Black Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Star Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	17% 10% 4 9% 5% 5% 5% 4 5% 6% 31%	131328
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	130820
Transparency	Day	52% 3 11% 21% 12% 1	134832
	Night	52% 3 11% 20% 12% 2	135403
Seeing	Day	26% 19% 28% 17% 7% 2	16658
	Night	25% 18% 27% 22% 7% 1	25812

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	9% 14% 6% 21% 10% 5% 5% 3 4 5% 19%	4661
	11th to 20th	Day	13% 11% 4 14% 8% 6% 4 4 4 5% 26%	4650
	21st to end	Day	13% 14% 6% 9% 6% 5% 4 3 4 5% 31%	5241
February	1st to 10th	Day	15% 12% 4 11% 7% 6% 6% 4 6% 5% 25%	1902
	11th to 20th	Day	13% 11% 6% 13% 8% 5% 4 4 4 6% 26%	2048
	21st to end	Day	17% 13% 4 12% 6% 4 4 3 4 4 28%	1730
March	1st to 10th	Day	19% 13% 6% 10% 5% 4 4 3 4 5% 28%	4576
	11th to 20th	Day	13% 11% 5% 10% 5% 5% 4 3 5% 5% 33%	4775
	21st to end	Day	21% 9% 4 10% 5% 4 4 3 4 5% 31%	6102
April	1st to 10th	Day	18% 13% 3 9% 5% 4 4 3 5% 5% 31%	2520
	11th to 20th	Day	16% 11% 3 7% 4 5% 6% 4 5% 6% 33%	2446
	21st to end	Day	18% 9% 4 8% 6% 5% 4 4 5% 5% 32%	2513
May	1st to 10th	Day	30% 12% 3 7% 4 4 4 3 4 6% 22%	5886
	11th to 20th	Day	18% 11% 3 9% 5% 5% 4 4 6% 6% 29%	6314
	21st to end	Day	20% 9% 3 7% 4 4 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	18% 7% 2 7% 6% 6% 5% 5% 7% 9% 30%	2800
	11th to 20th	Day	20% 6% 2 6% 5% 5% 4 5% 6% 9% 33%	2800
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 5% 8% 31%	2800
July	1st to 10th	Day	13% 7% 3 7% 6% 5% 5% 5% 7% 9% 33%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 34%	6360
	21st to end	Day	15% 7% 2 7% 4 4 5% 4 5% 7% 40%	6926
August	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	4484
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 8% 8% 33%	3886
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 8% 38%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	15% 6% 2 6% 4 3 4 3 5% 7% 46%	2284
	21st to end	Day	17% 6% 3 6% 4 4 4 4 6% 7% 39%	2200
October	1st to 10th	Day	21% 9% 2 7% 4 3 4 2 5% 5% 37%	2129
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 38%	1985
	21st to end	Day	21% 13% 4 8% 4 4 3 3 4 5% 30%	1980
November	1st to 10th	Day	12% 10% 4 7% 5% 3 3 3 4 6% 43%	1851
	11th to 20th	Day	16% 12% 3 7% 6% 3 5% 3 6% 6% 34%	1890
	21st to end	Day	17% 14% 5% 8% 5% 4 4 3 4 5% 30%	1774
December	1st to 10th	Day	15% 13% 5% 12% 6% 5% 5% 3 5% 5% 25%	2680
	11th to 20th	Day	18% 15% 6% 8% 5% 3 3 3 3 4 32%	3066
	21st to end	Day	17% 14% 5% 12% 4 4 3 3 5% 5% 28%	4048
January	1st to 10th	Night	12% 13% 7% 19% 8% 5% 4 3 4 5% 21%	8234
	11th to 20th	Night	12% 12% 4 17% 9% 7% 6% 4 5% 5% 18%	8235
	21st to end	Night	12% 12% 4 8% 6% 7% 7% 4 6% 6% 29%	8937
February	1st to 10th	Night	14% 11% 4 9% 7% 7% 5% 5% 6% 6% 26%	2716
	11th to 20th	Night	12% 10% 6% 12% 7% 6% 6% 5% 6% 5% 25%	2458
