DarkSkies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DarkSkies Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	16% 8% 3 7% 4 4 4 4 5% 7% 37%	143526
	Night	16% 8% 3 6% 4 4 4 3 5% 7% 39%	138371
Transparency	Day	44% 5% 12% 23% 14% 2	147035
	Night	43% 5% 12% 22% 16% 2	142925
Seeing	Day	24% 19% 32% 17% 6% 2	17228
	Night	22% 18% 33% 21% 5%	26094

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	9% 8% 3 11% 5% 6% 6% 5% 5% 6% 35%	4661
	11th to 20th	Day	12% 9% 3 7% 4 4 4 3 5% 5% 42%	4650
	21st to end	Day	13% 10% 3 5% 3 3 4 4 5% 6% 44%	5124
February	1st to 10th	Day	15% 8% 3 7% 5% 6% 5% 4 5% 7% 34%	1939
	11th to 20th	Day	12% 7% 3 8% 5% 5% 4 5% 6% 7% 36%	2048
	21st to end	Day	17% 11% 3 7% 5% 4 5% 3 4 4 36%	1730
March	1st to 10th	Day	18% 13% 3 7% 4 4 4 3 4 5% 34%	4576
	11th to 20th	Day	15% 8% 2 6% 5% 5% 5% 4 5% 6% 37%	4803
	21st to end	Day	23% 10% 3 6% 3 3 4 3 5% 7% 32%	6003
April	1st to 10th	Day	17% 11% 3 7% 5% 4 4 4 4 6% 35%	2520
	11th to 20th	Day	16% 9% 2 6% 5% 4 4 4 7% 6% 37%	2494
	21st to end	Day	19% 8% 3 6% 5% 5% 4 3 5% 6% 36%	2714
May	1st to 10th	Day	27% 9% 3 7% 4 4 4 4 5% 7% 27%	7639
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 3 5% 6% 34%	7938
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 6% 8% 30%	9060
June	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 5% 6% 9% 34%	2940
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 9% 35%	2940
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	2940
July	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	8280
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 7% 11% 42%	8280
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 41%	8993
August	1st to 10th	Day	16% 7% 2 7% 5% 6% 5% 4 6% 9% 34%	4502
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 4 6% 9% 36%	4102
	21st to end	Day	8% 5% 2 6% 4 5% 6% 5% 7% 10% 43%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 5% 8% 44%	2400
	11th to 20th	Day	14% 7% 2 4 3 3 3 3 5% 6% 50%	2225
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 5% 7% 42%	2200
October	1st to 10th	Day	22% 7% 3 5% 5% 4 3 3 4 4 40%	2200
	11th to 20th	Day	14% 7% 3 5% 3 4 4 4 5% 7% 44%	1934
	21st to end	Day	19% 8% 2 7% 5% 3 3 3 4 6% 39%	1980
November	1st to 10th	Day	13% 7% 2 8% 3 3 3 3 4 5% 48%	1851
	11th to 20th	Day	17% 8% 2 5% 3 4 3 3 4 5% 46%	1890
	21st to end	Day	17% 10% 2 5% 4 4 4 3 5% 5% 40%	1864
December	1st to 10th	Day	13% 9% 3 7% 4 4 4 4 5% 6% 42%	2680
	11th to 20th	Day	18% 9% 3 5% 3 4 4 3 4 5% 41%	3066
	21st to end	Day	18% 11% 3 7% 5% 4 4 3 4 6% 36%	4048
January	1st to 10th	Night	13% 8% 3 7% 4 4 5% 4 6% 7% 40%	8234
	11th to 20th	Night	13% 10% 3 9% 6% 5% 5% 3 5% 6% 36%	8235
	21st to end	Night	12% 8% 3 7% 5% 4 5% 4 5% 6% 41%	9057
February	1st to 10th	Night	14% 9% 3 7% 4 4 4 5% 6% 6% 38%	2675
	11th to 20th	Night	12% 9% 3 6% 5% 4 5% 5% 6% 7% 38%	2458
