Prairie State College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prairie State College clear sky chart from 2005-11-06 to 2026-01-17.

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% 9% 3 8% 5% 4 5% 4 6% 7% 36%	127252
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	124648
Transparency	Day	28% 39% 8% 15% 9% 1	130789
	Night	30% 38% 7% 14% 10% 1	129184
Seeing	Day	16% 10% 28% 28% 14% 4	17381
	Night	18% 10% 30% 29% 12% 1	27299

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% 12% 8% 13% 6% 5% 5% 4 6% 6% 24%	3697
	11th to 20th	Day	11% 13% 7% 14% 6% 5% 5% 6% 5% 4 25%	3668
	21st to end	Day	17% 19% 5% 9% 4 3 3 3 4 6% 28%	4056
February	1st to 10th	Day	15% 14% 5% 10% 6% 5% 4 3 5% 7% 25%	1885
	11th to 20th	Day	13% 12% 5% 9% 5% 6% 3 3 5% 6% 33%	1948
	21st to end	Day	14% 13% 5% 11% 4 4 5% 3 6% 6% 29%	1650
March	1st to 10th	Day	18% 12% 3 6% 4 3 3 2 4 5% 39%	4480
	11th to 20th	Day	8% 9% 4 7% 4 5% 5% 4 6% 8% 41%	4661
	21st to end	Day	17% 15% 6% 11% 6% 5% 5% 4 5% 6% 19%	5967
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 4 4 6% 31%	2400
	11th to 20th	Day	16% 9% 2 6% 6% 5% 4 4 5% 7% 35%	2374
	21st to end	Day	19% 9% 2 7% 5% 4 5% 4 5% 7% 33%	2525
May	1st to 10th	Day	18% 8% 3 6% 5% 5% 5% 4 6% 8% 33%	5862
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	6043
	21st to end	Day	14% 7% 2 7% 4 4 5% 4 5% 8% 40%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 5% 8% 45%	2800
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 6% 9% 45%	2800
	21st to end	Day	10% 5% 2 7% 5% 6% 6% 5% 8% 9% 38%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 6% 6% 4 8% 10% 36%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 8% 40%	6924
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 4 6% 8% 48%	4403
	11th to 20th	Day	9% 5% 2 7% 4 4 5% 4 7% 9% 45%	3894
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 5% 4 4 6% 8% 48%	2400
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 6% 8% 51%	2250
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 43%	2200
October	1st to 10th	Day	11% 6% 2 5% 4 5% 5% 3 4 7% 46%	2185
	11th to 20th	Day	18% 8% 3 6% 5% 4 5% 4 5% 7% 36%	1969
	21st to end	Day	19% 10% 3 7% 5% 3 4 3 6% 6% 34%	1980
November	1st to 10th	Day	12% 9% 3 6% 4 3 3 3 5% 7% 45%	1852
	11th to 20th	Day	16% 10% 3 9% 5% 4 4 3 4 7% 35%	1890
	21st to end	Day	17% 12% 3 9% 5% 4 4 3 4 5% 34%	1874
December	1st to 10th	Day	9% 11% 4 9% 4 4 5% 3 4 6% 40%	2690
	11th to 20th	Day	12% 15% 6% 11% 6% 5% 5% 4 6% 6% 23%	3088
	21st to end	Day	23% 22% 7% 13% 5% 4 4 3 4 4 11%	4070
January	1st to 10th	Night	14% 14% 6% 12% 5% 4 4 3 4 5% 28%	6488
	11th to 20th	Night	14% 15% 7% 13% 6% 6% 4 5% 6% 4 21%	6445
	21st to end	Night	19% 14% 6% 8% 4 4 4 3 4 4 31%	6907
February	1st to 10th	Night	16% 14% 4 9% 6% 4 4 3 4 5% 31%	2515
	11th to 20th	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 35%	2338
