Deneb Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deneb Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	2 2 1 3 3 3 3 3 5% 7% 68%	139712
	Night	3 3 1 4 3 3 4 3 5% 7% 63%	139001
Transparency	Day	17% 3 6% 14% 29% 31%	134720
	Night	20% 3 6% 14% 28% 29%	134998
Seeing	Day	7% 10% 21% 30% 23% 9%	16125
	Night	5% 6% 21% 41% 24% 2	26216

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	3 6% 2 5% 3 3 3 3 5% 6% 63%	5318
	11th to 20th	Day	1 1 1 1 1 1 2 3 5% 82%	5340
	21st to end	Day	7% 3 1 3 3 3 3 3 5% 6% 62%	5844
February	1st to 10th	Day	3 4 1 3 2 2 3 3 4 5% 69%	1890
	11th to 20th	Day	4 3 1 4 3 3 4 3 4 6% 66%	2037
	21st to end	Day	2 3 1 3 3 3 2 3 4 5% 70%	1856
March	1st to 10th	Day	2 3 1 3 2 2 3 2 5% 6% 69%	4977
	11th to 20th	Day	4 2 1 3 3 3 3 3 5% 5% 68%	5000
	21st to end	Day	2 4 1 4 4 3 4 3 5% 7% 64%	5486
April	1st to 10th	Day	1 2 1 2 2 2 2 3 4 6% 74%	2358
	11th to 20th	Day	2 2 2 2 1 2 2 3 4 80%	2371
	21st to end	Day	2 2 1 1 1 1 2 2 3 4 81%	2400
May	1st to 10th	Day	1 1 1 1 1 1 1 2 4 86%	5440
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 81%	5531
	21st to end	Day	1 1 1 1 2 2 2 3 4 85%	6423
June	1st to 10th	Day	3 2 2 2 2 3 3 4 7% 71%	2600
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 5% 76%	2600
	21st to end	Day	3 3 1 4 4 4 5% 4 7% 8% 56%	2600
July	1st to 10th	Day	3 2 1 3 3 3 4 4 6% 9% 61%	5879
	11th to 20th	Day	2 1 1 3 3 4 5% 4 8% 11% 57%	5880
	21st to end	Day	3 4 2 5% 5% 6% 6% 7% 10% 13% 40%	6401
August	1st to 10th	Day	2 2 1 4 3 4 4 4 8% 9% 59%	5661
	11th to 20th	Day	2 2 1 4 3 4 5% 4 7% 10% 57%	5366
	21st to end	Day	3 2 1 4 4 5% 5% 5% 8% 10% 54%	5984
September	1st to 10th	Day	7% 5% 2 5% 3 4 4 5% 7% 7% 52%	2400
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 8% 58%	2360
	21st to end	Day	3 2 1 3 2 3 2 2 5% 6% 72%	2120
October	1st to 10th	Day	2 2 1 2 2 2 2 2 4 4 79%	3270
	11th to 20th	Day	3 2 1 4 2 2 3 2 3 5% 74%	3250
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 86%	3548
November	1st to 10th	Day	1 2 1 3 3 3 3 2 4 6% 74%	2059
	11th to 20th	Day	1 2 1 3 3 3 4 2 4 6% 70%	2100
	21st to end	Day	6% 4 2 3 3 2 3 3 5% 7% 63%	2100
December	1st to 10th	Day	4 3 1 4 4 2 3 3 5% 6% 67%	3155
	11th to 20th	Day	2 4 2 5% 3 3 2 2 3 6% 68%	3499
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 71%	4609
January	1st to 10th	Night	4 6% 2 5% 4 4 4 4 5% 6% 57%	7605
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 81%	7650
	21st to end	Night	6% 5% 2 5% 3 3 4 3 5% 6% 58%	8370
February	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 7% 67%	2730
	11th to 20th	Night	5% 4 2 3 2 3 2 2 4 6% 66%	2468
	21st to end	Night	2 3 1 3 2 2 4 3 5% 6% 68%	1945
March	1st to 10th	Night	2 2 1 2 2 2 3 3 4 7% 71%	4962
	11th to 20th	Night	4 3 1 3 2 2 3 3 4 6% 68%	5000
