Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Sky Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	14% 5% 2 6% 4 4 4 4 6% 8% 44%	126724
	Night	15% 5% 2 6% 4 4 4 4 5% 7% 44%	123644
Transparency	Day	38% 4 11% 21% 19% 6%	130448
	Night	39% 4 10% 19% 21% 7%	127989
Seeing	Day	17% 8% 21% 26% 19% 10%	16424
	Night	17% 6% 21% 31% 21% 4	26447

By Month

Percentage Cloud Cover SE (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	16% 4 1 4 2 3 3 3 4 6% 53%	4056
	11th to 20th	Day	12% 5% 1 5% 3 2 2 2 3 3 60%	4060
	21st to end	Day	21% 5% 3 5% 3 2 3 3 4 6% 44%	4466
February	1st to 10th	Day	17% 5% 2 5% 4 4 4 3 5% 7% 43%	1945
	11th to 20th	Day	22% 6% 2 5% 4 3 3 3 5% 5% 43%	1948
	21st to end	Day	20% 5% 2 6% 4 4 3 3 5% 5% 43%	1650
March	1st to 10th	Day	21% 7% 2 6% 4 4 3 3 4 6% 40%	4796
	11th to 20th	Day	11% 4 2 5% 3 3 3 3 4 6% 55%	4980
	21st to end	Day	18% 5% 2 5% 4 4 4 3 4 5% 46%	5465
April	1st to 10th	Day	13% 4 1 4 4 4 4 4 7% 8% 47%	2200
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 3 5% 7% 49%	2369
	21st to end	Day	12% 4 1 5% 4 4 4 3 6% 8% 51%	2600
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 9% 37%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 5% 7% 9% 30%	5861
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 42%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 6% 6% 6% 8% 11% 34%	2600
	11th to 20th	Day	5% 4 2 5% 5% 5% 7% 7% 10% 12% 38%	2600
	21st to end	Day	7% 4 2 6% 6% 6% 5% 5% 8% 11% 40%	2600
July	1st to 10th	Day	6% 5% 2 7% 5% 5% 6% 6% 8% 11% 39%	5900
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 9% 13% 37%	5900
	21st to end	Day	8% 6% 3 8% 7% 8% 7% 6% 9% 11% 27%	6424
August	1st to 10th	Day	13% 7% 2 6% 5% 5% 6% 5% 7% 11% 33%	4234
	11th to 20th	Day	9% 5% 2 6% 4 6% 6% 4 8% 10% 40%	4174
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	4212
September	1st to 10th	Day	8% 4 2 7% 5% 5% 6% 5% 7% 9% 42%	2200
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	2200
	21st to end	Day	15% 6% 2 5% 3 3 4 3 5% 8% 46%	2200
October	1st to 10th	Day	14% 5% 2 4 2 3 3 3 3 6% 55%	2196
	11th to 20th	Day	12% 4 2 4 3 3 2 2 4 5% 59%	2000
	21st to end	Day	14% 5% 1 4 3 3 2 3 4 4 58%	2200
November	1st to 10th	Day	14% 5% 2 3 3 3 2 2 4 5% 57%	1961
	11th to 20th	Day	14% 5% 1 3 3 3 3 3 4 7% 53%	1863
	21st to end	Day	16% 5% 2 4 4 3 3 3 5% 7% 50%	1800
December	1st to 10th	Day	20% 5% 1 3 2 3 2 2 4 6% 50%	2930
	11th to 20th	Day	20% 5% 2 5% 4 4 4 4 5% 5% 42%	3373
	21st to end	Day	25% 4 1 3 2 2 3 3 4 6% 46%	4466
January	1st to 10th	Night	15% 3 1 3 3 3 3 3 3 5% 57%	5889
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 5% 53%	5900
	21st to end	Night	19% 6% 2 4 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 4 6% 48%	2450
	11th to 20th	Night	22% 6% 2 5% 4 3 3 3 4 4 44%	2338
	21st to end	Night	18% 6% 2 5% 3 4 3 3 5% 5% 44%	1980
