CHAPTER 8
Atmospheric Pressure and Altimetry
ATMOSPHERIC PRESSURE
ATMOSPHERIC PRESSURE
ATMOSPHERIC PRESSURE
ATMOSPHERIC PRESSURE
ATMOSPHERIC PRESSURE
STANDARD ATMOSPHERE
ATMOSPHERIC PRESSURE
PRESSURE & ALTITUDE
TEMPERATURE
TEMPERATURE
TEMPERATURE
PRESSURE
PRESSURE
Altitude Correction
PRESSURE
Density
Density
ALTIMETRY
ALTIMETRY
PRESSURE or TEMPERATURE CHANGES in FLIGHT
LOW PRESSURE
HIGH PRESSURE
Chapter 9
Chapter 9
Global Circulations and Jet Streams
GLOBAL CIRCULATION PATTERNS
Weather books love to show the single-cell model
This is an over simplification of the heat engine at work
Heat rises at the equator, cools at the poles, sets up an advective wind whereby it flows back to the equator heats and rises then repeats the process
THE THREE CELL MODEL
But we have an earth that spins, land masses, a tilting axis and ocean currents so that results in a three cell model
The three cells have names
The Hadley cell
The Ferrel cell
The Polar cell
GLOBAL CIRCULATION PATTERNS
This 3 cell model sets up some semi-permanent highs and lows
At the equator we have a semi-permanent low
Horizontal pressure gradients are weak here so winds are light
This is referred to as the Doldrums
Convective “Hot Towers” form here often
Where the northeast trade winds converge with the southwest trade winds is known as the Intertropical Convergence Zone or ITCZ
At 30°we have a semi-permanent high
This belt is referred to as the Subtropical High
Sometimes referred to as the Horse Latitudes
Winds are light here so sailing ships were stuck here for days or weeks forcing the sailors to either eat their horses or toss them overboard
This is where we find large deserts like the Sahara
This is also where the trade winds originate or Westerlies
At 60°we again have a semi-permanent low
This is where warmer air meets the Polar Front producing the Subpolar Low
North of the polar front we have the Polar Easterlies
Remember from our discussion on lows and highs
Descending air is characteristic of a high pressure area
Rising air is a characteristic of a low pressure area
GLOBAL CIRCULATION PATTERNS
Remember Coriolis and Pressure Gradient Force result in the curved flow of the
Polar Easterlies
Westerlies
NE trade winds
SE trade winds
10 years of weather in 9 minutes
DISCOVERY OF THE JET STREAM
As World War II was approaching its conclusion, the United States introduced the first high-altitude bomber plane called the B-29. It could fly at altitudes well above 20,000 feet.
When the B-29s were being put into service from a Pacific island base, two air force meteorologists were assigned the task of producing a wind forecast for aircraft operations at such altitudes.
DISCOVERY OF THE JET STREAM
To make their prediction, the meteorologists used primarily surface observations and what is known in meteorology as the “thermal wind” relationship.
In plain language, this relationship implies “that if you stand with your back to the wind, and the air is colder to the left and warmer to the right, the wind will get stronger on your back as you ascend in the atmosphere.”
Using this relationship, the meteorologists then predicted a 168-knot wind from the west. Their commanding officer could not believe the estimate. However, on the next day, the B-29 pilots reported wind speeds of 170 knots from the west!
The jet stream was discovered.
JET STREAM FACTS
Narrow river of wind
50-150 knots
100-400 miles wide
1 mile thick
Subtropical jet 33,000 to 43,000
Polar Front jet 25,000 to 35,000
JET STREAM FACTS
Stronger in winter than summer
Farther south in winter than summer
Occurs at breaks in the tropopause
Has to be 50 kts or greater to be classified as a Jet Stream
THE JET STREAM
The subtropical jet will form between breaks between the Hadley and Ferrel cells
The polar jet will form between the Ferrel and Polar cells
CAUSE OF THE JET STREAM
Great contrast of temperatures in adjacent air frontal zones
Rapid change in temp causes rapid change in pressure
Steep pressure gradient intensifies wind speed and causes the Jet
Temp difference is greatest in winter between polar and tropical air masses
This causes the jet to be more intense in winter than summer
THE JET STREAM
Why does the jet blow west to east?
