Dark Skies Unlimited Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Skies Unlimited Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	3 3 1 4 3 4 4 4 6% 7% 60%	134840
	Night	2 3 1 3 2 3 3 3 5% 8% 67%	134259
Transparency	Day	20% 5% 9% 16% 25% 24%	129928
	Night	16% 4 9% 17% 27% 26%	130175
Seeing	Day	12% 16% 26% 25% 16% 4	16287
	Night	9% 16% 26% 29% 17% 3	26566

By Month

Percentage Cloud Cover SW (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	1 4 2 3 2 2 3 3 5% 6% 67%	4188
	11th to 20th	Day	1 1 1 3 2 2 2 2 4 5% 77%	4154
	21st to end	Day	1 3 1 4 2 2 3 3 6% 9% 66%	4510
February	1st to 10th	Day	1 3 1 3 2 3 3 3 4 6% 69%	1988
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 7% 67%	1949
	21st to end	Day	2 1 3 2 2 3 3 4 6% 73%	1650
March	1st to 10th	Day	1 6% 2 4 2 3 3 2 3 6% 69%	4701
	11th to 20th	Day	3 6% 1 3 2 3 3 2 4 7% 66%	5000
	21st to end	Day	1 2 1 3 3 3 4 3 6% 8% 65%	5718
April	1st to 10th	Day	1 3 1 3 2 2 4 3 4 6% 71%	2400
	11th to 20th	Day	2 3 1 2 2 2 3 3 4 6% 71%	2371
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	2400
May	1st to 10th	Day	1 2 1 3 2 2 3 2 4 7% 72%	5440
	11th to 20th	Day	2 2 1 2 2 3 4 3 5% 5% 70%	5405
	21st to end	Day	1 1 1 1 2 3 3 5% 7% 76%	6314
June	1st to 10th	Day	1 2 1 3 4 4 5% 5% 7% 10% 58%	2800
	11th to 20th	Day	2 2 1 2 3 3 4 4 7% 8% 66%	2800
	21st to end	Day	7% 4 2 5% 5% 6% 6% 7% 8% 8% 42%	2765
July	1st to 10th	Day	7% 5% 2 6% 6% 7% 8% 6% 8% 9% 37%	5879
	11th to 20th	Day	3 3 1 5% 5% 6% 8% 7% 9% 10% 43%	5880
	21st to end	Day	6% 4 2 7% 7% 8% 9% 7% 10% 10% 29%	6317
August	1st to 10th	Day	4 3 2 6% 5% 6% 7% 6% 8% 10% 42%	5348
	11th to 20th	Day	4 3 2 7% 7% 7% 7% 6% 8% 10% 39%	5366
	21st to end	Day	6% 4 2 6% 5% 6% 6% 6% 8% 9% 41%	5984
September	1st to 10th	Day	10% 6% 2 7% 4 5% 4 5% 8% 8% 43%	2400
	11th to 20th	Day	8% 5% 2 7% 5% 4 5% 3 5% 7% 49%	2387
	21st to end	Day	5% 4 1 4 3 3 4 3 5% 7% 63%	2200
October	1st to 10th	Day	4 4 1 4 2 3 3 3 4 4 68%	3317
	11th to 20th	Day	4 4 1 4 2 2 3 3 4 5% 67%	3250
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 85%	3548
November	1st to 10th	Day	2 2 2 2 2 2 2 3 4 77%	2059
	11th to 20th	Day	1 2 1 3 2 2 3 2 5% 7% 71%	1902
	21st to end	Day	3 3 1 2 3 2 3 3 4 7% 68%	1890
December	1st to 10th	Day	3 5% 1 4 2 2 3 2 4 6% 67%	3030
	11th to 20th	Day	1 3 1 4 3 2 3 3 5% 7% 69%	3221
	21st to end	Day	1 2 1 2 2 2 2 3 4 6% 76%	4309
January	1st to 10th	Night	1 5% 2 4 2 3 3 3 4 6% 66%	5997
	11th to 20th	Night	1 1 1 3 2 2 3 3 3 5% 77%	5958
	21st to end	Night	1 3 1 3 2 3 3 3 5% 8% 67%	6468
February	1st to 10th	Night	2 1 3 2 3 3 3 5% 7% 70%	2408
	11th to 20th	Night	2 2 1 2 1 3 3 3 5% 7% 71%	2338
	21st to end	Night	1 2 1 2 2 2 2 3 5% 7% 73%	1980
March	1st to 10th	Night	2 1 3 2 2 3 2 4 6% 75%	5166
	11th to 20th	Night	3 10% 2 3 2 2 2 2 2 5% 67%	5000
