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 2024-11-25.

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	3 3 1 4 3 4 4 4 6% 7% 60%	104560
	Night	3 3 1 3 2 3 3 3 5% 8% 66%	103617
Transparency	Day	21% 5% 10% 17% 25% 22%	104326
	Night	17% 4 10% 18% 27% 23%	103441
Seeing	Day	12% 16% 26% 25% 16% 5%	14802
	Night	8% 16% 26% 29% 17% 3	25010

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 3 2 2 3 3 5% 7% 70%	2858
	11th to 20th	Day	1 1 2 2 2 2 3 4 6% 76%	2860
	21st to end	Day	2 4 1 4 3 3 4 4 7% 11% 56%	3146
February	1st to 10th	Day	1 3 2 3 2 3 3 3 4 6% 69%	1888
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 7% 67%	1849
	21st to end	Day	1 2 1 3 2 2 3 3 4 6% 73%	1570
March	1st to 10th	Day	1 6% 2 5% 3 4 4 3 4 6% 62%	3275
	11th to 20th	Day	4 9% 2 4 3 4 4 3 4 7% 57%	3490
	21st to end	Day	1 2 1 3 3 3 4 3 7% 10% 63%	3992
April	1st to 10th	Day	1 2 1 3 2 2 4 3 4 6% 72%	2280
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 7% 70%	2251
	21st to end	Day	2 2 1 2 2 2 2 2 3 6% 76%	2280
May	1st to 10th	Day	1 1 2 2 2 2 2 4 6% 79%	3800
	11th to 20th	Day	3 3 1 3 3 4 6% 4 6% 7% 61%	3765
	21st to end	Day	1 1 1 2 2 2 2 2 4 5% 79%	4401
June	1st to 10th	Day	1 2 1 4 4 4 5% 5% 7% 10% 57%	2660
	11th to 20th	Day	2 2 1 3 3 3 4 4 7% 7% 66%	2660
	21st to end	Day	7% 4 2 5% 5% 6% 6% 7% 8% 8% 43%	2627
July	1st to 10th	Day	6% 4 2 7% 6% 7% 8% 6% 9% 10% 35%	4110
	11th to 20th	Day	4 3 2 6% 5% 6% 7% 6% 8% 8% 45%	4110
	21st to end	Day	8% 5% 2 6% 5% 7% 8% 7% 10% 10% 31%	4396
August	1st to 10th	Day	4 3 2 5% 5% 6% 7% 7% 9% 11% 42%	3784
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 5% 7% 8% 45%	3726
	21st to end	Day	6% 4 2 6% 5% 6% 7% 7% 9% 10% 38%	4180
September	1st to 10th	Day	10% 6% 2 6% 4 5% 4 5% 8% 8% 43%	2280
	11th to 20th	Day	8% 4 2 7% 4 4 6% 4 5% 7% 49%	2268
	21st to end	Day	5% 4 1 4 2 3 4 3 5% 7% 63%	2090
October	1st to 10th	Day	4 4 1 3 2 3 3 3 4 5% 68%	3215
	11th to 20th	Day	4 4 1 4 2 2 3 3 4 6% 67%	3150
	21st to end	Day	1 1 1 2 1 1 1 1 2 3 85%	3438
November	1st to 10th	Day	2 2 1 2 2 2 2 2 4 5% 76%	1959
	11th to 20th	Day	1 2 1 3 2 2 3 2 5% 7% 72%	1812
	21st to end	Day	3 3 1 2 3 3 3 3 4 7% 67%	1746
December	1st to 10th	Day	3 3 1 3 2 2 4 3 5% 7% 66%	1710
	11th to 20th	Day	1 4 2 5% 3 3 4 3 7% 8% 60%	1998
	21st to end	Day	1 2 1 3 2 2 3 3 5% 7% 71%	2936
January	1st to 10th	Night	1 2 1 3 3 3 4 3 4 6% 71%	4097
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 5% 74%	4110
	21st to end	Night	2 3 1 4 3 3 4 4 7% 10% 59%	4521
February	1st to 10th	Night	2 1 3 2 3 3 3 5% 7% 69%	2288
	11th to 20th	Night	2 2 1 2 2 3 3 3 5% 6% 70%	2218
	21st to end	Night	1 2 1 1 2 2 2 3 5% 7% 73%	1884
