Blazing Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blazing Star Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	6% 7% 3 6% 4 4 4 4 6% 7% 49%	131251
	Night	8% 9% 3 7% 4 4 5% 4 6% 7% 43%	128508
Transparency	Day	35% 5% 10% 24% 19% 7%	130978
	Night	39% 5% 12% 24% 15% 5%	128358
Seeing	Day	10% 13% 25% 29% 18% 6%	15644
	Night	11% 7% 20% 38% 21% 3	24661

By Month

Percentage Cloud Cover NW (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	14% 18% 6% 15% 6% 5% 4 4 5% 5% 18%	3639
	11th to 20th	Day	8% 12% 3 7% 5% 5% 5% 5% 6% 8% 35%	3616
	21st to end	Day	15% 15% 7% 9% 5% 4 4 4 6% 6% 25%	4004
February	1st to 10th	Day	8% 14% 3 10% 6% 6% 6% 5% 7% 7% 29%	1725
	11th to 20th	Day	9% 12% 4 8% 4 5% 5% 4 5% 8% 36%	1755
	21st to end	Day	6% 11% 4 9% 6% 5% 5% 5% 7% 9% 32%	1649
March	1st to 10th	Day	5% 7% 3 8% 6% 5% 6% 5% 8% 9% 38%	4518
	11th to 20th	Day	5% 7% 2 7% 6% 5% 5% 4 6% 7% 47%	4822
	21st to end	Day	9% 8% 3 8% 6% 6% 7% 5% 8% 10% 30%	5872
April	1st to 10th	Day	8% 7% 3 7% 6% 6% 7% 5% 7% 9% 37%	2400
	11th to 20th	Day	5% 7% 2 7% 5% 5% 5% 5% 8% 8% 44%	2372
	21st to end	Day	4 7% 3 7% 5% 5% 5% 5% 6% 9% 44%	2660
May	1st to 10th	Day	4 5% 2 5% 4 4 4 4 6% 7% 55%	6320
	11th to 20th	Day	4 5% 2 7% 5% 5% 6% 4 7% 9% 46%	6277
	21st to end	Day	6% 6% 2 6% 5% 5% 5% 4 6% 8% 48%	6903
June	1st to 10th	Day	8% 6% 2 5% 5% 4 5% 5% 7% 8% 47%	2800
	11th to 20th	Day	5% 5% 2 6% 4 5% 4 5% 7% 8% 50%	2800
	21st to end	Day	4 4 2 5% 4 3 4 3 6% 7% 59%	2800
July	1st to 10th	Day	1 1 1 2 2 2 3 2 3 5% 78%	6320
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 79%	6320
	21st to end	Day	3 2 1 2 2 2 3 3 4 5% 73%	6874
August	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 7% 72%	4541
	11th to 20th	Day	2 2 1 3 3 4 4 3 6% 8% 67%	4314
	21st to end	Day	4 4 1 4 3 3 4 3 5% 6% 62%	4319
September	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 7% 67%	2400
	11th to 20th	Day	4 4 1 5% 3 4 5% 4 6% 8% 56%	2390
	21st to end	Day	6% 4 1 5% 4 3 4 3 5% 7% 58%	2170
October	1st to 10th	Day	3 4 2 5% 4 3 4 4 5% 8% 60%	3280
	11th to 20th	Day	5% 6% 2 6% 5% 5% 5% 4 7% 8% 47%	3239
	21st to end	Day	9% 5% 3 8% 4 5% 5% 4 6% 6% 44%	3200
November	1st to 10th	Day	11% 11% 3 8% 6% 5% 5% 4 7% 7% 34%	1807
	11th to 20th	Day	9% 14% 4 11% 5% 4 6% 4 6% 6% 31%	1680
	21st to end	Day	9% 14% 7% 10% 6% 6% 4 4 6% 7% 28%	1680
December	1st to 10th	Day	8% 9% 5% 10% 6% 5% 4 6% 7% 7% 32%	2700
	11th to 20th	Day	13% 19% 8% 13% 6% 5% 5% 4 5% 5% 18%	3081
	21st to end	Day	11% 20% 6% 15% 7% 6% 6% 3 4 5% 17%	4004
January	1st to 10th	Night	14% 22% 8% 14% 6% 7% 4 3 4 5% 14%	6306
	11th to 20th	Night	10% 11% 3 7% 5% 5% 7% 6% 10% 9% 28%	6282
	21st to end	Night	14% 17% 8% 11% 4 4 4 4 8% 6% 20%	6952
February	1st to 10th	Night	10% 12% 4 10% 6% 5% 6% 5% 7% 8% 28%	2673
	11th to 20th	Night	10% 9% 3 7% 5% 4 5% 5% 7% 9% 35%	2532
