1. Announcements Please see the exam key to be posted later today before coming to see me about your exam. The key will have the point values for written questions. Perfect student exam will also be posted. Assignment #2 due Wednesday. Hand in during class or no later than 5pm to my office mailbox. If labs are not graded yet, will be done by Wednesday. Sorry for the delay.

2. NATS 101 Section 4: Lecture 28 Thunderstorms and Severe Weather Part III: Western U.S. Hazards and Lightning

3. So far I’ve kept the discussion of severe weather mainly focused to the central and eastern part of the U.S. There are some unique considerations for severe thunderstorm hazards in the western U.S., particularly here in Arizona.

4. Average number of days with thunderstorms Average number of days with hail More thunderstorms in other parts of the country… BUT, the thunderstorms that do occur HERE are a lot different!

5. Why are thunderstorms in the western U.S. different? The topography is the major factor in generating thunderstorms, by mountain-valley circulations. Atmospheric conditions in which thunderstorms form are generally drier with less vertical shear. More hail formation Dry thunderstorms which only have strong wind and lightning. Much less likely to develop supercell thunderstorms Steep terrain and dry soils enhance (flash) flood potential

6. Severe weather dangers particular to the Southwest U.S. Flash flooding Areas of steep topography, such as canyons Dry arroyos and sheet flood zones Urban areas with poor drainage Hail (though it is not too big…) Dust storms (haboobs) Microbursts Lightning and lightning induced wildfire NEARLY ALL OF THESE OCCUR DURING THE MONSOON SEASON OR RIGHT BEFORE

7. Conditions for enhanced monsoon thunderstorms An upper-level disturbance (X) traveling around the monsoon ridge. Low level-moisture surging up the Gulf of California RESULT Thunderstorms which originate on the Mogollon Rim intensify and move westward toward low deserts and the Colorado River Valley. THIS IS WHEN IT RAINS IN YUMA! GULFSURGE X Shortwavedisturbance

8. An active monsoon day…

9. So what happens when the monsoon gets active?

10. Flash Flooding in Arizona ARROYOSCANYONS AND DRY RIVERBEDS

11. WHEN THE STREETS OR ARROYOS FLOOD, DON’T TRY TO CROSS THEM!! LAS VEGAS FLOOD, July 1999

12. Though on the periphery of the monsoon, Colorado is another notorious place for flash flooding in mountain canyons in the summer

13. Big Thompson Flood July 31, 1976

14. Fort Collins Flood July 29, 1997 Storm rainfall (inches)

15. Microburst Precipitation in the downdraft part of the thunderstorm evaporates (partially or fully) before it hits the ground. Cooled air sinks rapidly toward the surface. Wet microburst on the west side of Tucson, near Ryan Field Dry microburst near Denver, CO.

16. (Williams) Delta Flight 191 Crashed August 2, 1985 Cause: Microburst related wind shear and pilot error Microburst Aviation Hazard

17. Haboob: Dust or sand storm Phoenix, Arizona Caused by rapid movement of air associated with the gust front of a thunderstorm. Common during the monsoon, particularly just as it starts because the preceding months are dry. Also common in the Middle East (e.g. Iraq)—and they’re much bigger there!

18. Lightning Discharge of electricity, or spark that usually occurs in thunderstorms. Only about 20% actually are cloud-to-ground. TEMPERATURE WITHIN THE BOLT = 30,000°C (5X HOTTER THAN SUN)

19. To get a clue what causes lightning, let’s first look at where lightning occurs throughout the world.

20. Global lightning distribution NASA Imagery Observation: Nearly all the thunderstorms which produce lightning occur over land, or close to it. Very little lightning occurs over the oceans.

21. Difference in convective clouds Ocean vs. Land Maritime Convective Clouds Few cloud condensation nuclei and virtually no ice condensation nuclei Mostly warm clouds (no ice) with a relatively few big drops. Continental Convective Clouds A lot of cloud AND ice condensation nuclei. Mostly cold clouds with smaller and more numerous drops and ice particles. Therefore… THE KEY TO CLOUD ELECTRIFICATION IS THE PRESENCE OF ICE PHASE PROCESSES IN CONVECTIVE CLOUDS!

