Stellar Skies Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellar Skies Dark Sky Site clear sky chart from 2005-11-06 to 2026-05-17.

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% 5% 2 5% 4 4 4 3 5% 7% 53%	132288
	Night	9% 5% 2 5% 3 3 3 3 5% 7% 56%	130853
Transparency	Day	30% 5% 10% 21% 22% 12%	136093
	Night	28% 4 10% 21% 23% 15%	135112
Seeing	Day	11% 7% 23% 30% 22% 7%	16893
	Night	11% 6% 28% 38% 15% 1	26906

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	8% 6% 2 5% 3 3 2 2 3 5% 60%	5289
	11th to 20th	Day	10% 6% 2 3 2 2 3 3 4 5% 60%	5310
	21st to end	Day	19% 6% 2 5% 4 3 3 2 4 5% 46%	6330
February	1st to 10th	Day	12% 7% 2 5% 2 3 3 3 3 6% 52%	2100
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 6% 50%	2049
	21st to end	Day	13% 6% 2 4 2 3 3 2 4 5% 56%	1730
March	1st to 10th	Day	8% 6% 3 6% 3 3 3 3 5% 6% 54%	4842
	11th to 20th	Day	10% 5% 2 6% 3 3 3 3 4 6% 55%	5090
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 7% 51%	5583
April	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 49%	2310
	11th to 20th	Day	13% 7% 3 6% 4 4 3 3 5% 5% 47%	2416
	21st to end	Day	10% 6% 2 6% 4 4 3 4 5% 6% 50%	2636
May	1st to 10th	Day	13% 7% 3 6% 4 4 4 4 6% 7% 43%	5992
	11th to 20th	Day	10% 6% 3 6% 4 4 5% 4 6% 7% 47%	5941
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 4 7% 9% 40%	6434
June	1st to 10th	Day	4 4 1 5% 4 4 4 4 6% 9% 55%	2600
	11th to 20th	Day	4 3 2 4 3 4 5% 4 7% 9% 56%	2600
	21st to end	Day	5% 4 1 4 5% 4 6% 4 6% 9% 52%	2600
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 6% 5% 6% 7% 40%	5900
	11th to 20th	Day	3 2 1 4 4 4 5% 4 6% 10% 56%	5900
	21st to end	Day	4 3 1 5% 4 5% 5% 4 7% 10% 51%	6422
August	1st to 10th	Day	1 1 1 2 3 3 3 3 4 8% 70%	4120
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 8% 64%	3846
	21st to end	Day	3 3 1 5% 5% 4 5% 4 7% 9% 54%	4249
September	1st to 10th	Day	9% 5% 2 5% 4 5% 4 4 5% 6% 50%	2200
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 3 5% 6% 50%	2200
	21st to end	Day	8% 4 2 4 4 4 4 3 5% 7% 54%	2200
October	1st to 10th	Day	4 3 1 4 3 4 4 3 5% 7% 61%	2200
	11th to 20th	Day	8% 4 2 4 2 3 3 3 3 5% 63%	2110
	21st to end	Day	10% 6% 2 4 2 3 3 2 3 4 63%	2200
November	1st to 10th	Day	12% 6% 2 5% 3 3 2 2 4 6% 55%	2011
	11th to 20th	Day	7% 4 2 4 3 3 4 3 5% 6% 60%	1890
	21st to end	Day	12% 6% 3 6% 3 3 3 3 4 5% 52%	1890
December	1st to 10th	Day	17% 7% 3 7% 4 3 3 2 3 4 47%	3030
	11th to 20th	Day	8% 7% 3 7% 4 5% 5% 3 5% 6% 47%	3481
	21st to end	Day	10% 5% 2 4 3 2 2 2 3 5% 63%	4587
January	1st to 10th	Night	10% 6% 2 4 3 2 2 2 3 5% 61%	7634
	11th to 20th	Night	7% 2 1 3 2 2 2 2 3 6% 69%	7680
	21st to end	Night	21% 6% 3 6% 3 3 3 3 3 5% 44%	7926
February	1st to 10th	Night	10% 6% 2 5% 3 3 3 2 4 6% 55%	2520
	11th to 20th	Night	14% 6% 2 5% 3 3 2 3 4 6% 53%	2458
	21st to end	Night	13% 6% 2 4 3 3 3 3 5% 5% 55%	2076
