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 2026-03-03.

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% 7% 3 7% 4 4 4 4 6% 8% 42%	131060
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	130370
Transparency	Day	39% 6% 12% 25% 16% 2	134624
	Night	40% 5% 11% 24% 17% 3	134871
Seeing	Day	15% 10% 25% 26% 18% 6%	17362
	Night	16% 9% 29% 31% 14% 1	27322

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% 10% 5% 13% 6% 5% 4 4 5% 6% 31%	4691
	11th to 20th	Day	8% 9% 3 7% 7% 7% 4 4 5% 6% 40%	4967
	21st to end	Day	17% 14% 4 8% 4 4 4 3 4 5% 34%	5812
February	1st to 10th	Day	16% 11% 4 8% 3 4 3 4 5% 6% 35%	1901
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 6% 6% 36%	2048
	21st to end	Day	14% 10% 3 8% 4 3 4 3 5% 5% 40%	1730
March	1st to 10th	Day	17% 11% 2 5% 3 2 3 2 3 4 47%	4500
	11th to 20th	Day	9% 8% 2 4 3 3 4 3 6% 8% 50%	4953
	21st to end	Day	14% 8% 3 7% 6% 5% 5% 4 6% 8% 36%	5821
April	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 41%	2400
	11th to 20th	Day	16% 8% 3 7% 4 4 4 2 4 5% 42%	2374
	21st to end	Day	16% 9% 2 6% 5% 4 4 3 5% 7% 38%	2400
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 8% 10% 30%	5635
	11th to 20th	Day	15% 8% 3 7% 4 4 5% 4 6% 8% 37%	6174
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	6935
June	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 47%	2800
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 6% 9% 48%	2800
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 10% 47%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	6360
	11th to 20th	Day	5% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	6707
August	1st to 10th	Day	6% 3 2 4 4 4 4 4 6% 10% 52%	4172
	11th to 20th	Day	12% 5% 2 6% 3 5% 5% 4 8% 9% 42%	3849
	21st to end	Day	5% 4 2 4 4 4 5% 4 6% 10% 52%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 7% 51%	2400
	11th to 20th	Day	8% 6% 2 5% 4 4 4 3 5% 8% 51%	2350
	21st to end	Day	8% 4 2 6% 4 4 5% 4 5% 7% 51%	2200
October	1st to 10th	Day	10% 4 2 4 3 3 3 3 5% 6% 56%	2035
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 6% 52%	2000
	21st to end	Day	19% 7% 2 5% 3 3 3 2 4 6% 46%	2141
November	1st to 10th	Day	11% 6% 2 4 4 3 4 3 5% 5% 54%	1852
	11th to 20th	Day	15% 8% 2 6% 4 3 3 3 5% 6% 46%	1890
	21st to end	Day	16% 11% 3 7% 4 4 3 2 4 6% 39%	1843
December	1st to 10th	Day	9% 10% 3 6% 3 3 3 3 4 5% 49%	2690
	11th to 20th	Day	14% 7% 5% 9% 5% 5% 4 3 4 5% 37%	3088
	21st to end	Day	25% 16% 5% 10% 5% 4 3 3 4 4 23%	4070
January	1st to 10th	Night	13% 10% 5% 10% 5% 4 4 3 5% 5% 37%	8232
	11th to 20th	Night	9% 10% 4 11% 5% 5% 5% 4 4 5% 39%	8008
	21st to end	Night	17% 11% 4 9% 4 4 4 3 5% 6% 33%	8402
February	1st to 10th	Night	15% 11% 5% 8% 5% 4 4 3 5% 6% 35%	2717
	11th to 20th	Night	15% 9% 3 8% 4 3 4 3 4 7% 40%	2458
	21st to end	Night	15% 9% 3 7% 5% 4 3 3 5% 5% 42%	2076
