Northwest Suburban Astronomers
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northwest Suburban Astronomers 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 5% 7% 37%	149571
	Night	13% 10% 4 8% 4 4 4 3 5% 6% 39%	142827
Transparency	Day	44% 6% 12% 23% 13% 2	153094
	Night	45% 5% 11% 23% 14% 2	147381
Seeing	Day	17% 15% 31% 23% 10% 3	18577
	Night	18% 12% 31% 27% 11% 1	28341

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% 8% 11% 6% 5% 4 4 6% 5% 26%	4691
	11th to 20th	Day	8% 11% 7% 13% 5% 4 6% 6% 5% 5% 31%	4710
	21st to end	Day	13% 15% 5% 10% 4 4 3 3 4 6% 32%	5273
February	1st to 10th	Day	13% 16% 6% 8% 6% 6% 5% 4 4 7% 27%	1931
	11th to 20th	Day	13% 12% 4 8% 5% 5% 5% 3 5% 5% 36%	2048
	21st to end	Day	14% 13% 4 11% 5% 4 4 4 5% 6% 31%	1730
March	1st to 10th	Day	16% 11% 4 6% 4 3 3 2 3 5% 42%	4580
	11th to 20th	Day	10% 7% 4 6% 4 5% 5% 4 6% 8% 41%	4786
	21st to end	Day	20% 14% 6% 12% 6% 5% 4 3 4 5% 22%	6099
April	1st to 10th	Day	18% 14% 3 7% 6% 4 4 3 4 6% 30%	2520
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 7% 34%	2494
	21st to end	Day	18% 9% 3 8% 5% 5% 4 4 5% 7% 33%	2644
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 36%	7616
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 8% 39%	8044
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 8% 39%	9047
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 5% 8% 45%	2940
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 44%	2940
	21st to end	Day	11% 6% 2 6% 5% 4 5% 5% 7% 9% 40%	2940
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	8280
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 10% 47%	8280
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	8990
August	1st to 10th	Day	15% 6% 3 6% 4 4 4 4 6% 8% 40%	6366
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 4 6% 8% 37%	5539
	21st to end	Day	7% 5% 2 7% 4 4 4 4 6% 9% 48%	6080
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 47%	2520
	11th to 20th	Day	12% 4 1 4 2 4 4 3 5% 8% 51%	2384
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	2310
October	1st to 10th	Day	13% 6% 3 6% 4 3 3 4 5% 6% 48%	2257
	11th to 20th	Day	20% 7% 3 7% 5% 4 4 3 5% 6% 37%	2079
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 5% 38%	2052
November	1st to 10th	Day	13% 9% 2 6% 4 4 4 3 5% 7% 44%	1852
	11th to 20th	Day	15% 14% 3 8% 5% 3 4 3 4 7% 33%	1890
	21st to end	Day	18% 13% 3 9% 4 4 4 3 3 5% 34%	1811
December	1st to 10th	Day	9% 11% 4 8% 5% 4 4 4 5% 6% 40%	2690
	11th to 20th	Day	11% 15% 6% 12% 5% 5% 5% 5% 5% 6% 25%	3088
	21st to end	Day	23% 24% 7% 11% 5% 4 4 2 3 4 13%	4070
January	1st to 10th	Night	12% 13% 7% 12% 6% 4 4 3 4 4 31%	8232
	11th to 20th	Night	11% 14% 7% 14% 7% 6% 5% 5% 5% 4 23%	8280
	21st to end	Night	14% 14% 6% 8% 4 4 4 3 5% 5% 33%	8938
February	1st to 10th	Night	14% 14% 5% 8% 5% 5% 4 3 4 6% 32%	2688
	11th to 20th	Night	13% 11% 4 8% 5% 4 4 3 5% 6% 37%	2458
