1. Thunderstorms
&
Tornadoes

2. What are the stages of development in a Thunderstorm?
Three stages have been identified in ordinary thunderstorms:
Cumulus stage: an unstable atmosphere and vertical updrafts
Mature stage
Dissapating stage:weakening updrafts and loss of the fuel source.

3. Mature Stage Thunderstorm
Video: Hail.asf

4. Dissipating Stage of Thunderstorm

5. Multicell Storms
Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward.
Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

6. Severe Thunderstorms Storms producing a minimum of
3/4 inch hail and/or
wind gusts of 50 knots and/or
tornado winds, classify as severe.
In ordinary storms, the downdraft and falling precipitation cut off the updraft.
In severe storms, winds aloft push the rain ahead and the updraft is not weakened and the storm can continue maturing.

7. Diagram of a developing Thunderstorm

8. Gust Front Shelf Cloud
When unstable air is prevalent near the base of the thunderstorm, the warm rising air along the forward edge of the gust front is likely to generate a shelf, or arcus, cloud.

9. Trailing Stratified Clouds

10. Microbursts from Dense Air
Dry air entrained into the thunderstorm will evaporate and cool the falling mix of precipitation and air, which may create dry, and in humid areas wet, microbursts of strong winds.

11. Pre-Frontal Squall Line Storms on Radar

12. Mesoscale Convective Complex
An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC).
With weak upper level winds, such MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.
They often form beneath a ridge of high pressure.

13. What are Dryline Thunderstorms
Abrupt geographic changes from moist to dry dew-point temperature, called drylines

14. Thunderstorm Movement
Figure 15.17
Middle troposphere winds often direct individual cells of a thunderstorm movement, but due to dying storm downdrafts spawning new storms, the storm system tends to be right-moving relative to the upper level winds.
In this figure, upper level winds move storms to the northeast, but downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell.

15. What causes lightning during a thunderstorm?
Charged layers in the cloud are formed by the collision of hailstones and ice with the cumulonimbus cloud.
When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance a lightning bolt forms.
Video: Lightning_Strikes.asf

16. What causes thunder to form?
The rapid heating of air by a lightning bolt creates an explosive shock wave called thunder, which requires approximately 3 seconds to travel about 1 mile.

17. Types of Lightning
Nearly 90% of lightning is the negative cloud-to-ground type, but positive cloud-to-ground lightning can generate more current and more damage.
Several names, such as forked, bead, ball, and sheet lightning describe forms of the flash.
Distant, unseen lightning is often called heat lightning.

18. Lightning Rods & Fulgurite
Metal rods that are grounded by wires provide a low resistance path for lightning into the earth, which is a poor conductor.
The fusion of sand particles into root like tubes, called fulgurite, may result.

19. What is a tornado? Video: All_About_Tornadoes_and_Tornado_Alley.asf
A rotating column of air ranging in width from a few yards to more than a mile and whirling at destructively high speeds, usually accompanied by a funnel-shaped downward extension of a cumulonimbus cloud.

20. When do tornadoes occur?
Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift.

21. Tornado Wind Speed
As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed.
For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side.

22. Suction Vortices & Damage
A system of tornadoes with smaller whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado.
Damage from tornadoes may include its low pressure centers causing buildings to explode out and the lifting of structures.
Human protection may be greatest in internal and basement rooms of a house.

23. What scale measures tornado intensity?
Once the storm is observed, or has passed, the Fujita scale of F0 – F5 is used to classify tornadoes according to their rotational speed based on damage done by the storm.

24. Atmospheric Conditions
A specific pattern of events often coincide during the formation of tornadoes and severe thunderstorms.
This may include when an open-wave mid-latitude cyclone mixes together cold dry air with warm moist air at the surface, and 850 mb warm moist and 700 mb cold dry air aloft flow north and north east, as shown in this figure.
Further, at the 500 mb level a trough of low pressure pressure forms to the west of the surface low, and the 300 mb polar jet swings over the region.
Figure 15.34

25. Thunderstorm Sounding
Temperature and dew point have typical vertical profile in the warm sector before a tornado occurs, including the shallow inversion at 800 mb that acts like a cap on the moist air below.
The cold dry air above warm humid air produces convective instability and lifting.

26. How does a tornado begin to spin (vorticity)?
Spinning horizontal vortex tubes created by surface wind may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado.

27. What is a “supercell”?
A thunderstorm with a persistent rotating updraft. While rare, it produces the most severe weather such as tornadoes, strong winds, and hail.

28. Overhead view of a Super Cell

29. Rotating Clouds

30. NonSupercell Tornadoes
If a pre-existing wall cloud was not present, than any tornado formed is not from a supercell storm, and is often called a funnel cloud, or may be a gustnado if the form along a gust front.

31. Landspouts Landspouts, which form over land but look like waterspouts.
They form when surface winds converge along a boundary where opposite blowing wind creates a horizontal rotational spin.
If a storm passes above, its updraft may lift and stretch the horizontal spinning air, causing it to narrow and increase in rotational speed.
Landspouts

32. Doppler Radar Analysis
A single Doppler radar unit can uncover many features of thunderstorm rotation and movement, but cannot detect winds parallel to the antenna.
As such, data from two or more units might be combined to provide a complete view of the storm.
Dopplar lidar (light beam rather than microwave beam) provides more details on the storm features, and will help measure wind speeds in smaller tornadoes.

33. NEXRAD Wind Analysis
NEXt Generation Weather RADar (NEXRAD) is operated by the National Weather Service and uses Doppler measurement to detect winds moving toward (green) and away (blue) from the antenna, which indicates areas of rotation and strong shear.

34. What is a waterspout?
Warm, shallow coastal water is often home to waterspouts, which are much smaller than an average tornado, but similar in shape and appearance.
The waterspout does not draw water into its core, but is a condensed cloud of vapor.
A waterspout may, however, lift swirling spray from the water as it touches the water surface.