Black Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Skies Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	15% 11% 4 9% 5% 5% 4 3 5% 7% 33%	127263
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	121684
Transparency	Day	50% 7% 12% 20% 10% 1	130847
	Night	55% 7% 11% 17% 9% 1	126188
Seeing	Day	22% 17% 31% 19% 8% 3	16060
	Night	23% 15% 29% 23% 9% 1	24827

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% 14% 9% 13% 7% 5% 5% 3 4 6% 23%	3397
	11th to 20th	Day	8% 16% 8% 20% 9% 7% 7% 4 4 4 11%	3824
	21st to end	Day	11% 18% 8% 16% 8% 6% 6% 4 5% 5% 14%	4466
February	1st to 10th	Day	12% 18% 6% 16% 9% 6% 4 3 3 4 19%	1800
	11th to 20th	Day	11% 16% 6% 13% 7% 5% 5% 4 5% 5% 24%	1753
	21st to end	Day	15% 16% 6% 11% 6% 5% 5% 4 5% 5% 21%	1522
March	1st to 10th	Day	17% 12% 5% 10% 6% 5% 4 3 4 5% 29%	4927
	11th to 20th	Day	11% 13% 6% 10% 6% 6% 6% 4 5% 7% 27%	4980
	21st to end	Day	17% 13% 5% 9% 5% 3 3 3 5% 5% 32%	5465
April	1st to 10th	Day	19% 13% 3 8% 5% 4 5% 3 4 5% 31%	2360
	11th to 20th	Day	19% 12% 3 6% 5% 4 4 3 4 5% 36%	2491
	21st to end	Day	20% 10% 3 6% 4 4 4 3 5% 6% 35%	2600
May	1st to 10th	Day	16% 10% 3 8% 4 4 4 3 5% 6% 36%	5924
	11th to 20th	Day	12% 9% 3 8% 4 4 4 4 5% 7% 39%	6318
	21st to end	Day	20% 8% 3 6% 4 4 4 3 6% 7% 36%	6949
June	1st to 10th	Day	18% 7% 3 6% 4 4 4 4 5% 8% 38%	2800
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 6% 9% 40%	2986
	21st to end	Day	16% 7% 3 7% 4 4 4 3 6% 8% 39%	3000
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	6819
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 5% 7% 49%	6457
	21st to end	Day	8% 5% 2 6% 4 4 4 3 6% 8% 50%	6570
August	1st to 10th	Day	13% 7% 2 7% 4 5% 5% 4 5% 8% 40%	4234
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 9% 39%	4174
	21st to end	Day	9% 4 2 6% 5% 5% 4 4 6% 9% 45%	4543
September	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 41%	2400
	11th to 20th	Day	13% 6% 3 5% 4 4 4 3 6% 9% 45%	2250
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 4 4 6% 36%	2057
October	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 6% 7% 33%	1900
	11th to 20th	Day	23% 10% 4 10% 5% 5% 4 4 5% 6% 23%	1900
	21st to end	Day	27% 14% 4 9% 5% 5% 4 3 4 5% 19%	2090
November	1st to 10th	Day	17% 17% 4 9% 6% 4 4 3 5% 7% 25%	1631
	11th to 20th	Day	17% 15% 5% 11% 6% 6% 6% 4 4 7% 19%	1600
	21st to end	Day	18% 23% 6% 10% 6% 6% 6% 3 5% 5% 12%	1600
December	1st to 10th	Day	15% 26% 7% 13% 7% 6% 5% 3 5% 5% 9%	2600
	11th to 20th	Day	15% 20% 9% 15% 6% 5% 3 3 4 5% 15%	2986
	21st to end	Day	22% 16% 6% 14% 8% 5% 4 3 3 3 15%	3890
January	1st to 10th	Night	12% 20% 10% 17% 6% 5% 4 3 4 4 15%	6495
	11th to 20th	Night	9% 16% 8% 17% 9% 7% 6% 4 6% 6% 12%	6060
	21st to end	Night	12% 16% 7% 15% 7% 6% 5% 3 5% 6% 18%	6490
February	1st to 10th	Night	11% 19% 6% 15% 9% 5% 5% 3 4 4 17%	2600
	11th to 20th	Night	12% 17% 6% 13% 7% 5% 5% 3 4 5% 22%	2533
