Blue Moon Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Moon Astronomical Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	10% 7% 2 6% 4 4 4 4 5% 7% 46%	132516
	Night	10% 6% 2 5% 3 3 4 3 5% 7% 51%	130940
Transparency	Day	35% 7% 10% 23% 19% 5%	136128
	Night	31% 6% 10% 23% 22% 8%	135405
Seeing	Day	15% 13% 27% 26% 15% 4	17414
	Night	15% 11% 26% 30% 16% 2	27288

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% 11% 4 6% 4 4 4 3 4 6% 45%	5289
	11th to 20th	Day	6% 7% 2 6% 4 3 3 2 4 5% 59%	5310
	21st to end	Day	19% 10% 2 5% 2 2 3 3 4 4 46%	5812
February	1st to 10th	Day	17% 10% 3 6% 5% 4 3 3 4 4 41%	1890
	11th to 20th	Day	14% 9% 3 7% 5% 4 5% 3 4 5% 40%	1918
	21st to end	Day	16% 10% 4 6% 4 3 3 3 4 6% 42%	1891
March	1st to 10th	Day	15% 8% 2 5% 3 4 4 3 5% 6% 45%	5046
	11th to 20th	Day	9% 6% 2 5% 4 4 4 3 6% 6% 51%	5090
	21st to end	Day	13% 8% 3 7% 5% 5% 4 3 5% 7% 40%	5583
April	1st to 10th	Day	12% 5% 3 7% 6% 4 4 3 6% 7% 43%	2389
	11th to 20th	Day	14% 7% 3 6% 5% 4 4 3 5% 6% 43%	2494
	21st to end	Day	15% 8% 3 8% 6% 5% 4 3 4 6% 38%	2520
May	1st to 10th	Day	14% 9% 3 8% 5% 4 5% 3 5% 6% 38%	5939
	11th to 20th	Day	12% 6% 3 8% 5% 5% 5% 4 6% 7% 40%	5992
	21st to end	Day	14% 8% 3 9% 6% 5% 5% 4 6% 7% 35%	6577
June	1st to 10th	Day	8% 6% 3 6% 5% 5% 6% 5% 7% 9% 42%	2601
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 5% 7% 8% 43%	2600
	21st to end	Day	3 4 1 4 5% 4 6% 5% 8% 9% 51%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 9% 38%	5900
	11th to 20th	Day	4 4 2 4 4 4 4 4 6% 8% 57%	5900
	21st to end	Day	5% 4 2 5% 5% 6% 7% 6% 9% 10% 42%	6422
August	1st to 10th	Day	3 3 1 4 4 4 5% 5% 6% 8% 56%	4234
	11th to 20th	Day	8% 4 2 5% 4 4 6% 5% 7% 10% 45%	4105
	21st to end	Day	5% 3 2 5% 5% 5% 7% 5% 7% 8% 47%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 5% 6% 49%	2375
	11th to 20th	Day	12% 4 2 5% 4 4 5% 4 7% 9% 44%	2200
	21st to end	Day	8% 4 2 5% 4 3 4 4 6% 7% 53%	2197
October	1st to 10th	Day	6% 4 1 4 4 3 4 4 4 7% 58%	2000
	11th to 20th	Day	12% 4 2 4 4 3 3 2 3 4 59%	2000
	21st to end	Day	18% 6% 2 4 3 3 3 3 4 5% 50%	2200
November	1st to 10th	Day	9% 5% 2 5% 4 3 3 3 5% 7% 54%	2058
	11th to 20th	Day	8% 6% 2 4 4 4 3 3 4 6% 56%	2084
	21st to end	Day	13% 8% 2 6% 4 3 4 3 4 5% 49%	1890
December	1st to 10th	Day	14% 6% 2 6% 4 3 3 2 4 6% 51%	3030
	11th to 20th	Day	9% 8% 3 6% 4 5% 5% 4 6% 6% 44%	3481
	21st to end	Day	13% 11% 4 8% 4 4 4 3 4 5% 38%	4587
January	1st to 10th	Night	10% 9% 3 8% 4 3 3 3 4 5% 47%	7634
	11th to 20th	Night	6% 5% 2 5% 3 3 2 2 3 5% 65%	7680
	21st to end	Night	17% 11% 4 7% 3 3 3 2 4 5% 42%	8402
February	1st to 10th	Night	15% 9% 3 6% 3 3 3 3 5% 6% 44%	2730
	11th to 20th	Night	14% 9% 3 7% 4 3 3 3 5% 6% 43%	2606
	21st to end	Night	12% 8% 3 5% 4 3 4 3 4 6% 49%	1913
March	1st to 10th	Night	18% 9% 2 6% 3 3 3 2 4 5% 45%	5029
