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 2025-01-07.

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% 10% 4 9% 5% 4 4 4 5% 7% 35%	104566
	Night	15% 12% 5% 10% 6% 5% 4 3 5% 6% 29%	104593
Transparency	Day	49% 4 13% 23% 10% 1	108060
	Night	55% 4 12% 19% 9% 1	109174
Seeing	Day	17% 6% 24% 31% 17% 4	15737
	Night	20% 9% 26% 29% 14% 2	25346

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	14% 17% 10% 20% 8% 6% 5% 3 5% 4 8%	2555
	11th to 20th	Day	14% 17% 6% 19% 15% 7% 4 3 3 5% 7%	2530
	21st to end	Day	13% 21% 8% 14% 7% 6% 5% 4 5% 5% 11%	2783
February	1st to 10th	Day	9% 14% 5% 13% 9% 5% 5% 4 5% 7% 25%	1648
	11th to 20th	Day	12% 13% 5% 12% 7% 5% 4 4 6% 7% 26%	1775
	21st to end	Day	11% 13% 5% 11% 7% 5% 5% 4 5% 6% 28%	1570
March	1st to 10th	Day	12% 12% 3 8% 5% 4 4 3 6% 7% 38%	3118
	11th to 20th	Day	8% 13% 4 12% 7% 6% 4 4 4 6% 32%	3443
	21st to end	Day	14% 11% 8% 10% 5% 5% 5% 3 6% 6% 28%	3872
April	1st to 10th	Day	18% 11% 3 7% 5% 5% 5% 4 5% 7% 31%	2280
	11th to 20th	Day	18% 10% 3 7% 4 4 4 3 5% 6% 36%	2404
	21st to end	Day	15% 9% 3 8% 5% 4 4 4 6% 7% 35%	2470
May	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 6% 7% 41%	4108
	11th to 20th	Day	12% 6% 3 6% 5% 5% 4 4 5% 7% 44%	4198
	21st to end	Day	13% 6% 2 4 4 3 4 4 5% 9% 46%	5026
June	1st to 10th	Day	13% 5% 2 6% 4 4 4 5% 6% 9% 43%	2850
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 7% 9% 41%	2850
	21st to end	Day	15% 7% 2 6% 4 3 3 3 5% 8% 44%	2850
July	1st to 10th	Day	16% 7% 2 6% 3 4 3 3 5% 7% 44%	4635
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 6% 8% 52%	4440
	21st to end	Day	10% 5% 2 5% 3 3 3 4 6% 8% 52%	4814
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 6% 9% 45%	4210
	11th to 20th	Day	12% 4 2 5% 4 4 4 3 6% 7% 48%	4044
	21st to end	Day	12% 5% 3 6% 4 4 3 3 4 6% 49%	4476
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 45%	2223
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 5% 9% 44%	2090
	21st to end	Day	15% 7% 3 6% 5% 4 4 4 6% 7% 39%	2090
October	1st to 10th	Day	18% 10% 3 6% 4 3 5% 3 5% 8% 35%	2057
	11th to 20th	Day	20% 15% 3 9% 5% 5% 5% 4 5% 6% 22%	1822
	21st to end	Day	23% 15% 3 8% 5% 5% 5% 4 6% 6% 20%	1881
November	1st to 10th	Day	17% 15% 4 10% 5% 5% 5% 3 6% 7% 23%	1762
	11th to 20th	Day	15% 18% 6% 12% 6% 5% 7% 4 4 5% 18%	1750
	21st to end	Day	20% 20% 9% 12% 6% 4 4 3 4 4 13%	1565
December	1st to 10th	Day	14% 26% 8% 14% 7% 5% 5% 3 4 4 10%	2280
	11th to 20th	Day	14% 18% 7% 18% 9% 7% 5% 3 4 4 10%	2621
	21st to end	Day	25% 20% 9% 14% 5% 5% 5% 2 3 3 10%	3476
January	1st to 10th	Night	15% 16% 7% 19% 7% 5% 4 3 4 5% 14%	4516
	11th to 20th	Night	12% 16% 8% 18% 13% 6% 5% 3 4 4 12%	4440
	21st to end	Night	15% 20% 7% 14% 7% 5% 5% 3 5% 5% 14%	4884
February	1st to 10th	Night	10% 16% 5% 11% 9% 6% 5% 4 6% 5% 22%	2530
	11th to 20th	Night	10% 13% 5% 13% 8% 5% 5% 4 5% 6% 25%	2287
