Dark Skies Unlimited Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Skies Unlimited Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	4 3 2 4 4 3 4 5% 6% 8% 59%	74692
	Night	3 3 1 3 3 2 4 4 5% 8% 65%	75070
Transparency	Day	22% 5% 9% 18% 25% 20%	74421
	Night	18% 4 10% 18% 27% 22%	74863
Seeing	Day	11% 16% 26% 26% 16% 5%	13622
	Night	8% 16% 26% 29% 18% 4	23925

By Month

Percentage Cloud Cover SW (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	1 3 1 4 4 2 3 5% 4 7% 67%	1708
	11th to 20th	Day	2 1 1 2 3 1 3 4 5% 6% 72%	1647
	21st to end	Day	3 4 2 4 4 2 4 4 4 7% 64%	1782
February	1st to 10th	Day	1 3 2 3 3 3 3 4 4 6% 70%	1789
	11th to 20th	Day	2 3 1 3 3 2 4 4 4 7% 66%	1749
	21st to end	Day	1 2 1 3 3 1 3 3 3 7% 73%	1480
March	1st to 10th	Day	1 3 2 3 4 3 4 5% 6% 7% 62%	1875
	11th to 20th	Day	2 3 2 4 3 3 5% 4 4 6% 64%	1980
	21st to end	Day	1 2 1 2 3 2 3 4 5% 9% 68%	2268
April	1st to 10th	Day	1 2 1 3 2 1 3 3 4 6% 73%	2160
	11th to 20th	Day	3 3 1 2 2 2 3 3 4 7% 70%	2131
	21st to end	Day	2 2 1 2 2 1 2 3 3 6% 76%	2160
May	1st to 10th	Day	3 2 1 2 3 2 3 3 4 8% 69%	2160
	11th to 20th	Day	5% 3 1 3 4 3 6% 5% 5% 6% 58%	2125
	21st to end	Day	2 1 1 2 3 2 3 4 5% 7% 69%	2474
June	1st to 10th	Day	1 2 1 3 4 3 5% 6% 7% 11% 57%	2520
	11th to 20th	Day	2 1 1 2 3 2 3 5% 6% 8% 66%	2520
	21st to end	Day	6% 4 2 4 5% 4 7% 8% 8% 8% 43%	2489
July	1st to 10th	Day	8% 5% 3 6% 8% 6% 7% 8% 7% 9% 34%	2340
	11th to 20th	Day	6% 4 2 6% 7% 5% 8% 8% 9% 10% 34%	2340
	21st to end	Day	7% 4 2 6% 7% 5% 8% 9% 11% 11% 29%	2511
August	1st to 10th	Day	7% 4 2 7% 8% 5% 10% 9% 8% 9% 30%	2149
	11th to 20th	Day	8% 4 3 6% 9% 5% 8% 7% 10% 10% 31%	2123
	21st to end	Day	7% 5% 2 6% 7% 6% 8% 7% 8% 9% 34%	2376
September	1st to 10th	Day	10% 5% 3 5% 5% 3 5% 6% 8% 8% 42%	2160
	11th to 20th	Day	8% 4 2 6% 6% 3 6% 5% 5% 7% 48%	2149
	21st to end	Day	5% 4 1 4 3 2 4 4 5% 7% 62%	1980
October	1st to 10th	Day	6% 3 1 4 3 3 3 4 4 6% 61%	1832
	11th to 20th	Day	4 3 1 3 2 1 3 3 3 4 72%	1800
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	1980
November	1st to 10th	Day	2 2 1 1 3 2 2 3 3 4 76%	1859
	11th to 20th	Day	2 1 3 2 2 3 3 5% 7% 71%	1722
	21st to end	Day	3 3 1 2 3 2 3 4 4 8% 66%	1710
December	1st to 10th	Day	3 3 1 3 3 2 4 5% 5% 7% 66%	1710
	11th to 20th	Day	1 4 2 4 4 2 3 4 7% 8% 60%	1998
	21st to end	Day	1 2 1 2 2 1 3 4 5% 7% 71%	2936
January	1st to 10th	Night	1 3 1 3 3 2 3 4 4 5% 70%	2457
	11th to 20th	Night	2 3 2 4 3 2 4 3 4 6% 68%	2391
	21st to end	Night	3 3 2 3 2 2 3 3 5% 7% 68%	2574
February	1st to 10th	Night	2 1 3 2 2 4 4 5% 7% 69%	2167
	11th to 20th	Night	2 2 1 2 2 2 4 4 5% 6% 70%	2098
	21st to end	Night	1 2 1 2 1 2 2 4 5% 7% 74%	1776
March	1st to 10th	Night	1 3 1 2 3 2 3 4 5% 8% 67%	2082
	11th to 20th	Night	2 3 2 3 3 2 3 4 4 7% 67%	1980
