Cottonstone Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cottonstone Mountain clear sky chart from 2005-11-06 to 2025-12-18.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	127161
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123868
Transparency	Day	39% 6% 11% 20% 18% 6%	130767
	Night	38% 5% 10% 19% 21% 7%	128331
Seeing	Day	17% 11% 26% 25% 16% 6%	16930
	Night	16% 9% 25% 30% 17% 2	27022

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	15% 5% 2 5% 3 2 3 2 2 4 56%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 3 2 3 4 58%	4060
	21st to end	Day	15% 8% 3 7% 4 4 4 3 4 4 45%	4466
February	1st to 10th	Day	14% 7% 3 5% 4 4 4 4 7% 7% 42%	1800
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 3 5% 44%	1833
	21st to end	Day	18% 8% 3 6% 4 4 3 3 4 5% 42%	1768
March	1st to 10th	Day	20% 7% 3 6% 4 4 4 3 5% 6% 40%	4935
	11th to 20th	Day	9% 6% 2 5% 4 3 3 3 5% 6% 54%	4980
	21st to end	Day	16% 7% 2 5% 4 4 3 3 4 5% 47%	5465
April	1st to 10th	Day	11% 4 1 4 3 4 4 4 5% 8% 52%	2260
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 48%	2374
	21st to end	Day	12% 5% 2 5% 5% 4 5% 4 6% 7% 46%	2453
May	1st to 10th	Day	9% 7% 3 8% 6% 5% 5% 5% 7% 10% 35%	5862
	11th to 20th	Day	19% 7% 2 7% 5% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	12% 5% 2 7% 4 5% 5% 4 6% 8% 42%	6433
June	1st to 10th	Day	9% 5% 2 6% 6% 6% 5% 5% 8% 10% 38%	2600
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 5% 8% 10% 42%	2600
	21st to end	Day	6% 5% 2 5% 6% 5% 7% 5% 8% 11% 40%	2600
July	1st to 10th	Day	10% 6% 2 8% 6% 6% 6% 5% 7% 11% 33%	5900
	11th to 20th	Day	9% 6% 2 8% 6% 6% 7% 6% 9% 13% 28%	5900
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 6% 9% 11% 31%	6424
August	1st to 10th	Day	21% 8% 3 7% 5% 6% 6% 5% 8% 11% 21%	4234
	11th to 20th	Day	9% 5% 2 7% 6% 7% 6% 6% 9% 10% 33%	4126
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 9% 50%	4312
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	2350
	11th to 20th	Day	12% 5% 3 8% 5% 5% 3 3 6% 8% 42%	2200
	21st to end	Day	18% 6% 2 6% 5% 5% 6% 4 5% 6% 37%	2200
October	1st to 10th	Day	13% 6% 2 5% 4 3 3 3 4 6% 50%	2014
	11th to 20th	Day	12% 4 2 5% 3 3 3 2 3 5% 59%	2000
	21st to end	Day	13% 5% 2 5% 3 2 3 2 3 5% 57%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 5% 55%	2057
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 6% 53%	2026
	21st to end	Day	13% 5% 2 6% 3 5% 3 3 5% 5% 50%	1890
December	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 5% 52%	3020
	11th to 20th	Day	17% 6% 3 7% 4 4 3 3 5% 6% 44%	3436
	21st to end	Day	19% 6% 2 5% 3 3 3 3 4 6% 45%	4466
January	1st to 10th	Night	13% 6% 2 4 3 2 3 2 3 4 57%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	5900
	21st to end	Night	16% 8% 2 5% 3 4 3 2 4 6% 46%	6490
February	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 4 6% 46%	2600
	11th to 20th	Night	21% 7% 1 6% 4 3 3 2 5% 5% 43%	2449
