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 2024-01-27.

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% 5% 2 5% 5% 3 5% 5% 5% 7% 45%	74765
	Night	12% 5% 2 4 4 3 4 4 5% 7% 50%	75282
Transparency	Day	37% 6% 11% 23% 19% 5%	74489
	Night	34% 5% 10% 23% 22% 6%	75079
Seeing	Day	14% 6% 20% 25% 23% 11%	14241
	Night	15% 5% 24% 33% 20% 3	24185

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	19% 8% 2 4 4 2 2 4 3 6% 46%	1707
	11th to 20th	Day	18% 8% 3 5% 5% 3 4 4 4 5% 42%	1710
	21st to end	Day	18% 5% 3 4 4 2 3 3 4 6% 49%	1836
February	1st to 10th	Day	21% 7% 3 5% 4 2 3 3 4 6% 42%	1620
	11th to 20th	Day	17% 8% 3 6% 5% 3 4 4 3 5% 42%	1726
	21st to end	Day	24% 8% 3 6% 5% 2 3 3 3 6% 36%	1484
March	1st to 10th	Day	18% 7% 2 5% 5% 2 5% 3 4 6% 44%	1980
	11th to 20th	Day	18% 7% 4 6% 5% 3 4 3 4 5% 41%	1980
	21st to end	Day	17% 7% 3 6% 5% 3 4 4 4 6% 41%	2201
April	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 6% 46%	2160
	11th to 20th	Day	15% 5% 3 5% 5% 2 4 4 4 7% 46%	2134
	21st to end	Day	16% 5% 2 6% 5% 3 5% 4 5% 6% 42%	2160
May	1st to 10th	Day	16% 6% 2 5% 5% 3 5% 5% 5% 7% 40%	2160
	11th to 20th	Day	11% 5% 2 7% 6% 3 4 5% 5% 9% 42%	2230
	21st to end	Day	8% 4 3 6% 6% 5% 6% 6% 8% 10% 38%	2593
June	1st to 10th	Day	8% 6% 2 5% 6% 4 6% 5% 6% 9% 44%	2520
	11th to 20th	Day	5% 3 2 4 5% 4 5% 6% 7% 9% 49%	2520
	21st to end	Day	4 3 2 4 5% 3 5% 6% 7% 10% 50%	2479
July	1st to 10th	Day	5% 4 3 5% 8% 4 8% 8% 8% 11% 36%	2340
	11th to 20th	Day	6% 4 2 5% 7% 4 7% 6% 7% 11% 42%	2340
	21st to end	Day	6% 4 2 4 6% 4 6% 8% 6% 11% 43%	2546
August	1st to 10th	Day	6% 5% 2 6% 6% 4 6% 7% 8% 11% 41%	2309
	11th to 20th	Day	10% 5% 2 5% 5% 4 6% 6% 6% 8% 43%	2125
	21st to end	Day	7% 4 2 5% 6% 4 6% 5% 8% 9% 44%	2376
September	1st to 10th	Day	10% 5% 2 5% 6% 4 5% 5% 6% 7% 47%	2160
	11th to 20th	Day	13% 5% 2 3 4 3 4 5% 5% 7% 48%	2160
	21st to end	Day	10% 4 1 4 5% 3 3 4 4 7% 56%	1905
October	1st to 10th	Day	8% 3 2 4 4 2 4 3 6% 6% 58%	1800
	11th to 20th	Day	14% 5% 2 3 3 2 3 3 4 6% 56%	1800
	21st to end	Day	19% 5% 2 4 4 1 2 3 4 5% 52%	1980
November	1st to 10th	Day	12% 5% 2 4 3 2 2 3 4 5% 58%	1858
	11th to 20th	Day	11% 4 2 5% 4 3 3 4 4 7% 55%	1771
	21st to end	Day	20% 7% 3 4 3 2 4 4 3 5% 45%	1710
December	1st to 10th	Day	19% 6% 2 5% 4 2 4 4 4 5% 46%	1520
	11th to 20th	Day	15% 7% 3 5% 5% 2 4 5% 4 5% 44%	1918
	21st to end	Day	15% 8% 3 6% 6% 3 4 6% 5% 6% 39%	2947
January	1st to 10th	Night	19% 6% 2 4 3 2 3 3 4 5% 48%	2458
	11th to 20th	Night	18% 6% 3 5% 4 2 3 4 3 5% 47%	2470
	21st to end	Night	18% 5% 2 4 3 3 3 3 4 5% 51%	2665
February	1st to 10th	Night	21% 7% 3 5% 4 2 3 4 4 5% 43%	2340
	11th to 20th	Night	19% 7% 3 5% 4 2 4 3 3 5% 45%	2119
	21st to end	Night	21% 6% 3 4 4 2 3 3 4 5% 45%	1776
March	1st to 10th	Night	18% 6% 2 4 3 2 4 4 5% 6% 46%	1990