	21st to end	Night	14% 14% 5% 9% 8% 6% 5% 4 5% 6% 26%	1884
March	1st to 10th	Night	16% 10% 3 6% 4 3 3 3 4 7% 41%	3722
	11th to 20th	Night	9% 12% 4 9% 5% 4 4 3 5% 7% 38%	3515
	21st to end	Night	15% 9% 6% 9% 5% 4 5% 3 6% 6% 33%	3788
April	1st to 10th	Night	19% 12% 3 7% 5% 5% 5% 5% 5% 6% 30%	1900
	11th to 20th	Night	19% 9% 3 6% 5% 4 5% 4 4 5% 36%	1724
	21st to end	Night	17% 9% 2 8% 5% 4 5% 4 5% 6% 36%	1710
May	1st to 10th	Night	17% 8% 3 7% 6% 5% 4 4 5% 6% 37%	2833
	11th to 20th	Night	12% 8% 2 9% 6% 4 4 4 5% 6% 40%	2695
	21st to end	Night	17% 7% 2 5% 4 3 4 4 5% 7% 42%	2559
June	1st to 10th	Night	13% 7% 2 6% 4 5% 4 5% 7% 9% 40%	1330
	11th to 20th	Night	12% 5% 1 6% 6% 4 5% 4 7% 8% 41%	1330
	21st to end	Night	14% 7% 3 4 5% 5% 4 4 5% 7% 44%	1330
July	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 5% 8% 43%	2330
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 7% 52%	2530
	21st to end	Night	11% 5% 2 5% 3 3 4 3 6% 7% 51%	2750
August	1st to 10th	Night	10% 6% 3 6% 5% 4 4 4 5% 8% 44%	2720
	11th to 20th	Night	14% 4 2 5% 3 3 3 2 5% 8% 51%	2785
	21st to end	Night	11% 5% 2 5% 3 3 4 3 4 6% 52%	3178
September	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 4 6% 48%	1937
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 8% 44%	2090
	21st to end	Night	18% 7% 3 7% 3 4 5% 3 6% 6% 39%	2090
October	1st to 10th	Night	19% 8% 2 6% 5% 4 4 3 5% 5% 39%	2123
	11th to 20th	Night	23% 11% 3 8% 5% 5% 4 4 5% 6% 25%	2356
	21st to end	Night	26% 14% 3 9% 5% 5% 4 4 4 4 22%	2717
November	1st to 10th	Night	17% 16% 4 8% 5% 4 5% 3 5% 7% 26%	2536
	11th to 20th	Night	16% 18% 5% 12% 7% 6% 5% 3 5% 5% 18%	2646
	21st to end	Night	20% 21% 7% 13% 6% 5% 4 3 4 4 13%	2830
December	1st to 10th	Night	13% 23% 7% 15% 8% 5% 4 3 5% 5% 12%	4910
	11th to 20th	Night	14% 19% 7% 20% 8% 5% 4 3 3 4 13%	5586
	21st to end	Night	25% 20% 10% 13% 6% 5% 4 2 3 3 10%	7502

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	79% 4 10% 6% 1	2556
	11th to 20th	Day	82% 5% 7% 5% 1	2530
	21st to end	Day	75% 3 11% 7% 3	2783
February	1st to 10th	Day	60% 4 13% 19% 5%	1648
	11th to 20th	Day	58% 5% 13% 15% 8% 1	1784
	21st to end	Day	56% 3 12% 21% 7%	1570
March	1st to 10th	Day	46% 3 16% 22% 11% 1	3120
	11th to 20th	Day	52% 3 14% 23% 7% 1	3443
	21st to end	Day	57% 3 13% 17% 8% 2	3872
April	1st to 10th	Day	54% 3 10% 21% 11% 1	2280
	11th to 20th	Day	49% 3 10% 22% 14% 2	2404
	21st to end	Day	48% 3 11% 19% 16% 4	2470
May	1st to 10th	Day	44% 4 8% 26% 16% 2	4120
	11th to 20th	Day	42% 4 9% 30% 13% 2	4202
	21st to end	Day	36% 4 14% 25% 19% 3	5026
June	1st to 10th	Day	38% 5% 12% 29% 14% 2	2850
	11th to 20th	Day	39% 5% 13% 27% 15% 1	2850
	21st to end	Day	39% 4 15% 26% 13% 3	2850
July	1st to 10th	Day	40% 4 17% 24% 13% 1	4564
	11th to 20th	Day	28% 6% 16% 38% 11% 1	4365
	21st to end	Day	30% 5% 12% 35% 18% 1	4662
