Bois Blanc Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bois Blanc Island Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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% 10% 4 9% 5% 5% 4 3 5% 7% 35%	131169
	Night	15% 12% 5% 11% 6% 5% 4 3 5% 6% 29%	131558
Transparency	Day	49% 5% 12% 23% 10% 1	134646
	Night	55% 5% 12% 19% 9% 1	136155
Seeing	Day	17% 7% 23% 31% 18% 4	17482
	Night	20% 9% 26% 29% 14% 2	27124

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	12% 17% 9% 22% 8% 6% 5% 3 4 4 11%	4671
	11th to 20th	Day	11% 19% 7% 22% 12% 7% 4 3 3 4 9%	4679
	21st to end	Day	9% 14% 6% 16% 8% 7% 7% 4 5% 5% 18%	5156
February	1st to 10th	Day	9% 14% 5% 12% 9% 5% 5% 3 5% 7% 25%	1822
	11th to 20th	Day	12% 13% 5% 12% 7% 6% 4 4 5% 6% 25%	1968
	21st to end	Day	11% 13% 5% 12% 7% 5% 5% 4 5% 6% 27%	1730
March	1st to 10th	Day	13% 10% 2 10% 5% 4 4 3 6% 7% 36%	4578
	11th to 20th	Day	13% 11% 3 11% 7% 6% 5% 4 5% 6% 29%	5038
	21st to end	Day	19% 11% 7% 10% 5% 4 4 3 5% 6% 26%	5673
April	1st to 10th	Day	17% 10% 3 7% 5% 5% 5% 4 5% 7% 31%	2520
	11th to 20th	Day	18% 10% 3 7% 4 4 4 3 5% 6% 37%	2660
	21st to end	Day	14% 9% 3 8% 5% 4 4 4 6% 7% 35%	2730
May	1st to 10th	Day	16% 8% 3 6% 3 4 4 3 5% 6% 43%	5901
	11th to 20th	Day	12% 7% 3 7% 5% 5% 4 4 5% 6% 43%	6034
	21st to end	Day	12% 8% 3 6% 4 4 4 3 5% 8% 45%	7222
June	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 6% 8% 44%	3000
	11th to 20th	Day	11% 6% 3 7% 4 5% 5% 4 7% 9% 40%	3000
	21st to end	Day	15% 7% 2 6% 4 3 4 3 5% 8% 43%	3000
July	1st to 10th	Day	15% 5% 2 5% 3 4 3 3 5% 7% 48%	6638
	11th to 20th	Day	9% 4 2 5% 3 3 4 3 5% 8% 53%	6360
	21st to end	Day	9% 4 2 5% 3 3 3 4 6% 8% 52%	6926
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 45%	4343
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 6% 7% 47%	4174
	21st to end	Day	12% 5% 2 6% 4 4 3 3 4 6% 49%	4617
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 44%	2340
	11th to 20th	Day	12% 5% 2 6% 4 5% 4 4 5% 9% 44%	2200
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 6% 7% 39%	2200
October	1st to 10th	Day	18% 9% 3 6% 4 3 5% 3 5% 9% 35%	2165
	11th to 20th	Day	20% 15% 3 9% 5% 6% 5% 4 5% 6% 22%	1918
	21st to end	Day	22% 14% 3 8% 5% 5% 6% 4 6% 6% 21%	1980
November	1st to 10th	Day	17% 15% 4 10% 5% 5% 5% 3 6% 8% 23%	1852
	11th to 20th	Day	15% 18% 5% 11% 6% 5% 7% 4 5% 6% 18%	1837
	21st to end	Day	20% 21% 9% 13% 6% 5% 5% 3 4 4 12%	1643
December	1st to 10th	Day	14% 26% 8% 15% 7% 5% 5% 3 4 4 10%	2350
	11th to 20th	Day	14% 18% 7% 19% 9% 7% 5% 3 4 4 10%	2691
	21st to end	Day	25% 20% 9% 14% 5% 5% 5% 2 3 3 10%	3553
January	1st to 10th	Night	13% 15% 7% 23% 9% 5% 5% 3 4 5% 12%	8236
	11th to 20th	Night	10% 19% 8% 19% 11% 7% 5% 4 5% 4 9%	8236
	21st to end	Night	9% 13% 6% 15% 8% 7% 7% 5% 6% 6% 19%	9058
February	1st to 10th	Night	10% 15% 5% 11% 9% 6% 5% 4 6% 5% 23%	2796
