Milwaukee Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Milwaukee Astronomical Society 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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	144090
	Night	12% 10% 4 8% 5% 4 4 4 5% 6% 39%	138464
Transparency	Day	45% 3 11% 25% 14% 1	147589
	Night	46% 3 11% 24% 15% 2	143042
Seeing	Day	18% 12% 31% 24% 11% 4	18421
	Night	18% 10% 30% 28% 12% 1	28192

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	10% 13% 6% 14% 6% 6% 5% 4 6% 5% 25%	4691
	11th to 20th	Day	7% 11% 7% 12% 4 4 6% 6% 6% 6% 31%	4710
	21st to end	Day	10% 12% 7% 10% 6% 4 5% 3 4 5% 34%	5197
February	1st to 10th	Day	10% 15% 7% 10% 5% 5% 5% 4 5% 7% 29%	1895
	11th to 20th	Day	12% 13% 3 8% 4 5% 5% 4 5% 5% 36%	2048
	21st to end	Day	11% 15% 4 11% 5% 5% 4 5% 5% 6% 29%	1730
March	1st to 10th	Day	13% 10% 3 6% 4 4 3 3 5% 6% 42%	4580
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	4776
	21st to end	Day	21% 11% 4 10% 5% 4 4 3 4 5% 26%	6099
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 5% 4 5% 6% 29%	2520
	11th to 20th	Day	14% 11% 4 7% 5% 5% 5% 4 5% 7% 33%	2494
	21st to end	Day	17% 9% 3 8% 5% 4 4 4 6% 7% 33%	2658
May	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	7616
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 7% 8% 40%	8213
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 39%	9047
June	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 6% 8% 45%	2940
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 3 7% 9% 43%	2940
	21st to end	Day	11% 6% 2 7% 5% 4 5% 4 7% 9% 41%	2940
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 39%	8280
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 7% 10% 48%	8280
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	8990
August	1st to 10th	Day	10% 6% 2 6% 4 5% 6% 5% 7% 10% 40%	4674
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 8% 42%	4045
	21st to end	Day	8% 6% 2 7% 5% 4 5% 4 6% 9% 45%	4420
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 5% 8% 44%	2400
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 9% 49%	2280
	21st to end	Day	12% 7% 3 6% 4 4 4 4 6% 8% 43%	2200
October	1st to 10th	Day	13% 6% 3 6% 3 3 3 3 5% 6% 47%	2130
	11th to 20th	Day	18% 9% 3 7% 5% 6% 5% 3 5% 7% 32%	1970
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 6% 38%	1980
November	1st to 10th	Day	12% 9% 3 7% 4 5% 4 3 5% 6% 43%	1852
	11th to 20th	Day	12% 11% 5% 10% 6% 4 5% 4 4 8% 31%	1890
	21st to end	Day	17% 13% 3 8% 4 4 3 3 4 6% 35%	1757
December	1st to 10th	Day	8% 8% 5% 10% 5% 5% 5% 4 6% 7% 37%	2690
	11th to 20th	Day	12% 16% 5% 10% 6% 4 5% 4 6% 8% 25%	3088
	21st to end	Day	21% 21% 8% 12% 5% 4 4 3 4 4 16%	4070
January	1st to 10th	Night	11% 13% 7% 12% 6% 4 4 3 4 4 33%	8232
	11th to 20th	Night	9% 14% 7% 14% 6% 6% 6% 5% 5% 5% 23%	8280
	21st to end	Night	10% 13% 6% 9% 6% 5% 5% 4 6% 6% 31%	9016
February	1st to 10th	Night	12% 14% 4 9% 6% 5% 4 3 4 5% 34%	2723
