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 2024-01-16.

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	13% 7% 3 6% 5% 3 5% 5% 6% 7% 41%	74861
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	74950
Transparency	Day	41% 5% 11% 24% 16% 3	74568
	Night	41% 4 10% 24% 18% 4	74764
Seeing	Day	16% 8% 23% 24% 20% 10%	14275
	Night	18% 7% 23% 31% 19% 3	24186

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	17% 8% 4 9% 4 3 5% 3 5% 5% 36%	1517
	11th to 20th	Day	19% 11% 5% 8% 6% 4 4 4 5% 6% 29%	1485
	21st to end	Day	14% 9% 4 7% 6% 2 5% 4 4 6% 38%	1819
February	1st to 10th	Day	19% 10% 5% 6% 5% 3 4 4 4 6% 35%	1800
	11th to 20th	Day	19% 9% 4 7% 5% 3 4 4 5% 5% 33%	1748
	21st to end	Day	16% 12% 3 7% 6% 3 4 4 4 6% 34%	1480
March	1st to 10th	Day	16% 10% 3 5% 5% 3 4 4 5% 5% 41%	1800
	11th to 20th	Day	21% 8% 4 7% 5% 3 4 4 4 6% 34%	1849
	21st to end	Day	18% 9% 4 7% 6% 3 5% 5% 5% 6% 32%	2366
April	1st to 10th	Day	11% 7% 3 5% 5% 3 4 5% 5% 8% 44%	2160
	11th to 20th	Day	15% 7% 4 6% 5% 3 4 5% 5% 7% 40%	2134
	21st to end	Day	17% 6% 3 5% 5% 3 4 4 5% 7% 41%	2178
May	1st to 10th	Day	16% 6% 3 6% 6% 3 5% 5% 6% 8% 37%	2340
	11th to 20th	Day	16% 7% 3 6% 6% 3 4 4 5% 7% 39%	2301
	21st to end	Day	8% 5% 3 6% 7% 4 6% 6% 7% 9% 38%	2558
June	1st to 10th	Day	9% 4 2 5% 5% 3 5% 5% 6% 9% 46%	2520
	11th to 20th	Day	4 3 2 4 5% 4 6% 7% 7% 10% 48%	2520
	21st to end	Day	5% 3 2 5% 6% 4 6% 6% 7% 10% 45%	2520
July	1st to 10th	Day	9% 5% 3 5% 7% 4 7% 7% 7% 9% 37%	2520
	11th to 20th	Day	6% 4 2 5% 5% 3 6% 6% 7% 10% 46%	2520
	21st to end	Day	7% 4 2 5% 6% 3 6% 7% 8% 11% 40%	2663
August	1st to 10th	Day	7% 5% 2 5% 5% 3 6% 6% 8% 10% 43%	2205
	11th to 20th	Day	9% 6% 3 5% 6% 4 5% 6% 7% 9% 42%	2125
	21st to end	Day	8% 6% 2 4 5% 4 4 4 6% 9% 48%	2376
September	1st to 10th	Day	10% 5% 2 5% 6% 3 4 5% 7% 7% 45%	2160
	11th to 20th	Day	11% 5% 2 5% 6% 3 5% 4 6% 8% 47%	2000
	21st to end	Day	11% 4 2 4 5% 2 4 4 4 7% 53%	1980
October	1st to 10th	Day	10% 4 2 4 4 2 4 4 3 7% 56%	1980
	11th to 20th	Day	14% 4 2 5% 4 2 3 4 4 7% 50%	1854
	21st to end	Day	19% 7% 2 4 5% 2 3 3 4 5% 46%	1818
November	1st to 10th	Day	9% 5% 3 5% 5% 3 3 4 4 5% 55%	1672
	11th to 20th	Day	16% 5% 2 4 5% 3 3 4 4 6% 46%	1710
	21st to end	Day	18% 8% 3 5% 3 3 4 5% 5% 6% 43%	1710
December	1st to 10th	Day	18% 10% 3 6% 5% 2 3 5% 4 5% 38%	1710
	11th to 20th	Day	15% 10% 5% 8% 6% 3 5% 4 5% 6% 33%	1980
	21st to end	Day	18% 12% 5% 6% 6% 4 4 5% 4 6% 28%	2783
January	1st to 10th	Night	16% 10% 3 7% 6% 3 4 4 4 6% 38%	2648
	11th to 20th	Night	19% 11% 5% 8% 6% 2 4 4 4 5% 31%	2603
	21st to end	Night	16% 8% 3 7% 5% 2 4 4 5% 7% 37%	2528
February	1st to 10th	Night	22% 11% 6% 6% 5% 3 4 3 4 5% 33%	2160
	11th to 20th	Night	19% 8% 4 7% 5% 4 4 4 4 5% 38%	2098
	21st to end	Night	21% 10% 4 6% 5% 3 3 4 4 5% 35%	1776
