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 2024-01-14.

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% 9% 4 6% 6% 3 5% 5% 5% 7% 36%	74825
	Night	14% 9% 4 7% 5% 3 4 4 5% 7% 38%	74959
Transparency	Day	31% 38% 8% 14% 8% 1	74526
	Night	31% 37% 8% 14% 9% 1	74779
Seeing	Day	16% 9% 27% 29% 15% 5%	14605
	Night	18% 9% 29% 29% 13% 1	24580

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	15% 13% 5% 9% 7% 4 5% 4 6% 6% 28%	1517
	11th to 20th	Day	14% 17% 8% 11% 7% 3 5% 4 4 6% 22%	1464
	21st to end	Day	14% 17% 8% 11% 6% 3 4 4 4 6% 23%	1616
February	1st to 10th	Day	17% 14% 6% 9% 7% 4 5% 4 5% 6% 25%	1696
	11th to 20th	Day	14% 11% 5% 7% 7% 5% 5% 4 5% 6% 32%	1748
	21st to end	Day	16% 13% 6% 11% 5% 3 4 4 5% 6% 27%	1480
March	1st to 10th	Day	17% 13% 5% 6% 7% 3 5% 4 5% 5% 29%	1800
	11th to 20th	Day	13% 12% 4 6% 6% 4 5% 5% 5% 8% 34%	1843
	21st to end	Day	21% 12% 5% 7% 6% 4 4 5% 5% 6% 25%	2376
April	1st to 10th	Day	16% 11% 4 7% 6% 3 5% 5% 4 7% 31%	2160
	11th to 20th	Day	17% 9% 3 5% 6% 4 5% 5% 5% 7% 35%	2134
	21st to end	Day	20% 9% 3 6% 6% 4 4 5% 5% 6% 33%	2272
May	1st to 10th	Day	17% 8% 3 6% 6% 4 6% 6% 6% 8% 31%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 5% 4 5% 9% 34%	2371
	21st to end	Day	12% 5% 3 7% 7% 3 6% 5% 6% 7% 39%	2725
June	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 5% 5% 9% 45%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 5% 6% 8% 46%	2520
	21st to end	Day	10% 4 3 6% 6% 4 7% 6% 7% 9% 37%	2520
July	1st to 10th	Day	8% 5% 3 4 5% 3 5% 5% 6% 10% 45%	2520
	11th to 20th	Day	9% 5% 2 4 5% 3 6% 5% 7% 10% 44%	2520
	21st to end	Day	6% 4 3 5% 6% 4 5% 7% 7% 9% 44%	2743
August	1st to 10th	Day	7% 4 2 4 6% 3 6% 6% 7% 9% 45%	2422
	11th to 20th	Day	9% 4 3 6% 6% 3 5% 5% 6% 10% 44%	2155
	21st to end	Day	7% 4 2 5% 5% 3 5% 6% 6% 9% 47%	2376
September	1st to 10th	Day	13% 5% 2 4 5% 3 5% 5% 6% 8% 44%	2160
	11th to 20th	Day	12% 5% 2 4 4 2 4 4 6% 8% 49%	2026
	21st to end	Day	11% 4 2 5% 5% 3 5% 5% 6% 9% 44%	1980
October	1st to 10th	Day	13% 7% 3 5% 4 4 5% 4 5% 7% 45%	1967
	11th to 20th	Day	18% 8% 3 5% 6% 3 5% 5% 5% 8% 34%	1773
	21st to end	Day	20% 10% 4 6% 6% 2 3 4 5% 6% 32%	1782
November	1st to 10th	Day	13% 7% 3 5% 4 2 3 4 6% 7% 46%	1672
	11th to 20th	Day	16% 8% 4 7% 6% 3 4 4 4 8% 36%	1710
	21st to end	Day	18% 10% 4 7% 6% 2 4 4 4 5% 35%	1696
December	1st to 10th	Day	16% 13% 5% 8% 5% 3 5% 5% 4 6% 28%	1520
	11th to 20th	Day	13% 12% 6% 9% 7% 3 4 5% 5% 6% 27%	1918
	21st to end	Day	21% 22% 8% 13% 7% 2 3 4 4 3 13%	2783
January	1st to 10th	Night	15% 13% 5% 8% 7% 3 4 4 3 6% 33%	2648
	11th to 20th	Night	17% 14% 7% 10% 6% 3 5% 4 4 5% 26%	2575
	21st to end	Night	15% 13% 8% 9% 7% 4 4 4 5% 5% 26%	2739
February	1st to 10th	Night	18% 14% 5% 9% 7% 3 5% 3 4 5% 29%	2264
	11th to 20th	Night	15% 10% 4 8% 6% 4 4 4 5% 7% 35%	2098
