Twentymile Rock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twentymile Rock clear sky chart from 2005-11-06 to 2026-01-02.

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	5% 7% 2 7% 4 4 4 3 5% 7% 53%	135735
	Night	6% 8% 3 7% 5% 4 4 4 5% 7% 47%	133002
Transparency	Day	31% 6% 11% 21% 20% 12%	130441
	Night	36% 7% 13% 20% 16% 9%	129298
Seeing	Day	8% 8% 22% 29% 23% 9%	16694
	Night	9% 6% 23% 40% 20% 2	26364

By Month

Percentage Cloud Cover SW (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	6% 16% 6% 15% 8% 7% 6% 5% 5% 6% 21%	3624
	11th to 20th	Day	5% 8% 4 14% 10% 7% 6% 3 5% 5% 33%	3640
	21st to end	Day	9% 13% 5% 10% 5% 4 4 3 5% 6% 36%	4492
February	1st to 10th	Day	6% 11% 5% 15% 7% 6% 5% 5% 5% 6% 29%	1800
	11th to 20th	Day	7% 12% 4 12% 7% 6% 7% 5% 7% 7% 26%	1795
	21st to end	Day	5% 10% 4 11% 6% 6% 5% 4 7% 6% 36%	1650
March	1st to 10th	Day	5% 11% 4 10% 5% 5% 5% 4 5% 8% 38%	4709
	11th to 20th	Day	4 10% 3 7% 5% 4 4 4 5% 7% 48%	5000
	21st to end	Day	5% 10% 4 10% 7% 7% 6% 5% 7% 8% 33%	5486
April	1st to 10th	Day	7% 9% 3 8% 5% 5% 5% 5% 5% 7% 41%	2281
	11th to 20th	Day	5% 10% 3 5% 5% 4 4 4 6% 7% 48%	2567
	21st to end	Day	7% 8% 2 7% 5% 4 4 4 6% 9% 43%	2600
May	1st to 10th	Day	5% 8% 2 5% 3 3 3 3 5% 8% 55%	5880
	11th to 20th	Day	7% 6% 3 7% 4 4 5% 4 5% 7% 50%	5841
	21st to end	Day	5% 5% 2 4 3 3 4 3 5% 7% 60%	6761
June	1st to 10th	Day	6% 4 1 6% 4 4 5% 5% 7% 8% 49%	2979
	11th to 20th	Day	6% 5% 2 5% 3 4 4 4 6% 7% 55%	3000
	21st to end	Day	2 2 1 3 2 3 3 3 4 7% 70%	3000
July	1st to 10th	Day	1 2 1 3 2 3 3 3 4 6% 73%	6603
	11th to 20th	Day	1 2 1 3 2 2 3 3 5% 7% 72%	6320
	21st to end	Day	2 2 1 3 3 2 3 3 5% 7% 69%	6455
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 7% 63%	5779
	11th to 20th	Day	2 1 1 2 2 2 3 3 5% 6% 72%	5798
	21st to end	Day	4 4 1 4 3 3 3 3 4 5% 65%	6468
September	1st to 10th	Day	4 2 1 3 3 2 3 2 4 5% 71%	2310
	11th to 20th	Day	4 4 1 4 3 3 3 3 5% 7% 63%	2200
	21st to end	Day	6% 4 1 4 3 2 3 2 4 6% 65%	2200
October	1st to 10th	Day	7% 4 2 4 3 3 2 2 3 4 67%	3445
	11th to 20th	Day	3 5% 2 5% 3 4 4 3 4 6% 62%	3250
	21st to end	Day	5% 6% 2 5% 4 4 5% 4 6% 8% 49%	3337
November	1st to 10th	Day	8% 8% 4 7% 5% 3 3 3 5% 7% 45%	1854
	11th to 20th	Day	6% 9% 3 8% 5% 5% 5% 5% 6% 8% 41%	1794
	21st to end	Day	7% 10% 3 10% 5% 4 4 3 6% 7% 41%	1680
December	1st to 10th	Day	5% 7% 3 9% 5% 4 5% 4 6% 5% 47%	2700
	11th to 20th	Day	8% 15% 7% 13% 8% 6% 5% 4 5% 9% 22%	2862
	21st to end	Day	8% 16% 5% 11% 6% 6% 5% 4 5% 6% 30%	3575
January	1st to 10th	Night	7% 13% 6% 13% 6% 5% 5% 5% 6% 7% 27%	6385
	11th to 20th	Night	7% 12% 5% 13% 8% 6% 5% 4 5% 7% 29%	6320
	21st to end	Night	7% 13% 4 9% 5% 4 4 3 4 7% 39%	6469
February	1st to 10th	Night	8% 12% 5% 10% 6% 6% 6% 5% 6% 6% 31%	2600
	11th to 20th	Night	9% 12% 4 9% 6% 5% 5% 3 5% 7% 35%	2489
