Jonathan W. Slemmer Aviation Weather Center, Kansas City, Missouri

Transcription

1 P11.9 AVIATION ADVISORY CLIMATOLOGIES Jonathan W. Slemmer Aviation Weather Center, Kansas City, Missouri 1. INTRODUCTION Weather is a signifiant omponent of air traffi elays (Roenhuis 24) resulting in sustantial osts 1. The Convetive SIGMET (CSIG) Climatology (Slemmer et al. 24) has aie Feeral Aviation Aministration (FAA) evaluation of air traffi management performane regaring elays ue to thunerstorms. Aitional evelopment of the CSIG limatology has een one. In aition, the National Climati Data Center (NCDC) has taken over the routine generation of monthly onvetive frequenies (CF) that are provie to the FAA. The CSIG Climatology metho has also een applie to other aviation avisories issue y the Aviation Weather Center (AWC). Examples of these limatologies will e presente along with a isussion on their potential value in further supporting air traffi management with the intent of reuing elays. 2. DATA AND METHODOLOGY ASCII text files of eah type of aviation avisory are proesse to alulate their respetive frequeny of ourrene inluing Convetive SIGMETs (thunerstorms), Non-onvetive SIGMETs (severe iing, severe turulene), International SIGMETs (thunerstorms, tropial storms, severe turulene, volani ash), an AIRMETs (moerate iing, moerate turulene, IFR, mountain osuration). All ata is proesse in a similar manner as esrie in Slemmer (24). Aitional iagnostis were evelope suh as a mean CSIG frequeny over pre-efine areas, the frequeny of when CSIGs are near major airports, average CSIG sizes for eah month an year aross the Continental Unite States (CONUS), an the CF istriution of ifferent CSIG shapes (area, line, an isolate irles) aross the CONUS. Corresponing author aress: Jonathan W. Slemmer, NOAA/National Weather Servie, National Center for Environmental Preition, Aviation Weather Center, 722 NW 11 st Terrae, Room 118, Kansas City, MO ; Jonathan.Slemmer@noaa.gov 3. ADDITIONAL CSIG CLIMATOLOGY INFORMATION 3.1 Routine CSIG Climatology Proution Routine proution of the CSIG monthly maps is performe at NCDC an an e foun at The FAA is using this information to assess air traffi management performane. Monthly maps showing the istriution of CF an graphs showing the average CF for the map area are generate for the CONUS, four regions of the CONUS, 2 Air Route Traffi Control Centers plus the Northeastern oastal waters, an for areas within a 75 nautial mile (nm) raius aroun 2 major airports (Fig. 1a). Anomaly maps are also generate for the same areas y taking the ifferene etween a month s CF an that month s 1995 to 24 mean CF (Fig. 1). In aition, hourly an weekly CF maps an graphs may soon e ae to this we-site. The CONUS CF maps agree well with Chara (25). 3.2 CSIGs near Major Airports Figure 2 shows the frequeny of how many major airports ha at least one CSIG within a 75 nm raius for eah hour of July from 1995 to 24. From aroun minight to the late morning hours, only a few major airports ha CSIGs near them. By the late afternoon when the greatest frequeny usually ours, aroun one-thir of these airports ha CSIGs near them aout two-thirs of the time. In rare instanes, nearly three-quarters of the airports will have ative CSIGs then. During July most airports have peak frequenies aroun 22 UTC when maximum heating ourre, ut the iurnal ranges an trens vary. Atlanta (ATL) CSIG ourrenes (Fig. 3a) were strongly riven y the iurnal yle with nearly half the time having CSIGs near the airport aroun peak heating an very few ourrenes in the late morning. ATL ha the least CSIG ourrene near 1 Data from the Air Transport Assoiation, MIT Linoln Las, an National Researh Counil availale at the following Cooperative Program for Operational Meteorology, Euation, an Training (COMET) we-site:

2 Figure 1. June 24 Convetive SIGMET Frequeny (CF) maps an graphs (ar graph y month from January 1999 to May 25): a) Chiago O Hare (ORD) CF map an CF average value ) Chiago O Hare (ORD) anomaly CF map an CF average anomaly value. the airport of these four airports in the early morning. New York La Guaria (LGA) has a similar pattern as ATL although less extreme (Fig. 3). Chiago O Hare s (ORD) CSIG ourrene near the airport is more evenly istriute throughout the ay ue to more noturnal onvetion events (Fig. 3). Dallas/Fort Worth (DFW) ha the least overall CSIG ourrene uring July of these four airports sine a sutropial high usually persists over the region uring July resulting in a suppression of onvetion (Fig. 3). There also appeare to e a seon well efine peak in CSIG ourrenes in the late morning whih may e a result of noturnal onvetion that originate further to the northwest having finally arrive into the area. 3.3 CSIG an Weather Anomalies Correlations etween weather patterns an assoiate thunerstorm evelopment have een one using the NCEP/NCAR reanalysis ata ( These orrelations may e partiularly useful when attempting to antiipate future aviation impats, espeially as the auray of long range foreasting inreases. As an example, CF s in May 24 (Fig. 4a) were muh higher than normal over the southern Great Lakes region ue to anomalous riging along the East oast an anomalous troughing over western Canaa (Fig. 4). A strong front persiste over the southern Great Lakes with aove normal lift an moisture present (not shown) resulting in muh aove Frequeny of the Numer of Major Airports (2) with CSIG(s) in the Viinity Hour (Z) Figure 2. Numer of major airports with ative CSIG(s) within 75 nm y hour for July (1995 to 24).

3 75 Atlanta (ATL) CSIG Frequeny witin 75 nm 75 New York La Guaria (LGA) CSIG Frequeny within 9 nm Hour (Z) Hour (UTC) 75 Chiago O'Hare (ORD) CSIG Frequeny within 75 nm 75 Dallas/Fort Worth (DFW) CSIG Frequeny within 75 nm Hour (Z) Hour (UTC) Figure 3. Frequeny of CSIGs within 75 nm of a major airport y hour for July (1995 to 24) a) Atlanta (ATL) ) New York La Guaria (LGA) ) Chiago O Hare (ORD) ) Dallas Fort Worth (DFW). normal onvetion. May 25 ha muh elow normal onvetion aross the eastern CONUS (Fig. 4). The height anomalies were nearly opposite as ompare to May 24 with muh of the East uner more stale northwest flow (not shown) ue to anomalous troughing along the East Coast (Fig. 4). By sujetively oing similar omparisons, it was foun that large anomalies in CF values frequently orrelate well with the anomalous weather patterns (not shown). 3.4 CSIG Issuanes an Size The numer of annual CSIG issuanes has een a Figure 4. Anomalous CF an 5-m height maps a) CONUS May 24 anomalous CF ) CONUS May 24 anomalous 5-m heights ) CONUS May 25 anomalous CF ) CONUS May 25 anomalous 5-m heights.

4 Numer of CSIG Issuane CONUS CSIG Issuanes CONUS Convetive Frequeny Perent Convetive Frequeny Square Nautial Miles Average CSIG Size (Marh - Otoer) Year Year Figure 5. a) Numer of annual CSIG Issuanes an average CF values aross the CONUS from 1999 to 24. ) Average annual CSIG size (square nm) from Marh through Otoer (1999 to 24). steaily inreasing in reent years. It was initially thought that the numer of CSIG issuanes was inreasing eause etter tehnology was allowing the foreaster to issue smaller, more preise CSIGs (Fig. 5a re ar graph). During this perio (1999 to 24), the average CF values were also inreasing at a similar tren (Fig. 5a lue line). Consequently, the numer of CSIG issuanes was mostly inreasing uring this perio eause there was more onvetion to apture. Figure 5 shows that the overall CSIG size has not hange muh from 1999 to 24 uring the onvetive season (Marh through Otoer). This information is signifiant eause the inrease in the CONUS CF in reent years may also have resulte in greater air traffi weather relate elays sine thunerstorm frequeny an e a major omponent of elays epening on time an loation of ourrene. Although 25 has yet to e assesse ompletely, initial results suggest that the CF in the Northeast an surrouning areas whih are partiularly sensitive to weather impats will e lower in 25 as ompare to reent years whih may result in overall fewer weather relate aviation elays. This information also provies some insight into the harateristis of CSIGs suh as how CSIG size relates to where an when onvetion ours. Figure 6 shows the numer of CSIG issuanes an average CSIG size y month from January 1999 to Novemer 24. The numer of monthly CSIG issuanes reahes a maximum in the summer an minimum in the winter. In general, the average CSIG size also follows this pattern with a maximum in the summer an