March	1st to 10th	Night	18% 11% 3 6% 4 3 3 3 4 4 41%	5398
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 4 7% 51%	5036
	21st to end	Night	17% 9% 3 7% 5% 4 4 4 5% 6% 35%	5107
April	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 5% 6% 46%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 46%	1975
	21st to end	Night	18% 8% 3 6% 5% 4 4 3 5% 5% 40%	2000
May	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 6% 9% 32%	4260
	11th to 20th	Night	13% 7% 2 6% 4 3 4 4 7% 8% 42%	3731
	21st to end	Night	12% 7% 3 6% 5% 4 5% 5% 6% 8% 39%	3942
June	1st to 10th	Night	8% 6% 2 5% 4 5% 4 5% 6% 9% 48%	1600
	11th to 20th	Night	6% 4 2 4 4 3 5% 3 5% 9% 54%	1600
	21st to end	Night	5% 4 1 4 3 5% 4 4 8% 10% 52%	1600
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 7% 9% 48%	3618
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 4 8% 10% 46%	3617
	21st to end	Night	6% 5% 2 7% 5% 5% 7% 5% 8% 11% 39%	4160
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 7% 9% 56%	2984
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 4 5% 7% 49%	3200
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 10% 58%	3586
September	1st to 10th	Night	10% 4 2 5% 3 3 3 3 5% 7% 54%	2000
	11th to 20th	Night	7% 5% 2 6% 4 4 4 3 5% 9% 51%	2030
	21st to end	Night	10% 4 2 5% 4 3 4 3 5% 6% 53%	2200
October	1st to 10th	Night	9% 3 2 4 3 4 3 3 4 6% 58%	2360
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 5% 6% 54%	2400
	21st to end	Night	18% 7% 2 6% 3 3 4 3 4 5% 46%	2696
November	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 5% 6% 54%	2666
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 6% 48%	2730
	21st to end	Night	18% 8% 3 6% 4 3 3 3 4 5% 42%	2772
December	1st to 10th	Night	13% 8% 3 7% 5% 3 3 3 4 5% 45%	4720
	11th to 20th	Night	15% 7% 3 7% 4 4 4 3 5% 7% 41%	5339
	21st to end	Night	26% 16% 5% 9% 4 4 4 3 4 4 21%	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	53% 5% 11% 22% 8% 1	4692
	11th to 20th	Day	44% 3 12% 23% 15% 2	4967
	21st to end	Day	54% 3 9% 18% 12% 4	5812
February	1st to 10th	Day	49% 5% 13% 23% 9% 1	1901
	11th to 20th	Day	48% 5% 9% 21% 14% 3	2058
	21st to end	Day	46% 5% 9% 22% 15% 3	1730
March	1st to 10th	Day	42% 3 5% 23% 24% 2	4500
	11th to 20th	Day	32% 5% 10% 27% 21% 5%	4953
	21st to end	Day	45% 6% 15% 25% 8% 1	5823
April	1st to 10th	Day	41% 6% 12% 24% 16% 1	2400
	11th to 20th	Day	45% 4 10% 24% 14% 3	2374
	21st to end	Day	45% 6% 13% 23% 12% 1	2400
May	1st to 10th	Day	46% 9% 13% 21% 10%	5670
	11th to 20th	Day	44% 8% 11% 24% 13% 1	6178
	21st to end	Day	41% 7% 11% 22% 19% 1	6949
June	1st to 10th	Day	32% 8% 14% 27% 16% 3	2800
	11th to 20th	Day	30% 8% 14% 23% 22% 3	2800
	21st to end	Day	30% 9% 17% 23% 17% 5%	2800
July	1st to 10th	Day	33% 6% 10% 30% 19% 2	6291
	11th to 20th	Day	36% 9% 15% 25% 14% 1	6285
	21st to end	Day	35% 8% 17% 29% 10% 1	6516
August	1st to 10th	Day	26% 7% 14% 37% 15%	4158
