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 2024-12-24.

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% 5% 5% 5% 4 6% 8% 40%	105647
	Night	8% 8% 3 8% 5% 5% 5% 4 6% 7% 41%	103114
Transparency	Day	41% 7% 15% 26% 11% 1	105386
	Night	40% 7% 14% 26% 12% 1	102965
Seeing	Day	12% 9% 33% 30% 13% 3	16259
	Night	11% 9% 35% 34% 10% 1	26285

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 12% 4 14% 15% 7% 5% 4 6% 6% 23%	2456
	11th to 20th	Day	3 8% 3 11% 9% 10% 9% 11% 11% 9% 17%	2540
	21st to end	Day	5% 11% 4 13% 8% 8% 6% 5% 6% 6% 27%	2794
February	1st to 10th	Day	7% 13% 4 13% 8% 7% 7% 5% 6% 7% 24%	1547
	11th to 20th	Day	6% 12% 4 12% 8% 7% 7% 5% 7% 6% 25%	1789
	21st to end	Day	6% 12% 5% 14% 8% 7% 6% 4 5% 7% 26%	1570
March	1st to 10th	Day	8% 12% 5% 12% 7% 5% 5% 3 4 7% 33%	3130
	11th to 20th	Day	5% 9% 5% 10% 7% 7% 7% 5% 6% 9% 30%	3377
	21st to end	Day	15% 14% 5% 13% 5% 5% 4 3 5% 6% 24%	3959
April	1st to 10th	Day	10% 10% 3 8% 6% 4 5% 4 5% 7% 38%	2280
	11th to 20th	Day	16% 11% 4 9% 5% 5% 3 3 5% 6% 33%	2382
	21st to end	Day	16% 11% 3 7% 5% 4 4 4 5% 7% 35%	2470
May	1st to 10th	Day	16% 9% 2 6% 4 4 4 4 5% 8% 37%	4141
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 3 6% 8% 40%	4388
	21st to end	Day	13% 7% 2 7% 5% 4 4 4 6% 7% 40%	5069
June	1st to 10th	Day	13% 7% 2 4 4 3 5% 4 6% 8% 44%	2850
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	2850
	21st to end	Day	8% 5% 2 6% 4 5% 4 5% 6% 9% 46%	2850
July	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 5% 7% 10% 44%	4695
	11th to 20th	Day	4 3 2 4 4 3 4 4 6% 8% 58%	4430
	21st to end	Day	4 3 2 5% 4 4 5% 4 6% 9% 53%	4802
August	1st to 10th	Day	10% 5% 2 5% 4 5% 4 5% 6% 8% 48%	4381
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 4 6% 8% 42%	4041
	21st to end	Day	7% 5% 2 5% 4 3 3 3 5% 6% 56%	4420
September	1st to 10th	Day	10% 4 1 5% 3 4 3 4 5% 6% 55%	2280
	11th to 20th	Day	10% 6% 2 5% 3 3 4 4 5% 7% 52%	2101
	21st to end	Day	10% 7% 2 6% 4 3 3 2 5% 6% 52%	2090
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 5% 7% 55%	3458
	11th to 20th	Day	9% 7% 2 5% 3 2 3 3 6% 8% 52%	2876
	21st to end	Day	10% 8% 3 8% 5% 4 6% 4 6% 8% 38%	3091
November	1st to 10th	Day	11% 11% 4 9% 5% 4 5% 3 5% 7% 35%	1763
	11th to 20th	Day	7% 10% 5% 12% 7% 7% 4 4 5% 6% 34%	1736
	21st to end	Day	5% 10% 5% 13% 7% 5% 4 4 7% 7% 32%	1425
December	1st to 10th	Day	5% 12% 4 11% 9% 6% 8% 5% 8% 9% 23%	1400
	11th to 20th	Day	7% 13% 5% 12% 8% 5% 5% 4 7% 9% 26%	1753
	21st to end	Day	10% 10% 4 10% 7% 5% 5% 5% 7% 7% 30%	2463
January	1st to 10th	Night	6% 12% 4 10% 7% 7% 6% 5% 6% 8% 29%	4481
	11th to 20th	Night	4 7% 3 10% 8% 7% 11% 9% 9% 8% 23%	4430
	21st to end	Night	5% 10% 4 14% 7% 8% 6% 5% 5% 6% 30%	4873
February	1st to 10th	Night	7% 12% 5% 11% 7% 7% 7% 5% 6% 6% 27%	2632
	11th to 20th	Night	6% 11% 4 9% 6% 5% 6% 5% 7% 7% 34%	2293
