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 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	14% 9% 3 7% 4 4 4 4 5% 7% 37%	119813
	Night	14% 10% 4 7% 4 4 4 3 5% 6% 38%	119195
Transparency	Day	44% 6% 12% 23% 13% 2	123381
	Night	45% 5% 11% 22% 14% 2	123768
Seeing	Day	17% 14% 31% 24% 10% 3	16722
	Night	18% 12% 31% 27% 11% 1	26597

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% 8% 12% 6% 6% 5% 4 5% 4 27%	3617
	11th to 20th	Day	10% 12% 7% 12% 4 4 4 7% 6% 5% 30%	3620
	21st to end	Day	15% 18% 6% 7% 4 3 3 3 4 6% 32%	4071
February	1st to 10th	Day	13% 16% 5% 8% 6% 6% 5% 4 4 6% 27%	1839
	11th to 20th	Day	13% 12% 4 9% 5% 5% 5% 3 5% 5% 35%	1948
	21st to end	Day	14% 12% 4 11% 5% 4 4 4 5% 6% 31%	1650
March	1st to 10th	Day	16% 11% 4 6% 4 3 3 3 3 5% 42%	4480
	11th to 20th	Day	10% 7% 4 6% 4 5% 5% 4 6% 8% 41%	4684
	21st to end	Day	20% 14% 6% 12% 5% 5% 4 3 5% 5% 21%	5967
April	1st to 10th	Day	18% 14% 3 7% 6% 4 4 3 4 6% 31%	2400
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 5% 7% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 5% 4 4 5% 6% 33%	2518
May	1st to 10th	Day	18% 9% 2 6% 4 4 5% 4 5% 8% 35%	5862
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	6181
	21st to end	Day	16% 7% 2 6% 4 4 4 4 6% 8% 39%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 5% 8% 45%	2800
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 7% 9% 44%	2800
	21st to end	Day	11% 6% 2 6% 6% 4 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 8% 9% 33%	4511
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 49%	4440
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 8% 41%	4813
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	4337
	11th to 20th	Day	11% 5% 2 5% 4 5% 5% 4 6% 8% 44%	3767
	21st to end	Day	8% 5% 2 7% 4 4 4 4 6% 9% 47%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 4 7% 48%	2280
	11th to 20th	Day	12% 4 1 3 3 4 5% 3 5% 7% 52%	2157
	21st to end	Day	13% 5% 2 6% 4 4 5% 3 6% 8% 42%	2090
October	1st to 10th	Day	13% 6% 3 6% 5% 3 3 4 5% 6% 46%	2043
	11th to 20th	Day	21% 7% 3 7% 5% 4 4 3 5% 6% 36%	1881
	21st to end	Day	21% 9% 3 6% 4 4 4 4 4 5% 36%	1881
November	1st to 10th	Day	13% 9% 2 7% 4 4 4 3 5% 7% 43%	1762
	11th to 20th	Day	15% 14% 3 8% 5% 4 4 4 5% 7% 33%	1800
	21st to end	Day	18% 12% 3 9% 4 4 4 3 3 5% 35%	1725
December	1st to 10th	Day	9% 11% 3 8% 4 4 4 4 5% 7% 41%	2610
	11th to 20th	Day	11% 15% 6% 12% 5% 4 5% 5% 5% 6% 25%	3008
	21st to end	Day	23% 24% 7% 11% 5% 4 4 2 3 4 13%	3982
January	1st to 10th	Night	13% 13% 7% 12% 5% 4 4 3 4 5% 32%	6348
	11th to 20th	Night	13% 15% 7% 12% 6% 5% 4 5% 5% 4 23%	6360
	21st to end	Night	17% 15% 7% 8% 4 3 3 2 4 5% 32%	6904
February	1st to 10th	Night	14% 14% 5% 8% 5% 5% 4 3 4 5% 32%	2560
	11th to 20th	Night	13% 11% 4 8% 5% 4 4 3 5% 6% 36%	2338
	21st to end	Night	17% 12% 3 9% 4 3 3 3 4 5% 37%	1980
