Nebula Knoll Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nebula Knoll 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	17% 8% 2 6% 4 4 4 4 6% 7% 37%	126948
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 38%	124102
Transparency	Day	29% 39% 7% 14% 10% 2	130455
	Night	29% 39% 7% 14% 10% 2	128682
Seeing	Day	23% 17% 31% 20% 8% 2	16323
	Night	21% 12% 31% 27% 8% 1	25192

By Month

Percentage Cloud Cover NE (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	12% 7% 3 6% 5% 6% 5% 5% 7% 7% 37%	3597
	11th to 20th	Day	12% 8% 3 6% 4 3 4 3 5% 6% 45%	3570
	21st to end	Day	13% 9% 3 6% 4 4 4 3 4 6% 45%	3960
February	1st to 10th	Day	15% 9% 2 7% 5% 5% 5% 4 5% 7% 36%	1781
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	1948
	21st to end	Day	15% 11% 4 7% 5% 5% 5% 4 4 6% 35%	1650
March	1st to 10th	Day	19% 12% 2 7% 5% 5% 4 4 4 5% 33%	4476
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 5% 6% 38%	4794
	21st to end	Day	25% 9% 2 5% 4 3 3 3 5% 7% 33%	5735
April	1st to 10th	Day	19% 10% 3 6% 5% 3 3 3 5% 6% 37%	2400
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 6% 7% 38%	2427
	21st to end	Day	21% 8% 3 7% 5% 5% 5% 3 4 5% 34%	2600
May	1st to 10th	Day	33% 11% 3 7% 4 4 4 3 4 5% 22%	5886
	11th to 20th	Day	19% 8% 3 9% 6% 5% 5% 4 6% 7% 29%	6075
	21st to end	Day	19% 7% 2 7% 4 5% 6% 4 6% 7% 33%	6949
June	1st to 10th	Day	21% 8% 2 7% 5% 4 5% 3 6% 8% 31%	2800
	11th to 20th	Day	17% 6% 3 6% 5% 5% 4 5% 6% 7% 37%	2800
	21st to end	Day	19% 7% 3 6% 5% 5% 5% 4 6% 8% 33%	2800
July	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 5% 6% 10% 32%	6360
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 5% 7% 10% 39%	6360
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 6% 2 8% 5% 6% 5% 4 6% 8% 36%	4334
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 6% 8% 35%	4175
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4358
September	1st to 10th	Day	12% 4 2 5% 3 4 4 4 7% 8% 48%	2390
	11th to 20th	Day	14% 4 2 5% 3 3 3 3 4 6% 54%	2200
	21st to end	Day	15% 6% 2 5% 3 4 4 3 5% 6% 46%	2200
October	1st to 10th	Day	20% 7% 3 6% 4 3 4 3 4 6% 40%	2200
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 7% 43%	1920
	21st to end	Day	22% 7% 2 6% 4 4 4 3 4 5% 38%	1980
November	1st to 10th	Day	14% 6% 2 5% 4 3 4 3 5% 6% 47%	1851
	11th to 20th	Day	18% 5% 2 4 3 4 4 4 5% 7% 45%	1890
	21st to end	Day	20% 8% 2 6% 3 3 4 3 5% 6% 39%	1874
December	1st to 10th	Day	15% 10% 2 6% 4 4 5% 4 7% 7% 35%	2680
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 5% 6% 43%	3042
	21st to end	Day	21% 10% 2 7% 4 4 5% 4 5% 6% 35%	3960
January	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 6% 42%	6349
	11th to 20th	Night	11% 9% 3 9% 6% 5% 5% 4 6% 7% 35%	6315
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 6% 41%	6996
February	1st to 10th	Night	14% 10% 3 7% 4 5% 4 4 5% 7% 37%	2616
