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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. Thcloud 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 •
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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-
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. does and hail. Thunderstorms typically extend 4 to 8 miles above the freezing level. Thus, if the level is closer to the ground, as it is thunderstorm is recorded at a weather in fall, spring, and winter, the height of station if one or more peals of thunder the thunderstorm is not as great,_ are heard. particularly since there is much less of it below the freezing level. A Look Inside. Visual and radar ob- servations of thousands of thunderstorms In that area of a thunderstorm above studied by the Water Survey give great the freezing level, we find the charge insight as to the dimensions of these centers from which lightning is storms. In the summer, a typical Illinois generated, and the zone of the storm thunderstorm is from 8 to 12 miles tall where upcoming air is condensed and rain and about 8 to 1S miles wide. It is often or sometimes hail is formed. This zone shaped like a large cylinder with a diam- ranging from 1 to 3 miles above the eter about the same size as its height. freezing level is truly the "rain factory" of a thunderstorm. It is also a very dan- A radar cross section of the inside of a thunderstorm will show varying rain in- gerous place for any aircraft to fly be- cause it is an area of strong air motions tensities. High intensities indicate heavy rain rates and sometimes indicate hail. and large volumes of water, ice, hail, and lightning. Figure 1 reveals that the height of thun- derstorm echoes relates to storm produc- What Feeds The Storm? A thunder- t ion on a given day. When echoes exceed storm can be thought of as a large cloud 50,000 feet in Illinois and other parts of machine that behaves like a vacuum the Midwest, the probability of severe cleaner. A thunderstorm feeds on warm hail or tornadoes increases greatly. Ra- air and moisture, and much of the air is dars with doppler capability and multiple pulled up from near the ground.
5 ~ 3:00 PM w w so:------...... 0 40 (/) J a z ~ 0:c ~ I ~ 10 I ~ :c ~ w "L_ :c tracks 35 40 30 35 40 30 35 40 30 35 40 ot NAUTICAL MILES lo11s FIGURE 3. Radar cross sections through WA a thunderstorm show the growth and dis- MO sipation of 5 convective cells inside the storm during a little over 20 minutes. Al ...... wa~m As a cumulus cloud grows (see Figure ~1r \ 3), it entrains warm moist air near the ·-, surface. As this air rises, the cloud THE grows vertically and the moist humid air "FLYING WEDGE" / inside condenses to form tiny cloud drop- .. _ -~ lets. As the cloud grows higher and goes FIGURE 4. How conflicting air mosses meet, with cold ai above the freezing level, some of this the warm moist air and often producing severe thunderst eostem two-thirds of the U.S. moisture is converted to ice particles and large water drops, both of which lead to the formation of rain. Sometime af- which are ter the cloud goes above the freezing change, a thunderstorm can last 3 or 4 masses of level, we begin to find the development hours and travel 100 miles. However, moisture bo of electrical charges inside the cloud. most thunderstorms in Illinois last from are a primar When these become strong enough, light- 30 to 90 minutes and cover anywhere occur. Whyl ning discharges occur. Then we have a from 20 to SO miles in their lifetimes. thunderstorm. Thunderstorms typically consist of 3 or Meteorol more active cells (a cell is a rising col- moving air r The size, duration, and violence of umn of air and moisture), with one cell ing into othi a thunderstorm are partially determined dissipating, one at maturity, and one or triggers of by its source of warm moist surface air. more youthful cells growing along its \ of the atmo If there are large amounts of this avail- flanks. represents v. able, and nothing limiting the vertical \ hitting warn growth of the thunderstorm in the atmo- Along its ofl sphere, the storm can grow rapidly ( 10 Meteorological Factors lifts the war minutes to go 2 miles up) and reach very Causing ThWlderstorms ulus clouds tall heights. Most thunderstorms need a good sup- Thus, we ge1 Some thunderstorms in Illinois have ply of warm moist air near the ground, disturbance tops of '12 to 13 miles above the surface. but clouds will not develop into thunder- miles to ge If the supply of air and the wind field storms unless there are other key atmo- Often they aloft that is pushing a storm do not spheric conditions present. Fronts, times they a1 6 ,------~------~
