Boulder Knolls Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boulder Knolls Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	5% 3 1 3 3 3 3 3 4 6% 67%	135793
	Night	7% 6% 2 4 3 3 3 3 4 6% 59%	133198
Transparency	Day	19% 3 6% 16% 27% 28%	130889
	Night	25% 4 7% 15% 23% 25%	129076
Seeing	Day	7% 7% 26% 37% 17% 5%	17306
	Night	8% 3 11% 35% 35% 8%	27714

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	6% 3 1 3 2 2 3 2 3 4 70%	4107
	11th to 20th	Day	5% 4 2 3 2 2 2 3 3 6% 68%	4353
	21st to end	Day	11% 4 1 5% 3 3 5% 4 6% 9% 49%	5016
February	1st to 10th	Day	10% 5% 2 6% 3 3 3 3 3 5% 57%	2000
	11th to 20th	Day	10% 5% 2 3 3 3 4 2 6% 7% 53%	1950
	21st to end	Day	11% 7% 2 5% 4 3 4 4 5% 6% 50%	1650
March	1st to 10th	Day	12% 5% 3 6% 4 4 5% 4 7% 9% 43%	4518
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 7% 55%	4770
	21st to end	Day	10% 6% 2 4 3 3 3 3 5% 7% 54%	5788
April	1st to 10th	Day	7% 5% 2 4 4 3 3 3 5% 7% 57%	2400
	11th to 20th	Day	5% 5% 1 4 3 3 3 4 5% 6% 62%	2369
	21st to end	Day	7% 4 1 3 3 3 2 2 5% 7% 63%	2400
May	1st to 10th	Day	6% 5% 2 4 3 2 3 2 3 4 66%	5704
	11th to 20th	Day	3 5% 1 5% 4 4 4 3 5% 8% 59%	5841
	21st to end	Day	3 5% 2 4 4 3 3 2 4 6% 62%	6423
June	1st to 10th	Day	4 6% 2 5% 2 3 3 2 4 6% 63%	2600
	11th to 20th	Day	2 3 1 4 2 2 2 2 2 6% 73%	2600
	21st to end	Day	2 2 1 3 3 2 3 2 3 4 76%	2600
July	1st to 10th	Day	1 1 1 1 2 1 1 2 3 85%	5879
	11th to 20th	Day	1 2 1 3 4 2 3 2 3 5% 74%	5880
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 80%	6124
August	1st to 10th	Day	1 1 1 1 1 2 2 3 4 84%	5346
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 4 86%	5364
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 79%	5984
September	1st to 10th	Day	2 2 1 2 2 2 3 3 3 5% 75%	2386
	11th to 20th	Day	2 3 1 2 2 1 2 1 3 5% 78%	2200
	21st to end	Day	3 2 1 4 2 2 2 2 3 5% 75%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	3600
	11th to 20th	Day	4 4 1 3 2 2 2 2 3 5% 73%	3563
	21st to end	Day	3 2 1 3 2 2 3 3 5% 7% 71%	3396
November	1st to 10th	Day	5% 3 1 2 2 3 2 2 5% 6% 69%	1854
	11th to 20th	Day	5% 3 2 3 2 3 3 3 4 5% 67%	1890
	21st to end	Day	7% 5% 2 4 2 3 4 3 5% 8% 58%	1890
December	1st to 10th	Day	4 3 1 2 2 2 2 3 5% 8% 68%	3040
	11th to 20th	Day	8% 4 2 4 4 4 5% 4 6% 7% 52%	3499
	21st to end	Day	9% 5% 2 5% 5% 6% 5% 4 7% 8% 45%	4609
January	1st to 10th	Night	5% 4 1 3 3 3 3 2 4 7% 66%	5880
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 4 6% 61%	5634
	21st to end	Night	14% 8% 2 4 3 3 4 3 5% 6% 49%	5984
February	1st to 10th	Night	10% 7% 1 4 3 3 3 3 5% 5% 57%	2400
	11th to 20th	Night	11% 7% 2 5% 5% 3 3 2 4 5% 53%	2338
	21st to end	Night	12% 8% 2 5% 3 3 4 3 4 5% 51%	1980
March	1st to 10th	Night	21% 11% 2 5% 3 3 3 3 5% 6% 39%	5376
