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 2025-02-21.

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 7% 5% 5% 5% 4 5% 7% 35%	108776
	Night	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	109798
Transparency	Day	46% 7% 13% 22% 11% 1	112366
	Night	49% 6% 12% 20% 11% 1	114371
Seeing	Day	20% 15% 31% 21% 10% 3	14753
	Night	21% 12% 30% 25% 10% 1	24145

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	13% 13% 4 9% 5% 4 4 4 6% 7% 32%	3597
	11th to 20th	Day	11% 18% 7% 14% 7% 6% 5% 4 4 4 20%	3570
	21st to end	Day	14% 17% 6% 12% 6% 5% 5% 3 5% 5% 21%	4463
February	1st to 10th	Day	13% 15% 5% 12% 7% 5% 5% 4 5% 5% 24%	1800
	11th to 20th	Day	13% 13% 4 12% 7% 5% 5% 4 4 5% 28%	1753
	21st to end	Day	19% 13% 3 10% 6% 5% 4 4 4 5% 27%	1521
March	1st to 10th	Day	21% 11% 3 8% 5% 4 4 3 4 5% 31%	3294
	11th to 20th	Day	11% 14% 5% 10% 5% 5% 5% 3 5% 5% 32%	3480
	21st to end	Day	15% 8% 2 6% 4 4 3 3 4 6% 44%	3906
April	1st to 10th	Day	19% 11% 3 7% 6% 4 4 3 5% 6% 32%	2280
	11th to 20th	Day	21% 8% 2 6% 4 4 3 3 5% 6% 39%	2254
	21st to end	Day	20% 10% 3 7% 6% 4 5% 3 4 6% 33%	2422
May	1st to 10th	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 34%	4321
	11th to 20th	Day	15% 10% 3 8% 4 5% 5% 4 5% 9% 34%	4398
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	17% 6% 3 7% 5% 5% 5% 4 7% 9% 33%	2660
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 40%	2660
	21st to end	Day	16% 7% 2 6% 5% 4 4 4 6% 9% 38%	2660
July	1st to 10th	Day	11% 4 2 6% 4 4 5% 4 6% 10% 42%	4440
	11th to 20th	Day	11% 4 2 4 3 4 4 4 6% 10% 46%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 9% 43%	4795
August	1st to 10th	Day	16% 7% 3 5% 4 4 5% 4 5% 8% 38%	4104
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 7% 9% 39%	4044
	21st to end	Day	12% 4 2 5% 4 4 5% 5% 7% 9% 43%	4213
September	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 45%	2280
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 7% 50%	2280
	21st to end	Day	15% 6% 3 7% 4 5% 4 3 5% 7% 40%	1994
October	1st to 10th	Day	16% 8% 3 7% 5% 4 4 4 5% 7% 36%	1900
	11th to 20th	Day	20% 7% 3 7% 6% 4 4 4 5% 7% 33%	1900
	21st to end	Day	24% 10% 3 6% 5% 4 5% 4 4 6% 28%	2043
November	1st to 10th	Day	15% 9% 3 6% 5% 5% 4 4 5% 7% 36%	1721
	11th to 20th	Day	15% 10% 4 8% 5% 5% 4 4 5% 7% 33%	1600
	21st to end	Day	20% 12% 4 8% 5% 5% 5% 4 6% 6% 26%	1600
December	1st to 10th	Day	20% 13% 4 9% 7% 6% 5% 4 4 6% 21%	2600
	11th to 20th	Day	17% 15% 6% 10% 5% 5% 4 3 4 4 27%	2986
	21st to end	Day	26% 12% 3 8% 5% 4 4 3 4 5% 24%	3960
January	1st to 10th	Night	14% 13% 6% 12% 6% 5% 5% 3 4 5% 26%	6349
	11th to 20th	Night	13% 14% 5% 14% 8% 6% 6% 4 5% 6% 18%	6315