	11th to 20th	Night	12% 9% 3 5% 5% 5% 5% 4 6% 6% 40%	2338
	21st to end	Night	12% 9% 3 6% 5% 5% 5% 4 5% 7% 39%	1980
March	1st to 10th	Night	21% 10% 3 6% 5% 4 4 3 5% 5% 33%	5364
	11th to 20th	Night	13% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	5164
	21st to end	Night	25% 8% 2 5% 3 3 4 4 6% 7% 33%	5194
April	1st to 10th	Night	19% 9% 3 7% 5% 5% 4 3 5% 7% 36%	2000
	11th to 20th	Night	18% 8% 3 7% 5% 4 3 3 6% 6% 38%	1921
	21st to end	Night	21% 8% 3 6% 3 4 3 4 5% 6% 37%	1800
May	1st to 10th	Night	34% 10% 2 7% 4 4 4 3 5% 5% 22%	4054
	11th to 20th	Night	14% 9% 3 9% 6% 6% 6% 4 7% 7% 28%	3860
	21st to end	Night	19% 7% 3 7% 4 4 4 5% 6% 7% 33%	3926
June	1st to 10th	Night	20% 7% 2 7% 5% 4 4 4 7% 6% 34%	1600
	11th to 20th	Night	17% 8% 2 6% 3 4 4 4 5% 7% 39%	1600
	21st to end	Night	19% 6% 2 5% 5% 4 4 4 5% 8% 38%	1600
July	1st to 10th	Night	13% 6% 3 6% 4 5% 5% 5% 7% 10% 38%	3600
	11th to 20th	Night	12% 7% 3 8% 4 5% 5% 4 6% 8% 38%	3600
	21st to end	Night	11% 4 2 5% 4 4 4 3 5% 7% 50%	3926
August	1st to 10th	Night	21% 7% 3 7% 5% 4 4 4 4 7% 34%	2781
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 8% 37%	2872
	21st to end	Night	10% 5% 1 5% 4 4 4 3 5% 9% 49%	3537
September	1st to 10th	Night	12% 6% 2 4 3 3 4 4 5% 6% 52%	2010
	11th to 20th	Night	13% 6% 2 4 3 3 3 2 5% 7% 52%	2200
	21st to end	Night	14% 6% 3 5% 4 3 3 3 5% 6% 47%	2200
October	1st to 10th	Night	18% 7% 2 6% 5% 4 4 4 4 6% 41%	2200
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 7% 43%	2476
	21st to end	Night	22% 7% 2 6% 4 3 4 2 5% 6% 37%	2860
November	1st to 10th	Night	15% 6% 3 6% 3 4 4 3 5% 7% 45%	2667
	11th to 20th	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2730
	21st to end	Night	16% 8% 2 5% 5% 4 4 3 5% 7% 42%	2743
December	1st to 10th	Night	15% 11% 3 6% 4 4 4 4 5% 6% 38%	4720
	11th to 20th	Night	22% 7% 2 5% 4 3 3 3 4 5% 40%	5307
	21st to end	Night	20% 10% 3 7% 4 4 3 3 4 5% 36%	6996

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	31% 29% 5% 22% 12% 1	3598
	11th to 20th	Day	28% 28% 9% 21% 12% 3	3600
	21st to end	Day	29% 30% 6% 16% 15% 4	3960
February	1st to 10th	Day	18% 62% 5% 8% 5% 2	1781
	11th to 20th	Day	18% 61% 3 7% 8% 2	1957
	21st to end	Day	17% 62% 3 8% 8% 2	1650
March	1st to 10th	Day	41% 30% 5% 8% 9% 8%	4479
	11th to 20th	Day	32% 30% 12% 13% 9% 3	4794
	21st to end	Day	39% 29% 3 12% 13% 4	5735
April	1st to 10th	Day	20% 62% 2 7% 6% 2	2400
	11th to 20th	Day	17% 63% 4 8% 7% 1	2427
	21st to end	Day	21% 61% 3 6% 8%	2600
May	1st to 10th	Day	50% 32% 7% 9% 3	5897
	11th to 20th	Day	40% 33% 10% 12% 4 1	6079
	21st to end	Day	36% 33% 9% 13% 9%	6949
June	1st to 10th	Day	18% 62% 3 9% 7%	2800
	11th to 20th	Day	16% 62% 5% 8% 8% 1	2800
	21st to end	Day	17% 62% 3 10% 7% 1	2800
July	1st to 10th	Day	35% 28% 10% 16% 10%	6290
	11th to 20th	Day	28% 30% 11% 18% 12% 1	6285
	21st to end	Day	30% 27% 9% 20% 14% 1	6728