March	1st to 10th	Night	16% 8% 4 6% 4 2 4 4 4 5% 44%	2160
	11th to 20th	Night	17% 10% 3 6% 4 3 4 4 4 7% 38%	2110
	21st to end	Night	20% 9% 3 5% 6% 3 4 4 5% 7% 34%	2003
April	1st to 10th	Night	12% 6% 2 4 5% 3 4 4 5% 7% 47%	1800
	11th to 20th	Night	15% 6% 2 6% 4 3 3 4 5% 6% 45%	1775
	21st to end	Night	16% 6% 3 5% 6% 2 4 5% 6% 8% 39%	1779
May	1st to 10th	Night	17% 7% 3 5% 4 3 4 4 5% 8% 41%	1620
	11th to 20th	Night	13% 5% 3 6% 5% 3 5% 5% 6% 8% 41%	1592
	21st to end	Night	6% 3 2 5% 5% 3 6% 6% 8% 12% 44%	1711
June	1st to 10th	Night	11% 5% 2 4 4 3 4 5% 6% 9% 47%	1440
	11th to 20th	Night	5% 2 1 4 4 3 4 5% 7% 11% 54%	1440
	21st to end	Night	3 2 2 4 5% 3 6% 4 7% 13% 51%	1440
July	1st to 10th	Night	7% 4 2 4 4 3 5% 5% 6% 10% 49%	1440
	11th to 20th	Night	7% 5% 2 4 4 3 4 5% 5% 10% 53%	1440
	21st to end	Night	8% 3 2 4 5% 4 6% 6% 6% 9% 47%	1651
August	1st to 10th	Night	7% 4 2 4 5% 3 3 6% 6% 10% 51%	1729
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 6% 6% 10% 48%	1776
	21st to end	Night	6% 4 2 4 4 3 4 4 5% 10% 53%	1980
September	1st to 10th	Night	10% 4 2 5% 5% 3 3 5% 4 6% 54%	1800
	11th to 20th	Night	8% 6% 3 4 4 4 3 4 6% 7% 51%	1941
	21st to end	Night	11% 5% 2 3 3 2 3 4 4 6% 56%	1980
October	1st to 10th	Night	9% 4 2 4 4 3 3 4 4 6% 58%	1980
	11th to 20th	Night	14% 5% 2 4 3 3 4 4 4 5% 52%	2101
	21st to end	Night	18% 8% 2 5% 4 2 4 3 4 5% 45%	2537
November	1st to 10th	Night	12% 4 2 5% 4 2 3 3 4 5% 55%	2406
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 4 6% 50%	2470
	21st to end	Night	16% 6% 3 5% 4 3 3 3 4 6% 46%	2470
December	1st to 10th	Night	19% 10% 4 6% 5% 2 3 4 5% 6% 36%	2470
	11th to 20th	Night	17% 9% 3 6% 5% 3 4 4 5% 5% 38%	3212
	21st to end	Night	18% 13% 5% 8% 6% 4 5% 5% 4 5% 27%	4884

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	50% 4 8% 24% 11% 2	1518
	11th to 20th	Day	57% 3 9% 20% 10%	1485
	21st to end	Day	47% 3 10% 27% 11% 2	1819
February	1st to 10th	Day	50% 3 9% 24% 12% 2	1800
	11th to 20th	Day	53% 4 8% 22% 11% 3	1758
	21st to end	Day	52% 3 10% 18% 12% 5%	1480
March	1st to 10th	Day	46% 4 7% 23% 18% 2	1800
	11th to 20th	Day	53% 3 10% 22% 11% 2	1849
	21st to end	Day	53% 5% 11% 20% 10% 2	2366
April	1st to 10th	Day	39% 4 12% 25% 18% 2	2160
	11th to 20th	Day	44% 4 8% 24% 17% 3	2134
	21st to end	Day	43% 4 11% 28% 12% 2	2178
May	1st to 10th	Day	45% 6% 13% 25% 10% 1	2340
	11th to 20th	Day	46% 4 12% 22% 15% 1	2305
	21st to end	Day	39% 7% 18% 21% 12% 2	2558
June	1st to 10th	Day	33% 6% 13% 25% 20% 2	2520
	11th to 20th	Day	29% 7% 14% 28% 20% 3	2520
	21st to end	Day	32% 7% 15% 25% 17% 5%	2520
July	1st to 10th	Day	41% 7% 13% 23% 13% 4	2451
	11th to 20th	Day	32% 6% 16% 26% 16% 3	2445
	21st to end	Day	35% 7% 17% 27% 12% 2	2515
August	1st to 10th	Day	34% 7% 15% 29% 15% 1	2192
	11th to 20th	Day	37% 6% 13% 28% 14% 2	2125
