1

NOAA Technical Memorandum GLERL 133 

____________________________________________________________________________ 

Great Lakes Weekly Ice Cover Statistics 

Raymond A. Assel 

NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 

Great Lakes Environmental Research Laboratory 

Ann Arbor, Michigan 

March 2005 

____________________________________________________________________________ 

UNITED STATES NATIONAL OCEANIC AND 

DEPARTMENT OF COMMERCE ATMOSPHERIC ADMINISTRATION 

Carlos M. Gutierrez Conrad C. Lautenbacher, Jr. 

Secretary Administrator 
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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. 1350. 

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, 2205 Commonwealth Blvd., Ann Arbor, MI 48105. 

pubs.glerl@noaa.gov. 
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TABLE OF CONTENTS

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

INTRODUCTION .................................................................................................................... 5 

DATA USED TO CALCULATE WEEKLY STATISTICS .............................................................. 6 

METHODS ............................................................................................................................ 7 

RESULTS .............................................................................................................................. 9

ICE COVER FOR SPECIFIC BATHYMETRY RANGES .............................................................. 14 

LAKE ERIE ICE COVER AND BATHYMETRY ........................................................................ 14 

LAKE SUPERIOR ICE COVER AND BATHYMETRY................................................................ 16 

LAKE HURON ICE COVER AND BATHYMETRY.................................................................... 18 

LAKE MICHIGAN ICE COVER AND BATHYMETRY............................................................... 20 

LAKE ONTARIO ICE COVER AND BATHYMETRY................................................................. 22 

SUMMARY.......................................................................................................................... 24 

REFERENCES ...................................................................................................................... 24 

APPENDIX 1. DAILY ANOMALY DATA ............................................................................... 26 

APPENDIX 2. LAKE DEPTH, LAKE ID GRIDS AND SPATIAL AVERAGE ICE COVER. ........... 27 

LIST OF FIGURES 

Figure 1. Place Names. ....................................................................................................... 5 

Figure 2. Ice charts digitized each of the 30-winter seasons: 1973 – 2002. ....................... 6 

Figure 3. Number of winters with data for each weekly period. ........................................ 7 

Figure 4a. Median (left) and synoptic median (right) ice covers for January 15, February 

1, March 1, and April 1......................................................................................... 10 

Figure 4b. First (left) and third (right) quartiles of ice cover for January 15, February 1, 

March 1, and April 1............................................................................................. 11 

Figure 4c. Composite minimum and maximum ice cover for January 15, February 1, 

March 1, and April 1............................................................................................. 12 

Figure 5. Ice cover anomaly: January 10, February 28, and March 31, 1994 (blue, above 

median), 1998 (red, below median) ...................................................................... 13 

Figure 6. Lake Erie Bathymetry....................................................................................... 14 

Figure 7. Average Lake Erie ice cover for a) the entire lake for first quartile (Q25), 

median (Q50), and the third quartile (Q75), and for discrete depth ranges for b) 

Q50, c) Q75, and d) Q25....................................................................................... 15 

Figure 8. Lake Superior Bathymetry. ............................................................................... 16 

Figure 9. Average Lake Superior ice cover for a) the entire lake for the first quartile 

(Q25), median (Q50), and the third quartile (Q75), and for discrete depth ranges 

for b) Q50, c), Q75, and d) Q25............................................................................ 17 

Figure 10. Lake Huron Bathymetry.................................................................................. 18 
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Figure 11. Average Lake Huron ice cover for a) the entire lake for the first quartile (Q25), 

median (Q50), and third quartile (Q75), and for discrete depth ranges for b) Q50, 

c) Q75, and d) Q25................................................................................................ 19 

Figure 12. Lake Michigan Bathymetry............................................................................. 20 

Figure 13. Average Lake Michigan ice cover for a) the entire lake for the first quartile 

(Q25), median (Q50), and third quartile (Q75), and for discrete depth ranges for 

b) Q50, c) Q75, and d) Q25. ................................................................................. 21 

Figure 14. Lake Ontario Bathymetry................................................................................ 22 

Figure 15. Average Lake Ontario ice cover for a) the entire lake for the first quartile 

(Q25), median (Q50), and the third quartile (Q75), and for discrete depth ranges 

for b) Q25, c) Q50, and d) Q75............................................................................ 23 

LIST OF TABLES 

Table 1 Distribution of Ice Chart Grids for December 15 – 21.......................................... 8 

Table 2 Distribution of Severe and Mild Winters........................................................... 23 
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Great Lakes Weekly Ice Cover Statistics 

Raymond A. Assel 

ABSTRACT

Data and methods used to calculate weekly ice cover statistics for a 30-winter (1973-2002) base 

period are documented. Spatial distribution patterns of the maximum, minimum, first quartile, 

median, and third quartile ice cover statistics are presented and discussed within the context of 

lake bathymetry ranges. Mild and severe winters are identified for each Great Lake. These data 

are available free of charge on CD-ROM, DVD, and are also on the Internet at: 

http://www.glerl.noaa.gov/data/ice/atlas/weekly_stats/weeklystats.html. Anomaly ice charts and 

other data given here for the first time are available in this report. 