	21st to end	Night	16% 14% 5% 11% 7% 6% 4 3 5% 6% 23%	2105
March	1st to 10th	Night	15% 12% 5% 10% 5% 4 5% 4 4 6% 29%	4913
	11th to 20th	Night	9% 13% 6% 9% 6% 6% 5% 4 6% 7% 30%	4979
	21st to end	Night	17% 13% 6% 9% 5% 4 4 4 5% 5% 28%	5467
April	1st to 10th	Night	20% 13% 4 9% 5% 3 4 3 5% 6% 29%	2040
	11th to 20th	Night	19% 11% 3 7% 4 4 5% 3 5% 5% 35%	1876
	21st to end	Night	17% 10% 4 8% 5% 5% 4 3 6% 6% 31%	1800
May	1st to 10th	Night	16% 9% 3 7% 5% 5% 5% 4 5% 7% 35%	4017
	11th to 20th	Night	12% 7% 2 7% 5% 4 4 4 6% 7% 42%	3575
	21st to end	Night	19% 6% 3 6% 5% 4 3 3 5% 7% 40%	3926
June	1st to 10th	Night	15% 8% 2 7% 5% 4 5% 5% 6% 8% 36%	1600
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 7% 10% 42%	1409
	21st to end	Night	15% 6% 3 7% 4 5% 4 4 7% 7% 38%	1400
July	1st to 10th	Night	12% 6% 3 7% 4 4 5% 4 6% 7% 42%	3160
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	3504
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 48%	4279
August	1st to 10th	Night	10% 6% 3 6% 4 4 5% 3 6% 8% 46%	2915
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 4 7% 44%	2875
	21st to end	Night	14% 5% 2 6% 4 3 4 4 5% 8% 45%	3320
September	1st to 10th	Night	10% 6% 3 7% 5% 4 5% 3 6% 8% 44%	2000
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 5% 6% 47%	2104
	21st to end	Night	16% 8% 2 7% 5% 4 4 3 5% 7% 40%	2121
October	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 4 5% 36%	2280
	11th to 20th	Night	23% 13% 3 8% 4 5% 4 4 5% 6% 25%	2280
	21st to end	Night	24% 13% 4 10% 5% 5% 4 3 5% 6% 21%	2508
November	1st to 10th	Night	18% 15% 3 9% 5% 5% 4 3 4 5% 28%	2664
	11th to 20th	Night	16% 19% 5% 11% 6% 5% 4 4 5% 5% 22%	2800
	21st to end	Night	20% 20% 6% 11% 6% 5% 4 4 4 6% 13%	2800
December	1st to 10th	Night	16% 25% 8% 14% 7% 5% 4 3 4 4 10%	4580
	11th to 20th	Night	14% 20% 8% 18% 8% 5% 4 3 4 4 11%	5208
	21st to end	Night	20% 21% 7% 16% 6% 5% 3 3 3 3 13%	7001

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	62% 6% 10% 13% 8%	3398
	11th to 20th	Day	74% 6% 10% 6% 4	3857
	21st to end	Day	70% 7% 9% 8% 6%	4466
February	1st to 10th	Day	71% 6% 7% 13% 3 1	1800
	11th to 20th	Day	62% 7% 10% 14% 6% 1	1762
	21st to end	Day	66% 6% 9% 14% 5%	1522
March	1st to 10th	Day	57% 6% 13% 11% 13% 1	4930
	11th to 20th	Day	56% 7% 13% 17% 7% 1	4980
	21st to end	Day	53% 7% 8% 20% 11% 1	5465
April	1st to 10th	Day	57% 5% 9% 19% 9% 1	2360
	11th to 20th	Day	51% 5% 8% 22% 13% 1	2491
	21st to end	Day	51% 6% 10% 19% 12% 3	2600
May	1st to 10th	Day	46% 7% 11% 19% 11% 5%	5933
	11th to 20th	Day	44% 8% 11% 18% 18% 1	6322
	21st to end	Day	47% 6% 11% 20% 13% 3	6949
June	1st to 10th	Day	44% 7% 11% 24% 14%	2800
	11th to 20th	Day	38% 9% 13% 25% 15% 1	2986
	21st to end	Day	44% 7% 14% 23% 12% 1	3000
July	1st to 10th	Day	39% 7% 15% 26% 12% 1	6746
	11th to 20th	Day	34% 6% 12% 34% 14% 1	6379