	11th to 20th	Day	37% 6% 15% 26% 17%	3885
	21st to end	Day	27% 7% 15% 33% 15% 3	4311
September	1st to 10th	Day	33% 5% 11% 26% 21% 4	2400
	11th to 20th	Day	31% 5% 9% 26% 26% 3	2350
	21st to end	Day	31% 6% 9% 26% 25% 3	2200
October	1st to 10th	Day	19% 3 6% 19% 45% 8%	3308
	11th to 20th	Day	23% 3 8% 21% 37% 8%	3240
	21st to end	Day	34% 3 7% 27% 27% 3	3443
November	1st to 10th	Day	31% 4 13% 27% 20% 5%	1852
	11th to 20th	Day	40% 4 9% 24% 17% 6%	1890
	21st to end	Day	47% 5% 10% 25% 14%	1843
December	1st to 10th	Day	37% 4 14% 29% 15%	2690
	11th to 20th	Day	49% 4 10% 22% 13% 2	3085
	21st to end	Day	66% 4 11% 12% 6% 1	4070
January	1st to 10th	Night	50% 4 14% 24% 7%	8233
	11th to 20th	Night	48% 3 8% 22% 17% 3	8008
	21st to end	Night	52% 4 11% 17% 14% 1	8402
February	1st to 10th	Night	51% 4 10% 22% 11% 1	2717
	11th to 20th	Night	45% 4 10% 22% 17% 2	2459
	21st to end	Night	44% 4 11% 24% 14% 3	2076
March	1st to 10th	Night	46% 4 6% 22% 20% 2	5398
	11th to 20th	Night	35% 4 10% 24% 22% 5%	5036
	21st to end	Night	49% 5% 10% 24% 11% 1	5107
April	1st to 10th	Night	38% 5% 10% 26% 16% 3	2000
	11th to 20th	Night	38% 5% 10% 24% 19% 3	1975
	21st to end	Night	46% 5% 9% 22% 16% 2	2000
May	1st to 10th	Night	46% 9% 14% 19% 11% 1	4290
	11th to 20th	Night	38% 7% 13% 25% 15% 2	3732
	21st to end	Night	40% 8% 12% 23% 16% 2	3950
June	1st to 10th	Night	30% 9% 14% 25% 19% 4	1600
	11th to 20th	Night	27% 6% 11% 27% 25% 4	1600
	21st to end	Night	25% 8% 15% 28% 18% 7%	1600
July	1st to 10th	Night	33% 5% 11% 29% 20% 2	3580
	11th to 20th	Night	30% 9% 16% 30% 13% 2	3572
	21st to end	Night	35% 8% 20% 25% 11% 1	4040
August	1st to 10th	Night	23% 8% 16% 35% 16% 1	2974
	11th to 20th	Night	34% 6% 12% 25% 22% 1	3230
	21st to end	Night	22% 6% 14% 35% 20% 3	3586
September	1st to 10th	Night	31% 5% 9% 27% 22% 6%	2000
	11th to 20th	Night	30% 6% 8% 27% 24% 5%	2030
	21st to end	Night	31% 6% 9% 21% 28% 5%	2200
October	1st to 10th	Night	17% 3 7% 18% 47% 8%	3854
	11th to 20th	Night	23% 2 8% 22% 38% 7%	3899
	21st to end	Night	32% 4 8% 24% 26% 6%	4344
November	1st to 10th	Night	32% 4 12% 24% 23% 6%	2666
	11th to 20th	Night	37% 4 9% 25% 21% 4	2730
	21st to end	Night	45% 4 9% 24% 16% 1	2772
December	1st to 10th	Night	42% 4 10% 29% 15% 1	4720
	11th to 20th	Night	43% 4 13% 23% 16%	5341
	21st to end	Night	66% 4 11% 13% 5% 1	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	22% 21% 32% 19% 6%	330
	11th to 20th	Day	24% 21% 37% 16% 3	330
	21st to end	Day	19% 19% 42% 15% 4	363
February	1st to 10th	Day	24% 13% 41% 16% 5%	330
	11th to 20th	Day	25% 23% 31% 16% 3 1	330
	21st to end	Day	22% 19% 38% 19% 1	273
March	1st to 10th	Day	17% 18% 43% 19% 3	306
	11th to 20th	Day	26% 21% 32% 14% 7%	300
	21st to end	Day	28% 15% 33% 18% 5%	482
April	1st to 10th	Day	18% 17% 35% 21% 8% 1	500
	11th to 20th	Day	20% 17% 29% 21% 12% 1	492
