Iowa City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Iowa City clear sky chart from 2005-11-06 to 2026-04-17.

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% 5% 5% 4 6% 7% 38%	131988
	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 40%	130477
Transparency	Day	43% 6% 12% 24% 14% 2	135554
	Night	42% 5% 12% 23% 15% 2	134988
Seeing	Day	15% 10% 29% 27% 14% 4	17732
	Night	16% 9% 31% 31% 11% 1	27725

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% 4 17% 8% 7% 6% 4 5% 6% 23%	4691
	11th to 20th	Day	6% 9% 4 10% 8% 7% 7% 5% 7% 7% 29%	4890
	21st to end	Day	10% 13% 5% 12% 7% 6% 6% 3 4 5% 29%	5812
February	1st to 10th	Day	12% 12% 5% 10% 7% 5% 5% 4 5% 8% 28%	1890
	11th to 20th	Day	14% 12% 3 7% 5% 5% 4 3 5% 7% 33%	1886
	21st to end	Day	12% 10% 4 11% 6% 4 3 3 5% 6% 36%	1730
March	1st to 10th	Day	19% 11% 3 5% 3 2 2 2 3 5% 46%	4767
	11th to 20th	Day	9% 6% 3 6% 4 4 5% 3 5% 7% 48%	5090
	21st to end	Day	20% 12% 4 9% 5% 5% 5% 4 5% 6% 26%	5788
April	1st to 10th	Day	16% 13% 3 8% 5% 5% 4 3 4 5% 33%	2520
	11th to 20th	Day	16% 9% 4 7% 6% 5% 4 4 6% 7% 33%	2446
	21st to end	Day	21% 9% 3 7% 5% 4 4 3 5% 6% 35%	2400
May	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	6073
	11th to 20th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	6319
	21st to end	Day	14% 7% 3 7% 5% 4 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	9% 5% 2 6% 5% 4 6% 5% 7% 9% 44%	2800
	11th to 20th	Day	10% 5% 2 7% 4 5% 4 4 7% 8% 45%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 40%	2800
July	1st to 10th	Day	8% 7% 3 7% 5% 5% 5% 4 7% 9% 39%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 4 4 4 6% 8% 44%	6360
	21st to end	Day	10% 6% 3 7% 4 5% 6% 5% 6% 10% 37%	6751
August	1st to 10th	Day	6% 5% 2 6% 4 4 5% 4 8% 10% 44%	4234
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 8% 9% 41%	4072
	21st to end	Day	8% 5% 2 5% 4 3 5% 4 6% 8% 51%	4312
September	1st to 10th	Day	12% 3 2 5% 3 3 4 4 5% 8% 50%	2400
	11th to 20th	Day	10% 3 2 5% 4 4 4 3 5% 9% 50%	2400
	21st to end	Day	10% 6% 2 5% 4 4 4 4 5% 8% 47%	2122
October	1st to 10th	Day	13% 7% 2 6% 4 3 3 3 4 5% 50%	2000
	11th to 20th	Day	15% 7% 3 7% 5% 3 4 3 5% 7% 43%	2000
	21st to end	Day	16% 8% 3 7% 4 4 3 3 5% 6% 41%	2165
November	1st to 10th	Day	11% 9% 3 5% 4 4 4 4 6% 6% 44%	1852
	11th to 20th	Day	15% 11% 3 7% 4 3 4 2 5% 8% 36%	1795
	21st to end	Day	16% 11% 3 7% 4 3 4 3 4 6% 38%	1680
December	1st to 10th	Day	9% 8% 3 9% 4 3 4 3 4 7% 47%	2690
	11th to 20th	Day	14% 11% 4 13% 7% 5% 5% 4 5% 5% 26%	3088
	21st to end	Day	23% 22% 8% 11% 5% 4 3 2 3 4 16%	4070
January	1st to 10th	Night	10% 12% 6% 14% 5% 4 4 3 4 5% 32%	8232
	11th to 20th	Night	8% 12% 5% 11% 7% 7% 6% 4 6% 5% 29%	8085
	21st to end	Night	11% 12% 5% 11% 5% 4 3 3 4 7% 35%	8402
February	1st to 10th	Night	13% 12% 4 8% 5% 4 4 4 5% 5% 34%	2730
	11th to 20th	Night	12% 10% 3 7% 5% 5% 4 3 5% 6% 40%	2619
