Mount Chestnut Oak Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Chestnut Oak Observing Site clear sky chart from 2005-11-06 to 2025-07-21.

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	15% 9% 3 7% 5% 4 5% 4 6% 7% 36%	120141
	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	119013
Transparency	Day	46% 4 10% 22% 15% 2	123883
	Night	45% 4 10% 22% 16% 3	123407
Seeing	Day	20% 12% 24% 22% 15% 7%	15664
	Night	20% 9% 23% 29% 17% 3	25708

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	16% 17% 7% 12% 3 3 4 3 5% 6% 25%	4056
	11th to 20th	Day	17% 14% 6% 13% 4 4 3 2 2 3 32%	4060
	21st to end	Day	20% 11% 4 6% 4 3 4 3 4 6% 34%	4466
February	1st to 10th	Day	18% 12% 5% 9% 4 3 3 3 4 5% 32%	1975
	11th to 20th	Day	24% 9% 5% 8% 4 3 3 3 3 6% 30%	1948
	21st to end	Day	20% 12% 3 8% 4 4 4 3 4 5% 34%	1650
March	1st to 10th	Day	19% 13% 4 8% 4 4 3 3 4 6% 33%	4562
	11th to 20th	Day	13% 8% 3 6% 4 4 4 3 5% 6% 44%	4980
	21st to end	Day	15% 8% 2 7% 4 4 5% 4 5% 7% 39%	5465
April	1st to 10th	Day	14% 8% 3 7% 5% 4 4 3 6% 8% 39%	2323
	11th to 20th	Day	15% 8% 3 7% 5% 3 4 3 5% 6% 42%	2392
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 6% 37%	2600
May	1st to 10th	Day	19% 10% 3 8% 5% 6% 5% 4 6% 8% 25%	5862
	11th to 20th	Day	16% 9% 4 9% 6% 5% 5% 4 6% 6% 29%	5861
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 9% 32%	6433
June	1st to 10th	Day	12% 6% 2 6% 6% 5% 5% 6% 6% 10% 37%	2600
	11th to 20th	Day	7% 5% 2 6% 6% 5% 5% 5% 7% 9% 41%	2600
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	2600
July	1st to 10th	Day	8% 5% 2 6% 6% 6% 6% 5% 7% 10% 38%	4250
	11th to 20th	Day	9% 5% 2 7% 6% 5% 6% 5% 7% 10% 39%	4250
	21st to end	Day	21% 10% 4 10% 6% 5% 6% 4 5% 7% 22%	4466
August	1st to 10th	Day	8% 5% 2 7% 4 5% 5% 5% 7% 10% 41%	4104
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 11% 36%	3961
	21st to end	Day	6% 3 1 4 3 4 5% 4 7% 10% 53%	4180
September	1st to 10th	Day	14% 6% 2 6% 6% 4 5% 4 6% 6% 41%	2138
	11th to 20th	Day	14% 6% 2 6% 3 4 5% 4 6% 8% 43%	2090
	21st to end	Day	16% 6% 2 5% 3 4 3 3 5% 7% 45%	2090
October	1st to 10th	Day	12% 4 2 4 3 3 3 3 5% 8% 52%	2090
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 4 6% 51%	1952
	21st to end	Day	18% 7% 2 5% 3 3 3 3 4 7% 45%	2006
November	1st to 10th	Day	12% 6% 2 4 4 3 4 2 5% 6% 51%	1762
	11th to 20th	Day	16% 8% 2 5% 4 4 3 4 4 7% 44%	1800
	21st to end	Day	17% 10% 3 6% 3 4 3 3 4 5% 41%	1800
December	1st to 10th	Day	17% 10% 4 8% 5% 3 3 3 4 5% 38%	2930
	11th to 20th	Day	22% 14% 5% 8% 4 4 4 3 5% 6% 26%	3373
	21st to end	Day	18% 14% 5% 9% 5% 5% 5% 3 5% 6% 25%	4466
January	1st to 10th	Night	16% 16% 6% 13% 5% 4 3 2 4 4 26%	5889
	11th to 20th	Night	20% 12% 4 8% 7% 4 3 2 3 3 33%	5900
	21st to end	Night	19% 10% 4 7% 4 3 3 2 3 5% 38%	6490
February	1st to 10th	Night	20% 10% 5% 9% 4 4 4 4 5% 5% 30%	2423
	11th to 20th	Night	24% 10% 4 7% 4 4 3 3 4 3 32%	2338
