Cedar Knoll Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cedar Knoll Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	12% 8% 3 7% 5% 4 5% 4 6% 7% 40%	131277
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	131080
Transparency	Day	41% 5% 11% 25% 16% 3	134920
	Night	41% 4 10% 24% 18% 3	135510
Seeing	Day	16% 9% 23% 24% 19% 9%	17305
	Night	17% 8% 24% 31% 18% 2	27196

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% 10% 6% 12% 5% 5% 5% 3 5% 5% 31%	4691
	11th to 20th	Day	11% 8% 3 9% 6% 4 4 3 4 6% 42%	4710
	21st to end	Day	18% 11% 5% 8% 5% 4 4 3 4 5% 34%	5936
February	1st to 10th	Day	17% 11% 4 7% 4 4 4 3 4 6% 36%	2100
	11th to 20th	Day	18% 10% 4 8% 4 4 3 3 5% 5% 35%	2048
	21st to end	Day	15% 11% 3 8% 6% 4 4 3 5% 5% 35%	1730
March	1st to 10th	Day	17% 12% 4 7% 4 3 3 3 4 5% 38%	4580
	11th to 20th	Day	12% 9% 3 6% 3 3 3 2 4 6% 48%	4756
	21st to end	Day	12% 9% 3 7% 5% 4 4 3 5% 6% 41%	6088
April	1st to 10th	Day	12% 7% 2 6% 4 4 4 4 6% 7% 43%	2520
	11th to 20th	Day	14% 8% 3 6% 4 4 4 4 5% 7% 40%	2446
	21st to end	Day	16% 7% 3 6% 4 4 4 4 5% 7% 41%	2421
May	1st to 10th	Day	17% 9% 3 9% 6% 5% 6% 5% 6% 8% 26%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	6516
June	1st to 10th	Day	10% 5% 1 6% 4 4 5% 4 7% 9% 45%	2800
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 7% 10% 47%	2800
	21st to end	Day	5% 4 2 5% 6% 5% 6% 5% 8% 10% 45%	2800
July	1st to 10th	Day	8% 6% 3 6% 5% 5% 5% 4 6% 10% 41%	6360
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 12% 40%	6360
	21st to end	Day	6% 7% 3 9% 6% 6% 7% 6% 8% 11% 31%	6708
August	1st to 10th	Day	5% 4 1 4 3 4 5% 4 7% 10% 53%	4007
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	3849
	21st to end	Day	5% 4 2 5% 4 4 3 3 6% 9% 56%	4312
September	1st to 10th	Day	9% 6% 2 6% 5% 4 4 4 6% 7% 46%	2400
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 3 6% 8% 47%	2222
	21st to end	Day	12% 5% 2 6% 5% 3 4 3 4 7% 50%	2200
October	1st to 10th	Day	9% 4 2 4 3 3 4 3 4 7% 57%	2200
	11th to 20th	Day	12% 4 2 5% 4 3 3 3 4 7% 53%	2060
	21st to end	Day	19% 7% 2 5% 4 3 3 2 4 5% 47%	2020
November	1st to 10th	Day	9% 6% 3 6% 5% 3 3 3 4 5% 52%	1852
	11th to 20th	Day	16% 7% 2 5% 4 4 2 3 4 7% 46%	1890
	21st to end	Day	17% 9% 3 6% 3 3 4 3 4 6% 41%	1890
December	1st to 10th	Day	14% 10% 3 7% 4 3 3 3 4 5% 45%	3020
	11th to 20th	Day	15% 10% 5% 10% 5% 5% 4 3 5% 5% 34%	3192
	21st to end	Day	19% 14% 5% 9% 5% 5% 4 4 4 6% 25%	4070
January	1st to 10th	Night	16% 12% 4 9% 4 4 3 3 4 5% 37%	8232
	11th to 20th	Night	14% 10% 4 10% 4 4 4 2 3 4 41%	8280
	21st to end	Night	18% 10% 4 7% 4 4 4 3 4 6% 37%	8258
February	1st to 10th	Night	20% 12% 5% 7% 4 3 4 3 4 4 33%	2520
	11th to 20th	Night	20% 8% 3 8% 4 4 3 3 4 5% 38%	2458
	21st to end	Night	19% 9% 3 8% 4 3 3 4 4 5% 38%	2076
March	1st to 10th	Night	18% 11% 4 8% 4 3 4 3 4 5% 36%	5493
