Local Winds and Air Masses 9&10

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BBCC Aviation Meteorology

Chapter 9

Local Winds

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 day
  • Synoptic 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

LawrenceLarryRichard 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)
  • https://www.youtube.com/watch?v=GluAM2tNRKo
  • https://www.youtube.com/watch?v=2H8Y5ptn9Cs

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
  • http://www.youtube.com/watch?v=XH_M4jItiKw&desktop_uri=%2Fwatch%3Fv%3DXH_M4jItiKw&app=desktop

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

 

Chapter 10

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
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