	11th to 20th	Night	10% 7% 2 4 3 3 3 3 5% 6% 55%	5090
	21st to end	Night	10% 8% 3 6% 4 4 4 3 6% 7% 44%	5588
April	1st to 10th	Night	11% 6% 2 5% 5% 4 4 3 6% 7% 48%	2231
	11th to 20th	Night	11% 7% 2 5% 4 4 3 3 6% 6% 49%	2075
	21st to end	Night	14% 6% 2 4 3 4 3 3 5% 8% 47%	2100
May	1st to 10th	Night	11% 7% 3 7% 5% 5% 4 4 6% 7% 39%	4189
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 4 7% 10% 42%	4143
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 8% 42%	4542
June	1st to 10th	Night	5% 4 2 5% 3 3 4 4 7% 9% 55%	1800
	11th to 20th	Night	6% 4 2 4 3 2 4 4 5% 9% 58%	1800
	21st to end	Night	3 3 1 3 2 3 3 3 6% 10% 64%	1800
July	1st to 10th	Night	6% 4 1 5% 4 4 5% 5% 8% 12% 45%	4078
	11th to 20th	Night	4 2 1 3 2 2 3 2 5% 8% 66%	4077
	21st to end	Night	3 3 1 5% 3 5% 5% 5% 7% 10% 54%	4449
August	1st to 10th	Night	2 2 1 4 3 3 4 3 5% 9% 65%	2926
	11th to 20th	Night	4 3 2 4 3 3 4 3 6% 8% 60%	2951
	21st to end	Night	3 3 1 5% 3 4 4 3 6% 9% 60%	3586
September	1st to 10th	Night	7% 4 1 5% 3 4 4 3 5% 7% 57%	2020
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 6% 9% 51%	2200
	21st to end	Night	8% 4 1 4 3 3 4 2 5% 6% 59%	2200
October	1st to 10th	Night	6% 3 1 4 2 2 3 3 5% 8% 62%	2381
	11th to 20th	Night	10% 4 2 4 3 3 2 2 4 5% 61%	2400
	21st to end	Night	17% 6% 2 5% 2 2 3 2 3 4 54%	2640
November	1st to 10th	Night	10% 5% 1 5% 3 2 3 3 4 5% 58%	2460
	11th to 20th	Night	7% 6% 2 4 3 2 2 3 5% 7% 58%	2531
	21st to end	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	2730
December	1st to 10th	Night	11% 7% 3 5% 3 4 3 3 4 6% 52%	4380
	11th to 20th	Night	9% 7% 3 8% 4 5% 4 3 5% 5% 48%	4946
	21st to end	Night	11% 11% 5% 8% 4 4 4 3 4 5% 42%	6633

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	40% 6% 9% 24% 20% 2	5290
	11th to 20th	Day	29% 4 10% 19% 32% 7%	5310
	21st to end	Day	42% 3 3 19% 24% 8%	5812
February	1st to 10th	Day	46% 5% 10% 17% 19% 4	1890
	11th to 20th	Day	47% 4 7% 19% 19% 4	1927
	21st to end	Day	45% 3 7% 15% 21% 9%	1891
March	1st to 10th	Day	39% 6% 8% 15% 17% 15%	5046
	11th to 20th	Day	32% 7% 9% 16% 20% 17%	5090
	21st to end	Day	42% 7% 10% 24% 16% 1	5585
April	1st to 10th	Day	40% 6% 11% 20% 19% 5%	2389
	11th to 20th	Day	42% 6% 10% 20% 18% 5%	2494
	21st to end	Day	47% 7% 9% 18% 17% 2	2520
May	1st to 10th	Day	44% 9% 10% 23% 13% 1	5977
	11th to 20th	Day	40% 9% 11% 19% 18% 2	5996
	21st to end	Day	46% 10% 11% 16% 15% 3	6590
June	1st to 10th	Day	36% 9% 14% 24% 15% 2	2601
	11th to 20th	Day	36% 9% 14% 21% 18% 2	2600
	21st to end	Day	25% 9% 16% 29% 18% 3	2600
July	1st to 10th	Day	37% 9% 17% 27% 9% 1	5836
	11th to 20th	Day	24% 7% 12% 38% 18% 1	5831
	21st to end	Day	29% 12% 16% 30% 12% 1	6240
August	1st to 10th	Day	22% 7% 13% 29% 25% 5%	4220
	11th to 20th	Day	32% 8% 16% 33% 11% 1	4144
	21st to end	Day	30% 9% 13% 32% 14% 2	4311
