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 2025-01-22.

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	8% 5% 2 5% 4 4 4 3 5% 7% 53%	105244
	Night	9% 5% 2 5% 3 3 3 3 5% 7% 56%	104244
Transparency	Day	30% 5% 10% 21% 22% 12%	109088
	Night	28% 4 10% 21% 23% 14%	108527
Seeing	Day	10% 7% 22% 30% 23% 7%	15155
	Night	11% 6% 28% 39% 16% 1	25207

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	10% 9% 2 5% 3 2 2 2 3 5% 58%	2937
	11th to 20th	Day	10% 6% 2 3 2 2 3 3 6% 5% 58%	2940
	21st to end	Day	13% 6% 2 5% 3 3 3 3 4 6% 51%	3414
February	1st to 10th	Day	12% 8% 2 6% 2 4 3 2 3 6% 52%	1900
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 7% 50%	1849
	21st to end	Day	14% 6% 2 4 2 3 4 2 4 4 55%	1570
March	1st to 10th	Day	11% 8% 3 6% 4 4 3 3 5% 6% 47%	3302
	11th to 20th	Day	15% 7% 3 8% 5% 4 4 3 5% 7% 38%	3480
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 48%	3814
April	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 5% 7% 50%	2090
	11th to 20th	Day	12% 7% 3 6% 4 4 3 3 5% 5% 48%	2183
	21st to end	Day	10% 6% 2 5% 4 4 3 4 4 5% 52%	2384
May	1st to 10th	Day	14% 8% 3 7% 4 4 4 4 5% 7% 41%	4082
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 5% 6% 46%	4082
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 4 7% 9% 41%	4477
June	1st to 10th	Day	4 4 1 5% 4 4 4 4 6% 9% 55%	2470
	11th to 20th	Day	4 3 2 4 3 4 5% 4 7% 9% 56%	2470
	21st to end	Day	6% 4 1 4 5% 4 6% 5% 6% 9% 50%	2470
July	1st to 10th	Day	3 4 2 5% 5% 5% 6% 5% 6% 8% 52%	4120
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 9% 62%	4120
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	4465
August	1st to 10th	Day	1 1 1 2 3 3 3 3 4 8% 70%	3993
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 8% 64%	3726
	21st to end	Day	3 3 1 5% 4 4 5% 4 7% 9% 54%	4119
September	1st to 10th	Day	9% 5% 2 5% 4 5% 4 4 5% 6% 50%	2090
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 3 4 6% 49%	2090
	21st to end	Day	8% 4 2 4 4 4 4 4 6% 7% 54%	2090
October	1st to 10th	Day	4 3 1 4 3 4 4 3 5% 7% 61%	2090
	11th to 20th	Day	9% 4 2 4 3 3 3 3 3 5% 61%	2005
	21st to end	Day	10% 6% 2 4 2 3 3 2 2 4 62%	2090
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 4 6% 53%	1913
	11th to 20th	Day	7% 4 2 4 3 3 4 3 5% 7% 60%	1800
	21st to end	Day	12% 6% 3 5% 3 3 3 3 4 5% 53%	1800
December	1st to 10th	Day	17% 7% 3 7% 4 3 3 2 3 5% 47%	2940
	11th to 20th	Day	8% 7% 2 7% 4 5% 5% 3 5% 6% 47%	3391
	21st to end	Day	10% 5% 2 4 3 2 2 2 3 4 63%	4488
January	1st to 10th	Night	12% 7% 2 4 2 2 2 2 3 5% 59%	4238
	11th to 20th	Night	10% 3 1 3 2 2 2 2 3 4 68%	4250
	21st to end	Night	14% 6% 2 6% 4 3 4 3 4 6% 48%	4290
February	1st to 10th	Night	11% 7% 2 6% 3 3 3 2 4 5% 54%	2280
	11th to 20th	Night	14% 6% 2 5% 3 3 2 3 3 6% 53%	2218
	21st to end	Night	13% 6% 2 4 3 3 3 3 5% 5% 54%	1884
