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 2025-07-21.

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 4 4 5% 7% 39%	120471
	Night	13% 9% 3 8% 4 4 4 3 5% 6% 41%	118881
Transparency	Day	43% 4 12% 24% 15% 2	124095
	Night	43% 4 11% 24% 16% 2	123400
Seeing	Day	16% 10% 28% 27% 15% 4	16791
	Night	16% 9% 31% 31% 12% 1	26633

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% 5% 17% 8% 7% 5% 4 5% 5% 24%	3617
	11th to 20th	Day	8% 9% 3 11% 7% 6% 6% 5% 6% 6% 33%	3804
	21st to end	Day	12% 15% 5% 12% 6% 5% 4 3 4 5% 28%	4488
February	1st to 10th	Day	14% 11% 6% 10% 7% 4 5% 5% 5% 7% 27%	1800
	11th to 20th	Day	14% 10% 4 8% 4 6% 6% 3 5% 8% 33%	1795
	21st to end	Day	14% 9% 4 8% 6% 5% 3 4 4 6% 38%	1650
March	1st to 10th	Day	18% 11% 3 5% 3 3 2 2 4 4 46%	4698
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 6% 8% 48%	4980
	21st to end	Day	20% 11% 4 9% 5% 5% 5% 4 6% 7% 26%	5644
April	1st to 10th	Day	16% 11% 3 8% 5% 5% 4 3 5% 5% 34%	2400
	11th to 20th	Day	17% 8% 3 8% 5% 5% 4 3 4 6% 37%	2374
	21st to end	Day	21% 8% 3 7% 4 4 5% 3 5% 6% 34%	2400
May	1st to 10th	Day	14% 7% 2 8% 5% 5% 5% 4 5% 8% 36%	6010
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 45%	6319
	21st to end	Day	13% 7% 3 7% 4 5% 5% 4 6% 7% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 5% 6% 9% 44%	2800
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 3 6% 8% 47%	2800
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 7% 9% 41%	2800
July	1st to 10th	Day	8% 8% 3 7% 5% 5% 6% 4 7% 9% 37%	4580
	11th to 20th	Day	6% 4 1 5% 4 5% 5% 4 6% 9% 51%	4580
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 12% 40%	4660
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 5% 8% 11% 45%	4104
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	3931
	21st to end	Day	7% 5% 2 6% 4 4 4 4 5% 9% 50%	4180
September	1st to 10th	Day	13% 4 2 4 3 4 4 3 6% 7% 51%	2280
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 6% 9% 52%	2280
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 7% 48%	2014
October	1st to 10th	Day	12% 7% 2 5% 5% 4 3 3 4 6% 49%	1900
	11th to 20th	Day	15% 7% 2 6% 4 3 3 3 4 6% 45%	1900
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 6% 41%	2071
November	1st to 10th	Day	11% 7% 3 6% 4 4 4 3 6% 7% 44%	1762
	11th to 20th	Day	16% 10% 3 6% 4 4 4 3 5% 7% 37%	1715
	21st to end	Day	16% 11% 3 6% 3 4 3 4 5% 6% 40%	1600
December	1st to 10th	Day	8% 10% 3 7% 4 3 4 3 4 7% 48%	2610
	11th to 20th	Day	14% 10% 5% 14% 6% 6% 5% 4 4 6% 26%	3008
	21st to end	Day	24% 20% 7% 12% 5% 3 3 2 3 4 17%	3982
January	1st to 10th	Night	11% 11% 6% 15% 5% 4 4 3 4 5% 31%	6348
	11th to 20th	Night	10% 13% 5% 11% 7% 6% 5% 3 6% 5% 28%	6159
	21st to end	Night	14% 13% 5% 11% 5% 4 4 3 4 5% 32%	6490
February	1st to 10th	Night	15% 12% 4 8% 5% 5% 4 4 5% 6% 34%	2600
	11th to 20th	Night	13% 9% 3 7% 5% 4 4 3 6% 6% 39%	2490