March	1st to 10th	Night	17% 10% 4 7% 3 3 3 3 3 5% 42%	5373
	11th to 20th	Night	10% 7% 2 6% 4 4 4 3 5% 7% 48%	5318
	21st to end	Night	19% 13% 5% 10% 5% 4 4 3 4 5% 28%	4962
April	1st to 10th	Night	16% 9% 3 6% 5% 5% 5% 4 4 5% 38%	2000
	11th to 20th	Night	19% 10% 2 6% 4 4 4 4 4 5% 38%	1975
	21st to end	Night	18% 8% 3 6% 4 3 5% 4 4 6% 39%	1880
May	1st to 10th	Night	18% 7% 3 7% 4 4 4 3 5% 7% 37%	4033
	11th to 20th	Night	13% 6% 2 6% 3 4 5% 4 6% 9% 42%	3711
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 5% 41%	3942
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 5% 5% 8% 47%	1600
	11th to 20th	Night	10% 5% 2 7% 5% 5% 4 4 5% 8% 46%	1600
	21st to end	Night	10% 6% 2 6% 4 4 5% 5% 7% 9% 43%	1600
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 5% 5% 6% 8% 40%	2570
	11th to 20th	Night	6% 4 1 5% 4 4 5% 4 6% 8% 54%	2530
	21st to end	Night	6% 4 2 6% 5% 5% 6% 5% 8% 8% 46%	2750
August	1st to 10th	Night	7% 4 2 5% 4 5% 5% 5% 6% 8% 49%	2599
	11th to 20th	Night	10% 5% 1 5% 4 4 4 4 6% 8% 51%	3060
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 8% 53%	3476
September	1st to 10th	Night	11% 5% 2 6% 3 4 3 2 4 6% 53%	1900
	11th to 20th	Night	10% 4 2 4 3 4 3 2 4 7% 56%	2004
	21st to end	Night	12% 6% 2 5% 4 5% 4 4 5% 7% 47%	2090
October	1st to 10th	Night	12% 5% 3 5% 3 4 3 3 4 6% 52%	2135
	11th to 20th	Night	17% 8% 2 5% 4 3 3 3 4 6% 45%	2299
	21st to end	Night	21% 9% 2 5% 4 4 3 3 5% 6% 38%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 4 3 3 4 5% 48%	2536
	11th to 20th	Night	14% 11% 3 8% 5% 4 4 3 5% 6% 36%	2600
	21st to end	Night	16% 11% 4 8% 5% 3 4 3 5% 4 37%	2670
December	1st to 10th	Night	11% 10% 3 7% 5% 4 4 3 4 6% 43%	4580
	11th to 20th	Night	17% 16% 4 9% 5% 4 4 3 5% 5% 29%	5199
	21st to end	Night	23% 19% 6% 10% 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	59% 4 12% 18% 8%	3618
	11th to 20th	Day	54% 4 11% 11% 15% 4	3620
	21st to end	Day	55% 3 11% 21% 9%	4071
February	1st to 10th	Day	57% 5% 12% 14% 10% 1	1839
	11th to 20th	Day	51% 5% 10% 22% 10% 2	1958
	21st to end	Day	53% 7% 10% 18% 10% 2	1650
March	1st to 10th	Day	46% 5% 9% 25% 11% 4	4480
	11th to 20th	Day	39% 5% 14% 30% 11% 1	4684
	21st to end	Day	64% 6% 12% 11% 6% 1	5969
April	1st to 10th	Day	54% 6% 10% 19% 11%	2400
	11th to 20th	Day	48% 6% 11% 20% 12% 3	2374
	21st to end	Day	50% 7% 13% 18% 11% 1	2518
May	1st to 10th	Day	45% 7% 11% 22% 12% 2	5897
	11th to 20th	Day	40% 8% 12% 25% 15% 1	6185
	21st to end	Day	40% 6% 13% 24% 13% 3	6949
June	1st to 10th	Day	36% 7% 13% 25% 18% 2	2800
	11th to 20th	Day	35% 8% 13% 23% 19% 3	2800
	21st to end	Day	39% 9% 13% 22% 14% 2	2800
July	1st to 10th	Day	44% 7% 14% 22% 11% 2	4442
	11th to 20th	Day	29% 8% 16% 30% 16% 1	4365
	21st to end	Day	38% 7% 16% 16% 20% 2	4662
August	1st to 10th	Day	36% 8% 15% 27% 12% 1	4322
	11th to 20th	Day	34% 8% 12% 31% 15% 1	3804