	21st to end	Night	15% 13% 4 10% 6% 4 5% 4 5% 5% 28%	2076
March	1st to 10th	Night	19% 10% 4 12% 6% 5% 5% 3 5% 5% 27%	5484
	11th to 20th	Night	15% 10% 4 8% 6% 5% 4 3 5% 6% 32%	5384
	21st to end	Night	20% 14% 5% 10% 5% 4 4 3 4 4 29%	5072
April	1st to 10th	Night	19% 12% 3 8% 5% 4 5% 4 5% 6% 29%	2100
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 3 4 6% 34%	2036
	21st to end	Night	20% 10% 3 6% 4 3 4 4 5% 5% 36%	1885
May	1st to 10th	Night	31% 14% 3 6% 4 4 3 3 4 6% 23%	4054
	11th to 20th	Night	18% 13% 3 7% 5% 5% 4 3 4 7% 29%	3621
	21st to end	Night	24% 11% 3 7% 4 4 4 3 5% 7% 30%	3926
June	1st to 10th	Night	20% 7% 2 7% 5% 4 4 3 6% 7% 37%	1600
	11th to 20th	Night	18% 7% 3 6% 4 4 5% 4 6% 8% 36%	1600
	21st to end	Night	15% 6% 2 5% 4 5% 5% 5% 6% 7% 41%	1600
July	1st to 10th	Night	14% 8% 3 6% 5% 4 4 4 6% 9% 38%	3600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 5% 6% 8% 35%	3600
	21st to end	Night	13% 8% 2 6% 4 4 4 3 4 6% 47%	3926
August	1st to 10th	Night	25% 8% 2 6% 3 4 4 3 5% 6% 35%	2619
	11th to 20th	Night	14% 7% 3 7% 5% 4 5% 4 5% 7% 40%	3158
	21st to end	Night	9% 8% 2 6% 3 4 4 3 5% 8% 47%	3586
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 6% 45%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 7% 52%	2115
	21st to end	Night	17% 7% 3 7% 4 4 3 3 4 6% 43%	2200
October	1st to 10th	Night	20% 8% 2 6% 4 4 3 3 5% 6% 39%	2270
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 3 5% 5% 41%	2413
	21st to end	Night	20% 12% 3 7% 5% 3 4 3 5% 6% 32%	2860
November	1st to 10th	Night	14% 10% 2 6% 4 3 4 3 5% 6% 41%	2667
	11th to 20th	Night	17% 10% 4 9% 5% 4 5% 3 4 5% 34%	2730
	21st to end	Night	13% 13% 3 9% 4 5% 5% 4 6% 6% 32%	2840
December	1st to 10th	Night	17% 14% 5% 13% 6% 5% 4 4 5% 6% 22%	4720
	11th to 20th	Night	23% 13% 5% 9% 5% 4 3 3 3 4 29%	5348
	21st to end	Night	18% 14% 6% 12% 5% 4 4 3 4 4 26%	7150

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	66% 2 8% 18% 5%	4662
	11th to 20th	Day	59% 3 8% 14% 14% 2	4680
	21st to end	Day	55% 2 7% 22% 13% 1	5241
February	1st to 10th	Day	58% 4 10% 16% 10% 1	1902
	11th to 20th	Day	56% 4 8% 19% 11% 1	2057
	21st to end	Day	58% 3 9% 18% 11% 1	1730
March	1st to 10th	Day	59% 2 10% 14% 11% 4	4579
	11th to 20th	Day	52% 2 15% 21% 8% 3	4775
	21st to end	Day	56% 3 6% 14% 19% 3	6102
April	1st to 10th	Day	55% 3 9% 20% 11% 2	2520
	11th to 20th	Day	50% 4 10% 20% 14% 1	2446
	21st to end	Day	53% 3 9% 20% 13% 3	2513
May	1st to 10th	Day	65% 3 9% 15% 8% 1	5897
	11th to 20th	Day	55% 3 13% 20% 9% 1	6318
	21st to end	Day	53% 4 11% 22% 10% 1	6949
June	1st to 10th	Day	49% 5% 12% 23% 10%	2800
	11th to 20th	Day	48% 4 15% 22% 11% 1	2800
	21st to end	Day	50% 5% 13% 21% 11% 1	2800
July	1st to 10th	Day	48% 4 13% 22% 12%	6290
	11th to 20th	Day	44% 4 16% 24% 12% 1	6285
	21st to end	Day	44% 3 12% 26% 15%	6728
August	1st to 10th	Day	51% 4 14% 21% 9% 1	4470