	21st to end	Night	13% 10% 4 8% 5% 3 4 3 4 4 41%	2076
March	1st to 10th	Night	18% 8% 3 6% 3 3 3 3 4 5% 45%	5338
	11th to 20th	Night	10% 7% 2 6% 3 4 3 3 4 7% 51%	5090
	21st to end	Night	18% 12% 5% 10% 5% 5% 5% 3 5% 5% 27%	5383
April	1st to 10th	Night	16% 10% 3 7% 4 3 4 3 4 6% 40%	2100
	11th to 20th	Night	15% 9% 3 7% 4 3 4 4 4 7% 40%	2035
	21st to end	Night	17% 8% 2 5% 4 5% 4 5% 5% 7% 38%	2000
May	1st to 10th	Night	16% 7% 2 7% 4 3 4 3 5% 6% 43%	3838
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 7% 46%	3596
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 7% 42%	3942
June	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 7% 8% 48%	1600
	11th to 20th	Night	7% 5% 3 6% 4 4 4 4 7% 10% 44%	1600
	21st to end	Night	10% 5% 2 6% 3 4 5% 3 6% 8% 48%	1600
July	1st to 10th	Night	10% 4 2 7% 4 4 5% 4 7% 11% 41%	3618
	11th to 20th	Night	7% 4 2 6% 3 3 4 4 6% 9% 52%	3617
	21st to end	Night	9% 6% 2 7% 4 4 4 4 7% 8% 43%	4117
August	1st to 10th	Night	4 3 2 5% 4 5% 5% 4 7% 8% 53%	2926
	11th to 20th	Night	6% 4 2 6% 4 4 4 4 5% 7% 54%	2982
	21st to end	Night	7% 4 2 5% 4 3 4 4 6% 8% 53%	3586
September	1st to 10th	Night	9% 4 2 4 3 3 3 4 5% 7% 55%	2000
	11th to 20th	Night	10% 5% 2 4 4 4 4 3 5% 7% 52%	2000
	21st to end	Night	7% 5% 2 6% 4 3 4 3 5% 7% 54%	2255
October	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 7% 53%	2400
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 6% 48%	2400
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 3 5% 43%	2673
November	1st to 10th	Night	11% 7% 2 4 3 3 3 3 5% 6% 52%	2666
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 3 5% 5% 42%	2822
	21st to end	Night	16% 9% 3 8% 4 4 4 3 4 6% 39%	2940
December	1st to 10th	Night	8% 8% 3 7% 5% 4 4 3 5% 6% 47%	4720
	11th to 20th	Night	14% 11% 4 11% 6% 4 4 3 4 6% 32%	5339
	21st to end	Night	23% 17% 7% 12% 6% 5% 4 3 3 4 16%	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	60% 6% 12% 15% 7%	4692
	11th to 20th	Day	51% 5% 10% 19% 14% 2	4890
	21st to end	Day	56% 4 12% 20% 7%	5812
February	1st to 10th	Day	54% 6% 14% 19% 7% 1	1890
	11th to 20th	Day	50% 5% 13% 21% 9% 2	1895
	21st to end	Day	49% 5% 12% 22% 9% 2	1730
March	1st to 10th	Day	43% 4 9% 26% 15% 3	4767
	11th to 20th	Day	35% 5% 10% 32% 17% 1	5090
	21st to end	Day	58% 7% 14% 16% 5% 1	5790
April	1st to 10th	Day	53% 4 11% 22% 10%	2520
	11th to 20th	Day	48% 6% 11% 22% 11% 2	2446
	21st to end	Day	49% 5% 11% 20% 13% 2	2400
May	1st to 10th	Day	44% 7% 11% 20% 18% 1	6111
	11th to 20th	Day	38% 6% 11% 27% 14% 3	6323
	21st to end	Day	43% 7% 12% 24% 14% 1	6949
June	1st to 10th	Day	34% 9% 15% 24% 16% 2	2800
	11th to 20th	Day	35% 7% 11% 22% 21% 3	2800
	21st to end	Day	39% 8% 13% 23% 15% 2	2800
July	1st to 10th	Day	38% 7% 14% 28% 12% 1	6291
	11th to 20th	Day	35% 8% 13% 28% 15% 1	6285
	21st to end	Day	38% 10% 17% 20% 14% 1	6558
August	1st to 10th	Day	31% 9% 18% 28% 13% 1	4220
