Thunderstorms 19

CHAPTER 19

THUNDERSTORMS

RECIPE FOR A THUNDERSTORM

1. Water Vapor
2. Unstable Lapse Rate
3. An initial upward boost or lifting action

LIKE A PERPETUAL MOTION MACHINE

1. Initial updraft
2. Cooling forces condensation
3. Latent heat is released warming the air
4. Creates buoyancy and furthers the air’s upward movement

LIFE CYCLE

1. Cumulus
2. Mature
3. Dissipating

CUMULUS

Characterized by continuous updrafts
Up to 3000’ per minute
Soon droplets collide forming big droplets that fall and create cool downdrafts

MATURE

Characterized by the start of rain from the cloud base
Downdrafts have started
Cold rain within the cloud creates downdrafts
This cold rain offsets the compressional warming
Cooler than ambient air then accelerates the downdraft
Updrafts reach max topping at 6000’ min.

DISSIPATING STAGE

Characterized by downdrafts
Anvil head forms
Rain ends
Cloud dissipates

THUNDERSTORM TYPES

There are 3 main classifications of thunderstorms
–Single cell or Airmass Thunderstorms
–Multicell or Steady State Thunderstorms
–Supercell

DIMENSIONS

Vary with degree of instability and moisture content
Range from 5 miles to 30 miles in diameter
Usually the more moisture the lower the cloud base will be
Tops can reach 65,000 feet or more

DIMENSIONS

Mesoscale Convective Complexes are the big dog
Cover an area of 39,000 square miles (200 by 200 miles) for 6 hours or longer
These systems are bigger than normal thunderstorms but just smaller than extratropical tropical cyclones
Gravity waves can be seen rippling through the top of the echo at right

SINGLE CELL OR AIR MASS

Result from surface heating
Occur at random
Last a short time
Middle to late afternoon
As soon as the rain starts and cools down the surface that caused the storm, it will dissipate

MULTICELL OR STEADY STATE THUNDERSTORMS

Form in a line
Last several hours
Dumps heavy rain and hail
Associated with wx systems
e.g. fast moving cold fronts
Produce strong gusty winds and sometimes tornadoes
The rain falls outside the updrafts so no cooling
It does not slow down

MULTICELL STEADY STATE THUNDERSTORM

SUPERCELL

A supercell thunderstorm is a convective storm that consists primarily of a single, quasi-steady rotating updraft
These can persists for an extended period of time
It has a very organized internal structure that enables it to produce especially dangerous weather
Updraft speeds may reach 9,000 feet per minute (100 knots).
Nearly all supercells produce severe weather and about 25 percent produce a tornado.
A supercell may persist for many hours (or longer).
New cells will continue to form as long as the three necessary ingredients exist

Thunderstorm Movement

Movement is dictated by 2 processes
Advection is the movement of the storm because of wind
–Check the winds at about 18,000 feet to predict movement
Propagation is the movement due to old cell dissipation and new cell development

Shelf Cloud

HAZARDS ASSOCIATED WITH THUNDERSTORMS

1. Tornadoes
2. Turbulence
3. Icing
4. Hail
5. Microburst
6. Lightning
7. Low ceilings
8. Visibility in rain

TORNADO

TORNADOES

Associated with violent thunderstorms
Usually multicell or supercell thunderstorms
Fast moving cold fronts or squall lines
Preceded by a funnel cloud
They usually move from southwest to northeast

HOW DO TORNADOES FORM?

A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
Rising air within the thunderstorm’s updraft tilts the rotating air from horizontal to vertical.
Most strong and violent tornadoes form within this area of rotation.
In this photo, the rotation is in the area of the lower cloud base. Spotters refer to this area as a rotating wall cloud.

