Iowa SkyNet Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Iowa SkyNet Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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% 8% 3 7% 6% 3 5% 5% 5% 7% 38%	78862
	Night	13% 8% 4 7% 5% 3 4 4 5% 6% 40%	81316
Transparency	Day	46% 5% 12% 23% 13% 2	78567
	Night	44% 4 12% 23% 14% 2	81131
Seeing	Day	16% 9% 28% 27% 16% 4	14890
	Night	17% 9% 31% 31% 12% 1	24953

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	9% 10% 6% 15% 9% 6% 6% 5% 6% 6% 22%	2527
	11th to 20th	Day	12% 12% 4 10% 11% 5% 6% 6% 6% 5% 24%	2658
	21st to end	Day	15% 18% 7% 15% 7% 4 3 3 4 5% 19%	3135
February	1st to 10th	Day	14% 11% 6% 9% 8% 4 4 6% 5% 7% 28%	1710
	11th to 20th	Day	14% 9% 4 6% 5% 5% 6% 4 5% 8% 34%	1702
	21st to end	Day	14% 9% 4 7% 7% 3 4 4 4 6% 36%	1570
March	1st to 10th	Day	14% 10% 4 8% 5% 3 4 3 5% 5% 38%	1983
	11th to 20th	Day	12% 10% 4 6% 7% 3 5% 4 6% 7% 36%	2090
	21st to end	Day	22% 11% 4 7% 6% 3 5% 4 4 6% 28%	2374
April	1st to 10th	Day	16% 10% 4 7% 5% 3 4 4 5% 5% 34%	2184
	11th to 20th	Day	17% 8% 3 7% 6% 3 4 4 4 7% 37%	2134
	21st to end	Day	22% 8% 3 6% 5% 2 5% 4 5% 6% 34%	2160
May	1st to 10th	Day	18% 7% 3 6% 6% 4 5% 5% 5% 8% 31%	2390
	11th to 20th	Day	18% 7% 3 7% 6% 3 5% 4 5% 6% 36%	2479
	21st to end	Day	14% 6% 3 7% 6% 4 6% 6% 6% 8% 34%	2725
June	1st to 10th	Day	10% 4 2 5% 5% 3 5% 5% 6% 9% 45%	2520
	11th to 20th	Day	8% 4 2 6% 5% 4 5% 4 6% 8% 48%	2520
	21st to end	Day	11% 5% 2 6% 6% 4 5% 6% 8% 9% 39%	2520
July	1st to 10th	Day	8% 4 2 4 4 3 4 5% 6% 10% 51%	2520
	11th to 20th	Day	9% 4 2 4 4 4 5% 6% 6% 9% 47%	2520
	21st to end	Day	6% 4 2 4 5% 3 5% 6% 8% 11% 47%	2656
August	1st to 10th	Day	6% 5% 2 5% 6% 4 6% 8% 8% 10% 41%	2328
	11th to 20th	Day	9% 5% 3 6% 5% 3 4 6% 6% 9% 44%	2247
	21st to end	Day	9% 5% 2 6% 5% 3 3 5% 5% 8% 51%	2376
September	1st to 10th	Day	14% 4 2 3 4 3 4 5% 6% 7% 49%	2160
	11th to 20th	Day	10% 3 2 4 4 2 3 4 6% 8% 53%	2160
	21st to end	Day	10% 4 2 5% 5% 3 4 5% 6% 7% 48%	1909
October	1st to 10th	Day	13% 7% 2 4 6% 3 4 4 4 6% 47%	1800
	11th to 20th	Day	16% 7% 3 5% 6% 3 4 4 4 6% 43%	1800
	21st to end	Day	18% 8% 2 6% 5% 3 4 4 5% 6% 41%	1962
November	1st to 10th	Day	11% 7% 3 5% 5% 3 5% 4 5% 7% 46%	1672
	11th to 20th	Day	16% 9% 3 5% 5% 3 4 4 5% 7% 37%	1630
	21st to end	Day	17% 9% 4 5% 4 3 3 4 4 6% 40%	1520
December	1st to 10th	Day	14% 13% 4 8% 6% 3 5% 4 4 6% 34%	1520
	11th to 20th	Day	13% 10% 6% 8% 6% 4 5% 4 4 7% 34%	1918
	21st to end	Day	22% 17% 8% 11% 6% 3 3 4 3 4 18%	2783
January	1st to 10th	Night	12% 11% 7% 13% 7% 3 4 4 4 5% 29%	4428
	11th to 20th	Night	12% 15% 6% 10% 6% 3 5% 4 8% 5% 24%	4309
	21st to end	Night	18% 16% 6% 11% 7% 3 3 4 4 5% 24%	4532
February	1st to 10th	Night	15% 11% 5% 7% 5% 3 5% 4 5% 6% 34%	2470
	11th to 20th	Night	13% 8% 4 6% 6% 3 4 4 6% 6% 40%	2363