	21st to end	Night	2 3 1 4 3 4 4 3 5% 7% 65%	5489
April	1st to 10th	Night	1 3 1 2 3 3 2 3 4 5% 73%	2040
	11th to 20th	Night	3 3 1 3 2 2 3 2 4 5% 73%	1977
	21st to end	Night	2 2 1 2 2 2 3 2 4 4 77%	2000
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 78%	4540
	11th to 20th	Night	2 2 1 3 3 3 3 2 4 5% 73%	4380
	21st to end	Night	1 1 2 2 2 2 2 3 3 83%	4474
June	1st to 10th	Night	3 2 1 3 3 3 2 3 6% 7% 67%	1800
	11th to 20th	Night	2 2 1 3 3 2 3 2 4 6% 71%	1800
	21st to end	Night	4 3 2 5% 5% 5% 6% 6% 9% 8% 47%	1800
July	1st to 10th	Night	3 3 2 5% 5% 5% 6% 5% 9% 11% 44%	4100
	11th to 20th	Night	2 3 1 7% 5% 6% 8% 6% 10% 14% 38%	4100
	21st to end	Night	4 6% 3 9% 7% 7% 7% 6% 8% 12% 31%	4472
August	1st to 10th	Night	3 4 2 6% 6% 6% 7% 6% 9% 12% 39%	4150
	11th to 20th	Night	4 5% 2 8% 7% 8% 8% 6% 9% 12% 32%	4483
	21st to end	Night	3 5% 3 8% 6% 7% 7% 6% 9% 9% 38%	4993
September	1st to 10th	Night	6% 5% 2 5% 5% 4 5% 5% 7% 8% 49%	2000
	11th to 20th	Night	5% 4 2 4 4 4 4 4 6% 8% 55%	2035
	21st to end	Night	2 2 1 4 2 3 3 3 6% 6% 67%	2278
October	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 76%	3920
	11th to 20th	Night	3 2 1 3 2 2 2 3 3 5% 73%	3898
	21st to end	Night	1 1 1 1 1 2 2 2 3 5% 82%	4252
November	1st to 10th	Night	2 2 1 2 2 2 2 2 5% 6% 75%	2460
	11th to 20th	Night	3 2 1 2 2 3 4 2 4 6% 72%	2520
	21st to end	Night	6% 3 1 4 3 2 3 2 5% 6% 64%	2520
December	1st to 10th	Night	4 2 1 3 2 2 3 2 4 6% 72%	4238
	11th to 20th	Night	2 2 1 4 3 4 4 3 4 6% 67%	4941
	21st to end	Night	4 3 1 3 2 2 2 2 4 6% 71%	6611

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	24% 2 5% 15% 37% 17%	5321
	11th to 20th	Day	8% 1 3 13% 46% 29%	5340
	21st to end	Day	23% 3 3 15% 31% 24%	5844
February	1st to 10th	Day	18% 2 4 14% 26% 35%	1890
	11th to 20th	Day	22% 2 4 15% 31% 26%	2047
	21st to end	Day	17% 2 3 9% 32% 37%	1856
March	1st to 10th	Day	15% 2 3 5% 33% 42%	4977
	11th to 20th	Day	17% 3 4 13% 30% 33%	5000
	21st to end	Day	19% 2 5% 8% 33% 31%	5486
April	1st to 10th	Day	12% 1 4 9% 34% 39%	2358
	11th to 20th	Day	11% 1 2 6% 26% 54%	2374
	21st to end	Day	9% 1 3 8% 28% 51%	2400
May	1st to 10th	Day	6% 1 2 10% 29% 53%	5440
	11th to 20th	Day	8% 1 4 8% 24% 54%	5534
	21st to end	Day	7% 1 2 6% 14% 70%	6424
June	1st to 10th	Day	13% 3 5% 9% 27% 43%	2600
	11th to 20th	Day	11% 2 3 6% 20% 59%	2600
	21st to end	Day	23% 5% 9% 13% 21% 30%	2600
July	1st to 10th	Day	19% 4 11% 20% 25% 22%	5816
	11th to 20th	Day	19% 4 14% 28% 26% 9%	5811
	21st to end	Day	31% 6% 17% 23% 15% 7%	6217
August	1st to 10th	Day	24% 6% 13% 26% 25% 7%	4123
	11th to 20th	Day	20% 6% 10% 17% 34% 12%	3882
	21st to end	Day	28% 5% 13% 25% 22% 8%	4311
September	1st to 10th	Day	29% 5% 9% 17% 22% 17%	2400
	11th to 20th	Day	23% 4 8% 17% 26% 21%	2360