INTRODUCTION

This report is part of a series of reports (Assel 2003a; Assel and Norton 2001; Assel et al. 2002; 

Assel et al. 2003; Assel 2003b; Assel 2004) documenting and supplementing data and products 

published as a NOAA Great Lakes Ice Atlas (Assel 2003c, available free of charge on CD-ROM 

or DVD and for browsing on the Internet at: http://www.glerl.noaa.gov/data/ice/atlas/). Here, 

data and methods used to calculate weekly ice cover statistics for a 30 winter (1973-2002) base 

period are described and discussed. ASCII files, graphics, and computer animations of ice cover 

statistics are available in Assel 2003c. Most place names cited in the text are shown in Figure 1.

Figure 1. Place Names. 
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DATA USED TO CALCULATE WEEKLY STATISTICS

The input data, Assel et al. (2002), consists of 1210 ASCII files of ice concentration. Each file 

consists of a grid of 510 rows by 516 columns with 38,193 over-water “grid cells” embedded in 

this matrix. Each over-water grid cell is coded with a number between 0 and 100, representing 

the percentage of that cell that is covered by ice. The latitude and longitude coordinate of the 

center of each grid cell is given in two fixed formatted text files, Latgrid.zip and Longrid.zip at: 

http://www.glerl.noaa.gov/data/ice/atlas/daily_ice_cover/daily_grids/dailygrids.html. Each grid 

cell has a nominal spatial resolution of 2.55 km x 2.55 km; the actual size of each grid cell varies 

with latitude. The temporal distribution of the data by winter season shows that there is a bias for 

a greater number of observations for the winters after 1989 (Fig. 2). This is because National Ice 

Center ice charts were digitized after 1989. Prior to winter 1989 only Canadian Ice Service ice 

charts were available. Thus, there is a potential bias in the calculation of weekly statistics that is 

addressed in the next section of this report. 

Figure 2. Ice charts digitized each of the 30-winter seasons: 1973 – 2002. 
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METHODS

The data was partitioned into 173 overlapping weekly periods. Week 1 is December 1-7, week 2 

is December 2-8, and so on to week 173, which is May 22-28, (Fig. 3). Thus, for any day in the 

winter season there is a weekly grid representative of that 7-day period. The number of winters 

with ice charts for each weekly period varied over the winter season. The statistics, described 

below, are most robust between the weeks with beginning dates from December 27 to April 17 

because 90% to100% of the 30 winters had observations for each 7-day period between those 

dates. 

Maximum and minimum values of ice concentration were found for each over-water grid cell for 

each weekly period using all the data for that week. Once this was done, the aggregated data for 

a given week was searched for the number of grids for each winter. If there was more than one 

grid for a given winter, a median grid was calculated and used to represent that winter for that 

weekly period. In this way no single winter (Fig. 2) would have greater influence than any other 

winter in computing the statistics (first quartile, median, third quartile, and standard deviation) of 

ice concentration for each grid cell for a give week. For example, 42 ice chart grids spanning 21 

individual winter seasons were used for the week of December 15 - 21 (Table 1). These charts 

are in Assel (2003c). In years where there was more than one ice chart, a median grid was 

calculated. The Decembers in which this was done for the week of December 15-21 include: 

1988 (3 charts), 1989 (4 charts), 1991 (4 charts), 1992 (4 charts), 1993 (3 charts), 1994 (4 

Figure 3. Number of winters with data for each weekly period. 
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charts), 1996 (2 charts), 1997 (2 charts), 1998 (2 charts), 1999 (2 charts), and 2001 (2 charts). 

The Decembers that had only one ice chart for that week include: 1972, 1976, 1977, 1978, 

1981,1982, 1983, 1985, 1995, and 2000. 

Grid No. Year Month Day Grid No. Year Month Day 

1 1972 12 20 20 1992 12 16 

 21 1992 12 18 

2 1976 12 16 22 1992 12 20 

 23 1992 12 21 

3 1977 12 21 

 24 1993 12 15 

4 1978 12 18 25 1993 12 17 

 26 1993 12 20 

5 1981 12 21 

 27 1994 12 16 

6 1982 12 21 28 1994 12 19 

 29 1994 12 19 

7 1983 12 20 30 1994 12 21 

 

8 1985 12 21 31 1995 12 18 

9 1988 12 16 32 1996 12 17 

10 1988 12 19 33 1996 12 20 

11 1988 12 21 

 34 1997 12 16 

12 1989 12 15 35 1997 12 19 

13 1989 12 17 

14 1989 12 18 36 1998 12 15 

15 1989 12 20 37 1998 12 18 

 

16 1991 12 15 38 1999 12 17 

17 1991 12 16 39 1999 12 21 

18 1991 12 18 

19 1991 12 20 40 2000 12 21 

 

 41 2001 12 17 

 42 2001 12 20 

Median and First and Third Quartiles. The first quartile is defined as the 25% point in the 

cumulative frequency distribution (cfd), the median is the 50% point in the cfd, and the third 

quartile is the 75% point in the cfd. The difference between the maximum and minimum defines 

the range of ice cover under the current climate. The median, with half the observations below 

and half above, defines the typical value of ice cover each week for the 30 winters. The first 

quartile demarcates the boundary between the typical value and below-typical ice cover, and the 

third quartiles demarcates the boundary between the typical value and above-typical ice cover for 

the 30-winter base period. The standard deviation also provides a measure of the variation of ice 

cover (about the mean). The mean was not used in this analysis as the typical value because an 

earlier study indicated that ice cover frequency distributions are skewed (Assel et al., 2003), so 

the median is more representative of usually observed ice cover. 