FUJITA SCALE

WEAK TORNADO

VIOLENT TORNADO

TORNADO ALLEY

EAST WENATCHEE TORNADO

EAST WENATCHEE HAIL

GEG TORNADO

DAMAGE

HAIL 4” DEEP

WATERSPOUT

CUMULONIMBUS MAMMATUS

Signify areas of high turbulence
Very frequent lightning and roll clouds
Hazardous turbulence in all thunderstorms

TURBULENCE

Can exceed aircraft structural integrity
Normal category 3.8 G’s
Slow to Va
Hold a constant attitude and accept variations in altitude and airspeed

ICING

Supercooled water, freezing rain
Clear, Mixed, Rime
Builds very fast and can accompany a downdraft

HAIL

Very hazardous to aircraft
Large drops freeze others latch on and freeze
If updrafts are strong enough a large hailstone may build up
Located in, around, downwind and under the anvil top

Largest Hailstone Ever!

On September 03, 1970 in Coffeyville, Kansas a hailstone fell with a diameter of a 5.7 inches, a circumference of 17.5 inches and a weight of 1.67 pounds. It held the record for the world’s largest hailstone for over 30 years until…….June 22, 2003 when a storm pounded Aurora, Nebraska dropping tennis-sized hail at will.

The largest hailstone recovered from that storm officially measured 7 inches in diameter and had a circumference of 18.75 inches, this after a good chunk of it was broken off upon impact and some of it was lost to melting before it could be measured.

The Aurora hailstone still holds the record for circumference but on July 23, 2010 one fell in Vivian, South Dakota that measured 8” in diameter and 18.62 in circumference and weighed 1 lb 15 oz

Small hail, up to about the size of a pea, can wipe out a field of ripening grain or tear a vegetable garden to shreds. Large hail, the size of a tennis ball or larger, can fall at speeds faster than 100 miles per hour and can batter rooftops, shatter windows and “total” automobiles.

MICROBURST

Up to 6,000 feet per minute downdraft
Calm wind shears to a headwind
Headwind shears to downdraft
Which then shears to a tailwind
What to look for:
–Sharply defined column of showers
–Roll cloud coming off the point at which it strikes the ground
–Lots of dust, trees sharply bent over

MICROBURST?

DEFINITELY!!

Microburst From Dense Air

Dry air entrained into the thunderstorm will evaporate and cool the falling mix of precipitation and air, which may create dry, and in humid areas wet microbursts of strong winds.

Microburst Formation

Microbursts may form in areas of virga when the rain’s evaporative process cools the air to such a point that it becomes extremely heavy and accelerates downward.
The bases of the clouds producing these “Dry Microbursts” may be as high as 15,000 feet.

LLWAS and TDWR

LLWAS and TDWR (Terminal Doppler Weather Radar) are systems designed to provide pilots with information on hazardous wind shear and microburst activity in the vicinity of an airport.
Not all airports will have this capability, but more than half of the towered airports will have the capability to provide some level of alert.
At airports equipped with LLWAS, controllers are provided with gust front wind shear information. Controllers will provide this information to pilots by giving the pilot the airport wind followed by the boundary wind.
EXAMPLE – Wind shear alert, airport wind 230 at 8, south boundary wind 170 at 20.
Airports equipped with LLWAS “network expansion,” LLWAS systems integrated with TDWR and TDWR systems provide the capability of detecting microburst alerts and wind shear alerts.
EXAMPLE – Runway 17 arrival microburst alert, 40 knot loss 3 mile final.
An airport equipped with the LLWAS is so indicated in the Airport/Facility Directory under Weather Data Sources for that particular airport.

Microburst

Characteristics of microbursts include:
Size:
–The microburst downdraft is typically less than 1 mile in diameter.
–It descends from the cloud base to about 1,000 – 3,000 feet above the ground.
–In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.
Intensity:
–The downdrafts can be as strong as 6,000 feet per minute.
–Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst.
–These strong horizontal winds occur within a few hundred feet of the ground.
–Some extreme microbursts have clocked in at 150 kts.

Microburst

Visual Signs:
–Microbursts can be found almost anywhere that there is convective activity.
–They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga.
–When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence.
Duration:
–An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation.
–The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approximately 2-4 minutes.
–Sometimes microbursts are concentrated into a line structure, and under these conditions, activity may continue for as long as an hour.
–Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.