	21st to end	Night	14% 9% 3 9% 4 3 3 2 4 5% 43%	1980
March	1st to 10th	Night	18% 8% 3 6% 4 3 3 3 4 5% 45%	5199
	11th to 20th	Night	11% 6% 2 6% 3 4 4 3 4 6% 52%	4980
	21st to end	Night	17% 12% 5% 10% 5% 5% 5% 4 5% 5% 28%	5284
April	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 43%	2000
	11th to 20th	Night	15% 8% 2 5% 5% 4 4 3 5% 6% 42%	1975
	21st to end	Night	17% 7% 2 6% 4 5% 5% 3 6% 7% 38%	2000
May	1st to 10th	Night	17% 7% 2 7% 4 4 4 3 5% 7% 41%	3896
	11th to 20th	Night	12% 4 2 5% 4 5% 5% 4 6% 7% 45%	3596
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 8% 42%	3942
June	1st to 10th	Night	8% 5% 2 5% 4 4 5% 4 7% 9% 49%	1600
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 7% 10% 47%	1600
	21st to end	Night	8% 5% 3 6% 5% 4 3 4 6% 8% 49%	1600
July	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	2610
	11th to 20th	Night	4 4 1 3 3 3 3 3 6% 9% 61%	2610
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 7% 8% 49%	2900
August	1st to 10th	Night	4 4 1 5% 3 4 4 4 7% 10% 54%	2836
	11th to 20th	Night	7% 4 2 5% 4 3 3 3 5% 7% 55%	2891
	21st to end	Night	7% 4 2 5% 4 4 4 4 5% 9% 55%	3476
September	1st to 10th	Night	10% 4 1 4 3 2 3 3 4 7% 57%	1900
	11th to 20th	Night	10% 5% 1 5% 4 3 4 4 5% 7% 53%	1900
	21st to end	Night	7% 5% 1 5% 4 4 4 4 5% 7% 54%	2142
October	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 6% 53%	2280
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	16% 8% 3 7% 4 5% 3 3 4 5% 42%	2526
November	1st to 10th	Night	10% 7% 2 4 3 3 3 3 4 7% 54%	2536
	11th to 20th	Night	13% 9% 3 7% 4 4 4 3 5% 6% 42%	2680
	21st to end	Night	16% 9% 2 7% 5% 4 4 3 4 6% 39%	2800
December	1st to 10th	Night	8% 9% 3 7% 4 4 4 3 4 5% 49%	4580
	11th to 20th	Night	14% 10% 4 11% 6% 4 5% 3 5% 6% 32%	5199
	21st to end	Night	23% 18% 7% 11% 5% 4 4 3 4 4 18%	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	61% 4 10% 16% 8%	3618
	11th to 20th	Day	52% 3 9% 18% 15% 3	3804
	21st to end	Day	60% 3 11% 19% 7% 1	4488
February	1st to 10th	Day	57% 3 14% 18% 7% 1	1800
	11th to 20th	Day	50% 4 13% 22% 10% 1	1804
	21st to end	Day	48% 4 12% 23% 11% 2	1650
March	1st to 10th	Day	45% 2 9% 25% 17% 2	4698
	11th to 20th	Day	35% 3 10% 32% 18% 2	4980
	21st to end	Day	58% 4 15% 18% 5% 1	5646
April	1st to 10th	Day	51% 4 11% 24% 10%	2400
	11th to 20th	Day	48% 4 10% 24% 12% 2	2374
	21st to end	Day	50% 4 11% 21% 13% 1	2400
May	1st to 10th	Day	46% 4 10% 20% 18% 1	6048
	11th to 20th	Day	40% 4 10% 28% 15% 3	6323
	21st to end	Day	44% 5% 10% 25% 15% 1	6949
June	1st to 10th	Day	35% 6% 16% 26% 15% 2	2800
	11th to 20th	Day	35% 6% 11% 24% 21% 4	2800
	21st to end	Day	37% 8% 14% 23% 16% 2	2800
July	1st to 10th	Day	43% 6% 11% 28% 11% 2	4511
	11th to 20th	Day	30% 7% 14% 31% 18% 2	4505
	21st to end	Day	33% 10% 19% 22% 16% 1	4513
August	1st to 10th	Day	31% 8% 16% 30% 14% 1	4090
