Grand Marais
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Marais clear sky chart from 2005-11-06 to 2025-09-19.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	13% 9% 3 7% 4 4 4 4 6% 7% 38%	126805
	Night	12% 10% 4 8% 5% 4 4 4 5% 6% 37%	122538
Transparency	Day	44% 8% 15% 22% 11% 1	130235
	Night	46% 7% 14% 21% 11% 1	127194
Seeing	Day	18% 16% 34% 22% 8% 2	17411
	Night	17% 13% 31% 27% 10% 1	27222

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	7% 12% 5% 13% 6% 4 4 3 4 5% 38%	3617
	11th to 20th	Day	5% 11% 6% 17% 8% 5% 3 3 5% 7% 28%	3589
	21st to end	Day	6% 14% 8% 13% 7% 6% 5% 4 6% 6% 25%	3982
February	1st to 10th	Day	7% 11% 5% 10% 6% 7% 5% 3 7% 7% 33%	1765
	11th to 20th	Day	8% 10% 4 9% 5% 5% 5% 4 6% 7% 37%	1754
	21st to end	Day	8% 14% 5% 11% 5% 4 4 4 6% 6% 33%	1635
March	1st to 10th	Day	10% 10% 3 7% 4 4 5% 3 5% 6% 43%	4478
	11th to 20th	Day	8% 11% 4 10% 5% 5% 5% 4 5% 7% 35%	4883
	21st to end	Day	19% 14% 4 8% 4 4 4 3 4 5% 33%	5826
April	1st to 10th	Day	13% 11% 3 7% 5% 5% 6% 4 7% 7% 33%	2400
	11th to 20th	Day	17% 12% 4 9% 5% 3 4 3 5% 6% 31%	2374
	21st to end	Day	20% 10% 3 7% 4 5% 4 3 5% 5% 34%	2654
May	1st to 10th	Day	15% 7% 2 5% 3 3 3 3 4 5% 50%	6348
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 8% 42%	6319
	21st to end	Day	13% 7% 2 6% 4 5% 5% 3 6% 8% 41%	6949
June	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 9% 41%	2800
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 6% 9% 40%	2800
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 7% 7% 38%	2800
July	1st to 10th	Day	10% 4 2 5% 3 4 5% 4 7% 10% 45%	6359
	11th to 20th	Day	12% 5% 2 5% 4 4 4 5% 7% 9% 45%	6360
	21st to end	Day	10% 4 2 5% 4 4 4 4 7% 10% 46%	6926
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	4563
	11th to 20th	Day	12% 4 2 4 3 4 4 4 6% 10% 47%	4298
	21st to end	Day	13% 5% 2 5% 3 4 5% 5% 7% 9% 42%	4345
September	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 3 6% 8% 45%	2400
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 9% 41%	2362
	21st to end	Day	15% 7% 3 6% 4 4 4 4 5% 8% 39%	2044
October	1st to 10th	Day	16% 7% 4 7% 5% 5% 4 4 6% 8% 34%	1900
	11th to 20th	Day	17% 12% 3 7% 4 5% 4 4 5% 6% 33%	1891
	21st to end	Day	22% 13% 4 6% 5% 3 4 3 4 5% 32%	1881
November	1st to 10th	Day	15% 13% 3 7% 5% 5% 5% 4 5% 6% 33%	1707
	11th to 20th	Day	16% 16% 5% 8% 5% 6% 3 4 5% 5% 27%	1600
	21st to end	Day	15% 12% 5% 8% 4 5% 4 3 6% 7% 31%	1600
December	1st to 10th	Day	13% 15% 4 9% 5% 5% 5% 4 6% 6% 28%	2610
	11th to 20th	Day	10% 20% 5% 9% 5% 4 3 3 4 5% 32%	3004
	21st to end	Day	19% 22% 7% 12% 3 3 3 3 4 3 21%	3982
January	1st to 10th	Night	8% 12% 5% 11% 6% 4 4 3 5% 5% 37%	6348
	11th to 20th	Night	6% 10% 6% 16% 9% 5% 5% 5% 7% 7% 25%	6316
	21st to end	Night	7% 14% 7% 12% 6% 5% 4 3 5% 7% 30%	6996
February	1st to 10th	Night	8% 12% 4 9% 5% 5% 4 4 7% 6% 35%	2633
	11th to 20th	Night	8% 10% 3 10% 5% 6% 4 5% 6% 8% 37%	2533
