Twin Oaks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Oaks Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	113883
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	114747
Transparency	Day	40% 5% 11% 23% 17% 4	117663
	Night	39% 5% 10% 23% 19% 5%	119092
Seeing	Day	17% 9% 23% 24% 19% 9%	15270
	Night	17% 8% 22% 30% 20% 4	25378

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% 10% 3 7% 4 3 3 3 4 5% 42%	4056
	11th to 20th	Day	16% 6% 4 7% 4 3 3 2 4 6% 44%	4060
	21st to end	Day	21% 8% 3 4 4 3 3 3 4 6% 40%	4696
February	1st to 10th	Day	18% 9% 3 7% 4 4 4 4 6% 5% 37%	2000
	11th to 20th	Day	21% 9% 2 7% 4 4 4 3 4 7% 34%	1948
	21st to end	Day	19% 9% 2 7% 4 4 3 3 5% 5% 39%	1650
March	1st to 10th	Day	19% 10% 3 5% 5% 5% 5% 4 5% 6% 34%	4480
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 6% 49%	4975
	21st to end	Day	14% 7% 2 7% 5% 4 4 3 5% 6% 43%	5465
April	1st to 10th	Day	12% 5% 2 7% 5% 5% 4 5% 6% 7% 42%	2373
	11th to 20th	Day	12% 7% 3 7% 4 3 3 4 6% 6% 45%	2374
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 5% 8% 42%	2461
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 7% 37%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 4 6% 8% 34%	4081
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 4 6% 8% 41%	4476
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 10% 37%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 43%	2470
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 6% 9% 11% 38%	2470
July	1st to 10th	Day	6% 6% 2 6% 6% 7% 8% 7% 9% 11% 32%	4120
	11th to 20th	Day	9% 6% 1 6% 6% 6% 7% 6% 8% 10% 34%	4120
	21st to end	Day	14% 11% 4 10% 8% 8% 7% 5% 7% 8% 19%	4466
August	1st to 10th	Day	5% 4 2 5% 5% 6% 6% 6% 7% 12% 42%	4104
	11th to 20th	Day	6% 4 1 6% 6% 5% 6% 5% 9% 11% 41%	3729
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 5% 7% 49%	4180
September	1st to 10th	Day	11% 5% 2 6% 6% 6% 5% 4 7% 8% 40%	2161
	11th to 20th	Day	14% 6% 2 6% 5% 5% 4 4 6% 8% 40%	2090
	21st to end	Day	15% 5% 1 5% 4 4 4 3 5% 7% 46%	2090
October	1st to 10th	Day	11% 5% 2 4 3 3 4 3 5% 6% 54%	2090
	11th to 20th	Day	10% 5% 2 5% 3 4 3 3 4 6% 55%	2024
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 7% 51%	1991
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 5% 54%	1762
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 5% 7% 47%	1800
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 7% 44%	1800
December	1st to 10th	Day	17% 8% 2 6% 4 3 3 3 4 5% 45%	2930
	11th to 20th	Day	17% 11% 3 8% 5% 5% 4 3 4 5% 34%	3373
	21st to end	Day	20% 7% 3 6% 4 3 4 4 6% 8% 35%	4466
January	1st to 10th	Night	15% 9% 2 7% 4 3 4 3 4 5% 43%	5889
	11th to 20th	Night	17% 8% 3 6% 4 3 3 2 4 5% 46%	5900
	21st to end	Night	18% 9% 2 5% 4 4 4 3 4 6% 43%	6242
February	1st to 10th	Night	16% 7% 3 7% 6% 4 4 3 5% 6% 40%	2400
	11th to 20th	Night	21% 8% 4 7% 5% 4 4 3 4 4 37%	2338
