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 2025-03-27.

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 7% 5% 4 4 4 6% 7% 38%	108914
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 39%	110602
Transparency	Day	44% 5% 11% 23% 15% 2	112518
	Night	44% 4 10% 23% 16% 2	115139
Seeing	Day	16% 8% 27% 26% 17% 6%	16130
	Night	17% 8% 29% 31% 13% 1	26142

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	14% 11% 7% 14% 6% 5% 5% 4 5% 7% 23%	3617
	11th to 20th	Day	10% 11% 6% 14% 7% 8% 7% 5% 6% 5% 21%	3620
	21st to end	Day	18% 17% 6% 10% 6% 4 3 2 4 5% 25%	4369
February	1st to 10th	Day	15% 13% 6% 9% 5% 5% 4 3 5% 6% 29%	1855
	11th to 20th	Day	14% 11% 4 10% 6% 5% 4 3 5% 5% 32%	1948
	21st to end	Day	16% 13% 4 9% 5% 3 4 3 5% 5% 32%	1650
March	1st to 10th	Day	18% 17% 4 8% 4 4 2 2 4 4 34%	3220
	11th to 20th	Day	9% 9% 3 6% 5% 3 4 4 6% 7% 44%	3415
	21st to end	Day	17% 12% 4 9% 5% 5% 6% 3 5% 6% 28%	4229
April	1st to 10th	Day	14% 9% 3 8% 5% 4 4 4 6% 8% 35%	2280
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 5% 5% 40%	2254
	21st to end	Day	20% 7% 3 7% 4 4 4 4 6% 7% 35%	2298
May	1st to 10th	Day	16% 7% 3 6% 5% 5% 5% 5% 8% 9% 31%	4082
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 3 5% 8% 37%	4234
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	4824
June	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 7% 9% 45%	2660
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 6% 8% 50%	2660
	21st to end	Day	8% 6% 2 7% 6% 5% 6% 4 7% 8% 42%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 10% 38%	4440
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 45%	4440
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 8% 10% 41%	4813
August	1st to 10th	Day	8% 4 2 4 4 4 4 4 6% 10% 51%	4093
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 6% 8% 10% 41%	3729
	21st to end	Day	5% 4 2 4 4 5% 5% 4 8% 11% 49%	4180
September	1st to 10th	Day	12% 6% 3 6% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 7% 51%	2180
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 7% 49%	2090
October	1st to 10th	Day	11% 4 2 6% 4 4 4 3 4 7% 51%	2014
	11th to 20th	Day	17% 6% 3 5% 3 3 4 3 5% 6% 44%	1900
	21st to end	Day	17% 9% 3 7% 4 4 4 2 5% 7% 38%	1938
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 5% 7% 49%	1762
	11th to 20th	Day	16% 10% 3 6% 4 3 3 4 4 7% 40%	1800
	21st to end	Day	16% 10% 4 8% 5% 4 3 3 4 5% 37%	1780
December	1st to 10th	Day	9% 10% 4 7% 5% 4 3 3 4 7% 44%	2610
	11th to 20th	Day	14% 15% 5% 10% 5% 5% 6% 3 5% 7% 25%	3008
	21st to end	Day	27% 18% 6% 13% 4 4 3 3 3 5% 15%	3982
January	1st to 10th	Night	15% 14% 7% 11% 4 3 4 3 4 5% 29%	6348
	11th to 20th	Night	12% 16% 6% 12% 6% 6% 6% 3 4 4 24%	6360
	21st to end	Night	19% 14% 6% 9% 5% 4 4 3 3 5% 29%	6605
February	1st to 10th	Night	17% 12% 5% 9% 5% 4 4 3 5% 7% 30%	2544
