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 2025-05-11.

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% 10% 3 8% 5% 4 4 4 6% 7% 36%	113634
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	115507
Transparency	Day	46% 4 11% 23% 14% 2	117205
	Night	47% 3 11% 23% 15% 2	120079
Seeing	Day	18% 13% 31% 24% 11% 4	16411
	Night	18% 11% 30% 28% 11% 1	26395

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% 12% 6% 6% 5% 4 6% 5% 26%	3617
	11th to 20th	Day	10% 13% 7% 13% 5% 5% 4 6% 6% 4 26%	3620
	21st to end	Day	16% 20% 5% 8% 4 3 3 3 5% 6% 29%	4074
February	1st to 10th	Day	16% 14% 5% 10% 6% 5% 5% 3 5% 5% 26%	1865
	11th to 20th	Day	14% 11% 5% 8% 6% 5% 4 2 5% 6% 33%	1948
	21st to end	Day	15% 14% 4 9% 5% 4 4 4 5% 7% 30%	1650
March	1st to 10th	Day	17% 12% 3 7% 3 3 3 2 4 5% 40%	4480
	11th to 20th	Day	8% 8% 4 6% 5% 5% 5% 4 6% 8% 41%	4652
	21st to end	Day	19% 14% 6% 12% 6% 5% 5% 4 5% 5% 19%	5967
April	1st to 10th	Day	17% 14% 3 9% 5% 5% 4 3 5% 5% 31%	2400
	11th to 20th	Day	17% 10% 2 6% 5% 5% 5% 4 5% 6% 35%	2374
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 6% 7% 33%	2519
May	1st to 10th	Day	17% 8% 2 7% 5% 4 5% 4 6% 8% 34%	4212
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 8% 41%	4257
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 44%	4824
June	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 7% 45%	2660
	11th to 20th	Day	9% 6% 2 6% 4 5% 4 4 7% 8% 45%	2660
	21st to end	Day	11% 6% 2 7% 6% 5% 5% 5% 8% 9% 36%	2660
July	1st to 10th	Day	11% 7% 3 8% 6% 5% 6% 5% 7% 8% 33%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 47%	4440
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	4813
August	1st to 10th	Day	9% 5% 3 6% 4 4 5% 5% 6% 8% 44%	4300
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 44%	3765
	21st to end	Day	7% 5% 2 6% 4 5% 5% 3 6% 10% 48%	4180
September	1st to 10th	Day	12% 5% 1 6% 5% 4 4 3 5% 8% 46%	2280
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 6% 8% 51%	2147
	21st to end	Day	11% 7% 2 6% 4 4 4 4 6% 8% 42%	2090
October	1st to 10th	Day	12% 7% 3 7% 4 4 4 3 5% 6% 46%	2060
	11th to 20th	Day	18% 9% 3 7% 5% 5% 3 4 4 6% 37%	1877
	21st to end	Day	20% 9% 3 7% 5% 3 3 4 5% 6% 34%	1881
November	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 5% 6% 45%	1762
	11th to 20th	Day	16% 11% 3 8% 5% 4 4 3 5% 7% 33%	1800
	21st to end	Day	18% 11% 3 10% 5% 3 4 2 4 5% 34%	1760
December	1st to 10th	Day	9% 11% 3 8% 5% 4 4 3 4 6% 42%	2610
	11th to 20th	Day	12% 17% 6% 11% 6% 5% 6% 4 5% 6% 24%	3008
	21st to end	Day	25% 22% 7% 12% 5% 5% 3 3 4 4 11%	3982
January	1st to 10th	Night	14% 14% 6% 12% 5% 4 4 3 4 5% 29%	6348
	11th to 20th	Night	14% 15% 7% 12% 7% 6% 4 4 6% 4 22%	6360
	21st to end	Night	18% 14% 6% 8% 4 4 4 3 4 4 32%	6886
February	1st to 10th	Night	17% 14% 4 8% 6% 4 4 3 4 6% 31%	2535
	11th to 20th	Night	14% 12% 4 8% 5% 4 5% 3 5% 5% 36%	2338
