NOAA Technical Memorandum GLERL-157

ANALYSIS OF GREAT LAKES ICE COVER CLIMATOLOGY: 

WINTERS 2006-2011

Raymond A. Assel1 

Jia Wang2

Anne H. Clites2

Xuezhi Bai1

1

Cooperative Institute for Limnology and Ecosystems Research, 4840 S. State Rd., Ann Arbor, 

MI 48108

2

NOAA, Great Lakes Environmental Research Laboratory, 4840 S. State Rd., Ann Arbor, MI 

48108

April 2013

UNITED STATES

DEPARTMENT OF COMMERCE

Rebecca Blank

Acting Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Kathryn Sullivan

Acting, Under Secretary for Oceans & 

Atmosphere

NOAA Administrator

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S.DEPARTMENT OF COMMERCE
2

NOTICE

Mention of a commercial company or product does not constitute an endorsement 

by the NOAA. Use of information from this publication concerning proprietary 

products or the tests of such products for publicity or advertising purposes is not 

authorized. This is GLERL Contribution No. 1658.

This publication is available as a PDF file and can be downloaded from GLERL’s 

web site: www.glerl.noaa.gov. Hard copies can be requested from GLERL

Information Services, 4840 S. State Rd., Ann Arbor, MI 48108.

pubs.glerl@noaa.gov.

NOAA’s Mission – To understand and predict changes in Earth’s environment 

and conserve and manage coastal and marine resources to meet our nation’s 

economic, social, and environmental needs. 

NOAA’s Mission Goals:

•	 Protect, restore and manage the use of coastal and ocean resources through an

ecosystem approach to management

•	 Understand climate variability and change to enhance society’s ability to plan

and respond

•	 Serve society’s needs for weather and water information

•	 Support the Nation’s commerce with information for safe, efficient, and

environmentally sound transportation

•	 Provide critical support for NOAA’s Mission
3

TABLE OF CONTENTS

ABSTRACT.............................................................................................................................................5

1.	 INTRODUCTION .............................................................................................................................5

2. DATA .................................................................................................................................................5

3. ANALYSIS METHODS....................................................................................................................5

4. 	 PRODUCTS AND DISCUSSION.....................................................................................................6

5. 	 DATES OF FIRST (LAST) ICE AND ICE DURATION..................................................................6

6. 	 SEASONAL PROGRESSION OF ICE COVER ...........................................................................11

6.1 The 2006 Great Lakes Ice Cycle ...............................................................................................11

6.2 The 2007 Great Lakes Ice Cycle ...............................................................................................12

6.3 The 2008 Great Lakes Ice Cycle................................................................................................13

6.4 The 2009 Great Lakes Ice Cycle ...............................................................................................14

6.5 The 2010 Great Lakes Ice Cycle ...............................................................................................16

6.6 The 2011 Great Lakes Ice Cycle ...............................................................................................17

7.	 CONCLUSIONS..............................................................................................................................19

8. REFERENCES ................................................................................................................................19

APPENDIX 1. DATES OF FIRST ICE, LAST ICE, AND ICE DURATION ......................................21

APPENDIX 2. LAKE AVERAGE ICE COVER TIME SERIES......................................................... 23

APPENDIX 3. ICE COVER AND ICE COVER ANOMALY ICE CHARTS .....................................24

APPENDIX 4. SURFACE ANALYSIS MARCH 13-14, 2010, FEBRUARY 13-14, 2011..................25

LIST OF FIGURES

Figure 1. 	 Dates of first reported ice ≥ 10% ...........................................................................................8

Figure 2. 	Date of last reported ice ≥ 10%..............................................................................................9

Figure 3. 	 Duration of ice cover ≥ 10% ................................................................................................10

Figure 4. 	 Daily lake-averaged ice cover winter 2006......................................................................... 11

Figure 5. 	Maximum ice cover and ice anomaly charts winter 2006................................................... 11

Figure 6. 	Daily lake-averaged ice cover winter 2007..........................................................................12 

Figure 7. 	Maximum ice cover and ice anomaly charts winter 2007....................................................12 

Figure 8. 	Daily lake-averaged ice cover winter 2008..........................................................................13 

Figure 9. 	Maximum ice cover and ice anomaly charts winter 2008....................................................13

Figure 10. 	Daily lake-averaged ice cover winter 2009..........................................................................14 

Figure 11. January 26, 2009 ice cove anomalies...................................................................................14

Figure 12.	Maximum ice cover and ice anomaly charts winter 2009....................................................15 

Figure 13. 	Daily lake-averaged ice cover winter 2010..........................................................................16 

Figure 14. 	Maximum ice cover and ice anomaly charts winter 2010....................................................16 

Figure 15. 	Daily lake-averaged ice cover winter 2011..........................................................................17 

Figure 16. 	Maximum ice cover and ice anomaly charts winter 2011....................................................17 

LIST OF TABLES

Table 1. Annual Maximum Ice Cover (AMIC) ranked lowest to highest winters 1973 - 2011.............18
4
5

Analysis of Great Lakes Ice Cover Climatology: Winters 2006-2011

Raymond A. Assel 

Jia Wang

Anne H. Clites

Xuezhi Bai

ABSTRACT

A 33-winter ice concentration climatology (Assel 2003a, Assel 2005a) was recently updated for

winters 2006-2011 (Wang et al. 2012a). This report provides an analysis of the 2006-2011 ice

cycles within the context of: dates of first (last) ice, ice duration, ice cover distribution, ice cover

anomalies, and seasonal progression of lake-averaged ice cover. Analysis data are available as

ASCII and graphic files.

1. INTRODUCTION

The annual formation and loss of ice cover on the Great Lakes each winter, i.e. the annual ice cycle, affects

the lake’s ecosystem, the regional economy of the Great Lakes, and is an index of regional winter climate.

A 30-winter (1973-2002) climatology of Great Lakes ice cover (Assel 2003a) was updated for total ice

concentration for winters 2003-2005 (Assel 2005a) and more recently for winters 2006-2011 (Wang et al., 2012a).

