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 2024-08-15.

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	9% 6% 2 5% 4 4 4 3 5% 7% 51%	95386
	Night	9% 5% 2 5% 3 3 3 3 5% 7% 56%	93392
Transparency	Day	32% 5% 10% 21% 21% 11%	95162
	Night	29% 5% 10% 21% 22% 13%	93222
Seeing	Day	10% 7% 22% 30% 23% 7%	14661
	Night	11% 6% 28% 39% 16% 1	24558

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%	2847
	11th to 20th	Day	10% 6% 2 3 2 3 3 3 5% 5% 58%	2850
	21st to end	Day	13% 6% 2 5% 3 3 3 3 4 6% 51%	3405
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	2 2 1 3 3 3 4 4 4 9% 66%	2399
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 6% 8% 50%	2172
	21st to end	Day	4 3 1 5% 5% 5% 6% 4 7% 9% 50%	2343
September	1st to 10th	Day	9% 5% 2 5% 4 6% 4 4 5% 6% 50%	1980
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 3 4 6% 48%	1980
	21st to end	Day	8% 4 2 4 4 4 4 4 6% 7% 53%	1980
October	1st to 10th	Day	4 3 2 5% 3 4 4 3 5% 8% 59%	1980
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 3 5% 61%	1900
	21st to end	Day	11% 6% 2 4 2 3 3 2 2 4 61%	1980
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 2 4 6% 54%	1816
	11th to 20th	Day	7% 4 2 4 3 3 4 3 6% 7% 58%	1710
	21st to end	Day	12% 7% 3 6% 3 3 3 3 4 5% 52%	1710
December	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 4 5% 53%	1710
	11th to 20th	Day	10% 7% 2 7% 4 4 4 3 5% 6% 49%	2161
	21st to end	Day	12% 7% 2 4 2 2 2 2 3 5% 60%	3135
January	1st to 10th	Night	12% 7% 2 4 2 2 2 2 3 5% 59%	4108
	11th to 20th	Night	11% 3 1 3 2 2 2 2 3 4 68%	4120
	21st to end	Night	14% 6% 2 6% 4 3 4 3 4 6% 48%	4276
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	2 1 2 2 2 3 3 5% 8% 74%	1754
	11th to 20th	Night	5% 4 2 4 4 4 4 4 7% 9% 54%	1828
	21st to end	Night	4 3 1 4 3 3 4 3 8% 11% 56%	2011
September	1st to 10th	Night	6% 5% 1 5% 3 4 4 4 6% 8% 55%	1980
	11th to 20th	Night	8% 5% 2 5% 3 4 4 4 6% 10% 49%	1980
	21st to end	Night	7% 4 2 5% 3 4 3 4 5% 7% 57%	1980
October	1st to 10th	Night	4 3 2 4 3 3 3 3 4 7% 64%	1980
	11th to 20th	Night	10% 3 1 4 3 3 3 3 4 5% 60%	2040
	21st to end	Night	11% 5% 2 4 2 2 3 2 3 4 61%	2376
November	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 4 5% 59%	2258
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 61%	2470
	21st to end	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	2470
December	1st to 10th	Night	10% 5% 2 4 2 3 3 3 4 6% 56%	2470
	11th to 20th	Night	10% 7% 3 7% 4 4 3 3 4 5% 50%	3036
	21st to end	Night	13% 7% 2 4 2 2 3 2 4 5% 57%	4532