	21st to end	Night	3 11% 4 10% 7% 6% 5% 5% 6% 7% 35%	1980
March	1st to 10th	Night	5% 12% 4 9% 5% 4 4 3 6% 8% 41%	5183
	11th to 20th	Night	3 7% 3 6% 5% 5% 5% 4 6% 9% 46%	5000
	21st to end	Night	5% 8% 3 8% 6% 5% 6% 5% 8% 9% 37%	5489
April	1st to 10th	Night	7% 9% 3 8% 5% 4 4 4 5% 7% 43%	2114
	11th to 20th	Night	6% 9% 3 7% 4 4 5% 4 6% 6% 45%	1781
	21st to end	Night	7% 7% 4 6% 5% 5% 4 4 6% 6% 47%	1800
May	1st to 10th	Night	7% 7% 3 6% 4 4 5% 3 4 6% 52%	4100
	11th to 20th	Night	6% 6% 2 7% 4 5% 5% 4 6% 8% 49%	4074
	21st to end	Night	6% 5% 2 5% 4 4 4 4 6% 9% 51%	4122
June	1st to 10th	Night	5% 4 2 7% 4 5% 5% 5% 7% 8% 48%	1416
	11th to 20th	Night	7% 4 2 5% 4 4 3 3 5% 7% 55%	1400
	21st to end	Night	3 2 1 3 3 3 3 4 5% 6% 67%	1400
July	1st to 10th	Night	2 2 1 3 2 3 4 3 5% 5% 71%	3352
	11th to 20th	Night	1 1 1 3 2 2 3 3 5% 7% 71%	3640
	21st to end	Night	2 2 1 3 3 3 4 4 5% 7% 66%	4415
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 62%	4033
	11th to 20th	Night	1 2 1 3 3 3 3 3 5% 6% 69%	4051
	21st to end	Night	7% 4 2 4 3 2 3 3 5% 6% 62%	4509
September	1st to 10th	Night	4 3 1 4 2 3 3 3 4 5% 70%	2084
	11th to 20th	Night	6% 3 1 4 3 4 3 3 6% 7% 60%	2200
	21st to end	Night	7% 4 1 3 3 3 3 2 5% 6% 64%	2200
October	1st to 10th	Night	5% 3 1 4 2 2 2 2 3 4 71%	3750
	11th to 20th	Night	8% 7% 2 5% 4 3 4 3 4 5% 56%	3898
	21st to end	Night	6% 6% 3 7% 5% 5% 5% 4 6% 7% 47%	4458
November	1st to 10th	Night	9% 8% 3 5% 4 5% 5% 4 6% 6% 46%	2665
	11th to 20th	Night	7% 9% 3 7% 5% 5% 5% 4 6% 7% 40%	2804
	21st to end	Night	6% 8% 4 9% 6% 4 4 5% 5% 7% 42%	2940
December	1st to 10th	Night	5% 8% 3 7% 5% 4 5% 4 5% 7% 47%	4710
	11th to 20th	Night	7% 16% 5% 10% 6% 5% 5% 5% 6% 8% 27%	5548
	21st to end	Night	9% 13% 5% 11% 7% 6% 6% 4 6% 6% 28%	7623

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	62% 11% 10% 12% 4	3624
	11th to 20th	Day	52% 8% 17% 13% 7% 2	3640
	21st to end	Day	50% 7% 11% 21% 8% 3	4492
February	1st to 10th	Day	53% 11% 15% 15% 5% 1	1800
	11th to 20th	Day	51% 10% 16% 17% 5% 1	1804
	21st to end	Day	45% 10% 16% 19% 9% 2	1650
March	1st to 10th	Day	42% 8% 16% 22% 9% 3	4709
	11th to 20th	Day	36% 6% 12% 31% 15% 1	5000
	21st to end	Day	45% 9% 19% 21% 6%	5486
April	1st to 10th	Day	39% 9% 16% 21% 13% 2	2281
	11th to 20th	Day	34% 6% 13% 27% 17% 3	2570
	21st to end	Day	37% 7% 12% 27% 16% 1	2600
May	1st to 10th	Day	27% 6% 10% 27% 24% 6%	5880
	11th to 20th	Day	32% 6% 8% 25% 22% 7%	5844
	21st to end	Day	23% 6% 8% 33% 26% 4	6762
June	1st to 10th	Day	29% 6% 14% 29% 20% 2	2979
	11th to 20th	Day	26% 5% 11% 20% 24% 14%	3000
	21st to end	Day	14% 3 7% 20% 35% 21%	3000
July	1st to 10th	Day	13% 2 4 10% 26% 45%	6533
	11th to 20th	Day	12% 1 5% 10% 29% 43%	6245
	21st to end	Day	15% 2 5% 11% 35% 33%	6269
August	1st to 10th	Day	21% 3 7% 15% 30% 25%	4208
	11th to 20th	Day	13% 3 5% 20% 37% 22%	4197
	21st to end	Day	16% 3 5% 18% 29% 28%	4663