The information presented here updates analysis products given in Assel (2003a, 2005a) including: dates of first

ice, dates of last ice, and ice cover duration. Line plots and ice charts portray the seasonal and spatial patterns 

and trends of ice cover over each winter season. Analysis methods are the same as in Assel (2005a). Analysis

products are available as fixed formatted ASCII grid and graphical files described in Appendix 1, Appendix 2, and 

Appendix 3. This report makes access and use of these data feasible to others interested in such information. 

2. DATA

The ice cycle analysis presented here is based on 389 digitized ice charts: 46 (2006), 46 (2007), 55 (2008), 45

(2009), 46 (2010), and 151 (2011). In general, ice charts began in early December and ended in early May. 

Wang et al. (2012a) provides details of the distribution of dates of ice charts for each winter season. The original

ASCII ice chart files were produced by the National Ice Center (NIC) and the Canadian Ice Service (CIS). The

data consists of grids of over-water cells embedded in a land mask (a 510 x 516 matrix at a nominal spatial

resolution of 2.55 km for winter 2006, and a 1024 x 1024 matrix at a nominal spatial resolution of 1.275 km at

approximately 450N for winters 2007-2011). The resolutions vary with latitude, see Wang et al. 2012a for details.

Ice charts were downloaded from http://www.natice.noaa.gov/products/great_lakes.html and quality controlled for 

location of over-water grid cells, to be consistent with NOAA GLERL CoastWatch land mask (http://coastwatch.

glerl.noaa.gov/ftp/masks/new/), and ice concentration codes, consistent with Assel (2003a, 2005a). Fixed

formatted ASCII and ARC/INFO shape files: names, structure, and ice concentration codes, are available in Wang

et al. (2012a) for those interested in these data. 

3. ANALYSIS METHODS

An analysis for threshold ice concentration values of dates of first (last) ice each winter and ice duration (the

difference between these dates) was made for each winter season. The original ice charts had ice concentration

recorded to the nearest 10%. The analysis was made on nine overlapping threshold ice concentration values in

10% increments starting from ice concentration ≥10%, and ending at ice concentrations ≥90% (i.e., ≥10%, ≥20%,

≥30%, ≥40%, ≥50%, ≥60%, ≥70%, ≥80%, ≥90%). A grid search of over-water cells, cell by cell, was made on
6

the original ice chart grids for each winter to determine the first (last) observed date for each of the nine threshold

values of ice concentration at each grid cell. These dates of observed first (last) ice may not be the actual date 

that ice first formed and actual last date of ice because this analysis is limited by the dates of the first and last ice 

charts of each winter and the fact that daily observations were not made. However, it is likely that for most of the

Great Lakes surface, the actual date of first ice occurred somewhere between the date of the first ice chart, and the

ice chart that has the observed threshold value of ice concentration. This is true because on the date of the first ice 

chart, most of the Great Lakes are usually open water or ice free. A similar comment is true for the date of last ice

cover. Additional information and details on dates of first (last) ice and ice duration for the 30-winter base period

(1973-2002) are given in Assel (2003b).

Spatial lake-averaged ice cover is calculated for each Great Lake and for the Great Lakes as a whole for each ice

chart given in Wang et al. (2012a) for each winter season. A grid encoding the grid cells for each lake was used to

calculate the lake-averaged ice cover for each lake. That grid, given as a fixed formatted mask text file (GLmask2.

txt for the 510 x 516 land mask and CW1024a.txt for the 1024 x 1024 land mask) is documented and available 

in Appendix A-3 of Wang et al. (2012a). All grid cell ice concentrations for a given lake for a given date are

summed and a lake average is then calculated by dividing the sum by the total number of grid cells for that lake. 

Line plots of lake-averaged ice cover for a given lake and winter season represent general linear trends in the daily

lake averaged time series. It should be cautioned that the true lake-averaged ice cover trend may vary from those 

given by the line plots as weather conditions can affect ice cover greatly between consecutive observed ice charts.

Assel (2005a,b) produced ice anomaly charts for each winter from 1973 to 2005. Ice anomaly charts were

produced for each ice chart of the winter by calculating the difference (for each over water grid cell) between

the 30-winter (1973-2002) weekly median ice concentration grid and the ice concentration grid for a date that

corresponds to the mid-week day of the weekly median (e.g., the 30-winter weekly median ice chart for January

12-18 would be used to calculate the ice anomaly chart for an ice chart dated January 15). In a similar manner, ice

anomaly charts were produced from the ice charts given in Wang et al. (2012a) for the six winters of the update

period. Ice anomaly charts provide a metric of the trends in ice cover over any date in a given winter season

relative to the 30-winter base period. Positive (negative) differences occur when the ice chart for a given winter

has ice cover concentration greater (less) then the 30-winter median.

4. PRODUCTS AND DISCUSSION 

Analysis products are summarized in a series of line plots (for lake averaged ice cover) and charts for: 1) dates

of first (last) ice and ice duration, 2) daily ice concentration, and 3) daily ice concentration anomalies. ASCII 

text files containing integer codes for ice are also produced for all the data given in the graphic plot file products. 

Data and file documentation for: 1) the dates of first (last) ice and ice duration are given in Appendix 1; plots and

tables of daily lake-averages are given in Appendix 2; daily ice cover anomaly plots and ASCII grids are given in

Appendix 3. Seasonal and spatial trends are discussed below using selected data from these appendices. Many of

the geographic locations cited in the remainder of this report are given in the Great Lakes place name chart for the 

reader’s convenience.

5. DATES OF FIRST (LAST) ICE AND ICE DURATION

Ice concentration ≥ 10% (the boundary between ice and open water) for dates of first (last) ice and ice duration,

respectively are portrayed in Figures 1-3, and trends are summarized.
7

Great Lakes Place Names
8

Figure 1. Dates of first reported ice ≥ 10%.

In general, shallow nearshore areas formed ice in December and January; deeper shore areas and most midlake areas formed ice in February and March, (Figure 1), April (2011) at some mid-lake areas of Lake Superior.

