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 2025-07-06.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	119271
	Night	14% 11% 4 8% 5% 4 4 4 5% 7% 34%	119563
Transparency	Day	48% 7% 12% 20% 12% 2	122871
	Night	48% 6% 13% 21% 12% 2	124088
Seeing	Day	16% 12% 30% 24% 13% 5%	16198
	Night	18% 10% 31% 29% 11% 1	25766

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	10% 16% 6% 14% 6% 4 4 3 4 5% 27%	3617
	11th to 20th	Day	13% 17% 6% 16% 9% 8% 5% 3 5% 5% 14%	3620
	21st to end	Day	19% 17% 6% 11% 6% 5% 5% 3 4 5% 18%	4438
February	1st to 10th	Day	14% 14% 6% 14% 8% 5% 5% 4 5% 5% 22%	1800
	11th to 20th	Day	13% 14% 5% 11% 8% 5% 5% 5% 5% 6% 24%	1847
	21st to end	Day	14% 15% 5% 12% 7% 5% 4 4 4 5% 25%	1650
March	1st to 10th	Day	15% 12% 5% 10% 6% 4 4 3 4 6% 31%	4636
	11th to 20th	Day	11% 12% 4 9% 6% 4 4 3 6% 8% 32%	4980
	21st to end	Day	18% 14% 4 10% 5% 4 5% 3 5% 6% 27%	5465
April	1st to 10th	Day	16% 13% 4 8% 5% 4 4 4 5% 7% 29%	2297
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 7% 35%	2569
	21st to end	Day	16% 9% 3 9% 5% 5% 4 3 5% 6% 34%	2600
May	1st to 10th	Day	18% 9% 3 8% 5% 4 4 4 5% 6% 34%	5862
	11th to 20th	Day	16% 8% 3 7% 4 5% 5% 4 6% 8% 34%	5861
	21st to end	Day	14% 6% 3 8% 5% 4 5% 4 5% 6% 40%	6540
June	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 10% 39%	2980
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 42%	3000
	21st to end	Day	9% 7% 2 7% 5% 5% 5% 5% 8% 9% 38%	3000
July	1st to 10th	Day	10% 5% 3 7% 5% 5% 6% 5% 8% 9% 37%	4528
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 7% 10% 45%	4440
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 7% 9% 40%	4515
August	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	4104
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	4044
	21st to end	Day	9% 6% 3 7% 5% 6% 6% 4 7% 9% 39%	4532
September	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 6% 9% 39%	2155
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2090
	21st to end	Day	15% 8% 3 6% 5% 4 5% 4 7% 9% 34%	2090
October	1st to 10th	Day	13% 7% 3 8% 5% 5% 4 4 5% 6% 40%	2009
	11th to 20th	Day	17% 10% 3 8% 5% 5% 5% 3 5% 7% 33%	1900
	21st to end	Day	19% 11% 3 8% 5% 4 5% 4 6% 7% 29%	1954
November	1st to 10th	Day	13% 10% 3 9% 6% 5% 4 4 6% 8% 34%	1762
	11th to 20th	Day	13% 11% 4 10% 7% 5% 4 4 6% 6% 29%	1785
	21st to end	Day	17% 14% 4 9% 7% 4 5% 5% 4 5% 26%	1600
December	1st to 10th	Day	14% 14% 6% 10% 6% 5% 5% 4 7% 7% 22%	2600
	11th to 20th	Day	17% 22% 5% 8% 5% 3 4 3 5% 5% 23%	2763
	21st to end	Day	24% 22% 7% 10% 5% 4 3 3 4 4 13%	3638
January	1st to 10th	Night	13% 12% 5% 11% 5% 4 4 3 5% 6% 31%	6348
	11th to 20th	Night	11% 15% 6% 14% 8% 7% 6% 4 5% 6% 18%	6360
	21st to end	Night	16% 16% 8% 9% 5% 5% 5% 3 5% 5% 23%	6517
February	1st to 10th	Night	14% 16% 5% 11% 7% 6% 5% 4 5% 5% 22%	2600
	11th to 20th	Night	13% 14% 5% 10% 6% 4 5% 4 5% 6% 28%	2434