	21st to end	Night	17% 12% 3 10% 4 3 3 3 4 5% 37%	2076
March	1st to 10th	Night	17% 10% 4 7% 3 3 3 3 3 5% 41%	5493
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 47%	5436
	21st to end	Night	19% 12% 5% 10% 5% 4 4 3 4 5% 29%	5072
April	1st to 10th	Night	17% 9% 3 6% 5% 5% 5% 3 4 5% 37%	2100
	11th to 20th	Night	19% 9% 2 6% 4 4 4 4 4 5% 38%	2075
	21st to end	Night	18% 7% 2 6% 4 3 4 4 4 6% 40%	1974
May	1st to 10th	Night	15% 7% 3 7% 4 4 4 4 5% 7% 40%	5237
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 10% 44%	4822
	21st to end	Night	16% 7% 3 7% 4 4 4 3 6% 7% 42%	5139
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 5% 5% 8% 47%	1680
	11th to 20th	Night	10% 5% 2 7% 4 4 4 4 5% 8% 46%	1680
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 7% 8% 42%	1680
July	1st to 10th	Night	10% 6% 2 6% 5% 5% 5% 5% 6% 8% 43%	4708
	11th to 20th	Night	6% 5% 2 5% 4 3 4 3 6% 8% 54%	4707
	21st to end	Night	9% 4 2 6% 4 5% 5% 4 7% 7% 47%	5117
August	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 5% 6% 44%	3798
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 3 5% 7% 42%	4491
	21st to end	Night	7% 4 2 5% 4 4 4 3 5% 8% 54%	5046
September	1st to 10th	Night	10% 5% 2 6% 3 4 3 3 5% 7% 52%	2100
	11th to 20th	Night	10% 4 2 5% 3 4 3 2 4 7% 56%	2215
	21st to end	Night	12% 6% 2 5% 4 5% 4 3 5% 7% 46%	2310
October	1st to 10th	Night	12% 5% 3 5% 3 4 3 3 4 5% 53%	2360
	11th to 20th	Night	17% 8% 2 5% 4 3 3 3 4 6% 45%	2541
	21st to end	Night	20% 8% 2 6% 4 4 3 3 5% 6% 38%	2965
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 3 4 5% 47%	2666
	11th to 20th	Night	14% 12% 3 8% 5% 4 4 3 5% 6% 37%	2730
	21st to end	Night	16% 11% 4 8% 5% 3 4 3 4 4 36%	2804
December	1st to 10th	Night	11% 10% 3 7% 5% 4 4 3 5% 6% 42%	4720
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 5% 5% 28%	5339
	21st to end	Night	23% 19% 6% 10% 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	60% 4 15% 15% 6%	4692
	11th to 20th	Day	55% 4 10% 16% 13% 3	4710
	21st to end	Day	55% 3 11% 22% 9%	5273
February	1st to 10th	Day	57% 5% 12% 15% 9% 1	1931
	11th to 20th	Day	51% 5% 10% 22% 10% 2	2058
	21st to end	Day	53% 7% 10% 18% 10% 2	1730
March	1st to 10th	Day	46% 5% 9% 25% 11% 4	4580
	11th to 20th	Day	40% 5% 14% 30% 10% 1	4786
	21st to end	Day	64% 6% 12% 11% 6% 1	6101
April	1st to 10th	Day	55% 6% 10% 19% 11%	2520
	11th to 20th	Day	48% 6% 11% 20% 11% 3	2494
	21st to end	Day	50% 7% 13% 19% 11% 1	2644
May	1st to 10th	Day	44% 7% 11% 24% 12% 1	7651
	11th to 20th	Day	41% 8% 12% 23% 16% 1	8048
	21st to end	Day	40% 6% 12% 22% 15% 4	9061
June	1st to 10th	Day	36% 7% 13% 24% 18% 2	2940
	11th to 20th	Day	35% 8% 13% 22% 19% 3	2940
	21st to end	Day	39% 9% 13% 23% 14% 2	2940
July	1st to 10th	Day	40% 6% 14% 24% 15% 1	8211
	11th to 20th	Day	31% 7% 14% 31% 17% 1	8205
	21st to end	Day	38% 6% 14% 22% 18% 2	8794
