Edwardsville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edwardsville Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	13% 7% 3 5% 5% 3 4 5% 5% 7% 42%	74777
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	75021
Transparency	Day	40% 7% 12% 24% 15% 2	74483
	Night	41% 6% 11% 23% 17% 3	74836
Seeing	Day	15% 9% 25% 27% 18% 7%	14469
	Night	16% 8% 29% 31% 14% 2	24421

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% 5% 9% 6% 2 4 5% 4 6% 35%	1517
	11th to 20th	Day	16% 11% 4 7% 7% 3 5% 4 5% 5% 32%	1547
	21st to end	Day	13% 11% 5% 8% 6% 2 4 4 4 6% 37%	1782
February	1st to 10th	Day	17% 10% 5% 7% 4 3 4 4 5% 6% 35%	1629
	11th to 20th	Day	16% 8% 4 6% 5% 4 5% 5% 6% 6% 35%	1748
	21st to end	Day	16% 10% 4 7% 5% 3 3 4 5% 5% 37%	1480
March	1st to 10th	Day	16% 9% 3 5% 5% 3 4 5% 4 5% 41%	1800
	11th to 20th	Day	19% 10% 3 6% 5% 3 4 5% 5% 6% 36%	1967
	21st to end	Day	21% 10% 4 6% 6% 4 5% 5% 5% 6% 28%	2314
April	1st to 10th	Day	12% 6% 2 5% 6% 3 5% 5% 6% 7% 43%	2160
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 3 5% 4 42%	2134
	21st to end	Day	17% 9% 2 5% 6% 3 4 4 5% 7% 39%	2160
May	1st to 10th	Day	17% 6% 2 6% 6% 3 5% 5% 7% 8% 36%	2250
	11th to 20th	Day	16% 5% 2 6% 5% 3 4 6% 6% 8% 39%	2418
	21st to end	Day	10% 6% 3 6% 6% 3 5% 6% 6% 10% 38%	2725
June	1st to 10th	Day	8% 4 2 5% 5% 3 5% 5% 6% 10% 48%	2520
	11th to 20th	Day	7% 3 2 4 5% 4 5% 6% 6% 9% 49%	2520
	21st to end	Day	5% 4 2 4 5% 4 6% 6% 7% 11% 46%	2520
July	1st to 10th	Day	10% 3 1 4 5% 4 6% 6% 7% 11% 42%	2520
	11th to 20th	Day	4 4 2 4 4 3 5% 5% 7% 9% 52%	2520
	21st to end	Day	8% 4 2 4 6% 3 5% 6% 6% 9% 47%	2663
August	1st to 10th	Day	8% 4 2 5% 6% 4 7% 6% 7% 10% 43%	2296
	11th to 20th	Day	9% 5% 2 5% 5% 4 6% 6% 8% 9% 41%	2125
	21st to end	Day	7% 5% 2 4 5% 3 5% 5% 6% 10% 49%	2376
September	1st to 10th	Day	13% 5% 2 4 4 3 3 3 5% 7% 50%	2160
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 5% 8% 50%	2116
	21st to end	Day	7% 3 2 5% 4 3 4 5% 5% 7% 53%	1980
October	1st to 10th	Day	11% 4 2 4 4 3 4 4 5% 6% 54%	1832
	11th to 20th	Day	15% 5% 2 4 4 2 4 4 5% 6% 50%	1800
	21st to end	Day	19% 7% 2 5% 4 2 3 3 4 6% 46%	1927
November	1st to 10th	Day	11% 5% 3 3 3 2 4 4 4 6% 55%	1672
	11th to 20th	Day	15% 7% 3 5% 5% 2 3 3 5% 6% 46%	1710
	21st to end	Day	15% 10% 3 7% 5% 2 4 3 3 7% 40%	1668
December	1st to 10th	Day	14% 11% 5% 7% 4 2 4 5% 5% 7% 37%	1520
	11th to 20th	Day	15% 9% 4 6% 5% 3 5% 4 5% 6% 37%	1918
	21st to end	Day	22% 14% 4 8% 6% 3 4 4 5% 5% 25%	2783
January	1st to 10th	Night	14% 9% 4 8% 7% 3 5% 4 5% 5% 37%	2648
	11th to 20th	Night	16% 10% 5% 8% 7% 4 3 3 3 5% 35%	2507
	21st to end	Night	15% 11% 4 7% 6% 3 4 4 5% 7% 34%	2574
February	1st to 10th	Night	16% 10% 6% 8% 5% 3 4 4 4 5% 33%	2329
	11th to 20th	Night	15% 8% 3 7% 5% 2 4 4 4 6% 40%	2098
	21st to end	Night	17% 10% 3 7% 5% 3 3 3 4 5% 39%	1776