	21st to end	Day	33% 5% 12% 27% 18% 5%	2376
September	1st to 10th	Day	36% 6% 10% 26% 19% 4	2160
	11th to 20th	Day	37% 5% 8% 25% 23% 3	2000
	21st to end	Day	33% 3 7% 25% 26% 5%	1980
October	1st to 10th	Day	30% 3 9% 25% 28% 5%	1980
	11th to 20th	Day	35% 4 7% 25% 22% 7%	1854
	21st to end	Day	42% 3 6% 26% 19% 4	1818
November	1st to 10th	Day	32% 4 9% 27% 23% 6%	1672
	11th to 20th	Day	40% 2 8% 24% 21% 5%	1710
	21st to end	Day	43% 4 6% 27% 17% 3	1710
December	1st to 10th	Day	49% 3 11% 23% 12% 1	1710
	11th to 20th	Day	52% 4 8% 24% 10% 1	1977
	21st to end	Day	57% 3 12% 18% 10%	2783
January	1st to 10th	Night	49% 3 8% 22% 15% 2	2648
	11th to 20th	Night	57% 3 8% 20% 11% 1	2603
	21st to end	Night	48% 3 11% 23% 13% 2	2528
February	1st to 10th	Night	56% 3 8% 21% 11% 1	2160
	11th to 20th	Night	50% 4 6% 22% 15% 3	2099
	21st to end	Night	52% 3 8% 18% 14% 4	1776
March	1st to 10th	Night	44% 4 7% 19% 21% 5%	2160
	11th to 20th	Night	48% 4 11% 22% 13% 3	2110
	21st to end	Night	50% 4 12% 21% 11% 2	2003
April	1st to 10th	Night	36% 4 10% 27% 20% 3	1800
	11th to 20th	Night	39% 4 9% 21% 20% 7%	1775
	21st to end	Night	43% 5% 9% 24% 17% 3	1779
May	1st to 10th	Night	42% 5% 10% 26% 15% 2	1620
	11th to 20th	Night	40% 7% 13% 23% 16% 1	1593
	21st to end	Night	31% 8% 16% 26% 16% 3	1712
June	1st to 10th	Night	33% 6% 11% 25% 22% 3	1440
	11th to 20th	Night	24% 5% 18% 25% 24% 4	1440
	21st to end	Night	24% 8% 16% 27% 18% 7%	1440
July	1st to 10th	Night	30% 6% 15% 28% 17% 5%	1400
	11th to 20th	Night	29% 5% 15% 29% 17% 5%	1392
	21st to end	Night	32% 6% 13% 30% 16% 2	1559
August	1st to 10th	Night	29% 6% 16% 27% 21% 1	1718
	11th to 20th	Night	31% 6% 13% 29% 19% 2	1776
	21st to end	Night	27% 5% 13% 29% 21% 5%	1980
September	1st to 10th	Night	32% 4 9% 28% 22% 5%	1800
	11th to 20th	Night	32% 4 7% 27% 25% 5%	1941
	21st to end	Night	30% 3 8% 25% 27% 7%	1980
October	1st to 10th	Night	29% 3 8% 23% 30% 7%	1980
	11th to 20th	Night	36% 2 8% 22% 25% 7%	2101
	21st to end	Night	42% 3 6% 23% 19% 6%	2537
November	1st to 10th	Night	33% 3 8% 25% 26% 6%	2406
	11th to 20th	Night	36% 2 7% 24% 23% 8%	2470
	21st to end	Night	41% 3 8% 25% 20% 3	2470
December	1st to 10th	Night	50% 3 11% 22% 12% 2	2470
	11th to 20th	Night	49% 3 8% 23% 16% 2	3214
	21st to end	Night	60% 3 9% 17% 10% 2	4884

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	25% 14% 34% 20% 6% 1	270
	11th to 20th	Day	26% 17% 37% 16% 4 1	257
	21st to end	Day	18% 17% 44% 16% 4	264
February	1st to 10th	Day	28% 19% 33% 15% 4 1	240
	11th to 20th	Day	33% 18% 35% 12% 2	240
	21st to end	Day	32% 15% 31% 16% 6% 1	198
March	1st to 10th	Day	25% 15% 42% 13% 5%	240
	11th to 20th	Day	26% 16% 33% 17% 6% 2	289
	21st to end	Day	26% 14% 31% 20% 8% 1	462
April	1st to 10th	Day	17% 14% 31% 26% 10% 1	420
	11th to 20th	Day	20% 12% 28% 23% 15% 2	412