	21st to end	Night	1 1 1 2 2 2 3 2 5% 8% 74%	5244
April	1st to 10th	Night	2 2 1 2 2 2 3 4 5% 8% 70%	2000
	11th to 20th	Night	2 2 1 2 2 2 3 2 5% 7% 73%	1977
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 77%	2000
May	1st to 10th	Night	3 4 1 3 2 2 2 3 4 6% 71%	4540
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 5% 74%	4510
	21st to end	Night	2 1 1 2 2 2 2 4 4 80%	4561
June	1st to 10th	Night	2 3 2 4 4 4 4 5% 7% 8% 59%	1600
	11th to 20th	Night	2 2 1 2 2 2 3 2 6% 6% 72%	1600
	21st to end	Night	5% 4 2 4 4 6% 7% 5% 7% 9% 48%	1627
July	1st to 10th	Night	9% 6% 1 4 3 3 4 4 7% 10% 49%	4100
	11th to 20th	Night	2 2 1 3 3 5% 6% 5% 9% 13% 51%	4100
	21st to end	Night	5% 4 1 4 4 4 5% 5% 11% 16% 40%	4553
August	1st to 10th	Night	2 2 1 3 3 4 4 4 9% 13% 54%	4464
	11th to 20th	Night	3 3 1 4 4 4 4 4 9% 14% 51%	4483
	21st to end	Night	4 3 1 4 3 4 5% 4 7% 13% 52%	4993
September	1st to 10th	Night	6% 5% 2 4 4 4 4 3 7% 9% 51%	2000
	11th to 20th	Night	7% 4 1 4 3 3 4 3 6% 8% 59%	2011
	21st to end	Night	6% 3 1 3 2 2 3 3 4 6% 67%	2200
October	1st to 10th	Night	3 2 1 3 2 1 2 2 3 5% 76%	3854
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 5% 70%	3898
	21st to end	Night	2 1 1 1 1 1 1 1 3 5% 83%	4252
November	1st to 10th	Night	3 3 1 3 2 1 2 2 4 5% 75%	2460
	11th to 20th	Night	1 1 1 2 3 2 3 3 5% 7% 72%	2693
	21st to end	Night	2 4 1 4 3 3 3 2 4 6% 67%	2730
December	1st to 10th	Night	2 6% 1 4 2 2 3 2 4 6% 67%	4380
	11th to 20th	Night	1 2 1 3 2 3 4 3 6% 7% 70%	5189
	21st to end	Night	1 2 1 3 2 2 2 3 4 6% 75%	6925

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	17% 3 7% 14% 31% 28%	4189
	11th to 20th	Day	10% 3 5% 16% 38% 29%	4154
	21st to end	Day	16% 4 8% 13% 31% 28%	4510
February	1st to 10th	Day	16% 3 7% 16% 28% 30%	1988
	11th to 20th	Day	18% 4 7% 19% 26% 26%	1959
	21st to end	Day	14% 2 6% 11% 29% 38%	1650
March	1st to 10th	Day	16% 4 5% 7% 19% 48%	4701
	11th to 20th	Day	15% 8% 6% 9% 30% 33%	5000
	21st to end	Day	16% 4 9% 14% 31% 26%	5718
April	1st to 10th	Day	15% 4 5% 16% 34% 27%	2400
	11th to 20th	Day	14% 3 4 13% 30% 35%	2374
	21st to end	Day	11% 3 5% 16% 34% 31%	2400
May	1st to 10th	Day	12% 4 6% 16% 26% 36%	5440
	11th to 20th	Day	12% 6% 7% 14% 24% 38%	5408
	21st to end	Day	8% 4 8% 11% 18% 52%	6315
June	1st to 10th	Day	18% 8% 11% 18% 25% 20%	2800
	11th to 20th	Day	14% 5% 6% 16% 24% 37%	2800
	21st to end	Day	32% 9% 12% 16% 17% 14%	2765
July	1st to 10th	Day	37% 10% 15% 19% 12% 7%	5816
	11th to 20th	Day	29% 9% 20% 21% 17% 4	5811
	21st to end	Day	41% 10% 21% 18% 9% 1	6134
August	1st to 10th	Day	29% 8% 15% 30% 15% 3	3892
	11th to 20th	Day	32% 8% 13% 21% 23% 3	3882
	21st to end	Day	35% 9% 18% 26% 10% 1	4311
September	1st to 10th	Day	35% 6% 13% 18% 18% 10%	2400
	11th to 20th	Day	32% 7% 8% 18% 26% 9%	2387
