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 2026-10-29.

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 5% 4 6% 7% 37%	149479
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	142911
Transparency	Day	44% 6% 12% 23% 13% 2	153004
	Night	45% 5% 11% 23% 14% 2	147463
Seeing	Day	17% 12% 30% 27% 11% 3	18564
	Night	18% 11% 29% 28% 12% 1	28331

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	13% 12% 7% 11% 6% 5% 5% 4 6% 5% 26%	4691
	11th to 20th	Day	8% 10% 6% 13% 6% 5% 6% 6% 6% 5% 29%	4710
	21st to end	Day	13% 17% 5% 9% 4 4 4 3 4 6% 32%	5234
February	1st to 10th	Day	13% 13% 5% 10% 6% 6% 5% 4 5% 6% 26%	1950
	11th to 20th	Day	13% 12% 4 9% 5% 5% 4 4 5% 6% 34%	2048
	21st to end	Day	13% 14% 4 9% 5% 5% 4 4 6% 6% 30%	1730
March	1st to 10th	Day	16% 10% 3 7% 4 3 4 3 4 5% 41%	4580
	11th to 20th	Day	9% 7% 4 8% 5% 5% 5% 4 6% 9% 41%	4754
	21st to end	Day	19% 13% 7% 12% 6% 5% 4 4 4 5% 20%	6099
April	1st to 10th	Day	18% 13% 3 6% 6% 4 4 3 5% 6% 31%	2520
	11th to 20th	Day	15% 10% 3 6% 5% 5% 5% 4 5% 7% 35%	2494
	21st to end	Day	18% 9% 3 8% 4 5% 5% 4 5% 6% 33%	2656
May	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 4 6% 8% 36%	7616
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 4 6% 8% 39%	7982
	21st to end	Day	14% 7% 2 6% 4 4 4 3 6% 8% 41%	9047
June	1st to 10th	Day	10% 5% 3 5% 4 5% 5% 4 6% 8% 45%	2940
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 6% 9% 44%	2940
	21st to end	Day	12% 6% 3 6% 4 5% 5% 4 7% 9% 40%	2940
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 8% 39%	8280
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 10% 48%	8280
	21st to end	Day	10% 6% 3 6% 4 5% 5% 4 7% 9% 41%	8990
August	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 8% 41%	6320
	11th to 20th	Day	15% 7% 2 7% 4 4 5% 3 6% 8% 39%	5581
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	6080
September	1st to 10th	Day	12% 6% 2 4 4 4 5% 3 6% 7% 47%	2520
	11th to 20th	Day	12% 5% 2 4 2 3 3 4 6% 8% 51%	2372
	21st to end	Day	13% 6% 3 6% 4 5% 4 5% 7% 8% 40%	2310
October	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 5% 6% 46%	2276
	11th to 20th	Day	19% 7% 3 7% 5% 5% 4 3 5% 6% 37%	2065
	21st to end	Day	18% 10% 2 6% 4 4 5% 3 5% 6% 36%	2052
November	1st to 10th	Day	12% 9% 3 7% 4 3 5% 3 5% 8% 42%	1852
	11th to 20th	Day	14% 13% 3 9% 5% 4 4 3 5% 7% 33%	1890
	21st to end	Day	17% 12% 3 8% 5% 4 4 3 3 5% 35%	1832
December	1st to 10th	Day	9% 11% 4 8% 5% 4 4 4 5% 7% 38%	2690
	11th to 20th	Day	12% 16% 6% 11% 6% 5% 4 4 6% 6% 25%	3088
	21st to end	Day	23% 22% 7% 11% 5% 4 4 2 4 5% 14%	4070
January	1st to 10th	Night	12% 13% 6% 12% 6% 5% 4 3 4 5% 30%	8232
	11th to 20th	Night	12% 14% 6% 14% 6% 7% 5% 4 5% 5% 23%	8280
	21st to end	Night	14% 15% 5% 8% 4 4 4 3 5% 5% 32%	8964
February	1st to 10th	Night	15% 15% 4 7% 6% 4 4 3 5% 5% 32%	2668
	11th to 20th	Night	13% 11% 4 9% 4 5% 5% 3 5% 6% 36%	2458
