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 2024-12-02.

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% 2 6% 4 4 4 4 6% 8% 42%	101462
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 43%	98251
Transparency	Day	38% 6% 12% 25% 16% 2	105067
	Night	40% 5% 11% 24% 18% 3	102774
Seeing	Day	14% 9% 25% 27% 18% 6%	15714
	Night	16% 8% 29% 31% 14% 1	25550

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% 5% 11% 5% 5% 4 4 5% 7% 31%	2527
	11th to 20th	Day	14% 9% 3 8% 8% 8% 4 5% 4 5% 32%	2661
	21st to end	Day	22% 18% 6% 7% 3 3 3 3 3 4 28%	3135
February	1st to 10th	Day	16% 11% 4 7% 4 5% 3 4 5% 6% 35%	1719
	11th to 20th	Day	15% 9% 3 7% 4 5% 4 4 6% 6% 36%	1848
	21st to end	Day	15% 11% 3 8% 4 3 4 3 5% 6% 39%	1570
March	1st to 10th	Day	20% 12% 3 6% 3 3 3 3 3 4 40%	3120
	11th to 20th	Day	12% 8% 2 5% 3 3 4 4 5% 7% 47%	3467
	21st to end	Day	16% 9% 2 7% 6% 5% 5% 4 6% 7% 34%	4061
April	1st to 10th	Day	12% 7% 2 6% 5% 4 5% 4 6% 7% 42%	2280
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 3 4 5% 42%	2254
	21st to end	Day	16% 9% 2 6% 5% 4 4 3 5% 7% 38%	2280
May	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 5% 8% 10% 32%	3925
	11th to 20th	Day	17% 8% 2 7% 4 3 4 4 6% 8% 37%	4296
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 5% 8% 45%	4824
June	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 7% 9% 47%	2660
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 49%	2660
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 10% 46%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 41%	4440
	11th to 20th	Day	4 4 2 6% 4 5% 5% 5% 8% 11% 47%	4440
	21st to end	Day	7% 5% 2 6% 6% 5% 5% 5% 8% 10% 41%	4666
August	1st to 10th	Day	6% 3 2 5% 4 4 5% 4 6% 10% 52%	4044
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 8% 9% 41%	3729
	21st to end	Day	5% 4 2 4 4 4 5% 4 6% 10% 52%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 5% 7% 51%	2280
	11th to 20th	Day	8% 6% 2 5% 4 4 4 3 5% 8% 50%	2233
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 7% 51%	2090
October	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 6% 56%	1933
	11th to 20th	Day	14% 5% 1 5% 3 3 4 3 5% 6% 51%	1900
	21st to end	Day	18% 7% 2 5% 3 3 3 2 4 6% 46%	2034
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 5% 5% 54%	1762
	11th to 20th	Day	15% 8% 2 6% 4 3 3 3 5% 6% 46%	1800
	21st to end	Day	15% 11% 3 7% 4 4 3 3 4 6% 39%	1755
December	1st to 10th	Day	13% 12% 4 7% 4 3 4 4 5% 6% 37%	1528
	11th to 20th	Day	15% 9% 4 7% 4 4 5% 3 5% 6% 37%	1918
	21st to end	Day	22% 15% 4 10% 4 4 3 3 5% 5% 25%	2783
January	1st to 10th	Night	15% 10% 6% 10% 5% 4 4 3 5% 4 34%	4428
	11th to 20th	Night	13% 13% 5% 12% 6% 6% 6% 4 3 4 29%	4308
	21st to end	Night	25% 15% 5% 9% 4 3 3 2 4 5% 25%	4532
February	1st to 10th	Night	16% 11% 5% 8% 5% 4 4 4 4 6% 34%	2459
