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 2024-11-03.

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% 5% 4 4 4 5% 7% 37%	101303
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 38%	97752
Transparency	Day	45% 7% 12% 22% 13% 2	101092
	Night	46% 6% 12% 22% 13% 2	97635
Seeing	Day	17% 14% 31% 24% 11% 4	15824
	Night	18% 12% 31% 27% 12% 1	25652

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% 14% 9% 9% 5% 5% 4 3 4 4 29%	2527
	11th to 20th	Day	13% 14% 7% 14% 4 4 5% 8% 7% 5% 19%	2530
	21st to end	Day	19% 24% 8% 8% 4 3 3 2 4 5% 20%	2844
February	1st to 10th	Day	13% 16% 5% 7% 5% 6% 5% 4 4 7% 28%	1747
	11th to 20th	Day	13% 12% 4 8% 5% 5% 5% 3 5% 5% 36%	1848
	21st to end	Day	14% 13% 5% 11% 5% 4 4 4 4 6% 31%	1570
March	1st to 10th	Day	15% 14% 4 7% 4 3 4 3 4 5% 36%	3120
	11th to 20th	Day	10% 7% 4 7% 4 5% 5% 4 6% 8% 39%	3268
	21st to end	Day	20% 13% 5% 12% 5% 5% 5% 3 5% 5% 21%	4165
April	1st to 10th	Day	18% 14% 3 7% 6% 4 4 3 5% 6% 31%	2280
	11th to 20th	Day	17% 9% 3 7% 5% 5% 4 4 5% 7% 34%	2254
	21st to end	Day	19% 9% 3 7% 5% 5% 4 4 5% 6% 33%	2392
May	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 4 5% 8% 35%	4082
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 5% 7% 42%	4303
	21st to end	Day	12% 6% 2 6% 4 4 4 4 7% 8% 42%	4824
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 5% 8% 45%	2660
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 7% 9% 45%	2660
	21st to end	Day	12% 6% 2 6% 6% 4 5% 5% 7% 9% 38%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 6% 5% 8% 9% 33%	4440
	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 3 4 4 5% 36%	1881
November	1st to 10th	Day	12% 8% 2 6% 4 4 4 3 5% 6% 44%	1690
	11th to 20th	Day	15% 13% 3 8% 4 3 4 4 5% 7% 34%	1710
	21st to end	Day	18% 12% 3 8% 4 4 4 3 3 5% 36%	1639
December	1st to 10th	Day	14% 15% 4 9% 5% 4 4 4 4 5% 31%	1520
	11th to 20th	Day	11% 14% 5% 10% 5% 4 4 5% 5% 7% 29%	1918
	21st to end	Day	20% 24% 8% 12% 6% 4 3 2 3 4 15%	2783
January	1st to 10th	Night	14% 15% 8% 10% 5% 3 3 2 3 4 31%	4428
	11th to 20th	Night	16% 17% 7% 13% 5% 3 4 6% 6% 4 19%	4440
	21st to end	Night	22% 19% 8% 9% 4 3 3 2 4 5% 22%	4820
February	1st to 10th	Night	14% 14% 5% 8% 5% 5% 4 3 4 5% 32%	2432
	11th to 20th	Night	13% 11% 4 8% 5% 4 5% 3 5% 6% 36%	2218
	21st to end	Night	18% 12% 3 10% 4 3 3 2 4 5% 36%	1884
March	1st to 10th	Night	18% 10% 3 7% 3 3 3 3 4 5% 40%	3733
	11th to 20th	Night	9% 7% 3 7% 5% 4 4 3 5% 7% 47%	3720
	21st to end	Night	21% 12% 5% 10% 5% 4 4 3 4 5% 28%	3466
April	1st to 10th	Night	16% 9% 3 6% 5% 5% 5% 4 4 5% 38%	1900
	11th to 20th	Night	19% 10% 2 5% 4 4 4 4 4 4 39%	1875
	21st to end	Night	19% 7% 2 6% 4 3 5% 4 4 6% 39%	1786
May	1st to 10th	Night	16% 7% 3 8% 5% 5% 5% 4 6% 7% 36%	2813
	11th to 20th	Night	15% 6% 2 5% 3 4 5% 4 7% 9% 39%	2582
	21st to end	Night	13% 8% 2 6% 4 3 4 3 6% 6% 44%	2743
