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 2024-01-17.

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% 4 7% 6% 3 5% 5% 5% 7% 35%	74588
	Night	16% 11% 5% 9% 7% 3 5% 4 5% 6% 29%	75232
Transparency	Day	49% 4 13% 22% 10% 1	74291
	Night	56% 4 11% 19% 9% 1	75064
Seeing	Day	18% 6% 24% 31% 17% 4	14416
	Night	20% 9% 26% 29% 14% 2	24073

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	13% 15% 8% 14% 10% 5% 7% 5% 5% 5% 12%	1510
	11th to 20th	Day	15% 16% 8% 14% 11% 6% 6% 4 3 5% 11%	1488
	21st to end	Day	12% 14% 8% 13% 10% 4 6% 6% 5% 6% 16%	1584
February	1st to 10th	Day	10% 13% 6% 11% 10% 4 6% 5% 5% 7% 24%	1562
	11th to 20th	Day	13% 12% 5% 10% 9% 4 5% 4 6% 6% 27%	1679
	21st to end	Day	11% 13% 5% 9% 8% 4 5% 5% 5% 7% 28%	1480
March	1st to 10th	Day	13% 10% 3 7% 7% 4 4 4 5% 6% 36%	1798
	11th to 20th	Day	10% 10% 4 8% 6% 4 4 4 4 6% 38%	1957
	21st to end	Day	16% 8% 4 6% 5% 3 4 4 6% 7% 37%	2209
April	1st to 10th	Day	18% 9% 4 6% 5% 4 6% 5% 5% 7% 30%	2160
	11th to 20th	Day	18% 10% 4 6% 4 4 4 4 5% 6% 36%	2276
	21st to end	Day	15% 8% 4 8% 6% 3 4 4 6% 8% 34%	2340
May	1st to 10th	Day	15% 7% 4 5% 4 3 4 5% 5% 7% 40%	2340
	11th to 20th	Day	15% 7% 2 5% 6% 3 5% 4 4 7% 42%	2362
	21st to end	Day	11% 6% 3 4 5% 3 5% 5% 6% 9% 45%	2830
June	1st to 10th	Day	13% 5% 2 5% 5% 3 4 5% 6% 9% 44%	2700
	11th to 20th	Day	11% 5% 3 6% 5% 3 5% 5% 7% 9% 41%	2700
	21st to end	Day	14% 6% 3 5% 4 2 4 4 5% 8% 45%	2700
July	1st to 10th	Day	7% 4 2 4 4 3 5% 4 6% 8% 54%	2631
	11th to 20th	Day	10% 5% 2 4 4 2 5% 4 6% 8% 49%	2520
	21st to end	Day	9% 5% 2 5% 5% 3 3 5% 6% 8% 49%	2743
August	1st to 10th	Day	10% 5% 2 4 5% 2 4 5% 6% 9% 47%	2392
	11th to 20th	Day	9% 4 2 4 5% 3 5% 5% 6% 8% 49%	2303
	21st to end	Day	12% 5% 2 5% 4 3 4 3 5% 8% 49%	2546
September	1st to 10th	Day	14% 7% 2 5% 5% 3 4 4 5% 7% 44%	2106
	11th to 20th	Day	13% 4 2 6% 5% 3 4 6% 5% 9% 43%	1980
	21st to end	Day	15% 7% 4 6% 6% 3 4 5% 6% 8% 37%	1980
October	1st to 10th	Day	19% 10% 3 6% 4 3 4 5% 4 9% 33%	1949
	11th to 20th	Day	22% 15% 3 8% 6% 4 4 6% 5% 6% 21%	1727
	21st to end	Day	23% 14% 4 7% 6% 4 6% 6% 5% 5% 19%	1782
November	1st to 10th	Day	17% 14% 4 8% 6% 4 6% 5% 5% 7% 23%	1672
	11th to 20th	Day	15% 17% 6% 11% 8% 4 6% 6% 4 5% 17%	1663
	21st to end	Day	20% 19% 9% 12% 7% 4 5% 4 4 4 13%	1487
December	1st to 10th	Day	14% 22% 11% 15% 9% 4 5% 3 3 4 10%	1330
	11th to 20th	Day	17% 18% 7% 14% 11% 6% 5% 4 4 4 11%	1671
	21st to end	Day	24% 20% 9% 14% 8% 4 4 3 3 3 9%	2431
January	1st to 10th	Night	15% 15% 7% 15% 9% 5% 5% 4 5% 5% 14%	2651
	11th to 20th	Night	15% 16% 8% 11% 13% 4 5% 4 4 4 14%	2617
	21st to end	Night	13% 14% 7% 12% 10% 5% 6% 5% 5% 6% 18%	2772
February	1st to 10th	Night	11% 15% 7% 10% 11% 5% 6% 6% 5% 5% 21%	2397
	11th to 20th	Night	11% 12% 6% 12% 9% 4 6% 5% 5% 7% 25%	2163