	21st to end	Day	15% 3 6% 17% 33% 26%	2120
October	1st to 10th	Day	11% 2 4 9% 24% 49%	3270
	11th to 20th	Day	16% 2 3 13% 30% 37%	3250
	21st to end	Day	8% 1 2 10% 32% 48%	3548
November	1st to 10th	Day	14% 2 4 16% 33% 31%	2059
	11th to 20th	Day	17% 2 3 17% 31% 29%	2100
	21st to end	Day	22% 2 5% 18% 31% 22%	2100
December	1st to 10th	Day	19% 2 5% 21% 34% 19%	3155
	11th to 20th	Day	21% 2 3 14% 40% 20%	3498
	21st to end	Day	14% 2 6% 16% 41% 20%	4609
January	1st to 10th	Night	28% 2 4 16% 37% 13%	7605
	11th to 20th	Night	9% 2 2 13% 44% 30%	7650
	21st to end	Night	27% 3 3 14% 27% 25%	8370
February	1st to 10th	Night	17% 3 5% 15% 27% 34%	2730
	11th to 20th	Night	20% 3 4 14% 32% 27%	2468
	21st to end	Night	15% 3 5% 10% 30% 36%	1945
March	1st to 10th	Night	14% 1 4 6% 37% 37%	4962
	11th to 20th	Night	17% 3 4 15% 32% 30%	5000
	21st to end	Night	19% 2 5% 12% 32% 29%	5489
April	1st to 10th	Night	14% 2 4 9% 32% 38%	2040
	11th to 20th	Night	15% 2 3 8% 25% 46%	1977
	21st to end	Night	12% 2 4 8% 27% 48%	2000
May	1st to 10th	Night	11% 1 2 11% 22% 52%	4537
	11th to 20th	Night	13% 2 5% 9% 20% 50%	4381
	21st to end	Night	9% 1 2 5% 13% 69%	4474
June	1st to 10th	Night	16% 4 6% 10% 24% 41%	1800
	11th to 20th	Night	15% 2 5% 7% 16% 55%	1800
	21st to end	Night	29% 6% 11% 14% 14% 26%	1800
July	1st to 10th	Night	30% 5% 16% 19% 17% 12%	4055
	11th to 20th	Night	32% 5% 17% 20% 19% 6%	4046
	21st to end	Night	44% 5% 18% 16% 9% 8%	4344
August	1st to 10th	Night	39% 6% 15% 22% 13% 6%	3012
	11th to 20th	Night	34% 6% 11% 13% 26% 10%	3243
	21st to end	Night	43% 5% 15% 18% 13% 6%	3597
September	1st to 10th	Night	31% 6% 10% 17% 19% 17%	2000
	11th to 20th	Night	26% 5% 8% 17% 22% 21%	2035
	21st to end	Night	16% 4 9% 18% 29% 25%	2278
October	1st to 10th	Night	12% 3 5% 8% 22% 49%	3920
	11th to 20th	Night	15% 3 3 13% 23% 44%	3898
	21st to end	Night	9% 1 2 9% 30% 49%	4252
November	1st to 10th	Night	12% 3 5% 14% 32% 34%	2460
	11th to 20th	Night	16% 2 3 16% 32% 31%	2520
	21st to end	Night	22% 3 6% 15% 31% 23%	2520
December	1st to 10th	Night	16% 2 4 22% 38% 18%	4238
	11th to 20th	Night	19% 3 5% 12% 34% 27%	4941
	21st to end	Night	16% 2 4 16% 41% 21%	6611

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	12% 12% 24% 36% 14% 3	330
	11th to 20th	Day	8% 8% 24% 38% 18% 5%	330
	21st to end	Day	10% 10% 25% 33% 18% 5%	363
February	1st to 10th	Day	7% 11% 22% 34% 21% 5%	330
	11th to 20th	Day	15% 12% 28% 30% 13% 2	330
	21st to end	Day	13% 28% 25% 24% 7% 4	399
March	1st to 10th	Day	13% 23% 30% 25% 8% 2	509
	11th to 20th	Day	16% 16% 28% 25% 12% 3	500
	21st to end	Day	14% 25% 31% 19% 9% 2	550
April	1st to 10th	Day	8% 18% 34% 25% 12% 4	500
	11th to 20th	Day	8% 27% 30% 24% 9% 2	492
	21st to end	Day	8% 22% 32% 25% 12% 1	500
May	1st to 10th	Day	5% 20% 35% 25% 14% 1	503
