Fond du Lac
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fond du Lac clear sky chart from 2005-11-06 to 2025-10-04.

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% 9% 3 8% 5% 4 4 4 5% 7% 38%	126642
	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 38%	123015
Transparency	Day	44% 5% 12% 25% 13% 1	130129
	Night	46% 4 11% 23% 14% 2	127602
Seeing	Day	16% 11% 30% 25% 13% 4	17233
	Night	17% 10% 32% 29% 11% 1	27021

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% 11% 6% 17% 8% 6% 5% 4 5% 5% 26%	3617
	11th to 20th	Day	8% 12% 7% 12% 5% 4 8% 6% 6% 6% 26%	3620
	21st to end	Day	13% 13% 6% 12% 4 3 4 3 3 6% 33%	3997
February	1st to 10th	Day	10% 14% 5% 10% 5% 5% 5% 3 5% 7% 29%	1800
	11th to 20th	Day	12% 12% 3 8% 5% 5% 5% 4 4 6% 36%	1913
	21st to end	Day	12% 15% 4 8% 6% 5% 4 4 4 6% 31%	1650
March	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 45%	4480
	11th to 20th	Day	11% 7% 2 8% 5% 4 4 3 5% 7% 42%	4691
	21st to end	Day	22% 12% 4 9% 4 4 3 3 4 4 29%	5967
April	1st to 10th	Day	16% 15% 3 7% 5% 4 4 3 5% 7% 31%	2400
	11th to 20th	Day	19% 11% 3 6% 5% 4 4 4 5% 6% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 5% 6% 33%	2535
May	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 7% 44%	5990
	11th to 20th	Day	12% 5% 2 7% 4 4 4 4 6% 8% 44%	6319
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 8% 41%	6935
June	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 4 7% 8% 44%	2800
	11th to 20th	Day	11% 7% 3 7% 5% 4 4 4 6% 9% 41%	2800
	21st to end	Day	11% 6% 2 5% 4 5% 4 4 7% 9% 43%	2800
July	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 40%	6360
	11th to 20th	Day	8% 4 1 5% 4 5% 5% 4 7% 10% 47%	6360
	21st to end	Day	9% 6% 2 5% 4 5% 5% 4 6% 9% 44%	6924
August	1st to 10th	Day	9% 6% 2 7% 4 5% 6% 4 6% 9% 42%	4563
	11th to 20th	Day	13% 5% 2 5% 3 4 5% 4 6% 8% 44%	3982
	21st to end	Day	10% 7% 2 6% 4 5% 4 4 5% 8% 43%	4312
September	1st to 10th	Day	11% 6% 2 7% 4 4 4 4 5% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 8% 49%	2295
	21st to end	Day	13% 8% 2 7% 4 5% 4 3 5% 8% 41%	2200
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 44%	2028
	11th to 20th	Day	19% 11% 3 7% 5% 4 5% 3 5% 7% 30%	1872
	21st to end	Day	17% 12% 3 8% 4 5% 3 3 5% 6% 35%	1881
November	1st to 10th	Day	14% 10% 3 6% 4 4 3 3 5% 7% 41%	1762
	11th to 20th	Day	11% 14% 4 10% 7% 6% 5% 4 5% 6% 28%	1800
	21st to end	Day	15% 14% 4 8% 5% 4 3 3 5% 5% 33%	1615
December	1st to 10th	Day	7% 10% 7% 12% 6% 6% 7% 6% 7% 7% 26%	2610
	11th to 20th	Day	10% 16% 6% 14% 7% 5% 5% 4 4 6% 23%	3008
	21st to end	Day	18% 21% 7% 13% 6% 4 3 3 4 5% 15%	3982
January	1st to 10th	Night	9% 11% 7% 12% 7% 4 3 3 4 5% 34%	6348
	11th to 20th	Night	10% 13% 7% 14% 6% 5% 6% 5% 5% 4 25%	6360
	21st to end	Night	10% 16% 7% 12% 5% 5% 3 3 4 5% 31%	6978
February	1st to 10th	Night	11% 14% 5% 9% 5% 5% 4 3 4 5% 34%	2600
