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 2026-08-30.

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 7% 5% 5% 5% 4 6% 8% 39%	143441
	Night	14% 7% 2 6% 4 4 4 4 6% 8% 42%	138788
Transparency	Day	40% 7% 12% 22% 16% 3	147177
	Night	39% 6% 12% 22% 17% 4	143110
Seeing	Day	16% 9% 23% 24% 19% 9%	17099
	Night	17% 8% 22% 30% 20% 4	27058

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	18% 9% 3 7% 4 3 3 3 4 5% 41%	5258
	11th to 20th	Day	13% 6% 4 7% 4 3 2 2 3 5% 51%	5280
	21st to end	Day	21% 8% 3 6% 4 3 3 3 4 6% 38%	6079
February	1st to 10th	Day	17% 9% 3 7% 4 4 4 4 5% 5% 38%	2100
	11th to 20th	Day	21% 9% 2 7% 4 4 4 3 4 7% 35%	2048
	21st to end	Day	18% 9% 2 7% 4 4 3 3 5% 5% 39%	1730
March	1st to 10th	Day	18% 10% 3 5% 5% 5% 5% 4 5% 6% 34%	4580
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 4 5% 48%	5085
	21st to end	Day	14% 7% 2 7% 4 4 4 3 5% 6% 44%	5586
April	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 5% 6% 7% 41%	2492
	11th to 20th	Day	11% 6% 3 7% 4 3 3 4 6% 6% 46%	2494
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 5% 8% 41%	2652
May	1st to 10th	Day	16% 8% 3 6% 4 5% 5% 5% 7% 9% 33%	7616
	11th to 20th	Day	13% 8% 3 8% 6% 5% 5% 4 6% 8% 33%	7626
	21st to end	Day	20% 10% 4 8% 5% 4 5% 4 5% 6% 30%	8391
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	2730
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	2730
	21st to end	Day	6% 4 2 6% 5% 5% 7% 6% 9% 11% 39%	2730
July	1st to 10th	Day	5% 5% 2 6% 6% 6% 7% 6% 9% 12% 36%	7680
	11th to 20th	Day	8% 6% 2 8% 6% 7% 8% 7% 10% 12% 26%	7680
	21st to end	Day	11% 8% 3 8% 6% 7% 7% 5% 8% 11% 26%	8340
August	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 6% 7% 12% 40%	4364
	11th to 20th	Day	6% 4 2 6% 5% 5% 7% 5% 9% 11% 41%	3969
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 6% 7% 49%	4420
September	1st to 10th	Day	10% 5% 2 7% 6% 6% 5% 4 7% 8% 40%	2275
	11th to 20th	Day	14% 6% 2 6% 5% 5% 4 4 6% 8% 41%	2200
	21st to end	Day	14% 5% 1 5% 5% 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	11% 5% 2 4 3 3 4 3 5% 6% 53%	2200
	11th to 20th	Day	10% 5% 2 5% 3 4 3 3 4 6% 56%	2130
	21st to end	Day	15% 5% 2 4 3 3 3 3 5% 7% 50%	2096
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 2 4 5% 54%	1852
	11th to 20th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 48%	1890
	21st to end	Day	16% 7% 1 5% 4 4 4 4 5% 7% 44%	1890
December	1st to 10th	Day	16% 8% 2 6% 4 3 3 3 5% 5% 45%	3020
	11th to 20th	Day	17% 11% 3 8% 5% 5% 4 3 4 5% 35%	3463
	21st to end	Day	19% 7% 3 6% 4 3 4 4 6% 8% 35%	4565
January	1st to 10th	Night	17% 9% 2 6% 4 3 4 3 4 5% 43%	7635
	11th to 20th	Night	14% 7% 3 6% 4 3 3 2 4 5% 48%	7680
	21st to end	Night	18% 10% 3 6% 4 4 4 3 4 6% 40%	8084
February	1st to 10th	Night	15% 7% 3 7% 6% 4 4 3 5% 7% 40%	2520
	11th to 20th	Night	21% 8% 4 7% 5% 4 4 3 4 4 37%	2458
	21st to end	Night	21% 8% 2 7% 4 3 4 3 5% 5% 36%	2076