	11th to 20th	Day	5% 15% 30% 34% 14% 2	528
	21st to end	Day	3 13% 34% 32% 16% 2	573
June	1st to 10th	Day	5% 6% 29% 35% 20% 5%	530
	11th to 20th	Day	4 7% 25% 36% 25% 4	530
	21st to end	Day	4 3 20% 35% 27% 12%	530
July	1st to 10th	Day	4 2 8% 32% 37% 17%	526
	11th to 20th	Day	4 1 3 30% 41% 21%	524
	21st to end	Day	5% 1 5% 28% 38% 23%	575
August	1st to 10th	Day	3 1 4 26% 39% 27%	527
	11th to 20th	Day	4 2 8% 29% 38% 19%	517
	21st to end	Day	4 1 6% 28% 41% 20%	583
September	1st to 10th	Day	9% 3 10% 38% 29% 12%	530
	11th to 20th	Day	6% 1 15% 33% 35% 9%	530
	21st to end	Day	4 4 16% 37% 28% 11%	450
October	1st to 10th	Day	5% 4 18% 32% 28% 13%	330
	11th to 20th	Day	7% 6% 22% 30% 23% 12%	330
	21st to end	Day	6% 8% 17% 28% 27% 15%	363
November	1st to 10th	Day	5% 8% 24% 28% 25% 9%	330
	11th to 20th	Day	5% 8% 21% 29% 30% 8%	330
	21st to end	Day	12% 11% 24% 32% 16% 5%	330
December	1st to 10th	Day	8% 8% 23% 32% 24% 5%	330
	11th to 20th	Day	7% 11% 23% 30% 22% 7%	330
	21st to end	Day	9% 13% 23% 32% 18% 5%	363
January	1st to 10th	Night	7% 7% 20% 46% 18% 2	840
	11th to 20th	Night	4 5% 24% 45% 19% 2	840
	21st to end	Night	9% 9% 22% 42% 17% 2	924
February	1st to 10th	Night	4 10% 23% 43% 19% 2	840
	11th to 20th	Night	9% 10% 27% 42% 11% 1	824
	21st to end	Night	5% 13% 33% 40% 9% 1	692
March	1st to 10th	Night	8% 12% 30% 39% 10% 1	808
	11th to 20th	Night	6% 14% 27% 39% 13% 1	800
	21st to end	Night	5% 14% 31% 38% 12% 1	880
April	1st to 10th	Night	5% 14% 35% 36% 10% 1	640
	11th to 20th	Night	6% 17% 37% 31% 9% 1	593
	21st to end	Night	4 15% 36% 36% 9% 1	600
May	1st to 10th	Night	4 10% 37% 41% 8%	600
	11th to 20th	Night	3 8% 35% 40% 13%	591
	21st to end	Night	3 8% 35% 44% 10%	650
June	1st to 10th	Night	4 3 27% 48% 18% 1	600
	11th to 20th	Night	2 2 27% 51% 17% 1	600
	21st to end	Night	5% 3 18% 43% 30% 1	600
July	1st to 10th	Night	4 1 10% 42% 39% 4	585
	11th to 20th	Night	5% 2 5% 33% 48% 5%	582
	21st to end	Night	5% 2 6% 32% 47% 7%	657
August	1st to 10th	Night	5% 3 4 30% 53% 6%	597
	11th to 20th	Night	6% 2 7% 29% 50% 6%	594
	21st to end	Night	6% 2 5% 29% 51% 7%	660
September	1st to 10th	Night	8% 2 7% 36% 46% 2	600
	11th to 20th	Night	7% 2 7% 45% 38% 2	635
	21st to end	Night	4 2 10% 46% 36% 3	800
October	1st to 10th	Night	5% 2 15% 47% 29% 2	800
	11th to 20th	Night	5% 3 17% 47% 26% 2	800
	21st to end	Night	3 3 15% 43% 32% 4	880
November	1st to 10th	Night	3 3 16% 42% 31% 5%	820
	11th to 20th	Night	4 3 17% 45% 28% 2	840
	21st to end	Night	8% 5% 20% 44% 21% 3	840
December	1st to 10th	Night	7% 4 19% 45% 23% 2	840
	11th to 20th	Night	4 6% 21% 48% 18% 2	840
	21st to end	Night	6% 7% 22% 45% 17% 2	924

Page generated at 2026-02-27 23:40:26. Version ac07

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