Quite often our most violent thunder- storms come with "squall lines." A squall line is a rapidly moving, organized line of active thunderstorms typically 100 to 150 miles ahead of a cold front. The squall line forms as the warm unstable air ahead of the cold front is forced rap- idly upward. Illinois typically has 10 to 15 squall lines a year and they always produce thunderstorms. Furthermore, many of these squall line storms produce severe weather such as tornadoes. Fig- ure 5 shows radar echoes aligned along a squall line. Some thunderstorms occur without frontal passages. They are labeled "air mass thunderstorms." These occur on a few days during the summer as a result
EXPLANATION TIME. CST r masses meet, with cold air wedging up D 1,,s,.m. Joliet 0 producing severe thw,derstorms in the l!lllllm 1:33a.m. ~ l:48a.m. ~ which are the boundaries between air masses of differing temperature and moisture both at the surface and aloft, are a primary place where thunderstorms occur. Why? Peoria 0 If Meteorologists know that fast or slow moving air masses (see Figure 4), wedg- I I O ------I b ing into other air masses, are the major triggers of changes in vertical motions I u· /-R~toul t of the atmosphere. A cold front simply 0$.0 I represents where a wedge of cold air is Ch•mpa,gn 270' o,0- 0 \ \ hitting warmer (usually more moist) air. 60 nm -40--20' - -. Along its often wiggly edge, the cold air Decatur lifts the warm air and helps trigger cum- Springfield 0 I 0 ' ulus clouds and then thunderstorms. I ____ / Thus, we generally have an atmospheric disturbance extending over hundreds of FIGURE 5. Radar echoes aligned along miles to get thunderstorms in II linois. a squall line showing a time sequence in Often they are cold fronts, but other one storm. Squall line thunderstorms times they are warm fronts. usually bring severe weather.
7 I/ Temporal Characteristics of Storms 7 Thunderstorms occur with frontal conditions during all times of the year, but they are most prevalent in summer. 6 Illinois typically has thunderstorms somewhere in the state during half of all 5 summer days. On the average, Illinois 1-z has thunderstorms somewhere in the w state on 90 days of each year. ~4 w Figure 6 illustrates the monthly fre- a. quency of thunderstorms at Chicago, 3 Springfield, and Cairo. This shows that locations farther south in Illinois have an earlier peak in thunderstorm activity 2 in the spring; they also experience more winter season thunderstorms than does 1 northern Illinois. 12 J There is also a notable variation in thunderstorm occurrences with time of the day. Figure 7 shows the hourly fre- quency of thunderstorms in Illinois. The importance of surface heating is reflec- ted in the late afternoon maximum and the cool mid-morning minimum. The thunderstorm daily distribution also shows a tendency for a second peak (/) of nocturnal (night-time) thunderstorms. >- cf An increase in the low-level winds often 0 occurs in the central plains during the a: FIGURE 6. This shows the average num- w summer night-time hours. This wind pat- 0 ber of thunderstorm days in a year across tern, or low- level jet as it is often z Illinois, and the monthly frequency of ::> called, can lead to the initiation of J: thunder days at Chicago, Springfield, and I- Cairo. thunderstorms in the central Midwest u. during the late night hours. Thus Illinois, 0 a: of strong localized heating when there and particularly the western sections of w the state, experiences a secondary peak Cll is sufficient moisture aloft. A hot mug- ~ gy summer day can lead to development of thunderstorm activity during those :> hours. z of isolated cumulus clouds over par ts of 3 Illinois, and a few of these reach suffi- Another dimension of the temporal cient height to become thunderstorms. aspect of thunderstorms is their frequen- l These are usually very isolated with pos- cies over long periods. Figure 8 shows sibly one or two in a county and no the number of days with thunderstorms others in Illinois. They do not persist at two weather stations in or near Illinois very lonq. As soon as the atmosphere over the 80-year period from 1901 i starts cooling off in the late afternoon, through 1980. In the northern part of the F/( air mass thunderstorms begin to die. state, thunderstorms have been gradually Se e"l 8 ere r--- I