March	1st to 10th	Night	21% 9% 2 6% 5% 4 4 4 5% 7% 33%	5493
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 49%	5095
	21st to end	Night	15% 6% 3 7% 5% 5% 5% 3 5% 4 42%	5588
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 6% 8% 48%	2145
	11th to 20th	Night	11% 6% 1 5% 3 3 4 3 5% 8% 51%	2075
	21st to end	Night	12% 6% 2 4 4 3 5% 3 5% 7% 48%	1965
May	1st to 10th	Night	13% 8% 3 7% 5% 5% 6% 4 7% 9% 34%	5237
	11th to 20th	Night	15% 7% 3 7% 5% 5% 4 4 6% 8% 36%	5234
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 6% 8% 31%	5761
June	1st to 10th	Night	8% 5% 2 6% 5% 6% 5% 5% 7% 10% 40%	1890
	11th to 20th	Night	5% 3 1 4 4 4 4 5% 8% 12% 50%	1890
	21st to end	Night	5% 4 2 5% 5% 5% 5% 4 8% 12% 45%	1890
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 9% 13% 38%	5278
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 6% 9% 14% 33%	5277
	21st to end	Night	10% 8% 3 8% 6% 5% 6% 5% 8% 12% 30%	5736
August	1st to 10th	Night	5% 3 2 6% 5% 5% 5% 5% 8% 11% 46%	3005
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 7% 10% 50%	3295
	21st to end	Night	6% 3 2 5% 4 4 4 4 6% 9% 54%	3676
September	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 42%	2104
	11th to 20th	Night	13% 5% 2 7% 4 4 4 4 6% 8% 43%	2200
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 6% 51%	2200
October	1st to 10th	Night	11% 3 1 4 3 2 3 3 5% 7% 57%	2200
	11th to 20th	Night	11% 4 1 5% 4 3 3 3 4 6% 55%	2270
	21st to end	Night	15% 6% 2 4 4 2 2 3 4 5% 54%	2740
November	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 6% 52%	2666
	11th to 20th	Night	12% 6% 2 4 4 3 4 3 5% 6% 51%	2730
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 6% 44%	2730
December	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 5% 43%	4380
	11th to 20th	Night	20% 8% 3 6% 3 3 4 3 4 6% 40%	4952
	21st to end	Night	21% 6% 2 6% 5% 5% 5% 4 6% 7% 34%	6633

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	45% 5% 8% 25% 16% 1	5259
	11th to 20th	Day	37% 5% 5% 20% 29% 5%	5280
	21st to end	Day	48% 4 7% 22% 15% 4	6079
February	1st to 10th	Day	47% 4 8% 22% 17% 3	2100
	11th to 20th	Day	50% 4 6% 19% 17% 3	2058
	21st to end	Day	47% 2 9% 16% 20% 6%	1730
March	1st to 10th	Day	50% 4 8% 14% 17% 8%	4580
	11th to 20th	Day	36% 4 6% 20% 22% 11%	5085
	21st to end	Day	42% 3 8% 30% 15% 2	5586
April	1st to 10th	Day	38% 6% 12% 24% 18% 3	2492
	11th to 20th	Day	37% 4 9% 21% 24% 6%	2494
	21st to end	Day	39% 5% 11% 28% 16% 2	2652
May	1st to 10th	Day	45% 7% 13% 21% 12% 1	7651
	11th to 20th	Day	46% 8% 15% 21% 10% 1	7630
	21st to end	Day	54% 6% 8% 17% 13% 2	8405
June	1st to 10th	Day	37% 6% 17% 24% 14% 2	2730
	11th to 20th	Day	31% 8% 18% 26% 15% 2	2730
	21st to end	Day	34% 8% 20% 21% 13% 3	2730
July	1st to 10th	Day	34% 13% 23% 21% 9% 1	7615
	11th to 20th	Day	39% 16% 25% 15% 3 1	7611
	21st to end	Day	47% 11% 19% 17% 6%	8156
August	1st to 10th	Day	31% 9% 16% 33% 10% 1	4351
