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11/16/2011 Thunderstorms Thunderstorms and Tornados Chapter 10 — A storm that generates lightning and thunder. ◦ Gusty winds, heavy rain, and hail ◦ Warm humid air rising – Unstable environment – Unequal heating of earth’s surface Air-Mass Thunderstorms — Stages of Development Occur within Maritime Tropical air-masses ◦ Unstable air ◦ Summer days Stages of Development — Cumulus Stage – Warm humid air rising – Reaches dew point – Condensation - Clouds – Addition of latent heat increases development – – – – – Updrafts dominate Precipitation is generated Updrafts can’t support precip. Downdrafts are initiated from precip. Entrainment – influx of cool dry air Stages of Development — Mature Stage ◦ ◦ ◦ ◦ ◦ ◦ Equal updrafts and downdrafts Creates anvil shape High winds Lightning Heavy precip. Possibly hail – Causes some precip to evap. (cooling process) – Downdrafts – Precipitation -Cold 1 11/16/2011 Stages of Development — Severe Thunderstorms — ◦ High Winds - 93 km/h ◦ Hail - 1.9 cm diameter ◦ Tornados Dissipating Stage ◦ Entrainment increases ◦ Downdrafts dominate Severe Thunderstorms Wind Shear gives strength — 10,000 out of 100,000 per year (US) Severe Thunderstorms — ◦ Change in wind direction and/or speed between different heights — Criteria (Any one of the followi ng) Roll Cloud ◦ Spinning occurring between updrafts and downdrafts. Tilted updrafts ◦ Storm can keep feeding itself ◦ Maintains strength ◦ Builds upward — Overshooting top — Gust front ◦ Extends in stratosphere ◦ Pushes warm air up ◦ (Mini cold front) Supercell Thunderstorms Squall Lines 2,000 – 3,000 per year (US) — Very large, very dangerous T-storms — Typically spawn tornados — — — — — — — Line of thunderstorms Occur in the warm section of a cyclone In front of a cold front Can form along a dryline Caused by divergence aloft Warm humid air gets forced upward 2 11/16/2011 Squall Lines — Mesoscale Convective Complexes Mammatus Sky — ◦ Dark cloud rolls — — — — — — Microburst — — — — — — Downdrafts or downbursts Created by evaporative cooling Cold air – very dense At surface air pushes outward 160 kph An example of wind sheer Many individual T-storms organized into a cluster 100,000 sq. km Slow moving Originate as air-mass T-storms Lots of warm humid air needed Instable air (rising) Gust fronts Lightning and Thunder — Sheet lightning ◦ Bright – illuminates the sky ◦ Can’t see bolt ◦ Bolt within the clouds ◦ Change in wind direction and/or speed between different heights Lightning and Thunder — Lightning and Thunder Cloud to Ground Lightning — ◦ More dangerous — What causes lightning? Meteorologists unsure ◦ Friction between updrafts and downdrafts ◦ Formation of hail — Negative charges at — Positive charges at — Negative charges ◦ Base of cloud ◦ Top of cloud ◦ On ground 3 11/16/2011 Lightning and Thunder — Flash Lightning and Thunder — ◦ Bright streak of light ◦ Composed of 3-5 strokes Step-leader ◦ Ionizes the air ◦ (loses electrons) ◦ Forms path for lightning – To travel ◦ No lightning yet Lightning and Thunder — Return Stroke Lightning and Thunder — ◦ Path gets built upward (like having a wire) ◦ Carries discharge downward (electricity flows) Additional strokes from these charges ◦ Dart-leaders Lightning and Thunder Tornadoes Lightning superheats air — 33,000 C — Air expands explosively and produces thunder — 5 sec. = 1 mile — — — — Violent windstorms that take the form of a rotating column of air (vortex) that extends downward from a cumulonimbus cloud Heat lightning – lightning far away Can’t hear 4 11/16/2011 Tornadoes — — — — — Low pressure in the center Air spirals upward Air merges with cloud above Dark color from soil and debris Up to 300 mph Multiple Vortex Tornadoes Multiple tornados grouped together Each suction vortex may form and die quickly — Rotates around center — — Mesocyclone How Tornadoes Form Vertical cylinder of rotating air — 3-10km wide — — ◦ Changes in wind speed and/or direction ◦ Horizontal rolling motion — How Tornadoes Form — A) Wind Shear B) Updraft turns spinning vertical Wall Cloud C)Wall cloud forms ◦ Funnel Cloud Forms ◦ Tornado – when it touches ground 5 11/16/2011 Funnel Cloud Tornado Where Tornadoes Form Typical Profile of a Tornado Cold Fronts — Squall Lines — Supercell Thunderstorms — Diameter 150-600 m Forward movement 45 kph — Path – 26 km long — — Tornado Forecasting Tornado Destruction — Fujita Intensity Scale (F-scale) Very difficult to predict — Tornado Warning — Associated with severe weather ◦ Typically there has been a sighting ◦ High probability of danger — Tornado Watch — ◦ Conditions are right for a tornado 6 11/16/2011 Doppler Radar Transmits short pulses of electromagnetic energy — Waves bounce off storm — Return to radar — Signal indicates — ◦ Rainfall intensity ◦ Distance from radar ◦ Increase in frequency – Storm approaching ◦ Decrease in frequency – Storm Moving away Detects motion — Tornadoes — ◦ 11 min lead time 7
