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 2024-01-27.

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 4 4 3 4 4 5% 7% 53%	74999
	Night	9% 4 2 4 4 2 3 4 5% 7% 56%	75072
Transparency	Day	31% 5% 10% 21% 22% 11%	74724
	Night	28% 5% 11% 21% 22% 13%	74867
Seeing	Day	10% 7% 22% 30% 24% 7%	13830
	Night	11% 6% 27% 39% 16% 1	23918

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	9% 6% 2 4 3 2 2 3 4 5% 59%	1707
	11th to 20th	Day	14% 5% 2 3 4 3 3 4 5% 5% 54%	1710
	21st to end	Day	12% 5% 2 3 3 2 4 4 3 6% 57%	1997
February	1st to 10th	Day	13% 7% 2 5% 3 3 3 3 3 6% 53%	1800
	11th to 20th	Day	13% 7% 2 4 4 2 3 4 4 7% 50%	1749
	21st to end	Day	14% 5% 3 4 3 2 4 3 4 5% 55%	1480
March	1st to 10th	Day	13% 7% 3 5% 4 3 4 4 5% 6% 46%	1894
	11th to 20th	Day	13% 5% 3 6% 4 2 4 4 4 6% 50%	1980
	21st to end	Day	11% 5% 3 4 4 2 4 3 4 6% 55%	2178
April	1st to 10th	Day	10% 5% 3 5% 4 2 4 4 5% 8% 50%	1980
	11th to 20th	Day	12% 6% 3 6% 4 2 3 4 5% 5% 49%	2066
	21st to end	Day	10% 5% 3 5% 4 3 3 4 4 5% 53%	2258
May	1st to 10th	Day	11% 4 2 5% 5% 3 5% 5% 6% 7% 46%	2340
	11th to 20th	Day	11% 7% 4 6% 5% 3 6% 5% 6% 7% 42%	2302
	21st to end	Day	10% 6% 2 5% 5% 3 6% 5% 6% 9% 41%	2531
June	1st to 10th	Day	4 4 1 4 4 3 4 4 6% 9% 56%	2340
	11th to 20th	Day	4 3 2 3 3 3 5% 5% 7% 9% 57%	2340
	21st to end	Day	6% 3 2 3 6% 3 6% 6% 7% 10% 49%	2340
July	1st to 10th	Day	6% 5% 2 6% 6% 4 7% 6% 7% 9% 42%	2340
	11th to 20th	Day	4 2 1 2 3 3 5% 5% 6% 9% 60%	2340
	21st to end	Day	3 2 2 4 5% 3 5% 5% 7% 10% 55%	2546
August	1st to 10th	Day	2 2 1 3 3 2 3 4 4 9% 67%	2273
	11th to 20th	Day	6% 5% 2 6% 5% 4 5% 5% 6% 8% 49%	2123
	21st to end	Day	4 3 2 5% 6% 3 6% 6% 7% 9% 50%	2343
September	1st to 10th	Day	9% 4 2 5% 4 4 5% 5% 5% 6% 50%	1980
	11th to 20th	Day	10% 5% 2 5% 6% 3 6% 4 4 6% 48%	1980
	21st to end	Day	8% 4 2 4 4 3 4 4 6% 8% 53%	1980
October	1st to 10th	Day	4 3 2 4 4 3 5% 4 5% 8% 59%	1980
	11th to 20th	Day	9% 4 3 4 3 2 3 4 3 5% 61%	1900
	21st to end	Day	11% 6% 2 4 3 2 3 2 3 4 61%	1980
November	1st to 10th	Day	11% 6% 2 5% 3 2 4 3 4 6% 54%	1816
	11th to 20th	Day	7% 4 2 4 4 2 4 4 5% 7% 58%	1710
	21st to end	Day	12% 7% 3 5% 4 2 3 3 3 5% 52%	1710
December	1st to 10th	Day	11% 7% 2 4 3 3 4 4 3 5% 54%	1710
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 4 4 6% 49%	2161
	21st to end	Day	12% 6% 2 4 3 2 2 2 3 5% 60%	3135
January	1st to 10th	Night	10% 5% 2 4 3 2 3 3 3 6% 59%	2458
	11th to 20th	Night	12% 4 2 4 3 2 3 2 4 5% 58%	2470
	21st to end	Night	12% 6% 3 5% 4 2 4 3 4 5% 54%	2517
February	1st to 10th	Night	11% 6% 2 4 4 2 3 3 4 6% 55%	2160
	11th to 20th	Night	15% 5% 2 4 3 2 2 4 3 6% 53%	2098
	21st to end	Night	13% 5% 3 4 3 3 3 3 5% 5% 54%	1776
March	1st to 10th	Night	12% 6% 2 4 4 3 4 4 5% 8% 48%	2065
	11th to 20th	Night	11% 5% 3 4 5% 3 5% 4 4 6% 50%	1980
	21st to end	Night	7% 4 2 4 4 2 3 4 5% 8% 57%	2178