March	1st to 10th	Night	16% 9% 3 6% 5% 3 4 4 3 5% 42%	2160
	11th to 20th	Night	19% 8% 3 6% 5% 3 4 4 5% 7% 37%	2008
	21st to end	Night	21% 8% 3 6% 5% 3 4 5% 5% 6% 34%	2041
April	1st to 10th	Night	13% 6% 3 5% 5% 3 3 4 4 7% 47%	1800
	11th to 20th	Night	15% 6% 2 5% 4 2 4 4 5% 7% 45%	1775
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 5% 5% 40%	1800
May	1st to 10th	Night	16% 6% 3 5% 5% 3 4 4 5% 8% 41%	1710
	11th to 20th	Night	13% 7% 3 6% 5% 2 4 4 5% 7% 45%	1473
	21st to end	Night	7% 6% 3 5% 7% 4 5% 7% 8% 9% 41%	1551
June	1st to 10th	Night	8% 5% 2 3 4 4 5% 5% 5% 9% 50%	1440
	11th to 20th	Night	6% 3 2 3 5% 3 4 4 6% 9% 55%	1440
	21st to end	Night	5% 3 2 4 4 3 4 6% 7% 10% 52%	1440
July	1st to 10th	Night	9% 2 2 3 3 4 4 5% 8% 9% 50%	1440
	11th to 20th	Night	5% 3 2 4 4 3 4 5% 6% 9% 54%	1440
	21st to end	Night	7% 4 2 5% 4 2 5% 5% 6% 10% 50%	1651
August	1st to 10th	Night	7% 4 2 3 4 3 4 6% 8% 9% 51%	1641
	11th to 20th	Night	6% 5% 2 5% 5% 3 6% 5% 5% 8% 50%	1776
	21st to end	Night	6% 3 2 4 4 3 4 5% 6% 8% 57%	1980
September	1st to 10th	Night	11% 4 3 4 3 2 3 4 4 7% 54%	1800
	11th to 20th	Night	7% 4 2 5% 5% 2 4 5% 5% 9% 52%	1826
	21st to end	Night	9% 4 2 4 4 2 4 4 6% 7% 55%	1980
October	1st to 10th	Night	10% 3 2 4 3 3 3 5% 4 7% 56%	2124
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 5% 6% 52%	2160
	21st to end	Night	19% 7% 2 5% 4 2 4 4 3 5% 46%	2426
November	1st to 10th	Night	10% 5% 2 4 4 3 4 3 4 6% 55%	2406
	11th to 20th	Night	14% 5% 3 5% 4 2 4 4 5% 6% 49%	2470
	21st to end	Night	18% 7% 3 5% 4 2 3 4 4 5% 44%	2508
December	1st to 10th	Night	17% 10% 4 6% 5% 3 4 5% 5% 6% 34%	2660
	11th to 20th	Night	17% 7% 3 5% 6% 2 4 3 4 6% 41%	3279
	21st to end	Night	21% 16% 5% 8% 6% 4 5% 4 4 5% 21%	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	51% 5% 10% 21% 12% 1	1518
	11th to 20th	Day	54% 5% 12% 23% 6%	1547
	21st to end	Day	49% 5% 12% 24% 10% 1	1782
February	1st to 10th	Day	51% 5% 13% 21% 9% 1	1629
	11th to 20th	Day	48% 6% 10% 21% 12% 3	1758
	21st to end	Day	49% 6% 9% 22% 13% 3	1480
March	1st to 10th	Day	45% 5% 9% 25% 14% 2	1800
	11th to 20th	Day	50% 6% 11% 19% 12% 2	1967
	21st to end	Day	56% 7% 12% 17% 8% 1	2314
April	1st to 10th	Day	39% 6% 13% 25% 16% 1	2160
	11th to 20th	Day	46% 4 9% 24% 13% 3	2134
	21st to end	Day	45% 6% 12% 23% 12% 1	2160
May	1st to 10th	Day	44% 9% 15% 23% 8% 1	2250
	11th to 20th	Day	41% 9% 13% 22% 14% 1	2422
	21st to end	Day	40% 9% 16% 20% 13% 2	2725
June	1st to 10th	Day	31% 8% 14% 27% 16% 3	2520
	11th to 20th	Day	30% 8% 15% 23% 22% 3	2520
	21st to end	Day	30% 9% 18% 22% 16% 5%	2520
July	1st to 10th	Day	33% 10% 14% 26% 13% 4	2451
	11th to 20th	Day	27% 8% 16% 28% 19% 3	2445
	21st to end	Day	32% 8% 13% 28% 16% 2	2515
August	1st to 10th	Day	33% 10% 13% 27% 16% 1	2282
	11th to 20th	Day	35% 8% 16% 26% 14% 1	2125
