Blue Dome
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Dome clear sky chart from 2005-11-06 to 2026-08-15.

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	3 5% 2 6% 4 5% 5% 4 7% 8% 50%	152202
	Night	2 5% 2 6% 4 4 5% 4 6% 8% 52%	147056
Transparency	Day	28% 9% 14% 22% 19% 8%	146905
	Night	27% 8% 15% 24% 19% 7%	143360
Seeing	Day	7% 13% 31% 31% 15% 4	17509
	Night	6% 7% 27% 40% 18% 3	26945

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	6% 5% 12% 7% 7% 7% 5% 7% 8% 35%	4681
	11th to 20th	Day	1 3 1 4 4 4 5% 5% 8% 10% 54%	4740
	21st to end	Day	1 8% 4 8% 4 5% 5% 4 6% 7% 48%	5578
February	1st to 10th	Day	1 6% 3 9% 7% 5% 5% 5% 8% 10% 40%	1890
	11th to 20th	Day	1 8% 4 9% 7% 6% 6% 5% 9% 11% 34%	1901
	21st to end	Day	2 6% 4 9% 6% 7% 7% 5% 8% 9% 37%	1730
March	1st to 10th	Day	1 7% 3 8% 6% 6% 6% 5% 7% 9% 44%	4750
	11th to 20th	Day	1 9% 4 8% 5% 4 4 4 5% 8% 49%	5110
	21st to end	Day	2 8% 4 10% 6% 7% 7% 5% 8% 10% 33%	5607
April	1st to 10th	Day	3 9% 3 8% 6% 4 5% 5% 7% 9% 42%	2424
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 4 6% 8% 45%	2697
	21st to end	Day	5% 8% 3 8% 5% 5% 5% 4 6% 9% 41%	2730
May	1st to 10th	Day	3 6% 2 5% 4 4 4 4 6% 9% 54%	7626
	11th to 20th	Day	6% 8% 3 6% 5% 4 5% 4 6% 7% 47%	7596
	21st to end	Day	6% 7% 3 7% 5% 5% 4 4 7% 8% 45%	9024
June	1st to 10th	Day	7% 6% 2 7% 5% 4 6% 5% 8% 10% 40%	3150
	11th to 20th	Day	7% 6% 3 6% 5% 4 5% 4 6% 8% 47%	3150
	21st to end	Day	2 3 2 5% 4 4 5% 4 6% 7% 58%	3150
July	1st to 10th	Day	1 2 1 4 3 4 5% 5% 7% 8% 61%	8621
	11th to 20th	Day	1 2 1 4 4 5% 5% 5% 8% 10% 55%	8220
	21st to end	Day	2 2 1 4 4 5% 6% 5% 8% 8% 53%	8591
August	1st to 10th	Day	3 2 1 4 4 6% 7% 5% 8% 9% 52%	5909
	11th to 20th	Day	2 1 2 2 3 4 4 6% 8% 66%	5851
	21st to end	Day	5% 4 2 4 4 3 4 4 5% 6% 60%	6468
September	1st to 10th	Day	4 3 1 4 3 3 4 3 4 6% 64%	2316
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 7% 53%	2200
	21st to end	Day	6% 4 2 4 3 3 3 3 4 7% 60%	2200
October	1st to 10th	Day	5% 5% 2 5% 3 3 3 3 3 5% 64%	3475
	11th to 20th	Day	3 4 2 4 3 3 4 3 5% 6% 63%	3250
	21st to end	Day	2 6% 2 5% 3 4 6% 5% 7% 8% 53%	3269
November	1st to 10th	Day	4 6% 2 9% 6% 5% 5% 4 7% 8% 43%	1854
	11th to 20th	Day	2 6% 2 8% 5% 6% 6% 5% 8% 9% 43%	1798
	21st to end	Day	2 7% 2 8% 5% 4 5% 4 6% 7% 48%	1680
December	1st to 10th	Day	1 3 1 5% 5% 5% 5% 3 6% 6% 61%	2676
	11th to 20th	Day	1 8% 5% 11% 6% 6% 7% 6% 8% 7% 34%	2715
	21st to end	Day	1 9% 5% 11% 6% 6% 6% 5% 7% 7% 38%	3575
January	1st to 10th	Night	1 6% 3 10% 6% 6% 6% 5% 7% 9% 40%	8254
	11th to 20th	Night	1 3 2 6% 4 4 5% 4 6% 9% 56%	8220
	21st to end	Night	1 7% 3 7% 4 4 5% 4 6% 7% 51%	8620
February	1st to 10th	Night	1 7% 4 10% 6% 5% 6% 5% 7% 8% 41%	2730
	11th to 20th	Night	1 8% 4 11% 6% 5% 6% 5% 6% 7% 41%	2604
	21st to end	Night	2 5% 3 8% 9% 8% 6% 4 8% 9% 39%	2076
March	1st to 10th	Night	1 10% 4 8% 4 4 4 3 5% 8% 48%	5364
