Ch 14 - Thunderstorms Flashcards
Thunderstorm (TS) Defintion
When a CB is now producing thunder and lightening.
Must have 3 things:
- Very High relative humidity (lots of moisture)
- Instability over a minimum of 10,000ft
- A trigger for the uplift; Convection (thermal activity), Orographic uplift (mountains), Cold front passing
Heat and Air Mass for a TS
Must have absolute instability; ELR > DALR and SALR
2-3 hours after midday will be the most severe TS (thermal TS)
Frontal TS
Form just ahead of a the cold front and are the quickest moving (in terms of horizontal distance)
Orographic TS
Air rises up over the mountain in unstable conditions, continues to rise and condense and eventually will become a TS if it continues getting enough moisture and the instability continues up over 10,000ft
Stages of Development for a TS (1)
Stage 1 - The forming stage
Categorised as having only updrafts - low pressure on surface
0C< Water droplets and low level
0C> SCWD and ice crystals
Turbulence and Ice - Mod/Sev in cloud, light -mod out of cloud
No precip
Lasts 15-20 Mins
Stages of development for a TS (2)
Mature stage
Mix of both updrafts and downdrafts
Precip; Hail (GR), +RA, SN, SH
Might not reach the ground - Virga
Lightening and thunder starts (up to 5000K) and can interfere with; instruments, structural damage, NIHL, Temporary Blindness
15 - 30 minutes
Can cause microbursts
<5km and lasts less than 5 min
Macrobursts
More than 5km and more than 5 minutes
Stages of Development for a TS (3)
Disappearing or decaying Stage
Mainly downdrafts that kill the TS - takes away the fuel
Wider band of precip, heavy rains stops and becomes showers
Cloud base gets higher and can form an anvil
TS will always move in the same direction as the wind at 10000ft
Downdrafts eventually stops
Total time 1.5 hrs - 2.5 hours total
Types of TS
Single Cell - Initial/forming stage, Maturing stage, dissipating, Big CB
Multi-cell TS - along a squall line, lots of single cell TSs, along a cold front and will move with it, can be bigger and can influence 50-60km ahead of it and will be taller - will be found along ITCZ - Mesoscale Convective systems (MCS) - Organised complex of TSs - on/in/around ITCZ and can last for hours or more, average height of 45,000ft but can be bigger
Supercell - Must have Windshear, change in direction and or speed of the wind, tilts the TS, separates the updraughts and downdraughts and becomes self propagating, constantly pulling up cold air and moisture - only system to produce tornadoes
Tropical Squall lines
Produced by MCSs
100-1000km long
EQ regions they can be ever larger, higher and more destructive Lasting for days.
+RA
Big danger - biggest and wettest precipitation on weather return and icing very possible. Higher around or total avoidance
Tornadoes
Only come from super cell TS, happen when 4 different air masses converge and create that rotation, cold and hot air above, windshear creates the roll
Horizontal movement - 20-40kts Wind speed in funnel 200kts Associated with CBs Most likely in spring/early summer NH US tornado alley - instability Life span - few minutes to 30 Average diameter - 100-150m Measured with the fajita scale
Funnel Cloud
The same process as a tornado but it doesn’t reach the ground
The uk is a hotspot for funnel clouds
Water Spouts
A tornado over water
Dust Devils
Up from the ground, sand rises in hot air, calm weather days, up to 650ft, 10-100ft diameter and nowhere near as dangerous as tornadoes
Hazards with TSs
Hail - structural damage up to 45000ft Virga Lightening - st elmos cage Icing Windshear Water ingestion of engines Instrument Error
Airborne Weather Radar Display
Darker section = stronger return - more warning; chance of wet hail, large rain drops (intense precip)
Dry Hail
Dry Snow
+RA
Can mask what lies behind it
Avoid by 20 miles if above FL200 all sides
Below FL200, 10 miles avoidance on all sides
