Decatur Area Astronomy Club
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Decatur Area Astronomy Club clear sky chart from 2005-11-06 to 2026-07-31.

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 8% 5% 4 4 4 6% 8% 37%	138226
	Night	13% 9% 3 7% 5% 4 4 3 5% 7% 39%	135068
Transparency	Day	45% 4 11% 23% 15% 2	141785
	Night	45% 4 10% 23% 16% 2	139586
Seeing	Day	16% 8% 27% 26% 17% 6%	18089
	Night	17% 8% 29% 31% 13% 1	27864

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% 11% 6% 13% 6% 5% 5% 3 5% 7% 27%	4691
	11th to 20th	Day	8% 11% 5% 15% 8% 8% 6% 5% 5% 5% 24%	4710
	21st to end	Day	16% 16% 6% 11% 8% 4 3 3 4 5% 24%	5665
February	1st to 10th	Day	15% 13% 6% 9% 5% 5% 4 3 5% 6% 29%	1948
	11th to 20th	Day	14% 11% 4 10% 7% 5% 4 3 4 5% 32%	2048
	21st to end	Day	15% 13% 4 10% 5% 3 4 4 5% 5% 32%	1730
March	1st to 10th	Day	18% 15% 4 7% 3 3 2 2 3 4 39%	4580
	11th to 20th	Day	8% 10% 3 5% 5% 3 4 4 6% 8% 44%	4844
	21st to end	Day	17% 11% 4 9% 5% 5% 5% 3 5% 7% 29%	6091
April	1st to 10th	Day	16% 9% 3 8% 5% 4 4 4 6% 7% 34%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 5% 6% 39%	2494
	21st to end	Day	19% 8% 3 7% 4 4 4 4 6% 7% 35%	2540
May	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 7% 8% 32%	7616
	11th to 20th	Day	15% 7% 3 7% 5% 4 4 3 5% 7% 40%	7919
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 5% 7% 8% 33%	9047
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	2940
	11th to 20th	Day	9% 4 2 6% 4 5% 4 3 6% 8% 48%	2940
	21st to end	Day	8% 6% 2 7% 6% 4 6% 5% 7% 9% 41%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 5% 4 4 6% 11% 43%	6500
	11th to 20th	Day	8% 7% 3 7% 5% 5% 5% 5% 7% 10% 37%	6500
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 7% 10% 38%	7065
August	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 10% 51%	4223
	11th to 20th	Day	8% 5% 2 5% 5% 5% 5% 5% 8% 10% 42%	3849
	21st to end	Day	5% 4 2 4 4 5% 5% 4 8% 11% 48%	4312
September	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 7% 52%	2295
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 7% 49%	2200
October	1st to 10th	Day	11% 4 2 6% 4 4 4 3 4 6% 51%	2120
	11th to 20th	Day	17% 6% 3 5% 4 4 4 3 5% 6% 44%	2000
	21st to end	Day	17% 9% 3 7% 4 4 4 2 5% 6% 39%	2040
November	1st to 10th	Day	11% 7% 2 5% 4 3 3 3 5% 7% 50%	1852
	11th to 20th	Day	16% 10% 3 6% 4 3 3 4 4 7% 40%	1890
	21st to end	Day	17% 11% 4 8% 4 4 3 3 4 5% 37%	1869
December	1st to 10th	Day	9% 10% 5% 7% 5% 4 3 3 4 7% 43%	2690
	11th to 20th	Day	15% 14% 5% 10% 5% 5% 6% 3 5% 7% 25%	3088
	21st to end	Day	26% 18% 6% 13% 4 4 3 3 3 5% 15%	4070
January	1st to 10th	Night	13% 14% 6% 11% 4 3 4 3 4 5% 32%	8232
	11th to 20th	Night	10% 14% 5% 12% 6% 6% 6% 3 5% 5% 27%	8280
	21st to end	Night	16% 12% 6% 9% 5% 4 4 3 4 5% 31%	8545
February	1st to 10th	Night	16% 11% 5% 9% 5% 4 4 3 5% 6% 31%	2671
