Belleville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Belleville clear sky chart from 2005-11-06 to 2026-06-16.

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	15% 9% 3 8% 5% 5% 5% 4 5% 7% 35%	138134
	Night	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	133844
Transparency	Day	47% 6% 13% 22% 11% 1	141677
	Night	49% 6% 12% 20% 11% 1	138388
Seeing	Day	20% 15% 30% 21% 10% 3	16481
	Night	21% 12% 30% 26% 10% 1	25758

By Month

Percentage Cloud Cover NE (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	15% 13% 4 9% 5% 5% 5% 4 6% 7% 28%	4661
	11th to 20th	Day	12% 19% 7% 16% 7% 6% 5% 3 4 4 17%	4650
	21st to end	Day	11% 16% 6% 12% 6% 6% 6% 4 5% 6% 22%	5777
February	1st to 10th	Day	13% 14% 5% 12% 6% 5% 5% 4 6% 5% 25%	1890
	11th to 20th	Day	13% 13% 4 12% 7% 5% 5% 4 4 5% 27%	1843
	21st to end	Day	18% 14% 3 11% 6% 5% 4 4 4 5% 26%	1677
March	1st to 10th	Day	20% 11% 4 9% 5% 4 4 3 4 5% 33%	4829
	11th to 20th	Day	11% 11% 4 9% 5% 5% 5% 4 6% 6% 32%	5090
	21st to end	Day	20% 9% 3 7% 4 4 3 3 4 5% 39%	5722
April	1st to 10th	Day	19% 11% 3 7% 6% 4 4 3 4 6% 32%	2520
	11th to 20th	Day	21% 9% 2 6% 4 5% 3 3 5% 5% 38%	2494
	21st to end	Day	20% 9% 3 7% 6% 4 5% 3 4 6% 34%	2677
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 4 4 5% 6% 32%	8035
	11th to 20th	Day	14% 9% 3 8% 4 4 4 4 5% 7% 37%	8221
	21st to end	Day	19% 6% 2 5% 4 4 4 4 5% 8% 38%	9060
June	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 4 7% 9% 34%	2940
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 40%	2870
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 6% 9% 38%	2800
July	1st to 10th	Day	10% 5% 2 7% 4 4 5% 4 6% 10% 43%	6360
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 6% 9% 47%	6360
	21st to end	Day	7% 4 2 6% 4 4 5% 4 7% 9% 47%	6893
August	1st to 10th	Day	16% 7% 3 5% 4 4 5% 4 5% 8% 40%	4234
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 7% 9% 39%	4174
	21st to end	Day	12% 4 2 5% 4 4 5% 5% 6% 9% 43%	4346
September	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 46%	2400
	11th to 20th	Day	10% 5% 2 5% 3 3 4 3 5% 7% 51%	2400
	21st to end	Day	15% 6% 3 7% 4 5% 4 3 6% 7% 40%	2098
October	1st to 10th	Day	16% 8% 3 7% 4 4 4 4 5% 7% 38%	2000
	11th to 20th	Day	19% 7% 3 8% 6% 4 5% 4 5% 7% 33%	2000
	21st to end	Day	23% 10% 3 6% 5% 4 5% 3 4 6% 29%	2150
November	1st to 10th	Day	16% 9% 3 6% 5% 5% 4 4 5% 7% 36%	1809
	11th to 20th	Day	14% 10% 4 8% 5% 4 4 4 5% 7% 32%	1680
	21st to end	Day	20% 12% 4 9% 5% 4 5% 3 6% 6% 26%	1680
December	1st to 10th	Day	20% 14% 4 9% 7% 6% 5% 4 4 6% 22%	2680
	11th to 20th	Day	17% 15% 6% 10% 5% 5% 4 3 4 4 27%	3066
	21st to end	Day	25% 12% 3 8% 5% 5% 4 3 5% 5% 24%	4048
January	1st to 10th	Night	14% 14% 6% 12% 6% 5% 5% 3 5% 5% 24%	8234
	11th to 20th	Night	13% 16% 6% 15% 8% 6% 6% 4 5% 5% 17%	8235
	21st to end	Night	14% 13% 5% 10% 6% 6% 5% 4 5% 6% 27%	8401
February	1st to 10th	Night	15% 15% 4 11% 6% 5% 4 4 4 5% 27%	2730