	11th to 20th	Day	37% 8% 15% 26% 14% 1	4111
	21st to end	Day	31% 7% 13% 31% 16% 2	4311
September	1st to 10th	Day	31% 6% 10% 27% 22% 4	2400
	11th to 20th	Day	30% 6% 13% 30% 18% 3	2400
	21st to end	Day	34% 6% 10% 30% 19% 2	2122
October	1st to 10th	Day	23% 3 6% 29% 38% 2	3260
	11th to 20th	Day	28% 3 11% 23% 28% 6%	3240
	21st to end	Day	39% 4 10% 28% 15% 3	3479
November	1st to 10th	Day	38% 6% 14% 25% 14% 3	1852
	11th to 20th	Day	47% 4 14% 21% 13% 1	1795
	21st to end	Day	47% 6% 14% 24% 10%	1680
December	1st to 10th	Day	38% 4 14% 36% 8% 1	2690
	11th to 20th	Day	58% 5% 12% 14% 9% 1	3085
	21st to end	Day	73% 4 9% 10% 3	4070
January	1st to 10th	Night	54% 5% 16% 18% 7%	8233
	11th to 20th	Night	53% 5% 13% 15% 14% 1	8085
	21st to end	Night	50% 4 13% 20% 11% 1	8402
February	1st to 10th	Night	50% 5% 12% 22% 11% 1	2730
	11th to 20th	Night	44% 5% 12% 25% 12% 2	2620
	21st to end	Night	45% 4 11% 23% 15% 2	2076
March	1st to 10th	Night	43% 3 7% 27% 18% 2	5338
	11th to 20th	Night	34% 4 9% 31% 18% 4	5090
	21st to end	Night	58% 7% 12% 15% 7% 1	5383
April	1st to 10th	Night	45% 4 9% 26% 14% 2	2100
	11th to 20th	Night	44% 5% 10% 22% 16% 3	2035
	21st to end	Night	44% 6% 12% 22% 13% 3	2000
May	1st to 10th	Night	42% 6% 10% 21% 19% 2	3865
	11th to 20th	Night	35% 8% 13% 23% 18% 3	3597
	21st to end	Night	39% 6% 11% 27% 15% 1	3950
June	1st to 10th	Night	31% 8% 13% 27% 17% 3	1600
	11th to 20th	Night	32% 6% 13% 25% 20% 4	1600
	21st to end	Night	33% 7% 16% 22% 18% 5%	1600
July	1st to 10th	Night	35% 8% 14% 29% 13% 2	3580
	11th to 20th	Night	27% 7% 16% 30% 20% 1	3572
	21st to end	Night	35% 8% 18% 23% 14% 2	3999
August	1st to 10th	Night	26% 8% 16% 32% 16% 2	2916
	11th to 20th	Night	28% 8% 14% 32% 17% 1	3009
	21st to end	Night	28% 7% 12% 30% 18% 5%	3586
September	1st to 10th	Night	28% 6% 11% 27% 26% 3	2000
	11th to 20th	Night	32% 5% 9% 32% 19% 4	2000
	21st to end	Night	30% 5% 9% 28% 25% 3	2255
October	1st to 10th	Night	23% 3 6% 26% 37% 3	3920
	11th to 20th	Night	27% 3 10% 22% 32% 6%	3899
	21st to end	Night	46% 4 8% 21% 18% 4	4309
November	1st to 10th	Night	33% 5% 14% 27% 17% 4	2666
	11th to 20th	Night	44% 4 12% 25% 13% 2	2822
	21st to end	Night	47% 4 11% 26% 11% 1	2940
December	1st to 10th	Night	37% 4 13% 34% 11% 1	4720
	11th to 20th	Night	53% 5% 12% 22% 6% 1	5341
	21st to end	Night	71% 5% 9% 10% 4	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	20% 13% 33% 25% 7% 2	330
	11th to 20th	Day	18% 18% 42% 18% 4	330
	21st to end	Day	18% 14% 42% 20% 4 1	363
February	1st to 10th	Day	16% 19% 38% 21% 5%	330
	11th to 20th	Day	23% 18% 33% 20% 6%	330
	21st to end	Day	18% 19% 43% 16% 3	273
March	1st to 10th	Day	20% 19% 42% 13% 6%	330
	11th to 20th	Day	21% 19% 32% 21% 6% 1	330
	21st to end	Day	30% 16% 34% 15% 4 1	452
April	1st to 10th	Day	23% 17% 36% 19% 5% 1	540
	11th to 20th	Day	21% 19% 30% 20% 9% 1	516
