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 2026-02-16.

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	12% 8% 3 8% 5% 4 4 4 6% 7% 38%	131296
	Night	11% 8% 3 8% 5% 4 4 3 5% 7% 41%	129865
Transparency	Day	43% 6% 13% 24% 13% 1	134895
	Night	42% 5% 12% 24% 14% 2	134343
Seeing	Day	15% 10% 31% 27% 13% 3	17589
	Night	15% 10% 34% 31% 10% 1	27596

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	6% 13% 7% 23% 8% 5% 5% 3 4 4 22%	4691
	11th to 20th	Day	5% 9% 4 12% 7% 7% 7% 5% 8% 8% 28%	5167
	21st to end	Day	8% 11% 6% 12% 5% 6% 7% 4 6% 6% 29%	5812
February	1st to 10th	Day	9% 12% 5% 11% 7% 6% 4 5% 6% 7% 27%	1890
	11th to 20th	Day	10% 12% 5% 8% 5% 4 5% 5% 6% 7% 34%	1799
	21st to end	Day	14% 10% 4 9% 5% 4 4 4 5% 5% 35%	1575
March	1st to 10th	Day	15% 8% 3 7% 4 3 3 2 3 5% 46%	4931
	11th to 20th	Day	11% 6% 2 6% 4 3 3 3 5% 9% 45%	4980
	21st to end	Day	21% 13% 6% 8% 5% 5% 4 3 5% 5% 25%	5462
April	1st to 10th	Day	19% 10% 4 7% 4 4 4 4 5% 7% 32%	2400
	11th to 20th	Day	18% 9% 4 7% 6% 5% 4 3 5% 5% 34%	2374
	21st to end	Day	21% 8% 4 7% 4 4 4 3 6% 6% 34%	2552
May	1st to 10th	Day	15% 7% 2 7% 4 4 4 4 6% 7% 39%	5862
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 6% 48%	6314
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 7% 36%	6935
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 8% 43%	2800
	11th to 20th	Day	11% 6% 2 7% 4 4 5% 4 6% 9% 43%	2800
	21st to end	Day	11% 6% 2 8% 5% 4 4 4 7% 7% 40%	2800
July	1st to 10th	Day	8% 6% 2 6% 4 5% 5% 4 7% 9% 41%	6360
	11th to 20th	Day	8% 6% 2 6% 4 4 4 3 5% 7% 51%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 9% 41%	6479
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 8% 45%	4234
	11th to 20th	Day	12% 5% 2 7% 5% 4 5% 5% 7% 9% 38%	4171
	21st to end	Day	7% 5% 2 6% 5% 4 5% 4 6% 8% 49%	4411
September	1st to 10th	Day	11% 6% 2 4 3 3 4 3 5% 7% 53%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 48%	2348
	21st to end	Day	11% 5% 2 6% 4 5% 3 4 5% 6% 49%	2115
October	1st to 10th	Day	15% 6% 2 4 5% 3 4 3 5% 6% 47%	2000
	11th to 20th	Day	15% 7% 2 7% 5% 4 3 3 4 6% 44%	2000
	21st to end	Day	16% 9% 3 6% 4 3 4 3 5% 7% 40%	2200
November	1st to 10th	Day	11% 7% 3 6% 4 3 4 3 6% 7% 45%	1866
	11th to 20th	Day	13% 10% 4 7% 5% 4 4 4 6% 7% 36%	1680
	21st to end	Day	13% 11% 4 7% 5% 4 4 4 5% 7% 37%	1680
December	1st to 10th	Day	7% 7% 2 8% 5% 5% 4 3 6% 9% 42%	2690
	11th to 20th	Day	13% 12% 4 15% 8% 5% 5% 3 5% 6% 24%	3088
	21st to end	Day	20% 18% 9% 15% 5% 4 3 3 3 5% 16%	4070
January	1st to 10th	Night	8% 12% 5% 18% 7% 6% 4 3 5% 6% 26%	8232
	11th to 20th	Night	6% 9% 3 10% 8% 7% 8% 5% 7% 8% 29%	7820
	21st to end	Night	8% 10% 5% 11% 5% 5% 4 3 6% 7% 36%	8402
February	1st to 10th	Night	10% 12% 5% 11% 7% 4 4 4 5% 7% 31%	2730
	11th to 20th	Night	10% 10% 4 9% 4 5% 4 3 5% 7% 39%	2599
	21st to end	Night	12% 9% 3 9% 6% 3 4 3 5% 5% 41%	2055
