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 2024-03-28.

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	16% 8% 3 6% 5% 3 5% 5% 5% 7% 37%	79127
	Night	16% 9% 3 6% 5% 3 4 4 5% 7% 39%	80887
Transparency	Day	47% 4 10% 22% 15% 2	78850
	Night	46% 4 10% 21% 16% 3	80683
Seeing	Day	20% 11% 23% 22% 16% 7%	13877
	Night	20% 9% 22% 29% 17% 3	24048

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% 16% 6% 8% 5% 3 4 4 5% 7% 25%	2836
	11th to 20th	Day	20% 15% 6% 12% 6% 4 3 3 2 3 26%	2840
	21st to end	Day	25% 13% 5% 6% 5% 2 3 4 3 5% 28%	3124
February	1st to 10th	Day	19% 11% 5% 9% 5% 3 3 4 4 5% 32%	1876
	11th to 20th	Day	24% 8% 5% 6% 6% 3 3 4 3 6% 32%	1848
	21st to end	Day	22% 12% 4 7% 5% 3 3 4 3 6% 32%	1570
March	1st to 10th	Day	20% 10% 3 5% 5% 3 3 4 4 6% 37%	1928
	11th to 20th	Day	21% 9% 4 6% 5% 3 4 4 5% 6% 33%	2090
	21st to end	Day	20% 10% 3 6% 5% 2 4 4 5% 6% 33%	2255
April	1st to 10th	Day	14% 6% 3 6% 5% 3 4 4 6% 8% 41%	2090
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 4 4 6% 42%	2150
	21st to end	Day	17% 6% 4 6% 5% 3 5% 4 6% 6% 37%	2340
May	1st to 10th	Day	18% 6% 2 5% 6% 3 5% 5% 6% 8% 36%	2340
	11th to 20th	Day	17% 8% 4 6% 7% 3 5% 4 5% 6% 36%	2301
	21st to end	Day	10% 6% 2 5% 6% 4 6% 6% 7% 10% 38%	2531
June	1st to 10th	Day	12% 6% 2 5% 6% 3 6% 6% 6% 10% 37%	2340
	11th to 20th	Day	7% 5% 3 5% 7% 4 5% 6% 7% 9% 43%	2340
	21st to end	Day	9% 5% 2 5% 5% 4 7% 6% 7% 10% 39%	2340
July	1st to 10th	Day	12% 6% 3 6% 6% 5% 7% 7% 6% 10% 33%	2340
	11th to 20th	Day	10% 4 2 6% 7% 4 7% 7% 8% 10% 35%	2340
	21st to end	Day	12% 5% 3 6% 6% 4 7% 7% 6% 9% 36%	2547
August	1st to 10th	Day	11% 5% 3 6% 6% 4 7% 7% 8% 9% 35%	2327
	11th to 20th	Day	12% 6% 3 6% 7% 4 7% 6% 7% 10% 34%	2262
	21st to end	Day	10% 5% 2 5% 6% 4 5% 6% 8% 11% 39%	2376
September	1st to 10th	Day	15% 6% 2 5% 7% 3 5% 5% 6% 7% 40%	2026
	11th to 20th	Day	15% 6% 2 5% 4 3 4 5% 5% 8% 43%	1980
	21st to end	Day	15% 6% 2 4 4 3 4 4 5% 7% 47%	1980
October	1st to 10th	Day	13% 4 2 4 4 2 4 3 5% 9% 51%	1980
	11th to 20th	Day	14% 6% 2 5% 4 2 3 3 4 6% 50%	1850
	21st to end	Day	19% 7% 2 4 3 2 3 3 4 7% 44%	1901
November	1st to 10th	Day	12% 5% 2 3 4 2 4 3 5% 6% 53%	1672
	11th to 20th	Day	16% 8% 2 4 4 2 3 4 4 7% 45%	1710
	21st to end	Day	18% 8% 3 5% 4 3 3 4 4 6% 43%	1710
December	1st to 10th	Day	24% 11% 5% 6% 5% 2 3 4 4 5% 32%	1710
	11th to 20th	Day	17% 12% 4 5% 5% 3 5% 5% 6% 7% 32%	2153
	21st to end	Day	19% 16% 4 6% 5% 3 4 5% 6% 7% 26%	3124
January	1st to 10th	Night	18% 18% 5% 7% 6% 3 3 3 4 5% 27%	4109
	11th to 20th	Night	22% 12% 4 6% 9% 3 3 3 3 4 32%	4120
	21st to end	Night	23% 13% 4 6% 5% 3 3 3 4 5% 31%	4532
February	1st to 10th	Night	20% 9% 5% 8% 5% 3 4 4 5% 6% 31%	2302
	11th to 20th	Night	24% 10% 5% 6% 5% 3 4 4 4 4 33%	2218
	21st to end	Night	25% 12% 4 7% 5% 3 4 4 4 4 29%	1884
