Thunderstorms Flashcards by Adrian Lewczuk

A weather phenomenon produced by a cumulonimbus (CB) cloud is:
* always accompanied by
* usually accompanied by

thunder, lightning

strong gusts of wind, heavy rain

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Severe weather phenomena produced by a CB cloud (10%) can be extremely dangerous and may produce:

hail, tornadoes

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Individual thunderstorms are generally no more than ___ NM in diameter, but tend to develop in…

10 NM

clusters of two or more

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Thunderstorm activity may cover a very large area and CBs associated with thunderstorms may extend vertically as high as

70 000 ft

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There are approx. ___ thunderstorms each year worldwide. At any time, there are around ___ thunderstorms in progress.

16 million

2000

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Thunderstorms in Canada vary significantly in intensity regionally, but they share the same 3 requirements for formation:

Unstable air
Lifting action to get air moving upward
High moisture content in the air

These factors are also required for the formation of harmless cumulus clouds, so their intensity is a key factor.

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Describe the 2 types of thunderstorms

Frontal Thunderstorms
* Caused by humid or unstable air being lifted by the frontal surface
* Clouds form in a solid line several hundred miles long, in various stages of development
* Most often associated with cold fronts, but can also occur with warm fronts

Air Mass Thunderstorms
* Form when warm, moist, and unstable conditions exist in the atmosphere
* All thunderstorms are severe but since these usually happen in isolation they are considered “ordinary” or “garden variety”
* Can form individually (single-cell) or in large clusters (supercell)

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Cold Front Thunderstorms
* occur:
* dissipate with:
* clouds are:

Cold Front
* Occur day or night
* Dissipate with movement of the front
* Clouds are in a line parallel to the front

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Warm Front Thunderstorms:
* occur:
* dissipate with:
* often:

Warm Front
* Occur day or night
* Dissipate with movement of the front
* Often embedded in stratiform clouds (less visible than cold front thunderstorms)

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Air mass thunderstorms usually form as the result of

(These are the 3 kinds of air mass thunderstorms)

convection, advection, or orographic lift

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Convection Thunderstorms
* Caused by
* Usually occur
* Dissipate
* Clouds are

Caused by uneven heating of the ground
Usually occur in the afternoon/early evening
Dissipate late in the evening or overnight
Clouds scattered over a large area

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Advection Thunderstorms (Over Land)
* Caused by
* Usually occur
* Dissipate
* Clouds are

Caused by cold air going over a warm surface
Usually occur in the afternoon
Dissipate in the late evening/overnight
Clouds scattered over a large area and the storm has a northerly flow

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Advection Thunderstorms (Over Water)
* Caused by
* Usually occur
* Dissipate
* Clouds are

Caused by cold air going over a warm surface
Usually occur in the morning
Dissipate with a southerly flow
Clouds tend to be close together

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Orographic Thunderstorms
* Caused by
* Usually occur
* Dissipate
* Clouds are

Caused by air being lifted by sloping ground
May occur day or night
Dissipate with a change in wind direction
Clouds form in a stationary line

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What are the 3 stages of thunderstorm formation?

Cumulus
Mature
Dissipating

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What happens in the Cumulus Stage of thunderstorm formation?

Formation of a cumulus cloud
Gradually builds up into a towering cumulus
Cloud mostly has updrafts that support the water droplets within it
No precipitation

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What happens in the Mature Stage of thunderstorm formation?

Onset of precipitation
Highest incidence of precipitation
Possibility of lightning, microbursts, gust fronts, hail, and tornadoes
Updrafts within the cloud may attain 6000 FPM
As the water droplets grow, they start falling and dragging air down with them, producing downdrafts in the 2000 FPM range
Violent turbulence is associated with these updrafts and downdrafts

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What happens in the Dissipating Stage of thunderstorm formation?

