UW-Whitewater Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UW-Whitewater Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 9% 3 8% 5% 4 5% 4 6% 7% 38%	127043
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	123574
Transparency	Day	44% 5% 12% 25% 13% 1	130538
	Night	46% 5% 11% 23% 14% 2	128153
Seeing	Day	16% 11% 29% 26% 13% 5%	17313
	Night	18% 9% 31% 30% 11% 1	27134

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	8% 12% 7% 15% 8% 7% 5% 4 5% 6% 25%	3617
	11th to 20th	Day	8% 12% 6% 12% 5% 3 5% 7% 7% 6% 28%	3620
	21st to end	Day	11% 14% 8% 10% 4 3 3 2 4 6% 34%	4102
February	1st to 10th	Day	12% 14% 5% 11% 6% 5% 5% 4 5% 6% 29%	1800
	11th to 20th	Day	13% 12% 3 7% 6% 5% 5% 3 5% 5% 36%	1921
	21st to end	Day	11% 14% 4 10% 7% 5% 4 3 4 6% 31%	1650
March	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 5% 6% 44%	4480
	11th to 20th	Day	11% 6% 3 7% 5% 5% 4 3 5% 7% 43%	4704
	21st to end	Day	20% 12% 4 11% 6% 4 4 3 4 5% 27%	5962
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 5% 3 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 6% 4 5% 4 5% 7% 33%	2374
	21st to end	Day	18% 10% 3 7% 5% 4 4 4 5% 6% 34%	2496
May	1st to 10th	Day	14% 8% 3 7% 4 4 4 4 6% 7% 39%	5944
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 4 6% 7% 40%	6319
	21st to end	Day	15% 7% 2 6% 4 4 5% 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 6% 4 6% 9% 43%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 7% 9% 42%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	11% 7% 3 7% 4 5% 5% 5% 7% 9% 38%	6360
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 46%	6360
	21st to end	Day	9% 7% 3 7% 4 5% 5% 4 6% 9% 42%	6924
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4563
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 6% 9% 46%	3866
	21st to end	Day	8% 7% 2 6% 4 4 5% 4 6% 9% 45%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 5% 4 3 7% 8% 44%	2400
	11th to 20th	Day	11% 5% 1 5% 3 3 3 3 5% 9% 51%	2305
	21st to end	Day	11% 7% 2 6% 5% 5% 4 3 5% 7% 45%	2200
October	1st to 10th	Day	13% 7% 2 5% 4 4 3 3 5% 7% 47%	2090
	11th to 20th	Day	17% 11% 3 7% 4 5% 5% 3 4 7% 35%	2000
	21st to end	Day	17% 10% 3 7% 4 4 4 3 5% 6% 37%	1984
November	1st to 10th	Day	12% 8% 3 9% 4 4 4 4 4 6% 44%	1852
	11th to 20th	Day	12% 12% 4 9% 6% 5% 4 4 5% 6% 32%	1863
	21st to end	Day	16% 13% 3 9% 5% 4 3 3 4 6% 35%	1640
December	1st to 10th	Day	8% 9% 5% 9% 6% 5% 6% 4 5% 7% 37%	2610
	11th to 20th	Day	12% 14% 6% 13% 7% 5% 5% 4 5% 7% 23%	3008
	21st to end	Day	19% 20% 9% 13% 5% 4 3 2 4 5% 14%	3982
January	1st to 10th	Night	9% 12% 6% 14% 7% 5% 4 3 4 5% 32%	6348
	11th to 20th	Night	10% 13% 6% 15% 6% 5% 5% 5% 6% 5% 22%	6360
	21st to end	Night	12% 15% 7% 11% 5% 4 4 3 5% 5% 29%	6862
February	1st to 10th	Night	12% 14% 5% 10% 5% 5% 4 3 4 6% 32%	2600
	11th to 20th	Night	12% 11% 3 7% 5% 5% 5% 4 5% 5% 38%	2363