August	1st to 10th	Day	40% 7% 13% 27% 13% 1	6351
	11th to 20th	Day	42% 7% 12% 26% 12% 1	5576
	21st to end	Day	32% 6% 13% 31% 16% 1	6079
September	1st to 10th	Day	35% 7% 13% 25% 17% 3	2520
	11th to 20th	Day	30% 6% 12% 31% 19% 2	2384
	21st to end	Day	39% 5% 11% 26% 17% 1	2310
October	1st to 10th	Day	26% 4 7% 38% 24% 1	3608
	11th to 20th	Day	35% 5% 11% 27% 22% 1	3307
	21st to end	Day	43% 4 9% 28% 13% 3	3252
November	1st to 10th	Day	41% 4 13% 23% 16% 2	1852
	11th to 20th	Day	52% 4 12% 22% 10% 1	1890
	21st to end	Day	54% 5% 11% 23% 8%	1811
December	1st to 10th	Day	45% 4 18% 26% 7% 1	2690
	11th to 20th	Day	58% 5% 12% 18% 6% 1	3085
	21st to end	Day	76% 4 6% 9% 5%	4070
January	1st to 10th	Night	57% 4 10% 20% 9%	8233
	11th to 20th	Night	63% 4 10% 11% 11% 1	8280
	21st to end	Night	54% 3 12% 21% 9%	8938
February	1st to 10th	Night	54% 5% 11% 17% 12% 2	2688
	11th to 20th	Night	49% 5% 11% 21% 13% 1	2459
	21st to end	Night	50% 4 11% 21% 10% 4	2076
March	1st to 10th	Night	46% 4 11% 24% 11% 3	5493
	11th to 20th	Night	36% 5% 12% 30% 14% 3	5436
	21st to end	Night	58% 5% 9% 17% 10%	5072
April	1st to 10th	Night	48% 6% 11% 22% 12% 1	2100
	11th to 20th	Night	47% 7% 8% 19% 16% 4	2075
	21st to end	Night	44% 7% 11% 24% 13% 2	1974
May	1st to 10th	Night	43% 5% 11% 22% 16% 3	5264
	11th to 20th	Night	35% 7% 16% 24% 16% 3	4823
	21st to end	Night	41% 5% 10% 20% 19% 5%	5149
June	1st to 10th	Night	32% 8% 12% 25% 20% 3	1680
	11th to 20th	Night	35% 5% 11% 24% 22% 2	1680
	21st to end	Night	38% 7% 16% 20% 17% 2	1680
July	1st to 10th	Night	37% 7% 14% 25% 14% 3	4670
	11th to 20th	Night	27% 6% 14% 29% 22% 2	4662
	21st to end	Night	33% 7% 13% 24% 20% 2	5002
August	1st to 10th	Night	38% 7% 13% 25% 16% 1	3789
	11th to 20th	Night	41% 6% 14% 24% 14% 1	4520
	21st to end	Night	29% 5% 12% 36% 15% 3	5046
September	1st to 10th	Night	32% 6% 11% 28% 21% 2	2100
	11th to 20th	Night	29% 4 8% 32% 25% 2	2215
	21st to end	Night	37% 6% 10% 28% 17% 2	2310
October	1st to 10th	Night	23% 4 7% 38% 27% 2	3789
	11th to 20th	Night	36% 4 8% 23% 27% 2	4052
	21st to end	Night	41% 4 9% 25% 17% 3	4715
November	1st to 10th	Night	39% 5% 9% 26% 17% 3	2666
	11th to 20th	Night	48% 5% 11% 24% 11% 2	2730
	21st to end	Night	51% 5% 11% 22% 10%	2804
December	1st to 10th	Night	44% 4 14% 26% 10% 1	4720
	11th to 20th	Night	58% 5% 12% 15% 10% 1	5341
	21st to end	Night	69% 5% 8% 12% 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% 17% 42% 17% 2 1	330
	11th to 20th	Day	24% 26% 35% 13% 2	330
	21st to end	Day	26% 21% 35% 14% 4	363
February	1st to 10th	Day	21% 24% 38% 15% 2	330
	11th to 20th	Day	26% 25% 33% 14% 2	330
	21st to end	Day	25% 26% 39% 7% 3	273
March	1st to 10th	Day	22% 28% 38% 12% 1	330
	11th to 20th	Day	28% 21% 35% 13% 3	361
	21st to end	Day	26% 20% 32% 16% 4 1	594
April	1st to 10th	Day	28% 19% 31% 18% 4	540
	11th to 20th	Day	22% 25% 25% 19% 7% 2	532