	21st to end	Night	8% 14% 5% 9% 5% 4 4 3 6% 8% 33%	1995
March	1st to 10th	Night	8% 8% 3 7% 5% 5% 5% 4 6% 7% 43%	5362
	11th to 20th	Night	9% 13% 4 9% 5% 6% 5% 4 7% 8% 30%	5117
	21st to end	Night	20% 12% 3 8% 5% 4 4 3 4 5% 32%	5104
April	1st to 10th	Night	15% 11% 3 8% 5% 4 4 4 6% 5% 36%	2000
	11th to 20th	Night	17% 11% 2 8% 4 4 4 3 5% 6% 36%	1975
	21st to end	Night	19% 10% 3 6% 4 5% 4 3 5% 5% 35%	1742
May	1st to 10th	Night	17% 5% 2 5% 4 4 4 3 5% 5% 47%	3613
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 44%	3596
	21st to end	Night	14% 7% 2 5% 4 4 5% 3 6% 7% 42%	3949
June	1st to 10th	Night	13% 6% 3 6% 4 4 5% 4 6% 8% 41%	1600
	11th to 20th	Night	15% 7% 2 5% 4 4 4 5% 7% 7% 41%	1600
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 7% 7% 44%	1600
July	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 9% 50%	3620
	11th to 20th	Night	10% 4 2 4 4 3 4 3 5% 7% 55%	3620
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 6% 8% 47%	3948
August	1st to 10th	Night	10% 4 2 5% 4 5% 5% 4 6% 8% 48%	2597
	11th to 20th	Night	11% 4 2 5% 4 3 4 4 6% 7% 49%	2758
	21st to end	Night	18% 5% 2 5% 3 3 3 3 4 7% 46%	3553
September	1st to 10th	Night	12% 4 2 5% 3 3 4 4 6% 6% 51%	2000
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 5% 6% 48%	1980
	21st to end	Night	17% 6% 2 5% 3 3 4 3 5% 8% 42%	2128
October	1st to 10th	Night	15% 8% 3 7% 4 4 5% 3 5% 6% 41%	2280
	11th to 20th	Night	16% 8% 3 5% 5% 4 4 3 5% 6% 39%	2289
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 33%	2717
November	1st to 10th	Night	16% 9% 3 7% 5% 4 4 3 7% 7% 33%	2590
	11th to 20th	Night	17% 14% 4 7% 5% 4 4 3 5% 6% 32%	2800
	21st to end	Night	14% 13% 4 7% 5% 4 3 3 6% 6% 33%	2800
December	1st to 10th	Night	12% 14% 5% 10% 6% 5% 5% 4 7% 7% 25%	4580
	11th to 20th	Night	10% 14% 5% 12% 6% 5% 4 3 5% 6% 29%	5203
	21st to end	Night	21% 20% 7% 12% 3 3 3 2 3 3 24%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	49% 7% 8% 20% 12% 4	3618
	11th to 20th	Day	55% 6% 9% 23% 7%	3620
	21st to end	Day	56% 11% 10% 14% 9%	3982
February	1st to 10th	Day	49% 7% 12% 19% 11% 1	1765
	11th to 20th	Day	45% 6% 14% 21% 12% 1	1763
	21st to end	Day	50% 7% 14% 13% 15% 2	1635
March	1st to 10th	Day	42% 6% 16% 17% 13% 6%	4480
	11th to 20th	Day	48% 8% 14% 15% 14% 1	4883
	21st to end	Day	55% 6% 8% 16% 14% 2	5826
April	1st to 10th	Day	50% 7% 12% 17% 12% 2	2400
	11th to 20th	Day	55% 6% 12% 17% 9% 1	2374
	21st to end	Day	53% 6% 10% 18% 13% 1	2654
May	1st to 10th	Day	37% 6% 16% 25% 15% 2	6357
	11th to 20th	Day	41% 8% 13% 21% 15% 2	6323
	21st to end	Day	39% 10% 12% 21% 16% 1	6949
June	1st to 10th	Day	38% 9% 17% 22% 12% 1	2800
	11th to 20th	Day	40% 8% 15% 24% 12%	2800
	21st to end	Day	43% 9% 16% 22% 9% 1	2800
July	1st to 10th	Day	35% 8% 19% 27% 11%	6291
	11th to 20th	Day	33% 7% 23% 31% 6%	6285
	21st to end	Day	31% 9% 21% 31% 8%	6728