	21st to end	Night	17% 14% 6% 10% 6% 5% 5% 3 5% 5% 23%	6490
February	1st to 10th	Night	15% 15% 4 11% 6% 5% 4 4 4 5% 26%	2600
	11th to 20th	Night	15% 12% 5% 10% 5% 5% 4 3 6% 6% 29%	2533
	21st to end	Night	17% 11% 3 7% 6% 4 4 4 6% 7% 31%	1933
March	1st to 10th	Night	19% 10% 4 9% 5% 4 4 3 5% 6% 32%	3543
	11th to 20th	Night	17% 12% 4 9% 5% 4 4 4 5% 6% 29%	3479
	21st to end	Night	16% 8% 3 5% 3 3 3 3 5% 7% 44%	3724
April	1st to 10th	Night	20% 9% 3 7% 4 4 4 3 5% 6% 34%	1900
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 3 5% 5% 37%	1875
	21st to end	Night	19% 8% 3 6% 4 3 4 4 5% 6% 35%	1773
May	1st to 10th	Night	21% 8% 3 7% 4 4 5% 3 5% 6% 34%	2620
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 4 6% 7% 37%	2495
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 6% 8% 45%	2738
June	1st to 10th	Night	14% 6% 3 7% 6% 4 6% 5% 6% 9% 37%	1520
	11th to 20th	Night	10% 5% 2 6% 5% 5% 4 3 7% 8% 43%	1520
	21st to end	Night	12% 8% 2 7% 5% 4 5% 3 6% 7% 41%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	2520
	11th to 20th	Night	13% 5% 2 4 4 3 5% 4 5% 8% 47%	2520
	21st to end	Night	9% 6% 2 5% 4 4 4 3 6% 8% 49%	2755
August	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 5% 8% 46%	2825
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 8% 47%	2785
	21st to end	Night	9% 4 2 4 4 4 4 5% 6% 10% 48%	3410
September	1st to 10th	Night	12% 5% 3 6% 4 3 4 3 6% 8% 46%	1900
	11th to 20th	Night	12% 3 2 4 3 3 4 3 5% 7% 52%	1900
	21st to end	Night	17% 6% 2 5% 4 3 5% 4 5% 7% 42%	2182
October	1st to 10th	Night	15% 7% 3 7% 4 5% 4 4 5% 7% 39%	2280
	11th to 20th	Night	20% 8% 3 7% 5% 5% 5% 3 5% 6% 33%	2280
	21st to end	Night	22% 9% 3 7% 5% 4 4 4 4 7% 31%	2555
November	1st to 10th	Night	18% 7% 3 6% 5% 3 3 3 5% 7% 39%	2575
	11th to 20th	Night	17% 10% 3 6% 5% 5% 5% 4 6% 7% 33%	2800
	21st to end	Night	21% 11% 2 7% 5% 5% 5% 4 6% 6% 28%	2800
December	1st to 10th	Night	19% 14% 4 7% 5% 5% 5% 4 6% 7% 23%	4580
	11th to 20th	Night	19% 15% 4 9% 4 4 3 3 5% 6% 28%	5208
	21st to end	Night	27% 15% 4 9% 4 4 4 3 5% 5% 22%	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	51% 5% 13% 19% 9% 1	3598
	11th to 20th	Day	67% 5% 12% 9% 6% 1	3600
	21st to end	Day	64% 6% 9% 11% 10% 1	4463
February	1st to 10th	Day	61% 7% 12% 15% 5% 1	1800
	11th to 20th	Day	58% 7% 9% 15% 10% 1	1762
	21st to end	Day	59% 5% 11% 14% 10% 1	1521
March	1st to 10th	Day	54% 6% 11% 19% 9% 1	3297
	11th to 20th	Day	50% 7% 15% 16% 10% 2	3480
	21st to end	Day	41% 5% 9% 25% 16% 3	3906
April	1st to 10th	Day	53% 5% 8% 20% 11% 3	2280
	11th to 20th	Day	47% 4 9% 23% 15% 2	2254
	21st to end	Day	51% 5% 10% 18% 13% 3	2422
May	1st to 10th	Day	48% 7% 10% 22% 11% 2	4334
