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 2024-11-12.

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% 3 7% 4 4 4 3 5% 7% 52%	105187
	Night	6% 8% 3 7% 5% 4 4 4 5% 7% 47%	102677
Transparency	Day	31% 6% 11% 21% 19% 12%	104936
	Night	35% 7% 12% 19% 16% 10%	102516
Seeing	Day	8% 9% 22% 29% 23% 9%	15229
	Night	10% 5% 22% 40% 21% 2	24806

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% 14% 6% 15% 7% 6% 5% 4 5% 7% 23%	2516
	11th to 20th	Day	7% 10% 6% 14% 8% 6% 5% 4 5% 5% 31%	2540
	21st to end	Day	12% 18% 6% 12% 6% 4 4 3 4 5% 26%	3128
February	1st to 10th	Day	6% 11% 5% 15% 6% 5% 5% 6% 5% 6% 29%	1710
	11th to 20th	Day	7% 11% 4 12% 7% 6% 7% 5% 7% 7% 27%	1703
	21st to end	Day	5% 10% 4 11% 6% 6% 5% 4 7% 7% 36%	1570
March	1st to 10th	Day	4 11% 4 11% 6% 5% 5% 4 5% 8% 36%	3277
	11th to 20th	Day	4 10% 3 6% 4 4 3 3 4 6% 53%	3490
	21st to end	Day	7% 13% 5% 11% 7% 7% 5% 5% 6% 7% 28%	3825
April	1st to 10th	Day	7% 9% 3 8% 4 5% 5% 5% 5% 8% 40%	2167
	11th to 20th	Day	5% 10% 3 5% 5% 4 4 4 6% 6% 47%	2437
	21st to end	Day	7% 8% 2 7% 5% 4 4 4 6% 9% 43%	2470
May	1st to 10th	Day	6% 10% 3 6% 4 4 4 3 5% 8% 46%	4110
	11th to 20th	Day	4 4 1 5% 3 3 4 4 5% 7% 60%	4071
	21st to end	Day	7% 7% 2 5% 4 4 4 4 5% 7% 51%	4717
June	1st to 10th	Day	6% 4 1 6% 5% 4 5% 5% 7% 8% 49%	2830
	11th to 20th	Day	6% 5% 2 5% 3 4 4 4 6% 7% 54%	2850
	21st to end	Day	2 2 1 3 2 3 3 3 4 7% 71%	2850
July	1st to 10th	Day	1 1 1 2 2 2 2 2 4 6% 76%	4620
	11th to 20th	Day	2 2 1 3 2 3 3 2 5% 6% 71%	4420
	21st to end	Day	2 2 1 4 3 3 3 3 5% 7% 68%	4495
August	1st to 10th	Day	4 3 1 4 3 3 4 4 5% 7% 61%	4092
	11th to 20th	Day	2 1 1 3 2 3 3 3 5% 7% 70%	4028
	21st to end	Day	2 3 1 3 2 3 3 2 4 5% 71%	4521
September	1st to 10th	Day	4 2 1 3 3 2 3 2 4 5% 71%	2195
	11th to 20th	Day	5% 4 1 4 2 3 3 3 5% 7% 62%	2090
	21st to end	Day	6% 4 1 3 3 2 3 2 4 6% 65%	2090
October	1st to 10th	Day	7% 4 2 4 3 3 2 2 3 4 67%	3340
	11th to 20th	Day	3 5% 2 5% 3 3 4 3 4 5% 63%	3150
	21st to end	Day	5% 6% 2 6% 4 4 5% 4 6% 8% 49%	3234
November	1st to 10th	Day	9% 8% 4 8% 5% 3 3 3 5% 7% 45%	1764
	11th to 20th	Day	6% 9% 3 7% 5% 5% 5% 5% 6% 8% 41%	1633
	21st to end	Day	7% 11% 3 10% 5% 4 4 3 5% 7% 42%	1520
December	1st to 10th	Day	8% 12% 5% 12% 8% 5% 4 4 5% 6% 32%	1520
	11th to 20th	Day	9% 14% 4 12% 9% 6% 5% 5% 5% 9% 21%	1772
	21st to end	Day	6% 12% 5% 11% 6% 5% 4 3 5% 6% 35%	2442
January	1st to 10th	Night	6% 13% 6% 13% 6% 5% 5% 5% 6% 8% 27%	4456
	11th to 20th	Night	9% 16% 6% 11% 7% 6% 4 3 5% 7% 27%	4420
	21st to end	Night	11% 18% 6% 12% 6% 5% 5% 3 4 6% 24%	4522
February	1st to 10th	Night	8% 12% 5% 10% 7% 6% 6% 5% 6% 6% 31%	2470
	11th to 20th	Night	9% 12% 4 9% 6% 5% 5% 3 5% 7% 36%	2362
	21st to end	Night	3 11% 5% 10% 7% 6% 5% 5% 6% 7% 34%	1884
March	1st to 10th	Night	4 10% 3 9% 5% 5% 5% 4 6% 8% 40%	3595
