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 2026-04-17.

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	9% 8% 3 8% 6% 5% 5% 4 6% 7% 40%	140150
	Night	7% 8% 3 8% 6% 5% 5% 4 6% 7% 39%	139390
Transparency	Day	43% 6% 14% 25% 11% 1	134749
	Night	42% 6% 15% 25% 11% 1	135800
Seeing	Day	13% 9% 33% 30% 13% 3	18024
	Night	11% 9% 35% 34% 10% 1	28132

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 9% 4 14% 14% 9% 7% 5% 7% 6% 22%	4585
	11th to 20th	Day	2 7% 3 13% 9% 9% 10% 9% 12% 9% 18%	4740
	21st to end	Day	4 8% 3 13% 9% 8% 5% 4 6% 7% 33%	5188
February	1st to 10th	Day	7% 13% 5% 13% 7% 6% 6% 5% 6% 7% 24%	1711
	11th to 20th	Day	6% 11% 4 12% 7% 7% 7% 5% 7% 6% 26%	1983
	21st to end	Day	6% 12% 5% 13% 9% 7% 6% 4 6% 7% 26%	1730
March	1st to 10th	Day	7% 10% 4 12% 6% 4 4 3 4 7% 38%	4600
	11th to 20th	Day	4 9% 5% 10% 7% 7% 6% 5% 7% 9% 30%	4944
	21st to end	Day	13% 14% 6% 13% 6% 5% 4 3 5% 6% 25%	5790
April	1st to 10th	Day	11% 10% 3 8% 6% 4 5% 4 5% 7% 38%	2520
	11th to 20th	Day	15% 10% 4 9% 6% 5% 3 3 5% 6% 33%	2585
	21st to end	Day	16% 11% 3 7% 5% 4 4 4 5% 7% 34%	2600
May	1st to 10th	Day	12% 7% 2 5% 3 4 4 3 5% 7% 48%	5934
	11th to 20th	Day	20% 8% 3 7% 3 4 4 3 5% 7% 36%	6298
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	7281
June	1st to 10th	Day	13% 7% 2 5% 4 3 5% 4 6% 8% 44%	3000
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	3000
	21st to end	Day	9% 5% 2 6% 4 4 4 5% 6% 9% 46%	3000
July	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 4 7% 9% 47%	6716
	11th to 20th	Day	8% 4 2 5% 4 3 4 4 6% 8% 52%	6340
	21st to end	Day	7% 5% 2 7% 4 5% 5% 5% 6% 9% 45%	6903
August	1st to 10th	Day	11% 5% 3 7% 5% 5% 4 4 6% 7% 43%	6190
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 6% 8% 43%	5821
	21st to end	Day	6% 5% 2 5% 3 3 3 3 4 6% 59%	6336
September	1st to 10th	Day	9% 4 1 5% 3 4 3 4 5% 6% 55%	2400
	11th to 20th	Day	11% 6% 2 5% 3 3 3 4 5% 7% 50%	2211
	21st to end	Day	10% 7% 2 5% 4 3 3 2 5% 6% 53%	2200
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 5% 7% 55%	3567
	11th to 20th	Day	10% 7% 2 5% 3 2 3 2 6% 8% 52%	2967
	21st to end	Day	10% 8% 3 8% 5% 4 5% 4 6% 7% 39%	3190
November	1st to 10th	Day	11% 11% 4 9% 5% 4 5% 4 5% 7% 35%	1853
	11th to 20th	Day	7% 9% 5% 11% 7% 7% 4 4 5% 6% 35%	1823
	21st to end	Day	5% 10% 5% 13% 7% 5% 4 4 7% 7% 32%	1496
December	1st to 10th	Day	5% 11% 5% 16% 9% 6% 6% 4 6% 7% 26%	2360
	11th to 20th	Day	5% 12% 5% 17% 9% 6% 6% 4 6% 8% 22%	2713
	21st to end	Day	8% 11% 4 12% 9% 5% 5% 4 6% 6% 31%	3575
January	1st to 10th	Night	4 10% 4 13% 11% 9% 8% 6% 7% 8% 22%	8319
	11th to 20th	Night	3 7% 3 11% 8% 8% 11% 8% 10% 8% 24%	8250
	21st to end	Night	3 7% 3 12% 8% 8% 7% 5% 7% 7% 34%	9026
February	1st to 10th	Night	7% 12% 5% 11% 7% 7% 7% 5% 6% 6% 26%	2908
	11th to 20th	Night	6% 10% 4 9% 6% 5% 6% 5% 7% 7% 36%	2541
