CHAPTER 3
ATMOSPHERIC
PRESSURE
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• http://www.avcanada.ca/forums2/viewtopic.ph
p?f=54&t=70372
Weight of Earth’s Atmosphere
• Given: Weight per square inch = 14.7 lbs
• Given: Earth’s surface area = 196,000,000
sq miles (statute)
• Weight per square foot = 2116.8 lbs or
1.0584 tons
• Weight per square mile = 29,506,498 tons
• Total weight of atmosphere =
5,783,000,000,000,000 tons
14.7 lbs. per square inch
Volume of Earth’s Atmosphere
• Given: 99% is contained within 31 miles of
the surface
• Total volume = 6,076,000,000 cubic miles
ATMOSPHERIC PRESSURE
• Evangelista Toricelli who was a
student of Galileo invented the
barometer in 1643.
• There are 2 common types of
Barometers: Mercurial and Aneroid
(without liquid) Wafer type.
• Baro =Greek for weight
• Aneroid = not wet
BAROMETERS
• Any instrument that measures pressure is
called a barometer
• Aneroid Barometers work similar to Altimeter
Aneroid barometer
Mercury Barometer
BAROMETERS
• Mercury Barometers need to be
corrected before any of the pressure
readings can be used for maps.
• Elevation must be corrected (set for
sea level)
• Temperature (corrected to 0 degrees
C)
• Acceleration of gravity (45 degree
latitude)
http://www.engineeringtoolbox.com/accel
eration-gravity-latitude-d_1554.html
BAROMETERS ELEVATION
BAROMETERS ELEVATION
MERCURY BAROMETERS
• Atmospheric pressure forces
mercury from the open dish upward
into the evacuated glass tube. The
height of the mercury column is a
measure of atmospheric pressure.
MERCURY BAROMETERS
• Standard sea level pressure =
29.92 inches of mercury or 1013.25
hectopascals (=millibars)
• pressure = force per unit area
AIR HAS WEIGHT
• Air is heavy. It is a lot heavier than most people
realize. One cubic yard of air at sea level
pressure and at a temperature of 70 F weighs
almost exactly 2 pounds. The air in a room 12 ft x
14 ft with an 8-foot ceiling weighs almost exactly
100 pounds.
• To come to the point quickly, the average air
pressure at sea level is about 14.7 psi. It varies
from day to day, but this is the average pressure
in pounds per square inch, in round numbers.
Water barometer
• Water can be used as the liquid to make a pretty good barometer, but there
is a practical difficulty in that the arms of the U tube need to be about 34
feet long. That is, atmospheric pressure is able to push a column of water
upward a distance of 34 feet above the level at which the pressure is
applied.
• The first barometer was developed by Evangelista Torricelli in 1643, in the
era of Issac Newton. One of his earlier models used water as the liquid.
Story has it that he mounted the thing (34 feet tall) in the village square so
the townspeople could see it. But they made him take it down because
every time the liquid dropped down, the weather turned bad. And they
didn't need any more bad weather.
• After Mr. Torricelli had developed a workable and portable mercury
barometer, he and his friends read the thing very carefully while down in
the valley, and then carried it up to the top of a nearby mountain. Ahh ha!
The barometer indicated a lower pressure at the top of the mountain than
it did down in the valley. This gave the first indication that the atmosphere
of the earth had a finite depth rather than extending upward forever.
• Atmospheric pressure = 14.7 psi = 30 in/Hg = 34 feet of water
• All these numbers are approximate, of course.
PRESSURE VARIATION
• Pressure Varies with
• Altitude - Pressure drops at an
average of 1 inch/ 1000’ as we go
up in the atmosphere
• Also with: Temperature
Approximately 1 inch per 1000ft
•
•
•
•
•
•
•
•
Here is a table that gives the atmospheric pressure at various altitudes. The
altitude is given in feet and the pressure is in inches of mercury.
Altitude
Pressure
Altitude
Pressure
0,000
29.92
20,000
13.75
1,000
28.86
25,000
11.10
2,000
27.82
30,000
8.886
3,000
26.82
35.000
7.041
4,000
25.84
40,000
5.538
5,000
24.89
45,000
4.355
10,000
20.58
50,000
3.425
15,000
16.88
60,000
2.118
18,000 *
14.94
100,000
0.329
* This is almost exactly one-half the sea-level value.
To convert in/Hg to psi, multiply by 0.491.
