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## NATS 101 - 06 Lecture 2 Density, Pressure

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### NATS 101 - 06 Lecture 2 Density, Pressure & Temperature Climate and Weather – PowerPoint PPT presentation

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Title: NATS 101 - 06 Lecture 2 Density, Pressure

1
NATS 101 - 06Lecture 2Density, Pressure
TemperatureClimate and Weather

2
Two Important Concepts
• Lets introduce two new concepts...
• Density
• Pressure

3
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)

4
Density Change
• Density (?) changes by altering either
• a) molecules in a constant volume
• b) volume occupied by the same molecules

5
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

6
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
7
Density and Pressure Variation
• Key Points
• Both decrease rapidly with height
• Air is compressible, i.e. its density varies

Ahrens, Fig. 1.5
8
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.

9
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
10
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.

11
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
12
A Thinning Atmosphere
density Rapid decrease
NASA photo gallery
13
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
14
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

15
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
16
Equation for Pressure Variation
• We can Quantify Pressure Change with Height

17
What is Pressure at 2.8 km?(Summit of Mt. Lemmon)
• Use Equation for Pressure Change

18
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

19
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
20
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
21
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

22
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

23
Climate and Weather
• Climate is what you expect.
• Weather is what you get.
• -Robert A. Heinlein

24
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

25
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
26
Sky Cover and Weather Symbols
Ahrens, p 431
Ahrens, p 431
27
Pressure Tendency
• Change in pressure over the past 3 hours is also
plotted.
• Also called barometric tendency

Ahrens, p 432
28
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
29
SLP pressure
temperature dew point
cloud cover
Ohio State website
wind
30
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

31
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

32
Surface Map Symbols
• Fronts
• Mark the boundary between different air
masseslater
• Significant weather occurs near fronts
• Current US Map

Ahrens, p 432
33
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