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 2024-10-25.

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% 39%	100831
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 43%	97945
Transparency	Day	41% 5% 12% 23% 16% 3	100640
	Night	40% 5% 11% 23% 18% 4	97805
Seeing	Day	17% 8% 22% 24% 20% 10%	14677
	Night	17% 8% 22% 30% 20% 4	24671

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	17% 11% 3 6% 5% 3 4 4 5% 6% 35%	2836
	11th to 20th	Day	21% 7% 4 8% 4 3 3 3 4 6% 37%	2840
	21st to end	Day	24% 10% 2 4 3 3 4 3 4 6% 37%	3284
February	1st to 10th	Day	19% 10% 3 6% 4 4 4 4 5% 5% 37%	1900
	11th to 20th	Day	21% 9% 2 7% 4 4 4 3 4 7% 35%	1848
	21st to end	Day	20% 9% 2 7% 4 4 3 3 5% 5% 37%	1570
March	1st to 10th	Day	24% 12% 3 5% 4 4 3 3 4 5% 30%	3120
	11th to 20th	Day	15% 6% 3 6% 4 4 3 3 3 5% 49%	3475
	21st to end	Day	15% 7% 2 8% 5% 4 5% 3 6% 6% 38%	3815
April	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 5% 6% 7% 42%	2254
	11th to 20th	Day	13% 7% 3 7% 4 4 3 4 6% 6% 44%	2254
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 5% 8% 43%	2399
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%	1934
November	1st to 10th	Day	11% 5% 2 5% 4 3 3 2 4 5% 56%	1672
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 4 7% 48%	1710
	21st to end	Day	16% 7% 2 5% 4 4 3 4 5% 7% 44%	1710
December	1st to 10th	Day	22% 8% 3 7% 5% 4 3 4 5% 5% 35%	1710
	11th to 20th	Day	14% 7% 3 8% 5% 5% 4 4 4 6% 40%	2153
	21st to end	Day	19% 9% 3 6% 5% 4 4 4 6% 8% 33%	3124
January	1st to 10th	Night	18% 9% 3 6% 3 3 4 3 5% 6% 39%	4109
	11th to 20th	Night	18% 9% 3 7% 5% 4 3 3 4 5% 40%	4120
	21st to end	Night	22% 12% 2 5% 4 4 4 3 3 5% 36%	4368
February	1st to 10th	Night	16% 7% 2 8% 6% 4 3 3 5% 6% 40%	2280
	11th to 20th	Night	20% 8% 4 7% 5% 4 4 3 4 4 38%	2218
	21st to end	Night	22% 9% 2 7% 4 3 4 3 5% 5% 35%	1884
March	1st to 10th	Night	25% 9% 3 6% 5% 3 3 4 5% 6% 32%	3733
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 48%	3485
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 3 5% 4 40%	3817
April	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 8% 49%	1941
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 8% 49%	1875
	21st to end	Night	12% 6% 2 4 4 3 5% 3 6% 7% 49%	1778
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% 5% 2 4 3 2 2 3 4 5% 55%	2525
November	1st to 10th	Night	13% 5% 2 5% 3 2 3 3 5% 6% 54%	2406
	11th to 20th	Night	12% 7% 2 4 4 3 4 3 4 6% 50%	2470
	21st to end	Night	16% 7% 2 5% 4 4 4 4 5% 6% 44%	2470
December	1st to 10th	Night	19% 8% 3 6% 5% 4 5% 4 4 6% 36%	2470
	11th to 20th	Night	15% 7% 2 6% 3 3 4 3 5% 7% 44%	3042
	21st to end	Night	19% 7% 2 6% 5% 5% 6% 5% 6% 9% 30%	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	47% 5% 6% 20% 20% 2	2837
	11th to 20th	Day	50% 4 6% 18% 20% 1	2840
	21st to end	Day	50% 3 7% 23% 17% 1	3284
February	1st to 10th	Day	49% 3 7% 21% 17% 3	1900
	11th to 20th	Day	50% 4 6% 19% 17% 4	1858
	21st to end	Day	49% 2 9% 15% 19% 6%	1570
March	1st to 10th	Day	56% 3 8% 13% 13% 8%	3120
	11th to 20th	Day	39% 3 7% 23% 22% 6%	3475
	21st to end	Day	47% 3 9% 26% 13% 2	3815
April	1st to 10th	Day	38% 5% 11% 23% 19% 4	2254
	11th to 20th	Day	39% 4 8% 19% 24% 6%	2254
	21st to end	Day	39% 5% 10% 28% 16% 2	2399
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	31% 5% 9% 21% 26% 8%	2090
