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 2024-02-25.

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	14% 6% 3 5% 6% 3 5% 5% 6% 8% 39%	78841
	Night	14% 6% 3 5% 5% 3 4 5% 5% 7% 43%	80468
Transparency	Day	42% 5% 11% 22% 17% 3	78564
	Night	40% 5% 11% 22% 18% 4	80264
Seeing	Day	17% 8% 22% 24% 20% 10%	13717
	Night	17% 8% 22% 29% 20% 4	23853

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 5% 6% 2 4 5% 5% 6% 36%	2836
	11th to 20th	Day	21% 6% 4 7% 5% 3 4 3 4 6% 37%	2840
	21st to end	Day	24% 10% 3 4 4 2 4 4 4 6% 37%	3284
February	1st to 10th	Day	19% 9% 3 6% 5% 3 4 5% 5% 5% 37%	1900
	11th to 20th	Day	21% 8% 3 6% 5% 3 4 4 4 7% 35%	1848
	21st to end	Day	19% 9% 3 6% 5% 2 3 4 5% 5% 37%	1520
March	1st to 10th	Day	15% 6% 3 4 5% 4 5% 4 5% 7% 43%	1800
	11th to 20th	Day	21% 7% 3 6% 5% 3 4 4 4 6% 37%	1975
	21st to end	Day	15% 7% 3 8% 6% 3 5% 5% 6% 6% 36%	2178
April	1st to 10th	Day	12% 5% 2 5% 6% 3 5% 6% 5% 7% 43%	2135
	11th to 20th	Day	13% 6% 3 6% 4 3 3 4 6% 6% 44%	2134
	21st to end	Day	13% 5% 3 5% 5% 4 5% 5% 5% 8% 43%	2272
May	1st to 10th	Day	11% 5% 3 5% 5% 4 5% 5% 6% 7% 44%	2340
	11th to 20th	Day	11% 7% 3 6% 6% 4 6% 5% 6% 8% 38%	2301
	21st to end	Day	11% 5% 2 4 6% 4 6% 6% 7% 9% 40%	2531
June	1st to 10th	Day	9% 5% 2 5% 6% 4 6% 7% 6% 11% 38%	2340
	11th to 20th	Day	5% 3 2 5% 5% 3 7% 7% 9% 11% 43%	2340
	21st to end	Day	6% 4 2 5% 6% 4 7% 7% 8% 11% 38%	2340
July	1st to 10th	Day	8% 6% 3 6% 7% 4 9% 8% 8% 10% 31%	2340
	11th to 20th	Day	11% 5% 2 5% 7% 4 8% 7% 8% 11% 31%	2340
	21st to end	Day	7% 6% 3 7% 7% 5% 9% 7% 9% 11% 30%	2547
August	1st to 10th	Day	8% 5% 3 6% 7% 6% 8% 8% 8% 11% 30%	2327
	11th to 20th	Day	10% 5% 2 7% 9% 5% 7% 6% 8% 9% 32%	2125
	21st to end	Day	12% 6% 3 6% 8% 4 7% 7% 7% 9% 31%	2376
September	1st to 10th	Day	11% 5% 2 5% 7% 4 6% 5% 7% 8% 39%	2048
	11th to 20th	Day	14% 6% 2 5% 6% 4 4 5% 6% 8% 41%	1980
	21st to end	Day	13% 4 2 4 5% 3 4 4 5% 7% 48%	1980
October	1st to 10th	Day	12% 5% 2 4 3 2 4 4 4 5% 54%	1980
	11th to 20th	Day	11% 5% 2 4 4 3 3 4 5% 6% 54%	1918
	21st to end	Day	14% 4 2 4 4 2 3 4 5% 7% 50%	1887
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 4 5% 56%	1672
	11th to 20th	Day	13% 5% 2 5% 5% 2 4 4 4 7% 49%	1710
	21st to end	Day	16% 7% 2 4 4 3 4 4 5% 7% 44%	1710
December	1st to 10th	Day	22% 8% 3 7% 5% 3 3 4 5% 5% 35%	1710
	11th to 20th	Day	14% 7% 3 7% 6% 4 4 5% 4 6% 40%	2153
	21st to end	Day	19% 9% 3 6% 5% 2 4 5% 5% 9% 33%	3124
January	1st to 10th	Night	18% 9% 3 5% 4 2 4 4 5% 6% 39%	4109
	11th to 20th	Night	18% 8% 4 6% 5% 3 3 4 4 5% 40%	4120
	21st to end	Night	22% 12% 3 5% 5% 3 4 3 3 5% 36%	4368
February	1st to 10th	Night	16% 7% 3 7% 6% 3 4 4 5% 6% 41%	2280
	11th to 20th	Night	21% 7% 4 6% 6% 3 4 4 4 4 38%	2218
	21st to end	Night	23% 8% 3 7% 5% 2 5% 4 5% 5% 35%	1831
March	1st to 10th	Night	14% 7% 2 5% 5% 2 4 5% 5% 8% 44%	2160
	11th to 20th	Night	16% 7% 3 6% 6% 3 5% 4 5% 7% 38%	1985
	21st to end	Night	14% 7% 4 5% 6% 5% 4 5% 4 5% 41%	2178
April	1st to 10th	Night	12% 4 2 4 3 3 4 4 5% 8% 50%	1839
	11th to 20th	Night	12% 5% 2 5% 4 2 4 3 4 9% 50%	1775
	21st to end	Night	12% 5% 2 4 4 2 5% 4 5% 8% 49%	1685
