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 2024-02-15.

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	14% 8% 4 6% 5% 3 5% 5% 5% 7% 37%	78318
	Night	15% 9% 4 6% 5% 3 4 4 4 6% 39%	80808
Transparency	Day	45% 6% 12% 22% 13% 2	78019
	Night	46% 5% 11% 22% 14% 2	80628
Seeing	Day	16% 7% 26% 26% 18% 7%	14623
	Night	17% 8% 29% 31% 13% 1	24642

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% 12% 9% 12% 7% 3 5% 4 5% 6% 24%	2527
	11th to 20th	Day	14% 11% 6% 10% 9% 5% 8% 8% 6% 5% 18%	2530
	21st to end	Day	22% 20% 7% 9% 5% 3 4 3 3 5% 18%	3044
February	1st to 10th	Day	15% 12% 7% 8% 6% 3 5% 4 4 6% 29%	1762
	11th to 20th	Day	14% 10% 4 9% 8% 4 5% 4 5% 6% 31%	1788
	21st to end	Day	17% 13% 5% 8% 6% 2 5% 4 5% 6% 28%	1480
March	1st to 10th	Day	16% 10% 4 8% 6% 3 4 4 5% 6% 35%	1800
	11th to 20th	Day	15% 10% 4 6% 7% 3 4 4 6% 7% 33%	1880
	21st to end	Day	21% 12% 5% 7% 6% 3 6% 4 5% 5% 26%	2368
April	1st to 10th	Day	14% 8% 4 7% 5% 3 5% 4 6% 8% 36%	2160
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 4 5% 6% 39%	2134
	21st to end	Day	21% 7% 3 6% 5% 3 4 4 5% 6% 35%	2177
May	1st to 10th	Day	17% 7% 3 6% 6% 3 5% 5% 7% 7% 34%	2340
	11th to 20th	Day	18% 6% 3 5% 6% 3 5% 4 5% 7% 37%	2384
	21st to end	Day	13% 5% 3 6% 6% 4 6% 6% 6% 9% 37%	2725
June	1st to 10th	Day	9% 4 2 5% 5% 3 5% 5% 6% 9% 45%	2520
	11th to 20th	Day	9% 4 2 5% 5% 3 5% 4 6% 8% 49%	2520
	21st to end	Day	8% 5% 3 6% 6% 3 6% 6% 7% 9% 42%	2520
July	1st to 10th	Day	9% 4 2 4 5% 4 6% 6% 6% 10% 43%	2520
	11th to 20th	Day	7% 5% 2 5% 5% 3 5% 6% 7% 11% 45%	2520
	21st to end	Day	9% 4 2 4 4 3 5% 6% 7% 9% 48%	2743
August	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 7% 10% 47%	2331
	11th to 20th	Day	8% 4 2 5% 5% 4 5% 6% 7% 9% 45%	2125
	21st to end	Day	7% 4 2 3 5% 3 5% 5% 7% 9% 50%	2376
September	1st to 10th	Day	13% 5% 3 5% 4 3 4 4 5% 7% 46%	2160
	11th to 20th	Day	10% 5% 2 4 5% 3 4 5% 4 7% 51%	2066
	21st to end	Day	10% 4 2 4 4 3 4 4 6% 8% 51%	1980
October	1st to 10th	Day	12% 4 3 5% 4 3 4 4 4 7% 49%	1908
	11th to 20th	Day	18% 6% 3 5% 4 3 4 4 5% 6% 43%	1800
	21st to end	Day	18% 9% 3 6% 5% 3 4 3 5% 7% 37%	1836
November	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 5% 8% 50%	1672
	11th to 20th	Day	17% 8% 4 5% 5% 2 3 4 4 7% 41%	1710
	21st to end	Day	17% 9% 5% 6% 5% 3 3 4 3 5% 39%	1691
December	1st to 10th	Day	16% 11% 5% 7% 6% 3 5% 4 6% 7% 29%	1520
	11th to 20th	Day	15% 12% 4 10% 5% 3 5% 4 4 8% 30%	1918
	21st to end	Day	21% 18% 7% 14% 5% 3 4 4 4 5% 16%	2783
January	1st to 10th	Night	15% 15% 7% 9% 5% 3 4 3 4 5% 29%	4428
	11th to 20th	Night	14% 17% 6% 9% 6% 5% 7% 5% 4 4 22%	4440
	21st to end	Night	24% 16% 8% 8% 4 3 3 3 3 5% 22%	4619
February	1st to 10th	Night	17% 11% 5% 9% 6% 3 4 3 4 7% 31%	2417
	11th to 20th	Night	14% 9% 4 7% 7% 4 5% 4 5% 6% 36%	2153