	11th to 20th	Night	15% 13% 5% 10% 5% 5% 4 3 6% 5% 28%	2663
	21st to end	Night	16% 11% 4 8% 6% 5% 4 4 6% 7% 30%	2128
March	1st to 10th	Night	19% 10% 3 9% 5% 5% 4 3 5% 6% 31%	5214
	11th to 20th	Night	13% 10% 4 8% 5% 4 5% 4 6% 7% 34%	5089
	21st to end	Night	20% 9% 3 6% 3 3 3 3 5% 6% 38%	5450
April	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 5% 6% 33%	2100
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 3 5% 5% 37%	2075
	21st to end	Night	18% 8% 3 6% 4 4 4 4 5% 6% 36%	1959
May	1st to 10th	Night	24% 8% 3 7% 4 4 5% 3 5% 6% 30%	4850
	11th to 20th	Night	14% 8% 3 7% 4 4 4 4 5% 8% 39%	4639
	21st to end	Night	18% 5% 2 6% 4 4 5% 4 5% 7% 39%	5114
June	1st to 10th	Night	14% 6% 3 7% 5% 4 5% 5% 6% 9% 38%	1680
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 3 7% 8% 44%	1640
	21st to end	Night	12% 8% 2 7% 5% 4 5% 4 6% 7% 41%	1600
July	1st to 10th	Night	10% 5% 3 7% 4 5% 5% 5% 7% 8% 42%	3600
	11th to 20th	Night	11% 5% 2 5% 4 3 5% 4 5% 8% 49%	3600
	21st to end	Night	9% 5% 2 5% 4 4 3 3 6% 9% 51%	3942
August	1st to 10th	Night	10% 6% 2 6% 3 4 5% 3 5% 8% 47%	2915
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 8% 47%	2875
	21st to end	Night	9% 4 2 4 4 4 4 5% 7% 10% 48%	3518
September	1st to 10th	Night	11% 5% 3 6% 4 3 4 3 6% 8% 47%	2000
	11th to 20th	Night	11% 3 2 4 3 3 4 3 5% 7% 53%	2000
	21st to end	Night	18% 6% 2 6% 4 3 5% 4 4 6% 42%	2298
October	1st to 10th	Night	15% 7% 3 7% 4 5% 4 4 6% 6% 41%	2400
	11th to 20th	Night	20% 8% 3 8% 5% 4 5% 3 5% 6% 34%	2400
	21st to end	Night	22% 8% 3 7% 5% 4 4 4 4 7% 32%	2690
November	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 5% 7% 39%	2707
	11th to 20th	Night	17% 10% 3 6% 5% 5% 5% 4 6% 7% 33%	2940
	21st to end	Night	21% 11% 3 7% 5% 5% 5% 4 6% 6% 28%	2940
December	1st to 10th	Night	19% 14% 4 7% 5% 5% 5% 4 6% 7% 23%	4720
	11th to 20th	Night	19% 15% 4 9% 4 4 3 3 5% 6% 28%	5348
	21st to end	Night	27% 15% 4 9% 4 4 4 3 5% 5% 22%	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	55% 5% 14% 17% 8% 1	4662
	11th to 20th	Day	71% 4 10% 8% 6% 1	4680
	21st to end	Day	62% 6% 10% 12% 10% 1	5777
February	1st to 10th	Day	59% 7% 12% 15% 6% 1	1890
	11th to 20th	Day	58% 7% 9% 15% 10% 1	1852
	21st to end	Day	60% 5% 10% 15% 9% 1	1677
March	1st to 10th	Day	53% 5% 11% 15% 15% 1	4832
	11th to 20th	Day	49% 7% 19% 14% 10% 2	5090
	21st to end	Day	48% 4 7% 21% 17% 2	5722
April	1st to 10th	Day	54% 4 8% 19% 12% 3	2520
	11th to 20th	Day	49% 4 9% 23% 15% 2	2494
	21st to end	Day	51% 5% 10% 19% 13% 3	2677
May	1st to 10th	Day	50% 6% 11% 20% 10% 2	8045
	11th to 20th	Day	45% 6% 13% 21% 14% 1	8225
	21st to end	Day	43% 6% 12% 25% 12% 1	9061
June	1st to 10th	Day	45% 8% 12% 20% 14% 1	2940
	11th to 20th	Day	38% 6% 14% 26% 13% 2	2870
	21st to end	Day	41% 7% 14% 23% 13% 1	2800
July	1st to 10th	Day	36% 6% 15% 29% 14% 1	6290
	11th to 20th	Day	32% 6% 16% 30% 15% 1	6285
