THUNDERSTORMS Flashcards
What fuels a TS?
Latent heat released when condensation occurs due to water vapour condensing in thermal updraughts.
What causes downdraughts in TS?
Heavy rain formed within the cloud entrains (drags) col air downwards.
What causes Thunder & lightening?
When electro-potentials are created in TS development, at a critical value these cause lightening, which in turn heats & expands the air so rapidly that thunder is heard.
% of cloud-ground lightening strikes vs cloud-cloud?
10% vs. 90%.
3 conditions required for TS development?
- A trigger mech to initiate lifting
- Adequte moistue at lower levels
- Conditional instability through depth in the atmosphere.
Why is moisture important in TS development? (2)
- determines the high of the CB base. (greater the temp/dew point sep, the higher the base).
- Moisture is fuel (moisture condenses, latent heat released, further destabilises already unstable atmosphere).
3 trigger mechanisms of TS development?
- Convection: air gaining positive bouyancy due to being in contact with warm surface)
- Widespread ascent: frontal lifting/troughs/convergence zones
- Orography: mechanical lifting of air by terrain
List the 6 types of TS
Convective localised (stationary) TS
Convective travelling TS
Orographic TS
Nocturnal tropical TS
Frontal/convergence TS
Upper trough TS
Warm front embedded TS
Describe Convective localised (stationary) TS & the type of hazards it produces
Upright stance, may slowly move during formation process.
Little to no wind shear aloft & so cloud develops vertically with no tilt.
Downdrafts are therefore lined up with updrafts & the two compete to destroy each other. (Anvil only part unaffected)
Produces any of the hazards except tornadoes, waterspouts & funnel clouds. (need windshear)
Describe convective, travelling TS & the type of hazards it produces
Associated with increasing horizontal windshear with altitude. causes the cloud to tilt in direction of travel
Results in separation of up & down drafts.
As these don’t cx each other out, a travelling CB will last longer than an upright CB.
Otherwise known as super cell CBs. all hazards may exist.
What are the two types of TS?
Frontal & airmass
Describe orographic TS
How are EMBD CB formed in this process?
Form when moist air is forced to rise up on the windward side of a mountain range
Conditionally unstable environment, numerous small & stable layers. These act to form multilayered clouds alongside CBs = EMBD
Describe Nocturnal tropical TS
form when the tops of Cu clouds cool at night due to release of longwave radiation.
Instability is increased as the lapse rate is steepened & thus the Cu clouds grow to become active CBs
Describe the formation of Frontal type TS
How does it eventuate?
Lifting of warm unstable air at the frontal surface.
Or, forced lifting where moist air converges across a trough or within a col during the summer.
Frontal forms a line of TS along the frontal SFC, which move with the front. these TS are often embedded in Ac, As and Sc cloud
How do convergence storms eventuate?
Trough vs. Col?
tend to form lines or clusters.
If formed on a trough, they will likely travel & heavy shower activity will accompany.
Col: individual TS, seperated & haphazard pattern - tend to be upright & slow moving.
How do upper troughs form
Cold air is advected into area aloft in mid-latitudes. Steepens the lapse rate through depth & therefore creates instability.
Trigger to initiate lifting is still required, but cold air aloft ensures the TS will develop
How do warm front embedded TS form?
Warm sectors are generally stable & TS are rare. the warm air approaching a front may become unstable & isol embd CBs may form.
What are the 3 stages of TS development
- Cumulus/developing
- Mature
- Anvil/dissipating
Describe the cumulus stage
Hazards?
Updrafts only
Mass latent heat release due condensing of water vapour.
Light to mod turb, mod-severe icing (numerous SCWD being carried aloft by water droplets)
Most other hazards yet to materialise
Describe the mature stage
Hazards?
Any or all of the 8 hazards now exist
Characterised by onset of heavy precip & anvil will start to form at top (dropletss spread out beneath inversion/tropopause)
Describe anvil/dissipating stage
Anvil starts to glaciate (clear signs of Ci/Cs development).
Updrafts cease, hazards weaken & disappear.
Bottom of cloud may evaporate
Total CB life cycle?
1.5-2hrs max
30mins per stage
List the 8 hazards associated with TS
Sev turb
Sev icing
Microbursts
First gust
Electrical phenomena
Tornadoes
Hail
Poor vis
Worst turb in TS?
Where to avoid?
In the centre of the TS
AVOID FLYING BENEATH CLOUD BASE (downdrafts strongest under centre)
Worst icing in TS?
temps between 0 and -12deg C
What is a microburst?
localised sev wind pattern, driven by extremely strong downdrafts from dense rain-cooled air.
Can be >100kts in the vertical & 4km horizontal extent.
Intensify for 5mins after SFC contact & dissipate 10-20mins after.
When to expect dry microburst
Why are they worse than wet?
High based TS (up to FL250).
ID’d by a region on virga or curls of dust.
Worse as evaporating moisture results in further cooling & therefore acceleration of downdrafting air
What is a gust front
Why is it hazardous
Strong downdraft of air that spreads horizontally at base of TS.
Characterised by sudden wind change & lowered temp. (similar to a cold front)
A/C LDG or T/O may experience low level WS, combined with changes in air density may result iin sig changes in lift.
How is lightening created
rapid movement of large qtys of moist air creates large static charges. Resulting electrical discharge = lightening.
Hazards of lightening?
Damage to A/C skin & avionics equipment & compasses s
How is a water spout created?
When a tornado occurs over water, the extremly low internal pressure sucks water high into the air, causing a waterspouts.
How do hailstones grow?
Carried in cycles of up & downdrafts in the cloud.
Growth occurs due to coalescence with SCWDs.
Opaque layers: rime ice (higher cloud levels)
Clear layers: glaze ice (lower layers)
Each layer indicates the number of times it has cycled the CB.
One it grows too heavy, it is ejected from the cycle.
Characteristics of Multi-cell TS?
Tend to move west to east
New cells form on east side as gust fronts from existing CBs trigger new cells on LE.
On western side, CBs are entering dissipating stage.
How are multi-celled TS formed?
Result from a large area of instability, CB cells are triggered rapidly at differing times in close prox to each other. Cells form overlapping CBs at different stages. Most active @ middle of cluster.
8 characteristics of super-cell CBs?
- Leaning over as it develops
- Rain & microbursts ahead of cloud mass
- Sep of up and down drafts
- strong updraft continues to punch through tropopause = massive release of energy (latent heat)
- Interaction b/t up & down drafting air = rotating eddy.
- eddy can get caught in updrafts & tilts fwd
- Small % of super-cells have rotating wall clouds
- <10% of rotating wall clouds produce tornadoes/water spouts/funnel clouds
What wavelength mcrowaves does wx radar use
3-10cm
How does radar ID TS?
energy sent from radar unit, reflectd by raindrops/snow/hail back to the unit. strong return signal indicates numerous/larger raindrops.
Range height indicator (RHI) assesses height of cells