September	1st to 10th	Day	16% 3 7% 18% 32% 24%	2310
	11th to 20th	Day	21% 5% 8% 25% 28% 13%	2200
	21st to end	Day	21% 3 8% 27% 30% 10%	2200
October	1st to 10th	Day	23% 3 7% 20% 34% 12%	3445
	11th to 20th	Day	24% 4 11% 37% 18% 7%	3250
	21st to end	Day	32% 6% 14% 29% 17% 2	3337
November	1st to 10th	Day	39% 6% 19% 24% 12% 1	1854
	11th to 20th	Day	40% 8% 20% 23% 7% 2	1794
	21st to end	Day	42% 8% 20% 21% 8% 1	1680
December	1st to 10th	Day	39% 8% 21% 23% 8% 1	2700
	11th to 20th	Day	61% 9% 15% 11% 3	2860
	21st to end	Day	55% 8% 16% 16% 5%	3575
January	1st to 10th	Night	55% 11% 12% 16% 6%	6385
	11th to 20th	Night	55% 8% 15% 15% 5% 2	6320
	21st to end	Night	48% 8% 13% 18% 10% 4	6469
February	1st to 10th	Night	52% 9% 15% 16% 6% 1	2600
	11th to 20th	Night	48% 8% 16% 20% 7% 1	2490
	21st to end	Night	47% 10% 17% 16% 8% 2	1980
March	1st to 10th	Night	40% 9% 15% 21% 11% 4	5183
	11th to 20th	Night	32% 8% 15% 28% 16% 1	5000
	21st to end	Night	38% 9% 20% 25% 8% 1	5489
April	1st to 10th	Night	39% 8% 14% 21% 15% 3	2114
	11th to 20th	Night	37% 7% 13% 23% 17% 2	1781
	21st to end	Night	35% 5% 13% 26% 19% 1	1800
May	1st to 10th	Night	31% 6% 9% 25% 24% 4	4097
	11th to 20th	Night	29% 7% 11% 24% 22% 6%	4075
	21st to end	Night	26% 7% 10% 24% 26% 6%	4122
June	1st to 10th	Night	29% 7% 15% 22% 24% 4	1416
	11th to 20th	Night	27% 5% 10% 18% 25% 14%	1400
	21st to end	Night	15% 4 7% 16% 31% 27%	1400
July	1st to 10th	Night	15% 2 5% 8% 23% 46%	3314
	11th to 20th	Night	12% 2 5% 9% 26% 46%	3592
	21st to end	Night	16% 2 5% 12% 29% 36%	4289
August	1st to 10th	Night	18% 3 7% 13% 28% 31%	2928
	11th to 20th	Night	18% 3 4 18% 32% 25%	2928
	21st to end	Night	17% 2 5% 14% 27% 34%	3245
September	1st to 10th	Night	17% 4 7% 13% 31% 28%	2084
	11th to 20th	Night	23% 5% 9% 21% 25% 18%	2200
	21st to end	Night	21% 4 8% 23% 32% 13%	2200
October	1st to 10th	Night	19% 3 8% 18% 35% 19%	3750
	11th to 20th	Night	32% 3 7% 31% 18% 8%	3898
	21st to end	Night	34% 6% 13% 27% 18% 2	4458
November	1st to 10th	Night	36% 7% 16% 25% 15% 2	2665
	11th to 20th	Night	41% 8% 18% 23% 8% 2	2804
	21st to end	Night	41% 8% 20% 21% 9% 1	2940
December	1st to 10th	Night	38% 7% 22% 26% 6% 1	4710
	11th to 20th	Night	54% 10% 19% 14% 2	5549
	21st to end	Night	55% 9% 14% 14% 7%	7623

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	14% 8% 21% 27% 21% 8%	202
	11th to 20th	Day	12% 7% 12% 41% 23% 8%	200
	21st to end	Day	12% 6% 21% 34% 21% 6%	327
February	1st to 10th	Day	12% 14% 26% 29% 17% 2	300
	11th to 20th	Day	12% 9% 26% 34% 16% 2	340
	21st to end	Day	9% 12% 26% 33% 17% 3	414
March	1st to 10th	Day	9% 9% 30% 33% 17% 2	500
	11th to 20th	Day	10% 11% 23% 33% 20% 3	500
	21st to end	Day	10% 15% 29% 28% 15% 2	550
April	1st to 10th	Day	11% 15% 25% 26% 17% 6%	500
	11th to 20th	Day	9% 18% 33% 21% 15% 4	492