	21st to end	Night	15% 10% 3 7% 5% 4 4 3 5% 6% 38%	2076
March	1st to 10th	Night	20% 9% 2 5% 4 3 4 3 5% 7% 37%	5484
	11th to 20th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 38%	5375
	21st to end	Night	23% 10% 3 5% 4 3 5% 3 5% 6% 34%	5166
April	1st to 10th	Night	18% 9% 2 7% 5% 4 4 5% 6% 6% 35%	2100
	11th to 20th	Night	18% 8% 3 5% 4 4 4 3 5% 6% 39%	2075
	21st to end	Night	19% 6% 3 7% 4 4 3 3 5% 6% 40%	1906
May	1st to 10th	Night	27% 11% 3 6% 4 4 4 3 5% 7% 27%	5259
	11th to 20th	Night	15% 11% 4 8% 5% 5% 5% 4 5% 7% 31%	4964
	21st to end	Night	21% 9% 3 7% 5% 4 5% 3 5% 6% 31%	5114
June	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 4 5% 40%	1680
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 4 6% 7% 37%	1680
	21st to end	Night	15% 6% 3 6% 3 4 4 4 6% 8% 41%	1680
July	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 6% 9% 38%	4680
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 9% 45%	4680
	21st to end	Night	15% 6% 2 5% 4 3 3 3 4 7% 48%	5084
August	1st to 10th	Night	21% 7% 2 7% 4 4 5% 3 5% 6% 35%	2832
	11th to 20th	Night	13% 8% 3 9% 5% 4 5% 4 6% 7% 37%	3031
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 8% 58%	3586
September	1st to 10th	Night	12% 7% 3 5% 4 4 3 3 6% 7% 48%	2000
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 7% 52%	2174
	21st to end	Night	16% 6% 3 6% 4 4 4 3 4 6% 44%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 4 6% 42%	2200
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 6% 7% 45%	2460
	21st to end	Night	20% 9% 3 6% 3 4 4 3 5% 6% 39%	2860
November	1st to 10th	Night	14% 7% 2 6% 3 3 4 3 4 6% 47%	2667
	11th to 20th	Night	16% 7% 2 5% 5% 4 4 3 4 6% 45%	2730
	21st to end	Night	14% 8% 3 6% 4 4 4 4 6% 7% 42%	2751
December	1st to 10th	Night	15% 11% 3 5% 4 4 3 3 5% 7% 39%	4720
	11th to 20th	Night	21% 9% 3 6% 4 3 3 3 4 5% 39%	5348
	21st to end	Night	18% 10% 3 7% 3 3 3 3 5% 6% 38%	7150

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	45% 3 10% 26% 14% 1	4662
	11th to 20th	Day	42% 4 10% 21% 19% 4	4680
	21st to end	Day	39% 4 9% 25% 21% 2	5124
February	1st to 10th	Day	48% 5% 13% 18% 14% 2	1939
	11th to 20th	Day	44% 7% 10% 22% 15% 3	2057
	21st to end	Day	50% 5% 9% 19% 14% 3	1730
March	1st to 10th	Day	51% 4 10% 14% 12% 8%	4579
	11th to 20th	Day	43% 7% 15% 24% 10% 2	4803
	21st to end	Day	50% 3 7% 16% 18% 5%	6003
April	1st to 10th	Day	50% 4 9% 18% 15% 3	2520
	11th to 20th	Day	43% 7% 12% 19% 16% 2	2494
	21st to end	Day	48% 5% 8% 19% 17% 3	2714
May	1st to 10th	Day	56% 5% 12% 15% 11% 1	7651
	11th to 20th	Day	50% 4 15% 20% 11% 1	7942
	21st to end	Day	51% 5% 10% 22% 11% 1	9061
June	1st to 10th	Day	45% 8% 14% 22% 11% 1	2940
	11th to 20th	Day	43% 7% 13% 20% 14% 2	2940
	21st to end	Day	46% 7% 13% 21% 13% 1	2940
July	1st to 10th	Day	45% 5% 16% 21% 12% 1	8210
	11th to 20th	Day	34% 4 17% 31% 14% 1	8205
	21st to end	Day	41% 3 12% 28% 15% 1	8795