	21st to end	Night	7% 11% 4 9% 5% 5% 5% 5% 8% 7% 33%	1980
March	1st to 10th	Night	5% 7% 2 7% 5% 6% 7% 6% 8% 11% 36%	5376
	11th to 20th	Night	6% 8% 3 7% 4 4 4 3 5% 6% 50%	5175
	21st to end	Night	8% 9% 3 8% 7% 7% 6% 5% 7% 9% 31%	5102
April	1st to 10th	Night	6% 7% 3 6% 6% 6% 5% 5% 6% 8% 42%	2000
	11th to 20th	Night	4 9% 2 6% 5% 4 4 4 5% 6% 49%	1979
	21st to end	Night	6% 6% 2 6% 4 4 4 4 5% 8% 51%	1736
May	1st to 10th	Night	4 3 1 4 3 3 4 4 6% 7% 61%	3640
	11th to 20th	Night	5% 5% 2 5% 4 5% 4 4 6% 8% 52%	3616
	21st to end	Night	5% 3 2 3 3 3 5% 4 7% 11% 54%	3972
June	1st to 10th	Night	6% 4 2 5% 4 5% 4 4 7% 9% 50%	1600
	11th to 20th	Night	5% 5% 1 5% 4 5% 5% 4 7% 6% 54%	1600
	21st to end	Night	3 3 1 3 4 3 5% 4 5% 7% 65%	1600
July	1st to 10th	Night	1 2 1 2 2 1 1 2 3 4 82%	3640
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 84%	3640
	21st to end	Night	1 1 1 2 2 2 3 3 4 5% 75%	3969
August	1st to 10th	Night	2 2 1 3 4 3 3 3 5% 6% 69%	2609
	11th to 20th	Night	2 2 1 3 2 3 3 3 5% 7% 68%	2735
	21st to end	Night	4 3 1 4 4 3 3 3 4 6% 66%	3600
September	1st to 10th	Night	2 2 1 4 3 3 3 3 5% 8% 67%	2000
	11th to 20th	Night	4 3 1 5% 3 3 4 2 6% 7% 62%	2010
	21st to end	Night	5% 4 1 4 3 3 4 4 6% 8% 59%	2228
October	1st to 10th	Night	4 4 1 4 3 4 5% 4 6% 8% 57%	3920
	11th to 20th	Night	5% 5% 2 6% 4 5% 5% 4 7% 8% 50%	3918
	21st to end	Night	9% 6% 3 9% 5% 4 4 4 6% 7% 44%	4600
November	1st to 10th	Night	14% 9% 3 7% 5% 4 5% 4 7% 8% 33%	2708
	11th to 20th	Night	11% 12% 3 6% 6% 5% 5% 5% 7% 7% 33%	2940
	21st to end	Night	11% 14% 5% 10% 5% 6% 5% 4 6% 7% 26%	2940
December	1st to 10th	Night	7% 10% 4 10% 5% 5% 8% 7% 10% 7% 27%	4700
	11th to 20th	Night	16% 17% 7% 15% 7% 6% 6% 4 5% 6% 11%	5278
	21st to end	Night	16% 20% 5% 13% 7% 5% 5% 3 4 4 18%	6952

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	67% 6% 7% 12% 7%	3639
	11th to 20th	Day	44% 6% 17% 25% 7% 1	3640
	21st to end	Day	61% 4 9% 20% 6%	4004
February	1st to 10th	Day	56% 7% 13% 16% 5% 2	1725
	11th to 20th	Day	49% 6% 16% 18% 9% 1	1764
	21st to end	Day	48% 8% 15% 19% 9% 1	1649
March	1st to 10th	Day	39% 7% 10% 23% 20%	4518
	11th to 20th	Day	36% 4 11% 34% 14% 2	4822
	21st to end	Day	46% 7% 17% 19% 10% 1	5872
April	1st to 10th	Day	42% 6% 14% 24% 12% 2	2400
	11th to 20th	Day	36% 5% 14% 23% 17% 5%	2372
	21st to end	Day	36% 6% 11% 28% 16% 3	2660
May	1st to 10th	Day	27% 5% 6% 25% 31% 6%	6320
	11th to 20th	Day	31% 7% 10% 30% 17% 4	6280
	21st to end	Day	34% 5% 12% 31% 16% 2	6904
June	1st to 10th	Day	35% 5% 9% 26% 19% 7%	2800
	11th to 20th	Day	31% 5% 9% 29% 21% 5%	2800
	21st to end	Day	24% 4 9% 30% 25% 8%	2800
July	1st to 10th	Day	11% 3 6% 21% 29% 30%	6251
	11th to 20th	Day	12% 2 3 21% 31% 33%	6245
	21st to end	Day	14% 3 4 26% 33% 19%	6681
August	1st to 10th	Day	14% 3 5% 22% 38% 18%	4524
	11th to 20th	Day	16% 4 11% 36% 27% 7%	4356
	21st to end	Day	24% 3 8% 29% 29% 7%	4318