Table 1 Distribution of Ice Chart Grids for December 15 – 21. 
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Synoptic Median. A synoptic median was also calculated for each lake. The synoptic median is 

defined as the original ice chart in the 7-day base period under consideration for which the sum 

of squares of the differences (observed minus median) for all grid cells for a given lake was 

smallest for all ice charts during a given weekly period. Thus, the "synoptic median" is the 

observed ice chart for a given lake that comes closest to the statistical median ice chart. The 

synoptic ice chart may be of more use to numerical models in which real world data, rather than 

statistics, are desirable. Note that for a given week, the synoptic median may occur on a different 

date for each Great Lake. The synoptic median for each lake could be combined to produce an 

“all lakes regional synoptic median” for a given week since all occur for the same weekly period. 

RESULTS

One ASCII grid was produced for each week for each statistic (maximum, minimum, median, 

first quartile, third quartile, standard deviation, and synoptic median) making a total of 1211 

grids (7 statistics x 173 weekly periods). These grids have the same format and ice concentration 

codes described earlier under the “Data” section of this report. Ice concentration for a given 

statistic for a given week was plotted to portray spatial patterns. The spatial patterns for January 

15, February 1, March 1, and April 1 are shown in Fig. 4 to illustrate the seasonal progression of 

the typical ice cover (represented by the median and synoptic median, Fig. 4a), the variation in 

the typical ice cover for each of the above dates in the winter season over the 30 winters 

(represented by the first and third quartiles, Fig. 4b), and the composite extremes (represented by 

the maximum and minimum, Fig. 4c) over the 30-winter base period. Ice concentration in Fig. 4

is color coded in 10% increments from 0%-10% (dark blue) to 90%-100% (red). The ASCII 

grids, the graphic plot files, and the computer animations for all 173 days of each of the seven 

statistics are not shown here for the sake of brevity but as shown below by the example for the 

median statistic all of these statistics are available on the Internet under the general link

http://www.glerl.noaa.gov/data/ice/atlas/weekly_stats/weeklystats.html. 

If you click on the link given above you will find the following text for links to the median 

statistic on that Internet page. 

Median (the 50% point in the frequency distribution) 

Computer Animation: AVI format | FLC format

ASCII Grid Files: medgrid30.zip

Graphic Files: medplotpng30.zip 

Preview Selected Graphic Files at Weekly Intervals 

By clicking on the appropriate icon shown in the above example for the median, you can 

download the selected data. The ASCII grids and graphic files are compressed, medgrid30.zip 

and medplotpng30.zip, respectively. Once you have downloaded these files to your computer, 

you need to un-zip them to access the 173 weekly files. The computer animation can be 

downloaded in a similar manner. You can also preview selected graphic files by placing your 

cursor over the “Graphic Files” icon on the “Preview Selected” line. The individual ice charts in 

Figure 4 were obtained from the median, first quartile, third quartile, maximum, and minimum 

graphic files. The individual ice charts were reduced in size to produce Figure 4.
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Figure 4a. Median (left) and synoptic median (right) ice covers for January 15, February 1, March 1, and April 1. 
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Figure 4b. First (left) and third (right) quartiles of ice cover for January 15, February 1, March 1, and April 1. 
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Figure 4c. Composite minimum and maximum ice cover for January 15, February 1, March 1, and April 1. 
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ANOMALY ICE CHARTS

Daily anomaly ice charts were calculated by subtracting the 30-winter weekly median (for the 

mid-week day) from the same day for each of the 30 winter seasons, e.g. the March 1-7 median 

would be subtracted from the March 4 ice chart for each winter. Anomaly charts provide a way 

to compare ice cover for any date of a given winter season with the long-term median. Anomaly 

graphs and ASCII Grids are given as Appendix 1. These data are not in Assel (2003c) and are 

made available here for the first time. Spatial anomaly patterns for a mild (Assel et al. 2000), and 

a severe winter (Assel et al. 1996) are vividly portrayed in Fig. 5. 

Figure 5. Ice cover anomaly: January 10, February 28, and March 31, 1994 (blue, above median), 1998 (red, below 

median). 
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ICE COVER FOR SPECIFIC BATHYMETRY RANGES

Average ice cover for five discrete lake depth ranges was calculated from the weekly median, 

first quartile, and third quartile statistics for each Great Lake. The depth ranges for Lakes 

Superior, Michigan, Huron, and Ontario are 1-20 m, 21-50 m, 51-100 m, 101-200 m, and over 

200 m. The depth ranges for Lake Erie, which is the shallowest of the Great Lakes, are 1-10 m, 

11-15 m, 16-20 m, 21-30 m, and over 30 m. These data, which provide information of the 

variation of ice concentration for lake surface areas associated with each discrete depth range, 

are not in Assel (2003c). A grid mask coded for each Great Lake and a second grid mask with 

water depths were used to make these calculations. ASCII files of the grids and the ice data are 

included as Appendix 2. The ice data in Figs. 7, 9, 11, 13, and 15 was smoothed using a 7-day 

moving average. Most of the place names used in the discussion below can be found in Fig. 1. 