	21st to end	Night	21% 8% 2 7% 4 3 4 3 5% 5% 36%	1980
March	1st to 10th	Night	21% 9% 2 6% 5% 4 4 4 5% 7% 33%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	4985
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 3 5% 4 42%	5467
April	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 6% 8% 48%	2043
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 4 8% 50%	1975
	21st to end	Night	12% 6% 2 4 4 3 5% 3 6% 7% 48%	1832
May	1st to 10th	Night	12% 8% 3 7% 5% 5% 5% 4 5% 8% 39%	2813
	11th to 20th	Night	15% 6% 2 6% 5% 5% 4 4 6% 8% 39%	2812
	21st to end	Night	8% 5% 2 6% 4 4 5% 5% 8% 10% 43%	3079
June	1st to 10th	Night	8% 5% 2 7% 5% 6% 5% 5% 7% 10% 40%	1710
	11th to 20th	Night	4 3 1 4 4 4 4 5% 8% 12% 51%	1710
	21st to end	Night	6% 4 2 6% 5% 5% 5% 4 8% 12% 44%	1710
July	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 6% 9% 11% 35%	2840
	11th to 20th	Night	9% 5% 2 5% 4 4 4 5% 8% 12% 42%	2840
	21st to end	Night	13% 11% 3 10% 6% 5% 5% 5% 7% 9% 25%	3087
August	1st to 10th	Night	4 3 2 5% 4 4 5% 5% 8% 11% 49%	2825
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 7% 9% 51%	3095
	21st to end	Night	6% 3 2 4 4 4 4 4 6% 9% 55%	3476
September	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	1999
	11th to 20th	Night	13% 6% 2 7% 4 4 4 4 6% 8% 42%	2090
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 6% 52%	2090
October	1st to 10th	Night	11% 3 1 4 3 2 3 2 5% 6% 58%	2090
	11th to 20th	Night	12% 5% 1 5% 4 3 3 3 4 6% 54%	2156
	21st to end	Night	14% 6% 2 4 3 2 2 3 4 5% 55%	2603
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 6% 52%	2536
	11th to 20th	Night	13% 7% 2 4 4 3 4 3 5% 6% 50%	2600
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 6% 44%	2600
December	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 4 5% 44%	4250
	11th to 20th	Night	20% 9% 3 6% 3 3 4 3 4 6% 40%	4822
	21st to end	Night	21% 6% 3 6% 4 5% 5% 4 5% 7% 34%	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	43% 5% 7% 24% 20% 1	4057
	11th to 20th	Day	42% 5% 5% 21% 25% 2	4060
	21st to end	Day	46% 4 8% 23% 17% 3	4696
February	1st to 10th	Day	48% 4 8% 22% 16% 3	2000
	11th to 20th	Day	51% 4 6% 19% 17% 4	1958
	21st to end	Day	48% 2 9% 16% 20% 6%	1650
March	1st to 10th	Day	50% 4 7% 14% 17% 8%	4480
	11th to 20th	Day	36% 4 6% 20% 23% 11%	4975
	21st to end	Day	42% 3 8% 29% 15% 2	5465
April	1st to 10th	Day	38% 5% 12% 23% 18% 4	2373
	11th to 20th	Day	38% 4 8% 20% 24% 6%	2374
	21st to end	Day	39% 5% 10% 28% 16% 2	2461
May	1st to 10th	Day	45% 6% 11% 23% 14% 2	4120
	11th to 20th	Day	46% 5% 13% 24% 11% 1	4085
	21st to end	Day	38% 6% 11% 23% 20% 2	4489
June	1st to 10th	Day	38% 6% 17% 24% 13% 2	2470
	11th to 20th	Day	30% 7% 19% 27% 16% 2	2470
	21st to end	Day	35% 7% 20% 20% 14% 4	2470
July	1st to 10th	Day	38% 11% 21% 20% 8% 1	4055
	11th to 20th	Day	37% 10% 23% 21% 6% 2	4051
	21st to end	Day	60% 8% 13% 14% 5%	4324