	11th to 20th	Night	12% 10% 3 7% 5% 4 4 4 5% 6% 39%	2458
	21st to end	Night	17% 11% 3 7% 5% 4 4 3 5% 6% 36%	2076
March	1st to 10th	Night	15% 9% 3 7% 4 4 4 3 4 5% 42%	5493
	11th to 20th	Night	10% 7% 2 5% 4 4 4 3 6% 8% 46%	5434
	21st to end	Night	19% 12% 5% 9% 5% 5% 4 3 4 5% 29%	5072
April	1st to 10th	Night	16% 9% 3 7% 4 4 6% 4 5% 6% 36%	2100
	11th to 20th	Night	19% 11% 2 7% 4 4 4 3 4 4 37%	2075
	21st to end	Night	17% 8% 3 6% 4 4 4 4 5% 6% 39%	1960
May	1st to 10th	Night	13% 7% 3 6% 4 5% 5% 4 6% 8% 40%	5237
	11th to 20th	Night	11% 6% 2 8% 4 4 5% 4 6% 8% 42%	4675
	21st to end	Night	14% 8% 2 6% 5% 4 4 4 5% 7% 42%	5139
June	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	1680
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	1680
	21st to end	Night	10% 6% 2 6% 6% 4 4 4 6% 8% 45%	1680
July	1st to 10th	Night	10% 7% 3 7% 4 4 4 3 6% 8% 43%	4708
	11th to 20th	Night	5% 4 2 5% 3 3 4 4 6% 9% 56%	4707
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 9% 50%	5117
August	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 6% 8% 50%	2702
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 6% 8% 48%	3220
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	3676
September	1st to 10th	Night	8% 4 2 6% 4 4 3 3 5% 7% 53%	2000
	11th to 20th	Night	9% 4 1 6% 3 3 4 3 5% 7% 54%	2100
	21st to end	Night	11% 7% 2 6% 5% 4 4 3 4 7% 48%	2200
October	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 5% 6% 50%	2268
	11th to 20th	Night	17% 8% 3 6% 4 4 3 3 4 6% 43%	2430
	21st to end	Night	18% 9% 3 6% 4 4 4 3 5% 7% 37%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 4 6% 45%	2666
	11th to 20th	Night	14% 10% 3 8% 5% 5% 5% 4 5% 6% 35%	2730
	21st to end	Night	16% 11% 4 6% 5% 4 4 4 5% 6% 35%	2861
December	1st to 10th	Night	9% 10% 4 7% 5% 4 5% 4 5% 6% 41%	4720
	11th to 20th	Night	15% 16% 5% 9% 6% 4 4 3 5% 4 27%	5339
	21st to end	Night	20% 20% 6% 10% 5% 4 4 3 4 5% 19%	7150

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% 3 14% 16% 7%	4692
	11th to 20th	Day	53% 2 9% 19% 16% 2	4710
	21st to end	Day	54% 2 8% 23% 13%	5197
February	1st to 10th	Day	54% 4 12% 19% 10% 1	1895
	11th to 20th	Day	49% 5% 9% 24% 11% 2	2058
	21st to end	Day	56% 3 10% 19% 10% 2	1730
March	1st to 10th	Day	44% 3 10% 26% 12% 5%	4580
	11th to 20th	Day	43% 4 12% 27% 13% 2	4776
	21st to end	Day	60% 4 12% 13% 10% 1	6101
April	1st to 10th	Day	56% 5% 10% 19% 10%	2520
	11th to 20th	Day	50% 4 11% 21% 12% 3	2494
	21st to end	Day	49% 5% 13% 20% 12% 1	2658
May	1st to 10th	Day	44% 4 8% 28% 15% 1	7651
	11th to 20th	Day	43% 4 10% 27% 16% 1	8217
	21st to end	Day	43% 3 10% 25% 16% 4	9061
June	1st to 10th	Day	36% 4 14% 27% 17% 2	2940
	11th to 20th	Day	37% 4 14% 25% 18% 2	2940
	21st to end	Day	39% 5% 14% 25% 15% 2	2940
July	1st to 10th	Day	42% 2 12% 30% 12% 2	8211
	11th to 20th	Day	32% 2 13% 34% 19% 1	8205
	21st to end	Day	38% 3 13% 25% 20% 1	8794