	21st to end	Night	19% 9% 4 7% 6% 3 3 4 4 5% 36%	1776
March	1st to 10th	Night	16% 10% 4 7% 5% 3 4 4 4 5% 37%	2160
	11th to 20th	Night	17% 7% 4 7% 5% 3 4 4 5% 7% 38%	2096
	21st to end	Night	22% 9% 3 6% 5% 3 4 4 4 7% 33%	1986
April	1st to 10th	Night	12% 7% 4 5% 6% 3 5% 5% 4 7% 42%	1800
	11th to 20th	Night	17% 6% 3 5% 4 2 4 3 5% 6% 45%	1775
	21st to end	Night	21% 7% 3 5% 5% 3 4 5% 4 6% 36%	1782
May	1st to 10th	Night	15% 7% 3 6% 5% 3 5% 5% 6% 8% 39%	1620
	11th to 20th	Night	16% 6% 3 6% 4 3 3 4 5% 8% 42%	1507
	21st to end	Night	9% 5% 3 7% 7% 3 5% 5% 6% 9% 40%	1551
June	1st to 10th	Night	10% 5% 2 4 4 3 5% 4 6% 7% 51%	1440
	11th to 20th	Night	8% 3 3 4 5% 3 5% 4 5% 9% 51%	1440
	21st to end	Night	6% 4 3 6% 6% 4 4 5% 7% 10% 46%	1440
July	1st to 10th	Night	10% 3 2 5% 5% 3 4 4 7% 10% 47%	1440
	11th to 20th	Night	5% 4 2 3 4 2 4 5% 6% 10% 57%	1440
	21st to end	Night	4 4 2 4 4 3 4 5% 7% 9% 55%	1575
August	1st to 10th	Night	7% 3 2 4 4 3 4 4 6% 10% 53%	1602
	11th to 20th	Night	6% 4 2 5% 4 3 5% 4 5% 9% 52%	1776
	21st to end	Night	6% 4 1 4 5% 3 4 4 5% 8% 56%	1980
September	1st to 10th	Night	12% 4 2 5% 4 3 4 4 5% 8% 51%	1800
	11th to 20th	Night	8% 3 2 4 4 2 4 4 6% 7% 56%	1874
	21st to end	Night	9% 5% 2 4 5% 3 4 4 5% 7% 53%	1980
October	1st to 10th	Night	11% 4 2 4 4 3 4 4 5% 6% 53%	2049
	11th to 20th	Night	15% 7% 2 4 3 2 4 4 4 6% 50%	2160
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 4 5% 40%	2519
November	1st to 10th	Night	11% 5% 3 5% 5% 2 3 4 4 5% 53%	2406
	11th to 20th	Night	13% 6% 3 6% 4 3 4 4 5% 7% 44%	2470
	21st to end	Night	18% 9% 4 6% 5% 2 4 4 3 5% 40%	2484
December	1st to 10th	Night	17% 11% 4 7% 5% 3 5% 4 5% 6% 32%	2660
	11th to 20th	Night	17% 9% 6% 8% 6% 3 4 3 3 5% 36%	3279
	21st to end	Night	24% 18% 7% 10% 6% 3 4 4 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% 4 8% 20% 6% 1	2528
	11th to 20th	Day	65% 5% 12% 12% 6%	2530
	21st to end	Day	69% 4 9% 13% 5%	3044
February	1st to 10th	Day	57% 4 11% 17% 9% 2	1762
	11th to 20th	Day	54% 5% 10% 20% 9% 1	1798
	21st to end	Day	56% 5% 9% 19% 9% 2	1480
March	1st to 10th	Day	52% 5% 10% 22% 10% 1	1800
	11th to 20th	Day	50% 5% 13% 22% 8% 1	1880
	21st to end	Day	60% 6% 10% 17% 6% 1	2368
April	1st to 10th	Day	46% 6% 12% 22% 14%	2160
	11th to 20th	Day	46% 5% 10% 24% 11% 4	2134
	21st to end	Day	48% 6% 12% 20% 11% 2	2177
May	1st to 10th	Day	48% 7% 15% 22% 8%	2340
	11th to 20th	Day	46% 8% 13% 19% 13% 1	2388
	21st to end	Day	43% 9% 13% 19% 14% 3	2725
June	1st to 10th	Day	33% 9% 13% 25% 16% 3	2520
	11th to 20th	Day	33% 7% 11% 25% 21% 4	2520
	21st to end	Day	37% 9% 14% 22% 16% 3	2520
July	1st to 10th	Day	35% 8% 13% 27% 14% 4	2451
	11th to 20th	Day	32% 9% 15% 27% 14% 2	2445
	21st to end	Day	30% 9% 15% 29% 16% 2	2591
August	1st to 10th	Day	30% 10% 16% 30% 12% 2	2316
	11th to 20th	Day	32% 9% 14% 28% 15% 2	2125