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% 15% 29% 19%	2848
	11th to 20th	Day	29% 2 6% 15% 30% 17%	2850
	21st to end	Day	34% 3 7% 13% 29% 13%	3405
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	16% 5% 15% 37% 25% 3	2385
	11th to 20th	Day	30% 9% 13% 30% 14% 4	2172
	21st to end	Day	27% 10% 13% 29% 16% 5%	2343
September	1st to 10th	Day	33% 7% 10% 20% 20% 9%	1980
	11th to 20th	Day	36% 6% 13% 17% 22% 5%	1980
	21st to end	Day	29% 6% 9% 22% 23% 12%	1980
October	1st to 10th	Day	25% 5% 11% 22% 25% 12%	1980
	11th to 20th	Day	28% 4 8% 16% 25% 19%	1900
	21st to end	Day	30% 3 4 15% 26% 21%	1980
November	1st to 10th	Day	33% 3 7% 23% 22% 13%	1816
	11th to 20th	Day	26% 4 8% 21% 23% 18%	1710
	21st to end	Day	36% 3 5% 17% 23% 16%	1710
December	1st to 10th	Day	33% 4 7% 20% 24% 12%	1710
	11th to 20th	Day	36% 4 9% 12% 23% 16%	2159
	21st to end	Day	30% 2 5% 16% 29% 18%	3135
January	1st to 10th	Night	31% 2 4 14% 30% 20%	4108
	11th to 20th	Night	23% 2 4 19% 27% 25%	4120
	21st to end	Night	37% 3 6% 12% 26% 15%	4276
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	10% 4 18% 38% 27% 2	1744
	11th to 20th	Night	25% 7% 18% 30% 15% 4	1828
	21st to end	Night	20% 7% 21% 31% 16% 5%	2011
September	1st to 10th	Night	27% 7% 12% 23% 21% 10%	1980
	11th to 20th	Night	30% 7% 18% 22% 18% 5%	1980
	21st to end	Night	26% 5% 11% 21% 22% 14%	1980
October	1st to 10th	Night	21% 3 13% 27% 23% 14%	1980
	11th to 20th	Night	27% 4 8% 16% 24% 21%	2040
	21st to end	Night	28% 4 5% 16% 25% 22%	2376
November	1st to 10th	Night	28% 3 9% 19% 25% 15%	2258
	11th to 20th	Night	25% 4 8% 21% 23% 18%	2470
	21st to end	Night	35% 4 7% 15% 23% 16%	2470
December	1st to 10th	Night	30% 4 9% 19% 24% 14%	2470
	11th to 20th	Night	37% 3 6% 13% 22% 19%	3037
	21st to end	Night	31% 3 5% 13% 29% 18%	4532

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% 13% 39% 26% 8% 1	270
	11th to 20th	Day	17% 12% 31% 29% 9% 2	270
	21st to end	Day	13% 13% 38% 26% 10%	297
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	8% 2 6% 29% 41% 15%	454
	21st to end	Day	4 1 6% 23% 43% 22%	462
September	1st to 10th	Day	12% 3 17% 39% 29%	270
	11th to 20th	Day	7% 1 5% 21% 42% 24%	270
	21st to end	Day	13% 1 6% 26% 34% 20%	270
October	1st to 10th	Day	4 1 12% 31% 43% 8%	270
	11th to 20th	Day	10% 4 24% 35% 23% 4	270
	21st to end	Day	19% 12% 23% 25% 20% 3	297
November	1st to 10th	Day	17% 7% 23% 34% 14% 4	270
	11th to 20th	Day	13% 12% 21% 34% 14% 6%	270
	21st to end	Day	19% 13% 34% 21% 9% 3	270
December	1st to 10th	Day	14% 8% 29% 33% 13% 4	270
	11th to 20th	Day	13% 15% 35% 27% 10%	270
	21st to end	Day	19% 10% 33% 26% 11% 1	297
January	1st to 10th	Night	14% 8% 35% 35% 8% 1	950
	11th to 20th	Night	15% 8% 31% 38% 7%	950
	21st to end	Night	15% 11% 32% 35% 6%	893
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	7% 5% 44% 40% 4	549
	21st to end	Night	5% 1 4 44% 42% 5%	594
September	1st to 10th	Night	8% 1 9% 40% 39% 3	540
	11th to 20th	Night	11% 1 6% 42% 38% 2	540
	21st to end	Night	9% 1 11% 42% 34% 3	540
October	1st to 10th	Night	6% 2 13% 49% 28% 2	540
	11th to 20th	Night	12% 3 22% 44% 17% 1	600
	21st to end	Night	14% 7% 28% 32% 18% 2	792
November	1st to 10th	Night	11% 6% 22% 43% 16% 1	778
	11th to 20th	Night	7% 7% 27% 42% 15% 1	950
	21st to end	Night	17% 8% 28% 33% 12% 1	950
December	1st to 10th	Night	13% 6% 28% 43% 9%	950
	11th to 20th	Night	17% 7% 31% 38% 6%	950
	21st to end	Night	14% 8% 34% 36% 8%	1045

Page generated at 2024-08-12 08:52:52. Version ac07

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