Blue Moon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Moon Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 7% 3 6% 4 4 4 4 6% 8% 42%	126057
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	123463
Transparency	Day	39% 6% 11% 24% 18% 3	129719
	Night	38% 5% 10% 23% 19% 5%	127876
Seeing	Day	16% 10% 23% 24% 18% 8%	16583
	Night	17% 9% 25% 29% 18% 3	26273

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	15% 11% 5% 10% 4 2 3 2 4 6% 37%	3617
	11th to 20th	Day	15% 8% 2 7% 3 2 2 2 3 5% 50%	3868
	21st to end	Day	19% 8% 2 5% 3 3 3 3 5% 7% 43%	4620
February	1st to 10th	Day	19% 11% 4 6% 4 4 3 3 4 6% 36%	2000
	11th to 20th	Day	20% 9% 3 8% 4 3 3 3 4 5% 38%	1948
	21st to end	Day	21% 9% 3 6% 4 4 4 3 4 4 37%	1650
March	1st to 10th	Day	16% 13% 3 8% 4 4 4 3 4 4 36%	4480
	11th to 20th	Day	13% 8% 2 6% 4 4 3 2 4 5% 48%	4730
	21st to end	Day	12% 7% 3 7% 4 4 4 3 5% 6% 44%	5967
April	1st to 10th	Day	14% 6% 3 6% 4 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	15% 7% 2 6% 5% 3 4 3 4 5% 44%	2374
	21st to end	Day	15% 7% 2 7% 4 4 5% 4 5% 8% 41%	2400
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 5% 7% 8% 28%	5671
	11th to 20th	Day	15% 8% 3 8% 6% 5% 5% 4 6% 8% 31%	5861
	21st to end	Day	16% 7% 3 6% 4 4 4 4 5% 8% 38%	6493
June	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 10% 42%	2800
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 7% 10% 47%	2800
	21st to end	Day	6% 4 1 5% 4 5% 6% 5% 7% 11% 46%	2800
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 7% 11% 40%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 6% 9% 13% 36%	6360
	21st to end	Day	10% 7% 3 7% 4 5% 6% 5% 8% 11% 34%	6481
August	1st to 10th	Day	4 3 1 4 3 4 5% 5% 7% 10% 55%	3905
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 46%	3849
	21st to end	Day	6% 4 2 4 4 4 4 4 5% 8% 57%	4312
September	1st to 10th	Day	10% 6% 1 6% 4 5% 5% 3 6% 8% 46%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 3 4 5% 7% 48%	2260
	21st to end	Day	12% 7% 2 4 3 3 5% 4 5% 5% 49%	2200
October	1st to 10th	Day	9% 4 2 4 3 3 3 3 5% 6% 58%	2123
	11th to 20th	Day	13% 5% 2 4 3 3 3 4 5% 6% 54%	1910
	21st to end	Day	18% 7% 2 4 3 2 3 2 3 4 50%	2005
November	1st to 10th	Day	13% 6% 2 5% 2 3 2 3 4 5% 55%	1762
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 4 6% 50%	1800
	21st to end	Day	17% 8% 3 4 3 3 3 3 5% 6% 45%	1800
December	1st to 10th	Day	17% 9% 2 5% 3 3 4 3 4 5% 45%	2930
	11th to 20th	Day	20% 9% 3 7% 4 4 3 3 4 5% 38%	3139
	21st to end	Day	16% 12% 3 7% 5% 5% 5% 3 6% 6% 32%	3982
January	1st to 10th	Night	17% 11% 3 9% 3 3 3 3 4 6% 38%	6348
	11th to 20th	Night	16% 9% 3 7% 4 3 3 2 3 3 46%	6110
	21st to end	Night	22% 8% 3 6% 3 3 3 3 4 5% 41%	6335
February	1st to 10th	Night	21% 10% 3 7% 5% 4 3 3 5% 5% 34%	2400
	11th to 20th	Night	21% 7% 3 8% 4 4 4 3 3 4 38%	2338
	21st to end	Night	19% 8% 3 6% 4 4 4 2 4 5% 40%	1980
