Blue Mountain Star Party
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Mountain Star Party clear sky chart from 2005-11-06 to 2026-03-03.

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	7% 10% 3 7% 4 4 4 3 5% 6% 47%	139868
	Night	8% 11% 4 7% 4 4 4 3 5% 6% 44%	138613
Transparency	Day	41% 2 8% 22% 18% 9%	134611
	Night	44% 2 9% 22% 17% 6%	134871
Seeing	Day	15% 16% 27% 24% 13% 4	16028
	Night	16% 10% 17% 34% 20% 3	25289

By Month

Percentage Cloud Cover SW (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% 21% 8% 13% 7% 5% 5% 4 4 5% 18%	4722
	11th to 20th	Day	7% 11% 6% 8% 4 3 4 3 5% 6% 43%	4740
	21st to end	Day	7% 11% 4 9% 5% 4 4 3 4 7% 41%	5187
February	1st to 10th	Day	13% 18% 6% 11% 6% 4 3 4 5% 6% 24%	1853
	11th to 20th	Day	11% 21% 6% 10% 6% 4 4 3 4 5% 25%	1960
	21st to end	Day	9% 18% 8% 11% 6% 5% 4 3 6% 5% 25%	1730
March	1st to 10th	Day	7% 13% 5% 11% 7% 6% 5% 5% 8% 9% 26%	4539
	11th to 20th	Day	5% 13% 6% 11% 7% 7% 5% 3 5% 4 35%	4709
	21st to end	Day	8% 20% 6% 12% 7% 6% 6% 4 5% 6% 20%	5961
April	1st to 10th	Day	11% 19% 6% 10% 6% 5% 4 4 5% 6% 26%	2400
	11th to 20th	Day	9% 15% 4 10% 6% 4 4 3 5% 6% 33%	2369
	21st to end	Day	12% 16% 4 7% 5% 5% 4 3 4 5% 33%	2545
May	1st to 10th	Day	9% 11% 3 6% 4 4 4 3 5% 6% 45%	6183
	11th to 20th	Day	11% 12% 4 7% 4 4 4 3 5% 6% 40%	6277
	21st to end	Day	8% 10% 2 6% 4 4 4 3 6% 8% 46%	6902
June	1st to 10th	Day	15% 9% 2 5% 5% 4 4 3 5% 7% 40%	2800
	11th to 20th	Day	12% 9% 2 5% 4 4 4 3 6% 7% 44%	2800
	21st to end	Day	6% 4 2 4 2 3 4 2 5% 6% 62%	2800
July	1st to 10th	Day	2 2 1 2 2 2 3 2 4 5% 76%	6319
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 4 82%	6320
	21st to end	Day	1 1 1 2 2 2 3 3 4 7% 73%	6880
August	1st to 10th	Day	4 4 2 5% 3 3 3 3 5% 7% 62%	6210
	11th to 20th	Day	3 2 1 3 2 2 2 1 3 5% 76%	5695
	21st to end	Day	4 3 1 4 3 4 4 3 4 6% 63%	5984
September	1st to 10th	Day	4 2 1 3 3 3 3 3 4 7% 67%	2400
	11th to 20th	Day	8% 5% 2 5% 4 3 3 3 5% 5% 57%	2339
	21st to end	Day	8% 8% 2 6% 4 3 3 3 4 5% 53%	2200
October	1st to 10th	Day	9% 8% 2 4 3 2 2 2 3 4 61%	3359
	11th to 20th	Day	7% 9% 3 7% 4 3 4 3 7% 9% 45%	3239
	21st to end	Day	12% 13% 4 9% 5% 5% 5% 3 5% 6% 32%	3200
November	1st to 10th	Day	15% 15% 3 8% 6% 4 5% 4 5% 7% 28%	1854
	11th to 20th	Day	13% 18% 4 11% 6% 5% 4 4 6% 6% 23%	1812
