Local Winds and Air Masses 9&10

CHAPTER 10
WIND

Wind causality
Unequal heating of the earth’s surface causes temperature imbalances
Differences in temperature cause differences in pressure
Since the universe is all about equilibrium, those differences in pressure cause the flow
Flow is always from high pressure to low pressure
The bigger the pressure differential, the faster the winds
F=ma (Newton’s second law)
Wind is named according to where it is coming from
Wind from 360 degrees is from the north (a north wind)
There are 4 main forces behind wind flow
Pressure gradient force
Coriolis force
Centripetal force
Friction

  • CONVECTION
    A really good illustration of the temp/pressure/wind example is the convection model
    Warm air rises
    Cold air sinks
    With convection, warm air rises cools then sinks
    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.

PRESSURE GRADIENT
Pressure gradient = difference in pressure/distance
A common condition is when a high or a low pressure system sets up a flow pattern
This force is at 90° to the isobars
The closer the isobars, the stronger the pressure gradient force and the stronger the wind
In fact wind speed is directly proportional to PGF

  • CORIOLIS FORCE
    This force describes the apparent force due to the rotation of the earth
    Really not a force by the way, more of an effect
    All free moving objects such as ocean currents, old artillery projectiles, air molecules and aircraft seem to deflect from a straight line path because the earth rotates under them.
    This deflection is to the right when viewed in the direction of travel in the northern hemisphere

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
When viewing wx maps the flow will be roughly parallel to the isobars
Near the ground, the deflection depends on surface friction
Apparent Coriolis Force
Earth’s rotation transforms straight line motion into curved motion for an outside viewer.
The Coriolis force explains this apparent curvature of winds to the right due to the earth’s rotation under the winds.
The earth rotates at about 15° longitude an hour.
Thus if say a missile were airborne for an hour flying from the north pole toward the equator, it would appear to deflect toward the southwest.
Actual and Observed Paths
Latitude effects Coriolis by increasing the angular deviation at the higher latitudes.
Since the earth is a sphere, a straight line will only be congruent with the equator.
Anywhere north of the equator will have an ever greater degree of divergence from the starting line of latitude.
Since the 60° circle is smaller than the 30° circle, a tangent line drawn to it will represent a greater angle.
Therefore greater Coriolis angle due to latitude would be present the higher the latitude.

CORIOLIS and friction
http://www.youtube.com/watch?NR=1&feature=endscreen&v=__SlJtnpCD8
Surface friction slows the wind allowing the pressure gradient force to over power Coriolis
Over water 10º to the isobars
Surface friction is less so it only causes a small decrease in Coriolis causing the flow to be mostly parallel to the isobars
Over land 45º to the isobars
Surface friction is much greater decreasing Coriolis allowing PGF to turn the flow at a greater angle to the isobars
The magnitude varies with the speed of the wind and the latitude
As wind speed increases Coriolis increases
As latitude nears the poles, Coriolis increases
Under a no wind, no course correction scenario your airplane (a Cub @ about 60-70mph) will move 1,500 feet to the right for every 100 miles

Geostrophic winds
Geostrophic winds are winds blowing parallel to the contours
Geo means earth
Strophic means turning
As pressure gradient starts to move the air, Coriolis starts working
Eventually as wind speed reaches its max Coriolis and PGF equalize and the wind blows straight along the isobars at altitude
This is useful for determining wind speed and direction for which there is no means for direct measurement

Meridional and Zonal Flow
Wind direction and speed are indicated by lines, barbs, and flags, and appear as an archer’s arrow.
Upper level winds that travel a north-south path are meridional, and those traveling a west-east path are zonal.
Zonal flow can enhance a flights groundspeed from west to east in the US.
Since the gradient force over the northwest is larger in winter, this can lead to greater meridional wind force.

Curved winds (the low)
The flow around a Low is counterclockwise or cyclonic
At point 1 PGF is moving air towards the center of the low
Coriolis force deflects and at point 2 the air is parallel to the isobars
If the wind were geostrophic at point 3 it would continue northward along straight line isobars, but that’s not the case because the isobars are curved
Since it is moving parallel to curved isobars along a curved path the wind is known as a Gradient wind
This is where centripetal force comes into play
Centripetal force acts 90° to the Gradient wind keeping it on a curved path

Curved winds (the low)
Acceleration occurs when there is both a change in speed or direction
Since the wind is changing direction it has an acceleration force
Newton’s law explains that if an object is accelerating there must be a force acting on it (F=ma)
In the low’s case there is an imbalance between a greater PGF and Coriolis
The resulting imbalance produces centripetal force acting toward the center of the low
Centripetal force is effected by velocity and the radius of the path

Curved winds (the low)
When winds are light centripetal force is relatively weak
In the case of hurricanes and tornadoes this force is very strong and plays a large roll in the destructive forces
So circulation is
Inward toward the center
Upward
Counterclockwise
Memorize these!