	21st to end	Night	18% 13% 4 8% 4 3 4 3 4 5% 34%	1980
March	1st to 10th	Night	16% 11% 4 7% 4 4 3 2 4 5% 40%	5373
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	5313
	21st to end	Night	17% 11% 6% 9% 6% 6% 5% 4 4 5% 28%	4990
April	1st to 10th	Night	15% 9% 3 7% 4 5% 4 4 5% 7% 38%	2000
	11th to 20th	Night	17% 9% 3 7% 5% 4 3 3 5% 7% 38%	1975
	21st to end	Night	17% 8% 3 8% 5% 5% 4 3 5% 6% 36%	1873
May	1st to 10th	Night	18% 10% 3 7% 5% 4 4 4 5% 7% 34%	4033
	11th to 20th	Night	13% 7% 3 5% 4 4 4 4 6% 9% 42%	3843
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	9% 6% 2 6% 4 5% 4 3 5% 7% 49%	1600
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 9% 45%	1600
	21st to end	Night	9% 6% 2 5% 6% 5% 6% 4 6% 8% 43%	1600
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 3 6% 9% 49%	3617
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 4 6% 8% 44%	3947
August	1st to 10th	Night	6% 5% 2 6% 5% 4 5% 4 7% 9% 47%	2751
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 5% 8% 51%	3138
	21st to end	Night	8% 4 2 5% 4 3 4 4 5% 9% 52%	3586
September	1st to 10th	Night	10% 6% 2 6% 3 3 3 3 4 6% 53%	2000
	11th to 20th	Night	8% 4 2 4 3 4 4 3 6% 8% 55%	2130
	21st to end	Night	10% 6% 2 6% 5% 4 5% 3 6% 7% 46%	2200
October	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 4 6% 52%	2210
	11th to 20th	Night	16% 7% 2 6% 4 3 3 3 3 6% 46%	2430
	21st to end	Night	20% 9% 2 6% 4 4 4 3 5% 6% 37%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 5% 7% 45%	2666
	11th to 20th	Night	14% 9% 4 8% 5% 4 4 3 5% 6% 37%	2730
	21st to end	Night	18% 11% 3 8% 4 4 4 3 4 6% 35%	2741
December	1st to 10th	Night	12% 9% 3 8% 4 4 3 3 5% 6% 42%	4720
	11th to 20th	Night	16% 15% 4 8% 5% 4 4 3 5% 5% 29%	5339
	21st to end	Night	24% 18% 6% 11% 5% 5% 3 3 4 5% 17%	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	45% 28% 9% 12% 6%	3698
	11th to 20th	Day	44% 27% 8% 7% 11% 4	3668
	21st to end	Day	42% 28% 9% 14% 6% 1	4056
February	1st to 10th	Day	23% 61% 4 6% 4 1	1885
	11th to 20th	Day	21% 61% 3 10% 5% 1	1958
	21st to end	Day	19% 62% 5% 9% 4 2	1650
March	1st to 10th	Day	34% 29% 5% 19% 10% 3	4480
	11th to 20th	Day	26% 30% 10% 27% 7%	4661
	21st to end	Day	47% 33% 8% 8% 4	5969
April	1st to 10th	Day	24% 62% 3 8% 3	2400
	11th to 20th	Day	17% 63% 5% 9% 4 2	2374
	21st to end	Day	18% 62% 5% 7% 5% 2	2525
May	1st to 10th	Day	33% 32% 9% 15% 9% 2	5897
	11th to 20th	Day	29% 32% 8% 20% 10% 1	6047
	21st to end	Day	28% 30% 9% 20% 12% 2	6949
June	1st to 10th	Day	12% 63% 5% 11% 9% 1	2800
	11th to 20th	Day	16% 63% 5% 10% 5% 1	2800
	21st to end	Day	14% 62% 6% 10% 6% 1	2800
July	1st to 10th	Day	31% 30% 11% 18% 10% 1	6291
	11th to 20th	Day	26% 30% 13% 21% 10%	6285
	21st to end	Day	29% 29% 12% 18% 12% 1	6728
August	1st to 10th	Day	21% 38% 11% 20% 10%	4388