	21st to end	Night	18% 11% 4 8% 4 3 3 3 4 5% 36%	2076
March	1st to 10th	Night	17% 10% 4 7% 4 3 3 3 4 4 40%	5493
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 5% 7% 47%	5466
	21st to end	Night	18% 12% 5% 9% 5% 5% 5% 4 4 5% 29%	5077
April	1st to 10th	Night	17% 9% 3 6% 5% 5% 4 4 5% 6% 36%	2100
	11th to 20th	Night	19% 8% 3 6% 3 5% 4 3 6% 5% 37%	2075
	21st to end	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	1963
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 5% 7% 39%	5237
	11th to 20th	Night	11% 6% 3 6% 4 4 5% 4 6% 9% 43%	4882
	21st to end	Night	15% 7% 3 7% 4 4 4 3 5% 7% 41%	5139
June	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 8% 47%	1680
	11th to 20th	Night	11% 6% 1 6% 4 4 5% 4 6% 9% 44%	1680
	21st to end	Night	9% 7% 3 6% 4 5% 5% 5% 7% 8% 41%	1680
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4708
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 8% 53%	4707
	21st to end	Night	9% 4 2 5% 5% 5% 5% 4 6% 7% 47%	5117
August	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 4 5% 7% 43%	3839
	11th to 20th	Night	13% 7% 3 7% 4 4 5% 3 5% 7% 41%	4461
	21st to end	Night	7% 4 2 5% 4 4 4 4 5% 8% 52%	5046
September	1st to 10th	Night	10% 5% 2 5% 5% 3 4 3 5% 7% 51%	2100
	11th to 20th	Night	10% 5% 2 5% 3 3 4 2 5% 7% 55%	2226
	21st to end	Night	12% 7% 3 6% 4 4 5% 3 5% 6% 45%	2310
October	1st to 10th	Night	12% 5% 3 5% 5% 3 3 3 5% 6% 51%	2341
	11th to 20th	Night	17% 8% 3 5% 4 3 3 3 4 5% 44%	2553
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 7% 38%	2965
November	1st to 10th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 46%	2666
	11th to 20th	Night	15% 11% 4 8% 4 4 3 4 5% 6% 37%	2730
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 5% 4 36%	2783
December	1st to 10th	Night	12% 11% 3 7% 4 4 4 3 5% 7% 40%	4720
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 5% 5% 28%	5339
	21st to end	Night	24% 19% 6% 9% 5% 4 4 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	59% 4 16% 14% 6%	4692
	11th to 20th	Day	54% 6% 10% 16% 11% 3	4710
	21st to end	Day	54% 4 12% 22% 8%	5234
February	1st to 10th	Day	57% 6% 12% 15% 9% 1	1950
	11th to 20th	Day	50% 5% 10% 23% 9% 2	2058
	21st to end	Day	54% 7% 12% 17% 9% 1	1730
March	1st to 10th	Day	45% 5% 10% 24% 11% 4	4580
	11th to 20th	Day	38% 8% 13% 31% 10% 1	4754
	21st to end	Day	64% 7% 11% 11% 6% 1	6101
April	1st to 10th	Day	52% 7% 10% 19% 11%	2520
	11th to 20th	Day	47% 7% 11% 21% 11% 3	2494
	21st to end	Day	48% 8% 12% 19% 11% 2	2656
May	1st to 10th	Day	44% 7% 11% 25% 12% 2	7651
	11th to 20th	Day	41% 8% 12% 23% 15% 1	7986
	21st to end	Day	40% 5% 12% 22% 16% 4	9061
June	1st to 10th	Day	37% 7% 13% 24% 18% 2	2940
	11th to 20th	Day	36% 6% 13% 24% 19% 3	2940
	21st to end	Day	39% 9% 13% 23% 14% 2	2940
July	1st to 10th	Day	39% 8% 13% 24% 15% 2	8211
	11th to 20th	Day	31% 7% 14% 31% 17% 1	8205
	21st to end	Day	37% 7% 14% 22% 18% 2	8794