Curved winds (the high)
The flow around a High is clockwise or anticyclonic
The same principle applies to the high except the greater Coriolis force is producing the imbalance
Centripetal force still pulls inward as per Newton’s law which causes the Gradient wind to result

Curved winds (the high)
Normally we associate bad weather with the low since the air is being pulled inward, upward and forced aloft
Normally we associate good weather with the high since the flow is
Outward
Downward
Clockwise

LENTICULAR CLOUDS
Sure sign of high winds at altitude

SCALES OF WIND
There is a wind scale hierarchy:
Microscale
A few yards in diameter, sway branches, kick up dust etc.
Last only a few minutes or less
Mesoscale
1 to about 50 miles in diameter, sea breeze, mountain & valley breezes, thunderstorms
Lasts several hours to a daySynoptic scale
The size of a normal High or Low, weather map scale
Lasts for days sometimes weeks
Planetary (global) scale
Circle the globe
Macroscale
A term that combines the synoptic and global scales together

LOCAL WINDS
Mesoscale winds or local winds are usually effected mostly by pressure gradient force
Due to their smaller scale, Coriolis has little effect on their motion
Remember our Coriolis Cub example of 100 miles 1,500 feet
The FAA includes the following as local winds
Sea breeze
Land breeze
Lake breeze
Lake effect
Valley breeze
Mountain-plains wind circulation
Mountain breeze
All of these winds have a common driving force of surface heating and cooling

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
The key concept here is that these winds depend on temperature differential to exist
Thus, the sea breeze will be stronger all things being equal

SEA BREEZE FRONT
At the leading edge of the cooler moister marine air, cumulus clouds may form if unstable air exists
If stable air exists, stratiform clouds may form
A peninsula, like Florida or an island is the perfect setup for this type of phenomena
Convergence of airflow from 2 directions enhance the upward lift

SEA BREEZES CAN BE DANGEROUS
Lawrence “Larry” Richard Walters (April 19, 1949 – October 6, 1993), nicknamed “Lawnchair Larry” or the “Lawn Chair Pilot”, was an American truck driver[1] who took flight on July 2, 1982, in a homemade airship. Dubbed Inspiration I, the “flying machine” consisting of an ordinary patio chair with 45 helium-filled weather balloons attached to it.
Walters rose to an altitude of over 15,000 feet (4,600 m) and floated from his point of origin in San Pedro, California, into controlled airspace near Los Angeles International Airport.
In mid-1982, Walters and his girlfriend, Carol Van Deusen, purchased 45 eight-foot weather balloons and obtained helium tanks from California Toy Time Balloons. They used a forged requisition from his employer, FilmFair Studios, saying the balloons were for a television commercial.
Walters attached the balloons to his lawn chair, filled them with helium, put on a parachute, and strapped himself into the chair in the backyard of a home at 1633 W. 7th St. in San Pedro.
He took his pellet gun, a CB radio, sandwiches, beer, and a camera
When his friends cut the cord that tied his lawn chair to his Jeep, Walters’s lawn chair rose rapidly to a height of about 16,000 feet and was spotted by two commercial airlines.
At first, he did not dare shoot any balloons, fearing that he might unbalance the load and cause himself to spill out.
He slowly drifted over Long Beach and crossed the primary approach corridor of Long Beach Airport.
He was in contact with REACT, a Citizen band radio monitoring organization, who recorded their conversation:
REACT: What information do you wish me to tell the airport at this time as to your location and your difficulty?
Larry: Ah, the difficulty is, ah, this was an unauthorized balloon launch, and, uh, I know I’m in a federal airspace, and, uh, I’m sure my ground crew has alerted the proper authority. But, uh, just call them and tell them I’m okay.
After 45 minutes in the sky, he shot several balloons, and then accidentally dropped his pellet gun overboard. He descended slowly, until the balloons’ dangling cables got caught in a power line, causing a 20-minute electricity blackout in a Long Beach neighborhood.
Walters was able to climb to the ground.
He was immediately arrested by waiting members of the Long Beach Police Department.
Regional safety inspector Neal Savoy was reported to have said, “We know he broke some part of the Federal Aviation Act, and as soon as we decide which part it is, some type of charge will be filed. If he had a pilot’s license, we’d suspend that. But he doesn’t.”
Walters initially was fined $4,000 for violations under U.S. Federal Aviation Regulations, including operating an aircraft within an airport traffic area “without establishing and maintaining two-way communications with the control tower.”
Walters appealed, and the fine was reduced to $1,500.[3] A charge of operating a “civil aircraft for which there is not currently in effect an airworthiness certificate” was dropped, as it was not applicable to his class of aircraft.