May	1st to 10th	Night	10% 6% 3 4 4 3 5% 5% 5% 7% 48%	1620
	11th to 20th	Night	11% 4 2 5% 6% 4 5% 6% 7% 8% 44%	1592
	21st to end	Night	9% 5% 2 5% 4 3 5% 7% 8% 10% 43%	1744
June	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 6% 7% 10% 42%	1620
	11th to 20th	Night	5% 3 1 3 4 3 4 6% 8% 12% 52%	1620
	21st to end	Night	5% 4 2 5% 5% 3 5% 6% 8% 12% 45%	1620
July	1st to 10th	Night	9% 6% 2 5% 6% 4 6% 7% 9% 10% 37%	1620
	11th to 20th	Night	9% 4 2 4 5% 3 5% 6% 7% 13% 41%	1620
	21st to end	Night	6% 5% 2 6% 6% 3 6% 6% 9% 12% 39%	1771
August	1st to 10th	Night	6% 4 2 5% 6% 4 6% 7% 9% 12% 40%	1611
	11th to 20th	Night	9% 5% 3 5% 7% 4 5% 6% 7% 9% 41%	1771
	21st to end	Night	10% 5% 3 5% 6% 3 6% 6% 7% 10% 40%	1980
September	1st to 10th	Night	9% 6% 2 5% 6% 4 5% 5% 6% 9% 42%	1893
	11th to 20th	Night	13% 5% 2 6% 5% 3 4 5% 5% 8% 44%	1980
	21st to end	Night	10% 6% 2 4 3 2 3 4 5% 6% 53%	1980
October	1st to 10th	Night	12% 3 1 3 4 2 2 3 5% 6% 58%	1980
	11th to 20th	Night	13% 5% 2 5% 5% 2 3 4 5% 6% 51%	2042
	21st to end	Night	15% 5% 2 4 4 2 2 4 4 5% 54%	2466
November	1st to 10th	Night	13% 5% 2 4 3 2 3 4 4 6% 54%	2406
	11th to 20th	Night	12% 6% 2 4 4 2 3 4 4 7% 51%	2470
	21st to end	Night	16% 6% 2 4 4 3 4 4 5% 6% 44%	2470
December	1st to 10th	Night	20% 7% 3 6% 5% 3 5% 5% 4 6% 36%	2470
	11th to 20th	Night	15% 7% 3 5% 4 3 4 4 5% 7% 44%	3042
	21st to end	Night	20% 6% 3 6% 6% 4 5% 6% 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% 18% 6%	1520
March	1st to 10th	Day	42% 3 9% 19% 20% 6%	1800
	11th to 20th	Day	49% 4 10% 18% 15% 4	1975
	21st to end	Day	47% 4 11% 23% 13% 1	2178
April	1st to 10th	Day	38% 5% 12% 23% 18% 4	2135
	11th to 20th	Day	39% 4 9% 20% 23% 6%	2134
	21st to end	Day	39% 5% 10% 28% 17% 2	2272
May	1st to 10th	Day	37% 5% 12% 27% 15% 3	2340
	11th to 20th	Day	43% 5% 15% 22% 13% 1	2305
	21st to end	Day	38% 7% 14% 25% 14% 2	2531
June	1st to 10th	Day	37% 6% 17% 24% 14% 1	2340
	11th to 20th	Day	30% 7% 19% 27% 15% 2	2340
	21st to end	Day	36% 7% 20% 20% 13% 4	2340
July	1st to 10th	Day	43% 9% 16% 19% 10% 2	2275
	11th to 20th	Day	41% 8% 21% 16% 10% 4	2271
	21st to end	Day	41% 9% 18% 22% 9% 1	2405
August	1st to 10th	Day	42% 9% 20% 20% 8% 1	2314
	11th to 20th	Day	43% 7% 16% 20% 13% 1	2125
	21st to end	Day	46% 7% 15% 17% 11% 3	2376
September	1st to 10th	Day	40% 6% 13% 19% 17% 5%	2048
	11th to 20th	Day	41% 6% 11% 21% 20% 2	1980
	21st to end	Day	36% 4 10% 22% 22% 7%	1980
October	1st to 10th	Day	32% 4 9% 22% 25% 8%	1980
	11th to 20th	Day	32% 3 8% 25% 24% 8%	1918
	21st to end	Day	34% 3 7% 23% 28% 5%	1887
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 6% 17% 16% 5%	1831
March	1st to 10th	Night	40% 4 9% 18% 22% 8%	2160
	11th to 20th	Night	45% 5% 10% 21% 16% 3	1985
	21st to end	Night	46% 3 9% 25% 16% 2	2178
April	1st to 10th	Night	33% 3 11% 25% 23% 4	1839
	11th to 20th	Night	33% 3 8% 25% 22% 9%	1775
	21st to end	Night	34% 4 13% 28% 19% 2	1685
May	1st to 10th	Night	35% 5% 13% 24% 22% 3	1620
	11th to 20th	Night	37% 6% 19% 24% 14% 1	1593
	21st to end	Night	34% 8% 15% 24% 16% 2	1745
June	1st to 10th	Night	37% 7% 17% 23% 14% 2	1620