	21st to end	Day	22% 3 8% 24% 31% 13%	2200
October	1st to 10th	Day	18% 4 6% 12% 29% 30%	3317
	11th to 20th	Day	19% 4 8% 18% 29% 23%	3250
	21st to end	Day	7% 1 3 14% 40% 34%	3548
November	1st to 10th	Day	12% 2 4 21% 34% 26%	2059
	11th to 20th	Day	14% 2 6% 21% 31% 26%	1902
	21st to end	Day	17% 2 10% 17% 28% 27%	1890
December	1st to 10th	Day	19% 4 5% 17% 30% 25%	3030
	11th to 20th	Day	16% 3 7% 13% 35% 27%	3219
	21st to end	Day	11% 3 6% 14% 36% 31%	4309
January	1st to 10th	Night	19% 3 6% 12% 37% 23%	5997
	11th to 20th	Night	11% 4 5% 13% 35% 33%	5958
	21st to end	Night	16% 3 9% 15% 29% 28%	6468
February	1st to 10th	Night	14% 3 8% 17% 27% 32%	2408
	11th to 20th	Night	15% 4 7% 17% 31% 26%	2338
	21st to end	Night	12% 3 8% 14% 27% 36%	1980
March	1st to 10th	Night	10% 3 5% 8% 28% 45%	5166
	11th to 20th	Night	18% 7% 5% 9% 29% 33%	5000
	21st to end	Night	11% 3 7% 18% 34% 26%	5244
April	1st to 10th	Night	13% 3 6% 17% 35% 26%	2000
	11th to 20th	Night	12% 2 7% 16% 29% 33%	1977
	21st to end	Night	12% 3 6% 18% 32% 29%	2000
May	1st to 10th	Night	14% 4 6% 16% 21% 39%	4537
	11th to 20th	Night	11% 5% 6% 16% 28% 33%	4511
	21st to end	Night	9% 2 6% 9% 15% 58%	4562
June	1st to 10th	Night	21% 6% 11% 19% 22% 22%	1600
	11th to 20th	Night	14% 3 5% 18% 22% 39%	1600
	21st to end	Night	30% 7% 15% 18% 14% 16%	1627
July	1st to 10th	Night	29% 6% 19% 27% 14% 4	4055
	11th to 20th	Night	21% 7% 22% 30% 16% 3	4046
	21st to end	Night	28% 8% 29% 24% 10% 1	4424
August	1st to 10th	Night	21% 7% 18% 33% 18% 3	3244
	11th to 20th	Night	20% 6% 16% 29% 27% 2	3243
	21st to end	Night	26% 8% 23% 27% 15% 1	3597
September	1st to 10th	Night	29% 6% 16% 18% 19% 12%	2000
	11th to 20th	Night	24% 5% 13% 19% 27% 11%	2011
	21st to end	Night	20% 3 11% 22% 32% 13%	2200
October	1st to 10th	Night	13% 3 6% 12% 33% 33%	3854
	11th to 20th	Night	16% 4 9% 14% 32% 26%	3898
	21st to end	Night	8% 2 3 14% 37% 36%	4252
November	1st to 10th	Night	14% 2 5% 18% 31% 30%	2460
	11th to 20th	Night	12% 4 5% 19% 32% 28%	2693
	21st to end	Night	19% 3 7% 17% 30% 24%	2730
December	1st to 10th	Night	17% 6% 6% 17% 33% 22%	4380
	11th to 20th	Night	14% 3 6% 14% 30% 33%	5190
	21st to end	Night	12% 3 4 17% 31% 32%	6925

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	18% 26% 34% 16% 5% 1	330
	11th to 20th	Day	18% 22% 33% 18% 8% 2	318
	21st to end	Day	13% 26% 28% 22% 10% 1	330
February	1st to 10th	Day	17% 26% 29% 20% 6% 1	300
	11th to 20th	Day	23% 28% 30% 13% 5% 1	300
	21st to end	Day	28% 31% 24% 12% 4	249
March	1st to 10th	Day	23% 27% 33% 13% 4	384
	11th to 20th	Day	25% 25% 31% 12% 6% 1	500
	21st to end	Day	24% 30% 32% 10% 3 1	550
April	1st to 10th	Day	18% 29% 36% 13% 3 1	500
	11th to 20th	Day	19% 31% 33% 13% 3 1	492
	21st to end	Day	17% 29% 36% 14% 4	500
May	1st to 10th	Day	10% 27% 41% 18% 4	503
