Chapter 10 Thunderstorms II Met 10 Damaging winds from thunderstorms A downburst is a downdraft that spreads out horizontally from the base of a thunderstorm A downburst with winds extending only 4 km or less is termed a microburst Microbursts are capable of blowing down trees Even with their small size microbursts can have winds as strong as 146 knots 168 mph Microbursts are responsible for several airline crashes Flying into a microburst At position a the pilot encounters a headwind at position b a strong downdraft and at position c a tailwind that reduces lift and causes the aircraft to lose altitude Dust clouds rising in response to a microburst near Denver Colorado Tornadoes Tornadoes A tornado is a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground A tornado s circulation is present on the ground either as a funnel shaped could or as a swirling cloud of dust and debris Sometimes called twisters or cyclones tornadoes can assume a variety of shapes and forms that range from twisting rope like funnels cylindricalshaped funnels A funnel cloud is a tornado that has not reached the ground Tornadoes A majority of North American tornadoes rotate counterclockwise about their central core of low pressure The diameter of most tornadoes is between 100 and 600 m 300 2000 ft although some are a few meters wide and others have diameters exceeding 1 mile Winds in tornadoes are very destructive Most have winds that are 125 knots or less but the most powerful have winds up to 220 knots Tornado Winds Table 10 2 p 288 Total destruction caused by an F5 tornado Oklahoma on May 3 1999 Fig 10 32 p 288 The total wind speed of a tornado is greater on one side than on the other When facing an on rushing tornado the strongest winds will be on your left side A powerful multi vortex tornado with three suction vortices Fig 10 31 p 287 Tornado Occurrence Average number of tornadoes during each month in the United States Fig 10 29 p 285 Tornado incidence by state number of tornadoes reported by each state during a 25 year period The lower figure is the average annual number of tornadoes per 10 000 square miles Fig 10 28 p 285 Table 10 1 p 287 1 Spinning vortex tubes created by wind shear 2 The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm Features associated with a tornado breeding supercell thunderstorm A tornado spawning supercell thunderstorm A hook echo in its rainfall pattern on a Doppler radar screen The colors red and orange represent the heaviest precipitation Fig 10 36 p 290 A classic tornadic supercell thunderstorm showing updrafts and downdrafts Fig 10 37 p 291 Fig 10 38 p 291 Fig 10 39 p 292 Nonsupercell Tornadoes a Along the boundary of converging winds the air rises and condenses into a cumulus congestus cloud At the surface the converging winds along the boundary create a region of counterclockwise spin b As the cloud moves over the area of rotation the updraft draws the spinning air up into the cloud producing a nonsupercell tornado or landspout Fig 10 40 p 293 Doppler radar display of winds associated with the supercell storm that moved through parts of Oklahoma City during the afternoon of May 3 1999 The close packing of the horizontal winds blowing toward the radar green and blue shades and those blowing away from the radar yellow and red shades indicate strong cyclonic rotation and the presence of a tornado Fig 10 41 p 294 Graduate students from the University of Oklahoma use a portable Doppler radar to probe a tornado near Hodges Oklahoma Figure 3 p 295 Lightning and Thunder Lightning and Thunder Lightning is simply a discharge of electricity a giant spark which usually occurs in mature thunderstorms Lightning may take place within a cloud from one cloud to another from a cloud to surrounding air or from cloud to the ground Majority of lightning strikes occur within the cloud while only 20 strike the ground A lightning stroke can heat the air to 30 000 C This extreme heating causes the air to expand explosively thus initiating a shock wave that becomes a booming sound wave called thunder that travels outward in all directions from the flash The lightning stroke can travel in a number of directions Within a cloud from one cloud to another cloud from a cloud to the air or from a cloud to the ground Notice that the cloud to ground lightning can travel out away from the cloud then turn downward striking the ground many miles from the thunderstorm Electrification of Clouds Clouds become electrified during the formation of precipitation when regions of separate charge exist within tiny cloud droplets and larger precipitation particles When falling precipitation collides with smaller particles the larger precipitation particles become negatively charged and the smaller particles become positively charged Updrafts within the cloud then sweep the smaller positively charged particles into the upper reaches of the cloud while larger negative charged particles settle toward the lower and middle parts of the cloud The generalized charge distribution in a mature thunderstorm Fig 10 21 p 279 The Lightning Stroke Because unlike charges attract one another the negative charge at the bottom of the cloud causes a region of the ground beneath it to become positively charged As the thunderstorm moves along this region of positive charge follows the cloud like a shadow The positive charge is most dense on protruding objects such as trees poles and buildings The difference in charges causes and electrical potential between the cloud and ground The development of a lightning stroke a When the negative charge near the bottom of the cloud becomes large enough a flow of electrons the stepped leader rushes toward the earth b As electrons approach the ground a region of positive charge moves up into the air through any conducting object such as trees buildings and even humans c When the downward flow of electrons meets the upward surge of positive charge a strong electric current a bright return stroke carries positive charge upward into the cloud A cloud to ground lightning flash hitting a 65 foot sycamore tree It should be apparent why one should not seek shelter under a tree during a thunderstorm Lightning Brush Lightning protection Cloud to ground lightning strikes in the vicinity of Chicago Illinois Detected by the National Lightning Detection Network The four marks on the road surface represent