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 2024-12-24.

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% 7% 3 7% 5% 4 4 4 6% 7% 41%	101410
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 43%	98732
Transparency	Day	40% 4 10% 25% 17% 3	105097
	Night	40% 4 10% 24% 19% 4	103185
Seeing	Day	16% 8% 23% 24% 19% 10%	15570
	Night	17% 8% 23% 31% 18% 2	25404

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% 12% 7% 10% 4 4 5% 3 5% 5% 32%	2527
	11th to 20th	Day	17% 9% 3 9% 8% 6% 5% 3 4 5% 30%	2530
	21st to end	Day	24% 14% 6% 7% 4 3 4 3 4 5% 26%	3183
February	1st to 10th	Day	18% 11% 4 6% 4 4 4 3 5% 6% 36%	1900
	11th to 20th	Day	18% 10% 4 8% 4 4 3 3 5% 5% 36%	1848
	21st to end	Day	16% 12% 3 8% 6% 4 4 3 5% 6% 35%	1570
March	1st to 10th	Day	23% 14% 3 7% 4 3 2 2 4 4 33%	3120
	11th to 20th	Day	14% 6% 3 6% 3 3 3 2 4 6% 49%	3251
	21st to end	Day	12% 8% 3 7% 5% 4 5% 4 5% 6% 40%	4155
April	1st to 10th	Day	12% 8% 3 6% 4 4 4 4 5% 8% 43%	2280
	11th to 20th	Day	14% 7% 4 6% 4 4 4 4 5% 7% 40%	2254
	21st to end	Day	16% 7% 3 5% 4 4 4 4 5% 7% 41%	2300
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 5% 7% 9% 31%	4082
	11th to 20th	Day	18% 7% 3 6% 4 4 4 4 5% 8% 37%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 7% 41%	4531
June	1st to 10th	Day	10% 5% 1 6% 4 4 5% 4 7% 9% 45%	2660
	11th to 20th	Day	4 4 1 5% 5% 5% 6% 5% 7% 10% 48%	2660
	21st to end	Day	5% 4 2 6% 6% 5% 6% 5% 8% 10% 45%	2660
July	1st to 10th	Day	9% 6% 3 6% 5% 5% 6% 4 6% 9% 40%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 6% 4 8% 10% 47%	4440
	21st to end	Day	8% 9% 4 11% 8% 6% 7% 5% 6% 8% 28%	4666
August	1st to 10th	Day	5% 4 2 4 3 4 4 4 7% 10% 53%	3884
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	3729
	21st to end	Day	5% 4 2 5% 4 4 3 3 6% 9% 56%	4180
September	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 7% 46%	2280
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 3 6% 8% 47%	2111
	21st to end	Day	11% 5% 2 6% 5% 3 4 3 4 7% 50%	2090
October	1st to 10th	Day	9% 4 2 4 3 3 4 3 3 7% 57%	2090
	11th to 20th	Day	13% 4 2 5% 4 3 3 3 4 7% 52%	1957
	21st to end	Day	18% 7% 2 5% 4 3 3 2 4 5% 47%	1919
November	1st to 10th	Day	10% 5% 3 6% 5% 3 3 3 4 5% 53%	1762
	11th to 20th	Day	16% 7% 2 5% 4 4 2 3 4 6% 46%	1800
	21st to end	Day	17% 9% 3 6% 3 3 4 4 4 6% 42%	1800
December	1st to 10th	Day	18% 10% 3 8% 4 3 3 4 4 5% 38%	1800
	11th to 20th	Day	15% 10% 5% 10% 5% 5% 4 3 5% 5% 33%	2063
	21st to end	Day	18% 13% 5% 7% 5% 5% 5% 4 5% 6% 28%	2807
January	1st to 10th	Night	17% 11% 3 7% 4 4 4 3 5% 6% 36%	4428
	11th to 20th	Night	16% 12% 4 10% 5% 5% 5% 3 3 4 32%	4440
	21st to end	Night	24% 13% 5% 6% 3 3 3 3 4 7% 29%	4472
February	1st to 10th	Night	21% 12% 5% 7% 4 3 4 3 4 5% 34%	2280
	11th to 20th	Night	18% 8% 3 7% 4 5% 3 3 4 5% 39%	2218
	21st to end	Night	20% 10% 3 8% 5% 3 3 4 4 5% 36%	1884