	11th to 20th	Day	30% 3 8% 25% 25% 9%	2024
	21st to end	Day	33% 3 6% 23% 29% 5%	1934
November	1st to 10th	Day	33% 3 7% 21% 29% 7%	1672
	11th to 20th	Day	37% 3 5% 24% 25% 6%	1710
	21st to end	Day	41% 3 7% 25% 20% 4	1710
December	1st to 10th	Day	52% 3 6% 22% 14% 3	1710
	11th to 20th	Day	45% 4 8% 21% 18% 3	2150
	21st to end	Day	48% 5% 8% 24% 12% 3	3124
January	1st to 10th	Night	47% 2 6% 24% 19% 2	4109
	11th to 20th	Night	46% 4 6% 20% 19% 5%	4120
	21st to end	Night	53% 3 5% 20% 18% 2	4368
February	1st to 10th	Night	45% 3 9% 21% 18% 3	2280
	11th to 20th	Night	51% 2 7% 18% 16% 6%	2219
	21st to end	Night	51% 4 7% 17% 17% 5%	1884
March	1st to 10th	Night	53% 4 9% 14% 15% 5%	3733
	11th to 20th	Night	35% 5% 7% 24% 19% 10%	3485
	21st to end	Night	47% 2 7% 23% 18% 2	3817
April	1st to 10th	Night	34% 3 11% 25% 22% 4	1941
	11th to 20th	Night	34% 3 8% 25% 22% 8%	1875
	21st to end	Night	34% 5% 13% 28% 19% 2	1778
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	28% 3 8% 24% 27% 10%	2090
	11th to 20th	Night	33% 3 7% 23% 24% 10%	2156
	21st to end	Night	33% 3 4 24% 27% 9%	2525
November	1st to 10th	Night	32% 3 7% 24% 25% 9%	2406
	11th to 20th	Night	36% 2 5% 23% 26% 8%	2470
	21st to end	Night	42% 2 6% 23% 22% 4	2470
December	1st to 10th	Night	50% 4 8% 19% 16% 3	2470
	11th to 20th	Night	40% 4 8% 21% 25% 2	3044
	21st to end	Night	49% 6% 8% 21% 14% 1	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	27% 22% 27% 20% 3 1	270
	11th to 20th	Day	28% 20% 30% 14% 6% 1	270
	21st to end	Day	24% 15% 45% 13% 3	297
February	1st to 10th	Day	26% 17% 33% 18% 6% 1	270
	11th to 20th	Day	36% 17% 31% 13% 2 1	270
	21st to end	Day	35% 17% 38% 9% 1	225
March	1st to 10th	Day	25% 17% 35% 16% 6% 1	270
	11th to 20th	Day	24% 17% 29% 23% 6% 1	455
	21st to end	Day	20% 14% 37% 21% 6% 1	506
April	1st to 10th	Day	19% 14% 31% 22% 12% 3	460
	11th to 20th	Day	16% 14% 32% 23% 12% 2	452
	21st to end	Day	17% 9% 33% 25% 13% 3	460
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% 20% 31% 17% 5%	421
November	1st to 10th	Day	19% 9% 25% 28% 14% 5%	270
	11th to 20th	Day	14% 11% 37% 27% 9% 1	270
	21st to end	Day	15% 10% 39% 23% 12% 1	270
December	1st to 10th	Day	30% 12% 31% 20% 6% 1	270
	11th to 20th	Day	17% 17% 37% 21% 7% 1	270
	21st to end	Day	29% 14% 32% 19% 5% 1	297
January	1st to 10th	Night	26% 15% 34% 20% 5%	760
	11th to 20th	Night	22% 17% 30% 25% 6%	760
	21st to end	Night	22% 13% 33% 29% 2	836
February	1st to 10th	Night	20% 14% 33% 28% 6%	760
	11th to 20th	Night	25% 18% 32% 22% 2	743
	21st to end	Night	29% 15% 33% 20% 3	628
March	1st to 10th	Night	20% 12% 33% 29% 6%	760
	11th to 20th	Night	20% 12% 33% 27% 8% 1	760
	21st to end	Night	19% 12% 32% 29% 8%	836
April	1st to 10th	Night	16% 10% 31% 31% 11%	611
	11th to 20th	Night	17% 9% 31% 31% 12% 1	563
	21st to end	Night	14% 6% 27% 38% 13% 1	570
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	17% 6% 20% 37% 17% 2	812
November	1st to 10th	Night	15% 8% 20% 34% 20% 2	740
	11th to 20th	Night	15% 10% 32% 31% 11% 1	760
	21st to end	Night	18% 11% 28% 30% 12% 1	760
December	1st to 10th	Night	25% 13% 26% 27% 9%	760
	11th to 20th	Night	19% 10% 32% 30% 9% 1	760
	21st to end	Night	25% 13% 26% 29% 7%	836

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