2001 Chicago, O'Hare International Airport, Illinois (ORD)

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2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT, ILLINOIS (ORD)

Chicago is located along the southwest shore of Lake Michigan and occupies a plain which, for the most part, is only some tens of feet above the lake. Lake Michigan averages 579 feet above sea level. Natural water drainage over most of the city would be into Lake Michigan, and from areas west of the city is into the Mississippi River System. But actual drainage over most of the city is artificially channeled also into the Mississippi system. Topography does not significantly affect air flow in or near the city except that lesser frictional drag over Lake Michigan causes winds to be frequently stronger along the lakeshore, and often permits air masses moving from the north to reach shore areas an hour or more before affecting western parts of the city.

Chicago is in a region of frequently changeable weather. The climate is predominately continental, ranging from relatively warm in summer to relatively cold in winter. However, the continentality is partially modified by Lake Michigan, and to a lesser extent by other Great Lakes. In late autumn and winter, air masses that are initially very cold often reach the city only after being tempered by passage over one or more of the lakes. Similarly, in late spring and summer, air masses reaching the city from the north, northeast, or east are cooler because of movement over the Great Lakes. Very low winter temperatures most often occur in air that flows southward to the west of Lake Superior before reaching the Chicago area. In summer the higher temperatures are with south or southwest flow and are therefore not influenced by the lakes, the only modifying effect being a local lake breeze. Strong south or southwest flow may overcome the lake breeze and cause high temperatures to extend over the entire city.

During the warm season, when the lake is cold relative to land, there is frequently a lake breeze that reduces daytime temperature near the shore, sometimes by 10 degrees or more below temperatures farther inland. When the breeze off the lake is light this effect usually reaches inland only a mile or two, but with stronger on–shore winds the whole city is cooled. On the other hand, temperatures at night are warmer near the lake so that 24–hour averages on the whole are only slightly different in various parts of the city and suburbs.

At the O'Hare International Airport temperatures of 96 degrees or higher occur in about half the summers, while about half the winters have a minimum as low as −15 degrees. The average occurrence of the first temperature as low as 32 degrees in the fall is mid–October and the average occurrence of the last temperature as low as 32 degrees in the spring is late April.

Precipitation falls mostly from air that has passed over the Gulf of Mexico. But in winter there is sometimes snowfall, light inland but locally heavy near the lakeshore, with Lake Michigan as the principal moisture source. The heavy lakeshore snow occurs when initially colder air moves from the north with a long trajectory over Lake Michigan and impinges on the Chicago lakeshore. In this situation the air mass is warmed and its moisture content increased up to a height of several thousand feet. Snowfall is produced by upward currents that become stronger, because of frictional effects, when the air moves from the lake onto land. This type of snowfall therefore tends to be heavier and to extend farther inland in south–shore areas of the city and in Indiana suburbs, where the angle between wind–flow and shoreline is greatest. The effect of Lake Michigan, both on winter temperatures and lake–produced snowfall, is enhanced by non–freezing of much of the lake during the winter, even though areas and harbors are often ice–choked.

Summer thunderstorms are often locally heavy and variable, parts of the city may receive substantial rainfall and other parts none. Longer periods of continuous precipitation are mostly in autumn, winter, and spring. About one–half the precipitation in winter, and about 10 percent of the yearly total precipitation, falls as snow. Snowfall from month to month and year to year is greatly variable. There is a 50 percent likelihood that the first and last l–inch snowfall of a season will occur by December 5 and March 20, respectively.

Channeling of winds between tall buildings often causes locally stronger gusts in the central business area. However, the nickname, windy city, is a misnomer as the average wind speed is not greater than in many other parts of the U.S.