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 5% 37%	5373
	11th to 20th	Night	13% 7% 2 5% 3 3 4 3 5% 6% 49%	5272
	21st to end	Night	13% 7% 3 7% 4 4 3 4 5% 8% 42%	4962
April	1st to 10th	Night	13% 6% 3 5% 4 3 4 3 5% 6% 48%	2000
	11th to 20th	Night	16% 6% 2 6% 3 4 3 2 5% 7% 47%	1975
	21st to end	Night	13% 7% 2 5% 4 4 4 4 6% 7% 43%	2000
May	1st to 10th	Night	18% 10% 3 8% 4 3 4 3 4 7% 35%	4224
	11th to 20th	Night	14% 9% 4 8% 6% 4 5% 3 5% 6% 35%	4032
	21st to end	Night	15% 7% 2 8% 4 4 4 4 6% 8% 37%	4347
June	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 43%	1600
	11th to 20th	Night	5% 4 1 4 4 3 4 5% 6% 9% 57%	1600
	21st to end	Night	3 2 1 5% 3 4 6% 4 7% 13% 53%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	3618
	11th to 20th	Night	4 3 1 5% 4 5% 6% 5% 8% 13% 46%	3617
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 4 8% 13% 37%	4363
August	1st to 10th	Night	3 3 1 3 3 3 4 3 6% 8% 62%	3244
	11th to 20th	Night	4 3 2 5% 4 4 4 4 7% 10% 53%	3200
	21st to end	Night	5% 3 1 4 3 3 3 4 5% 9% 61%	3586
September	1st to 10th	Night	9% 4 2 4 4 3 4 3 4 8% 54%	2000
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 8% 52%	2120
	21st to end	Night	10% 4 1 5% 4 4 3 3 5% 7% 55%	2200
October	1st to 10th	Night	9% 4 1 4 2 3 3 3 5% 6% 62%	2124
	11th to 20th	Night	12% 5% 1 4 3 3 4 3 4 5% 56%	2266
	21st to end	Night	17% 6% 2 5% 2 3 3 3 4 5% 50%	2590
November	1st to 10th	Night	13% 6% 2 5% 3 3 2 2 4 6% 54%	2536
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2600
	21st to end	Night	16% 7% 2 5% 4 3 3 3 4 5% 48%	2600
December	1st to 10th	Night	19% 8% 2 6% 3 3 3 3 3 5% 45%	4250
	11th to 20th	Night	20% 9% 2 6% 3 3 3 3 4 5% 43%	5057
	21st to end	Night	19% 10% 4 9% 5% 4 4 3 5% 6% 31%	6996

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	49% 4 9% 21% 16% 1	3618
	11th to 20th	Day	39% 3 6% 27% 25% 1	3868
	21st to end	Day	41% 3 8% 22% 20% 6%	4620
February	1st to 10th	Day	50% 3 9% 20% 16% 2	2000
	11th to 20th	Day	50% 4 7% 20% 16% 4	1958
	21st to end	Day	50% 3 8% 18% 15% 6%	1650
March	1st to 10th	Day	52% 4 4 17% 15% 9%	4480
	11th to 20th	Day	39% 4 6% 23% 20% 8%	4730
	21st to end	Day	41% 4 9% 25% 19% 1	5969
April	1st to 10th	Day	39% 5% 9% 23% 21% 3	2400
	11th to 20th	Day	42% 4 8% 21% 20% 5%	2374
	21st to end	Day	41% 6% 10% 26% 17% 2	2400
May	1st to 10th	Day	50% 8% 13% 19% 10% 1	5706
	11th to 20th	Day	50% 7% 14% 19% 10% 1	5865
	21st to end	Day	44% 6% 10% 23% 15% 2	6507
June	1st to 10th	Day	36% 7% 13% 22% 19% 2	2800
	11th to 20th	Day	31% 7% 15% 27% 18% 3	2800
	21st to end	Day	30% 9% 15% 25% 18% 4	2800
July	1st to 10th	Day	35% 9% 15% 28% 12% 2	6291
	11th to 20th	Day	32% 11% 21% 27% 7% 1	6285
	21st to end	Day	40% 9% 22% 22% 6%	6294
August	1st to 10th	Day	22% 6% 17% 30% 23% 2	3893
	11th to 20th	Day	32% 6% 16% 26% 17% 2	3885
