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 2025-04-11.

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 6% 3 7% 5% 5% 5% 4 6% 8% 49%	116907
	Night	3 6% 2 7% 5% 4 5% 4 6% 8% 50%	120193
Transparency	Day	29% 9% 15% 22% 18% 8%	116653
	Night	29% 9% 15% 23% 17% 7%	120036
Seeing	Day	7% 12% 30% 32% 16% 4	15637
	Night	5% 7% 26% 40% 18% 3	25329

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	5% 4 13% 7% 8% 8% 5% 7% 8% 34%	3611
	11th to 20th	Day	1 4 2 6% 5% 5% 7% 6% 9% 11% 45%	3640
	21st to end	Day	2 11% 5% 9% 4 4 4 4 5% 6% 47%	4300
February	1st to 10th	Day	2 7% 3 9% 7% 5% 5% 5% 8% 9% 40%	1800
	11th to 20th	Day	1 8% 4 9% 7% 6% 6% 5% 9% 11% 34%	1808
	21st to end	Day	2 7% 4 10% 6% 7% 7% 5% 8% 9% 35%	1650
March	1st to 10th	Day	1 7% 3 8% 6% 6% 6% 5% 6% 9% 44%	4647
	11th to 20th	Day	1 9% 4 8% 5% 4 4 4 5% 7% 49%	5000
	21st to end	Day	2 8% 4 10% 6% 7% 7% 5% 8% 10% 33%	5486
April	1st to 10th	Day	4 9% 3 8% 6% 4 6% 4 7% 9% 41%	2308
	11th to 20th	Day	4 7% 3 7% 5% 5% 5% 4 6% 9% 44%	2439
	21st to end	Day	6% 8% 3 7% 5% 5% 4 4 6% 9% 43%	2470
May	1st to 10th	Day	5% 9% 3 6% 5% 5% 5% 4 6% 9% 44%	4110
	11th to 20th	Day	5% 6% 2 6% 4 4 5% 3 6% 7% 51%	4071
	21st to end	Day	7% 9% 3 7% 5% 5% 4 4 6% 7% 44%	4831
June	1st to 10th	Day	7% 7% 2 7% 5% 4 6% 5% 8% 10% 38%	2850
	11th to 20th	Day	7% 6% 3 7% 6% 4 5% 4 6% 7% 45%	2850
	21st to end	Day	2 3 1 5% 4 4 4 4 6% 7% 59%	2850
July	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 6% 69%	4626
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 7% 8% 56%	4420
	21st to end	Day	3 3 1 5% 4 4 6% 5% 8% 8% 53%	4615
August	1st to 10th	Day	4 3 1 5% 4 6% 7% 5% 7% 8% 49%	4092
	11th to 20th	Day	3 2 1 2 3 3 4 4 6% 8% 64%	4028
	21st to end	Day	2 3 1 4 3 3 3 3 5% 6% 64%	4521
September	1st to 10th	Day	4 3 1 4 3 3 4 3 4 6% 65%	2200
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 7% 52%	2090
	21st to end	Day	6% 4 2 4 4 3 3 3 5% 7% 60%	2090
October	1st to 10th	Day	5% 5% 2 5% 3 3 3 3 3 5% 64%	3369
	11th to 20th	Day	2 4 2 4 3 3 3 3 5% 6% 64%	3150
	21st to end	Day	2 6% 2 5% 3 4 6% 5% 7% 8% 52%	3168
November	1st to 10th	Day	4 7% 2 10% 6% 5% 5% 4 7% 7% 43%	1764
	11th to 20th	Day	1 6% 2 8% 5% 6% 6% 5% 8% 9% 43%	1713
	21st to end	Day	2 8% 3 7% 5% 5% 5% 4 6% 7% 48%	1600
December	1st to 10th	Day	1 3 2 5% 4 4 5% 3 5% 6% 61%	2597
	11th to 20th	Day	1 9% 5% 11% 6% 6% 7% 6% 8% 7% 33%	2645
	21st to end	Day	1 9% 5% 11% 6% 6% 6% 5% 7% 7% 37%	3498
January	1st to 10th	Night	1 5% 3 10% 6% 6% 6% 5% 7% 9% 42%	6376
	11th to 20th	Night	1 4 2 8% 5% 5% 6% 4 7% 8% 49%	6320
	21st to end	Night	1 9% 4 9% 5% 4 5% 4 5% 6% 48%	6666
February	1st to 10th	Night	1 7% 4 10% 6% 5% 6% 5% 7% 8% 41%	2600
	11th to 20th	Night	1 8% 4 10% 6% 4 6% 5% 6% 8% 41%	2477
	21st to end	Night	2 5% 3 8% 9% 8% 6% 4 8% 8% 39%	1980
March	1st to 10th	Night	1 10% 4 8% 4 4 4 3 5% 8% 48%	5247