September	1st to 10th	Day	17% 3 8% 31% 32% 9%	2400
	11th to 20th	Day	26% 4 11% 27% 24% 7%	2390
	21st to end	Day	28% 3 15% 30% 18% 5%	2170
October	1st to 10th	Day	24% 4 13% 37% 21% 2	3280
	11th to 20th	Day	35% 5% 17% 23% 19% 1	3239
	21st to end	Day	39% 5% 13% 25% 15% 4	3200
November	1st to 10th	Day	49% 7% 15% 18% 10% 1	1807
	11th to 20th	Day	54% 7% 14% 17% 7% 1	1680
	21st to end	Day	58% 7% 14% 12% 8% 1	1680
December	1st to 10th	Day	49% 7% 18% 20% 5%	2700
	11th to 20th	Day	71% 6% 9% 10% 5%	3084
	21st to end	Day	71% 6% 8% 10% 5%	4004
January	1st to 10th	Night	70% 7% 10% 10% 4	6306
	11th to 20th	Night	43% 7% 19% 23% 8%	6320
	21st to end	Night	61% 4 11% 18% 6%	6952
February	1st to 10th	Night	52% 8% 15% 16% 7% 1	2673
	11th to 20th	Night	44% 9% 17% 19% 9% 1	2534
	21st to end	Night	47% 9% 13% 21% 10% 1	1980
March	1st to 10th	Night	36% 6% 12% 28% 16% 2	5376
	11th to 20th	Night	35% 4 13% 31% 16% 1	5176
	21st to end	Night	47% 8% 17% 21% 7% 1	5102
April	1st to 10th	Night	40% 5% 15% 27% 11% 2	2000
	11th to 20th	Night	34% 5% 12% 28% 18% 3	1979
	21st to end	Night	33% 5% 11% 31% 18% 2	1736
May	1st to 10th	Night	23% 4 7% 24% 38% 4	3637
	11th to 20th	Night	29% 5% 9% 32% 23% 2	3616
	21st to end	Night	25% 5% 17% 37% 16% 1	3972
June	1st to 10th	Night	32% 6% 9% 29% 20% 5%	1600
	11th to 20th	Night	29% 5% 12% 30% 22% 3	1600
	21st to end	Night	21% 4 11% 32% 26% 5%	1600
July	1st to 10th	Night	10% 2 5% 24% 30% 30%	3600
	11th to 20th	Night	6% 2 4 24% 31% 33%	3592
	21st to end	Night	12% 4 5% 27% 31% 22%	3857
August	1st to 10th	Night	16% 4 6% 23% 29% 21%	2601
	11th to 20th	Night	16% 4 11% 37% 25% 7%	2759
	21st to end	Night	21% 3 8% 29% 29% 9%	3600
September	1st to 10th	Night	17% 3 8% 29% 32% 11%	2000
	11th to 20th	Night	22% 4 10% 28% 28% 8%	2010
	21st to end	Night	25% 4 13% 34% 18% 6%	2228
October	1st to 10th	Night	24% 5% 10% 41% 19% 1	3920
	11th to 20th	Night	33% 6% 14% 28% 19% 1	3918
	21st to end	Night	40% 5% 12% 28% 13% 3	4600
November	1st to 10th	Night	48% 6% 17% 18% 9% 2	2708
	11th to 20th	Night	49% 8% 15% 18% 8% 2	2940
	21st to end	Night	57% 6% 13% 15% 7% 1	2940
December	1st to 10th	Night	43% 7% 20% 25% 4	4700
	11th to 20th	Night	74% 7% 9% 7% 3	5274
	21st to end	Night	69% 6% 8% 10% 7% 1	6952

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	24% 11% 20% 28% 15% 5%	200
	11th to 20th	Day	20% 7% 21% 29% 18% 6%	200
	21st to end	Day	20% 7% 18% 33% 18% 5%	220
February	1st to 10th	Day	11% 11% 32% 30% 13% 2	325
	11th to 20th	Day	12% 12% 31% 30% 11% 4	395
	21st to end	Day	10% 16% 31% 26% 14% 2	414
March	1st to 10th	Day	10% 14% 30% 28% 16% 2	500
	11th to 20th	Day	11% 17% 22% 27% 19% 3	500
	21st to end	Day	11% 21% 27% 27% 12% 1	550
April	1st to 10th	Day	14% 20% 26% 26% 10% 4	500
	11th to 20th	Day	9% 26% 27% 23% 11% 4	492
	21st to end	Day	10% 18% 28% 23% 16% 5%	500