	11th to 20th	Night	11% 13% 5% 14% 8% 5% 5% 4 5% 6% 25%	2534
	21st to end	Night	14% 14% 5% 9% 7% 6% 5% 4 5% 6% 26%	2076
March	1st to 10th	Night	17% 10% 3 7% 5% 3 3 3 5% 7% 36%	5482
	11th to 20th	Night	12% 11% 4 10% 5% 5% 5% 4 5% 7% 32%	5140
	21st to end	Night	21% 10% 5% 8% 4 4 4 3 4 5% 31%	5544
April	1st to 10th	Night	19% 12% 3 7% 5% 5% 5% 4 4 6% 30%	2100
	11th to 20th	Night	20% 9% 4 6% 4 4 5% 4 4 5% 36%	1908
	21st to end	Night	17% 9% 2 8% 5% 4 5% 4 5% 6% 35%	1890
May	1st to 10th	Night	16% 8% 3 6% 5% 5% 4 4 5% 5% 40%	4062
	11th to 20th	Night	14% 8% 3 8% 5% 4 4 4 5% 5% 40%	3863
	21st to end	Night	17% 8% 3 6% 3 3 4 3 5% 6% 41%	3674
June	1st to 10th	Night	13% 7% 3 6% 4 5% 4 5% 7% 9% 40%	1400
	11th to 20th	Night	12% 5% 1 7% 6% 4 5% 5% 7% 8% 40%	1400
	21st to end	Night	14% 7% 3 5% 5% 4 4 4 5% 7% 42%	1400
July	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 8% 45%	3336
	11th to 20th	Night	10% 4 2 6% 4 4 4 3 5% 6% 53%	3620
	21st to end	Night	9% 4 2 4 3 3 4 3 6% 8% 54%	3948
August	1st to 10th	Night	10% 6% 3 6% 5% 4 4 4 5% 8% 45%	2807
	11th to 20th	Night	14% 4 2 5% 3 3 3 2 5% 8% 50%	2875
	21st to end	Night	11% 5% 2 5% 3 3 4 3 4 6% 53%	3279
September	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 4 7% 47%	2039
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 8% 45%	2200
	21st to end	Night	18% 7% 3 7% 3 4 5% 3 6% 6% 39%	2200
October	1st to 10th	Night	19% 7% 2 6% 5% 4 4 3 5% 6% 40%	2235
	11th to 20th	Night	23% 11% 4 9% 5% 4 5% 3 5% 6% 25%	2480
	21st to end	Night	25% 13% 3 9% 5% 5% 4 4 4 5% 24%	2860
November	1st to 10th	Night	17% 16% 4 8% 5% 4 5% 3 5% 7% 26%	2666
	11th to 20th	Night	15% 18% 5% 12% 7% 6% 5% 3 5% 5% 19%	2779
	21st to end	Night	21% 21% 7% 13% 5% 4 3 3 4 4 14%	2972
December	1st to 10th	Night	13% 23% 8% 15% 8% 5% 4 3 5% 5% 12%	5060
	11th to 20th	Night	14% 18% 7% 21% 8% 5% 4 3 3 4 13%	5736
	21st to end	Night	25% 20% 10% 13% 6% 5% 4 2 3 3 10%	7667

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	73% 7% 10% 7% 3 1	4672
	11th to 20th	Day	79% 6% 5% 7% 2	4710
	21st to end	Day	67% 6% 10% 14% 4	5156
February	1st to 10th	Day	59% 4 12% 20% 5%	1822
	11th to 20th	Day	59% 5% 13% 15% 8% 1	1977
	21st to end	Day	58% 3 12% 21% 6%	1730
March	1st to 10th	Day	47% 5% 16% 17% 14% 3	4580
	11th to 20th	Day	54% 3 14% 19% 7% 1	5038
	21st to end	Day	59% 3 11% 18% 7% 1	5673
April	1st to 10th	Day	53% 4 10% 22% 11% 1	2520
	11th to 20th	Day	49% 3 10% 22% 15% 1	2660
	21st to end	Day	48% 3 12% 19% 15% 3	2730
May	1st to 10th	Day	43% 4 8% 24% 18% 3	5910
	11th to 20th	Day	45% 4 7% 26% 16% 2	6038
	21st to end	Day	39% 4 13% 24% 16% 4	7222
June	1st to 10th	Day	37% 5% 12% 30% 14% 2	3000
	11th to 20th	Day	40% 5% 13% 27% 14% 1	3000
	21st to end	Day	40% 4 15% 26% 13% 2	3000
July	1st to 10th	Day	36% 5% 15% 30% 13% 1	6568
	11th to 20th	Day	28% 6% 18% 38% 10% 1	6285
	21st to end	Day	30% 5% 14% 35% 15%	6728
