Transcript lecture02erk

NATS 101
Intro to Weather and Climate
Section 05: 2:00PM TTh ILC 150
Dr. E. Robert Kursinski
TAs: Mike Stovern &
April Chiriboga
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NATS 101 - 05
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NATS 101 - 05
Lecture 2
Density, Pressure &
Temperature
Climate and Weather
Two Important Concepts
Let’s introduce two new
concepts...
Density
Pressure
What is Density?
Density () = Mass (M) per unit Volume (V)
 = M/V
 = Greek letter “rho”
Typical Units: kg/m3, gm/cm3
Mass =
# molecules (mole)  molecular mass
(gm/mole)
Avogadro number (6.023x1023 molecules/mole)
Density Change
Density () changes by altering either
a) # molecules in a constant volume
b) volume occupied by the same # molecules
a
b
What is Pressure?
Pressure (p) = Force (F) per unit Area (A)
Typical Units: pounds per square inch
(psi), millibars (mb), inches
Hg
Average pressure at sea-level:
14.7 psi
1013 mb
29.92 in. Hg
Pressure
Can be thought of as weight of air above you.
(Note that pressure acts in all directions!)
So as elevation increases, pressure decreases.
Top
Bottom
Higher elevation
Less air above
Lower pressure
Lower elevation
More air above
Higher pressure
Density and Pressure
Variation
Key Points
1. Both decrease
rapidly with height
2. Air is
compressible, i.e.
its density varies
Ahrens, Fig. 1.5
Why rapid change with
height?
Consider a spring with 10 kg bricks on top of it
The spring compresses a little more with each
addition of a brick. The spring is compressible.
10 kg
10 kg
10 kg
10 kg
10 kg
10 kg
Why rapid change with
height?
Now consider several 10 kg
springs piled on top of each
other.
Topmost spring compresses
the least!
Bottom spring compresses the
most!
The total mass above you
decreases rapidly w/height.
 mass
 mass
 mass
 mass
Why rapid change with
height?
Finally, consider piled-up
parcels of air, each with
the same # molecules.
The bottom parcel is
squished the most.
Its density is the highest.
Density decreases most
rapidly at bottom.
Why rapid change with
height?
Each parcel has the same
mass (i.e. same number
of molecules), so the
height of a parcel
represents the same
change in pressure p.
Thus, pressure must
decrease most rapidly
near the bottom.
p
p
p
p
A Thinning
Atmosphere
Top
Bottom
NASA photo gallery
Lower density,
Gradual drop
Higher density
Rapid decrease
Pressure Decreases
Exponentially with Height
1 mb
10 mb
48 km
32 km
100 mb 16 km
Ahrens, Fig. 1.5
Logarithmic Decrease
• For each 16 km
increase inaltitude,
pressure drops
by factor of 10.
48 km - 1 mb
32 km - 10 mb
16 km - 100 mb
0 km - 1000 mb
Exponential Variation
Logarithmic
Decrease
• For each 5.5 km
height increase,
pressure drops
by factor of 2.
16.5 km - 125 mb
11 km - 250 mb
5.5 km - 500 mb
0 km - 1000 mb
Water versus Air
Pressure variation in water acts more like
bricks, close to incompressible, instead of
like springs.
Top
Bottom
Air:
Lower density,
Gradual drop
Higher density
Rapid decrease
Top
Water:
Constant drop
Constant drop
Bottom
Equation for Pressure
Variation
We can Quantify Pressure Change with Height
p (at elevation zin km)  pMSL  10
 Z /(16 km)
where
z is elevation in kilometers (km)
p is pressure in millibars (mb)
at elevation z in meters (km)
pMSL is pressure (mb) at mean sea level
What is Pressure at 2.8 km?
(Summit of Mt. Lemmon)
Use Equation for Pressure Change
 Z /(16 km)
p (at elevation Zin km)  pMSL 10
set Z = 2.8 km, pMSL  1013 mb
p (2.8 km)  1013mb 10  (2.8 km) /(16 km)
0.175
p (2.8 km)  1013mb 10
p (2.8 km)  1013mb  0.668  677 mb
What is Pressure at
Tucson?
Use Equation for Pressure Change
p(at elevation Zin km)  pMSL 10
set Z = 800 m, pMSL  1013 mb
 Z /(16 km)
Let’s get cocky…
How about Denver? Z=1,600 m
How about Mt. Everest? Z=8,700 m
You try these examples at home for practice
Temperature (T) Profile
inversion
isothermal
6.5oC/km
Ahrens, Fig. 1.7
• More complex than
pressure or density
• Layers based on
the Environmental
Lapse Rate (ELR),
the rate at which
temperature
decreases with
height.