May	1st to 10th	Day	8% 14% 28% 31% 16% 3	503
	11th to 20th	Day	13% 14% 23% 26% 19% 5%	528
	21st to end	Day	9% 14% 21% 29% 19% 7%	573
June	1st to 10th	Day	10% 16% 28% 25% 17% 5%	530
	11th to 20th	Day	8% 17% 33% 26% 12% 3	530
	21st to end	Day	6% 11% 24% 29% 22% 8%	530
July	1st to 10th	Day	5% 9% 24% 37% 22% 3	526
	11th to 20th	Day	3 10% 27% 29% 24% 6%	524
	21st to end	Day	3 9% 19% 36% 23% 10%	575
August	1st to 10th	Day	5% 5% 22% 34% 24% 9%	527
	11th to 20th	Day	3 7% 22% 32% 25% 11%	517
	21st to end	Day	4 11% 22% 31% 21% 10%	583
September	1st to 10th	Day	4 9% 22% 28% 24% 12%	530
	11th to 20th	Day	7% 10% 22% 30% 23% 8%	530
	21st to end	Day	9% 13% 23% 30% 20% 4	530
October	1st to 10th	Day	8% 15% 22% 28% 18% 8%	530
	11th to 20th	Day	11% 13% 22% 27% 21% 7%	520
	21st to end	Day	16% 11% 25% 29% 15% 3	363
November	1st to 10th	Day	16% 12% 22% 31% 18% 1	305
	11th to 20th	Day	16% 16% 22% 20% 18% 8%	220
	21st to end	Day	13% 7% 20% 33% 20% 6%	220
December	1st to 10th	Day	20% 9% 18% 21% 19% 14%	220
	11th to 20th	Day	20% 8% 21% 34% 13% 4	214
	21st to end	Day	16% 5% 21% 36% 17% 4	220
January	1st to 10th	Night	22% 9% 23% 28% 16% 3	800
	11th to 20th	Night	20% 8% 17% 37% 17% 3	800
	21st to end	Night	18% 4 18% 36% 20% 3	880
February	1st to 10th	Night	15% 6% 23% 39% 14% 2	800
	11th to 20th	Night	14% 5% 24% 36% 18% 2	784
	21st to end	Night	11% 10% 26% 33% 16% 3	660
March	1st to 10th	Night	13% 6% 21% 40% 18% 2	800
	11th to 20th	Night	9% 9% 25% 35% 21% 2	800
	21st to end	Night	12% 9% 27% 38% 13% 1	880
April	1st to 10th	Night	9% 10% 22% 36% 21% 2	800
	11th to 20th	Night	8% 10% 29% 34% 17% 2	791
	21st to end	Night	9% 9% 30% 32% 18% 2	536
May	1st to 10th	Night	5% 6% 26% 41% 20% 2	400
	11th to 20th	Night	10% 5% 18% 39% 26% 2	394
	21st to end	Night	9% 5% 14% 42% 28% 3	433
June	1st to 10th	Night	7% 7% 21% 43% 21% 1	400
	11th to 20th	Night	5% 8% 26% 42% 19% 1	400
	21st to end	Night	2 6% 20% 43% 27% 2	400
July	1st to 10th	Night	2 3 15% 47% 30% 2	390
	11th to 20th	Night	2 4 20% 47% 27% 1	388
	21st to end	Night	1 3 15% 45% 35% 2	438
August	1st to 10th	Night	2 3 12% 39% 42% 3	398
	11th to 20th	Night	3 2 11% 38% 40% 6%	489
	21st to end	Night	4 4 16% 41% 31% 4	872
September	1st to 10th	Night	3 4 18% 37% 34% 5%	800
	11th to 20th	Night	6% 5% 15% 31% 35% 9%	800
	21st to end	Night	6% 6% 18% 39% 25% 6%	800
October	1st to 10th	Night	11% 6% 17% 39% 26% 3	800
	11th to 20th	Night	8% 7% 22% 37% 23% 4	800
	21st to end	Night	14% 8% 23% 37% 16% 3	880
November	1st to 10th	Night	20% 6% 22% 40% 12%	820
	11th to 20th	Night	17% 8% 23% 33% 15% 3	840
	21st to end	Night	17% 8% 18% 35% 18% 4	840
December	1st to 10th	Night	17% 6% 18% 37% 19% 3	840
	11th to 20th	Night	19% 6% 20% 40% 14% 1	828
	21st to end	Night	21% 8% 19% 40% 10% 1	880

Page generated at 2025-12-14 22:13:49. Version ac07

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