March	1st to 10th	Night	15% 8% 2 5% 3 2 3 3 4 6% 50%	4963
	11th to 20th	Night	9% 6% 2 6% 4 3 4 3 6% 9% 49%	4980
	21st to end	Night	20% 11% 4 11% 4 4 4 3 5% 5% 29%	5467
April	1st to 10th	Night	14% 8% 3 7% 5% 4 3 4 5% 6% 42%	2000
	11th to 20th	Night	15% 9% 3 6% 5% 4 4 3 5% 7% 39%	1975
	21st to end	Night	18% 8% 2 6% 4 3 3 2 5% 7% 42%	1845
May	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 49%	4053
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 4 5% 52%	3599
	21st to end	Night	16% 8% 2 6% 5% 3 4 3 4 7% 42%	3942
June	1st to 10th	Night	8% 4 3 6% 4 3 5% 4 7% 8% 48%	1600
	11th to 20th	Night	10% 5% 1 6% 4 4 5% 4 6% 10% 45%	1600
	21st to end	Night	10% 5% 2 5% 4 3 4 3 6% 9% 49%	1600
July	1st to 10th	Night	7% 5% 2 5% 4 5% 5% 4 6% 10% 47%	3618
	11th to 20th	Night	6% 5% 2 5% 4 3 4 3 5% 9% 54%	3617
	21st to end	Night	7% 4 2 6% 4 4 5% 4 8% 10% 46%	4376
August	1st to 10th	Night	5% 4 2 5% 5% 3 5% 4 6% 9% 51%	2926
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 8% 55%	2888
	21st to end	Night	8% 5% 2 5% 4 3 3 2 4 7% 56%	3490
September	1st to 10th	Night	9% 5% 2 5% 4 3 3 2 4 7% 57%	2000
	11th to 20th	Night	10% 5% 2 4 4 4 3 3 6% 8% 53%	2049
	21st to end	Night	9% 5% 1 4 3 3 4 4 5% 5% 57%	2264
October	1st to 10th	Night	12% 5% 3 5% 3 3 3 2 4 7% 52%	2400
	11th to 20th	Night	15% 6% 2 6% 3 4 4 3 4 7% 46%	2400
	21st to end	Night	15% 9% 3 5% 3 3 4 3 4 6% 44%	2640
November	1st to 10th	Night	12% 6% 2 6% 3 3 3 3 5% 7% 49%	2646
	11th to 20th	Night	12% 9% 4 8% 4 4 5% 3 5% 6% 41%	2940
	21st to end	Night	13% 9% 3 8% 4 4 3 3 4 7% 41%	2940
December	1st to 10th	Night	6% 8% 3 9% 5% 6% 5% 4 6% 8% 42%	4720
	11th to 20th	Night	12% 10% 5% 12% 6% 5% 4 4 5% 5% 33%	5339
	21st to end	Night	19% 17% 7% 16% 5% 4 3 3 4 4 17%	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	66% 5% 10% 13% 6%	4692
	11th to 20th	Day	52% 4 10% 19% 14% 1	5167
	21st to end	Day	55% 4 12% 21% 7% 1	5812
February	1st to 10th	Day	53% 6% 16% 17% 7% 1	1890
	11th to 20th	Day	47% 7% 12% 21% 11% 2	1808
	21st to end	Day	48% 7% 13% 19% 11% 2	1575
March	1st to 10th	Day	41% 5% 11% 27% 14% 2	4931
	11th to 20th	Day	35% 6% 10% 31% 16% 2	4980
	21st to end	Day	60% 7% 14% 15% 4 1	5464
April	1st to 10th	Day	51% 6% 12% 22% 9% 1	2400
	11th to 20th	Day	51% 5% 11% 19% 13% 1	2374
	21st to end	Day	51% 6% 10% 21% 11% 1	2552
May	1st to 10th	Day	40% 8% 12% 22% 17% 1	5900
	11th to 20th	Day	36% 6% 11% 27% 17% 2	6318
	21st to end	Day	44% 8% 12% 24% 12% 1	6949
June	1st to 10th	Day	36% 8% 15% 25% 15% 2	2800
	11th to 20th	Day	36% 8% 14% 23% 18% 2	2800
	21st to end	Day	40% 9% 15% 21% 13% 2	2800
July	1st to 10th	Day	36% 6% 15% 32% 9% 1	6291
	11th to 20th	Day	32% 6% 12% 32% 16% 1	6285
	21st to end	Day	35% 9% 17% 25% 13% 1	6295
August	1st to 10th	Day	35% 8% 16% 26% 14% 1	4220
	11th to 20th	Day	41% 8% 15% 24% 12% 1	4210
