Indiana Dunes State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Indiana Dunes State Park clear sky chart from 2005-11-06 to 2025-09-04.

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% 10% 4 8% 5% 4 4 4 5% 7% 35%	125895
	Night	15% 12% 5% 9% 5% 4 4 3 5% 6% 33%	123037
Transparency	Day	46% 5% 12% 22% 12% 1	129434
	Night	50% 6% 10% 20% 13% 1	127572
Seeing	Day	18% 12% 30% 24% 12% 4	16932
	Night	20% 12% 29% 26% 12% 1	26712

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	12% 16% 11% 18% 7% 5% 3 4 5% 5% 14%	3617
	11th to 20th	Day	12% 17% 8% 15% 7% 8% 5% 7% 5% 5% 13%	3620
	21st to end	Day	18% 19% 5% 9% 4 3 3 3 4 7% 24%	3982
February	1st to 10th	Day	15% 15% 6% 12% 7% 5% 4 4 4 6% 22%	1914
	11th to 20th	Day	14% 13% 4 12% 7% 5% 4 3 5% 5% 29%	1948
	21st to end	Day	14% 16% 5% 11% 5% 4 4 3 4 6% 28%	1650
March	1st to 10th	Day	18% 11% 5% 10% 4 3 3 3 4 5% 36%	4480
	11th to 20th	Day	8% 9% 5% 8% 5% 4 5% 3 6% 7% 41%	4699
	21st to end	Day	16% 16% 6% 12% 6% 5% 4 4 5% 5% 22%	5875
April	1st to 10th	Day	16% 12% 4 7% 5% 4 3 4 5% 7% 33%	2400
	11th to 20th	Day	15% 11% 3 7% 4 4 4 3 4 8% 37%	2374
	21st to end	Day	17% 9% 3 7% 4 4 4 4 6% 7% 33%	2560
May	1st to 10th	Day	18% 8% 3 6% 4 5% 5% 4 6% 9% 34%	5862
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 41%	5953
	21st to end	Day	14% 8% 3 6% 4 3 4 3 5% 8% 42%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 6% 7% 45%	2800
	11th to 20th	Day	9% 5% 1 6% 5% 5% 5% 5% 6% 8% 46%	2800
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 42%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 10% 39%	6360
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 11% 44%	6360
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 7% 9% 42%	6924
August	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 6% 8% 49%	4342
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 5% 8% 48%	3982
	21st to end	Day	7% 4 2 6% 4 4 5% 3 6% 9% 50%	4312
September	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 3 5% 7% 47%	2316
	11th to 20th	Day	11% 5% 2 5% 3 5% 4 4 6% 8% 48%	2113
	21st to end	Day	12% 7% 4 6% 4 4 5% 4 6% 9% 40%	2090
October	1st to 10th	Day	13% 7% 3 6% 4 4 4 3 5% 6% 46%	2090
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 34%	1862
	21st to end	Day	21% 11% 4 8% 4 5% 5% 3 5% 7% 27%	1881
November	1st to 10th	Day	13% 11% 2 7% 4 4 5% 3 6% 5% 39%	1762
	11th to 20th	Day	17% 15% 5% 9% 6% 4 4 3 5% 6% 27%	1800
	21st to end	Day	19% 16% 5% 11% 5% 3 3 3 4 5% 28%	1800
December	1st to 10th	Day	10% 16% 5% 16% 8% 4 3 3 4 5% 26%	2642
	11th to 20th	Day	14% 19% 9% 14% 5% 4 4 3 6% 6% 16%	3008
	21st to end	Day	23% 21% 7% 14% 6% 4 4 3 3 3 10%	3982
January	1st to 10th	Night	15% 19% 9% 15% 7% 4 3 3 4 4 16%	6348
	11th to 20th	Night	14% 20% 8% 15% 6% 5% 5% 5% 4 6% 12%	6360
	21st to end	Night	19% 16% 7% 9% 4 4 4 3 4 5% 26%	6996
February	1st to 10th	Night	17% 15% 6% 9% 6% 4 5% 4 4 6% 25%	2482
