Transcript Notes

NATS 101-06
Lecture 11
Air Pressure
Review
• ELR-Environmental Lapse Rate
Temp change w/height measured by a
thermometer hanging from a balloon
DAR and MAR are Temp change w/height for
an air parcel (i.e. the air inside balloon)
• Why Do Supercooled Water Droplets Exist?
Freezing needs embryo ice crystal
First one, in pure water, is difficult to make
Review
• Updraft velocity and raindrop size
Modulates time a raindrop suspended in cloud
• Ice Crystal Process
SVP over ice is less than over SC water droplets
• Accretion-Splintering-Aggregation
Accretion-supercooled droplets freeze on contact
with ice crystals
Splintering-big ice crystals fragment into many
smaller ones
Aggregation-ice crystals adhere on snowflakes,
which upon melting, become raindrops!
Warm Cloud Precipitation
Terminal
Fall Speed
(5 m/s)
Updraft
(5 m/s)
Ahrens, Fig. 5.16
As cloud droplet ascends,
it grows larger by
collision-coalescence
Cloud droplet reaches the
height where the
updraft speed equals
terminal fall speed
As drop falls, it grows by
collision-coalescence to
size of a large raindrop
Ice Crystal
Process
Effect maximized around -15oC
Ahrens, Fig. 5.19
Since SVP for a water
droplet is higher than
for ice crystal, vapor
next to droplet will
diffuse towards ice
Ice crystals grow at the
expense of water drops,
which freeze on contact
As the ice crystals grow,
they begin to fall
Accretion-Aggregation Process
Small ice
particles will
adhere to ice
crystals
Supercooled
water droplets
will freeze on
contact with ice
snowflake
ice crystal
Ahrens, Fig. 5.17
Accretion
Splintering
Aggregation
(Riming)
Also known as the Bergeron Process after the
meteorologist who first recognized the
importance of ice in the precipitation process
What is Air Pressure?
Recoil
Force
Pressure = Force/Area
What is a Force?
It’s like a push/shove
In an air filled container,
pressure is due to
molecules pushing the
sides outward by
recoiling off them
Air Pressure
Recoil
Force
Concept applies to
an “air parcel”
surrounded by
more air parcels,
but molecules create
pressure through
rebounding off air
molecules in other
neighboring parcels
Air Pressure
Recoil
Force
At any point, pressure
is the same in all
directions
But pressure can vary
from one point to
another point
Higher density
at the same temperature
creates higher pressure
by more collisions
among molecules of
average same speed
Higher temperatures
at the same density
creates higher pressure
by collisions amongst
faster moving molecules
Ideal Gas Law
• Relation between pressure, temperature and
density is quantified by the Ideal Gas Law
P(mb) = constant  (kg/m3)  T(K)
• Where P is pressure in millibars
• Where  is density in kilograms/(meter)3
• Where T is temperature in Kelvin
Ideal Gas Law
• Ideal Gas Law describes relation between 3
variables: temperature, density and pressure
P(mb) = constant  (kg/m3)  T(K)
P(mb) = 2.87  (kg/m3)  T(K)
• If you change one variable, the other two
will change. It is easiest to understand the
concept if one variable is held constant
while varying the other two
Ideal Gas Law
P = constant    T (constant)
With T constant, Ideal Gas Law reduces to
 P varies with  
Denser air has a higher pressure than less
dense air at the same temperature
Why? You give the physical reason!
Ideal Gas Law
P = constant   (constant)  T
With  constant, Ideal Gas Law reduces to
 P varies with T 
Warmer air has a higher pressure than
colder air at the same density
Why? You should be able to answer the
underlying physics!
Ideal Gas Law
P (constant) = constant    T
With P constant, Ideal Gas Law reduces to
 T varies with 1/ 
Colder air is more dense ( big, 1/ small)
than warmer air at the same pressure
Why? Again, you reason the mechanism!