August	1st to 10th	Day	40% 7% 12% 27% 13% 1	6305
	11th to 20th	Day	42% 7% 12% 27% 12% 1	5619
	21st to end	Day	31% 7% 13% 32% 16% 2	6079
September	1st to 10th	Day	35% 6% 14% 25% 17% 3	2520
	11th to 20th	Day	30% 6% 13% 30% 19% 2	2372
	21st to end	Day	39% 7% 12% 25% 17% 1	2310
October	1st to 10th	Day	27% 4 7% 38% 23% 1	3640
	11th to 20th	Day	37% 4 11% 27% 20% 1	3281
	21st to end	Day	44% 4 9% 27% 13% 3	3252
November	1st to 10th	Day	41% 5% 13% 23% 16% 2	1852
	11th to 20th	Day	52% 5% 13% 20% 10% 1	1890
	21st to end	Day	54% 4 12% 21% 8%	1832
December	1st to 10th	Day	45% 4 20% 24% 6%	2690
	11th to 20th	Day	59% 6% 12% 18% 5%	3085
	21st to end	Day	74% 6% 6% 9% 4	4070
January	1st to 10th	Night	57% 4 11% 20% 8%	8233
	11th to 20th	Night	62% 4 11% 11% 10% 1	8280
	21st to end	Night	54% 4 13% 20% 9%	8964
February	1st to 10th	Night	55% 5% 11% 17% 11% 1	2668
	11th to 20th	Night	49% 5% 11% 22% 12% 1	2459
	21st to end	Night	51% 4 11% 20% 10% 3	2076
March	1st to 10th	Night	47% 4 11% 24% 11% 3	5493
	11th to 20th	Night	37% 5% 12% 29% 14% 3	5466
	21st to end	Night	58% 5% 9% 18% 9%	5077
April	1st to 10th	Night	47% 6% 12% 22% 12% 1	2100
	11th to 20th	Night	47% 5% 9% 19% 16% 3	2075
	21st to end	Night	45% 6% 11% 23% 13% 2	1963
May	1st to 10th	Night	44% 5% 10% 22% 16% 3	5264
	11th to 20th	Night	36% 7% 14% 24% 15% 3	4883
	21st to end	Night	42% 5% 8% 20% 19% 5%	5149
June	1st to 10th	Night	34% 7% 12% 25% 20% 3	1680
	11th to 20th	Night	35% 6% 11% 24% 22% 2	1680
	21st to end	Night	37% 8% 16% 20% 17% 1	1680
July	1st to 10th	Night	37% 7% 14% 25% 14% 3	4670
	11th to 20th	Night	28% 6% 13% 29% 22% 2	4662
	21st to end	Night	34% 7% 13% 24% 20% 2	5002
August	1st to 10th	Night	38% 7% 13% 25% 16% 1	3830
	11th to 20th	Night	41% 7% 13% 24% 14% 1	4489
	21st to end	Night	29% 5% 12% 37% 15% 2	5046
September	1st to 10th	Night	32% 6% 10% 29% 20% 3	2100
	11th to 20th	Night	30% 4 9% 29% 26% 2	2226
	21st to end	Night	38% 6% 10% 27% 17% 2	2310
October	1st to 10th	Night	23% 3 7% 37% 27% 2	3757
	11th to 20th	Night	37% 4 9% 23% 26% 2	4076
	21st to end	Night	41% 4 10% 25% 18% 3	4715
November	1st to 10th	Night	40% 5% 11% 25% 17% 3	2666
	11th to 20th	Night	48% 5% 13% 23% 11% 2	2730
	21st to end	Night	51% 5% 12% 22% 9%	2783
December	1st to 10th	Night	43% 5% 16% 25% 9% 2	4720
	11th to 20th	Night	57% 5% 12% 15% 10% 1	5341
	21st to end	Night	69% 5% 9% 12% 5%	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% 15% 39% 20% 3 1	330
	11th to 20th	Day	22% 23% 36% 17% 2	330
	21st to end	Day	23% 18% 36% 18% 4	363
February	1st to 10th	Day	21% 19% 41% 17% 3	330
	11th to 20th	Day	22% 21% 35% 18% 2	330
	21st to end	Day	21% 22% 43% 12% 2	273
March	1st to 10th	Day	20% 22% 40% 17% 1	330
	11th to 20th	Day	22% 20% 37% 16% 4	372
	21st to end	Day	26% 20% 31% 17% 5% 1	594
April	1st to 10th	Day	25% 18% 32% 20% 4	540
	11th to 20th	Day	18% 22% 32% 19% 8% 2	532
