WWIN Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WWIN Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	12% 6% 2 6% 4 4 4 4 5% 7% 46%	131516
	Night	13% 6% 2 5% 4 3 3 3 5% 6% 49%	131846
Transparency	Day	37% 5% 10% 24% 19% 5%	135123
	Night	34% 4 9% 23% 22% 7%	136317
Seeing	Day	14% 7% 22% 25% 22% 10%	17418
	Night	15% 5% 25% 33% 19% 2	27261

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% 8% 3 6% 4 3 3 3 4 5% 47%	5289
	11th to 20th	Day	10% 7% 2 6% 4 3 3 3 3 5% 54%	5310
	21st to end	Day	21% 6% 2 6% 3 2 2 2 3 4 49%	5812
February	1st to 10th	Day	22% 8% 2 6% 3 3 2 3 4 6% 41%	1890
	11th to 20th	Day	17% 9% 3 6% 4 4 3 3 4 5% 42%	2022
	21st to end	Day	21% 7% 3 7% 4 3 3 3 4 6% 40%	1735
March	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 4 5% 37%	5046
	11th to 20th	Day	10% 5% 2 6% 4 3 4 3 5% 6% 52%	5090
	21st to end	Day	13% 8% 3 7% 4 4 3 3 4 5% 45%	5662
April	1st to 10th	Day	14% 6% 2 6% 3 4 4 4 6% 6% 46%	2520
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 45%	2494
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 3 6% 7% 39%	2520
May	1st to 10th	Day	17% 7% 3 7% 4 5% 5% 4 5% 7% 36%	5525
	11th to 20th	Day	13% 7% 3 8% 5% 4 5% 4 6% 7% 38%	5804
	21st to end	Day	14% 7% 3 8% 5% 6% 5% 4 7% 8% 34%	6753
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 44%	2801
	11th to 20th	Day	5% 5% 2 5% 5% 5% 5% 5% 7% 9% 47%	2800
	21st to end	Day	4 4 2 4 5% 4 4 6% 7% 10% 50%	2753
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 6% 5% 8% 11% 38%	5900
	11th to 20th	Day	5% 3 2 4 4 4 5% 5% 7% 11% 52%	5900
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 6% 9% 42%	6422
August	1st to 10th	Day	3 3 1 4 3 3 4 4 6% 9% 61%	4186
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 42%	3849
	21st to end	Day	5% 3 2 5% 5% 5% 5% 4 7% 8% 51%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 4 3 5% 7% 47%	2400
	11th to 20th	Day	14% 5% 2 4 4 4 4 4 5% 8% 47%	2400
	21st to end	Day	10% 4 1 5% 4 3 3 3 4 7% 54%	2115
October	1st to 10th	Day	8% 3 2 4 3 3 4 3 5% 6% 61%	2000
	11th to 20th	Day	13% 5% 1 4 2 3 3 2 3 5% 59%	2000
	21st to end	Day	19% 5% 2 5% 3 2 3 2 4 5% 52%	2200
November	1st to 10th	Day	14% 5% 2 4 3 3 2 2 4 5% 57%	2058
	11th to 20th	Day	11% 4 2 5% 3 3 3 3 4 7% 55%	1957
	21st to end	Day	19% 7% 2 5% 3 3 4 3 3 6% 45%	1890
December	1st to 10th	Day	17% 7% 3 5% 4 3 4 3 3 4 47%	2690
	11th to 20th	Day	19% 8% 3 6% 4 3 3 3 4 4 44%	3088
	21st to end	Day	19% 8% 3 7% 4 3 3 4 4 5% 41%	4323
January	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 6% 47%	7634
	11th to 20th	Night	11% 4 1 4 3 2 3 2 3 4 63%	7680
	21st to end	Night	23% 7% 2 6% 3 3 3 2 3 4 44%	8402
February	1st to 10th	Night	21% 7% 3 5% 3 3 3 3 4 5% 44%	2730
	11th to 20th	Night	20% 7% 2 6% 4 3 3 3 4 5% 45%	2482
	21st to end	Night	19% 6% 2 4 4 4 3 2 4 5% 48%	2076