NORMALS, MEANS, AND EXTREMES

CHICAGO, IL (ORD)

LATITUDE:    LONGITUDE:    ELEVATION (FT):    TIME ZONE:    WBAN: 94846

41 59′ 10″ N  87 54′ 51″ W  GRND: 655  BARO: 658  CENTRAL (UTC + 6)

ELEMENTPORJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECYEAR
TEMPERATURE FNORMAL DAILY MAXIMUM3029.033.545.858.670.179.683.781.874.863.348.434.058.5
MEAN DAILY MAXIMUM4329.534.345.458.470.179.583.681.974.763.048.034.758.6
HIGHEST DAILY MAXIMUM43657288919310410410199917871104
YEAR OF OCCURRENCE198920001986198019771988199519911985196319781982JUL 1995
MEAN OF EXTREME MAXS.4349.554.572.080.587.593.095.092.989.981.768.155.976.7
NORMAL DAILY MINIMUM3012.917.228.538.647.757.562.661.653.942.231.619.139.5
MEAN DAILY MINIMUM4314.018.728.038.648.257.462.962.153.942.431.820.139.8
LOWEST DAILY MINIMUM43-27-19-872436404128171-25-27
YEAR OF OCCURRENCE198519961962198219661972196519651974198119761983JAN 1985
MEAN OF EXTREME MINS.43-9.0-2.910.023.134.143.550.250.838.627.215.3-1.923.2
NORMAL DRY BULB3021.025.437.248.658.968.673.271.764.452.840.026.649.0
MEAN DRY BULB4321.726.536.848.459.268.573.271.964.352.839.727.449.2
MEAN WET BULB1822.526.234.043.053.362.166.765.958.347.734.826.645.1
MEAN DEW POINT1817.821.027.936.247.257.062.361.953.542.330.021.839.9
NORMAL NO. DAYS WITH:
MAXIMUM 90300.00.00.0*0.83.86.74.21.50.10.00.017.1
MAXIMUM 323017.612.73.90.10.00.00.00.00.00.01.711.947.9
MINIMUM 323028.725.021.07.80.90.00.00.00.25.316.526.7132.1
MINIMUM 0307.03.20.20.00.00.00.00.00.00.00.02.913.3
H/CNORMAL HEATING DEG. DAYS3013641109862492235355198439175011906536
NORMAL COOLING DEG. DAYS30000046143259226661200752
RHNORMAL (PERCENT)3072727065646668717169727670
HOUR 00 LST3075767672737579818176777877
HOUR 06 LST3076777977777882858581808080
HOUR 12 LST3067656155535557575755647060
HOUR 18 LST3071696457545558616364707463
SPERCENT POSSIBLE SUNSHINE1644495150586766625955384354
W/OMEAN NO. DAYS WITH:
HEAVY FOG (VISBY 1/4 MI)441.71.82.20.91.20.60.50.70.40.81.31.914.0
THUNDERSTORMS440.30.52.04.05.16.65.96.04.21.71.10.638.0
CLOUDINESSMEAN:
SUNRISE-SUNSET (OKTAS)385.55.45.85.55.04.74.44.44.54.75.75.65.1
MIDNIGHT-MIDNIGHT (OKTAS)325.35.05.15.04.54.34.04.04.24.35.45.54.7
MEAN NO. DAYS WITH:
CLEAR396.86.04.96.07.27.38.08.68.58.65.25.782.8
PARTLY CLOUDY396.26.58.57.69.911.512.111.29.58.56.25.9103.6
CLOUDY3918.015.717.616.413.911.210.110.411.213.317.718.5174.0
PRMEAN STATION PRESSURE (IN)2929.3029.3029.3029.2029.2029.2029.3029.3029.3029.3129.3129.3129.28
MEAN SEA-LEVEL PRES. (IN)1830.1030.1030.0629.9629.9730.0130.0030.0430.0630.0830.0630.1130.05
WINDSMEAN SPEED (MPH)3811.611.411.912.010.59.48.68.49.010.211.111.010.4
PREVAIL. DIR (TENS OF DEGS)3327292904042123231819212721
MAXIMUM 2-MINUTE:
SPEED (MPH)537384143373937413743413243
DIR. (TENS OF DEGS)09260226283001242807233307
YEAR OF OCCURRENCE199920011998199719981998199720001997199719982000OCT 1997
MAXIMUM 5-SECOND:
SPEED (MPH)547564756544861594852574161
DIR. (TENS OF DEGS)09240225273020252107242720
YEAR OF OCCURRENCE199919991998199719981998199920002001199719982001JUL 1999
PRECIPITATIONNORMAL (IN)301.531.362.693.643.323.783.664.223.822.412.922.4735.82
MAXIMUM MONTHLY (IN)434.475.565.917.697.149.968.3317.1011.448.548.228.5617.10
YEAR OF OCCURRENCE199919971976198319701993198219871961200119851982AUG 1987
MINIMUM MONTHLY (IN)430.100.120.630.970.300.951.180.510.020.160.440.230.02
YEAR OF OCCURRENCE198119691981197119921991197719691979196419991962SEP 1979
MAXIMUM IN 24 HOURS (IN)432.003.782.392.783.453.792.909.353.004.622.994.539.35
YEAR OF OCCURRENCE196019971985198319811994199319871978196919901982AUG 1987
NORMAL NO. DAYS WITH:
PRECIPITATION 0.013011.19.412.412.610.910.19.89.59.59.210.911.6127.0
PRECIPITATION 1.00300.20.10.40.70.50.91.01.21.10.30.50.57.4
SNOWFALLNORMAL (IN)3010.78.46.51.80.10.00.00.00.00.51.98.838.7
MAXIMUM MONTHLY (IN)4234.326.224.711.11.6TTTT6.610.435.335.3
YEAR OF OCCURRENCE197919941965197519661992199519891967196719591978DEC 1978
MAXIMUM IN 24 HOURS (IN)4218.611.110.610.91.6TTTT6.65.811.018.6
YEAR OF OCCURRENCE199920001970197519661992199519891967196719751969JAN 1999