	21st to end	Night	23% 12% 3 7% 5% 4 4 3 4 4 31%	1980
March	1st to 10th	Night	19% 13% 3 7% 5% 4 4 3 4 5% 34%	5291
	11th to 20th	Night	12% 7% 2 4 3 4 3 4 5% 7% 48%	4980
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	5467
April	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 6% 7% 43%	2093
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 6% 5% 44%	1955
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 39%	1800
May	1st to 10th	Night	18% 9% 3 9% 5% 5% 4 4 6% 9% 27%	4033
	11th to 20th	Night	18% 10% 3 8% 6% 5% 5% 3 7% 7% 28%	4032
	21st to end	Night	13% 7% 3 7% 5% 4 5% 4 6% 9% 38%	4421
June	1st to 10th	Night	10% 7% 2 5% 4 4 5% 4 6% 9% 44%	1800
	11th to 20th	Night	5% 3 2 7% 4 6% 4 4 7% 11% 47%	1800
	21st to end	Night	7% 5% 1 6% 5% 5% 6% 4 9% 11% 41%	1800
July	1st to 10th	Night	8% 4 2 7% 5% 5% 5% 5% 9% 9% 41%	2930
	11th to 20th	Night	8% 5% 2 6% 4 3 4 4 7% 10% 47%	2930
	21st to end	Night	14% 9% 3 10% 6% 5% 7% 6% 6% 8% 25%	3087
August	1st to 10th	Night	9% 5% 3 6% 4 4 5% 4 6% 9% 46%	2825
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 4 7% 9% 41%	2865
	21st to end	Night	4 3 1 3 3 3 3 3 6% 9% 62%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 5% 8% 47%	2023
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 3 5% 7% 46%	2090
	21st to end	Night	14% 6% 3 5% 3 3 3 4 5% 6% 50%	2090
October	1st to 10th	Night	10% 5% 1 4 2 4 3 2 4 6% 58%	2090
	11th to 20th	Night	16% 6% 2 5% 3 3 3 2 5% 5% 51%	2228
	21st to end	Night	16% 7% 2 5% 3 3 3 4 4 6% 47%	2589
November	1st to 10th	Night	12% 7% 3 5% 4 3 3 3 4 6% 51%	2536
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 4 7% 46%	2600
	21st to end	Night	17% 11% 3 5% 4 3 3 3 4 6% 42%	2600
December	1st to 10th	Night	20% 14% 3 9% 4 3 2 2 4 4 35%	4250
	11th to 20th	Night	23% 12% 4 7% 3 3 3 3 4 5% 33%	4822
	21st to end	Night	17% 15% 4 9% 6% 6% 6% 4 5% 7% 23%	6490

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	61% 2 7% 20% 9% 1	4057
	11th to 20th	Day	59% 2 4 17% 16% 2	4060
	21st to end	Day	52% 2 7% 24% 13% 2	4466
February	1st to 10th	Day	57% 2 7% 19% 14% 2	1975
	11th to 20th	Day	58% 3 7% 20% 11% 2	1958
	21st to end	Day	54% 3 7% 18% 14% 4	1650
March	1st to 10th	Day	54% 3 5% 15% 18% 6%	4562
	11th to 20th	Day	41% 3 8% 23% 22% 4	4980
	21st to end	Day	45% 4 9% 29% 13% 1	5465
April	1st to 10th	Day	44% 4 10% 24% 15% 3	2323
	11th to 20th	Day	44% 3 8% 21% 20% 4	2392
	21st to end	Day	47% 4 11% 26% 12% 2	2600
May	1st to 10th	Day	56% 5% 11% 19% 9% 1	5897
	11th to 20th	Day	54% 5% 13% 18% 10%	5865
	21st to end	Day	46% 5% 14% 18% 15% 2	6447
June	1st to 10th	Day	42% 6% 12% 23% 15% 2	2600
	11th to 20th	Day	38% 5% 15% 23% 17% 2	2600
	21st to end	Day	37% 6% 15% 20% 18% 3	2600
July	1st to 10th	Day	38% 8% 20% 21% 12% 1	4185
	11th to 20th	Day	38% 5% 16% 31% 8% 2	4181
	21st to end	Day	61% 5% 11% 15% 8% 1	4324
August	1st to 10th	Day	36% 6% 16% 29% 13%	4091
