Mound City Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mound City Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	3 5% 2 6% 4 4 4 4 6% 7% 54%	140163
	Night	2 5% 2 6% 4 4 4 4 6% 8% 54%	139211
Transparency	Day	27% 6% 12% 20% 22% 13%	134911
	Night	26% 7% 14% 24% 20% 10%	135468
Seeing	Day	8% 17% 32% 28% 12% 3	16910
	Night	7% 7% 25% 39% 20% 2	27391

By Month

Percentage Cloud Cover SW (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	1 7% 4 11% 7% 6% 6% 5% 7% 8% 38%	5318
	11th to 20th	Day	1 6% 4 11% 4 4 3 3 4 6% 55%	5340
	21st to end	Day	1 7% 5% 8% 4 3 3 3 4 7% 57%	5844
February	1st to 10th	Day	3 12% 4 10% 5% 5% 5% 5% 7% 6% 37%	1890
	11th to 20th	Day	3 13% 4 10% 7% 5% 6% 5% 7% 6% 34%	1898
	21st to end	Day	3 11% 4 10% 6% 5% 6% 5% 6% 8% 38%	1730
March	1st to 10th	Day	2 8% 5% 11% 6% 5% 6% 4 6% 7% 40%	4974
	11th to 20th	Day	2 7% 2 7% 6% 5% 6% 5% 7% 10% 43%	5110
	21st to end	Day	2 12% 4 10% 7% 6% 6% 5% 7% 8% 33%	5607
April	1st to 10th	Day	3 10% 4 7% 6% 6% 5% 4 6% 8% 42%	2247
	11th to 20th	Day	4 9% 4 8% 5% 5% 5% 4 6% 7% 42%	2497
	21st to end	Day	4 8% 3 6% 6% 5% 5% 4 6% 8% 45%	2600
May	1st to 10th	Day	6% 9% 3 6% 5% 5% 5% 4 6% 7% 43%	5880
	11th to 20th	Day	4 4 2 5% 4 5% 5% 4 8% 10% 50%	5841
	21st to end	Day	3 4 2 5% 4 4 5% 5% 7% 9% 52%	6423
June	1st to 10th	Day	3 3 1 4 4 4 5% 4 6% 9% 57%	2730
	11th to 20th	Day	3 4 1 4 3 3 4 3 4 6% 65%	2800
	21st to end	Day	2 2 1 2 2 3 4 4 5% 5% 70%	2669
July	1st to 10th	Day	2 1 1 2 2 3 3 3 4 5% 74%	5879
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 8% 9% 59%	5880
	21st to end	Day	2 2 1 3 3 3 4 3 5% 6% 70%	6401
August	1st to 10th	Day	2 1 2 2 2 3 3 5% 7% 73%	5779
	11th to 20th	Day	1 1 1 3 2 3 3 3 4 6% 73%	5798
	21st to end	Day	2 2 1 4 4 4 5% 5% 7% 8% 59%	6380
September	1st to 10th	Day	3 2 1 3 3 3 3 3 5% 7% 66%	2284
	11th to 20th	Day	4 3 1 3 3 3 4 3 5% 7% 63%	2200
	21st to end	Day	4 3 1 3 3 3 3 3 4 6% 67%	2200
October	1st to 10th	Day	3 4 1 4 2 3 3 3 4 4 68%	3408
	11th to 20th	Day	5% 5% 1 4 4 3 4 3 6% 7% 57%	3250
	21st to end	Day	3 7% 2 4 3 3 4 4 6% 6% 59%	3548
November	1st to 10th	Day	5% 6% 3 6% 4 5% 4 4 6% 7% 50%	1874
	11th to 20th	Day	5% 9% 3 4 4 4 5% 3 6% 7% 49%	1890
	21st to end	Day	2 7% 4 7% 6% 5% 5% 4 6% 7% 48%	1706
December	1st to 10th	Day	2 5% 2 5% 3 4 3 3 6% 8% 60%	2700
	11th to 20th	Day	2 7% 2 8% 5% 5% 5% 5% 7% 9% 45%	3106
	21st to end	Day	2 8% 4 10% 7% 5% 7% 5% 7% 8% 36%	4482
January	1st to 10th	Night	1 7% 4 11% 6% 6% 5% 5% 7% 8% 41%	7605
	11th to 20th	Night	1 6% 4 11% 5% 4 4 4 6% 5% 51%	7650
	21st to end	Night	1 7% 4 8% 4 3 4 3 6% 8% 53%	8370
February	1st to 10th	Night	3 10% 4 9% 5% 5% 6% 4 7% 10% 38%	2730
	11th to 20th	Night	2 10% 3 8% 6% 5% 5% 4 7% 8% 41%	2604
	21st to end	Night	2 8% 3 8% 6% 5% 5% 5% 7% 8% 43%	2076
March	1st to 10th	Night	2 9% 3 9% 6% 6% 6% 4 7% 8% 40%	5138