	21st to end	Night	15% 13% 6% 9% 8% 4 5% 5% 5% 6% 25%	1776
March	1st to 10th	Night	12% 10% 5% 8% 6% 3 4 3 5% 6% 39%	2149
	11th to 20th	Night	12% 9% 4 9% 6% 3 5% 4 5% 6% 36%	2001
	21st to end	Night	15% 8% 4 6% 6% 3 5% 4 5% 6% 38%	2162
April	1st to 10th	Night	19% 10% 3 6% 6% 4 6% 6% 5% 6% 29%	1800
	11th to 20th	Night	18% 9% 3 5% 5% 3 5% 5% 4 5% 37%	1633
	21st to end	Night	16% 8% 3 7% 6% 3 5% 5% 5% 6% 36%	1620
May	1st to 10th	Night	14% 7% 3 6% 6% 4 5% 4 6% 7% 36%	1620
	11th to 20th	Night	15% 9% 3 7% 6% 3 4 4 5% 6% 37%	1527
	21st to end	Night	12% 5% 2 5% 4 3 4 5% 5% 8% 46%	1444
June	1st to 10th	Night	13% 6% 2 5% 5% 3 4 5% 6% 9% 41%	1260
	11th to 20th	Night	12% 4 2 5% 6% 3 6% 5% 7% 9% 42%	1260
	21st to end	Night	13% 6% 3 4 5% 3 4 5% 5% 7% 44%	1260
July	1st to 10th	Night	9% 3 1 4 4 3 4 4 5% 10% 52%	1324
	11th to 20th	Night	9% 3 3 6% 7% 3 5% 5% 5% 7% 48%	1440
	21st to end	Night	9% 5% 3 5% 4 3 5% 4 6% 7% 49%	1575
August	1st to 10th	Night	8% 5% 3 5% 5% 2 3 5% 5% 8% 51%	1544
	11th to 20th	Night	9% 3 3 5% 4 3 4 4 5% 8% 52%	1598
	21st to end	Night	12% 6% 2 5% 4 2 4 5% 4 7% 48%	1808
September	1st to 10th	Night	13% 6% 2 5% 5% 2 4 4 4 6% 47%	1835
	11th to 20th	Night	16% 5% 2 5% 5% 3 4 4 5% 8% 43%	1980
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 6% 7% 37%	1980
October	1st to 10th	Night	20% 8% 2 5% 5% 3 4 4 5% 5% 38%	2011
	11th to 20th	Night	24% 11% 4 7% 6% 3 4 5% 5% 6% 24%	2232
	21st to end	Night	27% 13% 4 8% 6% 4 5% 4 4 4 22%	2574
November	1st to 10th	Night	17% 15% 5% 7% 6% 3 4 5% 5% 7% 25%	2406
	11th to 20th	Night	16% 17% 6% 11% 8% 4 6% 5% 5% 5% 18%	2513
	21st to end	Night	20% 20% 8% 12% 6% 3 4 4 4 4 14%	2688
December	1st to 10th	Night	14% 20% 9% 14% 10% 4 4 3 4 4 13%	2850
	11th to 20th	Night	17% 16% 8% 17% 9% 4 4 4 4 4 13%	3526
	21st to end	Night	25% 21% 9% 12% 7% 4 4 3 3 3 10%	5236

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% 6% 10% 9% 2	1511
	11th to 20th	Day	75% 7% 7% 7% 2	1488
	21st to end	Day	68% 6% 10% 12% 5%	1584
February	1st to 10th	Day	61% 4 13% 18% 4	1562
	11th to 20th	Day	57% 5% 13% 16% 9% 1	1688
	21st to end	Day	57% 3 12% 21% 7%	1480
March	1st to 10th	Day	49% 4 13% 21% 12% 2	1800
	11th to 20th	Day	48% 3 13% 25% 9% 2	1957
	21st to end	Day	47% 5% 12% 22% 11% 3	2209
April	1st to 10th	Day	54% 3 10% 21% 11% 1	2160
	11th to 20th	Day	49% 3 10% 22% 15% 2	2276
	21st to end	Day	49% 3 11% 19% 15% 4	2340
May	1st to 10th	Day	43% 4 10% 26% 14% 4	2340
	11th to 20th	Day	44% 4 8% 27% 15% 2	2366
	21st to end	Day	37% 5% 16% 27% 13% 3	2830
June	1st to 10th	Day	36% 5% 13% 30% 14% 2	2700
	11th to 20th	Day	39% 5% 13% 27% 15% 2	2700
	21st to end	Day	38% 4 15% 26% 13% 3	2700
July	1st to 10th	Day	28% 5% 16% 32% 19% 1	2560
	11th to 20th	Day	33% 4 13% 33% 15% 2	2445
	21st to end	Day	33% 5% 16% 29% 16% 1	2591
August	1st to 10th	Day	33% 4 19% 30% 12% 1	2377