	21st to end	Day	33% 6% 16% 26% 17% 2	4410
September	1st to 10th	Day	31% 6% 14% 27% 20% 2	2400
	11th to 20th	Day	33% 8% 14% 30% 15%	2348
	21st to end	Day	34% 6% 11% 29% 19% 1	2115
October	1st to 10th	Day	23% 4 7% 29% 35% 2	3260
	11th to 20th	Day	32% 4 11% 27% 24% 2	3240
	21st to end	Day	41% 4 10% 29% 13% 3	3538
November	1st to 10th	Day	37% 6% 15% 23% 17% 2	1866
	11th to 20th	Day	46% 6% 14% 22% 11% 1	1680
	21st to end	Day	46% 6% 15% 22% 10% 1	1680
December	1st to 10th	Day	39% 4 15% 34% 7% 1	2690
	11th to 20th	Day	61% 5% 13% 13% 7%	3085
	21st to end	Day	72% 4 8% 12% 3	4070
January	1st to 10th	Night	59% 5% 12% 16% 7% 1	8233
	11th to 20th	Night	50% 5% 13% 16% 15% 1	7820
	21st to end	Night	48% 5% 14% 22% 12%	8402
February	1st to 10th	Night	51% 6% 12% 20% 10% 1	2730
	11th to 20th	Night	44% 5% 13% 24% 13% 2	2600
	21st to end	Night	45% 5% 11% 21% 15% 3	2055
March	1st to 10th	Night	35% 5% 9% 30% 18% 2	4963
	11th to 20th	Night	32% 4 10% 32% 17% 4	4980
	21st to end	Night	58% 7% 13% 15% 6% 1	5467
April	1st to 10th	Night	41% 7% 11% 25% 13% 2	2000
	11th to 20th	Night	45% 5% 11% 19% 17% 2	1975
	21st to end	Night	42% 6% 13% 23% 14% 2	1845
May	1st to 10th	Night	35% 6% 10% 27% 21% 1	4080
	11th to 20th	Night	34% 5% 11% 26% 18% 6%	3600
	21st to end	Night	43% 7% 10% 26% 13% 2	3950
June	1st to 10th	Night	31% 9% 14% 28% 15% 3	1600
	11th to 20th	Night	32% 8% 14% 21% 20% 6%	1600
	21st to end	Night	31% 7% 15% 26% 17% 4	1600
July	1st to 10th	Night	32% 6% 16% 31% 14% 1	3580
	11th to 20th	Night	28% 5% 16% 33% 16% 2	3572
	21st to end	Night	30% 9% 19% 27% 15% 1	4249
August	1st to 10th	Night	29% 7% 15% 32% 14% 2	2916
	11th to 20th	Night	29% 6% 16% 33% 13% 2	2915
	21st to end	Night	30% 5% 16% 28% 19% 3	3490
September	1st to 10th	Night	29% 5% 11% 28% 25% 2	2000
	11th to 20th	Night	29% 6% 14% 30% 21% 1	2049
	21st to end	Night	27% 6% 11% 28% 25% 2	2264
October	1st to 10th	Night	22% 3 7% 27% 39% 3	3920
	11th to 20th	Night	27% 4 10% 28% 25% 6%	3899
	21st to end	Night	42% 4 10% 26% 16% 2	4252
November	1st to 10th	Night	34% 6% 14% 26% 17% 3	2646
	11th to 20th	Night	43% 6% 13% 24% 12% 2	2940
	21st to end	Night	45% 5% 12% 25% 11% 2	2940
December	1st to 10th	Night	40% 5% 14% 32% 8% 1	4720
	11th to 20th	Night	51% 6% 13% 24% 5%	5341
	21st to end	Night	70% 5% 10% 11% 5%	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	16% 11% 38% 25% 9% 2	330
	11th to 20th	Day	18% 15% 45% 18% 3	330
	21st to end	Day	16% 15% 40% 24% 4 1	363
February	1st to 10th	Day	13% 20% 41% 22% 3 1	330
	11th to 20th	Day	15% 22% 35% 23% 5% 1	315
	21st to end	Day	21% 14% 43% 19% 3	249
March	1st to 10th	Day	17% 14% 46% 17% 5% 1	300
	11th to 20th	Day	21% 16% 33% 24% 5% 1	300
	21st to end	Day	30% 19% 31% 18% 2	330
April	1st to 10th	Day	26% 14% 36% 19% 5%	500
	11th to 20th	Day	23% 17% 31% 21% 7% 1	492
	21st to end	Day	24% 14% 34% 18% 8% 2	652