LAKE ERIE ICE COVER AND BATHYMETRY

Lake Erie has a mean depth of 19 m. This lake is divided into a west basin, from Toledo to Pt. 

Pelee, a central basin from Pt. Pelee to Long Point, and an east basin from Long Pt. to Buffalo, 

see Fig. 1 and Fig. 6. Depths of 1-10 m occur primarily in the west basin of the lake, depths of 

11-15 m and 16-20 m occur primarily in the shore regions of the central and eastern basins of the 

lake, and depths of 21-30 m and depths of over 30 m occur primarily in the off shore waters of 

the central and eastern portions of the lake, with the deepest waters east of Point Pelee. 

Note the general shape of the lake-averaged ice cover for each curve in Fig. 7a is mirrored in its 

depth decomposition in Figs 7b, 7c, and 7d. A major difference of the Q75 curve relative to Q50 

and Q25 (Fig. 7a) is earlier ice formation, longer period near seasonal maximum ice extent (mid 

January to early March, when the lake is over 90% covered), and more extensive ice cover in 

Figure 6. Lake Erie Bathymetry. 
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March and April. This pattern is also seen in Figs 4a-4b. For Q50, the period leading up to 

seasonal maximum ice starts late-December and culminates in mid-February with maximum ice 

cover at about 90%. The period of ice loss starts in the second half of February and is virtually 

complete by the end of March. For Q25, the period of ice formation starts the first half of 

January with a rapid increase in ice in February leading to an abbreviated maximum ice extent, 

about 60%, near mid February. The ice loss period runs from mid-February to mid-March. These 

patterns are also seen in Figs 4a-4b. 

The spatial progression of ice cover is from the shallow west basin and lake perimeter, to the 

deeper areas of the central basin, to the deepest area of the lake located off of Long Point in the 

eastern lake basin. The depth decomposition of these patterns (Fig 7b, Fig 7c, and Fig 7d) reflect 

the general seasonal shift of ice cover from the shallow west basin where ice forms first to the 

deepest east lake basin where the ice lasts longest in the spring. This is shown by the greater ice 

concentrations in deeper depth ranges (16-20 m, 20-30 m, > 30 m relative to the 1-10 m and 11-

15 m depth ranges) after the period of maximum ice extent. More information on Lake Erie ice 

cover is given in Assel (2004). The severe and mild Lake Erie ice cycles are given in Table 2. 

Lake-averaged maximum ice extent has varied from 5% to over 99%. Total observed ice cover 

variation is 10% or less to 90% or more from January 15 to March 1, Fig. 4c. 

Figure 7. Average Lake Erie ice cover for a) the entire lake for first quartile (Q25), median (Q50), and the third 

quartile (Q75), and for discrete depth ranges for b) Q50, c) Q75, and d) Q25. 

a c

b d 
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LAKE SUPERIOR ICE COVER AND BATHYMETRY

Lake Superior, with a mean depth of 148 m, is the deepest of the Great Lakes. The 1-20 m, 21-50 

m, and 51-100 m depth ranges occur most frequently along and extend out from the entire 

southern shore of the lake, in shoal areas associated with Isle Royale and a shoal area (Caribou 

Island) south of Michipicoten Island, and in the three large bays (Thunder, Black, and Nipigon 

Bays) along the north central lake shore (see Fig. 1, Fig. 8). These then are the shore and 

nearshore waters. The 101-200 m and > 200 m depth ranges are the offshore waters of Lake 

Superior. The deepest waters lay north and east of the Keweenaw Peninsula (Fig. 1), making the 

eastern basin of this lake the last to form ice because of its greater heat storage. There is also a 

deep trough off the northwest shore that, in conjunction with prevailing westerly winds, results in 

lower ice concentrations due to upwelling of warmer waters during the winter. 

The annual lake-averaged ice cycle for Lake Superior (Fig. 9a) consists of a period of increasing 

shore ice extent in December and January leading to seasonal maximum ice extent (midFebruary for Q25, first week in March for Q50 and Q75), followed by a period of decreasing ice 

extent, leading to a loss of all ice cover, mid-to-late March for Q25, late March to early April for 

Q50, and mid-to-late April for Q75. 

The maximum lake-averaged ice cover for Q25 (Fig 9a, Fig. 4b) is less than 20% with (Fig. 9d), 

less than 80% for the 1-20 m range, 60% for the 21-50 m range, and 20% for the 51-100 m 

range, depths > 100 m have less than 5% ice cover. The maximum lake-averaged ice cover for 

Q75 (Fig. 9a, Fig 4b) is over 80% with (Fig. 9c) ice cover 80% or greater starting about mid 

January for the 1-20 m depths, late January for the 21-50 m depths, the first week of February for 

Figure 8. Lake Superior Bathymetry. 
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the 51-100 m depths, the second half of February for 101-200 m depths and in early March for 

depths > 200 m. The period of maximum lake-averaged ice extent for Q50 (Fig. 9a, Fig 4a) is 

over 60% with (Fig. 9b) ice cover over 80% by the last week of January for 1-20 m depths, over 

80% by mid February for 21-50 m depths, over 70% by the last week of February for 51-100 m 

depths, and over 50% in early March for 101-200 m depths. More detailed information on Lake 

Superior ice cover for mild, typical, and severe ice cycles is given in Assel (submitted, 2005b). 

