NATS 101 - 06

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NATS 101 - 06

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NATS 101 - 06Lecture 2Density, Pressure
TemperatureClimate and Weather

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Two Important Concepts

• Lets 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/m3, gm/cm3
• Mass
• molecules (mole) ? molecular mass (gm/mole)
• Avogadro number (6.023x1023 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

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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.

Higher elevation Less air above Lower
pressure Lower elevation More air above Higher
pressure
Top
Bottom
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Density and Pressure Variation

• Key Points
• Both decrease rapidly with height
• 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.

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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
Lower density, Gradual drop Higher
density Rapid decrease
NASA photo gallery
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Pressure Decreases Exponentially with Height

• Logarithmic Decrease
• For each 16 km increase in altitude, pressure
  drops by factor of 10.
• 48 km - 1 mb 32 km - 10 mb 16 km - 100
  mb 0 km - 1000 mb

1 mb
48 km
10 mb
32 km
100 mb
16 km
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.

Air Lower density, Gradual drop Higher
density Rapid decrease
Top
Top
Water Constant drop Constant drop
Bottom
Bottom
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Equation for Pressure Variation

• We can Quantify Pressure Change with Height

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What is Pressure at 2.8 km?(Summit of Mt. Lemmon)

• Use Equation for Pressure Change

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What is Pressure at Tucson?

• Use Equation for Pressure Change
• Lets get cocky
• How about Denver? Z1,600 m
• How about Mt. Everest? Z8,700 m
• You try these examples at home for practice

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Temperature (T) Profile

• More complex than pressure or density
• Layers based on the Environmental Lapse Rate
  (ELR), the rate at which temperature decreases
  with height.

Ahrens, Fig. 1.7
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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
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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 H2O vapor, weather of public
  interest

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Summary

• Many gases make up air
• N2 and O2 account for 99
• Trace gases CO2, H2O, O3, etc.
• Some are very importantmore 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

• 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

Ahrens, p 431
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Sky Cover and Weather Symbols
Ahrens, p 431
Ahrens, p 431
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Pressure Tendency

• Change in pressure over the past 3 hours is also
  plotted.
• Also called barometric tendency

Ahrens, p 432
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Wind Barbs

• Direction
• Wind is going towards
• Westerly ? from the West
• Speed (accumulated)
• Each flag is 50 knots
• Each full barb is 10 knots
• Each half barb is 5 knots

65 kts from west
Ahrens, p 432
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SLP pressure
temperature dew point
cloud cover
Ohio State website
wind
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Practice Surface Station

• Temperate (oF)
• 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

• 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

• Fronts
• Mark the boundary between different air
  masseslater
• Significant weather occurs near fronts
• Current US Map

Ahrens, p 432
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Radiosonde

• Weather balloons, or radiosondes, sample
  atmospheric to 10 mb.
• They measure temperature moisture pressure
• They are tracked to get winds

Ahrens, Fig. 1
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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

• 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

stratosphere
tropopause
troposphere
temperature profile
moisture profile
wind profile
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Upper-Air Model
Responsible for boxed parameters

• Conditions at specific pressure level
• Wind
• Temperature (?C)
• Moisture (Later)
• Height above MSL
• UA 500mb Analysis

Ahrens, p 431
Ahrens, p 427
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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 TucsonMonthly 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 TucsonProbability of Last Freeze
Cool Site Western Region Climate Center
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Climate of TucsonProbability of Rain
Cool Site Western Region Climate Center
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Climate of TucsonExtreme Rainfall
Cool Site Western Region Climate Center
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Climate of TucsonSnow!
Cool Site Western Region Climate Center
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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

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Reading Assignment

• Ahrens
• Pages 25-42
• Problems 2.1-2.4, 2.7, 2.9-2.12
• (2.1 ? Chapter 2, Problem 1)
• Dont Forgot the 4x 6 Index Cards