	21st to end	Day	28% 9% 13% 25% 18% 7%	2376
September	1st to 10th	Day	38% 6% 10% 23% 19% 4	2160
	11th to 20th	Day	32% 6% 9% 24% 23% 5%	2066
	21st to end	Day	32% 5% 9% 28% 23% 3	1980
October	1st to 10th	Day	35% 5% 11% 25% 20% 3	1908
	11th to 20th	Day	42% 4 9% 22% 21% 2	1800
	21st to end	Day	48% 4 10% 22% 15% 2	1836
November	1st to 10th	Day	33% 5% 14% 26% 18% 4	1672
	11th to 20th	Day	44% 4 10% 23% 15% 3	1710
	21st to end	Day	49% 5% 10% 22% 13% 1	1691
December	1st to 10th	Day	54% 5% 12% 19% 9%	1520
	11th to 20th	Day	54% 5% 9% 24% 7% 1	1915
	21st to end	Day	71% 5% 9% 10% 4	2783
January	1st to 10th	Night	56% 4 12% 19% 8%	4428
	11th to 20th	Night	66% 3 8% 15% 7% 1	4440
	21st to end	Night	65% 4 9% 15% 6%	4619
February	1st to 10th	Night	56% 4 11% 18% 10% 2	2417
	11th to 20th	Night	49% 4 12% 21% 12% 3	2154
	21st to end	Night	51% 5% 10% 21% 10% 3	1776
March	1st to 10th	Night	49% 5% 10% 21% 14% 1	2160
	11th to 20th	Night	46% 6% 13% 21% 13% 2	2096
	21st to end	Night	52% 7% 11% 19% 10% 1	1986
April	1st to 10th	Night	43% 4 11% 24% 16% 3	1800
	11th to 20th	Night	42% 4 10% 24% 16% 5%	1775
	21st to end	Night	48% 6% 10% 19% 12% 4	1782
May	1st to 10th	Night	43% 7% 13% 25% 11% 1	1620
	11th to 20th	Night	41% 7% 13% 22% 15% 1	1508
	21st to end	Night	40% 8% 15% 17% 16% 4	1552
June	1st to 10th	Night	31% 6% 12% 27% 18% 5%	1440
	11th to 20th	Night	30% 6% 12% 25% 22% 5%	1440
	21st to end	Night	32% 7% 17% 22% 17% 4	1440
July	1st to 10th	Night	31% 8% 14% 28% 16% 4	1400
	11th to 20th	Night	22% 7% 15% 31% 22% 3	1392
	21st to end	Night	25% 7% 16% 30% 19% 4	1487
August	1st to 10th	Night	27% 8% 16% 30% 18% 2	1593
	11th to 20th	Night	29% 7% 12% 30% 20% 2	1776
	21st to end	Night	26% 8% 13% 25% 21% 7%	1980
September	1st to 10th	Night	32% 6% 10% 25% 20% 8%	1800
	11th to 20th	Night	27% 6% 8% 27% 28% 4	1874
	21st to end	Night	30% 5% 10% 26% 24% 5%	1980
October	1st to 10th	Night	31% 6% 11% 24% 24% 5%	2049
	11th to 20th	Night	35% 4 9% 27% 21% 4	2160
	21st to end	Night	46% 4 8% 22% 17% 2	2519
November	1st to 10th	Night	34% 5% 10% 26% 20% 5%	2406
	11th to 20th	Night	40% 5% 10% 26% 18% 2	2470
	21st to end	Night	48% 4 11% 22% 15% 1	2484
December	1st to 10th	Night	52% 6% 11% 20% 10% 1	2660
	11th to 20th	Night	52% 3 11% 22% 11% 1	3281
	21st to end	Night	72% 5% 6% 11% 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	25% 13% 33% 20% 8% 1	270
	11th to 20th	Day	26% 14% 37% 18% 5%	270
	21st to end	Day	20% 13% 37% 23% 6%	297
February	1st to 10th	Day	27% 12% 35% 21% 5%	270
	11th to 20th	Day	25% 14% 33% 21% 5% 1	252
	21st to end	Day	26% 11% 42% 18% 3	198
March	1st to 10th	Day	19% 21% 36% 18% 5%	240
	11th to 20th	Day	30% 10% 38% 15% 6%	240
	21st to end	Day	30% 9% 35% 19% 7% 1	454
April	1st to 10th	Day	19% 12% 36% 24% 9% 1	420
	11th to 20th	Day	20% 13% 29% 22% 14% 1	412
