Morton Grove
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morton Grove clear sky chart from 2005-11-06 to 2025-07-06.

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 7% 5% 4 4 4 6% 7% 37%	119747
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 38%	119255
Transparency	Day	44% 6% 12% 23% 13% 2	123317
	Night	46% 5% 12% 22% 14% 2	123826
Seeing	Day	17% 12% 30% 27% 12% 3	16709
	Night	18% 11% 29% 28% 12% 1	26587

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% 7% 11% 6% 6% 6% 4 6% 5% 27%	3617
	11th to 20th	Day	10% 12% 6% 12% 5% 5% 5% 6% 6% 6% 28%	3620
	21st to end	Day	16% 19% 5% 7% 3 2 3 2 4 6% 32%	4046
February	1st to 10th	Day	14% 14% 5% 10% 6% 6% 5% 4 5% 6% 26%	1857
	11th to 20th	Day	13% 12% 4 9% 5% 5% 4 4 5% 6% 33%	1948
	21st to end	Day	13% 13% 4 9% 5% 5% 4 4 6% 6% 31%	1650
March	1st to 10th	Day	16% 10% 3 7% 4 3 4 3 4 5% 41%	4480
	11th to 20th	Day	8% 7% 4 7% 5% 5% 5% 4 6% 9% 41%	4651
	21st to end	Day	19% 14% 7% 12% 6% 5% 4 4 4 5% 20%	5967
April	1st to 10th	Day	18% 13% 4 6% 6% 4 4 3 5% 6% 32%	2400
	11th to 20th	Day	15% 10% 3 6% 5% 5% 5% 4 5% 7% 35%	2374
	21st to end	Day	18% 9% 3 8% 4 5% 5% 4 5% 6% 34%	2530
May	1st to 10th	Day	18% 8% 3 6% 5% 4 4 4 6% 8% 35%	5862
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	6133
	21st to end	Day	15% 7% 2 6% 4 4 4 3 6% 9% 41%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 4 6% 7% 46%	2800
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 10% 44%	2800
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	11% 8% 3 7% 5% 5% 6% 5% 7% 8% 34%	4511
	11th to 20th	Day	7% 4 2 4 4 4 4 4 7% 9% 51%	4440
	21st to end	Day	9% 6% 3 7% 4 5% 5% 5% 7% 8% 42%	4813
August	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 7% 8% 45%	4306
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	3794
	21st to end	Day	7% 5% 3 6% 4 4 5% 3 6% 8% 49%	4180
September	1st to 10th	Day	12% 6% 2 4 4 4 5% 3 6% 7% 47%	2280
	11th to 20th	Day	11% 5% 1 3 2 4 3 4 6% 8% 51%	2146
	21st to end	Day	12% 6% 3 6% 4 5% 4 5% 7% 8% 40%	2090
October	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 4 6% 45%	2060
	11th to 20th	Day	19% 7% 3 7% 5% 5% 4 3 5% 6% 36%	1869
	21st to end	Day	20% 10% 3 6% 5% 4 5% 3 5% 6% 34%	1881
November	1st to 10th	Day	12% 9% 3 7% 4 3 5% 3 5% 8% 42%	1762
	11th to 20th	Day	14% 13% 3 9% 5% 5% 4 4 5% 7% 32%	1800
	21st to end	Day	18% 12% 3 7% 5% 4 4 3 3 5% 35%	1745
December	1st to 10th	Day	9% 10% 4 8% 5% 4 4 4 5% 7% 39%	2610
	11th to 20th	Day	12% 16% 6% 11% 6% 5% 4 4 6% 6% 24%	3008
	21st to end	Day	23% 23% 7% 10% 5% 4 4 3 4 5% 14%	3982
January	1st to 10th	Night	13% 13% 6% 12% 6% 5% 4 3 4 5% 31%	6348
	11th to 20th	Night	14% 14% 6% 13% 6% 6% 5% 4 6% 5% 22%	6360
	21st to end	Night	18% 16% 6% 8% 4 3 3 2 4 4 32%	6930
February	1st to 10th	Night	15% 16% 4 6% 6% 4 4 3 5% 5% 31%	2541
	11th to 20th	Night	13% 12% 4 9% 4 5% 5% 3 5% 5% 36%	2338