	21st to end	Day	8% 12% 27% 28% 18% 6%	500
May	1st to 10th	Day	7% 12% 26% 25% 22% 8%	503
	11th to 20th	Day	13% 11% 22% 25% 20% 9%	528
	21st to end	Day	10% 7% 20% 27% 24% 12%	716
June	1st to 10th	Day	6% 9% 21% 27% 26% 11%	730
	11th to 20th	Day	10% 11% 25% 30% 19% 5%	730
	21st to end	Day	4 5% 24% 33% 25% 9%	730
July	1st to 10th	Day	3 3 19% 36% 28% 10%	652
	11th to 20th	Day	2 5% 20% 34% 31% 8%	524
	21st to end	Day	3 3 19% 31% 29% 15%	575
August	1st to 10th	Day	5% 5% 19% 31% 29% 12%	527
	11th to 20th	Day	4 3 19% 26% 31% 18%	517
	21st to end	Day	6% 5% 16% 29% 27% 17%	583
September	1st to 10th	Day	3 4 17% 26% 32% 18%	530
	11th to 20th	Day	5% 7% 21% 29% 24% 15%	530
	21st to end	Day	8% 6% 17% 24% 28% 16%	530
October	1st to 10th	Day	12% 6% 18% 25% 23% 17%	440
	11th to 20th	Day	5% 8% 24% 22% 21% 20%	330
	21st to end	Day	12% 10% 23% 22% 25% 7%	363
November	1st to 10th	Day	12% 12% 22% 25% 23% 7%	330
	11th to 20th	Day	9% 7% 19% 35% 25% 5%	330
	21st to end	Day	13% 5% 18% 37% 22% 4	330
December	1st to 10th	Day	12% 9% 19% 33% 20% 7%	330
	11th to 20th	Day	19% 8% 19% 33% 20% 1	269
	21st to end	Day	17% 12% 16% 30% 19% 7%	242
January	1st to 10th	Night	13% 7% 20% 39% 19% 1	830
	11th to 20th	Night	12% 7% 20% 39% 20% 4	800
	21st to end	Night	14% 4 21% 40% 17% 3	880
February	1st to 10th	Night	14% 7% 29% 37% 13% 1	800
	11th to 20th	Night	14% 5% 27% 37% 14% 2	784
	21st to end	Night	7% 9% 28% 40% 15% 2	660
March	1st to 10th	Night	9% 8% 25% 42% 15% 1	800
	11th to 20th	Night	8% 6% 26% 42% 17% 1	800
	21st to end	Night	10% 8% 27% 39% 15% 1	880
April	1st to 10th	Night	10% 8% 27% 38% 16% 1	774
	11th to 20th	Night	10% 8% 34% 36% 11% 1	593
	21st to end	Night	10% 6% 27% 39% 17% 2	600
May	1st to 10th	Night	8% 8% 29% 36% 19% 1	600
	11th to 20th	Night	8% 6% 27% 35% 23% 2	591
	21st to end	Night	12% 6% 21% 38% 21% 2	650
June	1st to 10th	Night	6% 6% 20% 39% 26% 3	416
	11th to 20th	Night	7% 5% 25% 46% 17% 1	400
	21st to end	Night	5% 3 19% 48% 23% 3	400
July	1st to 10th	Night	3 1 16% 48% 30% 2	390
	11th to 20th	Night	2 13% 57% 26% 1	388
	21st to end	Night	3 1 16% 47% 31% 3	626
August	1st to 10th	Night	4 3 18% 44% 30% 1	597
	11th to 20th	Night	2 2 18% 42% 34% 2	594
	21st to end	Night	5% 2 21% 41% 30% 2	660
September	1st to 10th	Night	5% 1 17% 40% 32% 4	732
	11th to 20th	Night	7% 3 15% 40% 29% 6%	800
	21st to end	Night	8% 3 17% 37% 30% 6%	800
October	1st to 10th	Night	9% 4 19% 38% 29% 2	800
	11th to 20th	Night	9% 6% 21% 36% 25% 3	800
	21st to end	Night	7% 6% 23% 41% 20% 3	880
November	1st to 10th	Night	11% 6% 24% 38% 19% 2	820
	11th to 20th	Night	11% 6% 26% 39% 16% 2	914
	21st to end	Night	10% 6% 21% 41% 20% 2	1050
December	1st to 10th	Night	12% 8% 21% 43% 15% 2	1050
	11th to 20th	Night	15% 9% 21% 40% 14% 2	1050
	21st to end	Night	18% 7% 26% 36% 12% 1	1155

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