August	1st to 10th	Day	46% 6% 17% 22% 9% 1	4488
	11th to 20th	Day	44% 5% 16% 26% 9% 1	4141
	21st to end	Day	34% 5% 12% 31% 16% 1	4312
September	1st to 10th	Day	38% 6% 12% 25% 18% 2	2400
	11th to 20th	Day	35% 5% 9% 26% 23% 2	2225
	21st to end	Day	41% 5% 9% 23% 19% 3	2200
October	1st to 10th	Day	43% 4 7% 24% 19% 3	3590
	11th to 20th	Day	33% 4 8% 28% 23% 4	3111
	21st to end	Day	35% 4 10% 30% 17% 3	3180
November	1st to 10th	Day	39% 5% 12% 25% 17% 2	1851
	11th to 20th	Day	39% 5% 10% 22% 21% 3	1890
	21st to end	Day	44% 6% 14% 24% 11% 2	1864
December	1st to 10th	Day	41% 4 14% 28% 11% 1	2680
	11th to 20th	Day	42% 4 14% 19% 20% 2	3066
	21st to end	Day	49% 6% 7% 19% 14% 5%	4048
January	1st to 10th	Night	41% 3 12% 29% 14% 2	8234
	11th to 20th	Night	47% 4 11% 18% 15% 5%	8280
	21st to end	Night	44% 3 9% 19% 21% 4	9057
February	1st to 10th	Night	44% 6% 14% 21% 13% 2	2675
	11th to 20th	Night	42% 6% 12% 21% 15% 3	2460
	21st to end	Night	46% 5% 10% 23% 13% 4	2076
March	1st to 10th	Night	46% 4 10% 17% 17% 5%	5494
	11th to 20th	Night	44% 7% 17% 18% 12% 3	5376
	21st to end	Night	50% 4 6% 14% 22% 5%	5166
April	1st to 10th	Night	48% 6% 11% 18% 15% 3	2100
	11th to 20th	Night	44% 7% 10% 20% 18% 2	2075
	21st to end	Night	45% 5% 10% 19% 17% 4	1906
May	1st to 10th	Night	55% 5% 13% 15% 11% 1	5265
	11th to 20th	Night	49% 6% 14% 20% 10% 1	4965
	21st to end	Night	52% 5% 12% 21% 9%	5115
June	1st to 10th	Night	45% 7% 9% 25% 13% 1	1680
	11th to 20th	Night	44% 8% 13% 21% 13% 1	1680
	21st to end	Night	39% 9% 14% 24% 13% 1	1680
July	1st to 10th	Night	43% 5% 15% 23% 14%	4640
	11th to 20th	Night	35% 4 18% 24% 18% 1	4632
	21st to end	Night	38% 3 12% 31% 15% 1	4967
August	1st to 10th	Night	49% 7% 13% 20% 11% 1	2822
	11th to 20th	Night	45% 6% 15% 25% 9%	3058
	21st to end	Night	26% 5% 15% 33% 20% 1	3586
September	1st to 10th	Night	36% 7% 12% 26% 19% 1	2000
	11th to 20th	Night	33% 5% 11% 25% 25% 2	2174
	21st to end	Night	41% 5% 9% 26% 16% 3	2200
October	1st to 10th	Night	39% 4 14% 25% 17% 2	3590
	11th to 20th	Night	28% 4 11% 27% 26% 4	3996
	21st to end	Night	35% 3 12% 32% 14% 2	4610
November	1st to 10th	Night	37% 6% 11% 22% 19% 3	2667
	11th to 20th	Night	40% 4 11% 22% 20% 3	2730
	21st to end	Night	40% 6% 15% 26% 13% 1	2751
December	1st to 10th	Night	44% 4 12% 30% 11% 1	4720
	11th to 20th	Night	46% 4 15% 13% 20% 2	5348
	21st to end	Night	46% 5% 8% 21% 16% 4	7150

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	30% 30% 25% 15% 1	220
	11th to 20th	Day	33% 29% 30% 6% 2	220
	21st to end	Day	31% 29% 30% 8% 2	242
February	1st to 10th	Day	31% 31% 28% 8% 1	401
	11th to 20th	Day	32% 29% 31% 8%	536
	21st to end	Day	31% 24% 33% 10% 2	446
March	1st to 10th	Day	31% 24% 35% 8% 2	540
	11th to 20th	Day	29% 30% 29% 8% 3 1	540
	21st to end	Day	33% 25% 30% 7% 4 1	594
April	1st to 10th	Day	35% 25% 27% 9% 5%	540