	11th to 20th	Night	3 6% 2 5% 4 4 4 4 5% 8% 54%	3490
	21st to end	Night	6% 10% 4 8% 5% 5% 6% 5% 8% 8% 35%	3828
April	1st to 10th	Night	7% 9% 3 8% 5% 3 4 4 5% 7% 43%	2009
	11th to 20th	Night	6% 10% 3 7% 4 4 5% 4 6% 6% 45%	1691
	21st to end	Night	7% 7% 4 6% 5% 5% 4 4 6% 6% 47%	1710
May	1st to 10th	Night	7% 9% 3 7% 4 4 5% 4 4 5% 47%	2860
	11th to 20th	Night	3 4 1 5% 3 4 4 4 6% 8% 57%	2834
	21st to end	Night	8% 7% 2 5% 4 5% 4 4 6% 8% 47%	2870
June	1st to 10th	Night	5% 4 2 7% 4 5% 5% 5% 7% 8% 48%	1345
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 5% 8% 53%	1330
	21st to end	Night	2 2 1 3 3 3 3 4 5% 6% 67%	1330
July	1st to 10th	Night	2 2 1 3 2 2 3 3 4 5% 73%	2336
	11th to 20th	Night	1 1 1 3 2 3 4 4 5% 8% 68%	2540
	21st to end	Night	2 2 1 3 3 4 3 4 5% 7% 66%	3066
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 62%	2847
	11th to 20th	Night	2 2 1 4 3 3 4 3 4 6% 67%	2811
	21st to end	Night	4 3 1 3 2 2 3 2 4 5% 72%	3146
September	1st to 10th	Night	4 3 1 4 2 3 3 3 4 5% 70%	1979
	11th to 20th	Night	7% 3 1 4 3 4 3 3 5% 7% 60%	2090
	21st to end	Night	7% 4 1 3 3 3 3 2 5% 6% 64%	2090
October	1st to 10th	Night	5% 3 1 4 2 2 2 2 3 4 71%	3635
	11th to 20th	Night	8% 7% 2 5% 4 3 4 3 4 5% 56%	3778
	21st to end	Night	6% 6% 3 7% 5% 5% 5% 4 6% 7% 47%	4320
November	1st to 10th	Night	9% 8% 3 6% 4 4 5% 4 6% 6% 46%	2535
	11th to 20th	Night	7% 9% 3 7% 5% 5% 5% 4 6% 7% 42%	2549
	21st to end	Night	7% 8% 4 8% 6% 4 4 5% 5% 6% 43%	2660
December	1st to 10th	Night	8% 12% 5% 10% 6% 5% 4 4 6% 7% 33%	2660
	11th to 20th	Night	8% 15% 4 10% 6% 5% 6% 4 6% 7% 28%	3415
	21st to end	Night	7% 12% 5% 11% 7% 5% 5% 4 5% 6% 32%	5214

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% 13% 12% 12% 3 1	2516
	11th to 20th	Day	53% 10% 16% 13% 6% 1	2540
	21st to end	Day	63% 9% 12% 12% 4 1	3128
February	1st to 10th	Day	52% 11% 16% 15% 5% 1	1710
	11th to 20th	Day	50% 10% 17% 17% 5% 1	1712
	21st to end	Day	45% 11% 17% 18% 8% 2	1570
March	1st to 10th	Day	44% 10% 16% 23% 7% 1	3277
	11th to 20th	Day	32% 7% 14% 31% 14% 1	3490
	21st to end	Day	51% 11% 17% 16% 5% 1	3825
April	1st to 10th	Day	40% 9% 17% 20% 13% 2	2167
	11th to 20th	Day	35% 7% 14% 27% 16% 2	2440
	21st to end	Day	36% 7% 12% 27% 16% 1	2470
May	1st to 10th	Day	33% 8% 10% 24% 21% 5%	4110
	11th to 20th	Day	21% 6% 9% 31% 23% 10%	4074
	21st to end	Day	30% 8% 11% 30% 19% 2	4717
June	1st to 10th	Day	29% 6% 14% 29% 19% 2	2830
	11th to 20th	Day	27% 6% 11% 20% 23% 14%	2850
	21st to end	Day	13% 3 7% 21% 34% 21%	2850
July	1st to 10th	Day	10% 2 4 10% 26% 47%	4549
	11th to 20th	Day	14% 1 4 9% 23% 48%	4345
	21st to end	Day	16% 2 5% 11% 31% 33%	4353
August	1st to 10th	Day	21% 3 7% 15% 29% 25%	4078
	11th to 20th	Day	14% 3 5% 20% 36% 22%	4067
	21st to end	Day	15% 3 5% 19% 29% 29%	4520
September	1st to 10th	Day	16% 3 7% 17% 32% 25%	2195