August	1st to 10th	Day	33% 6% 16% 32% 12%	4195
	11th to 20th	Day	35% 5% 16% 32% 12% 1	4083
	21st to end	Day	36% 5% 18% 32% 8% 1	4475
September	1st to 10th	Day	39% 5% 14% 29% 12% 1	2223
	11th to 20th	Day	36% 4 18% 32% 10%	2090
	21st to end	Day	44% 5% 17% 25% 9%	2090
October	1st to 10th	Day	34% 5% 13% 32% 16%	3421
	11th to 20th	Day	53% 5% 12% 24% 6%	2999
	21st to end	Day	58% 6% 12% 20% 4	3081
November	1st to 10th	Day	58% 4 12% 21% 5%	1762
	11th to 20th	Day	66% 5% 12% 11% 4	1750
	21st to end	Day	75% 4 12% 8% 2	1565
December	1st to 10th	Day	75% 7% 11% 6% 2	2280
	11th to 20th	Day	76% 6% 9% 8% 2	2621
	21st to end	Day	81% 4 4 7% 3	3476
January	1st to 10th	Night	72% 4 11% 11% 3	4516
	11th to 20th	Night	78% 4 9% 7% 2	4440
	21st to end	Night	74% 4 11% 8% 3	4884
February	1st to 10th	Night	63% 3 11% 16% 6%	2530
	11th to 20th	Night	58% 6% 12% 17% 6% 1	2289
	21st to end	Night	59% 4 12% 17% 8%	1884
March	1st to 10th	Night	45% 2 17% 24% 11%	3733
	11th to 20th	Night	47% 3 15% 24% 9% 1	3516
	21st to end	Night	51% 4 14% 21% 9% 1	3788
April	1st to 10th	Night	55% 4 8% 19% 13% 1	1900
	11th to 20th	Night	50% 3 8% 24% 12% 2	1724
	21st to end	Night	50% 3 9% 20% 14% 3	1710
May	1st to 10th	Night	48% 5% 9% 23% 14% 2	2840
	11th to 20th	Night	45% 3 8% 28% 15% 1	2696
	21st to end	Night	42% 3 14% 20% 15% 6%	2560
June	1st to 10th	Night	40% 4 13% 24% 18% 1	1330
	11th to 20th	Night	38% 5% 12% 27% 15% 3	1330
	21st to end	Night	40% 5% 13% 28% 11% 3	1330
July	1st to 10th	Night	39% 4 15% 25% 15% 2	2292
	11th to 20th	Night	31% 5% 16% 31% 16% 1	2482
	21st to end	Night	32% 5% 14% 26% 21% 1	2662
August	1st to 10th	Night	38% 4 15% 28% 13% 1	2711
	11th to 20th	Night	33% 4 16% 32% 13% 1	2812
	21st to end	Night	32% 5% 14% 34% 14% 1	3178
September	1st to 10th	Night	38% 5% 12% 29% 15% 1	1937
	11th to 20th	Night	38% 3 17% 30% 10% 1	2090
	21st to end	Night	45% 4 17% 25% 8%	2090
October	1st to 10th	Night	38% 4 12% 33% 12%	3539
	11th to 20th	Night	45% 5% 17% 22% 11%	3905
	21st to end	Night	56% 6% 13% 21% 5%	4467
November	1st to 10th	Night	56% 5% 11% 21% 7%	2536
	11th to 20th	Night	67% 5% 12% 11% 6%	2646
	21st to end	Night	74% 4 10% 10% 2	2830
December	1st to 10th	Night	72% 8% 11% 8% 2	4910
	11th to 20th	Night	74% 6% 8% 9% 3	5585
	21st to end	Night	81% 4 5% 8% 2	7502

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	20% 11% 34% 26% 9% 1	285
	11th to 20th	Day	22% 11% 31% 26% 8% 1	270
	21st to end	Day	24% 7% 25% 30% 13% 1	297
February	1st to 10th	Day	17% 11% 36% 24% 9% 1	270
	11th to 20th	Day	16% 11% 42% 20% 9% 1	270
	21st to end	Day	16% 15% 37% 26% 6%	225
March	1st to 10th	Day	16% 9% 33% 29% 14%	270
	11th to 20th	Day	17% 8% 31% 31% 11% 1	436
	21st to end	Day	24% 8% 27% 30% 9% 2	506
April	1st to 10th	Day	22% 9% 28% 32% 8% 2	460
	11th to 20th	Day	23% 9% 29% 27% 11% 2	452
	21st to end	Day	20% 6% 32% 29% 12% 2	460