	11th to 20th	Night	14% 10% 4 8% 5% 5% 4 3 4 6% 37%	2458
	21st to end	Night	17% 9% 3 8% 5% 3 3 3 4 5% 39%	2076
March	1st to 10th	Night	18% 14% 3 6% 3 3 3 3 4 4 38%	5493
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 6% 7% 46%	5380
	21st to end	Night	19% 10% 3 8% 5% 5% 4 3 5% 6% 31%	5078
April	1st to 10th	Night	13% 9% 3 7% 5% 4 4 4 4 6% 40%	2100
	11th to 20th	Night	16% 7% 2 6% 4 3 3 3 5% 5% 45%	2075
	21st to end	Night	20% 7% 2 6% 4 4 5% 3 5% 6% 37%	2079
May	1st to 10th	Night	17% 10% 3 7% 5% 4 4 3 5% 6% 35%	5237
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 7% 10% 41%	4954
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	5139
June	1st to 10th	Night	10% 5% 2 4 3 4 4 4 7% 7% 49%	1680
	11th to 20th	Night	9% 4 2 6% 4 4 4 3 5% 9% 50%	1680
	21st to end	Night	6% 5% 2 7% 5% 4 5% 4 8% 9% 45%	1680
July	1st to 10th	Night	9% 4 2 6% 5% 4 5% 3 6% 9% 44%	3698
	11th to 20th	Night	5% 5% 2 6% 5% 4 5% 5% 7% 11% 45%	3697
	21st to end	Night	9% 7% 3 6% 4 5% 5% 4 7% 9% 41%	4027
August	1st to 10th	Night	6% 4 2 5% 3 4 4 4 6% 10% 52%	2913
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 51%	3200
	21st to end	Night	6% 4 1 5% 4 4 4 3 5% 9% 55%	3586
September	1st to 10th	Night	11% 4 1 5% 3 3 3 3 5% 7% 54%	2000
	11th to 20th	Night	7% 3 2 5% 3 3 3 4 6% 7% 56%	2083
	21st to end	Night	9% 6% 2 5% 5% 3 4 4 5% 7% 51%	2200
October	1st to 10th	Night	10% 4 1 4 4 4 4 3 4 5% 56%	2277
	11th to 20th	Night	14% 7% 2 4 3 3 4 3 4 6% 51%	2400
	21st to end	Night	18% 7% 3 7% 4 4 4 3 4 6% 41%	2799
November	1st to 10th	Night	11% 6% 3 5% 4 3 4 3 4 5% 52%	2666
	11th to 20th	Night	13% 7% 3 7% 4 4 3 4 5% 7% 43%	2730
	21st to end	Night	18% 10% 5% 6% 5% 3 4 3 3 5% 38%	2746
December	1st to 10th	Night	12% 9% 3 8% 4 3 3 3 5% 6% 43%	4720
	11th to 20th	Night	16% 10% 5% 10% 6% 5% 4 3 4 6% 33%	5339
	21st to end	Night	27% 17% 6% 10% 5% 4 4 3 3 5% 17%	7150

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	58% 2 10% 21% 7% 1	4692
	11th to 20th	Day	62% 3 9% 13% 11% 2	4710
	21st to end	Day	64% 2 8% 18% 6% 2	5665
February	1st to 10th	Day	57% 4 12% 18% 8% 1	1948
	11th to 20th	Day	54% 5% 9% 20% 10% 2	2058
	21st to end	Day	53% 4 9% 21% 10% 2	1730
March	1st to 10th	Day	52% 2 6% 20% 17% 3	4580
	11th to 20th	Day	37% 4 11% 31% 16% 1	4844
	21st to end	Day	55% 5% 12% 21% 7% 1	6093
April	1st to 10th	Day	48% 6% 11% 21% 13%	2520
	11th to 20th	Day	46% 5% 10% 24% 11% 4	2494
	21st to end	Day	47% 6% 13% 21% 11% 2	2540
May	1st to 10th	Day	49% 5% 11% 23% 12% 1	7651
	11th to 20th	Day	45% 5% 10% 23% 16% 2	7923
	21st to end	Day	47% 4 11% 20% 15% 1	9061
June	1st to 10th	Day	35% 8% 13% 25% 16% 3	2940
	11th to 20th	Day	33% 6% 10% 26% 21% 3	2940
	21st to end	Day	37% 8% 14% 22% 16% 3	2940
July	1st to 10th	Day	37% 3 11% 31% 16% 2	6431
	11th to 20th	Day	42% 4 14% 25% 14% 1	6425
	21st to end	Day	40% 4 20% 24% 11% 1	6869
August	1st to 10th	Day	29% 6% 16% 35% 13% 1	4208