	21st to end	Day	33% 6% 13% 25% 21% 2	4179
September	1st to 10th	Day	35% 7% 13% 23% 18% 3	2280
	11th to 20th	Day	29% 6% 12% 31% 19% 3	2157
	21st to end	Day	39% 5% 11% 26% 17% 1	2090
October	1st to 10th	Day	25% 4 7% 37% 25% 1	3394
	11th to 20th	Day	35% 5% 12% 27% 21%	3109
	21st to end	Day	44% 4 9% 28% 12% 3	3081
November	1st to 10th	Day	41% 5% 14% 23% 16% 2	1762
	11th to 20th	Day	52% 4 12% 21% 10% 1	1800
	21st to end	Day	53% 5% 11% 23% 8%	1725
December	1st to 10th	Day	44% 4 18% 27% 7%	2610
	11th to 20th	Day	58% 5% 12% 17% 6% 1	3005
	21st to end	Day	75% 5% 6% 10% 5%	3982
January	1st to 10th	Night	57% 3 9% 20% 10%	6348
	11th to 20th	Night	63% 4 9% 10% 13% 1	6360
	21st to end	Night	56% 3 13% 16% 11% 1	6904
February	1st to 10th	Night	54% 5% 11% 17% 12% 2	2560
	11th to 20th	Night	49% 5% 11% 20% 14% 1	2339
	21st to end	Night	51% 4 11% 20% 10% 4	1980
March	1st to 10th	Night	46% 4 11% 24% 11% 3	5373
	11th to 20th	Night	36% 5% 12% 30% 14% 3	5318
	21st to end	Night	58% 5% 9% 17% 10%	4962
April	1st to 10th	Night	47% 6% 11% 22% 13% 1	2000
	11th to 20th	Night	46% 7% 8% 19% 16% 4	1975
	21st to end	Night	44% 7% 11% 23% 13% 2	1880
May	1st to 10th	Night	47% 5% 13% 17% 15% 3	4060
	11th to 20th	Night	37% 8% 16% 23% 14% 3	3712
	21st to end	Night	43% 5% 11% 21% 16% 4	3950
June	1st to 10th	Night	32% 8% 12% 26% 19% 3	1600
	11th to 20th	Night	35% 5% 11% 25% 21% 2	1600
	21st to end	Night	37% 7% 16% 21% 18% 2	1600
July	1st to 10th	Night	40% 8% 16% 20% 12% 4	2530
	11th to 20th	Night	26% 7% 16% 31% 18% 2	2482
	21st to end	Night	32% 9% 15% 18% 23% 4	2662
August	1st to 10th	Night	31% 8% 15% 28% 17% 1	2590
	11th to 20th	Night	30% 6% 15% 29% 19% 1	3089
	21st to end	Night	30% 5% 12% 32% 17% 4	3476
September	1st to 10th	Night	32% 6% 10% 26% 23% 3	1900
	11th to 20th	Night	28% 5% 8% 32% 25% 2	2004
	21st to end	Night	37% 6% 11% 29% 16% 2	2090
October	1st to 10th	Night	23% 4 7% 38% 26% 2	3564
	11th to 20th	Night	36% 4 8% 24% 26% 2	3810
	21st to end	Night	41% 4 9% 25% 17% 3	4467
November	1st to 10th	Night	39% 5% 9% 26% 17% 3	2536
	11th to 20th	Night	48% 5% 11% 23% 11% 2	2600
	21st to end	Night	50% 5% 12% 22% 10%	2670
December	1st to 10th	Night	43% 4 15% 27% 10% 1	4580
	11th to 20th	Night	58% 5% 12% 15% 10% 1	5201
	21st to end	Night	69% 5% 8% 12% 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	21% 17% 41% 18% 2 1	300
	11th to 20th	Day	23% 24% 36% 14% 2	300
	21st to end	Day	26% 20% 35% 15% 4	330
February	1st to 10th	Day	22% 24% 38% 14% 2	300
	11th to 20th	Day	26% 24% 32% 14% 3	300
	21st to end	Day	25% 25% 38% 8% 3	249
March	1st to 10th	Day	21% 28% 38% 12% 1	300
	11th to 20th	Day	26% 21% 35% 14% 4	330
	21st to end	Day	26% 19% 32% 17% 5% 1	550
April	1st to 10th	Day	27% 19% 32% 19% 4	500
	11th to 20th	Day	22% 25% 25% 20% 7% 2	492
	21st to end	Day	23% 20% 30% 19% 8% 1	500
