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 2024-03-18.

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% 5% 3 5% 5% 5% 7% 36%	78560
	Night	15% 10% 4 6% 5% 3 4 4 5% 6% 37%	81280
Transparency	Day	47% 7% 12% 21% 12% 2	78261
	Night	48% 6% 12% 21% 12% 2	81100
Seeing	Day	16% 11% 29% 28% 12% 3	14834
	Night	18% 10% 29% 28% 13% 1	24904

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	14% 12% 8% 8% 7% 4 5% 4 4 4 28%	2527
	11th to 20th	Day	13% 13% 6% 13% 6% 3 5% 8% 8% 6% 18%	2530
	21st to end	Day	19% 25% 6% 8% 5% 1 4 3 3 5% 22%	2819
February	1st to 10th	Day	14% 12% 5% 8% 7% 4 5% 5% 5% 6% 27%	1764
	11th to 20th	Day	14% 11% 5% 8% 5% 3 5% 5% 5% 6% 34%	1848
	21st to end	Day	13% 13% 5% 9% 5% 3 5% 5% 5% 6% 30%	1570
March	1st to 10th	Day	15% 12% 5% 8% 6% 4 5% 4 4 6% 31%	1900
	11th to 20th	Day	13% 8% 4 6% 6% 3 6% 5% 5% 8% 36%	1917
	21st to end	Day	19% 11% 5% 9% 7% 4 5% 4 5% 6% 26%	2376
April	1st to 10th	Day	17% 10% 5% 6% 6% 3 4 4 5% 6% 32%	2160
	11th to 20th	Day	16% 10% 3 5% 6% 4 5% 5% 5% 7% 35%	2134
	21st to end	Day	20% 9% 3 7% 5% 4 5% 5% 5% 6% 33%	2276
May	1st to 10th	Day	18% 8% 4 6% 7% 3 4 4 6% 8% 32%	2340
	11th to 20th	Day	19% 8% 3 5% 5% 3 4 4 5% 9% 35%	2405
	21st to end	Day	13% 6% 4 6% 6% 4 6% 5% 6% 8% 38%	2725
June	1st to 10th	Day	10% 5% 2 4 5% 3 6% 5% 5% 8% 46%	2520
	11th to 20th	Day	11% 5% 2 5% 4 3 5% 4 6% 9% 45%	2520
	21st to end	Day	12% 5% 3 6% 5% 3 5% 6% 6% 10% 39%	2520
July	1st to 10th	Day	8% 4 2 4 5% 3 5% 6% 7% 9% 47%	2520
	11th to 20th	Day	10% 5% 3 4 4 3 5% 5% 6% 8% 49%	2520
	21st to end	Day	7% 4 2 4 5% 4 6% 6% 6% 10% 45%	2743
August	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 5% 8% 9% 44%	2446
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 5% 6% 9% 45%	2164
	21st to end	Day	8% 4 2 4 4 4 5% 5% 7% 9% 48%	2376
September	1st to 10th	Day	13% 6% 2 4 5% 3 5% 4 6% 7% 45%	2160
	11th to 20th	Day	12% 5% 2 3 3 3 4 4 6% 8% 50%	2034
	21st to end	Day	11% 5% 3 6% 5% 3 4 6% 7% 9% 40%	1980
October	1st to 10th	Day	15% 8% 2 6% 5% 3 4 4 4 5% 44%	1952
	11th to 20th	Day	20% 7% 4 7% 6% 4 5% 3 5% 6% 34%	1771
	21st to end	Day	20% 10% 3 6% 5% 3 4 4 4 7% 33%	1782
November	1st to 10th	Day	13% 8% 3 5% 5% 2 5% 4 5% 8% 43%	1672
	11th to 20th	Day	15% 11% 4 8% 5% 3 4 5% 5% 7% 33%	1710
	21st to end	Day	18% 11% 3 6% 5% 3 5% 3 3 5% 37%	1658
December	1st to 10th	Day	16% 14% 6% 7% 5% 3 5% 4 5% 7% 28%	1520
	11th to 20th	Day	12% 13% 6% 8% 7% 3 4 5% 6% 7% 29%	1918
	21st to end	Day	20% 20% 8% 10% 7% 3 3 3 3 5% 16%	2783
January	1st to 10th	Night	15% 14% 8% 9% 6% 3 4 4 3 5% 29%	4428
	11th to 20th	Night	17% 16% 6% 12% 6% 3 4 6% 6% 4 18%	4440
	21st to end	Night	23% 18% 8% 8% 5% 3 3 3 3 4 22%	4846
February	1st to 10th	Night	16% 14% 5% 6% 7% 3 5% 4 4 6% 32%	2414
	11th to 20th	Night	13% 10% 4 8% 5% 4 5% 4 5% 5% 36%	2218
