Thunderstorms Flashcards
What cloud formation are thunderstorms associated with?
Cb
What is the definition of a thunderstorm?
One or more sudden electrical discharges in the form of a flash of light (lightning) and a rumbling sound (thunder)
3 requirements for a thunderstorm to form
- Deeply unstable air
- A lot of moisture
- A trigger to cause the air to rise
What is the key feature of the initial stage?
The cell only contains updraughts (30m/s/60kt
Lasts 15-20 minutes
What are the key features of the mature stage?
Updraughts (50m/s/10,000ft/min) and downdraughts (2,000ft/min or 6,000ft/min in a microburst) coexist
Rain begins to fall
Lasts between 15-30 minutes
What can a Cb do when the updraughts are strong enough?
Overshoot the anvil and punch through the tropopause
What happens in the dissipating stage?
The downdraught cuts off the updraught
The storm no longer has a supply of warm moist air
Light rain and weak outflow winds may remain for awhile, before leaving behind just a remnant anvil top
May last 1.5 to 2 hours
4 thunderstorm triggers
- Convection
- Frontal
- Orographic
- Low level convergence
4 thunderstorm types
- Single cell
- Thunderstorm cluster (multi cell)
- Squall line
- Supercell
Characteristics of a single cell
Form on warm and humid days
May be severe with hail and microburst winds
Multi cell thunderstorm characteristics
Multiple cells covering a large area
Individual cells can move in different directions to the cluster
Supercell characteristics
A single long-lived thunderstorm
Responsible for tornadoes and large hailstones
Squall line characteristics
A line of thunderstorms
What is a mesoscale convective system (MCS)?
Collection of thunderstorms that act as a system. Can spread across an entire state and last more than 12 hours. May appear on radar as a solid line, broken line or cluster
What is a mesoscale convective complex (MCC)?
Is a type of MCS. Large, circular, long lived cluster of showers and thunderstorms. Area of cloud top at least 100,000km^2 at a temperature of -32 degrees or a cloud top of 50,000km^2 at a temperature of -52 degrees
What is a gust front?
Downdraught air flows underneath the warm air inflow. Can spread up to 20-30km/15-20nm in front of the cell.
What are roll and shelf clouds?
Roll shaped clouds associated with gust fronts
What is a microburst?
Rapidly descending air that lasts up to 5 min with a diameter less than 4km, can occur multiple times in a single cell. Can be 50kt or 6,000ft/min
What is a macroburst?
Descending air that lasts between 5-20 mins normally greater than 4km in diameter, can occur more than once in a single cell
Biggest danger of downbursts
At low level during take-off and approach
What are the dangers of Virga?
Can generate intense downdraughts that can be difficult to detect
What is the danger of hail?
Can cause damage to the airframe, in particular the nose which is made from a weaker material in order to reduce radar interference
Where’s the best place to fly if approaching a cell?
On the windward side
Where’s the greatest risk of hail?
In the anvil and beneath the anvil, but danger anywhere near the system. Hail can be shot out of the top of the cell by intense updraughts
Greatest risk of rain
High ingestion leading to engine flame out
How do aircraft protect against lightning?
They have a faradays cage which allows the charge to flow though the aircraft to discharge points
How can lightning effect crew?
May cause temporary blindness and deafness
What are the 4 types of lightning?
- Cloud to air
- Cloud to ground
- Intra cloud
- Cloud to cloud
What is St. Elmo’s fire?
Visible discharge of the surface of the aircraft, appears as a blue plasma
Which airframes are at particular risk to lightning?
Ones made from composite materials that do not conduct charge as well
Which pressure instruments can show errors in a thunderstorm?
- Altimeter
- VSI
- Airspeed indicator
Which magnetic instruments can show errors in a thunderstorm?
- ADF (navigation)
2. Magnetic compass
What causes a tornado?
Strong inward convergence and updraught intensify and narrow a column of air, this increases speed of rotation and makes the pressure fall sharply
What are the characteristics of a tornado?
Diameter 100m-150m
Winds can exceed 200kt
Most common in spring and summer
What is a water spout?
Tornado over water
What is a dust devil (PO)?
A small tornado in hot, calm, dry conditions
Can be 10-100ft wide and can reach up to 650ft
Avoidance criteria for thunderstorms
5000ft vertical separation
20nm lateral seperation (can be 10nm below 20,00ft if using weather radar)
What is a stormscope display?
Simple lightning detector
Scans 120 degrees in front within a 200nm radius to show potential storm activity
What are the actions if unable to avoid a thunderstorm?
Secure items, passengers and crew
Plan, set and maintain speed and attitude
Ensure all anti-icing equipment is on
Use weather radar to help navigate