	21st to end	Night	17% 11% 3 11% 6% 5% 4 3 4 6% 28%	1980
March	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 35%	5217
	11th to 20th	Night	9% 8% 3 8% 5% 4 5% 5% 7% 8% 38%	4980
	21st to end	Night	14% 10% 4 8% 4 5% 4 4 4 5% 38%	5467
April	1st to 10th	Night	18% 11% 3 7% 4 4 4 3 6% 6% 34%	2120
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 4 5% 7% 37%	1779
	21st to end	Night	16% 9% 2 7% 5% 4 4 4 5% 9% 36%	1800
May	1st to 10th	Night	18% 9% 4 7% 5% 4 4 4 5% 7% 33%	4033
	11th to 20th	Night	15% 9% 3 8% 5% 5% 4 3 4 6% 40%	4032
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 8% 44%	4298
June	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 7% 8% 46%	1415
	11th to 20th	Night	10% 6% 1 6% 4 3 5% 5% 8% 9% 43%	1400
	21st to end	Night	8% 4 2 6% 4 4 5% 3 5% 9% 49%	1400
July	1st to 10th	Night	7% 5% 2 5% 3 4 5% 3 7% 12% 47%	2534
	11th to 20th	Night	9% 4 2 4 4 3 5% 4 6% 8% 52%	2530
	21st to end	Night	5% 3 2 6% 5% 6% 6% 4 7% 10% 47%	3026
August	1st to 10th	Night	10% 6% 3 6% 4 4 4 3 6% 8% 47%	2825
	11th to 20th	Night	9% 6% 3 6% 4 4 4 4 6% 7% 47%	2785
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 8% 47%	3124
September	1st to 10th	Night	10% 6% 3 6% 4 5% 4 4 5% 7% 48%	2002
	11th to 20th	Night	9% 5% 2 5% 3 3 5% 4 5% 7% 54%	2090
	21st to end	Night	14% 6% 2 6% 5% 4 4 4 5% 6% 43%	2090
October	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 6% 8% 45%	2170
	11th to 20th	Night	19% 8% 2 7% 4 5% 4 3 6% 6% 35%	2280
	21st to end	Night	21% 9% 3 8% 5% 5% 5% 4 5% 6% 30%	2641
November	1st to 10th	Night	13% 9% 3 5% 4 4 4 3 5% 7% 42%	2536
	11th to 20th	Night	15% 11% 4 8% 5% 5% 5% 5% 6% 6% 29%	2613
	21st to end	Night	19% 14% 4 8% 6% 4 4 3 5% 7% 27%	2800
December	1st to 10th	Night	13% 14% 6% 11% 6% 5% 5% 4 6% 7% 24%	4580
	11th to 20th	Night	14% 18% 6% 11% 6% 5% 4 3 5% 5% 22%	5435
	21st to end	Night	22% 21% 6% 10% 5% 4 4 3 4 5% 16%	7322

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	59% 6% 10% 15% 7% 2	3618
	11th to 20th	Day	72% 5% 10% 6% 5%	3620
	21st to end	Day	68% 5% 12% 8% 6% 1	4438
February	1st to 10th	Day	64% 6% 10% 14% 7%	1800
	11th to 20th	Day	59% 6% 11% 14% 9% 1	1856
	21st to end	Day	60% 6% 11% 14% 7% 2	1650
March	1st to 10th	Day	54% 5% 13% 17% 9% 1	4636
	11th to 20th	Day	49% 6% 14% 21% 8% 2	4980
	21st to end	Day	57% 5% 12% 15% 9% 2	5465
April	1st to 10th	Day	54% 6% 11% 19% 10% 1	2297
	11th to 20th	Day	47% 6% 12% 19% 14% 2	2569
	21st to end	Day	49% 7% 13% 18% 12% 1	2600
May	1st to 10th	Day	48% 7% 12% 17% 13% 5%	5897
	11th to 20th	Day	46% 8% 11% 20% 13% 2	5865
	21st to end	Day	42% 7% 9% 19% 19% 5%	6555
June	1st to 10th	Day	38% 7% 13% 25% 15% 2	2980
	11th to 20th	Day	37% 8% 14% 24% 15% 2	3000
	21st to end	Day	38% 8% 14% 24% 15% 1	3000
July	1st to 10th	Day	40% 7% 15% 20% 15% 2	4459
	11th to 20th	Day	31% 7% 13% 34% 14% 1	4365
	21st to end	Day	37% 8% 15% 18% 20% 1	4372
August	1st to 10th	Day	39% 8% 14% 21% 16% 1	4091