March	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 4 5% 44%	5040
	11th to 20th	Night	9% 5% 3 6% 4 3 4 3 4 7% 52%	5090
	21st to end	Night	12% 6% 2 7% 5% 3 3 3 5% 7% 45%	5507
April	1st to 10th	Night	13% 5% 2 6% 3 3 3 3 5% 7% 49%	2100
	11th to 20th	Night	13% 4 1 4 4 3 4 4 5% 8% 51%	2075
	21st to end	Night	13% 5% 2 6% 4 3 4 4 5% 8% 46%	2100
May	1st to 10th	Night	18% 8% 3 7% 4 4 4 4 5% 6% 36%	4590
	11th to 20th	Night	18% 8% 3 9% 4 4 4 3 5% 6% 36%	4306
	21st to end	Night	16% 7% 3 7% 5% 5% 4 4 5% 6% 38%	4355
June	1st to 10th	Night	5% 4 2 5% 4 4 4 5% 8% 10% 52%	1600
	11th to 20th	Night	3 3 1 5% 4 4 4 3 6% 8% 58%	1600
	21st to end	Night	2 3 2 3 4 3 5% 4 6% 8% 61%	1637
July	1st to 10th	Night	6% 4 2 6% 5% 5% 6% 5% 6% 11% 44%	4078
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 12% 55%	4077
	21st to end	Night	8% 5% 2 5% 4 4 4 4 6% 9% 48%	4449
August	1st to 10th	Night	2 1 1 3 2 2 3 3 5% 8% 70%	2957
	11th to 20th	Night	3 2 1 5% 3 4 4 5% 6% 11% 55%	3200
	21st to end	Night	4 3 2 4 4 4 5% 4 6% 9% 56%	3586
September	1st to 10th	Night	7% 4 2 4 4 4 3 4 5% 8% 55%	2000
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 6% 53%	2000
	21st to end	Night	8% 5% 1 4 3 3 3 3 5% 6% 58%	2259
October	1st to 10th	Night	8% 4 2 4 2 3 4 3 4 5% 63%	2400
	11th to 20th	Night	10% 4 2 4 3 3 3 2 4 5% 61%	2400
	21st to end	Night	16% 5% 1 4 3 3 3 2 3 3 54%	2640
November	1st to 10th	Night	12% 4 2 5% 3 3 3 2 5% 6% 56%	2460
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 5% 6% 57%	2659
	21st to end	Night	15% 6% 2 5% 4 4 3 3 4 5% 51%	2730
December	1st to 10th	Night	16% 6% 2 5% 3 4 3 3 3 5% 51%	4717
	11th to 20th	Night	17% 9% 3 6% 4 3 3 2 3 4 45%	5339
	21st to end	Night	19% 7% 2 6% 4 4 4 3 4 4 42%	6891

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	41% 2 10% 29% 16% 2	5290
	11th to 20th	Day	34% 3 6% 21% 30% 6%	5310
	21st to end	Day	42% 2 5% 18% 26% 7%	5812
February	1st to 10th	Day	46% 4 9% 22% 16% 3	1890
	11th to 20th	Day	45% 3 8% 21% 17% 5%	2032
	21st to end	Day	47% 4 6% 17% 18% 8%	1735
March	1st to 10th	Day	50% 4 6% 15% 14% 11%	5046
	11th to 20th	Day	33% 4 7% 20% 24% 12%	5090
	21st to end	Day	42% 4 8% 25% 20% 2	5664
April	1st to 10th	Day	38% 6% 9% 23% 20% 4	2520
	11th to 20th	Day	39% 4 10% 21% 18% 7%	2494
	21st to end	Day	43% 6% 11% 21% 17% 2	2520
May	1st to 10th	Day	46% 7% 10% 23% 14% 1	5560
	11th to 20th	Day	43% 7% 13% 23% 12% 1	5808
	21st to end	Day	46% 7% 12% 19% 14% 2	6767
June	1st to 10th	Day	35% 7% 12% 26% 17% 2	2801
	11th to 20th	Day	31% 7% 13% 28% 19% 3	2800
	21st to end	Day	26% 7% 14% 28% 20% 5%	2753
July	1st to 10th	Day	35% 8% 16% 26% 13% 1	5836
	11th to 20th	Day	25% 7% 17% 35% 15% 2	5831
	21st to end	Day	37% 6% 15% 29% 12%	6240
August	1st to 10th	Day	19% 6% 16% 32% 24% 4	4172
	11th to 20th	Day	35% 7% 15% 31% 10% 1	3885
	21st to end	Day	29% 7% 12% 28% 21% 3	4311