minimum in the winter. Septemer 24 ha the largest average monthly CSIG size eause large CSIGs typially overe onvetion assoiate with three hurrianes that impate the Southeast while the rest of the CONUS ha very little onvetion. July 21 ha the seon largest average CSIG size eause the West ha muh aove normal onvetion while the East ha muh elow normal. The typial CSIG in the West is typially large ue to the more sattere nature of the onvetion as ompare to the East. M o nt hly M e a ns : J a n ,F e ,M a r ,A pr ,M a y ,J un ,J ul ,A ug ,S e p ,O t ,N o v ,D e Average CSIG Size (Sq NM) Numer of CSIG Issuanes Average CSIG Size (Sq NM) /99 2/99 3/99 4/99 5/99 6/99 7/99 8/99 9/99 1/99 11/99 12/99 1/ 2/ 3/ 4/ 5/ 6/ 7/ 8/ 9/ 1/ 11/ 12/ 1/ 2/ 3/1 4/1 5/1 6/1 7/1 8/1 9/1 1/1 11/1 12/1 1/2 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 1/2 11/2 12/2 1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 1/3 11/3 12/3 1/4 2/4 3/4 4/4 5/4 6/4 7/4 8/4 9/4 1/4 11/4 2 1 Numer of CSIG Issuanes Month Figure 6. Numer of monthly CSIG Issuanes an average CSIG size (square nm) aross the CONUS from January 1999 to Novemer 24.

5 Figure 7. CONUS CF istriution for ifferent CSIG shapes from Marh through Otoer (1995 to 25) a) areas ) lines ) isolate irles ) Frequeny CSIG is a line when a CSIG ourre. 3.5 CF Distriution of Different Shapes CSIGs an assume three ifferent shapes when esriing onvetion whih are areas, lines, or irles. Areas were the most ommon an represent approximately 87 perent of the CONUS CF istriution uring the onvetive season (Marh through Otoer) from 1995 to 25. Figure 7a shows the CF istriution of CSIG area shapes aross the CONUS uring this time. Similarly, Figure 7 shows the CF istriution of CSIG line shapes whih represent approximately 13 perent of the CONUS CF istriution. Only aout.1 perent of the CONUS CF istriution is represente y irles whih are issue for either isolate severe or isolate emee onvetion (Figure 7). The Blak Hills of South Dakota an portions of the Front Range extening from near the Wyoming/Neraska/Colorao orer region into East Central Colorao ha the highest overall isolate frequenies. The Plains were also a region of higher frequenies proaly ue to isolate severe storm initiation along ry lines. In some ases, the area aroun large ities also ha relatively high isolate frequenies. Figure 7 shows the CONUS istriution of how often a CSIG is a line when a CSIG ours. The greatest line frequeny ours in the Northeast ranging from aout 2 to 4 perent of the time whih suggests that when onvetion ours it is frequently assoiate with fronts. The least line frequeny ours in portions of the Southwest where onvetion is typially more sattere an not assoiate with fronts. Floria has higher lines frequenies along oasts where lan an sea reeze interations are the greatest versus the inlan areas. 4. OTHER AVIATION ADVISORY CLIMATOLOGIES The CSIG limatology metho has also een applie to other aviation avisories issue y the AWC suh as Non-onvetive SIGMETs (severe iing an severe turulene), International SIGMETs (thunerstorms, severe turulene, tropial storms, an volani ash), an AIRMETs (moerate iing, moerate turulene, IFR, an mountain osuration). For aitional information on riteria use for issuing these prouts, refer to Sine these prouts are issue for perios of either four or six hours, they ten to e a len of oth oserve an foreaste onitions, whereas the CSIG is in essene an hourly oservation of signifiant onvetion. Due to the infrequent numer of issuanes assoiate with some of these prouts, some of the frequeny maps will appear loky sine the ata is not smoothe an iniviual prouts an oasionally e seen. Amenments, orretions, an anellations were ignore in orer to simplify the ata proessing. In aition, these prout issuanes, espeially AIRMETs, an e greatly influene y the quantity of pilot reports. For example, the same