March	1st to 10th	Night	3 2 3 2 2 3 3 4 7% 70%	3591
	11th to 20th	Night	5% 13% 3 4 2 3 2 3 3 5% 58%	3490
	21st to end	Night	1 1 2 2 2 3 3 6% 9% 71%	3653
April	1st to 10th	Night	1 2 2 2 2 3 4 5% 8% 70%	1900
	11th to 20th	Night	2 2 1 2 2 2 3 2 5% 7% 72%	1877
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 76%	1900
May	1st to 10th	Night	1 2 1 2 2 1 2 2 4 5% 79%	3170
	11th to 20th	Night	3 3 1 2 1 2 3 2 4 6% 72%	3140
	21st to end	Night	2 1 1 1 2 2 2 2 3 4 83%	3177
June	1st to 10th	Night	2 3 2 4 4 4 4 5% 7% 9% 57%	1520
	11th to 20th	Night	2 2 1 2 2 2 3 2 5% 6% 72%	1520
	21st to end	Night	5% 3 2 4 4 7% 7% 5% 7% 8% 48%	1545
July	1st to 10th	Night	7% 5% 2 4 3 4 5% 5% 9% 12% 45%	2860
	11th to 20th	Night	3 2 1 3 4 5% 6% 6% 8% 10% 53%	2860
	21st to end	Night	6% 6% 1 5% 4 4 5% 5% 10% 15% 38%	3164
August	1st to 10th	Night	3 2 1 4 3 5% 5% 4 10% 14% 49%	3155
	11th to 20th	Night	3 3 1 4 4 3 3 4 7% 10% 59%	3113
	21st to end	Night	4 4 1 4 3 4 6% 5% 8% 15% 46%	3487
September	1st to 10th	Night	6% 5% 2 4 4 4 4 3 7% 9% 51%	1900
	11th to 20th	Night	7% 4 1 4 3 3 3 2 6% 7% 59%	1910
	21st to end	Night	6% 2 1 3 2 2 2 3 4 6% 68%	2090
October	1st to 10th	Night	3 2 1 3 2 1 2 2 3 5% 77%	3736
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 5% 70%	3778
	21st to end	Night	2 1 1 1 1 1 1 1 2 5% 83%	4120
November	1st to 10th	Night	3 3 1 3 2 1 2 2 4 5% 74%	2340
	11th to 20th	Night	1 1 1 2 3 2 3 3 4 7% 73%	2565
	21st to end	Night	2 4 1 4 3 3 4 2 4 6% 68%	2529
December	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 9% 67%	2470
	11th to 20th	Night	1 3 1 4 3 4 4 3 6% 8% 64%	3195
	21st to end	Night	2 3 1 4 2 2 2 3 5% 5% 71%	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	14% 3 9% 15% 32% 26%	2859
	11th to 20th	Day	10% 3 6% 20% 37% 23%	2860
	21st to end	Day	21% 5% 11% 17% 30% 15%	3146
February	1st to 10th	Day	17% 3 7% 17% 28% 28%	1888
	11th to 20th	Day	18% 4 7% 18% 26% 27%	1859
	21st to end	Day	14% 2 6% 11% 29% 36%	1570
March	1st to 10th	Day	20% 5% 8% 10% 23% 35%	3275
	11th to 20th	Day	21% 10% 8% 12% 32% 16%	3490
	21st to end	Day	15% 5% 12% 18% 30% 20%	3992
April	1st to 10th	Day	14% 3 5% 16% 35% 27%	2280
	11th to 20th	Day	15% 3 4 13% 31% 35%	2254
	21st to end	Day	12% 3 5% 17% 35% 28%	2280
May	1st to 10th	Day	9% 2 4 11% 24% 49%	3800
	11th to 20th	Day	16% 8% 9% 13% 25% 29%	3768
	21st to end	Day	8% 3 4 11% 18% 56%	4401
June	1st to 10th	Day	18% 8% 11% 18% 25% 19%	2660
	11th to 20th	Day	14% 4 6% 16% 23% 37%	2660
	21st to end	Day	31% 9% 12% 17% 17% 15%	2627
July	1st to 10th	Day	36% 11% 18% 16% 12% 7%	4046
	11th to 20th	Day	31% 8% 14% 21% 22% 4	4041
	21st to end	Day	38% 10% 18% 23% 10% 1	4257