September	1st to 10th	Day	35% 7% 10% 21% 19% 7%	2400
	11th to 20th	Day	35% 6% 11% 20% 23% 6%	2400
	21st to end	Day	31% 4 10% 20% 27% 8%	2115
October	1st to 10th	Day	15% 2 6% 15% 35% 27%	3260
	11th to 20th	Day	18% 2 4 20% 38% 18%	3240
	21st to end	Day	30% 4 4 28% 28% 7%	3538
November	1st to 10th	Day	31% 3 7% 26% 25% 8%	2058
	11th to 20th	Day	31% 4 9% 22% 26% 10%	1957
	21st to end	Day	42% 3 7% 20% 23% 5%	1890
December	1st to 10th	Day	42% 4 5% 25% 20% 3	2690
	11th to 20th	Day	44% 3 6% 23% 19% 4	3085
	21st to end	Day	45% 4 8% 25% 14% 3	4323
January	1st to 10th	Night	40% 2 8% 29% 19% 2	7635
	11th to 20th	Night	27% 3 6% 19% 35% 10%	7680
	21st to end	Night	46% 3 7% 17% 21% 6%	8402
February	1st to 10th	Night	44% 4 7% 21% 19% 5%	2730
	11th to 20th	Night	43% 3 7% 21% 19% 6%	2483
	21st to end	Night	40% 4 8% 17% 23% 9%	2076
March	1st to 10th	Night	42% 4 7% 14% 20% 13%	5040
	11th to 20th	Night	33% 4 7% 24% 18% 14%	5090
	21st to end	Night	38% 4 9% 25% 21% 3	5507
April	1st to 10th	Night	35% 4 10% 23% 20% 8%	2100
	11th to 20th	Night	31% 5% 9% 24% 21% 10%	2075
	21st to end	Night	36% 6% 10% 22% 22% 3	2100
May	1st to 10th	Night	47% 6% 12% 21% 12% 2	4620
	11th to 20th	Night	49% 6% 11% 19% 13% 1	4307
	21st to end	Night	45% 7% 12% 19% 13% 4	4363
June	1st to 10th	Night	26% 7% 14% 29% 22% 2	1600
	11th to 20th	Night	23% 6% 14% 29% 25% 4	1600
	21st to end	Night	21% 5% 13% 30% 24% 7%	1637
July	1st to 10th	Night	33% 7% 16% 25% 18% 1	4035
	11th to 20th	Night	21% 6% 18% 36% 16% 3	4026
	21st to end	Night	32% 6% 15% 30% 17%	4320
August	1st to 10th	Night	13% 5% 16% 35% 26% 5%	2947
	11th to 20th	Night	22% 6% 19% 36% 15% 2	3230
	21st to end	Night	24% 6% 11% 27% 28% 4	3586
September	1st to 10th	Night	27% 7% 11% 24% 22% 9%	2000
	11th to 20th	Night	31% 6% 9% 21% 25% 7%	2000
	21st to end	Night	27% 4 10% 20% 29% 9%	2259
October	1st to 10th	Night	15% 3 6% 15% 30% 31%	3920
	11th to 20th	Night	17% 2 4 15% 38% 24%	3899
	21st to end	Night	28% 4 5% 21% 31% 13%	4252
November	1st to 10th	Night	31% 3 7% 22% 27% 10%	2460
	11th to 20th	Night	29% 3 6% 25% 26% 11%	2659
	21st to end	Night	37% 3 6% 20% 28% 6%	2730
December	1st to 10th	Night	37% 3 6% 31% 19% 4	4717
	11th to 20th	Night	45% 2 5% 21% 21% 6%	5341
	21st to end	Night	45% 3 9% 22% 17% 3	6891

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	24% 12% 34% 21% 7% 2	330
	11th to 20th	Day	22% 11% 43% 21% 2 1	330
	21st to end	Day	19% 12% 45% 18% 5%	363
February	1st to 10th	Day	28% 10% 37% 18% 7% 1	330
	11th to 20th	Day	26% 20% 36% 14% 4	330
	21st to end	Day	29% 17% 33% 14% 7%	273
March	1st to 10th	Day	25% 15% 32% 20% 6% 1	330
	11th to 20th	Day	22% 19% 35% 22% 2	330
	21st to end	Day	21% 15% 39% 18% 7% 1	391
April	1st to 10th	Day	18% 13% 33% 24% 11% 1	540
	11th to 20th	Day	22% 13% 31% 23% 10% 1	532
	21st to end	Day	18% 9% 32% 26% 12% 2	540