6 Figure 8. Marh 22 to 24 Non-onvetive SIGMET frequenies a) severe high-level turulene ) severe low level turulene. area of weather may generate more pilot reports of more signifiant weather epening on the time of ay ( UTC versus 12 UTC) an the loation (Northeast versus the Northern Plains). This aitional aviation avisory limatology information may prove useful to the FAA in further assessing air traffi management performane. 4.1 Non-Convetive SIGMETs Non-onvetive SIGMETs are issue for a perio of four hours aross the CONUS for severe iing an turulene. Figures 8a- shows the frequenies of high-level an low-level turulene SIGMETs for Marh 22 to 24. High-level turulene SIGMETs were most ommonly issue over the entral Rokies an the Northeast (Fig. 8a). Low-level turulene tene to e most ommonly issue in the Paifi Northwest, Southern California, along an in the lee of the Rokies, an along an in the lee of the Appalahians (Fig. 8). 4.2 International SIGMETs International SIGMETs issue y the AWC over muh of the northwestern Atlanti, portions of the northeastern Cariean an the entral Gulf of Mexio, an the North Paifi Oean. International SIGMETs of thunerstorms an turulene are issue for a perio of four hours while tropial storms an volani ash are issue for a perio six hours. Figure 9a shows the tropial storm issuanes for August 25 within the Atlanti Basin omain whih is within the five areas enote y the thik lak lines. The evelopment of Hurriane Katrina an e seen in the western Bahamas an its susequent path through the eastern an entral Gulf of Mexio. In aition, a ouple of tropial storm paths an e seen in the Atlanti. Figure 9 shows the tropial storm issuanes for August 21 to 25 within the Paifi Basin omain whih is within the two areas enote y the thik lak lines. The most ommon loation for tropial systems in the Paifi Basin omain uring these five years is in the southeastern orner where eastern Paifi tropial storms our. Also, sometimes western Paifi tropial storms an trak into the extreme western portion of the a Figure 9. International SIGMET frequenies a) August 25 Atlanti tropial storms ) August 21 to 25 Paifi tropial storms.

7 Figure 1. IFR Airmet frequenies for August 21 to 23 a) UTC ) 6 UTC ) 12 UTC ) 18 UTC. Paifi Basin omain. All of these tropial storms are relatively small ompare to the tropial storms in the Atlanti Basin sine they are typially weakening as the storms usually enounter ool sea surfae temperatures in the Paifi Basin omain. 4.3 AIRMETs AIRMETs are typially issue for a perio of six hours aross the CONUS for moerate turulene an iing, IFR onitions, an mountain osuration. The synopti hour was use to esrie the time of issuane whih is asially entere aroun the typial AIRMET issuane times. Figures 1a- shows the iurnal nature of wiesprea IFR onitions from August 21 to 23. At UTC (Fig. 1a), IFR AIRMET frequenies ourre aout half the time in the California oastal waters ut is less in the Paifi Northwest an very little exists in the East with the highest ourrene in the Northeast oastal waters. At 6 UTC (Fig. 1), IFR AIRMET frequenies have greatly inrease in the East with a maximum of aroun 75 perent frequeny in the entral Appalahians an IFR onitions are also inreasing in the western oastal waters. By 12 UTC (Fig. 1), IFR AIRMETs were often issue East of the Mississippi River an ourre almost 1 perent of the time along the orer etween Virginia an West Virginia. IFR AIRMETs have also peake aross muh of the western oastal waters an have move into the inlan valleys. By 18 UTC (Fig. 1), IFR AIRMET frequenies have greatly erease in the East an have generally erease a little along the West Coast. IFR AIRMET frequenies were rarely issue if at all for the interior West, High Plains, an southern Floria. Figures 11a-f shows the AIRMET frequenies for 12 UTC for the three year perio from May 21 through April 24. The Northeast has relatively high AIRMET frequenies whih may e ue to the higher traffi volume whih results in more pilot reports of potentially more averse weather onitions. Moerate iing was most ommon in the Great Lakes/Northeast an also in the Paifi Northwest ourring as frequently as 3 to 45 perent of the year (Fig. 11a). A notieale erease in iing AIRMET frequeny ourre in the lee of the Rokies. Iing frequeny results tene to agree well with finings generate from pilot reports uring the winter season (Young, et al. 23 an Bernstein, et al. 23). Moerate low-level turulene was most ommon in the Northeast, along an in the lee of the Rokies, Paifi Northwest, an southern California (Fig. 11). The omplex terrain in these areas ontriute to the higher low-level turulene frequenies of aroun 25 perent. Moerate high-level turulene tens to follow the jet stream aroun the rige along the West oast an the trough in the Great Lakes an is more pronoune in the Rokies at aroun 25 perent an in the Northeast at aroun 2 perent (Fig. 11). The interation with the high terrain in the Rokies ontriutes to the maximum in highlevel turulene frequeny ut reasons for the