	21st to end	Night	1 2 1 2 2 1 3 2 4 7% 75%	2086
April	1st to 10th	Night	2 2 1 1 2 2 3 5% 5% 8% 70%	1800
	11th to 20th	Night	3 1 1 2 2 2 3 3 5% 7% 72%	1777
	21st to end	Night	1 2 1 2 3 1 3 3 4 5% 76%	1800
May	1st to 10th	Night	2 2 1 3 2 1 3 3 4 7% 73%	1800
	11th to 20th	Night	4 3 1 3 2 1 4 3 4 7% 68%	1770
	21st to end	Night	3 1 1 2 2 2 3 4 6% 5% 71%	1793
June	1st to 10th	Night	2 3 2 3 4 3 4 5% 7% 9% 57%	1440
	11th to 20th	Night	2 2 1 2 2 2 4 3 5% 7% 72%	1440
	21st to end	Night	5% 3 2 4 4 5% 8% 7% 6% 9% 48%	1463
July	1st to 10th	Night	7% 4 2 4 4 3 5% 4 7% 11% 48%	1620
	11th to 20th	Night	4 3 1 3 4 5% 6% 7% 8% 12% 46%	1620
	21st to end	Night	4 3 1 4 4 3 5% 6% 9% 17% 43%	1804
August	1st to 10th	Night	5% 3 2 4 5% 4 5% 7% 10% 13% 42%	1790
	11th to 20th	Night	6% 4 2 5% 5% 3 4 5% 8% 13% 45%	1778
	21st to end	Night	4 3 2 4 4 3 5% 6% 7% 13% 48%	1980
September	1st to 10th	Night	6% 4 2 4 5% 3 4 4 7% 9% 51%	1800
	11th to 20th	Night	8% 4 1 3 3 2 4 3 6% 8% 58%	1809
	21st to end	Night	6% 2 1 3 3 2 3 3 4 6% 67%	1980
October	1st to 10th	Night	5% 3 1 3 4 2 4 3 5% 7% 63%	2124
	11th to 20th	Night	3 3 1 2 2 2 3 2 3 4 74%	2160
	21st to end	Night	3 2 1 2 2 2 2 2 4 5% 74%	2376
November	1st to 10th	Night	3 3 1 2 2 1 2 3 4 5% 75%	2220
	11th to 20th	Night	1 1 2 3 1 3 4 4 7% 73%	2436
	21st to end	Night	3 4 2 3 3 2 3 3 4 6% 67%	2470
December	1st to 10th	Night	3 3 1 2 2 2 3 3 4 9% 67%	2470
	11th to 20th	Night	1 2 1 4 3 3 4 4 5% 8% 64%	3195
	21st to end	Night	2 3 1 3 3 1 2 4 4 6% 72%	4744

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	18% 4 7% 18% 31% 22%	1709
	11th to 20th	Day	13% 4 7% 18% 35% 23%	1647
	21st to end	Day	21% 3 6% 15% 31% 24%	1782
February	1st to 10th	Day	17% 3 7% 17% 29% 28%	1789
	11th to 20th	Day	19% 4 7% 18% 26% 26%	1759
	21st to end	Day	14% 2 5% 11% 30% 37%	1480
March	1st to 10th	Day	20% 4 10% 16% 28% 22%	1875
	11th to 20th	Day	21% 4 7% 13% 30% 24%	1980
	21st to end	Day	15% 3 6% 18% 34% 23%	2268
April	1st to 10th	Day	14% 3 5% 16% 36% 26%	2160
	11th to 20th	Day	16% 3 4 13% 31% 33%	2134
	21st to end	Day	13% 3 5% 18% 35% 27%	2160
May	1st to 10th	Day	15% 2 6% 15% 28% 35%	2160
	11th to 20th	Day	23% 6% 9% 15% 24% 24%	2128
	21st to end	Day	14% 4 7% 19% 25% 31%	2474
June	1st to 10th	Day	18% 8% 12% 19% 25% 19%	2520
	11th to 20th	Day	14% 4 6% 16% 23% 36%	2520
	21st to end	Day	30% 9% 11% 16% 18% 16%	2489
July	1st to 10th	Day	41% 10% 14% 16% 14% 6%	2276
	11th to 20th	Day	35% 11% 17% 20% 14% 3	2271
	21st to end	Day	37% 12% 24% 19% 8%	2371
August	1st to 10th	Day	41% 11% 17% 22% 7% 2	2136
	11th to 20th	Day	42% 9% 17% 20% 10% 3	2123
	21st to end	Day	39% 11% 16% 20% 13% 2	2376
September	1st to 10th	Day	35% 7% 14% 18% 18% 9%	2160
	11th to 20th	Day	33% 7% 8% 18% 26% 8%	2149