August	1st to 10th	Day	29% 9% 16% 35% 10% 1	4091
	11th to 20th	Day	31% 8% 18% 27% 15% 2	3765
	21st to end	Day	32% 6% 12% 30% 17% 2	4179
September	1st to 10th	Day	39% 6% 13% 18% 17% 6%	2161
	11th to 20th	Day	41% 5% 11% 21% 20% 2	2090
	21st to end	Day	37% 4 11% 21% 21% 7%	2090
October	1st to 10th	Day	21% 5% 7% 20% 34% 12%	3480
	11th to 20th	Day	22% 2 5% 25% 33% 13%	3342
	21st to end	Day	28% 5% 8% 25% 27% 8%	3254
November	1st to 10th	Day	34% 4 7% 21% 27% 6%	1762
	11th to 20th	Day	37% 3 5% 24% 25% 6%	1800
	21st to end	Day	40% 4 7% 25% 20% 4	1800
December	1st to 10th	Day	41% 4 6% 25% 21% 3	2930
	11th to 20th	Day	53% 5% 6% 20% 13% 3	3370
	21st to end	Day	46% 5% 12% 26% 10% 2	4466
January	1st to 10th	Night	44% 3 8% 24% 19% 2	5889
	11th to 20th	Night	41% 5% 6% 21% 23% 5%	5900
	21st to end	Night	45% 4 6% 21% 18% 5%	6242
February	1st to 10th	Night	45% 3 9% 21% 18% 3	2400
	11th to 20th	Night	52% 3 7% 17% 15% 5%	2339
	21st to end	Night	49% 4 7% 18% 18% 5%	1980
March	1st to 10th	Night	51% 4 7% 16% 18% 4	5373
	11th to 20th	Night	34% 4 6% 20% 22% 13%	4985
	21st to end	Night	46% 2 7% 26% 17% 3	5467
April	1st to 10th	Night	35% 4 11% 24% 22% 4	2043
	11th to 20th	Night	33% 3 8% 25% 23% 8%	1975
	21st to end	Night	35% 5% 13% 27% 18% 2	1832
May	1st to 10th	Night	43% 5% 13% 23% 14% 1	2840
	11th to 20th	Night	43% 6% 15% 24% 13%	2813
	21st to end	Night	33% 8% 15% 26% 17% 2	3087
June	1st to 10th	Night	38% 7% 16% 22% 14% 2	1710
	11th to 20th	Night	23% 7% 22% 30% 16% 2	1710
	21st to end	Night	31% 9% 19% 22% 14% 5%	1710
July	1st to 10th	Night	38% 11% 22% 20% 8% 2	2795
	11th to 20th	Night	33% 9% 24% 25% 6% 3	2786
	21st to end	Night	54% 8% 15% 15% 6% 1	2988
August	1st to 10th	Night	27% 8% 15% 35% 14% 1	2814
	11th to 20th	Night	27% 8% 23% 27% 14% 1	3125
	21st to end	Night	26% 7% 15% 33% 18% 1	3476
September	1st to 10th	Night	38% 6% 16% 20% 14% 5%	1999
	11th to 20th	Night	39% 6% 12% 19% 21% 2	2090
	21st to end	Night	33% 5% 10% 23% 23% 6%	2090
October	1st to 10th	Night	19% 4 7% 24% 31% 16%	3480
	11th to 20th	Night	21% 2 5% 22% 34% 16%	3577
	21st to end	Night	24% 4 5% 28% 29% 10%	4277
November	1st to 10th	Night	34% 4 7% 23% 24% 9%	2536
	11th to 20th	Night	36% 2 5% 23% 26% 8%	2600
	21st to end	Night	42% 2 6% 24% 22% 4	2600
December	1st to 10th	Night	44% 4 6% 21% 23% 2	4250
	11th to 20th	Night	47% 4 7% 21% 19% 2	4824
	21st to end	Night	47% 6% 10% 24% 13% 1	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	27% 23% 29% 18% 3 1	300
	11th to 20th	Day	26% 20% 33% 14% 5% 1	300
	21st to end	Day	23% 15% 46% 13% 3	330
February	1st to 10th	Day	23% 19% 35% 17% 6% 1	300
	11th to 20th	Day	37% 18% 31% 12% 2 1	300
	21st to end	Day	32% 17% 38% 11% 2	249
March	1st to 10th	Day	24% 18% 35% 16% 5% 1	300
	11th to 20th	Day	24% 17% 29% 23% 6% 1	495
	21st to end	Day	20% 15% 38% 21% 6% 1	550
April	1st to 10th	Day	19% 14% 32% 22% 11% 2	500