Lake Superior ice cycles classified as severe and mild are shown in Table 2. The severe ice 

cycles have ice cover similar to or greater than Q75 (Fig. 9c), and the mild ice cycles have ice 

cover similar to or less than Q25 (Fig. 9d). Lake-averaged maximum ice cover has varied from 

11% to over 99%. The composite maximum and minimum ice charts (Fig. 4c) show that with the 

exception of portions of the east lake basin in January, ice cover on this lake varied from less 

than 10% to 90% over most grid cells from February 1 to April 1. 

Figure 9. Average Lake Superior ice cover for a) the entire lake for the first quartile (Q25), median (Q50), and the 

third quartile (Q75), and for discrete depth ranges for b) Q50, c), Q75, and d) Q25. 

a c

b d 
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LAKE HURON ICE COVER AND BATHYMETRY

Lake Huron has a mean depth of 58 m, and is the second shallowest of the Great Lakes. Lake 

depths (Fig. 10) of 1-20 m and 21-50 m, include the entire perimeter of the lake, extend off 

shore, and include the Straits of Mackinac, Thunder Bay, Saginaw Bay, the southern basin of the 

lake, Georgian Bay, and the North Channel (Fig. 1). Depths of 51-100 m occur in deeper areas of 

Georgian Bay and south of a line from Kincardine, Ontario to Alpena, Michigan. The deepest 

mid lake area, depths of 101-200 m, occurs northeast of that line. That is the last area of the lake 

to form ice and the first to lose it. 

Ice first forms in the shallow areas of the perimeter of Lake Huron in December and January, 

reaches its maximum extent from mid-to-late February, decreases in extent in March, and by late 

March to mid-April covers less than 20% of the lakes surface (Fig. 11a, Fig. 4a). The largest 

increase in ice extent occurs in January, and the largest decrease occurs in March. The seasonal 

maximum lake-averaged ice extent can vary from approximately 80% (Q75) to 30% (Q25), Fig. 

11a, Fig 4b. The maximum ice cover in the shore zone (1-20 m depth range) varies from an 

average of over 90% for Q75 (Fig. 11c), to over 65% for Q25 (Fig. 11d), and is over 80% for 

Q50 (Fig. 11b). Shore areas include Saginaw Bay, shallow areas of Thunder Bay, North 

Channel, and Georgian Bay, and much of the rest of the lake’s perimeter. The duration of 

maximum ice extent for the 1-20 m depths is approximately mid-January to mid-March for Q75 

and Q50 and less than a week around mid-February for Q25. Mid lake depths (51-100 m) have 

maximum ice extent that ranges from an average of over 70% for Q75 (Fig. 11c) to between 5-

10% for Q25 (Fig. 11d), and is over 30% for Q50 (Fig. 11b). These mid-lake depths cover most 

of the southern and western portion of the lake (Fig. 10) and portions of Georgian Bay, North 

Channel. The duration of maximum ice in these mid-lake areas is approximately mid-February 

Figure 10. Lake Huron Bathymetry. 

Kincardine

Alpena 
19

to early to mid March. The deepest area of the lake (101-200 m depths) is located in the north 

and east mid-lake area (Fig. 10). The maximum ice cover for the 101-200 m depths occurs the 

second half of February to early March and varies from an average between 45%-50% for Q75 

(Fig. 11c) to less than 5% for Q25 (Fig. 11d), and less than 15% for Q50 (Fig. 11b). The severe 

and mild Lake Huron ice cycles are identified in Table 2. Lake-averaged maximum ice cover 

has varied from 29% to 98%. The total variation in seasonal ice cover for any grid cell on Lake 

Huron is portrayed in Fig. 4c, which shows that ice concentration has varied from 10% or less to 

90% or more over most of the lakes surface from mid January until the end of March. 

Figure 11. Average Lake Huron ice cover for a) the entire lake for the first quartile (Q25), median (Q50), and third 

quartile (Q75), and for discrete depth ranges for b) Q50, c) Q75, and d) Q25. 

a c 

b d 
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LAKE MICHIGAN ICE COVER AND BATHYMETRY

Lake Michigan has a mean depth of 85 m. The 

shore areas (Fig. 1, Fig. 12), i.e., depths of 1-20 m 

and 21-50 m, line the lake’s perimeter and include 

Green Bay and the lake from the Straits of 

Mackinac to Beaver Island and the Fox Islands in 

the north. The relatively narrow strip of 1-20 m 

and 21-50 m depths along the west and east shores 

becomes wider south of about 43o N. Depths of 51-

100 m are more prevalent in mid lake area of the 

southern lake and depths of 101-200 m are more 

prevalent in mid- lake areas of the northern portion 

of Lake Michigan. However, because of its large 

north to south extent, air temperatures tend to be 

lower in the northern half of the lake. Maximum 

Lake Michigan ice cover (Fig. 13a) is much less 

than that of Lakes Erie, Superior, and Huron (Fig. 

7a, Fig 9a, and Fig 11a) because Lake Michigan 

has more heat storage capacity (due to greater 

depth) relative to Lake Erie and less heat loss 

potential (due to milder air temperatures) relative 

to Lakes Huron and Superior. Maximum lakeaveraged ice cover occurs near mid February (Fig. 