	21st to end	Night	16% 6% 2 4 4 2 4 3 5% 6% 48%	1860
March	1st to 10th	Night	19% 6% 3 7% 4 4 3 3 4 5% 43%	4918
	11th to 20th	Night	11% 4 2 4 3 3 4 3 5% 6% 53%	4980
	21st to end	Night	15% 5% 2 5% 4 4 4 3 4 7% 48%	5467
April	1st to 10th	Night	14% 6% 1 5% 4 3 3 3 6% 8% 48%	2138
	11th to 20th	Night	12% 5% 2 6% 3 3 4 3 5% 6% 51%	1975
	21st to end	Night	12% 5% 2 5% 5% 4 5% 4 5% 7% 47%	1963
May	1st to 10th	Night	14% 6% 2 7% 4 4 5% 4 6% 9% 40%	4033
	11th to 20th	Night	18% 8% 3 7% 4 4 5% 3 6% 9% 34%	4032
	21st to end	Night	14% 5% 2 6% 3 4 4 4 6% 9% 43%	4421
June	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 5% 7% 11% 36%	1800
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 38%	1800
	21st to end	Night	8% 5% 3 6% 6% 4 5% 5% 9% 13% 37%	1800
July	1st to 10th	Night	12% 7% 3 8% 6% 5% 6% 5% 8% 10% 28%	4058
	11th to 20th	Night	10% 7% 3 9% 6% 6% 7% 6% 8% 11% 26%	4057
	21st to end	Night	13% 9% 3 8% 5% 6% 6% 5% 8% 10% 27%	4427
August	1st to 10th	Night	23% 8% 3 7% 6% 5% 5% 4 7% 9% 22%	2915
	11th to 20th	Night	9% 6% 3 7% 6% 5% 6% 5% 8% 10% 35%	2875
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 10% 51%	3586
September	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 7% 8% 47%	2045
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 6% 44%	2200
	21st to end	Night	17% 6% 3 8% 4 4 4 4 5% 8% 37%	2200
October	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 6% 50%	2385
	11th to 20th	Night	13% 5% 2 4 2 3 3 3 4 5% 55%	2400
	21st to end	Night	12% 6% 2 4 3 3 3 2 4 5% 56%	2640
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 2 3 5% 54%	2461
	11th to 20th	Night	11% 7% 2 5% 4 3 4 3 4 6% 52%	2590
	21st to end	Night	13% 5% 2 5% 3 3 3 4 4 7% 51%	2730
December	1st to 10th	Night	14% 7% 2 4 3 3 3 3 4 5% 51%	4380
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 5% 41%	4914
	21st to end	Night	19% 5% 2 5% 3 3 4 3 4 5% 47%	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	35% 2 4 19% 34% 7%	4057
	11th to 20th	Day	32% 2 4 15% 37% 9%	4060
	21st to end	Day	43% 3 3 19% 23% 9%	4466
February	1st to 10th	Day	40% 4 10% 21% 20% 5%	1800
	11th to 20th	Day	44% 3 7% 18% 22% 7%	1842
	21st to end	Day	46% 3 7% 16% 21% 7%	1768
March	1st to 10th	Day	45% 4 6% 15% 19% 12%	4935
	11th to 20th	Day	31% 3 7% 15% 26% 18%	4980
	21st to end	Day	40% 4 6% 21% 21% 7%	5465
April	1st to 10th	Day	30% 5% 10% 23% 26% 7%	2260
	11th to 20th	Day	35% 4 10% 21% 22% 8%	2374
	21st to end	Day	36% 5% 12% 24% 19% 4	2453
May	1st to 10th	Day	42% 7% 18% 21% 11% 2	5897
	11th to 20th	Day	51% 6% 13% 19% 10% 1	5865
	21st to end	Day	39% 6% 12% 20% 20% 3	6447
June	1st to 10th	Day	38% 7% 15% 24% 15% 1	2600
	11th to 20th	Day	34% 9% 17% 23% 15% 2	2600
	21st to end	Day	34% 10% 16% 24% 16% 2	2600
July	1st to 10th	Day	43% 9% 16% 22% 10% 1	5835
	11th to 20th	Day	42% 13% 27% 14% 3	5831
	21st to end	Day	43% 10% 17% 21% 9%	6240
August	1st to 10th	Day	55% 8% 18% 14% 5%	4221