	11th to 20th	Day	32% 5% 17% 32% 13% 1	2303
	21st to end	Day	35% 4 18% 30% 12% 2	2546
September	1st to 10th	Day	40% 5% 14% 29% 12% 1	2106
	11th to 20th	Day	37% 4 19% 31% 9%	1980
	21st to end	Day	45% 5% 18% 25% 7%	1980
October	1st to 10th	Day	48% 5% 13% 25% 8%	1949
	11th to 20th	Day	62% 6% 12% 16% 3	1727
	21st to end	Day	64% 6% 11% 15% 4	1782
November	1st to 10th	Day	58% 4 12% 21% 5%	1672
	11th to 20th	Day	67% 6% 12% 10% 4	1663
	21st to end	Day	76% 3 12% 7% 2	1487
December	1st to 10th	Day	77% 4 8% 7% 3	1330
	11th to 20th	Day	78% 5% 8% 8% 2	1671
	21st to end	Day	84% 3 5% 4 4	2431
January	1st to 10th	Night	71% 5% 11% 11% 3	2651
	11th to 20th	Night	74% 5% 10% 8% 3	2617
	21st to end	Night	67% 4 9% 13% 6%	2772
February	1st to 10th	Night	64% 3 10% 16% 6%	2397
	11th to 20th	Night	58% 5% 13% 17% 6% 1	2165
	21st to end	Night	60% 3 11% 17% 7%	1776
March	1st to 10th	Night	48% 4 13% 22% 13% 1	2160
	11th to 20th	Night	49% 4 10% 23% 11% 2	2002
	21st to end	Night	47% 4 11% 22% 14% 2	2162
April	1st to 10th	Night	55% 4 9% 19% 13% 1	1800
	11th to 20th	Night	49% 3 8% 24% 13% 3	1633
	21st to end	Night	50% 3 9% 20% 14% 4	1620
May	1st to 10th	Night	47% 4 10% 21% 14% 4	1620
	11th to 20th	Night	48% 3 8% 23% 16% 2	1528
	21st to end	Night	37% 5% 13% 26% 15% 5%	1445
June	1st to 10th	Night	38% 4 13% 25% 18% 1	1260
	11th to 20th	Night	38% 5% 12% 27% 15% 3	1260
	21st to end	Night	39% 5% 13% 28% 12% 3	1260
July	1st to 10th	Night	29% 4 14% 31% 20% 2	1286
	11th to 20th	Night	35% 4 12% 30% 18% 1	1392
	21st to end	Night	34% 5% 15% 27% 17% 1	1487
August	1st to 10th	Night	32% 4 18% 32% 12% 2	1535
	11th to 20th	Night	30% 4 19% 32% 13% 1	1598
	21st to end	Night	35% 5% 17% 27% 15% 1	1808
September	1st to 10th	Night	38% 5% 12% 28% 16% 1	1835
	11th to 20th	Night	40% 3 18% 30% 8% 1	1980
	21st to end	Night	46% 4 18% 25% 6%	1980
October	1st to 10th	Night	47% 4 12% 25% 10% 1	2011
	11th to 20th	Night	60% 5% 13% 17% 4	2232
	21st to end	Night	66% 4 10% 15% 4	2574
November	1st to 10th	Night	56% 5% 11% 20% 7%	2406
	11th to 20th	Night	67% 4 12% 11% 5%	2513
	21st to end	Night	74% 4 10% 9% 2	2688
December	1st to 10th	Night	74% 5% 10% 10% 2	2850
	11th to 20th	Night	75% 5% 9% 7% 3	3525
	21st to end	Night	82% 3 6% 6% 2	5236

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	21% 10% 34% 25% 9% 1	267
	11th to 20th	Day	24% 12% 29% 27% 8% 1	258
	21st to end	Day	25% 7% 26% 30% 12% 1	264
February	1st to 10th	Day	18% 13% 38% 25% 7% 1	240
	11th to 20th	Day	17% 11% 42% 20% 8% 2	240
	21st to end	Day	18% 16% 35% 26% 6%	198
March	1st to 10th	Day	16% 9% 34% 27% 14%	240
	11th to 20th	Day	17% 8% 31% 31% 11% 2	397
	21st to end	Day	25% 8% 24% 30% 10% 2	462
April	1st to 10th	Day	22% 8% 28% 31% 9% 2	420
	11th to 20th	Day	23% 9% 27% 27% 11% 2	412
	21st to end	Day	20% 6% 32% 28% 13% 2	420