	21st to end	Day	23% 14% 34% 18% 9% 1	500
May	1st to 10th	Day	19% 11% 37% 24% 7% 2	689
	11th to 20th	Day	19% 13% 27% 27% 12% 2	725
	21st to end	Day	15% 9% 28% 28% 14% 5%	789
June	1st to 10th	Day	8% 6% 28% 33% 20% 5%	730
	11th to 20th	Day	12% 6% 28% 33% 18% 3	730
	21st to end	Day	12% 8% 31% 31% 14% 5%	730
July	1st to 10th	Day	8% 4 17% 33% 25% 13%	721
	11th to 20th	Day	9% 3 24% 34% 23% 7%	718
	21st to end	Day	7% 4 23% 37% 24% 6%	794
August	1st to 10th	Day	6% 3 23% 37% 23% 7%	726
	11th to 20th	Day	9% 5% 19% 34% 25% 8%	655
	21st to end	Day	8% 4 18% 34% 27% 8%	583
September	1st to 10th	Day	12% 7% 18% 31% 25% 7%	530
	11th to 20th	Day	8% 4 18% 38% 23% 9%	530
	21st to end	Day	12% 8% 26% 24% 24% 5%	422
October	1st to 10th	Day	17% 8% 27% 29% 14% 5%	330
	11th to 20th	Day	18% 20% 35% 18% 9%	330
	21st to end	Day	26% 15% 30% 23% 5% 1	363
November	1st to 10th	Day	16% 11% 42% 19% 8% 3	330
	11th to 20th	Day	22% 16% 34% 19% 7% 2	330
	21st to end	Day	23% 12% 35% 23% 6%	330
December	1st to 10th	Day	14% 13% 42% 23% 6% 2	330
	11th to 20th	Day	17% 15% 35% 25% 8%	330
	21st to end	Day	30% 15% 36% 16% 3	363
January	1st to 10th	Night	15% 12% 39% 27% 7%	1050
	11th to 20th	Night	17% 16% 36% 26% 6%	1050
	21st to end	Night	18% 12% 39% 26% 5%	1155
February	1st to 10th	Night	20% 12% 38% 26% 4	1050
	11th to 20th	Night	17% 14% 36% 29% 5%	985
	21st to end	Night	20% 12% 41% 22% 3 1	692
March	1st to 10th	Night	20% 12% 38% 23% 6% 1	776
	11th to 20th	Night	18% 12% 34% 29% 7%	630
	21st to end	Night	22% 13% 36% 24% 4 1	693
April	1st to 10th	Night	20% 14% 40% 22% 3	630
	11th to 20th	Night	20% 14% 32% 27% 7%	611
	21st to end	Night	20% 10% 32% 29% 8% 1	600
May	1st to 10th	Night	17% 7% 38% 29% 7% 1	600
	11th to 20th	Night	18% 8% 28% 34% 11%	591
	21st to end	Night	14% 4 32% 33% 16% 1	650
June	1st to 10th	Night	10% 4 25% 44% 16% 1	600
	11th to 20th	Night	9% 4 28% 45% 15%	600
	21st to end	Night	13% 3 29% 38% 17% 1	600
July	1st to 10th	Night	9% 2 11% 42% 32% 4	585
	11th to 20th	Night	5% 3 21% 43% 26% 3	582
	21st to end	Night	7% 3 19% 43% 27% 2	657
August	1st to 10th	Night	5% 3 16% 46% 29% 2	597
	11th to 20th	Night	6% 3 15% 43% 29% 5%	594
	21st to end	Night	8% 2 15% 44% 28% 3	660
September	1st to 10th	Night	10% 5% 19% 38% 25% 3	600
	11th to 20th	Night	12% 2 18% 40% 24% 5%	600
	21st to end	Night	8% 7% 26% 36% 20% 2	750
October	1st to 10th	Night	15% 6% 26% 32% 19% 2	800
	11th to 20th	Night	18% 11% 31% 30% 10%	800
	21st to end	Night	21% 12% 33% 28% 7%	880
November	1st to 10th	Night	16% 9% 37% 29% 9%	820
	11th to 20th	Night	18% 14% 39% 23% 6%	932
	21st to end	Night	21% 12% 33% 26% 6%	1050
December	1st to 10th	Night	18% 12% 38% 26% 5% 1	1050
	11th to 20th	Night	21% 11% 34% 28% 6%	1050
	21st to end	Night	26% 14% 34% 23% 3	1155

Page generated at 2026-04-14 01:44:25. Version ac07

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