1. Static Stability and Cloud Development

2. Cloud Formation What is static stability?
It describes what would happen to an air parcel if it was forced upwards.
It could keep rising
It could return back to it’s original/starting height
How is air is cooled to the saturation point?
If atmosphere is stable:
If atmosphere is unstable:
Generally through vertical lifting
Flat-layered type clouds form (stratus)
Cumulus-type clouds form (puffy clouds)

3. Stratus

4. Cumulus

6. Parcel rising and sinking in the atmosphere

7. How does air get forced upward?
Topography
Surface heating/convection
Convergence of surface air
Lifting associated with weather fronts

9. Convection

10. How to create an unstable atmosphere?
Get lots of warm air underneath cold air
Heating the surface considerably (convection)
Wind/advection brings warm air in from another region
Add moisture near the surface
moisture makes air more buoyant and able to rise
Bring in cold air to upper atmosphere
troughs of low pressure do this

11. How to create a stable atmosphere
Get cold air below warmer air.
This is called a temeprature inversion, and this prohibits convection
A cool ocean nearby can accomplish this by creating a marine layer
Remove moisture near the surface
“drying out” the air makes air less buoyant and less likely to rise
Bring in warm air to upper atmosphere
ridges of high pressure do this

12. Thunderstorms/
Lightning and
Tornadoes

14. Thunderstorms
Thunderstorms are generally classified into one of two groups:
Air Mass Thunderstorms
Mesoscale Convective Systems
These are not-so-intense storms that are
short-lived and localized.
These systems lead to the generation of
numerous thunderstorms.

15. Air Mass Thunderstorms
These thunderstorms form within a single air mass and are not tied to fronts or mid-latitude cyclones.
Daily solar heating is primarily responsible for rising air motion,
producing rising cumulus clouds.
These storms may go through growing, mature and dissipation stage
In just an hour or two
Often associated with summer storms.

16. Air Mass Thunderstorms
Air mass thunderstorms are “self extinguishing” –
their natural evolution forces their dissipation
An updraft is required to release the latent heat that
drives the thunderstorms. In the later stages, rainfall
will lead to air cooling and a downdraft. This largely
kills the updraft and thus the thunderstorm

18. Severe Thunderstorms
As the name suggests, these thunderstorms are part of a mesoscale system. The horizontal scale of up to a few hundred kilometers.
The actual structure of an these storms can vary considerably.

19. Severe Thunderstorms
As the name suggests, these thunderstorms are part of a mesoscale system. The horizontal scale of up to a few hundred kilometers.
The actual structure of an these storms can vary considerably.
mesoscale convective complexes
squall line thunderstorms
supercell thunderstorms

20. Severe Thunderstorms

21. Severe Thunderstorms
Change in wind with altitude important to formation:
“Wind Shear”

22. Squall-line Thunderstorms

23. Severe Thunderstorms Characteristics
Definition of Severe Thunderstorm:
Capable of producing large hail
Strong gusty surface winds
Flash floods
Tornadoes
¾ inch hail or
Surface wind gusts of 50 knots

24. Supercell Thunderstorms

25. Average # of days thunderstorms observed

26. Average number of days hail observed

27. Lightning and Thunder
Lightning is the discharge of electricity that occurs within a thunderstorm.
The extreme heating associated with lightning causes air to expand rapidly, and produces sound waves we recognize as thunder.
If you want to estimate the distance of an approaching thunderstorm:
Count the number of seconds between a lightning
strike and the sound of thunder
For every five seconds, the storm is 1 mile away

29. Normal charge separation in a mature thunderstorm
Charge separation not fully understood, but
rapid convection (vertical motion) is certainly important

30. Separation of Charge in Clouds
One theory suggests that the separation of charge is due to collisions between ice particles. After exchanging charge, the heavier ice crystals settle toward the cloud base. - - ++

31. Development of lightning stroke

32. Cloud to Ground Lightning Strike
Positive charge is drawn up to the stepped leader. Usually through the highest conducting object.
The return stroke can travel at 1 ´ 108 ms-1 (roughly 1/3 the speed of light.) This return stroke is visible to the human eye.
There are commonly three or four strokes along a common path, which is why it is sometimes appears like the lightning flickers.

34. Why one shouldn’t shelter under a tree during a thunderstorm…

35. Lightning Facts
It is estimated that globally 5000 people are killed by lightning annually.
At any given moment there are ~ 1000 thunderstorms occurring over the globe. Two thirds of all lightning strikes occur within the tropics.
Most lightning strikes are NOT cloud to ground strikes (20%) - rather cloud to cloud strikes are most common.
Aircraft are usually not damaged by lightning strikes.

36. Tornadoes … … are also called twisters or cyclones.
… are rapidly rotating winds that blow around a small area of intense low pressure.
… come in many shapes, but mostly look like funnels or tubes.
… often descend from large cumulonimbus clouds.
A funnel cloud is a tornado that doesn’t hit the ground.
A waterspout is a tornado-like storm that occurs over the ocean

37. Tornado Characteristics
Majority of tornadoes rotate counter-clockwise (cyclonic)
Most tornadoes only last a few minutes
Most tornadoes are ~ 100 – 600 m ( ft) in diameter

39. Power of the Wind
The force, or power associated with the wind is proportional to the cube of the wind speed.
This means if the wind speed doubles, the force increases by the factor of eight.
A wind speed of 100 mph is 8 times more powerful than a 50 mph wind

40. Enhanced Fujita Tornado Damage Scale
EF Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Winds of mph
EFl Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken. Winds of mph EF Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. Winds of mph

41. Enhanced Fujita Tornado Damage Scale
EF Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance. Winds of mph
EF Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated. Winds of mph
EF Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); steel reinforced concrete structure badly damaged; high-rise buildings have significant structural deformation; incredible phenomena will occur. Winds over 200 mph
So far only one EF5 tornado has been recorded since the Enhanced Fujita Scale was introduced on February 1, 2007.

42. Tornado Formation Formed in association with severe thunderstorms
Conditionally unstable atmosphere is important
Multiple tornadoes can come from a single storm (like a supercell storm)
Example: May 4-5, 2003, during a 24 hour period, there were over 80 reported tornadoes.

44. Recipe for a tornado Strong wind shear - Can enhance rotation
- Caused mainly by jet stream
Warm moist air below dry colder air
- Large instability
- Explosive growth due to latent heat release
Supercell storms good candidate for tornadoes (they already have rotation).

45. Tornado occurrence?
Tornadoes possible everywhere in the world, but most are in the U.S. (tornado alley Texas –Nebraska
3/4 of the tornadoes occur from March to July, with the maximum in _____.
Jet stream is still a large influence
Most often occur in the _______________
Least frequent ______________
May
late afternoon (4-6pm)
before sunrise

46. Tornado incidence by state
25 year total

47. Why is Tornado Alley the most likely place to get tornadoes?
Perfect location for the mixing of air masses
Warm, moist gulf air to the south
Cold, dry to the north/northeast
Rockies mountains to west/northwest
Downslope flow is cool and dry
Right latitude for the polar jet stream

48. On Radar, the presence of a hook echo indicates a mesocyclone; a region in a thunderstorm very likely to spawn a tornado