August	1st to 10th	Day	38% 4 15% 31% 12% 1	4659
	11th to 20th	Day	38% 4 13% 33% 12%	4083
	21st to end	Day	36% 3 13% 27% 20% 1	4419
September	1st to 10th	Day	37% 6% 11% 29% 16% 2	2400
	11th to 20th	Day	33% 4 12% 33% 17% 2	2280
	21st to end	Day	38% 5% 11% 32% 14%	2200
October	1st to 10th	Day	25% 3 6% 42% 24% 1	3468
	11th to 20th	Day	37% 3 9% 29% 21% 1	3186
	21st to end	Day	42% 3 9% 30% 13% 2	3180
November	1st to 10th	Day	44% 3 13% 25% 14% 1	1852
	11th to 20th	Day	52% 4 12% 22% 9% 1	1890
	21st to end	Day	52% 3 12% 25% 7%	1757
December	1st to 10th	Day	47% 2 20% 24% 7% 1	2690
	11th to 20th	Day	57% 3 12% 18% 9% 1	3085
	21st to end	Day	74% 2 6% 11% 7%	4070
January	1st to 10th	Night	56% 2 9% 22% 11%	8233
	11th to 20th	Night	63% 3 8% 10% 15% 1	8280
	21st to end	Night	55% 2 11% 21% 11%	9016
February	1st to 10th	Night	53% 4 10% 19% 12% 2	2723
	11th to 20th	Night	45% 4 10% 24% 15% 2	2459
	21st to end	Night	49% 3 10% 22% 12% 3	2076
March	1st to 10th	Night	46% 3 12% 23% 13% 3	5493
	11th to 20th	Night	36% 3 12% 31% 13% 4	5434
	21st to end	Night	59% 3 9% 16% 12% 1	5072
April	1st to 10th	Night	47% 5% 10% 22% 15% 1	2100
	11th to 20th	Night	51% 3 6% 21% 16% 4	2075
	21st to end	Night	45% 4 13% 22% 15% 1	1960
May	1st to 10th	Night	43% 3 10% 25% 17% 2	5264
	11th to 20th	Night	40% 4 12% 26% 16% 2	4676
	21st to end	Night	43% 3 8% 22% 18% 7%	5149
June	1st to 10th	Night	35% 5% 12% 28% 19% 2	1680
	11th to 20th	Night	37% 4 11% 25% 21% 2	1680
	21st to end	Night	37% 6% 14% 24% 17% 2	1680
July	1st to 10th	Night	41% 2 14% 27% 13% 3	4670
	11th to 20th	Night	27% 2 13% 33% 24% 2	4662
	21st to end	Night	31% 2 13% 29% 23% 2	5002
August	1st to 10th	Night	33% 3 15% 28% 19% 2	2693
	11th to 20th	Night	35% 3 11% 35% 14% 1	3248
	21st to end	Night	30% 3 13% 30% 20% 3	3676
September	1st to 10th	Night	31% 6% 11% 27% 23% 2	2000
	11th to 20th	Night	31% 3 10% 34% 20% 2	2100
	21st to end	Night	38% 4 8% 32% 17% 1	2200
October	1st to 10th	Night	26% 2 6% 39% 25% 1	3710
	11th to 20th	Night	37% 3 10% 26% 22% 2	3953
	21st to end	Night	43% 3 8% 27% 18% 2	4610
November	1st to 10th	Night	41% 4 9% 27% 17% 2	2666
	11th to 20th	Night	48% 4 12% 22% 11% 2	2730
	21st to end	Night	50% 4 14% 22% 10% 1	2861
December	1st to 10th	Night	44% 2 16% 26% 8% 3	4720
	11th to 20th	Night	60% 3 9% 16% 11% 1	5341
	21st to end	Night	69% 2 8% 14% 6%	7150

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	21% 17% 36% 21% 5%	330
	11th to 20th	Day	22% 22% 35% 16% 4 1	330
	21st to end	Day	23% 17% 38% 15% 6%	363
February	1st to 10th	Day	18% 19% 42% 15% 5% 1	330
	11th to 20th	Day	22% 28% 32% 15% 3	330
	21st to end	Day	26% 23% 40% 10% 1	273
March	1st to 10th	Day	23% 25% 41% 10% 2	330
	11th to 20th	Day	29% 21% 31% 14% 5% 1	354
	21st to end	Day	27% 15% 32% 18% 6% 1	594
April	1st to 10th	Day	26% 16% 34% 18% 5%	540
	11th to 20th	Day	24% 22% 27% 17% 8% 2	532
