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 2024-11-05.

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% 9% 3 7% 4 4 5% 4 6% 8% 38%	101262
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	97845
Transparency	Day	43% 4 12% 24% 15% 2	104853
	Night	43% 3 11% 24% 17% 2	102395
Seeing	Day	16% 10% 28% 26% 15% 5%	15690
	Night	17% 10% 30% 29% 13% 1	25505

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% 13% 8% 14% 5% 4 5% 3 4 7% 24%	2527
	11th to 20th	Day	14% 11% 7% 11% 7% 8% 8% 6% 6% 5% 18%	2530
	21st to end	Day	21% 22% 7% 9% 4 4 4 2 3 4 20%	3075
February	1st to 10th	Day	15% 13% 6% 11% 5% 5% 4 3 4 6% 29%	1756
	11th to 20th	Day	14% 10% 4 9% 7% 5% 4 3 5% 6% 32%	1848
	21st to end	Day	17% 13% 4 9% 5% 5% 4 3 4 6% 31%	1570
March	1st to 10th	Day	19% 16% 4 8% 4 3 3 2 3 4 34%	3120
	11th to 20th	Day	10% 9% 3 6% 4 3 4 4 5% 8% 45%	3336
	21st to end	Day	17% 12% 4 9% 5% 5% 4 3 5% 6% 29%	4150
April	1st to 10th	Day	14% 10% 3 6% 5% 4 4 3 6% 6% 36%	2280
	11th to 20th	Day	16% 9% 3 6% 4 4 4 3 4 6% 40%	2254
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 6% 37%	2281
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 5% 7% 10% 30%	4082
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 6% 8% 36%	4239
	21st to end	Day	8% 6% 2 6% 5% 4 5% 4 6% 9% 44%	4824
June	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 9% 44%	2660
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 49%	2660
	21st to end	Day	8% 6% 2 7% 6% 6% 5% 5% 7% 10% 41%	2660
July	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 10% 39%	4440
	11th to 20th	Day	5% 4 2 7% 5% 5% 5% 5% 7% 11% 45%	4440
	21st to end	Day	8% 5% 2 6% 4 4 6% 5% 8% 11% 39%	4813
August	1st to 10th	Day	8% 4 2 4 4 4 4 4 6% 10% 51%	4063
	11th to 20th	Day	10% 5% 2 6% 4 5% 6% 5% 8% 10% 40%	3729
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 10% 48%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 48%	2280
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 5% 8% 50%	2190
	21st to end	Day	9% 6% 2 5% 4 4 4 4 6% 7% 50%	2090
October	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 6% 51%	2003
	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	10% 7% 3 4 4 3 3 3 4 8% 51%	1708
	11th to 20th	Day	15% 10% 4 6% 3 3 3 4 4 7% 41%	1710
	21st to end	Day	16% 12% 3 7% 5% 3 4 2 4 5% 39%	1691
December	1st to 10th	Day	15% 12% 5% 8% 7% 5% 4 4 5% 6% 30%	1520
	11th to 20th	Day	14% 13% 4 12% 5% 5% 4 3 5% 7% 29%	1918
	21st to end	Day	21% 19% 6% 14% 5% 4 4 2 4 6% 16%	2783
January	1st to 10th	Night	15% 15% 6% 10% 4 4 4 3 5% 5% 29%	4428
	11th to 20th	Night	13% 17% 6% 11% 6% 6% 8% 3 4 3 23%	4440
	21st to end	Night	23% 17% 7% 9% 4 4 3 3 3 6% 21%	4587
February	1st to 10th	Night	16% 12% 4 9% 5% 5% 3 3 5% 6% 30%	2423