LAKE BREEZE
This process is similar to a sea breeze
Usually occur during summer
They have the best chance of forming in light synoptic wind conditions
They can be strong enough to cause thunderstorm formation
The deeper the lake the larger the temperature differential and the stronger the wind
Most prevalent around the Great Lakes but may occur around any large lake like the Great Salt Lake

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.

MOUNTAIN-PLAINS WINDS
Diurnal – change of temperature from day to night
This happens east of the Rockies
It’s basically half of the valley breeze model
Since the mountain warms faster, warm air ascends along the slope causing cool air from the plains to rush toward the mountain range
Watch for cumuliform clouds and thunderstorms in the afternoon

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 an easterly wind in the Columbia gorge down by Portland.
Another smaller scale example is the Waterville plateau into Ephrata
Even though the air warms through adiabatic compression it is not enough to offset the temp differential.
These winds have been known to reach hurricane speeds in some parts of the world like the artic ice shelf area
Anabatic winds are winds that flow in the opposite direction (valley breeze)

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 and decompressed
This causes a loss of moisture and gain in latent heat
The leeward side then sees warm dry air through adiabatic compression.
Other names for this wind are the Santa Anna in Cali and Foehn (pronounced Fain) in the Alps

THE HABOOB
The Haboob forms as cold downdrafts along the leading edge of a thunderstorm lift dust or sand into a huge tumbling dark cloud
About 24 occur in the African Sudan each year
In the U.S. they can occur in the desert southwest
Wind Maps
Leave it to an artist to develop a useful wind map
http://hint.fm/wind/
This one has an app and is more useful to the aviation side
https://www.windy.com/?47.190,-119.307,6,i:pressure
Here’s another
https://earth.nullschool.net/

Adverse winds

Types Of Adverse Winds
Types
Crosswinds
Gusts
Tailwinds
Variable wind
Sudden wind shift
The book says you’re most at risk during takeoff and landing
It also says little planes are affected most

Adverse Winds
Crosswind
This wind blows from one side or the other
Breaks off the landing gear
Gust
This is where the wind speeds up suddenly (10kts or more)
Breaks or bends the spar
Tailwind
This wind blows up your tail
Runs you off the end of the runway or into the trees on takeoff
A 10% increase in landing speed will result in at least a 21% greater landing distance
Variable wind/Wind shift
This is when the wind is variable and is shifty
Wind shear
This one is trickier than the rest, it pretends it’s your friend then whammy

Computing X-wind & Headwind
For x-wind use the sin of the angle between the wind and the runway
Wind 280@20 runway 36
360-280=80
Sin(80)=.984807753 x 20=19.69kts
For the headwind use the cos of the angle between the wind and the runway
Wind 280@20 runway 36
360-280=80
Cos(80)=.1736481777 x 20=3.47kts

More Adverse Winds
Air biscuit
Air monkey
Backdoor trumpet
Back draft
Backend blow out
Barking rats
Blow fish
Blue angel
Bork
Brown speckled mallard
Brown mist
Bull snort
Burners
After burners
Butt cheek squeak
Can O’ Cheddar
Donald Duck
Mickey Mouse (pretty much all the Disney characters)
Double flutterblast
Drive by
Fartrogen dioxide
Crunchy frog
Walmart Special

Chapter 11

Air Masses and Fronts

Definitions

  • Air Mass – Body of air that has fairly uniform temperature and moisture
  • Cold air mass is generally defined as being colder than the ground it is moving over
  • Likewise a warm air mass is warmer than the ground it moves over
  • Source Region – The region which an air mass acquires its particular properties of temp and moisture
  • There are 5 source regions:
  • –cA, cP, cT, mP and mT
  • –mA seldom forms, if ever

Definitions

  • Front – Zone between 2 different air masses
  • Aloft the upward extension of the front is the frontal surface or zone
  • Frontolysis – when a front dissipates or merges into the adjacent air mass (it DIES)
  • Frontogenesis – formation of a front (it is CREATED)

Air Mass Modification

  • When an air mass moves away from its source region, it takes on the characteristics of  its new region
  • Degree of Modification depends on 3 factors
  • –Speed of the airmass
  • –Nature of the region
  • –Temp. difference between the new surface and the air mass

Air Mass Modification

  • 4 ways air masses are modified
  • –Warming from below (instability and showers)
  • –Cooling from below (stable, if cooled to much fog)
  • –Subtraction of water vapor (condensation and precipitation)
  • –Addition of water vapor (cold air over warm water i.e. evaporation)