May	1st to 10th	Day	22% 4 28% 29% 14% 3	463
	11th to 20th	Day	16% 5% 21% 37% 17% 4	552
	21st to end	Day	13% 3 21% 37% 20% 6%	734
June	1st to 10th	Day	13% 2 25% 36% 21% 3	680
	11th to 20th	Day	13% 2 23% 38% 20% 4	680
	21st to end	Day	18% 5% 17% 38% 19% 4	680
July	1st to 10th	Day	8% 2 16% 40% 26% 7%	671
	11th to 20th	Day	10% 4 16% 35% 27% 7%	668
	21st to end	Day	13% 2 13% 35% 28% 9%	739
August	1st to 10th	Day	11% 3 16% 33% 29% 8%	554
	11th to 20th	Day	11% 3 11% 33% 28% 15%	479
	21st to end	Day	15% 3 16% 34% 25% 7%	539
September	1st to 10th	Day	16% 3 17% 29% 26% 9%	490
	11th to 20th	Day	13% 3 16% 31% 26% 11%	490
	21st to end	Day	18% 7% 18% 29% 22% 7%	490
October	1st to 10th	Day	21% 7% 26% 23% 17% 5%	490
	11th to 20th	Day	25% 9% 28% 26% 11% 1	412
	21st to end	Day	32% 6% 25% 28% 8% 2	330
November	1st to 10th	Day	20% 15% 28% 25% 10% 2	300
	11th to 20th	Day	21% 14% 34% 21% 8% 2	275
	21st to end	Day	25% 19% 35% 19% 3	200
December	1st to 10th	Day	27% 12% 35% 20% 6% 1	200
	11th to 20th	Day	22% 20% 32% 19% 8% 1	200
	21st to end	Day	26% 18% 34% 16% 5% 1	220
January	1st to 10th	Night	23% 14% 32% 25% 5% 1	981
	11th to 20th	Night	23% 12% 32% 24% 8%	950
	21st to end	Night	20% 10% 33% 28% 8% 1	1045
February	1st to 10th	Night	16% 11% 37% 28% 7%	820
	11th to 20th	Night	16% 11% 35% 27% 10%	743
	21st to end	Night	19% 9% 35% 29% 7%	628
March	1st to 10th	Night	17% 8% 34% 31% 9% 1	760
	11th to 20th	Night	17% 9% 33% 32% 9% 1	760
	21st to end	Night	18% 7% 27% 35% 12% 1	819
April	1st to 10th	Night	23% 6% 28% 33% 8% 1	570
	11th to 20th	Night	24% 5% 25% 32% 12% 2	563
	21st to end	Night	22% 4 27% 32% 14% 1	570
May	1st to 10th	Night	18% 4 22% 37% 17% 2	570
	11th to 20th	Night	19% 4 16% 39% 20% 2	561
	21st to end	Night	14% 3 20% 33% 25% 4	504
June	1st to 10th	Night	14% 4 20% 36% 25% 1	380
	11th to 20th	Night	13% 2 19% 38% 26% 2	380
	21st to end	Night	16% 4 15% 37% 25% 3	380
July	1st to 10th	Night	11% 3 12% 43% 28% 4	435
	11th to 20th	Night	8% 3 14% 37% 31% 8%	552
	21st to end	Night	11% 3 12% 40% 30% 4	624
August	1st to 10th	Night	12% 4 14% 39% 27% 5%	567
	11th to 20th	Night	11% 2 16% 30% 32% 8%	564
	21st to end	Night	14% 3 14% 39% 26% 4	627
September	1st to 10th	Night	15% 4 18% 30% 27% 6%	607
	11th to 20th	Night	16% 4 18% 31% 24% 7%	760
	21st to end	Night	20% 7% 17% 28% 22% 6%	760
October	1st to 10th	Night	23% 6% 22% 30% 17% 2	760
	11th to 20th	Night	29% 9% 23% 30% 8% 1	760
	21st to end	Night	30% 14% 22% 27% 6% 1	836
November	1st to 10th	Night	23% 12% 29% 26% 9% 1	780
	11th to 20th	Night	25% 16% 32% 20% 6% 1	800
	21st to end	Night	31% 15% 30% 19% 4	830
December	1st to 10th	Night	23% 16% 36% 19% 5% 1	1000
	11th to 20th	Night	25% 17% 34% 16% 6% 2	1000
	21st to end	Night	32% 13% 30% 19% 6%	1100

Page generated at 2025-01-04 03:23:57. Version ac07

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