April	1st to 10th	Night	7% 4 2 4 4 2 4 6% 5% 8% 54%	1980
	11th to 20th	Night	10% 6% 3 4 4 3 4 4 4 6% 52%	1840
	21st to end	Night	8% 5% 3 4 3 2 3 4 5% 7% 57%	1698
May	1st to 10th	Night	8% 4 2 5% 4 2 4 4 6% 8% 54%	1620
	11th to 20th	Night	9% 5% 2 4 5% 3 4 5% 6% 10% 47%	1593
	21st to end	Night	8% 4 3 4 4 4 4 5% 5% 11% 49%	1745
June	1st to 10th	Night	2 2 1 3 3 3 4 5% 6% 10% 61%	1620
	11th to 20th	Night	3 2 1 3 3 2 4 3 6% 10% 64%	1620
	21st to end	Night	4 3 2 3 4 2 4 4 5% 9% 60%	1620
July	1st to 10th	Night	4 4 2 4 4 4 4 4 6% 12% 52%	1620
	11th to 20th	Night	3 1 1 1 3 2 3 3 5% 10% 68%	1620
	21st to end	Night	2 1 1 3 2 3 3 4 5% 13% 62%	1772
August	1st to 10th	Night	2 1 1 2 1 3 4 4 8% 75%	1661
	11th to 20th	Night	5% 3 2 4 4 3 4 5% 8% 9% 53%	1778
	21st to end	Night	4 3 1 3 3 2 4 4 8% 11% 56%	2011
September	1st to 10th	Night	6% 4 2 4 4 3 4 4 6% 8% 55%	1980
	11th to 20th	Night	8% 5% 2 4 4 3 4 5% 6% 9% 49%	1980
	21st to end	Night	7% 4 2 4 4 3 4 4 5% 8% 57%	1980
October	1st to 10th	Night	4 3 1 3 3 2 3 3 4 8% 64%	1980
	11th to 20th	Night	10% 3 2 3 4 2 3 4 4 5% 60%	2040
	21st to end	Night	11% 5% 2 4 3 1 3 3 3 5% 61%	2376
November	1st to 10th	Night	10% 5% 2 4 3 2 3 3 4 5% 59%	2258
	11th to 20th	Night	6% 4 2 4 4 2 3 4 4 6% 61%	2470
	21st to end	Night	13% 6% 2 4 4 3 4 4 4 6% 51%	2470
December	1st to 10th	Night	10% 5% 2 4 3 2 4 4 4 6% 57%	2470
	11th to 20th	Night	10% 7% 3 6% 5% 3 4 4 4 5% 50%	3036
	21st to end	Night	13% 6% 2 3 3 2 3 3 3 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	29% 3 6% 18% 30% 14%	1708
	11th to 20th	Day	33% 3 5% 17% 25% 16%	1710
	21st to end	Day	31% 3 8% 14% 27% 17%	1997
February	1st to 10th	Day	34% 3 7% 16% 20% 21%	1800
	11th to 20th	Day	36% 3 6% 19% 20% 16%	1759
	21st to end	Day	34% 3 5% 15% 23% 20%	1480
March	1st to 10th	Day	39% 5% 7% 14% 21% 14%	1894
	11th to 20th	Day	36% 5% 8% 14% 19% 18%	1980
	21st to end	Day	32% 4 10% 17% 23% 14%	2178
April	1st to 10th	Day	34% 5% 10% 17% 19% 14%	1980
	11th to 20th	Day	37% 4 8% 16% 21% 13%	2066
	21st to end	Day	34% 5% 6% 21% 23% 11%	2258
May	1st to 10th	Day	36% 6% 10% 16% 21% 9%	2340
	11th to 20th	Day	41% 6% 10% 19% 19% 5%	2306
	21st to end	Day	38% 8% 13% 22% 17% 2	2531
June	1st to 10th	Day	24% 7% 14% 29% 21% 5%	2340
	11th to 20th	Day	23% 7% 16% 28% 20% 7%	2340
	21st to end	Day	29% 9% 12% 23% 21% 6%	2340
July	1st to 10th	Day	34% 9% 15% 26% 14% 1	2276
	11th to 20th	Day	19% 7% 16% 26% 27% 5%	2271
	21st to end	Day	20% 8% 19% 35% 17% 1	2405
August	1st to 10th	Day	16% 5% 15% 37% 24% 3	2259
	11th to 20th	Day	31% 9% 13% 29% 14% 5%	2123
	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	29% 2 5% 16% 27% 21%	2458
	11th to 20th	Night	30% 2 5% 20% 25% 19%	2470
	21st to end	Night	34% 3 5% 14% 24% 21%	2517
February	1st to 10th	Night	32% 3 9% 13% 22% 20%	2160
	11th to 20th	Night	34% 2 7% 17% 22% 18%	2098