	21st to end	Night	6% 11% 3 9% 6% 5% 5% 4 6% 7% 39%	2076
March	1st to 10th	Night	7% 9% 3 10% 7% 4 4 3 5% 8% 39%	5494
	11th to 20th	Night	7% 8% 4 9% 6% 5% 5% 4 6% 9% 38%	5242
	21st to end	Night	12% 12% 4 10% 5% 5% 5% 4 6% 7% 29%	5375
April	1st to 10th	Night	8% 8% 3 8% 6% 5% 5% 5% 6% 7% 39%	2100
	11th to 20th	Night	12% 9% 4 7% 5% 3 4 5% 6% 7% 37%	1895
	21st to end	Night	14% 9% 3 6% 4 4 3 5% 7% 7% 38%	1800
May	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 4 6% 7% 47%	4043
	11th to 20th	Night	18% 7% 3 6% 4 4 5% 3 6% 7% 38%	3617
	21st to end	Night	13% 5% 2 5% 3 4 4 4 6% 9% 45%	3616
June	1st to 10th	Night	8% 5% 2 5% 4 4 4 5% 7% 9% 48%	1400
	11th to 20th	Night	9% 5% 2 6% 4 4 6% 5% 6% 9% 44%	1400
	21st to end	Night	5% 4 1 4 3 4 5% 4 7% 8% 54%	1400
July	1st to 10th	Night	4 3 1 4 3 4 5% 5% 7% 9% 54%	3259
	11th to 20th	Night	6% 4 2 4 3 3 3 4 6% 8% 57%	3640
	21st to end	Night	6% 5% 2 7% 4 4 5% 4 7% 10% 46%	3970
August	1st to 10th	Night	6% 4 2 5% 3 3 4 4 6% 8% 55%	3622
	11th to 20th	Night	7% 5% 2 6% 4 4 4 3 5% 7% 53%	4028
	21st to end	Night	4 4 2 4 3 3 4 3 5% 7% 62%	4625
September	1st to 10th	Night	6% 4 1 4 3 3 3 3 6% 7% 61%	2000
	11th to 20th	Night	8% 4 1 5% 3 4 4 3 6% 9% 52%	2188
	21st to end	Night	11% 6% 2 4 3 3 4 2 5% 6% 54%	2200
October	1st to 10th	Night	7% 4 2 4 3 3 4 2 5% 6% 59%	3590
	11th to 20th	Night	10% 7% 2 5% 3 3 4 4 6% 7% 48%	4169
	21st to end	Night	10% 9% 3 7% 4 3 4 4 6% 7% 44%	4610
November	1st to 10th	Night	8% 8% 3 8% 5% 5% 5% 5% 7% 8% 38%	2665
	11th to 20th	Night	7% 10% 4 10% 7% 6% 5% 4 5% 7% 35%	2794
	21st to end	Night	6% 7% 4 10% 7% 6% 5% 4 6% 7% 36%	3119
December	1st to 10th	Night	4 9% 5% 12% 8% 7% 7% 5% 7% 7% 29%	5050
	11th to 20th	Night	7% 10% 4 12% 8% 7% 7% 5% 7% 8% 25%	5714
	21st to end	Night	8% 10% 4 11% 9% 5% 5% 5% 5% 6% 31%	7645

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	60% 5% 13% 16% 6%	4587
	11th to 20th	Day	56% 5% 14% 15% 9%	4740
	21st to end	Day	51% 4 15% 18% 12%	5188
February	1st to 10th	Day	58% 6% 12% 14% 8% 1	1711
	11th to 20th	Day	54% 8% 11% 17% 9% 1	1993
	21st to end	Day	56% 6% 15% 16% 7%	1730
March	1st to 10th	Day	47% 5% 15% 24% 9%	4600
	11th to 20th	Day	46% 8% 17% 22% 7%	4944
	21st to end	Day	60% 6% 13% 15% 6%	5790
April	1st to 10th	Day	45% 7% 15% 21% 12% 1	2520
	11th to 20th	Day	51% 6% 12% 20% 11% 1	2588
	21st to end	Day	48% 7% 15% 23% 7% 1	2600
May	1st to 10th	Day	33% 7% 13% 30% 17% 1	5934
	11th to 20th	Day	46% 7% 10% 24% 12% 1	6302
	21st to end	Day	36% 8% 13% 28% 15%	7282
June	1st to 10th	Day	37% 8% 16% 28% 10% 1	3000
	11th to 20th	Day	40% 8% 15% 26% 10% 1	3000
	21st to end	Day	33% 8% 20% 26% 12% 1	3000
July	1st to 10th	Day	34% 5% 18% 32% 9% 3	6644
	11th to 20th	Day	31% 4 13% 36% 14% 2	6265
	21st to end	Day	36% 4 20% 30% 10% 1	6705
August	1st to 10th	Day	34% 6% 14% 29% 16% 2	4505
	11th to 20th	Day	40% 4 15% 31% 8% 1	4210
