Decatur(IL)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Decatur(IL) clear sky chart from 2005-11-06 to 2025-10-04.

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% 9% 3 7% 5% 4 4 4 6% 8% 38%	126473
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 39%	123134
Transparency	Day	44% 4 12% 24% 15% 2	130019
	Night	44% 4 11% 24% 16% 2	127662
Seeing	Day	16% 10% 28% 26% 15% 5%	16968
	Night	17% 10% 30% 29% 13% 1	26719

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	13% 11% 7% 15% 6% 5% 5% 4 5% 6% 23%	3617
	11th to 20th	Day	10% 11% 7% 12% 8% 8% 8% 5% 5% 4 21%	3620
	21st to end	Day	18% 18% 6% 10% 6% 4 3 3 3 4 26%	4414
February	1st to 10th	Day	15% 12% 6% 11% 5% 5% 4 3 4 6% 29%	1849
	11th to 20th	Day	14% 11% 4 9% 7% 5% 5% 3 5% 6% 31%	1948
	21st to end	Day	16% 13% 4 9% 5% 4 3 3 4 6% 33%	1650
March	1st to 10th	Day	18% 14% 3 7% 3 3 3 2 3 4 40%	4480
	11th to 20th	Day	8% 11% 3 6% 3 3 4 4 5% 8% 45%	4766
	21st to end	Day	17% 12% 4 9% 5% 5% 4 4 5% 6% 29%	5938
April	1st to 10th	Day	15% 10% 3 6% 5% 4 4 3 6% 6% 36%	2400
	11th to 20th	Day	16% 10% 3 6% 4 4 4 3 4 6% 40%	2374
	21st to end	Day	19% 8% 3 6% 4 4 4 4 6% 6% 36%	2401
May	1st to 10th	Day	18% 9% 3 7% 4 5% 5% 4 6% 9% 30%	5862
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 3 5% 8% 38%	6089
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 8% 37%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 9% 44%	2800
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 4 6% 8% 48%	2800
	21st to end	Day	7% 6% 2 7% 5% 6% 5% 5% 7% 10% 41%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 5% 4 4 6% 11% 44%	6360
	11th to 20th	Day	7% 6% 3 8% 6% 5% 6% 5% 7% 11% 36%	6360
	21st to end	Day	10% 6% 2 7% 4 4 6% 5% 8% 11% 37%	6924
August	1st to 10th	Day	8% 4 2 4 4 4 4 4 6% 10% 51%	4191
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 5% 8% 10% 41%	3849
	21st to end	Day	5% 4 2 5% 5% 4 5% 4 7% 10% 48%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 5% 8% 51%	2305
	21st to end	Day	8% 6% 2 5% 4 4 4 4 6% 7% 50%	2200
October	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 7% 51%	2036
	11th to 20th	Day	16% 7% 2 5% 3 4 5% 3 4 6% 44%	1900
	21st to end	Day	17% 9% 3 8% 4 4 4 3 4 6% 39%	1952
November	1st to 10th	Day	11% 7% 3 5% 4 3 3 3 4 8% 50%	1762
	11th to 20th	Day	15% 10% 4 6% 3 3 3 4 4 7% 41%	1800
	21st to end	Day	16% 12% 4 8% 5% 3 4 2 4 5% 38%	1779
December	1st to 10th	Day	9% 9% 4 7% 5% 4 4 3 5% 6% 45%	2610
	11th to 20th	Day	13% 15% 5% 11% 5% 4 5% 4 5% 7% 26%	3008
	21st to end	Day	27% 18% 6% 12% 4 3 3 2 4 5% 15%	3982
January	1st to 10th	Night	15% 14% 7% 11% 4 4 4 3 4 5% 29%	6348
	11th to 20th	Night	11% 15% 6% 13% 6% 5% 7% 3 4 5% 24%	6360
	21st to end	Night	18% 14% 6% 9% 4 4 4 3 4 6% 29%	6559
February	1st to 10th	Night	16% 12% 5% 10% 5% 5% 4 3 5% 6% 30%	2550
	11th to 20th	Night	15% 10% 4 9% 5% 4 3 3 5% 6% 36%	2338