December in winters 2009 (December 2008) and 2011 (December 2010) had largest shore and nearshore areas

ice cover. Earliest most extensive mid-lake ice formed January 2009. Largest areas of open water (≤ 10% ice)

occurred in winter 2006 (all lakes) and in winter 2010 (Lakes Superior and Huron).

 
9

Figure 2. Dates of last reported ice ≥ 10% 

Last reports of mid-lake ice cover ≥ 10% occurred in March over much of the Great Lakes (Figure 2), with last

dates in February in western Lake Superior (2010), southern half of Lake Michigan (2011), north-central shore

of Lake Erie (2010), and lake-ward of northeast shore of Lake Ontario (2009, 2011). Last reports of ice cover in

April were most prevalent over Lake Superior (2011, 2009), Huron (2011), and western Lake Erie (2008). Bay

ice lasted into May in northern Lake Superior (2007, 2008, 2009, and 2011) and North Channel of Lake Huron 

(2008).
10

Figure 3. Duration of ice cover ≥ 10%.

Winter 2006 had the smallest area of ice cover duration for ice cover concentration ≥ 10%, and winter 2009 had

the largest for the Great Lakes (Figure 3). Shallow shore areas of the Great Lakes had higher ice cover duration

(> 90 days) compared to more exposed mid-lake areas (< 60 days most winters). The highest ice cover durations

occurred in portions of Thunder, Black, Nipigon, and Whitefish Bays (Lake Superior, exceeded 150 days some 

winters), Green Bay (Lake Michigan exceeded 120 days some winters), St. Marys River (exceeded 135 days some

winters), North Channel, Georgian Bay, and Saginaw Bay of Lake Huron (exceeded 120 days some winters),

Lake St. Clair (exceeded 105 days some winters) the western basin of Lake Erie (exceeded 90 days some winters),

and the southern Lake Erie shore (exceeded 105 days one winter).
11

6. SEASONAL PROGRESSION OF ICE COVER

The daily lake–averaged ice cover portrays the seasonal progression of the ice cycles. Charts of ice cover and 

charts of ice cover anomaly portray ice cover near the time of maximum ice cover for the Great Lakes as a whole

are presented below. Detailed data are made available in Appendix 2 and Appendix 3.

6.1 The 2006 Great Lakes Ice Cycle 

Ice cover was limited primarily to areas along the shores and in large bay of the Great Lakes in winter 2006

(Figure 4). Ice cover extent was near its seasonal maximum value in early March (Figure 5, Table 1). The seasonal 

maximum value ranked as the third (Great Lakes, Lake Erie) fourth (Lakes Superior, Michigan, Huron), and 

eighth (Lake Ontario) lowest over the 39 winters from 1973 to 2011. Near the time of the annual maximum

ice cover, Lake Superior ice cover extended from the Apostle Islands to the western shore of the Keweenaw

Peninsula. Along the north shore, it extended lake-ward from the three large bays along the north shore to Isle

Royale and covered Whitefish Bay in the east. Lake Michigan’s Green Bay and the Straits to Bois Blanc Island

had extensive ice cover; ice lined the perimeter of Lake Huron with high concentrations in the North Channel,

the eastern half of Georgian Bay, and Saginaw Bay. Ice cover was extensive on Lake St. Clair. Lake Erie had

disjointed ice cover in the western and central lake basins, and Lake Ontario had substantial ice cover in the

north-eastern portion of that lake. Large negative ice cover anomalies extended over all of Lake Superior (with the

exception of the mid-lake area of the eastern basin), over the western half of Georgian Bay and southern portions

of the southern end of Lake Huron, and again over virtually all of Lake Erie. Lake Ontario had positive ice cover 

anomalies in the north-eastern quadrant of that lake in early March. Most of the Great Lakes ice had dissipated by

the end of March (Figure 2). Ice in some bay areas lasted into April.

Figure 4. Daily lake-averaged ice cover winter 2006.

Figure 5. Maximum ice cover and ice anomaly charts winter 2006. 
12

6.2 The 2007 Great Lakes Ice Cycle

Ice cover was more extensive in winter 2007 (Figure 6) relative to the previous year (Figure 4). Ice cover was

confined primarily to shore and near shore in January. A large increase in ice occurred in February to reach near 

Annual Maximum Ice Cover (AMIC) values. Large decreases occurred in the last week of that month and early

March. Lower air temperatures late in the first week of March resulted in the Great Lakes being at or near their

AMIC near the beginning of the second week of March (Figure 7). The AMIC of the Great Lakes (55.2%) ranked

18th out of 39 winters and so can be thought of as typical, i.e., not extreme (Table 1). At that time, extensive ice

covers were located in (1) the west basin and lake ward of the shore of the east basin of Lake Superior, (2) Green

Bay, (3) much of the lake north of the north end of Grand Traverse Bay and the perimeter of the south shore 

in Lake Michigan, (4) much of Lake Huron with the exception of the deeper mid-lake area between Alpena,

Michigan and the Bruce Peninsula to the east, (5) all of Lake Erie, and (6) the northeast end of Lake Ontario. This

ice cover had positive anomalies for much of the north shore of Lake Superior and the perimeters of the remaining 

Great Lakes and a negative anomaly lake-ward of the eastern basin and west of the Keweenaw Peninsula to the

Apostle Islands in Lake Superior. The ice cover was short lived as warmer air temperatures brought a dramatic

reduction in ice cover for Lake Superior and the northern third of Lakes Michigan and Huron by March 12. 

By mid-March, the ice dissipation period was well under way over the entire Great Lakes, and by the end of

the March, the bulk of the ice was located in the three large bays along the north shore and Whitefish Bay in

Lake Superior, and the North Channel and the southern portion of Georgian Bay in Lake Huron; ice of lower

concentration remained in the southern end of Lake Huron and in the eastern half of Lake Erie.

Figure 6. Daily lake-averaged ice cover winter 2007.