	21st to end	Night	18% 11% 4 8% 5% 4 3 3 4 5% 35%	1980
March	1st to 10th	Night	15% 12% 4 7% 4 3 3 3 4 5% 40%	5373
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	5349
	21st to end	Night	18% 11% 6% 10% 5% 5% 5% 3 4 5% 27%	4962
April	1st to 10th	Night	15% 10% 3 6% 4 4 4 3 6% 7% 38%	2000
	11th to 20th	Night	18% 8% 3 6% 4 4 3 4 4 6% 39%	1975
	21st to end	Night	16% 8% 3 8% 5% 4 4 4 5% 7% 35%	1880
May	1st to 10th	Night	16% 7% 3 7% 5% 4 5% 4 6% 7% 35%	2903
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 4 6% 8% 40%	2646
	21st to end	Night	10% 8% 2 7% 5% 4 4 4 5% 7% 44%	2743
June	1st to 10th	Night	9% 5% 2 5% 5% 4 4 4 6% 7% 49%	1520
	11th to 20th	Night	7% 6% 2 5% 5% 4 4 4 6% 10% 45%	1520
	21st to end	Night	11% 6% 3 6% 4 5% 6% 4 6% 9% 42%	1520
July	1st to 10th	Night	11% 5% 2 7% 5% 6% 6% 4 6% 8% 40%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 5% 7% 55%	2530
	21st to end	Night	5% 5% 2 6% 4 5% 6% 5% 7% 9% 45%	2750
August	1st to 10th	Night	7% 5% 2 7% 4 5% 5% 4 6% 8% 47%	2635
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 5% 7% 52%	3061
	21st to end	Night	8% 5% 2 6% 3 3 4 3 5% 8% 52%	3476
September	1st to 10th	Night	10% 5% 2 7% 3 3 4 2 5% 7% 52%	1900
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 5% 7% 55%	2014
	21st to end	Night	10% 6% 2 9% 3 3 4 5% 6% 7% 46%	2090
October	1st to 10th	Night	11% 5% 3 5% 4 4 3 3 5% 7% 50%	2118
	11th to 20th	Night	17% 8% 3 5% 4 3 4 2 3 6% 45%	2302
	21st to end	Night	20% 9% 3 7% 3 3 4 3 5% 7% 36%	2717
November	1st to 10th	Night	13% 8% 3 5% 4 3 3 3 5% 6% 47%	2536
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 5% 6% 35%	2600
	21st to end	Night	17% 11% 3 8% 5% 4 3 3 4 6% 36%	2635
December	1st to 10th	Night	12% 9% 3 7% 4 3 4 3 5% 6% 42%	4580
	11th to 20th	Night	17% 16% 4 9% 5% 4 4 3 4 5% 29%	5199
	21st to end	Night	24% 20% 6% 9% 5% 5% 4 3 3 5% 16%	6996

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 13% 17% 8%	3618
	11th to 20th	Day	59% 2 10% 11% 14% 4	3620
	21st to end	Day	59% 2 11% 19% 8% 1	4074
February	1st to 10th	Day	61% 4 11% 15% 9% 2	1865
	11th to 20th	Day	54% 3 10% 22% 10% 2	1958
	21st to end	Day	55% 4 11% 21% 8% 2	1650
March	1st to 10th	Day	50% 2 7% 25% 12% 4	4480
	11th to 20th	Day	41% 2 12% 33% 10% 1	4652
	21st to end	Day	67% 3 12% 12% 6% 1	5969
April	1st to 10th	Day	55% 4 9% 20% 12%	2400
	11th to 20th	Day	49% 4 10% 22% 12% 3	2374
	21st to end	Day	50% 4 12% 20% 12% 2	2519
May	1st to 10th	Day	48% 4 10% 26% 11% 1	4250
	11th to 20th	Day	43% 3 11% 28% 14%	4261
	21st to end	Day	38% 3 12% 23% 21% 4	4837
June	1st to 10th	Day	36% 4 13% 27% 18% 2	2660
	11th to 20th	Day	36% 5% 11% 26% 20% 3	2660
	21st to end	Day	42% 6% 14% 22% 13% 3	2660
July	1st to 10th	Day	47% 5% 12% 21% 13% 2	4371
	11th to 20th	Day	31% 6% 16% 29% 17% 1	4365
	21st to end	Day	39% 6% 16% 17% 20% 2	4662