March	1st to 10th	Night	20% 5% 2 5% 4 3 4 4 6% 7% 40%	5056
	11th to 20th	Night	10% 3 1 4 3 3 4 4 6% 8% 55%	4980
	21st to end	Night	18% 4 2 5% 4 4 4 3 4 6% 47%	5467
April	1st to 10th	Night	14% 4 2 5% 3 4 4 3 5% 7% 50%	2200
	11th to 20th	Night	12% 5% 2 5% 3 3 2 3 5% 7% 51%	1994
	21st to end	Night	11% 5% 2 5% 4 4 4 4 4 7% 50%	1800
May	1st to 10th	Night	13% 6% 3 8% 5% 5% 4 4 7% 9% 36%	4033
	11th to 20th	Night	23% 6% 3 5% 4 4 4 5% 6% 8% 31%	4032
	21st to end	Night	14% 5% 2 6% 4 4 4 3 7% 9% 41%	4421
June	1st to 10th	Night	9% 7% 2 7% 5% 6% 7% 5% 8% 11% 34%	1800
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 5% 7% 11% 40%	1800
	21st to end	Night	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	1800
July	1st to 10th	Night	10% 8% 3 8% 6% 7% 7% 5% 7% 9% 29%	4058
	11th to 20th	Night	10% 6% 2 8% 6% 6% 7% 7% 9% 11% 29%	4057
	21st to end	Night	12% 9% 4 10% 6% 6% 6% 5% 7% 10% 25%	4427
August	1st to 10th	Night	14% 7% 3 8% 5% 6% 6% 5% 6% 10% 31%	2915
	11th to 20th	Night	13% 8% 4 9% 6% 6% 5% 5% 7% 10% 27%	2875
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 4 6% 9% 42%	3633
September	1st to 10th	Night	10% 7% 2 7% 6% 6% 5% 4 7% 9% 38%	2200
	11th to 20th	Night	17% 7% 2 6% 5% 4 5% 4 6% 7% 35%	2200
	21st to end	Night	16% 5% 2 6% 4 4 4 3 6% 5% 44%	2200
October	1st to 10th	Night	13% 4 2 5% 3 4 4 3 5% 5% 51%	2201
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 5% 55%	2400
	21st to end	Night	14% 3 1 4 3 3 2 3 4 5% 58%	2640
November	1st to 10th	Night	15% 4 2 5% 3 2 3 2 3 5% 56%	2554
	11th to 20th	Night	16% 6% 2 4 3 2 2 2 4 6% 53%	2692
	21st to end	Night	17% 4 2 4 3 3 3 4 5% 5% 49%	2600
December	1st to 10th	Night	18% 5% 2 4 3 3 3 3 4 5% 51%	4250
	11th to 20th	Night	22% 6% 2 6% 4 3 3 3 3 5% 44%	4822
	21st to end	Night	23% 5% 2 4 3 3 4 3 4 5% 44%	6490

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	34% 2 4 23% 31% 7%	4057
	11th to 20th	Day	31% 2 5% 16% 37% 10%	4060
	21st to end	Day	43% 1 4 17% 24% 11%	4466
February	1st to 10th	Day	41% 3 7% 22% 20% 6%	1945
	11th to 20th	Day	43% 3 6% 20% 22% 7%	1958
	21st to end	Day	43% 3 6% 16% 22% 9%	1650
March	1st to 10th	Day	47% 2 5% 15% 18% 13%	4796
	11th to 20th	Day	32% 3 6% 13% 26% 20%	4980
	21st to end	Day	42% 2 5% 23% 21% 6%	5465
April	1st to 10th	Day	35% 4 10% 21% 25% 6%	2200
	11th to 20th	Day	36% 3 7% 22% 22% 10%	2369
	21st to end	Day	33% 3 10% 29% 21% 4	2600
May	1st to 10th	Day	44% 4 14% 22% 13% 3	5897
	11th to 20th	Day	50% 4 15% 18% 11% 1	5865
	21st to end	Day	39% 4 13% 21% 20% 3	6447
June	1st to 10th	Day	41% 6% 18% 24% 9% 2	2600
	11th to 20th	Day	33% 8% 20% 25% 13% 2	2600
	21st to end	Day	33% 8% 18% 22% 14% 4	2600
July	1st to 10th	Day	36% 10% 18% 27% 8% 1	5835
	11th to 20th	Day	34% 10% 31% 19% 4 1	5831
	21st to end	Day	47% 10% 18% 19% 6%	6240
August	1st to 10th	Day	44% 6% 16% 27% 7%	4221
	11th to 20th	Day	36% 7% 22% 19% 14% 2	4213
