Blackstar II Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackstar II Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	127421
	Night	16% 10% 3 8% 5% 4 4 4 5% 6% 33%	124604
Transparency	Day	48% 7% 11% 20% 12% 2	130926
	Night	47% 7% 12% 20% 13% 2	129185
Seeing	Day	25% 20% 28% 17% 7% 2	16385
	Night	23% 20% 27% 22% 8% 1	25518

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% 10% 6% 17% 9% 7% 5% 3 5% 5% 24%	3677
	11th to 20th	Day	13% 12% 4 13% 8% 6% 5% 4 4 5% 27%	3650
	21st to end	Day	15% 12% 7% 9% 5% 4 5% 4 4 5% 32%	4115
February	1st to 10th	Day	14% 11% 5% 10% 7% 5% 6% 4 6% 7% 26%	1824
	11th to 20th	Day	12% 9% 5% 12% 7% 5% 5% 4 7% 6% 28%	1948
	21st to end	Day	18% 11% 3 10% 7% 4 5% 3 5% 4 30%	1650
March	1st to 10th	Day	18% 12% 4 10% 6% 4 4 3 5% 5% 29%	4476
	11th to 20th	Day	13% 9% 5% 9% 6% 5% 5% 4 5% 6% 34%	4670
	21st to end	Day	20% 9% 3 10% 5% 5% 4 3 4 6% 31%	5970
April	1st to 10th	Day	17% 13% 3 9% 6% 4 4 3 5% 5% 32%	2400
	11th to 20th	Day	16% 10% 3 6% 6% 5% 5% 4 5% 6% 34%	2374
	21st to end	Day	17% 9% 3 8% 5% 6% 4 4 5% 5% 33%	2522
May	1st to 10th	Day	32% 11% 3 7% 4 4 4 3 4 6% 22%	5886
	11th to 20th	Day	18% 11% 4 9% 5% 5% 4 4 5% 6% 29%	6286
	21st to end	Day	20% 9% 3 7% 4 5% 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	17% 7% 3 6% 6% 5% 5% 4 7% 9% 30%	2800
	11th to 20th	Day	18% 8% 2 6% 4 5% 5% 4 7% 9% 33%	2800
	21st to end	Day	17% 7% 3 8% 5% 6% 5% 4 6% 8% 32%	2800
July	1st to 10th	Day	12% 8% 3 8% 6% 5% 6% 5% 6% 9% 33%	6360
	11th to 20th	Day	10% 6% 2 8% 5% 6% 6% 5% 8% 11% 34%	6360
	21st to end	Day	15% 7% 3 6% 4 4 5% 4 6% 7% 40%	6926
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 8% 31%	4472
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 4 8% 8% 33%	3898
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 4 8% 8% 38%	4312
September	1st to 10th	Day	12% 7% 2 7% 4 5% 5% 3 5% 6% 43%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 5% 7% 47%	2275
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 5% 8% 38%	2200
October	1st to 10th	Day	20% 9% 3 6% 5% 3 4 3 4 4 37%	2140
	11th to 20th	Day	16% 9% 3 6% 4 5% 5% 3 5% 7% 39%	1976
	21st to end	Day	20% 13% 3 7% 5% 3 4 4 5% 5% 31%	1980
November	1st to 10th	Day	11% 9% 3 7% 4 4 3 3 5% 6% 44%	1851
	11th to 20th	Day	15% 9% 4 7% 4 4 4 5% 6% 6% 36%	1890
	21st to end	Day	16% 13% 3 7% 5% 5% 4 4 5% 5% 32%	1790
December	1st to 10th	Day	14% 11% 4 10% 6% 5% 4 4 6% 8% 28%	2680
	11th to 20th	Day	18% 13% 4 9% 4 4 3 3 3 5% 33%	3066
	21st to end	Day	16% 13% 4 11% 6% 4 3 3 5% 6% 29%	4048
January	1st to 10th	Night	12% 9% 5% 15% 8% 6% 5% 4 4 5% 25%	6489
	11th to 20th	Night	11% 10% 4 15% 9% 6% 6% 4 5% 6% 23%	6455
	21st to end	Night	13% 12% 4 8% 5% 5% 5% 4 6% 7% 32%	7079
February	1st to 10th	Night	14% 9% 4 9% 6% 6% 6% 5% 6% 7% 28%	2576
	11th to 20th	Night	12% 10% 3 11% 7% 6% 6% 4 7% 6% 29%	2338