	11th to 20th	Night	14% 9% 4 9% 5% 5% 4 3 5% 6% 37%	2218
	21st to end	Night	18% 10% 4 8% 5% 3 4 3 4 5% 36%	1884
March	1st to 10th	Night	19% 14% 3 6% 3 3 4 3 4 5% 36%	3733
	11th to 20th	Night	12% 7% 2 5% 4 4 3 3 5% 7% 48%	3654
	21st to end	Night	20% 12% 4 7% 4 4 4 3 4 5% 33%	3480
April	1st to 10th	Night	13% 8% 3 7% 6% 3 4 4 5% 6% 43%	1900
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 45%	1875
	21st to end	Night	21% 7% 2 6% 5% 4 4 4 4 6% 36%	1897
May	1st to 10th	Night	17% 9% 3 6% 5% 3 4 4 5% 8% 35%	2813
	11th to 20th	Night	16% 7% 2 5% 4 4 4 4 5% 7% 41%	2644
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 6% 9% 44%	2743
June	1st to 10th	Night	8% 5% 2 5% 3 4 5% 4 5% 10% 50%	1520
	11th to 20th	Night	7% 4 2 6% 4 3 4 5% 5% 7% 53%	1520
	21st to end	Night	7% 5% 2 6% 5% 4 4 4 8% 10% 46%	1520
July	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	2530
	11th to 20th	Night	3 3 1 4 4 4 4 4 6% 10% 55%	2530
	21st to end	Night	7% 6% 3 6% 4 5% 5% 4 7% 11% 42%	2750
August	1st to 10th	Night	6% 3 2 5% 4 3 4 4 7% 9% 53%	2867
	11th to 20th	Night	7% 4 2 6% 5% 4 5% 4 6% 8% 50%	3100
	21st to end	Night	5% 4 1 5% 3 4 4 4 6% 9% 55%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 7% 52%	1900
	11th to 20th	Night	7% 4 2 4 3 3 4 3 6% 9% 55%	1971
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 7% 52%	2090
October	1st to 10th	Night	9% 4 2 5% 4 4 4 3 4 6% 55%	2175
	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	10% 6% 2 5% 4 3 3 3 5% 6% 52%	2470
	11th to 20th	Night	12% 8% 3 6% 4 3 4 3 5% 6% 46%	2470
	21st to end	Night	18% 9% 3 6% 4 3 3 3 4 5% 40%	2488
December	1st to 10th	Night	17% 11% 3 9% 5% 4 4 4 5% 6% 31%	2660
	11th to 20th	Night	17% 10% 5% 10% 4 4 3 3 3 5% 37%	3279
	21st to end	Night	24% 20% 6% 10% 5% 4 4 3 3 4 17%	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	61% 3 7% 21% 7% 1	2528
	11th to 20th	Day	68% 1 12% 13% 6%	2530
	21st to end	Day	70% 2 8% 14% 6%	3075
February	1st to 10th	Day	58% 3 11% 18% 9% 2	1756
	11th to 20th	Day	53% 3 10% 22% 11% 1	1858
	21st to end	Day	55% 4 8% 22% 9% 2	1570
March	1st to 10th	Day	56% 3 7% 18% 14% 2	3120
	11th to 20th	Day	38% 3 13% 26% 18% 1	3336
	21st to end	Day	57% 3 13% 22% 5% 1	4150
April	1st to 10th	Day	47% 4 11% 24% 14%	2280
	11th to 20th	Day	46% 3 10% 25% 12% 4	2254
	21st to end	Day	48% 3 13% 21% 12% 2	2281
May	1st to 10th	Day	48% 4 14% 26% 7%	4120
	11th to 20th	Day	46% 4 12% 28% 9%	4243
	21st to end	Day	36% 5% 11% 23% 22% 2	4837
June	1st to 10th	Day	35% 5% 15% 26% 16% 4	2660
	11th to 20th	Day	32% 5% 10% 27% 22% 3	2660
	21st to end	Day	37% 6% 15% 23% 17% 3	2660
July	1st to 10th	Day	40% 4 12% 29% 13% 2	4371
	11th to 20th	Day	31% 7% 15% 28% 18% 1	4365
	21st to end	Day	36% 7% 19% 22% 15% 1	4662