	21st to end	Night	34% 4 7% 12% 20% 23%	1776
March	1st to 10th	Night	35% 4 10% 14% 25% 12%	2065
	11th to 20th	Night	36% 5% 6% 16% 19% 18%	1980
	21st to end	Night	27% 5% 13% 19% 22% 15%	2178
April	1st to 10th	Night	29% 6% 10% 20% 18% 17%	1980
	11th to 20th	Night	34% 4 9% 16% 20% 17%	1840
	21st to end	Night	29% 5% 10% 22% 24% 11%	1698
May	1st to 10th	Night	30% 5% 11% 22% 22% 10%	1620
	11th to 20th	Night	32% 7% 13% 26% 17% 5%	1594
	21st to end	Night	31% 8% 14% 30% 15% 2	1746
June	1st to 10th	Night	19% 8% 15% 33% 21% 4	1620
	11th to 20th	Night	18% 5% 17% 31% 22% 8%	1620
	21st to end	Night	22% 6% 14% 28% 25% 5%	1620
July	1st to 10th	Night	25% 10% 19% 30% 13% 2	1575
	11th to 20th	Night	12% 6% 18% 31% 27% 5%	1566
	21st to end	Night	14% 5% 25% 37% 18% 1	1673
August	1st to 10th	Night	10% 5% 18% 38% 27% 3	1651
	11th to 20th	Night	25% 8% 18% 29% 15% 4	1778
	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	14% 13% 40% 25% 8%	282
February	1st to 10th	Day	20% 16% 33% 23% 8%	240
	11th to 20th	Day	24% 11% 37% 23% 5%	240
	21st to end	Day	21% 11% 41% 23% 4	198
March	1st to 10th	Day	18% 13% 40% 23% 6%	240
	11th to 20th	Day	13% 16% 42% 21% 8%	240
	21st to end	Day	12% 15% 41% 24% 8% 1	264
April	1st to 10th	Day	15% 15% 38% 27% 4	240
	11th to 20th	Day	13% 16% 42% 22% 6%	344
	21st to end	Day	14% 13% 41% 25% 7% 1	518
May	1st to 10th	Day	13% 8% 38% 32% 9%	603
	11th to 20th	Day	13% 9% 30% 33% 13% 2	625
	21st to end	Day	14% 5% 24% 39% 16% 1	679
June	1st to 10th	Day	6% 3 15% 40% 30% 6%	630
	11th to 20th	Day	3 2 17% 47% 26% 5%	630
	21st to end	Day	5% 14% 37% 34% 10%	630
July	1st to 10th	Day	6% 1 6% 35% 38% 14%	621
	11th to 20th	Day	2 1 4 32% 50% 12%	618
	21st to end	Day	2 5% 28% 48% 17%	684
August	1st to 10th	Day	2 1 3 30% 46% 18%	571
	11th to 20th	Day	8% 2 6% 29% 41% 15%	437
	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	16% 11% 33% 34% 6%	877
February	1st to 10th	Night	15% 10% 39% 30% 6%	720
	11th to 20th	Night	18% 8% 39% 31% 4	704
	21st to end	Night	18% 12% 44% 25% 2	592
March	1st to 10th	Night	16% 10% 39% 32% 3	625
	11th to 20th	Night	15% 10% 36% 34% 5%	540
	21st to end	Night	8% 13% 45% 30% 4	594
April	1st to 10th	Night	9% 7% 50% 31% 4	540
	11th to 20th	Night	14% 9% 48% 27% 3	533
	21st to end	Night	10% 10% 49% 24% 7%	540
May	1st to 10th	Night	10% 8% 44% 34% 4	540
	11th to 20th	Night	10% 5% 34% 39% 11% 1	531
	21st to end	Night	9% 5% 36% 36% 14%	584
June	1st to 10th	Night	4 1 18% 56% 19% 1	540
	11th to 20th	Night	3 1 23% 59% 14%	540
	21st to end	Night	3 2 15% 51% 27% 1	540
July	1st to 10th	Night	5% 3 9% 47% 34% 3	525
	11th to 20th	Night	3 6% 53% 35% 2	522
	21st to end	Night	4 6% 48% 40% 2	591
August	1st to 10th	Night	2 4 51% 42% 2	537
	11th to 20th	Night	7% 5% 44% 40% 4	534
	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

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