	11th to 20th	Night	12% 7% 3 5% 3 3 3 3 5% 7% 49%	5423
	21st to end	Night	14% 8% 3 8% 5% 4 4 3 5% 7% 38%	5124
April	1st to 10th	Night	13% 7% 2 5% 5% 4 4 4 5% 7% 46%	2100
	11th to 20th	Night	14% 6% 2 6% 4 3 4 3 5% 7% 45%	2035
	21st to end	Night	15% 7% 3 6% 5% 3 3 4 6% 8% 38%	1976
May	1st to 10th	Night	17% 10% 3 7% 4 4 4 4 6% 9% 31%	4033
	11th to 20th	Night	14% 8% 3 9% 6% 5% 5% 4 7% 7% 33%	4032
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	4336
June	1st to 10th	Night	11% 5% 3 5% 4 4 4 4 6% 9% 46%	1600
	11th to 20th	Night	6% 3 2 5% 4 4 4 4 7% 11% 52%	1600
	21st to end	Night	3 2 1 4 5% 5% 5% 4 8% 13% 51%	1600
July	1st to 10th	Night	6% 6% 2 6% 4 5% 5% 4 6% 9% 47%	3618
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 48%	3617
	21st to end	Night	6% 6% 3 8% 6% 6% 6% 5% 8% 11% 35%	4151
August	1st to 10th	Night	5% 3 1 5% 3 3 3 4 5% 8% 60%	3138
	11th to 20th	Night	4 4 2 6% 4 4 5% 5% 6% 9% 51%	3200
	21st to end	Night	4 3 1 3 4 3 4 4 5% 9% 61%	3586
September	1st to 10th	Night	9% 4 2 6% 4 4 3 4 4 6% 54%	2000
	11th to 20th	Night	8% 6% 3 5% 3 4 3 3 6% 7% 51%	2157
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 6% 54%	2200
October	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 6% 59%	2200
	11th to 20th	Night	13% 5% 2 5% 2 3 4 2 4 5% 56%	2335
	21st to end	Night	18% 8% 2 5% 4 3 3 3 4 6% 46%	2819
November	1st to 10th	Night	12% 5% 2 6% 3 3 3 3 4 6% 53%	2666
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 4 6% 50%	2730
	21st to end	Night	17% 7% 3 6% 4 3 2 3 4 6% 45%	2730
December	1st to 10th	Night	17% 9% 3 8% 4 3 3 2 4 4 42%	4380
	11th to 20th	Night	18% 10% 3 6% 4 3 4 3 5% 6% 39%	5227
	21st to end	Night	22% 13% 4 9% 5% 5% 4 3 4 4 26%	7150

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	55% 4 13% 21% 7% 1	4692
	11th to 20th	Day	45% 3 6% 28% 17% 2	4710
	21st to end	Day	54% 2 9% 19% 11% 4	5936
February	1st to 10th	Day	49% 3 10% 24% 12% 1	2100
	11th to 20th	Day	52% 3 8% 21% 12% 3	2058
	21st to end	Day	51% 3 9% 19% 14% 5%	1730
March	1st to 10th	Day	50% 3 4 20% 18% 4	4580
	11th to 20th	Day	40% 3 8% 27% 20% 3	4756
	21st to end	Day	43% 4 10% 27% 15% 1	6090
April	1st to 10th	Day	40% 4 13% 24% 17% 2	2520
	11th to 20th	Day	43% 4 9% 24% 16% 3	2446
	21st to end	Day	43% 4 10% 28% 12% 2	2421
May	1st to 10th	Day	53% 6% 13% 17% 10% 1	5897
	11th to 20th	Day	48% 6% 13% 23% 11% 1	5865
	21st to end	Day	42% 6% 12% 20% 17% 3	6530
June	1st to 10th	Day	34% 6% 13% 25% 20% 3	2800
	11th to 20th	Day	31% 7% 13% 27% 21% 2	2800
	21st to end	Day	31% 8% 15% 25% 17% 5%	2800
July	1st to 10th	Day	37% 6% 11% 29% 16% 2	6291
	11th to 20th	Day	32% 10% 18% 29% 10% 1	6285
	21st to end	Day	42% 9% 19% 22% 7% 1	6516
August	1st to 10th	Day	24% 6% 16% 33% 19% 1	3994
	11th to 20th	Day	34% 7% 13% 28% 16% 1	3885
	21st to end	Day	25% 5% 11% 36% 20% 3	4311