	11th to 20th	Night	14% 8% 3 8% 4 4 4 3 4 6% 41%	2218
	21st to end	Night	16% 10% 3 8% 5% 4 3 3 5% 5% 39%	1884
March	1st to 10th	Night	21% 11% 2 5% 4 3 3 3 3 4 41%	3733
	11th to 20th	Night	14% 6% 2 5% 3 3 2 2 4 7% 51%	3522
	21st to end	Night	18% 9% 3 7% 5% 4 4 4 5% 6% 35%	3569
April	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 45%	1875
	21st to end	Night	18% 8% 3 6% 5% 4 4 3 5% 5% 40%	1900
May	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 6% 9% 37%	2970
	11th to 20th	Night	14% 7% 2 6% 4 3 4 4 6% 7% 44%	2602
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 5% 7% 8% 41%	2743
June	1st to 10th	Night	8% 5% 2 4 4 5% 4 4 5% 9% 49%	1520
	11th to 20th	Night	6% 4 2 3 4 3 5% 3 6% 9% 55%	1520
	21st to end	Night	5% 4 1 4 3 4 4 4 8% 10% 52%	1520
July	1st to 10th	Night	12% 5% 2 4 3 4 4 3 7% 10% 46%	2530
	11th to 20th	Night	5% 3 2 5% 3 3 4 4 6% 9% 56%	2530
	21st to end	Night	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	2893
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 7% 8% 56%	2892
	11th to 20th	Night	10% 5% 2 5% 4 5% 5% 4 5% 7% 48%	3100
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 10% 57%	3476
September	1st to 10th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	1900
	11th to 20th	Night	7% 5% 2 6% 4 4 4 3 5% 9% 51%	1928
	21st to end	Night	9% 5% 2 5% 4 3 4 3 6% 7% 53%	2090
October	1st to 10th	Night	9% 3 2 4 3 4 3 3 4 7% 58%	2242
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 5% 6% 54%	2280
	21st to end	Night	18% 7% 2 5% 3 3 4 3 4 5% 46%	2561
November	1st to 10th	Night	11% 6% 1 5% 3 4 4 3 4 6% 55%	2536
	11th to 20th	Night	14% 6% 3 5% 4 3 3 3 5% 6% 48%	2600
	21st to end	Night	18% 8% 3 6% 4 3 3 3 4 5% 43%	2640
December	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 5% 6% 34%	2687
	11th to 20th	Night	17% 7% 3 7% 5% 4 4 2 4 6% 41%	3279
	21st to end	Night	21% 17% 5% 10% 5% 5% 4 3 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	50% 6% 11% 23% 8% 1	2528
	11th to 20th	Day	53% 5% 13% 19% 10%	2661
	21st to end	Day	61% 4 8% 17% 9% 1	3135
February	1st to 10th	Day	49% 5% 13% 22% 9% 1	1719
	11th to 20th	Day	47% 6% 10% 21% 13% 3	1858
	21st to end	Day	47% 5% 9% 21% 14% 3	1570
March	1st to 10th	Day	48% 5% 6% 22% 16% 3	3120
	11th to 20th	Day	35% 7% 12% 25% 16% 4	3467
	21st to end	Day	46% 7% 17% 22% 7%	4061
April	1st to 10th	Day	40% 6% 12% 25% 16% 1	2280
	11th to 20th	Day	44% 4 10% 24% 14% 3	2254
	21st to end	Day	45% 6% 13% 23% 12% 1	2280
May	1st to 10th	Day	43% 10% 15% 24% 8% 1	3960
	11th to 20th	Day	44% 8% 11% 25% 11% 1	4300
	21st to end	Day	35% 8% 12% 23% 21% 1	4837
June	1st to 10th	Day	32% 8% 14% 27% 16% 3	2660
	11th to 20th	Day	29% 8% 14% 23% 23% 3	2660
	21st to end	Day	30% 9% 17% 23% 16% 5%	2660
July	1st to 10th	Day	38% 7% 10% 29% 13% 3	4371