	11th to 20th	Night	1 7% 2 6% 4 5% 6% 5% 8% 9% 46%	5110
	21st to end	Night	1 8% 4 7% 5% 5% 4 4 6% 7% 48%	5610
April	1st to 10th	Night	4 10% 3 6% 5% 4 4 3 5% 7% 49%	2176
	11th to 20th	Night	3 6% 1 4 5% 4 6% 6% 7% 7% 50%	1848
	21st to end	Night	5% 7% 2 6% 4 4 4 4 7% 8% 49%	1800
May	1st to 10th	Night	3 7% 2 7% 5% 5% 4 4 5% 7% 50%	4100
	11th to 20th	Night	2 3 1 4 3 3 4 4 6% 8% 62%	4074
	21st to end	Night	3 4 2 4 4 3 3 3 5% 8% 61%	4474
June	1st to 10th	Night	3 4 1 4 4 4 4 4 6% 8% 59%	1660
	11th to 20th	Night	3 3 1 4 2 2 4 4 4 6% 68%	1600
	21st to end	Night	1 1 1 3 2 3 3 4 4 8% 71%	1721
July	1st to 10th	Night	1 1 1 3 4 3 4 3 4 4 71%	4100
	11th to 20th	Night	2 2 1 4 3 4 5% 4 7% 10% 59%	4100
	21st to end	Night	1 1 1 3 3 3 4 4 5% 7% 68%	4472
August	1st to 10th	Night	2 1 2 2 3 3 3 5% 7% 71%	4033
	11th to 20th	Night	1 2 1 4 3 4 4 4 5% 6% 67%	4051
	21st to end	Night	2 2 1 4 3 4 5% 5% 7% 7% 61%	4596
September	1st to 10th	Night	2 3 1 3 3 3 3 3 5% 6% 69%	2112
	11th to 20th	Night	4 2 1 3 3 3 4 4 4 8% 64%	2200
	21st to end	Night	5% 3 1 3 3 3 3 3 3 6% 70%	2200
October	1st to 10th	Night	3 3 1 2 2 2 2 3 3 4 74%	3784
	11th to 20th	Night	4 5% 1 4 3 3 4 4 7% 7% 58%	3898
	21st to end	Night	3 4 2 4 3 2 3 3 6% 8% 61%	4252
November	1st to 10th	Night	3 5% 2 5% 5% 3 4 4 5% 8% 55%	2641
	11th to 20th	Night	4 8% 2 4 4 3 4 4 6% 10% 51%	2730
	21st to end	Night	3 7% 3 7% 4 5% 5% 3 6% 9% 49%	2888
December	1st to 10th	Night	1 6% 3 5% 3 3 3 3 4 8% 62%	4710
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 7% 9% 49%	5321
	21st to end	Night	3 9% 3 11% 7% 6% 7% 5% 7% 9% 33%	6777

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	38% 12% 19% 19% 10% 1	5321
	11th to 20th	Day	30% 6% 14% 28% 16% 6%	5340
	21st to end	Day	28% 5% 19% 24% 22% 2	5844
February	1st to 10th	Day	45% 7% 15% 19% 12% 2	1890
	11th to 20th	Day	45% 7% 14% 20% 12% 1	1907
	21st to end	Day	43% 6% 14% 23% 11% 2	1730
March	1st to 10th	Day	38% 8% 14% 21% 14% 7%	4974
	11th to 20th	Day	30% 10% 18% 28% 13% 1	5110
	21st to end	Day	45% 8% 19% 18% 8% 1	5607
April	1st to 10th	Day	39% 6% 15% 20% 15% 4	2247
	11th to 20th	Day	40% 5% 13% 23% 17% 2	2500
	21st to end	Day	37% 6% 12% 26% 19% 2	2600
May	1st to 10th	Day	34% 6% 12% 20% 22% 5%	5880
	11th to 20th	Day	25% 8% 13% 26% 25% 3	5844
	21st to end	Day	24% 5% 13% 24% 30% 4	6424
June	1st to 10th	Day	22% 6% 10% 23% 30% 9%	2730
	11th to 20th	Day	21% 4 7% 17% 30% 22%	2800
	21st to end	Day	15% 3 6% 11% 32% 34%	2669
July	1st to 10th	Day	12% 4 4 7% 17% 57%	5816
	11th to 20th	Day	15% 5% 8% 13% 27% 31%	5811
	21st to end	Day	14% 4 5% 9% 25% 43%	6217
August	1st to 10th	Day	15% 5% 8% 21% 27% 25%	4208
	11th to 20th	Day	16% 5% 7% 16% 32% 24%	4197
	21st to end	Day	16% 5% 7% 14% 29% 30%	4599
September	1st to 10th	Day	18% 4 7% 17% 28% 27%	2284
	11th to 20th	Day	20% 4 8% 18% 28% 21%	2200