LAKE EFFECT

  • This happens when air moves over a warm lake
  • Absorbs moisture
  • Then moves over colder land
  • The airmass cools and presto, fog, rain or snow
  • Most prevalent in the winter around the Great Lakes

Fronts

  • Discontinuities are changes in properties from 1 front to another
  • –Temperature – usually a temp change
  • –Dew Point – Temp dew point spread will usually change
  • –Wind always changes across a front usually both in speed and direction
  • Beware of wind shear in this area
  • –Pressure
  • Cold front passage pressure rises
  • Warm front passage pressure falls
  • Fronts are classified by which type of air mass replaces the other

Frontal Symbology

Types of Fronts

Types of Fronts

The Cold Front

Cold Front Characteristics

  • Much steeper slope than a warm front
  • –A cold front moving at 25kts has a slope of 1:50 (vertical:horizontal)
  • Wind shifts from southwesterly to northwesterly
  • Lowest pressure as the front passes then rising
  • Freezing level lowers
  • Faster moving than the warm front
  • May be associated with Squall lines and thunderstorms

The Warm Front

  • Warm Front Characteristics
  • Much shallower slope than a cold front
  • –The average slope is 1:150 to 1:200 (vertical:horizontal)
  • Wind shifts to south or southwesterly
  • Pressure levels off at passage followed by a slight rise then fall
  • Freezing level rises
  • Slower moving than the cold front
  • Creates a frontal inversion which may produce freezing rain

The Stationary Front

The Occluded Front

The type of wx associated  with the warm and cold front occlusion

  • Clouds are similar to a warm front
  • Weather is similar to a cold front
  • Heavy often showery precip then clearing sky
  • The most violent wx occurs where the cold front is overtaking the warm front (greatest temp. differential)

The Wave Cyclone

The Wave Cyclone

  • They usually form on slow moving cold fronts or stationary fronts
  • A. Winds blowing parallel cause a disturbance (remember stationary, so winds are parallel)
  • B. Wave starts
  • C. Start of a Cyclonic (clockwise) circulation

The Wave Cyclone

  • D. At the peak pressure falls it then transforms into a Low which reinforces the cylconic circulatory pattern
  • E. Cold front catches up to the warm front they form an occluded front
  • F. As it grows in length the circulatory pattern diminishes, the air masses start to mix
  • G. The fronts merge, break off & disappear

Frontal Weather

  • Weather along the front depends upon numerous things:
  • –The amount of moisture
  • Must be present for clouds and precip to form
  • Dry cold front meeting a moist warm front
  • –Degree of stability
  • Stable stratiform
  • Unstable Cumuliform

Frontal Weather

  • –The slope of the front
  • Shallow gives large areas of precipitation and or fog
  • Steep front gives thinner line of precip but usually more intense cumiliform

Frontal Weather

  • -The speed of the front
  • Faster it moves the more energy and intensity experienced usually
  • Dry fronts may only have cirroform clouds

Frontal Weather

  • –Upper wind flow pattern (Jet Stream)
  • When perpendicular the front moves same direction
  • Parallel to the front it moves very little
  • Watch the Jet it is the primary moving force

The Dryline

  • Drylines are not cold fronts or warm fronts but represent a boundary between moisture properties
  • Dew point temps may drop by as much as 9°C which leads to their nickname of a Dew Point Front
  • They typically occur in west Texas but may also be found in India and Central west Africa
  • To the west of the dryline the air is typically hot and dry
  • To the east it is typically hot and humid
  • –The pips point to the humid side
  • Birds and insects congregate along the line enabling Doppler Radar to pick it up at times
  • Severe thunderstorms and tornadoes will sometimes form on the east side of the dryline
  • Note the difference of the dew point is dramatic, but the temps are close to the same on the map

The Picture Upstairs

  • Long Waves are spaced about 3000 nm apart and are set up as semi permanent lows located roughly around Siberia, the Aleutians and Greenland
  • Short Waves are much closer at about 1000 nm apart and often associate with surface lows.
  • Short Waves form before the surface low appears and are usually upwind from the where the surface low is located on the map.

The Picture Upstairs

  • Look for the worst weather on the East side of the front just ahead of the upper level trough
  • If you’re flying with a continual left crosswind, you are heading for the center of a low pressure
  • As we learned earlier, low pressure means bad weather and you’re heading for it.

Upper Air Fronts

  • These fronts do not touch the ground
  • They form when the tropopause dips toward the ground and folds under the polar jet
  • On the north side the air sinks
  • On the south side the air rises
  • This aids the development of storms