	21st to end	Day	31% 7% 13% 33% 16%	6382
August	1st to 10th	Day	40% 8% 15% 25% 12% 1	4221
	11th to 20th	Day	41% 7% 17% 25% 9%	4213
	21st to end	Day	34% 7% 14% 34% 10% 1	4543
September	1st to 10th	Day	41% 7% 14% 27% 10% 1	2400
	11th to 20th	Day	36% 7% 15% 29% 12%	2250
	21st to end	Day	48% 7% 12% 24% 8%	2057
October	1st to 10th	Day	45% 6% 13% 27% 9% 1	3160
	11th to 20th	Day	50% 6% 15% 21% 8%	3140
	21st to end	Day	59% 8% 14% 16% 4	3428
November	1st to 10th	Day	60% 6% 11% 16% 5%	1631
	11th to 20th	Day	64% 8% 12% 12% 4	1600
	21st to end	Day	73% 7% 9% 9% 1	1600
December	1st to 10th	Day	75% 8% 11% 6% 1	2600
	11th to 20th	Day	70% 8% 9% 11% 2	2986
	21st to end	Day	74% 6% 7% 8% 6%	3890
January	1st to 10th	Night	71% 6% 6% 11% 5%	6495
	11th to 20th	Night	69% 9% 9% 8% 5%	6102
	21st to end	Night	66% 6% 10% 9% 7% 1	6490
February	1st to 10th	Night	70% 7% 9% 12% 3	2600
	11th to 20th	Night	64% 6% 10% 15% 6%	2535
	21st to end	Night	62% 6% 10% 15% 6%	2105
March	1st to 10th	Night	54% 6% 11% 15% 11% 3	4923
	11th to 20th	Night	52% 6% 18% 16% 8% 1	4980
	21st to end	Night	55% 7% 9% 17% 11% 2	5467
April	1st to 10th	Night	56% 6% 9% 18% 10% 1	2040
	11th to 20th	Night	50% 5% 7% 22% 14% 2	1876
	21st to end	Night	52% 6% 9% 15% 16% 1	1800
May	1st to 10th	Night	47% 6% 12% 16% 15% 4	4024
	11th to 20th	Night	39% 7% 12% 22% 18% 2	3576
	21st to end	Night	44% 6% 11% 24% 13% 1	3927
June	1st to 10th	Night	44% 6% 12% 22% 16% 2	1600
	11th to 20th	Night	36% 6% 13% 26% 18% 1	1409
	21st to end	Night	43% 8% 13% 23% 13% 1	1400
July	1st to 10th	Night	38% 7% 13% 26% 14% 2	3125
	11th to 20th	Night	35% 5% 13% 30% 16% 1	3459
	21st to end	Night	33% 6% 9% 35% 16% 1	4152
August	1st to 10th	Night	36% 6% 15% 27% 16% 1	2904
	11th to 20th	Night	39% 7% 17% 23% 13% 1	2902
	21st to end	Night	36% 6% 12% 33% 11% 1	3320
September	1st to 10th	Night	36% 8% 16% 27% 13% 1	2000
	11th to 20th	Night	36% 6% 14% 28% 15%	2104
	21st to end	Night	44% 5% 14% 26% 10%	2121
October	1st to 10th	Night	38% 7% 12% 25% 17%	3800
	11th to 20th	Night	49% 6% 16% 20% 8%	3779
	21st to end	Night	52% 8% 15% 21% 5%	4120
November	1st to 10th	Night	56% 6% 12% 19% 7%	2664
	11th to 20th	Night	62% 8% 13% 11% 4 1	2800
	21st to end	Night	70% 8% 11% 10% 1	2800
December	1st to 10th	Night	75% 8% 10% 5% 1	4580
	11th to 20th	Night	76% 7% 9% 7% 2	5208
	21st to end	Night	76% 5% 7% 8% 4	7001

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% 31% 34% 12% 5%	200
	11th to 20th	Day	20% 28% 34% 13% 4	257
	21st to end	Day	26% 22% 33% 16% 2	330
February	1st to 10th	Day	29% 33% 27% 8% 3	300
	11th to 20th	Day	26% 30% 37% 6% 1	300
	21st to end	Day	37% 25% 33% 4	249
March	1st to 10th	Day	23% 23% 38% 13% 3	495
	11th to 20th	Day	23% 25% 34% 14% 3 1	500
	21st to end	Day	33% 21% 27% 14% 5%	550
April	1st to 10th	Day	30% 21% 32% 12% 5% 1	500
	11th to 20th	Day	35% 19% 23% 15% 6% 3	492