	21st to end	Day	19% 3 7% 21% 36% 14%	2200
October	1st to 10th	Day	19% 4 5% 16% 37% 19%	3408
	11th to 20th	Day	25% 5% 8% 29% 27% 7%	3250
	21st to end	Day	23% 6% 13% 27% 27% 5%	3548
November	1st to 10th	Day	31% 5% 14% 24% 21% 5%	1874
	11th to 20th	Day	32% 5% 15% 28% 18% 2	1890
	21st to end	Day	35% 5% 15% 26% 15% 4	1706
December	1st to 10th	Day	22% 6% 15% 42% 13% 2	2700
	11th to 20th	Day	31% 11% 18% 26% 12% 1	3104
	21st to end	Day	41% 10% 13% 16% 18% 2	4482
January	1st to 10th	Night	38% 13% 20% 19% 10% 1	7605
	11th to 20th	Night	31% 7% 16% 30% 14% 2	7650
	21st to end	Night	27% 9% 16% 26% 21% 2	8370
February	1st to 10th	Night	40% 8% 18% 22% 10% 2	2730
	11th to 20th	Night	38% 7% 15% 22% 16% 1	2605
	21st to end	Night	36% 6% 15% 28% 12% 3	2076
March	1st to 10th	Night	35% 8% 15% 24% 14% 3	5138
	11th to 20th	Night	28% 11% 19% 27% 14% 1	5110
	21st to end	Night	32% 8% 21% 25% 12% 1	5610
April	1st to 10th	Night	33% 5% 17% 22% 18% 5%	2176
	11th to 20th	Night	29% 6% 16% 25% 21% 3	1848
	21st to end	Night	31% 6% 16% 27% 19% 1	1800
May	1st to 10th	Night	30% 6% 14% 23% 25% 2	4097
	11th to 20th	Night	17% 8% 13% 32% 25% 5%	4075
	21st to end	Night	20% 4 11% 31% 33% 2	4474
June	1st to 10th	Night	22% 4 11% 24% 29% 9%	1660
	11th to 20th	Night	16% 4 7% 20% 31% 22%	1600
	21st to end	Night	13% 2 6% 13% 31% 35%	1721
July	1st to 10th	Night	16% 3 5% 7% 15% 54%	4055
	11th to 20th	Night	18% 5% 7% 12% 26% 31%	4046
	21st to end	Night	15% 4 7% 11% 26% 36%	4344
August	1st to 10th	Night	17% 4 10% 20% 24% 25%	2928
	11th to 20th	Night	22% 5% 8% 16% 26% 22%	2928
	21st to end	Night	14% 4 9% 14% 29% 29%	3308
September	1st to 10th	Night	16% 4 7% 18% 26% 29%	2112
	11th to 20th	Night	17% 4 11% 21% 26% 21%	2200
	21st to end	Night	17% 4 6% 20% 37% 16%	2200
October	1st to 10th	Night	15% 3 7% 20% 34% 22%	3784
	11th to 20th	Night	23% 4 9% 30% 27% 7%	3898
	21st to end	Night	19% 5% 12% 32% 29% 3	4252
November	1st to 10th	Night	27% 5% 14% 28% 21% 5%	2641
	11th to 20th	Night	27% 6% 15% 32% 17% 3	2730
	21st to end	Night	32% 5% 15% 27% 18% 4	2888
December	1st to 10th	Night	21% 6% 16% 40% 15% 2	4710
	11th to 20th	Night	29% 9% 25% 26% 11%	5322
	21st to end	Night	42% 10% 17% 18% 12% 1	6777

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	10% 17% 36% 21% 12% 3	330
	11th to 20th	Day	5% 15% 35% 28% 13% 2	330
	21st to end	Day	5% 14% 35% 31% 13% 2	363
February	1st to 10th	Day	15% 19% 38% 20% 7% 1	330
	11th to 20th	Day	10% 22% 38% 24% 5% 1	383
	21st to end	Day	9% 26% 36% 23% 6% 1	446
March	1st to 10th	Day	11% 22% 39% 20% 7% 1	540
	11th to 20th	Day	10% 26% 33% 24% 6% 1	540
	21st to end	Day	14% 29% 32% 21% 4	594
April	1st to 10th	Day	10% 26% 31% 21% 9% 2	505
	11th to 20th	Day	14% 27% 34% 17% 7% 1	492
	21st to end	Day	11% 27% 30% 23% 8% 1	500
May	1st to 10th	Day	11% 22% 35% 22% 9% 1	503