September	1st to 10th	Day	34% 8% 10% 22% 18% 9%	2375
	11th to 20th	Day	34% 8% 11% 24% 18% 6%	2200
	21st to end	Day	30% 6% 10% 19% 27% 7%	2197
October	1st to 10th	Day	16% 4 7% 12% 33% 29%	3260
	11th to 20th	Day	20% 3 4 18% 37% 18%	3240
	21st to end	Day	28% 6% 8% 28% 23% 8%	3538
November	1st to 10th	Day	30% 5% 10% 23% 25% 8%	2058
	11th to 20th	Day	30% 5% 8% 23% 25% 10%	2084
	21st to end	Day	37% 6% 7% 19% 26% 6%	1890
December	1st to 10th	Day	36% 4 8% 28% 21% 4	3030
	11th to 20th	Day	38% 6% 7% 21% 25% 4	3479
	21st to end	Day	45% 8% 8% 20% 16% 3	4587
January	1st to 10th	Night	37% 5% 8% 27% 20% 2	7635
	11th to 20th	Night	23% 5% 7% 17% 36% 12%	7680
	21st to end	Night	44% 4 6% 15% 22% 8%	8402
February	1st to 10th	Night	41% 5% 9% 22% 18% 6%	2730
	11th to 20th	Night	42% 5% 7% 21% 19% 6%	2607
	21st to end	Night	37% 4 9% 18% 21% 11%	1913
March	1st to 10th	Night	41% 6% 6% 14% 20% 13%	5029
	11th to 20th	Night	31% 5% 7% 21% 19% 18%	5090
	21st to end	Night	36% 8% 11% 24% 18% 3	5588
April	1st to 10th	Night	35% 7% 10% 20% 19% 9%	2231
	11th to 20th	Night	35% 6% 10% 20% 20% 9%	2075
	21st to end	Night	35% 7% 12% 23% 22% 2	2100
May	1st to 10th	Night	38% 12% 12% 24% 12% 3	4216
	11th to 20th	Night	34% 10% 14% 24% 16% 2	4144
	21st to end	Night	38% 9% 15% 20% 13% 5%	4551
June	1st to 10th	Night	24% 8% 16% 29% 20% 3	1800
	11th to 20th	Night	23% 6% 13% 33% 22% 3	1800
	21st to end	Night	16% 7% 15% 30% 27% 5%	1800
July	1st to 10th	Night	28% 10% 18% 30% 12% 2	4035
	11th to 20th	Night	17% 6% 13% 38% 26% 1	4026
	21st to end	Night	21% 10% 18% 33% 17% 1	4320
August	1st to 10th	Night	15% 7% 14% 30% 28% 7%	2916
	11th to 20th	Night	20% 7% 20% 35% 17% 2	2978
	21st to end	Night	21% 7% 15% 32% 23% 2	3586
September	1st to 10th	Night	26% 7% 9% 26% 23% 10%	2020
	11th to 20th	Night	29% 8% 12% 22% 22% 7%	2200
	21st to end	Night	26% 4 9% 23% 29% 9%	2200
October	1st to 10th	Night	13% 3 7% 14% 28% 35%	3901
	11th to 20th	Night	18% 4 5% 15% 37% 22%	3899
	21st to end	Night	27% 4 6% 26% 26% 11%	4252
November	1st to 10th	Night	29% 5% 8% 21% 27% 11%	2460
	11th to 20th	Night	26% 4 9% 23% 27% 11%	2531
	21st to end	Night	35% 5% 8% 19% 26% 7%	2730
December	1st to 10th	Night	33% 5% 8% 29% 21% 4	4380
	11th to 20th	Night	37% 6% 6% 20% 22% 9%	4947
	21st to end	Night	40% 9% 8% 20% 19% 4	6633

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	25% 22% 35% 15% 3	330
	11th to 20th	Day	23% 28% 36% 11% 2 1	330
	21st to end	Day	25% 28% 32% 10% 5%	363
February	1st to 10th	Day	29% 22% 32% 15% 2	330
	11th to 20th	Day	30% 30% 30% 8% 2	330
	21st to end	Day	34% 26% 29% 8% 2	273
March	1st to 10th	Day	26% 27% 29% 17% 2	330
	11th to 20th	Day	32% 25% 32% 9% 2	330
	21st to end	Day	27% 23% 38% 10% 2	363
April	1st to 10th	Day	26% 23% 32% 12% 6%	409
	11th to 20th	Day	26% 20% 35% 14% 5%	532
	21st to end	Day	21% 23% 31% 19% 5% 1	540