August	1st to 10th	Day	28% 5% 17% 36% 13% 1	4048
	11th to 20th	Day	35% 6% 15% 27% 15% 1	3765
	21st to end	Day	29% 6% 16% 31% 14% 4	4179
September	1st to 10th	Day	37% 4 8% 26% 20% 5%	2280
	11th to 20th	Day	33% 4 9% 27% 23% 5%	2190
	21st to end	Day	33% 4 8% 29% 23% 3	2090
October	1st to 10th	Day	24% 2 8% 23% 40% 3	3328
	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	33% 3 14% 28% 19% 4	1708
	11th to 20th	Day	44% 3 10% 25% 15% 3	1710
	21st to end	Day	50% 3 11% 23% 13%	1691
December	1st to 10th	Day	54% 3 11% 21% 10%	1520
	11th to 20th	Day	55% 3 8% 25% 7% 1	1915
	21st to end	Day	71% 3 9% 11% 5%	2783
January	1st to 10th	Night	57% 3 11% 21% 9%	4428
	11th to 20th	Night	67% 2 8% 16% 7%	4440
	21st to end	Night	67% 2 8% 16% 6%	4587
February	1st to 10th	Night	55% 3 11% 19% 10% 2	2423
	11th to 20th	Night	48% 3 10% 23% 14% 2	2219
	21st to end	Night	50% 3 10% 23% 12% 3	1884
March	1st to 10th	Night	52% 3 9% 23% 12% 1	3733
	11th to 20th	Night	36% 4 10% 26% 21% 3	3654
	21st to end	Night	54% 3 10% 24% 8% 1	3480
April	1st to 10th	Night	43% 3 11% 26% 16% 2	1900
	11th to 20th	Night	41% 3 9% 26% 17% 4	1875
	21st to end	Night	49% 3 11% 20% 13% 3	1897
May	1st to 10th	Night	48% 3 16% 24% 9% 1	2840
	11th to 20th	Night	43% 3 14% 29% 11% 1	2645
	21st to end	Night	37% 5% 12% 22% 21% 3	2751
June	1st to 10th	Night	31% 4 14% 28% 19% 5%	1520
	11th to 20th	Night	31% 4 11% 26% 24% 5%	1520
	21st to end	Night	31% 5% 17% 24% 18% 5%	1520
July	1st to 10th	Night	37% 5% 13% 26% 16% 3	2490
	11th to 20th	Night	22% 5% 14% 35% 22% 3	2482
	21st to end	Night	33% 7% 18% 23% 16% 3	2662
August	1st to 10th	Night	25% 6% 17% 33% 18% 1	2858
	11th to 20th	Night	31% 5% 13% 31% 19% 1	3130
	21st to end	Night	26% 5% 15% 32% 17% 4	3476
September	1st to 10th	Night	30% 3 9% 27% 22% 8%	1900
	11th to 20th	Night	27% 4 9% 27% 28% 5%	1971
	21st to end	Night	32% 3 8% 28% 24% 5%	2090
October	1st to 10th	Night	21% 3 7% 24% 43% 3	3630
	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	34% 3 10% 28% 20% 6%	2470
	11th to 20th	Night	40% 3 9% 28% 19% 2	2470
	21st to end	Night	48% 2 10% 24% 16% 1	2488
December	1st to 10th	Night	52% 4 12% 20% 11% 1	2660
	11th to 20th	Night	52% 2 11% 24% 11% 1	3281
	21st to end	Night	73% 2 6% 12% 6% 1	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	26% 17% 34% 16% 7%	270
	11th to 20th	Day	27% 19% 37% 14% 3	270
	21st to end	Day	20% 19% 37% 21% 3	297
February	1st to 10th	Day	24% 16% 40% 17% 3	270
	11th to 20th	Day	24% 19% 35% 17% 3 1	270
	21st to end	Day	27% 13% 46% 12% 2	225
March	1st to 10th	Day	24% 20% 36% 17% 3	270
	11th to 20th	Day	32% 14% 36% 14% 4	270
	21st to end	Day	31% 13% 33% 16% 5%	491
April	1st to 10th	Day	21% 13% 37% 22% 6%	460
	11th to 20th	Day	20% 17% 31% 20% 11% 1	452