	21st to end	Night	19% 11% 4 7% 5% 2 3 3 4 5% 35%	1884
March	1st to 10th	Night	17% 11% 4 7% 6% 3 4 4 4 5% 35%	2280
	11th to 20th	Night	12% 7% 3 5% 5% 3 5% 5% 5% 7% 42%	2220
	21st to end	Night	20% 10% 4 7% 5% 3 5% 4 4 7% 32%	1985
April	1st to 10th	Night	17% 8% 3 5% 6% 4 5% 4 5% 7% 38%	1800
	11th to 20th	Night	20% 8% 3 5% 4 4 5% 4 6% 5% 37%	1775
	21st to end	Night	19% 8% 3 5% 5% 2 4 5% 5% 7% 37%	1684
May	1st to 10th	Night	16% 8% 3 7% 5% 4 4 6% 5% 7% 34%	1620
	11th to 20th	Night	16% 7% 3 5% 4 3 5% 4 5% 7% 41%	1487
	21st to end	Night	12% 6% 3 5% 6% 3 4 5% 5% 8% 42%	1551
June	1st to 10th	Night	10% 5% 2 4 5% 3 5% 5% 6% 9% 47%	1440
	11th to 20th	Night	11% 6% 1 5% 5% 2 5% 5% 4 9% 46%	1440
	21st to end	Night	9% 6% 3 5% 5% 3 5% 6% 7% 8% 42%	1440
July	1st to 10th	Night	8% 3 1 3 5% 4 5% 5% 8% 9% 50%	1440
	11th to 20th	Night	8% 4 2 5% 4 3 4 5% 6% 8% 52%	1440
	21st to end	Night	7% 3 1 4 5% 2 5% 6% 6% 10% 50%	1575
August	1st to 10th	Night	6% 4 1 4 6% 3 5% 6% 6% 9% 51%	1489
	11th to 20th	Night	7% 3 2 4 5% 3 5% 5% 6% 8% 52%	1733
	21st to end	Night	9% 4 2 4 4 2 5% 4 5% 8% 54%	1980
September	1st to 10th	Night	11% 5% 2 4 6% 3 4 4 5% 7% 50%	1800
	11th to 20th	Night	10% 5% 2 4 4 2 4 3 4 7% 55%	1908
	21st to end	Night	12% 7% 2 5% 5% 3 5% 4 5% 7% 47%	1980
October	1st to 10th	Night	13% 5% 3 4 5% 2 4 3 5% 6% 48%	2006
	11th to 20th	Night	18% 8% 3 4 5% 2 4 3 5% 5% 43%	2187
	21st to end	Night	22% 9% 2 6% 5% 3 3 4 4 7% 36%	2574
November	1st to 10th	Night	12% 7% 2 5% 5% 2 4 4 4 5% 48%	2406
	11th to 20th	Night	14% 10% 4 7% 4 3 4 4 5% 7% 37%	2470
	21st to end	Night	17% 9% 3 6% 6% 3 4 4 5% 5% 38%	2517
December	1st to 10th	Night	17% 12% 3 6% 6% 3 5% 5% 5% 7% 30%	2660
	11th to 20th	Night	15% 14% 4 8% 5% 3 4 4 5% 6% 32%	3279
	21st to end	Night	24% 17% 6% 8% 6% 3 4 4 4 5% 18%	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	59% 6% 13% 16% 6%	2528
	11th to 20th	Day	63% 6% 12% 14% 5%	2530
	21st to end	Day	67% 4 8% 17% 5%	2819
February	1st to 10th	Day	57% 6% 12% 15% 9% 2	1764
	11th to 20th	Day	50% 6% 10% 23% 10% 1	1858
	21st to end	Day	54% 7% 11% 18% 9% 2	1570
March	1st to 10th	Day	54% 6% 12% 19% 7% 2	1900
	11th to 20th	Day	46% 6% 14% 23% 10% 2	1917
	21st to end	Day	59% 6% 11% 17% 5% 1	2376
April	1st to 10th	Day	51% 8% 11% 19% 11%	2160
	11th to 20th	Day	48% 7% 10% 20% 12% 4	2134
	21st to end	Day	50% 7% 12% 17% 12% 2	2276
May	1st to 10th	Day	50% 7% 12% 22% 8%	2340
	11th to 20th	Day	47% 7% 12% 22% 11% 1	2409
	21st to end	Day	44% 7% 13% 20% 15% 2	2725
June	1st to 10th	Day	36% 7% 13% 23% 18% 2	2520
	11th to 20th	Day	36% 7% 12% 23% 20% 3	2520
	21st to end	Day	40% 9% 14% 22% 14% 2	2520
July	1st to 10th	Day	31% 9% 13% 28% 15% 4	2451
	11th to 20th	Day	32% 8% 14% 28% 16% 2	2445
	21st to end	Day	32% 10% 16% 23% 16% 4	2591
August	1st to 10th	Day	33% 9% 15% 29% 11% 2	2431
	11th to 20th	Day	35% 8% 14% 26% 15% 1	2164