	11th to 20th	Day	21% 5% 9% 25% 27% 13%	2090
	21st to end	Day	21% 3 8% 28% 29% 10%	2090
October	1st to 10th	Day	23% 3 7% 19% 35% 12%	3340
	11th to 20th	Day	23% 4 11% 37% 19% 7%	3150
	21st to end	Day	32% 6% 15% 28% 17% 1	3234
November	1st to 10th	Day	40% 6% 18% 23% 13% 1	1764
	11th to 20th	Day	40% 9% 20% 22% 7% 2	1633
	21st to end	Day	42% 8% 20% 20% 8% 1	1520
December	1st to 10th	Day	53% 9% 14% 16% 7% 1	1520
	11th to 20th	Day	59% 10% 13% 14% 3	1770
	21st to end	Day	48% 9% 17% 19% 6% 1	2442
January	1st to 10th	Night	53% 13% 14% 14% 5%	4456
	11th to 20th	Night	57% 11% 13% 13% 5% 1	4420
	21st to end	Night	63% 11% 13% 9% 4 1	4522
February	1st to 10th	Night	52% 10% 15% 17% 6% 1	2470
	11th to 20th	Night	47% 9% 17% 20% 7% 1	2363
	21st to end	Night	48% 10% 17% 16% 7% 2	1884
March	1st to 10th	Night	38% 12% 17% 22% 10% 1	3595
	11th to 20th	Night	26% 10% 19% 29% 15% 1	3490
	21st to end	Night	39% 11% 21% 24% 4 1	3828
April	1st to 10th	Night	39% 8% 15% 21% 14% 3	2009
	11th to 20th	Night	37% 8% 13% 23% 16% 2	1691
	21st to end	Night	35% 5% 13% 26% 19% 1	1710
May	1st to 10th	Night	34% 8% 10% 24% 18% 4	2860
	11th to 20th	Night	20% 8% 13% 25% 26% 9%	2835
	21st to end	Night	29% 9% 12% 25% 21% 3	2870
June	1st to 10th	Night	29% 7% 16% 22% 23% 3	1345
	11th to 20th	Night	28% 6% 10% 17% 25% 13%	1330
	21st to end	Night	15% 4 7% 17% 29% 28%	1330
July	1st to 10th	Night	13% 3 5% 9% 22% 48%	2298
	11th to 20th	Night	13% 2 4 10% 21% 49%	2492
	21st to end	Night	17% 2 5% 15% 26% 35%	2970
August	1st to 10th	Night	18% 3 7% 13% 27% 32%	2838
	11th to 20th	Night	18% 3 5% 18% 32% 25%	2838
	21st to end	Night	16% 3 5% 14% 27% 35%	3146
September	1st to 10th	Night	17% 4 6% 13% 31% 29%	1979
	11th to 20th	Night	23% 5% 9% 21% 25% 18%	2090
	21st to end	Night	21% 4 7% 24% 31% 13%	2090
October	1st to 10th	Night	18% 3 8% 17% 35% 19%	3635
	11th to 20th	Night	32% 3 7% 31% 19% 8%	3778
	21st to end	Night	34% 6% 13% 26% 19% 2	4320
November	1st to 10th	Night	37% 7% 16% 24% 15% 2	2535
	11th to 20th	Night	40% 8% 18% 24% 8% 2	2549
	21st to end	Night	40% 8% 20% 20% 10% 1	2660
December	1st to 10th	Night	50% 10% 18% 14% 7% 2	2660
	11th to 20th	Night	54% 11% 16% 16% 3	3416
	21st to end	Night	51% 10% 14% 14% 9% 1	5214

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% 20% 26% 23% 9%	180
	11th to 20th	Day	12% 7% 13% 38% 22% 8%	180
	21st to end	Day	13% 6% 21% 34% 20% 5%	294
February	1st to 10th	Day	9% 14% 27% 30% 18% 2	270
	11th to 20th	Day	11% 10% 27% 34% 16% 3	308
	21st to end	Day	9% 12% 26% 34% 16% 2	382
March	1st to 10th	Day	8% 10% 31% 33% 16% 2	460
	11th to 20th	Day	8% 10% 22% 35% 21% 3	460
	21st to end	Day	10% 15% 29% 29% 16% 1	506
April	1st to 10th	Day	11% 15% 25% 27% 17% 5%	460
	11th to 20th	Day	9% 18% 33% 21% 14% 4	452
	21st to end	Day	9% 13% 26% 28% 17% 7%	460