Figure 7. Maximum ice cover and ice anomaly charts winter 2007.
13

6.3 The 2008 Great Lakes Ice Cycle

Winter 2008 was similar to winter 2007. Ice formed in the shore areas and the west basin of Lake Erie in

December and became more extensive in January (Figure 1). Mid-lake ice formations began in February. The

maximum ice cover for the individual Great Lakes occurred on February 21 (Lake Michigan), March 10 (Lakes 

Superior, Erie, Ontario), and March 17 (Lake Huron). The seasonal maximum ice cover for the Great Lakes 

as a whole occurred on March 10 (Figure 8, Figure 9). The AMIC, 53.7%, ranked 17th from the lowest and

was similar to winter 2007 (Table 1). At the time of maximum ice cover for the Great Lakes as a unit, not for 

individual Great Lakes, Lake Erie was over 90% ice covered, Lake Superior was 62% ice covered -- the midlake east basin of Lake Superior had open water, Lake Huron was 57% ice covered -- ice extended out from the

entire lake perimeter and covered the North Channel and Georgian Bay, Lake Michigan was 31% ice covered

-- with mid-lake ice limited primarily north of Traverse City and shore ice more extensive along the west shore,

Lake Ontario had 14% ice cover with the ice limited primarily to the northeastern end of the lake and much more

limited ice along the lake’s perimeter. At the time of maximum ice extent, the largest areas of positive ice cover

anomalies were located in the western basin of Lake Superior, along much of the perimeter of Lake Huron, most

of the west basin and extending to the north shore west of Long Point in Lake Erie, and in the extreme northeast

end of Lake Ontario. Ice cover dissipated over the next month, by April 10, the bulk of the ice cover was located

in the large bays of Lakes Superior and the northern bays in Lakes Huron and Michigan.

Figure 8. Daily lake-averaged ice cover winter 2008.

Figure 9. Maximum ice cover and ice anomaly charts winter 2008.
14

6.4 The 2009 Great Lakes Ice Cycle

Early winter ice cover was anomalously high in winter 2009. Ice formed in the mid-lake areas in January (Figure 

1, Figure 10). Lake Erie was over 90% ice covered by mid month, and anomalies continued to be positive through

the last week of January, e.g. Figure 11. Lake-averaged ice covers were at or near seasonal maximums on four

of the Great Lakes from January 29 to February 2: January 29 (Lake Michigan, 52.3%), February 2 (Lake Huron

73%, [not maximum], Lake Erie 95.5%, and Ontario 25.9%). The seasonal maximum ice cover for the Great

Lakes as a whole took place about one month later, March 2 (73.9%), and was ranked 31st from the lowest ice

cover or the 9th from the highest ice cover over the 39 winters under study (Table 1). On March 2, lake-averaged

ice cover on the individual Great Lakes was 93.6% on Lake Superior, 42.1% on Lake Michigan, 85.1% on Lake

Huron, 95.0% on Lake Erie, and 8.4% on Lake Ontario. Ice cover anomalies on March 2 were positive and most

extensive in the east basin of Lake Superior, the main body of Lake Huron, and portions of the north end and the 

south shores of Lake Michigan (Figure 12). Ice covers dissipated over mid-lake areas during March. Ice lasted

to early April in western Lake Superior. By April 27, the only significant ice cover was in three bays along Lake

Superior’s north shore.

Figure 10. Daily lake-averaged ice cover winter 2009.

Figure 11. January 29, 2009 ice cover anomalies.
15

Figure 12. Maximum ice cover and ice anomaly charts winter 2009.
16

Figure 13. Daily lake-averaged ice cover winter 2010.

Figure 14. Maximum ice cover and ice anomaly charts winter 2010.

6.5 The 2010 Great Lakes Ice Cycle

The 2010 ice cycle was mild, the mid-lake areas of the Great Lakes remained open water most of the winter. Ice

formed in the western basin of Lake Erie and the shore regions of large bays of the Great Lakes in December, in

deeper areas of large bays and in general along the perimeter of the Great Lakes and the central basin of Lake Erie 

in January, in the mid-lake west basin of Lake Superior, most of Georgian Bay, and deeper areas of central and

east Lake Erie in February, and much of Lake Michigan in March (Figure 1). The Great Lakes AMIC (27.4%)

ranked as the 7th from the lowest over the 39 winters 1973-2011 (Figure 13, Figure 14, Table 1) and occurred on 

February 8. Lakes Michigan (23.5%), Huron (34.6%), Erie (93.1%) and Ontario (13.1%) were at or near their

seasonal maximum value on February 8. Lake Superior was at its seasonal maximum value (27.3%) two weeks

later on February 22. At the time of maximum ice extent of the entire Great Lakes (Figure 14) ice cover was

limited primarily to the perimeter of the Great Lakes, with the exception of Lake Erie. The nearshore regions of

Lake Superior and northern Lake Huron had negative ice cover anomalies, while the southern shore region of

Lake Michigan had positive ice cover anomalies (Figure 14). Ice cover declined over the Great Lakes during the 

remainder of February and into March. A dramatic loss of ice cover occurred on Lake Erie on March 14 (Figure 

13) due to an intense low pressure system passing in the southern Great Lakes region that brought warmer

temperatures and high winds, see Appendix 4 for surface temperature, wind and pressure fields March 13-14. By

the end of March, the Great Lakes were ice free with the exception of very limited amounts of ice in the shore

areas and bays of Lake Superior and the North Channel and Georgian Bay in Lake Huron.
17

6.6 The 2011 Great Lakes Ice Cycle 

The temporal variability of daily lake averaged ice cover is better defined in 2011 (Figure 15) relative to the 

other five winters (Figures 4, 6, 8, 10, 13) because of the greater number of observations that winter (151). Large

areas of initial ice formation occurred in December and were unusually extensive for the southern end of Lake

Michigan and southern shore of Lake Erie relative to the other winters (Figure 1). Lake Erie was near 70% ice

covered by mid-January and at its maximum ice cover by February 1. The Great Lakes as a whole and Lake

Michigan reached maximum ice cover (40.1%, 29.4% respectively) on February 11, this was the 11th from the

lowest maximum ice cover (Table 1); Lake Superior had 26% ice cover that date. At maximum ice cover negative

ice anomalies occurred in the Great Lakes (Figure 16) primarily in: Lake Superior, northern Lake Michigan, Lake

Huron (Georgian Bay); notable positive ice anomalies occurred: north of the Apostle Islands in Lake Superior,

along the south half of the west shore, and along the south shore Lake Michigan (south of 44th parallel), lake-ward

of the west shore of Lake Huron, and along the south shore of Lake Ontario. A large decline in ice cover occurred

on the Great Lakes in mid February (Figure 15) due to the passage of a strong low pressure system February 13-

14, see Appendix 4. Ice cover increased after that time and maximum ice covers for Lakes Huron (63.8%) and

Ontario (31.1%) occurred on February 24, Lake Superior was at 29.2% (March 8) but did not reach it seasonal

maximum of 33.6% until April 2 due to extensive short lived new ice formation in late March-early April. In

general ice covers declined during March; by mid April the Great Lakes were virtually ice free except for the

three large north shore bays of Lake Superior, where it lasted until May 10.