	11th to 20th	Day	37% 9% 14% 28% 10% 1	4083
	21st to end	Day	38% 7% 13% 28% 12% 1	4531
September	1st to 10th	Day	39% 9% 14% 23% 13% 1	2155
	11th to 20th	Day	34% 6% 14% 31% 14% 1	2090
	21st to end	Day	45% 7% 14% 23% 10% 1	2090
October	1st to 10th	Day	34% 5% 11% 32% 17% 2	3334
	11th to 20th	Day	42% 5% 11% 24% 17%	3140
	21st to end	Day	47% 5% 14% 23% 11%	3191
November	1st to 10th	Day	48% 6% 12% 22% 10% 1	1762
	11th to 20th	Day	53% 7% 13% 17% 9%	1785
	21st to end	Day	60% 5% 13% 16% 5% 1	1600
December	1st to 10th	Day	59% 8% 18% 11% 4	2600
	11th to 20th	Day	63% 5% 11% 13% 8%	2759
	21st to end	Day	74% 5% 8% 8% 5%	3638
January	1st to 10th	Night	52% 6% 12% 20% 8% 3	6348
	11th to 20th	Night	65% 7% 11% 11% 6% 1	6360
	21st to end	Night	62% 5% 11% 15% 7%	6517
February	1st to 10th	Night	62% 7% 9% 14% 7% 1	2600
	11th to 20th	Night	56% 6% 10% 18% 9% 1	2436
	21st to end	Night	57% 5% 12% 17% 8% 1	1980
March	1st to 10th	Night	48% 5% 15% 21% 10% 1	5217
	11th to 20th	Night	40% 7% 17% 23% 12% 1	4980
	21st to end	Night	47% 6% 12% 20% 14% 1	5467
April	1st to 10th	Night	49% 6% 10% 21% 12% 2	2120
	11th to 20th	Night	44% 7% 11% 19% 18% 2	1779
	21st to end	Night	44% 7% 11% 19% 16% 3	1800
May	1st to 10th	Night	49% 6% 10% 17% 12% 5%	4060
	11th to 20th	Night	44% 7% 11% 23% 12% 2	4033
	21st to end	Night	37% 7% 11% 24% 16% 5%	4305
June	1st to 10th	Night	34% 8% 11% 27% 19% 2	1415
	11th to 20th	Night	34% 9% 14% 23% 18% 3	1400
	21st to end	Night	33% 6% 14% 27% 19% 1	1400
July	1st to 10th	Night	30% 5% 16% 27% 16% 6%	2494
	11th to 20th	Night	27% 7% 14% 34% 17% 1	2482
	21st to end	Night	29% 11% 18% 20% 21% 2	2929
August	1st to 10th	Night	35% 7% 14% 27% 15% 2	2814
	11th to 20th	Night	33% 7% 16% 30% 12% 2	2812
	21st to end	Night	34% 8% 13% 30% 14% 1	3124
September	1st to 10th	Night	36% 7% 13% 27% 15% 2	2002
	11th to 20th	Night	29% 7% 12% 31% 19% 1	2090
	21st to end	Night	41% 7% 11% 29% 12% 1	2090
October	1st to 10th	Night	26% 4 13% 30% 27% 1	3625
	11th to 20th	Night	44% 5% 10% 24% 16% 1	3779
	21st to end	Night	43% 5% 12% 27% 12% 1	4341
November	1st to 10th	Night	40% 7% 13% 25% 13% 1	2536
	11th to 20th	Night	52% 7% 14% 17% 9% 1	2613
	21st to end	Night	57% 5% 15% 17% 5% 1	2800
December	1st to 10th	Night	59% 6% 19% 13% 2	4580
	11th to 20th	Night	61% 6% 11% 13% 8% 1	5438
	21st to end	Night	71% 5% 10% 9% 5%	7322

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	20% 17% 41% 17% 5% 1	300
	11th to 20th	Day	20% 18% 43% 16% 3 1	300
	21st to end	Day	25% 19% 34% 17% 4 1	330
February	1st to 10th	Day	22% 21% 40% 14% 3 1	300
	11th to 20th	Day	24% 19% 37% 18% 2	300
	21st to end	Day	23% 24% 37% 15% 1	249
March	1st to 10th	Day	23% 20% 36% 16% 3 1	365
	11th to 20th	Day	21% 14% 35% 21% 8% 1	500
	21st to end	Day	26% 13% 33% 22% 6% 1	550
April	1st to 10th	Day	20% 18% 34% 20% 7% 1	500
	11th to 20th	Day	25% 15% 30% 17% 8% 5%	492