May	1st to 10th	Day	17% 10% 29% 30% 12% 1	543
	11th to 20th	Day	14% 6% 25% 27% 21% 7%	691
	21st to end	Day	10% 4 20% 33% 24% 9%	844
June	1st to 10th	Day	10% 4 14% 27% 31% 15%	731
	11th to 20th	Day	5% 3 13% 29% 37% 13%	730
	21st to end	Day	5% 2 10% 36% 30% 17%	730
July	1st to 10th	Day	5% 2 8% 28% 38% 20%	721
	11th to 20th	Day	5% 2 6% 29% 36% 22%	718
	21st to end	Day	7% 2 7% 25% 38% 21%	794
August	1st to 10th	Day	5% 2 8% 32% 37% 16%	704
	11th to 20th	Day	11% 3 6% 25% 34% 20%	519
	21st to end	Day	10% 2 6% 21% 36% 25%	583
September	1st to 10th	Day	9% 2 6% 27% 37% 18%	530
	11th to 20th	Day	12% 2 10% 19% 36% 22%	530
	21st to end	Day	12% 5% 15% 28% 24% 16%	395
October	1st to 10th	Day	9% 2 21% 28% 30% 10%	330
	11th to 20th	Day	11% 8% 30% 25% 20% 7%	330
	21st to end	Day	24% 9% 28% 23% 12% 4	363
November	1st to 10th	Day	17% 8% 31% 26% 15% 3	330
	11th to 20th	Day	10% 7% 38% 32% 12% 2	330
	21st to end	Day	24% 13% 30% 21% 10% 3	330
December	1st to 10th	Day	19% 14% 35% 20% 10% 2	330
	11th to 20th	Day	25% 9% 36% 19% 10% 1	330
	21st to end	Day	25% 14% 35% 22% 3 1	363
January	1st to 10th	Night	22% 8% 33% 31% 6%	1050
	11th to 20th	Night	20% 8% 40% 27% 4	1050
	21st to end	Night	21% 7% 39% 29% 3	1155
February	1st to 10th	Night	24% 8% 34% 29% 6%	1050
	11th to 20th	Night	24% 11% 37% 25% 3	847
	21st to end	Night	22% 14% 36% 26% 2	692
March	1st to 10th	Night	20% 7% 35% 33% 5%	641
	11th to 20th	Night	18% 10% 37% 29% 5% 1	630
	21st to end	Night	18% 7% 38% 30% 8%	693
April	1st to 10th	Night	17% 8% 35% 31% 9%	630
	11th to 20th	Night	15% 7% 35% 31% 11%	623
	21st to end	Night	16% 6% 34% 31% 11% 1	630
May	1st to 10th	Night	15% 6% 22% 45% 12%	630
	11th to 20th	Night	15% 4 26% 35% 17% 2	621
	21st to end	Night	10% 3 19% 37% 26% 5%	683
June	1st to 10th	Night	5% 2 11% 39% 40% 4	600
	11th to 20th	Night	3 2 8% 43% 40% 5%	600
	21st to end	Night	3 1 8% 40% 43% 5%	600
July	1st to 10th	Night	5% 2 6% 34% 46% 9%	585
	11th to 20th	Night	4 1 5% 29% 51% 9%	582
	21st to end	Night	6% 6% 33% 44% 10%	657
August	1st to 10th	Night	4 4 37% 47% 8%	597
	11th to 20th	Night	6% 1 5% 35% 45% 8%	594
	21st to end	Night	7% 1 6% 29% 48% 8%	660
September	1st to 10th	Night	8% 1 8% 33% 45% 5%	600
	11th to 20th	Night	11% 2 8% 32% 43% 5%	600
	21st to end	Night	10% 2 12% 38% 34% 4	729
October	1st to 10th	Night	8% 2 15% 40% 30% 5%	800
	11th to 20th	Night	12% 3 24% 40% 20% 1	800
	21st to end	Night	18% 5% 26% 33% 17% 1	880
November	1st to 10th	Night	13% 5% 25% 37% 17% 2	820
	11th to 20th	Night	13% 5% 32% 38% 12%	840
	21st to end	Night	18% 7% 31% 31% 13%	840
December	1st to 10th	Night	20% 7% 31% 36% 7%	1047
	11th to 20th	Night	22% 7% 31% 33% 7%	1050
	21st to end	Night	22% 9% 32% 32% 5%	1155

Page generated at 2026-05-29 00:53:26. Version ac07

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