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 2024-01-16.

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	14% 9% 3 6% 5% 3 5% 5% 6% 7% 37%	74779
	Night	14% 10% 4 6% 5% 3 4 4 5% 7% 37%	75030
Transparency	Day	46% 8% 14% 21% 10% 1	74485
	Night	47% 8% 14% 21% 10% 1	74858
Seeing	Day	17% 15% 34% 23% 8% 2	14970
	Night	17% 13% 31% 27% 10% 1	25013

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	11% 13% 5% 8% 7% 3 4 4 5% 6% 33%	1517
	11th to 20th	Day	11% 12% 7% 13% 9% 4 4 4 5% 6% 26%	1480
	21st to end	Day	10% 13% 6% 12% 8% 4 5% 4 7% 7% 25%	1584
February	1st to 10th	Day	8% 10% 5% 9% 7% 4 6% 5% 6% 8% 33%	1588
	11th to 20th	Day	9% 10% 4 7% 6% 4 4 5% 6% 7% 38%	1574
	21st to end	Day	9% 14% 5% 11% 6% 3 4 5% 5% 5% 34%	1466
March	1st to 10th	Day	10% 8% 5% 8% 5% 4 5% 5% 4 6% 41%	1798
	11th to 20th	Day	14% 9% 4 7% 5% 3 4 4 5% 6% 39%	1939
	21st to end	Day	16% 10% 3 7% 5% 4 5% 4 5% 7% 34%	2317
April	1st to 10th	Day	14% 10% 3 6% 6% 4 6% 5% 7% 7% 32%	2160
	11th to 20th	Day	18% 12% 4 8% 5% 3 4 5% 5% 5% 32%	2134
	21st to end	Day	20% 10% 3 6% 5% 3 4 4 4 5% 35%	2388
May	1st to 10th	Day	16% 8% 3 6% 6% 3 5% 5% 5% 7% 36%	2520
	11th to 20th	Day	16% 6% 2 6% 5% 3 5% 5% 5% 9% 38%	2479
	21st to end	Day	19% 5% 2 5% 5% 3 5% 4 5% 8% 38%	2725
June	1st to 10th	Day	14% 5% 2 6% 5% 3 4 5% 6% 9% 42%	2520
	11th to 20th	Day	14% 4 3 5% 4 3 5% 5% 6% 9% 41%	2520
	21st to end	Day	17% 6% 2 5% 5% 3 5% 5% 6% 8% 39%	2520
July	1st to 10th	Day	7% 3 2 4 4 2 5% 4 7% 11% 50%	2520
	11th to 20th	Day	11% 4 2 5% 5% 4 5% 6% 6% 9% 43%	2520
	21st to end	Day	9% 4 2 5% 5% 3 5% 6% 6% 9% 47%	2743
August	1st to 10th	Day	8% 4 2 4 5% 3 6% 6% 7% 11% 43%	2507
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 5% 6% 10% 44%	2371
	21st to end	Day	13% 4 1 4 3 3 4 5% 6% 8% 47%	2394
September	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 4 6% 8% 46%	2160
	11th to 20th	Day	14% 6% 2 4 5% 3 4 5% 6% 8% 41%	2147
	21st to end	Day	16% 7% 3 6% 5% 3 4 5% 5% 8% 37%	1937
October	1st to 10th	Day	16% 8% 4 6% 6% 4 5% 5% 6% 8% 33%	1800
	11th to 20th	Day	17% 11% 4 7% 4 4 4 5% 5% 7% 31%	1792
	21st to end	Day	22% 12% 4 5% 6% 2 3 4 4 5% 32%	1782
November	1st to 10th	Day	15% 11% 5% 6% 6% 3 5% 5% 5% 6% 34%	1620
	11th to 20th	Day	16% 14% 7% 7% 6% 4 5% 4 5% 5% 27%	1520
	21st to end	Day	15% 11% 5% 7% 6% 4 4 4 5% 7% 32%	1520
December	1st to 10th	Day	11% 12% 7% 9% 6% 3 7% 5% 7% 6% 28%	1520
	11th to 20th	Day	13% 14% 6% 10% 7% 3 4 4 5% 6% 27%	1914
	21st to end	Day	25% 22% 7% 6% 4 2 2 2 3 3 23%	2783
January	1st to 10th	Night	10% 11% 6% 8% 7% 3 4 5% 5% 7% 35%	2648
	11th to 20th	Night	11% 11% 7% 11% 8% 4 4 4 6% 7% 27%	2591
	21st to end	Night	12% 15% 6% 8% 7% 4 5% 4 5% 8% 26%	2772
February	1st to 10th	Night	8% 11% 5% 8% 7% 4 5% 4 7% 7% 34%	2370
	11th to 20th	Night	9% 9% 3 8% 5% 4 4 5% 6% 8% 39%	2273