	21st to end	Night	17% 12% 3 7% 4 3 3 3 5% 5% 37%	1980
March	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 4 5% 43%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 6% 8% 48%	5287
	21st to end	Night	20% 14% 5% 9% 5% 4 4 3 3 4 29%	4966
April	1st to 10th	Night	16% 10% 4 5% 4 5% 4 3 5% 7% 37%	2000
	11th to 20th	Night	17% 10% 3 7% 4 4 4 3 5% 5% 38%	1975
	21st to end	Night	18% 8% 3 7% 4 3 4 4 5% 6% 38%	1902
May	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 8% 39%	3953
	11th to 20th	Night	10% 6% 3 6% 4 4 5% 3 6% 7% 44%	3596
	21st to end	Night	15% 9% 3 6% 4 4 4 4 6% 7% 38%	3942
June	1st to 10th	Night	9% 4 2 5% 5% 4 6% 4 7% 9% 47%	1600
	11th to 20th	Night	11% 5% 1 6% 4 4 4 4 7% 10% 44%	1600
	21st to end	Night	9% 6% 3 7% 4 3 4 3 6% 9% 47%	1600
July	1st to 10th	Night	12% 7% 2 6% 4 4 5% 4 5% 9% 42%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 5% 9% 51%	3617
	21st to end	Night	7% 5% 2 7% 3 5% 5% 5% 6% 7% 48%	3947
August	1st to 10th	Night	9% 4 2 5% 3 4 4 4 7% 8% 49%	2597
	11th to 20th	Night	9% 7% 2 6% 4 4 4 4 5% 8% 47%	3177
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 9% 49%	3586
September	1st to 10th	Night	8% 5% 2 6% 4 3 4 3 6% 7% 53%	2000
	11th to 20th	Night	11% 4 2 4 4 4 4 3 5% 7% 52%	2074
	21st to end	Night	12% 6% 2 6% 4 4 3 2 4 7% 49%	2200
October	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 4 5% 51%	2307
	11th to 20th	Night	16% 9% 2 7% 3 4 3 3 5% 5% 43%	2400
	21st to end	Night	19% 9% 2 6% 4 4 4 3 5% 7% 37%	2855
November	1st to 10th	Night	12% 9% 2 6% 4 3 3 4 5% 6% 46%	2666
	11th to 20th	Night	13% 12% 4 9% 5% 5% 4 3 4 6% 36%	2692
	21st to end	Night	16% 11% 4 8% 5% 4 4 3 4 6% 34%	2756
December	1st to 10th	Night	9% 9% 3 8% 5% 5% 5% 4 5% 7% 40%	4580
	11th to 20th	Night	16% 17% 5% 10% 5% 5% 4 3 4 5% 26%	5199
	21st to end	Night	19% 19% 6% 13% 6% 4 4 3 4 5% 17%	6996

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	61% 4 12% 18% 5%	3618
	11th to 20th	Day	54% 4 12% 12% 17% 2	3620
	21st to end	Day	53% 3 11% 21% 12%	4102
February	1st to 10th	Day	56% 4 12% 17% 10% 1	1800
	11th to 20th	Day	50% 4 10% 24% 10% 2	1931
	21st to end	Day	56% 4 9% 19% 11% 2	1650
March	1st to 10th	Day	41% 4 11% 28% 12% 4	4480
	11th to 20th	Day	41% 4 11% 30% 13% 1	4704
	21st to end	Day	60% 4 11% 15% 9% 2	5964
April	1st to 10th	Day	55% 5% 10% 19% 11%	2400
	11th to 20th	Day	50% 4 10% 21% 12% 2	2374
	21st to end	Day	50% 6% 12% 19% 12% 1	2496
May	1st to 10th	Day	43% 6% 10% 24% 16% 2	5979
	11th to 20th	Day	41% 7% 9% 27% 16% 1	6323
	21st to end	Day	41% 6% 11% 27% 13% 2	6949
June	1st to 10th	Day	37% 5% 13% 27% 16% 2	2800
	11th to 20th	Day	37% 6% 13% 25% 18% 2	2800
	21st to end	Day	40% 6% 13% 23% 16% 2	2800
July	1st to 10th	Day	40% 6% 12% 30% 11% 1	6291
	11th to 20th	Day	34% 6% 14% 31% 15% 1	6285
	21st to end	Day	37% 6% 16% 22% 17% 2	6728