	11th to 20th	Day	46% 8% 15% 21% 9% 1	4402
	21st to end	Day	35% 8% 15% 26% 15% 2	4837
June	1st to 10th	Day	45% 8% 13% 19% 13% 2	2660
	11th to 20th	Day	38% 6% 14% 26% 14% 2	2660
	21st to end	Day	42% 7% 14% 23% 13% 1	2660
July	1st to 10th	Day	35% 6% 14% 30% 14% 1	4370
	11th to 20th	Day	32% 6% 17% 35% 10%	4365
	21st to end	Day	34% 9% 15% 30% 11%	4642
August	1st to 10th	Day	43% 7% 14% 23% 12% 1	4091
	11th to 20th	Day	39% 8% 17% 28% 9%	4083
	21st to end	Day	35% 7% 14% 34% 10% 1	4213
September	1st to 10th	Day	35% 7% 16% 29% 12% 1	2280
	11th to 20th	Day	32% 6% 14% 32% 15% 1	2280
	21st to end	Day	42% 6% 14% 27% 10%	1994
October	1st to 10th	Day	38% 5% 14% 31% 12%	3160
	11th to 20th	Day	41% 6% 17% 25% 10% 1	3140
	21st to end	Day	49% 6% 13% 23% 9%	3345
November	1st to 10th	Day	46% 7% 15% 22% 10% 1	1721
	11th to 20th	Day	50% 7% 14% 18% 10% 1	1600
	21st to end	Day	59% 6% 13% 15% 7%	1600
December	1st to 10th	Day	63% 6% 17% 10% 4	2600
	11th to 20th	Day	60% 7% 11% 14% 8%	2986
	21st to end	Day	60% 7% 9% 18% 7%	3960
January	1st to 10th	Night	60% 6% 11% 15% 7%	6349
	11th to 20th	Night	66% 7% 12% 9% 5% 1	6360
	21st to end	Night	62% 6% 10% 11% 10% 1	6490
February	1st to 10th	Night	60% 5% 12% 17% 6% 1	2600
	11th to 20th	Night	55% 5% 11% 19% 9% 1	2535
	21st to end	Night	52% 6% 13% 20% 9% 1	1933
March	1st to 10th	Night	52% 6% 9% 22% 10% 1	3553
	11th to 20th	Night	53% 7% 14% 17% 9% 1	3480
	21st to end	Night	39% 5% 9% 26% 19% 3	3724
April	1st to 10th	Night	50% 5% 9% 21% 13% 3	1900
	11th to 20th	Night	48% 6% 9% 19% 17% 1	1875
	21st to end	Night	47% 6% 10% 17% 15% 3	1773
May	1st to 10th	Night	50% 6% 11% 17% 14% 2	2626
	11th to 20th	Night	45% 6% 15% 23% 10% 2	2496
	21st to end	Night	36% 7% 12% 22% 20% 2	2739
June	1st to 10th	Night	43% 7% 11% 23% 14% 2	1520
	11th to 20th	Night	36% 7% 13% 22% 20% 3	1520
	21st to end	Night	42% 7% 13% 23% 14% 2	1520
July	1st to 10th	Night	36% 8% 12% 23% 19% 1	2480
	11th to 20th	Night	34% 6% 13% 33% 14%	2472
	21st to end	Night	31% 8% 17% 32% 11% 1	2667
August	1st to 10th	Night	36% 6% 14% 29% 14% 1	2814
	11th to 20th	Night	35% 6% 18% 28% 12% 1	2812
	21st to end	Night	30% 7% 14% 36% 11% 1	3410
September	1st to 10th	Night	35% 7% 16% 27% 14% 1	1900
	11th to 20th	Night	31% 6% 14% 32% 17% 1	1900
	21st to end	Night	41% 6% 13% 26% 14% 1	2182
October	1st to 10th	Night	36% 6% 11% 27% 20% 1	3800
	11th to 20th	Night	42% 5% 15% 29% 8%	3779
	21st to end	Night	46% 5% 14% 26% 9%	4203
November	1st to 10th	Night	45% 5% 14% 23% 12% 1	2575
	11th to 20th	Night	48% 6% 14% 20% 10% 2	2800
	21st to end	Night	54% 8% 16% 16% 5% 1	2800
December	1st to 10th	Night	59% 7% 14% 16% 4	4580
	11th to 20th	Night	58% 6% 12% 13% 9% 1	5208