	21st to end	Night	9% 13% 6% 9% 6% 4 4 4 6% 8% 33%	1790
March	1st to 10th	Night	10% 9% 3 7% 7% 4 7% 6% 6% 6% 38%	2149
	11th to 20th	Night	12% 10% 4 5% 4 4 5% 6% 5% 6% 40%	2033
	21st to end	Night	14% 10% 4 6% 5% 3 5% 6% 4 7% 38%	2038
April	1st to 10th	Night	15% 10% 3 8% 5% 3 4 6% 6% 5% 35%	1800
	11th to 20th	Night	18% 11% 3 7% 5% 3 5% 4 5% 6% 35%	1775
	21st to end	Night	20% 10% 3 6% 5% 3 5% 4 5% 6% 33%	1568
May	1st to 10th	Night	20% 7% 3 5% 6% 3 4 5% 5% 6% 36%	1440
	11th to 20th	Night	17% 4 3 5% 5% 3 5% 5% 5% 7% 41%	1416
	21st to end	Night	18% 6% 3 4 5% 4 5% 5% 5% 8% 39%	1551
June	1st to 10th	Night	14% 5% 3 5% 4 3 5% 5% 6% 9% 41%	1440
	11th to 20th	Night	15% 6% 2 5% 4 3 4 5% 7% 8% 41%	1440
	21st to end	Night	15% 5% 2 4 4 3 5% 5% 6% 8% 44%	1440
July	1st to 10th	Night	8% 5% 2 4 4 3 4 5% 5% 7% 54%	1440
	11th to 20th	Night	9% 4 2 4 6% 4 5% 5% 6% 6% 50%	1440
	21st to end	Night	8% 4 2 4 4 2 5% 4 6% 9% 52%	1575
August	1st to 10th	Night	9% 4 2 5% 5% 3 4 5% 5% 7% 52%	1432
	11th to 20th	Night	10% 5% 2 4 4 3 3 5% 6% 7% 52%	1527
	21st to end	Night	15% 4 2 3 3 2 3 4 5% 7% 51%	1961
September	1st to 10th	Night	12% 4 2 4 4 2 4 5% 6% 7% 51%	1800
	11th to 20th	Night	12% 5% 2 5% 5% 3 4 5% 5% 6% 49%	1800
	21st to end	Night	18% 6% 2 5% 4 3 4 4 6% 8% 40%	2015
October	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 4 5% 6% 40%	2160
	11th to 20th	Night	17% 8% 3 5% 5% 4 4 4 5% 7% 38%	2168
	21st to end	Night	20% 10% 3 6% 6% 4 4 4 5% 6% 33%	2574
November	1st to 10th	Night	15% 9% 3 6% 6% 3 5% 4 7% 7% 34%	2457
	11th to 20th	Night	17% 13% 4 6% 5% 3 4 4 5% 6% 32%	2660
	21st to end	Night	15% 12% 4 6% 6% 4 4 4 5% 7% 33%	2660
December	1st to 10th	Night	11% 13% 6% 9% 6% 3 4 5% 6% 9% 29%	2660
	11th to 20th	Night	12% 13% 5% 9% 6% 3 4 4 6% 8% 30%	3283
	21st to end	Night	23% 22% 6% 8% 4 2 3 2 3 3 23%	4884

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	51% 7% 12% 21% 8%	1518
	11th to 20th	Day	60% 7% 9% 18% 6%	1480
	21st to end	Day	57% 7% 10% 14% 11% 1	1584
February	1st to 10th	Day	48% 7% 12% 19% 12% 2	1588
	11th to 20th	Day	45% 6% 15% 21% 12% 1	1583
	21st to end	Day	51% 6% 14% 13% 15% 2	1466
March	1st to 10th	Day	45% 6% 16% 18% 13% 2	1800
	11th to 20th	Day	48% 6% 12% 19% 14% 1	1939
	21st to end	Day	51% 8% 12% 15% 12% 2	2317
April	1st to 10th	Day	50% 7% 12% 17% 12% 2	2160
	11th to 20th	Day	55% 6% 12% 17% 10% 1	2134
	21st to end	Day	53% 6% 10% 17% 13% 1	2388
May	1st to 10th	Day	48% 8% 14% 20% 10% 1	2520
	11th to 20th	Day	45% 7% 14% 23% 10% 1	2483
	21st to end	Day	44% 9% 13% 21% 12% 1	2725
June	1st to 10th	Day	39% 9% 17% 22% 12% 1	2520
	11th to 20th	Day	39% 8% 15% 24% 13%	2520
	21st to end	Day	41% 10% 17% 23% 8% 1	2520
July	1st to 10th	Day	28% 12% 14% 29% 16% 1	2451
	11th to 20th	Day	35% 9% 15% 29% 11% 1	2445
	21st to end	Day	32% 11% 17% 27% 11% 1	2591
August	1st to 10th	Day	32% 13% 19% 26% 9% 1	2492
