Moraine Valley Community College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Moraine Valley Community College Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	13% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	148976
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	142409
Transparency	Day	46% 4 11% 23% 14% 2	152500
	Night	46% 3 11% 23% 15% 2	146962
Seeing	Day	18% 13% 31% 23% 11% 4	18380
	Night	18% 11% 30% 28% 11% 1	28123

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% 13% 7% 12% 6% 5% 5% 4 6% 6% 24%	4691
	11th to 20th	Day	8% 12% 7% 13% 6% 5% 6% 5% 5% 4 27%	4710
	21st to end	Day	14% 17% 5% 10% 5% 4 3 3 5% 6% 29%	5276
February	1st to 10th	Day	15% 14% 5% 11% 6% 5% 5% 3 5% 5% 26%	1958
	11th to 20th	Day	14% 12% 5% 9% 6% 4 4 3 5% 6% 34%	2048
	21st to end	Day	15% 14% 4 10% 5% 4 4 4 5% 6% 30%	1730
March	1st to 10th	Day	17% 12% 3 7% 3 3 3 2 4 5% 40%	4580
	11th to 20th	Day	9% 8% 4 6% 5% 5% 5% 4 6% 8% 41%	4755
	21st to end	Day	18% 14% 6% 12% 6% 5% 5% 4 5% 5% 20%	6099
April	1st to 10th	Day	17% 14% 3 9% 5% 5% 4 3 5% 5% 30%	2520
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 6% 34%	2494
	21st to end	Day	19% 8% 3 8% 5% 4 4 3 6% 7% 33%	2645
May	1st to 10th	Day	16% 9% 2 7% 5% 5% 5% 4 6% 8% 33%	7616
	11th to 20th	Day	13% 6% 2 8% 5% 5% 5% 4 6% 8% 38%	7942
	21st to end	Day	13% 7% 3 7% 4 4 5% 4 6% 8% 39%	9047
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 7% 45%	2940
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 7% 8% 44%	2940
	21st to end	Day	10% 6% 2 6% 6% 5% 6% 5% 8% 9% 37%	2940
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	8280
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 11% 45%	8280
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	8990
August	1st to 10th	Day	12% 6% 3 7% 4 4 5% 4 6% 8% 40%	6300
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	5537
	21st to end	Day	6% 4 2 6% 4 5% 4 4 7% 11% 48%	6080
September	1st to 10th	Day	12% 5% 2 6% 5% 4 4 3 5% 8% 47%	2520
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 6% 8% 52%	2296
	21st to end	Day	11% 6% 3 6% 4 5% 4 4 6% 8% 42%	2200
October	1st to 10th	Day	12% 6% 3 7% 4 4 4 3 5% 6% 47%	2168
	11th to 20th	Day	18% 9% 3 7% 5% 5% 3 3 4 6% 37%	1976
	21st to end	Day	19% 9% 3 7% 5% 4 3 4 5% 6% 35%	1980
November	1st to 10th	Day	12% 9% 3 5% 4 4 3 3 5% 6% 45%	1852
	11th to 20th	Day	16% 11% 3 8% 5% 4 3 3 5% 7% 33%	1890
	21st to end	Day	18% 12% 3 10% 5% 3 4 2 3 5% 34%	1848
December	1st to 10th	Day	9% 11% 3 9% 6% 4 4 3 4 6% 41%	2690
	11th to 20th	Day	12% 17% 6% 11% 5% 5% 6% 4 5% 6% 24%	3088
	21st to end	Day	24% 22% 7% 12% 5% 5% 3 3 4 4 11%	4070
January	1st to 10th	Night	13% 14% 6% 12% 5% 4 4 3 4 4 29%	8232
	11th to 20th	Night	11% 14% 6% 14% 7% 6% 4 4 5% 4 22%	8280
	21st to end	Night	15% 13% 5% 8% 4 5% 5% 3 5% 5% 31%	8920
February	1st to 10th	Night	16% 13% 4 8% 6% 4 4 3 4 6% 31%	2661
