface

3) The PGF on an isobaric surface corresponds to the downhill direction

Contour MapsHow we display

atmospheric fields

Portray undulations of 3D surface on 2D map

A familiar example is a USGS Topographic Map

It’s a useful way to display atmospheric quantities such as temperatures, dew points, pressures, wind speeds, etc.

Gedlezman, p15

Contour Maps “To successfully isopleth the 50-degree isotherm, imagine that you're a competitor in a roller-blading contest and that you're wearing number "50". You can win the contest only if you roller-blade through gates marked by a flag numbered slightly less than than 50 and a flag numbered slightly greater than 50.”

https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p02.html

Click “interactive exercise”

https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p03.html

https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p04.html

Click “interactive isotherm map”

From

Online Picture

570 dam contour570 dam contour

576 dam contour576 dam contour

570 and 576 dam contours570 and 576 dam contours

All contours at All contours at 6 dam spacing6 dam spacing

All contours at All contours at 6 dam spacing6 dam spacing

-20 C and –15 C -20 C and –15 C Temp contoursTemp contours

-20 C, –15 C, -10 C -20 C, –15 C, -10 C Temp contoursTemp contours

All contours at All contours at 55oo C spacing C spacing

Height contours Height contours Temps shadedTemps shaded

Region of Region of HighHigh Heights Heights RIDGERIDGE

and and WarmthWarmth

Region of Region of LowLow Heights Heights TROUGHTROUGH

and and ColdCold

PGFWind

Key Concepts for Today

• Station Pressure and Surface Analyses

Reduced to Mean Sea Level Pressure (SLP) PGF Corresponds to Pressure Differences

• Upper-Air Maps

On Isobaric (Constant Pressure) Surfaces PGF Corresponds to Height Sloping Downhill

• Contour Analysis

Surface Maps-Analyze Isobars of SLP Upper Air Maps-Analyze Height Contours

Key Concepts for Today

• Wind Direction and PGF

Winds more than 1 to 2 km above the ground are perpendicular to PGF!

Analogous a marble rolling not downhill, but at a constant elevation with lower altitudes to the left of the marble’s direction

Assignment

Topic – Newton’s Laws

Reading - Ahrens pg 150-157

Problems - 6.12, 6.13, 6.17, 6.19, 6.22