March	1st to 10th	Night	8% 6% 2 5% 4 4 4 3 6% 7% 51%	3555
	11th to 20th	Night	14% 8% 3 6% 4 4 4 4 5% 6% 42%	3480
	21st to end	Night	8% 6% 2 5% 4 4 4 3 5% 7% 52%	3817
April	1st to 10th	Night	7% 5% 1 4 4 3 4 5% 5% 8% 54%	2090
	11th to 20th	Night	10% 7% 2 5% 4 4 3 4 5% 6% 50%	1943
	21st to end	Night	9% 6% 2 4 3 2 4 3 6% 6% 56%	1792
May	1st to 10th	Night	17% 8% 2 5% 3 3 3 3 5% 7% 45%	2823
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	2823
	21st to end	Night	8% 5% 2 5% 3 5% 4 5% 6% 8% 48%	3090
June	1st to 10th	Night	2 2 1 4 3 4 5% 3 6% 9% 60%	1710
	11th to 20th	Night	3 2 1 4 2 3 3 3 6% 9% 63%	1710
	21st to end	Night	3 4 1 4 3 3 4 3 5% 9% 61%	1710
July	1st to 10th	Night	2 3 1 4 3 3 3 3 5% 10% 61%	2850
	11th to 20th	Night	2 1 1 2 2 2 2 3 6% 9% 70%	2850
	21st to end	Night	3 4 2 6% 4 4 4 3 6% 11% 53%	3098
August	1st to 10th	Night	1 1 1 3 2 2 2 3 4 6% 76%	2928
	11th to 20th	Night	3 2 1 3 2 3 3 4 7% 10% 63%	3113
	21st to end	Night	2 3 1 4 2 3 4 3 8% 11% 58%	3544
September	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 6% 7% 54%	2090
	11th to 20th	Night	8% 5% 2 5% 3 4 4 4 6% 9% 50%	2090
	21st to end	Night	7% 4 2 5% 3 4 3 4 5% 7% 58%	2090
October	1st to 10th	Night	4 3 1 3 3 2 3 2 4 7% 66%	2090
	11th to 20th	Night	10% 3 1 3 3 3 3 3 4 5% 61%	2154
	21st to end	Night	11% 5% 2 4 2 2 3 2 3 4 61%	2508
November	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 4 5% 58%	2380
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 62%	2600
	21st to end	Night	12% 6% 2 5% 3 3 3 3 4 6% 52%	2600
December	1st to 10th	Night	17% 5% 2 5% 3 3 3 3 5% 6% 48%	4250
	11th to 20th	Night	12% 8% 3 7% 3 3 3 3 3 5% 50%	4816
	21st to end	Night	10% 6% 2 4 2 2 3 2 3 5% 60%	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	31% 2 5% 16% 28% 18%	2938
	11th to 20th	Day	29% 2 6% 16% 30% 18%	2940
	21st to end	Day	34% 3 7% 13% 29% 13%	3414
February	1st to 10th	Day	35% 3 6% 16% 20% 21%	1900
	11th to 20th	Day	35% 3 6% 19% 20% 17%	1859
	21st to end	Day	34% 3 5% 15% 24% 20%	1570
March	1st to 10th	Day	38% 4 6% 11% 22% 18%	3302
	11th to 20th	Day	45% 6% 10% 12% 16% 10%	3480
	21st to end	Day	32% 5% 12% 15% 22% 14%	3814
April	1st to 10th	Day	34% 5% 10% 17% 19% 16%	2090
	11th to 20th	Day	38% 4 8% 16% 21% 13%	2183
	21st to end	Day	34% 5% 6% 21% 23% 11%	2384
May	1st to 10th	Day	42% 6% 11% 14% 18% 9%	4120
	11th to 20th	Day	38% 6% 7% 17% 24% 8%	4086
	21st to end	Day	36% 9% 14% 23% 12% 6%	4489
June	1st to 10th	Day	24% 7% 14% 29% 21% 5%	2470
	11th to 20th	Day	23% 7% 15% 27% 20% 8%	2470
	21st to end	Day	28% 9% 12% 23% 21% 6%	2470
July	1st to 10th	Day	27% 8% 16% 38% 10% 1	4056
	11th to 20th	Day	18% 6% 14% 35% 23% 4	4051
	21st to end	Day	29% 7% 15% 32% 16% 1	4325
August	1st to 10th	Day	13% 4 12% 35% 33% 2	3979
	11th to 20th	Day	19% 6% 16% 40% 17% 3	3762
	21st to end	Day	23% 10% 17% 27% 20% 3	4118