13a) and varies from about 40% for Q75 to about 

10-15% for Q25, and is about 20% for Q50. The 

average ice cover for 1-20 m depths is over 80% 

from about mid-January to the end of February for 

Q75 (Fig. 13c). The Q75 21-50 m and the 51-100 m 

depth ranges have average ice covers of over 60% 

and over 15%, respectively, for most of February. 

At the other extreme (Q25, Fig. 13d) the 1-20 m depth range has ice cover over 40% from late 

January to early March, the 21-50 m depth range has ice cover over 20% in February and early 

March, and the average ice cover is virtually nil for depths over 50 m. For Q50, (Fig. 13b) the 1-

20 m depths have ice that averages over 60% from late January to late February, the 21-50 m 

depths have ice that averages over 30% from late January to early March, and the 51-100 m 

depths have ice that averages over 5% for most of February. The ice cover in the northern 

portion of the lake is usually more extensive than in the southern portion due to lower air 

temperatures and the large shallow areas located there, (Fig. 4a-4b). Lake-averaged maximum 

ice cover in Lake Michigan has varied from 13% to over 96%, the years of extreme ice cycles 

are shown in Table 2. The composite maximum and minimum ice charts (Fig. 4c) show that ice 

cover can vary over most of the lake’s surface from 10% or less to 90% or more during much of 

the winter. 

Figure 12. Lake Michigan Bathymetry. 
21

Figure 13. Average Lake Michigan ice cover for a) the entire lake for the first quartile (Q25), median (Q50), and 

third quartile (Q75), and for discrete depth ranges for b) Q50, c) Q75, and d) Q25. 

a 

b 

c 

d 
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LAKE ONTARIO ICE COVER AND BATHYMETRY

The mean depth of Lake Ontario is 86 m. The 1-20 m and 21-50 m depths line the perimeter of 

the lake (Fig. 14), and include much of the northeast end of the lake from Stoney Point to Prince 

Edward Point (Fig. 1).

Lake Ontario’s typical (Q50) ice cover (Fig. 15a) is less than Lake Michigan’s (Q50, Fig. 13a) 

and tends to be located in the northeast quadrant of the lake (Fig. 4a). Lake Ontario’s mean depth 

is similar to that of Lake Michigan, giving it a substantial heat storage capacity. However, unlike 

Lake Michigan, Lake Ontario is located substantially south of 45o

 N, and so it is exposed to 

milder air temperatures than northern portions of Lakes Michigan and Huron and all of Lake 

Superior. For these reasons, maximum lake-averaged ice cover is lower on this lake than Lakes 

Erie, Superior, Huron, and Michigan. Maximum lake-averaged ice cover for Q75 (Fig. 15a) is 

less than 30%, for Q50 is less than 20%, and for Q25 is less than 10%. The ice cover is usually 

limited to the 1-20 m and 21-50 m depths where the average ice cover varies between 50-60% 

and 40-50% respectively for Q75 (Fig. 15c), between 40-50% and 20-30% respectively for Q50 

(Fig. 15b), and between 30-40% and 10-15% respectively for Q25 (Fig. 15d), during the month 

of February. Only in Q75 (Fig. 15c) is there substantial depth-averaged ice cover for the 51-100 

m depths, maximum value ranging from 10% to 15% in mid February. The ice cover is virtually 

gone by the end of March (Figs. 4a-4b) with the exception of some ice in the shore areas located 

in the northeast quadrant of the lake, e.g., the Bay of Quinte. Lake Ontario ice cycles classified 

as severe and mild are given in Table 2. Lake-averaged maximum ice extent has varied from 6% 

to 90%. The composite maximum ice cover charts (Fig. 4c) show that in a few extreme winters 

(e.g.,1979, 1994) extensive ice cover has formed in the deep mid-lake area of this lake. 

Figure 14. Lake Ontario Bathymetry. 
23

Figure 15. Average Lake Ontario ice cover for a) the entire lake for the first quartile (Q25), median (Q50), and the 

third quartile (Q75), and for discrete depth ranges for b) Q25, c) Q50, and d) Q75. 

Table 2 Distribution of Severe and Mild Winters* 

Severe** Mild*** 

Erie 1996 1986 1984 1982 1978 1977 2002 1998 1991 1987 1983 1985 

Superior 1996 1994 1986 1979 1977 1974 2002 2000 1999 1998 1995 1987 

Huron 1996 1994 1982 1979 1978 1977 2002 2000 1999 1998 1987 1983 

Michigan 1994 1986 1982 1979 1978 1977 2002 2000 1998 1995 1987 1983 

Ontario 1994 1986 1982 1979 1978 1977 2002 1999 1998 1987 1983 1975 

* Listed in order of severity given in Assel (submitted, 2005a). 

** Severe ice cycles occurred on all Great Lakes in 1977, on four Great Lakes in 1978, 1979, and 1994. 

*** Mild ice cycles occurred on all Great Lakes in 1987, 1998 and 2002, on four Great Lakes in 1983.

a 

b 

c 

d 
24

SUMMARY

Data and methods used to calculate the weekly ice cover statistics for a 30-winter base period 

(1973-2002) are described. The entire data set was not presented here for the sake of brevity, but 

it is available at http://www.glerl.noaa.gov/data/ice/atlas/weekly_stats/weeklystats.html. 