May	1st to 10th	Day	17% 21% 37% 21% 4 1	726
	11th to 20th	Day	17% 15% 35% 23% 9% 2	775
	21st to end	Day	12% 11% 40% 22% 11% 4	844
June	1st to 10th	Day	10% 6% 24% 35% 19% 5%	731
	11th to 20th	Day	6% 6% 29% 36% 20% 4	730
	21st to end	Day	3 5% 27% 37% 22% 6%	730
July	1st to 10th	Day	7% 2 19% 37% 28% 8%	721
	11th to 20th	Day	4 1 18% 39% 30% 8%	718
	21st to end	Day	4 2 13% 36% 34% 11%	794
August	1st to 10th	Day	6% 2 14% 43% 31% 5%	726
	11th to 20th	Day	11% 1 13% 37% 30% 8%	675
	21st to end	Day	9% 2 10% 31% 35% 13%	583
September	1st to 10th	Day	10% 3 11% 36% 33% 8%	505
	11th to 20th	Day	5% 3 22% 38% 26% 6%	330
	21st to end	Day	11% 8% 22% 33% 20% 5%	330
October	1st to 10th	Day	4 8% 32% 35% 18% 2	330
	11th to 20th	Day	13% 15% 36% 26% 9% 1	330
	21st to end	Day	31% 12% 32% 20% 5% 1	363
November	1st to 10th	Day	12% 22% 36% 22% 8%	330
	11th to 20th	Day	17% 17% 40% 20% 4 2	330
	21st to end	Day	24% 25% 27% 18% 5%	330
December	1st to 10th	Day	25% 22% 29% 18% 5% 1	330
	11th to 20th	Day	24% 23% 32% 15% 5%	330
	21st to end	Day	24% 21% 37% 15% 3	363
January	1st to 10th	Night	19% 16% 31% 29% 5%	1050
	11th to 20th	Night	19% 21% 32% 24% 4	1050
	21st to end	Night	18% 19% 32% 27% 4	1155
February	1st to 10th	Night	23% 16% 31% 25% 5%	1050
	11th to 20th	Night	23% 21% 33% 21% 1	972
	21st to end	Night	24% 15% 35% 22% 3	529
March	1st to 10th	Night	24% 13% 31% 28% 5%	630
	11th to 20th	Night	22% 15% 32% 25% 5%	630
	21st to end	Night	18% 16% 37% 23% 5%	693
April	1st to 10th	Night	17% 20% 31% 27% 6%	630
	11th to 20th	Night	16% 19% 32% 23% 9%	623
	21st to end	Night	16% 16% 34% 25% 8% 1	630
May	1st to 10th	Night	11% 12% 31% 37% 9%	630
	11th to 20th	Night	14% 12% 28% 31% 15% 1	621
	21st to end	Night	10% 7% 31% 29% 22% 1	683
June	1st to 10th	Night	7% 4 18% 35% 35% 2	600
	11th to 20th	Night	6% 2 25% 40% 26% 2	600
	21st to end	Night	3 1 20% 35% 36% 5%	600
July	1st to 10th	Night	4 1 12% 36% 42% 5%	585
	11th to 20th	Night	4 1 10% 35% 43% 6%	582
	21st to end	Night	3 1 8% 39% 42% 7%	657
August	1st to 10th	Night	2 1 11% 39% 41% 5%	597
	11th to 20th	Night	7% 1 9% 36% 42% 5%	594
	21st to end	Night	6% 1 9% 38% 39% 7%	660
September	1st to 10th	Night	8% 2 9% 41% 37% 3	600
	11th to 20th	Night	13% 2 11% 33% 37% 4	600
	21st to end	Night	10% 2 17% 35% 33% 5%	600
October	1st to 10th	Night	8% 2 21% 37% 29% 3	781
	11th to 20th	Night	13% 9% 29% 35% 13% 1	800
	21st to end	Night	22% 11% 25% 27% 14% 1	880
November	1st to 10th	Night	13% 10% 31% 34% 11%	820
	11th to 20th	Night	13% 14% 31% 33% 10%	851
	21st to end	Night	19% 16% 29% 27% 8%	1050
December	1st to 10th	Night	19% 13% 32% 30% 6%	1050
	11th to 20th	Night	18% 15% 32% 30% 5%	1050
	21st to end	Night	19% 19% 30% 27% 6%	1155

Page generated at 2026-05-29 00:53:25. Version ac07

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