May	1st to 10th	Day	22% 16% 33% 20% 7% 2	503
	11th to 20th	Day	22% 12% 31% 25% 8% 2	665
	21st to end	Day	17% 13% 32% 22% 13% 4	789
June	1st to 10th	Day	11% 11% 35% 28% 12% 3	730
	11th to 20th	Day	14% 8% 35% 30% 11% 3	730
	21st to end	Day	13% 9% 35% 29% 10% 4	730
July	1st to 10th	Day	9% 5% 26% 31% 20% 10%	697
	11th to 20th	Day	10% 6% 31% 31% 15% 7%	668
	21st to end	Day	6% 7% 30% 36% 15% 5%	739
August	1st to 10th	Day	9% 5% 25% 34% 19% 8%	629
	11th to 20th	Day	11% 6% 22% 33% 20% 8%	479
	21st to end	Day	7% 5% 25% 32% 23% 8%	539
September	1st to 10th	Day	13% 7% 27% 29% 19% 4	490
	11th to 20th	Day	9% 5% 21% 33% 22% 10%	490
	21st to end	Day	14% 15% 24% 24% 16% 7%	490
October	1st to 10th	Day	16% 13% 29% 24% 14% 3	443
	11th to 20th	Day	23% 24% 30% 19% 4	300
	21st to end	Day	32% 17% 32% 14% 4	330
November	1st to 10th	Day	19% 17% 36% 20% 7% 1	300
	11th to 20th	Day	23% 26% 32% 13% 5%	300
	21st to end	Day	26% 17% 37% 18% 1 1	300
December	1st to 10th	Day	16% 27% 36% 17% 3 1	300
	11th to 20th	Day	19% 27% 36% 14% 4	300
	21st to end	Day	28% 24% 33% 14% 2	330
January	1st to 10th	Night	18% 19% 40% 20% 3	1000
	11th to 20th	Night	21% 21% 35% 19% 4 1	1000
	21st to end	Night	22% 15% 37% 23% 4	1100
February	1st to 10th	Night	23% 17% 35% 22% 4	960
	11th to 20th	Night	21% 18% 35% 23% 3	783
	21st to end	Night	23% 18% 38% 18% 3	660
March	1st to 10th	Night	21% 15% 36% 22% 7%	800
	11th to 20th	Night	17% 12% 40% 24% 7% 1	786
	21st to end	Night	23% 14% 29% 25% 8% 1	660
April	1st to 10th	Night	22% 14% 34% 24% 6% 1	600
	11th to 20th	Night	22% 14% 32% 23% 7% 1	593
	21st to end	Night	21% 13% 29% 29% 7% 1	600
May	1st to 10th	Night	17% 11% 32% 31% 8% 1	600
	11th to 20th	Night	18% 7% 28% 31% 15% 1	591
	21st to end	Night	15% 6% 26% 34% 17% 1	650
June	1st to 10th	Night	11% 5% 27% 40% 17%	600
	11th to 20th	Night	12% 5% 30% 39% 13% 1	600
	21st to end	Night	11% 6% 30% 33% 18% 1	600
July	1st to 10th	Night	8% 2 21% 38% 28% 4	570
	11th to 20th	Night	8% 4 24% 36% 24% 4	552
	21st to end	Night	6% 1 21% 44% 26% 1	624
August	1st to 10th	Night	5% 2 17% 44% 28% 4	567
	11th to 20th	Night	8% 3 17% 36% 32% 4	564
	21st to end	Night	9% 3 19% 37% 30% 2	627
September	1st to 10th	Night	11% 7% 21% 32% 25% 4	570
	11th to 20th	Night	10% 4 21% 32% 28% 4	674
	21st to end	Night	13% 9% 26% 28% 20% 3	760
October	1st to 10th	Night	14% 8% 27% 30% 18% 3	760
	11th to 20th	Night	21% 13% 32% 24% 9%	760
	21st to end	Night	25% 15% 30% 22% 8%	836
November	1st to 10th	Night	19% 14% 35% 22% 9% 1	780
	11th to 20th	Night	21% 20% 33% 21% 5% 1	800
	21st to end	Night	22% 17% 35% 21% 6%	870
December	1st to 10th	Night	22% 18% 36% 20% 4	1000
	11th to 20th	Night	24% 15% 35% 23% 3	1000
	21st to end	Night	30% 17% 32% 18% 2	1100

Page generated at 2025-07-03 00:30:13. Version ac07

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