	21st to end	Day	20% 17% 33% 20% 9% 1	540
May	1st to 10th	Day	21% 12% 34% 24% 7% 1	543
	11th to 20th	Day	20% 10% 31% 30% 8% 1	689
	21st to end	Day	15% 8% 30% 29% 15% 3	844
June	1st to 10th	Day	12% 7% 30% 34% 14% 3	780
	11th to 20th	Day	13% 8% 31% 31% 14% 2	780
	21st to end	Day	13% 8% 34% 32% 11% 4	780
July	1st to 10th	Day	10% 3 25% 34% 21% 8%	771
	11th to 20th	Day	11% 4 26% 35% 18% 5%	768
	21st to end	Day	8% 5% 27% 40% 17% 4	849
August	1st to 10th	Day	9% 4 20% 39% 20% 7%	704
	11th to 20th	Day	13% 7% 21% 33% 20% 7%	559
	21st to end	Day	7% 6% 22% 38% 21% 6%	627
September	1st to 10th	Day	15% 6% 23% 33% 19% 4	570
	11th to 20th	Day	11% 5% 20% 34% 23% 8%	570
	21st to end	Day	16% 10% 26% 26% 18% 5%	570
October	1st to 10th	Day	18% 10% 26% 29% 14% 3	536
	11th to 20th	Day	20% 20% 30% 23% 7% 1	360
	21st to end	Day	26% 16% 35% 20% 3 1	387
November	1st to 10th	Day	17% 18% 32% 24% 7% 1	330
	11th to 20th	Day	26% 21% 32% 16% 5% 1	330
	21st to end	Day	26% 15% 40% 18% 2	330
December	1st to 10th	Day	18% 23% 35% 18% 5%	330
	11th to 20th	Day	21% 25% 36% 15% 4	330
	21st to end	Day	27% 21% 35% 15% 2	363
January	1st to 10th	Night	19% 17% 40% 21% 3	1050
	11th to 20th	Night	22% 20% 35% 18% 4 1	1050
	21st to end	Night	23% 14% 37% 23% 4	1155
February	1st to 10th	Night	22% 13% 37% 22% 5%	988
	11th to 20th	Night	19% 16% 35% 26% 3	823
	21st to end	Night	24% 14% 39% 20% 3	692
March	1st to 10th	Night	22% 13% 36% 21% 7%	840
	11th to 20th	Night	19% 11% 37% 27% 7%	840
	21st to end	Night	24% 12% 31% 24% 8% 1	698
April	1st to 10th	Night	22% 15% 31% 26% 6% 1	630
	11th to 20th	Night	23% 12% 30% 24% 9% 1	623
	21st to end	Night	21% 10% 29% 29% 11% 1	630
May	1st to 10th	Night	18% 8% 30% 33% 10% 1	630
	11th to 20th	Night	18% 7% 25% 33% 16% 1	621
	21st to end	Night	14% 5% 21% 37% 20% 2	683
June	1st to 10th	Night	12% 4 21% 43% 20% 1	630
	11th to 20th	Night	13% 5% 25% 39% 17% 1	630
	21st to end	Night	11% 4 27% 36% 21% 1	630
July	1st to 10th	Night	9% 3 14% 41% 28% 5%	615
	11th to 20th	Night	9% 3 18% 40% 26% 3	612
	21st to end	Night	8% 2 19% 42% 28% 2	690
August	1st to 10th	Night	7% 2 16% 41% 29% 4	627
	11th to 20th	Night	8% 3 17% 36% 30% 5%	624
	21st to end	Night	9% 3 19% 38% 29% 2	693
September	1st to 10th	Night	12% 4 20% 33% 27% 4	630
	11th to 20th	Night	11% 6% 18% 32% 29% 5%	756
	21st to end	Night	15% 8% 24% 27% 22% 4	840
October	1st to 10th	Night	15% 7% 26% 32% 19% 2	840
	11th to 20th	Night	21% 13% 29% 27% 9% 1	840
	21st to end	Night	24% 13% 30% 25% 8%	913
November	1st to 10th	Night	19% 14% 35% 21% 11% 1	820
	11th to 20th	Night	21% 20% 33% 21% 5%	840
	21st to end	Night	22% 16% 35% 21% 5%	893
December	1st to 10th	Night	23% 17% 35% 21% 4	1050
	11th to 20th	Night	23% 17% 34% 23% 3	1050
	21st to end	Night	31% 17% 33% 18% 2	1155

Page generated at 2026-10-26 03:37:47. Version ac07

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