Blue Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Star Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	11% 6% 2 5% 4 4 4 3 5% 7% 48%	132284
	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	130845
Transparency	Day	34% 6% 10% 23% 20% 7%	135962
	Night	31% 5% 10% 21% 22% 11%	135231
Seeing	Day	12% 7% 21% 27% 24% 9%	17046
	Night	13% 7% 26% 35% 17% 2	26991

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	13% 6% 2 5% 3 3 3 3 5% 7% 49%	5289
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 5% 6% 57%	5310
	21st to end	Day	25% 9% 3 8% 4 2 2 2 3 4 37%	5812
February	1st to 10th	Day	20% 8% 4 6% 3 2 3 3 4 6% 42%	1985
	11th to 20th	Day	19% 8% 2 7% 4 2 4 2 4 6% 41%	2049
	21st to end	Day	19% 6% 2 5% 3 2 2 2 4 5% 49%	1824
March	1st to 10th	Day	16% 9% 2 6% 4 3 3 3 5% 6% 43%	5046
	11th to 20th	Day	13% 6% 1 6% 4 4 4 2 4 5% 52%	5090
	21st to end	Day	15% 8% 3 5% 4 3 3 3 5% 6% 45%	5583
April	1st to 10th	Day	14% 8% 2 5% 5% 5% 3 4 5% 7% 43%	2310
	11th to 20th	Day	14% 7% 3 6% 4 5% 4 3 6% 8% 40%	2428
	21st to end	Day	13% 6% 3 6% 5% 3 4 5% 5% 7% 44%	2551
May	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 5% 7% 36%	5992
	11th to 20th	Day	12% 7% 3 8% 5% 5% 4 3 5% 6% 43%	5941
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	6434
June	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 6% 11% 49%	2600
	11th to 20th	Day	5% 4 1 4 5% 5% 5% 4 7% 11% 51%	2600
	21st to end	Day	4 3 1 4 4 5% 5% 5% 7% 11% 50%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	5900
	11th to 20th	Day	3 3 1 4 3 3 4 4 6% 11% 58%	5900
	21st to end	Day	6% 6% 2 6% 4 4 5% 4 6% 9% 48%	6422
August	1st to 10th	Day	2 1 1 2 2 2 3 2 5% 8% 72%	4234
	11th to 20th	Day	3 2 1 3 2 3 3 3 6% 10% 63%	4171
	21st to end	Day	4 2 1 4 4 5% 5% 5% 8% 11% 50%	4189
September	1st to 10th	Day	9% 4 3 5% 4 4 4 4 6% 7% 50%	2200
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 5% 6% 9% 46%	2200
	21st to end	Day	6% 4 1 4 3 4 4 4 5% 8% 57%	2200
October	1st to 10th	Day	5% 4 1 4 3 3 3 3 5% 7% 63%	2110
	11th to 20th	Day	11% 4 2 4 3 2 3 2 4 6% 60%	2000
	21st to end	Day	13% 5% 1 5% 2 2 2 2 4 4 59%	2200
November	1st to 10th	Day	12% 5% 1 6% 4 2 3 2 4 5% 54%	2058
	11th to 20th	Day	9% 6% 2 5% 3 2 3 3 4 6% 55%	2068
	21st to end	Day	17% 7% 2 5% 4 4 3 3 4 5% 47%	1890
December	1st to 10th	Day	20% 6% 2 4 3 3 3 3 5% 5% 48%	3030
	11th to 20th	Day	18% 8% 3 6% 4 4 3 3 5% 6% 40%	3481
	21st to end	Day	15% 9% 3 7% 4 3 3 2 4 5% 46%	4587
January	1st to 10th	Night	14% 5% 2 5% 3 3 3 2 4 5% 54%	7634
	11th to 20th	Night	9% 4 1 3 2 2 2 2 4 7% 63%	7680
	21st to end	Night	26% 7% 3 7% 3 3 3 3 4 5% 36%	8402
February	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 5% 48%	2653
	11th to 20th	Night	19% 6% 3 5% 4 3 3 2 4 5% 45%	2458
	21st to end	Night	18% 6% 2 5% 3 3 3 3 3 4 50%	1982
March	1st to 10th	Night	19% 7% 2 6% 4 4 3 3 4 6% 42%	5029