September	1st to 10th	Day	35% 6% 10% 25% 20% 5%	2400
	11th to 20th	Day	35% 5% 9% 25% 23% 3	2222
	21st to end	Day	36% 4 7% 23% 26% 4	2200
October	1st to 10th	Day	18% 2 6% 20% 46% 7%	3590
	11th to 20th	Day	22% 3 8% 27% 32% 8%	3339
	21st to end	Day	31% 3 6% 28% 27% 6%	3245
November	1st to 10th	Day	34% 4 8% 26% 22% 5%	1852
	11th to 20th	Day	40% 2 8% 24% 21% 5%	1890
	21st to end	Day	44% 4 6% 26% 17% 3	1890
December	1st to 10th	Day	43% 3 9% 27% 17% 1	3020
	11th to 20th	Day	52% 4 9% 22% 11% 2	3189
	21st to end	Day	59% 4 12% 18% 7%	4070
January	1st to 10th	Night	50% 3 12% 26% 9% 1	8233
	11th to 20th	Night	49% 2 5% 23% 18% 3	8280
	21st to end	Night	50% 3 8% 21% 16% 2	8258
February	1st to 10th	Night	55% 4 8% 20% 12% 1	2520
	11th to 20th	Night	51% 4 6% 23% 15% 3	2459
	21st to end	Night	50% 3 8% 20% 15% 4	2076
March	1st to 10th	Night	50% 3 6% 17% 20% 4	5493
	11th to 20th	Night	36% 3 10% 23% 20% 8%	5423
	21st to end	Night	45% 4 11% 24% 14% 2	5124
April	1st to 10th	Night	38% 4 11% 26% 18% 3	2100
	11th to 20th	Night	39% 4 10% 21% 20% 6%	2035
	21st to end	Night	44% 5% 9% 23% 17% 2	1976
May	1st to 10th	Night	49% 5% 13% 23% 9% 1	4060
	11th to 20th	Night	48% 7% 12% 21% 11% 1	4033
	21st to end	Night	40% 6% 13% 24% 14% 2	4344
June	1st to 10th	Night	34% 6% 11% 25% 21% 3	1600
	11th to 20th	Night	26% 5% 16% 23% 25% 4	1600
	21st to end	Night	23% 8% 16% 27% 19% 6%	1600
July	1st to 10th	Night	32% 7% 12% 26% 20% 2	3580
	11th to 20th	Night	30% 9% 18% 29% 12% 2	3572
	21st to end	Night	39% 9% 19% 23% 9% 1	4030
August	1st to 10th	Night	21% 6% 14% 39% 19% 1	3127
	11th to 20th	Night	27% 6% 16% 30% 19% 1	3230
	21st to end	Night	20% 5% 10% 37% 25% 3	3586
September	1st to 10th	Night	31% 4 9% 26% 23% 5%	2000
	11th to 20th	Night	31% 5% 8% 27% 25% 4	2157
	21st to end	Night	32% 4 8% 23% 26% 7%	2200
October	1st to 10th	Night	19% 2 5% 15% 49% 10%	3590
	11th to 20th	Night	21% 2 6% 24% 39% 8%	3795
	21st to end	Night	31% 3 6% 23% 28% 10%	4544
November	1st to 10th	Night	34% 3 8% 25% 25% 5%	2666
	11th to 20th	Night	36% 2 7% 25% 22% 8%	2730
	21st to end	Night	42% 3 9% 25% 18% 3	2730
December	1st to 10th	Night	46% 3 9% 23% 18% 1	4380
	11th to 20th	Night	46% 3 9% 23% 17% 1	5229
	21st to end	Night	62% 3 8% 18% 7% 1	7150

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	23% 17% 33% 21% 6% 1	330
	11th to 20th	Day	23% 20% 37% 16% 3 1	330
	21st to end	Day	21% 17% 44% 14% 4	363
February	1st to 10th	Day	26% 18% 33% 19% 4 1	330
	11th to 20th	Day	29% 19% 37% 12% 2	330
	21st to end	Day	29% 16% 34% 16% 5%	273
March	1st to 10th	Day	26% 14% 40% 14% 5%	330
	11th to 20th	Day	24% 19% 35% 16% 4 1	388
	21st to end	Day	24% 14% 35% 19% 6% 1	594
April	1st to 10th	Day	19% 15% 31% 24% 9% 1	540
	11th to 20th	Day	20% 16% 28% 22% 13% 2	516
	21st to end	Day	20% 12% 33% 21% 11% 4	500