March	1st to 10th	Night	23% 10% 4 7% 4 3 3 3 4 5% 35%	3733
	11th to 20th	Night	13% 7% 2 5% 3 4 3 3 4 6% 48%	3708
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 5% 7% 41%	3503
April	1st to 10th	Night	12% 7% 2 5% 5% 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	15% 7% 2 6% 3 3 4 3 5% 6% 45%	1875
	21st to end	Night	16% 8% 3 6% 5% 3 3 4 6% 8% 39%	1877
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 5% 9% 37%	2813
	11th to 20th	Night	14% 7% 2 6% 4 3 5% 4 6% 7% 42%	2812
	21st to end	Night	7% 5% 2 6% 4 5% 6% 4 7% 10% 44%	3020
June	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 9% 46%	1520
	11th to 20th	Night	5% 3 1 5% 3 3 4 3 8% 11% 53%	1520
	21st to end	Night	3 2 1 4 5% 5% 5% 4 8% 13% 50%	1520
July	1st to 10th	Night	7% 6% 2 5% 4 5% 5% 4 6% 9% 48%	2530
	11th to 20th	Night	6% 5% 1 5% 3 3 4 4 5% 8% 56%	2530
	21st to end	Night	9% 7% 4 10% 6% 7% 7% 5% 7% 7% 31%	2889
August	1st to 10th	Night	5% 3 1 5% 3 3 3 4 5% 8% 60%	3041
	11th to 20th	Night	5% 4 2 6% 4 4 4 5% 6% 9% 51%	3100
	21st to end	Night	4 3 1 3 4 3 4 4 5% 9% 61%	3476
September	1st to 10th	Night	10% 4 2 6% 4 4 3 4 4 5% 54%	1900
	11th to 20th	Night	8% 6% 3 5% 3 4 3 3 6% 7% 50%	2049
	21st to end	Night	12% 5% 2 4 3 4 3 3 4 6% 54%	2090
October	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 6% 59%	2090
	11th to 20th	Night	14% 5% 2 5% 2 3 4 2 4 5% 54%	2218
	21st to end	Night	17% 8% 2 5% 4 3 3 3 4 6% 46%	2678
November	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 5% 53%	2536
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 49%	2600
	21st to end	Night	16% 7% 3 6% 4 3 3 3 4 6% 46%	2600
December	1st to 10th	Night	19% 10% 3 7% 4 3 4 3 5% 6% 37%	2600
	11th to 20th	Night	17% 10% 3 7% 4 4 4 3 5% 5% 37%	3349
	21st to end	Night	18% 13% 5% 10% 5% 5% 5% 4 4 5% 28%	4933

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	54% 4 9% 23% 9% 1	2528
	11th to 20th	Day	58% 2 8% 22% 8% 2	2530
	21st to end	Day	62% 2 8% 20% 7% 1	3183
February	1st to 10th	Day	49% 3 9% 24% 13% 2	1900
	11th to 20th	Day	51% 4 8% 22% 12% 4	1858
	21st to end	Day	52% 3 9% 17% 14% 5%	1570
March	1st to 10th	Day	55% 3 5% 19% 13% 5%	3120
	11th to 20th	Day	39% 2 9% 29% 17% 4	3251
	21st to end	Day	44% 4 11% 23% 17% 1	4155
April	1st to 10th	Day	40% 4 12% 25% 17% 2	2280
	11th to 20th	Day	43% 4 7% 25% 17% 3	2254
	21st to end	Day	43% 4 10% 28% 12% 2	2300
May	1st to 10th	Day	49% 5% 14% 21% 10% 1	4120
	11th to 20th	Day	46% 4 11% 25% 13% 1	4085
	21st to end	Day	40% 5% 12% 19% 21% 1	4544
June	1st to 10th	Day	34% 6% 13% 25% 20% 3	2660
	11th to 20th	Day	29% 7% 13% 27% 21% 3	2660
	21st to end	Day	31% 7% 15% 25% 17% 5%	2660
July	1st to 10th	Day	39% 7% 12% 27% 13% 2	4371
	11th to 20th	Day	28% 8% 15% 34% 13% 2	4365
	21st to end	Day	51% 7% 13% 20% 8% 1	4518
August	1st to 10th	Day	24% 6% 16% 33% 20% 1	3871