	11th to 20th	Night	1 5% 2 7% 5% 5% 5% 4 6% 9% 51%	5110
	21st to end	Night	2 7% 3 8% 5% 5% 5% 5% 9% 12% 39%	5610
April	1st to 10th	Night	4 9% 2 6% 6% 5% 5% 5% 6% 8% 43%	2192
	11th to 20th	Night	4 6% 2 6% 5% 4 6% 5% 7% 8% 47%	1871
	21st to end	Night	6% 7% 2 7% 6% 5% 5% 5% 5% 8% 44%	1890
May	1st to 10th	Night	5% 5% 2 4 3 3 4 3 6% 7% 58%	5321
	11th to 20th	Night	6% 6% 2 5% 4 4 4 4 6% 8% 50%	5296
	21st to end	Night	6% 5% 2 6% 4 4 4 4 7% 8% 50%	5149
June	1st to 10th	Night	6% 4 2 7% 5% 5% 5% 6% 7% 9% 45%	1470
	11th to 20th	Night	8% 3 2 5% 4 3 4 4 6% 6% 54%	1470
	21st to end	Night	2 4 2 4 4 3 4 3 5% 7% 62%	1470
July	1st to 10th	Night	1 1 2 3 3 3 3 6% 9% 69%	4320
	11th to 20th	Night	1 1 1 2 3 2 4 3 6% 8% 69%	4740
	21st to end	Night	1 1 1 2 3 4 4 4 7% 8% 64%	5486
August	1st to 10th	Night	2 2 1 2 3 3 5% 4 7% 10% 61%	4123
	11th to 20th	Night	1 1 1 2 2 2 3 3 5% 6% 74%	4087
	21st to end	Night	6% 4 1 4 3 3 3 3 4 6% 63%	4509
September	1st to 10th	Night	3 3 1 4 3 3 3 3 4 5% 68%	2080
	11th to 20th	Night	5% 3 1 5% 3 3 4 3 5% 7% 60%	2200
	21st to end	Night	6% 4 1 4 3 3 3 3 3 6% 63%	2200
October	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 6% 69%	3720
	11th to 20th	Night	3 6% 2 4 3 4 4 3 5% 7% 58%	3898
	21st to end	Night	3 5% 2 5% 4 5% 5% 4 7% 9% 52%	4527
November	1st to 10th	Night	4 6% 2 6% 5% 4 6% 5% 7% 8% 46%	2665
	11th to 20th	Night	1 7% 2 8% 5% 5% 5% 5% 7% 9% 46%	2798
	21st to end	Night	1 5% 2 7% 6% 5% 5% 5% 7% 8% 48%	2940
December	1st to 10th	Night	1 4 2 5% 4 4 4 4 5% 7% 59%	4727
	11th to 20th	Night	1 7% 4 11% 6% 7% 7% 6% 8% 10% 33%	5696
	21st to end	Night	2 8% 4 11% 7% 5% 6% 5% 8% 8% 37%	7623

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	42% 12% 18% 17% 10%	4682
	11th to 20th	Day	23% 9% 24% 24% 13% 6%	4740
	21st to end	Day	35% 8% 15% 20% 17% 6%	5578
February	1st to 10th	Day	37% 15% 20% 19% 8% 1	1890
	11th to 20th	Day	40% 13% 19% 18% 9% 2	1910
	21st to end	Day	39% 14% 17% 18% 10% 3	1730
March	1st to 10th	Day	32% 10% 19% 23% 12% 5%	4750
	11th to 20th	Day	29% 10% 16% 29% 14% 1	5110
	21st to end	Day	39% 15% 22% 18% 5%	5607
April	1st to 10th	Day	36% 10% 20% 22% 11% 1	2424
	11th to 20th	Day	34% 8% 18% 24% 14% 2	2700
	21st to end	Day	37% 8% 16% 27% 12%	2730
May	1st to 10th	Day	23% 9% 13% 33% 19% 3	7626
	11th to 20th	Day	31% 7% 12% 22% 19% 9%	7599
	21st to end	Day	31% 9% 13% 31% 15% 1	9025
June	1st to 10th	Day	34% 9% 17% 27% 11% 2	3150
	11th to 20th	Day	31% 7% 11% 20% 22% 9%	3150
	21st to end	Day	22% 6% 10% 20% 32% 10%	3150
July	1st to 10th	Day	16% 6% 8% 16% 27% 27%	8551
	11th to 20th	Day	17% 7% 9% 19% 24% 24%	8145
	21st to end	Day	20% 8% 9% 15% 31% 17%	8400
August	1st to 10th	Day	24% 7% 10% 15% 30% 14%	4338
	11th to 20th	Day	14% 5% 9% 22% 36% 14%	4250
	21st to end	Day	19% 6% 8% 20% 31% 16%	4663
September	1st to 10th	Day	19% 4 9% 19% 32% 16%	2316