March	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 6% 7% 53%	5226
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 4 6% 54%	5090
	21st to end	Night	9% 6% 2 5% 3 3 3 3 5% 6% 55%	5588
April	1st to 10th	Night	8% 5% 1 4 4 4 4 5% 5% 8% 53%	2310
	11th to 20th	Night	11% 7% 2 5% 4 4 3 4 4 6% 49%	2149
	21st to end	Night	10% 6% 2 4 3 3 3 3 6% 6% 54%	1980
May	1st to 10th	Night	16% 8% 2 5% 3 3 4 3 5% 7% 45%	4143
	11th to 20th	Night	7% 5% 2 6% 5% 4 4 4 6% 7% 50%	4107
	21st to end	Night	10% 6% 3 6% 4 5% 4 4 6% 9% 44%	4443
June	1st to 10th	Night	3 3 1 4 3 4 5% 3 6% 9% 60%	1800
	11th to 20th	Night	3 3 1 4 3 3 3 3 6% 9% 62%	1800
	21st to end	Night	3 4 1 4 3 3 4 3 5% 9% 61%	1800
July	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 6% 11% 48%	4078
	11th to 20th	Night	2 1 1 2 3 3 3 3 6% 10% 65%	4077
	21st to end	Night	2 3 2 5% 3 4 4 4 6% 11% 55%	4449
August	1st to 10th	Night	1 1 1 3 2 2 2 2 4 6% 76%	3021
	11th to 20th	Night	3 2 1 3 2 3 3 4 7% 10% 63%	3213
	21st to end	Night	2 3 1 4 2 3 4 3 8% 11% 58%	3656
September	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 6% 7% 53%	2200
	11th to 20th	Night	8% 5% 2 5% 3 4 4 4 6% 10% 50%	2200
	21st to end	Night	7% 4 2 5% 3 4 3 4 5% 8% 58%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 4 7% 67%	2200
	11th to 20th	Night	9% 3 1 3 3 3 3 3 4 5% 63%	2268
	21st to end	Night	10% 5% 2 4 2 2 3 2 3 4 62%	2640
November	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 4 5% 60%	2502
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 62%	2730
	21st to end	Night	13% 6% 2 5% 3 4 4 3 4 6% 51%	2730
December	1st to 10th	Night	17% 5% 2 5% 3 3 3 3 5% 6% 48%	4380
	11th to 20th	Night	12% 8% 3 7% 3 3 3 2 3 5% 50%	4946
	21st to end	Night	10% 6% 2 4 2 2 3 2 3 5% 60%	6633

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	28% 2 6% 23% 29% 12%	5290
	11th to 20th	Day	28% 2 5% 18% 29% 20%	5310
	21st to end	Day	41% 2 6% 13% 25% 13%	6330
February	1st to 10th	Day	35% 3 6% 16% 20% 22%	2100
	11th to 20th	Day	35% 4 7% 19% 20% 15%	2059
	21st to end	Day	33% 2 5% 16% 26% 19%	1730
March	1st to 10th	Day	31% 4 6% 13% 20% 26%	4842
	11th to 20th	Day	32% 4 7% 10% 15% 33%	5090
	21st to end	Day	31% 4 10% 18% 23% 14%	5585
April	1st to 10th	Day	35% 5% 10% 17% 18% 16%	2310
	11th to 20th	Day	39% 4 8% 15% 20% 13%	2416
	21st to end	Day	36% 5% 7% 21% 21% 10%	2636
May	1st to 10th	Day	39% 6% 10% 16% 20% 9%	6030
	11th to 20th	Day	36% 6% 7% 15% 24% 12%	5945
	21st to end	Day	39% 7% 12% 23% 13% 6%	6447
June	1st to 10th	Day	25% 7% 14% 29% 21% 5%	2600
	11th to 20th	Day	23% 6% 15% 27% 22% 7%	2600
	21st to end	Day	27% 8% 13% 25% 21% 6%	2600
July	1st to 10th	Day	40% 8% 15% 29% 8%	5836
	11th to 20th	Day	22% 7% 16% 33% 19% 3	5831
	21st to end	Day	25% 7% 17% 30% 20% 1	6240
August	1st to 10th	Day	13% 4 12% 36% 33% 2	4106
	11th to 20th	Day	19% 6% 16% 40% 17% 3	3882