	11th to 20th	Day	38% 5% 15% 27% 14% 1	4000
	21st to end	Day	26% 5% 13% 40% 15% 2	4179
September	1st to 10th	Day	43% 4 11% 19% 17% 5%	2138
	11th to 20th	Day	41% 3 10% 22% 21% 2	2090
	21st to end	Day	40% 3 9% 24% 22% 3	2090
October	1st to 10th	Day	25% 3 6% 22% 40% 4	3480
	11th to 20th	Day	26% 2 10% 24% 31% 7%	3218
	21st to end	Day	32% 2 8% 29% 21% 10%	3282
November	1st to 10th	Day	36% 2 9% 24% 25% 4	1762
	11th to 20th	Day	42% 2 6% 25% 21% 5%	1800
	21st to end	Day	46% 2 8% 22% 18% 3	1800
December	1st to 10th	Day	50% 2 5% 21% 21% 1	2930
	11th to 20th	Day	60% 3 8% 18% 10% 1	3370
	21st to end	Day	60% 4 10% 20% 6% 1	4466
January	1st to 10th	Night	62% 3 8% 18% 9% 1	5889
	11th to 20th	Night	57% 2 4 16% 16% 4	5900
	21st to end	Night	51% 2 7% 19% 19% 3	6490
February	1st to 10th	Night	56% 3 10% 18% 12% 1	2423
	11th to 20th	Night	58% 2 6% 18% 13% 3	2339
	21st to end	Night	58% 3 7% 15% 14% 4	1980
March	1st to 10th	Night	53% 3 6% 19% 14% 5%	5291
	11th to 20th	Night	36% 3 8% 27% 20% 6%	4980
	21st to end	Night	48% 3 10% 25% 13% 1	5467
April	1st to 10th	Night	41% 3 10% 23% 19% 3	2093
	11th to 20th	Night	42% 3 9% 21% 21% 6%	1955
	21st to end	Night	44% 3 11% 27% 14% 2	1800
May	1st to 10th	Night	53% 5% 12% 20% 8% 1	4060
	11th to 20th	Night	55% 4 14% 19% 8%	4033
	21st to end	Night	42% 6% 13% 23% 14% 1	4429
June	1st to 10th	Night	36% 5% 14% 26% 14% 3	1800
	11th to 20th	Night	32% 6% 18% 24% 18% 2	1800
	21st to end	Night	35% 7% 16% 21% 17% 4	1800
July	1st to 10th	Night	36% 7% 19% 25% 12% 1	2885
	11th to 20th	Night	32% 5% 17% 32% 12% 3	2876
	21st to end	Night	54% 7% 12% 18% 8% 1	2988
August	1st to 10th	Night	34% 6% 14% 30% 15% 2	2814
	11th to 20th	Night	38% 5% 14% 29% 13% 1	2892
	21st to end	Night	20% 4 14% 39% 21% 3	3476
September	1st to 10th	Night	35% 5% 14% 20% 19% 6%	2023
	11th to 20th	Night	38% 5% 9% 26% 20% 3	2090
	21st to end	Night	36% 3 7% 23% 27% 4	2090
October	1st to 10th	Night	27% 2 8% 21% 36% 6%	3480
	11th to 20th	Night	25% 2 7% 25% 31% 9%	3701
	21st to end	Night	27% 3 7% 25% 28% 10%	4263
November	1st to 10th	Night	37% 2 6% 24% 24% 6%	2536
	11th to 20th	Night	40% 2 7% 21% 24% 6%	2600
	21st to end	Night	45% 2 6% 25% 19% 3	2600
December	1st to 10th	Night	55% 2 7% 19% 15% 1	4250
	11th to 20th	Night	56% 2 7% 20% 13% 1	4824
	21st to end	Night	60% 4 9% 18% 9%	6490

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	30% 21% 32% 13% 3	300
	11th to 20th	Day	39% 24% 25% 9% 4	300
	21st to end	Day	29% 28% 30% 10% 2	330
February	1st to 10th	Day	29% 19% 36% 13% 2	300
	11th to 20th	Day	41% 25% 26% 7% 1	300
	21st to end	Day	35% 22% 35% 6% 1	249
March	1st to 10th	Day	35% 18% 33% 10% 4	330
	11th to 20th	Day	28% 21% 28% 18% 5%	500
	21st to end	Day	29% 18% 31% 15% 6% 1	550
April	1st to 10th	Day	28% 17% 27% 19% 9% 1	500
	11th to 20th	Day	22% 19% 29% 18% 9% 3	492
	21st to end	Day	23% 14% 31% 20% 10% 3	500
