NATS 101 Lecture 15 Surface 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 )

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 Phoenix Elevation 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, p mb 496 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 Maps How 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

Rules of Contouring (Gedzelman, p15-16) “ Every point on a given contour line has the same value of height above sea level.” “Every contour line separates regions with greater values than on the line itself from regions with smaller values than on the line itself.” “The closer the contour lines, the steeper the slope or larger the gradient.” “The shape of the contours indicates the shape of the map features.”

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.” Click “interactive exercise” Click “interactive isotherm map” From Online Picture

570 dam contour

576 dam contour

570 and 576 dam contours

All contours at 6 dam spacing

-20 C and –15 C Temp contours

-20 C, –15 C, -10 C Temp contours

All contours at 5 o C spacing

Height contours Temps shaded Region of High Heights  RIDGE  and Warmth Region of Low Heights  TROUGH  and Cold

PGF Wind

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 Problems , 6.13, 6.17, 6.19, 6.22

Review Ideal Gas Law…By Far

Example: Ideal Gas Law Relation between pressure, temperature and density is quantified by the Ideal Gas Law, which can be qualitatively written as Pressure = Constant x Density x Temperature It can be expressed in equation form as P(mb) = 2.87   (kg/m 3 )  T(K)

Ideal Gas Law P = constant    T With T constant, Ideal Gas Law reduces to P varies with  Dense air has a higher pressure than less dense air at the same temperature

Ideal Gas Law P = constant    T With  constant, Ideal Gas Law reduces to P varies with T Warm air has a higher pressure than cold air at the same density

Ideal Gas Law P = constant    T With P constant, Ideal Gas Law reduces to T varies with 1/  Cold air is more dense (  bigger, 1/  smaller) than warmer air at the same pressure

Example: Ideal Gas Law If Pressure at Sea Level averages 1013 mb and Temperature at Sea Level averages 288 K, what is the average Density at Sea Level? Answer can be found using the Ideal Gas Law P(mb) = 2.87   (kg/m 3 )  T(K)  (kg/m 3 ) = P(mb) / {2.87  T(K)}  (kg/m 3 ) = (1013 mb) / (2.87  288 K)  (kg/m 3 ) = 1.23 kg/m 3  2.00 lbs/yard 3

Summary Ideal Gas Law Relates Pressure = Density  Temperature P varies with  P varies with T T varies with 1/ 

Pressure-Temperature-Density Pressure (vertical scale highly distorted) Decreases more rapidly with height in cold air than in warm air Isobaric surfaces will slope downward toward cold air Slope increases with height to tropopause, near 300 mb in winter COLD 8.5 km COLD 9.5 km WARM 300 mb 1000 mb 400 mb 500 mb 600 mb 700 mb 800 mb 900 mb Minneapolis Same pressure Houston Same pressure 200 mb 100 mb

Pressure-Temperature-Density Pressure Higher along horizontal red line in warm air than in cold air Pressure difference is a non-zero force Pressure Gradient Force or PGF (red arrow) Air will accelerate from column 2 towards 1 Pressure falls at bottom of column 2, rises at 1 COLD 8.5 km COLD 9.5 km WARM 300 mb 1000 mb 400 mb 500 mb 600 mb 700 mb 800 mb 900 mb H L LH PGF 200 mb 100 mb

Pressure-Height Remember Pressure falls very rapidly with height close to sea level 3,000 m 701 mb 2,500 m747 mb 2,000 m 795 mb 1,500 m846 mb 1,000 m899 mb 500 m955 mb 0 m1013 mb Vertical pressure changes from differences in station elevation dominate horizontal changes Consequently………. Vertical pressure changes from differences in station elevation dominate horizontal changes

Measuring Air Pressure Mercury Barometer Air pressure at sea level can support nearly 30 inches of Hg Hg level responds to changes in pressure Pressure can support nearly 30 feet of water Ahrens, Fig. 6.4

Recording Aneroid Barometer Aneroid cell is partially evacuated Contracts as pressure rises Expands as pressure falls Changes recorded by revolving drum Ahrens, Fig. 6.6

Pressure-Height Remember Pressure falls very rapidly with height near sea-level 3,000 m 701 mb 2,500 m747 mb 2,000 m 795 mb 1,500 m846 mb 1,000 m899 mb 500 m955 mb 0 m1013 mb Vertical pressure changes from differences in station elevation dominate horizontal changes Consequently………. Vertical pressure changes from differences in station elevation dominate horizontal changes

Station Pressure Pressure is recorded at stations with different altitudes Station pressure differences reflect altitude differences Wind is forced by horizontal pressure differences Adjust station pressures to standard level: Mean Sea Level Ahrens, Fig. 6.7

Reduction to Sea-Level-Pressure Mean Sea Level Station pressures are adjusted to Mean Sea Level Make altitude correction of 1 mb per 10 m elevation Ahrens, Fig. 6.7

Correction for Tucson Elevation of Tucson Airport is ~800 m Station pressure at Tucson runs ~930 mb So, SLP for Tucson would be SLP = 930 mb + (1 mb / 10 m)  800 m SLP = 930 mb + 80 mb = 1010 mb

Correction for Denver Elevation of Denver CO is ~1600 m Station pressure at Denver runs ~850 mb So SLP for Denver would be SLP = 850 mb + (1 mb / 10 m)  1600 m SLP = 850 mb mb = 1010 mb Actual pressure corrections take into account temperature and pressure-height variations, but 1 mb / 10 m is a good approximation

Sea Level Pressure Values Ahrens, Fig. 6.3

Pop Quiz #2 Elevation of Phoenix AZ is ~340 m Elevation of Tucson AZ is ~800 m The station pressure at Phoenix was ~976 mb at 8 am today The station pressure at Tucson was ~932 mb at 8 am today Which station had the highest SLP?

Correction for Phoenix Elevation of PHX Airport is ~340 m Station pressure at PHX was ~976 mb So, SLP for PHX would be SLP = 976 mb + (1 mb / 10 m)  340 m SLP = 976 mb + 34 mb = 1010 mb

Correction for Tucson Elevation of TUS Airport is ~800 m Station pressure at TUS was ~932 mb So, SLP for TUS would be SLP = 932 mb + (1 mb / 10 m)  800 m SLP = 932 mb + 80 mb = 1012 mb So SLP at TUS was higher!