	21st to end	Night	7% 11% 3 9% 6% 5% 5% 4 6% 7% 38%	1884
March	1st to 10th	Night	8% 11% 4 10% 8% 5% 4 4 5% 7% 35%	3734
	11th to 20th	Night	7% 7% 4 7% 5% 4 5% 4 6% 9% 42%	3586
	21st to end	Night	14% 14% 4 9% 5% 5% 5% 4 5% 7% 30%	3673
April	1st to 10th	Night	6% 8% 3 8% 6% 5% 5% 5% 6% 7% 40%	1900
	11th to 20th	Night	12% 9% 4 7% 5% 4 5% 5% 6% 7% 37%	1745
	21st to end	Night	14% 9% 3 5% 4 4 3 5% 7% 7% 39%	1710
May	1st to 10th	Night	12% 7% 2 6% 4 5% 4 4 7% 7% 42%	2826
	11th to 20th	Night	12% 6% 3 6% 4 4 5% 4 6% 7% 43%	2517
	21st to end	Night	12% 4 2 6% 4 4 5% 5% 6% 10% 41%	2518
June	1st to 10th	Night	8% 5% 2 5% 4 4 4 5% 7% 9% 48%	1330
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 6% 9% 44%	1330
	21st to end	Night	4 4 1 4 3 4 5% 4 7% 8% 55%	1330
July	1st to 10th	Night	4 3 1 4 4 4 5% 6% 8% 10% 50%	2271
	11th to 20th	Night	4 3 1 4 4 3 3 3 5% 8% 61%	2540
	21st to end	Night	3 3 2 6% 4 4 4 5% 7% 10% 53%	2761
August	1st to 10th	Night	4 4 1 5% 3 3 3 4 6% 7% 60%	2558
	11th to 20th	Night	6% 4 2 6% 5% 4 4 3 6% 8% 50%	2798
	21st to end	Night	5% 5% 2 4 3 3 3 3 5% 7% 60%	3231
September	1st to 10th	Night	6% 4 1 4 3 3 3 3 5% 7% 62%	1900
	11th to 20th	Night	7% 4 1 5% 3 4 4 3 5% 9% 55%	2078
	21st to end	Night	11% 6% 2 4 3 3 4 2 5% 6% 53%	2090
October	1st to 10th	Night	7% 5% 2 4 3 3 4 2 5% 6% 59%	3480
	11th to 20th	Night	9% 6% 2 5% 3 3 4 4 6% 8% 49%	4041
	21st to end	Night	10% 8% 3 7% 4 3 4 4 6% 7% 44%	4467
November	1st to 10th	Night	8% 8% 3 8% 5% 5% 5% 5% 7% 8% 38%	2535
	11th to 20th	Night	6% 10% 4 10% 7% 6% 5% 4 5% 7% 36%	2661
	21st to end	Night	6% 7% 4 10% 6% 6% 5% 5% 6% 7% 37%	2970
December	1st to 10th	Night	6% 10% 5% 11% 8% 6% 6% 5% 7% 7% 29%	3000
	11th to 20th	Night	6% 12% 4 12% 7% 6% 6% 5% 7% 8% 29%	3664
	21st to end	Night	11% 12% 4 10% 7% 5% 5% 4 5% 5% 32%	5277

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	58% 9% 16% 12% 4 1	2457
	11th to 20th	Day	54% 9% 15% 15% 6%	2540
	21st to end	Day	55% 7% 11% 19% 8%	2794
February	1st to 10th	Day	57% 7% 13% 15% 7% 1	1547
	11th to 20th	Day	55% 8% 12% 16% 7% 1	1799
	21st to end	Day	57% 6% 14% 15% 8%	1570
March	1st to 10th	Day	50% 8% 12% 22% 8%	3130
	11th to 20th	Day	45% 12% 19% 19% 5%	3377
	21st to end	Day	59% 9% 13% 12% 7%	3959
April	1st to 10th	Day	44% 7% 15% 21% 12% 1	2280
	11th to 20th	Day	51% 6% 12% 19% 10% 1	2385
	21st to end	Day	47% 8% 15% 23% 7% 1	2470
May	1st to 10th	Day	40% 10% 15% 26% 9% 1	4141
	11th to 20th	Day	38% 9% 13% 28% 10% 2	4392
	21st to end	Day	39% 10% 17% 28% 6%	5069
June	1st to 10th	Day	37% 9% 16% 28% 10% 1	2850
	11th to 20th	Day	39% 9% 15% 26% 11% 1	2850
	21st to end	Day	32% 9% 20% 26% 12% 1	2850
July	1st to 10th	Day	34% 7% 19% 32% 8% 1	4622
	11th to 20th	Day	23% 6% 12% 38% 19% 2	4355
	21st to end	Day	28% 5% 19% 34% 13% 1	4651
August	1st to 10th	Day	33% 6% 13% 29% 16% 2	4366