8 e f Figure 11. AIRMET frequenies for 12 UTC from May 21 to April 24 a) iing ) moerate low-level turulene ) moerate high-level turulene ) IFR e) mountain osuration f) omposite of all AIRMETs. relatively high frequeny in the Northeast are less lear an may e ue to more frequent pilot reports or a more ative storm trak region. A previous limatology of high-level turulene (Sharman, et al. 23) was iffiult to ompare to this stuy, however it i agree well with the entral Rokies having the highest frequeny. Wiesprea IFR onitions were most ommon along the Gulf Coast, the Appalahians, the Great Lakes, an the West Coast with aout a 3 to 5 perent frequeny (Fig. 11). The highest frequeny (55 perent) was loate in West Virginia an along the Georgia/Floria orer. Other notale regions of higher IFR frequeny were in the Saramento Valley an northern portions of the valleys in the inter-mountain region. IFR AIRMETs are rarely issue for the Southwest. Mountain osuration AIRMETs were issue frequently for many of the signifiant mountainous regions suh as the Appalahians, northern/entral Rokies, the Paifi Northwest, an the California oastal mountains (Fig. 11e). The southern Rokies an interior Southwest ha a notale lak of mountain osuration AIRMET frequenies. Figure 11f is a omposite of the other five figures showing the overall AIRMET frequenies. The highest frequenies of AIRMETs were along the Appalahians followe y the Paifi Northwest, entral Rokies, an Great Lakes. The lowest frequenies of AIRMETs were in southern Floria followe y the esert Southwest an the High Plains. Sine mountain osuration has a relatively high frequeny, many mountainous regions partiularly stanout. A potentially more useful way of assessing overall weather impats woul e to give ifferent weights to the parameters rather than having equal weights for all parameters whih oul also inlue SIGMETs. One of the parameters esriing where iing an turulene is ourring in their respetive avisories is a vertial range one in inrements of a thousan feet. Thus, the AIRMET frequeny of iing or turulene an e alulate in the vertial every thousan feet. Figures 12a- shows the frequeny of moerate turulene at various flight levels for Deemer through Feruary of 22 to 24. Central Colorao ha the most onsistently high overall frequenies ranging from 4 to 5 perent throughout. At 5 feet MSL (Fig. 12a), the Northeast an Mi-Atlanti ha a large area of

9 Figure 12. Turulene AIRMET frequenies at speifi flight levels for Deemer through Feruary (22 to 24) a) 5, ft MSL ) 15, ft MSL ) 25, ft MSL ) 35, ft MSL. frequenies from 4 to 5 perent an high frequenies also ourre over the Paifi Northwest, Southern California, an along an in the lee of the Rokies. At 15, feet MSL (Fig. 12), the geographial istriution of the higher frequenies ha not hange muh ut all areas exept for the Rokies ha seen ereases, espeially the Northeast whih ha only a 1 to 15 perent frequeny. At 25, feet MSL (Fig. 12), the influene of the jet stream generate more wiesprea an higher frequenies aross the muh of the CONUS an the pattern of the higher frequenies is suggestive of the typial weak rige in the West an weak trough in the Great Lakes. At 35, feet MSL (Fig. 12), not muh has hange from 25, ut the frequenies were slightly lower in the Northeast an slightly higher in the South an West possily iniative of a lower tropopause in the Northeast uring the winter. Turulene AIRMET frequenies erease quikly with inreasing altitue at aroun 4, feet MSL (not shown). Figures 13a- shows the frequeny of moerate iing at various flight levels for Deemer through Feruary of 22 to 24. Two regions that onsistently ha the highest frequenies are in the a Figure 13. Iing AIRMET frequenies at speifi flight levels for Deemer through Feruary (22 to 24) a) 3, ft MSL ) 9, ft MSL ) 15, ft MSL ) 21, ft MSL.