	21st to end	Night	17% 9% 3 7% 5% 3 4 3 4 6% 38%	1980
March	1st to 10th	Night	18% 14% 4 6% 3 3 3 3 4 4 38%	5373
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 48%	5224
	21st to end	Night	18% 11% 4 8% 5% 4 4 4 5% 6% 32%	4990
April	1st to 10th	Night	13% 8% 3 7% 6% 3 4 4 5% 6% 42%	2000
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 45%	1975
	21st to end	Night	20% 7% 2 7% 5% 4 4 4 5% 6% 36%	1997
May	1st to 10th	Night	18% 10% 3 7% 5% 3 4 4 4 7% 35%	4033
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 10% 39%	3799
	21st to end	Night	13% 8% 3 7% 4 4 5% 4 5% 8% 40%	3942
June	1st to 10th	Night	9% 5% 2 5% 4 4 5% 3 6% 10% 50%	1600
	11th to 20th	Night	8% 5% 2 6% 4 3 4 5% 5% 7% 52%	1600
	21st to end	Night	6% 5% 2 5% 5% 4 4 4 7% 10% 47%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 10% 44%	3618
	11th to 20th	Night	5% 5% 2 6% 5% 5% 5% 5% 8% 10% 44%	3617
	21st to end	Night	8% 7% 3 6% 4 5% 5% 4 7% 10% 40%	3947
August	1st to 10th	Night	6% 3 2 5% 4 3 4 5% 7% 9% 53%	2958
	11th to 20th	Night	6% 4 2 6% 5% 4 5% 4 7% 8% 49%	3200
	21st to end	Night	5% 3 1 5% 3 4 5% 4 6% 9% 55%	3586
September	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 8% 53%	2000
	11th to 20th	Night	7% 4 2 4 3 3 4 3 6% 9% 55%	2075
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 7% 52%	2200
October	1st to 10th	Night	9% 4 2 5% 4 4 4 3 4 7% 55%	2209
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 6% 51%	2280
	21st to end	Night	18% 8% 2 6% 4 4 3 3 5% 6% 42%	2646
November	1st to 10th	Night	11% 7% 2 5% 4 3 4 3 5% 6% 51%	2536
	11th to 20th	Night	12% 8% 3 6% 4 4 4 3 5% 6% 45%	2600
	21st to end	Night	18% 10% 4 7% 4 3 3 3 4 5% 39%	2619
December	1st to 10th	Night	12% 9% 3 8% 4 4 3 3 4 6% 44%	4580
	11th to 20th	Night	16% 9% 4 10% 5% 4 4 3 4 6% 33%	5199
	21st to end	Night	27% 18% 5% 10% 5% 4 4 3 4 4 16%	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% 3 7% 20% 7% 1	3618
	11th to 20th	Day	65% 2 10% 12% 9% 2	3620
	21st to end	Day	64% 2 8% 18% 7% 1	4414
February	1st to 10th	Day	57% 3 12% 18% 9% 1	1849
	11th to 20th	Day	55% 3 10% 21% 11% 1	1958
	21st to end	Day	53% 3 8% 22% 11% 2	1650
March	1st to 10th	Day	51% 2 6% 21% 18% 3	4480
	11th to 20th	Day	37% 3 10% 31% 17% 1	4766
	21st to end	Day	56% 3 12% 22% 7% 1	5940
April	1st to 10th	Day	48% 4 11% 23% 13%	2400
	11th to 20th	Day	46% 3 10% 25% 11% 3	2374
	21st to end	Day	48% 3 13% 22% 11% 2	2401
May	1st to 10th	Day	50% 4 12% 22% 11% 1	5897
	11th to 20th	Day	45% 4 11% 26% 12% 2	6093
	21st to end	Day	42% 5% 11% 23% 16% 2	6949
June	1st to 10th	Day	35% 4 15% 26% 16% 4	2800
	11th to 20th	Day	33% 5% 10% 27% 21% 3	2800
	21st to end	Day	36% 6% 15% 22% 18% 3	2800
July	1st to 10th	Day	35% 4 13% 30% 17% 2	6291
	11th to 20th	Day	40% 7% 14% 24% 14% 1	6285
	21st to end	Day	38% 7% 19% 24% 11% 1	6728
August	1st to 10th	Day	28% 5% 17% 36% 12% 1	4176