Summary
• Ideal Gas Law Relates
Temperature-Density-Pressure
Pressure-Temperature-Density
300 mb
500 mb
9.0 km
9.0 km
400 mb
600 mb
700 mb
800 mb
900 mb
Minneapolis
Houston
Pressure
Decreases with height
at same rate in air of
same temperature
Isobaric Surfaces
Slopes are horizontal
Pressure-Temperature-Density
WARM
8.5 km
9.5 km
COLD
Minneapolis
Houston
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
Pressure-Temperature-Density
WARM
L
H
8.5 km
PGF
H
PGF
Minneapolis
SFC pressure rises
9.5 km
COLD
L
Houston
SFC pressure falls
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
Animation
Summary
• Ideal Gas Law Implies
Pressure decreases more rapidly with
height in cold air than in warm air.
• Consequently…..
Horizontal temperature differences lead
to horizontal pressure differences!
And horizontal pressure differences lead
to air motion…or the wind!
Review: Pressure-Height
Remember
• Pressure falls very
rapidly with height
near sea-level
3,000 m 701 mb
2,500 m 747 mb
2,000 m 795 mb
1,500 m 846 mb
Consequently……….
1,000 m 899 mb
955 mb
Vertical pressure changes from 500 m
1013 mb
differences in station elevation 0 m
1 mb per 10 m height
dominate horizontal changes
Station Pressure
Ahrens, Fig. 6.7
Pressure is recorded at stations with different altitudes
Station pressure differences reflect altitude differences
Wind is forced by horizontal pressure differences
Horizontal pressure variations are 1 mb per 100 km
Adjust station pressures to one standard level:
Mean Sea Level
Reduction to Sea-Level-Pressure
Ahrens, Fig. 6.7
Station pressures are adjusted to Sea Level Pressure
Make altitude correction of 1 mb per 10 m elevation
Correction for Tucson
Elevation of Tucson AZ 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 + 160 mb = 1010 mb
Actual pressure corrections take into account
temperature and pressure-height variations,
but 1 mb / 10 m is a good approximation
You Try at Home for Phoenix
Elevation of Phoenix AZ is ~340 m
Assume the station pressure at Phoenix was
~977 mb at 3pm yesterday
So SLP for Phoenix would be?
Sea Level Pressure Values
882 mb Hurricane Wilma October 2005
Ahrens, Fig. 6.3
Summary
• Because horizontal pressure differences
are the force that drives the wind
Station pressures are adjusted to one
standard level…Mean Sea Level…to
remove the dominating impact of
different elevations on pressure change
PGF
Ahrens, Fig. 6.7
Key Points for Today
• Air Pressure
Force / Area (Recorded with Barometer)
• Ideal Gas Law
Relates Temperature, Density and Pressure
• Pressure Changes with Height
Decreases more rapidly in cold air than warm
• Station Pressure
Reduced to Sea Level Pressure
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
(Constant height)
496 mb
504 mb
Some generalities:
1) The
2)
3)
Warm/Cold
High/Low
PGF on
heights
temps
an isobaric
on
onan
ansurface
isobar
isobaric
surface correspond
corresponds
to the downhill
to Warm/Cold
High/Low
direction
temps
pressures
on
a constant
on aheight
constant
surface
height surface
Ahrens, Fig. 2, p141
Contour Maps
Display 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 50degree isotherm, imagine that
you're a competitor in a rollerblading 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”
From
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”
570 dam contour
576 dam contour
570 and 576 dam contours
All contours at
6 dam spacing
All contours at
6 dam spacing
-20 C and –15 C
Temp contours
-20 C, –15 C, -10 C
Temp contours
All contours at
5o C spacing
Height contours
Temp shading
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
• Reading - Ahrens pg 148-149
include Focus on Special Topic: Isobaric Maps
• Problems - 6.9, 6.10
Assignment
Topic – Newton’s Laws
Reading - Ahrens pg 150-157
Problems - 6.12, 6.13, 6.17, 6.19, 6.22