	21st to end	Night	19% 12% 4 7% 5% 3 4 4 4 5% 33%	1776
March	1st to 10th	Night	15% 12% 4 7% 6% 4 4 4 4 4 34%	2160
	11th to 20th	Night	12% 8% 3 6% 6% 4 4 4 6% 7% 42%	2117
	21st to end	Night	19% 10% 4 7% 6% 3 5% 4 5% 5% 31%	1994
April	1st to 10th	Night	14% 8% 3 6% 5% 3 5% 4 6% 7% 38%	1800
	11th to 20th	Night	18% 8% 3 6% 6% 3 4 3 5% 7% 38%	1775
	21st to end	Night	18% 7% 3 7% 6% 3 5% 4 5% 6% 36%	1687
May	1st to 10th	Night	16% 8% 3 5% 5% 3 5% 5% 6% 8% 35%	1620
	11th to 20th	Night	16% 7% 3 6% 4 3 4 5% 5% 7% 40%	1520
	21st to end	Night	10% 5% 4 6% 7% 3 5% 6% 7% 8% 40%	1551
June	1st to 10th	Night	9% 4 3 5% 5% 3 5% 4 5% 8% 50%	1440
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 4 6% 9% 46%	1440
	21st to end	Night	10% 5% 3 5% 6% 4 5% 6% 5% 8% 43%	1440
July	1st to 10th	Night	7% 4 2 4 5% 3 5% 4 7% 10% 50%	1440
	11th to 20th	Night	7% 4 2 4 4 3 4 4 6% 9% 53%	1440
	21st to end	Night	4 3 2 4 6% 3 5% 5% 7% 10% 52%	1575
August	1st to 10th	Night	5% 4 2 5% 5% 4 4 5% 7% 10% 49%	1516
	11th to 20th	Night	7% 4 2 4 4 3 6% 4 6% 9% 52%	1742
	21st to end	Night	8% 4 2 4 4 2 4 5% 5% 8% 54%	1980
September	1st to 10th	Night	11% 7% 2 5% 4 3 3 4 5% 7% 50%	1800
	11th to 20th	Night	9% 4 2 4 3 3 4 4 5% 8% 54%	1916
	21st to end	Night	9% 6% 3 5% 6% 3 5% 4 6% 7% 47%	1980
October	1st to 10th	Night	12% 5% 2 5% 5% 3 4 5% 4 6% 50%	1988
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 3 6% 45%	2186
	21st to end	Night	22% 9% 3 5% 5% 3 3 4 4 6% 37%	2574
November	1st to 10th	Night	13% 6% 3 6% 4 3 3 4 4 7% 46%	2406
	11th to 20th	Night	14% 8% 4 7% 6% 3 5% 4 5% 6% 37%	2470
	21st to end	Night	18% 10% 3 6% 5% 3 4 4 4 6% 37%	2479
December	1st to 10th	Night	16% 11% 5% 9% 6% 3 4 4 5% 6% 31%	2660
	11th to 20th	Night	15% 14% 5% 8% 5% 3 5% 4 4 5% 32%	3279
	21st to end	Night	24% 17% 7% 10% 6% 4 4 4 4 5% 16%	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	37% 41% 8% 8% 5%	1518
	11th to 20th	Day	40% 42% 6% 8% 3	1464
	21st to end	Day	41% 37% 7% 10% 4	1616
February	1st to 10th	Day	40% 37% 9% 9% 4 1	1696
	11th to 20th	Day	35% 37% 8% 13% 6% 1	1758
	21st to end	Day	40% 37% 6% 12% 4 1	1480
March	1st to 10th	Day	38% 37% 7% 13% 4	1800
	11th to 20th	Day	32% 38% 8% 15% 6% 1	1843
	21st to end	Day	42% 37% 7% 11% 3	2376
April	1st to 10th	Day	32% 38% 7% 13% 10%	2160
	11th to 20th	Day	33% 37% 6% 13% 8% 2	2134
	21st to end	Day	36% 37% 9% 12% 6% 1	2272
May	1st to 10th	Day	34% 38% 7% 16% 5%	2340
	11th to 20th	Day	30% 38% 7% 13% 10% 1	2375
	21st to end	Day	30% 40% 6% 12% 10% 2	2725
June	1st to 10th	Day	26% 39% 8% 16% 10% 1	2520
	11th to 20th	Day	20% 39% 7% 17% 16% 2	2520
	21st to end	Day	27% 40% 10% 11% 10% 2	2520
July	1st to 10th	Day	23% 39% 8% 17% 11% 2	2451
	11th to 20th	Day	23% 39% 9% 16% 10% 2	2445
	21st to end	Day	23% 41% 12% 14% 9% 1	2591
August	1st to 10th	Day	24% 39% 10% 18% 8% 1	2407