	21st to end	Night	19% 11% 4 8% 4 3 3 2 4 5% 36%	1980
March	1st to 10th	Night	17% 10% 3 7% 4 3 3 3 4 4 41%	5373
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	5348
	21st to end	Night	19% 12% 6% 9% 5% 5% 5% 3 4 5% 28%	4967
April	1st to 10th	Night	17% 9% 3 5% 5% 5% 4 4 5% 6% 37%	2000
	11th to 20th	Night	19% 9% 3 6% 3 5% 4 3 6% 5% 38%	1975
	21st to end	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	1870
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 5% 7% 35%	4033
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 6% 8% 41%	3758
	21st to end	Night	18% 8% 3 6% 4 4 3 3 4 5% 41%	3942
June	1st to 10th	Night	10% 6% 2 5% 5% 4 4 4 6% 8% 47%	1600
	11th to 20th	Night	11% 6% 1 6% 4 4 5% 4 5% 9% 44%	1600
	21st to end	Night	9% 7% 3 6% 4 5% 5% 5% 6% 8% 42%	1600
July	1st to 10th	Night	12% 5% 2 7% 5% 5% 5% 4 8% 9% 39%	2570
	11th to 20th	Night	6% 3 2 5% 4 4 5% 5% 6% 8% 53%	2530
	21st to end	Night	6% 4 2 5% 5% 5% 6% 5% 7% 9% 46%	2750
August	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 4 6% 8% 48%	2626
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 5% 7% 51%	3031
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 9% 51%	3476
September	1st to 10th	Night	11% 5% 2 5% 5% 3 4 3 5% 7% 51%	1900
	11th to 20th	Night	10% 5% 2 4 4 3 4 2 5% 8% 55%	2014
	21st to end	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	2090
October	1st to 10th	Night	13% 5% 3 5% 5% 3 3 3 5% 6% 50%	2118
	11th to 20th	Night	17% 8% 3 5% 4 3 3 3 4 5% 44%	2309
	21st to end	Night	21% 9% 2 6% 4 3 3 3 4 7% 37%	2717
November	1st to 10th	Night	13% 8% 2 6% 4 4 4 3 5% 5% 47%	2536
	11th to 20th	Night	15% 11% 4 8% 4 4 3 4 5% 6% 36%	2600
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 5% 4 36%	2650
December	1st to 10th	Night	12% 10% 3 7% 4 4 4 3 5% 7% 41%	4580
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 5% 5% 28%	5199
	21st to end	Night	24% 19% 6% 9% 5% 4 4 3 4 5% 17%	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	58% 5% 13% 17% 7%	3618
	11th to 20th	Day	54% 6% 12% 11% 13% 4	3620
	21st to end	Day	54% 4 12% 21% 9%	4046
February	1st to 10th	Day	58% 6% 12% 14% 9% 2	1857
	11th to 20th	Day	51% 5% 10% 23% 9% 2	1958
	21st to end	Day	53% 7% 11% 18% 9% 2	1650
March	1st to 10th	Day	45% 5% 10% 24% 11% 4	4480
	11th to 20th	Day	38% 8% 13% 31% 10% 1	4651
	21st to end	Day	65% 7% 11% 11% 6% 1	5969
April	1st to 10th	Day	52% 7% 11% 19% 11%	2400
	11th to 20th	Day	47% 7% 10% 21% 12% 3	2374
	21st to end	Day	49% 8% 13% 18% 11% 2	2530
May	1st to 10th	Day	45% 7% 12% 22% 12% 2	5897
	11th to 20th	Day	40% 8% 12% 25% 14% 1	6137
	21st to end	Day	40% 6% 12% 25% 15% 3	6949
June	1st to 10th	Day	36% 7% 13% 24% 18% 2	2800
	11th to 20th	Day	36% 6% 13% 23% 19% 3	2800
	21st to end	Day	39% 9% 13% 22% 15% 2	2800
July	1st to 10th	Day	43% 9% 13% 22% 11% 2	4442
	11th to 20th	Day	28% 7% 17% 30% 17% 1	4365
	21st to end	Day	37% 9% 16% 16% 20% 2	4662