	21st to end	Day	32% 8% 14% 35% 11%	6695
August	1st to 10th	Day	42% 7% 15% 24% 12% 1	4221
	11th to 20th	Day	38% 7% 16% 28% 9% 1	4213
	21st to end	Day	34% 7% 14% 34% 10% 1	4346
September	1st to 10th	Day	35% 7% 16% 30% 12% 1	2400
	11th to 20th	Day	31% 5% 14% 32% 16% 1	2400
	21st to end	Day	43% 6% 14% 27% 10%	2098
October	1st to 10th	Day	38% 5% 14% 31% 12%	3260
	11th to 20th	Day	42% 6% 17% 25% 10% 1	3240
	21st to end	Day	49% 6% 13% 22% 10%	3452
November	1st to 10th	Day	46% 7% 15% 22% 10% 1	1809
	11th to 20th	Day	50% 7% 14% 18% 10% 1	1680
	21st to end	Day	59% 6% 13% 15% 7%	1680
December	1st to 10th	Day	63% 6% 16% 10% 4	2680
	11th to 20th	Day	60% 7% 11% 14% 8%	3066
	21st to end	Day	60% 6% 9% 17% 7%	4048
January	1st to 10th	Night	61% 6% 11% 14% 7%	8234
	11th to 20th	Night	68% 6% 10% 9% 6%	8280
	21st to end	Night	58% 6% 10% 14% 12% 1	8401
February	1st to 10th	Night	59% 5% 12% 17% 6% 1	2730
	11th to 20th	Night	56% 5% 10% 19% 9% 1	2665
	21st to end	Night	53% 6% 12% 20% 9% 1	2128
March	1st to 10th	Night	53% 5% 8% 17% 12% 4	5224
	11th to 20th	Night	47% 7% 17% 19% 9% 2	5090
	21st to end	Night	46% 5% 8% 23% 17% 2	5450
April	1st to 10th	Night	50% 5% 9% 21% 13% 2	2100
	11th to 20th	Night	48% 5% 9% 20% 17% 1	2075
	21st to end	Night	47% 6% 10% 19% 15% 3	1959
May	1st to 10th	Night	54% 6% 8% 17% 12% 3	4856
	11th to 20th	Night	43% 6% 15% 22% 14% 1	4640
	21st to end	Night	43% 6% 12% 21% 16% 2	5115
June	1st to 10th	Night	42% 7% 11% 23% 16% 2	1680
	11th to 20th	Night	36% 7% 13% 22% 20% 3	1640
	21st to end	Night	41% 7% 13% 23% 14% 2	1600
July	1st to 10th	Night	36% 8% 15% 22% 19% 1	3560
	11th to 20th	Night	34% 6% 12% 32% 15% 1	3552
	21st to end	Night	30% 7% 14% 33% 15% 1	3825
August	1st to 10th	Night	36% 6% 14% 29% 14% 1	2904
	11th to 20th	Night	35% 6% 17% 28% 13% 1	2902
	21st to end	Night	30% 7% 14% 36% 11% 1	3518
September	1st to 10th	Night	34% 7% 16% 29% 13% 1	2000
	11th to 20th	Night	30% 6% 14% 33% 17% 1	2000
	21st to end	Night	42% 6% 13% 25% 14% 1	2298
October	1st to 10th	Night	36% 5% 11% 27% 20% 1	3920
	11th to 20th	Night	42% 5% 15% 28% 9%	3899
	21st to end	Night	45% 5% 14% 26% 10%	4338
November	1st to 10th	Night	45% 5% 14% 23% 12% 1	2707
	11th to 20th	Night	49% 6% 14% 20% 9% 2	2940
	21st to end	Night	55% 8% 16% 16% 5% 1	2940
December	1st to 10th	Night	59% 7% 14% 16% 4	4720
	11th to 20th	Night	58% 6% 12% 14% 9% 1	5348
	21st to end	Night	64% 5% 10% 11% 9%	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	23% 21% 32% 15% 7% 1	220
	11th to 20th	Day	25% 24% 28% 17% 4 1	220
	21st to end	Day	20% 23% 34% 16% 5% 1	362
February	1st to 10th	Day	26% 23% 32% 14% 4 1	330
	11th to 20th	Day	26% 25% 35% 12% 2	330
	21st to end	Day	33% 19% 35% 12% 2	393
March	1st to 10th	Day	23% 18% 37% 17% 4	540
	11th to 20th	Day	27% 21% 30% 17% 4	540
	21st to end	Day	31% 16% 30% 15% 6% 1	594
April	1st to 10th	Day	25% 22% 33% 14% 5% 1	540
	11th to 20th	Day	28% 19% 27% 16% 7% 3	532