	11th to 20th	Night	15% 13% 5% 9% 7% 5% 5% 4 5% 5% 28%	2338
	21st to end	Night	18% 14% 5% 10% 5% 3 4 3 4 4 30%	1980
March	1st to 10th	Night	15% 12% 6% 8% 5% 4 4 3 4 5% 34%	5373
	11th to 20th	Night	11% 8% 4 6% 4 4 4 3 5% 6% 44%	5261
	21st to end	Night	17% 13% 7% 8% 5% 4 4 4 4 5% 29%	5082
April	1st to 10th	Night	15% 11% 4 6% 4 4 4 4 5% 7% 35%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 5% 4 4 4 6% 38%	1975
	21st to end	Night	17% 9% 4 8% 5% 5% 4 3 4 6% 36%	1839
May	1st to 10th	Night	19% 10% 3 7% 4 4 5% 3 5% 7% 33%	4033
	11th to 20th	Night	15% 7% 2 6% 3 4 4 3 5% 8% 42%	3929
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 5% 41%	3942
June	1st to 10th	Night	10% 6% 1 5% 4 4 4 3 6% 10% 48%	1600
	11th to 20th	Night	9% 6% 3 7% 5% 4 4 3 6% 10% 44%	1600
	21st to end	Night	9% 7% 2 7% 5% 5% 5% 5% 5% 8% 42%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 4 6% 4 7% 9% 43%	3618
	11th to 20th	Night	8% 5% 2 6% 5% 4 4 4 6% 8% 48%	3617
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 4 7% 8% 44%	3947
August	1st to 10th	Night	9% 4 2 6% 5% 4 5% 4 7% 9% 46%	2809
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 8% 51%	3063
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 7% 51%	3586
September	1st to 10th	Night	11% 5% 2 6% 3 4 4 2 5% 7% 51%	1930
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 8% 53%	2047
	21st to end	Night	12% 7% 2 5% 4 4 5% 4 6% 7% 44%	2090
October	1st to 10th	Night	12% 6% 2 5% 4 5% 4 3 5% 6% 48%	2090
	11th to 20th	Night	21% 9% 2 7% 3 4 3 3 4 6% 39%	2312
	21st to end	Night	23% 11% 3 6% 4 3 4 3 4 7% 33%	2717
November	1st to 10th	Night	14% 13% 4 7% 4 4 4 3 4 6% 38%	2536
	11th to 20th	Night	14% 17% 4 10% 5% 4 4 4 5% 6% 27%	2600
	21st to end	Night	19% 15% 5% 11% 5% 3 4 3 4 6% 27%	2600
December	1st to 10th	Night	12% 13% 6% 16% 8% 5% 5% 3 6% 5% 22%	4542
	11th to 20th	Night	15% 19% 8% 14% 6% 5% 4 3 5% 6% 15%	5199
	21st to end	Night	23% 22% 7% 11% 6% 5% 4 3 4 4 11%	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	70% 7% 9% 11% 4	3618
	11th to 20th	Day	68% 9% 8% 7% 6% 1	3620
	21st to end	Day	61% 3 13% 17% 6%	3982
February	1st to 10th	Day	62% 6% 12% 12% 7% 1	1914
	11th to 20th	Day	56% 6% 11% 19% 8% 1	1958
	21st to end	Day	58% 5% 11% 17% 7% 2	1650
March	1st to 10th	Day	51% 5% 8% 23% 10% 2	4480
	11th to 20th	Day	39% 7% 12% 32% 8% 1	4699
	21st to end	Day	63% 6% 12% 12% 6% 1	5877
April	1st to 10th	Day	51% 5% 11% 21% 12%	2400
	11th to 20th	Day	46% 5% 11% 22% 13% 3	2374
	21st to end	Day	49% 5% 14% 18% 12% 2	2560
May	1st to 10th	Day	47% 6% 11% 23% 11% 2	5897
	11th to 20th	Day	41% 6% 12% 27% 13% 1	5957
	21st to end	Day	40% 5% 11% 25% 17% 1	6949
June	1st to 10th	Day	37% 6% 11% 25% 18% 2	2800
	11th to 20th	Day	35% 5% 12% 26% 19% 3	2800
	21st to end	Day	37% 7% 17% 21% 16% 2	2800
July	1st to 10th	Day	39% 5% 13% 26% 15% 2	6291
	11th to 20th	Day	36% 4 15% 30% 14% 1	6285