September	1st to 10th	Day	33% 7% 10% 20% 20% 11%	2090
	11th to 20th	Day	35% 6% 12% 17% 23% 6%	2090
	21st to end	Day	28% 6% 9% 22% 23% 13%	2090
October	1st to 10th	Day	14% 3 7% 13% 27% 36%	3480
	11th to 20th	Day	20% 3 6% 20% 26% 24%	3310
	21st to end	Day	23% 2 4 20% 38% 14%	3428
November	1st to 10th	Day	34% 3 6% 21% 22% 13%	1913
	11th to 20th	Day	25% 4 8% 21% 22% 20%	1800
	21st to end	Day	35% 3 5% 19% 22% 16%	1800
December	1st to 10th	Day	42% 4 5% 16% 22% 11%	2940
	11th to 20th	Day	38% 4 8% 12% 23% 15%	3389
	21st to end	Day	27% 2 5% 18% 28% 21%	4488
January	1st to 10th	Night	31% 2 4 15% 30% 19%	4238
	11th to 20th	Night	23% 2 4 19% 27% 26%	4250
	21st to end	Night	38% 3 6% 12% 26% 15%	4290
February	1st to 10th	Night	33% 3 8% 13% 22% 20%	2280
	11th to 20th	Night	33% 2 7% 17% 22% 18%	2218
	21st to end	Night	34% 4 6% 13% 21% 23%	1884
March	1st to 10th	Night	31% 4 9% 14% 26% 15%	3555
	11th to 20th	Night	43% 6% 9% 15% 16% 11%	3480
	21st to end	Night	32% 5% 12% 18% 22% 12%	3817
April	1st to 10th	Night	29% 5% 10% 20% 18% 19%	2090
	11th to 20th	Night	35% 4 9% 15% 19% 17%	1943
	21st to end	Night	30% 5% 10% 21% 23% 10%	1792
May	1st to 10th	Night	40% 4 11% 20% 16% 10%	2850
	11th to 20th	Night	31% 6% 11% 24% 18% 10%	2824
	21st to end	Night	32% 7% 13% 28% 13% 6%	3099
June	1st to 10th	Night	20% 7% 15% 33% 21% 4	1710
	11th to 20th	Night	19% 5% 16% 30% 22% 8%	1710
	21st to end	Night	22% 6% 13% 29% 25% 5%	1710
July	1st to 10th	Night	19% 8% 21% 41% 11% 1	2805
	11th to 20th	Night	11% 6% 17% 41% 22% 3	2796
	21st to end	Night	25% 5% 20% 35% 14% 1	2999
August	1st to 10th	Night	11% 4 13% 35% 36% 2	2918
	11th to 20th	Night	16% 5% 23% 37% 15% 3	3143
	21st to end	Night	18% 10% 22% 28% 19% 3	3544
September	1st to 10th	Night	29% 7% 12% 22% 20% 11%	2090
	11th to 20th	Night	30% 7% 17% 22% 18% 6%	2090
	21st to end	Night	25% 5% 11% 21% 22% 15%	2090
October	1st to 10th	Night	12% 2 7% 15% 23% 40%	3480
	11th to 20th	Night	19% 3 8% 18% 29% 22%	3575
	21st to end	Night	21% 2 5% 22% 34% 15%	4120
November	1st to 10th	Night	29% 3 9% 18% 24% 16%	2380
	11th to 20th	Night	24% 4 8% 21% 23% 20%	2600
	21st to end	Night	34% 4 8% 16% 23% 16%	2600
December	1st to 10th	Night	37% 4 6% 18% 22% 13%	4250
	11th to 20th	Night	39% 3 6% 12% 24% 18%	4817
	21st to end	Night	28% 3 5% 17% 28% 19%	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	14% 13% 39% 25% 8% 1	300
	11th to 20th	Day	16% 13% 33% 28% 8% 2	300
	21st to end	Day	13% 14% 38% 25% 10%	300
February	1st to 10th	Day	19% 19% 32% 21% 8%	270
	11th to 20th	Day	21% 12% 38% 23% 6%	270
	21st to end	Day	20% 11% 41% 24% 3	225
March	1st to 10th	Day	17% 13% 39% 24% 6%	270
	11th to 20th	Day	13% 16% 44% 21% 7%	270
	21st to end	Day	11% 15% 42% 24% 7% 1	297
April	1st to 10th	Day	15% 16% 38% 26% 4 1	270
	11th to 20th	Day	13% 17% 44% 21% 6%	381