	11th to 20th	Day	35% 6% 16% 27% 15% 1	3885
	21st to end	Day	28% 6% 16% 31% 15% 4	4311
September	1st to 10th	Day	36% 4 9% 26% 21% 4	2400
	11th to 20th	Day	32% 4 9% 27% 25% 4	2305
	21st to end	Day	32% 4 8% 28% 24% 3	2200
October	1st to 10th	Day	24% 2 8% 23% 40% 3	3361
	11th to 20th	Day	29% 3 7% 23% 34% 3	3140
	21st to end	Day	39% 3 7% 24% 24% 2	3190
November	1st to 10th	Day	34% 2 14% 27% 18% 4	1762
	11th to 20th	Day	44% 3 10% 24% 15% 4	1800
	21st to end	Day	51% 3 11% 22% 13%	1779
December	1st to 10th	Day	41% 3 14% 31% 11%	2610
	11th to 20th	Day	58% 4 10% 22% 6% 1	3005
	21st to end	Day	73% 3 10% 10% 4	3982
January	1st to 10th	Night	57% 3 9% 23% 8%	6348
	11th to 20th	Night	63% 2 8% 14% 10% 2	6360
	21st to end	Night	59% 2 8% 20% 10% 1	6559
February	1st to 10th	Night	55% 3 11% 18% 10% 2	2550
	11th to 20th	Night	50% 3 10% 22% 14% 2	2339
	21st to end	Night	48% 3 10% 23% 13% 3	1980
March	1st to 10th	Night	51% 2 7% 22% 15% 3	5373
	11th to 20th	Night	36% 3 9% 28% 19% 3	5224
	21st to end	Night	53% 4 9% 24% 8% 1	4990
April	1st to 10th	Night	44% 3 10% 25% 16% 2	2000
	11th to 20th	Night	41% 3 9% 26% 16% 4	1975
	21st to end	Night	49% 4 11% 21% 12% 3	1997
May	1st to 10th	Night	50% 3 12% 23% 10% 2	4060
	11th to 20th	Night	41% 4 13% 27% 13% 2	3800
	21st to end	Night	42% 5% 12% 22% 16% 2	3950
June	1st to 10th	Night	31% 4 14% 28% 18% 4	1600
	11th to 20th	Night	32% 4 11% 26% 23% 4	1600
	21st to end	Night	31% 5% 17% 25% 18% 5%	1600
July	1st to 10th	Night	35% 5% 12% 28% 18% 2	3580
	11th to 20th	Night	31% 7% 16% 28% 16% 2	3572
	21st to end	Night	36% 8% 17% 24% 13% 2	3832
August	1st to 10th	Night	25% 6% 17% 33% 18% 1	2949
	11th to 20th	Night	31% 5% 14% 31% 18% 1	3230
	21st to end	Night	26% 5% 15% 32% 18% 4	3586
September	1st to 10th	Night	30% 3 9% 27% 22% 8%	2000
	11th to 20th	Night	26% 4 9% 27% 29% 5%	2075
	21st to end	Night	32% 3 8% 27% 25% 5%	2200
October	1st to 10th	Night	21% 3 7% 24% 43% 3	3664
	11th to 20th	Night	24% 2 7% 27% 36% 4	3779
	21st to end	Night	34% 3 9% 25% 24% 5%	4358
November	1st to 10th	Night	35% 3 10% 27% 20% 5%	2536
	11th to 20th	Night	41% 2 9% 27% 19% 2	2600
	21st to end	Night	49% 2 10% 23% 15% 1	2619
December	1st to 10th	Night	42% 3 12% 29% 13% 1	4580
	11th to 20th	Night	52% 3 11% 22% 10% 1	5201
	21st to end	Night	72% 3 9% 12% 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	26% 17% 35% 16% 6%	300
	11th to 20th	Day	24% 22% 38% 14% 2	300
	21st to end	Day	19% 20% 39% 20% 3	330
February	1st to 10th	Day	24% 16% 41% 15% 3	300
	11th to 20th	Day	25% 21% 35% 15% 3 1	300
	21st to end	Day	27% 15% 46% 11% 2	249
March	1st to 10th	Day	24% 19% 38% 16% 3	300
	11th to 20th	Day	32% 16% 34% 14% 4	300
	21st to end	Day	33% 15% 32% 15% 5%	534
April	1st to 10th	Day	22% 14% 35% 22% 6%	500
	11th to 20th	Day	22% 18% 29% 20% 10% 1	492
	21st to end	Day	25% 13% 38% 17% 7% 1	500