	21st to end	Night	15% 9% 4 7% 6% 3 3 3 4 5% 42%	1884
March	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 4 6% 43%	2214
	11th to 20th	Night	14% 6% 3 8% 5% 4 5% 4 4 7% 40%	2090
	21st to end	Night	20% 10% 5% 9% 6% 3 5% 5% 5% 5% 27%	2223
April	1st to 10th	Night	16% 8% 3 6% 4 2 4 4 5% 6% 41%	1820
	11th to 20th	Night	17% 8% 3 4 5% 3 4 4 5% 6% 41%	1775
	21st to end	Night	18% 7% 3 5% 5% 3 5% 4 5% 6% 39%	1800
May	1st to 10th	Night	16% 6% 3 6% 5% 3 4 4 5% 9% 39%	1562
	11th to 20th	Night	18% 5% 2 5% 4 4 4 6% 5% 7% 40%	1416
	21st to end	Night	10% 6% 2 5% 7% 3 5% 4 5% 9% 42%	1551
June	1st to 10th	Night	8% 4 2 5% 4 3 5% 5% 7% 9% 49%	1440
	11th to 20th	Night	8% 4 2 6% 5% 2 5% 5% 7% 9% 48%	1440
	21st to end	Night	8% 4 3 4 5% 3 4 5% 6% 8% 49%	1440
July	1st to 10th	Night	6% 4 1 4 4 2 5% 5% 6% 10% 53%	1440
	11th to 20th	Night	4 4 2 2 3 1 3 4 6% 9% 61%	1440
	21st to end	Night	7% 3 2 4 5% 3 5% 5% 7% 8% 52%	1658
August	1st to 10th	Night	4 4 2 4 5% 3 6% 5% 7% 9% 51%	1611
	11th to 20th	Night	7% 4 2 4 5% 3 4 5% 5% 7% 54%	1654
	21st to end	Night	8% 3 1 3 4 2 4 4 4 8% 58%	1980
September	1st to 10th	Night	11% 4 2 4 4 1 4 4 4 7% 55%	1800
	11th to 20th	Night	11% 5% 1 4 4 2 3 4 5% 7% 54%	1800
	21st to end	Night	7% 4 2 4 4 3 4 4 5% 8% 54%	2028
October	1st to 10th	Night	12% 5% 2 4 4 2 4 4 4 6% 52%	2160
	11th to 20th	Night	14% 7% 2 4 4 2 4 4 5% 7% 47%	2160
	21st to end	Night	16% 7% 3 6% 5% 4 4 3 4 6% 42%	2393
November	1st to 10th	Night	10% 6% 2 3 3 2 3 3 4 7% 55%	2406
	11th to 20th	Night	13% 8% 3 6% 5% 3 4 4 5% 6% 43%	2546
	21st to end	Night	16% 8% 3 6% 6% 3 4 4 4 6% 40%	2660
December	1st to 10th	Night	14% 10% 4 7% 6% 3 5% 4 4 6% 37%	2660
	11th to 20th	Night	14% 9% 5% 8% 6% 3 4 4 4 6% 37%	3279
	21st to end	Night	21% 15% 7% 9% 6% 3 4 4 4 4 21%	4884

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	62% 4 11% 15% 7%	2528
	11th to 20th	Day	61% 3 11% 18% 7%	2658
	21st to end	Day	70% 3 9% 12% 5%	3135
February	1st to 10th	Day	56% 3 14% 19% 7% 1	1710
	11th to 20th	Day	49% 4 13% 23% 10% 1	1711
	21st to end	Day	49% 4 12% 22% 10% 2	1570
March	1st to 10th	Day	50% 3 14% 21% 11% 1	1983
	11th to 20th	Day	48% 4 14% 22% 12% 1	2090
	21st to end	Day	58% 4 12% 18% 6% 1	2374
April	1st to 10th	Day	51% 4 11% 24% 10%	2184
	11th to 20th	Day	49% 4 9% 25% 11% 2	2134
	21st to end	Day	50% 5% 10% 21% 13% 1	2160
May	1st to 10th	Day	51% 5% 12% 22% 9% 1	2390
	11th to 20th	Day	49% 5% 12% 22% 12% 1	2483
	21st to end	Day	46% 7% 12% 19% 13% 2	2725
June	1st to 10th	Day	35% 6% 16% 26% 15% 2	2520
	11th to 20th	Day	34% 6% 11% 23% 22% 4	2520
	21st to end	Day	38% 8% 15% 22% 16% 2	2520
July	1st to 10th	Day	29% 6% 14% 33% 15% 3	2451
	11th to 20th	Day	33% 9% 14% 26% 15% 3	2445
	21st to end	Day	29% 8% 18% 28% 16% 1	2508
August	1st to 10th	Day	34% 10% 16% 27% 12% 1	2314
	11th to 20th	Day	35% 7% 13% 29% 15% 2	2247