March	1st to 10th	Night	17% 10% 3 6% 5% 3 4 5% 4 5% 37%	2252
	11th to 20th	Night	18% 8% 3 6% 5% 4 4 5% 5% 7% 36%	2090
	21st to end	Night	16% 10% 4 6% 7% 3 5% 4 4 7% 35%	2256
April	1st to 10th	Night	13% 6% 3 5% 5% 3 4 4 5% 7% 46%	1885
	11th to 20th	Night	16% 6% 3 4 5% 3 4 4 5% 6% 44%	1757
	21st to end	Night	17% 6% 2 5% 5% 3 4 5% 5% 7% 40%	1620
May	1st to 10th	Night	16% 5% 3 6% 5% 4 5% 5% 5% 8% 37%	1620
	11th to 20th	Night	16% 6% 3 5% 5% 3 5% 5% 8% 8% 36%	1592
	21st to end	Night	5% 5% 2 6% 7% 4 5% 6% 8% 10% 42%	1744
June	1st to 10th	Night	9% 5% 2 4 5% 3 5% 5% 7% 9% 45%	1620
	11th to 20th	Night	5% 3 2 5% 5% 4 5% 5% 7% 10% 49%	1620
	21st to end	Night	7% 4 2 5% 5% 4 6% 6% 8% 12% 40%	1620
July	1st to 10th	Night	11% 5% 2 6% 6% 3 5% 6% 8% 9% 41%	1620
	11th to 20th	Night	7% 4 2 5% 5% 3 5% 6% 7% 11% 46%	1620
	21st to end	Night	8% 4 2 7% 7% 3 6% 6% 7% 10% 39%	1771
August	1st to 10th	Night	10% 5% 2 4 4 2 5% 5% 6% 9% 47%	1611
	11th to 20th	Night	8% 6% 3 5% 6% 3 5% 6% 7% 9% 41%	1636
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 7% 10% 48%	1980
September	1st to 10th	Night	12% 5% 2 5% 5% 3 4 5% 5% 8% 47%	1916
	11th to 20th	Night	13% 6% 3 4 4 3 4 4 6% 7% 46%	1980
	21st to end	Night	12% 5% 3 4 4 2 3 4 5% 6% 52%	1980
October	1st to 10th	Night	11% 5% 2 4 3 2 3 3 4 6% 58%	1980
	11th to 20th	Night	17% 5% 2 4 4 2 3 3 5% 6% 49%	2110
	21st to end	Night	17% 6% 3 5% 4 3 3 4 4 6% 45%	2452
November	1st to 10th	Night	11% 7% 3 4 4 3 3 4 4 6% 52%	2406
	11th to 20th	Night	16% 6% 3 5% 4 2 4 3 3 7% 47%	2470
	21st to end	Night	17% 9% 3 4 4 2 3 4 4 6% 43%	2470
December	1st to 10th	Night	23% 13% 3 6% 5% 3 3 3 5% 6% 30%	2470
	11th to 20th	Night	19% 11% 5% 7% 4 2 3 3 4 5% 35%	3042
	21st to end	Night	18% 14% 4 8% 7% 4 6% 5% 5% 7% 22%	4532

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	59% 3 7% 19% 11% 1	2837
	11th to 20th	Day	65% 3 5% 16% 9%	2840
	21st to end	Day	61% 3 6% 22% 8% 1	3124
February	1st to 10th	Day	57% 2 7% 19% 14% 2	1876
	11th to 20th	Day	56% 3 7% 21% 11% 2	1858
	21st to end	Day	56% 3 7% 16% 13% 4	1570
March	1st to 10th	Day	49% 3 7% 21% 18% 3	1928
	11th to 20th	Day	53% 3 8% 21% 12% 3	2090
	21st to end	Day	52% 4 10% 22% 11% 1	2255
April	1st to 10th	Day	42% 4 10% 25% 15% 4	2090
	11th to 20th	Day	45% 3 8% 20% 21% 4	2150
	21st to end	Day	47% 4 10% 25% 12% 2	2340
May	1st to 10th	Day	46% 5% 11% 26% 10% 2	2340
	11th to 20th	Day	49% 4 12% 21% 12% 1	2305
	21st to end	Day	41% 6% 16% 23% 13% 2	2531
June	1st to 10th	Day	42% 6% 11% 24% 15% 2	2340
	11th to 20th	Day	37% 5% 15% 24% 17% 2	2340
	21st to end	Day	38% 6% 15% 21% 17% 3	2340
July	1st to 10th	Day	45% 7% 15% 18% 13% 2	2275
	11th to 20th	Day	41% 6% 17% 19% 13% 3	2271
	21st to end	Day	44% 5% 14% 23% 12% 1	2405
August	1st to 10th	Day	43% 6% 17% 21% 12%	2314
	11th to 20th	Day	44% 5% 15% 22% 14% 1	2262