	21st to end	Day	30% 5% 15% 25% 22% 3	4212
September	1st to 10th	Day	36% 6% 15% 19% 20% 4	2200
	11th to 20th	Day	41% 5% 11% 21% 19% 3	2200
	21st to end	Day	38% 4 12% 21% 20% 6%	2200
October	1st to 10th	Day	28% 3 6% 18% 34% 10%	3573
	11th to 20th	Day	21% 1 5% 16% 30% 26%	3240
	21st to end	Day	29% 2 4 22% 29% 14%	3538
November	1st to 10th	Day	31% 2 7% 20% 29% 10%	1961
	11th to 20th	Day	34% 2 5% 22% 27% 9%	1863
	21st to end	Day	36% 2 7% 22% 26% 7%	1800
December	1st to 10th	Day	38% 2 5% 18% 27% 10%	2930
	11th to 20th	Day	45% 3 6% 18% 19% 9%	3370
	21st to end	Day	42% 2 6% 28% 20% 2	4466
January	1st to 10th	Night	32% 2 5% 19% 31% 10%	5889
	11th to 20th	Night	35% 2 4 17% 32% 10%	5900
	21st to end	Night	39% 2 4 17% 30% 8%	6490
February	1st to 10th	Night	39% 3 8% 22% 22% 7%	2450
	11th to 20th	Night	45% 2 5% 18% 23% 7%	2339
	21st to end	Night	41% 3 7% 18% 21% 9%	1980
March	1st to 10th	Night	43% 2 6% 14% 27% 7%	5056
	11th to 20th	Night	28% 3 7% 14% 30% 18%	4980
	21st to end	Night	40% 2 5% 22% 21% 10%	5467
April	1st to 10th	Night	36% 3 8% 21% 25% 7%	2200
	11th to 20th	Night	33% 3 6% 24% 24% 10%	1994
	21st to end	Night	35% 3 9% 27% 21% 4	1800
May	1st to 10th	Night	44% 4 15% 22% 13% 1	4060
	11th to 20th	Night	52% 4 15% 18% 11% 1	4033
	21st to end	Night	40% 4 12% 24% 17% 2	4429
June	1st to 10th	Night	41% 8% 16% 20% 12% 2	1800
	11th to 20th	Night	36% 7% 18% 24% 13% 2	1800
	21st to end	Night	35% 8% 19% 18% 15% 4	1800
July	1st to 10th	Night	47% 9% 18% 19% 6% 1	4015
	11th to 20th	Night	44% 12% 25% 16% 3 1	4006
	21st to end	Night	52% 8% 18% 16% 5% 1	4297
August	1st to 10th	Night	46% 8% 21% 20% 5%	2904
	11th to 20th	Night	50% 9% 16% 16% 8% 1	2902
	21st to end	Night	38% 7% 13% 21% 19% 2	3633
September	1st to 10th	Night	40% 7% 14% 19% 16% 3	2200
	11th to 20th	Night	45% 6% 12% 18% 16% 2	2200
	21st to end	Night	42% 4 11% 19% 18% 6%	2200
October	1st to 10th	Night	30% 4 10% 15% 33% 9%	3604
	11th to 20th	Night	21% 3 5% 17% 30% 25%	3899
	21st to end	Night	29% 2 4 21% 27% 17%	4252
November	1st to 10th	Night	34% 2 6% 20% 27% 10%	2554
	11th to 20th	Night	34% 2 4 18% 29% 12%	2692
	21st to end	Night	36% 4 5% 21% 26% 8%	2600
December	1st to 10th	Night	37% 2 5% 19% 30% 8%	4250
	11th to 20th	Night	45% 2 8% 16% 20% 8%	4824
	21st to end	Night	44% 2 6% 26% 20% 3	6490

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	23% 13% 36% 22% 4	300
	11th to 20th	Day	25% 18% 29% 22% 5% 1	300
	21st to end	Day	23% 17% 39% 16% 4 1	330
February	1st to 10th	Day	22% 15% 33% 25% 5%	300
	11th to 20th	Day	35% 15% 34% 15% 1	300
	21st to end	Day	26% 22% 36% 13% 3	249
March	1st to 10th	Day	21% 14% 29% 26% 9% 1	434
	11th to 20th	Day	20% 12% 39% 21% 7% 1	500
	21st to end	Day	24% 12% 32% 22% 9% 1	550
April	1st to 10th	Day	20% 14% 29% 24% 13% 2	500
	11th to 20th	Day	15% 12% 33% 24% 14% 3	492
	21st to end	Day	18% 8% 23% 27% 18% 6%	500