August	1st to 10th	Day	33% 7% 16% 32% 12%	4328
	11th to 20th	Day	35% 5% 16% 33% 11% 1	4213
	21st to end	Day	36% 5% 18% 32% 8% 1	4616
September	1st to 10th	Day	40% 5% 14% 28% 11% 1	2340
	11th to 20th	Day	36% 4 18% 32% 10%	2200
	21st to end	Day	44% 5% 17% 25% 9%	2200
October	1st to 10th	Day	34% 5% 14% 32% 15%	3529
	11th to 20th	Day	53% 5% 12% 24% 6%	3095
	21st to end	Day	58% 6% 12% 20% 4	3180
November	1st to 10th	Day	57% 4 12% 22% 5%	1852
	11th to 20th	Day	66% 6% 13% 12% 4	1837
	21st to end	Day	75% 4 11% 8% 2	1643
December	1st to 10th	Day	74% 7% 10% 6% 2	2350
	11th to 20th	Day	75% 6% 9% 8% 2	2691
	21st to end	Day	81% 4 4 7% 3	3553
January	1st to 10th	Night	72% 5% 9% 10% 4	8236
	11th to 20th	Night	78% 5% 7% 7% 3	8280
	21st to end	Night	63% 6% 11% 14% 6%	9058
February	1st to 10th	Night	62% 3 11% 16% 7% 1	2796
	11th to 20th	Night	58% 6% 13% 17% 6% 1	2536
	21st to end	Night	59% 4 12% 18% 7%	2076
March	1st to 10th	Night	48% 3 16% 20% 11% 1	5493
	11th to 20th	Night	52% 4 17% 18% 8% 1	5141
	21st to end	Night	56% 3 11% 19% 10% 1	5544
April	1st to 10th	Night	55% 4 8% 19% 13% 1	2100
	11th to 20th	Night	51% 3 8% 24% 13% 2	1908
	21st to end	Night	50% 3 10% 20% 14% 3	1890
May	1st to 10th	Night	45% 5% 8% 22% 16% 4	4069
	11th to 20th	Night	47% 4 6% 26% 16% 1	3864
	21st to end	Night	44% 4 12% 22% 12% 6%	3675
June	1st to 10th	Night	40% 4 12% 24% 19% 1	1400
	11th to 20th	Night	39% 5% 12% 27% 14% 3	1400
	21st to end	Night	41% 5% 13% 27% 11% 3	1400
July	1st to 10th	Night	38% 5% 14% 26% 16% 1	3298
	11th to 20th	Night	31% 5% 16% 32% 16% 1	3572
	21st to end	Night	29% 5% 15% 33% 18% 1	3832
August	1st to 10th	Night	38% 5% 15% 28% 13% 1	2798
	11th to 20th	Night	34% 4 16% 32% 13% 1	2902
	21st to end	Night	32% 5% 15% 34% 14% 1	3279
September	1st to 10th	Night	38% 5% 13% 28% 15% 1	2039
	11th to 20th	Night	37% 4 18% 30% 11% 1	2200
	21st to end	Night	45% 4 17% 25% 8%	2200
October	1st to 10th	Night	38% 4 12% 33% 12% 1	3651
	11th to 20th	Night	46% 5% 16% 22% 11%	4029
	21st to end	Night	55% 6% 13% 21% 5%	4610
November	1st to 10th	Night	56% 5% 11% 20% 7%	2666
	11th to 20th	Night	66% 5% 12% 11% 6%	2779
	21st to end	Night	74% 4 10% 9% 3	2972
December	1st to 10th	Night	72% 8% 11% 8% 2	5060
	11th to 20th	Night	74% 6% 8% 9% 3	5735
	21st to end	Night	81% 4 5% 7% 2	7667

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	19% 12% 33% 27% 8% 1	327
	11th to 20th	Day	21% 12% 31% 26% 8% 1	330
	21st to end	Day	21% 8% 27% 31% 13% 1	363
February	1st to 10th	Day	16% 12% 37% 24% 9% 1	330
	11th to 20th	Day	17% 12% 38% 23% 9% 2	330
	21st to end	Day	18% 13% 35% 27% 7%	273
March	1st to 10th	Day	16% 9% 32% 30% 13%	330
	11th to 20th	Day	19% 8% 29% 32% 10% 1	514
	21st to end	Day	25% 8% 28% 29% 9% 2	594
April	1st to 10th	Day	21% 9% 29% 31% 9% 2	540
	11th to 20th	Day	22% 9% 27% 29% 11% 2	532
	21st to end	Day	19% 6% 31% 29% 13% 2	540