	11th to 20th	Day	15% 15% 33% 23% 12% 2	492
	21st to end	Day	18% 10% 32% 25% 13% 3	480
May	1st to 10th	Day	14% 10% 31% 25% 14% 6%	463
	11th to 20th	Day	14% 5% 22% 28% 21% 10%	487
	21st to end	Day	15% 4 18% 26% 22% 15%	529
June	1st to 10th	Day	13% 3 16% 30% 24% 13%	490
	11th to 20th	Day	7% 3 12% 29% 31% 18%	490
	21st to end	Day	9% 2 14% 32% 23% 20%	490
July	1st to 10th	Day	10% 2 9% 23% 30% 25%	485
	11th to 20th	Day	12% 1 7% 24% 33% 23%	484
	21st to end	Day	11% 2 8% 23% 32% 23%	532
August	1st to 10th	Day	12% 3 9% 23% 34% 19%	486
	11th to 20th	Day	12% 3 9% 31% 34% 11%	479
	21st to end	Day	14% 2 8% 20% 35% 22%	539
September	1st to 10th	Day	10% 2 8% 25% 36% 18%	490
	11th to 20th	Day	17% 3 9% 24% 31% 16%	490
	21st to end	Day	19% 3 13% 25% 31% 9%	490
October	1st to 10th	Day	14% 3 18% 26% 28% 11%	490
	11th to 20th	Day	11% 7% 28% 30% 21% 3	490
	21st to end	Day	18% 9% 21% 30% 18% 4	440
November	1st to 10th	Day	20% 9% 24% 28% 14% 5%	300
	11th to 20th	Day	15% 12% 36% 28% 8% 1	300
	21st to end	Day	14% 11% 39% 23% 11% 1	300
December	1st to 10th	Day	29% 13% 33% 20% 5% 1	300
	11th to 20th	Day	19% 16% 36% 21% 6% 1	300
	21st to end	Day	30% 14% 29% 20% 7% 1	330
January	1st to 10th	Night	25% 16% 34% 20% 5%	800
	11th to 20th	Night	22% 17% 31% 25% 6%	800
	21st to end	Night	21% 13% 34% 29% 2	880
February	1st to 10th	Night	20% 14% 33% 28% 6%	800
	11th to 20th	Night	26% 20% 32% 21% 2	783
	21st to end	Night	28% 15% 33% 21% 3	660
March	1st to 10th	Night	20% 13% 33% 29% 6%	800
	11th to 20th	Night	20% 12% 32% 28% 8% 1	800
	21st to end	Night	19% 12% 32% 29% 8%	880
April	1st to 10th	Night	16% 11% 31% 31% 11%	643
	11th to 20th	Night	16% 9% 31% 32% 11% 1	593
	21st to end	Night	15% 6% 28% 38% 13% 1	587
May	1st to 10th	Night	12% 4 25% 36% 21% 2	570
	11th to 20th	Night	15% 4 18% 32% 26% 4	561
	21st to end	Night	12% 2 11% 34% 35% 6%	617
June	1st to 10th	Night	10% 2 14% 35% 33% 7%	570
	11th to 20th	Night	5% 2 11% 35% 36% 12%	570
	21st to end	Night	8% 2 8% 33% 39% 11%	570
July	1st to 10th	Night	11% 3 7% 26% 41% 13%	555
	11th to 20th	Night	11% 1 5% 27% 41% 15%	552
	21st to end	Night	10% 1 7% 26% 43% 13%	624
August	1st to 10th	Night	8% 1 5% 29% 47% 11%	567
	11th to 20th	Night	11% 1 7% 33% 41% 6%	564
	21st to end	Night	12% 2 6% 25% 43% 12%	627
September	1st to 10th	Night	12% 1 9% 31% 39% 9%	669
	11th to 20th	Night	17% 2 11% 27% 37% 7%	760
	21st to end	Night	15% 3 15% 31% 33% 4	760
October	1st to 10th	Night	14% 4 16% 31% 28% 8%	760
	11th to 20th	Night	15% 8% 23% 32% 21% 1	760
	21st to end	Night	16% 7% 20% 38% 17% 2	836
November	1st to 10th	Night	16% 8% 21% 33% 20% 2	780
	11th to 20th	Night	16% 10% 32% 31% 11% 1	800
	21st to end	Night	18% 12% 29% 30% 11% 1	800
December	1st to 10th	Night	25% 14% 26% 27% 9%	800
	11th to 20th	Night	20% 10% 32% 30% 9% 1	800
	21st to end	Night	25% 13% 26% 29% 7%	880

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