	11th to 20th	Night	14% 12% 4 8% 5% 4 5% 3 5% 5% 36%	2458
	21st to end	Night	17% 11% 4 8% 5% 4 3 3 4 5% 35%	2076
March	1st to 10th	Night	15% 12% 4 7% 4 3 3 3 4 5% 40%	5493
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	5467
	21st to end	Night	18% 11% 6% 10% 5% 5% 5% 3 4 6% 27%	5072
April	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 6% 7% 36%	2100
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 4 6% 39%	2075
	21st to end	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 36%	1974
May	1st to 10th	Night	15% 7% 3 7% 5% 4 5% 4 6% 7% 37%	5237
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 9% 45%	4919
	21st to end	Night	13% 8% 3 7% 5% 4 4 4 5% 7% 40%	5139
June	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 6% 7% 49%	1680
	11th to 20th	Night	8% 6% 3 6% 4 4 4 4 6% 10% 45%	1680
	21st to end	Night	10% 6% 3 5% 4 5% 6% 4 6% 9% 42%	1680
July	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 4 6% 8% 44%	4708
	11th to 20th	Night	5% 6% 2 6% 4 4 4 3 5% 9% 52%	4707
	21st to end	Night	7% 5% 2 7% 4 5% 6% 4 7% 9% 45%	5117
August	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	3861
	11th to 20th	Night	12% 7% 3 7% 5% 5% 4 4 5% 7% 41%	4492
	21st to end	Night	7% 5% 2 6% 3 4 4 3 5% 9% 54%	5046
September	1st to 10th	Night	9% 5% 2 7% 4 3 4 3 5% 8% 52%	2100
	11th to 20th	Night	8% 4 2 4 3 3 4 3 5% 7% 56%	2150
	21st to end	Night	10% 6% 2 9% 4 3 4 5% 6% 7% 46%	2200
October	1st to 10th	Night	11% 5% 3 5% 4 4 3 3 5% 7% 51%	2230
	11th to 20th	Night	16% 8% 2 6% 4 3 4 3 3 6% 45%	2423
	21st to end	Night	19% 9% 3 7% 3 3 4 4 5% 7% 36%	2860
November	1st to 10th	Night	13% 9% 3 5% 4 3 3 3 5% 6% 47%	2666
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 5% 6% 36%	2730
	21st to end	Night	17% 11% 4 8% 5% 4 3 3 4 5% 35%	2767
December	1st to 10th	Night	12% 10% 3 8% 4 4 4 3 5% 6% 42%	4720
	11th to 20th	Night	17% 16% 4 9% 5% 4 4 3 4 6% 29%	5339
	21st to end	Night	24% 20% 6% 9% 5% 5% 4 3 3 5% 16%	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	60% 2 16% 14% 6% 1	4692
	11th to 20th	Day	59% 2 9% 15% 12% 3	4710
	21st to end	Day	58% 2 11% 20% 8% 1	5276
February	1st to 10th	Day	60% 3 12% 15% 8% 2	1958
	11th to 20th	Day	53% 3 10% 22% 10% 2	2058
	21st to end	Day	55% 4 11% 21% 8% 2	1730
March	1st to 10th	Day	50% 2 7% 25% 12% 4	4580
	11th to 20th	Day	42% 2 12% 33% 10% 1	4755
	21st to end	Day	67% 3 12% 12% 6% 1	6101
April	1st to 10th	Day	56% 4 8% 20% 12%	2520
	11th to 20th	Day	49% 4 11% 21% 11% 3	2494
	21st to end	Day	50% 4 12% 20% 11% 2	2645
May	1st to 10th	Day	48% 4 10% 25% 12% 1	7651
	11th to 20th	Day	44% 5% 11% 23% 16% 1	7946
	21st to end	Day	43% 4 11% 22% 16% 4	9061
June	1st to 10th	Day	37% 4 13% 26% 18% 2	2940
	11th to 20th	Day	36% 5% 11% 26% 20% 3	2940
	21st to end	Day	41% 6% 14% 22% 14% 3	2940
July	1st to 10th	Day	40% 6% 13% 24% 16% 1	8211
	11th to 20th	Day	33% 6% 13% 31% 17% 1	8205