Figure 16. Maximum ice cover and ice anomaly charts winter 2011.

Figure 15. Daily lake averaged ice cover, winter 2011.
18

Table 1. Annual Maximum Ice Cover (AMIC) Lowest to Highest Winters 1973 - 2011

Great Lakes Lake Superior Lake Michigan Lake Huron Lake Erie Lake Ontario

Rank Year AMIC Year AMIC Year AMIC Year AMIC Year AMIC Year AMIC Rank

1 2002 11.8 2002 10.3 2002 12.4 2002 26.1 1998 5.4 2002 4.0 1 

2 1998 14 1998 11.1 1998 15.1 1998 28.6 2002 14.4 1998 6.2 2 

3 2006 20 1987 14.5 2006 15.8 1983 31.8 2006 21.9 1987 8.4 3 

4 1983 22.1 2006 16.6 1987 20.0 2006 31.9 1983 40.8 1983 12.5 4 

5 1999 26.2 1999 18.3 1999 23.0 1999 36.0 1991 44.4 2010 13.1 5 

6 1987 27.3 1983 20.2 2010 23.5 2010 36.9 1999 74.8 1991 13.6 6 

7 2010 27.4 2010 27.3 1983 23.6 2000 41.9 1975 80.1 2008 14 7 

8 2001 34.4 2000 33.5 1995 23.7 1987 43.5 1990 85.5 2006 14.1 8 

9 2000 34.8 2011 33.6 1991 25.8 2001 45.7 1987 88.0 1975 14.9 9 

10 1976 39.9 1995 43.9 2000 27.2 1991 50.5 1974 88.5 1976 15.2 10 

11 2011 40.1 2001 46.7 1975 28.1 1976 52.5 2000 90.7 1989 17.7 11 

12 2005 43.3 1976 49.9 2011 29.4 1975 55.1 1988 91.5 1999 17.9 12 

13 1995 46.4 2007 51.8 2005 29.4 2005 58.9 2005 93.0 2001 17.9 13 

14 1975 50.2 2004 52.2 2001 29.5 2008 59.5 2010 93.1 1992 21.1 14 

15 2004 51.7 2005 54.5 1976 29.5 1995 59.7 2008 93.4 1988 21.1 15 

16 1988 53.6 2008 62.7 1988 32.7 1988 60.1 1980 93.4 1995 21.7 16 

17 2008 53.7 1975 64.9 1973 33.0 1974 61.4 2001 94.0 2000 22.3 17 

18 2007 55.2 1988 67.3 2008 33.5 2011 63.8 1992 94.1 2007 23.8 18 

19 1991 56.5 1973 69.8 1989 36.3 2004 64.5 1989 95 1974 24.5 19 

20 1997 58.4 1974 73.8 2004 36.4 1997 65.3 1993 95.2 1997 25.6 20 

21 1974 58.7 1980 77.2 1993 36.4 1973 66.7 2004 95.4 2009 25.9 21 

22 1973 60.6 1993 80.3 2007 37.2 1980 70.3 1976 95.4 1980 30.6 22 

23 1980 62.8 1992 80.8 1997 37.8 2007 71.4 1986 95.5 2011 31.1 23 

24 1984 65.2 1990 81.7 1980 38.6 1986 73.5 2009 95.5 1993 34.3 24 

25 1992 67.8 1985 81.9 1974 39.4 1990 75.3 1973 95.7 1990 35.1 25 

26 1993 68.2 1981 84.7 1985 41.3 1992 76 2003 95.7 2005 37.8 26 

27 1990 69.2 1982 85.3 1992 42.2 1984 77.6 1984 95.7 1985 38.2 27 

28 1985 69.8 1989 87.5 1984 43.3 1993 79.4 1995 95.8 2004 38.5 28 

29 1989 71.2 1984 88.2 1990 46.9 2009 85.1 2007 95.8 1984 40.7 29 

30 1981 71.3 1997 89.0 2003 48.0 1989 86.3 2011 95.8 1986 43.7 30 

31 2009 73.9 1986 90.7 2009 52.3 1985 87.4 1985 96.0 1996 45.1 31 

32 1982 74.8 1991 91.3 1981 53.8 1982 93.3 1981 96.0 1977 46.7 32 

33 1986 76.4 1978 92.5 1982 60.2 1981 93.8 1982 99.1 1981 47.6 33 

34 2003 79.1 2009 93.6 1978 66.6 1977 95.0 1997 99.6 2003 49.6 34 

35 1996 81.7 2003 95.5 1986 66.8 1978 95.8 1977 99.8 1982 50.7 35 

36 1978 83 1977 96.0 1996 75.0 2003 96.2 1994 99.8 1978 57.7 36 

37 1977 89.9 1994 96.7 1994 90.6 1979 96.4 1979 100.0 1973 62.6 37 

38 1979 94.7 1979 97.1 1979 92.3 1996 98.2 1996 100.0 1994 79.7 38 

39 1994 94.8 1996 100.0 1977 93.1 1994 98.5 1978 100.0 1979 86.2 39 

Rank Year AMIC Year AMIC Year AMIC Year AMIC Year AMIC Year AMIC Rank

Great Lakes Lake Superior Lake Michigan Lake Huron Lake Erie Lake Ontario

The sources of AMIC are lake averages from Assel (2003a), Assel (2005a), and calculated from original ice charts from 

Wang et al. (2012a). AMIC values between 1989 and 1995 in Assel (2003a) were previously calculated by averaging 

CIS and NIC daily lake-averaged data. Here we used the original ice charts to calculate AMIC. Note that AMIC values for 

the basin were calculated using the maximum ice cover for each lake on different days, rather than the synoptic value. 