	21st to end	Day	23% 10% 17% 27% 20% 3	4248
September	1st to 10th	Day	33% 7% 10% 19% 20% 10%	2200
	11th to 20th	Day	34% 6% 13% 19% 23% 6%	2200
	21st to end	Day	28% 6% 8% 22% 23% 14%	2200
October	1st to 10th	Day	14% 3 7% 13% 28% 35%	3590
	11th to 20th	Day	20% 3 6% 21% 26% 24%	3415
	21st to end	Day	22% 2 4 19% 38% 14%	3538
November	1st to 10th	Day	33% 3 6% 20% 24% 14%	2011
	11th to 20th	Day	25% 4 8% 21% 23% 19%	1890
	21st to end	Day	35% 3 4 19% 23% 15%	1890
December	1st to 10th	Day	41% 4 5% 16% 22% 12%	3030
	11th to 20th	Day	37% 4 8% 11% 24% 15%	3479
	21st to end	Day	27% 2 5% 18% 28% 21%	4587
January	1st to 10th	Night	29% 2 5% 19% 31% 14%	7635
	11th to 20th	Night	20% 2 3 18% 31% 27%	7680
	21st to end	Night	45% 2 5% 9% 23% 16%	7926
February	1st to 10th	Night	31% 3 8% 14% 23% 21%	2520
	11th to 20th	Night	33% 3 8% 18% 21% 17%	2458
	21st to end	Night	33% 4 6% 13% 22% 22%	2076
March	1st to 10th	Night	28% 4 9% 17% 24% 18%	5226
	11th to 20th	Night	32% 4 6% 12% 16% 29%	5090
	21st to end	Night	30% 4 9% 18% 24% 15%	5588
April	1st to 10th	Night	30% 5% 10% 19% 16% 19%	2310
	11th to 20th	Night	37% 5% 9% 15% 19% 16%	2149
	21st to end	Night	32% 5% 11% 21% 22% 10%	1980
May	1st to 10th	Night	40% 4 10% 20% 15% 11%	4170
	11th to 20th	Night	32% 6% 10% 19% 17% 16%	4108
	21st to end	Night	37% 6% 13% 26% 12% 6%	4452
June	1st to 10th	Night	20% 7% 15% 33% 21% 3	1800
	11th to 20th	Night	19% 5% 16% 30% 22% 8%	1800
	21st to end	Night	21% 6% 14% 30% 24% 5%	1800
July	1st to 10th	Night	31% 8% 20% 32% 8% 1	4035
	11th to 20th	Night	14% 7% 20% 39% 19% 2	4026
	21st to end	Night	22% 7% 20% 30% 19% 1	4320
August	1st to 10th	Night	11% 3 13% 35% 36% 2	3011
	11th to 20th	Night	16% 5% 24% 38% 15% 2	3243
	21st to end	Night	18% 10% 22% 29% 19% 3	3656
September	1st to 10th	Night	29% 7% 11% 21% 21% 11%	2200
	11th to 20th	Night	29% 7% 17% 23% 18% 6%	2200
	21st to end	Night	25% 5% 11% 22% 22% 15%	2200
October	1st to 10th	Night	12% 2 7% 16% 24% 40%	3590
	11th to 20th	Night	19% 3 8% 19% 29% 22%	3689
	21st to end	Night	21% 2 5% 22% 34% 16%	4252
November	1st to 10th	Night	28% 3 8% 18% 26% 17%	2502
	11th to 20th	Night	24% 4 8% 21% 24% 19%	2730
	21st to end	Night	35% 4 7% 16% 23% 15%	2730
December	1st to 10th	Night	37% 4 6% 18% 22% 13%	4380
	11th to 20th	Night	38% 3 6% 12% 23% 18%	4947
	21st to end	Night	28% 3 5% 17% 28% 19%	6633

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	13% 12% 40% 25% 8% 1	330
	11th to 20th	Day	17% 13% 31% 29% 8% 2	330
	21st to end	Day	16% 12% 40% 23% 8%	363
February	1st to 10th	Day	18% 16% 33% 23% 8% 1	330
	11th to 20th	Day	21% 13% 37% 24% 5%	330
	21st to end	Day	18% 11% 41% 27% 4	273
March	1st to 10th	Day	17% 17% 36% 25% 6%	330
	11th to 20th	Day	13% 17% 42% 22% 6%	330
	21st to end	Day	10% 14% 40% 28% 7% 1	363
April	1st to 10th	Day	16% 15% 39% 25% 5% 1	330