	11th to 20th	Day	41% 10% 18% 20% 10% 1	4165
	21st to end	Day	29% 6% 13% 34% 16% 2	4312
September	1st to 10th	Day	35% 7% 15% 20% 19% 5%	2350
	11th to 20th	Day	40% 5% 12% 18% 21% 4	2200
	21st to end	Day	46% 6% 8% 20% 15% 5%	2200
October	1st to 10th	Day	32% 4 7% 19% 27% 11%	3274
	11th to 20th	Day	20% 1 5% 20% 27% 26%	3240
	21st to end	Day	26% 2 5% 25% 24% 17%	3538
November	1st to 10th	Day	33% 3 6% 20% 29% 9%	2057
	11th to 20th	Day	32% 3 7% 22% 28% 8%	2026
	21st to end	Day	36% 3 7% 23% 24% 7%	1890
December	1st to 10th	Day	36% 3 7% 20% 26% 9%	3020
	11th to 20th	Day	43% 3 9% 16% 23% 6%	3433
	21st to end	Day	41% 4 9% 25% 20% 2	4466
January	1st to 10th	Night	33% 2 5% 17% 35% 9%	5889
	11th to 20th	Night	34% 3 5% 15% 35% 8%	5900
	21st to end	Night	41% 2 5% 17% 27% 8%	6490
February	1st to 10th	Night	41% 4 8% 21% 21% 6%	2600
	11th to 20th	Night	45% 2 7% 17% 21% 7%	2451
	21st to end	Night	37% 3 9% 19% 24% 8%	1860
March	1st to 10th	Night	45% 4 5% 12% 25% 9%	4918
	11th to 20th	Night	31% 4 5% 14% 27% 19%	4980
	21st to end	Night	38% 4 7% 23% 22% 7%	5467
April	1st to 10th	Night	35% 4 9% 20% 24% 8%	2138
	11th to 20th	Night	35% 4 9% 22% 22% 8%	1975
	21st to end	Night	36% 4 12% 24% 19% 5%	1963
May	1st to 10th	Night	40% 6% 18% 20% 15% 1	4060
	11th to 20th	Night	47% 7% 15% 17% 13% 1	4033
	21st to end	Night	36% 7% 13% 25% 17% 2	4429
June	1st to 10th	Night	40% 9% 14% 22% 14% 2	1800
	11th to 20th	Night	40% 8% 16% 21% 13% 1	1800
	21st to end	Night	35% 12% 16% 22% 12% 3	1800
July	1st to 10th	Night	47% 10% 17% 15% 10% 1	4015
	11th to 20th	Night	48% 14% 22% 11% 3 1	4006
	21st to end	Night	49% 9% 17% 19% 6%	4297
August	1st to 10th	Night	56% 7% 17% 15% 4 1	2904
	11th to 20th	Night	40% 9% 23% 21% 7% 1	2902
	21st to end	Night	28% 7% 13% 29% 19% 3	3586
September	1st to 10th	Night	34% 7% 15% 22% 17% 5%	2045
	11th to 20th	Night	40% 5% 12% 19% 21% 3	2200
	21st to end	Night	45% 6% 11% 17% 16% 4	2200
October	1st to 10th	Night	36% 4 9% 20% 26% 5%	3905
	11th to 20th	Night	23% 2 6% 19% 29% 21%	3899
	21st to end	Night	28% 2 6% 23% 24% 17%	4252
November	1st to 10th	Night	34% 3 6% 20% 25% 11%	2461
	11th to 20th	Night	34% 3 6% 18% 27% 12%	2590
	21st to end	Night	33% 4 6% 23% 27% 7%	2730
December	1st to 10th	Night	36% 3 6% 19% 27% 8%	4380
	11th to 20th	Night	46% 4 9% 16% 17% 8%	4916
	21st to end	Night	39% 3 10% 22% 23% 2	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	26% 23% 37% 13% 1	300
	11th to 20th	Day	27% 23% 35% 11% 3	300
	21st to end	Day	22% 24% 41% 11% 3	330
February	1st to 10th	Day	23% 19% 41% 13% 3	300
	11th to 20th	Day	39% 20% 33% 7% 2	300
	21st to end	Day	27% 21% 32% 17% 3	367
March	1st to 10th	Day	23% 20% 30% 22% 5%	500
	11th to 20th	Day	23% 20% 38% 14% 5%	500
	21st to end	Day	25% 17% 34% 18% 5% 1	550
April	1st to 10th	Day	25% 17% 32% 18% 7% 1	500
	11th to 20th	Day	16% 16% 32% 22% 11% 2	492