	11th to 20th	Day	36% 5% 15% 27% 16% 1	3885
	21st to end	Day	29% 6% 16% 31% 15% 4	4311
September	1st to 10th	Day	36% 5% 9% 24% 21% 4	2400
	11th to 20th	Day	32% 5% 10% 24% 25% 5%	2295
	21st to end	Day	33% 5% 9% 26% 24% 3	2200
October	1st to 10th	Day	25% 3 7% 24% 38% 3	3458
	11th to 20th	Day	31% 2 8% 22% 34% 3	3240
	21st to end	Day	41% 2 7% 24% 24% 1	3278
November	1st to 10th	Day	34% 4 13% 27% 18% 4	1852
	11th to 20th	Day	45% 4 10% 23% 15% 3	1890
	21st to end	Day	51% 4 10% 21% 13% 1	1869
December	1st to 10th	Day	43% 3 13% 29% 11%	2690
	11th to 20th	Day	60% 4 9% 21% 6% 1	3085
	21st to end	Day	74% 4 9% 9% 4	4070
January	1st to 10th	Night	55% 2 12% 24% 7%	8233
	11th to 20th	Night	60% 2 7% 16% 13% 2	8280
	21st to end	Night	57% 2 9% 21% 9% 1	8545
February	1st to 10th	Night	55% 3 11% 19% 10% 2	2671
	11th to 20th	Night	49% 3 11% 21% 13% 2	2459
	21st to end	Night	48% 4 10% 23% 13% 3	2076
March	1st to 10th	Night	50% 3 7% 22% 14% 3	5493
	11th to 20th	Night	37% 4 9% 28% 18% 3	5380
	21st to end	Night	53% 5% 9% 23% 9% 1	5078
April	1st to 10th	Night	45% 4 10% 23% 15% 2	2100
	11th to 20th	Night	41% 4 10% 24% 16% 4	2075
	21st to end	Night	47% 5% 11% 21% 12% 3	2079
May	1st to 10th	Night	50% 5% 10% 23% 10% 2	5264
	11th to 20th	Night	39% 5% 11% 25% 18% 2	4955
	21st to end	Night	46% 4 11% 20% 17% 2	5149
June	1st to 10th	Night	32% 6% 12% 27% 18% 5%	1680
	11th to 20th	Night	32% 5% 11% 25% 23% 4	1680
	21st to end	Night	33% 6% 17% 23% 17% 4	1680
July	1st to 10th	Night	37% 4 11% 29% 18% 2	3660
	11th to 20th	Night	33% 4 15% 28% 18% 2	3652
	21st to end	Night	40% 4 17% 24% 13% 3	3912
August	1st to 10th	Night	29% 4 16% 32% 18% 1	2904
	11th to 20th	Night	31% 4 14% 31% 18% 1	3230
	21st to end	Night	27% 5% 14% 32% 18% 4	3586
September	1st to 10th	Night	31% 5% 9% 26% 22% 8%	2000
	11th to 20th	Night	26% 5% 9% 26% 30% 4	2083
	21st to end	Night	32% 5% 10% 25% 24% 4	2200
October	1st to 10th	Night	21% 3 6% 24% 42% 3	3719
	11th to 20th	Night	24% 2 8% 25% 36% 4	3899
	21st to end	Night	37% 2 8% 24% 24% 5%	4511
November	1st to 10th	Night	35% 4 9% 26% 20% 5%	2666
	11th to 20th	Night	40% 4 9% 26% 18% 2	2730
	21st to end	Night	49% 3 10% 21% 15% 1	2746
December	1st to 10th	Night	43% 3 11% 30% 12% 1	4720
	11th to 20th	Night	55% 2 11% 21% 10% 1	5341
	21st to end	Night	72% 3 8% 11% 4	7150

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	23% 15% 33% 21% 8% 1	330
	11th to 20th	Day	23% 17% 39% 17% 4	330
	21st to end	Day	18% 14% 40% 22% 5%	363
February	1st to 10th	Day	25% 12% 37% 22% 5%	330
	11th to 20th	Day	25% 15% 36% 18% 5% 1	330
	21st to end	Day	23% 12% 45% 18% 3	273
March	1st to 10th	Day	21% 19% 39% 17% 3 1	330
	11th to 20th	Day	27% 14% 39% 15% 5%	330
	21st to end	Day	27% 11% 37% 19% 5% 1	586
April	1st to 10th	Day	23% 12% 35% 21% 8% 1	540
	11th to 20th	Day	20% 16% 30% 21% 12% 1	532