Caused by precipitation during the mature stage which cools down the lower part of the cloud, breaking updrafts (its main source of energy)
Top of the cloud spreads out into an anvil form

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List the ten areas of concern when flying in or near a thunderstorm

Turbulence
Lightning
Gust Fronts
Tornadoes
Squall Lines
Icing
Microbursts
Hail
Low Ceiling & Low Visibility
Heavy Rain Showers

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Strongest turbulence in a thundercloud occurs during the mature stage in the transition zone between

the up and downdrafts, generally between 12 000 and 20 000 ft

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Below a thundercloud, downdrafts that race to the ground cause

Above the cloud, turbulence is also possible up to

low level wind shear

several thousand metres and outward up to 30km

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Is severe turbulence is present in all cumulonimbus clouds?

Yes

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What is a roll cloud?

A roll cloud is a low-level, horizontal, tube-shaped accessory cloud completely detached from the base of a CB cloud
* Associated with thunderstorms and turbulence
* Often appear on the leading edge of the storm and mark the position of eddies or swirls where the wind changes direction (known as a shear zone and is most prevalent with cold fronts and squall lines)
* Associated with the downdraft reaching the ground (a gust front) and often the first drastic change in surface wind conditions ahead of an approaching storm

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Gust fronts are the result of

strong downdrafts in the mature stage of a thunderstorm reaching the ground and spreading out horizontally

Usually cause significant turbulence. Nearly all thunderstorms produce some degree of gust front.

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Lightning is

The visible electric discharge produced by a thunderstorm

Lightning can be:

* In-cloud (or sheet) * Cloud-to-cloud * Cloud-to-ground * Distant

Lightning is always present in or near CBs and most prevalent in

the mature stage between +8 and -8 C (+/- 5000 ft from freezing level)

What is St. Elmo's fire?

an electrical discharge that appears as a bushy halo around some edges of the aircraft

Electricity from thunderstorms can cause:

* Punctures to the skin of aircraft * Damage to communication and navigational equipment * Temporary blindness of pilots * Errors in magnetic compasses * Disruption to low and medium radio frequencies * Ignition of fuel vapours (rare) * Static on low frequency bands (precipitation static) from static discharges on wind screens, wing tips, and propeller tips * Less frequently, St. Elmo’s fire

What is a tornado?

A violently rotating column of air from the thunderstorm to the ground

Tornadoes sometimes occur with isolated thunderstorms but much more frequently with

supercell thunderstorms (associated with cold fronts and squall lines)

Tornadoes can occur several kms outward from

the main lightning and precipitation

Tornadoes generate violent turbulence. An aircraft entering a tornado vortex is certain to Since the vortex extends well up into the cloud...

suffer structural damage. a hidden vortex could be encountered.

What are mamma clouds?

Cumulonimbus mammatus (mamma clouds) are: * often an indication of tornadoes and violent turbulence * round-shaped * hang under the cloud base or anvil

Squall lines are Generally develop... Can form... Can range... Can contain... Contain...

Non-frontal, narrow bands of highly active thunderstorms * Generally develop ahead of a cold front in moist and unstable air * Can form rapidly, reaching max intensity during the late afternoon or early evening * Can range from one to several hundred miles in length and may be broken or in a continuous line * Can contain severe thunderstorms with torrential rains, large hail, and tornadoes * Contains all the other hazards generally associated with thunderstorms

Hail occurs in the mature stage of thunderstorm cells that have

more intense updrafts

Hail occurs most often from the surface up to around

10 000 to 15 000 ft (3000 to 4500m), but can be as high as 45 000 ft

If the cloud is built up diagonally, hail that has been carried in the updraft may be

cast out into the clear air surrounding the cloud from the sides or from the overhanging anvil cloud

Hail can damage:

windshields, outside instrument sensors, even airframes

Microbursts are

Sudden and powerful downdrafts within a thunderstorm

Microbursts can cause sudden dangerous low-level wind shear, resulting in hazardous conditions for

takeoff and landing

When an aircraft flies at or below freezing temperatures and strikes supercooled water droplets, they

freeze and adhere

Icing is A major hazard to aviation, but also

a fact of life in Canada aviation

Clear and mixed icing is possible in all stages of thunderstorm cloud development, but severe clear icing is most likely to occur in

the upper half of a thunderstorm cloud in the mature stage, especially when temperatures are between 0 and -25 C

