Thunderstorms Flashcards
What are the requirements for the development of a thunderstorm?
- Lifting/Trigger action
- Conditionally unstable atmosphere through a deep layer
- CB cloud
- High moisture content
How do thermal thunderstorms form?
What is it related to?
When are they likely to occur?
- Where warm convective patterns warm the air forcing it rise.
- Usually diurnal and land-related, must be close to a water source for CB developments.
- In temperature latitudes, favours coastal regions during summer and usually early to mid-afternoon.
- In tropical latitudes, it can be a daily occurrence throughout the year, starting early afternoon until well after sunset.
How do convergence thunderstorms form?
Where they most likely to occur?
- Occurs when there are convergence patterns near the surface.
- Thermal/heat thunderstorm can fall in same category as convergence thunderstorm e.g. during a summer day when there is strong sea breeze activity, unstable conditions force moisture aloft.
- When the region is at the ITCZ, convergence thunderstorms are most common.
How do orographic thunderstorms form?
Does the time of day or season have an influence on when they form?
How long can they last?
- Where terrain gives moist unstable air sufficient lift for cloud formation to occur.
- Time of day or season has no influence.
- Can last for days.
- With high moisture content in an orthographic cloud, precipitation can be severe.
- Not unusual for mountains and adjacent plains to experience heavy hail.
How do nocturnal tropical thunderstorms form?
When do they form?
- Where oceanic areas are within 10º latitude of either side of the equator see pre-existing cumulus clouds becoming increasingly unstable at night due to temperature difference between air over the warm oceans and upper atmosphere.
- Sea temperatures are warm throughout the year.
- This type is most active towards DAWN when ELR has reached its steepest value.
- After daybreak, thunderstorms persist for some time but will cease to exist around mid-morning.
How do cold advection thunderstorms form?
Where are they likely to be found?
How intense are these thunderstorms?
- When a volume of cold air passes rapidly over a warm ocean area, it absorbs large amounts of water from the sea surface, while at the same time, its lower levels warm quickly.
- As ELR steepens and vertical mixing intensifies, it is perfect for the development of CB cloud.
- Can be experienced in Australia and NZ (during spring and early summer for NZ).
- Not severe.
How are frontal thunderstorms formed?
and what does it indicate?
How long is this severe feature?
Occurs when warm unstable air is lifted aloft the boundary between two air masses. Cloud base is lower than most air mass thunderstorms, and the number of CB clouds is greater.
What is the cross-sectional width of a frontal thunderstorm?
This thunderstorm is short-lived and its cross-sectional width is not more than 80 to 120 km.
What is the most severe feature of a frontal thunderstorm?
This thunderstorm is more severe, numerous and faster moving than other storms. Most severe of these are SQUALL LINES which often advance in front of the front.
How is a squall line formed?
Squall lines are created when the frontal uplift creates large numbers of linked storms along a sustained linear boundary.
What does squall lines indicate?
Squall lines indicate the severity of the front and is marked by a sudden wind change and very gusty conditions.
How long is a squall line and how long can it last for?
Squall lines can be several hundreds of kilometres long and can last for days.
Where is turbulence likely to be encountered with regards to flight in and around a thunderstorm?
The middle level of the storm.
Is turbulence the most serious aspect of flight in a thunderstorm?
Yes.
With regards to updraughts and downdraughts in turbulence, where are these most found common?
- Updraught = development stage
- Downdraught = decaying stage
Where are updraughts strongest with regards to turbulence?
Middle to the top of a CB cloud
Where are downdraughts strongest with regards to turbulence?
Middle of the cloud to the surface.
What is flight in mature stage thunderstorms likely to produce with respect to turbulence?
Moderate to severe turbulence in the boundaries between draughts.
What is the major cause of severe turbulence?
- Presence of gusts which are short-term and localised fluctuations in both vertical and horizontal airflow.
- They are likely found in the dividing sectors of strong up and downdraughts found in the mature stage.
At what height are icing conditions at their worst with regard to freezing level and thunderstorms? Why?
Above the freezing level, icing conditions are worst within the first 6000 to 8000 ft because within that band of height, supercooled water and droplet size combine to produce clear ice, which readily builds up and adheres strongly to the aircraft; and is difficult to get rid of.
What occurs at the top of thunderstorms with regards to icing conditions?
- Any supercooled water that remains will freeze to produce rime ice (easier to remove).
- This increases the risk of ice ingestion in turbine engines.
- Ice crystals found at a CB cloud top heights have a reduced adherence capacity.
When is icing a problem for aircraft?
When thunderstorm clouds are concentrated along a frontal line or within a large convergence zone.
What is a microburst with regards to a thunderstorm?
Involves downward spiralling of air usually within a small circumference.
What happens to microbursts as they hit the ground?
How long is a microbursts lifespan?
- They form a vortex ring.
- Relatively short.
What is a gust front with regards to a thunderstorm?
- Known as the sharp increase in wind speed often associated with the early mature stage of the thunderstorm cell.
- As downdraughts hit the surface, they split and spread out horizontally.
Where is the greatest speed achieved by the gust front?
The speed at which this flow travels horizontally is the greatest close to the CB cloud - speeds in excess of 45 knots have been recorded.
Where does a gust front form with regard to a CB cloud?
Some distance AHEAD of the CB.
How can a pilot see a gust front?
When the surface is covered by sand or dust, the outflow picks this material up and forms a great wall of dust, making it easy for a pilot to see the gust front.
Explain the development and severity of electrical phenomenon with regards to a thunderstorm?
- An aircraft flying through CB clouds and heavy precipitation may build up a potential charge large enough that discharges can occur between the aircraft and the cloud, or aircraft and precipitation and even the aircraft and the ground.
- Under all of these charged conditions, it’s possible that highly charged air, when rapidly accelerated through a propeller disc or past a nose cone, may become a visible discharge known as St Elmo’s Fire.
How are tornadoes formed with regards to a thunderstorm?
- Rotating funnels of air sucked up from below the base of a cumulonimbus cloud.
- Caused by extremely strong upward motion within the cloud.
- Drawn up into the cloud through convergence.
- As convergence develops, the already rotating air at the surface increases its rotational velocity.
How does rotation by wind shear cause a tornado to form?
- Air here rotates in the horizontal plane.
- If part of the air is drawn into the base of the CB, the rotation becomes vertical.
When have tornadoes been experienced in NZ?
During winter when surface temperatures are not high as opposed to the instability required by usually hot surface temperatures.
What is the lifespan of a tornado?
A few minutes.
Where is the largest risk for aircraft flying through a tornado?
Aircraft flying at low altitudes.
How big does hail have to be in order to pose as a serious threat to high-speed aircraft?
Stones larger than 1/2 an inch.
Where is the worst region encountering hail with regards to a thunderstorm?
- Between 8,000 to 25,000 ft in mid-latitude regions.
- Higher in tropical regions
How is hail formed with regard to a thunderstorm?
From the strong updraughts forcing supercooled water particles and ice above freezing layer, and the layers build up.
How is poor visibility formed with regard to a thunderstorm?
Can form from the precipitation below the thunderstorm.
List the hazards regarding poor visibility
- Reduced navigation ability.
- Hard to tell distance above ground.
- Depth perception is harder to judge. The sun makes it even harder.
- Visual distortion of ground features.
- Harder to see the horizon making it harder to complete accurate manoeuvers.
- May inadvertently enter cloud.