	11th to 20th	Night	1 5% 2 7% 5% 5% 5% 4 6% 9% 50%	5000
	21st to end	Night	3 7% 3 8% 5% 5% 5% 5% 9% 12% 39%	5489
April	1st to 10th	Night	5% 9% 2 6% 6% 5% 5% 5% 6% 8% 43%	2088
	11th to 20th	Night	4 6% 2 6% 5% 4 6% 5% 7% 8% 46%	1695
	21st to end	Night	6% 7% 2 7% 6% 5% 4 5% 5% 7% 45%	1710
May	1st to 10th	Night	7% 8% 2 6% 3 4 4 3 5% 7% 50%	2860
	11th to 20th	Night	5% 6% 2 6% 4 4 5% 4 5% 7% 53%	2834
	21st to end	Night	8% 6% 3 8% 5% 5% 5% 4 6% 8% 44%	2744
June	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 6% 7% 8% 44%	1330
	11th to 20th	Night	9% 3 2 6% 4 4 5% 4 6% 6% 51%	1330
	21st to end	Night	2 3 1 4 4 3 4 3 4 7% 64%	1330
July	1st to 10th	Night	1 1 2 3 2 3 3 5% 7% 74%	2316
	11th to 20th	Night	1 1 1 3 3 3 4 3 5% 7% 68%	2540
	21st to end	Night	2 2 1 3 3 3 4 4 6% 7% 65%	2940
August	1st to 10th	Night	3 2 1 3 4 3 5% 4 7% 9% 59%	2847
	11th to 20th	Night	1 2 1 3 3 3 4 3 6% 6% 68%	2811
	21st to end	Night	2 3 1 3 3 3 3 3 5% 5% 69%	3146
September	1st to 10th	Night	4 3 2 4 3 3 3 3 4 5% 68%	1976
	11th to 20th	Night	5% 4 1 5% 3 3 4 3 5% 7% 59%	2090
	21st to end	Night	6% 4 1 5% 3 3 3 3 3 6% 63%	2090
October	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 6% 69%	3606
	11th to 20th	Night	3 6% 2 4 3 4 4 3 5% 7% 59%	3778
	21st to end	Night	3 5% 2 5% 4 5% 5% 4 7% 9% 51%	4386
November	1st to 10th	Night	5% 6% 2 7% 5% 5% 6% 5% 7% 8% 46%	2535
	11th to 20th	Night	2 7% 2 7% 5% 5% 5% 5% 7% 9% 46%	2665
	21st to end	Night	2 6% 3 7% 6% 5% 5% 5% 7% 8% 48%	2800
December	1st to 10th	Night	1 4 2 5% 4 4 4 4 5% 7% 60%	4587
	11th to 20th	Night	1 7% 4 11% 6% 7% 7% 6% 8% 10% 32%	5546
	21st to end	Night	2 8% 4 11% 7% 5% 6% 5% 8% 8% 37%	7458

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	43% 13% 14% 19% 10%	3611
	11th to 20th	Day	30% 11% 23% 22% 12% 3	3640
	21st to end	Day	37% 8% 13% 21% 15% 6%	4300
February	1st to 10th	Day	38% 15% 20% 19% 7% 1	1800
	11th to 20th	Day	40% 14% 19% 18% 8% 1	1817
	21st to end	Day	40% 14% 16% 18% 9% 3	1650
March	1st to 10th	Day	32% 10% 19% 23% 12% 5%	4647
	11th to 20th	Day	30% 10% 16% 30% 14% 1	5000
	21st to end	Day	39% 15% 22% 18% 5%	5486
April	1st to 10th	Day	37% 10% 21% 21% 11% 1	2308
	11th to 20th	Day	34% 8% 19% 24% 13% 2	2442
	21st to end	Day	36% 8% 16% 28% 12%	2470
May	1st to 10th	Day	32% 11% 15% 25% 14% 3	4110
	11th to 20th	Day	26% 9% 15% 30% 14% 7%	4074
	21st to end	Day	34% 11% 14% 31% 9% 1	4831
June	1st to 10th	Day	35% 10% 18% 26% 10% 1	2850
	11th to 20th	Day	34% 8% 12% 20% 19% 8%	2850
	21st to end	Day	20% 6% 11% 21% 32% 11%	2850
July	1st to 10th	Day	13% 5% 7% 14% 25% 36%	4555
	11th to 20th	Day	18% 7% 9% 13% 22% 31%	4345
	21st to end	Day	20% 7% 9% 14% 29% 21%	4469
August	1st to 10th	Day	25% 7% 10% 15% 30% 13%	4078
	11th to 20th	Day	14% 5% 9% 22% 36% 14%	4067
	21st to end	Day	18% 6% 9% 21% 31% 16%	4520
September	1st to 10th	Day	19% 4 9% 19% 32% 17%	2200