Strong updrafts and downdrafts can rise or lower the \_\_\_ within a thunderstorm cloud

freezing level

A cluster of thunderstorm clouds could include several cells at

various stages of development

Within a thunderstorm cloud, visibility is generally... The ceiling and visibility also become greatly reduced in the...

near zero. precipitation between the cloud base and the ground

Heavy precipitation can saturate the air, rapidly causing low level clouds called \_\_\_ \_\_\_ and fog to form. As a result, ceiling and visibility are rapidly reduced turning VFR approach into IFR

stratus fractus

Aircraft will be restricted by the lower ceiling and visibility, but the situation may become even more severe when

other hazards are present such as: turbulence, hail, lightning, and low-level wind shear. These hazards make flying very difficult.

A thunderstorm contains vast amounts of large liquid water droplets. The heaviest rain showers are associated with

the mature stage of thunderstorm development and with the onset of the first downdrafts

Heavy rain showers associated with thunderstorms can contaminate the wing surface which may lead to

reduction in lift and possible early stall

Heavy rain can reduce visibility and cause refraction on the windscreen of the aircraft if encountered during approach and landing which produces the illusion that

the runway threshold is lower

Water lying on a runway can cause hydroplaning, which

destroys the braking action of the aircraft and can also lead to loss of control during takeoff

Flying near a CU cloud will result in... There are updrafts...

a bumpy flight beneath and in the cloud. below and in the cloud.

CU can develop into a

TCU and then a CB

Flying near a TCU cloud will result in

turbulence. There are strong updrafts below and in the cloud

A TCU may develop into a

Icing is possible in TCU at altitude above the

freezing level

Flying near an ACC cloud will result in

turbulence and icing. Precipitation may also be present.

Flying near a CB cloud will result in

severe turbulence, maybe also severe icing

CBs are accompanied by dangerous weather hazards such as:

thunderstorm activity, precipitation, wind, violent vertical currents, hail, lightning

Flying into CB clouds is

highly dangerous and even large airliners avoid crossing them

The only sensible defence against the hazards of a CB is to

avoid flying into one

In a CB, rapid changes in barometric pressure can cause

altitude readings on the altimeter to be unreliable

In a CB, strong surface winds and abrupt changes in wind speed and direction pose a serious hazard to aircraft during

takeoff and landing. Gusts in excess of 80 knots have been observed.

In a CB cloud, heaviest icing occurs

above freezing level where droplets are supercooled

Snow, ice pellets, or rain can be hazardous to aircraft attempting to land by

contaminating airfield and runway surfaces

Hail only forms in

thunderstorms

Hazards of a CB cloud include:

* Rapid changes in barometric pressure * Strong surface winds and abrupt changes in wind speed and direction * Moderate to severe icing, especially in higher levels of the cloud * Hazardous precipitation * Electrical disturbances * Turbulence * Extreme weather

Aircraft in the vicinity of a CB are at risk of being hit by

lightning

Aircraft flying near CBs may experience electrical disturbances affecting

communications and navigation systems

Static electricity may build up in the airframe, interfering with operation of the

radios and affecting compass behaviour (even permanently)

The electrical phenomenon known as St. Elmo’s Fire, while not a threat to safe flight, is an indication of

nearby CB activity

The interaction between strong updrafts and strong downdrafts causes

wind shear and severe turbulence within the cloud These vertical air currents (shear) are strong enough to cause an aircraft to stall or cause very serious structural damage.

Shear turbulence in a CB can be encountered up to

several thousand feet above and 20 miles laterally from a severe storm

Severe downdrafts known as microbursts, funnel clouds, and tornadoes are also features of CBs that can

cripple an aircraft

Strong surface winds, variable in direction, strength, are common at and can be particularly hazardous on

surface level near the CB takeoff or landing

Thunderstorms Flashcards

(77 cards)