	21st to end	Day	10% 18% 4 9% 5% 4 4 3 5% 5% 31%	1680
December	1st to 10th	Day	8% 13% 3 8% 3 3 3 2 4 5% 46%	2700
	11th to 20th	Day	10% 18% 5% 11% 6% 6% 6% 4 6% 6% 22%	3108
	21st to end	Day	12% 19% 5% 11% 6% 5% 5% 4 4 6% 23%	4092
January	1st to 10th	Night	12% 22% 7% 11% 5% 5% 5% 3 6% 6% 18%	8172
	11th to 20th	Night	8% 12% 5% 7% 4 4 3 2 4 7% 44%	8220
	21st to end	Night	6% 11% 4 8% 5% 5% 5% 4 6% 8% 38%	8994
February	1st to 10th	Night	12% 18% 6% 10% 5% 5% 4 4 5% 5% 26%	2766
	11th to 20th	Night	10% 21% 6% 9% 5% 4 3 3 4 6% 29%	2548
	21st to end	Night	9% 20% 6% 11% 5% 5% 5% 3 5% 6% 25%	2076
March	1st to 10th	Night	7% 11% 4 10% 6% 6% 6% 4 7% 7% 32%	5400
	11th to 20th	Night	6% 14% 6% 10% 6% 4 4 3 4 5% 38%	5275
	21st to end	Night	9% 19% 5% 10% 6% 5% 5% 5% 7% 8% 22%	5013
April	1st to 10th	Night	12% 15% 6% 9% 5% 5% 5% 3 4 6% 32%	2000
	11th to 20th	Night	11% 13% 3 7% 4 4 3 3 4 7% 40%	1979
	21st to end	Night	10% 12% 3 7% 6% 4 4 4 5% 7% 39%	1855
May	1st to 10th	Night	9% 10% 3 6% 4 4 3 3 4 5% 50%	3779
	11th to 20th	Night	11% 11% 2 6% 4 4 4 2 4 4 48%	3616
	21st to end	Night	11% 9% 2 4 3 3 3 3 4 6% 53%	3972
June	1st to 10th	Night	12% 7% 2 5% 4 4 3 4 4 7% 47%	1600
	11th to 20th	Night	13% 9% 3 5% 4 3 3 3 4 6% 48%	1600
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 5% 63%	1600
July	1st to 10th	Night	1 2 1 2 2 2 3 2 4 4 77%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 85%	3640
	21st to end	Night	3 2 1 2 1 2 2 3 5% 6% 74%	3970
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 9% 62%	3582
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 5% 75%	4155
	21st to end	Night	4 4 2 4 3 3 3 3 4 6% 66%	5015
September	1st to 10th	Night	4 3 1 3 2 3 2 2 3 6% 71%	2000
	11th to 20th	Night	6% 3 2 5% 3 4 3 3 5% 7% 59%	2060
	21st to end	Night	8% 6% 2 4 3 3 3 3 4 6% 57%	2200
October	1st to 10th	Night	7% 7% 2 4 2 2 2 2 2 4 65%	3837
	11th to 20th	Night	9% 7% 2 6% 4 3 5% 4 5% 8% 48%	3918
	21st to end	Night	9% 12% 4 8% 4 4 4 3 5% 7% 39%	4600
November	1st to 10th	Night	14% 14% 3 7% 5% 5% 5% 3 5% 7% 31%	2665
	11th to 20th	Night	15% 15% 4 9% 5% 5% 5% 4 5% 5% 28%	2804
	21st to end	Night	10% 15% 5% 8% 4 4 5% 4 5% 6% 32%	2940
December	1st to 10th	Night	9% 10% 4 6% 4 4 4 3 5% 6% 47%	4700
	11th to 20th	Night	9% 19% 6% 12% 7% 6% 6% 5% 6% 6% 19%	5316
	21st to end	Night	12% 18% 6% 10% 6% 5% 5% 4 5% 6% 24%	7106