June	1st to 10th	Night	10% 5% 2 5% 4 5% 4 5% 5% 8% 47%	1520
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 4 5% 8% 47%	1520
	21st to end	Night	10% 6% 2 7% 4 4 5% 5% 7% 9% 42%	1520
July	1st to 10th	Night	12% 6% 2 6% 5% 5% 5% 5% 6% 8% 39%	2530
	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	12% 8% 3 6% 4 4 3 3 4 5% 48%	2433
	11th to 20th	Night	14% 11% 3 8% 5% 4 4 3 5% 6% 37%	2470
	21st to end	Night	17% 10% 4 7% 5% 3 4 3 5% 4 38%	2536
December	1st to 10th	Night	16% 12% 4 8% 5% 4 4 4 4 6% 33%	2660
	11th to 20th	Night	15% 14% 4 9% 5% 3 3 3 5% 6% 32%	3279
	21st to end	Night	24% 18% 6% 10% 4 4 4 3 5% 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	60% 4 12% 17% 7%	2528
	11th to 20th	Day	62% 6% 12% 13% 7%	2530
	21st to end	Day	68% 3 7% 17% 5%	2844
February	1st to 10th	Day	57% 5% 12% 14% 10% 1	1747
	11th to 20th	Day	50% 5% 10% 22% 11% 2	1858
	21st to end	Day	54% 7% 10% 18% 9% 2	1570
March	1st to 10th	Day	49% 7% 10% 22% 10% 3	3120
	11th to 20th	Day	41% 7% 13% 28% 10% 1	3268
	21st to end	Day	63% 8% 13% 12% 4	4165
April	1st to 10th	Day	54% 6% 10% 19% 11%	2280
	11th to 20th	Day	48% 6% 10% 20% 12% 3	2254
	21st to end	Day	50% 7% 12% 17% 12% 1	2392
May	1st to 10th	Day	45% 8% 12% 23% 12% 1	4120
	11th to 20th	Day	40% 6% 12% 27% 14%	4307
	21st to end	Day	36% 7% 14% 21% 18% 4	4837
June	1st to 10th	Day	36% 7% 13% 24% 18% 2	2660
	11th to 20th	Day	35% 8% 12% 23% 20% 3	2660
	21st to end	Day	40% 9% 13% 22% 14% 2	2660
July	1st to 10th	Day	44% 7% 14% 22% 11% 2	4371
	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	38% 6% 11% 27% 16% 1	2043
	11th to 20th	Day	49% 4 12% 21% 14% 1	1881
	21st to end	Day	50% 5% 11% 23% 10% 2	1881
November	1st to 10th	Day	40% 5% 14% 23% 16% 2	1690
	11th to 20th	Day	51% 5% 13% 22% 9% 1	1710
	21st to end	Day	52% 5% 11% 23% 9%	1639
December	1st to 10th	Day	55% 5% 13% 18% 7% 1	1520
	11th to 20th	Day	54% 6% 14% 18% 7% 1	1915
	21st to end	Day	75% 5% 7% 10% 3	2783
January	1st to 10th	Night	58% 4 11% 18% 8% 1	4428
	11th to 20th	Night	66% 4 10% 13% 6%	4440
	21st to end	Night	66% 4 10% 14% 5% 1	4820
February	1st to 10th	Night	54% 5% 10% 17% 12% 2	2432
	11th to 20th	Night	48% 5% 12% 20% 14% 1	2219
	21st to end	Night	52% 4 10% 20% 10% 4	1884
March	1st to 10th	Night	47% 5% 12% 23% 9% 3	3733
	11th to 20th	Night	36% 6% 12% 27% 17% 2	3720
	21st to end	Night	58% 6% 10% 17% 7% 1	3466
April	1st to 10th	Night	46% 6% 11% 22% 13% 1	1900
	11th to 20th	Night	46% 7% 7% 19% 17% 4	1875
	21st to end	Night	45% 7% 11% 22% 13% 2	1786
May	1st to 10th	Night	46% 7% 12% 20% 14% 2	2840
	11th to 20th	Night	38% 8% 16% 26% 10% 1	2583
	21st to end	Night	38% 7% 12% 17% 20% 6%	2751
June	1st to 10th	Night	32% 8% 13% 25% 19% 3	1520
	11th to 20th	Night	34% 6% 11% 25% 22% 2	1520
	21st to end	Night	38% 7% 16% 20% 17% 2	1520
July	1st to 10th	Night	40% 8% 16% 20% 12% 5%	2490