MAXIMUM SNOW DEPTH (IN)412827201110000361728
YEAR OF OCCURRENCE19791967196519751966198919752000JAN 1979
NORMAL NO. DAYS WITH:
SNOWFALL 1.0303.32.72.00.40.*0.00.00.00.00.20.62.611.8
PRECIPITATION (inches) 2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT, IL (ORD)
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
19721.010.733.454.773.023.554.976.978.142.923.052.8945.47
19731.241.383.914.993.692.875.270.676.012.861.503.7138.10
19743.292.112.404.275.094.692.962.601.471.882.472.1235.35
19753.692.482.025.503.025.072.197.370.801.902.533.0539.62
19760.851.875.914.054.032.931.441.291.491.410.650.6426.56
19770.550.713.672.621.885.121.185.396.071.362.051.9632.56
19781.480.431.163.942.806.364.611.966.881.082.244.4137.35
19792.811.024.494.922.584.632.197.570.021.492.802.5837.10
19801.041.241.963.413.223.423.568.545.652.091.103.4338.66
19810.102.350.636.145.854.464.506.603.251.802.461.0539.19
19822.900.414.152.782.081.568.333.931.151.886.958.5644.68
19830.662.063.567.696.264.114.252.085.414.415.872.9949.35
19841.151.393.004.114.492.023.192.103.843.152.642.9234.00
19851.483.464.731.482.791.973.753.901.824.988.221.4940.07
19860.392.581.491.853.113.494.301.157.123.751.411.0931.73
19871.670.991.592.342.212.194.1917.100.941.592.773.7741.35
19881.881.292.152.081.191.052.743.293.795.056.452.4033.36
19890.820.771.671.371.592.015.897.313.911.492.160.4629.45
19901.972.253.091.796.854.502.257.751.034.105.601.9443.12
19911.410.623.544.005.200.951.322.812.517.363.591.7135.02
19920.871.392.672.210.301.353.773.564.311.795.412.4930.12
19933.830.824.524.571.839.964.455.744.472.191.521.0044.90
19941.772.561.092.200.586.091.624.051.043.233.751.6129.59
19953.210.411.435.794.471.403.173.491.044.203.680.5932.88
19961.580.710.952.596.954.803.951.452.732.321.481.2130.72
19971.385.561.571.762.693.813.044.501.692.751.461.5031.71
19982.671.704.293.563.022.901.756.882.345.222.001.2037.53
19994.471.641.737.514.464.953.732.303.271.070.442.6838.25
20001.351.971.185.154.024.323.582.263.591.122.712.1133.36
20011.122.571.302.823.342.612.9612.256.058.541.220.9945.77
POR=43 YRS1.751.532.553.663.323.793.554.233.482.732.712.1235.42
AVERAGE TEMPERATURE (F) 2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT, IL (
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
197219.623.634.044.861.065.773.673.863.549.337.723.947.5
197328.228.744.048.154.871.174.774.666.057.941.928.151.5
197424.827.438.652.356.865.573.670.060.552.840.630.249.4
197527.326.234.143.362.370.575.576.361.455.847.231.551.0
197619.935.242.852.355.970.174.070.862.748.332.419.448.7
197710.726.944.955.067.269.377.571.966.051.540.024.250.4
197815.716.831.947.558.367.672.072.468.851.440.825.847.4
197912.516.236.445.559.369.272.071.066.153.340.633.748.0
198023.421.532.646.559.765.375.775.766.048.439.928.048.6
198122.628.037.651.855.369.872.571.261.749.140.824.948.8
198212.221.535.144.564.362.174.168.862.153.239.136.047.8
198326.330.537.443.453.269.776.777.364.652.841.114.348.9
198417.133.929.545.855.570.370.372.861.154.737.931.048.3
198514.420.439.452.660.263.671.469.265.452.537.817.047.0
198622.824.040.451.559.566.374.968.566.853.736.030.649.6
198725.933.940.850.663.472.476.771.965.147.343.932.252.0
198819.822.738.148.261.071.776.876.865.946.141.727.749.7
198932.419.636.646.857.867.573.971.462.054.037.717.448.1
199033.931.341.349.956.269.671.771.965.951.644.728.651.4
199120.831.040.452.065.671.975.573.663.753.235.230.351.1
199228.133.337.546.156.964.969.367.062.750.438.328.648.6
199326.224.434.245.059.766.474.373.359.249.538.729.848.4
199415.922.138.551.158.270.273.468.766.854.744.434.849.9
199524.026.540.246.058.872.377.679.062.553.732.826.350.0
199623.426.030.845.255.068.069.972.363.551.933.427.747.3
199719.329.037.945.253.868.373.269.564.253.236.431.548.5
199829.638.739.049.864.869.374.573.567.755.544.834.753.5
199922.634.035.649.661.770.478.470.363.452.945.129.951.2
200025.334.144.247.262.067.371.172.464.756.137.016.049.8
200124.626.134.252.560.067.474.673.261.952.148.233.450.7
POR=43 YRS21.726.536.848.559.468.573.271.864.352.839.727.349.2