	21st to end	Day	25% 13% 37% 17% 7% 2	460
May	1st to 10th	Day	19% 12% 35% 25% 6% 2	463
	11th to 20th	Day	18% 10% 27% 27% 14% 4	580
	21st to end	Day	11% 7% 34% 28% 12% 8%	734
June	1st to 10th	Day	8% 6% 26% 35% 19% 7%	680
	11th to 20th	Day	9% 5% 26% 35% 18% 6%	680
	21st to end	Day	9% 8% 33% 28% 18% 5%	680
July	1st to 10th	Day	10% 4 16% 30% 24% 16%	671
	11th to 20th	Day	7% 4 21% 35% 25% 10%	668
	21st to end	Day	9% 3 17% 35% 27% 9%	739
August	1st to 10th	Day	8% 2 19% 31% 29% 10%	610
	11th to 20th	Day	11% 3 19% 30% 27% 10%	479
	21st to end	Day	7% 2 17% 32% 32% 9%	539
September	1st to 10th	Day	12% 6% 18% 29% 28% 7%	490
	11th to 20th	Day	10% 3 14% 31% 31% 12%	490
	21st to end	Day	12% 9% 24% 27% 18% 10%	490
October	1st to 10th	Day	14% 8% 26% 29% 18% 5%	403
	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	13% 13% 37% 23% 11% 4	282
	11th to 20th	Day	22% 17% 36% 16% 7% 2	270
	21st to end	Day	26% 11% 36% 21% 5%	270
December	1st to 10th	Day	16% 18% 41% 16% 6% 3	270
	11th to 20th	Day	19% 17% 39% 22% 3 1	270
	21st to end	Day	26% 19% 37% 15% 3	297
January	1st to 10th	Night	19% 15% 40% 22% 4	950
	11th to 20th	Night	22% 17% 36% 19% 6%	950
	21st to end	Night	21% 14% 36% 26% 3	1045
February	1st to 10th	Night	23% 13% 37% 24% 3	903
	11th to 20th	Night	20% 15% 39% 24% 2	743
	21st to end	Night	25% 15% 38% 19% 3	628
March	1st to 10th	Night	22% 14% 35% 25% 4	760
	11th to 20th	Night	20% 12% 35% 27% 5%	679
	21st to end	Night	25% 11% 34% 22% 7%	627
April	1st to 10th	Night	16% 16% 37% 26% 5%	570
	11th to 20th	Night	19% 12% 33% 27% 9%	563
	21st to end	Night	25% 9% 35% 23% 6% 1	570
May	1st to 10th	Night	18% 7% 36% 30% 8% 1	570
	11th to 20th	Night	17% 6% 27% 32% 16% 1	561
	21st to end	Night	11% 6% 27% 35% 19% 2	617
June	1st to 10th	Night	10% 4 24% 44% 19%	570
	11th to 20th	Night	9% 3 23% 45% 19% 1	570
	21st to end	Night	9% 4 25% 38% 22% 1	570
July	1st to 10th	Night	12% 3 12% 33% 34% 6%	555
	11th to 20th	Night	5% 1 19% 40% 31% 3	552
	21st to end	Night	6% 2 13% 46% 30% 2	624
August	1st to 10th	Night	9% 1 14% 41% 31% 4	567
	11th to 20th	Night	7% 2 14% 41% 32% 4	564
	21st to end	Night	5% 2 12% 41% 35% 4	627
September	1st to 10th	Night	11% 3 19% 37% 25% 4	570
	11th to 20th	Night	9% 2 16% 39% 29% 5%	641
	21st to end	Night	11% 6% 23% 31% 25% 3	760
October	1st to 10th	Night	10% 6% 27% 34% 20% 3	760
	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	14% 10% 40% 26% 10% 1	760
	11th to 20th	Night	18% 16% 33% 27% 6% 1	760
	21st to end	Night	24% 14% 32% 25% 6%	778
December	1st to 10th	Night	23% 12% 38% 22% 5%	950
	11th to 20th	Night	24% 10% 33% 27% 6% 1	950
	21st to end	Night	31% 15% 31% 21% 3	1045

Page generated at 2024-11-03 01:11:12. Version ac07

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