	11th to 20th	Day	28% 7% 11% 21% 23% 10%	2090
	21st to end	Day	23% 6% 12% 27% 24% 7%	2090
October	1st to 10th	Day	25% 5% 8% 22% 32% 9%	3369
	11th to 20th	Day	20% 5% 14% 35% 18% 7%	3150
	21st to end	Day	27% 9% 17% 26% 20% 1	3168
November	1st to 10th	Day	37% 10% 17% 20% 15% 1	1764
	11th to 20th	Day	35% 11% 21% 22% 9% 2	1713
	21st to end	Day	35% 9% 22% 22% 10% 3	1600
December	1st to 10th	Day	24% 9% 19% 24% 22% 2	2597
	11th to 20th	Day	45% 12% 18% 18% 6%	2644
	21st to end	Day	41% 12% 16% 19% 10% 3	3498
January	1st to 10th	Night	36% 10% 14% 25% 14% 1	6376
	11th to 20th	Night	30% 12% 14% 27% 12% 5%	6320
	21st to end	Night	36% 10% 14% 17% 16% 7%	6666
February	1st to 10th	Night	40% 13% 18% 19% 9% 2	2600
	11th to 20th	Night	39% 10% 19% 21% 9% 2	2478
	21st to end	Night	39% 12% 17% 20% 9% 3	1980
March	1st to 10th	Night	32% 9% 16% 22% 12% 9%	5247
	11th to 20th	Night	26% 9% 18% 30% 16% 1	5000
	21st to end	Night	30% 13% 23% 27% 6% 1	5489
April	1st to 10th	Night	35% 9% 18% 23% 13% 2	2088
	11th to 20th	Night	31% 9% 19% 25% 14% 2	1695
	21st to end	Night	34% 8% 16% 29% 12%	1710
May	1st to 10th	Night	30% 7% 17% 27% 17% 3	2860
	11th to 20th	Night	23% 10% 13% 30% 20% 3	2835
	21st to end	Night	32% 10% 15% 28% 15% 1	2744
June	1st to 10th	Night	32% 10% 14% 29% 14% 1	1330
	11th to 20th	Night	30% 8% 11% 22% 19% 10%	1330
	21st to end	Night	19% 4 10% 22% 32% 13%	1330
July	1st to 10th	Night	10% 4 7% 12% 26% 41%	2278
	11th to 20th	Night	12% 4 6% 14% 23% 41%	2492
	21st to end	Night	15% 4 7% 18% 31% 26%	2848
August	1st to 10th	Night	18% 6% 8% 18% 33% 17%	2838
	11th to 20th	Night	14% 5% 7% 24% 30% 20%	2838
	21st to end	Night	16% 5% 6% 22% 30% 21%	3146
September	1st to 10th	Night	19% 5% 7% 18% 31% 21%	1976
	11th to 20th	Night	23% 6% 10% 25% 25% 12%	2090
	21st to end	Night	23% 5% 9% 25% 31% 7%	2090
October	1st to 10th	Night	16% 5% 9% 24% 36% 10%	3606
	11th to 20th	Night	24% 5% 13% 28% 23% 6%	3778
	21st to end	Night	26% 8% 18% 29% 18% 1	4386
November	1st to 10th	Night	33% 9% 17% 24% 14% 2	2535
	11th to 20th	Night	31% 10% 20% 24% 12% 2	2665
	21st to end	Night	32% 10% 18% 25% 13% 3	2800
December	1st to 10th	Night	24% 9% 22% 27% 16% 1	4587
	11th to 20th	Night	44% 13% 17% 20% 5% 1	5547
	21st to end	Night	40% 11% 17% 18% 11% 4	7458

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% 7% 29% 35% 17% 8%	200
	11th to 20th	Day	4 6% 25% 42% 17% 8%	200
	21st to end	Day	6% 10% 27% 36% 17% 4	285
February	1st to 10th	Day	3 15% 43% 28% 9% 1	300
	11th to 20th	Day	4 10% 38% 30% 14% 4	353
	21st to end	Day	3 8% 37% 38% 11% 2	414
March	1st to 10th	Day	5% 9% 35% 37% 12% 1	500
	11th to 20th	Day	5% 13% 27% 38% 15% 2	500
	21st to end	Day	7% 13% 34% 29% 15% 2	550
April	1st to 10th	Day	6% 15% 29% 32% 15% 4	500
	11th to 20th	Day	5% 16% 38% 25% 12% 3	453
	21st to end	Day	7% 14% 25% 32% 18% 5%	460
