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 2026-10-29.

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	12% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	148859
	Night	13% 10% 4 8% 5% 5% 4 4 5% 7% 35%	143313
Transparency	Day	47% 6% 12% 21% 13% 1	152417
	Night	47% 6% 12% 21% 12% 1	147815
Seeing	Day	16% 12% 30% 24% 13% 5%	18009
	Night	18% 10% 31% 29% 11% 1	27480

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	11% 16% 7% 14% 7% 5% 4 3 4 4 25%	4691
	11th to 20th	Day	12% 18% 7% 16% 10% 8% 5% 3 4 4 13%	4710
	21st to end	Day	16% 15% 5% 11% 9% 6% 6% 3 4 6% 19%	5739
February	1st to 10th	Day	13% 14% 6% 14% 8% 5% 5% 3 5% 5% 23%	1890
	11th to 20th	Day	13% 14% 5% 11% 8% 5% 5% 5% 5% 6% 24%	1942
	21st to end	Day	13% 15% 5% 12% 7% 5% 4 4 4 5% 25%	1730
March	1st to 10th	Day	15% 12% 5% 10% 6% 4 4 3 4 6% 31%	4739
	11th to 20th	Day	12% 13% 5% 9% 6% 4 4 3 6% 8% 32%	5090
	21st to end	Day	18% 14% 4 10% 5% 5% 5% 3 5% 6% 27%	5586
April	1st to 10th	Day	16% 13% 4 8% 5% 4 4 4 5% 6% 29%	2412
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 5% 6% 35%	2699
	21st to end	Day	16% 8% 3 9% 5% 5% 4 3 6% 7% 34%	2730
May	1st to 10th	Day	15% 8% 3 9% 6% 5% 5% 4 5% 7% 34%	7616
	11th to 20th	Day	14% 8% 3 7% 4 5% 5% 4 6% 8% 35%	7626
	21st to end	Day	14% 6% 2 7% 4 4 4 3 5% 6% 44%	8526
June	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	3129
	11th to 20th	Day	10% 5% 2 7% 4 5% 5% 6% 6% 8% 41%	3150
	21st to end	Day	9% 7% 2 7% 5% 5% 5% 5% 8% 9% 39%	3150
July	1st to 10th	Day	9% 6% 3 7% 4 5% 5% 5% 7% 9% 39%	8324
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 7% 10% 50%	8280
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	8444
August	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	6014
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 5% 8% 11% 36%	5951
	21st to end	Day	8% 6% 3 8% 5% 6% 6% 5% 8% 10% 35%	6594
September	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 6% 6% 9% 40%	2381
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2310
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 35%	2310
October	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 5% 7% 43%	2221
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 3 5% 6% 35%	2100
	21st to end	Day	18% 11% 3 7% 5% 4 5% 4 6% 7% 31%	2133
November	1st to 10th	Day	13% 9% 3 9% 6% 5% 4 4 6% 8% 34%	1852
	11th to 20th	Day	13% 11% 4 11% 7% 5% 4 4 7% 6% 29%	1874
	21st to end	Day	16% 14% 5% 9% 7% 4 5% 4 5% 5% 26%	1680
December	1st to 10th	Day	14% 14% 6% 10% 6% 5% 5% 4 7% 7% 22%	2680
	11th to 20th	Day	16% 21% 5% 8% 5% 4 4 3 6% 5% 23%	2837
	21st to end	Day	24% 22% 7% 10% 5% 4 3 3 4 4 13%	3719
January	1st to 10th	Night	14% 14% 5% 11% 5% 5% 4 4 5% 6% 29%	8232
	11th to 20th	Night	9% 15% 7% 16% 8% 7% 6% 4 5% 6% 19%	8280
	21st to end	Night	13% 14% 8% 10% 6% 5% 6% 4 5% 5% 25%	8428
February	1st to 10th	Night	14% 15% 5% 11% 7% 6% 5% 4 5% 5% 24%	2730