	11th to 20th	Day	46% 3 16% 27% 7% 1	3923
	21st to end	Day	42% 3 12% 31% 11% 1	4312
September	1st to 10th	Day	42% 4 11% 27% 15% 1	2400
	11th to 20th	Day	39% 4 8% 25% 22% 2	2284
	21st to end	Day	45% 4 10% 24% 14% 2	2200
October	1st to 10th	Day	50% 2 6% 20% 20% 2	3467
	11th to 20th	Day	43% 2 10% 23% 17% 5%	3213
	21st to end	Day	41% 3 13% 26% 14% 3	3180
November	1st to 10th	Day	42% 4 14% 24% 14% 2	1851
	11th to 20th	Day	49% 4 9% 21% 16% 2	1890
	21st to end	Day	54% 3 13% 21% 8% 1	1774
December	1st to 10th	Day	57% 3 12% 21% 7%	2680
	11th to 20th	Day	55% 2 11% 17% 14% 1	3066
	21st to end	Day	59% 2 7% 14% 14% 4	4048
January	1st to 10th	Night	65% 1 9% 16% 7% 1	8234
	11th to 20th	Night	63% 3 10% 12% 10% 2	8280
	21st to end	Night	53% 2 10% 17% 17% 2	8937
February	1st to 10th	Night	54% 3 12% 20% 9% 2	2716
	11th to 20th	Night	55% 5% 10% 17% 12% 1	2460
	21st to end	Night	55% 3 11% 20% 9% 2	2076
March	1st to 10th	Night	57% 3 10% 16% 13% 2	5494
	11th to 20th	Night	51% 2 17% 20% 8% 3	5385
	21st to end	Night	58% 3 4 13% 19% 3	5072
April	1st to 10th	Night	54% 3 10% 20% 11% 2	2100
	11th to 20th	Night	52% 3 9% 21% 15% 1	2036
	21st to end	Night	49% 3 9% 20% 16% 3	1885
May	1st to 10th	Night	64% 2 11% 13% 8% 1	4060
	11th to 20th	Night	57% 3 12% 17% 8% 2	3622
	21st to end	Night	56% 3 12% 18% 10% 1	3927
June	1st to 10th	Night	46% 5% 11% 25% 12% 1	1600
	11th to 20th	Night	47% 4 16% 23% 10%	1600
	21st to end	Night	41% 6% 15% 25% 13% 1	1600
July	1st to 10th	Night	43% 4 15% 23% 15% 1	3560
	11th to 20th	Night	47% 4 14% 22% 12% 1	3552
	21st to end	Night	39% 3 14% 28% 16% 1	3809
August	1st to 10th	Night	52% 3 13% 23% 8%	2609
	11th to 20th	Night	43% 4 17% 28% 7%	3187
	21st to end	Night	35% 3 13% 35% 13% 1	3586
September	1st to 10th	Night	39% 4 12% 28% 16% 1	2000
	11th to 20th	Night	35% 4 10% 28% 23% 1	2115
	21st to end	Night	43% 4 10% 25% 17% 2	2200
October	1st to 10th	Night	46% 2 12% 23% 15% 2	3712
	11th to 20th	Night	39% 2 11% 24% 19% 5%	3924
	21st to end	Night	40% 3 13% 31% 12% 1	4610
November	1st to 10th	Night	43% 4 10% 23% 17% 2	2667
	11th to 20th	Night	50% 4 9% 20% 15% 2	2730
	21st to end	Night	51% 3 14% 23% 8%	2840
December	1st to 10th	Night	60% 3 10% 21% 5%	4720
	11th to 20th	Night	58% 3 12% 12% 14% 1	5348
	21st to end	Night	60% 3 6% 13% 13% 4	7150

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	33% 31% 27% 6% 3	220
	11th to 20th	Day	36% 29% 27% 6% 1	220
	21st to end	Day	28% 33% 26% 9% 3 1	263
February	1st to 10th	Day	37% 30% 26% 5% 1	342
	11th to 20th	Day	37% 27% 25% 9% 1	536
	21st to end	Day	31% 28% 30% 9% 1	446
March	1st to 10th	Day	33% 25% 29% 11% 1	540
	11th to 20th	Day	34% 26% 25% 10% 3	540
	21st to end	Day	36% 27% 22% 9% 3 2	594
April	1st to 10th	Day	41% 21% 21% 12% 4 1	540
	11th to 20th	Day	34% 25% 24% 11% 5% 1	516
	21st to end	Day	30% 22% 30% 12% 4 2	500