	11th to 20th	Day	11% 20% 33% 22% 10% 3	528
	21st to end	Day	9% 17% 32% 24% 14% 4	573
June	1st to 10th	Day	6% 14% 29% 31% 17% 4	660
	11th to 20th	Day	8% 15% 32% 28% 16% 1	730
	21st to end	Day	4 15% 31% 35% 12% 3	599
July	1st to 10th	Day	2 10% 35% 39% 11% 3	526
	11th to 20th	Day	3 6% 36% 37% 14% 4	524
	21st to end	Day	4 7% 31% 37% 17% 5%	575
August	1st to 10th	Day	4 11% 35% 31% 15% 3	527
	11th to 20th	Day	3 10% 30% 36% 15% 5%	517
	21st to end	Day	4 9% 31% 34% 18% 5%	583
September	1st to 10th	Day	4 10% 27% 34% 17% 8%	530
	11th to 20th	Day	7% 16% 30% 27% 16% 5%	530
	21st to end	Day	6% 15% 24% 32% 18% 5%	530
October	1st to 10th	Day	7% 16% 27% 26% 15% 8%	416
	11th to 20th	Day	10% 16% 24% 25% 19% 5%	330
	21st to end	Day	9% 15% 33% 30% 12% 2	363
November	1st to 10th	Day	12% 18% 28% 24% 15% 4	330
	11th to 20th	Day	6% 19% 29% 30% 12% 4	330
	21st to end	Day	8% 11% 31% 28% 18% 5%	330
December	1st to 10th	Day	5% 13% 34% 28% 15% 4	330
	11th to 20th	Day	6% 13% 35% 35% 9% 2	330
	21st to end	Day	12% 19% 29% 29% 9% 1	363
January	1st to 10th	Night	8% 8% 31% 33% 18% 2	840
	11th to 20th	Night	8% 7% 29% 39% 15% 3	840
	21st to end	Night	7% 6% 30% 39% 15% 2	924
February	1st to 10th	Night	9% 8% 32% 39% 10% 2	840
	11th to 20th	Night	7% 8% 34% 36% 13% 2	824
	21st to end	Night	6% 10% 32% 40% 11% 1	692
March	1st to 10th	Night	8% 9% 33% 37% 11% 2	840
	11th to 20th	Night	5% 9% 31% 38% 16% 2	840
	21st to end	Night	8% 12% 32% 35% 12% 1	924
April	1st to 10th	Night	11% 12% 24% 39% 13% 1	769
	11th to 20th	Night	7% 11% 33% 37% 12%	593
	21st to end	Night	10% 9% 28% 33% 19% 2	600
May	1st to 10th	Night	8% 7% 27% 41% 15% 1	600
	11th to 20th	Night	7% 6% 23% 38% 25% 2	591
	21st to end	Night	8% 6% 23% 37% 22% 3	650
June	1st to 10th	Night	5% 8% 19% 41% 25% 2	600
	11th to 20th	Night	5% 11% 24% 42% 19%	600
	21st to end	Night	2 3 22% 41% 30% 2	600
July	1st to 10th	Night	3 3 17% 36% 38% 3	585
	11th to 20th	Night	4 3 11% 48% 31% 3	582
	21st to end	Night	2 1 12% 38% 45% 2	657
August	1st to 10th	Night	5% 2 16% 44% 32% 1	597
	11th to 20th	Night	3 2 16% 44% 33% 3	594
	21st to end	Night	4 2 14% 46% 33% 2	660
September	1st to 10th	Night	4 3 15% 40% 34% 5%	712
	11th to 20th	Night	5% 5% 18% 38% 29% 6%	800
	21st to end	Night	6% 4 16% 33% 34% 7%	800
October	1st to 10th	Night	9% 5% 21% 39% 24% 3	800
	11th to 20th	Night	7% 6% 19% 41% 23% 4	800
	21st to end	Night	7% 7% 25% 37% 21% 4	880
November	1st to 10th	Night	6% 9% 24% 37% 21% 3	820
	11th to 20th	Night	8% 6% 28% 40% 15% 4	840
	21st to end	Night	7% 8% 31% 34% 17% 3	998
December	1st to 10th	Night	6% 7% 33% 38% 14% 1	1050
	11th to 20th	Night	10% 8% 31% 40% 10% 1	1050
	21st to end	Night	11% 13% 30% 35% 10% 1	999

Page generated at 2026-03-30 00:18:42. Version ac07

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