August	1st to 10th	Day	29% 38% 9% 16% 7%	4320
	11th to 20th	Day	33% 38% 9% 16% 5%	4214
	21st to end	Day	20% 38% 10% 17% 13% 1	4358
September	1st to 10th	Day	12% 58% 6% 15% 8% 1	2390
	11th to 20th	Day	16% 57% 3 12% 10% 2	2200
	21st to end	Day	17% 57% 3 13% 8% 1	2200
October	1st to 10th	Day	28% 38% 5% 15% 12% 1	3590
	11th to 20th	Day	19% 36% 7% 20% 16% 2	3097
	21st to end	Day	21% 36% 6% 21% 14% 2	3180
November	1st to 10th	Day	19% 53% 7% 11% 8% 1	1851
	11th to 20th	Day	17% 54% 4 9% 13% 2	1890
	21st to end	Day	20% 54% 6% 11% 8% 1	1874
December	1st to 10th	Day	28% 38% 9% 19% 6%	2680
	11th to 20th	Day	28% 32% 7% 14% 15% 5%	3042
	21st to end	Day	37% 28% 5% 14% 11% 6%	3960
January	1st to 10th	Night	29% 29% 6% 25% 9% 2	6349
	11th to 20th	Night	34% 28% 10% 15% 10% 2	6360
	21st to end	Night	32% 30% 7% 13% 13% 6%	6996
February	1st to 10th	Night	19% 62% 5% 7% 6% 1	2616
	11th to 20th	Night	16% 61% 4 8% 10% 2	2340
	21st to end	Night	17% 62% 3 10% 6% 3	1980
March	1st to 10th	Night	40% 30% 6% 7% 11% 5%	5374
	11th to 20th	Night	33% 31% 13% 11% 10% 2	5165
	21st to end	Night	38% 29% 3 10% 16% 4	5194
April	1st to 10th	Night	21% 62% 2 6% 8% 1	2000
	11th to 20th	Night	19% 63% 2 7% 7% 1	1921
	21st to end	Night	18% 62% 4 7% 8% 1	1800
May	1st to 10th	Night	48% 32% 8% 8% 4	4060
	11th to 20th	Night	38% 34% 9% 14% 5%	3861
	21st to end	Night	36% 32% 12% 14% 6% 1	3927
June	1st to 10th	Night	18% 63% 4 8% 6%	1600
	11th to 20th	Night	15% 63% 6% 9% 6% 1	1600
	21st to end	Night	17% 61% 4 13% 5% 1	1600
July	1st to 10th	Night	29% 29% 12% 19% 10%	3560
	11th to 20th	Night	32% 30% 10% 15% 14%	3552
	21st to end	Night	24% 27% 10% 27% 11%	3809
August	1st to 10th	Night	35% 38% 8% 12% 6%	2771
	11th to 20th	Night	32% 39% 10% 13% 6%	2899
	21st to end	Night	18% 38% 11% 20% 12% 1	3537
September	1st to 10th	Night	12% 59% 6% 14% 9%	2010
	11th to 20th	Night	15% 57% 4 12% 10% 2	2200
	21st to end	Night	16% 57% 5% 14% 8% 1	2200
October	1st to 10th	Night	28% 38% 8% 16% 10% 1	3590
	11th to 20th	Night	16% 36% 6% 19% 20% 3	4038
	21st to end	Night	23% 36% 5% 22% 12% 2	4610
November	1st to 10th	Night	19% 54% 4 14% 8% 1	2667
	11th to 20th	Night	18% 55% 3 11% 13% 1	2730
	21st to end	Night	18% 56% 7% 12% 7%	2743
December	1st to 10th	Night	29% 35% 6% 22% 8%	4720
	11th to 20th	Night	34% 31% 6% 10% 16% 3	5307
	21st to end	Night	38% 27% 5% 15% 13% 2	6996

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	26% 25% 28% 18% 3 1	200
	11th to 20th	Day	28% 20% 32% 17% 3 2	200
	21st to end	Day	20% 21% 40% 15% 4	220
February	1st to 10th	Day	26% 25% 32% 15% 2	350
	11th to 20th	Day	26% 24% 33% 14% 2	496
	21st to end	Day	28% 19% 31% 17% 4	414
March	1st to 10th	Day	28% 21% 32% 16% 3	500
	11th to 20th	Day	25% 27% 30% 13% 4 1	500
	21st to end	Day	29% 20% 31% 12% 6% 2	550
April	1st to 10th	Day	33% 24% 26% 11% 6% 1	500
	11th to 20th	Day	28% 26% 25% 13% 6% 1	492