August	1st to 10th	Day	35% 6% 15% 28% 14% 1	4285
	11th to 20th	Day	34% 5% 11% 33% 16% 1	3802
	21st to end	Day	32% 5% 12% 28% 20% 3	4179
September	1st to 10th	Day	36% 4 12% 24% 19% 4	2280
	11th to 20th	Day	31% 5% 11% 30% 21% 3	2147
	21st to end	Day	39% 5% 9% 27% 19% 1	2090
October	1st to 10th	Day	27% 2 7% 32% 29% 2	3424
	11th to 20th	Day	36% 3 11% 28% 23% 1	3093
	21st to end	Day	44% 3 9% 26% 15% 3	3081
November	1st to 10th	Day	40% 3 14% 24% 16% 2	1762
	11th to 20th	Day	52% 4 11% 21% 12% 1	1800
	21st to end	Day	54% 3 10% 23% 9%	1760
December	1st to 10th	Day	44% 3 16% 28% 8%	2610
	11th to 20th	Day	61% 4 11% 18% 6% 1	3005
	21st to end	Day	79% 2 6% 8% 4	3982
January	1st to 10th	Night	59% 2 9% 20% 10%	6348
	11th to 20th	Night	65% 2 10% 9% 12% 1	6360
	21st to end	Night	58% 2 12% 17% 11% 1	6886
February	1st to 10th	Night	56% 3 11% 18% 11% 2	2535
	11th to 20th	Night	51% 3 10% 22% 14% 1	2339
	21st to end	Night	52% 2 10% 21% 11% 3	1980
March	1st to 10th	Night	49% 2 10% 25% 12% 3	5373
	11th to 20th	Night	38% 3 11% 30% 15% 3	5349
	21st to end	Night	60% 2 9% 19% 9% 1	4962
April	1st to 10th	Night	46% 3 12% 25% 13% 2	2000
	11th to 20th	Night	47% 3 9% 21% 16% 4	1975
	21st to end	Night	49% 4 9% 22% 14% 2	1880
May	1st to 10th	Night	47% 3 12% 22% 13% 2	2930
	11th to 20th	Night	42% 4 14% 29% 10% 1	2647
	21st to end	Night	40% 4 11% 18% 21% 6%	2751
June	1st to 10th	Night	34% 4 13% 26% 20% 3	1520
	11th to 20th	Night	33% 4 13% 25% 23% 2	1520
	21st to end	Night	39% 6% 14% 22% 17% 2	1520
July	1st to 10th	Night	41% 5% 16% 22% 11% 5%	2490
	11th to 20th	Night	28% 5% 14% 28% 23% 2	2482
	21st to end	Night	33% 6% 17% 19% 22% 4	2662
August	1st to 10th	Night	34% 6% 15% 28% 16% 1	2626
	11th to 20th	Night	31% 5% 14% 28% 20% 1	3090
	21st to end	Night	30% 4 12% 33% 17% 4	3476
September	1st to 10th	Night	32% 3 12% 27% 24% 3	1900
	11th to 20th	Night	29% 3 8% 30% 27% 2	2014
	21st to end	Night	37% 4 10% 30% 17% 2	2090
October	1st to 10th	Night	22% 3 7% 38% 27% 2	3534
	11th to 20th	Night	37% 2 7% 25% 28% 2	3825
	21st to end	Night	41% 2 8% 26% 19% 3	4467
November	1st to 10th	Night	39% 3 8% 28% 19% 3	2536
	11th to 20th	Night	50% 3 11% 22% 13% 1	2600
	21st to end	Night	51% 3 11% 23% 11%	2635
December	1st to 10th	Night	43% 3 14% 28% 11% 2	4580
	11th to 20th	Night	58% 3 12% 16% 10% 1	5201
	21st to end	Night	72% 3 8% 11% 6%	6996

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% 17% 37% 19% 4	300
	11th to 20th	Day	25% 23% 34% 14% 3 1	300
	21st to end	Day	25% 20% 35% 17% 2 1	330
February	1st to 10th	Day	23% 21% 41% 13% 2	300
	11th to 20th	Day	26% 23% 32% 15% 2	300
	21st to end	Day	27% 19% 41% 11% 2	249
March	1st to 10th	Day	22% 26% 38% 12% 2	300
	11th to 20th	Day	25% 22% 34% 15% 4	337
	21st to end	Day	29% 19% 29% 16% 5% 1	550
April	1st to 10th	Day	27% 18% 31% 18% 6%	500
	11th to 20th	Day	25% 21% 25% 20% 9% 1	492