	11th to 20th	Day	13% 22% 34% 20% 8% 2	528
	21st to end	Day	7% 20% 41% 22% 10% 1	683
June	1st to 10th	Day	3 9% 32% 36% 18% 3	730
	11th to 20th	Day	3 8% 31% 38% 15% 4	730
	21st to end	Day	9% 5% 19% 34% 27% 7%	695
July	1st to 10th	Day	7% 3 11% 38% 30% 12%	526
	11th to 20th	Day	4 3 7% 38% 35% 14%	524
	21st to end	Day	4 2 6% 32% 42% 14%	575
August	1st to 10th	Day	5% 3 6% 33% 37% 17%	527
	11th to 20th	Day	10% 3 10% 34% 33% 11%	517
	21st to end	Day	8% 4 9% 36% 32% 12%	583
September	1st to 10th	Day	11% 5% 16% 37% 25% 6%	530
	11th to 20th	Day	5% 4 21% 42% 22% 6%	517
	21st to end	Day	6% 6% 30% 31% 22% 4	330
October	1st to 10th	Day	12% 11% 25% 32% 15% 5%	330
	11th to 20th	Day	12% 15% 31% 24% 15% 4	330
	21st to end	Day	12% 17% 29% 24% 12% 5%	363
November	1st to 10th	Day	15% 23% 31% 17% 12% 2	330
	11th to 20th	Day	14% 18% 30% 21% 15% 3	330
	21st to end	Day	17% 28% 28% 15% 11% 1	330
December	1st to 10th	Day	19% 22% 31% 21% 7% 1	330
	11th to 20th	Day	17% 22% 31% 23% 5% 2	330
	21st to end	Day	21% 26% 33% 15% 5%	363
January	1st to 10th	Night	11% 22% 31% 23% 10% 3	840
	11th to 20th	Night	8% 24% 31% 24% 10% 3	824
	21st to end	Night	10% 23% 30% 25% 10% 2	880
February	1st to 10th	Night	10% 22% 37% 21% 8% 3	800
	11th to 20th	Night	11% 24% 39% 20% 4 1	784
	21st to end	Night	14% 31% 36% 15% 3 1	660
March	1st to 10th	Night	14% 28% 28% 23% 6% 1	800
	11th to 20th	Night	15% 20% 31% 25% 10% 1	800
	21st to end	Night	12% 25% 38% 19% 6%	777
April	1st to 10th	Night	11% 24% 42% 20% 5%	600
	11th to 20th	Night	10% 25% 35% 24% 5%	593
	21st to end	Night	8% 25% 35% 25% 6%	600
May	1st to 10th	Night	5% 20% 42% 29% 4 1	600
	11th to 20th	Night	8% 14% 31% 33% 12% 2	591
	21st to end	Night	6% 13% 37% 33% 11% 1	650
June	1st to 10th	Night	3 5% 29% 44% 18% 1	600
	11th to 20th	Night	3 5% 31% 41% 18% 1	600
	21st to end	Night	6% 3 17% 41% 30% 3	600
July	1st to 10th	Night	7% 3 6% 41% 40% 3	585
	11th to 20th	Night	4 2 5% 34% 47% 8%	582
	21st to end	Night	4 3 3 28% 51% 11%	657
August	1st to 10th	Night	6% 1 3 29% 48% 13%	597
	11th to 20th	Night	5% 1 6% 33% 43% 13%	594
	21st to end	Night	5% 2 4 32% 48% 10%	660
September	1st to 10th	Night	7% 4 9% 37% 37% 7%	600
	11th to 20th	Night	8% 2 12% 44% 31% 4	600
	21st to end	Night	7% 4 17% 44% 25% 4	600
October	1st to 10th	Night	6% 9% 22% 40% 19% 4	760
	11th to 20th	Night	9% 10% 27% 35% 16% 3	800
	21st to end	Night	8% 14% 27% 29% 16% 6%	880
November	1st to 10th	Night	7% 14% 30% 29% 14% 5%	820
	11th to 20th	Night	6% 15% 29% 34% 13% 3	1013
	21st to end	Night	11% 21% 29% 25% 12% 3	1050
December	1st to 10th	Night	12% 20% 29% 27% 10% 2	1050
	11th to 20th	Night	10% 25% 31% 25% 9% 1	1050
	21st to end	Night	14% 23% 30% 24% 6% 2	1069

Page generated at 2026-01-13 23:14:27. Version ac07

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