	21st to end	Day	22% 3 8% 24% 31% 12%	1980
October	1st to 10th	Day	23% 5% 10% 18% 24% 21%	1832
	11th to 20th	Day	18% 3 5% 14% 29% 32%	1800
	21st to end	Day	12% 2 5% 21% 35% 24%	1980
November	1st to 10th	Day	13% 2 5% 22% 33% 25%	1859
	11th to 20th	Day	14% 2 7% 22% 30% 25%	1722
	21st to end	Day	18% 2 9% 17% 27% 26%	1710
December	1st to 10th	Day	18% 3 7% 24% 31% 17%	1710
	11th to 20th	Day	20% 4 11% 16% 29% 20%	1996
	21st to end	Day	13% 3 8% 16% 28% 31%	2936
January	1st to 10th	Night	17% 3 8% 16% 30% 25%	2457
	11th to 20th	Night	18% 6% 6% 13% 33% 24%	2391
	21st to end	Night	17% 2 7% 17% 31% 25%	2574
February	1st to 10th	Night	15% 3 8% 18% 27% 30%	2167
	11th to 20th	Night	16% 4 8% 17% 29% 27%	2098
	21st to end	Night	12% 3 7% 15% 28% 35%	1776
March	1st to 10th	Night	17% 3 8% 17% 27% 27%	2082
	11th to 20th	Night	18% 3 8% 14% 36% 21%	1980
	21st to end	Night	12% 3 7% 17% 38% 23%	2086
April	1st to 10th	Night	12% 3 6% 18% 36% 25%	1800
	11th to 20th	Night	14% 2 8% 16% 28% 31%	1777
	21st to end	Night	13% 2 7% 19% 34% 25%	1800
May	1st to 10th	Night	14% 3 5% 13% 30% 35%	1800
	11th to 20th	Night	18% 4 9% 20% 24% 25%	1771
	21st to end	Night	14% 3 8% 17% 26% 32%	1794
June	1st to 10th	Night	22% 6% 12% 18% 21% 21%	1440
	11th to 20th	Night	14% 2 5% 18% 22% 39%	1440
	21st to end	Night	29% 7% 15% 16% 14% 18%	1463
July	1st to 10th	Night	28% 8% 19% 22% 19% 5%	1575
	11th to 20th	Night	25% 10% 17% 28% 16% 5%	1566
	21st to end	Night	24% 11% 28% 24% 11% 1	1704
August	1st to 10th	Night	27% 9% 24% 27% 9% 3	1779
	11th to 20th	Night	29% 8% 23% 24% 13% 3	1778
	21st to end	Night	27% 8% 21% 25% 17% 2	1980
September	1st to 10th	Night	29% 6% 16% 18% 19% 12%	1800
	11th to 20th	Night	25% 5% 13% 19% 26% 11%	1809
	21st to end	Night	20% 3 11% 22% 32% 12%	1980
October	1st to 10th	Night	22% 5% 10% 18% 27% 19%	2124
	11th to 20th	Night	16% 3 5% 14% 29% 33%	2160
	21st to end	Night	15% 3 5% 18% 31% 29%	2376
November	1st to 10th	Night	14% 2 5% 20% 31% 28%	2220
	11th to 20th	Night	12% 4 5% 20% 33% 27%	2436
	21st to end	Night	20% 3 7% 17% 30% 23%	2470
December	1st to 10th	Night	17% 3 10% 19% 29% 22%	2470
	11th to 20th	Night	20% 3 10% 19% 27% 22%	3196
	21st to end	Night	15% 3 6% 18% 29% 29%	4744

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% 28% 34% 18% 4	270
	11th to 20th	Day	20% 21% 32% 18% 7% 2	249
	21st to end	Day	13% 27% 29% 21% 10%	264
February	1st to 10th	Day	9% 27% 33% 24% 6% 1	240
	11th to 20th	Day	23% 28% 30% 13% 5% 1	240
	21st to end	Day	30% 31% 22% 13% 4 1	198
March	1st to 10th	Day	19% 30% 35% 11% 4	315
	11th to 20th	Day	21% 25% 34% 13% 7% 1	420
	21st to end	Day	22% 33% 33% 9% 3 1	462
April	1st to 10th	Day	14% 32% 37% 13% 4 1	420
	11th to 20th	Day	18% 31% 32% 14% 3 1	412
	21st to end	Day	16% 30% 37% 13% 3	420