August	1st to 10th	Day	37% 7% 14% 30% 12% 1	4548
	11th to 20th	Day	35% 6% 13% 34% 12% 1	3902
	21st to end	Day	35% 5% 13% 28% 18% 1	4311
September	1st to 10th	Day	37% 6% 10% 28% 17% 2	2400
	11th to 20th	Day	30% 5% 12% 34% 17% 2	2305
	21st to end	Day	39% 5% 9% 30% 16% 1	2200
October	1st to 10th	Day	25% 3 6% 40% 25% 2	3402
	11th to 20th	Day	37% 3 9% 28% 22% 1	3240
	21st to end	Day	43% 3 9% 29% 14% 2	3184
November	1st to 10th	Day	42% 4 13% 24% 15% 2	1852
	11th to 20th	Day	52% 5% 12% 21% 10% 1	1863
	21st to end	Day	52% 4 13% 23% 7%	1640
December	1st to 10th	Day	47% 4 21% 22% 6%	2610
	11th to 20th	Day	60% 4 11% 17% 6% 1	3005
	21st to end	Day	75% 3 6% 10% 6%	3982
January	1st to 10th	Night	55% 4 8% 22% 11%	6348
	11th to 20th	Night	62% 4 9% 10% 15% 1	6360
	21st to end	Night	58% 3 12% 16% 11%	6862
February	1st to 10th	Night	53% 5% 10% 18% 12% 2	2600
	11th to 20th	Night	48% 5% 10% 22% 14% 1	2364
	21st to end	Night	49% 4 11% 21% 11% 3	1980
March	1st to 10th	Night	45% 4 12% 23% 11% 5%	5373
	11th to 20th	Night	35% 4 12% 33% 14% 3	5287
	21st to end	Night	60% 4 8% 16% 12% 1	4966
April	1st to 10th	Night	47% 5% 9% 22% 15% 1	2000
	11th to 20th	Night	49% 4 6% 21% 17% 3	1975
	21st to end	Night	46% 6% 11% 22% 14% 2	1902
May	1st to 10th	Night	43% 6% 12% 19% 18% 2	3980
	11th to 20th	Night	37% 8% 14% 22% 15% 3	3597
	21st to end	Night	43% 6% 10% 22% 15% 4	3950
June	1st to 10th	Night	32% 6% 13% 29% 19% 1	1600
	11th to 20th	Night	34% 7% 11% 24% 22% 3	1600
	21st to end	Night	35% 6% 14% 23% 19% 3	1600
July	1st to 10th	Night	39% 5% 12% 30% 11% 2	3580
	11th to 20th	Night	30% 5% 13% 31% 19% 1	3572
	21st to end	Night	33% 6% 14% 24% 21% 2	3832
August	1st to 10th	Night	31% 6% 14% 29% 18% 2	2588
	11th to 20th	Night	35% 5% 12% 33% 15% 1	3207
	21st to end	Night	31% 6% 12% 29% 19% 3	3586
September	1st to 10th	Night	30% 5% 13% 27% 23% 3	2000
	11th to 20th	Night	31% 5% 10% 33% 19% 2	2074
	21st to end	Night	36% 4 9% 31% 18% 1	2200
October	1st to 10th	Night	26% 4 6% 37% 26% 2	3775
	11th to 20th	Night	36% 3 10% 26% 22% 3	3899
	21st to end	Night	43% 4 8% 27% 17% 2	4605
November	1st to 10th	Night	40% 4 9% 27% 17% 2	2666
	11th to 20th	Night	50% 4 11% 22% 11% 2	2692
	21st to end	Night	51% 4 13% 21% 10% 1	2756
December	1st to 10th	Night	43% 5% 15% 27% 8% 2	4580
	11th to 20th	Night	62% 4 10% 15% 9% 1	5201
	21st to end	Night	70% 4 8% 12% 6%	6996

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	17% 15% 34% 25% 6% 2	300
	11th to 20th	Day	17% 18% 36% 23% 4 1	300
	21st to end	Day	22% 14% 39% 20% 4 2	330
February	1st to 10th	Day	15% 20% 45% 15% 5% 1	300
	11th to 20th	Day	19% 22% 38% 17% 4	300
	21st to end	Day	21% 17% 45% 14% 2	249
March	1st to 10th	Day	17% 17% 46% 14% 5%	300
	11th to 20th	Day	21% 21% 34% 16% 7% 1	300
	21st to end	Day	28% 13% 30% 22% 6% 2	545
April	1st to 10th	Day	23% 17% 33% 22% 5%	500
	11th to 20th	Day	24% 20% 24% 21% 9% 2	492