	11th to 20th	Day	34% 7% 13% 27% 17% 2	3765
	21st to end	Day	25% 5% 11% 36% 19% 3	4179
September	1st to 10th	Day	36% 5% 10% 25% 20% 5%	2280
	11th to 20th	Day	36% 5% 9% 25% 22% 3	2111
	21st to end	Day	36% 4 7% 24% 25% 4	2090
October	1st to 10th	Day	18% 2 6% 20% 46% 7%	3480
	11th to 20th	Day	23% 3 8% 27% 32% 8%	3236
	21st to end	Day	30% 3 6% 28% 27% 6%	3144
November	1st to 10th	Day	34% 4 8% 26% 23% 5%	1762
	11th to 20th	Day	40% 2 8% 24% 21% 5%	1800
	21st to end	Day	44% 4 6% 26% 17% 3	1800
December	1st to 10th	Day	49% 3 11% 23% 13% 1	1800
	11th to 20th	Day	53% 4 8% 24% 10% 1	2060
	21st to end	Day	56% 4 12% 18% 9%	2807
January	1st to 10th	Night	51% 3 8% 26% 11% 1	4428
	11th to 20th	Night	58% 3 6% 21% 10% 2	4440
	21st to end	Night	58% 2 8% 20% 10% 1	4472
February	1st to 10th	Night	55% 3 8% 21% 12% 1	2280
	11th to 20th	Night	49% 4 6% 22% 16% 3	2219
	21st to end	Night	52% 3 8% 17% 15% 4	1884
March	1st to 10th	Night	52% 3 6% 16% 18% 3	3733
	11th to 20th	Night	37% 3 12% 24% 14% 8%	3708
	21st to end	Night	45% 3 11% 25% 16% 1	3503
April	1st to 10th	Night	37% 4 10% 27% 19% 3	1900
	11th to 20th	Night	39% 3 9% 21% 21% 6%	1875
	21st to end	Night	43% 5% 9% 24% 17% 3	1877
May	1st to 10th	Night	46% 4 14% 25% 9% 1	2840
	11th to 20th	Night	41% 6% 12% 26% 15% 1	2813
	21st to end	Night	34% 6% 15% 26% 18% 2	3028
June	1st to 10th	Night	33% 6% 11% 25% 21% 3	1520
	11th to 20th	Night	24% 5% 17% 24% 25% 4	1520
	21st to end	Night	23% 8% 17% 27% 19% 7%	1520
July	1st to 10th	Night	32% 6% 13% 29% 17% 3	2490
	11th to 20th	Night	25% 5% 13% 36% 17% 3	2482
	21st to end	Night	48% 9% 13% 20% 10% 1	2797
August	1st to 10th	Night	21% 6% 13% 39% 20% 1	3030
	11th to 20th	Night	27% 6% 16% 30% 19% 1	3130
	21st to end	Night	20% 5% 10% 38% 24% 3	3476
September	1st to 10th	Night	32% 4 9% 27% 23% 5%	1900
	11th to 20th	Night	32% 5% 7% 27% 24% 5%	2049
	21st to end	Night	32% 4 8% 24% 26% 7%	2090
October	1st to 10th	Night	19% 3 5% 15% 50% 9%	3480
	11th to 20th	Night	22% 2 5% 24% 39% 8%	3678
	21st to end	Night	30% 3 6% 23% 28% 10%	4403
November	1st to 10th	Night	34% 3 8% 25% 25% 5%	2536
	11th to 20th	Night	37% 2 7% 24% 22% 8%	2600
	21st to end	Night	42% 3 8% 25% 19% 3	2600
December	1st to 10th	Night	49% 3 11% 22% 13% 2	2600
	11th to 20th	Night	48% 3 8% 23% 16% 2	3351
	21st to end	Night	60% 3 9% 17% 9% 2	4933

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% 39% 15% 4 1	270
	21st to end	Day	21% 16% 42% 16% 5%	297
February	1st to 10th	Day	27% 17% 30% 20% 4 1	270
	11th to 20th	Day	30% 18% 36% 13% 3	270
	21st to end	Day	33% 15% 32% 15% 5%	225
March	1st to 10th	Day	26% 15% 39% 15% 5%	270
	11th to 20th	Day	24% 19% 34% 17% 5% 2	322
	21st to end	Day	26% 13% 33% 19% 8% 1	506
April	1st to 10th	Day	19% 15% 30% 25% 10% 1	460
	11th to 20th	Day	21% 13% 28% 23% 14% 2	452
	21st to end	Day	21% 11% 32% 21% 11% 4	460