	21st to end	Night	16% 11% 4 9% 6% 5% 5% 4 6% 5% 30%	1980
March	1st to 10th	Night	18% 10% 3 9% 6% 5% 5% 4 6% 6% 29%	5364
	11th to 20th	Night	15% 9% 4 7% 5% 5% 5% 4 6% 7% 34%	5287
	21st to end	Night	19% 13% 4 9% 5% 5% 3 3 4 5% 30%	4962
April	1st to 10th	Night	17% 12% 3 8% 5% 5% 4 5% 6% 6% 30%	2000
	11th to 20th	Night	18% 12% 3 7% 4 3 4 4 5% 6% 35%	1975
	21st to end	Night	20% 10% 3 7% 4 3 4 4 5% 5% 36%	1876
May	1st to 10th	Night	33% 12% 3 6% 4 4 3 3 4 5% 23%	4054
	11th to 20th	Night	17% 13% 4 8% 4 5% 4 3 6% 6% 29%	3649
	21st to end	Night	22% 12% 3 7% 5% 3 4 3 5% 7% 30%	3926
June	1st to 10th	Night	18% 8% 3 6% 5% 4 3 4 5% 7% 38%	1600
	11th to 20th	Night	17% 8% 2 6% 5% 4 4 5% 5% 8% 37%	1600
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 5% 8% 41%	1600
July	1st to 10th	Night	13% 7% 3 7% 5% 4 4 4 6% 9% 39%	3600
	11th to 20th	Night	15% 8% 3 8% 4 4 4 4 6% 8% 36%	3600
	21st to end	Night	11% 7% 3 7% 4 4 3 3 5% 6% 47%	3926
August	1st to 10th	Night	23% 7% 3 7% 4 3 4 3 5% 6% 35%	2644
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 3 6% 7% 39%	3134
	21st to end	Night	9% 5% 2 6% 4 4 4 4 5% 8% 49%	3586
September	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 5% 2 4 6% 51%	2125
	21st to end	Night	17% 8% 2 5% 4 4 4 4 4 6% 42%	2200
October	1st to 10th	Night	20% 8% 3 6% 4 4 3 3 4 5% 40%	2255
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 3 5% 5% 42%	2422
	21st to end	Night	20% 10% 3 7% 4 4 4 3 5% 7% 34%	2860
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 4 5% 6% 43%	2667
	11th to 20th	Night	15% 8% 4 8% 6% 5% 4 4 5% 5% 36%	2730
	21st to end	Night	13% 11% 4 7% 5% 4 5% 4 6% 6% 34%	2827
December	1st to 10th	Night	16% 12% 5% 10% 6% 5% 5% 4 6% 7% 25%	4720
	11th to 20th	Night	23% 11% 4 8% 5% 4 4 3 4 4 30%	5348
	21st to end	Night	17% 12% 5% 11% 5% 4 4 4 5% 5% 27%	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	54% 9% 9% 22% 6% 1	3678
	11th to 20th	Day	52% 10% 9% 14% 12% 3	3680
	21st to end	Day	51% 6% 7% 21% 14% 1	4115
February	1st to 10th	Day	55% 7% 12% 15% 10% 1	1824
	11th to 20th	Day	50% 9% 9% 19% 12% 2	1957
	21st to end	Day	54% 7% 8% 18% 12% 1	1650
March	1st to 10th	Day	54% 5% 11% 14% 12% 5%	4479
	11th to 20th	Day	45% 10% 14% 21% 8% 3	4670
	21st to end	Day	51% 7% 6% 14% 19% 3	5970
April	1st to 10th	Day	53% 6% 7% 21% 11% 2	2400
	11th to 20th	Day	47% 7% 9% 21% 14% 2	2374
	21st to end	Day	49% 8% 8% 19% 13% 3	2522
May	1st to 10th	Day	62% 5% 10% 14% 8% 1	5897
	11th to 20th	Day	53% 6% 13% 18% 9% 1	6290
	21st to end	Day	49% 9% 11% 21% 9% 1	6949
June	1st to 10th	Day	47% 9% 13% 21% 10%	2800
	11th to 20th	Day	45% 8% 15% 21% 11% 1	2800
	21st to end	Day	48% 7% 14% 19% 11% 1	2800
July	1st to 10th	Day	46% 7% 12% 22% 12% 1	6290
	11th to 20th	Day	40% 8% 16% 23% 12% 1	6285
	21st to end	Day	42% 5% 12% 25% 15%	6728
August	1st to 10th	Day	48% 7% 14% 21% 9% 1	4458