May	1st to 10th	Day	9% 27% 40% 19% 4	423
	11th to 20th	Day	9% 22% 37% 22% 8% 2	448
	21st to end	Day	7% 20% 41% 21% 10% 1	583
June	1st to 10th	Day	4 8% 30% 37% 19% 3	630
	11th to 20th	Day	3 8% 30% 38% 17% 4	630
	21st to end	Day	8% 5% 19% 35% 28% 6%	599
July	1st to 10th	Day	7% 2 10% 37% 31% 12%	446
	11th to 20th	Day	4 3 5% 37% 36% 14%	444
	21st to end	Day	3 2 6% 31% 43% 15%	487
August	1st to 10th	Day	5% 3 5% 31% 39% 17%	447
	11th to 20th	Day	11% 3 10% 35% 31% 11%	437
	21st to end	Day	8% 4 8% 35% 33% 13%	495
September	1st to 10th	Day	11% 5% 14% 38% 26% 7%	450
	11th to 20th	Day	5% 3 19% 45% 23% 6%	439
	21st to end	Day	5% 6% 30% 32% 24% 4	270
October	1st to 10th	Day	13% 11% 26% 33% 13% 4	270
	11th to 20th	Day	11% 12% 31% 26% 16% 4	270
	21st to end	Day	12% 18% 28% 24% 12% 6%	297
November	1st to 10th	Day	13% 24% 31% 19% 11% 2	270
	11th to 20th	Day	10% 17% 31% 21% 17% 4	270
	21st to end	Day	17% 29% 25% 16% 11% 1	270
December	1st to 10th	Day	19% 21% 30% 22% 7% 1	270
	11th to 20th	Day	18% 23% 33% 21% 4 1	270
	21st to end	Day	21% 29% 32% 14% 5%	297
January	1st to 10th	Night	9% 21% 32% 24% 11% 3	760
	11th to 20th	Night	8% 26% 30% 24% 10% 3	736
	21st to end	Night	11% 24% 28% 25% 10% 2	792
February	1st to 10th	Night	8% 22% 36% 22% 9% 3	720
	11th to 20th	Night	9% 25% 40% 20% 5% 1	704
	21st to end	Night	14% 32% 35% 15% 3 1	592
March	1st to 10th	Night	12% 29% 28% 24% 6% 1	720
	11th to 20th	Night	12% 19% 30% 27% 11% 1	720
	21st to end	Night	11% 25% 39% 19% 7%	700
April	1st to 10th	Night	9% 24% 41% 20% 5%	540
	11th to 20th	Night	10% 25% 36% 24% 5%	533
	21st to end	Night	8% 26% 34% 25% 6%	540
May	1st to 10th	Night	5% 20% 43% 28% 4 1	540
	11th to 20th	Night	7% 13% 31% 34% 13% 2	531
	21st to end	Night	5% 13% 38% 31% 12% 1	584
June	1st to 10th	Night	4 5% 28% 43% 20% 1	540
	11th to 20th	Night	3 6% 30% 41% 19% 1	540
	21st to end	Night	6% 3 18% 41% 30% 3	540
July	1st to 10th	Night	7% 2 6% 41% 41% 4	525
	11th to 20th	Night	5% 2 5% 33% 48% 9%	522
	21st to end	Night	5% 2 3 26% 52% 12%	591
August	1st to 10th	Night	7% 1 2 28% 48% 14%	537
	11th to 20th	Night	5% 1 6% 31% 43% 14%	534
	21st to end	Night	5% 2 4 31% 48% 10%	594
September	1st to 10th	Night	6% 4 9% 34% 38% 8%	540
	11th to 20th	Night	9% 1 11% 41% 33% 4	540
	21st to end	Night	7% 4 18% 40% 27% 4	540
October	1st to 10th	Night	6% 10% 24% 36% 20% 4	684
	11th to 20th	Night	9% 10% 27% 35% 15% 3	720
	21st to end	Night	8% 15% 26% 27% 16% 7%	792
November	1st to 10th	Night	6% 15% 30% 29% 14% 6%	740
	11th to 20th	Night	4 15% 29% 35% 14% 3	916
	21st to end	Night	10% 22% 28% 24% 13% 3	950
December	1st to 10th	Night	11% 21% 28% 27% 11% 2	950
	11th to 20th	Night	10% 27% 31% 24% 9% 1	950
	21st to end	Night	14% 25% 29% 23% 6% 2	968

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