August	1st to 10th	Day	29% 8% 15% 31% 15% 3	3772
	11th to 20th	Day	32% 7% 13% 21% 24% 3	3762
	21st to end	Day	35% 10% 18% 26% 10% 1	4179
September	1st to 10th	Day	34% 7% 13% 18% 18% 10%	2280
	11th to 20th	Day	32% 7% 8% 18% 26% 10%	2268
	21st to end	Day	21% 3 8% 23% 31% 14%	2090
October	1st to 10th	Day	18% 4 6% 12% 29% 31%	3215
	11th to 20th	Day	18% 4 7% 18% 29% 23%	3150
	21st to end	Day	8% 1 3 14% 40% 34%	3438
November	1st to 10th	Day	13% 2 4 22% 33% 25%	1959
	11th to 20th	Day	13% 2 6% 21% 30% 27%	1812
	21st to end	Day	18% 2 9% 17% 27% 28%	1746
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	15% 3 8% 15% 33% 26%	4097
	11th to 20th	Night	13% 4 7% 17% 35% 24%	4110
	21st to end	Night	19% 4 13% 18% 30% 15%	4521
February	1st to 10th	Night	14% 3 9% 17% 27% 29%	2288
	11th to 20th	Night	15% 4 7% 16% 29% 28%	2218
	21st to end	Night	12% 3 8% 15% 28% 34%	1884
March	1st to 10th	Night	14% 4 7% 12% 30% 34%	3591
	11th to 20th	Night	25% 8% 7% 12% 32% 16%	3490
	21st to end	Night	10% 4 10% 21% 36% 18%	3653
April	1st to 10th	Night	12% 3 6% 17% 35% 26%	1900
	11th to 20th	Night	13% 2 8% 16% 28% 33%	1877
	21st to end	Night	12% 3 7% 18% 34% 26%	1900
May	1st to 10th	Night	8% 3 5% 8% 23% 52%	3170
	11th to 20th	Night	13% 5% 8% 16% 26% 32%	3141
	21st to end	Night	8% 2 4 10% 19% 57%	3178
June	1st to 10th	Night	22% 6% 11% 18% 22% 21%	1520
	11th to 20th	Night	14% 3 5% 18% 22% 39%	1520
	21st to end	Night	29% 7% 15% 17% 14% 17%	1545
July	1st to 10th	Night	29% 6% 21% 23% 15% 6%	2815
	11th to 20th	Night	22% 7% 16% 30% 21% 4	2806
	21st to end	Night	30% 8% 23% 28% 11% 1	3065
August	1st to 10th	Night	21% 7% 19% 34% 18% 3	3144
	11th to 20th	Night	20% 6% 15% 29% 28% 2	3143
	21st to end	Night	26% 7% 23% 27% 15% 1	3487
September	1st to 10th	Night	29% 6% 15% 18% 19% 13%	1900
	11th to 20th	Night	24% 5% 13% 19% 27% 12%	1910
	21st to end	Night	20% 3 11% 21% 32% 14%	2090
October	1st to 10th	Night	13% 3 6% 12% 33% 34%	3736
	11th to 20th	Night	16% 4 8% 14% 32% 25%	3778
	21st to end	Night	9% 2 3 15% 36% 35%	4120
November	1st to 10th	Night	14% 2 5% 19% 30% 29%	2340
	11th to 20th	Night	12% 4 5% 19% 32% 28%	2565
	21st to end	Night	19% 3 7% 17% 29% 25%	2529
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	19% 22% 33% 17% 7% 1	270
	21st to end	Day	12% 26% 29% 22% 10% 1	297
February	1st to 10th	Day	14% 27% 31% 22% 5% 1	270
	11th to 20th	Day	21% 27% 32% 13% 6% 1	270
	21st to end	Day	31% 31% 23% 12% 3	225
March	1st to 10th	Day	21% 29% 34% 12% 4	350
	11th to 20th	Day	23% 25% 33% 12% 6% 1	460
	21st to end	Day	24% 32% 32% 9% 3 1	506
April	1st to 10th	Day	16% 31% 37% 13% 3 1	460
	11th to 20th	Day	18% 31% 34% 13% 3 1	452