It is interesting to note that the pressure drops to one-half its sea-level value at
about 18,000 feet. The implication is that one-half of all the mass of the
atmosphere lies below this altitude. Further, almost (but not quite) a third of
the total lies below 10,000 feet.
http://www.challengers101.com/Pressure.html
Average decrease in pressure/1000ft
•
Average decrease
Altitude Range
per 1,000 feet Feet per in/Hg
• Sea level to 5,000 ft
1.006 in/Hg
994
5,000 to 10,000
0.862
1,160
10,000 to 15,000
0.740
1,350
Sea Level to 10,000 ft
0.934
1,070
• At atlitudes below 5,000 feet, you won't be in error too
much is you simply say 1 in/Hg per 1,000 feet, or, 1,000
feet for each change in pressure of 1 in/Hg. Or even, 10
feet for each 0.01 change in pressure.
• Stations then take the local pressure and
plot it on maps to follow the pressure
patterns.
• Lines of equal pressure are then
connected called isobars.
LOW PRESSURE
• Low = center of pressure
surrounded on all sides by
higher pressure also called a
cyclone. Cyclonic
• rotates counterclockwise
• area of rising air
• usually clouds present
• bad weather
HIGH PRESSURE
• High = a center of pressure
surrounded on all sides by
lower pressure also called
an Anticyclone.
Anticyclonic
• rotates clockwise
• area of descending air
• usually no clouds
• good weather
Other PRESSURE Definitions
• Trough - an elongated area of low
pressure with the lowest pressure along
a line marking maximum cyclonic
curvature.
• Ridge - an elongated area of high
pressure with the highest pressure
along a line marking maximum
anticyclonic curvature.
Other PRESSURE Definitions
• Col = the neutral are between two
highs and two lows (like a
mountain pass on a map
Surface/Upper Air Maps
• We will discuss more in detail latter on.
• You can find many different kinds of
weather maps for different pressure
analysis.
• 250, 500, 700 etc…
• These charts can be very useful in
determining the weather at specific altitudes
• Example 700mb chart is approximately
10,000 ft MSL
ALTIMETRY
• The Altimeter is basically an
aneroid barometer (measures height)
• Indicated altitude - read off a
correctly set altimeter
• Pressure altitude - altitude of the
29.92” line or read off altimeter
when set to 29.92
• Density altitude - pressure altitude
corrected for nonstandard temp.
ALTIMETRY
• Absolute altitude - the height above the
surface (AGL)
• True altitude - actual altitude above sea
level
TEMPERATURE
• Causes an airmass to expand or
contract
• This however does not necessarily
effect pressure with a given volume of
air
• therefore the pressure line will be
higher when warmer
• the pressure line will be lower when
colder
INDICATED ALTITUDE
• Temperature affects indicated altitude
• Cold temperature correction charts
DENSITY ALTITUDE
• High Density altitude refers to
height not density. Gives:
• reduced power
• reduced thrust
• reduced lift
• Use the same airspeeds but ground
speed is higher
DENSITY ALTITUDE
ICAO cold temperature error table
• http://www2.faa.gov/airports_airtraffic/air_traffic
/publications/ATpubs/AIM/Chap7/aim0702.html
PRESSURE CHANGES IN
FLIGHT (read Pages 18-19)
• When flying from High to Low
“Look out below”
• When flying from Low to High
“High in the sky”
• Above 18,000 feet the altimeter is
set to 29.92 and only pressure
altitudes are flown
Chapter #3
What causes a L or H pressure? 1
• Temperature
• In a closed container more temp
= more pressure
• You might think that the higher
the temp the higher the pressure
• But No!
What causes a L or H pressure? 1
• Usually the highest pressures
are found in cold regions
• Why?
• Because of Density
• Usually the higher density
offsets the lack of movement of
the molecules
What causes a L or H pressure? 2
• Convergence
• movement of air aloft is not always
at the same speed
• where it slows down it piles up into
a High pressure
• the piling up of air is called
convergence
What causes a L or H pressure? 3
• Divergence
• opposite of convergence
• the upper level wind speeds up and
stretches the air out creating a Low
pressure
• usually good wx under an upper level
divergence
What causes a L or H pressure? 4
• Thermal tides
• At an average altitude of 60 mi
(thermosphere) changes of over 500ºC
• the rapid warming and cooling of upper
air causes great density oscillations
• shows up as small pressure changes at
the surface because of the high altitude
CHAPTER 4 WIND
WIND
• Differences in temperature create
differences in pressure. These pressure
differences drive a complex system of
winds in a never ending attempt to
reach equilibrium. Wind also is a
transportation device for water vapor
and cloud condensation nuclei.