REFERENCE NOTES:

PAGE 1:

THE TEMPERATURE GRAPH SHOWS NORMAL MAXIMUM AND NORMAL MINIMUM DAILY TEMPERATURES (SOLID CURVES) AND THE ACTUAL DAILY HIGH AND LOW TEMPERATURES (VERTICAL BARS).

PAGE 2 AND 3:

H/C INDICATES HEATING AND COOLING DEGREE DAYS.

RH INDICATES RELATIVE HUMIDITY

W/O INDICATES WEATHER AND OBSTRUCTIONS

S INDICATES SUNSHINE.

PR INDICATES PRESSURE.

CLOUDINESS ON PAGE 3 IS THE SUM OF THE CEILOMETER AND SATELLITE DATA NOT TO EXCEED EIGHT EIGHTHS (OKTAS).

GENERAL:

T INDICATES TRACE PRECIPITATION, AN AMOUNT GREATER THAN ZERO BUT LESS THAN THE LOWEST REPORTABLE VALUE.

+ INDICATES THE VALUE ALSO OCCURS ON EARLIER DATES.

BLANK ENTRIES DENOTE MISSING OR UNREPORTED DATA.

NORMALS ARE 30–YEAR AVERAGES (1961–1990).

ASOS INDICATES AUTOMATED SURFACE OBSERVING SYSTEM.

PM INDICATES THE LAST DAY OF THE PREVIOUS MONTH.

POR (PERIOD OF RECORD) BEGINS WITH THE JANUARY DATA MONTH AND IS THE NUMBER OF YEARS USED TO COMPUTE THE MEAN. INDIVIDUAL MONTHS WITHIN THE POR MAY BE MISSING.