	11th to 20th	Night	15% 8% 4 6% 5% 2 5% 4 3 5% 43%	1980
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 5% 6% 44%	2153
April	1st to 10th	Night	12% 5% 2 5% 4 2 4 4 5% 7% 51%	1800
	11th to 20th	Night	12% 4 2 4 4 2 4 5% 5% 8% 51%	1775
	21st to end	Night	13% 4 3 5% 5% 2 3 4 5% 8% 48%	1800
May	1st to 10th	Night	13% 5% 3 3 5% 2 4 5% 5% 7% 49%	1800
	11th to 20th	Night	13% 6% 3 5% 5% 2 3 4 6% 7% 47%	1661
	21st to end	Night	6% 4 2 6% 5% 4 5% 5% 6% 10% 45%	1673
June	1st to 10th	Night	5% 4 2 4 4 3 4 5% 8% 9% 52%	1440
	11th to 20th	Night	2 3 2 3 4 3 4 4 5% 9% 61%	1440
	21st to end	Night	3 2 2 3 4 2 4 5% 6% 8% 61%	1473
July	1st to 10th	Night	3 3 2 4 5% 3 6% 6% 7% 12% 50%	1620
	11th to 20th	Night	4 3 1 4 5% 3 6% 6% 7% 12% 50%	1620
	21st to end	Night	4 2 1 3 4 3 5% 6% 7% 11% 53%	1772
August	1st to 10th	Night	3 2 1 4 5% 3 5% 6% 7% 11% 53%	1628
	11th to 20th	Night	5% 3 2 5% 5% 2 5% 5% 7% 11% 50%	1776
	21st to end	Night	6% 3 2 4 5% 4 5% 5% 6% 9% 50%	1980
September	1st to 10th	Night	7% 4 2 4 4 3 3 5% 5% 9% 55%	1800
	11th to 20th	Night	10% 4 2 4 4 4 4 4 5% 6% 54%	1800
	21st to end	Night	8% 4 1 3 3 2 4 3 5% 6% 59%	2031
October	1st to 10th	Night	9% 4 2 4 3 2 4 4 4 5% 62%	2160
	11th to 20th	Night	11% 4 2 4 3 2 3 3 4 5% 59%	2160
	21st to end	Night	17% 5% 2 3 4 2 4 3 3 3 55%	2376
November	1st to 10th	Night	11% 4 2 4 3 2 3 3 4 6% 58%	2220
	11th to 20th	Night	11% 5% 2 4 3 2 3 3 5% 6% 56%	2405
	21st to end	Night	15% 5% 2 5% 4 3 4 3 4 5% 50%	2470
December	1st to 10th	Night	18% 6% 2 5% 4 3 3 4 3 5% 47%	2657
	11th to 20th	Night	16% 6% 3 5% 5% 2 3 3 4 4 49%	3279
	21st to end	Night	16% 7% 3 6% 6% 3 4 4 4 5% 43%	4715

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	42% 3 7% 24% 19% 6%	1708
	11th to 20th	Day	45% 4 7% 22% 19% 2	1710
	21st to end	Day	38% 3 8% 24% 21% 5%	1836
February	1st to 10th	Day	45% 4 10% 22% 16% 3	1620
	11th to 20th	Day	46% 4 8% 20% 17% 5%	1736
	21st to end	Day	50% 4 6% 17% 16% 7%	1484
March	1st to 10th	Day	43% 4 9% 16% 19% 9%	1980
	11th to 20th	Day	48% 4 7% 20% 17% 3	1980
	21st to end	Day	45% 4 11% 25% 13% 2	2201
April	1st to 10th	Day	37% 6% 9% 24% 20% 3	2160
	11th to 20th	Day	40% 4 9% 22% 18% 7%	2134
	21st to end	Day	41% 6% 10% 22% 19% 2	2160
May	1st to 10th	Day	42% 8% 10% 18% 19% 3	2160
	11th to 20th	Day	39% 7% 14% 25% 14% 1	2234
	21st to end	Day	38% 9% 14% 26% 13% 1	2593
June	1st to 10th	Day	35% 7% 12% 27% 17% 2	2520
	11th to 20th	Day	29% 7% 13% 29% 19% 2	2520
	21st to end	Day	27% 7% 13% 27% 21% 5%	2479
July	1st to 10th	Day	35% 11% 16% 22% 14% 2	2276
	11th to 20th	Day	32% 9% 15% 24% 16% 4	2271
	21st to end	Day	31% 10% 16% 30% 12% 1	2405
August	1st to 10th	Day	32% 9% 17% 29% 12% 1	2295
	11th to 20th	Day	35% 9% 12% 25% 15% 3	2125
	21st to end	Day	33% 9% 14% 24% 16% 4	2376