	11th to 20th	Night	12% 11% 2 6% 3 3 3 3 5% 6% 47%	5230
	21st to end	Night	12% 14% 3 5% 3 3 3 3 4 6% 45%	5182
April	1st to 10th	Night	9% 8% 3 4 3 3 3 3 5% 6% 52%	2000
	11th to 20th	Night	8% 10% 2 5% 3 3 3 3 3 4 54%	1979
	21st to end	Night	8% 9% 3 5% 3 4 3 3 5% 5% 53%	2000
May	1st to 10th	Night	9% 11% 3 5% 4 2 2 2 3 4 56%	4270
	11th to 20th	Night	7% 14% 3 7% 6% 5% 3 2 4 5% 43%	4074
	21st to end	Night	5% 14% 3 6% 4 4 2 2 3 5% 50%	4474
June	1st to 10th	Night	3 11% 4 7% 4 5% 4 3 4 8% 46%	1800
	11th to 20th	Night	2 6% 3 5% 3 4 3 2 4 7% 60%	1800
	21st to end	Night	2 3 2 5% 3 4 2 3 4 7% 66%	1800
July	1st to 10th	Night	2 1 1 3 3 2 2 2 4 79%	4100
	11th to 20th	Night	1 3 1 5% 7% 5% 4 3 4 6% 62%	4100
	21st to end	Night	1 5% 1 4 4 3 3 2 4 5% 69%	4744
August	1st to 10th	Night	1 2 1 1 1 1 2 2 3 5% 81%	4486
	11th to 20th	Night	1 1 2 6% 3 3 2 3 5% 74%	4506
	21st to end	Night	2 2 1 3 3 3 2 2 3 5% 75%	5015
September	1st to 10th	Night	2 4 2 4 2 3 3 2 4 6% 67%	2012
	11th to 20th	Night	5% 4 1 3 3 2 2 2 3 4 72%	2200
	21st to end	Night	3 4 1 3 4 2 2 2 4 5% 71%	2200
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 76%	3600
	11th to 20th	Night	6% 10% 3 4 3 2 2 2 3 5% 61%	3579
	21st to end	Night	4 4 1 3 2 2 3 2 4 6% 69%	4408
November	1st to 10th	Night	5% 4 2 4 3 3 3 3 4 6% 66%	2665
	11th to 20th	Night	5% 4 2 5% 3 3 3 2 4 5% 64%	2730
	21st to end	Night	7% 4 2 4 4 4 4 4 5% 7% 55%	2730
December	1st to 10th	Night	5% 3 1 3 3 2 3 3 5% 5% 67%	4370
	11th to 20th	Night	9% 6% 2 4 3 3 4 4 6% 7% 50%	4941
	21st to end	Night	8% 7% 2 6% 4 4 4 4 7% 10% 44%	6611

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	19% 3 4 17% 26% 30%	4108
	11th to 20th	Day	19% 4 8% 19% 21% 30%	4353
	21st to end	Day	29% 5% 8% 9% 27% 22%	5016
February	1st to 10th	Day	30% 3 6% 13% 21% 27%	2000
	11th to 20th	Day	30% 4 7% 14% 26% 19%	1960
	21st to end	Day	35% 6% 6% 13% 20% 21%	1650
March	1st to 10th	Day	33% 7% 8% 18% 21% 13%	4518
	11th to 20th	Day	28% 5% 10% 25% 21% 11%	4770
	21st to end	Day	28% 5% 10% 31% 15% 10%	5788
April	1st to 10th	Day	27% 4 9% 24% 23% 13%	2400
	11th to 20th	Day	22% 5% 8% 19% 28% 19%	2372
	21st to end	Day	21% 4 8% 26% 30% 11%	2400
May	1st to 10th	Day	21% 4 7% 22% 34% 12%	5704
	11th to 20th	Day	19% 5% 8% 23% 35% 10%	5844
	21st to end	Day	20% 4 6% 22% 36% 12%	6424
June	1st to 10th	Day	22% 3 6% 24% 31% 15%	2600
	11th to 20th	Day	15% 3 4 12% 32% 35%	2600
	21st to end	Day	13% 2 3 8% 26% 48%	2600
July	1st to 10th	Day	6% 1 3 5% 21% 65%	5816
	11th to 20th	Day	10% 2 9% 12% 41% 26%	5811
	21st to end	Day	8% 2 6% 19% 29% 36%	5948
August	1st to 10th	Day	9% 2 4 17% 41% 27%	3890
	11th to 20th	Day	7% 1 5% 13% 26% 48%	3880
	21st to end	Day	8% 2 3 8% 29% 50%	4311
September	1st to 10th	Day	12% 3 4 9% 36% 36%	2386
	11th to 20th	Day	11% 2 3 10% 37% 37%	2200