Anomaly ice charts, the differences from the median, not given in Assel (2003c) were presented. 

Anomaly ice charts, Appendix1, provide a method to quantify ice cycle severity for any given 

winter and area of the Great Lakes. An overview of the typical value (median ice charts), 

variation of the typical value (first and third quartile) and climatic extremes (composite 

maximum and minimum) of ice cover was presented in graphical form (Figs 4a-4c). The ice 

cover between the first and third quartile show that most areas of the Great Lakes range from less 

than 10% ice cover to 80% or more over much of the winter season during the 30-winter base 

period. This variation is used to define the typical range of ice cover, mild winters have ice cover 

below the first quartile, and severe winters have ice cover above the third quartile. The spatialaveraged ice cover for five discrete depth ranges was calculated for each Great Lake for the 

median, first quartile, and third quartile for each week (Appendix 2). Plots of these data portray 

the depth dependent seasonal progression of ice cover for typical, severe, and mild winters (Fig. 

7, Fig. 9, Fig. 11, Fig. 13, and Fig. 15) for Lakes Erie, Superior, Huron, Michigan, and Ontario, 

respectively. All of these data provide a baseline of Great Lakes ice cover over the last three 

decades of the 20th century and first years of the 21st century. 

REFERENCES

Assel, R.A. (submitted, 2005a) Classification of Annual Great Lakes Ice Cycles: Winters 1973 – 

2002. J. Climate. 

Assel, R.A. (submitted, 2005b) Contemporary Lake Superior ice cover climatology. In: State of 

lake Superior in the 21st Century: health, integrity, and sustainability, AEHMS. 

Assel, R.A. 2004. 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, 36 pp. 

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-128/tm-128.pdf

Assel, R.A. 2003a. Great Lakes Ice Cover, First Ice, Last Ice, and Ice Duration: Winters 1973-

2002. NOAA TM ERL GLERL-125, Great Lakes Environmental Research Laboratory, 

Ann Arbor, MI. ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-125/tm-125.pdf

Assel, R. A. 2003b. Great Lakes monthly and seasonal accumulations of freezing degree days -- 

winters 1898-2002. NOAA Technical Memorandum GLERL-127. NOAA, Great Lakes 

Environmental Research Laboratory, Ann Arbor, MI, 36 pp. 

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-127

Assel, R.A. 2003c. NOAA Atlas. An Electronic Atlas of Great Lakes Ice Cover, Winters: 1973-

2002. Great Lakes Environmental Research Lab., Ann Arbor, MI. 

http://www.glerl.noaa.gov/data/ice/atlas/
25

Assel, R.A., Cronk, K. and Norton, D. C. 2003. Recent Trends in Great Lakes Ice Cover Climate 

Change 57(1):185-204. http://www.glerl.noaa.gov/pubs/fulltext/2003/20030001.pdf

Assel, R.A., Norton, D.C., and Cronk, K.C., 2002. A Great Lakes Ice Cover digital data set for 

winters 1973-2000. NOAA TM GLERL-121. Great Lakes Environmental Research 

Laboratory, Ann Arbor, MI. (available on the Internet at: 

ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-121/tm-121.pdf .

Assel, R.A. and D. C. Norton. 2001. Visualization of Great Lakes Ice Cycles. 2001. EOS, 

Transations, American Geophysical Union, 82(7) February 13, 2001. 

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

Assel, R.A., J. E. Janowiak, D. Boyce, C. O'Connors, F. H. Quinn, and D. C. Norton., 2000: 

Laurentian Great Lakes ice and weather conditions for the 1998 El Nino winter. Bull. 

Amer. Meteor. Soc., 81 (4) : 703-717). 

http://www.glerl.noaa.gov/pubs/fulltext/2000/20000001.pdf

Assel, R.A., J. Janowiak, D.Boyce, and S. Young. 1996. Comparison of 1994 Great Lakes 

Winter Weather and Ice Conditions with Previous Years. Bull. Amer. Meteor. Soc.,

77(1):71-88. http://www.glerl.noaa.gov/pubs/fulltext/1996/19960001.pdf
26

APPENDIX 1. DAILY ANOMALY DATA

Introduction. Ice cover anomalies grids are the difference between the weekly median ice concentration 

grid and the ice concentration grid for the daily ice chart in a given year with the date (month and day) 

that corresponds to the center day (fourth day) of the weekly median. Difference were calculated for each 

over-water grid cell. The first weekly median ice chart is for December 1-7. The last median ice chart is 

for the week of May 22-28. Thus, anomaly ice chart can be calculated for all days between December 4 

and May 25. The actual number of anomaly ice charts for a given winter depends upon the date of the 

first and the date of the last ice chart. It is not possible to calculate an ice anomaly chart prior to 

December 4 or prior to the date of the first observed ice chart if it is later than December 4, and after May 

25, or after the date of the last observed ice chart if it is earlier than May 25. The daily ice charts used to 

calculate the median are described in Assel (2003c) under “Daily Ice Cover Time Series”. 