	11th to 20th	Day	43% 8% 15% 26% 7%	3935
	21st to end	Day	39% 6% 13% 29% 12% 1	4312
September	1st to 10th	Day	41% 6% 12% 26% 14% 1	2400
	11th to 20th	Day	37% 7% 9% 25% 21% 2	2275
	21st to end	Day	43% 7% 11% 23% 15% 2	2200
October	1st to 10th	Day	47% 4 7% 20% 20% 2	3491
	11th to 20th	Day	40% 5% 10% 23% 18% 5%	3192
	21st to end	Day	39% 6% 12% 26% 15% 4	3180
November	1st to 10th	Day	40% 6% 14% 24% 13% 2	1851
	11th to 20th	Day	43% 9% 9% 20% 16% 2	1890
	21st to end	Day	49% 7% 13% 21% 8% 2	1790
December	1st to 10th	Day	49% 6% 14% 23% 8%	2680
	11th to 20th	Day	51% 6% 11% 16% 14% 1	3066
	21st to end	Day	54% 6% 8% 13% 14% 5%	4048
January	1st to 10th	Night	52% 7% 10% 19% 10% 1	6489
	11th to 20th	Night	51% 10% 11% 15% 10% 2	6500
	21st to end	Night	48% 6% 12% 15% 16% 3	7079
February	1st to 10th	Night	50% 9% 12% 19% 9% 2	2576
	11th to 20th	Night	48% 10% 11% 17% 13% 2	2340
	21st to end	Night	50% 8% 10% 21% 9% 3	1980
March	1st to 10th	Night	51% 6% 12% 16% 14% 2	5374
	11th to 20th	Night	44% 8% 16% 20% 8% 3	5288
	21st to end	Night	51% 7% 6% 13% 20% 3	4962
April	1st to 10th	Night	51% 6% 10% 19% 12% 2	2000
	11th to 20th	Night	49% 6% 10% 19% 16% 1	1975
	21st to end	Night	47% 6% 10% 18% 16% 3	1876
May	1st to 10th	Night	61% 6% 11% 12% 8% 1	4060
	11th to 20th	Night	54% 7% 13% 16% 8% 2	3650
	21st to end	Night	52% 8% 11% 18% 10% 1	3927
June	1st to 10th	Night	44% 7% 12% 25% 12% 1	1600
	11th to 20th	Night	44% 7% 16% 22% 10%	1600
	21st to end	Night	39% 9% 15% 23% 13% 1	1600
July	1st to 10th	Night	41% 6% 15% 22% 15%	3560
	11th to 20th	Night	44% 7% 14% 22% 12% 1	3552
	21st to end	Night	36% 5% 14% 28% 16% 1	3809
August	1st to 10th	Night	50% 6% 14% 22% 8% 1	2634
	11th to 20th	Night	42% 6% 17% 27% 8%	3162
	21st to end	Night	30% 6% 14% 35% 14% 1	3586
September	1st to 10th	Night	38% 6% 12% 26% 17% 1	2000
	11th to 20th	Night	34% 5% 11% 27% 22% 1	2125
	21st to end	Night	42% 6% 9% 25% 17% 2	2200
October	1st to 10th	Night	43% 4 12% 23% 16% 2	3684
	11th to 20th	Night	35% 4 12% 23% 20% 6%	3945
	21st to end	Night	38% 5% 14% 30% 13% 1	4610
November	1st to 10th	Night	40% 7% 10% 23% 18% 2	2667
	11th to 20th	Night	45% 8% 10% 19% 16% 2	2730
	21st to end	Night	45% 9% 13% 23% 9%	2827
December	1st to 10th	Night	50% 8% 10% 25% 6%	4720
	11th to 20th	Night	51% 9% 13% 11% 15% 1	5348
	21st to end	Night	53% 8% 7% 14% 13% 5%	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	30% 32% 27% 7% 3	220
	11th to 20th	Day	36% 31% 25% 6% 1	220
	21st to end	Day	32% 28% 27% 10% 2 1	258
February	1st to 10th	Day	36% 35% 22% 5% 2	324
	11th to 20th	Day	33% 32% 25% 9% 1	496
	21st to end	Day	31% 27% 29% 12% 1	414
March	1st to 10th	Day	31% 27% 28% 11% 2	500
	11th to 20th	Day	30% 29% 28% 9% 3 1	500
	21st to end	Day	35% 26% 24% 9% 4 2	550
April	1st to 10th	Day	37% 23% 24% 10% 5% 1	500
	11th to 20th	Day	33% 25% 26% 10% 4 2	492
	21st to end	Day	28% 23% 31% 13% 4 1	500