	11th to 20th	Day	28% 8% 14% 30% 18% 2	4365
	21st to end	Day	35% 9% 18% 25% 12% 1	4519
August	1st to 10th	Day	27% 7% 14% 37% 15%	4030
	11th to 20th	Day	37% 6% 14% 25% 17%	3765
	21st to end	Day	27% 7% 15% 33% 15% 3	4179
September	1st to 10th	Day	34% 5% 10% 26% 21% 5%	2280
	11th to 20th	Day	32% 6% 8% 25% 25% 3	2233
	21st to end	Day	31% 6% 10% 27% 24% 3	2090
October	1st to 10th	Day	19% 3 6% 19% 45% 8%	3206
	11th to 20th	Day	24% 3 8% 21% 37% 8%	3140
	21st to end	Day	33% 3 7% 28% 27% 3	3336
November	1st to 10th	Day	32% 4 13% 26% 20% 5%	1762
	11th to 20th	Day	40% 4 10% 24% 18% 6%	1800
	21st to end	Day	46% 5% 10% 25% 14%	1755
December	1st to 10th	Day	46% 6% 13% 23% 11%	1528
	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	52% 4 11% 23% 9% 1	4428
	11th to 20th	Night	58% 4 8% 20% 8% 1	4308
	21st to end	Night	62% 5% 9% 17% 7% 1	4532
February	1st to 10th	Night	52% 5% 9% 21% 12% 1	2459
	11th to 20th	Night	44% 4 11% 21% 17% 2	2219
	21st to end	Night	47% 4 11% 21% 14% 3	1884
March	1st to 10th	Night	47% 5% 8% 25% 14% 2	3733
	11th to 20th	Night	34% 5% 12% 24% 21% 4	3522
	21st to end	Night	49% 6% 12% 25% 8% 1	3569
April	1st to 10th	Night	38% 5% 10% 27% 16% 4	1900
	11th to 20th	Night	38% 5% 10% 24% 19% 3	1875
	21st to end	Night	46% 5% 9% 21% 16% 2	1900
May	1st to 10th	Night	42% 9% 16% 19% 12% 1	3000
	11th to 20th	Night	38% 8% 13% 27% 14% 1	2603
	21st to end	Night	35% 10% 12% 23% 17% 2	2751
June	1st to 10th	Night	29% 9% 14% 25% 19% 4	1520
	11th to 20th	Night	26% 6% 11% 27% 25% 4	1520
	21st to end	Night	25% 8% 15% 28% 18% 7%	1520
July	1st to 10th	Night	33% 6% 12% 30% 16% 3	2490
	11th to 20th	Night	23% 7% 12% 37% 19% 3	2482
	21st to end	Night	36% 8% 19% 23% 13% 1	2801
August	1st to 10th	Night	23% 8% 16% 35% 17% 1	2882
	11th to 20th	Night	35% 6% 11% 24% 22% 1	3130
	21st to end	Night	22% 6% 14% 35% 19% 3	3476
September	1st to 10th	Night	30% 5% 9% 27% 22% 7%	1900
	11th to 20th	Night	31% 6% 9% 27% 23% 5%	1928
	21st to end	Night	31% 6% 9% 22% 28% 5%	2090
October	1st to 10th	Night	17% 3 7% 19% 47% 8%	3736
	11th to 20th	Night	22% 2 8% 22% 38% 7%	3779
	21st to end	Night	32% 4 9% 25% 26% 6%	4209
November	1st to 10th	Night	32% 4 11% 24% 22% 6%	2536
	11th to 20th	Night	38% 4 9% 24% 21% 4	2600
	21st to end	Night	44% 4 10% 24% 17% 1	2640
December	1st to 10th	Night	49% 5% 11% 22% 12% 1	2687
	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	28% 16% 35% 18% 3	270
	21st to end	Day	20% 18% 40% 16% 5%	297
February	1st to 10th	Day	26% 14% 39% 16% 5%	270
	11th to 20th	Day	24% 21% 31% 19% 3 1	270
	21st to end	Day	23% 20% 36% 20% 1	225
March	1st to 10th	Day	18% 18% 43% 18% 3	270
	11th to 20th	Day	26% 19% 34% 13% 7%	270
	21st to end	Day	28% 15% 33% 18% 6%	441
April	1st to 10th	Day	15% 17% 37% 22% 8% 1	460
	11th to 20th	Day	19% 16% 30% 22% 12% 1	452