	21st to end	Day	31% 5% 11% 29% 19% 3	2376
September	1st to 10th	Day	34% 5% 10% 27% 19% 5%	2160
	11th to 20th	Day	29% 4 12% 32% 19% 3	2160
	21st to end	Day	34% 5% 10% 31% 19% 2	1909
October	1st to 10th	Day	38% 4 10% 25% 22% 1	1800
	11th to 20th	Day	42% 4 9% 25% 17% 3	1800
	21st to end	Day	46% 3 12% 23% 15% 2	1962
November	1st to 10th	Day	38% 4 14% 25% 16% 3	1672
	11th to 20th	Day	47% 3 13% 22% 14% 1	1630
	21st to end	Day	47% 4 14% 25% 10%	1520
December	1st to 10th	Day	53% 4 13% 21% 8% 1	1520
	11th to 20th	Day	51% 4 13% 21% 10% 1	1915
	21st to end	Day	72% 3 7% 12% 4	2783
January	1st to 10th	Night	60% 3 10% 18% 9% 1	4428
	11th to 20th	Night	60% 3 12% 13% 11% 1	4309
	21st to end	Night	64% 3 11% 16% 6% 1	4532
February	1st to 10th	Night	51% 3 10% 23% 11% 1	2470
	11th to 20th	Night	44% 5% 12% 25% 12% 2	2364
	21st to end	Night	46% 2 10% 25% 15% 2	1884
March	1st to 10th	Night	43% 4 12% 27% 13% 1	2214
	11th to 20th	Night	44% 4 13% 23% 13% 3	2090
	21st to end	Night	58% 4 12% 16% 8% 1	2223
April	1st to 10th	Night	44% 4 10% 25% 14% 2	1820
	11th to 20th	Night	44% 4 8% 23% 17% 3	1775
	21st to end	Night	45% 4 10% 23% 13% 4	1800
May	1st to 10th	Night	42% 5% 14% 26% 13% 1	1562
	11th to 20th	Night	42% 6% 14% 22% 15% 1	1417
	21st to end	Night	38% 7% 12% 23% 17% 3	1552
June	1st to 10th	Night	30% 6% 16% 28% 17% 3	1440
	11th to 20th	Night	30% 7% 13% 25% 21% 5%	1440
	21st to end	Night	32% 6% 15% 24% 19% 4	1440
July	1st to 10th	Night	27% 6% 16% 31% 16% 5%	1400
	11th to 20th	Night	22% 4 16% 34% 21% 3	1392
	21st to end	Night	28% 5% 14% 31% 19% 3	1566
August	1st to 10th	Night	27% 8% 18% 31% 14% 3	1601
	11th to 20th	Night	28% 6% 13% 30% 20% 2	1654
	21st to end	Night	26% 5% 11% 29% 22% 7%	1980
September	1st to 10th	Night	29% 5% 9% 28% 25% 4	1800
	11th to 20th	Night	31% 4 10% 31% 20% 5%	1800
	21st to end	Night	28% 5% 9% 32% 24% 3	2028
October	1st to 10th	Night	34% 4 11% 24% 24% 3	2160
	11th to 20th	Night	38% 3 11% 25% 20% 4	2160
	21st to end	Night	45% 4 11% 21% 18% 2	2393
November	1st to 10th	Night	30% 5% 15% 27% 19% 4	2406
	11th to 20th	Night	42% 3 12% 27% 14% 2	2546
	21st to end	Night	46% 3 12% 25% 13% 1	2660
December	1st to 10th	Night	49% 4 12% 24% 10% 1	2660
	11th to 20th	Night	49% 4 12% 26% 9% 1	3281
	21st to end	Night	67% 3 9% 15% 6%	4884

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% 14% 32% 22% 9% 2	270
	11th to 20th	Day	22% 16% 39% 18% 4	270
	21st to end	Day	20% 14% 36% 25% 5%	297
February	1st to 10th	Day	19% 20% 35% 21% 4	270
	11th to 20th	Day	20% 18% 32% 23% 7%	270
	21st to end	Day	22% 14% 43% 17% 4	225
March	1st to 10th	Day	18% 19% 43% 16% 5%	270
	11th to 20th	Day	21% 17% 33% 22% 7% 1	270
	21st to end	Day	33% 15% 31% 15% 6% 1	372
April	1st to 10th	Day	23% 15% 35% 20% 7% 1	428
	11th to 20th	Day	20% 16% 31% 22% 10% 1	412