	21st to end	Day	20% 14% 30% 25% 9% 2	500
May	1st to 10th	Day	19% 14% 32% 22% 11% 2	503
	11th to 20th	Day	19% 8% 29% 23% 14% 6%	527
	21st to end	Day	14% 10% 26% 26% 17% 8%	573
June	1st to 10th	Day	11% 9% 33% 30% 12% 5%	710
	11th to 20th	Day	11% 8% 33% 30% 14% 4	730
	21st to end	Day	11% 8% 29% 32% 14% 5%	730
July	1st to 10th	Day	7% 6% 22% 32% 22% 11%	697
	11th to 20th	Day	10% 5% 28% 31% 17% 8%	668
	21st to end	Day	7% 4 26% 30% 24% 10%	568
August	1st to 10th	Day	9% 5% 19% 28% 25% 14%	486
	11th to 20th	Day	10% 5% 20% 29% 23% 14%	479
	21st to end	Day	12% 5% 21% 26% 24% 12%	539
September	1st to 10th	Day	10% 7% 26% 25% 19% 12%	490
	11th to 20th	Day	10% 4 19% 24% 28% 16%	490
	21st to end	Day	16% 11% 23% 24% 17% 8%	490
October	1st to 10th	Day	15% 10% 29% 24% 16% 7%	490
	11th to 20th	Day	17% 17% 34% 23% 8% 2	490
	21st to end	Day	25% 15% 35% 19% 6% 1	403
November	1st to 10th	Day	16% 20% 35% 21% 7% 1	300
	11th to 20th	Day	24% 19% 35% 14% 7% 1	293
	21st to end	Day	26% 19% 38% 13% 5%	200
December	1st to 10th	Day	21% 21% 36% 16% 6% 1	200
	11th to 20th	Day	25% 22% 36% 15% 3 1	200
	21st to end	Day	28% 20% 33% 16% 4	256
January	1st to 10th	Night	21% 15% 38% 20% 5% 1	1000
	11th to 20th	Night	22% 17% 35% 21% 4 1	1000
	21st to end	Night	20% 12% 36% 25% 6%	894
February	1st to 10th	Night	22% 14% 41% 21% 3	800
	11th to 20th	Night	21% 16% 33% 25% 4	783
	21st to end	Night	23% 16% 38% 20% 2	660
March	1st to 10th	Night	21% 13% 36% 25% 5%	800
	11th to 20th	Night	16% 14% 38% 25% 6% 1	800
	21st to end	Night	22% 12% 32% 28% 6% 1	880
April	1st to 10th	Night	23% 14% 35% 25% 3	720
	11th to 20th	Night	21% 11% 33% 24% 8% 2	593
	21st to end	Night	20% 8% 32% 33% 7%	600
May	1st to 10th	Night	18% 6% 33% 32% 10% 1	600
	11th to 20th	Night	18% 7% 27% 34% 13% 1	591
	21st to end	Night	13% 4 25% 36% 20% 1	604
June	1st to 10th	Night	13% 3 30% 41% 13%	400
	11th to 20th	Night	13% 6% 26% 42% 13%	400
	21st to end	Night	9% 5% 24% 46% 16%	400
July	1st to 10th	Night	7% 2 17% 43% 26% 4	547
	11th to 20th	Night	9% 3 19% 43% 22% 4	552
	21st to end	Night	6% 2 21% 45% 23% 2	624
August	1st to 10th	Night	8% 4 15% 42% 29% 3	567
	11th to 20th	Night	7% 2 18% 40% 30% 3	564
	21st to end	Night	11% 3 19% 37% 28% 4	627
September	1st to 10th	Night	11% 5% 21% 35% 23% 5%	699
	11th to 20th	Night	11% 3 19% 36% 26% 5%	760
	21st to end	Night	17% 6% 23% 33% 18% 4	760
October	1st to 10th	Night	16% 9% 27% 29% 18% 2	760
	11th to 20th	Night	21% 14% 29% 26% 8% 1	760
	21st to end	Night	24% 13% 32% 27% 5%	836
November	1st to 10th	Night	18% 11% 32% 26% 11% 1	780
	11th to 20th	Night	19% 15% 37% 22% 7% 1	800
	21st to end	Night	24% 14% 37% 20% 5%	800
December	1st to 10th	Night	23% 13% 39% 20% 5%	800
	11th to 20th	Night	23% 14% 39% 20% 3	905
	21st to end	Night	29% 13% 34% 21% 2	1100

Page generated at 2025-07-03 01:30:11. Version ac07

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