	11th to 20th	Day	31% 9% 18% 27% 14% 1	4005
	21st to end	Day	31% 6% 12% 31% 17% 2	4419
September	1st to 10th	Day	39% 6% 13% 19% 17% 6%	2275
	11th to 20th	Day	40% 6% 11% 22% 19% 2	2200
	21st to end	Day	37% 4 11% 21% 20% 6%	2200
October	1st to 10th	Day	22% 5% 7% 21% 34% 12%	3590
	11th to 20th	Day	21% 2 5% 25% 33% 13%	3448
	21st to end	Day	29% 5% 8% 25% 27% 8%	3359
November	1st to 10th	Day	34% 4 7% 21% 27% 6%	1852
	11th to 20th	Day	36% 3 5% 25% 25% 5%	1890
	21st to end	Day	40% 4 7% 25% 20% 4	1890
December	1st to 10th	Day	42% 4 6% 25% 21% 3	3020
	11th to 20th	Day	52% 5% 6% 19% 13% 4	3460
	21st to end	Day	46% 5% 12% 26% 10% 2	4565
January	1st to 10th	Night	44% 4 12% 24% 15% 1	7636
	11th to 20th	Night	39% 5% 5% 20% 28% 4	7680
	21st to end	Night	47% 5% 7% 19% 16% 6%	8084
February	1st to 10th	Night	45% 3 9% 22% 18% 3	2520
	11th to 20th	Night	52% 3 7% 18% 15% 5%	2459
	21st to end	Night	50% 4 6% 18% 18% 5%	2076
March	1st to 10th	Night	50% 4 7% 16% 18% 4	5493
	11th to 20th	Night	34% 4 6% 21% 22% 13%	5095
	21st to end	Night	45% 3 7% 26% 17% 3	5588
April	1st to 10th	Night	34% 4 12% 24% 22% 4	2145
	11th to 20th	Night	32% 3 8% 26% 23% 8%	2075
	21st to end	Night	35% 5% 13% 27% 18% 2	1965
May	1st to 10th	Night	45% 7% 13% 21% 12% 1	5264
	11th to 20th	Night	46% 6% 15% 19% 13% 1	5235
	21st to end	Night	50% 8% 12% 19% 11% 1	5771
June	1st to 10th	Night	37% 7% 17% 22% 15% 3	1890
	11th to 20th	Night	24% 7% 21% 29% 16% 2	1890
	21st to end	Night	30% 9% 19% 24% 14% 4	1890
July	1st to 10th	Night	33% 13% 24% 21% 7% 1	5235
	11th to 20th	Night	38% 14% 27% 16% 4 2	5226
	21st to end	Night	44% 11% 23% 14% 7% 1	5606
August	1st to 10th	Night	29% 9% 15% 33% 13% 1	2994
	11th to 20th	Night	28% 8% 22% 27% 14% 1	3325
	21st to end	Night	26% 7% 15% 32% 19% 1	3676
September	1st to 10th	Night	39% 6% 16% 20% 15% 5%	2104
	11th to 20th	Night	38% 7% 13% 21% 21% 1	2200
	21st to end	Night	34% 5% 10% 23% 22% 6%	2200
October	1st to 10th	Night	19% 4 7% 24% 30% 15%	3590
	11th to 20th	Night	20% 2 5% 22% 34% 16%	3691
	21st to end	Night	25% 4 5% 28% 29% 10%	4414
November	1st to 10th	Night	33% 4 7% 23% 24% 8%	2666
	11th to 20th	Night	35% 2 6% 23% 26% 8%	2730
	21st to end	Night	42% 3 6% 24% 21% 4	2730
December	1st to 10th	Night	44% 4 7% 21% 22% 2	4380
	11th to 20th	Night	46% 4 8% 21% 19% 2	4954
	21st to end	Night	47% 6% 10% 24% 13% 1	6633

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% 28% 19% 3 1	330
	11th to 20th	Day	25% 21% 34% 14% 5% 1	330
	21st to end	Day	22% 17% 45% 12% 3	363
February	1st to 10th	Day	22% 19% 36% 17% 5% 1	330
	11th to 20th	Day	36% 18% 33% 11% 2 1	330
	21st to end	Day	30% 17% 38% 12% 3	273
March	1st to 10th	Day	23% 18% 38% 15% 5% 1	330
	11th to 20th	Day	23% 18% 29% 22% 6% 1	535
	21st to end	Day	19% 15% 38% 21% 6% 1	594
April	1st to 10th	Day	18% 14% 32% 23% 11% 2	540
	11th to 20th	Day	14% 15% 32% 23% 13% 2	532