	11th to 20th	Day	40% 6% 12% 27% 15% 1	3970
	21st to end	Day	31% 5% 14% 32% 16% 2	4179
September	1st to 10th	Day	32% 5% 10% 27% 21% 5%	2280
	11th to 20th	Day	30% 4 12% 32% 19% 3	2280
	21st to end	Day	34% 5% 9% 30% 19% 2	2014
October	1st to 10th	Day	23% 3 6% 23% 43% 2	3160
	11th to 20th	Day	27% 3 10% 23% 30% 7%	3140
	21st to end	Day	41% 2 9% 28% 17% 3	3397
November	1st to 10th	Day	39% 4 14% 25% 15% 3	1762
	11th to 20th	Day	47% 3 13% 21% 14% 1	1715
	21st to end	Day	47% 4 15% 25% 10%	1600
December	1st to 10th	Day	39% 3 14% 36% 9% 1	2610
	11th to 20th	Day	60% 3 10% 15% 10% 1	3005
	21st to end	Day	74% 3 8% 11% 3	3982
January	1st to 10th	Night	57% 3 12% 20% 7% 1	6348
	11th to 20th	Night	57% 3 12% 12% 15% 1	6159
	21st to end	Night	56% 3 10% 19% 12% 1	6490
February	1st to 10th	Night	51% 3 11% 23% 11% 1	2600
	11th to 20th	Night	45% 5% 12% 25% 12% 2	2491
	21st to end	Night	45% 2 10% 25% 16% 2	1980
March	1st to 10th	Night	44% 3 7% 27% 18% 2	5199
	11th to 20th	Night	35% 3 9% 31% 18% 5%	4980
	21st to end	Night	57% 4 15% 15% 7% 1	5284
April	1st to 10th	Night	43% 3 10% 27% 15% 2	2000
	11th to 20th	Night	43% 4 9% 23% 17% 3	1975
	21st to end	Night	45% 4 11% 23% 13% 3	2000
May	1st to 10th	Night	44% 4 10% 22% 19% 2	3923
	11th to 20th	Night	37% 5% 14% 24% 17% 3	3597
	21st to end	Night	40% 5% 9% 27% 18% 1	3950
June	1st to 10th	Night	30% 6% 16% 29% 16% 3	1600
	11th to 20th	Night	30% 7% 12% 26% 21% 4	1600
	21st to end	Night	33% 7% 14% 23% 20% 4	1600
July	1st to 10th	Night	38% 5% 14% 28% 13% 3	2570
	11th to 20th	Night	21% 4 16% 34% 23% 1	2562
	21st to end	Night	30% 7% 17% 25% 19% 2	2808
August	1st to 10th	Night	24% 6% 18% 33% 17% 2	2826
	11th to 20th	Night	28% 5% 15% 33% 18% 1	2918
	21st to end	Night	28% 6% 13% 31% 17% 5%	3476
September	1st to 10th	Night	28% 5% 8% 29% 26% 4	1900
	11th to 20th	Night	32% 4 10% 30% 20% 5%	1900
	21st to end	Night	29% 5% 9% 31% 24% 3	2142
October	1st to 10th	Night	23% 3 6% 24% 41% 3	3800
	11th to 20th	Night	27% 2 9% 23% 31% 8%	3779
	21st to end	Night	45% 3 7% 22% 19% 4	4150
November	1st to 10th	Night	32% 5% 15% 27% 19% 4	2536
	11th to 20th	Night	43% 3 12% 26% 14% 2	2680
	21st to end	Night	47% 3 12% 25% 12% 1	2800
December	1st to 10th	Night	38% 3 12% 36% 11% 1	4580
	11th to 20th	Night	53% 4 12% 23% 7% 1	5201
	21st to end	Night	71% 3 9% 13% 4	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	20% 14% 33% 22% 9% 2	300
	11th to 20th	Day	21% 17% 41% 17% 4	300
	21st to end	Day	19% 15% 39% 23% 4	330
February	1st to 10th	Day	18% 21% 35% 21% 4	300
	11th to 20th	Day	20% 18% 34% 21% 7%	300
	21st to end	Day	23% 14% 43% 17% 4	249
March	1st to 10th	Day	18% 18% 44% 15% 4	300
	11th to 20th	Day	20% 18% 32% 22% 6% 1	300
	21st to end	Day	32% 15% 33% 14% 5% 1	409
April	1st to 10th	Day	23% 16% 35% 19% 6% 1	500
	11th to 20th	Day	22% 17% 30% 21% 9% 1	492