May	1st to 10th	Day	7% 13% 28% 25% 21% 7%	463
	11th to 20th	Day	11% 11% 21% 26% 22% 9%	488
	21st to end	Day	11% 7% 20% 27% 23% 12%	665
June	1st to 10th	Day	7% 9% 22% 26% 25% 11%	680
	11th to 20th	Day	11% 11% 25% 29% 19% 5%	680
	21st to end	Day	4 5% 24% 33% 26% 9%	680
July	1st to 10th	Day	3 3 19% 35% 29% 11%	606
	11th to 20th	Day	2 5% 20% 34% 31% 7%	484
	21st to end	Day	3 3 18% 31% 29% 16%	531
August	1st to 10th	Day	5% 5% 19% 30% 30% 12%	487
	11th to 20th	Day	4 3 19% 25% 30% 19%	477
	21st to end	Day	5% 5% 17% 29% 26% 18%	539
September	1st to 10th	Day	3 4 18% 26% 32% 17%	490
	11th to 20th	Day	5% 7% 21% 28% 24% 15%	490
	21st to end	Day	8% 7% 17% 23% 29% 16%	490
October	1st to 10th	Day	11% 6% 18% 24% 24% 17%	405
	11th to 20th	Day	5% 9% 22% 20% 23% 21%	300
	21st to end	Day	12% 11% 24% 22% 25% 6%	330
November	1st to 10th	Day	13% 12% 21% 24% 23% 7%	300
	11th to 20th	Day	7% 7% 16% 38% 26% 5%	273
	21st to end	Day	15% 4 18% 36% 23% 4	270
December	1st to 10th	Day	12% 9% 19% 35% 19% 7%	270
	11th to 20th	Day	19% 8% 16% 35% 21% 2	221
	21st to end	Day	15% 12% 15% 31% 19% 8%	198
January	1st to 10th	Night	14% 7% 20% 38% 20% 2	784
	11th to 20th	Night	12% 7% 19% 38% 20% 4	760
	21st to end	Night	15% 4 20% 40% 17% 3	836
February	1st to 10th	Night	13% 6% 29% 38% 14% 1	760
	11th to 20th	Night	14% 5% 27% 37% 14% 2	744
	21st to end	Night	7% 9% 28% 40% 14% 2	628
March	1st to 10th	Night	8% 8% 25% 43% 14% 1	760
	11th to 20th	Night	8% 5% 26% 43% 18% 1	760
	21st to end	Night	10% 8% 26% 39% 14% 1	836
April	1st to 10th	Night	10% 8% 26% 39% 16% 1	736
	11th to 20th	Night	10% 7% 35% 36% 11% 1	563
	21st to end	Night	9% 6% 26% 39% 18% 2	570
May	1st to 10th	Night	8% 8% 30% 35% 18% 1	570
	11th to 20th	Night	7% 6% 26% 36% 24% 2	561
	21st to end	Night	12% 6% 21% 38% 21% 2	617
June	1st to 10th	Night	6% 6% 20% 39% 26% 3	395
	11th to 20th	Night	7% 5% 26% 44% 17% 1	380
	21st to end	Night	4 3 19% 48% 23% 3	380
July	1st to 10th	Night	3 1 16% 47% 31% 2	370
	11th to 20th	Night	2 12% 58% 27% 1	368
	21st to end	Night	3 1 15% 46% 31% 3	594
August	1st to 10th	Night	4 2 17% 44% 31% 1	567
	11th to 20th	Night	2 2 18% 42% 34% 2	564
	21st to end	Night	4 2 20% 41% 31% 2	627
September	1st to 10th	Night	6% 1 17% 40% 32% 4	695
	11th to 20th	Night	7% 3 14% 40% 29% 6%	760
	21st to end	Night	8% 3 17% 37% 29% 6%	760
October	1st to 10th	Night	9% 4 19% 38% 30% 2	760
	11th to 20th	Night	9% 5% 20% 36% 26% 4	760
	21st to end	Night	8% 6% 23% 41% 19% 3	836
November	1st to 10th	Night	12% 6% 23% 38% 19% 2	780
	11th to 20th	Night	11% 7% 25% 39% 17% 2	830
	21st to end	Night	11% 5% 21% 39% 21% 2	950
December	1st to 10th	Night	13% 7% 20% 44% 14% 2	950
	11th to 20th	Night	15% 8% 20% 40% 15% 2	950
	21st to end	Night	18% 7% 24% 37% 13% 1	1045

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