Upcoming Schedule • Friday, November 21: Chapter 14 (Thunderstorms) Teaching evaluations. • Monday, November 24: Chapter 14 (Lightning and Tornadoes): lightning, thunder, tornadoes, Doppler radar. • Wednesday, November 26: no class. • Monday, December 1: Chapter 15 (Hurricanes). • Wednesday, December 3: Third midterm exam. The exam will cover Chapters 11, 12, 13, 14 and 15. • Closed book • No textbooks, calculators or cheatsheets. • Alternate (anti-social) seating • Bring a picture ID to the exam. • Remember to: • Write the 5-digit test code in lines 76-80 • Write your name on the exam sheet and sign it • Turn in both the exam sheet and the answer sheet. • Friday, December 5: begin Chapter 16 (Climate Change). • Monday, December 8: finish Chapter 16 (Climate Change). • Wednesday, December 10: Q/A session. • Thursday, December 18, 10:00 am, NPB 1001. Final Exam (cumulative).
Thunderstorms: brief review • There are two basic types of thunderstorm cells: ordinary cells and supercells. • Ordinary cell thunderstorms tend to form where warm, humid air rises in a conditionally unstable atmosphere and where vertical wind shear is weak. They are usually short-lived and go through their life cycle of growth (cumulus stage), maturity (mature stage) and decay (dissipating stage) in less than an hour. They rarely produce severe weather. • As wind shear increases (and the winds aloft become stronger) thunderstorms are more likely to become severe and produce strong surface winds, large hail, heavy rain, and even tornadoes. • The cells that comprise a multicell thunderstorm can be ordinary or supercell. A squall line is a long line of multicell thunderstorms that may form along a frontal boundary or out ahead of it. A Mesoscale Convective Complex is a large circular cluster of multicell thunderstorms. • Flooding: typically in the spring, when the snow is melting and the rivers are full. Flash floods: floods that rise rapidly with little or no advance warning.
Lightning Compare to: Plasma ball in the lobby outside • What is it? An electrical discharge – a giant spark!
Electricity 101 • Particles may carry positive or negative charge. • Opposite charge particles attract each other • Same charge particles repel each other • Electrons are negatively charged. They are very mobile. • Nuclei are positively charged. • Molecules are made up of nuclei and electrons. They have no net charge. • Ions are molecules which have gained or lost electrons: they can have a negative or a positive net charge. • The motion of the charges results in electrical current. - + + +
Charging a Thundercloud • Raindrops, snow crystals and hail stones collide inside the cloud. During these collisions they may exchange electrons and ions. • The exact mechanism is not well understood, but the bottom line is: • Larger particles become negatively charged. • Smaller particles become positively charged. • Larger particles settle down to the bottom of the cloud. • Smaller particles are lifted to the top of the cloud by strong updrafts. • Note precipitation area: void of large particles -> positively charged.
Type of Discharges • Cloud-to-ground • 90% of the time: negative cloud to positive ground • 10% of the time: positive cloud to negative ground • Cloud-to-cloud: between oppositely charged regions of different clouds. • Cloud-to-atmosphere: self-explanatory • Base-to-top: discharge within the same cloud
If all the charges are in the cloud why does the lightning strike the ground? • The negative charges at the bottom of the cloud repel the electrons on the ground beneath the cloud. The ground just below the cloud is then positively charged (notice the tree). The ground away from the cloud will be negatively charged with excess electrons (not shown) which came from (used to be in) the tree • Once the accumulated charges become large enough some electrons will “jump” to the ground. A current starts to flow. • The molecules in the atmosphere below the cloud are bombarded with flying electrons. They become excited and then emit light (we see a spark).
Stages of Cloud-to-Ground Lightning. • The lightning is initiated with a flow of electrons from the base of the cloud towards the ground (stepped leader) • When the leader gets close to the ground a flow of positive charges surges upwards. • When they meet a strong current (return stroke) transfers a charge between the cloud and the ground. • The process can be repeated several times in the same channel (dart leader, return stroke…) 3 1 2
Lightning Varieties • Forked lightning: when the dart leader deviates from the original path of the stepped leader • Ribbon lightning: when a strong wind shifts the ionized channel between successive return strokes • Bead lightning: when the ionized channel breaks up • Ball lightning: a floating luminous sphere • Sheet lightning: when the flash is inside or obscured by a cloud • Dry lightning: when there is lightning but no precipitation • Heat lightning: distant lightning which is not heard. Since it happens very far, its short-wavelength light is scattered and it appears orange (recall setting sun).
Different Types of Lightning Ribbon lightning Forked lightning Bead lightning Heat lightning
Different Types of Lightning Cloud-to-ground Cloud discharge Sheet Ball
Other lightning phenomena • St. Elmo’s fire. Corona discharge at the top of antennas or ship masts. • Fulgurite: fused sand particles as a result of the strong heating at the location of the lightning stroke. • Cloud-to-upper atmosphere lightning • Elves, red sprites, blue jets
Thunder • We see the light emitted from the air molecules as they become excited from the electrical discharge. • The thunder is the sound that accompanies the lightning. It results from the fast expansion of the gas that is heated to high temperatures from the electrical discharge. • Sound propagates much slower than light V(sound) = 330 m/s V(light in vacuum) = 300,000,000 m/s • That is why we hear the thunder long after the lightning • How far is the lightning? Count the seconds between the flash and the thunder. Each second is worth 330m~1000ft. • What if there is no thunder? Sometimes the sound wave is reflected or absorbed while traveling through the air and may not reach us • Sonic boom: just a jet going supersonic.
In case of a thunderstorm: • If outdoors: • Seek shelter inside, preferably inside a building with a lightning rod • If not, stay inside the car • Do not stay under trees. • If hiking above the treeline, descend immediately. • It is NOT safe to be in a tent, small picnic shelters, near heavy machinery. • If indoors: • Stay away from water, plumbing, doors and windows • Do not use land line telephones. • Turn off, unplug, and stay away from appliances, computers, power tools, TVs. • Consider purchasing a heavy duty surge protector • Bring pets inside - especially dogs chained to trees • Lightning safety awareness week • June 24-30, 2007.
Tornadoes: Some Statistics • Tornado is a rapidly rotating column of air around an intense low pressure center that touches the ground. • The rotation is typically cyclonic (counterclockwise) in the NH. • In the US there are about 1000 tornadoes a year. • On average there are about 100 tornado casualties a year. • Most occur in the tornado alley that runs through the central plains • Tornado season: March-July • Spring in the south; • Summer in the north. • Conditions: • Unstable atmosphere: warm humid air below, cool dry air aloft • Strong wind shear. • Most likely time of the day: late afternoon (the atmosphere is most unstable) • Typical diameter: 100-600 m, extreme cases ~ 4 km • Typical wind speed: 40-300 mi/h • Typical duration: a few minutes to several hours.
Tornado Occurrences in the US • Number of tornadoes within a 25 year period • Upper figure: total per state • Lower figure: annual rate per state per 10,000 sq. miles
Severe Weather and Doppler Radar • A Doppler Radar uses radio waves • To measure precipitation it measures the intensity of the reflected radio waves. • To measure wind speed it measures the frequency of the reflected radio waves • Doppler effect: the change of the wave frequency as the object moves – the siren of an ambulance has a higher pitch when it is approaching. • Cannot measure the speed of parallel winds: need 2 radars • Doppler radar can reveal a Tornado Vortex Signature (TVS).
Not a real tornado: Funnel cloud Wall cloud Landspout Dust-tube tornado Waterspout (see active figure)
Most famous tornado of all times • It’s a twister! It’s a twister!