Highlighting indicates values from years 2006 through 2011.
19

7. CONCLUSIONS

The description of the dates of first (last) ice, ice duration, and ice cycles given above is only a small portion of 

the detailed data available in the appendices and provides a very limited look at the annual ice cycle for each 

winter and a comparison with a the 30-winter climatology. 

The differences in the timing of first (last) ice and ice duration for the relatively mild 2006 winter compared to the

relatively severe 2009 ice cycle suggests that there may be corresponding signals in the biological, geochemical, 

or physical process in the winter and early spring lake ecosystem that relate to the differences in ice cover during

these two winters. Schwab et al. (2006) and Eadie et al. (1996) provide evidence that the date of ice cover loss

in late winter - early spring affects sediment resuspension and the timing of the spring coastal plume in southern

Lake Michigan.

Recent research by Bai et al. (2010, 2012) and Wang et al. (2012b) provide evidence that these data are useful 

in the analysis of the inter-annual ice cover variability relative to teleconnection patterns in the ocean and

atmosphere and in the analysis of multi-decadal trends in Great Lakes ice cover over the last four decades. These

studies augment earlier work by Assel (2009, 2005b,c, 2004, 2003b, 1998), Assel et al. (2004, 2003), Assel and

Norton (2001), Assel and Rodionov (1998), and Rodionov et al. (2001).

8. REFERENCES

Assel, R.A. Contemporary Lake Superior ice cover climatology. In State of Lake Superior. M. Munawar and

I.F. Munawar (eds.). Aquatic Ecosystem Health and Management Society, Ecovision World Monograph Series,

Canada, 51-66 (2009). http://www.glerl.noaa.gov/pubs/fulltext/2009/20090048.pdf

Assel, R. A. Great Lakes ice cover climatology update: Winters 2003, 2004, and 2005. NOAA Technical

Memorandum GLERL-135. NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI, 21 pp.

(2005a). http://www.glerl.noaa.gov/ftp/publications/tech_reports/glerl-135/tm-135.pdf

Assel, R.A. Great Lakes weekly ice cover statistics. NOAA Technical Memorandum GLERL-133. NOAA Great

Lakes Environmental Research Laboratory, Ann Arbor, MI, 27 pp. (2005b). http://www.glerl.noaa.gov/ftp/

publications/tech_reports/glerl-133/tm-133.pdf

Assel, R. A. Classification of annual Great Lakes ice cycles: Winters of 1973-2002. Journal of Climate 18:4895-

4905 (2005c). http://www.glerl.noaa.gov/pubs/fulltext/2005/20050028.pdf

Assel, R. A. Lake Erie ice cover climatology -- basin averaged ice cover: winters 1898-2002. NOAA Technical

Memorandum GLERL-128. NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI, 15 pp.

(2004). http://www.glerl.noaa.gov/ftp/publications/tech_reports/glerl-128/tm-128.pdf

Assel, R.A. NOAAAtlas. An Electronic Atlas of Great Lakes Ice Cover, Winters: 1973-2002. Great Lakes

Environmental Research Lab., Ann Arbor, MI. (2003a). http://www.glerl.noaa.gov/data/ice/atlas/

Assel, R.A. Great Lakes ice cover, first ice, last ice, and ice duration: Winters 1973-2002. NOAA Technical

Memorandum ERL GLERL-125, Great Lakes Environmental Research Laboratory, Ann Arbor, MI. (2003b).

http://www.glerl.noaa.gov/ftp/publications/tech_reports/glerl-125/tm-125.pdf

Assel, R. The 1997 ENSO event and implications for North American Laurentian Great Lakes winter severity

and ice cover. Geophysical Research Letters 25(7):1031-1033 (1998). http://www.glerl.noaa.gov/pubs/

fulltext/1998/19980016.pdf
20

Assel, R.A. and D. C. Norton. Visualization of Great Lakes ice cycles. EOS Transactions 82(7):83 (2001). http://

www.agu.org/eos_elec/00259e.html

Assel, R.A., and S. Rodionov. Atmospheric teleconnections for annual maximum ice cover on the Laurentian 

Great Lakes. International Journal of Climatology 18:425-442 (1998).

http://www.glerl.noaa.gov/pubs/fulltext/1998/19980001.pdf

Assel, R. A., S. Drobot, and T. E. CROLEY, II. Improving 30-day Great Lakes ice cover outlooks. Journal of 

Hydrometeorology 5(4):713-717 (2004). http://www.glerl.noaa.gov/pubs/fulltext/2004/20040016.pdf

Assel, R. A., K. Cronk and D. C. Norton. Recent trends in Laurentian Great Lakes ice cover. Climatic Change

57:185-204 (2003). http://www.glerl.noaa.gov/pubs/fulltext/2003/20030001.pdf

Bai, X., J. Wang, C.E. Sellinger, A.H. Clites, and R.A. Assel. Interannual variability of Great Lakes ice 

cover and its relationship to NAO and ENSO. Journal of Geophysical Research 117(C03002):25 pp.