	21st to end	Day	19% 14% 34% 22% 9% 1	500
May	1st to 10th	Day	17% 12% 33% 27% 8% 2	503
	11th to 20th	Day	17% 8% 28% 27% 16% 4	658
	21st to end	Day	12% 7% 29% 29% 16% 6%	789
June	1st to 10th	Day	10% 6% 21% 31% 23% 9%	730
	11th to 20th	Day	9% 3 20% 36% 25% 7%	730
	21st to end	Day	5% 5% 26% 33% 24% 7%	730
July	1st to 10th	Day	11% 3 13% 26% 29% 17%	721
	11th to 20th	Day	5% 3 14% 37% 29% 11%	718
	21st to end	Day	8% 3 13% 33% 31% 12%	794
August	1st to 10th	Day	8% 2 14% 36% 29% 10%	690
	11th to 20th	Day	11% 3 13% 30% 30% 13%	519
	21st to end	Day	8% 2 12% 29% 34% 15%	583
September	1st to 10th	Day	10% 4 17% 27% 32% 12%	530
	11th to 20th	Day	8% 3 11% 27% 35% 16%	530
	21st to end	Day	10% 9% 20% 27% 25% 11%	530
October	1st to 10th	Day	9% 7% 23% 37% 18% 6%	365
	11th to 20th	Day	13% 14% 31% 27% 12% 3	330
	21st to end	Day	29% 17% 26% 21% 6%	363
November	1st to 10th	Day	15% 9% 36% 24% 12% 4	330
	11th to 20th	Day	18% 19% 32% 20% 9% 2	330
	21st to end	Day	26% 16% 24% 25% 8% 1	330
December	1st to 10th	Day	15% 19% 39% 17% 6% 4	330
	11th to 20th	Day	20% 17% 35% 23% 5%	330
	21st to end	Day	26% 17% 37% 15% 5%	363
January	1st to 10th	Night	18% 14% 39% 25% 4	1050
	11th to 20th	Night	22% 15% 37% 22% 4	1050
	21st to end	Night	20% 12% 40% 26% 2	1155
February	1st to 10th	Night	20% 14% 38% 25% 3	1037
	11th to 20th	Night	21% 16% 38% 24% 2	823
	21st to end	Night	21% 13% 40% 24% 1	692
March	1st to 10th	Night	22% 13% 38% 23% 4	809
	11th to 20th	Night	22% 10% 34% 29% 5%	630
	21st to end	Night	22% 12% 34% 27% 5%	660
April	1st to 10th	Night	17% 12% 39% 27% 6%	600
	11th to 20th	Night	17% 12% 32% 29% 10% 1	593
	21st to end	Night	21% 7% 37% 28% 7%	600
May	1st to 10th	Night	17% 7% 31% 34% 10% 1	600
	11th to 20th	Night	16% 5% 25% 37% 16% 1	591
	21st to end	Night	10% 4 24% 39% 20% 3	650
June	1st to 10th	Night	9% 3 20% 40% 28%	600
	11th to 20th	Night	9% 1 17% 49% 22% 3	600
	21st to end	Night	6% 2 19% 43% 29% 1	600
July	1st to 10th	Night	11% 2 9% 36% 36% 6%	585
	11th to 20th	Night	4 1 13% 45% 35% 3	582
	21st to end	Night	8% 2 11% 39% 37% 3	657
August	1st to 10th	Night	8% 2 10% 42% 35% 3	597
	11th to 20th	Night	7% 1 11% 41% 35% 5%	594
	21st to end	Night	6% 2 10% 38% 38% 5%	660
September	1st to 10th	Night	11% 3 11% 40% 30% 6%	600
	11th to 20th	Night	8% 2 15% 36% 34% 5%	630
	21st to end	Night	11% 5% 22% 31% 27% 5%	800
October	1st to 10th	Night	9% 2 25% 37% 23% 4	800
	11th to 20th	Night	16% 9% 28% 32% 15% 1	800
	21st to end	Night	22% 11% 31% 31% 6%	880
November	1st to 10th	Night	13% 7% 37% 32% 10% 1	820
	11th to 20th	Night	17% 10% 36% 29% 8%	840
	21st to end	Night	24% 11% 33% 25% 6%	882
December	1st to 10th	Night	21% 12% 34% 28% 4 1	1050
	11th to 20th	Night	23% 10% 32% 30% 5%	1050
	21st to end	Night	26% 14% 36% 22% 3	1155

Page generated at 2026-02-28 00:40:25. Version ac07

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