	11th to 20th	Day	28% 7% 11% 21% 25% 9%	2200
	21st to end	Day	23% 6% 12% 27% 26% 7%	2200
October	1st to 10th	Day	24% 5% 8% 23% 31% 9%	3475
	11th to 20th	Day	21% 6% 14% 35% 18% 7%	3250
	21st to end	Day	26% 9% 17% 27% 20% 2	3269
November	1st to 10th	Day	36% 10% 17% 21% 15% 1	1854
	11th to 20th	Day	35% 11% 21% 22% 8% 2	1798
	21st to end	Day	34% 9% 22% 22% 10% 2	1680
December	1st to 10th	Day	24% 10% 19% 24% 21% 2	2676
	11th to 20th	Day	45% 12% 18% 18% 6% 1	2714
	21st to end	Day	40% 12% 16% 19% 9% 4	3575
January	1st to 10th	Night	37% 11% 16% 23% 13%	8255
	11th to 20th	Night	23% 9% 17% 32% 12% 6%	8220
	21st to end	Night	31% 9% 17% 20% 17% 6%	8620
February	1st to 10th	Night	40% 12% 18% 19% 9% 2	2730
	11th to 20th	Night	40% 10% 19% 21% 9% 2	2605
	21st to end	Night	39% 12% 16% 20% 10% 3	2076
March	1st to 10th	Night	32% 9% 16% 22% 12% 8%	5364
	11th to 20th	Night	26% 9% 18% 30% 16% 1	5110
	21st to end	Night	29% 13% 22% 27% 7% 1	5610
April	1st to 10th	Night	35% 9% 17% 25% 12% 2	2192
	11th to 20th	Night	30% 9% 18% 26% 15% 2	1871
	21st to end	Night	33% 8% 16% 30% 12%	1890
May	1st to 10th	Night	22% 6% 12% 32% 24% 4	5319
	11th to 20th	Night	26% 9% 12% 25% 23% 5%	5297
	21st to end	Night	26% 8% 16% 30% 20% 1	5150
June	1st to 10th	Night	31% 10% 14% 29% 16% 1	1470
	11th to 20th	Night	27% 7% 11% 22% 22% 10%	1470
	21st to end	Night	20% 4 10% 21% 31% 13%	1470
July	1st to 10th	Night	10% 4 7% 15% 32% 33%	4282
	11th to 20th	Night	11% 4 6% 22% 27% 29%	4692
	21st to end	Night	14% 5% 8% 19% 34% 20%	5365
August	1st to 10th	Night	17% 6% 8% 18% 33% 18%	3018
	11th to 20th	Night	14% 5% 7% 24% 30% 20%	2964
	21st to end	Night	16% 5% 6% 22% 29% 21%	3245
September	1st to 10th	Night	18% 5% 7% 19% 31% 20%	2080
	11th to 20th	Night	22% 6% 9% 26% 25% 11%	2200
	21st to end	Night	23% 5% 9% 24% 32% 7%	2200
October	1st to 10th	Night	16% 5% 9% 24% 36% 10%	3720
	11th to 20th	Night	25% 5% 13% 28% 23% 6%	3898
	21st to end	Night	26% 8% 17% 29% 18% 1	4527
November	1st to 10th	Night	32% 9% 17% 25% 14% 2	2665
	11th to 20th	Night	32% 10% 20% 24% 12% 2	2798
	21st to end	Night	32% 10% 18% 25% 13% 3	2940
December	1st to 10th	Night	24% 9% 22% 27% 16% 1	4727
	11th to 20th	Night	43% 13% 17% 20% 6% 1	5697
	21st to end	Night	39% 11% 17% 18% 11% 4	7623

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	5% 10% 29% 34% 16% 7%	220
	11th to 20th	Day	4 7% 26% 41% 15% 7%	220
	21st to end	Day	6% 11% 27% 36% 16% 4	314
February	1st to 10th	Day	3 15% 42% 29% 9% 2	330
	11th to 20th	Day	4 10% 39% 30% 14% 3	386
	21st to end	Day	2 9% 40% 36% 11% 2	446
March	1st to 10th	Day	5% 11% 35% 36% 12% 1	540
	11th to 20th	Day	6% 15% 28% 35% 14% 2	540
	21st to end	Day	7% 15% 34% 28% 14% 2	594
April	1st to 10th	Day	6% 15% 29% 32% 15% 4	540
	11th to 20th	Day	6% 18% 38% 23% 11% 3	532
	21st to end	Day	7% 16% 27% 30% 17% 4	540