	11th to 20th	Day	40% 5% 15% 31% 8% 1	4080
	21st to end	Day	29% 5% 14% 36% 15% 2	4419
September	1st to 10th	Day	29% 7% 13% 33% 17% 1	2280
	11th to 20th	Day	31% 6% 16% 31% 15% 1	2101
	21st to end	Day	33% 7% 13% 30% 16% 2	2090
October	1st to 10th	Day	31% 3 10% 31% 20% 5%	3458
	11th to 20th	Day	31% 3 12% 41% 12%	2876
	21st to end	Day	44% 5% 12% 21% 17% 1	3091
November	1st to 10th	Day	48% 7% 13% 20% 10% 1	1763
	11th to 20th	Day	51% 6% 15% 19% 8% 1	1736
	21st to end	Day	50% 8% 16% 19% 8% 1	1425
December	1st to 10th	Day	54% 10% 15% 16% 4 2	1400
	11th to 20th	Day	51% 9% 16% 18% 5%	1750
	21st to end	Day	49% 7% 13% 21% 9% 1	2463
January	1st to 10th	Night	49% 11% 14% 20% 7%	4481
	11th to 20th	Night	51% 10% 15% 18% 5%	4430
	21st to end	Night	52% 8% 12% 20% 8%	4873
February	1st to 10th	Night	54% 6% 13% 16% 10%	2632
	11th to 20th	Night	46% 8% 14% 20% 10% 2	2293
	21st to end	Night	44% 6% 15% 20% 12% 2	1884
March	1st to 10th	Night	46% 7% 10% 27% 10%	3734
	11th to 20th	Night	34% 10% 20% 27% 8%	3586
	21st to end	Night	52% 9% 14% 17% 8% 1	3673
April	1st to 10th	Night	40% 7% 13% 23% 15% 2	1900
	11th to 20th	Night	44% 7% 12% 22% 14% 2	1745
	21st to end	Night	41% 8% 13% 25% 12% 1	1710
May	1st to 10th	Night	36% 10% 15% 26% 10% 3	2826
	11th to 20th	Night	35% 9% 16% 25% 14% 1	2518
	21st to end	Night	33% 11% 20% 26% 9%	2518
June	1st to 10th	Night	29% 9% 17% 31% 11% 3	1330
	11th to 20th	Night	34% 9% 16% 25% 12% 2	1330
	21st to end	Night	24% 9% 21% 31% 13% 2	1330
July	1st to 10th	Night	27% 7% 21% 32% 11% 2	2235
	11th to 20th	Night	23% 4 13% 42% 15% 3	2492
	21st to end	Night	27% 5% 22% 29% 16% 2	2674
August	1st to 10th	Night	23% 5% 14% 36% 19% 3	2550
	11th to 20th	Night	32% 5% 16% 35% 11% 1	2825
	21st to end	Night	25% 4 11% 42% 16% 2	3231
September	1st to 10th	Night	22% 6% 12% 35% 21% 3	1900
	11th to 20th	Night	26% 7% 19% 30% 17% 1	2078
	21st to end	Night	32% 6% 12% 31% 17% 2	2090
October	1st to 10th	Night	27% 4 8% 30% 27% 3	3480
	11th to 20th	Night	34% 3 11% 36% 14% 2	4041
	21st to end	Night	39% 4 11% 31% 13% 1	4467
November	1st to 10th	Night	40% 9% 14% 20% 15% 2	2535
	11th to 20th	Night	47% 6% 15% 20% 10% 1	2661
	21st to end	Night	43% 8% 19% 21% 8% 1	2970
December	1st to 10th	Night	50% 8% 18% 17% 6%	3000
	11th to 20th	Night	50% 9% 18% 18% 5%	3666
	21st to end	Night	51% 7% 10% 20% 11% 1	5277

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	9% 11% 37% 31% 9% 3	270
	11th to 20th	Day	9% 10% 43% 27% 9% 2	270
	21st to end	Day	11% 11% 37% 26% 12% 3	297
February	1st to 10th	Day	10% 10% 44% 30% 5%	270
	11th to 20th	Day	12% 8% 45% 26% 9%	270
	21st to end	Day	11% 8% 35% 35% 10%	225
March	1st to 10th	Day	18% 8% 40% 24% 9% 1	270
	11th to 20th	Day	10% 10% 36% 31% 12% 1	270
	21st to end	Day	13% 11% 35% 29% 10% 2	378
April	1st to 10th	Day	15% 13% 37% 28% 8%	460
	11th to 20th	Day	22% 15% 30% 23% 10% 1	583