	11th to 20th	Night	9% 10% 4 9% 5% 5% 5% 3 5% 5% 39%	2372
	21st to end	Night	15% 13% 3 7% 5% 4 4 3 5% 6% 35%	1980
March	1st to 10th	Night	14% 8% 2 6% 3 4 4 3 5% 6% 44%	5373
	11th to 20th	Night	9% 8% 2 7% 5% 4 5% 4 5% 8% 43%	5312
	21st to end	Night	20% 11% 4 9% 4 4 4 3 4 5% 31%	4962
April	1st to 10th	Night	18% 12% 3 7% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	19% 10% 3 6% 4 4 4 4 5% 5% 37%	1975
	21st to end	Night	15% 9% 3 8% 4 5% 3 3 6% 6% 38%	1864
May	1st to 10th	Night	15% 7% 3 7% 4 4 4 3 5% 8% 42%	3909
	11th to 20th	Night	9% 4 3 7% 5% 5% 5% 4 7% 8% 42%	3596
	21st to end	Night	18% 9% 2 7% 5% 4 4 3 4 6% 38%	3942
June	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 6% 8% 47%	1600
	11th to 20th	Night	11% 7% 2 5% 4 4 5% 4 6% 9% 43%	1600
	21st to end	Night	9% 6% 2 6% 4 3 4 5% 4 8% 48%	1600
July	1st to 10th	Night	12% 6% 2 5% 3 4 4 4 6% 8% 46%	3618
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 6% 8% 53%	3617
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 8% 52%	3947
August	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 6% 9% 49%	2597
	11th to 20th	Night	12% 6% 2 7% 4 4 3 4 5% 7% 45%	3063
	21st to end	Night	8% 5% 1 5% 4 4 4 4 6% 8% 49%	3586
September	1st to 10th	Night	9% 6% 2 5% 4 4 3 3 5% 7% 52%	2000
	11th to 20th	Night	12% 4 2 5% 4 4 5% 3 5% 6% 52%	2084
	21st to end	Night	12% 6% 2 6% 4 4 2 3 4 7% 49%	2200
October	1st to 10th	Night	12% 6% 3 7% 4 3 4 3 5% 6% 48%	2216
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 4 6% 39%	2306
	21st to end	Night	18% 11% 3 7% 4 4 4 4 6% 7% 34%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 4 3 4 5% 6% 45%	2536
	11th to 20th	Night	14% 12% 4 10% 6% 5% 5% 3 5% 6% 31%	2600
	21st to end	Night	15% 12% 4 8% 5% 5% 4 3 5% 6% 33%	2782
December	1st to 10th	Night	8% 10% 4 8% 6% 6% 5% 4 6% 8% 35%	4580
	11th to 20th	Night	13% 17% 7% 11% 6% 5% 4 3 4 4 26%	5199
	21st to end	Night	17% 17% 6% 13% 6% 5% 4 3 4 4 20%	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	60% 3 13% 17% 6%	3618
	11th to 20th	Day	56% 6% 9% 15% 14% 1	3620
	21st to end	Day	55% 3 10% 20% 12%	3997
February	1st to 10th	Day	53% 6% 11% 20% 10% 1	1800
	11th to 20th	Day	48% 6% 11% 23% 11% 2	1923
	21st to end	Day	53% 5% 11% 18% 10% 3	1650
March	1st to 10th	Day	39% 4 13% 24% 15% 5%	4480
	11th to 20th	Day	40% 5% 14% 26% 15% 1	4691
	21st to end	Day	60% 3 9% 17% 10% 1	5969
April	1st to 10th	Day	52% 6% 12% 20% 10% 1	2400
	11th to 20th	Day	50% 5% 8% 22% 13% 2	2374
	21st to end	Day	50% 7% 11% 20% 11% 1	2535
May	1st to 10th	Day	40% 4 12% 25% 16% 3	6025
	11th to 20th	Day	39% 6% 11% 27% 16% 1	6323
	21st to end	Day	41% 6% 9% 27% 16% 2	6949
June	1st to 10th	Day	37% 6% 13% 27% 15% 1	2800
	11th to 20th	Day	40% 5% 14% 25% 15% 1	2800
	21st to end	Day	36% 8% 14% 26% 14% 2	2800
July	1st to 10th	Day	40% 4 12% 33% 10% 1	6291
	11th to 20th	Day	33% 4 14% 33% 15% 1	6285
