Thunderstorms and Lightning

Review of last lecture Bergeron process: happens with coexistence of ice and super- cooled water. Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature.Bergeron process: happens with coexistence of ice and super- cooled water. Key: Saturation vapor pressure of ice < that of super-cooled water at the same temperature. Further growth of ice crystals (riming and aggregation)Extensive riming in strong updrafts (graupel, hail)Further growth of ice crystals (riming and aggregation)Extensive riming in strong updrafts (graupel, hail) Definition of airmasses. Bergeron classification of air masses (3 letters).Definition of airmasses. Bergeron classification of air masses (3 letters). Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line)Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line) Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation)Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation) The developmental stages and vertical structure of middle latitude cyclones (boundary between northern cold air and southern warm air, upper level low to the west of surface low)The developmental stages and vertical structure of middle latitude cyclones (boundary between northern cold air and southern warm air, upper level low to the west of surface low) The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks

The most common atmospheric circulation structure L H H L Heating Cooling or No Heating Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind Radiation Convection Latent/Sensible Conduction

Radiation Cloud/precipitation Shallow convection Boundary layer turbulence Mesoscale convective system Thunderstorm Tornado Heat wave Midlatitude cyclone Tropical cyclone Diurnal variation Madden-Julian Oscillation Tropical waves Annular modes 100,000yr100yr10yr1yr1mon1day1min1sec sec Global Climate System Globe Continent State City Football field 1 mm 1  m Spatial Scale Time Scale  m Composition Monsoon El Nino Biennial Oscillation Global warming Multi-decadal Oscillation Ice age Glacial cycle

Convective systems Tornadoes: about m, last 1 minute to 1 hour Tornadoes: about m, last 1 minute to 1 hour Thunderstorms: about 10 Km, last 10 minutes to a couple of hours. 3 types: ordinary, multicell, supercell Thunderstorms: about 10 Km, last 10 minutes to a couple of hours. 3 types: ordinary, multicell, supercell Mesoscale convective systems (MCSs): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 Km or more in at least one direction, and often last for several hours to a couple of days. Mesoscale convective systems (MCSs): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 Km or more in at least one direction, and often last for several hours to a couple of days.

Thunderstorms A storm containing lightning & thunder A storm containing lightning & thunder Convective; form when warm, humid air rises in conditionally unstable environment Convective; form when warm, humid air rises in conditionally unstable environment The warmer the rising air parcel is relative to environment, the more buoyant force is driving it upwards (stronger convection) The warmer the rising air parcel is relative to environment, the more buoyant force is driving it upwards (stronger convection) Trigger to start uplift: warming sfc, terrain (orography), converging sfc winds, frontal zones, divergence aloft (or combination) Trigger to start uplift: warming sfc, terrain (orography), converging sfc winds, frontal zones, divergence aloft (or combination)

Three stages have been identified in ordinary thunderstorms: a)an unstable atmosphere and vertical updrafts keep precipitation suspended b)MATURE: entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts c)DISSIPATING: weakening updrafts and loss of the fuel source after 15 to 30 minutes. Thunderstorms I. Ordinary 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. Thunderstorms II. Multicell Storm

A Multicell Thunderstorm

Storms producing a minimum of a) 3/4 inch hail and/or b) wind gusts of 50 knots and/or c) tornado winds, classify as severe. Thunderstorm III. Supercell Storm

Formation of supercell thunderstorms 1. Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. 2. Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a mesocyclone. 3. Most strong and violent tornadoes form within this area of strong rotation.

Vertical structure of a supercell thunderstorm In ordinary storms, the downdraft and falling precipitation cut off the updraft. But in supercell storms, winds aloft push the rain away and the updraft is not weakened and the storm can continue maturing and maintain its structure for hours. In ordinary storms, the downdraft and falling precipitation cut off the updraft. But in supercell storms, winds aloft push the rain away and the updraft is not weakened and the storm can continue maturing and maintain its structure for hours.

Cloud structure of a supercell thunderstorm

Video: All Alone in the Night - Time-lapse footage of the Earth as seen from the ISS AqZ5g AqZ5g AqZ5g AqZ5g

Lightning: Introduction Lightning is a discharge of electricity, a giant spark 80% of lightning occurs when clouds discharge electricity. This is referred to as cloud-to-cloud lightning; occurring when voltage gradient overcomes the electrical resistance of the air. 20% of lightning occurs when electrical discharge travels between the base of the cloud and the surface. This is referred to as cloud-to-ground lightning.

Video: Lightning in super slow motion weSU8 weSU8 weSU8 weSU8

1. Charge separation. Charge layers in the cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal when they collide with each other. 2. Stepped leader. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms. 3. Return stroke. A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud. 4. Dart leader. Not all of the first stroke neutralizes the negatively charged ions and results in another leader in 1/10 of a second Processes of Lightning Formation

Thunder Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder.Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder. It takes about 3 seconds for thunder to travel 1 kilometer (5 sec per mile).It takes about 3 seconds for thunder to travel 1 kilometer (5 sec per mile). A lag in lightning strike and thunder occurs due to sound traveling slower than light. When thunder is farther away, the echoing of sound waves off of objects (like buildings and hills) causes thunder to sound rumbling.

Global distribution of lightning strikes Significant difference between continent and ocean

Lightning Safety Practice the following when lightning is present: Always take cover in a building.Always take cover in a building. Do not stand under a tree or other tall object that might serve as a lightning rod.Do not stand under a tree or other tall object that might serve as a lightning rod. Avoid standing on mountain summits, ridges, rooftops, or other high areas.Avoid standing on mountain summits, ridges, rooftops, or other high areas. Avoid caves.Avoid caves. Avoid open water (pool, lake, or hot tub).Avoid open water (pool, lake, or hot tub).

Summary 1.The general size and lifetime of mesoscale convective systems, thunderstorms and tornadoes. 3 types of thunderstorms. 2.3 stages of the ordinary thunderstorms. 2.3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft. 3.Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions. 4.3 stages of the supercell thunderstorms. 4.3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened. 5.Two types of lightning (cloud-to-cloud 80%, cloud-to- ground 20%) 6.4 steps of lightning development. How fast does thunder travel?

Works cited hiking-rocky-mountains.htm hiking-rocky-mountains.htm hiking-rocky-mountains.htm hiking-rocky-mountains.htm der_acoustics der_acoustics der_acoustics der_acoustics