22. Thermoelectric effect in ice ICE BLOCK Idea: A separation in charge occurs when there is a temperature difference across ice. WARMER TEMPERATURE Higher number of negative ions NEGATIVE CHARGE COLDER TEMPERATURE Higher number of positive ions POSITIVE CHARGE + -

23. Charging of ice crystals Hailstone collides with a colder ice particle or supercooled cloud drop. When collision occurs: The warmer hailstone becomes negatively charged and FALLS. The colder ice particles become positively charged and RISE. NET RESULT: TOP PART OF CLOUD IS POSITIVELY CHARGED BOTTOM PART OF CLOUD IS NEGATIVELY CHARGED Consider a hailstone falling in a cloud updraft (Wallace and Hobbs)

24. Charge Distribution in Mature Thunderstorm TOP PART OF CLOUD (BELOW -20°C) POSITIVE CHARGE MIDDLE PART OF CLOUD (0°C to -20°C) NEGATIVE CHARGE BASE OF THE CLOUD (Around 0°C) WEAK POSITIVE CHARGE AT THE GROUND BELOW: POSITIVE CHARGE COLLECTS ON UPWARD PROTRUDING OBJECTS

25. Lightning Discharge Sequence Lead streamers of negative charge toward the ground from cloud Lead streamers of positive charge from the ground to cloud NEGATIVE STEPPED LEADER Streams meet, carrying positive charge upward to the cloud. POSITIVE STEPPED LEADER RETURN STROKE (BRIGHT FLASH)

26. Types of Lightning Discharges One more that IS NOT SHOWN here is POSITIVE cloud-to-ground lightning. Though rare, it is more dangerous because it has a higher current and longer flash duration. Within Cloud or Cloud-to-Cloud 80% Cloud-to-ground 20%

27. Thunder and the rumble Heating of the air causes it to expand and contract rapidly. The expansion and contraction of the air creates a sound wave, which we hear as thunder. The subsequent rumbling sound may result from Attenuation of the sound wave Echoes off of obstructions RULE OF THUMB TO GAUGE STORM DISTANCE: EACH 5s BETWEEN THE FLASH AND THUNDER = 1 MILE DISTANCE. (Williams)

28. U.S. Lightning Distribution (Courtesy Dr. Phil Krider)

29. Tucson Lightning Distribution Southern Arizona PHOENIX TUCSON Why more here? HIGHER ELEVATION CLOSER TO MTNS. MORE RAINFALL (NSSL)

30. Lightning and Wildfire Danger in Arizona NASA Image of Cave Creek fire in late June 2005 Lightning induced wildfire is a threat in Arizona, which is most acute right before the monsoon. Factors: Dry thunderstorms that produce lightning and wind but little or no rainfall. Late spring and early summer before the monsoon is the driest and hottest part of the year in Arizona.

31. What is the probability that one of us in this room will be struck by lightning in Tucson within the next year, assuming we’re all here for the monsoon? A)5% B)10% C)15% D)20% E)30%

32. THERE IS A LOT OF LIGHTNING IN TUCSON! A Startling Fact The probability of one of us in this room getting struck by lightning in Tucson in the next year is 30% (assuming we’re all here for the monsoon)

33. Weather Safety: Lightning (Williams)

34. Summary of Lecture 28 Thunderstorms that occur in the western U.S. are different because of the influence of the topography and the drier and less sheared environment in which they form. Severe weather dangers before and during the monsoon in Arizona include flash flooding, hail, dust storms, microbursts, and lightning. The two conditions that enhance monsoon thunderstorms in Arizona are an upper-level disturbance and a low-level moisture surge from the Gulf of California. Lightning is a discharge of electricity from a thunderstorm Cloud electrification is due to ice phase processes within the cloud, creating positive charge at the top and negative charge below. Lightning discharge occurs when lead streamers from cloud and ground meet, then there is a bright return stroke. Thunder is the sound wave caused by the heating of the air by the lightning.

35. Reading Assignment and Review Questions Reading: Chapter 15 Chapter 14 Questions Review: 10,16,17,18,19,20 (8 th ed.) 10,21,22,23,24,25 (9 th ed.) Thought: 6