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 6% 52%	5090
	21st to end	Night	14% 6% 3 6% 3 3 3 2 4 6% 49%	5588
April	1st to 10th	Night	12% 4 2 4 3 4 4 4 6% 7% 51%	2310
	11th to 20th	Night	12% 5% 2 4 4 5% 3 4 6% 6% 48%	2138
	21st to end	Night	11% 5% 2 5% 4 4 3 3 5% 9% 50%	2063
May	1st to 10th	Night	16% 8% 3 7% 4 3 4 3 6% 7% 38%	4143
	11th to 20th	Night	10% 6% 3 6% 5% 4 5% 4 6% 7% 45%	4107
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 46%	4443
June	1st to 10th	Night	5% 4 2 4 3 2 4 4 5% 9% 59%	1800
	11th to 20th	Night	3 2 1 4 3 3 3 3 5% 10% 65%	1800
	21st to end	Night	4 3 2 4 3 3 4 5% 7% 9% 58%	1800
July	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 4 8% 11% 44%	4078
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 11% 62%	4077
	21st to end	Night	4 4 2 5% 4 4 5% 4 7% 9% 53%	4449
August	1st to 10th	Night	1 1 1 1 1 2 2 4 7% 79%	2926
	11th to 20th	Night	3 2 1 3 2 2 2 3 6% 12% 62%	2888
	21st to end	Night	2 3 1 4 3 3 4 4 8% 11% 57%	3702
September	1st to 10th	Night	7% 4 2 5% 4 3 3 3 6% 8% 55%	2200
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 7% 55%	2200
	21st to end	Night	6% 3 1 4 3 3 4 3 5% 8% 61%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 3 4 7% 68%	2270
	11th to 20th	Night	11% 4 1 3 2 2 3 2 3 6% 63%	2400
	21st to end	Night	13% 4 2 4 2 2 2 2 3 4 61%	2640
November	1st to 10th	Night	11% 4 1 4 3 3 2 2 4 5% 59%	2460
	11th to 20th	Night	7% 5% 2 5% 4 4 3 2 4 5% 59%	2546
	21st to end	Night	15% 6% 2 5% 3 3 3 3 4 5% 50%	2730
December	1st to 10th	Night	19% 6% 2 5% 3 3 2 2 3 5% 49%	4380
	11th to 20th	Night	18% 8% 2 6% 4 3 2 2 4 5% 45%	4946
	21st to end	Night	14% 7% 2 6% 3 3 3 2 3 4 53%	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	35% 3 8% 25% 26% 4	5290
	11th to 20th	Day	29% 3 7% 23% 30% 9%	5310
	21st to end	Day	52% 2 5% 13% 17% 10%	5812
February	1st to 10th	Day	45% 4 5% 14% 22% 10%	1985
	11th to 20th	Day	46% 4 7% 17% 18% 8%	2059
	21st to end	Day	40% 3 7% 15% 22% 13%	1824
March	1st to 10th	Day	42% 4 6% 14% 21% 13%	5046
	11th to 20th	Day	34% 4 6% 15% 18% 22%	5090
	21st to end	Day	39% 5% 10% 22% 19% 6%	5585
April	1st to 10th	Day	41% 6% 9% 19% 17% 9%	2310
	11th to 20th	Day	41% 7% 10% 14% 20% 8%	2428
	21st to end	Day	39% 5% 10% 20% 21% 5%	2551
May	1st to 10th	Day	47% 6% 10% 23% 11% 2	6030
	11th to 20th	Day	43% 5% 9% 18% 22% 3	5945
	21st to end	Day	40% 9% 12% 20% 16% 3	6447
June	1st to 10th	Day	29% 8% 16% 30% 14% 3	2600
	11th to 20th	Day	27% 9% 15% 29% 16% 5%	2600
	21st to end	Day	26% 10% 14% 28% 19% 3	2600
July	1st to 10th	Day	35% 9% 24% 24% 7%	5836
	11th to 20th	Day	20% 6% 17% 36% 18% 2	5831
	21st to end	Day	31% 8% 14% 31% 16% 1	6240
August	1st to 10th	Day	11% 5% 12% 37% 31% 4	4220
	11th to 20th	Day	16% 6% 17% 42% 16% 2	4210
	21st to end	Day	25% 11% 19% 29% 14% 1	4188