	21st to end	Day	20% 12% 29% 27% 11% 2	650
May	1st to 10th	Day	17% 10% 31% 29% 12% 1	653
	11th to 20th	Day	15% 10% 28% 31% 12% 3	675
	21st to end	Day	15% 10% 31% 25% 15% 4	734
June	1st to 10th	Day	12% 11% 26% 30% 15% 6%	680
	11th to 20th	Day	13% 11% 38% 24% 11% 2	680
	21st to end	Day	8% 11% 30% 35% 14% 3	680
July	1st to 10th	Day	7% 4 27% 39% 20% 3	671
	11th to 20th	Day	7% 5% 32% 36% 17% 2	668
	21st to end	Day	4 4 27% 41% 19% 4	739
August	1st to 10th	Day	7% 5% 24% 39% 22% 4	676
	11th to 20th	Day	10% 4 25% 33% 23% 6%	665
	21st to end	Day	8% 8% 30% 33% 18% 4	688
September	1st to 10th	Day	10% 8% 32% 32% 14% 3	490
	11th to 20th	Day	13% 9% 30% 33% 12% 3	311
	21st to end	Day	16% 12% 34% 26% 11% 1	300
October	1st to 10th	Day	19% 12% 31% 24% 11% 3	300
	11th to 20th	Day	15% 13% 38% 25% 8% 2	300
	21st to end	Day	17% 13% 43% 19% 6% 2	330
November	1st to 10th	Day	18% 9% 38% 25% 8% 1	300
	11th to 20th	Day	10% 15% 38% 29% 7% 1	300
	21st to end	Day	11% 13% 40% 27% 8% 1	300
December	1st to 10th	Day	10% 9% 46% 25% 8% 1	300
	11th to 20th	Day	15% 11% 44% 24% 5% 1	300
	21st to end	Day	14% 12% 38% 26% 6% 4	306
January	1st to 10th	Night	11% 11% 41% 29% 8%	950
	11th to 20th	Night	12% 11% 39% 30% 7%	950
	21st to end	Night	10% 11% 40% 30% 9%	1045
February	1st to 10th	Night	12% 11% 36% 33% 7% 1	950
	11th to 20th	Night	11% 9% 42% 28% 8% 2	818
	21st to end	Night	11% 7% 34% 37% 9% 2	628
March	1st to 10th	Night	11% 11% 35% 31% 10% 2	760
	11th to 20th	Night	11% 10% 36% 31% 11% 1	637
	21st to end	Night	15% 7% 33% 32% 11% 1	627
April	1st to 10th	Night	8% 10% 41% 33% 7%	570
	11th to 20th	Night	15% 10% 34% 32% 9%	563
	21st to end	Night	18% 9% 29% 31% 12% 1	570
May	1st to 10th	Night	12% 7% 33% 34% 14%	570
	11th to 20th	Night	11% 7% 32% 35% 14%	561
	21st to end	Night	14% 8% 30% 36% 12%	617
June	1st to 10th	Night	10% 9% 27% 41% 12% 1	570
	11th to 20th	Night	11% 7% 37% 35% 9%	570
	21st to end	Night	6% 4 31% 48% 12%	570
July	1st to 10th	Night	6% 2 27% 47% 17% 1	555
	11th to 20th	Night	5% 3 33% 46% 13%	552
	21st to end	Night	4 4 26% 49% 16% 1	624
August	1st to 10th	Night	4 3 26% 51% 15% 1	567
	11th to 20th	Night	6% 2 30% 44% 17% 1	564
	21st to end	Night	5% 5% 32% 42% 15% 1	627
September	1st to 10th	Night	8% 4 29% 42% 15% 2	570
	11th to 20th	Night	7% 6% 35% 38% 12% 1	570
	21st to end	Night	13% 8% 35% 33% 11% 1	570
October	1st to 10th	Night	16% 10% 32% 33% 9%	570
	11th to 20th	Night	16% 11% 30% 33% 9% 1	907
	21st to end	Night	16% 11% 37% 27% 9% 1	1045
November	1st to 10th	Night	13% 11% 38% 29% 8%	975
	11th to 20th	Night	11% 12% 38% 29% 8% 1	1000
	21st to end	Night	10% 12% 40% 33% 5% 1	1000
December	1st to 10th	Night	11% 10% 39% 32% 8% 1	1000
	11th to 20th	Night	14% 13% 36% 30% 7% 1	1000
	21st to end	Night	17% 10% 35% 28% 9%	1063

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