May	1st to 10th	Day	23% 4 27% 29% 13% 3	423
	11th to 20th	Day	17% 5% 21% 37% 15% 4	508
	21st to end	Day	13% 3 21% 37% 19% 6%	679
June	1st to 10th	Day	13% 3 23% 37% 21% 3	630
	11th to 20th	Day	14% 2 23% 37% 20% 4	630
	21st to end	Day	17% 4 17% 37% 20% 4	630
July	1st to 10th	Day	7% 2 16% 41% 26% 7%	621
	11th to 20th	Day	11% 4 17% 36% 25% 7%	618
	21st to end	Day	13% 2 14% 37% 26% 8%	684
August	1st to 10th	Day	11% 3 15% 33% 29% 9%	511
	11th to 20th	Day	10% 3 11% 34% 28% 14%	439
	21st to end	Day	15% 3 17% 33% 25% 7%	495
September	1st to 10th	Day	17% 4 16% 29% 24% 9%	450
	11th to 20th	Day	14% 3 16% 33% 23% 10%	450
	21st to end	Day	18% 7% 19% 29% 21% 6%	450
October	1st to 10th	Day	23% 8% 24% 24% 18% 5%	450
	11th to 20th	Day	26% 9% 29% 25% 9% 1	377
	21st to end	Day	34% 6% 24% 26% 8% 2	297
November	1st to 10th	Day	20% 16% 28% 24% 11% 2	270
	11th to 20th	Day	21% 14% 36% 20% 7% 2	248
	21st to end	Day	26% 18% 34% 18% 3	180
December	1st to 10th	Day	26% 12% 36% 21% 6% 1	180
	11th to 20th	Day	22% 19% 33% 17% 8% 1	180
	21st to end	Day	26% 18% 35% 16% 4 2	198
January	1st to 10th	Night	24% 15% 31% 24% 5% 1	950
	11th to 20th	Night	24% 12% 32% 24% 8%	935
	21st to end	Night	19% 10% 33% 28% 8% 1	990
February	1st to 10th	Night	16% 12% 37% 27% 6%	777
	11th to 20th	Night	17% 11% 34% 27% 10%	703
	21st to end	Night	20% 9% 35% 28% 7%	592
March	1st to 10th	Night	16% 8% 35% 30% 10% 1	720
	11th to 20th	Night	17% 9% 32% 31% 9% 1	720
	21st to end	Night	18% 7% 27% 35% 12% 1	776
April	1st to 10th	Night	23% 7% 28% 33% 8% 1	540
	11th to 20th	Night	23% 5% 24% 33% 13% 2	533
	21st to end	Night	21% 5% 26% 33% 14% 1	540
May	1st to 10th	Night	17% 4 22% 37% 17% 2	540
	11th to 20th	Night	19% 4 16% 39% 19% 2	531
	21st to end	Night	14% 3 19% 34% 26% 4	477
June	1st to 10th	Night	14% 4 19% 38% 24% 1	360
	11th to 20th	Night	12% 3 19% 36% 28% 2	360
	21st to end	Night	16% 4 14% 37% 26% 3	360
July	1st to 10th	Night	10% 3 12% 44% 27% 4	411
	11th to 20th	Night	8% 3 14% 38% 29% 8%	522
	21st to end	Night	10% 3 13% 42% 28% 4	591
August	1st to 10th	Night	12% 3 14% 39% 27% 5%	537
	11th to 20th	Night	11% 1 17% 30% 33% 8%	534
	21st to end	Night	15% 3 14% 39% 25% 4	594
September	1st to 10th	Night	15% 4 17% 31% 27% 6%	575
	11th to 20th	Night	17% 5% 19% 32% 21% 7%	720
	21st to end	Night	21% 7% 18% 27% 21% 6%	720
October	1st to 10th	Night	23% 6% 20% 30% 18% 2	720
	11th to 20th	Night	30% 9% 24% 29% 7% 1	720
	21st to end	Night	31% 14% 22% 26% 6% 1	792
November	1st to 10th	Night	23% 12% 29% 25% 9% 1	740
	11th to 20th	Night	25% 17% 32% 20% 6% 1	760
	21st to end	Night	31% 15% 30% 19% 4	788
December	1st to 10th	Night	23% 16% 36% 19% 5% 1	950
	11th to 20th	Night	26% 17% 34% 16% 6% 2	950
	21st to end	Night	32% 13% 31% 19% 5%	1045

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