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	71% 2 9% 13% 5%	4723
	11th to 20th	Day	45% 1 7% 22% 20% 5%	4740
	21st to end	Day	47% 2 8% 17% 22% 4	5187
February	1st to 10th	Day	62% 4 12% 15% 7%	1853
	11th to 20th	Day	62% 4 10% 14% 9% 2	1969
	21st to end	Day	62% 3 10% 14% 10% 1	1730
March	1st to 10th	Day	57% 1 7% 19% 13% 4	4539
	11th to 20th	Day	54% 2 7% 27% 11%	4709
	21st to end	Day	67% 2 10% 17% 4	5961
April	1st to 10th	Day	61% 3 12% 17% 7% 1	2400
	11th to 20th	Day	53% 2 11% 19% 12% 2	2372
	21st to end	Day	55% 3 8% 21% 12%	2545
May	1st to 10th	Day	43% 1 6% 26% 22% 1	6183
	11th to 20th	Day	47% 1 9% 25% 14% 4	6280
	21st to end	Day	38% 2 14% 33% 11% 2	6904
June	1st to 10th	Day	45% 4 11% 28% 12% 1	2800
	11th to 20th	Day	42% 2 10% 27% 16% 3	2800
	21st to end	Day	25% 2 10% 34% 24% 6%	2800
July	1st to 10th	Day	14% 1 4 19% 26% 35%	6251
	11th to 20th	Day	11% 1 4 9% 32% 43%	6245
	21st to end	Day	14% 1 2 27% 36% 20%	6681
August	1st to 10th	Day	21% 2 3 14% 35% 26%	4524
	11th to 20th	Day	16% 1 3 29% 37% 15%	4121
	21st to end	Day	19% 1 6% 31% 23% 20%	4311
September	1st to 10th	Day	20% 1 5% 26% 30% 18%	2400
	11th to 20th	Day	30% 2 7% 25% 25% 11%	2339
	21st to end	Day	35% 2 7% 25% 23% 7%	2200
October	1st to 10th	Day	30% 1 8% 21% 31% 9%	3359
	11th to 20th	Day	39% 1 13% 28% 18% 2	3239
	21st to end	Day	55% 2 6% 25% 12% 1	3200
November	1st to 10th	Day	56% 3 11% 20% 9% 1	1854
	11th to 20th	Day	62% 3 12% 17% 5%	1812
	21st to end	Day	56% 3 10% 18% 11% 2	1680
December	1st to 10th	Day	43% 3 11% 22% 20% 1	2700
	11th to 20th	Day	64% 2 17% 11% 5%	3108
	21st to end	Day	66% 1 11% 19% 4	4092
January	1st to 10th	Night	69% 2 7% 18% 4	8172
	11th to 20th	Night	44% 1 6% 23% 21% 5%	8220
	21st to end	Night	47% 2 11% 17% 19% 5%	8994
February	1st to 10th	Night	61% 3 13% 15% 8% 1	2766
	11th to 20th	Night	60% 2 10% 18% 7% 3	2550
	21st to end	Night	62% 2 8% 17% 9% 2	2076
March	1st to 10th	Night	52% 1 6% 21% 14% 5%	5400
	11th to 20th	Night	51% 2 8% 30% 8%	5275
	21st to end	Night	61% 3 11% 21% 4	5013
April	1st to 10th	Night	56% 3 12% 22% 6% 1	2000
	11th to 20th	Night	47% 3 13% 24% 10% 2	1979
	21st to end	Night	46% 5% 13% 20% 15% 1	1855
May	1st to 10th	Night	39% 1 11% 25% 22% 1	3776
	11th to 20th	Night	42% 2 10% 30% 16%	3616
	21st to end	Night	35% 1 19% 34% 11%	3972
June	1st to 10th	Night	39% 5% 11% 29% 14% 2	1600
	11th to 20th	Night	40% 3 11% 29% 14% 4	1600
	21st to end	Night	24% 2 11% 33% 23% 7%	1600
July	1st to 10th	Night	13% 1 5% 20% 27% 34%	3600
	11th to 20th	Night	7% 1 5% 13% 32% 42%	3592
	21st to end	Night	14% 1 3 31% 31% 20%	3857
August	1st to 10th	Night	16% 1 4 17% 43% 18%	2601
	11th to 20th	Night	15% 1 4 35% 36% 9%	3003
	21st to end	Night	18% 1 7% 32% 23% 19%	3608
September	1st to 10th	Night	18% 1 7% 27% 32% 15%	2000
	11th to 20th	Night	27% 2 8% 27% 24% 12%	2060
	21st to end	Night	31% 2 8% 28% 22% 9%	2200
October	1st to 10th	Night	27% 1 7% 24% 34% 7%	3837
	11th to 20th	Night	38% 2 15% 20% 23% 2	3918
	21st to end	Night	47% 2 7% 27% 16% 2	4600
November	1st to 10th	Night	54% 4 10% 19% 12% 2	2665
	11th to 20th	Night	59% 3 11% 18% 9%	2804
	21st to end	Night	53% 3 10% 16% 16% 1	2940
December	1st to 10th	Night	41% 3 10% 24% 21% 2	4700
	11th to 20th	Night	67% 2 12% 13% 5% 1	5316
	21st to end	Night	64% 2 10% 17% 6% 1	7106