	21st to end	Day	39% 5% 14% 23% 17% 2	8794
August	1st to 10th	Day	41% 6% 12% 26% 15% 1	6285
	11th to 20th	Day	43% 5% 12% 27% 12% 1	5574
	21st to end	Day	30% 6% 13% 33% 17% 2	6079
September	1st to 10th	Day	35% 5% 12% 26% 19% 3	2520
	11th to 20th	Day	30% 4 11% 30% 22% 3	2296
	21st to end	Day	38% 5% 9% 27% 20% 1	2200
October	1st to 10th	Day	27% 2 7% 32% 29% 2	3532
	11th to 20th	Day	36% 3 11% 28% 22% 1	3192
	21st to end	Day	44% 3 9% 27% 15% 3	3180
November	1st to 10th	Day	40% 3 14% 25% 16% 2	1852
	11th to 20th	Day	51% 4 11% 22% 12% 1	1890
	21st to end	Day	55% 3 9% 23% 9%	1848
December	1st to 10th	Day	46% 2 16% 28% 8%	2690
	11th to 20th	Day	60% 4 11% 18% 6% 1	3085
	21st to end	Day	79% 2 6% 8% 4	4070
January	1st to 10th	Night	59% 2 11% 20% 8%	8233
	11th to 20th	Night	64% 3 10% 11% 11% 1	8280
	21st to end	Night	55% 3 11% 21% 9% 1	8920
February	1st to 10th	Night	56% 3 11% 18% 10% 2	2661
	11th to 20th	Night	50% 3 10% 23% 13% 1	2459
	21st to end	Night	52% 2 11% 21% 11% 3	2076
March	1st to 10th	Night	49% 2 10% 25% 11% 3	5493
	11th to 20th	Night	39% 3 11% 29% 15% 3	5467
	21st to end	Night	59% 3 9% 20% 9% 1	5072
April	1st to 10th	Night	47% 3 11% 24% 12% 1	2100
	11th to 20th	Night	47% 3 9% 21% 16% 4	2075
	21st to end	Night	48% 4 9% 23% 14% 2	1974
May	1st to 10th	Night	46% 3 11% 22% 15% 3	5264
	11th to 20th	Night	37% 4 14% 26% 16% 2	4920
	21st to end	Night	44% 3 9% 19% 19% 5%	5149
June	1st to 10th	Night	35% 4 12% 26% 20% 3	1680
	11th to 20th	Night	34% 4 12% 25% 23% 2	1680
	21st to end	Night	39% 6% 14% 22% 17% 2	1680
July	1st to 10th	Night	37% 5% 14% 26% 15% 3	4670
	11th to 20th	Night	30% 4 13% 28% 23% 2	4662
	21st to end	Night	35% 6% 13% 25% 19% 3	5002
August	1st to 10th	Night	39% 5% 13% 26% 16% 1	3852
	11th to 20th	Night	41% 6% 13% 24% 14% 1	4521
	21st to end	Night	29% 4 12% 37% 16% 3	5046
September	1st to 10th	Night	32% 3 12% 29% 22% 3	2100
	11th to 20th	Night	28% 3 8% 31% 28% 2	2150
	21st to end	Night	37% 4 9% 30% 18% 2	2200
October	1st to 10th	Night	22% 3 7% 38% 28% 2	3646
	11th to 20th	Night	37% 2 7% 25% 28% 2	3946
	21st to end	Night	41% 2 8% 26% 19% 3	4610
November	1st to 10th	Night	39% 3 8% 28% 19% 3	2666
	11th to 20th	Night	49% 3 11% 23% 12% 1	2730
	21st to end	Night	52% 3 11% 23% 11%	2767
December	1st to 10th	Night	44% 3 14% 27% 11% 2	4720
	11th to 20th	Night	58% 3 12% 16% 10% 1	5341
	21st to end	Night	72% 3 8% 11% 6%	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% 18% 37% 18% 4	330
	11th to 20th	Day	25% 25% 33% 13% 3 1	330
	21st to end	Day	25% 21% 35% 16% 2 1	363
February	1st to 10th	Day	22% 21% 42% 13% 3	330
	11th to 20th	Day	26% 23% 33% 15% 2	330
	21st to end	Day	27% 19% 42% 10% 2	273
March	1st to 10th	Day	23% 27% 36% 12% 2	330
	11th to 20th	Day	26% 22% 33% 14% 4	369
	21st to end	Day	29% 19% 31% 15% 5% 1	594
April	1st to 10th	Day	28% 18% 30% 17% 6%	540
	11th to 20th	Day	24% 23% 25% 19% 8% 1	532