	11th to 20th	Day	21% 38% 7% 23% 10%	3931
	21st to end	Day	21% 39% 9% 18% 13% 1	4311
September	1st to 10th	Day	15% 58% 6% 13% 8% 1	2400
	11th to 20th	Day	12% 58% 4 14% 10% 2	2250
	21st to end	Day	18% 58% 5% 11% 8% 1	2200
October	1st to 10th	Day	15% 36% 3 26% 19% 1	3562
	11th to 20th	Day	18% 38% 8% 17% 19%	3185
	21st to end	Day	27% 37% 6% 18% 11% 2	3180
November	1st to 10th	Day	21% 54% 8% 10% 6% 1	1852
	11th to 20th	Day	23% 54% 6% 10% 7% 1	1890
	21st to end	Day	28% 55% 5% 9% 3	1874
December	1st to 10th	Day	28% 35% 12% 19% 6%	2690
	11th to 20th	Day	41% 33% 9% 13% 3	3085
	21st to end	Day	62% 27% 4 6% 2	4070
January	1st to 10th	Night	45% 28% 6% 14% 7%	6488
	11th to 20th	Night	51% 27% 7% 6% 9% 1	6445
	21st to end	Night	41% 28% 9% 12% 9% 1	6907
February	1st to 10th	Night	21% 61% 4 7% 6% 1	2515
	11th to 20th	Night	20% 60% 3 11% 7%	2339
	21st to end	Night	18% 61% 4 8% 6% 1	1980
March	1st to 10th	Night	34% 29% 7% 19% 8% 2	5373
	11th to 20th	Night	25% 30% 8% 23% 12% 1	5313
	21st to end	Night	42% 31% 6% 15% 7%	4990
April	1st to 10th	Night	20% 62% 4 10% 4 1	2000
	11th to 20th	Night	17% 62% 5% 8% 6% 2	1975
	21st to end	Night	19% 62% 3 8% 7% 1	1873
May	1st to 10th	Night	38% 30% 8% 10% 11% 4	4060
	11th to 20th	Night	27% 32% 11% 19% 9% 2	3844
	21st to end	Night	33% 30% 6% 16% 13% 3	3950
June	1st to 10th	Night	13% 62% 4 11% 9% 1	1600
	11th to 20th	Night	17% 63% 4 9% 7% 1	1600
	21st to end	Night	14% 61% 6% 11% 8%	1600
July	1st to 10th	Night	30% 29% 10% 20% 8% 3	3580
	11th to 20th	Night	24% 29% 12% 18% 16% 1	3572
	21st to end	Night	28% 29% 11% 18% 13% 2	3832
August	1st to 10th	Night	22% 39% 10% 19% 9%	2742
	11th to 20th	Night	21% 38% 9% 19% 13%	3166
	21st to end	Night	20% 38% 8% 22% 11% 1	3586
September	1st to 10th	Night	13% 57% 6% 13% 11% 1	2000
	11th to 20th	Night	10% 57% 4 15% 12% 1	2130
	21st to end	Night	19% 58% 4 12% 7% 1	2200
October	1st to 10th	Night	11% 35% 5% 27% 20% 2	3613
	11th to 20th	Night	21% 36% 5% 15% 21% 1	3953
	21st to end	Night	23% 36% 6% 17% 15% 3	4610
November	1st to 10th	Night	23% 54% 5% 10% 6% 1	2666
	11th to 20th	Night	21% 54% 4 12% 7% 1	2730
	21st to end	Night	28% 54% 5% 10% 4	2741
December	1st to 10th	Night	27% 35% 9% 20% 8% 1	4720
	11th to 20th	Night	39% 32% 9% 11% 8% 1	5341
	21st to end	Night	54% 27% 7% 7% 4	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	21% 17% 37% 20% 5% 1	330
	11th to 20th	Day	22% 20% 33% 18% 5% 1	318
	21st to end	Day	23% 15% 36% 19% 6% 1	330
February	1st to 10th	Day	21% 18% 36% 22% 4	300
	11th to 20th	Day	23% 16% 37% 19% 4 1	300
	21st to end	Day	21% 15% 44% 17% 2	249
March	1st to 10th	Day	20% 14% 42% 22% 3	300
	11th to 20th	Day	21% 18% 32% 21% 7%	348
	21st to end	Day	23% 17% 31% 21% 6% 1	550
April	1st to 10th	Day	23% 13% 32% 25% 6%	500
	11th to 20th	Day	21% 14% 32% 21% 11% 2	492
	21st to end	Day	21% 14% 33% 22% 9% 2	500