Anomaly Ice Charts Grids. Anomaly ice chart grids are in compressed files. The file name encodes the 

winter and source of the anomaly charts (YYYY.zip and YYYYN.zip) where YYYY is the winter (1973-

2002). There are Canadian Ice Service (CIS) files for winters from 1973 to 1995 and National Ice Center 

(NIC) files for winters from 1989 to 2002. The NIC anomaly ice file names for 1989 to 1995 have an “N” 

after the year. Ice charts after 1995 are a mix of CIS and NIC ice charts. The anomaly grid files contained 

in the compressed files have names that encode the date and type of data, i.e., interpolated data, or 

observed data. 

Grid File Names: YYdddI.ANO and YYdddI.ANO

YY = winter season (for example, 73 for 1973, 74 for 1974, ...102 for 2002) 

ddd = day of the winter season (101=Dec 1, ...132=Jan 1, ...273=May 22) 

I = data indicator: I = 1 (interpolated data), I = 2 (observed data) 

Grid File Format: 

510 records, each record has 516 "grid cells" in integer (I3) format. 

Ice and Land Grid Cell Codes: Land grid cells have a value of 999. Ice cover anomalies can 

vary from –99 to +99. Over water cells with missing data have a code of 888. 

Anomaly Ice Chart Images and Animations. Daily Image files and computer animations of the daily 

image files of ice concentration anomalies over the Great Lakes for a given winter were produced from 

the daily anomaly ice chart grid files. Image files are arranged by winter season in compressed files 

(YYP.zip and YYPN.zip, where “YY”, “N” are defined above, and “P” is an indicator for plot files. 

Image File Names: YYdddI.GIF, where “YY”, “ddd”, and “I “are defined above. 

Animation file Names: YYYYN.AVI, where “YYYY” and”N”, are defined above.

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-133/appendix1/grids to download 

anomaly ice chart grid files. 

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-133/appendix1/animations to download 

computer animations for each winter. There are two animations for winters 1989, 1990, 1991, 1992, 

1993, 1994, and 1995 because there were separate NIC and CIS ice chart anomaly files those winters. 

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-133/appendix1/images to download 

images used to create the computer animations. 
27

APPENDIX 2. LAKE DEPTH, LAKE ID GRIDS AND SPATIAL AVERAGE ICE COVER. 

Lake Depth Grid. Depth24x.new is the name of a grid of lake depth in meters. There are 510 records. 

Each record has a format of 516 (I3), where I = integer format. Land cells are coded with –1. 

Lake ID Grid. GLmask2.txt is the name of a ASCII file that is a land mask grid with a single digit 

numeric code for each of the Great Lake and for some of the connecting channels. There are 510 records. 

Each record has a format of 516 I1. Land grid cells have a code of 0, the grid cell numeric codes for the 

Great Lakes are as follows: Superior = 4, Michigan = 5, Huron = 6, Erie = 7, and Ontario = 8, other 

numeric codes in this file can be neglected. 

Spatial Average Ice Cover. The spatial- averaged ice cover for five discrete depth ranges (1-20 m, 21-50 

m, 51-100 m, 101-200 m, and over 200 m for Lakes Superior, Michigan, Huron, and Ontario and 1-10 m, 

11-15 m, 16-20 m, 21-30 m, and over 30 m for Lake Erie) and for the entire surface area of each Great 

Lake (lake-averaged) was calculated for the median, first quartile, and third quartile statistics for each of 

the 173 overlapping weekly periods. These data are summarized in three tab delimited ASCII files for 

each Great Lake. 

 .

Lake File Names 

 3rd Quartile Median 1st Quartile

Lake Superior: SUP Q75.txt SUP Q50.txt SUP Q25.txt 

Lake Michigan: MIC Q75.txt MIC Q50.txt MIC Q25.txt 

Lake Huron: HUR Q75.txt HUR Q50.txt HUR Q25.txt 

Lake Erie: ERI Q75.txt ERI Q50.txt ERI Q25.txt 

Lake Ontario: ONT Q75.txt ONT Q50.txt ONT Q25.txt

File Structure

Record 1. Title: Lake Name upper limit of depth range Entire 

Record 2. Title: jdate 20 50 100 200 >200 Lake 

Records 3 - 175, tab delimited list of date, Julian date, ice data for 5 depth ranges, lake average ice data. 

The first 6 records of SUP Q75.txt are shown below as an example. 

Record 

1 L. Superior upper limit of depth range Entire 

2 jdate 20 50 100 200 >200 Lake 

3 Dec01qr3 335 12.6 0.7 0.1 0.0 0.0 1.1 

4 Dec02qr3 336 12.8 0.8 0.1 0.0 0.0 1.1 

5 Dec03qr3 337 13.2 0.8 0.1 0.0 0.0 1.1 

6 Dec04qr3 338 13.1 0.8 0.1 0.0 0.0 1.1 

Where “qr3” following the date is an indicator for Q75, in files for Q25 “qr1” is an indicator following 

the date, and in files for Q50 “med” is an indicator that follows the date. 

Click on ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-133/appendix2/ to download the depth 

grid file (Depth24x.new), the Lake ID grid file (GLmask2.txt), or any of the 15 spatial average ice cover 

files given in the table above (SUP Q75.txt, SUP Q50.txt, ...ONT Q75.txt,ONT 

Q50.txt,ONT Q25.txt).