	21st to end	Day	36% 4 14% 24% 19% 2	6728
August	1st to 10th	Day	38% 5% 14% 28% 13% 1	4548
	11th to 20th	Day	38% 4 11% 35% 11% 1	4021
	21st to end	Day	39% 4 12% 27% 16% 1	4311
September	1st to 10th	Day	38% 6% 12% 28% 15% 1	2400
	11th to 20th	Day	34% 4 15% 32% 14% 1	2295
	21st to end	Day	41% 5% 11% 31% 12% 1	2200
October	1st to 10th	Day	25% 3 7% 38% 26% 2	3353
	11th to 20th	Day	43% 4 9% 26% 17% 1	3088
	21st to end	Day	44% 3 12% 28% 12% 1	3081
November	1st to 10th	Day	43% 5% 15% 23% 13% 1	1762
	11th to 20th	Day	54% 5% 14% 19% 7% 1	1800
	21st to end	Day	53% 4 15% 21% 6%	1615
December	1st to 10th	Day	53% 6% 17% 19% 4	2610
	11th to 20th	Day	61% 3 10% 17% 7% 1	3005
	21st to end	Day	72% 3 6% 11% 7%	3982
January	1st to 10th	Night	54% 3 8% 24% 12% 1	6348
	11th to 20th	Night	61% 5% 7% 11% 15% 1	6360
	21st to end	Night	58% 3 11% 17% 11%	6978
February	1st to 10th	Night	52% 5% 9% 20% 12% 2	2600
	11th to 20th	Night	48% 5% 10% 24% 12% 2	2373
	21st to end	Night	50% 5% 12% 20% 11% 2	1980
March	1st to 10th	Night	41% 4 13% 25% 13% 3	5373
	11th to 20th	Night	38% 4 16% 25% 14% 3	5312
	21st to end	Night	57% 3 9% 17% 12% 1	4962
April	1st to 10th	Night	49% 5% 9% 18% 17% 1	2000
	11th to 20th	Night	48% 5% 8% 21% 15% 3	1975
	21st to end	Night	46% 6% 12% 22% 14% 1	1864
May	1st to 10th	Night	42% 4 12% 20% 19% 3	3936
	11th to 20th	Night	37% 7% 15% 22% 17% 2	3597
	21st to end	Night	48% 4 8% 25% 12% 4	3950
June	1st to 10th	Night	33% 7% 14% 29% 17% 1	1600
	11th to 20th	Night	37% 6% 12% 26% 18% 2	1600
	21st to end	Night	34% 7% 15% 25% 16% 3	1600
July	1st to 10th	Night	37% 3 14% 33% 10% 3	3580
	11th to 20th	Night	30% 3 16% 33% 17% 2	3572
	21st to end	Night	30% 4 16% 25% 22% 2	3832
August	1st to 10th	Night	33% 5% 15% 27% 19% 1	2588
	11th to 20th	Night	39% 4 11% 32% 14%	3090
	21st to end	Night	33% 4 14% 27% 20% 3	3586
September	1st to 10th	Night	31% 6% 12% 27% 23% 2	2000
	11th to 20th	Night	33% 4 11% 34% 17% 1	2084
	21st to end	Night	36% 5% 10% 31% 17% 1	2200
October	1st to 10th	Night	28% 3 7% 38% 23% 2	3671
	11th to 20th	Night	35% 3 12% 31% 17% 3	3829
	21st to end	Night	45% 4 10% 26% 16%	4467
November	1st to 10th	Night	40% 5% 11% 26% 16% 2	2536
	11th to 20th	Night	53% 5% 12% 18% 10% 1	2600
	21st to end	Night	51% 4 15% 22% 7% 1	2782
December	1st to 10th	Night	46% 4 19% 22% 6% 2	4580
	11th to 20th	Night	63% 3 9% 13% 11% 1	5201
	21st to end	Night	68% 5% 8% 14% 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	15% 14% 38% 25% 7% 1	300
	11th to 20th	Day	15% 19% 35% 25% 5%	300
	21st to end	Day	21% 13% 37% 24% 5% 1	330
February	1st to 10th	Day	14% 19% 42% 17% 7% 1	300
	11th to 20th	Day	18% 21% 38% 18% 6%	300
	21st to end	Day	20% 18% 46% 14% 2	249
March	1st to 10th	Day	18% 18% 44% 16% 4	300
	11th to 20th	Day	22% 18% 33% 18% 6% 3	310
	21st to end	Day	27% 11% 32% 23% 6% 2	550
April	1st to 10th	Day	26% 14% 33% 22% 5%	500