7 ,ntal rear, 6 mer. orms >fall 5 linois I-z the • w ~4 w , fre- ~ cago, 3 : -~ Moline , that . have 2 :tivity more 1 I does 12 AM 2 4 6 8 10 12 PM 2 4 6 8 10 12 TIME, CST :ion in FIGURE 7. Hourly occurrences of thwtderstorms at Moline, me of Springfield, and Evansville, Indiana. The peak hours for thwtderstorms ly fre- in nlinois are in late afternoon and early evening. s. The reflec- 560 1m and 540 I ' '\ I '\ /' ·ibution 520 I \d peak (/') / >- I ' ...... / '\ storms. <( ...... / 500 ' -./ \ Is often 0 a:: \ ing the w 480 0 \ Evansville ind pat- z s often ::> 460 ' J: v ,tion of I- \ u. 440 ,_ Midwest 0 Illinois, er ...... w 420 ...... :tions of a) -- 1ry peak ~ ::> 400 19 those z 380 temporal · frequen- 360 : 8 shows ~er storms 340'----~~-J..~~~-1-~~--''---~~---'--~~~-'-~~---L~~~~ 1901- 11- 21- 31- 41- 51- 61- 71- :ar II linois 1910 20 30 40 50 60 70 80 ·om 1901 FIGURE 8. Thwtderstorm frequencies over 80 years at Chicago, repre· 1art of the senting northern nunois, and at Evansville, Indiana, representing south- , gradually em nlinois. The recent increase in thunder days in the north and de- crease in the south are indicated.
9 provide days a year (see Figure 6), while in the Some te north they occur only about 40 days a Bloom1n9ton maps so d year. This difference relates to the the th regional difference in temperatures televisior (being warmer in the south), which are [However so critical to thunderstorms, and to the you may fact that a greater number of the thun- and othe derstorm-producing weather systems af- surge of fect southern Illinois than northern Illi- by lightn nois. Keep a ~ Areas of localized increases in storms storm.) are found in the Chicago and St. Louis If the areas. Large urban areas release more especially heat and particulates (tiny particles of developin~ o o,n,.,,, dust, soot, and many other materials) will issue than do rural areas, causing more tions (hig convection and leading to more summer ever, the FIGURE 9. This shows the pattem of thunderstorms. rainfall (in inches) from a single thunder- ning, whi storm which lasted a little over l l hours. weather-re The area of the heaviest rain (I to 2 What To Do About year. inches) also had hail. Thwlderstorms and Li~tning Most Ii Weather forecasts issued by the side activj increasing, whereas in the south, the National Weather Service will contain the garden number of thunderstorms has been indications of when the thunderstorm farm, or o decreasing. Water Survey studies show activity will occur. This is expressed in people kil that Illinois is astride two major U.S. a probabilistic manner, that is, "a SO% doors wher regions of temporal change in chance of thunderstorms today." This before the thunderstorm trends, up in the north and means that at any one place there is a storms. I down in the south, reflecting an ongoing S0-50 chance that you will have a thunderstor shift in climate. thunderstorm during the set "forecast begin to period," which is typically a 6- hour or threaten pe 1 2-hour period. proper co Vj Spatial Characteristics of Storms be alert to If the forecasters think that the thun- A thunderstorm traveling alone or in derstorms are going to be severe, the If condil a group of storms typically leaves an im- forecast will be issued with a statement seen befon print of rainfall on the ground that is S such as "Some of these thunderstorms light trave to 10 miles wide and 10 to SO miles long. may be severe, accompanied by maximum Figure 9 shows the rainfall pattern from dangerous lightning, heavy rain, hail, or miles, and t a single thunderstorm. Thunderstorms strong damaging winds." 20 to 30 m vary greatly in size due to the uniqueness faster. Thu of the atmospheric conditions on the What should one do in relation to heard, light days they develop. thunderstorm dangers? matter of m The average number of thunderstorms The first step is to be alert and keep So, the that occur across the state varies monitoring the forecasts on radio or step when I greatly. In the southern part of Illinois television. NOAA weather radio stations to seek shelt thunderstorms occur on an average of 60 on FM now cover all areas of Illinois and