	21st to end	Day	30% 8% 14% 26% 18% 4	2376
September	1st to 10th	Day	37% 7% 14% 22% 17% 4	2160
	11th to 20th	Day	31% 7% 13% 30% 17% 3	2034
	21st to end	Day	38% 8% 12% 24% 18% 1	1980
October	1st to 10th	Day	42% 6% 11% 25% 15% 2	1952
	11th to 20th	Day	51% 4 11% 21% 12% 1	1771
	21st to end	Day	52% 6% 10% 21% 10% 2	1782
November	1st to 10th	Day	41% 5% 14% 22% 16% 2	1672
	11th to 20th	Day	51% 5% 14% 20% 9% 1	1710
	21st to end	Day	53% 4 13% 21% 9%	1658
December	1st to 10th	Day	56% 5% 18% 15% 7% 1	1520
	11th to 20th	Day	54% 6% 14% 20% 6% 1	1915
	21st to end	Day	72% 7% 7% 10% 3	2783
January	1st to 10th	Night	59% 5% 12% 17% 6% 1	4428
	11th to 20th	Night	67% 5% 12% 12% 5%	4440
	21st to end	Night	67% 4 9% 14% 5%	4846
February	1st to 10th	Night	55% 5% 10% 17% 12% 1	2414
	11th to 20th	Night	49% 5% 11% 21% 13% 1	2219
	21st to end	Night	52% 4 10% 19% 10% 4	1884
March	1st to 10th	Night	53% 5% 12% 21% 9% 1	2280
	11th to 20th	Night	41% 6% 13% 22% 16% 3	2220
	21st to end	Night	53% 6% 12% 19% 8% 1	1985
April	1st to 10th	Night	46% 6% 13% 22% 13% 1	1800
	11th to 20th	Night	47% 5% 8% 20% 16% 4	1775
	21st to end	Night	47% 6% 11% 21% 13% 2	1684
May	1st to 10th	Night	48% 7% 11% 22% 12%	1620
	11th to 20th	Night	42% 7% 12% 23% 14% 2	1488
	21st to end	Night	40% 6% 14% 21% 17% 2	1552
June	1st to 10th	Night	33% 7% 13% 25% 19% 3	1440
	11th to 20th	Night	35% 6% 11% 24% 22% 2	1440
	21st to end	Night	36% 9% 16% 21% 16% 2	1440
July	1st to 10th	Night	27% 9% 16% 27% 16% 5%	1400
	11th to 20th	Night	29% 7% 13% 28% 20% 3	1392
	21st to end	Night	26% 9% 17% 25% 19% 5%	1487
August	1st to 10th	Night	27% 9% 17% 29% 16% 2	1480
	11th to 20th	Night	27% 8% 17% 28% 18% 2	1733
	21st to end	Night	29% 6% 11% 29% 20% 5%	1980
September	1st to 10th	Night	34% 6% 10% 26% 21% 3	1800
	11th to 20th	Night	31% 4 9% 30% 24% 2	1908
	21st to end	Night	36% 7% 11% 29% 16% 2	1980
October	1st to 10th	Night	37% 5% 10% 26% 19% 2	2006
	11th to 20th	Night	43% 4 11% 23% 18% 2	2187
	21st to end	Night	49% 5% 12% 22% 11% 1	2574
November	1st to 10th	Night	39% 5% 11% 25% 17% 3	2406
	11th to 20th	Night	48% 5% 13% 22% 11% 1	2470
	21st to end	Night	49% 5% 13% 23% 10%	2517
December	1st to 10th	Night	53% 6% 16% 17% 8% 1	2660
	11th to 20th	Night	53% 5% 14% 19% 8% 1	3281
	21st to end	Night	68% 6% 10% 12% 4	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% 15% 38% 22% 3 1	270
	11th to 20th	Day	21% 19% 36% 21% 3	270
	21st to end	Day	23% 15% 36% 22% 5%	297
February	1st to 10th	Day	21% 18% 40% 17% 3	270
	11th to 20th	Day	23% 20% 35% 20% 2	270
	21st to end	Day	19% 21% 43% 15% 2	225
March	1st to 10th	Day	17% 21% 42% 18% 1	270
	11th to 20th	Day	20% 17% 38% 19% 5%	297
	21st to end	Day	27% 17% 29% 19% 6% 1	462
April	1st to 10th	Day	20% 16% 34% 25% 5%	420
	11th to 20th	Day	17% 18% 32% 21% 9% 2	412