	11th to 20th	Day	32% 23% 31% 10% 3 1	532
	21st to end	Day	30% 20% 31% 14% 4 1	540
May	1st to 10th	Day	35% 23% 24% 14% 3 1	543
	11th to 20th	Day	21% 20% 40% 15% 3 1	567
	21st to end	Day	21% 16% 35% 20% 7% 1	617
June	1st to 10th	Day	18% 15% 37% 20% 8% 1	570
	11th to 20th	Day	19% 15% 38% 19% 7% 1	570
	21st to end	Day	18% 16% 38% 19% 8% 1	570
July	1st to 10th	Day	18% 9% 33% 28% 11% 2	565
	11th to 20th	Day	14% 10% 33% 32% 9% 2	564
	21st to end	Day	15% 11% 36% 29% 8% 2	620
August	1st to 10th	Day	13% 9% 34% 30% 12% 2	566
	11th to 20th	Day	17% 9% 33% 25% 14% 2	535
	21st to end	Day	13% 11% 34% 27% 11% 4	583
September	1st to 10th	Day	18% 12% 30% 23% 13% 5%	530
	11th to 20th	Day	18% 12% 26% 22% 14% 7%	530
	21st to end	Day	19% 14% 32% 20% 9% 5%	530
October	1st to 10th	Day	27% 13% 25% 20% 11% 3	530
	11th to 20th	Day	21% 21% 31% 20% 6% 2	484
	21st to end	Day	29% 18% 32% 16% 4 1	462
November	1st to 10th	Day	24% 23% 29% 16% 8% 1	425
	11th to 20th	Day	30% 24% 27% 15% 2 1	430
	21st to end	Day	34% 21% 28% 13% 4	404
December	1st to 10th	Day	27% 30% 29% 12% 3	220
	11th to 20th	Day	40% 25% 30% 5% 1	220
	21st to end	Day	31% 27% 29% 9% 4	242
January	1st to 10th	Night	24% 26% 36% 11% 2	840
	11th to 20th	Night	29% 24% 31% 13% 2	840
	21st to end	Night	21% 26% 36% 15% 2	924
February	1st to 10th	Night	23% 24% 38% 14% 1	840
	11th to 20th	Night	24% 27% 36% 12% 1	823
	21st to end	Night	28% 26% 33% 12% 2	692
March	1st to 10th	Night	27% 24% 35% 13% 2	840
	11th to 20th	Night	24% 25% 33% 15% 3	840
	21st to end	Night	26% 23% 32% 16% 3	924
April	1st to 10th	Night	27% 22% 33% 16% 2	840
	11th to 20th	Night	25% 23% 32% 16% 4	831
	21st to end	Night	24% 20% 35% 19% 2	646
May	1st to 10th	Night	28% 17% 35% 18% 2	630
	11th to 20th	Night	19% 18% 38% 21% 4	533
	21st to end	Night	21% 11% 36% 29% 4	455
June	1st to 10th	Night	18% 13% 35% 27% 7%	420
	11th to 20th	Night	20% 10% 35% 29% 5% 1	420
	21st to end	Night	16% 5% 42% 32% 5%	420
July	1st to 10th	Night	16% 6% 34% 36% 8%	410
	11th to 20th	Night	16% 5% 30% 38% 8% 1	408
	21st to end	Night	12% 5% 36% 37% 10%	460
August	1st to 10th	Night	13% 5% 34% 37% 11%	519
	11th to 20th	Night	13% 6% 31% 36% 12% 1	675
	21st to end	Night	16% 6% 29% 35% 14% 1	880
September	1st to 10th	Night	16% 8% 33% 32% 11% 1	800
	11th to 20th	Night	15% 8% 30% 29% 17% 2	800
	21st to end	Night	19% 11% 31% 26% 11% 2	800
October	1st to 10th	Night	25% 13% 28% 24% 10% 1	800
	11th to 20th	Night	19% 17% 32% 24% 7% 1	800
	21st to end	Night	26% 18% 32% 19% 4	880
November	1st to 10th	Night	20% 20% 35% 18% 6% 1	820
	11th to 20th	Night	23% 25% 30% 16% 5% 1	840
	21st to end	Night	24% 23% 34% 15% 3	840
December	1st to 10th	Night	27% 22% 34% 13% 3	840
	11th to 20th	Night	28% 23% 32% 14% 3	840
	21st to end	Night	28% 25% 31% 14% 2	924

Page generated at 2026-08-12 03:37:17. Version ac07

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