May	1st to 10th	Day	35% 21% 25% 12% 6% 1	503
	11th to 20th	Day	24% 19% 33% 18% 5% 1	527
	21st to end	Day	22% 14% 30% 23% 9% 2	573
June	1st to 10th	Day	18% 18% 32% 20% 10% 2	530
	11th to 20th	Day	23% 13% 30% 20% 12% 1	530
	21st to end	Day	21% 16% 33% 19% 8% 2	530
July	1st to 10th	Day	16% 10% 34% 26% 12% 2	525
	11th to 20th	Day	14% 8% 32% 33% 12% 2	524
	21st to end	Day	14% 11% 38% 25% 10% 2	576
August	1st to 10th	Day	13% 12% 27% 31% 12% 6%	526
	11th to 20th	Day	15% 9% 30% 25% 14% 7%	519
	21st to end	Day	14% 14% 28% 26% 14% 5%	583
September	1st to 10th	Day	19% 11% 26% 24% 14% 6%	530
	11th to 20th	Day	17% 11% 23% 21% 17% 10%	530
	21st to end	Day	21% 13% 28% 21% 11% 6%	530
October	1st to 10th	Day	26% 14% 25% 19% 12% 5%	530
	11th to 20th	Day	26% 18% 28% 18% 8% 2	522
	21st to end	Day	33% 19% 24% 19% 5% 1	462
November	1st to 10th	Day	27% 22% 25% 18% 8%	425
	11th to 20th	Day	30% 26% 24% 15% 4 1	430
	21st to end	Day	38% 24% 23% 14% 2 1	314
December	1st to 10th	Day	33% 29% 25% 10% 4	220
	11th to 20th	Day	40% 25% 25% 7% 2	220
	21st to end	Day	37% 22% 29% 10% 2	242
January	1st to 10th	Night	26% 30% 30% 13% 2	840
	11th to 20th	Night	31% 24% 27% 15% 3	840
	21st to end	Night	25% 25% 32% 15% 2	924
February	1st to 10th	Night	27% 25% 32% 15% 2	840
	11th to 20th	Night	27% 27% 31% 13% 1	823
	21st to end	Night	30% 27% 27% 14% 2	692
March	1st to 10th	Night	29% 24% 30% 15% 2	840
	11th to 20th	Night	27% 25% 28% 17% 2	840
	21st to end	Night	30% 22% 26% 17% 5%	924
April	1st to 10th	Night	30% 21% 30% 17% 3	840
	11th to 20th	Night	28% 23% 26% 17% 6%	816
	21st to end	Night	29% 20% 26% 20% 5%	685
May	1st to 10th	Night	31% 15% 29% 19% 6%	600
	11th to 20th	Night	22% 13% 34% 27% 4	414
	21st to end	Night	20% 8% 28% 31% 12% 1	433
June	1st to 10th	Night	20% 10% 29% 30% 11%	400
	11th to 20th	Night	19% 8% 30% 33% 10% 1	400
	21st to end	Night	15% 9% 27% 37% 12% 1	400
July	1st to 10th	Night	17% 5% 23% 40% 14% 1	390
	11th to 20th	Night	15% 5% 23% 41% 14% 1	388
	21st to end	Night	13% 5% 24% 42% 15%	438
August	1st to 10th	Night	14% 3 23% 41% 17% 1	418
	11th to 20th	Night	16% 5% 21% 34% 21% 2	763
	21st to end	Night	16% 6% 24% 33% 19% 2	880
September	1st to 10th	Night	17% 9% 27% 29% 16% 2	800
	11th to 20th	Night	16% 9% 24% 26% 21% 4	800
	21st to end	Night	21% 11% 24% 29% 14% 2	800
October	1st to 10th	Night	27% 9% 24% 25% 12% 3	800
	11th to 20th	Night	23% 15% 30% 23% 9% 1	800
	21st to end	Night	27% 19% 28% 21% 5%	880
November	1st to 10th	Night	22% 21% 28% 21% 7%	820
	11th to 20th	Night	26% 24% 27% 16% 6% 1	840
	21st to end	Night	27% 22% 27% 19% 4 1	840
December	1st to 10th	Night	30% 26% 25% 15% 4	840
	11th to 20th	Night	33% 22% 27% 15% 3	840
	21st to end	Night	31% 27% 26% 14% 2	924

Page generated at 2026-04-14 02:44:28. Version ac07

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