	21st to end	Day	23% 12% 33% 20% 10% 1	420
May	1st to 10th	Day	21% 13% 33% 24% 8% 2	585
	11th to 20th	Day	21% 11% 28% 25% 12% 3	625
	21st to end	Day	16% 9% 26% 26% 17% 7%	679
June	1st to 10th	Day	8% 4 25% 34% 22% 8%	630
	11th to 20th	Day	10% 5% 26% 35% 21% 3	630
	21st to end	Day	11% 9% 30% 31% 16% 3	630
July	1st to 10th	Day	7% 2 16% 35% 25% 14%	621
	11th to 20th	Day	11% 4 20% 34% 24% 7%	618
	21st to end	Day	6% 2 21% 36% 29% 6%	684
August	1st to 10th	Day	8% 2 19% 37% 26% 8%	626
	11th to 20th	Day	10% 4 16% 33% 30% 7%	561
	21st to end	Day	9% 2 18% 34% 28% 9%	495
September	1st to 10th	Day	16% 4 18% 31% 24% 7%	450
	11th to 20th	Day	9% 3 18% 39% 22% 10%	450
	21st to end	Day	12% 9% 27% 23% 25% 4	348
October	1st to 10th	Day	19% 8% 30% 26% 13% 5%	270
	11th to 20th	Day	15% 24% 32% 19% 10%	270
	21st to end	Day	28% 13% 30% 22% 6% 1	297
November	1st to 10th	Day	15% 7% 43% 21% 10% 3	270
	11th to 20th	Day	21% 15% 30% 22% 9% 2	270
	21st to end	Day	24% 11% 34% 26% 5%	270
December	1st to 10th	Day	14% 16% 38% 22% 8% 3	270
	11th to 20th	Day	16% 16% 36% 23% 9%	270
	21st to end	Day	25% 17% 39% 15% 3	297
January	1st to 10th	Night	15% 13% 39% 24% 7% 1	950
	11th to 20th	Night	18% 13% 37% 25% 6%	950
	21st to end	Night	20% 11% 36% 27% 6%	1045
February	1st to 10th	Night	22% 10% 37% 25% 5%	950
	11th to 20th	Night	18% 13% 35% 29% 4	889
	21st to end	Night	21% 13% 40% 22% 4 1	628
March	1st to 10th	Night	18% 12% 40% 23% 6% 1	694
	11th to 20th	Night	18% 11% 34% 30% 7%	570
	21st to end	Night	25% 11% 36% 23% 5% 1	627
April	1st to 10th	Night	21% 12% 38% 26% 3	546
	11th to 20th	Night	21% 12% 31% 29% 7%	533
	21st to end	Night	20% 9% 34% 29% 8% 1	540
May	1st to 10th	Night	18% 8% 36% 28% 9% 1	540
	11th to 20th	Night	19% 7% 30% 32% 12%	531
	21st to end	Night	12% 4 31% 33% 20%	584
June	1st to 10th	Night	10% 3 23% 46% 18% 1	540
	11th to 20th	Night	9% 4 25% 45% 17% 1	540
	21st to end	Night	10% 4 25% 42% 18% 1	540
July	1st to 10th	Night	6% 2 14% 41% 32% 3	525
	11th to 20th	Night	5% 2 19% 44% 26% 3	522
	21st to end	Night	8% 2 14% 48% 28% 2	591
August	1st to 10th	Night	6% 2 14% 46% 29% 3	537
	11th to 20th	Night	8% 2 14% 44% 27% 4	534
	21st to end	Night	10% 1 13% 44% 29% 3	594
September	1st to 10th	Night	13% 2 18% 39% 25% 3	540
	11th to 20th	Night	13% 2 19% 36% 25% 6%	540
	21st to end	Night	9% 6% 26% 36% 21% 2	670
October	1st to 10th	Night	15% 6% 28% 30% 18% 2	720
	11th to 20th	Night	19% 10% 30% 30% 9%	720
	21st to end	Night	21% 11% 34% 27% 8%	792
November	1st to 10th	Night	14% 10% 38% 28% 9% 1	740
	11th to 20th	Night	17% 13% 40% 24% 6%	836
	21st to end	Night	22% 10% 34% 27% 6%	950
December	1st to 10th	Night	19% 10% 38% 27% 5% 1	950
	11th to 20th	Night	21% 12% 32% 29% 6%	950
	21st to end	Night	25% 14% 33% 24% 3	1045

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