September	1st to 10th	Day	36% 7% 10% 21% 20% 6%	2160
	11th to 20th	Day	34% 6% 10% 18% 24% 6%	2160
	21st to end	Day	30% 4 9% 21% 27% 9%	1905
October	1st to 10th	Day	27% 4 10% 25% 27% 8%	1800
	11th to 20th	Day	31% 3 7% 20% 29% 10%	1800
	21st to end	Day	36% 4 5% 23% 22% 10%	1980
November	1st to 10th	Day	30% 3 7% 26% 25% 8%	1858
	11th to 20th	Day	31% 4 8% 20% 28% 9%	1771
	21st to end	Day	43% 3 6% 19% 23% 6%	1710
December	1st to 10th	Day	42% 6% 6% 24% 17% 5%	1520
	11th to 20th	Day	43% 4 7% 21% 21% 5%	1915
	21st to end	Day	44% 5% 11% 19% 16% 5%	2947
January	1st to 10th	Night	39% 3 7% 20% 26% 5%	2458
	11th to 20th	Night	41% 4 8% 21% 21% 5%	2470
	21st to end	Night	37% 3 6% 23% 25% 5%	2665
February	1st to 10th	Night	44% 4 8% 20% 19% 5%	2340
	11th to 20th	Night	43% 3 7% 22% 19% 6%	2120
	21st to end	Night	43% 3 7% 17% 21% 8%	1776
March	1st to 10th	Night	39% 3 9% 16% 21% 12%	1990
	11th to 20th	Night	44% 5% 8% 20% 18% 5%	1980
	21st to end	Night	41% 5% 8% 22% 20% 3	2153
April	1st to 10th	Night	33% 5% 10% 24% 21% 7%	1800
	11th to 20th	Night	31% 5% 7% 25% 22% 9%	1775
	21st to end	Night	35% 5% 10% 23% 24% 3	1800
May	1st to 10th	Night	35% 5% 12% 22% 20% 6%	1800
	11th to 20th	Night	36% 6% 12% 24% 20% 2	1662
	21st to end	Night	34% 8% 15% 25% 15% 2	1674
June	1st to 10th	Night	26% 7% 13% 29% 22% 3	1440
	11th to 20th	Night	19% 6% 14% 30% 26% 4	1440
	21st to end	Night	21% 6% 13% 28% 25% 7%	1473
July	1st to 10th	Night	24% 9% 20% 28% 16% 2	1575
	11th to 20th	Night	25% 8% 18% 26% 16% 6%	1566
	21st to end	Night	22% 10% 13% 35% 19% 1	1673
August	1st to 10th	Night	22% 8% 22% 33% 15% 1	1618
	11th to 20th	Night	26% 8% 15% 29% 18% 3	1776
	21st to end	Night	28% 8% 14% 27% 19% 4	1980
September	1st to 10th	Night	26% 7% 12% 24% 23% 8%	1800
	11th to 20th	Night	30% 6% 10% 20% 26% 8%	1800
	21st to end	Night	26% 4 9% 21% 30% 10%	2031
October	1st to 10th	Night	25% 5% 10% 24% 25% 12%	2160
	11th to 20th	Night	29% 3 6% 19% 32% 11%	2160
	21st to end	Night	36% 3 5% 18% 28% 10%	2376
November	1st to 10th	Night	29% 3 7% 23% 28% 10%	2220
	11th to 20th	Night	29% 3 6% 24% 27% 10%	2405
	21st to end	Night	37% 4 6% 20% 28% 5%	2470
December	1st to 10th	Night	42% 4 7% 20% 21% 7%	2657
	11th to 20th	Night	40% 3 6% 21% 25% 5%	3281
	21st to end	Night	43% 4 9% 19% 20% 4	4715

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	26% 11% 34% 20% 7% 2	270
	11th to 20th	Day	26% 12% 39% 20% 3 1	270
	21st to end	Day	20% 13% 41% 20% 5%	282
February	1st to 10th	Day	26% 10% 38% 18% 7% 2	240
	11th to 20th	Day	28% 19% 36% 14% 3	240
	21st to end	Day	36% 13% 31% 14% 6%	198
March	1st to 10th	Day	25% 14% 31% 23% 6% 1	240
	11th to 20th	Day	25% 19% 30% 24% 3	240
	21st to end	Day	24% 13% 36% 18% 9% 1	287
April	1st to 10th	Day	15% 13% 33% 26% 12% 1	420
	11th to 20th	Day	23% 9% 30% 25% 12% 1	412