(DOI:10.1029/2010JC006932) (2012). http://www.agu.org/pubs/crossref/2012/2010JC006932.shtml

Bai, X., J. Wang, C. Sellinger, A. Clites, and R. Assel, The impacts of ENSO and AO on the interannual variability

of Great Lakes ice cover, NOAA Technical Memorandum GLERL-152, 44 pp. (2010). http://www.glerl.noaa.

gov/ftp/publications/tech_reports/glerl-152/tm-152.pdf

Eadie, B.J., D.J. Schwab, G.A. Leshkevice, T.H. Johengen, R.A. Assel, N. Hawley, R.E. Holland, M.B. Lansing,

P.J. Lavrentyev, G.S. Miller, N.R. Morehead, J.A. Robbins, and P. Vanhoof. Development of recurrent coastal

plume in Lake Michigan observed for first time. EOS Transactions 77(35):337-338 (1996). http://www.glerl.noaa.

gov/pubs/fulltext/1996/19960010.pdf

Schwab, D. J., B. J. Eadie, R. A. Assel, and P. J. Roebber. Climatology of large sediment resuspension events in

southern Lake Michigan. Journal of Great Lakes Research 32:50-62 (2006). http://www.glerl.noaa.gov/pubs/

fulltext/2006/20060007.pdf

Rodionov, S., R. A. Assel, and L. R. Herche. Tree-structured modeling of the relationship between Great Lakes ice

cover and atmospheric circulation patterns. Journal of Great Lakes Research 27(4):486-502 (2001). http://www.

glerl.noaa.gov/pubs/fulltext/2001/20010011.pdf

Wang, J., R. A. Assel, S. Walterscheid, A.H. Clites, and X. Bai. Great Lakes ice cover climatology update: Winters

2006-2011 description of the digital ice cover data set. NOAA Technical Memorandum GLERL-155, 45 pp

(2012a). http://www.glerl.noaa.gov/ftp/publications/tech_reports/glerl-155/tm-155.pdf

Wang, J., X. Bai, H. Hu, A.H. Clites, M.C. Colton, and B.M. Lofgren. Temporal and spatial variability of Great

Lakes ice cover, 1973-2010. Journal of Climate 25(4):1318-1329 (DOI:10.1175/2011JCLI4066.1) (2012b). http://

journals.ametsoc.org/doi/pdf/10.1175/2011JCLI4066.1
21

APPENDIX 1. DATES OF FIRST ICE, LAST ICE, AND ICE DURATION

File names for the date of first reported ice (FST), last reported ice (LST), and ice duration (DUR) are listed

below. Download Appendix 1.

COMPRESSED UNCOMPRESSED 

Names No. files 

FSTYYGRID.ZIP FSTYYMM.DAT 9 

FSTYYPLOT.ZIP FSTYYMM.GIF 9 

LSTYYGRID.ZIP LSTYYMM.DAT 9

LSTYYPLOT.ZIP LSTYYMM.GIF 9

DURYYGRID.ZIP DURYYMM.DAT 9

DURYYPLOT.ZIP DURYYMM.GIF 9

Where: FST (LST) = first (last) reported ice file, DUR= ice cover duration in days. GRID = ASCII data file,

PLOT = ice chart plot file, YY = year (06, 07, 08, 09, 10, 11), MM = threshold, for first (last) reported date and

ice duration.

First Ice Cover Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

Last Ice Cover Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

Ice Duration Plots (ice concentration ≥: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%)

GRID File Structure 

Winter 2006: There are 510 records; each record has 516 “cells” in an I3 format, where I3 = an integer.

Winters 2007-2011: There are 1024 records; each record has 1024 “cells” in an I3 format, where I3 = an integer.

GRID Data Codes

Code = -1 (land), code = 0 (ice cover concentration is less than the threshold value), code = 1 to 182 (date that the

ice cover concentration threshold value was first (or last) observed, or for ice duration, this is the number of days

between first and last reported ice cover. If the first and last date are the same, the duration is 1, if there was no ice

cover for the given ice concentration value, the duration is 0. For example, if the date code for a single grid cell on 

a file of “first date ice cover greater or equal to 90%” was 63, that corresponds to a date of February 1; if the date

code for a single grid cell on a file of “last date ice cover greater or equal to 90%” was 122, that corresponds to a

date of April 1. See Table 1 below for a complete list of the first / last date codes and associated dates.
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Table A1. Summary of the first ice and lst ice date codes and associated calendar dates*