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provide continuous weather information. in the Some television stations display radar days a maps so you can watch the approach of to the the thunderstorm cells on the ratures television-transmitted radar patterns. ich are [However, in violent electrical storms, to the you may wish to unplug the television e thun- and other expensive appliances, as a ims af- surge of electrical power over lines hit 1rn llli- by lightning can burn out appliances. Keep a battery radio on to monitor the storms storm.) , Louis If the storm is nearing your area, and e more especially if very severe conditions are des of developing, the National Weather Service terials) will issue "warnings" for specific condi- more tions {high winds, tornadoes, etc.). How- .um mer ever, the danger will most often be light- ning, which is the leading cause of Multiple Thunderstornus weather-related deaths in the U.S. each Here we see patterns of rainfall (in year. inches) from a series of thunderstorms on a summer day. The Most lightning danger relates to out- data are from a network of 200 by the side activities, whether it's working in recording raingages (each 3 miles contain the garden, playing golf, working on the apart) in central ntinois that was op- !rstorm farm, or children playing outside. Most erated by the Water survey for 15 years. The thundersto,.ms vary in issed in people killed by lightning are out of doors where they assume they are safe size and are elongated, reflecting "a 50% their movement from the west. The " This before the approach of existing thunder- patterns show cores of heavier rain tre is a storms. However, newly developing in the center of these thunderstorms. ,ave a thunderstorms in clouds overhead may Localized hail also occurred in or orecast begin to produce lightning that will near the rain cores. iour or threaten people before they can get to proper cover. Everyone should always be alert to that possibility. inside. If you are outside, get inside a e thun- home or large building, or inside an all- If conditions permit, lightning will be re, the metal (not a convertible) vehicle. Avoid seen before thunder is heard because tement using the telephone, except in emergen- light travels faster than sound. The ·storms cies. id by maximum audibility of thunder is 6 ,ail, or miles, and thunderstorms typically move If caught outside, follow these rules: 20 to 30 miles an hour, and sometimes • Don't stand under trees or other faster. Thus, once the first thunder is tal I objects - they act as natural tion to heard, lightning can be near you in a lightning rods. matter of minutes or seconds. • Get off and away from open 1d keep So, the second and most important water, tractors and other metal dio or step when a thunderstorm threatens is farm equipment, or small metal tat ions to seek shelter. If you are at home, stay vehicles such as motorcycles, ois and 11 bicycles, and golf carts. Put down Where to Get More Information golf clubs. • Avoid projecting above the The Illinois State Water Survey has surrounding landscape, for made extensive studies of thunderstorms. example, a hilltop, open field, or If you desire more information about beach. these storms, please write to: Illinois • Stay away from wire fences, State Water S1.rvey, 2204 Griffith Drive, clotheslines, metal pipes, or rails. Champai~, Illinois 61120. Please • Groups of persons in open or ex- indicate what additional information posed areas such as hikers or concerning thunderstorms you would like campers should spread out, to have. staying several yards apart. • If you are caught out in an open If you desire more information about field and feel your hair stand on thunderstorm forecasts or what to do for end, lightning may be about to protection in relation to thunderstorms, strike you. Drop to your knees you can contact the National Weather and bend forward, putting your Service Forecast Office, O'Hare Office hands on your knees. DO NOT lie Building #2, Room 610, 10600 W. Higgins flat on the ground. Road, Rosemont, Illinois 60018.
This is the eleventh in a series of pamphlets describing in popular language our research findings about water resources and weather in Illinois and current issues concerning them.
December 1985
Stanley A. Changnon is a climatologist and Chief Emeritus of the Illi- nois State Water Survey. This brochure is based in part on his numer- ous studies concerning severe weather phenomena. Some of the infor- mation included was taken from materials supplied by the National Oceanic and Atmospheric Administration, National Weather Service. This brochure was edited and designed by J. Loreena Ivens, Head of the Survey's Communications Unit, and was reviewed by Steve Hilberg, Gail Taylor, and John Vogel. Graphics were provided by John Brother and Linda Riggtn.
State Water Survey Division Illinois Department of 2204 Griffith Drive Energy and Natural Resources Champaign, Illinois 61820 217 /333·2210 Publications office: 217 /333·8888