August	1st to 10th	Day	37% 10% 19% 23% 10% 1	4548
	11th to 20th	Day	34% 8% 23% 28% 7%	4340
	21st to end	Day	35% 9% 20% 26% 9%	4344
September	1st to 10th	Day	38% 9% 17% 28% 8%	2400
	11th to 20th	Day	40% 8% 24% 22% 5% 1	2362
	21st to end	Day	45% 9% 16% 23% 7%	2044
October	1st to 10th	Day	39% 8% 14% 28% 10% 1	3160
	11th to 20th	Day	48% 6% 12% 25% 8%	3119
	21st to end	Day	54% 6% 15% 17% 7%	3081
November	1st to 10th	Day	52% 8% 16% 16% 7% 1	1707
	11th to 20th	Day	60% 7% 12% 15% 6%	1600
	21st to end	Day	53% 9% 17% 14% 6% 2	1600
December	1st to 10th	Day	56% 9% 16% 15% 3	2610
	11th to 20th	Day	55% 5% 9% 11% 15% 4	3005
	21st to end	Day	68% 5% 7% 14% 6% 1	3982
January	1st to 10th	Night	48% 8% 12% 17% 12% 3	6348
	11th to 20th	Night	55% 8% 10% 17% 9%	6360
	21st to end	Night	53% 8% 12% 16% 10%	6996
February	1st to 10th	Night	47% 8% 13% 19% 12% 1	2633
	11th to 20th	Night	44% 7% 15% 21% 12% 2	2534
	21st to end	Night	49% 7% 11% 19% 12% 2	1995
March	1st to 10th	Night	41% 6% 14% 12% 18% 9%	5373
	11th to 20th	Night	50% 7% 15% 17% 10% 1	5118
	21st to end	Night	54% 6% 8% 16% 15% 1	5104
April	1st to 10th	Night	49% 7% 10% 17% 16% 1	2000
	11th to 20th	Night	50% 5% 11% 21% 11% 2	1975
	21st to end	Night	50% 6% 8% 21% 14% 1	1742
May	1st to 10th	Night	37% 5% 13% 26% 16% 2	3620
	11th to 20th	Night	40% 7% 11% 23% 15% 4	3597
	21st to end	Night	39% 9% 11% 25% 15% 1	3950
June	1st to 10th	Night	40% 9% 13% 22% 15% 2	1600
	11th to 20th	Night	40% 7% 13% 26% 14% 1	1600
	21st to end	Night	37% 9% 17% 25% 11% 2	1600
July	1st to 10th	Night	31% 7% 20% 27% 15% 1	3580
	11th to 20th	Night	29% 7% 23% 34% 7% 1	3572
	21st to end	Night	35% 6% 16% 32% 10% 1	3832
August	1st to 10th	Night	33% 7% 17% 29% 12% 1	2588
	11th to 20th	Night	33% 7% 19% 32% 8%	2782
	21st to end	Night	37% 7% 18% 29% 9%	3553
September	1st to 10th	Night	32% 8% 17% 31% 10% 1	2000
	11th to 20th	Night	34% 9% 22% 29% 5%	1980
	21st to end	Night	40% 8% 18% 26% 7% 1	2128
October	1st to 10th	Night	32% 7% 11% 32% 16% 1	3800
	11th to 20th	Night	44% 6% 13% 25% 13% 1	3800
	21st to end	Night	52% 7% 16% 18% 6%	4467
November	1st to 10th	Night	49% 9% 16% 18% 8%	2590
	11th to 20th	Night	54% 8% 15% 16% 7%	2800
	21st to end	Night	52% 8% 16% 16% 7% 1	2800
December	1st to 10th	Night	56% 8% 16% 16% 4	4580
	11th to 20th	Night	55% 7% 13% 14% 11%	5201
	21st to end	Night	67% 4 6% 14% 9%	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 18% 42% 18% 6%	300
	11th to 20th	Day	16% 17% 40% 20% 6% 1	300
	21st to end	Day	18% 21% 37% 20% 4	330
February	1st to 10th	Day	14% 21% 46% 13% 5% 1	300
	11th to 20th	Day	14% 29% 40% 11% 5% 1	300
	21st to end	Day	20% 24% 42% 12% 2	249
March	1st to 10th	Day	15% 23% 39% 20% 3	300
	11th to 20th	Day	16% 22% 40% 16% 5%	300
	21st to end	Day	23% 19% 34% 17% 6% 1	485
April	1st to 10th	Day	21% 17% 35% 20% 6% 1	500
	11th to 20th	Day	25% 17% 25% 23% 9% 1	492
	21st to end	Day	27% 16% 29% 21% 7% 1	615