WHEN THE POR FOR A NORMAL IS LESS THAN 30 YEARS, THE NORMAL IS PROVISIONAL AND IS BASED ON THE NUMBER OF YEARS INDICATED.

0.* OR * INDICATES THE VALUE OR MEAN–DAYS–WITH IS BETWEEN 0.00 AND 0.05.

CLOUDINESS FOR ASOS STATIONS DIFFERS FROM THE NON–ASOS OBSERVATION TAKEN BY A HUMAN OBSERVER. ASOS STATION CLOUDINESS IS BASED ON TIME–AVERAGED CEILOMETER DATA FOR CLOUDS AT OR BELOW 12,000 FEET AND ON SATELLITE DATA FOR CLOUDS ABOVE 12,000 FEET.

THE NUMBER OF DAYS WITH CLEAR, PARTLY CLOUDY, AND CLOUDY CONDITIONS FOR ASOS STATIONS IS THE SUM OF THE CEILOMETER AND SATELLITE DATA FOR THE SUNRISE TO SUNSET PERIOD.

GENERAL CONTINUED:

CLEAR INDICATES 0–2 OKTAS, PARTLY CLOUDY INDICATES 3–6 OKTAS, AND CLOUDY INDICATES 7 OR 8 OKTAS. WHEN AT LEAST ONE OF THE ELEMENTS (CEILOMETER OR SATELLITE) IS MISSING, THE DAILY CLOUDINESS IS NOT COMPUTED.

WIND DIRECTION IS RECORDED IN TENS OF DEGREES (2 DIGITS) CLOCKWISE FROM TRUE NORTH. "00" INDICATES CALM. "36" INDICATES TRUE NORTH.

RESULTANT WIND IS THE VECTOR AVERAGE OF THE SPEED AND DIRECTION.

AVERAGE TEMPERATURE IS THE SUM OF THE MEAN DAILY MAXIMUM AND MINIMUM TEMPERATURE DIVIDED BY 2.

SNOWFALL DATA COMPRISE ALL FORMS OF FROZEN PRECIPITATION, INCLUDING HAIL.

A HEATING (COOLING) DEGREE DAY IS THE DIFFERENCE BETWEEN THE AVERAGE DAILY TEMPERATURE AND 65 F.

DRY BULB IS THE TEMPERATURE OF THE AMBIENT AIR.

DEW POINT IS THE TEMPERATURE TO WHICH THE AIR MUST BE COOLED TO ACHIEVE 100 PERCENT RELATIVE HUMIDITY.

WET BULB IS THE TEMPERATURE THE AIR WOULD HAVE IF THE MOISTURE CONTENT WAS INCREASED TO 100 PERCENT RELATIVE HUMIDITY.

ON JULY 1, 1996, THE NATIONAL WEATHER SERVICE BEGAN USING THE "METAR" OBSERVATION CODE THAT WAS ALREADY EMPLOYED BY MOST OTHER NATIONS OF THE WORLD. THE MOST NOTICEABLE DIFFERENCE IN THIS ANNUAL PUBLICATION WILL BE THE CHANGE IN UNITS FROM TENTHS TO EIGHTS (OKTAS) FOR REPORTING THE AMOUNT OF SKY COVER.

