Transcript Lecture 3 - The University of Arizona Department of
NATS 101 - 06 Lecture 2
Density, Pressure & Temperature Climate and Weather
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Two Important Concepts
Let’s introduce two new concepts...
Density Pressure
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What is Density?
Density (
) = Mass (M) per unit Volume (V)
= M/V
= Greek letter “rho” Typical Units: kg/m 3 , gm/cm 3 Mass = # molecules (mole)
molecular mass (gm/mole) Avogadro number (6.023x10
23 molecules/mole)
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Density Change
Density (
) changes by altering either a) # molecules in a constant volume b) volume occupied by the same # molecules a b
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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
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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 6
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
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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 8
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
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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.
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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
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A Thinning Atmosphere
Top
Lower density, Gradual drop NASA photo gallery
Bottom
Higher density Rapid decrease
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Pressure Decreases Exponentially with Height Logarithmic Decrease 1 mb 10 mb 48 km 32 km
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For each 16 km increase in altitude, pressure drops by factor of 10. 100 mb 16 km 48 km - 1 mb 32 km - 10 mb 16 km - 100 mb 0 km - 1000 mb Ahrens, Fig. 1.5
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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
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Water versus Air
Pressure variation in water acts more like bricks, close to incompressible, instead of like springs.
Top
Air: Lower density, Gradual drop
Top
Water: Constant drop
Bottom
Higher density Rapid decrease
Bottom
Constant drop
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Equation for Pressure Variation
We can Quantify Pressure Change with Height
p
(at elevation z in km)
p
MSL 1 0
Z
/(16 km) where
z
is elevation in kilometers (km)
p
is pressure in millibars (mb) at elevation z in meters (km)
p
MSL is pre ssure (mb ) at mean sea leve l 16
What is Pressure at 2.8 km?
(
Summit of Mt. Lemmon
) Use Equation for Pressure Change
p
(at elevation Zin km)
p
MSL 10
Z p
MSL 10 13 mb /(16 km)
p
(2.8 km) (2.8 km) /(16 km)
p
(2.8 km) 1013mb 10 0.175
p
(2.8 km) 17
What is Pressure at Tucson?
Use Equation for Pressure Change
p
(at e levation Zin km)
p
M S L 10
Z Z
800 m ,
p
MS L 1013 mb Let’s get cocky… /(16 km) How about Denver? Z=1,600 m How about Mt. Everest? Z=8,700 m You try these examples at home for practice 18
Temperature (T) Profile
inversion
• •
More complex than pressure or density Layers based on the
Environmental Lapse Rate (ELR),
the rate at which temperature decreases with height. isothermal 6.5
o C/km
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Ahrens, Fig. 1.7
Higher Atmosphere
Ahrens, Fig. 1.8
• •
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.
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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” H 2 O vapor, weather of public interest
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Summary
• • •
Many gases make up air N 2 and O 2 account for ~99% Trace gases: CO 2 , H 2 O, O 3 , etc.
Some are very important…more later Pressure and Density Decrease rapidly with height Temperature Complex vertical structure
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Climate and Weather “Climate is what you expect. Weather is what you get.”
-Robert A. Heinlein
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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
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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
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Sky Cover and Weather Symbols
Ahrens, p 431 Ahrens, p 431
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Wind Barbs
Direction Wind is going towards Westerly
65 kts from west
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
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Ohio State website
SLP pressure temperature dew point wind cloud cover
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Practice Surface Station
72 111 58 Decimal point What are Temp, Dew Point, SLP, Cloud Cover, Wind Speed and Direction?
Ahrens, p 431 Temperate ( o F) Pressure (mb) Last Three Digits (tens, ones, tenths) Dew Point (later) Moisture Wind Barb Direction and Speed Cloud Cover Tenths total coverage
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Practice Surface Station
42 998 18 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
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Surface Map Symbols
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Fronts Mark the boundary between different air masses…later Ahrens, p 432 Significant weather occurs near fronts
Current US Map 32
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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
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Radiosonde for Tucson
stratosphere troposphere moisture profile tropopause 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 36
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)
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Climate of Tucson Monthly Averages
Individual months can show significant deviations from long-term, monthly means.
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Average and Record MAX and MIN Temperatures for Date
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Climate of Tucson
Probability of Last Freeze
Cool Site: Western Region Climate Center 44
Climate of Tucson
Probability of Rain
Cool Site: Western Region Climate Center 45
Climate of Tucson
Extreme Rainfall
Cool Site: Western Region Climate Center 46
Climate of Tucson Snow!
Cool Site: Western Region Climate Center 47
Summary
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Weather - atmospheric conditions at specific time and place Weather Maps
Instantaneous Values
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Climate - average weather and the range of extremes compiled over many years Statistical Quantities
Expected Values
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Reading Assignment
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Ahrens Pages 25-30
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Problems 2.1-2.4 (2.1
Chapter 2, Problem 1) Don’t Forgot the 4”x 6” Index Cards…
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