	21st to end	Day	24% 18% 35% 16% 7% 1	500
May	1st to 10th	Day	24% 16% 30% 21% 8% 2	503
	11th to 20th	Day	20% 13% 33% 22% 11% 3	585
	21st to end	Day	14% 11% 34% 24% 12% 5%	734
June	1st to 10th	Day	11% 10% 33% 28% 12% 5%	680
	11th to 20th	Day	12% 8% 34% 26% 16% 3	680
	21st to end	Day	13% 11% 35% 26% 10% 4	680
July	1st to 10th	Day	10% 5% 25% 32% 17% 11%	671
	11th to 20th	Day	12% 6% 29% 30% 15% 8%	668
	21st to end	Day	8% 7% 27% 34% 18% 6%	739
August	1st to 10th	Day	7% 4 26% 32% 20% 10%	605
	11th to 20th	Day	10% 5% 21% 30% 24% 10%	479
	21st to end	Day	7% 4 21% 39% 21% 8%	539
September	1st to 10th	Day	14% 6% 23% 31% 21% 5%	490
	11th to 20th	Day	10% 4 19% 33% 24% 9%	490
	21st to end	Day	14% 11% 25% 28% 17% 5%	490
October	1st to 10th	Day	17% 8% 29% 26% 15% 4	460
	11th to 20th	Day	21% 23% 31% 19% 5% 1	300
	21st to end	Day	31% 15% 34% 16% 3	330
November	1st to 10th	Day	16% 18% 36% 22% 6% 2	300
	11th to 20th	Day	24% 21% 35% 14% 6% 1	300
	21st to end	Day	27% 12% 41% 17% 3	300
December	1st to 10th	Day	22% 24% 35% 15% 4 1	300
	11th to 20th	Day	23% 18% 40% 16% 3	300
	21st to end	Day	29% 21% 36% 12% 2	330
January	1st to 10th	Night	21% 18% 35% 22% 4	1000
	11th to 20th	Night	23% 18% 34% 20% 4 1	1000
	21st to end	Night	22% 14% 35% 25% 4	1100
February	1st to 10th	Night	24% 14% 37% 22% 4	935
	11th to 20th	Night	21% 17% 34% 25% 3	783
	21st to end	Night	22% 18% 38% 18% 3	660
March	1st to 10th	Night	23% 14% 36% 21% 6%	800
	11th to 20th	Night	17% 13% 39% 24% 6% 1	800
	21st to end	Night	25% 13% 29% 24% 7% 1	660
April	1st to 10th	Night	21% 13% 35% 24% 6%	600
	11th to 20th	Night	23% 13% 32% 23% 8% 1	593
	21st to end	Night	23% 10% 32% 27% 8%	600
May	1st to 10th	Night	19% 8% 35% 30% 8%	600
	11th to 20th	Night	18% 4 29% 34% 14% 1	563
	21st to end	Night	14% 6% 27% 34% 19%	617
June	1st to 10th	Night	11% 4 26% 44% 14% 1	570
	11th to 20th	Night	10% 6% 28% 42% 13% 1	570
	21st to end	Night	12% 5% 26% 39% 17% 1	570
July	1st to 10th	Night	8% 3 18% 39% 27% 4	555
	11th to 20th	Night	7% 5% 21% 40% 25% 3	552
	21st to end	Night	5% 2 20% 47% 24% 2	624
August	1st to 10th	Night	6% 2 14% 47% 27% 3	567
	11th to 20th	Night	8% 3 14% 40% 31% 4	564
	21st to end	Night	9% 2 17% 39% 30% 3	627
September	1st to 10th	Night	11% 4 19% 33% 28% 4	570
	11th to 20th	Night	8% 5% 18% 36% 28% 4	684
	21st to end	Night	12% 8% 24% 34% 21% 3	760
October	1st to 10th	Night	14% 6% 26% 35% 17% 2	760
	11th to 20th	Night	20% 14% 30% 26% 9%	760
	21st to end	Night	25% 14% 29% 24% 8%	836
November	1st to 10th	Night	17% 14% 37% 21% 9% 2	780
	11th to 20th	Night	20% 19% 33% 23% 5% 1	800
	21st to end	Night	22% 14% 37% 21% 6%	835
December	1st to 10th	Night	21% 16% 37% 22% 4	1000
	11th to 20th	Night	24% 15% 34% 23% 4	1000
	21st to end	Night	31% 16% 32% 19% 2	1100

Page generated at 2025-05-08 08:34:00. Version ac07

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