	21st to end	Day	25% 13% 30% 21% 10% 1	540
May	1st to 10th	Day	20% 17% 31% 24% 6% 2	543
	11th to 20th	Day	22% 11% 33% 23% 8% 3	766
	21st to end	Day	17% 8% 34% 24% 11% 5%	844
June	1st to 10th	Day	10% 10% 34% 29% 12% 4	780
	11th to 20th	Day	16% 11% 30% 29% 10% 4	780
	21st to end	Day	15% 7% 35% 29% 11% 3	780
July	1st to 10th	Day	9% 5% 27% 31% 21% 8%	771
	11th to 20th	Day	9% 7% 29% 32% 16% 7%	768
	21st to end	Day	9% 3 31% 37% 14% 6%	849
August	1st to 10th	Day	10% 4 26% 32% 19% 9%	760
	11th to 20th	Day	13% 4 22% 28% 22% 11%	559
	21st to end	Day	11% 5% 23% 32% 22% 7%	619
September	1st to 10th	Day	14% 6% 25% 29% 19% 8%	530
	11th to 20th	Day	10% 2 23% 32% 21% 12%	530
	21st to end	Day	17% 10% 26% 26% 15% 7%	530
October	1st to 10th	Day	17% 12% 25% 27% 15% 4	460
	11th to 20th	Day	25% 17% 31% 21% 6% 1	330
	21st to end	Day	24% 15% 39% 16% 4 1	363
November	1st to 10th	Day	19% 16% 38% 19% 7% 1	330
	11th to 20th	Day	24% 19% 36% 15% 6%	330
	21st to end	Day	25% 15% 40% 15% 4	330
December	1st to 10th	Day	19% 22% 38% 14% 6% 1	330
	11th to 20th	Day	22% 21% 35% 17% 4 1	330
	21st to end	Day	29% 21% 34% 12% 3 1	363
January	1st to 10th	Night	19% 18% 36% 22% 5%	1050
	11th to 20th	Night	21% 17% 36% 19% 6%	1050
	21st to end	Night	20% 12% 38% 25% 5%	1155
February	1st to 10th	Night	20% 16% 36% 22% 6%	1043
	11th to 20th	Night	19% 17% 35% 24% 5%	823
	21st to end	Night	24% 15% 37% 19% 4	692
March	1st to 10th	Night	21% 13% 34% 24% 8% 1	840
	11th to 20th	Night	19% 14% 35% 25% 7%	818
	21st to end	Night	23% 10% 30% 29% 7% 1	693
April	1st to 10th	Night	21% 13% 35% 25% 6%	630
	11th to 20th	Night	26% 11% 30% 25% 7% 1	623
	21st to end	Night	23% 7% 31% 29% 9% 1	630
May	1st to 10th	Night	19% 10% 30% 32% 8% 1	630
	11th to 20th	Night	18% 7% 29% 32% 13% 1	621
	21st to end	Night	15% 6% 24% 34% 19% 2	683
June	1st to 10th	Night	11% 4 27% 40% 17% 1	630
	11th to 20th	Night	14% 5% 33% 31% 16% 1	630
	21st to end	Night	12% 4 25% 41% 17% 1	630
July	1st to 10th	Night	9% 2 20% 38% 28% 3	615
	11th to 20th	Night	8% 2 22% 39% 25% 3	612
	21st to end	Night	8% 2 19% 47% 23% 1	690
August	1st to 10th	Night	9% 2 16% 45% 26% 2	627
	11th to 20th	Night	11% 2 17% 38% 28% 4	624
	21st to end	Night	8% 2 19% 40% 28% 3	687
September	1st to 10th	Night	9% 4 22% 33% 26% 6%	600
	11th to 20th	Night	11% 5% 20% 31% 28% 6%	700
	21st to end	Night	15% 8% 23% 28% 21% 4	800
October	1st to 10th	Night	15% 6% 28% 31% 19% 2	800
	11th to 20th	Night	22% 10% 33% 26% 9%	800
	21st to end	Night	25% 11% 30% 25% 9% 1	880
November	1st to 10th	Night	19% 13% 36% 22% 9% 1	820
	11th to 20th	Night	19% 18% 35% 20% 6%	840
	21st to end	Night	23% 14% 35% 22% 5%	971
December	1st to 10th	Night	22% 16% 36% 21% 4	1050
	11th to 20th	Night	24% 14% 34% 22% 5% 1	1050
	21st to end	Night	30% 13% 35% 19% 3	1155

Page generated at 2026-08-27 03:07:11. Version ac07

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