	21st to end	Day	23% 13% 34% 20% 9% 1	500
May	1st to 10th	Day	19% 12% 34% 24% 8% 2	684
	11th to 20th	Day	21% 11% 27% 25% 13% 3	725
	21st to end	Day	15% 10% 27% 27% 15% 6%	789
June	1st to 10th	Day	8% 5% 26% 34% 20% 7%	730
	11th to 20th	Day	10% 5% 28% 34% 20% 3	730
	21st to end	Day	11% 8% 32% 31% 15% 3	730
July	1st to 10th	Day	8% 3 16% 36% 24% 13%	721
	11th to 20th	Day	11% 4 21% 34% 23% 7%	718
	21st to end	Day	7% 2 20% 36% 28% 6%	740
August	1st to 10th	Day	7% 3 21% 36% 25% 8%	676
	11th to 20th	Day	11% 4 17% 33% 28% 6%	607
	21st to end	Day	9% 2 18% 34% 29% 8%	539
September	1st to 10th	Day	15% 4 18% 32% 24% 7%	490
	11th to 20th	Day	9% 3 17% 38% 23% 10%	490
	21st to end	Day	13% 9% 27% 22% 25% 4	382
October	1st to 10th	Day	17% 8% 30% 26% 14% 5%	300
	11th to 20th	Day	14% 23% 32% 20% 11%	300
	21st to end	Day	27% 14% 30% 23% 5% 1	330
November	1st to 10th	Day	17% 8% 41% 22% 10% 2	300
	11th to 20th	Day	23% 15% 30% 21% 9% 2	300
	21st to end	Day	24% 12% 34% 25% 5%	300
December	1st to 10th	Day	13% 16% 41% 21% 7% 2	300
	11th to 20th	Day	18% 16% 36% 22% 8%	300
	21st to end	Day	28% 15% 39% 15% 3	330
January	1st to 10th	Night	16% 13% 39% 25% 7% 1	1000
	11th to 20th	Night	18% 15% 37% 25% 6%	1000
	21st to end	Night	20% 12% 36% 26% 5%	1100
February	1st to 10th	Night	21% 11% 38% 25% 5%	1000
	11th to 20th	Night	19% 15% 35% 28% 4	936
	21st to end	Night	20% 13% 41% 22% 3 1	660
March	1st to 10th	Night	19% 12% 40% 23% 5% 1	730
	11th to 20th	Night	18% 11% 34% 29% 7%	600
	21st to end	Night	25% 11% 36% 22% 5% 1	660
April	1st to 10th	Night	20% 14% 38% 25% 3	600
	11th to 20th	Night	20% 13% 32% 28% 7%	593
	21st to end	Night	20% 9% 34% 28% 7% 1	600
May	1st to 10th	Night	18% 8% 37% 29% 8% 1	600
	11th to 20th	Night	19% 6% 30% 32% 12%	591
	21st to end	Night	13% 4 31% 33% 18%	650
June	1st to 10th	Night	10% 4 24% 45% 17% 1	600
	11th to 20th	Night	8% 4 26% 44% 17% 1	600
	21st to end	Night	10% 6% 26% 41% 17% 1	600
July	1st to 10th	Night	7% 3 14% 42% 31% 3	585
	11th to 20th	Night	6% 2 19% 43% 27% 3	582
	21st to end	Night	8% 2 14% 47% 28% 2	624
August	1st to 10th	Night	6% 2 15% 45% 28% 3	567
	11th to 20th	Night	9% 2 15% 44% 26% 4	564
	21st to end	Night	10% 1 13% 44% 29% 3	627
September	1st to 10th	Night	13% 2 18% 39% 26% 3	570
	11th to 20th	Night	13% 2 18% 38% 24% 5%	570
	21st to end	Night	8% 6% 27% 36% 21% 2	708
October	1st to 10th	Night	15% 6% 28% 32% 18% 2	760
	11th to 20th	Night	19% 11% 30% 31% 10%	760
	21st to end	Night	20% 11% 34% 27% 7%	836
November	1st to 10th	Night	15% 10% 37% 28% 9% 1	780
	11th to 20th	Night	18% 13% 40% 23% 6%	880
	21st to end	Night	21% 11% 35% 27% 6%	1000
December	1st to 10th	Night	19% 11% 39% 26% 5% 1	1000
	11th to 20th	Night	21% 12% 32% 28% 6%	1000
	21st to end	Night	26% 14% 32% 25% 4	1100

Page generated at 2025-07-18 00:09:59. Version ac07

Copyright © 2025 Attilla Danko.