May	1st to 10th	Day	6% 14% 29% 29% 19% 3	463
	11th to 20th	Day	15% 11% 23% 29% 18% 4	488
	21st to end	Day	12% 10% 21% 28% 22% 7%	665
June	1st to 10th	Day	8% 11% 25% 31% 19% 6%	680
	11th to 20th	Day	14% 18% 29% 26% 12% 2	680
	21st to end	Day	4 11% 29% 35% 18% 3	680
July	1st to 10th	Day	4 10% 30% 31% 20% 4	612
	11th to 20th	Day	2 12% 37% 36% 12% 2	484
	21st to end	Day	5% 9% 36% 30% 16% 4	531
August	1st to 10th	Day	7% 9% 32% 32% 15% 4	487
	11th to 20th	Day	5% 10% 27% 33% 18% 6%	477
	21st to end	Day	5% 13% 31% 31% 15% 5%	539
September	1st to 10th	Day	5% 13% 30% 30% 16% 6%	490
	11th to 20th	Day	12% 12% 30% 27% 16% 4	490
	21st to end	Day	9% 14% 28% 30% 16% 3	490
October	1st to 10th	Day	12% 10% 29% 29% 15% 4	421
	11th to 20th	Day	6% 16% 24% 29% 19% 7%	300
	21st to end	Day	8% 14% 34% 33% 9% 2	330
November	1st to 10th	Day	7% 15% 33% 28% 14% 3	300
	11th to 20th	Day	4 13% 32% 37% 10% 4	300
	21st to end	Day	4 10% 30% 37% 16% 3	300
December	1st to 10th	Day	4 6% 28% 36% 19% 7%	295
	11th to 20th	Day	7% 10% 28% 39% 15% 2	200
	21st to end	Day	4 10% 32% 35% 14% 4	220
January	1st to 10th	Night	4 5% 30% 42% 16% 2	831
	11th to 20th	Night	4 10% 28% 40% 15% 4	800
	21st to end	Night	4 8% 30% 42% 14% 2	880
February	1st to 10th	Night	3 9% 38% 37% 11% 3	800
	11th to 20th	Night	4 6% 36% 40% 13% 2	784
	21st to end	Night	3 6% 33% 41% 14% 3	660
March	1st to 10th	Night	4 8% 30% 43% 14% 2	800
	11th to 20th	Night	5% 6% 31% 37% 18% 3	800
	21st to end	Night	8% 10% 26% 38% 16% 1	880
April	1st to 10th	Night	7% 11% 28% 36% 15% 3	788
	11th to 20th	Night	5% 8% 35% 36% 14% 2	565
	21st to end	Night	9% 6% 29% 36% 18% 2	570
May	1st to 10th	Night	5% 9% 29% 36% 18% 3	570
	11th to 20th	Night	10% 4 23% 33% 27% 4	561
	21st to end	Night	15% 5% 23% 29% 24% 4	546
June	1st to 10th	Night	7% 5% 19% 40% 25% 4	380
	11th to 20th	Night	8% 9% 23% 42% 16% 2	380
	21st to end	Night	2 3 18% 49% 25% 3	380
July	1st to 10th	Night	3 4 14% 46% 30% 4	370
	11th to 20th	Night	2 2 14% 57% 23% 1	368
	21st to end	Night	3 1 12% 48% 32% 4	520
August	1st to 10th	Night	5% 2 17% 45% 28% 3	567
	11th to 20th	Night	3 2 18% 45% 30% 3	564
	21st to end	Night	4 4 18% 41% 29% 4	627
September	1st to 10th	Night	7% 3 16% 40% 28% 6%	717
	11th to 20th	Night	6% 2 16% 38% 30% 7%	760
	21st to end	Night	8% 6% 19% 36% 24% 8%	760
October	1st to 10th	Night	9% 4 23% 39% 21% 4	760
	11th to 20th	Night	7% 7% 21% 40% 20% 6%	760
	21st to end	Night	6% 9% 27% 38% 16% 5%	836
November	1st to 10th	Night	7% 6% 25% 42% 17% 3	780
	11th to 20th	Night	4 9% 33% 40% 13% 1	865
	21st to end	Night	3 8% 32% 39% 15% 3	1000
December	1st to 10th	Night	4 8% 30% 44% 13% 2	1000
	11th to 20th	Night	5% 6% 29% 48% 11% 2	1000
	21st to end	Night	6% 10% 32% 39% 11% 2	1100

Page generated at 2025-04-08 07:17:27. Version ac07

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