	21st to end	Day	13% 2 4 12% 32% 36%	2200
October	1st to 10th	Day	12% 2 4 10% 16% 56%	3600
	11th to 20th	Day	15% 2 4 12% 25% 40%	3563
	21st to end	Day	15% 2 4 9% 32% 39%	3396
November	1st to 10th	Day	17% 3 5% 17% 22% 36%	1854
	11th to 20th	Day	20% 3 6% 13% 20% 37%	1890
	21st to end	Day	24% 4 7% 13% 19% 31%	1890
December	1st to 10th	Day	16% 2 6% 14% 25% 37%	3040
	11th to 20th	Day	29% 4 7% 15% 24% 21%	3498
	21st to end	Day	35% 5% 8% 23% 21% 8%	4609
January	1st to 10th	Night	20% 4 5% 15% 29% 28%	5880
	11th to 20th	Night	23% 5% 9% 18% 16% 30%	5634
	21st to end	Night	35% 4 7% 9% 23% 23%	5984
February	1st to 10th	Night	30% 3 7% 13% 21% 26%	2400
	11th to 20th	Night	34% 4 5% 14% 21% 23%	2339
	21st to end	Night	35% 5% 4 14% 21% 21%	1980
March	1st to 10th	Night	44% 6% 8% 12% 16% 13%	5376
	11th to 20th	Night	37% 6% 8% 22% 19% 8%	5230
	21st to end	Night	40% 7% 9% 18% 18% 9%	5182
April	1st to 10th	Night	31% 5% 9% 21% 20% 14%	2000
	11th to 20th	Night	31% 4 8% 16% 22% 18%	1979
	21st to end	Night	32% 6% 7% 23% 21% 12%	2000
May	1st to 10th	Night	31% 6% 8% 21% 25% 9%	4267
	11th to 20th	Night	34% 8% 7% 20% 25% 7%	4075
	21st to end	Night	33% 6% 7% 20% 22% 11%	4474
June	1st to 10th	Night	32% 6% 9% 21% 20% 13%	1800
	11th to 20th	Night	21% 3 5% 14% 28% 29%	1800
	21st to end	Night	19% 2 4 10% 28% 37%	1800
July	1st to 10th	Night	6% 1 8% 8% 18% 59%	4055
	11th to 20th	Night	13% 2 15% 13% 39% 17%	4046
	21st to end	Night	14% 3 10% 19% 27% 28%	4609
August	1st to 10th	Night	9% 2 5% 22% 42% 20%	3256
	11th to 20th	Night	9% 2 8% 15% 27% 39%	3256
	21st to end	Night	9% 2 7% 8% 28% 46%	3608
September	1st to 10th	Night	18% 2 5% 10% 30% 34%	2012
	11th to 20th	Night	17% 1 6% 8% 33% 35%	2200
	21st to end	Night	15% 3 6% 13% 35% 29%	2200
October	1st to 10th	Night	13% 2 4 10% 18% 53%	3600
	11th to 20th	Night	26% 3 4 8% 22% 37%	3579
	21st to end	Night	18% 2 5% 9% 28% 38%	4408
November	1st to 10th	Night	20% 3 5% 15% 19% 37%	2665
	11th to 20th	Night	23% 3 5% 13% 19% 37%	2730
	21st to end	Night	28% 4 8% 15% 15% 30%	2730
December	1st to 10th	Night	19% 3 6% 14% 25% 34%	4370
	11th to 20th	Night	31% 4 7% 12% 22% 24%	4941
	21st to end	Night	31% 6% 11% 23% 19% 10%	6611

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	16% 7% 29% 33% 14% 2	303
	11th to 20th	Day	13% 6% 28% 34% 17% 2	410
	21st to end	Day	12% 9% 30% 33% 13% 2	550
February	1st to 10th	Day	12% 11% 33% 29% 12% 3	500
	11th to 20th	Day	13% 14% 32% 30% 10% 1	495
	21st to end	Day	15% 16% 36% 24% 7% 1	414
March	1st to 10th	Day	14% 17% 34% 24% 10% 1	500
	11th to 20th	Day	16% 10% 33% 29% 11% 2	500
	21st to end	Day	11% 15% 37% 27% 8% 1	550
April	1st to 10th	Day	12% 11% 31% 29% 13% 3	500
	11th to 20th	Day	4 19% 35% 29% 11% 2	492
	21st to end	Day	8% 13% 31% 28% 15% 4	500