May	1st to 10th	Day	23% 10% 29% 28% 8% 2	503
	11th to 20th	Day	19% 9% 30% 27% 13% 3	605
	21st to end	Day	13% 6% 28% 31% 16% 5%	789
June	1st to 10th	Day	10% 8% 26% 34% 18% 4	730
	11th to 20th	Day	11% 5% 28% 32% 19% 4	730
	21st to end	Day	12% 6% 28% 34% 16% 4	730
July	1st to 10th	Day	8% 6% 21% 31% 22% 12%	721
	11th to 20th	Day	11% 3 21% 36% 19% 9%	718
	21st to end	Day	9% 2 23% 38% 22% 6%	794
August	1st to 10th	Day	6% 4 14% 37% 28% 10%	631
	11th to 20th	Day	10% 5% 18% 31% 25% 11%	519
	21st to end	Day	7% 2 16% 37% 29% 9%	583
September	1st to 10th	Day	14% 5% 18% 30% 26% 8%	530
	11th to 20th	Day	9% 4 13% 33% 28% 13%	530
	21st to end	Day	14% 8% 24% 26% 18% 9%	530
October	1st to 10th	Day	16% 5% 25% 31% 18% 5%	515
	11th to 20th	Day	18% 17% 28% 27% 9%	330
	21st to end	Day	26% 13% 32% 23% 5%	363
November	1st to 10th	Day	15% 13% 34% 25% 9% 2	330
	11th to 20th	Day	21% 20% 34% 18% 6% 1	330
	21st to end	Day	24% 14% 39% 20% 4	330
December	1st to 10th	Day	16% 21% 38% 19% 5% 1	330
	11th to 20th	Day	23% 15% 38% 20% 5%	330
	21st to end	Day	25% 18% 41% 14% 2	363
January	1st to 10th	Night	21% 16% 36% 24% 3	1050
	11th to 20th	Night	22% 16% 38% 19% 4	1031
	21st to end	Night	23% 13% 36% 25% 4	1100
February	1st to 10th	Night	24% 12% 39% 21% 4	915
	11th to 20th	Night	21% 13% 38% 25% 3	783
	21st to end	Night	24% 14% 38% 21% 3	660
March	1st to 10th	Night	20% 14% 37% 23% 6%	800
	11th to 20th	Night	17% 11% 40% 25% 7% 1	800
	21st to end	Night	23% 11% 31% 27% 8% 1	675
April	1st to 10th	Night	19% 13% 36% 26% 5% 1	600
	11th to 20th	Night	20% 11% 32% 27% 9% 1	593
	21st to end	Night	21% 8% 32% 30% 9% 1	600
May	1st to 10th	Night	21% 6% 30% 32% 12%	600
	11th to 20th	Night	18% 5% 26% 34% 16% 1	591
	21st to end	Night	14% 4 21% 38% 21% 2	650
June	1st to 10th	Night	10% 5% 21% 45% 19% 1	600
	11th to 20th	Night	11% 4 28% 38% 18% 2	600
	21st to end	Night	10% 4 24% 40% 21% 2	600
July	1st to 10th	Night	7% 3 14% 43% 27% 6%	585
	11th to 20th	Night	8% 2 17% 40% 29% 4	582
	21st to end	Night	5% 2 14% 48% 28% 3	657
August	1st to 10th	Night	6% 2 12% 46% 29% 6%	597
	11th to 20th	Night	8% 4 13% 37% 32% 5%	594
	21st to end	Night	9% 2 16% 38% 31% 4	660
September	1st to 10th	Night	12% 5% 16% 33% 30% 4	600
	11th to 20th	Night	9% 5% 14% 38% 30% 5%	730
	21st to end	Night	11% 6% 24% 31% 24% 4	800
October	1st to 10th	Night	13% 6% 24% 36% 20% 2	800
	11th to 20th	Night	21% 11% 32% 27% 9% 1	800
	21st to end	Night	25% 13% 31% 24% 7%	880
November	1st to 10th	Night	18% 11% 38% 24% 8% 1	820
	11th to 20th	Night	18% 18% 36% 22% 5%	840
	21st to end	Night	25% 14% 34% 22% 4	851
December	1st to 10th	Night	24% 16% 36% 21% 4	1050
	11th to 20th	Night	22% 16% 36% 22% 4	1050
	21st to end	Night	28% 15% 35% 20% 2	1155

Page generated at 2026-01-14 00:14:32. Version ac07

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