1. Unit 5 Section 1 Thunderstorms
Meteorology
Unit 5 Section 1
Thunderstorms

2. Thunderstorms Thunderstorms are convective storms
Form with rising warm air
Need unstable environmental conditions
As long as rising air parcel is warmer than the air surrounding it, it will rise
Warm air is less dense than cold air

3. Thunderstorms Each air parcel is called a “cell”
Involve wind shear – the change in wind speed and/or direction over a distance

4. Unstable Environmental Conditions
Unequal heating at the surface
Effect of terrain
Converging surface winds = contrasting temperatures and humidities
Diverging upper-level winds + converging surface winds + rising air
Usually many factors working together
Can form in warm or cold air temperatures

5. Severe Thunderstorms
To be considered severe, a storm must have at least one of the following:
Large hail >0.75 inches in diameter
Surface wind gusts of 58mph or more
Produces a tornado

6. Simple Thunderstorm Many names: “Scattered thunderstorms”
“Pop up storms”
“Ordinary cell thunderstorms”
“Air-mass
thunderstorms”

7. Simple Storm Rarely become severe Less than a kilometer wide
Predictable life cycle, complete in less than an hour
Form in regions with limited wind shear – wind speed and direction do NOT abruptly change

8. Simple Storm Life Cycle
1. Cumulus stage
2. Mature stage
3. Dissipating stage

9. Cumulus Stage Growth stage
Warm air rises, cools, and condenses into a cumulus cloud or cluster of clouds
Vertical Growth – Warm, humid air at the top of the cloud dissipates, which adds more moisture into the air, which cools and condenses, which adds more cloud to the top
Appears as a dome or tower
Can form a Towering Cumulus cloud in a matter of minutes
Not enough time for precipitation to form, no thunder and lightning

10. Cumulus Stage

11. Mature Stage Most intense stage
As the cloud builds above freezing level, cloud particles get larger and heavier and begin to fall
Drier air is drawn into the cloud, causing some raindrops to evaporate, which cools the air
Air is colder and heavier, forms a downdraft
Downdraft gets stronger if falling precipitation pulls air with it

12. Mature Stage
Mature storms have the original convective updrafts and downdrafts occurring at the same time
Top of cloud, containing ice crystals, reaches stable air and flattens out into an anvil shape
Severe turbulence in the middle of the cloud from updrafts and downdrafts
Lightning and thunder are present
Heavy rain and occasionally small hail

13. Too much talking. Not enough pictures.

14. Mature Stage

15. Mature Stage
Feel a downrush of cold air on the surface before precipitation
When downdraft of cold air reaches the surface, it spreads out horizontally
Gust front – surface boundary that separates the advancing cold air from the warm surface air
High wind shear, wind is rapidly changing speed and direction
Forces warm air back up into storm, which adds to updraft

16. Gust Front

17. Dissipating Stage Usually 15-30 minutes after start of mature stage
Updrafts weaken as gust front moves away
Downdrafts dominate, destroy humid updrafts = no fuel for storm
Light precipitation
Cloud disperses from ground up, the cirrus anvil is often the last thing to be seen

18. Life Cycle

19. Complex Thunderstorm “Multi-cell thunderstorm”
“Supercell thunderstorm”

20. Multi-cell Thunderstorm
Contain a number of cells, can be in different stages of development
Form in regions with moderate to strong vertical wind shear

21. Multi-cell Thunderstorm
Wind on one side of the storm is stronger, causing it to tilt
When tilted, downdrafts will not cut off updrafts = storm doesn’t lose fuel
Storm will survive longer
Can become intense and produce severe weather

22. Multi-cell Thunderstorm
Air at the gust front may form a shelf cloud that is attached to the base of the storm
Roll clouds may form just behind the gust front

24. Microbursts
Beneath an intense thunderstorm, downdraft may be very localized, forming a microburst
Can produce intense winds up to 170mph in a small area
Heavy damage to trees and structures
Can capsize boats and crash planes

25. Microbursts

26. Mesoscale Convective Complexes (MCCs)
Large, circular convective weather system made of individual multi-cell thunderstorms
Can be 1,000 larger than a simple thunderstorm
Can often cover an entire state
Move slowly and can exist for 12 hours or more

27. MCCs

28. MCCs
Pros – provide significant portion of seasonal rainfall over grain belt of USA
Cons – can produce severe weather, like hail, high winds, flash floods, and tornadoes

29. Supercell Thunderstorms
Supercell- A large, long-lasting thunderstorm with a single, violently rotating updraft
Form in regions with strong vertical wind shear
Tilted so storm doesn’t lose fuel
May exist for hours

30. Supercell

31. Supercell Thunderstorms
Wind shear may be strong enough to create rotation
Can produce updrafts of 100mph or more, damaging surface winds, and tornadoes
Strong updrafts suspend hailstones for long periods of time, allowing them to grow
Updraft can be so strong that precipitation can’t fall through it, creating a rain-free base

32. Supercell

33. Done.