	21st to end	Day	29% 8% 14% 24% 19% 6%	2376
September	1st to 10th	Day	34% 6% 11% 27% 18% 4	2160
	11th to 20th	Day	32% 5% 8% 26% 25% 3	2116
	21st to end	Day	28% 6% 10% 27% 25% 3	1980
October	1st to 10th	Day	32% 5% 10% 25% 25% 3	1832
	11th to 20th	Day	35% 4 10% 24% 22% 5%	1800
	21st to end	Day	41% 4 8% 28% 16% 3	1927
November	1st to 10th	Day	31% 4 14% 26% 20% 5%	1672
	11th to 20th	Day	39% 4 10% 24% 17% 5%	1710
	21st to end	Day	46% 5% 10% 25% 14%	1668
December	1st to 10th	Day	46% 6% 13% 23% 12%	1520
	11th to 20th	Day	48% 5% 10% 23% 12% 2	1915
	21st to end	Day	61% 5% 11% 14% 8% 1	2783
January	1st to 10th	Night	48% 5% 9% 23% 13% 1	2648
	11th to 20th	Night	53% 4 9% 24% 10% 1	2507
	21st to end	Night	50% 5% 13% 20% 11% 1	2574
February	1st to 10th	Night	53% 4 10% 20% 11% 1	2329
	11th to 20th	Night	45% 4 11% 21% 16% 2	2099
	21st to end	Night	48% 4 12% 22% 12% 3	1776
March	1st to 10th	Night	46% 4 9% 23% 16% 3	2160
	11th to 20th	Night	48% 6% 10% 21% 13% 3	2008
	21st to end	Night	52% 5% 13% 20% 9% 1	2041
April	1st to 10th	Night	37% 6% 10% 27% 17% 3	1800
	11th to 20th	Night	39% 5% 11% 23% 19% 4	1775
	21st to end	Night	46% 5% 9% 21% 17% 3	1800
May	1st to 10th	Night	42% 7% 12% 22% 15% 2	1710
	11th to 20th	Night	38% 8% 12% 23% 16% 2	1474
	21st to end	Night	37% 10% 14% 20% 16% 4	1552
June	1st to 10th	Night	29% 9% 14% 25% 20% 3	1440
	11th to 20th	Night	26% 6% 12% 27% 24% 4	1440
	21st to end	Night	25% 8% 15% 29% 17% 7%	1440
July	1st to 10th	Night	28% 8% 15% 30% 15% 5%	1400
	11th to 20th	Night	24% 8% 15% 30% 19% 4	1392
	21st to end	Night	28% 7% 15% 28% 20% 2	1559
August	1st to 10th	Night	27% 10% 14% 30% 16% 2	1631
	11th to 20th	Night	31% 8% 11% 27% 22% 1	1776
	21st to end	Night	24% 8% 13% 26% 24% 6%	1980
September	1st to 10th	Night	31% 6% 10% 27% 20% 7%	1800
	11th to 20th	Night	30% 6% 9% 27% 23% 5%	1826
	21st to end	Night	29% 6% 9% 22% 29% 5%	1980
October	1st to 10th	Night	28% 5% 10% 25% 25% 6%	2124
	11th to 20th	Night	33% 4 8% 27% 21% 7%	2160
	21st to end	Night	41% 5% 9% 21% 20% 4	2426
November	1st to 10th	Night	31% 4 12% 24% 22% 6%	2406
	11th to 20th	Night	37% 4 9% 25% 21% 4	2470
	21st to end	Night	43% 4 10% 24% 18% 1	2508
December	1st to 10th	Night	49% 5% 11% 23% 12% 1	2660
	11th to 20th	Night	44% 4 11% 25% 15% 1	3281
	21st to end	Night	64% 5% 10% 13% 7% 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	22% 21% 30% 20% 7%	270
	11th to 20th	Day	29% 16% 35% 18% 3	252
	21st to end	Day	19% 19% 42% 16% 5%	264
February	1st to 10th	Day	28% 15% 39% 14% 4	240
	11th to 20th	Day	26% 23% 28% 19% 3	240
	21st to end	Day	25% 20% 33% 22% 1 1	198
March	1st to 10th	Day	19% 18% 42% 18% 3	240
	11th to 20th	Day	30% 17% 33% 13% 8%	240
	21st to end	Day	30% 14% 32% 19% 6%	400
April	1st to 10th	Day	14% 16% 37% 23% 9% 1	420
	11th to 20th	Day	19% 15% 30% 22% 13% 1	412