September	1st to 10th	Day	31% 8% 12% 21% 19% 10%	2200
	11th to 20th	Day	33% 8% 13% 20% 18% 8%	2200
	21st to end	Day	25% 6% 9% 22% 24% 13%	2200
October	1st to 10th	Day	14% 2 7% 13% 27% 38%	3435
	11th to 20th	Day	17% 3 4 22% 31% 23%	3240
	21st to end	Day	25% 4 5% 26% 32% 10%	3538
November	1st to 10th	Day	33% 5% 8% 20% 25% 9%	2058
	11th to 20th	Day	31% 5% 8% 17% 25% 13%	2068
	21st to end	Day	41% 4 6% 19% 21% 9%	1890
December	1st to 10th	Day	39% 3 7% 20% 24% 6%	3030
	11th to 20th	Day	44% 5% 6% 12% 24% 10%	3479
	21st to end	Day	42% 3 5% 19% 25% 6%	4587
January	1st to 10th	Night	34% 2 8% 20% 29% 6%	7635
	11th to 20th	Night	23% 2 6% 18% 36% 15%	7680
	21st to end	Night	50% 3 5% 14% 16% 11%	8402
February	1st to 10th	Night	39% 4 6% 15% 21% 15%	2653
	11th to 20th	Night	41% 3 7% 16% 22% 10%	2458
	21st to end	Night	39% 3 7% 14% 20% 16%	1982
March	1st to 10th	Night	44% 4 6% 11% 21% 15%	5029
	11th to 20th	Night	33% 5% 7% 15% 18% 22%	5090
	21st to end	Night	38% 4 8% 19% 22% 8%	5588
April	1st to 10th	Night	32% 5% 11% 21% 18% 13%	2310
	11th to 20th	Night	35% 6% 11% 17% 18% 13%	2138
	21st to end	Night	33% 5% 11% 19% 25% 7%	2063
May	1st to 10th	Night	43% 7% 11% 22% 13% 4	4170
	11th to 20th	Night	37% 7% 13% 17% 24% 2	4108
	21st to end	Night	33% 7% 14% 28% 11% 7%	4452
June	1st to 10th	Night	22% 7% 15% 33% 20% 3	1800
	11th to 20th	Night	17% 6% 17% 33% 22% 6%	1800
	21st to end	Night	21% 9% 13% 28% 25% 4	1800
July	1st to 10th	Night	32% 7% 23% 31% 6% 1	4035
	11th to 20th	Night	17% 5% 22% 37% 16% 3	4026
	21st to end	Night	26% 7% 19% 31% 15% 3	4320
August	1st to 10th	Night	7% 4 12% 35% 38% 6%	2916
	11th to 20th	Night	15% 5% 19% 43% 16% 3	2915
	21st to end	Night	20% 8% 22% 30% 18% 2	3702
September	1st to 10th	Night	28% 6% 13% 21% 20% 11%	2200
	11th to 20th	Night	29% 6% 14% 24% 20% 8%	2200
	21st to end	Night	21% 5% 10% 22% 27% 15%	2200
October	1st to 10th	Night	11% 3 6% 15% 21% 44%	3712
	11th to 20th	Night	16% 2 5% 18% 33% 26%	3899
	21st to end	Night	22% 3 4 20% 34% 17%	4252
November	1st to 10th	Night	29% 4 7% 20% 26% 14%	2460
	11th to 20th	Night	29% 3 7% 20% 26% 15%	2546
	21st to end	Night	36% 4 7% 18% 24% 11%	2730
December	1st to 10th	Night	39% 3 6% 17% 27% 8%	4380
	11th to 20th	Night	42% 3 7% 13% 23% 12%	4947
	21st to end	Night	36% 4 6% 17% 27% 11%	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	19% 14% 34% 24% 8% 1	330
	11th to 20th	Day	18% 17% 35% 21% 7% 2	330
	21st to end	Day	24% 13% 39% 18% 7%	363
February	1st to 10th	Day	22% 16% 36% 20% 6%	330
	11th to 20th	Day	28% 20% 30% 17% 5%	330
	21st to end	Day	24% 21% 37% 15% 4	273
March	1st to 10th	Day	24% 16% 31% 22% 7%	330
	11th to 20th	Day	20% 14% 42% 19% 5% 1	330
	21st to end	Day	18% 12% 41% 21% 6% 2	363
April	1st to 10th	Day	22% 14% 35% 22% 7%	330
	11th to 20th	Day	20% 15% 36% 22% 8% 1	466