	21st to end	Day	23% 19% 31% 18% 8% 1	540
May	1st to 10th	Day	22% 17% 33% 20% 7% 1	543
	11th to 20th	Day	21% 14% 31% 24% 7% 2	712
	21st to end	Day	16% 13% 32% 22% 13% 4	844
June	1st to 10th	Day	11% 11% 36% 27% 11% 3	780
	11th to 20th	Day	14% 10% 34% 28% 10% 3	780
	21st to end	Day	13% 9% 36% 28% 10% 3	780
July	1st to 10th	Day	9% 5% 28% 30% 18% 9%	771
	11th to 20th	Day	10% 6% 31% 31% 15% 7%	768
	21st to end	Day	7% 8% 30% 36% 14% 5%	849
August	1st to 10th	Day	9% 5% 26% 35% 18% 8%	724
	11th to 20th	Day	11% 6% 22% 33% 19% 8%	559
	21st to end	Day	7% 6% 26% 32% 21% 8%	627
September	1st to 10th	Day	13% 7% 28% 28% 18% 5%	570
	11th to 20th	Day	10% 6% 21% 32% 21% 9%	570
	21st to end	Day	15% 13% 26% 24% 16% 6%	570
October	1st to 10th	Day	15% 12% 31% 25% 14% 3	517
	11th to 20th	Day	23% 21% 31% 20% 5% 1	360
	21st to end	Day	28% 17% 34% 16% 5%	387
November	1st to 10th	Day	21% 18% 35% 19% 7% 1	330
	11th to 20th	Day	26% 24% 32% 13% 5%	330
	21st to end	Day	28% 17% 37% 17% 1 1	330
December	1st to 10th	Day	16% 27% 36% 16% 4 1	330
	11th to 20th	Day	19% 28% 35% 15% 3	330
	21st to end	Day	28% 23% 34% 13% 2	363
January	1st to 10th	Night	18% 19% 40% 20% 3	1050
	11th to 20th	Night	21% 22% 35% 18% 4 1	1050
	21st to end	Night	22% 15% 37% 22% 3	1155
February	1st to 10th	Night	22% 17% 34% 22% 4	1008
	11th to 20th	Night	20% 18% 35% 24% 3	823
	21st to end	Night	23% 18% 38% 18% 3	692
March	1st to 10th	Night	21% 15% 35% 22% 7%	840
	11th to 20th	Night	17% 13% 39% 23% 7%	825
	21st to end	Night	23% 14% 30% 24% 7% 1	693
April	1st to 10th	Night	22% 15% 33% 23% 5%	630
	11th to 20th	Night	23% 15% 31% 23% 7% 1	623
	21st to end	Night	20% 13% 30% 29% 7% 1	630
May	1st to 10th	Night	17% 12% 33% 30% 7% 1	630
	11th to 20th	Night	17% 8% 29% 31% 14%	621
	21st to end	Night	15% 6% 26% 35% 17% 1	683
June	1st to 10th	Night	10% 5% 28% 40% 17%	630
	11th to 20th	Night	13% 6% 30% 38% 13%	630
	21st to end	Night	11% 6% 30% 34% 18% 1	630
July	1st to 10th	Night	8% 2 21% 38% 27% 4	615
	11th to 20th	Night	8% 4 24% 36% 24% 3	612
	21st to end	Night	7% 2 21% 44% 25% 1	690
August	1st to 10th	Night	6% 2 18% 43% 28% 3	627
	11th to 20th	Night	8% 4 18% 35% 31% 4	624
	21st to end	Night	8% 3 21% 36% 29% 2	693
September	1st to 10th	Night	10% 7% 23% 31% 25% 4	630
	11th to 20th	Night	11% 5% 21% 31% 28% 5%	745
	21st to end	Night	13% 9% 25% 29% 21% 3	840
October	1st to 10th	Night	14% 7% 27% 31% 18% 2	840
	11th to 20th	Night	21% 13% 32% 25% 9%	840
	21st to end	Night	24% 14% 30% 24% 9%	913
November	1st to 10th	Night	19% 15% 35% 21% 8% 1	820
	11th to 20th	Night	21% 21% 33% 20% 5% 1	840
	21st to end	Night	23% 17% 34% 21% 5%	914
December	1st to 10th	Night	22% 18% 36% 20% 4	1050
	11th to 20th	Night	24% 16% 34% 22% 3	1050
	21st to end	Night	31% 17% 32% 18% 2	1155

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

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