	21st to end	Day	16% 29% 37% 14% 4	460
May	1st to 10th	Day	10% 27% 40% 18% 4	463
	11th to 20th	Day	10% 22% 36% 22% 8% 2	488
	21st to end	Day	8% 20% 40% 21% 10% 1	633
June	1st to 10th	Day	4 8% 31% 36% 19% 3	680
	11th to 20th	Day	3 9% 31% 37% 16% 4	680
	21st to end	Day	8% 5% 19% 34% 28% 7%	647
July	1st to 10th	Day	7% 2 10% 38% 31% 12%	486
	11th to 20th	Day	4 3 7% 39% 34% 14%	484
	21st to end	Day	4 2 7% 31% 42% 15%	531
August	1st to 10th	Day	5% 2 6% 32% 38% 17%	487
	11th to 20th	Day	10% 3 10% 35% 31% 11%	477
	21st to end	Day	8% 3 9% 36% 32% 12%	539
September	1st to 10th	Day	11% 5% 16% 36% 25% 6%	490
	11th to 20th	Day	4 3 21% 44% 22% 6%	478
	21st to end	Day	6% 6% 30% 31% 24% 4	300
October	1st to 10th	Day	12% 10% 26% 33% 15% 5%	300
	11th to 20th	Day	10% 14% 30% 26% 16% 5%	300
	21st to end	Day	13% 17% 27% 24% 12% 6%	330
November	1st to 10th	Day	16% 23% 31% 18% 10% 2	300
	11th to 20th	Day	14% 16% 30% 20% 16% 4	300
	21st to end	Day	18% 29% 25% 15% 12% 1	282
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% 25% 30% 24% 10% 3	760
	21st to end	Night	10% 23% 29% 25% 10% 2	836
February	1st to 10th	Night	9% 22% 36% 21% 8% 3	760
	11th to 20th	Night	9% 25% 40% 20% 4 1	744
	21st to end	Night	14% 32% 35% 14% 3 1	628
March	1st to 10th	Night	13% 28% 28% 23% 6% 1	760
	11th to 20th	Night	13% 19% 30% 26% 10% 1	760
	21st to end	Night	11% 25% 38% 18% 6%	738
April	1st to 10th	Night	10% 25% 41% 20% 5%	570
	11th to 20th	Night	10% 25% 36% 24% 5%	563
	21st to end	Night	8% 26% 34% 25% 6%	570
May	1st to 10th	Night	5% 21% 43% 27% 4 1	570
	11th to 20th	Night	7% 13% 32% 34% 12% 2	561
	21st to end	Night	6% 14% 38% 30% 11% 1	617
June	1st to 10th	Night	4 5% 28% 43% 19% 1	570
	11th to 20th	Night	3 5% 30% 41% 18% 1	570
	21st to end	Night	6% 3 17% 40% 31% 3	570
July	1st to 10th	Night	6% 3 6% 41% 41% 3	555
	11th to 20th	Night	5% 1 5% 34% 47% 8%	552
	21st to end	Night	4 2 3 28% 51% 11%	624
August	1st to 10th	Night	7% 1 2 28% 48% 13%	567
	11th to 20th	Night	5% 1 5% 32% 43% 13%	564
	21st to end	Night	5% 2 4 32% 47% 10%	627
September	1st to 10th	Night	7% 4 9% 36% 37% 8%	570
	11th to 20th	Night	9% 1 11% 42% 32% 4	570
	21st to end	Night	7% 4 17% 42% 26% 4	570
October	1st to 10th	Night	6% 10% 23% 39% 19% 4	722
	11th to 20th	Night	8% 10% 27% 36% 16% 3	760
	21st to end	Night	8% 15% 26% 28% 16% 7%	836
November	1st to 10th	Night	8% 14% 30% 28% 13% 6%	780
	11th to 20th	Night	6% 15% 29% 34% 14% 3	965
	21st to end	Night	10% 22% 28% 25% 13% 3	973
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

Page generated at 2024-11-22 06:02:23. Version ac07

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