The earth’s rotation is responsible for developing the 3 primary cells
Recall the difference in rotational velocity along different points of latitude causes this
As air moves north it keeps it eastward flow
This is know as conservation of angular momentum
The Earth below the air, however, moves slower as that air travels toward the poles.
The result is that the air moves faster and faster in an easterly direction (relative to the Earth’s surface below) the farther it moves from the equator.
The farther the air moves northward the slower the earth’s rotation
This causes a shift in the speed of eastward movement of the air causing faster velocity
Because of this the jet moves from west to east in both the northern and southern hemispheres
CONSERVATION OF ANGULAR MOMENTUM
As we move northward air moves closer to axis of rotation of the earth
Air must move faster east which looks like a wind out of the west to an observer on the ground
The best example is an ice skater
CAUSE OF THE JET STREAM
THE JET STREAM
At 30 and 60 degrees latitude, temp changes are the greatest
As the temp difference increases so does the wind speed
THE JET STREAM
The actual appearance of the jet is a complex interaction between many variables:
Location of high an low pressure systems
Warm and cold air masses
Seasonal changes
They meander around the earth dipping and rising in altitude and latitude
They can split, form eddies, disappear and then reappear elsewhere
The jet will tend to be located where the strongest surface temp contrast exists
JET STREAKS
Jet streams form along the upper air boundaries of large masses of warm and cold air.
Often, the jet stream contains “jet streaks” of wind speeds faster than the surrounding regions.
These jet streaks can play a very important role in precipitation and storm formation.
Depending on the strength and position of a jet streak, a weak area of low pressure can quickly become a raging monster.
Highest velocities in jet streaks have hit 260 kts
JET STREAM FACTS
The curvature in the jet stream and upper-air flow can have significant implications for surface weather.
Curvature’s effects are the strongest in a highly amplified, meridional flow pattern with large troughs and ridges.
Meridional flow is north – south
Zonal flow is east – west
As the graphic shows, the anticyclonic curvature in the western USA ridge induces a clockwise circulation while the cyclonic curvature in the east trough tends to favor a counter-clockwise circulation.
JET STREAM FORECAST
Meteorologists use the jet position to forecast weather
The polar front jet is a critical piece when generating a forecast
Changes in it indicate changes in weather
Typically north of the polar jet is relatively cold air exists, warmer to the south
The polar jet can promote development of storms
Storms are most likely to develop under a jet streak
The polar jet generally steers storms in the direction of its travel
There are usually 2 jet streaks located somewhere over the US at any given time
JET STREAM CIRRUS
Rising air on the equatorial side can cause cirriform clouds
Descending air on the polar side causes them to dissipate
ODDS OF HITTING FORCAST TURBULENCE
Light 10-15% of the time
Moderate 2-3% of the time
Severe to Extreme 2/10ths of 1% of the time or 1 in 500 chance
AVOIDING CAT
Usually climbing or descending will get you out of it
Use FSS and/or Flight Watch on 122.00 find out pireps
Remember stress is cumulative (Turb + Manev = total stress)
Maintain constant attitude, slow to Va, accept variations
CAT IS STRONG AT TIMES
Apr 4, 1981 DC 10 at 37,000 over central Illinois
Dropped 2,000 feet
21 out of 154 injured including
Fractured hip
Jabbed nose with a fork then landed in the seat in front of him on it
CAT AND THE B-52
January 10 1964, USAF B-52H (S/N 61-023) took-off from Wichita, Kansas on a structural flight test mission. The all-Boeing air crew consisted of instructor pilot Charles Fisher, pilot Richard Curry, co-pilot Leo Coors, and navigator James Pittman. The aircraft was equipped with accelerometers and other sensors to record in-flight loads and stresses.
At approximately 345 KCAS, the Stratofortress and its crew experienced an extreme turbulence event that lasted roughly 9 seconds.
Amazingly, after flying 6 hours sans a vertical tail, the Stratofortress and her crew landed safely.
Safe recovery of crew and aircraft brought additional benefits. It was found that at least one gust in the severe CAT encounter registered at nearly 100 mph.
B-52H (61-023) was repaired and returned to the USAF inventory. The aircraft spent the latter part of its flying career as a member of the 2nd Bomb Wing at Barksdale AFB, Louisiana.
The venerable bird was retired from active service in July of 2008.