HEATING DEGREE DAYS (base 65 F) 2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT, IL (OR
YEARJULAUGSEPOCTNOVDECJANFEBMARAPRMAYJUNTOTAL
1972-73151010948181112691135101264550331106301
1973-740072244687113912401046812383266635952
1974-7501176384724107211601078951643152306371
1975-761014730353110331392859681411285175660
1976-7709119522973140816791060616332115416874
1977-7808424137411254152113461020518264467173
1978-791459418718120616221360879580233307110
1979-8016196238272296712811254995558198836537
1980-81037151174611401308103184639731366372
1981-8286135489719123616321213922608931187179
1982-837371523727728911194961847643364386278
1983-84160125383714156814798941095575300187167
1984-851911893208071046156312457874181831036681
1985-8606141380813148013021142765417202746722
1986-873296434386310601205866742432162145783
1987-8841974541629101113961221828503176406442
1988-890963583693114910031265882540261436491
1989-90051313448131471956938733491271336186
1990-9110510342560511201365945756393142135882
1991-920016336788710661137913847560284776301
1992-93937136449795112211961133948595184696673
1993-9403185479784108415161197817433253516802
1994-951236332261193212621074760561199255833
1995-96101503499581193128411241054589343587103
1996-9790119399940114814101003832587344536844
1997-9894774068521030109173281344987695619
1998-9900352895989331309860903456149345566
1999-001411036859110811224892640528148575644
2000-0163112286833151212481085948374205886700
2001-50128394496973
COOLING DEGREE DAYS (base 65 F) 2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT
YEARJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECANNUAL
197200006410628928972000820
1973000531893083011083200946
1974000102183274162481200610
19750000762033323584624101040
197600036617828619656800766
197700039191178395229760001108
1978000060132227243181200845
1979000261164241213992600806
19800001043101338342107200943
198100092015724820444000682
198200007938295161691400656
19830010418938538812210001099
198400051118419025477800729
1985000534271204142158000670
19860071737118318145123300768
19870006116241377238830101062
1988000559247373383961001164
1989002044121282207481100715
19900074381792262241371110836
19910001116722633427313210001153
19920001407915210675400457
199300002811829426619500730
199400023472122681431261000829
1995000013254398445818001199
199600004115416623579200677
199700004158265154594400684
199800130882053012671235001002
199900005220142217670220925
20000030631311992401121800766
200100075816830926244100849
SNOWFALL (inches) 2001 CHICAGO, O'HARE INTERNATIONAL AIRPORT, IL (ORD)
YEARJULAUGSEPOCTNOVDECJANFEBMARAPRMAYJUNTOTAL
1972-730.00.00.00.10.911.20.59.33.40.2T0.025.6
1973-740.00.00.00.0T18.87.49.61.4T0.00.037.2
1974-750.00.00.00.01.09.43.58.24.511.10.00.037.7
1975-760.00.00.00.06.46.810.01.61.90.8T0.027.5
1976-770.00.00.01.60.56.57.24.04.9T0.00.024.7
1977-780.00.00.00.05.212.721.97.94.50.20.00.052.4
1978-790.00.00.00.05.235.334.36.82.00.10.00.083.7
1979-800.00.00.00.04.00.96.214.711.64.20.00.041.6
1980-810.00.00.0T5.19.72.015.92.30.00.00.035.0
1981-820.00.00.0T3.64.921.14.814.310.60.00.059.3
1982-830.00.00.00.00.42.15.08.99.01.20.00.026.6
1983-840.00.00.00.01.016.517.21.99.72.70.00.049.0
1984-850.00.00.00.0T6.618.913.30.3T0.00.039.1
1985-860.00.00.00.01.15.26.910.94.10.80.00.029.0
1986-870.00.00.0T3.80.417.3T4.7T0.00.026.2
1987-880.00.00.00.11.018.75.415.51.9T0.00.042.6
1988-890.00.00.0T0.95.00.415.12.00.60.50.024.5
1989-900.0T0.06.33.95.43.213.61.30.1T0.033.8
1990-910.00.00.0TT3.211.13.35.9T0.00.023.5
1991-920.00.00.0T1.27.65.61.311.61.10.0T28.4
1992-93T0.00.00.30.25.715.28.013.83.70.00.046.9
1993-940.00.00.0T0.21.214.226.2TT0.00.041.8
1994-950.00.00.00.0T7.013.10.43.50.10.00.024.1
1995-96T0.00.0T3.99.95.90.33.9TT0.023.9
1996-970.00.0
1997-9811.0T8.20.00.0T
1998-990.00.00.00.00.21.029.61.918.20.00.00.050.9
1999-000.00.00.00.00.03.513.611.6T1.60.0T30.3
2000-010.00.00.0T0.130.91.52.24.20.3TT39.2
2001-T0.00.0T0.01.6
POR=41 YRSTTT0.41.98.410.87.86.51.60.1T37.5

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2001 Chicago, O'Hare International Airport, Illinois (ORD)

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    2001 Chicago, O'Hare International Airport, Illinois (ORD)