10 ORD (75nm raius) Low Tur AIRMET aroun UTC Monthly Means: Jan 3., Fe 31.2, Mar 37.5, Apr 41.2, May 29.4, Jun 15.8, Jul 2.3, Aug 4.6, Sep 2.5, Ot 32.9, Nov 35.5, De 22.8 Annual Means: , , , , All Years-25.3 ORD (75nm raius) High Tur AIRMET aroun UTC Monthly Means: Jan 26.1, Fe 24.1, Mar 41.5, Apr 27.2, May 28.1, Jun 27.7, Jul 17.5, Aug 12.9, Sep 17.1, Ot 22., Nov 28., De 34. Annual Means: , , , , All Years /1 6/1 7/1 8/1 9/1 1/1 11/1 12/1 1/2 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 1/2 11/2 12/2 1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 1/3 11/3 12/3 1/4 2/4 3/4 4/4 Month 5/1 6/1 7/1 8/1 9/1 1/1 11/1 12/1 1/2 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 1/2 11/2 12/2 1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 1/3 11/3 12/3 1/4 2/4 3/4 4/4 Month ORD (75nm raius) Iing AIRMET aroun UTC Monthly Means: Jan 4.8, Fe 36.4, Mar 43.7, Apr 31.1, May 29.3, Jun 1.4, Jul 3.3, Aug 2.6, Sep 12.1, Ot 35.5, Nov 34.5, De 39.4 Annual Means: , , , , All Years-25.8 ORD (75nm raius) IFR AIRMET aroun UTC Monthly Means: Jan 18., Fe 15.9, Mar 25.1, Apr 17.5, May 15.1, Jun 16.1, Jul 9.5, Aug 9.1, Sep 3.4, Ot 2.5, Nov 4.3, De 8.3 Annual Means: , , , , All Years /1 6/1 7/1 8/1 9/1 1/1 11/1 12/1 1/2 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 1/2 11/2 12/2 1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 1/3 11/3 12/3 1/4 2/4 3/4 4/4 5/1 6/1 7/1 8/1 9/1 1/1 11/1 12/1 1/2 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 1/2 11/2 12/2 1/3 2/3 3/3 4/3 5/3 6/3 7/3 8/3 9/3 1/3 11/3 12/3 1/4 2/4 3/4 4/4 Month Month Figure 14. Graphs showing the average AIRMET frequenies for the area within 75 nm of Chiago O Hare (ORD) y month for UTC from May 21 to April 24 a) moerate low-level turulene ) moerate high-level turulene ) moerate Iing ) IFR. Northwest an the Great Lakes region, although the Great Lakes region area migrates to the southeast with inreasingly higher altitues. At 3 feet MSL (Fig. 13a), the highest frequeny area extene from the upper Mi-West southeastwar into the Northeast with highest frequenies of aroun 4 to 5 perent in the lee of the Great Lakes. The far Northwest ha frequenies as high as 25 perent. At 9 feet MSL (Fig. 13), the geographial istriution of the area in the Great Lakes region ha not hange ut the frequenies are slightly less while the frequenies in the Northwest have inrease sustantially. At 15, feet MSL (Fig. 13), the Great Lakes region ha migrate to the southeast entere over the entral Appalahians as higher altitue temperatures ha eome too ol to support ie in the upper Mi-West an the highest frequenies have erease to aroun 3 perent. The area in the Northwest ha further expane to over muh of the West exept for the esert Southwest an peak frequenies are in western Oregon an Washington ranging from 5 to 6 perent. At 21, feet MSL (Fig. 13), temperatures were frequently too ol to support ie aross the CONUS, so a signifiant erease in frequenies ha ourre with peak frequenies of 1 to 15 perent in the West an Southeast. The former Great Lakes iing region in the lower altitues has shifte to the Southeast where freezing levels are higher. The same metho of alulating the average areal