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Thunderstorms and Lightning • • • Illinois' Most Awesome Weather by Stanley A

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Thunderstorms and Lightning • • • Illinois' Most Awesome Weather by Stanley A r DEPOSITORY APR 3 0 1986 UNJ\1£RSITY OF IU.lillOIS !\T UR13AtP l'}'-iAMPA'GN Thunderstorms and Lightning • • • Illinois' Most Awesome Weather By Stanley A. Changnon Illinois State Water Survey No other atmospheric event in Illinois carries with it the beauty, fear, and importance that thunderstorms produce. Thunderstorms are wonderfully complex natural machines that convert moisture and energy into rainfall, lightning, and many other severe weather phenomena that we all fear including tornadoes, heavy rainstorms, straight line winds, and hail. Thunderst1 ligh' QCS .C4 The Importance of Ttumde-rstorms 19( Thunderstorms are extremely impor- The Thundentorm As A Threat tant to the climate of the world and to the weat her of Illinois. A forecast for severe thunderstor ms causes many people to feel somewhat The atmosphere is a giant global cir- uneasy. Thunder stor ms produce all cuit. The electrical charge on the earth forms of what meteorologists call is generally negative and that in the "severe local storms." This includes higher atmosphere is generally positive. straight line winds that damage crops There is a leakage of current between and property, hail that ruins c,:ops, tor- these two "attracting forces." Thunder- nadoes, heavy rains that lead to flash storms help maintain the global electri- floods, and lightning-induced deaths and cal circuit by providing the connection, fires. which we see as lightning discharges be- Strai~t line winds result from air tween the atmosphere and the ground. brought down by the heavy rain in a These discharges in thunderstorms thunderstorm. This cold air sweeps out are occurring somewhere around the of a storm and reaches the ground, often world during every minute of every day, at a high speed, where it spreads out as helping to maintain the "electrical bal- a strong gust capable of doing damage ance" in the global circuit. to property and trees. Straight line Scientists at the Water Survey are at- winds can be nearly as destructive as tempting to unravel the great complexi- tornadoes. ties of the global circuit since it greatly Hail is formed high aloft in thunder- affects our understanding of how the storms by raindrops that are frozen. weather and climate of the earth func- These grow if strong winds going up in- tion. Breakthroughs in these studies will side a thunderstorm (called "updrafts") help us eventually to do a better job of hold the stones aloft where they capture predicting today's weather, as well as to more moisture inside the storm. understand better how our climate func- Although hail can break windows, dent tions. cars, and destroy crops, damaging hail Thunderstorms have immediate and in Illinois does not occur at any one place great importance to Illinois in many ways very often. Th<i - and not just because they sometimes Tornadoes often grow along the south represent a threat to life and limb. flanks of thunderstorms when volatile It$ Cl Thunderstorms bring, on the average, cloud unstable weather conditions with fast asceri 70% of all the annual precipitation moving frontal systems somehow lead to maist received in the state. In the summer the rotation of winds inside or alongside "supe months of June, July, and August, thun- above a thunderstorm, and to the formation of the 1 derstorms produce 90% of all rainfall. a funnel. People who live in Illinois are ra111d1 In light of these facts, thunderstorms aware of the great destruction of life Ol'1 tt are very critical and essential to the hy- and property that a severe tornado can devel, some drologic cycle and thus to the water re- cause, but as with hail, tornadoes do not to tht sources that the state of Illinois enjoys. occur frequently at any one place. releai crouda 2 Thunderstorms and lightni QC968 .C473 ANVIL WHERE ..IR AND ICE 1985 . TOP Of STOA,.. ARE LEAVING THE STORM ICE PARTICLES • lS tt 111 PQSI TIV[ Summer 111 Cl ~CTRICAl CHARGE Thrmderstorm I· )S RAIN AND H ..ll in Illinois r- DEVELOPING ;h 6 · td I ir a It :n 4 IS ie 1e fRFEZING LCVEL AT J MILES IS ,-l, ") 1• ·e~. lt ii ----~--==------ -- ·e ~ ( A ( f I ci HII o\l C IAR(j( Those Wonderful Weather Machines! h The workings of a mature thunder.Harm reveal Once the roin proce" has bequn, a shaft of cold Its complex machinery. /nit1allv air flows mta the ram de~cends rapidly. ft literally drags cold air cloud bottom bringing heat and moisture. The from aloft tn the storm downi.iard to the ground. ascending air po.