	21st to end	Day	39% 5% 14% 24% 15% 3	2376
September	1st to 10th	Day	44% 4 12% 20% 15% 5%	2026
	11th to 20th	Day	41% 3 10% 22% 21% 2	1980
	21st to end	Day	38% 3 8% 25% 23% 3	1980
October	1st to 10th	Day	32% 4 8% 26% 26% 5%	1980
	11th to 20th	Day	38% 3 9% 22% 26% 4	1850
	21st to end	Day	41% 3 7% 26% 19% 4	1901
November	1st to 10th	Day	34% 2 8% 25% 26% 5%	1672
	11th to 20th	Day	41% 2 6% 26% 20% 5%	1710
	21st to end	Day	45% 2 8% 23% 19% 4	1710
December	1st to 10th	Day	56% 2 7% 22% 11% 1	1710
	11th to 20th	Day	52% 4 10% 19% 14% 2	2150
	21st to end	Day	57% 4 9% 21% 7% 1	3124
January	1st to 10th	Night	60% 4 8% 16% 11% 2	4109
	11th to 20th	Night	59% 2 5% 16% 15% 3	4120
	21st to end	Night	58% 3 7% 19% 12% 1	4532
February	1st to 10th	Night	55% 3 10% 19% 13% 1	2302
	11th to 20th	Night	56% 2 6% 19% 13% 3	2219
	21st to end	Night	60% 3 7% 13% 13% 4	1884
March	1st to 10th	Night	49% 4 6% 18% 19% 4	2252
	11th to 20th	Night	49% 3 9% 24% 12% 3	2090
	21st to end	Night	51% 4 10% 23% 11% 1	2256
April	1st to 10th	Night	38% 3 10% 25% 20% 3	1885
	11th to 20th	Night	42% 2 9% 20% 21% 6%	1757
	21st to end	Night	44% 3 10% 27% 13% 2	1620
May	1st to 10th	Night	45% 4 11% 25% 12% 3	1620
	11th to 20th	Night	44% 4 16% 22% 12% 1	1593
	21st to end	Night	35% 7% 16% 27% 13% 2	1745
June	1st to 10th	Night	35% 5% 15% 27% 15% 3	1620
	11th to 20th	Night	30% 6% 19% 25% 18% 2	1620
	21st to end	Night	35% 6% 17% 22% 15% 5%	1620
July	1st to 10th	Night	38% 6% 18% 22% 14% 2	1575
	11th to 20th	Night	32% 5% 20% 21% 18% 5%	1566
	21st to end	Night	38% 6% 16% 25% 14% 1	1672
August	1st to 10th	Night	35% 5% 17% 27% 14% 2	1600
	11th to 20th	Night	38% 5% 17% 23% 15% 1	1636
	21st to end	Night	31% 6% 15% 27% 17% 3	1980
September	1st to 10th	Night	35% 5% 15% 21% 18% 6%	1916
	11th to 20th	Night	37% 5% 9% 26% 20% 3	1980
	21st to end	Night	34% 3 6% 24% 28% 4	1980
October	1st to 10th	Night	30% 3 10% 23% 28% 7%	1980
	11th to 20th	Night	37% 3 8% 23% 25% 4	2110
	21st to end	Night	41% 4 5% 23% 22% 5%	2452
November	1st to 10th	Night	36% 2 6% 25% 24% 7%	2406
	11th to 20th	Night	40% 2 6% 21% 24% 6%	2470
	21st to end	Night	44% 2 6% 26% 19% 3	2470
December	1st to 10th	Night	57% 2 9% 19% 10% 2	2470
	11th to 20th	Night	53% 2 7% 21% 15% 2	3044
	21st to end	Night	61% 4 9% 16% 10%	4532

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	29% 21% 32% 14% 4	270
	11th to 20th	Day	39% 24% 23% 10% 4	270
	21st to end	Day	30% 27% 31% 10% 3	297
February	1st to 10th	Day	29% 19% 34% 14% 3	270
	11th to 20th	Day	40% 24% 27% 7% 1	270
	21st to end	Day	38% 20% 35% 5% 1	225
March	1st to 10th	Day	34% 18% 35% 10% 3	298
	11th to 20th	Day	28% 21% 28% 18% 5%	460
	21st to end	Day	30% 17% 30% 16% 7% 1	490
April	1st to 10th	Day	25% 16% 26% 21% 11% 1	420
	11th to 20th	Day	23% 17% 28% 19% 10% 4	412
	21st to end	Day	24% 14% 29% 20% 10% 3	420