	21st to end	Day	18% 8% 33% 27% 12% 3	420
May	1st to 10th	Day	17% 10% 27% 31% 13% 2	423
	11th to 20th	Day	14% 4 22% 28% 23% 8%	553
	21st to end	Day	9% 3 18% 34% 26% 11%	679
June	1st to 10th	Day	11% 4 11% 26% 32% 17%	630
	11th to 20th	Day	5% 2 12% 29% 37% 14%	630
	21st to end	Day	5% 2 11% 37% 26% 18%	630
July	1st to 10th	Day	5% 2 8% 28% 38% 20%	621
	11th to 20th	Day	5% 2 6% 27% 35% 24%	618
	21st to end	Day	6% 1 6% 26% 38% 22%	684
August	1st to 10th	Day	5% 2 8% 30% 38% 17%	607
	11th to 20th	Day	13% 3 5% 25% 35% 19%	439
	21st to end	Day	10% 2 5% 17% 39% 27%	495
September	1st to 10th	Day	9% 2 5% 24% 39% 21%	450
	11th to 20th	Day	10% 2 11% 20% 36% 22%	450
	21st to end	Day	11% 3 14% 30% 26% 16%	329
October	1st to 10th	Day	10% 2 22% 26% 30% 10%	270
	11th to 20th	Day	13% 9% 32% 25% 16% 6%	270
	21st to end	Day	25% 10% 28% 23% 11% 5%	297
November	1st to 10th	Day	14% 8% 31% 26% 17% 4	270
	11th to 20th	Day	9% 7% 38% 33% 12% 2	270
	21st to end	Day	26% 13% 28% 19% 11% 3	270
December	1st to 10th	Day	20% 14% 34% 20% 11% 3	270
	11th to 20th	Day	27% 8% 34% 19% 12%	270
	21st to end	Day	25% 13% 37% 21% 3 1	297
January	1st to 10th	Night	22% 8% 33% 30% 6%	950
	11th to 20th	Night	22% 9% 37% 27% 5% 1	950
	21st to end	Night	22% 7% 37% 30% 4	1025
February	1st to 10th	Night	25% 8% 35% 27% 6%	900
	11th to 20th	Night	23% 10% 39% 26% 3	724
	21st to end	Night	25% 13% 34% 26% 2	592
March	1st to 10th	Night	20% 7% 32% 35% 5%	550
	11th to 20th	Night	19% 9% 37% 30% 5% 1	540
	21st to end	Night	19% 7% 37% 29% 8%	594
April	1st to 10th	Night	15% 8% 35% 33% 9% 1	540
	11th to 20th	Night	15% 6% 33% 32% 13% 1	533
	21st to end	Night	16% 5% 36% 31% 10% 2	540
May	1st to 10th	Night	15% 4 22% 46% 13% 1	540
	11th to 20th	Night	15% 3 25% 35% 19% 3	531
	21st to end	Night	8% 3 17% 39% 28% 5%	584
June	1st to 10th	Night	5% 2 10% 37% 42% 4	540
	11th to 20th	Night	2 2 7% 45% 41% 5%	540
	21st to end	Night	3 1 9% 40% 42% 6%	540
July	1st to 10th	Night	3 1 6% 34% 47% 9%	525
	11th to 20th	Night	5% 1 5% 28% 51% 10%	522
	21st to end	Night	5% 6% 34% 44% 10%	591
August	1st to 10th	Night	4 4 36% 47% 8%	537
	11th to 20th	Night	6% 1 5% 35% 45% 7%	534
	21st to end	Night	7% 2 6% 26% 51% 9%	594
September	1st to 10th	Night	8% 1 7% 32% 47% 5%	540
	11th to 20th	Night	11% 1 8% 33% 42% 5%	540
	21st to end	Night	10% 2 12% 37% 35% 4	655
October	1st to 10th	Night	9% 3 16% 38% 30% 5%	720
	11th to 20th	Night	13% 3 25% 40% 18% 2	720
	21st to end	Night	18% 5% 27% 31% 17% 1	792
November	1st to 10th	Night	13% 4 25% 38% 18% 2	740
	11th to 20th	Night	13% 5% 32% 37% 13%	760
	21st to end	Night	17% 8% 30% 31% 14%	760
December	1st to 10th	Night	21% 7% 29% 36% 7%	947
	11th to 20th	Night	23% 7% 29% 34% 7%	950
	21st to end	Night	22% 9% 33% 31% 5%	1045

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