Day

Code

Day

Code

Day

Code

Day

Code

Day

Code

Day

Code

001 Dec 1 032 Jan 1 063 Feb 1 091 Mar 1 122 Apr 1 152 May 1

002 Dec 2 033 Jan 2 064 Feb 2 092 Mar 2 123 Apr 2 153 May 2

003 Dec 3 034 Jan 3 065 Feb 3 093 Mar 3 124 Apr 3 154 May 3

004 Dec 4 035 Jan 4 066 Feb 4 094 Mar 4 125 Apr 4 155 May 4

005 Dec 5 036 Jan 5 067 Feb 5 095 Mar 5 126 Apr 5 156 May 5

006 Dec 6 037 Jan 6 068 Feb 6 096 Mar 6 127 Apr 6 157 May 6

007 Dec 7 038 Jan 7 069 Feb 7 097 Mar 7 128 Apr 7 158 May 7

008 Dec 039 Jan 8 070 Feb 8 098 Mar 8 129 Apr 8 159 May 8

009 Dec 9 040 Jan 9 071 Feb 9 099 Mar 9 130 Apr 9 160 May 9

010 Dec 10 041 Jan 10 072 Feb 10 100 Mar 10 131 Apr 10 161 May 10

011 Dec 11 042 Jan 11 073 Feb 11 101 Mar 11 132 Apr 11 162 May 11

012 Dec 12 043 Jan 12 074 Feb 12 102 Mar 12 133 Apr 12 163 May 12

013 Dec 13 044 Jan 13 075 Feb 13 103 Mar 13 134 Apr 13 164 May 13

014 Dec 14 045 Jan 14 076 Feb 14 104 Mar 14 135 Apr 14 165 May 14

015 Dec 15 046 Jan 15 077 Feb 15 105 Mar 15 136 Apr 15 166 May 15

016 Dec 16 047 Jan 16 078 Feb 16 106 Mar 16 137 Apr 16 167 May 16

017 Dec 17 048 Jan 17 079 Feb 17 107 Mar 17 138 Apr 17 168 May 17

018 Dec 18 049 Jan 18 080 Feb 18 108 Mar 18 139 Apr 18 169 May 18

019 Dec 19 050 Jan 19 081 Feb 19 109 Mar 19 140 Apr 19 170 May 19

020 Dec 20 051 Jan 20 082 Feb 20 110 Mar 20 141 Apr 20 171 May 20

021 Dec 21 052 Jan 21 083 Feb 21 111 Mar 21 142 Apr 21 172 May 21

022 Dec 22 053 Jan 22 084 Feb 22 112 Mar 22 143 Apr 22 173 May 22

023 Dec 23 054 Jan 23 085 Feb 23 113 Mar 23 144 Apr 23 174 May 23

024 Dec 24 055 Jan 24 086 Feb 24 114 Mar 24 145 Apr 24 175 May 24

025 Dec 25 056 Jan 25 087 Feb 25 115 Mar 25 146 Apr 25 176 May 25

026 Dec 26 057 Jan 26 088 Feb 26 116 Mar 26 147 Apr 26 177 May 26

027 Dec 27 058 Jan 27 089 Feb 27 117 Mar 27 148 Apr 27 178 May 27

028 Dec 28 059 Jan 28 090 Feb 28 118 Mar 28 149 Apr 28 179 May 28

029 Dec 29 060 Jan 29 119 Mar 29 150 Apr 29 180 May 29

030 Dec 30 061 Jan 30 120 Mar 30 151 Apr 30 181 May 30

031 Dec 31 062 Jan 31 121 Mar 31 May 31

* Note: For leap years, code 91 = Feb 29, code 92 = Mar 1 and so on.
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APPENDIX 2. LAKE AVERAGED ICE COVER TIME SERIES

Lake-averaged ice covers were calculated for each Great Lake each winter season for data given in Wang et al.

(2012a). The time series for each winter begins on the date of the first ice chart and ends on the date of the last ice

chart. Dates of first and last ice chart are summarized below for each winter season from Wang et al. (2012a).

 Table 1. Summary of Ice Charts Date Statistics

Winter 

Season

Number of 

Charts

Date of First 

Ice Chart

Date of Last 

Ice Chart

Days Between 

First and Last 

Ice Chart

Average Number 

of Days Between 

Ice Charts

2006 46 28-Nov 04-May 157 3.43 

2007 46 04-Dec 10-May 158 3.46 

2008 55 26-Nov 15-May 171 3.13 

2009 45 08-Dec 07-May 151 3.38 

2010 46 07-Dec 15-Apr 130 2.85 

2011 151 02-Dec 13-May 162 1.08 

Lake Averages - spatial averages of ice cover for each Great Lake. 

Composite line plots of lake averaged ice cover for all five Great Lakes (Superior, Michigan, Huron, Erie, and 

Ontario) for each winter are named seaYY.BMP, where YY is 06, 07, 08, 09, 10, and 11 for winters of 2006,

2007, 2008, 2009, 2010, and 2011 respectively. Note line plots of Lake St. Clair and the entire Great Lakes as a 

whole are not included in the composite plot each winter for the sake of clarity, these data are however given in

text files.

Six text files (avgYY.txt), where YY is as defined above, are fixed formatted ASCII files of lake-averaged ice

cover. The first record is a column heading listing the dates and lakes, the remaining records are the lake-averaged

ice cover for each of the five Great Lakes, Lake St. Clair, and the Great Lakes taken as a whole. The appropriate

data is under each of the header records. The header records for each winter season are:

Year Mo Da Jd Sup Mic Hur Stc Eri Ont GL

Where Year is the year, Mo is the month, Da is the day of the month, Jd is the Julian date, and Sup Mic, Hur, 

Eri, Stc, Ont, and GLs, are Lakes Superior, Michigan, Huron, Erie, St. Clair, Ontario, and the entire Great Lakes, 

respectively.

The number of over water grid cells used to calculate the lake-averaged ice cover for each great lake is

summarized below for the 510 x 516 and the 1024 x 1024 grid masks given in Wang et al. (2012a).

510 x 516 1024 x 1024

Superior 13850 54278

Michigan 8545 33510

Huron 9161 35402

St. Clair 157 575

Erie 3574 13897

Ontario 2801 10754

Great Lakes 38088 148416
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APPENDIX 3. ICE COVER AND ICE COVER ANOMALY ICE CHARTS

Introduction

Ice charts showing the distribution pattern of ice cover concentration over the Great Lakes are provided below in

a different format (color code) then given in Wang et al. (2012a). Also the daily median ice charts given in Assel 

(2005b) are used to calculate anomaly ice charts for the 2006-2011 winters as described in the report text. Click

on the blue text below to download ice charts and anomaly ice chart.

Ice Charts

Ice charts are in compressed files organized by winter season.

Compressed Image File Names: YYYYgif.zip, where YYYY is the winter (2006..., 2011), “gif” defines the

image format.

Image File Names: YYYddd2.GIF, where “YYY” = winter season, yyy=106 for 2006, 107 for 2007...111 for

2011; ddd = day of the winter season (101=Dec 1, ...132=Jan 1, ...282=May 31), 2 is an indicator for observed

ice chart.

Please note that one ice chart for winter 2006 and two ice charts for winter 2008 were not included because they

occurred in November when very little ice was observed on the Great Lakes. These charts are, however, available

in Wang et al. (2012a).

Anomaly Ice Charts

Charts that portray the spatial distribution patterns of ice concentration anomalies over the Great Lakes for a given 

date of a given winter. Anomaly ice charts are arranged by winter season in compressed files.

Compressed Image File Names: YYYYanogif.zip, where YYYY is the winter (2006..., 2011), “ano” is an

indicator for ice anomaly image data, and “gif” defines the image format.

Image File Names: YYYddd.GIF, where “YYY” and “ddd” are as defined above.
25

APPENDIX 4. SURFACE ANALYSIS MARCH 13-14, 2010, FEBRUARY 13-14, 2011

Figure 1. Air temperature (shaded, C), surface winds (vectors, m/s) and sea level pressure (contours, hPa) on 

March 13-14, 2010, and February 13-14 showing the passage of a cyclone from west to east. The data are from 

NCEP2. (www.cpc.ncep.noaa.gov/products/wesley/reanalysis2/)
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