	11th to 20th	Day	36% 10% 17% 26% 10%	2371
	21st to end	Day	34% 9% 19% 26% 12% 1	2394
September	1st to 10th	Day	38% 10% 16% 29% 8%	2160
	11th to 20th	Day	40% 9% 23% 22% 5% 1	2147
	21st to end	Day	47% 9% 17% 22% 5%	1937
October	1st to 10th	Day	50% 10% 13% 21% 6%	1800
	11th to 20th	Day	54% 8% 12% 19% 6% 1	1792
	21st to end	Day	57% 7% 12% 18% 7%	1782
November	1st to 10th	Day	52% 8% 15% 16% 7% 1	1620
	11th to 20th	Day	61% 7% 12% 15% 6%	1520
	21st to end	Day	52% 8% 18% 14% 7% 2	1520
December	1st to 10th	Day	54% 10% 14% 16% 5%	1520
	11th to 20th	Day	58% 7% 12% 12% 9% 3	1915
	21st to end	Day	68% 5% 7% 14% 5% 1	2783
January	1st to 10th	Night	49% 7% 15% 20% 9% 1	2648
	11th to 20th	Night	56% 7% 12% 17% 6% 1	2591
	21st to end	Night	57% 7% 9% 17% 10%	2772
February	1st to 10th	Night	48% 8% 13% 18% 12% 1	2370
	11th to 20th	Night	43% 7% 15% 21% 12% 2	2274
	21st to end	Night	50% 7% 11% 18% 12% 2	1790
March	1st to 10th	Night	47% 7% 14% 17% 13% 3	2160
	11th to 20th	Night	45% 6% 13% 18% 15% 2	2034
	21st to end	Night	46% 8% 13% 19% 12% 3	2038
April	1st to 10th	Night	49% 7% 11% 17% 15% 1	1800
	11th to 20th	Night	51% 5% 11% 20% 12% 2	1775
	21st to end	Night	51% 6% 8% 19% 14% 1	1568
May	1st to 10th	Night	48% 8% 11% 20% 12% 1	1440
	11th to 20th	Night	42% 6% 13% 23% 13% 2	1417
	21st to end	Night	43% 9% 10% 24% 13% 1	1552
June	1st to 10th	Night	39% 9% 14% 22% 14% 2	1440
	11th to 20th	Night	40% 7% 13% 24% 15% 1	1440
	21st to end	Night	37% 10% 17% 24% 11% 2	1440
July	1st to 10th	Night	29% 8% 15% 30% 17% 2	1400
	11th to 20th	Night	31% 10% 14% 30% 13% 1	1392
	21st to end	Night	27% 9% 19% 29% 14% 1	1487
August	1st to 10th	Night	31% 9% 16% 31% 11% 2	1423
	11th to 20th	Night	31% 9% 15% 34% 10%	1527
	21st to end	Night	34% 7% 18% 27% 14%	1961
September	1st to 10th	Night	32% 9% 17% 32% 11% 1	1800
	11th to 20th	Night	35% 9% 21% 30% 5%	1800
	21st to end	Night	42% 9% 18% 25% 6%	2015
October	1st to 10th	Night	45% 9% 14% 24% 7%	2160
	11th to 20th	Night	46% 9% 15% 23% 7% 1	2168
	21st to end	Night	53% 8% 14% 19% 6%	2574
November	1st to 10th	Night	49% 9% 15% 18% 8%	2457
	11th to 20th	Night	53% 9% 15% 15% 7%	2660
	21st to end	Night	52% 8% 15% 16% 8% 1	2660
December	1st to 10th	Night	52% 9% 14% 20% 4	2660
	11th to 20th	Night	53% 8% 15% 16% 7%	3281
	21st to end	Night	69% 4 7% 14% 6%	4884

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	17% 17% 40% 19% 7%	270
	11th to 20th	Day	16% 18% 35% 23% 7% 1	255
	21st to end	Day	18% 19% 38% 21% 5%	264
February	1st to 10th	Day	15% 18% 50% 13% 3 1	240
	11th to 20th	Day	15% 28% 41% 10% 5% 1	240
	21st to end	Day	19% 25% 42% 12% 2	198
March	1st to 10th	Day	12% 20% 43% 23% 3	240
	11th to 20th	Day	14% 23% 38% 18% 7%	240
	21st to end	Day	21% 18% 35% 17% 7% 1	403
April	1st to 10th	Day	20% 17% 34% 22% 6% 1	420
	11th to 20th	Day	24% 15% 25% 25% 10% 1	412