	11th to 20th	Day	22% 37% 10% 18% 12% 1	2155
	21st to end	Day	18% 38% 9% 18% 13% 4	2376
September	1st to 10th	Day	24% 38% 8% 16% 12% 3	2160
	11th to 20th	Day	23% 36% 8% 17% 14% 1	2026
	21st to end	Day	23% 38% 8% 18% 13% 1	1980
October	1st to 10th	Day	22% 38% 7% 18% 13% 1	1967
	11th to 20th	Day	35% 38% 7% 13% 7% 1	1773
	21st to end	Day	33% 37% 7% 14% 7% 1	1782
November	1st to 10th	Day	21% 40% 10% 17% 11% 1	1672
	11th to 20th	Day	30% 40% 8% 15% 7%	1710
	21st to end	Day	32% 40% 6% 15% 7%	1696
December	1st to 10th	Day	35% 41% 8% 12% 3	1520
	11th to 20th	Day	41% 33% 8% 15% 3	1915
	21st to end	Day	63% 25% 5% 5% 2	2783
January	1st to 10th	Night	38% 40% 7% 12% 3	2648
	11th to 20th	Night	38% 41% 8% 8% 5%	2575
	21st to end	Night	41% 37% 7% 10% 4	2739
February	1st to 10th	Night	39% 37% 6% 11% 5% 1	2264
	11th to 20th	Night	34% 38% 9% 11% 7% 1	2099
	21st to end	Night	38% 37% 7% 11% 5% 1	1776
March	1st to 10th	Night	35% 37% 8% 15% 6%	2160
	11th to 20th	Night	28% 38% 10% 12% 9% 2	2117
	21st to end	Night	37% 39% 8% 12% 5%	1994
April	1st to 10th	Night	28% 38% 9% 15% 10% 1	1800
	11th to 20th	Night	32% 36% 5% 14% 12% 2	1775
	21st to end	Night	33% 37% 7% 14% 8% 1	1687
May	1st to 10th	Night	30% 38% 9% 17% 7%	1620
	11th to 20th	Night	28% 38% 7% 14% 13% 1	1521
	21st to end	Night	26% 40% 8% 13% 10% 3	1552
June	1st to 10th	Night	22% 39% 8% 16% 12% 3	1440
	11th to 20th	Night	19% 37% 9% 15% 17% 2	1440
	21st to end	Night	27% 39% 9% 13% 10% 3	1440
July	1st to 10th	Night	20% 39% 9% 18% 11% 2	1400
	11th to 20th	Night	18% 39% 9% 17% 15% 1	1392
	21st to end	Night	18% 40% 12% 14% 11% 4	1487
August	1st to 10th	Night	22% 38% 10% 17% 12% 1	1507
	11th to 20th	Night	18% 37% 9% 17% 16% 2	1742
	21st to end	Night	17% 37% 7% 20% 14% 5%	1980
September	1st to 10th	Night	23% 37% 7% 16% 14% 4	1800
	11th to 20th	Night	21% 37% 5% 17% 18% 1	1916
	21st to end	Night	23% 37% 7% 19% 13% 2	1980
October	1st to 10th	Night	21% 37% 5% 18% 18% 1	1988
	11th to 20th	Night	30% 36% 6% 16% 10% 1	2186
	21st to end	Night	33% 37% 6% 15% 8% 1	2574
November	1st to 10th	Night	20% 40% 6% 18% 13% 1	2406
	11th to 20th	Night	31% 40% 8% 13% 7% 1	2470
	21st to end	Night	30% 40% 7% 15% 7%	2479
December	1st to 10th	Night	33% 40% 8% 13% 5%	2660
	11th to 20th	Night	40% 33% 8% 12% 7%	3281
	21st to end	Night	58% 26% 7% 7% 2	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	21% 16% 34% 22% 6% 1	270
	11th to 20th	Day	24% 14% 32% 22% 6% 1	249
	21st to end	Day	20% 14% 38% 22% 6% 1	264
February	1st to 10th	Day	23% 17% 35% 23% 2	240
	11th to 20th	Day	24% 14% 39% 18% 5% 1	240
	21st to end	Day	20% 15% 46% 16% 2 1	198
March	1st to 10th	Day	20% 14% 40% 23% 3	240
	11th to 20th	Day	22% 13% 33% 23% 8%	282
	21st to end	Day	24% 15% 29% 23% 7% 1	462
April	1st to 10th	Day	20% 13% 31% 29% 7%	420
	11th to 20th	Day	21% 12% 32% 21% 12% 3	412