	21st to end	Day	25% 5% 10% 32% 25% 2	4311
September	1st to 10th	Day	35% 7% 10% 25% 17% 5%	2400
	11th to 20th	Day	37% 5% 9% 23% 23% 3	2260
	21st to end	Day	35% 5% 7% 22% 24% 6%	2200
October	1st to 10th	Day	18% 3 6% 16% 45% 11%	3513
	11th to 20th	Day	22% 2 7% 25% 28% 16%	3150
	21st to end	Day	30% 2 5% 25% 32% 6%	3282
November	1st to 10th	Day	32% 4 8% 24% 27% 5%	1762
	11th to 20th	Day	37% 4 7% 21% 24% 7%	1800
	21st to end	Day	41% 3 7% 24% 20% 5%	1800
December	1st to 10th	Day	42% 4 5% 27% 19% 2	2930
	11th to 20th	Day	49% 4 7% 23% 14% 3	3136
	21st to end	Day	52% 4 9% 24% 10% 1	3982
January	1st to 10th	Night	49% 3 9% 21% 16% 1	6348
	11th to 20th	Night	45% 3 5% 21% 23% 3	6110
	21st to end	Night	46% 3 6% 22% 19% 4	6335
February	1st to 10th	Night	53% 3 9% 16% 16% 2	2400
	11th to 20th	Night	51% 4 7% 18% 16% 5%	2339
	21st to end	Night	48% 4 7% 18% 18% 5%	1980
March	1st to 10th	Night	48% 4 6% 15% 18% 9%	5373
	11th to 20th	Night	36% 4 7% 22% 17% 15%	5272
	21st to end	Night	40% 5% 9% 24% 19% 3	4962
April	1st to 10th	Night	37% 4 11% 25% 19% 5%	2000
	11th to 20th	Night	38% 4 7% 23% 22% 8%	1975
	21st to end	Night	40% 5% 10% 25% 18% 3	2000
May	1st to 10th	Night	49% 6% 12% 21% 11% 2	4254
	11th to 20th	Night	49% 7% 13% 19% 11% 1	4033
	21st to end	Night	43% 7% 12% 24% 13% 2	4355
June	1st to 10th	Night	37% 6% 12% 20% 23% 2	1600
	11th to 20th	Night	23% 5% 16% 31% 20% 4	1600
	21st to end	Night	23% 7% 18% 28% 18% 6%	1600
July	1st to 10th	Night	32% 10% 15% 27% 15% 2	3580
	11th to 20th	Night	25% 10% 21% 33% 9% 2	3572
	21st to end	Night	36% 9% 22% 24% 8%	4237
August	1st to 10th	Night	18% 6% 13% 40% 23% 1	3232
	11th to 20th	Night	24% 8% 17% 31% 18% 2	3230
	21st to end	Night	21% 6% 12% 33% 25% 4	3586
September	1st to 10th	Night	30% 5% 11% 27% 21% 6%	2000
	11th to 20th	Night	31% 6% 10% 22% 27% 4	2120
	21st to end	Night	30% 5% 7% 26% 24% 7%	2200
October	1st to 10th	Night	17% 3 5% 16% 44% 14%	3514
	11th to 20th	Night	19% 3 4 18% 37% 19%	3765
	21st to end	Night	26% 3 6% 26% 26% 13%	4263
November	1st to 10th	Night	33% 3 7% 24% 26% 7%	2536
	11th to 20th	Night	32% 3 7% 23% 26% 9%	2600
	21st to end	Night	39% 3 6% 24% 23% 5%	2600
December	1st to 10th	Night	42% 4 7% 22% 23% 2	4250
	11th to 20th	Night	46% 2 6% 24% 20% 3	5059
	21st to end	Night	53% 5% 8% 21% 10% 2	6996

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	26% 22% 34% 14% 3	300
	11th to 20th	Day	28% 23% 32% 13% 3 1	300
	21st to end	Day	24% 25% 38% 10% 3	330
February	1st to 10th	Day	28% 21% 31% 17% 3 1	300
	11th to 20th	Day	35% 25% 31% 7% 2	300
	21st to end	Day	34% 24% 29% 10% 2	249
March	1st to 10th	Day	28% 20% 35% 12% 4	300
	11th to 20th	Day	27% 24% 34% 13% 3	310
	21st to end	Day	26% 18% 32% 17% 7%	550
April	1st to 10th	Day	25% 16% 30% 22% 7% 1	500
	11th to 20th	Day	20% 18% 30% 22% 9% 1	492
	21st to end	Day	20% 16% 29% 23% 10% 2	500