~ses by the freezinq level but the nus "do...,ndra(t" rs dangerous to aircraft landing and moisture doe., not freeze nght ai.·ay anct becomes taking off, and as this air reaches the ground, it fans "supercooled.·• Ice particles appear ubout a mile out and can be ver-v damaging If the storm 1s large above rhe freezinq lf'Vel. The.~e parttdes capture and movtng fast. the water and beqtn to grow, ,·ol11de, and form lfatl is an occasional by-product of the up-<lown raindrops. :\ t this stage elpctrical charges appear cycle. If the updraft is strong, the small mushv ice on the droplets and eventually "chorqe centers" balls in the nrain factor-v zone" are suspended, allow· develop. These in rum lead to liqhtntng discharges, mg them to frpeze, and they form multiple layers sometimPs within the cloud, or between clouds, or as new upward surges of air furnish the moisture for to the ground as ~hown. ln this manner the .~trokes growth. When thpir we1pht exceeds the updraft relea.~e rhe electrical energy accumulated in the strength, down comes a volume of hail mrxed in with clouds. the rain. 3 Heavy rains from thunderstorms oc- 100 ,,,.- cur from two principal situations. First, ,,, ,,, - a series of average size thunderstorms 90 / cross the same place in a matter of a I I few hours, leading to several inches of I 80 THUNDER DAYS / rain. Second, a very large and slow mov- .... ing thunderstorm exists over an area and , I the storm generates heavy rainfall in a 70 I few hours at that location. Such heavy I rains can cause damaging flash floods I 60 I that may wash out a crop or flood base- I ments and streets. I I lightning is the most dangerous se- 50 I vere storm phenomenon produced by I thunderstorms, at least to human life. I 40 I Meteorologists classify lightning in I thunderstorms as "cloud to cloud" I FlGU lightning which is exchanged between JO I I weatl thunderstorms, or as "cloud-to-ground" I dual · lightning which goes from a charge I tee ti< 20 I center in a thunderstorm to some point I SEVERE does4 at the ground where a strong negative I THUNDER DAYS charge has been built up. 10 I TORNADO OA VS I y· The cloud-to-ground lightning is of ~ ............ ....,/ ··· thunde I .·· station particular concern to Illinois citizens. 0 /. 0 10 are hec Each year Illinois averages 10 deaths due 20 30 40 50 60 70 to lightning, more than by tornadoes. MAXIMUM ECHO HEIGHT. rhou,.nds of 1.. , Al Lightning also produces damages to FIGURE 1. This shows daily severe servati property of over a million dollars a year. weather probabilities as a function of studiec Lightning can start fires and cause dam- ma.rimum radar echo height. For exam- insight age to electrical circuits and home ap- ple, when echoes on any day exceed storms pliances. 30,000 feet, there is an 8096 chance of thunde thunderstorms occurring. When the and ab, Records show that lightning damages echoes exceed 50,000 feet, the chance to property and trees occur most often for severe weather increases rapidly. shaped in rural areas. The density of buildings eter at in major metropolitan areas collectively Ar help to cause leakage of charge between to do about lightning" as well as other a thunc the earth and the atmosphere. But in threats from thunderstorms. tensitii rural areas, there is greater prevalence rain r;; of cloud-to-ground lightning that hits What Are ThWlderstorms? Figure structures like farm houses, barns, de rs tor churches, and schools. Thunderstorms are very large cumu- tion on lus clouds that by definition lead to Lightning protection, such as light- 50,000 lightning discharges. It is the sudden the Mi ning rods, in rural areas is an important tremendous heat from the lightning consideration for rural structures. Later hail or stroke that causes the shock waves that dars wi in this brochure we will talk about "what we hear and refer to as "thunder." A 4 wavelengths like the Survey's radar (Figure 2) can better detect the beginnings of tornado and hail conditions. In the spring, fall, and winter seasons, thunderstorms typically are not quite as large as those in summer. This results because the freezing level is closer to the ground and the vertical air motions that help develop thunderstorms are not as strong in the colder seasons as they are in summer. The freezing level, that level of the atmosphere where the temperature is 32°F {0°C), is an important reference point in our thunderstorms. Much of a thunderstorm exists above that level in a zone where the rain is formed. In FIGURE 2. The Water Survey's research weather radar has doppler capability and summer, the freezing level is about 3 dual wavelengths that provide better de- miles above the earth, but in the fall and tection of severe storms such as torna- spring it is only about 1 mile up.
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