	21st to end	Day	21% 12% 33% 22% 9% 2	420
May	1st to 10th	Day	22% 10% 29% 28% 8% 3	423
	11th to 20th	Day	20% 7% 31% 26% 13% 3	517
	21st to end	Day	13% 5% 26% 33% 18% 6%	679
June	1st to 10th	Day	9% 5% 25% 36% 20% 5%	630
	11th to 20th	Day	11% 5% 27% 32% 21% 4	630
	21st to end	Day	13% 5% 27% 34% 17% 5%	630
July	1st to 10th	Day	7% 5% 19% 30% 25% 13%	621
	11th to 20th	Day	11% 3 18% 38% 21% 9%	618
	21st to end	Day	8% 2 22% 38% 24% 6%	684
August	1st to 10th	Day	7% 4 11% 37% 30% 11%	541
	11th to 20th	Day	9% 4 16% 31% 27% 13%	439
	21st to end	Day	7% 2 15% 36% 30% 10%	495
September	1st to 10th	Day	15% 4 16% 32% 26% 7%	450
	11th to 20th	Day	11% 4 14% 31% 28% 12%	450
	21st to end	Day	12% 8% 25% 26% 19% 10%	450
October	1st to 10th	Day	18% 3 25% 31% 17% 5%	437
	11th to 20th	Day	19% 17% 30% 25% 9%	270
	21st to end	Day	30% 12% 30% 23% 5%	297
November	1st to 10th	Day	14% 9% 34% 29% 11% 3	270
	11th to 20th	Day	20% 18% 34% 19% 8% 1	270
	21st to end	Day	22% 10% 40% 23% 4	270
December	1st to 10th	Day	17% 19% 37% 20% 6% 1	270
	11th to 20th	Day	20% 13% 39% 23% 6%	270
	21st to end	Day	21% 19% 42% 16% 2	297
January	1st to 10th	Night	21% 16% 35% 24% 3	950
	11th to 20th	Night	23% 15% 37% 21% 4 1	920
	21st to end	Night	22% 12% 36% 25% 4	990
February	1st to 10th	Night	25% 11% 39% 21% 3	824
	11th to 20th	Night	22% 11% 39% 24% 4	703
	21st to end	Night	26% 14% 36% 22% 3	592
March	1st to 10th	Night	20% 14% 37% 23% 6%	720
	11th to 20th	Night	17% 9% 40% 26% 7% 1	720
	21st to end	Night	23% 11% 31% 27% 9% 1	608
April	1st to 10th	Night	18% 12% 36% 27% 6% 1	540
	11th to 20th	Night	20% 10% 32% 28% 10% 1	533
	21st to end	Night	22% 7% 32% 29% 10% 1	540
May	1st to 10th	Night	21% 6% 30% 32% 11%	540
	11th to 20th	Night	19% 5% 26% 33% 16% 1	531
	21st to end	Night	14% 3 20% 39% 23% 2	584
June	1st to 10th	Night	10% 4 20% 45% 20% 1	540
	11th to 20th	Night	11% 3 26% 39% 19% 2	540
	21st to end	Night	11% 3 22% 40% 23% 2	540
July	1st to 10th	Night	7% 2 13% 44% 28% 7%	525
	11th to 20th	Night	8% 2 17% 39% 29% 5%	522
	21st to end	Night	5% 2 15% 47% 29% 2	591
August	1st to 10th	Night	6% 1 11% 46% 30% 6%	537
	11th to 20th	Night	8% 4 11% 37% 34% 6%	534
	21st to end	Night	9% 2 16% 36% 32% 4	594
September	1st to 10th	Night	13% 5% 15% 34% 29% 4	540
	11th to 20th	Night	10% 5% 14% 37% 28% 5%	656
	21st to end	Night	10% 5% 24% 31% 25% 4	720
October	1st to 10th	Night	14% 6% 25% 33% 20% 2	720
	11th to 20th	Night	21% 11% 33% 26% 8% 1	720
	21st to end	Night	26% 13% 31% 24% 7%	792
November	1st to 10th	Night	17% 10% 38% 24% 9% 1	740
	11th to 20th	Night	18% 17% 36% 23% 5%	760
	21st to end	Night	24% 12% 36% 23% 5%	769
December	1st to 10th	Night	24% 15% 36% 21% 4	950
	11th to 20th	Night	22% 14% 36% 24% 4	950
	21st to end	Night	28% 14% 35% 21% 2	1045

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