	11th to 20th	Night	13% 14% 4 11% 6% 4 4 4 5% 6% 28%	2559
	21st to end	Night	17% 12% 4 11% 6% 5% 4 3 4 6% 28%	2076
March	1st to 10th	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 35%	5334
	11th to 20th	Night	9% 9% 3 8% 5% 4 5% 5% 7% 8% 37%	5090
	21st to end	Night	14% 10% 4 8% 4 5% 4 4 4 5% 37%	5588
April	1st to 10th	Night	18% 11% 3 7% 4 4 4 3 6% 6% 34%	2226
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 8% 36%	1869
	21st to end	Night	15% 9% 2 7% 4 4 4 4 5% 9% 36%	1890
May	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 5% 7% 35%	5237
	11th to 20th	Night	13% 8% 3 8% 5% 4 4 3 4 7% 40%	5234
	21st to end	Night	15% 6% 2 5% 3 4 4 3 6% 7% 44%	5610
June	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 7% 8% 47%	1486
	11th to 20th	Night	10% 6% 1 6% 4 3 4 5% 7% 9% 43%	1470
	21st to end	Night	8% 4 2 6% 4 5% 5% 3 5% 9% 49%	1470
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 3 7% 10% 46%	4645
	11th to 20th	Night	6% 4 2 5% 4 3 5% 4 7% 10% 53%	4707
	21st to end	Night	7% 4 2 6% 4 5% 5% 4 6% 8% 49%	5624
August	1st to 10th	Night	15% 6% 3 7% 4 3 3 3 5% 7% 43%	4135
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 8% 42%	4095
	21st to end	Night	7% 5% 2 8% 5% 5% 5% 4 6% 9% 43%	4532
September	1st to 10th	Night	10% 5% 2 6% 4 5% 4 4 5% 7% 48%	2214
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 5% 7% 54%	2310
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 5% 7% 43%	2310
October	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 6% 8% 46%	2398
	11th to 20th	Night	18% 8% 2 7% 4 5% 4 3 5% 5% 37%	2520
	21st to end	Night	20% 9% 3 8% 5% 4 5% 4 5% 6% 33%	2881
November	1st to 10th	Night	13% 9% 3 5% 4 4 4 3 5% 7% 42%	2666
	11th to 20th	Night	14% 11% 4 8% 5% 5% 5% 5% 6% 7% 30%	2744
	21st to end	Night	18% 14% 4 9% 6% 4 4 3 5% 6% 27%	2940
December	1st to 10th	Night	13% 14% 6% 11% 6% 5% 5% 4 5% 7% 24%	4720
	11th to 20th	Night	14% 18% 6% 11% 6% 5% 4 4 5% 6% 23%	5580
	21st to end	Night	22% 21% 6% 10% 5% 4 4 3 4 5% 16%	7483

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	61% 6% 10% 15% 6% 1	4692
	11th to 20th	Day	73% 5% 9% 7% 6%	4710
	21st to end	Day	67% 4 10% 10% 8% 1	5739
February	1st to 10th	Day	63% 6% 9% 15% 7%	1890
	11th to 20th	Day	59% 6% 11% 15% 9% 1	1951
	21st to end	Day	60% 6% 11% 14% 7% 2	1730
March	1st to 10th	Day	54% 5% 13% 18% 9% 1	4739
	11th to 20th	Day	50% 6% 14% 20% 8% 2	5090
	21st to end	Day	57% 5% 12% 15% 9% 2	5586
April	1st to 10th	Day	54% 6% 11% 18% 10% 1	2412
	11th to 20th	Day	48% 6% 11% 19% 14% 2	2699
	21st to end	Day	49% 7% 13% 18% 12% 1	2730
May	1st to 10th	Day	47% 7% 12% 20% 12% 3	7651
	11th to 20th	Day	45% 7% 11% 21% 14% 2	7630
	21st to end	Day	39% 6% 8% 21% 22% 4	8541
June	1st to 10th	Day	37% 7% 13% 25% 16% 2	3129
	11th to 20th	Day	37% 8% 14% 24% 15% 2	3150
	21st to end	Day	38% 8% 14% 24% 15% 1	3150
July	1st to 10th	Day	38% 7% 13% 24% 16% 1	8255
	11th to 20th	Day	28% 6% 12% 33% 19% 1	8205