	21st to end	Day	20% 14% 34% 21% 10% 1	420
May	1st to 10th	Day	17% 12% 33% 28% 8% 3	423
	11th to 20th	Day	17% 7% 28% 28% 16% 4	564
	21st to end	Day	11% 5% 27% 30% 18% 7%	679
June	1st to 10th	Day	9% 5% 20% 31% 25% 10%	630
	11th to 20th	Day	8% 3 20% 36% 26% 7%	630
	21st to end	Day	5% 6% 26% 33% 23% 8%	630
July	1st to 10th	Day	11% 2 12% 27% 30% 18%	621
	11th to 20th	Day	5% 3 14% 37% 29% 12%	618
	21st to end	Day	9% 2 14% 33% 30% 12%	684
August	1st to 10th	Day	9% 1 13% 36% 29% 12%	594
	11th to 20th	Day	12% 2 12% 31% 30% 13%	439
	21st to end	Day	9% 2 12% 28% 34% 16%	495
September	1st to 10th	Day	12% 4 16% 25% 31% 12%	450
	11th to 20th	Day	8% 3 12% 28% 33% 16%	450
	21st to end	Day	8% 8% 21% 28% 25% 10%	450
October	1st to 10th	Day	11% 8% 24% 37% 17% 3	302
	11th to 20th	Day	14% 15% 34% 26% 9% 1	270
	21st to end	Day	31% 16% 27% 21% 6%	297
November	1st to 10th	Day	13% 7% 36% 24% 14% 5%	270
	11th to 20th	Day	18% 20% 31% 20% 10% 1	270
	21st to end	Day	23% 16% 27% 25% 9% 1	270
December	1st to 10th	Day	13% 19% 39% 17% 7% 4	270
	11th to 20th	Day	20% 16% 33% 26% 5%	270
	21st to end	Day	25% 18% 36% 15% 5%	297
January	1st to 10th	Night	18% 15% 38% 25% 4	950
	11th to 20th	Night	23% 13% 37% 22% 5%	923
	21st to end	Night	20% 13% 39% 27% 2	990
February	1st to 10th	Night	21% 13% 38% 24% 3	889
	11th to 20th	Night	20% 15% 38% 24% 2	703
	21st to end	Night	24% 13% 38% 24% 2	592
March	1st to 10th	Night	22% 12% 39% 23% 4	720
	11th to 20th	Night	23% 9% 35% 28% 5%	568
	21st to end	Night	23% 10% 34% 27% 6%	594
April	1st to 10th	Night	16% 11% 39% 27% 6%	540
	11th to 20th	Night	17% 11% 32% 29% 11% 1	533
	21st to end	Night	22% 6% 37% 27% 8%	540
May	1st to 10th	Night	17% 8% 30% 35% 10% 1	540
	11th to 20th	Night	16% 5% 24% 38% 16% 1	531
	21st to end	Night	9% 3 23% 39% 21% 3	584
June	1st to 10th	Night	8% 2 19% 40% 30%	540
	11th to 20th	Night	9% 1 17% 48% 22% 3	540
	21st to end	Night	6% 2 20% 41% 29% 1	540
July	1st to 10th	Night	10% 2 8% 36% 38% 7%	525
	11th to 20th	Night	4 1 12% 45% 34% 3	522
	21st to end	Night	8% 2 13% 38% 37% 3	591
August	1st to 10th	Night	8% 2 9% 42% 36% 3	537
	11th to 20th	Night	7% 1 11% 41% 36% 5%	534
	21st to end	Night	7% 2 11% 38% 38% 6%	594
September	1st to 10th	Night	12% 2 11% 39% 30% 6%	540
	11th to 20th	Night	8% 2 15% 36% 33% 5%	566
	21st to end	Night	9% 5% 23% 32% 27% 5%	720
October	1st to 10th	Night	10% 2 26% 34% 23% 4	720
	11th to 20th	Night	17% 10% 28% 32% 13% 1	720
	21st to end	Night	22% 10% 31% 30% 6% 1	792
November	1st to 10th	Night	13% 6% 36% 33% 11% 1	740
	11th to 20th	Night	17% 10% 35% 29% 8%	760
	21st to end	Night	23% 10% 34% 26% 7%	798
December	1st to 10th	Night	21% 12% 33% 28% 5% 1	950
	11th to 20th	Night	23% 9% 31% 31% 6%	950
	21st to end	Night	25% 14% 35% 23% 3	1045

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