August	1st to 10th	Day	35% 7% 15% 29% 13% 1	4291
	11th to 20th	Day	33% 7% 11% 33% 16% 1	3832
	21st to end	Day	32% 6% 13% 26% 20% 2	4179
September	1st to 10th	Day	35% 7% 13% 23% 18% 4	2280
	11th to 20th	Day	29% 6% 13% 30% 19% 3	2146
	21st to end	Day	38% 7% 11% 24% 18% 1	2090
October	1st to 10th	Day	27% 4 7% 37% 24% 1	3424
	11th to 20th	Day	36% 4 11% 27% 20%	3085
	21st to end	Day	45% 4 9% 27% 13% 3	3081
November	1st to 10th	Day	42% 5% 14% 23% 16% 2	1762
	11th to 20th	Day	52% 5% 13% 19% 10% 1	1800
	21st to end	Day	54% 4 12% 21% 8%	1745
December	1st to 10th	Day	44% 4 20% 25% 6%	2610
	11th to 20th	Day	59% 6% 12% 17% 5%	3005
	21st to end	Day	74% 6% 6% 10% 4	3982
January	1st to 10th	Night	57% 4 10% 20% 9%	6348
	11th to 20th	Night	63% 4 11% 9% 12% 1	6360
	21st to end	Night	57% 3 13% 16% 10% 1	6930
February	1st to 10th	Night	55% 5% 11% 17% 12% 1	2541
	11th to 20th	Night	50% 5% 11% 21% 12% 1	2339
	21st to end	Night	51% 4 11% 20% 10% 3	1980
March	1st to 10th	Night	47% 4 11% 24% 11% 3	5373
	11th to 20th	Night	36% 5% 12% 30% 14% 3	5348
	21st to end	Night	58% 5% 9% 18% 9% 1	4967
April	1st to 10th	Night	46% 6% 12% 22% 13% 1	2000
	11th to 20th	Night	47% 6% 8% 20% 16% 3	1975
	21st to end	Night	46% 6% 10% 22% 13% 2	1870
May	1st to 10th	Night	48% 6% 12% 16% 15% 3	4060
	11th to 20th	Night	39% 8% 14% 23% 13% 3	3759
	21st to end	Night	44% 6% 9% 21% 16% 4	3950
June	1st to 10th	Night	34% 7% 12% 26% 19% 3	1600
	11th to 20th	Night	36% 6% 12% 24% 21% 2	1600
	21st to end	Night	36% 9% 16% 21% 17% 2	1600
July	1st to 10th	Night	39% 8% 16% 21% 11% 5%	2530
	11th to 20th	Night	26% 7% 15% 31% 19% 2	2482
	21st to end	Night	32% 8% 16% 17% 23% 3	2662
August	1st to 10th	Night	32% 8% 15% 27% 17% 1	2617
	11th to 20th	Night	30% 7% 14% 29% 20% 1	3059
	21st to end	Night	30% 5% 12% 32% 17% 3	3476
September	1st to 10th	Night	33% 6% 10% 27% 22% 3	1900
	11th to 20th	Night	29% 4 9% 30% 26% 2	2014
	21st to end	Night	37% 6% 10% 28% 16% 2	2090
October	1st to 10th	Night	23% 3 8% 37% 27% 2	3534
	11th to 20th	Night	36% 4 9% 24% 26% 2	3832
	21st to end	Night	41% 4 9% 25% 17% 3	4467
November	1st to 10th	Night	39% 5% 11% 25% 17% 3	2536
	11th to 20th	Night	49% 5% 13% 22% 11% 2	2600
	21st to end	Night	51% 5% 12% 22% 9%	2650
December	1st to 10th	Night	43% 5% 17% 25% 9% 2	4580
	11th to 20th	Night	57% 5% 12% 15% 10% 1	5201
	21st to end	Night	69% 5% 9% 12% 5%	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	21% 15% 39% 21% 3 1	300
	11th to 20th	Day	21% 21% 36% 19% 2	300
	21st to end	Day	23% 17% 36% 19% 4	330
February	1st to 10th	Day	22% 19% 40% 16% 3	300
	11th to 20th	Day	23% 21% 35% 18% 2	300
	21st to end	Day	19% 22% 43% 14% 2	249
March	1st to 10th	Day	19% 21% 42% 18% 1	300
	11th to 20th	Day	20% 21% 37% 17% 4	340
	21st to end	Day	26% 19% 30% 18% 5% 1	550
April	1st to 10th	Day	23% 18% 33% 22% 4	500
	11th to 20th	Day	17% 21% 32% 20% 8% 2	492