frequeny of CSIGs has een applie to AIRMETs. Figures 14a- shows the average areal frequeny of AIRMETs within a 75 nm raius of Chiago O Hare (ORD) y month for UTC from May 21 to April 24. The peak in the frequenies for all AIRMETs ourre from Otoer through May with all AIRMETs exept for IFR with monthly averages aroun 3 to 35 perent frequeny an peak monthly frequenies as high as 5 to 6 perent. IFR frequenies uring this time ha an average monthly frequeny of 15 to 2 perent with peak monthly frequenies aroun 25 perent. Harly any AIRMETs were ever in effet uring the summer months with the exeption of high-level turulene whih ha aroun a 1 perent frequeny. 5. DISCUSSION When greater than normal aviation avisory frequenies our over sensitive air routes or near major airports, air traffi impats may inrease. The CSIG limatology is eing use y the FAA an other entities within the National Oeani an Atmospheri Aministration an the airlines to assist in evaluating air traffi management performane with the intent of improving future air traffi management eision making. Sine Slemmer (24), aitional tehniques have een evelope to analyze aviation avisory limatology information in orer to further support air traffi management. Resultant mitigation of air traffi weather impats oul lea to sustantial ost savings. In the future, the aviation avisory

11 limatologies oul e inorporate into an air traffi management matrix that woul generate reommenations on when the est times woul e to use or avoi partiular air traffi routes or major airports in orer to mitigate aviation weather impats. Correlations etween weather patterns an aviation avisory limatologies may also prove useful in preiting whih air traffi routes or major airports woul e the most impate y averse weather in the longer range, espeially as the auray of long range foreasting improves. 6. ACKNOWLEDGEMENTS The author wishes to thank Steve Silererg of the AWC for reviewing this manusript. The author also wishes to thank Jay Lawrimore an Jesse Enloe of NCDC for managing the routine proution of the CSIG Climatology. 7. REFERENCES Bernstein, B. C. an MDonough, F. 23: An Inferre Iing Climatology Part II: Applying a Version of IIDA to 14-years of Coinient Sounings an Surfae Oservations. 1 th Conferene on Aviation, Range, an Aerospae Meteorology. J1.6, pp. 4. Chara, J., an Liang, F. 25: Automate Twohour Thunerstorm Guiane Foreasts. Conferene on Meteorologial Appliations of Lightning Data. 3.4, pp. 6. Roenhuis, D. 24: Averse Weather an Air Traffi Delays. 11 th Conferene on Aviation, Range, an Aerospae Meteorology. P2.4, pp. 4. Sharman, R., Wolff, J., Fowler, T. L., an Brown, B. G. 23: Climatologies of Upper-level Turulene over the Continental U.S. an Oeans. 1 th Conferene on Aviation, Range, an Aerospae Meteorology. J1.8, pp. 4. Slemmer, J., an Silererg, S., 24: Convetive Signifiant Meteorologial Avisory (SIGMET) Climatology. 11 th Conferene on Aviation, Range, an Aerospae Meteorology. P5.14, pp. 6. Young, G. S., Brown, B. G., an MDonough, F. 23: An Inferre Iing Climatology Part I: Estimation from Pilot Reports an Surfae Conitions. 1 th Conferene on Aviation, Range, an Aerospae Meteorology. J1.5, pp. 5.