	21st to end	Day	29% 5% 14% 36% 15% 1	4559
September	1st to 10th	Day	29% 6% 14% 33% 17% 1	2400
	11th to 20th	Day	34% 6% 16% 30% 15% 1	2211
	21st to end	Day	31% 6% 13% 31% 16% 2	2200
October	1st to 10th	Day	30% 3 10% 32% 20% 5%	3567
	11th to 20th	Day	32% 3 12% 40% 12%	2967
	21st to end	Day	44% 5% 12% 22% 17% 1	3190
November	1st to 10th	Day	48% 7% 13% 21% 10% 1	1853
	11th to 20th	Day	50% 6% 15% 19% 8% 1	1823
	21st to end	Day	50% 7% 16% 19% 8% 1	1496
December	1st to 10th	Day	57% 6% 17% 15% 4 1	2360
	11th to 20th	Day	59% 7% 15% 15% 4	2710
	21st to end	Day	52% 5% 11% 21% 11%	3575
January	1st to 10th	Night	58% 7% 12% 18% 6%	8320
	11th to 20th	Night	52% 6% 14% 19% 9%	8250
	21st to end	Night	48% 5% 16% 21% 11%	9026
February	1st to 10th	Night	55% 6% 12% 16% 10%	2908
	11th to 20th	Night	45% 7% 13% 21% 12% 2	2541
	21st to end	Night	43% 6% 15% 22% 12% 2	2076
March	1st to 10th	Night	42% 5% 14% 27% 11%	5494
	11th to 20th	Night	40% 7% 18% 24% 10%	5242
	21st to end	Night	53% 6% 13% 21% 6% 1	5375
April	1st to 10th	Night	42% 6% 13% 23% 14% 2	2100
	11th to 20th	Night	44% 6% 12% 22% 14% 2	1895
	21st to end	Night	42% 8% 13% 24% 12% 1	1800
May	1st to 10th	Night	33% 7% 13% 31% 14% 2	4043
	11th to 20th	Night	43% 7% 13% 21% 16%	3618
	21st to end	Night	33% 9% 15% 26% 17%	3616
June	1st to 10th	Night	29% 9% 17% 31% 12% 3	1400
	11th to 20th	Night	35% 9% 17% 25% 12% 2	1400
	21st to end	Night	25% 9% 21% 31% 13% 1	1400
July	1st to 10th	Night	26% 5% 21% 35% 11% 2	3223
	11th to 20th	Night	26% 3 15% 40% 13% 2	3592
	21st to end	Night	34% 5% 24% 24% 11% 1	3856
August	1st to 10th	Night	24% 5% 14% 35% 18% 3	2631
	11th to 20th	Night	31% 5% 16% 35% 11% 1	2915
	21st to end	Night	25% 4 11% 42% 16% 2	3333
September	1st to 10th	Night	23% 6% 12% 36% 20% 3	2000
	11th to 20th	Night	28% 7% 19% 28% 16% 1	2188
	21st to end	Night	31% 6% 12% 32% 18% 2	2200
October	1st to 10th	Night	27% 4 8% 30% 27% 3	3590
	11th to 20th	Night	35% 3 11% 36% 14% 2	4169
	21st to end	Night	40% 4 11% 31% 13% 1	4610
November	1st to 10th	Night	41% 9% 14% 20% 14% 2	2665
	11th to 20th	Night	48% 6% 15% 20% 10% 1	2794
	21st to end	Night	44% 8% 19% 21% 7% 1	3119
December	1st to 10th	Night	52% 5% 22% 15% 5%	5050
	11th to 20th	Night	55% 6% 18% 16% 4	5716
	21st to end	Night	52% 5% 12% 21% 10% 1	7645

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	8% 10% 38% 31% 10% 4	330
	11th to 20th	Day	8% 13% 43% 26% 8% 2	330
	21st to end	Day	11% 14% 39% 23% 12% 2	363
February	1st to 10th	Day	12% 11% 45% 27% 5%	330
	11th to 20th	Day	12% 7% 43% 28% 10% 1	330
	21st to end	Day	11% 10% 34% 35% 10%	273
March	1st to 10th	Day	18% 7% 40% 26% 8% 1	330
	11th to 20th	Day	11% 10% 35% 32% 10% 1	330
	21st to end	Day	13% 12% 38% 27% 9% 2	452
April	1st to 10th	Day	17% 11% 37% 27% 7%	540
	11th to 20th	Day	21% 15% 31% 23% 9% 1	658
	21st to end	Day	20% 12% 29% 27% 11% 1	700