CONVECTION
• Warm air rises
• Cold air sinks
• With convection, warm air rises cools
then sinks. Uneven surface heating.
• The wind sets up an advection process
whereby the cool air is blown along the
ground until it is warmed then it rises
again and repeats the process.
CONVECTION (24)
PRESSURE GRADIENT
• Pressure gradient = difference
in pressure / distance
• Sets up a flow from high to low
• The closer the isobars, the
stronger the pressure gradient
force and the stronger the wind
PRESSURE GRADIENT
• Think of a Topographical map.
If you’re a ball on the top of a
steep mountain (high pressure
system) and you roll off into the
low lying are below (low
pressure system) the steeper the
gradient the faster the wind.
CORIOLIS FORCE
• This force describes the apparent
force due to the rotation of the
earth
• All free moving objects such as
ocean currents, artillery projectiles,
air molecules and aircraft seem to
deflect from a straight line path
because the earth rotates under
them.
On this non-rotating platform the
ball travels in a straight line from
one guy to another
On this counter-clockwise rotating
platform the ball seems to veer to
the right from the perspective of the
persons on the platform
CORIOLIS FORCE
• Flow would normally be 90º to isobars
except for Coriolis Force
• Causes a deflection of winds to the
right in the Northern Hemisphere
• To the left in the Southern Hemisphere
• The deflection turns the winds parallel
to the isobars at altitude
• Near the ground, the deflection
depends on surface friction
CORIOLIS FORCE
CORIOLIS FORCE
• Surface friction slows the wind allowing
the pressure gradient force to over power
Coriolis
• Over land 45º to the isobars
• Over water 10º to the isobars
• The magnitude varies with the speed of
the wind and the latitude
• As speed increases Coriolis increases
• As latitude nears the poles, Coriolis
increases
SURFACE FRICTION
SURFACE FRICTION
• Into a low on the surface out of a High
GLOBAL WIND
CIRCULATION PATTERNS
• 30º Latitude subtropical westerlies
• 60º Latitude polar easterlies
• Intertropical convergence zone
(ITCZ) - The boundary zone
separating the northeast trade
winds of the Northern Hemisphere
from the southeast trade winds of
the Southern Hemisphere (p28)
MOUNTAIN AND VALLEY
WINDS
• The slope warms during the day warming
the air causing it to rise.
• The slope cools at night cooling the air
causing it to sink.
DAYTIME
W
NIGHTIME
C
C
KATABATIC WIND
• Any wind blowing down an incline.
• A perfect example is when the Columbia
basin gets snow, causing cold air to form
near the surface creating an artificial High
• This pressure gradient then causes a wind in
the Columbia gorge down by Portland.
• Even though the air warms through
adiabatic compression it is not enough to
offset the temp differential.
CHINOOK WIND
• The Chinook is a warm dry wind that
descends downslope
• Temperature sometimes raises sharply
(36ºF)
• Air blowing up the windward side is
cooled by adiabatic expansion
• This causes a loss of moisture and gain in
heat (latent heat of fusion)
• The leeward side then sees warm dry air
through adiabatic compression.
CHINOOK WIND
• Moist and Dry are cool at different lapse
rates. Is a katabatic wind. Chapter 6 more
LAND AND SEA BREEZES
• Day - sea breeze (from sea to
land)
• Warm land, cool water
• Night - land breeze (from
land to sea)
• Cool land, warm water
LAND AND SEA BREEZES
WIND SHEAR
• It Can Happen
• Any altitude
• Any direction
• Any gradient
WIND SHEAR
• Two fluids moving in opposite
direction create friction and eddies
along a common shallow mixing
zone referred to as the shear zone.
WIND SHEAR
• Tailwind shearing to a calm or
headwind component
• initially the airspeed increases, the
aircraft pitches up, and the altitude
increases.
• Headwind shearing to a tailwind initially airspeed decreases, aircraft
pitches down, and altitude
decreases
WIND SHEAR
• Be careful with low level
temperature inversions. Wind just
above the inversion may be strong.
WIND SHEAR
• If climbing or landing a few knots
from the normal stall speed going
through the shear zone can induce
a stall.
• Check your winds a loft FD
forcast.