	21st to end	Day	18% 14% 33% 25% 10% 1	571
May	1st to 10th	Day	14% 11% 33% 32% 9% 1	753
	11th to 20th	Day	14% 8% 26% 30% 19% 3	756
	21st to end	Day	12% 5% 26% 33% 20% 5%	789
June	1st to 10th	Day	6% 3 16% 36% 29% 11%	730
	11th to 20th	Day	5% 2 15% 39% 31% 8%	730
	21st to end	Day	3 2 11% 33% 36% 16%	730
July	1st to 10th	Day	6% 2 9% 32% 37% 14%	721
	11th to 20th	Day	2 1 5% 32% 43% 17%	718
	21st to end	Day	4 2 4 27% 45% 19%	794
August	1st to 10th	Day	3 1 4 24% 49% 20%	726
	11th to 20th	Day	6% 1 3 23% 53% 15%	715
	21st to end	Day	7% 2 3 18% 40% 30%	512
September	1st to 10th	Day	8% 8% 22% 34% 28%	330
	11th to 20th	Day	6% 1 5% 24% 38% 26%	330
	21st to end	Day	8% 5% 7% 28% 30% 22%	330
October	1st to 10th	Day	3 1 14% 32% 38% 13%	330
	11th to 20th	Day	9% 6% 24% 28% 25% 8%	330
	21st to end	Day	20% 10% 21% 26% 17% 6%	363
November	1st to 10th	Day	15% 8% 26% 34% 13% 5%	330
	11th to 20th	Day	15% 8% 27% 31% 17% 2	330
	21st to end	Day	18% 16% 29% 23% 12% 1	330
December	1st to 10th	Day	21% 8% 34% 22% 13% 2	330
	11th to 20th	Day	24% 14% 30% 23% 8% 2	330
	21st to end	Day	20% 11% 37% 23% 7% 1	363
January	1st to 10th	Night	19% 9% 35% 30% 7%	1050
	11th to 20th	Night	20% 12% 32% 30% 5%	1050
	21st to end	Night	21% 10% 34% 30% 5%	1155
February	1st to 10th	Night	21% 12% 32% 30% 4	973
	11th to 20th	Night	24% 13% 35% 25% 3	824
	21st to end	Night	22% 14% 38% 23% 3	598
March	1st to 10th	Night	19% 13% 37% 26% 5%	630
	11th to 20th	Night	16% 11% 38% 29% 6%	630
	21st to end	Night	13% 12% 40% 29% 6%	693
April	1st to 10th	Night	13% 10% 43% 29% 6%	630
	11th to 20th	Night	15% 13% 38% 26% 7%	623
	21st to end	Night	13% 9% 40% 28% 8% 1	630
May	1st to 10th	Night	11% 8% 40% 33% 9%	630
	11th to 20th	Night	12% 5% 33% 34% 15% 1	609
	21st to end	Night	8% 3 30% 40% 17% 1	650
June	1st to 10th	Night	5% 3 16% 42% 31% 4	600
	11th to 20th	Night	4 1 17% 53% 24% 2	600
	21st to end	Night	5% 2 13% 39% 37% 6%	600
July	1st to 10th	Night	3 1 9% 46% 33% 7%	585
	11th to 20th	Night	2 6% 45% 40% 6%	582
	21st to end	Night	2 1 6% 44% 43% 4	657
August	1st to 10th	Night	2 1 5% 42% 45% 6%	597
	11th to 20th	Night	7% 1 4 42% 40% 6%	594
	21st to end	Night	3 1 5% 37% 47% 7%	660
September	1st to 10th	Night	8% 1 9% 37% 39% 6%	600
	11th to 20th	Night	11% 1 7% 34% 43% 4	600
	21st to end	Night	8% 1 11% 38% 38% 4	600
October	1st to 10th	Night	5% 2 15% 48% 26% 5%	670
	11th to 20th	Night	13% 3 23% 39% 20% 3	800
	21st to end	Night	15% 8% 26% 33% 16% 2	880
November	1st to 10th	Night	13% 4 25% 39% 17% 2	820
	11th to 20th	Night	9% 8% 32% 39% 12% 1	866
	21st to end	Night	18% 9% 31% 29% 12% 1	1050
December	1st to 10th	Night	18% 8% 33% 32% 8%	1050
	11th to 20th	Night	23% 7% 31% 33% 5%	1050
	21st to end	Night	20% 9% 32% 32% 6%	1155

Page generated at 2026-05-14 00:58:54. Version ac07

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