May	1st to 10th	Day	21% 15% 26% 25% 10% 3	503
	11th to 20th	Day	15% 10% 26% 26% 16% 7%	527
	21st to end	Day	12% 7% 22% 29% 17% 12%	594
June	1st to 10th	Day	11% 5% 22% 27% 22% 13%	730
	11th to 20th	Day	6% 2 17% 34% 29% 12%	730
	21st to end	Day	6% 3 17% 33% 26% 14%	730
July	1st to 10th	Day	9% 2 13% 29% 28% 19%	721
	11th to 20th	Day	8% 2 12% 28% 34% 16%	718
	21st to end	Day	8% 3 12% 28% 30% 19%	709
August	1st to 10th	Day	8% 3 11% 30% 32% 17%	526
	11th to 20th	Day	11% 2 14% 31% 31% 11%	519
	21st to end	Day	10% 3 8% 24% 37% 18%	583
September	1st to 10th	Day	8% 5% 13% 29% 34% 11%	530
	11th to 20th	Day	12% 1 13% 25% 35% 14%	530
	21st to end	Day	15% 6% 17% 31% 20% 10%	530
October	1st to 10th	Day	9% 4 19% 35% 24% 8%	502
	11th to 20th	Day	12% 12% 35% 25% 13% 2	310
	21st to end	Day	24% 15% 31% 21% 8% 1	330
November	1st to 10th	Day	16% 12% 34% 26% 11% 1	300
	11th to 20th	Day	12% 19% 37% 26% 6% 1	300
	21st to end	Day	25% 15% 31% 17% 10% 2	300
December	1st to 10th	Day	30% 19% 28% 17% 6% 1	300
	11th to 20th	Day	25% 19% 31% 19% 6%	300
	21st to end	Day	27% 21% 35% 16% 2	330
January	1st to 10th	Night	23% 17% 33% 24% 4	1000
	11th to 20th	Night	26% 18% 31% 21% 4	1000
	21st to end	Night	22% 14% 40% 23% 1	1036
February	1st to 10th	Night	26% 14% 36% 22% 3	800
	11th to 20th	Night	27% 16% 35% 21% 2	783
	21st to end	Night	26% 19% 34% 19% 2	660
March	1st to 10th	Night	21% 16% 32% 28% 3	800
	11th to 20th	Night	24% 13% 34% 23% 6% 1	746
	21st to end	Night	18% 12% 34% 28% 8%	660
April	1st to 10th	Night	17% 11% 36% 28% 8% 1	600
	11th to 20th	Night	18% 12% 28% 30% 11% 1	593
	21st to end	Night	15% 9% 36% 29% 10% 2	600
May	1st to 10th	Night	16% 7% 32% 29% 14% 2	600
	11th to 20th	Night	12% 7% 22% 36% 20% 2	591
	21st to end	Night	8% 4 21% 34% 28% 6%	650
June	1st to 10th	Night	11% 2 16% 37% 28% 5%	600
	11th to 20th	Night	5% 3 12% 39% 38% 4	600
	21st to end	Night	4 2 14% 37% 38% 6%	600
July	1st to 10th	Night	8% 2 8% 31% 42% 9%	585
	11th to 20th	Night	7% 2 9% 27% 46% 9%	582
	21st to end	Night	7% 10% 30% 44% 9%	657
August	1st to 10th	Night	5% 2 8% 32% 45% 9%	597
	11th to 20th	Night	6% 1 8% 39% 40% 6%	594
	21st to end	Night	9% 1 9% 31% 42% 9%	660
September	1st to 10th	Night	10% 2 9% 32% 42% 6%	600
	11th to 20th	Night	10% 3 11% 32% 38% 6%	720
	21st to end	Night	12% 2 17% 34% 31% 5%	800
October	1st to 10th	Night	10% 3 16% 35% 31% 4	773
	11th to 20th	Night	14% 7% 24% 34% 20% 1	760
	21st to end	Night	19% 8% 27% 30% 14% 2	836
November	1st to 10th	Night	16% 8% 26% 33% 16% 1	780
	11th to 20th	Night	14% 11% 32% 31% 11%	800
	21st to end	Night	20% 11% 29% 29% 11% 1	800
December	1st to 10th	Night	25% 12% 30% 26% 7%	800
	11th to 20th	Night	25% 11% 29% 28% 7%	910
	21st to end	Night	27% 14% 30% 25% 4	1100

Page generated at 2025-10-01 00:39:03. Version ac07

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