	11th to 20th	Day	15% 16% 43% 20% 5%	454
	21st to end	Day	15% 14% 40% 25% 6%	656
May	1st to 10th	Day	14% 8% 38% 30% 8%	753
	11th to 20th	Day	13% 10% 31% 32% 12% 2	756
	21st to end	Day	14% 5% 27% 38% 15% 1	789
June	1st to 10th	Day	6% 4 16% 41% 27% 5%	730
	11th to 20th	Day	3 3 18% 47% 24% 4	730
	21st to end	Day	4 1 14% 37% 35% 10%	730
July	1st to 10th	Day	8% 2 7% 35% 36% 13%	721
	11th to 20th	Day	2 1 6% 34% 46% 11%	718
	21st to end	Day	3 1 6% 30% 45% 16%	794
August	1st to 10th	Day	2 3 30% 46% 17%	664
	11th to 20th	Day	7% 2 5% 30% 41% 15%	517
	21st to end	Day	4 1 6% 24% 43% 22%	546
September	1st to 10th	Day	12% 2 6% 22% 35% 23%	330
	11th to 20th	Day	6% 1 6% 26% 41% 21%	330
	21st to end	Day	12% 1 6% 29% 32% 19%	330
October	1st to 10th	Day	3 1 11% 32% 42% 11%	330
	11th to 20th	Day	9% 3 23% 37% 23% 5%	330
	21st to end	Day	17% 12% 22% 27% 19% 4	363
November	1st to 10th	Day	18% 9% 22% 32% 15% 4	330
	11th to 20th	Day	11% 12% 22% 34% 15% 6%	330
	21st to end	Day	18% 13% 36% 20% 11% 3	330
December	1st to 10th	Day	14% 10% 31% 31% 11% 3	330
	11th to 20th	Day	12% 14% 35% 28% 10% 2	330
	21st to end	Day	17% 11% 33% 28% 11% 1	363
January	1st to 10th	Night	14% 8% 36% 34% 8% 1	1050
	11th to 20th	Night	14% 9% 32% 37% 7%	1050
	21st to end	Night	17% 12% 32% 33% 6%	987
February	1st to 10th	Night	15% 10% 40% 30% 5%	840
	11th to 20th	Night	18% 10% 40% 29% 4	824
	21st to end	Night	17% 12% 43% 26% 2	692
March	1st to 10th	Night	16% 12% 39% 31% 2	728
	11th to 20th	Night	14% 12% 37% 32% 5%	630
	21st to end	Night	9% 12% 45% 30% 4	693
April	1st to 10th	Night	10% 8% 48% 30% 4	630
	11th to 20th	Night	15% 12% 48% 23% 2	623
	21st to end	Night	13% 11% 46% 23% 7%	630
May	1st to 10th	Night	12% 8% 44% 32% 4	630
	11th to 20th	Night	11% 6% 34% 38% 10% 1	609
	21st to end	Night	10% 6% 37% 34% 12%	650
June	1st to 10th	Night	4 2 21% 54% 18% 1	600
	11th to 20th	Night	4 1 25% 57% 13%	600
	21st to end	Night	3 2 16% 52% 26% 1	600
July	1st to 10th	Night	5% 2 9% 47% 33% 3	585
	11th to 20th	Night	3 1 8% 53% 34% 2	582
	21st to end	Night	4 8% 47% 39% 2	657
August	1st to 10th	Night	2 4 50% 42% 2	597
	11th to 20th	Night	6% 5% 46% 39% 3	594
	21st to end	Night	5% 1 4 44% 42% 4	660
September	1st to 10th	Night	9% 1 8% 41% 38% 3	600
	11th to 20th	Night	10% 1 7% 45% 36% 2	600
	21st to end	Night	9% 1 11% 43% 34% 3	600
October	1st to 10th	Night	6% 2 12% 50% 30% 2	600
	11th to 20th	Night	10% 3 23% 45% 18% 1	668
	21st to end	Night	13% 7% 28% 33% 18% 1	880
November	1st to 10th	Night	11% 6% 23% 44% 16% 1	862
	11th to 20th	Night	8% 7% 28% 42% 15% 1	1050
	21st to end	Night	16% 9% 30% 33% 12% 1	1050
December	1st to 10th	Night	14% 7% 29% 41% 8%	1050
	11th to 20th	Night	17% 8% 30% 38% 7%	1050
	21st to end	Night	14% 8% 34% 36% 8%	1155

Page generated at 2026-05-14 00:58:59. Version ac07

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