	21st to end	Day	15% 13% 31% 26% 12% 3	500
May	1st to 10th	Day	13% 19% 33% 22% 10% 3	503
	11th to 20th	Day	18% 8% 24% 26% 18% 4	527
	21st to end	Day	17% 8% 21% 25% 22% 6%	573
June	1st to 10th	Day	11% 5% 24% 34% 21% 6%	530
	11th to 20th	Day	10% 4 21% 28% 27% 9%	530
	21st to end	Day	8% 4 15% 39% 26% 8%	530
July	1st to 10th	Day	10% 2 12% 31% 31% 14%	525
	11th to 20th	Day	9% 3 10% 32% 32% 14%	524
	21st to end	Day	8% 2 13% 35% 30% 12%	576
August	1st to 10th	Day	19% 5% 14% 28% 24% 11%	526
	11th to 20th	Day	10% 6% 11% 34% 24% 14%	519
	21st to end	Day	11% 3 14% 32% 25% 15%	583
September	1st to 10th	Day	11% 2 17% 32% 25% 13%	530
	11th to 20th	Day	15% 5% 15% 29% 25% 11%	530
	21st to end	Day	19% 6% 24% 31% 15% 5%	530
October	1st to 10th	Day	16% 7% 24% 25% 21% 7%	530
	11th to 20th	Day	13% 9% 28% 28% 18% 4	530
	21st to end	Day	16% 11% 28% 25% 15% 4	583
November	1st to 10th	Day	19% 10% 32% 26% 10% 3	535
	11th to 20th	Day	19% 16% 34% 21% 9% 1	466
	21st to end	Day	15% 19% 35% 23% 8% 1	330
December	1st to 10th	Day	27% 16% 38% 14% 4	330
	11th to 20th	Day	23% 15% 35% 20% 7% 1	321
	21st to end	Day	27% 14% 34% 16% 7% 2	330
January	1st to 10th	Night	23% 17% 30% 25% 5%	800
	11th to 20th	Night	19% 16% 33% 27% 5%	800
	21st to end	Night	21% 13% 38% 25% 3	880
February	1st to 10th	Night	20% 14% 36% 27% 4	800
	11th to 20th	Night	28% 17% 36% 18% 1	783
	21st to end	Night	20% 18% 34% 25% 4	660
March	1st to 10th	Night	19% 15% 34% 29% 4	800
	11th to 20th	Night	19% 16% 34% 23% 8%	800
	21st to end	Night	19% 14% 37% 24% 5%	880
April	1st to 10th	Night	18% 16% 34% 26% 6%	800
	11th to 20th	Night	17% 13% 32% 26% 12%	791
	21st to end	Night	15% 7% 31% 37% 10%	763
May	1st to 10th	Night	14% 10% 30% 35% 11% 1	600
	11th to 20th	Night	15% 6% 25% 33% 20% 1	591
	21st to end	Night	13% 5% 14% 40% 27% 2	650
June	1st to 10th	Night	13% 3 17% 39% 26% 2	600
	11th to 20th	Night	13% 4 15% 34% 29% 5%	600
	21st to end	Night	10% 3 11% 38% 36% 3	600
July	1st to 10th	Night	9% 2 12% 31% 38% 7%	585
	11th to 20th	Night	8% 3 9% 32% 40% 8%	582
	21st to end	Night	8% 2 11% 35% 38% 6%	657
August	1st to 10th	Night	15% 3 9% 32% 36% 6%	597
	11th to 20th	Night	11% 3 8% 34% 38% 6%	594
	21st to end	Night	10% 2 13% 31% 38% 7%	880
September	1st to 10th	Night	10% 2 13% 34% 35% 6%	800
	11th to 20th	Night	15% 4 14% 30% 33% 5%	800
	21st to end	Night	20% 5% 19% 34% 21% 2	800
October	1st to 10th	Night	15% 5% 20% 29% 26% 5%	800
	11th to 20th	Night	16% 9% 24% 32% 18% 2	800
	21st to end	Night	15% 10% 23% 32% 19% 1	880
November	1st to 10th	Night	16% 8% 26% 32% 15% 2	820
	11th to 20th	Night	14% 14% 30% 31% 10%	840
	21st to end	Night	15% 12% 31% 30% 12% 1	840
December	1st to 10th	Night	23% 12% 28% 31% 5%	840
	11th to 20th	Night	19% 11% 30% 31% 9%	829
	21st to end	Night	25% 12% 25% 29% 8%	880

Page generated at 2025-12-15 01:13:49. Version ac07

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