May	1st to 10th	Day	22% 4 26% 30% 16% 3	507
	11th to 20th	Day	17% 5% 20% 37% 17% 4	596
	21st to end	Day	14% 4 21% 36% 21% 5%	789
June	1st to 10th	Day	13% 2 25% 36% 21% 3	730
	11th to 20th	Day	13% 2 22% 39% 20% 3	730
	21st to end	Day	18% 5% 16% 39% 18% 4	730
July	1st to 10th	Day	9% 2 15% 40% 28% 7%	721
	11th to 20th	Day	10% 3 16% 36% 27% 7%	718
	21st to end	Day	12% 2 13% 36% 28% 8%	794
August	1st to 10th	Day	10% 3 15% 33% 31% 9%	597
	11th to 20th	Day	12% 3 10% 33% 27% 15%	519
	21st to end	Day	14% 3 15% 34% 25% 8%	583
September	1st to 10th	Day	17% 4 16% 29% 25% 9%	530
	11th to 20th	Day	12% 3 15% 30% 27% 13%	530
	21st to end	Day	17% 6% 18% 29% 23% 7%	530
October	1st to 10th	Day	21% 7% 25% 24% 18% 5%	530
	11th to 20th	Day	23% 9% 27% 29% 11% 1	448
	21st to end	Day	29% 7% 23% 28% 10% 2	363
November	1st to 10th	Day	20% 15% 27% 25% 11% 2	330
	11th to 20th	Day	21% 15% 34% 21% 8% 2	302
	21st to end	Day	24% 20% 35% 18% 4	220
December	1st to 10th	Day	26% 15% 32% 20% 6% 1	220
	11th to 20th	Day	20% 20% 34% 18% 7%	220
	21st to end	Day	26% 18% 34% 17% 4 1	242
January	1st to 10th	Night	23% 15% 32% 25% 5% 1	1050
	11th to 20th	Night	23% 13% 32% 24% 8%	1050
	21st to end	Night	18% 10% 34% 29% 7% 1	1155
February	1st to 10th	Night	16% 11% 37% 27% 8%	906
	11th to 20th	Night	17% 11% 34% 28% 10%	823
	21st to end	Night	19% 10% 34% 29% 8%	692
March	1st to 10th	Night	17% 8% 34% 31% 9% 1	840
	11th to 20th	Night	18% 9% 33% 32% 8% 1	840
	21st to end	Night	19% 7% 27% 35% 11% 1	905
April	1st to 10th	Night	23% 7% 28% 33% 8% 1	630
	11th to 20th	Night	25% 5% 23% 31% 13% 2	623
	21st to end	Night	22% 4 26% 34% 13% 1	630
May	1st to 10th	Night	17% 3 22% 37% 18% 2	603
	11th to 20th	Night	19% 4 16% 39% 20% 2	591
	21st to end	Night	15% 4 19% 34% 24% 4	531
June	1st to 10th	Night	14% 4 20% 37% 25% 1	400
	11th to 20th	Night	14% 2 18% 39% 26% 2	400
	21st to end	Night	17% 4 15% 37% 25% 3	400
July	1st to 10th	Night	10% 3 12% 42% 28% 3	459
	11th to 20th	Night	8% 2 14% 37% 31% 7%	582
	21st to end	Night	10% 3 13% 40% 30% 4	657
August	1st to 10th	Night	12% 3 14% 38% 28% 6%	597
	11th to 20th	Night	11% 2 16% 30% 32% 8%	594
	21st to end	Night	14% 3 13% 38% 27% 4	660
September	1st to 10th	Night	14% 4 18% 30% 27% 7%	639
	11th to 20th	Night	15% 4 18% 30% 24% 9%	800
	21st to end	Night	20% 7% 17% 28% 22% 7%	800
October	1st to 10th	Night	22% 6% 21% 30% 18% 2	800
	11th to 20th	Night	28% 10% 23% 30% 8% 1	800
	21st to end	Night	29% 13% 22% 28% 7% 1	880
November	1st to 10th	Night	23% 12% 29% 26% 9% 1	820
	11th to 20th	Night	24% 17% 33% 20% 6% 1	840
	21st to end	Night	32% 15% 31% 18% 4	872
December	1st to 10th	Night	22% 17% 36% 19% 5% 1	1050
	11th to 20th	Night	25% 17% 35% 17% 6% 1	1050
	21st to end	Night	32% 13% 31% 18% 5%	1155

Page generated at 2026-04-29 02:21:42. Version ac07

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