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NZ PPL - Meterology > Thunderstorms and Icing > Flashcards

Thunderstorms and Icing Flashcards

1
Q

What are the stages of a thunderstorm?

A

Thunderstorms are formed from Cumulonimbus clouds

  • Cumulus ( Growth )
  • Mature
  • Disappaiting
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1
Q

What are the conditions for a thunderstorm to form?

A

High relative humidty
Steep temp lapse through 10,000 feet
Trigger ( warm air rising )
- High surface temp
- Orograhpic features
- Frontal activity

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2
Q

Key impacts of thunder storm

A

Turbulence
Heavy Precipation
Electrical
Icing
Downdrafts
Instrument interference

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3
Q

In a thunderstorm where does the energy come from?

A

Latent heat from condensation.

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4
Q

With a thunderstorm - what is a cell?

A

The cumulonimbus is made up of a number of smaller clouds, cells, which are consistently forming and dispating.

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5
Q

With a thunderstorm - what happens in the growth stage?

A

Cells are rising, between 100 ft per minute to 6,000 ft per minute
Creates a strong updraft.
The freezing level inside the cloud will go upwards - due to the updrafts

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6
Q

With a thunderstorm - what happens in the mature stage?

A
  • Snowflake and rain drops form in the upper part of the cloud and fall as precip.
  • Causes some updrafts to become downdrafts
  • Squalls occur
  • Above the isotherm the cloud is still forming
  • Top is limited by the troposphere or a temp inversion.
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7
Q

What causes lightening and thunder

A

Lightening
* Ice crysals form in the top part.
* Creates + charge in the top and - in the bottom.
* Usually +/- 5,000 ft of the freezing level, creating a temp different of +10c to -10c.
* Discharges.

Thunder
* The electrical discharge causes heat and a rapid expansion of air.

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8
Q

Thunderstorm - what happens at the disappeting stage?

A

The precip causes the updrafts to slow.
Cloud distentgrates.

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9
Q

Dangers of a thunderstorm at the mature stage?

A

Unprectable and strong winds, updrafts and down drafts.

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10
Q

Thunderstorm - what conditions are needed for self propegation?

A

Wind, this means that the rains fall outside of the cloud.
Resulting in high humidy and turbelnece - allowing for the formation of cumulus clouds again.

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11
Q

How can you see the direction a thundercloud is moving?

A

The anvil shape. Upward is the blunt side.

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12
Q

What are the 3 types of icing ( and cause )

A

Clear Ice - super cooled droplets, close to 0c
Rine Ice - super cooled droplets, close to -25c
Hoar Ice - frost - caused by desublimaiton ( gas to solid )

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13
Q

How is ice formed?

A
  • Super cooled droplets
  • Touch a surface < 0c
  • Freeze ( 1/80th rule )
  • Type of ice depends on temp of air/surface
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14
Q

What is the 1/80th rule?

A

Approximately 1/80th of the super cooled water droplets will freeze on contact for each degree below 0.

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15
Q

Why does clear ice form?

A

When the temp is near zero, the supercooled droplets do not freeze on contact.
This combined with the latent heat, means the water slides across the surface
This results in a sheet of clear, heavy ice.
Parts breaking away is dangerous, ruins the laminar airflow.

16
Q

What factors impact the amount of ice that forms?

A

Air temp < 0
Plane surface temp < 0
Airspeed ( higher = less ice forming, 500kts not ice will form )
Shape ( thiner = more ice )

17
Q

Why does ice not form > 500kts?

A

Air over the surface means fricaiton keeps the surface > 0c

18
Q

Describe Clear Ice and how it is formed?

A

Sheet of clear ice.
Covers the whole surface
Formed because the super cooled droplets flow over the surface due to the latent heat created before freezing.
Temps close to zero result in clear ice
It is the most dangerous

19
Q

Describe Rime Ice and how it is formed?

A

Supercooled droplets touching the surface
Because the surface is much colder, more freezes on impact - causing buildup on the leading edge only.

20
Q

Describe Hoar ice and how it forms.

A

Hoar ice is a frost.
Forms when mositure in the air changes directly to solid ( de-sublimation )
On cold clear days.
And with signifcant temp. changes on ascent and descent.

21
Q

Under what temps and conditions does Carb icing occur?

A

Any temp from -15c to +38c
More likely 10c - 15c and with reduced throttle, and humidity
Less likely near freezing

22
Q

What to do with suspected carb ice or impact ice?

A

Carb Ice
Turn on carb heat
For at least 20 seconds

Impact Ice
Turn on alternative air ( it is heated )

In both cases if the engine continues to run rough, land as soon as pracitacable.

23
Q

What temps and layers in icing most likely to occur?

A

Clouds
< -25 - ice. Little to no icing
-25c - 12c - mostly ice with some water - icing
- 12 -> 0 Mostly super cooled water droplets with some ice - icing
- 0 - no icing

24
Q

What type if ice is expected to form in Stratus clouds?

A
  • Rime ice
  • small droplets
  • lower relative humity
  • Usually a narrow band of temp range
25
Q

What type of ice is likely to form in cumulus clouds?

A
  • Clear ice
  • Super cooled droplets
  • A lot of humidty
  • most serious
  • The most dangerous part is the center and updrafts
26
Q

Where is the risk of icing in warm front?

A

A layer ~2,000 above the freezing level.
Due to the graident of the front.

Where there is precip - icing is less likely.

27
Q

Where is the risk of icing in a cold front?

A

Icing is less likely than warm front. Fast aircraft can fly though, GA will still experience icing.

28
Q

What are the 2 causes of Precipiation?

A

Ice Cystal Process - cloud has a temp band less than -10c
Coalescence - cloud does not get below -10c

29
Q

How does the ice crystal process cause precipition?

A

As the water droplets move up, at -10c there is a mix of water and ice crystals, which is unstable.
The droplets evapourate, increasing the size of the ice crystals.
Eventually the updrafts can not keep a loft, they fall.
On the way down, they melt - becoming precip.

This is the common way that rain happens in the lower latitudes ( I.E NZ Winter )

30
Q

How does Coalenscence cause precip?

A

Large droplets rise slowly and accumulate more water.
Need to have
- a cloud of large vertical ascent
- updrafts

As the precip falls it hits other water droplets, increasing in size.

31
Q

What are the 4 factors for rain size and inesity?

A

Higher dew point
Vetical extent
Updrafts
Temp.

32
Q

Name the 8 types of precip ( and METER codes )

A
  • Snow - SN / BLSN ( blowing Snow )
  • Snow Grains - FZDZ ( freezing drizzle )
  • Rain - RA
  • Ice Pellets - PE
  • Drizzle - DZ
  • Sleet - SNRA ( Snow and rain )
  • Hail - GR, GS
  • Virga - not a code. This is when the precip evapourates before hitting the ground. Dangerous and when there is a wide Temp/Dew point spread
33
Q

3 Categories of preciptition

The conditions that cause rain

A

**Frontal Precip
**
* Wide spread - due to the size of the front.

Stable Air Mass
* Drizzle

Unstable Air Mass
* Sudden / Heavy Precip
* Strong Vertical Currents

NZ PPL - Meterology (9 decks)

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