1. AOS 100: Weather and Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell

2. Miscellaneous Homework due by the end of weather discussion
Extra Credit Offer:
- If you’re interested in an extra credit project, me with a topic you’re considering, and we’ll figure it out from there

3. Review of November 12th: Thunderstorms Continued
Air-mass thunderstorms (which are non-severe thunderstorms) have updrafts which are eventually cut off by the thunderstorm’s own downdrafts
Severe thunderstorms have strong wind shear, which tilts the updrafts and downdrafts so that this problem doesn’t occur
This allows severe thunderstorms to last longer, and become more intense

4. Review Continued
For our purposes, we can consider two types of severe weather:
A squall line is mostly linear continuous line of thunderstorms associated with speed wind shear, and they often produce strong straight line winds
Supercells are smaller storms associated with directional wind shear, and often cause tornados and large hail

5. Review Continued: Doppler RADAR
Doppler RADAR makes use of the “Doppler Effect” to determine whether objects are traveling towards or away from the radar site
Doppler RADAR measures the change in wavelength of the incoming signal (compared with the signal that was sent out)
This allows meteorologists to locate areas where air is moving towards the radar with adjacent air moving away from the radar
This usually indicates a tornado might be present or forming

6. A Supercell

7. A Supercell Reflectivity: Velocity:
We call this feature a “velocity couplet”, and it often signifies a tornado
From:

8. Classic Supercell

9. Classic Supercell

10. Classic Supercell – using doppler radar

13. Tornadoes
Tornadoes are caused by the creation of a rotating updraft

14. A Supercell (with tornado)

15. A Supercell (with tornado)
Figure 14.46
A classic tornadic supercell thunderstorm showing updrafts and downdrafts, along with surface air flowing counterclockwise and in toward the tornado. The flanking line is a line of cumulus clouds that form as surface air is lifted into the storm along the gust front.
Fig , p. 402

16. } Tornado Wall Cloud Figure 14.18
A supercell thunderstorm with a tornado sweeps over Texas.
Tornado
Wall Cloud

19. Tornado Damage
Figure 14.43
Total destruction caused by an EF5 tornado that devastated parts of Oklahoma on May 3, 1999.

20. Supercell Setup Supercells form at near the surface cold front
As you move upward in the atmosphere, the winds “veer”
Oftentimes, very fast winds occur at upper levels
These two factors help to increase the wind shear

21. A Squall Line
Dry air coming in behind the storm causes rain to evaporate, quickly cooling the air
This makes the air very dense, causing it to sink rapidly to the surface, which leads to strong straight-line winds

22. Approaching Squall Line
Cold Air

23. Incoming Dry Air
Squall Line Motion

24. Cold Air

25. A Squall Line
Dry air coming in behind the storm causes rain to evaporate, quickly cooling the air
This makes the air very dense, causing it to sink rapidly to the surface, which leads to strong straight-line winds