	21st to end	Day	20% 15% 34% 21% 9% 1	460
May	1st to 10th	Day	17% 11% 33% 27% 8% 3	463
	11th to 20th	Day	17% 7% 28% 28% 16% 4	611
	21st to end	Day	11% 6% 29% 30% 17% 7%	734
June	1st to 10th	Day	10% 6% 21% 31% 24% 9%	680
	11th to 20th	Day	8% 3 20% 37% 26% 7%	680
	21st to end	Day	5% 5% 27% 33% 22% 7%	680
July	1st to 10th	Day	12% 3 13% 27% 28% 17%	671
	11th to 20th	Day	5% 3 13% 37% 29% 12%	668
	21st to end	Day	8% 3 14% 34% 30% 12%	739
August	1st to 10th	Day	8% 1 15% 35% 29% 11%	642
	11th to 20th	Day	12% 3 14% 31% 28% 12%	479
	21st to end	Day	9% 1 12% 29% 34% 15%	539
September	1st to 10th	Day	11% 4 16% 26% 31% 12%	490
	11th to 20th	Day	9% 4 11% 28% 33% 16%	490
	21st to end	Day	10% 9% 21% 27% 24% 9%	490
October	1st to 10th	Day	10% 7% 24% 35% 17% 6%	333
	11th to 20th	Day	13% 14% 32% 27% 10% 2	300
	21st to end	Day	28% 17% 26% 22% 6%	330
November	1st to 10th	Day	15% 7% 36% 24% 13% 5%	300
	11th to 20th	Day	18% 20% 32% 19% 10% 2	300
	21st to end	Day	24% 17% 25% 25% 8% 1	300
December	1st to 10th	Day	13% 19% 40% 17% 7% 4	273
	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% 14% 36% 22% 5%	950
	21st to end	Night	21% 12% 38% 27% 2	1045
February	1st to 10th	Night	21% 14% 37% 25% 3	939
	11th to 20th	Night	20% 15% 39% 25% 2	743
	21st to end	Night	23% 13% 38% 24% 2	628
March	1st to 10th	Night	22% 12% 39% 23% 4	760
	11th to 20th	Night	22% 10% 35% 29% 5%	599
	21st to end	Night	22% 11% 34% 27% 6%	627
April	1st to 10th	Night	16% 13% 39% 26% 6%	570
	11th to 20th	Night	16% 12% 32% 29% 11% 1	563
	21st to end	Night	21% 7% 37% 27% 7%	570
May	1st to 10th	Night	17% 7% 31% 34% 10% 1	570
	11th to 20th	Night	16% 5% 24% 37% 17% 1	561
	21st to end	Night	9% 3 24% 40% 21% 3	617
June	1st to 10th	Night	8% 3 20% 40% 29%	570
	11th to 20th	Night	9% 1 17% 49% 22% 3	570
	21st to end	Night	6% 2 20% 42% 28% 1	570
July	1st to 10th	Night	10% 2 9% 36% 36% 6%	555
	11th to 20th	Night	4 1 13% 45% 34% 3	552
	21st to end	Night	8% 2 12% 39% 36% 3	624
August	1st to 10th	Night	8% 2 10% 41% 35% 3	567
	11th to 20th	Night	7% 1 12% 41% 34% 5%	564
	21st to end	Night	6% 1 10% 38% 38% 6%	627
September	1st to 10th	Night	12% 3 11% 39% 30% 6%	570
	11th to 20th	Night	9% 2 15% 36% 33% 5%	598
	21st to end	Night	10% 5% 23% 32% 26% 4	760
October	1st to 10th	Night	10% 2 26% 35% 24% 4	760
	11th to 20th	Night	16% 10% 28% 32% 14% 1	760
	21st to end	Night	21% 10% 31% 31% 6%	836
November	1st to 10th	Night	14% 6% 36% 33% 10% 1	780
	11th to 20th	Night	17% 11% 35% 29% 8%	800
	21st to end	Night	23% 11% 34% 25% 7%	840
December	1st to 10th	Night	21% 13% 34% 28% 5% 1	960
	11th to 20th	Night	23% 9% 31% 31% 6%	950
	21st to end	Night	25% 14% 35% 23% 3	1045

Page generated at 2024-11-29 09:43:07. Version ac07

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