	21st to end	Day	25% 12% 29% 23% 10% 1	500
May	1st to 10th	Day	19% 11% 31% 28% 8% 2	533
	11th to 20th	Day	22% 10% 27% 26% 12% 3	725
	21st to end	Day	16% 10% 29% 26% 14% 5%	789
June	1st to 10th	Day	10% 8% 30% 31% 15% 5%	730
	11th to 20th	Day	13% 8% 31% 29% 14% 5%	730
	21st to end	Day	14% 8% 29% 28% 16% 4	730
July	1st to 10th	Day	8% 3 19% 32% 26% 11%	721
	11th to 20th	Day	9% 5% 26% 30% 21% 8%	718
	21st to end	Day	8% 3 23% 37% 20% 9%	794
August	1st to 10th	Day	8% 4 20% 35% 22% 12%	726
	11th to 20th	Day	13% 4 17% 27% 24% 14%	534
	21st to end	Day	11% 3 20% 32% 24% 10%	583
September	1st to 10th	Day	13% 8% 23% 28% 21% 8%	530
	11th to 20th	Day	10% 3 21% 31% 23% 12%	530
	21st to end	Day	15% 10% 26% 23% 16% 10%	530
October	1st to 10th	Day	17% 9% 26% 25% 17% 5%	420
	11th to 20th	Day	21% 19% 31% 22% 7% 1	330
	21st to end	Day	27% 12% 35% 20% 5% 1	363
November	1st to 10th	Day	16% 18% 37% 19% 9% 1	330
	11th to 20th	Day	20% 18% 36% 17% 9%	321
	21st to end	Day	24% 12% 40% 20% 3	300
December	1st to 10th	Day	18% 17% 40% 18% 6% 1	300
	11th to 20th	Day	17% 19% 38% 18% 7% 1	300
	21st to end	Day	26% 16% 37% 18% 3 1	330
January	1st to 10th	Night	16% 15% 38% 24% 7% 1	1000
	11th to 20th	Night	21% 13% 36% 24% 5% 1	1000
	21st to end	Night	20% 10% 36% 27% 5%	1100
February	1st to 10th	Night	20% 11% 38% 26% 5%	1000
	11th to 20th	Night	19% 14% 36% 27% 4	808
	21st to end	Night	24% 12% 38% 23% 3	660
March	1st to 10th	Night	22% 10% 37% 24% 6% 1	800
	11th to 20th	Night	15% 13% 35% 29% 7%	738
	21st to end	Night	24% 9% 29% 29% 7% 1	660
April	1st to 10th	Night	21% 12% 38% 24% 5% 1	600
	11th to 20th	Night	22% 11% 32% 28% 6% 1	593
	21st to end	Night	22% 9% 32% 28% 9% 1	600
May	1st to 10th	Night	21% 7% 35% 31% 7% 1	600
	11th to 20th	Night	18% 6% 32% 32% 12% 1	591
	21st to end	Night	16% 5% 26% 36% 17%	650
June	1st to 10th	Night	10% 4 26% 45% 15% 1	600
	11th to 20th	Night	14% 4 32% 40% 11% 1	600
	21st to end	Night	12% 4 28% 40% 16% 1	600
July	1st to 10th	Night	10% 2 14% 41% 30% 3	585
	11th to 20th	Night	10% 2 20% 43% 23% 3	582
	21st to end	Night	7% 2 16% 50% 22% 2	657
August	1st to 10th	Night	7% 2 14% 46% 28% 3	597
	11th to 20th	Night	9% 3 14% 42% 28% 4	594
	21st to end	Night	11% 2 16% 43% 25% 3	660
September	1st to 10th	Night	8% 4 22% 36% 24% 5%	600
	11th to 20th	Night	12% 3 21% 35% 25% 4	674
	21st to end	Night	15% 6% 25% 31% 20% 2	800
October	1st to 10th	Night	14% 6% 31% 32% 16% 2	800
	11th to 20th	Night	21% 11% 31% 28% 8%	800
	21st to end	Night	24% 10% 31% 27% 7%	880
November	1st to 10th	Night	16% 12% 38% 25% 9% 1	820
	11th to 20th	Night	21% 15% 36% 23% 6%	829
	21st to end	Night	22% 12% 38% 21% 7%	956
December	1st to 10th	Night	22% 13% 39% 20% 5%	1000
	11th to 20th	Night	24% 12% 36% 23% 5% 1	1000
	21st to end	Night	30% 12% 33% 22% 3	1100

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