	21st to end	Day	32% 7% 14% 24% 22% 1	8259
August	1st to 10th	Day	41% 7% 14% 18% 18% 2	6001
	11th to 20th	Day	38% 8% 16% 29% 8% 1	5990
	21st to end	Day	40% 8% 16% 27% 9%	6593
September	1st to 10th	Day	39% 9% 14% 25% 12% 1	2381
	11th to 20th	Day	35% 6% 14% 30% 14% 1	2310
	21st to end	Day	45% 6% 13% 23% 11% 1	2310
October	1st to 10th	Day	32% 5% 11% 33% 17% 2	3546
	11th to 20th	Day	41% 5% 11% 24% 18% 1	3340
	21st to end	Day	46% 5% 13% 22% 12% 1	3370
November	1st to 10th	Day	48% 6% 12% 23% 10% 1	1852
	11th to 20th	Day	53% 7% 14% 17% 9%	1874
	21st to end	Day	59% 5% 12% 17% 5% 1	1680
December	1st to 10th	Day	59% 8% 18% 11% 4	2680
	11th to 20th	Day	63% 5% 11% 13% 8%	2833
	21st to end	Day	74% 5% 8% 8% 5%	3719
January	1st to 10th	Night	55% 6% 11% 18% 8% 2	8233
	11th to 20th	Night	65% 6% 10% 11% 8% 1	8280
	21st to end	Night	60% 5% 10% 18% 8%	8428
February	1st to 10th	Night	61% 6% 9% 15% 7% 1	2730
	11th to 20th	Night	56% 6% 10% 18% 9% 1	2561
	21st to end	Night	57% 5% 12% 17% 8% 1	2076
March	1st to 10th	Night	48% 5% 15% 21% 10% 1	5334
	11th to 20th	Night	41% 7% 17% 23% 12% 1	5090
	21st to end	Night	47% 6% 12% 20% 14% 1	5588
April	1st to 10th	Night	50% 5% 10% 21% 12% 2	2226
	11th to 20th	Night	45% 6% 11% 18% 18% 2	1869
	21st to end	Night	44% 7% 11% 20% 15% 3	1890
May	1st to 10th	Night	47% 6% 10% 18% 14% 4	5264
	11th to 20th	Night	43% 6% 11% 24% 14% 2	5235
	21st to end	Night	38% 6% 9% 23% 18% 5%	5619
June	1st to 10th	Night	33% 8% 10% 26% 21% 2	1486
	11th to 20th	Night	34% 8% 14% 23% 18% 3	1470
	21st to end	Night	33% 6% 13% 27% 19% 1	1470
July	1st to 10th	Night	32% 6% 15% 26% 17% 3	4607
	11th to 20th	Night	25% 6% 12% 36% 20% 1	4662
	21st to end	Night	31% 7% 13% 22% 25% 1	5496
August	1st to 10th	Night	40% 6% 12% 23% 17% 2	4124
	11th to 20th	Night	36% 7% 19% 28% 9% 1	4122
	21st to end	Night	36% 8% 16% 29% 11% 1	4532
September	1st to 10th	Night	36% 7% 13% 28% 15% 2	2214
	11th to 20th	Night	29% 7% 12% 32% 19% 1	2310
	21st to end	Night	40% 6% 11% 28% 13% 1	2310
October	1st to 10th	Night	25% 4 12% 31% 27% 1	3853
	11th to 20th	Night	44% 5% 10% 24% 17% 1	4019
	21st to end	Night	43% 5% 12% 27% 13% 1	4581
November	1st to 10th	Night	41% 7% 13% 25% 13% 1	2666
	11th to 20th	Night	51% 7% 14% 18% 9% 1	2744
	21st to end	Night	58% 5% 15% 17% 5% 1	2940
December	1st to 10th	Night	59% 6% 19% 14% 3	4720
	11th to 20th	Night	61% 6% 11% 13% 8% 1	5583
	21st to end	Night	71% 5% 10% 9% 5%	7483

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% 18% 41% 16% 4 1	330
	11th to 20th	Day	21% 20% 41% 15% 3 1	330
	21st to end	Day	24% 21% 33% 17% 4 1	363
February	1st to 10th	Day	21% 22% 39% 15% 3 1	330
	11th to 20th	Day	23% 18% 38% 19% 2	330
	21st to end	Day	21% 24% 40% 14% 1	273
March	1st to 10th	Day	24% 20% 36% 16% 3 1	398
	11th to 20th	Day	23% 14% 34% 20% 7% 1	540
	21st to end	Day	26% 13% 34% 21% 6% 1	594
April	1st to 10th	Day	21% 18% 34% 19% 7% 1	540