Higher Atmosphere
Molecular Composition
• Homosphere- gases
are well mixed. Below
80 km. Emphasis of
Course.
• Heterosphere- gases
separate by molecular
weight, with heaviest
near bottom. Lighter
gases (H, He) escape.
Ahrens, Fig. 1.8
Atmospheric Layers
Essentials
• Thermosphere-above 85 km
Temps warm w/height
Gases settle by molecular weight (Heterosphere)
• Mesosphere-50 to 85 km
Temps cool w/height
• Stratosphere-10 to 50 km
Temps warm w/height, very dry
• Troposphere-0 to 10 km (to the nearest 5 km)
Temps cool with height
Contains “all” H2O vapor, weather of public interest
Summary
• Many gases make up air
N2 and O2 account for ~99%
Trace gases: CO2, H2O, O3, etc.
Some are very important…more later
• Pressure and Density
Decrease rapidly with height
• Temperature
Complex vertical structure
Climate and Weather
“Climate is what you expect.
Weather is what you get.”
-Robert A. Heinlein
Weather
Weather – The state
of the atmosphere:
for a specific place
at a particular time
Weather Elements
1) Temperature
2) Pressure
3) Humidity
4) Wind
5) Visibility
6) Clouds
7) Significant Weather
Surface Station Model
Responsible for boxed parameters
Ahrens, p 431
Temperatures
Plotted F in U.S.
Sea Level Pressure
Leading 10 or 9 is
not plotted
Examples:
1013.8 plotted as 138
998.7 plotted as 987
1036.0 plotted as 360
Sky Cover and Weather
Symbols
Ahrens, p 431
Ahrens, p 431
Wind Barbs
Direction
Wind is going towards
65 kts from west
Westerly  from the West
Speed (accumulated)
Each flag is 50 knots
Each full barb is 10 knots
Each half barb is 5 knots
Ahrens, p 432
SLP pressure
temperature
dew point
cloud cover
Ohio State website
wind
Practice Surface Station
72
58
111
Decimal point
What are Temp, Dew Point,
SLP, Cloud Cover, Wind
Speed and Direction?
Ahrens, p 431
Temperate (oF)
Pressure (mb)
Last Three Digits
(tens, ones, tenths)
Dew Point (later)
Moisture
Wind Barb
Direction
and Speed
Cloud Cover Tenths total
coverage
Practice Surface Station
42
18
998
Decimal point
What are Temp, Dew Point,
SLP, Cloud Cover, Wind
Speed and Direction?
Ahrens, p 431
Sea Level Pressure
Leading 10 or 9 is
not plotted
Examples:
1013.8 plotted as 138
998.7 plotted as 987
1036.0 plotted as 360
Surface Map Symbols
• Fronts
Mark the boundary
between different
air masses…later
Significant weather
occurs near fronts
Current US Map
Ahrens, p 432
Radiosonde
Weather balloons, or
radiosondes, sample
atmospheric to 10 mb.
They measure
temperature
moisture
pressure
They are tracked to get
winds
Ahrens, Fig. 1
Radiosonde Distribution
Radiosondes released at
0000 and at 1200 GMT
for a global network of
stations.
Large gaps in network
over oceans and in
less affluent nations.
Stations ~400 km apart
over North America
Radiosonde for Tucson
stratosphere
tropopause
troposphere
moisture
profile
temperature
profile
wind
profile
Example of data taken
by weather balloon
released over Tucson
Temperature (red)
Moisture (green)
Winds (white)
Note variations of all
fields with height
UA Tucson 1200 RAOB
Climate
Climate - Average weather and range of
weather, computed over many years.
Whole year (mean annual precipitation for
Tucson, 1970-present)
Season (Winter: Dec-Jan-Feb)
Month (January rainfall in Tucson)
Date (Average, record high and low
temperatures for Jan 1 in Tucson)
Climate of Tucson
Monthly Averages
Individual months can show significant
deviations from long-term, monthly means.
Average and Record
MAX and MIN
Temperatures for Date
Climate of Tucson
Probability of Last Freeze
Cool Site: Western Region Climate Center
Climate of Tucson
Probability of Rain
Cool Site: Western Region Climate Center
Climate of Tucson
Extreme Rainfall
Cool Site: Western Region Climate Center
Climate of Tucson
Snow!
Cool Site: Western Region Climate Center
Summary
• Weather - atmospheric conditions at
specific time and place
Weather Maps  Instantaneous Values
• Climate - average weather and the range
of extremes compiled over many years
Statistical Quantities  Expected Values
Reading Assignment
• Ahrens
Pages 25-42
Problems 2.1-2.4, 2.7, 2.9-2.12
(2.1  Chapter 2, Problem 1)
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