	21st to end	Day	25% 10% 35% 20% 8% 2	420
May	1st to 10th	Day	20% 9% 34% 27% 9% 2	423
	11th to 20th	Day	21% 8% 27% 26% 15% 4	530
	21st to end	Day	13% 4 28% 29% 16% 9%	679
June	1st to 10th	Day	8% 4 23% 30% 27% 8%	630
	11th to 20th	Day	11% 4 23% 32% 23% 7%	630
	21st to end	Day	8% 5% 30% 31% 19% 7%	630
July	1st to 10th	Day	11% 2 11% 30% 27% 19%	621
	11th to 20th	Day	8% 3 16% 33% 28% 12%	618
	21st to end	Day	11% 1 15% 31% 31% 11%	684
August	1st to 10th	Day	8% 1 13% 32% 32% 13%	559
	11th to 20th	Day	9% 3 14% 27% 31% 16%	439
	21st to end	Day	6% 1 15% 30% 33% 15%	495
September	1st to 10th	Day	12% 5% 16% 26% 32% 9%	450
	11th to 20th	Day	9% 2 13% 29% 32% 16%	450
	21st to end	Day	12% 6% 22% 25% 23% 12%	450
October	1st to 10th	Day	16% 4 26% 29% 18% 7%	378
	11th to 20th	Day	17% 16% 35% 19% 11% 1	270
	21st to end	Day	32% 10% 34% 19% 4 1	297
November	1st to 10th	Day	13% 9% 34% 26% 14% 4	270
	11th to 20th	Day	21% 15% 33% 20% 7% 3	270
	21st to end	Day	22% 8% 37% 26% 8%	270
December	1st to 10th	Day	14% 15% 41% 19% 7% 4	270
	11th to 20th	Day	19% 13% 35% 26% 6% 1	270
	21st to end	Day	26% 17% 35% 17% 6%	297
January	1st to 10th	Night	18% 13% 39% 25% 4	950
	11th to 20th	Night	22% 14% 36% 23% 5%	950
	21st to end	Night	23% 10% 37% 26% 3	1045
February	1st to 10th	Night	23% 10% 39% 25% 4	897
	11th to 20th	Night	20% 13% 36% 28% 3	722
	21st to end	Night	24% 12% 39% 21% 3	592
March	1st to 10th	Night	23% 11% 35% 27% 4	720
	11th to 20th	Night	20% 10% 34% 30% 6%	656
	21st to end	Night	26% 9% 32% 25% 8%	594
April	1st to 10th	Night	16% 12% 37% 28% 6%	540
	11th to 20th	Night	20% 10% 33% 27% 11%	533
	21st to end	Night	26% 7% 31% 27% 7% 1	540
May	1st to 10th	Night	16% 6% 37% 32% 9% 1	540
	11th to 20th	Night	19% 4 25% 35% 16% 1	531
	21st to end	Night	11% 2 26% 36% 22% 3	584
June	1st to 10th	Night	11% 2 21% 44% 21% 1	540
	11th to 20th	Night	9% 2 21% 47% 19% 1	540
	21st to end	Night	7% 2 24% 42% 24% 1	540
July	1st to 10th	Night	11% 2 10% 36% 36% 5%	525
	11th to 20th	Night	7% 1 15% 43% 31% 3	522
	21st to end	Night	5% 1 12% 47% 33% 3	591
August	1st to 10th	Night	8% 1 9% 44% 35% 4	537
	11th to 20th	Night	6% 1 11% 43% 34% 5%	534
	21st to end	Night	6% 1 14% 35% 39% 4	594
September	1st to 10th	Night	15% 1 13% 38% 28% 5%	540
	11th to 20th	Night	9% 1 14% 43% 28% 5%	614
	21st to end	Night	11% 6% 22% 30% 29% 3	720
October	1st to 10th	Night	12% 4 23% 38% 20% 3	720
	11th to 20th	Night	18% 9% 31% 29% 12% 1	720
	21st to end	Night	23% 10% 35% 25% 7%	792
November	1st to 10th	Night	14% 8% 39% 28% 10% 1	740
	11th to 20th	Night	17% 14% 33% 28% 7% 1	760
	21st to end	Night	22% 11% 33% 28% 5%	774
December	1st to 10th	Night	24% 10% 36% 25% 5% 1	950
	11th to 20th	Night	22% 9% 34% 29% 6%	950
	21st to end	Night	30% 13% 31% 23% 3	1045

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