	21st to end	Day	22% 10% 31% 22% 10% 5%	420
May	1st to 10th	Day	21% 10% 25% 28% 12% 4	423
	11th to 20th	Day	19% 7% 23% 26% 18% 7%	447
	21st to end	Day	11% 3 17% 31% 22% 15%	512
June	1st to 10th	Day	11% 3 18% 25% 28% 14%	630
	11th to 20th	Day	5% 3 18% 29% 31% 14%	630
	21st to end	Day	6% 3 19% 34% 23% 15%	630
July	1st to 10th	Day	10% 3 13% 24% 27% 24%	621
	11th to 20th	Day	6% 4 12% 27% 33% 18%	618
	21st to end	Day	7% 2 14% 25% 32% 20%	604
August	1st to 10th	Day	10% 3 11% 26% 32% 19%	446
	11th to 20th	Day	14% 2 10% 29% 32% 13%	439
	21st to end	Day	11% 2 8% 19% 38% 21%	495
September	1st to 10th	Day	10% 2 12% 23% 35% 18%	450
	11th to 20th	Day	10% 2 9% 25% 34% 20%	450
	21st to end	Day	15% 4 18% 24% 26% 13%	450
October	1st to 10th	Day	11% 4 19% 31% 27% 9%	450
	11th to 20th	Day	13% 11% 29% 31% 10% 6%	324
	21st to end	Day	29% 7% 28% 25% 9% 2	297
November	1st to 10th	Day	12% 6% 35% 26% 15% 5%	270
	11th to 20th	Day	16% 17% 30% 26% 10% 1	270
	21st to end	Day	22% 10% 31% 24% 12% 1	270
December	1st to 10th	Day	20% 16% 33% 20% 10% 2	270
	11th to 20th	Day	24% 10% 32% 26% 7% 1	270
	21st to end	Day	27% 18% 32% 19% 4	297
January	1st to 10th	Night	23% 15% 29% 29% 5%	950
	11th to 20th	Night	26% 15% 33% 20% 5%	930
	21st to end	Night	21% 12% 37% 28% 3	922
February	1st to 10th	Night	28% 12% 31% 26% 3	720
	11th to 20th	Night	25% 12% 37% 24% 2	703
	21st to end	Night	26% 14% 36% 22% 2	592
March	1st to 10th	Night	22% 13% 29% 31% 5%	720
	11th to 20th	Night	23% 9% 32% 28% 7%	720
	21st to end	Night	25% 9% 25% 32% 8%	617
April	1st to 10th	Night	17% 9% 29% 34% 10%	540
	11th to 20th	Night	19% 9% 25% 32% 14% 1	533
	21st to end	Night	18% 7% 29% 34% 11% 1	540
May	1st to 10th	Night	20% 6% 25% 31% 16% 1	540
	11th to 20th	Night	16% 3 20% 35% 22% 3	531
	21st to end	Night	7% 2 17% 38% 33% 4	584
June	1st to 10th	Night	15% 1 13% 35% 31% 4	540
	11th to 20th	Night	6% 2 10% 43% 36% 3	540
	21st to end	Night	3 2 13% 40% 37% 5%	540
July	1st to 10th	Night	8% 2 6% 30% 43% 11%	525
	11th to 20th	Night	7% 3 6% 30% 47% 7%	522
	21st to end	Night	9% 1 10% 35% 36% 9%	591
August	1st to 10th	Night	9% 1 7% 31% 45% 7%	537
	11th to 20th	Night	7% 1 8% 33% 44% 6%	534
	21st to end	Night	8% 2 6% 28% 49% 7%	594
September	1st to 10th	Night	12% 3 8% 31% 40% 6%	540
	11th to 20th	Night	11% 3 9% 28% 41% 8%	681
	21st to end	Night	12% 3 17% 30% 31% 6%	720
October	1st to 10th	Night	11% 4 16% 34% 29% 6%	720
	11th to 20th	Night	16% 6% 22% 36% 18% 2	720
	21st to end	Night	22% 6% 28% 29% 13% 1	792
November	1st to 10th	Night	13% 7% 27% 36% 16% 1	740
	11th to 20th	Night	17% 9% 31% 30% 12%	760
	21st to end	Night	20% 10% 26% 31% 12%	760
December	1st to 10th	Night	24% 10% 27% 31% 7%	760
	11th to 20th	Night	26% 9% 28% 30% 7%	883
	21st to end	Night	27% 11% 30% 27% 5%	1045

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