May	1st to 10th	Day	37% 21% 24% 11% 4 3	503
	11th to 20th	Day	21% 24% 33% 16% 4 1	527
	21st to end	Day	21% 15% 32% 21% 10% 2	573
June	1st to 10th	Day	19% 17% 31% 21% 10% 1	530
	11th to 20th	Day	21% 16% 31% 21% 9% 1	530
	21st to end	Day	19% 18% 34% 19% 8% 2	530
July	1st to 10th	Day	16% 11% 35% 26% 10% 2	525
	11th to 20th	Day	14% 8% 35% 31% 10% 2	524
	21st to end	Day	14% 12% 38% 24% 9% 2	576
August	1st to 10th	Day	14% 10% 28% 31% 12% 5%	526
	11th to 20th	Day	16% 10% 28% 27% 13% 7%	519
	21st to end	Day	14% 14% 29% 25% 13% 5%	583
September	1st to 10th	Day	17% 12% 28% 22% 15% 5%	530
	11th to 20th	Day	15% 12% 25% 23% 14% 10%	530
	21st to end	Day	19% 15% 28% 22% 11% 6%	530
October	1st to 10th	Day	26% 15% 25% 18% 12% 4	530
	11th to 20th	Day	23% 21% 28% 20% 7% 2	516
	21st to end	Day	31% 19% 27% 16% 5% 1	462
November	1st to 10th	Day	26% 24% 24% 18% 8%	425
	11th to 20th	Day	30% 28% 24% 13% 5% 1	430
	21st to end	Day	36% 23% 23% 15% 2 1	330
December	1st to 10th	Day	33% 29% 25% 10% 3	220
	11th to 20th	Day	39% 25% 27% 7% 1	220
	21st to end	Day	31% 28% 28% 10% 2	242
January	1st to 10th	Night	25% 30% 30% 13% 2	840
	11th to 20th	Night	28% 26% 29% 15% 2	840
	21st to end	Night	22% 29% 30% 17% 2	924
February	1st to 10th	Night	25% 27% 32% 14% 1	801
	11th to 20th	Night	26% 31% 30% 13% 1	783
	21st to end	Night	28% 29% 27% 14% 2	660
March	1st to 10th	Night	28% 27% 27% 16% 3	800
	11th to 20th	Night	24% 28% 28% 17% 3 1	800
	21st to end	Night	26% 25% 26% 17% 5% 1	880
April	1st to 10th	Night	27% 22% 31% 16% 4	800
	11th to 20th	Night	26% 23% 28% 17% 5%	791
	21st to end	Night	28% 22% 25% 20% 4	676
May	1st to 10th	Night	30% 16% 27% 20% 7%	600
	11th to 20th	Night	21% 15% 32% 27% 5%	429
	21st to end	Night	22% 8% 28% 29% 12%	433
June	1st to 10th	Night	21% 10% 30% 29% 10% 1	400
	11th to 20th	Night	18% 9% 28% 34% 10% 1	400
	21st to end	Night	18% 10% 26% 34% 13% 1	400
July	1st to 10th	Night	15% 6% 25% 39% 14% 1	390
	11th to 20th	Night	16% 4 23% 40% 16% 1	388
	21st to end	Night	11% 7% 25% 41% 16%	438
August	1st to 10th	Night	14% 4 22% 40% 19%	430
	11th to 20th	Night	16% 7% 22% 32% 20% 3	751
	21st to end	Night	16% 7% 24% 32% 20% 2	880
September	1st to 10th	Night	16% 10% 26% 31% 16% 2	800
	11th to 20th	Night	14% 10% 22% 29% 20% 5%	800
	21st to end	Night	21% 12% 24% 28% 14% 2	800
October	1st to 10th	Night	26% 11% 26% 23% 12% 3	800
	11th to 20th	Night	23% 18% 26% 24% 9% 1	800
	21st to end	Night	26% 19% 29% 21% 4	880
November	1st to 10th	Night	21% 22% 28% 21% 8%	820
	11th to 20th	Night	25% 26% 25% 17% 7% 1	840
	21st to end	Night	23% 25% 28% 19% 4 1	840
December	1st to 10th	Night	26% 28% 28% 14% 4	840
	11th to 20th	Night	27% 27% 28% 15% 3	840
	21st to end	Night	28% 30% 25% 14% 3	924

Page generated at 2026-01-29 01:21:40. Version ac07

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