	21st to end	Day	38% 5% 16% 23% 16% 2	6728
August	1st to 10th	Day	33% 5% 15% 31% 14% 1	4327
	11th to 20th	Day	34% 5% 11% 33% 16% 1	4021
	21st to end	Day	30% 6% 13% 27% 20% 3	4311
September	1st to 10th	Day	35% 6% 13% 26% 17% 4	2316
	11th to 20th	Day	33% 5% 13% 28% 20% 2	2113
	21st to end	Day	39% 7% 10% 24% 18% 1	2090
October	1st to 10th	Day	28% 4 9% 34% 24% 1	3480
	11th to 20th	Day	41% 4 11% 21% 21%	3065
	21st to end	Day	48% 4 9% 25% 11% 2	3081
November	1st to 10th	Day	43% 6% 13% 21% 14% 1	1762
	11th to 20th	Day	58% 6% 10% 17% 9% 1	1800
	21st to end	Day	59% 6% 9% 19% 7%	1800
December	1st to 10th	Day	60% 4 14% 17% 3	2642
	11th to 20th	Day	67% 4 12% 14% 3	3005
	21st to end	Day	79% 5% 7% 8% 2	3982
January	1st to 10th	Night	71% 6% 7% 12% 4	6348
	11th to 20th	Night	71% 8% 7% 6% 7%	6360
	21st to end	Night	60% 4 11% 15% 8%	6996
February	1st to 10th	Night	60% 6% 10% 15% 8% 2	2482
	11th to 20th	Night	55% 7% 10% 18% 10% 1	2339
	21st to end	Night	57% 6% 9% 17% 10% 2	1980
March	1st to 10th	Night	51% 5% 9% 25% 8% 1	5373
	11th to 20th	Night	39% 6% 12% 28% 13% 2	5261
	21st to end	Night	56% 5% 9% 20% 9%	5082
April	1st to 10th	Night	47% 5% 12% 21% 13% 1	2000
	11th to 20th	Night	48% 5% 8% 20% 17% 3	1975
	21st to end	Night	49% 6% 10% 19% 14% 2	1839
May	1st to 10th	Night	51% 5% 9% 18% 14% 4	4060
	11th to 20th	Night	40% 6% 14% 25% 14% 2	3930
	21st to end	Night	46% 5% 9% 19% 19% 3	3950
June	1st to 10th	Night	31% 7% 13% 28% 19% 3	1600
	11th to 20th	Night	35% 6% 12% 24% 21% 2	1600
	21st to end	Night	38% 8% 15% 22% 15% 3	1600
July	1st to 10th	Night	37% 5% 12% 30% 13% 4	3580
	11th to 20th	Night	33% 5% 14% 22% 23% 2	3572
	21st to end	Night	36% 6% 15% 23% 17% 3	3832
August	1st to 10th	Night	34% 6% 14% 30% 16% 1	2800
	11th to 20th	Night	32% 5% 13% 27% 21% 1	3090
	21st to end	Night	32% 5% 10% 30% 20% 3	3586
September	1st to 10th	Night	34% 5% 12% 25% 19% 5%	1930
	11th to 20th	Night	30% 6% 8% 28% 26% 2	2047
	21st to end	Night	38% 6% 9% 27% 18% 3	2090
October	1st to 10th	Night	23% 5% 8% 35% 27% 2	3480
	11th to 20th	Night	44% 4 6% 20% 25% 1	3848
	21st to end	Night	42% 4 9% 25% 19% 1	4467
November	1st to 10th	Night	46% 6% 8% 22% 15% 2	2536
	11th to 20th	Night	55% 9% 9% 17% 9% 2	2600
	21st to end	Night	58% 6% 10% 19% 6%	2600
December	1st to 10th	Night	62% 5% 12% 17% 4	4542
	11th to 20th	Night	67% 8% 10% 11% 4	5201
	21st to end	Night	76% 5% 8% 8% 3	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	24% 20% 36% 17% 3	300
	11th to 20th	Day	30% 17% 37% 11% 3 1	300
	21st to end	Day	25% 21% 36% 16% 2	330
February	1st to 10th	Day	22% 26% 35% 13% 3	300
	11th to 20th	Day	26% 27% 31% 14% 3	300
	21st to end	Day	25% 27% 36% 10% 1	249
March	1st to 10th	Day	25% 23% 35% 15% 1	300
	11th to 20th	Day	25% 19% 34% 17% 5% 1	375
	21st to end	Day	29% 18% 31% 17% 5%	550
April	1st to 10th	Day	27% 16% 34% 17% 5% 1	500