	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	34% 6% 10% 29% 20% 2	2135
	11th to 20th	Night	41% 4 11% 23% 19% 2	2299
	21st to end	Night	47% 6% 11% 23% 12% 1	2717
November	1st to 10th	Night	38% 5% 9% 27% 18% 3	2433
	11th to 20th	Night	48% 5% 11% 23% 11% 1	2470
	21st to end	Night	49% 5% 12% 23% 11%	2536
December	1st to 10th	Night	52% 5% 13% 21% 8% 1	2660
	11th to 20th	Night	53% 5% 13% 18% 9% 1	3281
	21st to end	Night	68% 6% 9% 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% 17% 41% 19% 2 1	270
	11th to 20th	Day	23% 24% 35% 16% 3	270
	21st to end	Day	25% 19% 34% 16% 5%	297
February	1st to 10th	Day	21% 23% 38% 16% 2	270
	11th to 20th	Day	26% 23% 33% 15% 3	270
	21st to end	Day	25% 25% 38% 8% 4	225
March	1st to 10th	Day	20% 29% 37% 13% 1	270
	11th to 20th	Day	27% 18% 36% 15% 4	298
	21st to end	Day	27% 18% 31% 18% 5% 2	506
April	1st to 10th	Day	25% 19% 32% 20% 4	460
	11th to 20th	Day	21% 24% 25% 21% 8% 2	452
	21st to end	Day	23% 20% 29% 19% 8% 1	460
May	1st to 10th	Day	22% 16% 33% 20% 7% 2	463
	11th to 20th	Day	22% 11% 32% 25% 7% 2	618
	21st to end	Day	16% 12% 32% 23% 14% 4	734
June	1st to 10th	Day	11% 11% 34% 28% 12% 4	680
	11th to 20th	Day	14% 8% 34% 30% 11% 3	680
	21st to end	Day	13% 10% 34% 29% 10% 4	680
July	1st to 10th	Day	9% 5% 26% 30% 19% 10%	671
	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	17% 16% 37% 21% 8% 1	276
	11th to 20th	Day	23% 26% 32% 14% 6%	270
	21st to end	Day	27% 16% 37% 19% 1 1	270
December	1st to 10th	Day	17% 26% 37% 16% 3 1	270
	11th to 20th	Day	17% 26% 37% 15% 4	270
	21st to end	Day	25% 26% 35% 12% 2	297
January	1st to 10th	Night	18% 18% 40% 20% 4	950
	11th to 20th	Night	21% 21% 34% 19% 4 1	950
	21st to end	Night	21% 14% 36% 24% 4	1045
February	1st to 10th	Night	22% 16% 35% 22% 4	912
	11th to 20th	Night	20% 17% 35% 24% 3	743
	21st to end	Night	23% 19% 36% 19% 3	628
March	1st to 10th	Night	21% 15% 35% 22% 7%	760
	11th to 20th	Night	16% 12% 41% 24% 7% 1	747
	21st to end	Night	23% 14% 29% 25% 8% 1	627
April	1st to 10th	Night	21% 14% 34% 24% 6% 1	570
	11th to 20th	Night	22% 15% 31% 23% 7% 1	563
	21st to end	Night	22% 12% 29% 28% 7% 1	570
May	1st to 10th	Night	17% 11% 32% 31% 8% 1	570
	11th to 20th	Night	19% 7% 28% 31% 15%	561
	21st to end	Night	14% 6% 26% 34% 18% 1	617
June	1st to 10th	Night	10% 5% 27% 40% 18%	570
	11th to 20th	Night	12% 5% 29% 40% 14% 1	570
	21st to end	Night	12% 6% 29% 33% 19% 1	570
July	1st to 10th	Night	8% 2 20% 38% 28% 4	555
	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	17% 14% 36% 22% 9% 1	749
	11th to 20th	Night	20% 20% 33% 21% 5% 1	760
	21st to end	Night	22% 15% 36% 22% 6%	826
December	1st to 10th	Night	22% 17% 36% 20% 5%	950
	11th to 20th	Night	23% 15% 35% 23% 3	950
	21st to end	Night	30% 17% 32% 19% 2	1045

Page generated at 2024-10-31 02:39:28. Version ac07

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