May	1st to 10th	Day	16% 9% 31% 30% 12% 1	703
	11th to 20th	Day	17% 10% 28% 31% 11% 3	725
	21st to end	Day	15% 10% 30% 25% 16% 5%	789
June	1st to 10th	Day	13% 10% 27% 29% 15% 5%	730
	11th to 20th	Day	14% 11% 37% 25% 11% 2	730
	21st to end	Day	9% 11% 30% 34% 13% 3	730
July	1st to 10th	Day	7% 4 28% 38% 21% 3	721
	11th to 20th	Day	8% 6% 32% 36% 17% 2	718
	21st to end	Day	5% 4 27% 40% 19% 4	794
August	1st to 10th	Day	8% 5% 23% 39% 21% 4	726
	11th to 20th	Day	10% 4 26% 33% 22% 6%	715
	21st to end	Day	8% 7% 29% 33% 19% 4	740
September	1st to 10th	Day	10% 8% 31% 32% 15% 3	530
	11th to 20th	Day	17% 8% 29% 31% 12% 3	341
	21st to end	Day	15% 11% 33% 27% 12% 2	330
October	1st to 10th	Day	19% 11% 31% 25% 12% 2	330
	11th to 20th	Day	17% 14% 37% 23% 7% 2	330
	21st to end	Day	17% 13% 42% 19% 6% 2	363
November	1st to 10th	Day	17% 10% 39% 25% 8% 1	330
	11th to 20th	Day	11% 15% 38% 29% 7% 1	330
	21st to end	Day	12% 12% 40% 26% 8% 1	330
December	1st to 10th	Day	11% 9% 47% 25% 8% 1	330
	11th to 20th	Day	15% 14% 42% 22% 5% 1	330
	21st to end	Day	13% 10% 40% 26% 7% 3	363
January	1st to 10th	Night	11% 11% 41% 29% 8% 1	1050
	11th to 20th	Night	12% 12% 40% 29% 7%	1050
	21st to end	Night	9% 13% 41% 29% 8%	1155
February	1st to 10th	Night	13% 12% 36% 32% 7% 1	1050
	11th to 20th	Night	11% 8% 41% 29% 9% 1	906
	21st to end	Night	11% 8% 35% 36% 8% 2	692
March	1st to 10th	Night	11% 11% 37% 30% 9% 2	840
	11th to 20th	Night	11% 10% 37% 31% 10% 1	704
	21st to end	Night	16% 7% 35% 31% 10% 1	693
April	1st to 10th	Night	10% 10% 41% 32% 6%	630
	11th to 20th	Night	15% 10% 35% 31% 8%	611
	21st to end	Night	18% 9% 30% 31% 12% 1	600
May	1st to 10th	Night	12% 7% 34% 34% 14%	600
	11th to 20th	Night	13% 7% 33% 34% 14%	591
	21st to end	Night	15% 7% 29% 36% 13%	650
June	1st to 10th	Night	10% 9% 26% 42% 12% 1	600
	11th to 20th	Night	12% 7% 37% 36% 9%	600
	21st to end	Night	7% 4 31% 47% 12%	600
July	1st to 10th	Night	6% 2 26% 48% 17% 1	585
	11th to 20th	Night	5% 3 33% 46% 12%	582
	21st to end	Night	6% 4 26% 48% 16% 1	657
August	1st to 10th	Night	5% 3 26% 50% 15% 1	597
	11th to 20th	Night	6% 2 30% 44% 17% 1	594
	21st to end	Night	5% 5% 32% 42% 15% 1	660
September	1st to 10th	Night	8% 4 28% 42% 15% 2	600
	11th to 20th	Night	9% 6% 34% 38% 12% 1	600
	21st to end	Night	12% 8% 34% 33% 12% 1	600
October	1st to 10th	Night	15% 10% 33% 34% 9%	600
	11th to 20th	Night	18% 11% 30% 32% 9% 1	955
	21st to end	Night	16% 11% 37% 27% 9% 1	1100
November	1st to 10th	Night	13% 12% 39% 28% 8%	1025
	11th to 20th	Night	11% 12% 39% 29% 8% 1	1050
	21st to end	Night	10% 11% 40% 34% 5% 1	1050
December	1st to 10th	Night	11% 10% 39% 32% 7% 1	1050
	11th to 20th	Night	14% 14% 36% 29% 7% 1	1050
	21st to end	Night	16% 10% 37% 28% 9%	1155

Page generated at 2026-04-14 01:44:28. Version ac07

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