	21st to end	Day	16% 11% 32% 25% 13% 4	540
May	1st to 10th	Day	16% 10% 32% 24% 12% 5%	543
	11th to 20th	Day	12% 7% 25% 27% 20% 9%	567
	21st to end	Day	17% 6% 19% 24% 21% 13%	617
June	1st to 10th	Day	12% 3 17% 29% 25% 13%	570
	11th to 20th	Day	6% 4 15% 29% 29% 16%	570
	21st to end	Day	8% 2 14% 29% 25% 22%	570
July	1st to 10th	Day	8% 2 9% 23% 31% 26%	565
	11th to 20th	Day	12% 2 7% 23% 33% 24%	564
	21st to end	Day	11% 3 9% 23% 31% 23%	620
August	1st to 10th	Day	12% 4 9% 23% 33% 19%	566
	11th to 20th	Day	11% 3 8% 30% 34% 14%	559
	21st to end	Day	13% 2 9% 23% 34% 20%	619
September	1st to 10th	Day	10% 3 8% 26% 36% 17%	530
	11th to 20th	Day	16% 3 9% 24% 32% 16%	530
	21st to end	Day	18% 4 14% 26% 30% 8%	530
October	1st to 10th	Day	14% 3 18% 25% 28% 11%	530
	11th to 20th	Day	11% 7% 27% 31% 21% 3	530
	21st to end	Day	19% 9% 22% 30% 16% 4	479
November	1st to 10th	Day	19% 10% 25% 28% 13% 4	330
	11th to 20th	Day	14% 12% 37% 29% 8% 1	330
	21st to end	Day	15% 12% 39% 23% 10% 1	330
December	1st to 10th	Day	28% 14% 33% 19% 5% 1	330
	11th to 20th	Day	19% 15% 37% 22% 6% 1	330
	21st to end	Day	28% 15% 30% 20% 6% 1	363
January	1st to 10th	Night	25% 15% 34% 21% 5%	840
	11th to 20th	Night	21% 17% 31% 25% 6%	840
	21st to end	Night	21% 14% 34% 29% 2	924
February	1st to 10th	Night	19% 15% 33% 28% 5%	840
	11th to 20th	Night	26% 20% 31% 21% 2	823
	21st to end	Night	28% 16% 32% 20% 3	692
March	1st to 10th	Night	20% 13% 33% 29% 6%	840
	11th to 20th	Night	20% 12% 32% 28% 7% 1	840
	21st to end	Night	18% 12% 34% 29% 7%	924
April	1st to 10th	Night	16% 10% 31% 32% 11%	675
	11th to 20th	Night	16% 9% 30% 33% 12% 1	623
	21st to end	Night	15% 6% 26% 38% 13% 1	630
May	1st to 10th	Night	12% 5% 26% 36% 19% 2	630
	11th to 20th	Night	15% 5% 18% 35% 24% 4	621
	21st to end	Night	15% 3 12% 33% 32% 6%	683
June	1st to 10th	Night	11% 2 13% 36% 32% 6%	630
	11th to 20th	Night	6% 2 11% 36% 34% 11%	630
	21st to end	Night	7% 2 8% 32% 40% 12%	630
July	1st to 10th	Night	10% 3 6% 25% 43% 13%	615
	11th to 20th	Night	11% 1 6% 26% 41% 16%	612
	21st to end	Night	10% 2 7% 26% 42% 13%	690
August	1st to 10th	Night	9% 1 5% 28% 46% 11%	627
	11th to 20th	Night	11% 2 7% 31% 42% 7%	624
	21st to end	Night	11% 2 7% 28% 41% 11%	687
September	1st to 10th	Night	12% 2 9% 31% 38% 9%	704
	11th to 20th	Night	16% 2 10% 27% 38% 7%	800
	21st to end	Night	15% 3 15% 32% 33% 3	800
October	1st to 10th	Night	14% 4 16% 31% 28% 7%	800
	11th to 20th	Night	14% 8% 23% 34% 21% 1	800
	21st to end	Night	17% 7% 21% 38% 16% 2	880
November	1st to 10th	Night	16% 8% 21% 34% 19% 2	820
	11th to 20th	Night	16% 9% 32% 32% 10% 1	840
	21st to end	Night	18% 12% 29% 29% 11% 1	840
December	1st to 10th	Night	25% 14% 27% 27% 8%	840
	11th to 20th	Night	20% 10% 31% 30% 8%	840
	21st to end	Night	25% 13% 27% 29% 7%	924

Page generated at 2026-08-27 04:07:12. Version ac07

Copyright © 2026 Attilla Danko.