	21st to end	Day	24% 10% 35% 21% 9% 1	540
May	1st to 10th	Day	19% 10% 34% 26% 10% 2	543
	11th to 20th	Day	19% 8% 28% 25% 14% 5%	665
	21st to end	Day	13% 5% 30% 28% 16% 8%	844
June	1st to 10th	Day	10% 5% 25% 29% 24% 7%	780
	11th to 20th	Day	11% 5% 24% 31% 23% 6%	780
	21st to end	Day	8% 4 29% 32% 20% 6%	780
July	1st to 10th	Day	11% 2 12% 31% 27% 17%	771
	11th to 20th	Day	8% 3 16% 32% 27% 13%	768
	21st to end	Day	11% 2 16% 31% 31% 10%	845
August	1st to 10th	Day	8% 2 14% 33% 32% 12%	651
	11th to 20th	Day	9% 3 14% 27% 31% 15%	519
	21st to end	Day	5% 1 15% 32% 33% 14%	583
September	1st to 10th	Day	11% 5% 15% 28% 31% 10%	530
	11th to 20th	Day	8% 2 12% 28% 33% 17%	530
	21st to end	Day	13% 7% 21% 24% 22% 13%	530
October	1st to 10th	Day	14% 4 24% 30% 21% 7%	450
	11th to 20th	Day	15% 15% 33% 22% 13% 2	330
	21st to end	Day	29% 12% 34% 19% 5% 1	363
November	1st to 10th	Day	15% 12% 33% 25% 12% 4	330
	11th to 20th	Day	21% 16% 34% 20% 7% 2	330
	21st to end	Day	24% 9% 34% 26% 7%	330
December	1st to 10th	Day	13% 15% 43% 20% 6% 3	330
	11th to 20th	Day	20% 14% 34% 26% 5% 1	330
	21st to end	Day	28% 16% 34% 17% 5%	363
January	1st to 10th	Night	19% 13% 39% 25% 4	1050
	11th to 20th	Night	20% 15% 38% 22% 5%	1050
	21st to end	Night	22% 11% 39% 25% 3	1155
February	1st to 10th	Night	22% 10% 40% 24% 4	991
	11th to 20th	Night	20% 13% 36% 27% 3	823
	21st to end	Night	22% 13% 40% 22% 3	692
March	1st to 10th	Night	24% 11% 36% 25% 4	840
	11th to 20th	Night	19% 13% 35% 28% 5%	764
	21st to end	Night	25% 10% 33% 25% 7%	693
April	1st to 10th	Night	18% 13% 38% 27% 5%	630
	11th to 20th	Night	20% 10% 35% 26% 9%	623
	21st to end	Night	24% 7% 33% 29% 7%	630
May	1st to 10th	Night	17% 6% 38% 30% 9% 1	630
	11th to 20th	Night	17% 5% 26% 36% 16% 1	621
	21st to end	Night	12% 2 27% 36% 20% 3	683
June	1st to 10th	Night	12% 2 22% 43% 20% 1	630
	11th to 20th	Night	10% 2 21% 48% 17% 1	630
	21st to end	Night	8% 3 23% 43% 23% 1	630
July	1st to 10th	Night	11% 2 10% 38% 34% 5%	615
	11th to 20th	Night	7% 2 14% 43% 32% 3	612
	21st to end	Night	7% 1 12% 45% 33% 3	687
August	1st to 10th	Night	7% 1 10% 45% 33% 4	597
	11th to 20th	Night	6% 2 11% 45% 32% 4	594
	21st to end	Night	6% 2 13% 38% 38% 4	660
September	1st to 10th	Night	14% 2 14% 38% 29% 4	600
	11th to 20th	Night	8% 1 12% 43% 30% 5%	683
	21st to end	Night	12% 5% 21% 31% 28% 3	800
October	1st to 10th	Night	11% 4 22% 39% 21% 3	800
	11th to 20th	Night	18% 8% 30% 31% 12% 1	800
	21st to end	Night	22% 10% 36% 25% 7%	880
November	1st to 10th	Night	15% 8% 38% 29% 9% 1	820
	11th to 20th	Night	17% 14% 34% 28% 7% 1	840
	21st to end	Night	23% 12% 33% 26% 5%	856
December	1st to 10th	Night	23% 10% 37% 25% 5%	1050
	11th to 20th	Night	22% 9% 35% 28% 6%	1050
	21st to end	Night	31% 12% 31% 23% 3	1155

Page generated at 2026-07-28 02:25:21. Version ac07

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