	21st to end	Day	27% 14% 29% 21% 7% 1	523
May	1st to 10th	Day	19% 19% 36% 19% 5% 2	603
	11th to 20th	Day	19% 12% 36% 23% 8% 2	625
	21st to end	Day	20% 11% 34% 24% 9% 2	679
June	1st to 10th	Day	14% 14% 35% 26% 8% 3	630
	11th to 20th	Day	16% 12% 38% 22% 9% 2	630
	21st to end	Day	18% 9% 33% 27% 10% 3	630
July	1st to 10th	Day	9% 8% 37% 32% 12% 3	621
	11th to 20th	Day	11% 10% 34% 27% 13% 4	618
	21st to end	Day	10% 6% 37% 30% 13% 5%	684
August	1st to 10th	Day	11% 8% 33% 34% 12% 2	626
	11th to 20th	Day	13% 8% 27% 35% 14% 3	615
	21st to end	Day	18% 10% 27% 28% 14% 2	513
September	1st to 10th	Day	14% 12% 28% 30% 14% 2	450
	11th to 20th	Day	16% 10% 33% 27% 11% 3	450
	21st to end	Day	20% 17% 29% 21% 11% 1	407
October	1st to 10th	Day	21% 17% 31% 19% 11% 1	270
	11th to 20th	Day	18% 23% 36% 20% 3	270
	21st to end	Day	30% 23% 33% 13% 1	297
November	1st to 10th	Day	20% 22% 34% 18% 6%	270
	11th to 20th	Day	27% 27% 31% 11% 3 1	270
	21st to end	Day	25% 23% 30% 17% 4	270
December	1st to 10th	Day	22% 18% 37% 18% 4	270
	11th to 20th	Day	22% 24% 38% 14% 2	270
	21st to end	Day	25% 19% 39% 14% 2 1	297
January	1st to 10th	Night	16% 18% 43% 19% 4	950
	11th to 20th	Night	15% 19% 41% 19% 5% 1	926
	21st to end	Night	19% 16% 37% 23% 5%	990
February	1st to 10th	Night	15% 19% 39% 22% 5% 1	900
	11th to 20th	Night	14% 19% 37% 21% 8% 1	879
	21st to end	Night	16% 17% 37% 25% 5%	606
March	1st to 10th	Night	13% 16% 35% 28% 8% 1	720
	11th to 20th	Night	18% 14% 35% 26% 7% 1	594
	21st to end	Night	17% 13% 29% 27% 11% 3	594
April	1st to 10th	Night	18% 12% 33% 29% 6% 1	540
	11th to 20th	Night	21% 12% 26% 30% 11% 1	533
	21st to end	Night	23% 9% 25% 29% 12% 2	540
May	1st to 10th	Night	22% 11% 27% 30% 9% 2	540
	11th to 20th	Night	17% 8% 28% 31% 14% 3	531
	21st to end	Night	18% 9% 20% 37% 15% 1	584
June	1st to 10th	Night	16% 6% 24% 37% 16% 2	540
	11th to 20th	Night	17% 7% 23% 38% 14% 1	540
	21st to end	Night	17% 4 22% 37% 18% 2	540
July	1st to 10th	Night	9% 3 23% 44% 18% 2	525
	11th to 20th	Night	8% 5% 26% 39% 19% 2	522
	21st to end	Night	8% 3 26% 38% 22% 3	591
August	1st to 10th	Night	11% 3 27% 38% 20% 1	537
	11th to 20th	Night	12% 5% 20% 31% 29% 4	534
	21st to end	Night	16% 7% 21% 34% 20% 3	594
September	1st to 10th	Night	13% 4 24% 36% 19% 3	540
	11th to 20th	Night	14% 6% 26% 34% 17% 3	540
	21st to end	Night	21% 10% 25% 29% 13% 1	715
October	1st to 10th	Night	20% 11% 30% 26% 13% 1	720
	11th to 20th	Night	20% 16% 31% 25% 7% 1	720
	21st to end	Night	23% 15% 30% 24% 7%	792
November	1st to 10th	Night	20% 15% 32% 25% 6% 1	791
	11th to 20th	Night	22% 18% 32% 20% 8% 1	950
	21st to end	Night	22% 20% 33% 20% 5%	950
December	1st to 10th	Night	16% 19% 39% 20% 5%	950
	11th to 20th	Night	19% 20% 35% 19% 6% 1	950
	21st to end	Night	27% 17% 34% 18% 5%	1045

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