May	1st to 10th	Day	22% 15% 29% 18% 11% 4	423
	11th to 20th	Day	20% 11% 23% 26% 15% 6%	447
	21st to end	Day	15% 6% 20% 27% 20% 12%	485
June	1st to 10th	Day	14% 4 19% 30% 20% 12%	450
	11th to 20th	Day	9% 7% 20% 31% 23% 11%	450
	21st to end	Day	10% 5% 25% 27% 20% 12%	450
July	1st to 10th	Day	13% 4 16% 22% 24% 21%	445
	11th to 20th	Day	9% 5% 15% 29% 24% 17%	444
	21st to end	Day	11% 3 14% 30% 28% 13%	488
August	1st to 10th	Day	14% 5% 14% 24% 27% 15%	446
	11th to 20th	Day	14% 3 12% 31% 27% 12%	439
	21st to end	Day	13% 3 9% 27% 31% 17%	495
September	1st to 10th	Day	16% 3 11% 24% 31% 16%	450
	11th to 20th	Day	15% 4 12% 25% 30% 15%	450
	21st to end	Day	16% 9% 17% 26% 24% 10%	450
October	1st to 10th	Day	13% 6% 20% 28% 25% 8%	450
	11th to 20th	Day	18% 12% 26% 28% 13% 4	450
	21st to end	Day	24% 12% 26% 25% 11% 2	416
November	1st to 10th	Day	18% 9% 33% 23% 14% 4	270
	11th to 20th	Day	25% 15% 35% 20% 5% 1	270
	21st to end	Day	23% 16% 33% 19% 9% 1	270
December	1st to 10th	Day	31% 20% 26% 16% 6%	270
	11th to 20th	Day	25% 16% 32% 20% 6% 1	270
	21st to end	Day	29% 20% 31% 15% 5%	297
January	1st to 10th	Night	28% 17% 30% 22% 4	760
	11th to 20th	Night	33% 15% 27% 21% 4	760
	21st to end	Night	24% 17% 35% 23% 2	836
February	1st to 10th	Night	27% 11% 35% 24% 3	760
	11th to 20th	Night	32% 16% 32% 18% 2	743
	21st to end	Night	35% 15% 31% 17% 2	628
March	1st to 10th	Night	26% 14% 30% 25% 5%	760
	11th to 20th	Night	24% 14% 31% 24% 6% 1	760
	21st to end	Night	23% 13% 29% 28% 7%	821
April	1st to 10th	Night	17% 12% 31% 30% 10%	625
	11th to 20th	Night	20% 9% 29% 30% 11%	533
	21st to end	Night	20% 8% 32% 29% 10% 1	540
May	1st to 10th	Night	18% 6% 28% 30% 15% 3	540
	11th to 20th	Night	18% 5% 20% 33% 20% 4	531
	21st to end	Night	7% 3 17% 36% 31% 6%	584
June	1st to 10th	Night	9% 3 14% 41% 28% 5%	540
	11th to 20th	Night	6% 2 15% 38% 33% 6%	540
	21st to end	Night	9% 1 15% 40% 29% 5%	540
July	1st to 10th	Night	13% 3 8% 31% 37% 8%	525
	11th to 20th	Night	7% 2 10% 30% 45% 7%	522
	21st to end	Night	10% 2 8% 35% 38% 7%	591
August	1st to 10th	Night	12% 2 7% 31% 41% 7%	537
	11th to 20th	Night	10% 1 8% 37% 36% 7%	534
	21st to end	Night	9% 1 6% 35% 39% 11%	594
September	1st to 10th	Night	14% 2 9% 30% 39% 6%	656
	11th to 20th	Night	15% 2 12% 30% 35% 6%	720
	21st to end	Night	14% 4 17% 31% 29% 4	720
October	1st to 10th	Night	14% 5% 16% 30% 30% 5%	720
	11th to 20th	Night	19% 6% 22% 35% 16% 3	720
	21st to end	Night	20% 9% 25% 30% 14% 2	792
November	1st to 10th	Night	15% 7% 24% 35% 17% 2	740
	11th to 20th	Night	19% 11% 28% 32% 9% 1	760
	21st to end	Night	24% 11% 23% 32% 9% 1	760
December	1st to 10th	Night	32% 12% 24% 27% 4	760
	11th to 20th	Night	26% 13% 27% 28% 6% 1	760
	21st to end	Night	31% 12% 24% 28% 4	836

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