May	1st to 10th	Day	6% 14% 31% 29% 18% 3	543
	11th to 20th	Day	17% 14% 24% 26% 16% 4	568
	21st to end	Day	12% 11% 23% 28% 21% 6%	767
June	1st to 10th	Day	8% 12% 26% 30% 18% 6%	780
	11th to 20th	Day	12% 19% 31% 25% 11% 2	780
	21st to end	Day	5% 12% 31% 33% 16% 3	780
July	1st to 10th	Day	4 11% 32% 31% 19% 4	706
	11th to 20th	Day	2 12% 39% 35% 11% 2	564
	21st to end	Day	5% 10% 36% 30% 15% 4	619
August	1st to 10th	Day	7% 9% 34% 33% 14% 3	567
	11th to 20th	Day	4 11% 30% 31% 18% 6%	534
	21st to end	Day	5% 13% 31% 31% 15% 5%	583
September	1st to 10th	Day	4 12% 29% 32% 16% 6%	530
	11th to 20th	Day	11% 12% 30% 27% 16% 4	530
	21st to end	Day	9% 14% 28% 30% 15% 4	530
October	1st to 10th	Day	12% 11% 30% 28% 14% 4	457
	11th to 20th	Day	7% 17% 25% 27% 17% 7%	330
	21st to end	Day	8% 14% 35% 33% 9% 2	363
November	1st to 10th	Day	7% 16% 34% 27% 13% 3	330
	11th to 20th	Day	5% 14% 32% 35% 11% 3	330
	21st to end	Day	4 10% 32% 36% 15% 2	330
December	1st to 10th	Day	6% 7% 30% 34% 18% 6%	324
	11th to 20th	Day	9% 11% 29% 36% 13% 2	220
	21st to end	Day	5% 14% 31% 33% 14% 3	242
January	1st to 10th	Night	4 6% 31% 41% 15% 2	872
	11th to 20th	Night	4 11% 29% 39% 14% 4	840
	21st to end	Night	4 8% 32% 41% 14% 2	924
February	1st to 10th	Night	3 9% 38% 37% 10% 2	840
	11th to 20th	Night	4 6% 36% 39% 13% 2	824
	21st to end	Night	3 7% 34% 40% 13% 3	692
March	1st to 10th	Night	4 8% 31% 42% 13% 1	840
	11th to 20th	Night	6% 6% 33% 35% 17% 3	840
	21st to end	Night	8% 10% 28% 37% 15% 1	924
April	1st to 10th	Night	7% 11% 28% 36% 15% 3	827
	11th to 20th	Night	6% 9% 35% 36% 13% 1	623
	21st to end	Night	8% 8% 30% 36% 16% 2	630
May	1st to 10th	Night	5% 10% 28% 37% 18% 3	630
	11th to 20th	Night	11% 5% 24% 32% 24% 3	621
	21st to end	Night	14% 6% 24% 31% 22% 4	604
June	1st to 10th	Night	7% 6% 21% 39% 24% 3	420
	11th to 20th	Night	8% 10% 25% 40% 14% 2	420
	21st to end	Night	3 3 19% 49% 23% 3	420
July	1st to 10th	Night	2 4 15% 47% 29% 3	410
	11th to 20th	Night	2 2 17% 56% 22% 1	408
	21st to end	Night	3 1 14% 50% 29% 3	575
August	1st to 10th	Night	5% 3 20% 44% 25% 3	627
	11th to 20th	Night	3 2 19% 45% 28% 3	606
	21st to end	Night	5% 4 19% 41% 28% 4	660
September	1st to 10th	Night	6% 3 16% 41% 28% 6%	755
	11th to 20th	Night	6% 2 17% 38% 30% 7%	800
	21st to end	Night	8% 6% 19% 36% 25% 8%	800
October	1st to 10th	Night	9% 5% 25% 38% 21% 3	800
	11th to 20th	Night	7% 8% 23% 38% 19% 5%	800
	21st to end	Night	6% 9% 29% 37% 16% 4	880
November	1st to 10th	Night	7% 5% 27% 41% 16% 3	820
	11th to 20th	Night	4 9% 33% 40% 13% 1	908
	21st to end	Night	3 8% 33% 38% 15% 3	1050
December	1st to 10th	Night	3 9% 32% 42% 12% 2	1050
	11th to 20th	Night	5% 6% 31% 46% 11% 2	1050
	21st to end	Night	6% 10% 33% 38% 11% 2	1155

Page generated at 2026-08-12 01:37:21. Version ac07

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