May	1st to 10th	Day	21% 13% 33% 21% 10% 2	703
	11th to 20th	Day	23% 12% 28% 24% 11% 3	725
	21st to end	Day	19% 8% 29% 27% 15% 3	789
June	1st to 10th	Day	9% 8% 28% 33% 19% 3	730
	11th to 20th	Day	12% 6% 32% 33% 15% 3	730
	21st to end	Day	12% 8% 34% 31% 11% 4	730
July	1st to 10th	Day	8% 4 19% 35% 24% 9%	721
	11th to 20th	Day	7% 3 28% 35% 21% 6%	718
	21st to end	Day	7% 2 25% 38% 22% 5%	794
August	1st to 10th	Day	8% 4 22% 37% 24% 5%	726
	11th to 20th	Day	11% 6% 20% 33% 24% 6%	715
	21st to end	Day	9% 3 23% 37% 22% 6%	643
September	1st to 10th	Day	12% 6% 24% 33% 19% 7%	530
	11th to 20th	Day	9% 5% 25% 37% 19% 6%	478
	21st to end	Day	10% 11% 26% 33% 16% 4	330
October	1st to 10th	Day	18% 10% 26% 26% 16% 4	330
	11th to 20th	Day	19% 19% 38% 18% 6% 1	330
	21st to end	Day	21% 15% 37% 19% 7% 2	363
November	1st to 10th	Day	16% 13% 44% 19% 6% 2	330
	11th to 20th	Day	19% 21% 36% 18% 6%	330
	21st to end	Day	20% 17% 39% 21% 2 1	330
December	1st to 10th	Day	11% 16% 44% 21% 7% 1	330
	11th to 20th	Day	18% 15% 35% 24% 6% 1	330
	21st to end	Day	25% 14% 42% 16% 2 1	363
January	1st to 10th	Night	12% 14% 40% 27% 7%	1050
	11th to 20th	Night	15% 14% 39% 26% 6%	1050
	21st to end	Night	17% 12% 38% 27% 5%	1155
February	1st to 10th	Night	16% 13% 39% 27% 5%	1050
	11th to 20th	Night	15% 13% 41% 26% 4	1005
	21st to end	Night	18% 13% 39% 23% 5% 1	735
March	1st to 10th	Night	18% 16% 37% 21% 7% 1	620
	11th to 20th	Night	19% 9% 39% 25% 7% 1	600
	21st to end	Night	22% 14% 38% 19% 7% 1	660
April	1st to 10th	Night	19% 14% 41% 23% 4	600
	11th to 20th	Night	19% 15% 36% 24% 6%	593
	21st to end	Night	21% 11% 35% 25% 7% 1	600
May	1st to 10th	Night	17% 9% 33% 34% 7% 1	600
	11th to 20th	Night	17% 10% 29% 32% 11%	591
	21st to end	Night	16% 8% 28% 36% 12%	650
June	1st to 10th	Night	9% 6% 29% 43% 14% 1	600
	11th to 20th	Night	12% 5% 27% 46% 11%	600
	21st to end	Night	10% 4 30% 41% 15%	600
July	1st to 10th	Night	5% 2 18% 45% 27% 3	585
	11th to 20th	Night	6% 2 22% 45% 24% 2	582
	21st to end	Night	5% 3 17% 52% 21% 2	657
August	1st to 10th	Night	7% 2 18% 48% 24% 1	597
	11th to 20th	Night	9% 3 20% 40% 25% 3	594
	21st to end	Night	8% 4 23% 40% 23% 2	660
September	1st to 10th	Night	11% 5% 22% 39% 22% 2	600
	11th to 20th	Night	12% 3 25% 41% 19% 2	600
	21st to end	Night	11% 6% 31% 34% 17% 1	664
October	1st to 10th	Night	15% 8% 33% 31% 13% 1	800
	11th to 20th	Night	18% 11% 38% 26% 7%	800
	21st to end	Night	21% 11% 34% 27% 6%	880
November	1st to 10th	Night	16% 10% 37% 29% 7% 1	863
	11th to 20th	Night	16% 14% 43% 22% 5%	1050
	21st to end	Night	17% 14% 36% 26% 7%	1050
December	1st to 10th	Night	16% 13% 41% 25% 6%	1050
	11th to 20th	Night	20% 11% 36% 27% 6%	1050
	21st to end	Night	24% 12% 37% 22% 5%	1155

Page generated at 2026-02-13 00:42:13. Version ac07

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