	11th to 20th	Night	22% 7% 23% 31% 15% 2	1620
	21st to end	Night	30% 9% 20% 23% 14% 5%	1620
July	1st to 10th	Night	38% 8% 21% 18% 13% 3	1575
	11th to 20th	Night	34% 9% 21% 19% 11% 6%	1566
	21st to end	Night	34% 7% 22% 24% 10% 2	1672
August	1st to 10th	Night	34% 9% 19% 28% 10% 1	1600
	11th to 20th	Night	37% 8% 20% 19% 14% 2	1771
	21st to end	Night	38% 8% 18% 21% 13% 2	1980
September	1st to 10th	Night	38% 6% 17% 21% 15% 4	1893
	11th to 20th	Night	38% 6% 13% 20% 21% 2	1980
	21st to end	Night	32% 5% 9% 24% 24% 6%	1980
October	1st to 10th	Night	29% 3 8% 24% 27% 9%	1980
	11th to 20th	Night	35% 3 7% 22% 23% 9%	2042
	21st to end	Night	33% 3 4 24% 26% 9%	2466
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% 16% 39% 9% 1	210
March	1st to 10th	Day	23% 17% 38% 17% 5% 1	240
	11th to 20th	Day	25% 18% 28% 22% 6% 1	415
	21st to end	Day	21% 15% 35% 21% 7% 1	462
April	1st to 10th	Day	17% 13% 30% 24% 13% 3	420
	11th to 20th	Day	16% 14% 32% 23% 13% 2	412
	21st to end	Day	17% 10% 31% 25% 13% 4	420
May	1st to 10th	Day	13% 10% 31% 25% 13% 6%	423
	11th to 20th	Day	12% 5% 22% 28% 21% 11%	447
	21st to end	Day	15% 4 16% 25% 24% 16%	485
June	1st to 10th	Day	13% 2 16% 29% 25% 15%	450
	11th to 20th	Day	7% 4 12% 29% 28% 20%	450
	21st to end	Day	9% 2 15% 31% 22% 21%	450
July	1st to 10th	Day	10% 2 9% 22% 30% 27%	445
	11th to 20th	Day	13% 1 6% 24% 34% 23%	444
	21st to end	Day	10% 2 7% 22% 34% 26%	488
August	1st to 10th	Day	13% 3 8% 23% 33% 20%	446
	11th to 20th	Day	13% 3 7% 30% 35% 12%	439
	21st to end	Day	15% 2 9% 21% 34% 20%	495
September	1st to 10th	Day	11% 2 6% 24% 36% 20%	450
	11th to 20th	Day	16% 2 10% 24% 31% 17%	450
	21st to end	Day	16% 3 14% 26% 32% 9%	450
October	1st to 10th	Day	16% 3 17% 25% 28% 12%	450
	11th to 20th	Day	12% 8% 28% 29% 20% 3	450
	21st to end	Day	19% 9% 21% 31% 15% 5%	402
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	611
March	1st to 10th	Night	18% 13% 34% 29% 5%	720
	11th to 20th	Night	20% 12% 32% 26% 8% 1	720
	21st to end	Night	19% 12% 32% 29% 9%	792
April	1st to 10th	Night	15% 10% 31% 32% 12%	579
	11th to 20th	Night	16% 9% 31% 30% 12% 1	533
	21st to end	Night	14% 6% 27% 39% 13% 1	540
May	1st to 10th	Night	11% 5% 26% 37% 20% 2	540
	11th to 20th	Night	14% 4 18% 33% 26% 5%	531
	21st to end	Night	12% 2 10% 34% 36% 6%	584
June	1st to 10th	Night	9% 2 13% 35% 34% 7%	540
	11th to 20th	Night	5% 2 11% 35% 35% 13%	540
	21st to end	Night	8% 1 8% 34% 39% 11%	540
July	1st to 10th	Night	11% 2 7% 25% 42% 13%	525
	11th to 20th	Night	11% 1 5% 27% 40% 16%	522
	21st to end	Night	10% 1 7% 24% 45% 13%	591
August	1st to 10th	Night	8% 1 5% 28% 47% 12%	537
	11th to 20th	Night	12% 1 6% 32% 42% 7%	534
	21st to end	Night	12% 1 6% 26% 43% 11%	594
September	1st to 10th	Night	12% 1 7% 31% 40% 9%	633
	11th to 20th	Night	16% 2 11% 27% 37% 7%	720
	21st to end	Night	14% 3 15% 32% 33% 4	720
October	1st to 10th	Night	15% 4 16% 30% 28% 8%	720
	11th to 20th	Night	15% 7% 24% 32% 20% 1	720
	21st to end	Night	17% 7% 21% 38% 16% 2	792
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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