May	1st to 10th	Day	20% 11% 25% 28% 12% 4	463
	11th to 20th	Day	18% 7% 23% 25% 18% 8%	487
	21st to end	Day	11% 4 20% 30% 21% 14%	558
June	1st to 10th	Day	11% 4 20% 25% 26% 13%	680
	11th to 20th	Day	5% 3 18% 29% 31% 14%	680
	21st to end	Day	7% 3 19% 34% 22% 15%	680
July	1st to 10th	Day	10% 3 14% 25% 26% 22%	671
	11th to 20th	Day	6% 4 12% 29% 31% 17%	668
	21st to end	Day	8% 3 15% 25% 31% 18%	654
August	1st to 10th	Day	9% 3 12% 26% 32% 19%	486
	11th to 20th	Day	13% 3 12% 30% 31% 12%	479
	21st to end	Day	10% 2 8% 19% 38% 22%	539
September	1st to 10th	Day	10% 1 13% 24% 34% 18%	490
	11th to 20th	Day	10% 2 9% 26% 34% 20%	490
	21st to end	Day	16% 6% 18% 24% 24% 12%	490
October	1st to 10th	Day	10% 3 18% 31% 28% 10%	490
	11th to 20th	Day	13% 10% 29% 28% 12% 8%	357
	21st to end	Day	26% 7% 29% 26% 10% 2	330
November	1st to 10th	Day	13% 8% 35% 25% 14% 5%	300
	11th to 20th	Day	16% 18% 30% 25% 10% 1	300
	21st to end	Day	22% 11% 30% 23% 12% 1	300
December	1st to 10th	Day	20% 16% 33% 19% 10% 2	300
	11th to 20th	Day	24% 11% 31% 26% 7% 1	300
	21st to end	Day	26% 17% 32% 20% 4	306
January	1st to 10th	Night	23% 15% 29% 29% 5%	950
	11th to 20th	Night	27% 15% 33% 20% 5%	950
	21st to end	Night	22% 12% 36% 27% 3	974
February	1st to 10th	Night	27% 12% 32% 26% 3	760
	11th to 20th	Night	25% 12% 37% 24% 2	743
	21st to end	Night	26% 14% 36% 21% 2	628
March	1st to 10th	Night	22% 13% 29% 30% 5%	760
	11th to 20th	Night	23% 10% 32% 29% 6%	760
	21st to end	Night	24% 10% 26% 31% 8%	651
April	1st to 10th	Night	17% 11% 30% 33% 9%	570
	11th to 20th	Night	19% 9% 26% 31% 13% 1	563
	21st to end	Night	18% 7% 29% 34% 10% 1	570
May	1st to 10th	Night	20% 6% 25% 31% 17% 1	570
	11th to 20th	Night	16% 3 19% 36% 22% 3	561
	21st to end	Night	7% 3 17% 39% 31% 4	617
June	1st to 10th	Night	15% 2 14% 36% 31% 3	570
	11th to 20th	Night	6% 2 10% 43% 36% 2	570
	21st to end	Night	3 2 13% 41% 36% 5%	570
July	1st to 10th	Night	8% 3 6% 30% 41% 11%	555
	11th to 20th	Night	7% 4 6% 31% 46% 7%	552
	21st to end	Night	10% 1 10% 36% 35% 9%	624
August	1st to 10th	Night	9% 1 7% 31% 45% 7%	567
	11th to 20th	Night	6% 2 9% 35% 43% 6%	564
	21st to end	Night	7% 1 6% 28% 50% 8%	627
September	1st to 10th	Night	12% 3 8% 31% 39% 6%	570
	11th to 20th	Night	11% 3 9% 29% 42% 8%	719
	21st to end	Night	13% 4 17% 30% 30% 5%	760
October	1st to 10th	Night	11% 4 16% 35% 30% 6%	760
	11th to 20th	Night	15% 6% 22% 35% 19% 2	760
	21st to end	Night	21% 7% 28% 30% 13% 1	836
November	1st to 10th	Night	14% 8% 27% 36% 15% 1	780
	11th to 20th	Night	18% 9% 31% 30% 12%	800
	21st to end	Night	20% 10% 27% 31% 11%	800
December	1st to 10th	Night	23% 11% 28% 31% 6%	800
	11th to 20th	Night	26% 9% 29% 30% 6%	930
	21st to end	Night	26% 12% 30% 27% 5%	1063

Page generated at 2024-12-21 03:13:46. Version ac07

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