	11th to 20th	Night	14% 10% 4 8% 5% 5% 4 3 4 6% 36%	2338
	21st to end	Night	17% 9% 3 8% 5% 3 3 3 4 5% 39%	1980
March	1st to 10th	Night	18% 15% 3 6% 4 3 3 3 4 5% 36%	3853
	11th to 20th	Night	12% 7% 2 6% 4 3 4 3 5% 6% 48%	3796
	21st to end	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 32%	3538
April	1st to 10th	Night	12% 9% 3 7% 5% 4 4 5% 4 7% 41%	1900
	11th to 20th	Night	17% 7% 3 6% 4 3 4 3 5% 5% 45%	1875
	21st to end	Night	21% 7% 2 6% 4 4 5% 3 5% 6% 36%	1881
May	1st to 10th	Night	17% 10% 2 6% 5% 4 4 3 6% 7% 36%	2813
	11th to 20th	Night	17% 7% 2 6% 4 4 4 3 6% 8% 41%	2662
	21st to end	Night	10% 5% 2 7% 5% 4 4 3 6% 8% 44%	2743
June	1st to 10th	Night	9% 6% 2 4 3 4 4 4 7% 7% 50%	1520
	11th to 20th	Night	8% 4 2 5% 4 5% 4 3 5% 8% 51%	1520
	21st to end	Night	6% 5% 2 7% 5% 4 5% 4 7% 10% 45%	1520
July	1st to 10th	Night	12% 4 2 6% 5% 4 5% 3 7% 9% 43%	2530
	11th to 20th	Night	3 4 1 4 4 3 4 4 7% 10% 56%	2530
	21st to end	Night	6% 6% 2 6% 4 5% 5% 4 7% 9% 43%	2750
August	1st to 10th	Night	6% 4 2 5% 3 4 4 4 6% 10% 53%	2823
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 6% 9% 51%	3100
	21st to end	Night	6% 4 1 5% 4 4 4 3 5% 9% 55%	3476
September	1st to 10th	Night	11% 4 1 5% 3 3 4 3 4 8% 53%	1900
	11th to 20th	Night	7% 3 1 5% 3 3 3 4 6% 7% 56%	1979
	21st to end	Night	9% 6% 2 5% 5% 3 4 4 5% 7% 51%	2090
October	1st to 10th	Night	10% 4 1 4 4 4 4 3 5% 5% 55%	2163
	11th to 20th	Night	14% 7% 2 4 3 3 3 3 4 6% 51%	2280
	21st to end	Night	17% 7% 3 7% 4 4 4 3 4 6% 41%	2659
November	1st to 10th	Night	11% 6% 3 6% 4 3 4 3 4 5% 52%	2536
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 5% 7% 43%	2600
	21st to end	Night	17% 10% 5% 6% 5% 3 4 3 3 5% 39%	2615
December	1st to 10th	Night	12% 9% 3 7% 4 3 3 3 4 6% 44%	4580
	11th to 20th	Night	16% 10% 5% 10% 6% 5% 4 3 4 6% 33%	5199
	21st to end	Night	27% 17% 6% 10% 4 4 4 3 3 5% 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	62% 3 7% 19% 7% 1	3618
	11th to 20th	Day	64% 4 10% 11% 10% 2	3620
	21st to end	Day	64% 3 9% 17% 7% 1	4369
February	1st to 10th	Day	57% 4 11% 17% 9% 2	1855
	11th to 20th	Day	54% 5% 10% 20% 10% 1	1958
	21st to end	Day	53% 5% 10% 20% 10% 2	1650
March	1st to 10th	Day	55% 4 7% 17% 15% 2	3220
	11th to 20th	Day	36% 6% 14% 25% 18% 1	3415
	21st to end	Day	55% 7% 13% 20% 5% 1	4229
April	1st to 10th	Day	47% 6% 11% 21% 14%	2280
	11th to 20th	Day	45% 5% 10% 24% 12% 4	2254
	21st to end	Day	48% 6% 12% 20% 11% 2	2298
May	1st to 10th	Day	45% 9% 13% 25% 7%	4120
	11th to 20th	Day	45% 8% 11% 27% 9% 1	4238
	21st to end	Day	34% 8% 11% 21% 23% 3	4837
June	1st to 10th	Day	34% 9% 13% 25% 16% 4	2660
	11th to 20th	Day	32% 7% 10% 26% 22% 4	2660
	21st to end	Day	37% 8% 14% 22% 17% 3	2660
July	1st to 10th	Day	42% 5% 10% 28% 13% 2	4371
	11th to 20th	Day	33% 5% 15% 28% 18% 1	4365
	21st to end	Day	37% 5% 19% 21% 15% 1	4662