May	1st to 10th	Day	19% 19% 36% 18% 6% 2	703
	11th to 20th	Day	19% 14% 35% 23% 7% 2	725
	21st to end	Day	19% 13% 33% 25% 8% 2	789
June	1st to 10th	Day	14% 16% 36% 24% 7% 2	730
	11th to 20th	Day	18% 14% 37% 21% 8% 2	730
	21st to end	Day	19% 11% 34% 25% 9% 3	730
July	1st to 10th	Day	9% 9% 36% 31% 12% 3	721
	11th to 20th	Day	12% 11% 35% 26% 12% 4	718
	21st to end	Day	10% 8% 37% 29% 12% 4	794
August	1st to 10th	Day	12% 9% 33% 33% 12% 2	726
	11th to 20th	Day	13% 8% 29% 33% 14% 3	715
	21st to end	Day	17% 11% 30% 27% 13% 2	603
September	1st to 10th	Day	15% 13% 28% 29% 14% 2	530
	11th to 20th	Day	15% 10% 32% 28% 11% 3	522
	21st to end	Day	20% 17% 30% 22% 11% 1	444
October	1st to 10th	Day	24% 18% 29% 18% 10% 1	300
	11th to 20th	Day	20% 23% 36% 18% 2	300
	21st to end	Day	31% 22% 33% 13% 1	330
November	1st to 10th	Day	21% 22% 35% 17% 6%	300
	11th to 20th	Day	28% 27% 31% 10% 3 1	300
	21st to end	Day	24% 25% 31% 16% 4	300
December	1st to 10th	Day	22% 18% 39% 17% 4	300
	11th to 20th	Day	22% 25% 38% 13% 2	300
	21st to end	Day	23% 18% 40% 14% 4 1	330
January	1st to 10th	Night	16% 19% 43% 18% 4	1000
	11th to 20th	Night	15% 20% 41% 18% 5% 1	1000
	21st to end	Night	18% 17% 37% 23% 5%	1100
February	1st to 10th	Night	16% 18% 38% 21% 5% 1	1000
	11th to 20th	Night	13% 20% 37% 22% 8% 1	979
	21st to end	Night	15% 16% 38% 25% 5%	675
March	1st to 10th	Night	14% 17% 35% 27% 7% 1	800
	11th to 20th	Night	18% 14% 35% 26% 7% 1	660
	21st to end	Night	19% 14% 29% 25% 10% 3	660
April	1st to 10th	Night	19% 12% 34% 28% 6% 1	600
	11th to 20th	Night	20% 14% 27% 29% 10% 1	593
	21st to end	Night	23% 9% 26% 30% 11% 2	600
May	1st to 10th	Night	22% 10% 27% 30% 9% 2	600
	11th to 20th	Night	17% 9% 27% 30% 14% 3	591
	21st to end	Night	18% 9% 20% 37% 15% 1	650
June	1st to 10th	Night	16% 7% 25% 36% 15% 2	600
	11th to 20th	Night	17% 6% 25% 38% 13% 1	600
	21st to end	Night	17% 5% 23% 37% 16% 2	600
July	1st to 10th	Night	9% 3 25% 43% 18% 2	585
	11th to 20th	Night	8% 6% 26% 39% 19% 2	582
	21st to end	Night	9% 4 26% 37% 21% 3	657
August	1st to 10th	Night	11% 3 28% 37% 19% 2	597
	11th to 20th	Night	12% 5% 22% 31% 26% 3	594
	21st to end	Night	15% 7% 22% 33% 20% 3	660
September	1st to 10th	Night	14% 6% 25% 34% 20% 3	600
	11th to 20th	Night	14% 5% 24% 36% 18% 3	594
	21st to end	Night	20% 10% 25% 31% 14% 1	755
October	1st to 10th	Night	20% 12% 30% 25% 12% 1	760
	11th to 20th	Night	20% 16% 33% 25% 6% 1	760
	21st to end	Night	24% 15% 31% 23% 7%	836
November	1st to 10th	Night	21% 15% 31% 25% 6% 1	834
	11th to 20th	Night	22% 18% 31% 20% 8% 1	1000
	21st to end	Night	22% 21% 34% 19% 5%	1000
December	1st to 10th	Night	17% 19% 39% 19% 5%	1000
	11th to 20th	Night	20% 20% 35% 18% 6% 1	1000
	21st to end	Night	26% 17% 34% 18% 5%	1100

Page generated at 2025-09-16 00:49:14. Version ac07

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