	11th to 20th	Day	23% 21% 30% 16% 6% 3	492
	21st to end	Day	24% 15% 34% 18% 8% 1	500
May	1st to 10th	Day	24% 12% 34% 19% 8% 2	503
	11th to 20th	Day	21% 10% 32% 22% 14% 2	567
	21st to end	Day	15% 8% 32% 27% 13% 5%	789
June	1st to 10th	Day	12% 8% 30% 31% 15% 4	730
	11th to 20th	Day	11% 8% 32% 30% 16% 3	730
	21st to end	Day	13% 7% 32% 32% 12% 4	730
July	1st to 10th	Day	12% 4 25% 28% 21% 10%	721
	11th to 20th	Day	11% 5% 26% 31% 19% 7%	718
	21st to end	Day	10% 4 25% 37% 19% 6%	794
August	1st to 10th	Day	9% 2 19% 34% 24% 11%	596
	11th to 20th	Day	10% 4 25% 32% 21% 8%	519
	21st to end	Day	11% 4 20% 34% 22% 8%	583
September	1st to 10th	Day	14% 5% 23% 31% 20% 7%	502
	11th to 20th	Day	11% 5% 17% 33% 23% 11%	490
	21st to end	Day	18% 12% 20% 25% 18% 7%	490
October	1st to 10th	Day	16% 8% 29% 29% 15% 3	490
	11th to 20th	Day	23% 19% 30% 22% 7%	324
	21st to end	Day	32% 15% 35% 15% 3	330
November	1st to 10th	Day	17% 15% 40% 20% 6% 2	300
	11th to 20th	Day	27% 19% 33% 16% 4	300
	21st to end	Day	32% 15% 39% 13% 2	300
December	1st to 10th	Day	22% 24% 34% 15% 5%	300
	11th to 20th	Day	22% 23% 38% 13% 3	300
	21st to end	Day	27% 22% 36% 13% 1	330
January	1st to 10th	Night	24% 21% 35% 17% 3	1000
	11th to 20th	Night	25% 20% 35% 16% 4 1	1000
	21st to end	Night	25% 17% 36% 19% 3	1100
February	1st to 10th	Night	26% 14% 37% 19% 3	878
	11th to 20th	Night	22% 19% 34% 20% 4	783
	21st to end	Night	26% 17% 39% 15% 3	660
March	1st to 10th	Night	23% 16% 36% 19% 6% 1	800
	11th to 20th	Night	19% 13% 38% 24% 6%	800
	21st to end	Night	25% 12% 31% 26% 5%	715
April	1st to 10th	Night	21% 16% 35% 22% 6% 1	600
	11th to 20th	Night	23% 12% 31% 25% 8% 1	593
	21st to end	Night	21% 11% 31% 29% 8% 1	600
May	1st to 10th	Night	22% 6% 33% 28% 11% 1	600
	11th to 20th	Night	21% 4 27% 32% 15% 1	591
	21st to end	Night	13% 5% 22% 38% 20% 2	650
June	1st to 10th	Night	12% 5% 23% 41% 19% 1	600
	11th to 20th	Night	12% 4 28% 36% 18% 2	600
	21st to end	Night	10% 6% 25% 39% 19% 2	600
July	1st to 10th	Night	9% 2 16% 39% 29% 6%	585
	11th to 20th	Night	9% 4 20% 37% 26% 5%	582
	21st to end	Night	7% 2 17% 43% 30% 2	657
August	1st to 10th	Night	7% 2 12% 45% 29% 4	597
	11th to 20th	Night	9% 3 16% 37% 29% 6%	594
	21st to end	Night	9% 3 17% 40% 27% 4	660
September	1st to 10th	Night	12% 5% 17% 35% 27% 3	579
	11th to 20th	Night	10% 7% 17% 35% 27% 5%	717
	21st to end	Night	14% 6% 25% 29% 21% 4	760
October	1st to 10th	Night	15% 7% 28% 30% 19% 2	760
	11th to 20th	Night	25% 15% 29% 23% 8%	760
	21st to end	Night	27% 17% 29% 21% 6%	836
November	1st to 10th	Night	23% 16% 33% 18% 10% 1	780
	11th to 20th	Night	24% 22% 32% 17% 5%	800
	21st to end	Night	28% 18% 32% 17% 4	800
December	1st to 10th	Night	26% 21% 35% 16% 2	975
	11th to 20th	Night	25% 19% 35% 20% 2	1000
	21st to end	Night	30% 17% 35% 16% 2	1100

Page generated at 2025-09-01 00:51:07. Version ac07

Copyright © 2025 Attilla Danko.