August	1st to 10th	Day	29% 6% 16% 34% 13% 1	4078
	11th to 20th	Day	37% 5% 14% 27% 16% 1	3765
	21st to end	Day	29% 6% 17% 30% 14% 4	4179
September	1st to 10th	Day	37% 6% 9% 24% 21% 4	2280
	11th to 20th	Day	32% 5% 10% 24% 24% 5%	2180
	21st to end	Day	34% 5% 9% 27% 23% 3	2090
October	1st to 10th	Day	25% 3 7% 24% 38% 3	3352
	11th to 20th	Day	30% 2 8% 22% 34% 3	3140
	21st to end	Day	41% 2 7% 24% 24% 1	3176
November	1st to 10th	Day	34% 4 13% 26% 18% 4	1762
	11th to 20th	Day	45% 4 10% 23% 15% 4	1800
	21st to end	Day	50% 4 10% 22% 13% 1	1780
December	1st to 10th	Day	42% 3 13% 30% 11%	2610
	11th to 20th	Day	60% 4 9% 21% 6% 1	3005
	21st to end	Day	74% 4 9% 9% 4	3982
January	1st to 10th	Night	58% 3 9% 22% 8%	6348
	11th to 20th	Night	65% 2 8% 12% 10% 3	6360
	21st to end	Night	59% 3 8% 19% 10% 1	6605
February	1st to 10th	Night	56% 3 11% 18% 10% 2	2544
	11th to 20th	Night	50% 3 11% 21% 13% 2	2339
	21st to end	Night	48% 4 10% 22% 13% 3	1980
March	1st to 10th	Night	50% 5% 9% 22% 12% 1	3853
	11th to 20th	Night	35% 6% 11% 26% 20% 2	3796
	21st to end	Night	52% 6% 11% 23% 8% 1	3538
April	1st to 10th	Night	43% 4 11% 24% 16% 3	1900
	11th to 20th	Night	41% 4 10% 24% 16% 5%	1875
	21st to end	Night	48% 6% 11% 20% 13% 3	1881
May	1st to 10th	Night	46% 8% 14% 23% 9% 1	2840
	11th to 20th	Night	41% 7% 13% 28% 10% 1	2663
	21st to end	Night	35% 7% 13% 20% 22% 3	2751
June	1st to 10th	Night	31% 6% 12% 27% 18% 5%	1520
	11th to 20th	Night	31% 6% 12% 24% 23% 5%	1520
	21st to end	Night	32% 7% 17% 23% 17% 5%	1520
July	1st to 10th	Night	38% 5% 12% 26% 16% 3	2490
	11th to 20th	Night	24% 4 13% 34% 23% 2	2482
	21st to end	Night	36% 4 17% 23% 16% 4	2662
August	1st to 10th	Night	28% 4 16% 32% 18% 1	2814
	11th to 20th	Night	31% 4 14% 31% 19% 1	3130
	21st to end	Night	27% 5% 14% 32% 17% 4	3476
September	1st to 10th	Night	31% 5% 9% 25% 22% 8%	1900
	11th to 20th	Night	27% 6% 9% 26% 28% 5%	1979
	21st to end	Night	32% 5% 10% 26% 23% 5%	2090
October	1st to 10th	Night	21% 3 6% 23% 42% 3	3605
	11th to 20th	Night	24% 3 8% 26% 36% 4	3779
	21st to end	Night	36% 3 8% 24% 24% 5%	4371
November	1st to 10th	Night	35% 4 10% 26% 20% 5%	2536
	11th to 20th	Night	40% 5% 10% 25% 18% 2	2600
	21st to end	Night	49% 4 10% 21% 15% 1	2615
December	1st to 10th	Night	42% 3 11% 30% 12% 1	4580
	11th to 20th	Night	55% 3 11% 21% 11%	5201
	21st to end	Night	72% 3 8% 11% 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	24% 14% 33% 21% 8% 1	300
	11th to 20th	Day	24% 14% 40% 18% 4	300
	21st to end	Day	19% 12% 41% 22% 5%	330
February	1st to 10th	Day	27% 12% 36% 21% 4	300
	11th to 20th	Day	25% 15% 35% 18% 5% 1	300
	21st to end	Day	24% 12% 43% 18% 3	249
March	1st to 10th	Day	19% 20% 39% 18% 4 1	300
	11th to 20th	Day	26% 14% 37% 16% 6%	300
	21st to end	Day	28% 11% 35% 19% 6% 1	522
April	1st to 10th	Day	20% 12% 36% 23% 8% 1	460
	11th to 20th	Day	20% 13% 31% 22% 13% 1	452