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	30% 20% 20% 15% 12% 4	220
	11th to 20th	Day	17% 17% 28% 19% 13% 7%	220
	21st to end	Day	16% 16% 26% 26% 13% 2	242
February	1st to 10th	Day	25% 22% 27% 16% 7% 3	393
	11th to 20th	Day	23% 21% 27% 20% 7% 2	486
	21st to end	Day	26% 19% 27% 21% 7%	446
March	1st to 10th	Day	24% 21% 30% 20% 5% 1	509
	11th to 20th	Day	19% 24% 27% 20% 9% 1	500
	21st to end	Day	25% 20% 33% 17% 5%	550
April	1st to 10th	Day	25% 18% 28% 17% 9% 3	500
	11th to 20th	Day	21% 23% 27% 17% 8% 4	492
	21st to end	Day	21% 19% 30% 17% 11% 2	500
May	1st to 10th	Day	16% 19% 28% 24% 10% 3	503
	11th to 20th	Day	23% 15% 29% 19% 11% 4	528
	21st to end	Day	15% 13% 23% 27% 16% 6%	573
June	1st to 10th	Day	16% 13% 22% 26% 17% 5%	530
	11th to 20th	Day	17% 18% 30% 25% 10% 2	530
	21st to end	Day	5% 12% 32% 27% 18% 5%	530
July	1st to 10th	Day	2 11% 30% 34% 17% 4	526
	11th to 20th	Day	2 13% 31% 35% 15% 4	524
	21st to end	Day	1 10% 35% 30% 18% 7%	575
August	1st to 10th	Day	5% 9% 28% 34% 18% 6%	527
	11th to 20th	Day	3 7% 26% 35% 19% 10%	517
	21st to end	Day	6% 12% 28% 30% 17% 8%	583
September	1st to 10th	Day	6% 9% 26% 30% 21% 9%	530
	11th to 20th	Day	10% 10% 26% 29% 20% 5%	530
	21st to end	Day	12% 13% 24% 30% 16% 6%	530
October	1st to 10th	Day	14% 13% 24% 26% 19% 4	530
	11th to 20th	Day	14% 17% 25% 23% 16% 6%	520
	21st to end	Day	18% 18% 22% 23% 14% 4	363
November	1st to 10th	Day	24% 19% 21% 22% 14% 1	330
	11th to 20th	Day	23% 21% 22% 19% 12% 3	289
	21st to end	Day	18% 15% 27% 25% 13% 3	220
December	1st to 10th	Day	20% 16% 23% 19% 11% 11%	220
	11th to 20th	Day	28% 21% 28% 14% 9% 1	220
	21st to end	Day	27% 19% 23% 21% 8% 2	242
January	1st to 10th	Night	23% 15% 19% 25% 15% 2	840
	11th to 20th	Night	23% 12% 17% 28% 17% 2	840
	21st to end	Night	20% 9% 19% 34% 17% 2	924
February	1st to 10th	Night	21% 14% 20% 30% 13% 2	840
	11th to 20th	Night	19% 13% 20% 31% 16% 1	824
	21st to end	Night	22% 16% 21% 29% 10% 1	692
March	1st to 10th	Night	18% 14% 22% 32% 12% 2	808
	11th to 20th	Night	16% 16% 20% 29% 17% 2	800
	21st to end	Night	21% 15% 22% 29% 12% 1	880
April	1st to 10th	Night	20% 13% 17% 31% 16% 3	800
	11th to 20th	Night	16% 13% 22% 31% 15% 3	791
	21st to end	Night	15% 10% 21% 35% 18% 2	655
May	1st to 10th	Night	12% 9% 18% 38% 20% 2	461
	11th to 20th	Night	16% 6% 10% 32% 31% 3	394
	21st to end	Night	17% 6% 12% 36% 26% 3	433
June	1st to 10th	Night	13% 7% 15% 39% 24% 3	400
	11th to 20th	Night	17% 7% 14% 41% 20% 2	400
	21st to end	Night	5% 3 11% 51% 29% 2	400
July	1st to 10th	Night	2 2 10% 50% 33% 2	390
	11th to 20th	Night	1 1 10% 61% 26% 2	388
	21st to end	Night	2 1 7% 48% 40% 3	438
August	1st to 10th	Night	4 1 6% 45% 42% 3	398
	11th to 20th	Night	3 2 9% 39% 39% 8%	629
	21st to end	Night	5% 3 12% 44% 33% 4	880
September	1st to 10th	Night	5% 3 11% 43% 33% 5%	800
	11th to 20th	Night	7% 3 11% 38% 33% 8%	800
	21st to end	Night	12% 5% 11% 38% 26% 8%	800
October	1st to 10th	Night	13% 5% 13% 42% 23% 4	800
	11th to 20th	Night	16% 9% 14% 35% 21% 6%	800
	21st to end	Night	18% 11% 15% 33% 18% 5%	880
November	1st to 10th	Night	21% 13% 17% 34% 15% 1	820
	11th to 20th	Night	23% 11% 21% 30% 14% 2	840
	21st to end	Night	18% 14% 18% 29% 16% 5%	840
December	1st to 10th	Night	20% 10% 19% 29% 18% 2	840
	11th to 20th	Night	23% 13% 21% 30% 13% 2	840
	21st to end	Night	27% 16% 22% 24% 10% 1	924

Page generated at 2026-02-27 22:40:24. Version ac07

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