May	1st to 10th	Day	23% 16% 30% 17% 11% 4	503
	11th to 20th	Day	20% 12% 24% 25% 14% 5%	527
	21st to end	Day	17% 8% 22% 25% 18% 10%	573
June	1st to 10th	Day	14% 6% 22% 30% 19% 10%	530
	11th to 20th	Day	9% 7% 20% 30% 24% 10%	530
	21st to end	Day	9% 5% 23% 28% 21% 14%	530
July	1st to 10th	Day	11% 4 17% 25% 24% 19%	525
	11th to 20th	Day	10% 5% 16% 29% 25% 15%	524
	21st to end	Day	13% 4 15% 30% 26% 12%	532
August	1st to 10th	Day	14% 5% 15% 24% 26% 15%	486
	11th to 20th	Day	14% 4 14% 32% 25% 11%	479
	21st to end	Day	12% 3 8% 27% 32% 19%	539
September	1st to 10th	Day	14% 3 13% 24% 30% 15%	490
	11th to 20th	Day	16% 3 13% 25% 29% 14%	490
	21st to end	Day	18% 9% 17% 25% 22% 9%	490
October	1st to 10th	Day	13% 5% 20% 30% 24% 8%	490
	11th to 20th	Day	17% 12% 26% 26% 14% 5%	490
	21st to end	Day	22% 11% 27% 26% 11% 2	455
November	1st to 10th	Day	18% 10% 33% 24% 12% 3	300
	11th to 20th	Day	25% 15% 36% 19% 5% 1	300
	21st to end	Day	24% 17% 32% 18% 8% 1	300
December	1st to 10th	Day	30% 21% 28% 15% 6%	300
	11th to 20th	Day	27% 15% 31% 19% 6% 1	300
	21st to end	Day	29% 19% 31% 16% 5%	330
January	1st to 10th	Night	28% 17% 30% 22% 4	800
	11th to 20th	Night	32% 15% 27% 21% 4	800
	21st to end	Night	23% 16% 35% 23% 2	880
February	1st to 10th	Night	27% 12% 35% 23% 3	800
	11th to 20th	Night	32% 16% 32% 17% 2	783
	21st to end	Night	34% 15% 32% 18% 2	660
March	1st to 10th	Night	26% 14% 30% 25% 5%	800
	11th to 20th	Night	24% 13% 31% 25% 6% 1	800
	21st to end	Night	22% 13% 31% 27% 6%	880
April	1st to 10th	Night	19% 12% 32% 28% 9%	693
	11th to 20th	Night	20% 10% 30% 30% 11%	593
	21st to end	Night	19% 8% 33% 29% 10% 1	600
May	1st to 10th	Night	18% 7% 28% 30% 14% 3	600
	11th to 20th	Night	19% 5% 19% 34% 19% 4	591
	21st to end	Night	9% 4 18% 35% 28% 5%	650
June	1st to 10th	Night	10% 5% 13% 42% 26% 5%	600
	11th to 20th	Night	6% 3 15% 38% 32% 6%	600
	21st to end	Night	9% 1 14% 39% 31% 7%	600
July	1st to 10th	Night	12% 3 9% 31% 37% 7%	585
	11th to 20th	Night	7% 2 9% 30% 45% 7%	582
	21st to end	Night	11% 2 9% 35% 36% 7%	624
August	1st to 10th	Night	12% 3 8% 31% 41% 7%	567
	11th to 20th	Night	10% 2 8% 38% 35% 7%	564
	21st to end	Night	8% 1 6% 34% 40% 11%	627
September	1st to 10th	Night	14% 2 10% 31% 38% 6%	693
	11th to 20th	Night	15% 2 13% 30% 34% 6%	760
	21st to end	Night	16% 4 17% 31% 29% 4	760
October	1st to 10th	Night	13% 5% 16% 31% 30% 4	760
	11th to 20th	Night	18% 6% 22% 35% 17% 3	760
	21st to end	Night	19% 9% 25% 31% 14% 2	836
November	1st to 10th	Night	16% 7% 24% 35% 16% 2	780
	11th to 20th	Night	20% 12% 28% 32% 9% 1	800
	21st to end	Night	24% 11% 23% 32% 9% 1	800
December	1st to 10th	Night	32% 12% 25% 27% 4	800
	11th to 20th	Night	27% 13% 27% 27% 6% 1	800
	21st to end	Night	31% 12% 24% 29% 4	880

Page generated at 2025-07-18 01:10:02. Version ac07

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