	21st to end	Day	27% 16% 34% 15% 6% 1	540
May	1st to 10th	Day	22% 15% 32% 21% 7% 3	543
	11th to 20th	Day	17% 13% 35% 22% 10% 3	567
	21st to end	Day	16% 13% 32% 23% 11% 4	617
June	1st to 10th	Day	18% 12% 34% 25% 10% 2	570
	11th to 20th	Day	14% 11% 34% 26% 12% 3	550
	21st to end	Day	19% 8% 32% 26% 12% 2	530
July	1st to 10th	Day	9% 9% 26% 32% 20% 4	525
	11th to 20th	Day	15% 8% 25% 30% 16% 6%	524
	21st to end	Day	11% 7% 33% 32% 13% 5%	576
August	1st to 10th	Day	12% 7% 27% 29% 17% 7%	526
	11th to 20th	Day	13% 6% 25% 30% 18% 8%	519
	21st to end	Day	13% 10% 27% 26% 16% 7%	583
September	1st to 10th	Day	12% 11% 28% 26% 16% 7%	530
	11th to 20th	Day	9% 7% 25% 23% 21% 15%	530
	21st to end	Day	18% 11% 24% 27% 15% 5%	530
October	1st to 10th	Day	19% 16% 22% 22% 15% 6%	530
	11th to 20th	Day	21% 18% 32% 19% 8% 2	530
	21st to end	Day	28% 15% 28% 22% 5%	555
November	1st to 10th	Day	25% 18% 28% 20% 8% 1	383
	11th to 20th	Day	25% 18% 37% 14% 5% 1	220
	21st to end	Day	28% 20% 33% 15% 3 1	220
December	1st to 10th	Day	25% 22% 30% 18% 4	220
	11th to 20th	Day	30% 21% 33% 14% 3	220
	21st to end	Day	32% 20% 31% 13% 3 1	242
January	1st to 10th	Night	27% 18% 32% 19% 4	840
	11th to 20th	Night	29% 17% 30% 18% 6%	840
	21st to end	Night	21% 17% 34% 23% 5%	924
February	1st to 10th	Night	22% 18% 34% 23% 3	840
	11th to 20th	Night	22% 18% 35% 20% 4	823
	21st to end	Night	24% 18% 33% 21% 3	692
March	1st to 10th	Night	21% 15% 38% 23% 3	840
	11th to 20th	Night	24% 17% 32% 23% 4 1	840
	21st to end	Night	23% 14% 34% 22% 6% 1	924
April	1st to 10th	Night	24% 13% 35% 23% 4	840
	11th to 20th	Night	23% 15% 32% 21% 8% 1	831
	21st to end	Night	23% 11% 30% 25% 11% 1	699
May	1st to 10th	Night	25% 6% 26% 29% 12% 1	493
	11th to 20th	Night	15% 7% 34% 34% 10%	414
	21st to end	Night	13% 6% 33% 34% 13% 1	455
June	1st to 10th	Night	14% 5% 32% 34% 15% 1	420
	11th to 20th	Night	11% 7% 31% 32% 17% 1	410
	21st to end	Night	16% 5% 27% 39% 12% 1	400
July	1st to 10th	Night	6% 7% 25% 42% 19% 1	390
	11th to 20th	Night	12% 3 24% 38% 22% 1	388
	21st to end	Night	11% 3 26% 44% 15% 1	442
August	1st to 10th	Night	9% 4 21% 38% 26% 3	597
	11th to 20th	Night	9% 6% 19% 37% 25% 4	594
	21st to end	Night	11% 5% 26% 35% 22% 2	838
September	1st to 10th	Night	13% 5% 27% 31% 21% 3	800
	11th to 20th	Night	13% 5% 20% 29% 27% 7%	800
	21st to end	Night	20% 7% 21% 29% 19% 4	800
October	1st to 10th	Night	19% 11% 28% 22% 18% 3	800
	11th to 20th	Night	22% 12% 32% 25% 8% 1	800
	21st to end	Night	25% 13% 31% 25% 5%	880
November	1st to 10th	Night	22% 15% 26% 28% 9% 1	820
	11th to 20th	Night	22% 17% 31% 23% 6% 1	840
	21st to end	Night	29% 17% 28% 20% 6%	840
December	1st to 10th	Night	29% 17% 30% 20% 4	840
	11th to 20th	Night	26% 17% 36% 18% 3	840
	21st to end	Night	33% 16% 29% 19% 3	924

Page generated at 2026-06-13 02:31:39. Version ac07

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