May	1st to 10th	Day	19% 12% 34% 26% 7% 2	503
	11th to 20th	Day	18% 11% 26% 27% 13% 4	625
	21st to end	Day	12% 7% 35% 27% 11% 7%	789
June	1st to 10th	Day	9% 7% 26% 34% 18% 6%	730
	11th to 20th	Day	10% 5% 25% 36% 18% 5%	730
	21st to end	Day	8% 7% 32% 29% 18% 6%	730
July	1st to 10th	Day	10% 4 16% 32% 23% 16%	721
	11th to 20th	Day	8% 4 21% 34% 25% 9%	718
	21st to end	Day	10% 3 16% 35% 27% 9%	794
August	1st to 10th	Day	8% 3 20% 32% 29% 9%	656
	11th to 20th	Day	11% 3 18% 31% 27% 11%	519
	21st to end	Day	7% 2 18% 33% 31% 9%	583
September	1st to 10th	Day	11% 6% 18% 30% 28% 7%	530
	11th to 20th	Day	9% 3 14% 31% 31% 13%	530
	21st to end	Day	12% 8% 23% 27% 19% 11%	530
October	1st to 10th	Day	14% 7% 25% 29% 19% 5%	415
	11th to 20th	Day	15% 20% 31% 24% 9% 2	300
	21st to end	Day	30% 13% 36% 16% 5%	330
November	1st to 10th	Day	15% 12% 36% 23% 10% 3	300
	11th to 20th	Day	23% 17% 35% 16% 7% 2	300
	21st to end	Day	27% 13% 35% 20% 5%	300
December	1st to 10th	Day	15% 19% 43% 15% 5% 2	300
	11th to 20th	Day	22% 17% 37% 20% 3 1	300
	21st to end	Day	28% 18% 35% 16% 3	330
January	1st to 10th	Night	19% 16% 40% 22% 4	1000
	11th to 20th	Night	22% 18% 36% 19% 5%	1000
	21st to end	Night	21% 15% 36% 25% 3	1100
February	1st to 10th	Night	23% 14% 38% 23% 3	950
	11th to 20th	Night	21% 16% 37% 23% 2	783
	21st to end	Night	23% 15% 38% 20% 3	660
March	1st to 10th	Night	23% 14% 35% 25% 4	800
	11th to 20th	Night	20% 13% 35% 27% 5%	714
	21st to end	Night	26% 12% 34% 22% 7%	660
April	1st to 10th	Night	17% 16% 36% 26% 5%	600
	11th to 20th	Night	19% 12% 33% 27% 8%	593
	21st to end	Night	25% 9% 34% 25% 6% 1	600
May	1st to 10th	Night	18% 8% 36% 29% 8% 1	600
	11th to 20th	Night	16% 8% 27% 31% 16% 1	591
	21st to end	Night	12% 7% 27% 34% 19% 2	650
June	1st to 10th	Night	10% 4 24% 43% 18%	600
	11th to 20th	Night	10% 3 22% 45% 19% 1	600
	21st to end	Night	9% 4 25% 39% 22% 2	600
July	1st to 10th	Night	12% 3 12% 33% 35% 5%	585
	11th to 20th	Night	5% 2 19% 41% 30% 3	582
	21st to end	Night	6% 2 14% 46% 30% 2	657
August	1st to 10th	Night	8% 2 15% 41% 30% 3	597
	11th to 20th	Night	6% 2 14% 42% 32% 3	594
	21st to end	Night	5% 2 13% 42% 35% 4	660
September	1st to 10th	Night	11% 4 19% 38% 26% 4	600
	11th to 20th	Night	8% 2 16% 40% 29% 5%	675
	21st to end	Night	12% 6% 22% 32% 26% 3	800
October	1st to 10th	Night	10% 5% 27% 35% 20% 3	773
	11th to 20th	Night	18% 11% 28% 29% 13%	760
	21st to end	Night	21% 13% 35% 25% 6%	836
November	1st to 10th	Night	15% 10% 39% 26% 9% 1	780
	11th to 20th	Night	19% 16% 33% 26% 6% 1	800
	21st to end	Night	24% 15% 32% 24% 5%	819
December	1st to 10th	Night	23% 12% 38% 22% 4	1000
	11th to 20th	Night	24% 11% 33% 26% 6% 1	1000
	21st to end	Night	31% 15% 31% 21% 3	1100

Page generated at 2025-10-01 00:39:04. Version ac07

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