Microburst

There are 2 types of microbursts:
Symmetrical in which the downdraft hits the ground at a 90° angle causing a wind flow pattern of equal intensity around the contact area.
Asymmetrical in which the downdraft hits the ground at a lesser angle causing a wind flow pattern that has higher gusts on one side.

Flying into a Microburst

A pilot flying into a microburst must anticipate sudden and strong changes in wind direction and speed.
Initially a headwind is encountered that lifts the plane, followed by a strong downdraft, and when leaving the storm a tailwind causes a loss of altitude.
If encountered on takeoff a normal jet aircraft may have only 5 to 15 seconds for recognition and recovery.

Microburst Avoidance

Watch for thunderstorms in the forecast.
Any LLWS alerts.
Temperature Dew point spread of 17° to 28° C.
High wind gusts on the order of 40 to 50 kts or more.
Severe Weather Watch Reports.
Sigmets.
PIREPS.
The Mark IV eyeball.

PRECIPITATION STATIC

Ice, sand, dust, volcanic ash, and precip
All can build up a static charge
Windshield, engine nacelles, leading edges
Saint Elmo’s fire

JUDGEMENT

3/10 of the sky covered by T-storms is the limit
5/10 of the sky covered be on the ground
Avoid T-storm cells by 20 miles
Avoid the downwind side of the anvil (hail)
Make a 180

JUDGEMENT

Get as low as safety permits
Look for light areas of rain
Pink spots mean sun on the other side
Light gray means light rain
White probably hail, maybe snow – stay away
Don’t get above if cloud is boiling

JUDGEMENT

Don’t fly directly under the thunderstorm up drafts might pull you up, down drafts might put you on the ground
Stay in Clear area
If clear area moves over an airport – LAND!
If clear area moves over a good landing place – Consider Landing
If you decide to land, be careful of surface winds, soft fields

If you get into a Thunderstorm:

Use ATC for Vectors
Use airborne radar and fly in areas of light precip
Slow aircraft to Va
Maintain Attitude
Turn Auto Pilot Off
Avoid large control inputs
Don’t change your mind or heading, just your underwear.

Squall Lines

Squall lines generally form ahead of cold fronts and dry lines
Produce severe wx spanning several states
Travel quickly up to 60mph

Derecho

The Derecho is a squall line on steroids
Torrential rains tornadoes, flooding, hurricane force winds
Land based
Summer time storms form in midwest and move eastward

LIGHTNING

Got to have it to have a thunderstorm
More and More frequent T storm is more severe
Less and less T-storm is dissipating
Can hear it through the headsets
Can cause ADF needle to wander

LIGHTNING DEMYSTIFIED

CTC, CTG, GTC
Charges build up in the clouds and ground
Shoots toward ground in a series of steps 150 to 300 feet long then stops for 50 millionths of a second this is called a stepped leader
As it gets closer to the ground voltage increases

LIGHTNING DEMYSTIFIED

A return stroke surges upward along the path of the stepped leader
Process is repeated along the same path in a tenth of a millionth of a second this is called the dart leader
3 million volts per meter
Heats the air to 54,000º F causing the air to expand explosively

Stepped Leader

Electrons, which have negative charge, begin zigzagging downward
Attraction
As the stepped leader nears the ground, it draws a streamer of positive charge upward.

Flowing Charge

As the leader and the streamer come together, powerful electric current begins flowing.

Contact!

Intense wave of positive charge, a “return stroke” travels upward at 60,000 miles per second.

LIGHTNING DEMYSTIFIED

As a thunderstorm grows, electrical charges build up within the cloud.
Oppositely charged particles gather at the ground below.
The attraction between positive and negative charges quickly grows strong enough to overcome the air’s resistance to electrical flow.
Racing toward each other, they connect and complete the electrical circuit.
Charge from the ground then surges upward at nearly one-third the speed of light and we see a bright flash of lightning.

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