May	1st to 10th	Day	13% 12% 29% 26% 13% 8%	503
	11th to 20th	Day	19% 5% 20% 27% 20% 10%	527
	21st to end	Day	17% 8% 18% 24% 20% 13%	573
June	1st to 10th	Day	14% 5% 15% 31% 23% 12%	530
	11th to 20th	Day	7% 3 14% 30% 26% 20%	530
	21st to end	Day	9% 2 12% 31% 26% 21%	530
July	1st to 10th	Day	10% 3 8% 24% 32% 24%	525
	11th to 20th	Day	7% 2 7% 25% 34% 25%	524
	21st to end	Day	7% 2 6% 29% 35% 22%	576
August	1st to 10th	Day	17% 3 13% 25% 23% 19%	526
	11th to 20th	Day	12% 2 8% 25% 34% 19%	519
	21st to end	Day	14% 3 8% 26% 30% 20%	583
September	1st to 10th	Day	9% 2 12% 31% 30% 16%	530
	11th to 20th	Day	18% 4 10% 22% 30% 16%	530
	21st to end	Day	19% 5% 16% 26% 25% 9%	530
October	1st to 10th	Day	18% 4 15% 31% 23% 9%	530
	11th to 20th	Day	12% 7% 20% 32% 24% 5%	530
	21st to end	Day	19% 7% 22% 29% 19% 4	583
November	1st to 10th	Day	20% 7% 24% 27% 17% 5%	439
	11th to 20th	Day	17% 13% 29% 31% 9% 2	321
	21st to end	Day	12% 14% 30% 29% 12% 2	300
December	1st to 10th	Day	30% 11% 30% 22% 7%	300
	11th to 20th	Day	23% 12% 30% 25% 8% 1	300
	21st to end	Day	31% 14% 28% 17% 7% 3	330
January	1st to 10th	Night	21% 12% 31% 30% 5%	800
	11th to 20th	Night	21% 12% 31% 31% 5%	800
	21st to end	Night	20% 12% 36% 28% 3	880
February	1st to 10th	Night	18% 9% 30% 38% 6%	800
	11th to 20th	Night	26% 10% 34% 27% 4	783
	21st to end	Night	20% 11% 34% 31% 3	660
March	1st to 10th	Night	18% 10% 31% 35% 7%	800
	11th to 20th	Night	16% 10% 34% 31% 9%	800
	21st to end	Night	20% 9% 29% 31% 10%	880
April	1st to 10th	Night	17% 12% 27% 34% 11%	800
	11th to 20th	Night	15% 8% 27% 34% 16% 1	675
	21st to end	Night	14% 4 23% 36% 21% 2	600
May	1st to 10th	Night	13% 4 23% 37% 22% 1	600
	11th to 20th	Night	17% 5% 17% 33% 25% 3	591
	21st to end	Night	15% 2 11% 32% 34% 6%	650
June	1st to 10th	Night	13% 2 11% 33% 35% 6%	600
	11th to 20th	Night	11% 3 11% 27% 35% 14%	600
	21st to end	Night	9% 2 6% 34% 40% 9%	600
July	1st to 10th	Night	12% 3 7% 25% 39% 14%	585
	11th to 20th	Night	13% 3 4 22% 43% 16%	582
	21st to end	Night	9% 3 5% 25% 45% 12%	657
August	1st to 10th	Night	16% 3 7% 25% 36% 13%	597
	11th to 20th	Night	14% 3 8% 26% 38% 10%	594
	21st to end	Night	14% 2 5% 26% 40% 12%	704
September	1st to 10th	Night	11% 2 10% 30% 37% 10%	800
	11th to 20th	Night	18% 4 10% 26% 35% 6%	800
	21st to end	Night	17% 5% 12% 34% 29% 4	800
October	1st to 10th	Night	15% 5% 16% 31% 27% 6%	800
	11th to 20th	Night	14% 5% 22% 34% 23% 2	800
	21st to end	Night	16% 6% 21% 33% 23% 3	880
November	1st to 10th	Night	17% 5% 19% 37% 19% 3	820
	11th to 20th	Night	18% 8% 27% 32% 13% 2	829
	21st to end	Night	19% 7% 23% 36% 14% 1	800
December	1st to 10th	Night	25% 7% 26% 33% 9% 1	800
	11th to 20th	Night	21% 7% 27% 33% 12% 1	800
	21st to end	Night	27% 8% 25% 32% 8%	880

Page generated at 2025-11-15 00:55:54. Version ac07

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