	11th to 20th	Day	25% 16% 29% 17% 9% 5%	532
	21st to end	Day	19% 14% 30% 26% 9% 2	540
May	1st to 10th	Day	18% 14% 33% 22% 10% 2	543
	11th to 20th	Day	19% 10% 30% 23% 13% 6%	567
	21st to end	Day	14% 9% 27% 26% 17% 7%	617
June	1st to 10th	Day	10% 10% 32% 30% 13% 5%	759
	11th to 20th	Day	12% 9% 34% 28% 13% 4	780
	21st to end	Day	11% 8% 30% 33% 14% 5%	780
July	1st to 10th	Day	8% 6% 23% 32% 21% 11%	771
	11th to 20th	Day	10% 5% 27% 32% 18% 8%	768
	21st to end	Day	6% 4 26% 30% 23% 10%	660
August	1st to 10th	Day	9% 4 18% 30% 25% 13%	566
	11th to 20th	Day	10% 4 20% 31% 23% 13%	559
	21st to end	Day	12% 5% 21% 26% 24% 11%	627
September	1st to 10th	Day	10% 7% 26% 25% 19% 12%	570
	11th to 20th	Day	10% 4 18% 24% 27% 16%	570
	21st to end	Day	15% 11% 22% 26% 19% 8%	570
October	1st to 10th	Day	14% 9% 31% 23% 16% 7%	570
	11th to 20th	Day	16% 15% 35% 23% 8% 2	570
	21st to end	Day	23% 14% 34% 20% 8% 1	468
November	1st to 10th	Day	17% 20% 34% 22% 7% 1	330
	11th to 20th	Day	23% 20% 35% 13% 7% 1	322
	21st to end	Day	25% 20% 37% 12% 5%	220
December	1st to 10th	Day	20% 21% 35% 15% 7% 1	220
	11th to 20th	Day	25% 23% 34% 15% 3	220
	21st to end	Day	29% 19% 33% 15% 4	282
January	1st to 10th	Night	21% 16% 38% 20% 5% 1	1050
	11th to 20th	Night	22% 18% 36% 21% 3 1	1050
	21st to end	Night	19% 13% 37% 25% 6%	938
February	1st to 10th	Night	21% 14% 40% 22% 3	840
	11th to 20th	Night	21% 16% 34% 25% 4	823
	21st to end	Night	23% 17% 38% 21% 2	692
March	1st to 10th	Night	20% 13% 36% 25% 5%	840
	11th to 20th	Night	17% 15% 38% 25% 5% 1	840
	21st to end	Night	22% 13% 32% 27% 6% 1	924
April	1st to 10th	Night	23% 15% 35% 24% 3	756
	11th to 20th	Night	21% 12% 33% 24% 8% 2	623
	21st to end	Night	20% 7% 31% 34% 7%	630
May	1st to 10th	Night	17% 6% 34% 33% 10% 1	630
	11th to 20th	Night	18% 6% 29% 33% 13% 1	621
	21st to end	Night	13% 5% 25% 37% 19% 1	635
June	1st to 10th	Night	13% 3 29% 42% 13%	420
	11th to 20th	Night	13% 6% 27% 41% 13%	420
	21st to end	Night	9% 5% 24% 47% 16%	420
July	1st to 10th	Night	7% 3 16% 44% 27% 4	591
	11th to 20th	Night	8% 3 19% 44% 22% 4	612
	21st to end	Night	7% 3 20% 46% 22% 2	690
August	1st to 10th	Night	8% 4 15% 42% 29% 3	627
	11th to 20th	Night	8% 2 17% 41% 30% 3	624
	21st to end	Night	10% 3 18% 39% 26% 4	693
September	1st to 10th	Night	11% 5% 22% 35% 23% 4	773
	11th to 20th	Night	10% 4 18% 36% 27% 5%	840
	21st to end	Night	16% 5% 22% 34% 18% 4	840
October	1st to 10th	Night	15% 8% 27% 29% 18% 2	840
	11th to 20th	Night	21% 13% 30% 27% 8% 1	840
	21st to end	Night	22% 12% 31% 29% 5%	913
November	1st to 10th	Night	18% 12% 32% 26% 11% 1	820
	11th to 20th	Night	18% 16% 36% 22% 6% 1	840
	21st to end	Night	25% 14% 37% 19% 5%	840
December	1st to 10th	Night	22% 13% 39% 20% 5%	840
	11th to 20th	Night	23% 15% 40% 20% 2	950
	21st to end	Night	29% 14% 35% 21% 2	1155

Page generated at 2026-10-26 04:37:48. Version ac07

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