	21st to end	Day	25% 10% 36% 20% 8% 2	460
May	1st to 10th	Day	19% 9% 33% 27% 10% 2	463
	11th to 20th	Day	21% 7% 26% 26% 15% 5%	575
	21st to end	Day	12% 5% 30% 28% 16% 9%	734
June	1st to 10th	Day	9% 4 25% 30% 26% 7%	680
	11th to 20th	Day	11% 4 23% 32% 24% 6%	680
	21st to end	Day	8% 5% 31% 31% 19% 6%	680
July	1st to 10th	Day	12% 2 13% 30% 25% 17%	671
	11th to 20th	Day	7% 3 17% 33% 27% 13%	668
	21st to end	Day	11% 1 15% 31% 32% 10%	739
August	1st to 10th	Day	9% 1 14% 32% 32% 12%	605
	11th to 20th	Day	10% 3 16% 28% 29% 15%	479
	21st to end	Day	6% 1 14% 32% 33% 14%	539
September	1st to 10th	Day	11% 5% 15% 27% 32% 9%	490
	11th to 20th	Day	8% 2 12% 28% 33% 17%	490
	21st to end	Day	14% 7% 22% 24% 22% 11%	490
October	1st to 10th	Day	15% 4 25% 29% 20% 7%	414
	11th to 20th	Day	16% 15% 34% 20% 13% 2	300
	21st to end	Day	30% 11% 33% 19% 5% 1	330
November	1st to 10th	Day	15% 9% 32% 26% 13% 4	300
	11th to 20th	Day	21% 16% 33% 21% 7% 3	300
	21st to end	Day	23% 9% 35% 26% 7%	300
December	1st to 10th	Day	13% 16% 42% 19% 6% 3	300
	11th to 20th	Day	20% 14% 35% 25% 6% 1	300
	21st to end	Day	28% 16% 33% 18% 5%	330
January	1st to 10th	Night	19% 13% 39% 25% 4	1000
	11th to 20th	Night	21% 15% 37% 22% 5%	1000
	21st to end	Night	22% 10% 38% 26% 3	1100
February	1st to 10th	Night	22% 10% 40% 23% 4	944
	11th to 20th	Night	20% 14% 37% 26% 3	783
	21st to end	Night	22% 12% 40% 22% 3	660
March	1st to 10th	Night	23% 11% 35% 26% 4	800
	11th to 20th	Night	19% 11% 36% 29% 5%	728
	21st to end	Night	25% 10% 33% 25% 7%	647
April	1st to 10th	Night	17% 12% 37% 28% 6%	570
	11th to 20th	Night	20% 10% 34% 27% 10%	563
	21st to end	Night	25% 7% 32% 28% 7% 1	570
May	1st to 10th	Night	16% 6% 37% 31% 8% 1	570
	11th to 20th	Night	19% 4 24% 36% 16% 1	561
	21st to end	Night	11% 2 27% 36% 21% 3	617
June	1st to 10th	Night	11% 2 22% 44% 20% 1	570
	11th to 20th	Night	9% 2 21% 48% 19% 1	570
	21st to end	Night	7% 3 24% 42% 23% 1	570
July	1st to 10th	Night	12% 2 11% 37% 34% 5%	555
	11th to 20th	Night	7% 1 15% 43% 31% 3	552
	21st to end	Night	6% 1 12% 46% 33% 3	624
August	1st to 10th	Night	8% 1 10% 44% 34% 4	567
	11th to 20th	Night	6% 2 12% 43% 33% 4	564
	21st to end	Night	6% 1 13% 36% 39% 4	627
September	1st to 10th	Night	14% 1 13% 38% 29% 5%	570
	11th to 20th	Night	8% 1 13% 43% 30% 5%	649
	21st to end	Night	12% 6% 22% 31% 27% 3	760
October	1st to 10th	Night	11% 4 23% 38% 21% 3	760
	11th to 20th	Night	18% 9% 31% 29% 13% 1	760
	21st to end	Night	22% 10% 36% 25% 7%	836
November	1st to 10th	Night	15% 8% 38% 29% 10% 1	780
	11th to 20th	Night	18% 13% 33% 28% 7% 1	800
	21st to end	Night	23% 12% 34% 27% 5%	815
December	1st to 10th	Night	23% 10% 37% 25% 5% 1	1000
	11th to 20th	Night	23% 9% 35% 28% 6%	1000
	21st to end	Night	31% 13% 30% 23% 3	1100

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