	21st to end	Day	30% 19% 33% 10% 6% 2	500
May	1st to 10th	Day	26% 14% 33% 17% 7% 2	503
	11th to 20th	Day	17% 15% 32% 22% 11% 3	527
	21st to end	Day	18% 17% 30% 22% 10% 3	573
June	1st to 10th	Day	21% 15% 32% 25% 6% 1	530
	11th to 20th	Day	16% 13% 33% 26% 10% 3	716
	21st to end	Day	21% 12% 35% 22% 8% 2	730
July	1st to 10th	Day	11% 10% 34% 28% 14% 3	721
	11th to 20th	Day	15% 11% 27% 30% 12% 5%	566
	21st to end	Day	11% 10% 35% 26% 13% 5%	576
August	1st to 10th	Day	13% 7% 27% 29% 16% 7%	526
	11th to 20th	Day	16% 6% 25% 30% 14% 8%	519
	21st to end	Day	13% 13% 30% 26% 13% 5%	583
September	1st to 10th	Day	17% 11% 26% 25% 14% 7%	530
	11th to 20th	Day	11% 9% 28% 20% 20% 12%	522
	21st to end	Day	18% 17% 25% 26% 11% 4	490
October	1st to 10th	Day	24% 18% 26% 17% 12% 3	490
	11th to 20th	Day	24% 23% 31% 15% 6%	490
	21st to end	Day	35% 19% 30% 12% 3	539
November	1st to 10th	Day	33% 18% 31% 16% 3	236
	11th to 20th	Day	30% 24% 34% 12% 1 1	200
	21st to end	Day	29% 21% 41% 9% 2	200
December	1st to 10th	Day	30% 28% 28% 12% 3 1	200
	11th to 20th	Day	30% 33% 25% 10% 2 1	200
	21st to end	Day	31% 30% 29% 8% 2	220
January	1st to 10th	Night	27% 25% 30% 15% 3	868
	11th to 20th	Night	31% 18% 30% 17% 4	800
	21st to end	Night	21% 16% 34% 23% 6% 1	880
February	1st to 10th	Night	23% 24% 33% 18% 2	800
	11th to 20th	Night	23% 20% 34% 20% 4	783
	21st to end	Night	26% 22% 28% 20% 4	660
March	1st to 10th	Night	22% 22% 33% 20% 3	800
	11th to 20th	Night	22% 20% 34% 20% 4 1	800
	21st to end	Night	23% 16% 31% 23% 7%	880
April	1st to 10th	Night	29% 16% 32% 20% 4	800
	11th to 20th	Night	26% 14% 29% 22% 9%	692
	21st to end	Night	22% 15% 31% 22% 10%	600
May	1st to 10th	Night	21% 10% 25% 32% 11% 1	590
	11th to 20th	Night	18% 10% 31% 30% 10%	394
	21st to end	Night	13% 7% 32% 34% 14%	433
June	1st to 10th	Night	16% 9% 32% 33% 10% 1	400
	11th to 20th	Night	13% 9% 29% 37% 12%	400
	21st to end	Night	16% 7% 28% 34% 14% 1	400
July	1st to 10th	Night	8% 6% 20% 47% 17% 1	390
	11th to 20th	Night	11% 9% 25% 34% 20% 1	388
	21st to end	Night	10% 5% 28% 37% 20% 1	586
August	1st to 10th	Night	10% 6% 20% 36% 26% 2	597
	11th to 20th	Night	11% 5% 21% 36% 25% 2	594
	21st to end	Night	17% 7% 24% 28% 23% 1	718
September	1st to 10th	Night	12% 8% 29% 28% 19% 3	800
	11th to 20th	Night	15% 7% 23% 30% 21% 5%	793
	21st to end	Night	20% 10% 22% 30% 14% 3	760
October	1st to 10th	Night	22% 14% 29% 22% 12% 1	760
	11th to 20th	Night	31% 16% 26% 19% 8% 1	760
	21st to end	Night	30% 20% 29% 17% 3	836
November	1st to 10th	Night	29% 16% 29% 19% 6% 1	780
	11th to 20th	Night	27% 22% 30% 16% 5%	800
	21st to end	Night	34% 19% 30% 15% 3	800
December	1st to 10th	Night	34% 23% 28% 11% 4	800
	11th to 20th	Night	27% 23% 35% 11% 3 1	800
	21st to end	Night	33% 20% 29% 15% 2	885

Page generated at 2025-09-16 01:49:15. Version ac07

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