May	1st to 10th	Day	12% 12% 32% 28% 12% 4	503
	11th to 20th	Day	9% 8% 31% 36% 14% 2	528
	21st to end	Day	8% 11% 30% 34% 14% 3	573
June	1st to 10th	Day	2 6% 26% 39% 22% 5%	530
	11th to 20th	Day	3 3 26% 44% 18% 6%	530
	21st to end	Day	3 2 24% 44% 20% 8%	530
July	1st to 10th	Day	3 1 13% 48% 25% 10%	526
	11th to 20th	Day	2 10% 47% 28% 13%	524
	21st to end	Day	2 1 9% 49% 28% 11%	575
August	1st to 10th	Day	2 14% 49% 24% 11%	527
	11th to 20th	Day	2 1 11% 50% 24% 13%	517
	21st to end	Day	2 2 14% 49% 22% 11%	583
September	1st to 10th	Day	3 1 23% 46% 19% 8%	530
	11th to 20th	Day	4 5% 27% 44% 17% 3	530
	21st to end	Day	5% 6% 28% 38% 18% 5%	530
October	1st to 10th	Day	4 6% 29% 38% 18% 4	530
	11th to 20th	Day	7% 5% 32% 37% 17% 2	530
	21st to end	Day	4 5% 28% 37% 20% 6%	483
November	1st to 10th	Day	6% 7% 20% 38% 24% 5%	330
	11th to 20th	Day	6% 6% 25% 39% 20% 4	330
	21st to end	Day	9% 7% 30% 34% 15% 4	330
December	1st to 10th	Day	9% 5% 26% 35% 19% 5%	330
	11th to 20th	Day	6% 3 28% 35% 22% 5%	330
	21st to end	Day	19% 10% 25% 30% 13% 2	363
January	1st to 10th	Night	10% 3 20% 39% 22% 6%	804
	11th to 20th	Night	9% 3 17% 42% 22% 6%	800
	21st to end	Night	14% 3 16% 43% 20% 3	880
February	1st to 10th	Night	12% 4 16% 37% 24% 6%	800
	11th to 20th	Night	13% 4 18% 37% 22% 5%	784
	21st to end	Night	14% 6% 17% 38% 21% 4	660
March	1st to 10th	Night	14% 5% 17% 40% 21% 2	800
	11th to 20th	Night	13% 4 12% 39% 25% 7%	800
	21st to end	Night	11% 5% 15% 38% 26% 5%	880
April	1st to 10th	Night	13% 3 14% 42% 26% 4	800
	11th to 20th	Night	10% 4 16% 38% 27% 5%	791
	21st to end	Night	11% 4 10% 37% 31% 6%	800
May	1st to 10th	Night	11% 4 12% 37% 32% 5%	679
	11th to 20th	Night	9% 2 14% 34% 36% 5%	591
	21st to end	Night	10% 5% 12% 37% 33% 3	650
June	1st to 10th	Night	4 2 12% 39% 38% 4	600
	11th to 20th	Night	4 2 6% 40% 44% 4	600
	21st to end	Night	3 1 7% 37% 48% 6%	600
July	1st to 10th	Night	2 2 27% 58% 11%	585
	11th to 20th	Night	2 1 20% 64% 13%	582
	21st to end	Night	2 2 18% 60% 17%	776
August	1st to 10th	Night	2 1 2 25% 56% 16%	796
	11th to 20th	Night	1 2 17% 61% 19%	792
	21st to end	Night	2 1 17% 60% 21%	880
September	1st to 10th	Night	3 1 4 22% 57% 14%	800
	11th to 20th	Night	6% 1 4 33% 47% 10%	800
	21st to end	Night	4 1 7% 35% 41% 12%	800
October	1st to 10th	Night	5% 2 6% 43% 37% 7%	800
	11th to 20th	Night	8% 3 10% 39% 31% 9%	800
	21st to end	Night	7% 1 12% 34% 33% 13%	880
November	1st to 10th	Night	6% 3 10% 41% 32% 9%	820
	11th to 20th	Night	7% 2 13% 40% 28% 10%	840
	21st to end	Night	9% 3 15% 41% 25% 7%	840
December	1st to 10th	Night	10% 2 17% 42% 23% 7%	840
	11th to 20th	Night	13% 3 15% 41% 23% 4	840
	21st to end	Night	13% 5% 18% 37% 21% 6%	924

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