	11th to 20th	Day	27% 17% 25% 20% 9% 2	492
	21st to end	Day	23% 15% 29% 21% 11% 1	500
May	1st to 10th	Day	19% 12% 34% 24% 9% 2	553
	11th to 20th	Day	17% 11% 32% 24% 13% 3	725
	21st to end	Day	15% 10% 32% 23% 15% 5%	789
June	1st to 10th	Day	11% 9% 32% 29% 15% 3	730
	11th to 20th	Day	14% 9% 33% 26% 14% 4	730
	21st to end	Day	15% 10% 26% 31% 15% 4	730
July	1st to 10th	Day	8% 5% 22% 31% 25% 10%	721
	11th to 20th	Day	10% 6% 26% 33% 18% 8%	718
	21st to end	Day	7% 4 28% 34% 20% 7%	794
August	1st to 10th	Day	8% 4 23% 35% 21% 9%	726
	11th to 20th	Day	14% 4 17% 27% 26% 11%	546
	21st to end	Day	13% 6% 21% 29% 21% 9%	583
September	1st to 10th	Day	14% 9% 21% 27% 20% 8%	530
	11th to 20th	Day	11% 5% 21% 33% 21% 8%	530
	21st to end	Day	15% 12% 25% 23% 19% 5%	530
October	1st to 10th	Day	18% 12% 26% 27% 13% 4	407
	11th to 20th	Day	21% 18% 33% 20% 8%	300
	21st to end	Day	26% 13% 36% 18% 6% 1	330
November	1st to 10th	Day	20% 15% 34% 22% 8% 2	300
	11th to 20th	Day	17% 19% 37% 20% 6%	300
	21st to end	Day	25% 12% 40% 19% 4	300
December	1st to 10th	Day	17% 19% 41% 17% 6%	300
	11th to 20th	Day	21% 18% 39% 16% 6% 1	300
	21st to end	Day	26% 18% 35% 16% 3 1	330
January	1st to 10th	Night	16% 15% 38% 24% 7%	1000
	11th to 20th	Night	19% 14% 38% 23% 5% 1	1000
	21st to end	Night	18% 11% 38% 27% 5%	1100
February	1st to 10th	Night	19% 14% 37% 25% 6%	1000
	11th to 20th	Night	14% 17% 37% 25% 7%	817
	21st to end	Night	22% 14% 36% 23% 5%	660
March	1st to 10th	Night	21% 12% 34% 25% 8% 1	800
	11th to 20th	Night	16% 11% 39% 26% 6% 1	760
	21st to end	Night	23% 9% 27% 31% 9% 1	660
April	1st to 10th	Night	23% 11% 37% 23% 6%	600
	11th to 20th	Night	23% 11% 32% 27% 6% 1	593
	21st to end	Night	19% 10% 33% 30% 8% 1	600
May	1st to 10th	Night	18% 10% 36% 29% 7%	600
	11th to 20th	Night	15% 9% 35% 30% 10% 1	591
	21st to end	Night	14% 6% 28% 37% 14% 1	650
June	1st to 10th	Night	11% 5% 30% 39% 14%	600
	11th to 20th	Night	15% 4 33% 36% 11% 1	600
	21st to end	Night	13% 4 27% 40% 15% 1	600
July	1st to 10th	Night	9% 3 18% 42% 26% 2	585
	11th to 20th	Night	8% 3 23% 40% 23% 2	582
	21st to end	Night	8% 2 22% 45% 22% 1	657
August	1st to 10th	Night	9% 2 16% 49% 22% 2	597
	11th to 20th	Night	11% 2 17% 41% 26% 3	594
	21st to end	Night	11% 3 19% 45% 20% 2	660
September	1st to 10th	Night	11% 5% 24% 36% 20% 4	600
	11th to 20th	Night	13% 4 21% 35% 22% 4	684
	21st to end	Night	15% 7% 27% 31% 18% 2	800
October	1st to 10th	Night	15% 8% 29% 31% 16% 1	773
	11th to 20th	Night	22% 12% 34% 26% 7% 1	760
	21st to end	Night	25% 11% 30% 24% 10%	836
November	1st to 10th	Night	18% 12% 36% 25% 8% 1	780
	11th to 20th	Night	20% 16% 38% 21% 5%	800
	21st to end	Night	20% 14% 37% 22% 7%	982
December	1st to 10th	Night	19% 16% 39% 21% 5%	1000
	11th to 20th	Night	22% 13% 39% 21% 5% 1	1000
	21st to end	Night	24% 14% 35% 23% 3	1100

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