	21st to end	Day	21% 16% 31% 21% 10% 1	420
May	1st to 10th	Day	22% 10% 35% 24% 8% 2	423
	11th to 20th	Day	21% 6% 32% 31% 9% 2	551
	21st to end	Day	15% 6% 27% 30% 17% 4	679
June	1st to 10th	Day	12% 4 29% 36% 16% 3	630
	11th to 20th	Day	14% 5% 28% 34% 17% 2	630
	21st to end	Day	13% 7% 29% 36% 11% 4	630
July	1st to 10th	Day	9% 3 20% 35% 24% 9%	621
	11th to 20th	Day	12% 4 24% 37% 19% 5%	618
	21st to end	Day	7% 3 26% 41% 18% 4	684
August	1st to 10th	Day	9% 3 17% 41% 22% 8%	565
	11th to 20th	Day	11% 5% 18% 34% 23% 8%	439
	21st to end	Day	7% 4 20% 39% 23% 7%	495
September	1st to 10th	Day	16% 6% 20% 34% 21% 4	450
	11th to 20th	Day	11% 4 18% 35% 24% 8%	450
	21st to end	Day	14% 10% 26% 27% 18% 5%	450
October	1st to 10th	Day	21% 8% 24% 29% 14% 3	422
	11th to 20th	Day	21% 22% 30% 21% 6%	270
	21st to end	Day	30% 16% 33% 19% 2	297
November	1st to 10th	Day	15% 17% 33% 26% 9% 1	270
	11th to 20th	Day	22% 21% 33% 17% 6% 1	270
	21st to end	Day	25% 13% 40% 20% 2	270
December	1st to 10th	Day	18% 21% 37% 18% 5%	270
	11th to 20th	Day	18% 22% 39% 16% 5%	270
	21st to end	Day	25% 22% 35% 16% 2	297
January	1st to 10th	Night	19% 16% 40% 21% 3	950
	11th to 20th	Night	22% 19% 34% 20% 4 1	950
	21st to end	Night	22% 13% 36% 24% 4	1045
February	1st to 10th	Night	23% 12% 38% 22% 5%	894
	11th to 20th	Night	19% 14% 36% 27% 4	743
	21st to end	Night	25% 14% 37% 21% 3	628
March	1st to 10th	Night	21% 13% 36% 22% 8%	760
	11th to 20th	Night	18% 9% 37% 28% 7% 1	752
	21st to end	Night	24% 11% 29% 26% 9% 1	599
April	1st to 10th	Night	20% 13% 31% 29% 7% 1	540
	11th to 20th	Night	23% 11% 30% 26% 10% 1	533
	21st to end	Night	23% 7% 29% 29% 11% 1	540
May	1st to 10th	Night	19% 6% 29% 35% 10% 1	540
	11th to 20th	Night	20% 5% 24% 33% 18% 1	531
	21st to end	Night	13% 5% 20% 37% 23% 2	584
June	1st to 10th	Night	12% 2 19% 45% 21% 1	540
	11th to 20th	Night	14% 4 22% 40% 19% 1	540
	21st to end	Night	11% 3 25% 37% 23% 2	540
July	1st to 10th	Night	9% 2 11% 42% 30% 6%	525
	11th to 20th	Night	9% 2 18% 41% 26% 3	522
	21st to end	Night	7% 2 18% 42% 29% 2	591
August	1st to 10th	Night	6% 1 14% 42% 31% 5%	537
	11th to 20th	Night	7% 3 14% 37% 33% 6%	534
	21st to end	Night	9% 3 17% 37% 31% 3	594
September	1st to 10th	Night	13% 4 17% 34% 26% 5%	540
	11th to 20th	Night	12% 6% 18% 32% 28% 5%	648
	21st to end	Night	14% 8% 24% 27% 23% 4	720
October	1st to 10th	Night	16% 7% 24% 30% 20% 2	720
	11th to 20th	Night	22% 13% 30% 25% 9% 1	720
	21st to end	Night	26% 13% 30% 24% 7%	792
November	1st to 10th	Night	17% 14% 35% 22% 12% 1	740
	11th to 20th	Night	20% 19% 33% 22% 5% 1	760
	21st to end	Night	21% 15% 37% 22% 5%	807
December	1st to 10th	Night	23% 16% 35% 21% 5%	950
	11th to 20th	Night	23% 15% 35% 24% 4	950
	21st to end	Night	30% 17% 33% 18% 2	1045

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