May	1st to 10th	Day	21% 12% 24% 28% 11% 3	503
	11th to 20th	Day	18% 9% 23% 25% 18% 7%	527
	21st to end	Day	11% 5% 21% 30% 19% 13%	604
June	1st to 10th	Day	12% 4 20% 26% 26% 12%	730
	11th to 20th	Day	6% 3 18% 28% 31% 13%	730
	21st to end	Day	7% 3 18% 33% 23% 16%	730
July	1st to 10th	Day	10% 3 13% 24% 27% 22%	721
	11th to 20th	Day	6% 4 12% 29% 32% 17%	718
	21st to end	Day	8% 3 14% 25% 32% 19%	704
August	1st to 10th	Day	9% 3 11% 25% 33% 19%	526
	11th to 20th	Day	12% 3 12% 29% 32% 13%	519
	21st to end	Day	10% 3 9% 20% 38% 21%	583
September	1st to 10th	Day	10% 2 13% 25% 33% 17%	530
	11th to 20th	Day	9% 2 8% 25% 35% 21%	530
	21st to end	Day	17% 6% 18% 25% 24% 12%	530
October	1st to 10th	Day	11% 3 17% 31% 28% 10%	530
	11th to 20th	Day	12% 10% 28% 30% 13% 7%	390
	21st to end	Day	26% 9% 29% 25% 9% 2	363
November	1st to 10th	Day	14% 9% 35% 25% 13% 4	330
	11th to 20th	Day	16% 17% 32% 25% 10% 1	330
	21st to end	Day	23% 12% 29% 22% 12% 1	330
December	1st to 10th	Day	20% 16% 35% 18% 9% 2	330
	11th to 20th	Day	23% 12% 32% 25% 6% 1	330
	21st to end	Day	27% 17% 33% 19% 4	363
January	1st to 10th	Night	22% 15% 29% 29% 4	1050
	11th to 20th	Night	26% 15% 34% 20% 5%	1050
	21st to end	Night	21% 12% 37% 28% 3	1076
February	1st to 10th	Night	26% 12% 33% 26% 2	840
	11th to 20th	Night	26% 13% 35% 24% 2	823
	21st to end	Night	25% 14% 37% 23% 2	692
March	1st to 10th	Night	22% 14% 30% 30% 5%	840
	11th to 20th	Night	23% 10% 33% 28% 6%	840
	21st to end	Night	22% 11% 29% 30% 7%	719
April	1st to 10th	Night	17% 11% 31% 32% 9%	630
	11th to 20th	Night	18% 9% 28% 31% 13% 1	611
	21st to end	Night	18% 7% 29% 34% 11% 1	600
May	1st to 10th	Night	21% 7% 25% 31% 16% 1	600
	11th to 20th	Night	16% 4 20% 36% 21% 3	591
	21st to end	Night	8% 3 17% 38% 30% 4	650
June	1st to 10th	Night	14% 3 14% 37% 30% 3	600
	11th to 20th	Night	6% 3 11% 42% 36% 3	600
	21st to end	Night	3 2 13% 39% 38% 6%	600
July	1st to 10th	Night	8% 3 7% 30% 42% 10%	585
	11th to 20th	Night	7% 4 6% 31% 46% 6%	582
	21st to end	Night	9% 1 10% 34% 37% 9%	657
August	1st to 10th	Night	9% 1 7% 31% 45% 7%	597
	11th to 20th	Night	6% 2 9% 34% 44% 6%	594
	21st to end	Night	7% 2 6% 30% 48% 7%	660
September	1st to 10th	Night	12% 3 9% 32% 39% 6%	600
	11th to 20th	Night	10% 3 9% 29% 41% 7%	757
	21st to end	Night	14% 4 17% 31% 30% 5%	800
October	1st to 10th	Night	11% 4 15% 35% 30% 6%	800
	11th to 20th	Night	15% 6% 22% 36% 20% 2	800
	21st to end	Night	22% 7% 28% 29% 13% 1	880
November	1st to 10th	Night	13% 8% 28% 35% 14% 1	820
	11th to 20th	Night	17% 10% 31% 30% 11%	840
	21st to end	Night	21% 10% 27% 31% 11%	840
December	1st to 10th	Night	23% 11% 28% 32% 6%	840
	11th to 20th	Night	25% 9% 29% 30% 6%	977
	21st to end	Night	28% 12% 30% 26% 5%	1155

Page generated at 2026-04-14 01:44:27. Version ac07

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