	21st to end	Day	29% 22% 29% 12% 6% 1	500
May	1st to 10th	Day	35% 18% 26% 16% 3 1	503
	11th to 20th	Day	21% 19% 36% 17% 5% 2	527
	21st to end	Day	22% 17% 32% 22% 7% 1	573
June	1st to 10th	Day	21% 15% 38% 18% 8% 1	530
	11th to 20th	Day	19% 17% 33% 22% 7% 2	530
	21st to end	Day	22% 15% 36% 16% 10% 2	530
July	1st to 10th	Day	19% 10% 34% 25% 10% 3	525
	11th to 20th	Day	13% 8% 35% 29% 12% 3	524
	21st to end	Day	16% 9% 36% 29% 8% 2	576
August	1st to 10th	Day	13% 10% 27% 32% 16% 3	526
	11th to 20th	Day	16% 10% 28% 26% 15% 5%	519
	21st to end	Day	12% 10% 31% 28% 16% 3	583
September	1st to 10th	Day	15% 11% 28% 26% 14% 6%	530
	11th to 20th	Day	17% 11% 27% 20% 15% 10%	530
	21st to end	Day	18% 12% 32% 22% 11% 5%	530
October	1st to 10th	Day	25% 12% 23% 23% 12% 4	530
	11th to 20th	Day	21% 14% 29% 22% 12% 2	450
	21st to end	Day	31% 11% 31% 20% 7%	462
November	1st to 10th	Day	21% 18% 32% 22% 6% 1	425
	11th to 20th	Day	28% 17% 28% 21% 4 1	430
	21st to end	Day	30% 20% 29% 16% 4 1	414
December	1st to 10th	Day	26% 25% 26% 15% 8%	220
	11th to 20th	Day	32% 23% 31% 11% 3	214
	21st to end	Day	27% 21% 31% 14% 7%	220
January	1st to 10th	Night	20% 16% 39% 22% 2	800
	11th to 20th	Night	26% 16% 33% 22% 4	800
	21st to end	Night	17% 18% 38% 23% 4	880
February	1st to 10th	Night	22% 17% 38% 21% 2	800
	11th to 20th	Night	20% 18% 39% 18% 4	783
	21st to end	Night	22% 15% 36% 23% 4	660
March	1st to 10th	Night	26% 15% 36% 21% 3	800
	11th to 20th	Night	21% 17% 34% 23% 5%	800
	21st to end	Night	26% 15% 33% 20% 6%	880
April	1st to 10th	Night	25% 17% 33% 22% 4	800
	11th to 20th	Night	23% 16% 35% 21% 5%	737
	21st to end	Night	25% 16% 30% 25% 4	600
May	1st to 10th	Night	28% 11% 29% 28% 4 1	600
	11th to 20th	Night	22% 12% 34% 25% 7%	523
	21st to end	Night	22% 9% 27% 33% 8% 1	433
June	1st to 10th	Night	21% 7% 33% 29% 10%	400
	11th to 20th	Night	20% 7% 29% 31% 13% 1	400
	21st to end	Night	21% 4 31% 35% 9%	400
July	1st to 10th	Night	16% 3 25% 42% 13%	390
	11th to 20th	Night	14% 4 22% 41% 18% 1	388
	21st to end	Night	14% 3 22% 46% 14% 1	438
August	1st to 10th	Night	17% 4 21% 40% 17%	515
	11th to 20th	Night	15% 7% 27% 33% 16% 2	592
	21st to end	Night	15% 5% 25% 36% 17% 2	844
September	1st to 10th	Night	16% 5% 29% 34% 17% 1	800
	11th to 20th	Night	16% 7% 23% 31% 20% 4	800
	21st to end	Night	17% 8% 28% 29% 17% 2	800
October	1st to 10th	Night	22% 10% 23% 30% 14% 2	800
	11th to 20th	Night	19% 11% 32% 25% 11% 1	800
	21st to end	Night	28% 12% 27% 27% 6%	880
November	1st to 10th	Night	20% 13% 33% 26% 7% 1	820
	11th to 20th	Night	22% 17% 30% 25% 5% 1	840
	21st to end	Night	23% 18% 31% 23% 5%	840
December	1st to 10th	Night	24% 12% 38% 20% 5%	840
	11th to 20th	Night	24% 18% 30% 23% 3	829
	21st to end	Night	25% 14% 34% 25% 3	880

Page generated at 2025-12-15 01:13:54. Version ac07

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