	21st to end	Day	20% 17% 33% 20% 9% 1	500
May	1st to 10th	Day	22% 11% 34% 24% 8% 2	503
	11th to 20th	Day	20% 7% 31% 31% 9% 2	643
	21st to end	Day	16% 8% 29% 29% 15% 4	789
June	1st to 10th	Day	12% 7% 29% 35% 14% 3	730
	11th to 20th	Day	13% 6% 31% 33% 15% 2	730
	21st to end	Day	12% 7% 32% 33% 11% 4	730
July	1st to 10th	Day	9% 3 22% 34% 23% 8%	697
	11th to 20th	Day	11% 4 26% 35% 18% 5%	668
	21st to end	Day	7% 4 25% 41% 18% 4	739
August	1st to 10th	Day	9% 4 19% 40% 21% 8%	611
	11th to 20th	Day	11% 6% 21% 33% 22% 7%	479
	21st to end	Day	7% 4 19% 40% 23% 6%	539
September	1st to 10th	Day	14% 6% 21% 35% 20% 4	490
	11th to 20th	Day	10% 4 19% 35% 24% 8%	490
	21st to end	Day	16% 10% 26% 26% 18% 5%	490
October	1st to 10th	Day	19% 10% 24% 29% 14% 3	460
	11th to 20th	Day	20% 22% 30% 22% 6%	300
	21st to end	Day	29% 16% 34% 19% 2	330
November	1st to 10th	Day	17% 17% 33% 25% 8% 1	300
	11th to 20th	Day	23% 22% 33% 15% 6% 1	300
	21st to end	Day	25% 14% 40% 19% 2	300
December	1st to 10th	Day	18% 23% 35% 18% 5%	300
	11th to 20th	Day	20% 24% 37% 15% 5%	300
	21st to end	Day	26% 22% 34% 16% 2	330
January	1st to 10th	Night	19% 17% 40% 21% 3	1000
	11th to 20th	Night	22% 19% 35% 19% 4 1	1000
	21st to end	Night	22% 14% 36% 24% 4	1100
February	1st to 10th	Night	23% 13% 37% 22% 5%	941
	11th to 20th	Night	19% 15% 36% 26% 3	783
	21st to end	Night	25% 13% 38% 20% 3	660
March	1st to 10th	Night	22% 13% 36% 22% 8%	800
	11th to 20th	Night	18% 10% 37% 27% 7% 1	800
	21st to end	Night	24% 12% 30% 25% 9% 1	665
April	1st to 10th	Night	21% 14% 32% 27% 6% 1	600
	11th to 20th	Night	23% 12% 30% 25% 9% 1	593
	21st to end	Night	21% 10% 29% 29% 10% 1	600
May	1st to 10th	Night	19% 8% 29% 34% 10% 1	600
	11th to 20th	Night	18% 6% 25% 33% 17% 1	591
	21st to end	Night	14% 5% 21% 37% 21% 2	650
June	1st to 10th	Night	12% 4 20% 44% 20% 1	600
	11th to 20th	Night	13% 4 24% 40% 18% 1	600
	21st to end	Night	11% 4 27% 35% 22% 2	600
July	1st to 10th	Night	9% 3 13% 41% 29% 6%	570
	11th to 20th	Night	9% 3 18% 40% 26% 4	552
	21st to end	Night	7% 2 18% 42% 29% 2	624
August	1st to 10th	Night	6% 2 15% 42% 30% 5%	567
	11th to 20th	Night	7% 2 16% 37% 32% 6%	564
	21st to end	Night	9% 3 18% 38% 30% 3	627
September	1st to 10th	Night	13% 4 17% 34% 27% 5%	570
	11th to 20th	Night	11% 5% 17% 33% 28% 5%	684
	21st to end	Night	15% 8% 24% 27% 21% 4	760
October	1st to 10th	Night	15% 7% 25% 31% 20% 2	760
	11th to 20th	Night	22% 13% 30% 26% 9% 1	760
	21st to end	Night	25% 13% 30% 24% 7%	836
November	1st to 10th	Night	18% 13% 35% 22% 11% 1	780
	11th to 20th	Night	21% 20% 33% 22% 5% 1	800
	21st to end	Night	22% 16% 36% 21% 5%	850
December	1st to 10th	Night	23% 17% 35% 21% 4	1000
	11th to 20th	Night	23% 16% 34% 23% 4	1000
	21st to end	Night	30% 16% 33% 18% 2	1100

Page generated at 2025-07-03 00:30:14. Version ac07

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