	21st to end	Day	23% 17% 35% 16% 7% 1	540
May	1st to 10th	Day	24% 16% 30% 21% 7% 2	543
	11th to 20th	Day	20% 15% 32% 20% 10% 3	675
	21st to end	Day	15% 12% 34% 23% 11% 5%	844
June	1st to 10th	Day	12% 11% 34% 27% 12% 4	780
	11th to 20th	Day	13% 10% 35% 26% 14% 3	780
	21st to end	Day	12% 11% 36% 26% 11% 4	780
July	1st to 10th	Day	9% 5% 27% 33% 17% 10%	771
	11th to 20th	Day	11% 6% 29% 30% 16% 8%	768
	21st to end	Day	9% 8% 28% 33% 17% 5%	849
August	1st to 10th	Day	8% 4 26% 33% 20% 9%	697
	11th to 20th	Day	11% 6% 21% 30% 23% 9%	559
	21st to end	Day	7% 4 23% 37% 20% 8%	627
September	1st to 10th	Day	13% 6% 26% 30% 21% 5%	570
	11th to 20th	Day	10% 4 19% 32% 25% 9%	542
	21st to end	Day	13% 11% 25% 29% 17% 6%	530
October	1st to 10th	Day	16% 8% 29% 27% 16% 4	498
	11th to 20th	Day	20% 22% 32% 20% 5% 1	330
	21st to end	Day	29% 15% 36% 17% 3	363
November	1st to 10th	Day	17% 20% 35% 20% 5% 2	330
	11th to 20th	Day	25% 21% 33% 14% 5% 1	330
	21st to end	Day	27% 13% 40% 17% 3	330
December	1st to 10th	Day	22% 23% 35% 14% 4 1	330
	11th to 20th	Day	24% 19% 39% 15% 3	330
	21st to end	Day	30% 21% 36% 12% 1	363
January	1st to 10th	Night	21% 18% 35% 22% 4	1050
	11th to 20th	Night	23% 19% 34% 19% 4	1050
	21st to end	Night	23% 15% 35% 24% 3	1155
February	1st to 10th	Night	23% 14% 37% 22% 4	981
	11th to 20th	Night	20% 17% 34% 25% 3	823
	21st to end	Night	22% 19% 37% 19% 3	692
March	1st to 10th	Night	23% 15% 36% 21% 5%	840
	11th to 20th	Night	18% 14% 39% 24% 6% 1	840
	21st to end	Night	25% 14% 30% 24% 7% 1	693
April	1st to 10th	Night	22% 14% 34% 23% 6%	630
	11th to 20th	Night	24% 13% 31% 23% 8% 1	623
	21st to end	Night	22% 10% 32% 27% 8%	630
May	1st to 10th	Night	18% 9% 36% 29% 8%	630
	11th to 20th	Night	18% 5% 31% 33% 13% 1	621
	21st to end	Night	14% 5% 28% 35% 18%	683
June	1st to 10th	Night	11% 5% 26% 43% 14% 1	630
	11th to 20th	Night	11% 7% 29% 40% 12% 1	630
	21st to end	Night	12% 4 27% 39% 17% 1	630
July	1st to 10th	Night	9% 4 18% 39% 26% 4	615
	11th to 20th	Night	7% 5% 21% 40% 25% 3	612
	21st to end	Night	6% 2 21% 46% 23% 2	690
August	1st to 10th	Night	6% 2 15% 47% 27% 3	627
	11th to 20th	Night	8% 4 15% 39% 30% 4	624
	21st to end	Night	8% 2 18% 40% 29% 3	693
September	1st to 10th	Night	10% 5% 20% 33% 28% 4	630
	11th to 20th	Night	8% 5% 17% 37% 29% 4	729
	21st to end	Night	12% 7% 23% 35% 21% 3	800
October	1st to 10th	Night	13% 6% 26% 35% 18% 2	800
	11th to 20th	Night	20% 14% 30% 27% 9%	800
	21st to end	Night	25% 14% 30% 24% 7%	880
November	1st to 10th	Night	17% 14% 38% 20% 9% 1	820
	11th to 20th	Night	20% 19% 34% 22% 5%	840
	21st to end	Night	23% 14% 37% 20% 5%	877
December	1st to 10th	Night	22% 16% 37% 21% 4	1050
	11th to 20th	Night	24% 15% 34% 22% 4	1050
	21st to end	Night	32% 16% 31% 19% 2	1155

Page generated at 2026-09-11 03:11:53. Version ac07

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