	21st to end	Day	14% 14% 40% 25% 7% 1	564
May	1st to 10th	Day	13% 8% 39% 30% 9%	653
	11th to 20th	Day	14% 9% 30% 33% 12% 2	675
	21st to end	Day	14% 5% 26% 39% 15% 1	734
June	1st to 10th	Day	6% 4 15% 41% 29% 6%	680
	11th to 20th	Day	4 3 18% 47% 25% 4	680
	21st to end	Day	5% 1 14% 38% 34% 10%	680
July	1st to 10th	Day	6% 2 6% 35% 37% 14%	671
	11th to 20th	Day	2 1 5% 32% 48% 12%	668
	21st to end	Day	3 5% 30% 45% 16%	739
August	1st to 10th	Day	2 3 30% 46% 18%	617
	11th to 20th	Day	7% 2 5% 30% 41% 15%	477
	21st to end	Day	4 1 6% 24% 43% 21%	504
September	1st to 10th	Day	13% 1 5% 20% 36% 26%	300
	11th to 20th	Day	7% 1 6% 24% 41% 22%	300
	21st to end	Day	13% 1 6% 29% 32% 19%	300
October	1st to 10th	Day	4 1 11% 31% 42% 10%	300
	11th to 20th	Day	10% 3 23% 38% 22% 4	300
	21st to end	Day	18% 11% 22% 25% 20% 4	330
November	1st to 10th	Day	19% 8% 23% 33% 13% 4	300
	11th to 20th	Day	11% 13% 24% 32% 14% 6%	300
	21st to end	Day	18% 13% 36% 21% 9% 3	300
December	1st to 10th	Day	14% 9% 30% 32% 12% 3	300
	11th to 20th	Day	12% 15% 35% 27% 10% 1	300
	21st to end	Day	18% 12% 32% 26% 11% 1	330
January	1st to 10th	Night	14% 8% 36% 34% 8% 1	1000
	11th to 20th	Night	14% 9% 32% 38% 7%	1000
	21st to end	Night	16% 11% 32% 34% 6%	899
February	1st to 10th	Night	15% 11% 39% 30% 5%	760
	11th to 20th	Night	17% 9% 40% 30% 3	744
	21st to end	Night	18% 12% 44% 25% 1	628
March	1st to 10th	Night	16% 10% 39% 32% 2	659
	11th to 20th	Night	15% 11% 36% 34% 5%	570
	21st to end	Night	9% 13% 45% 29% 4	627
April	1st to 10th	Night	9% 8% 50% 30% 4	570
	11th to 20th	Night	14% 10% 48% 25% 2	563
	21st to end	Night	11% 10% 48% 24% 7%	570
May	1st to 10th	Night	11% 8% 45% 32% 4	570
	11th to 20th	Night	11% 5% 35% 39% 10% 1	561
	21st to end	Night	10% 6% 37% 35% 13%	617
June	1st to 10th	Night	4 1 19% 55% 19% 1	570
	11th to 20th	Night	3 1 24% 58% 14%	570
	21st to end	Night	3 2 16% 51% 27% 1	570
July	1st to 10th	Night	5% 3 9% 48% 33% 3	555
	11th to 20th	Night	3 1 7% 53% 34% 2	552
	21st to end	Night	4 7% 47% 39% 2	624
August	1st to 10th	Night	2 4 50% 42% 2	567
	11th to 20th	Night	6% 5% 45% 40% 4	564
	21st to end	Night	5% 1 4 45% 41% 4	627
September	1st to 10th	Night	9% 1 9% 40% 38% 3	570
	11th to 20th	Night	10% 1 6% 43% 37% 2	570
	21st to end	Night	9% 1 11% 42% 35% 3	570
October	1st to 10th	Night	6% 2 13% 49% 30% 2	570
	11th to 20th	Night	11% 3 23% 44% 17% 1	634
	21st to end	Night	14% 7% 28% 32% 19% 1	836
November	1st to 10th	Night	11% 6% 22% 43% 16% 1	820
	11th to 20th	Night	8% 7% 28% 41% 15% 1	1000
	21st to end	Night	16% 8% 29% 33% 13% 1	1000
December	1st to 10th	Night	14% 7% 28% 42% 9%	1000
	11th to 20th	Night	18% 8% 30% 38% 7%	1000
	21st to end	Night	14% 8% 34% 36% 8%	1100

Page generated at 2025-01-19 02:06:49. Version ac07

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