	21st to end	Night	64% 5% 10% 11% 9%	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	24% 22% 32% 14% 8% 2	200
	11th to 20th	Day	25% 23% 30% 18% 4 2	200
	21st to end	Day	21% 22% 36% 16% 4 1	329
February	1st to 10th	Day	27% 25% 33% 12% 2	300
	11th to 20th	Day	26% 27% 35% 10% 2	300
	21st to end	Day	34% 19% 35% 10% 2	333
March	1st to 10th	Day	23% 18% 38% 17% 4	460
	11th to 20th	Day	26% 20% 31% 18% 4	460
	21st to end	Day	31% 16% 31% 13% 7% 1	506
April	1st to 10th	Day	26% 21% 33% 13% 6% 1	460
	11th to 20th	Day	28% 20% 27% 15% 6% 3	452
	21st to end	Day	28% 18% 34% 13% 6% 1	460
May	1st to 10th	Day	22% 15% 30% 22% 8% 3	463
	11th to 20th	Day	18% 13% 35% 21% 11% 3	487
	21st to end	Day	14% 14% 33% 23% 12% 4	529
June	1st to 10th	Day	19% 13% 34% 24% 8% 1	490
	11th to 20th	Day	14% 12% 33% 25% 13% 4	490
	21st to end	Day	19% 9% 32% 26% 11% 2	490
July	1st to 10th	Day	9% 9% 27% 30% 21% 4	485
	11th to 20th	Day	15% 8% 26% 29% 16% 5%	484
	21st to end	Day	11% 8% 33% 30% 12% 5%	532
August	1st to 10th	Day	13% 7% 29% 29% 16% 7%	486
	11th to 20th	Day	11% 6% 27% 30% 18% 7%	479
	21st to end	Day	14% 11% 27% 26% 15% 7%	539
September	1st to 10th	Day	13% 10% 29% 27% 14% 7%	490
	11th to 20th	Day	10% 7% 27% 23% 20% 14%	490
	21st to end	Day	17% 12% 26% 25% 15% 5%	490
October	1st to 10th	Day	20% 17% 23% 21% 14% 6%	490
	11th to 20th	Day	22% 18% 31% 18% 9% 2	490
	21st to end	Day	29% 15% 29% 21% 5%	514
November	1st to 10th	Day	25% 18% 28% 21% 8% 1	355
	11th to 20th	Day	26% 18% 36% 15% 5% 2	200
	21st to end	Day	26% 21% 34% 15% 4 1	200
December	1st to 10th	Day	25% 22% 31% 18% 5% 1	200
	11th to 20th	Day	31% 22% 33% 13% 3	200
	21st to end	Day	33% 19% 30% 13% 4 1	220
January	1st to 10th	Night	27% 19% 32% 18% 4	800
	11th to 20th	Night	29% 16% 30% 19% 6%	800
	21st to end	Night	22% 16% 35% 23% 4	880
February	1st to 10th	Night	23% 19% 35% 21% 2	800
	11th to 20th	Night	22% 19% 36% 19% 4	783
	21st to end	Night	25% 18% 33% 20% 3	629
March	1st to 10th	Night	21% 15% 38% 23% 3	760
	11th to 20th	Night	25% 17% 31% 22% 4 1	760
	21st to end	Night	22% 14% 34% 22% 7% 1	836
April	1st to 10th	Night	24% 14% 35% 23% 4	760
	11th to 20th	Night	23% 15% 33% 20% 8% 1	751
	21st to end	Night	24% 11% 31% 23% 11% 1	633
May	1st to 10th	Night	23% 7% 27% 28% 13% 1	447
	11th to 20th	Night	15% 7% 34% 33% 11%	374
	21st to end	Night	11% 6% 33% 34% 14% 1	411
June	1st to 10th	Night	13% 6% 34% 33% 13% 1	380
	11th to 20th	Night	11% 7% 31% 32% 18% 1	380
	21st to end	Night	16% 5% 27% 39% 12% 1	380
July	1st to 10th	Night	6% 7% 25% 41% 20% 1	370
	11th to 20th	Night	12% 3 25% 37% 22% 1	368
	21st to end	Night	11% 2 26% 45% 15% 1	420
August	1st to 10th	Night	9% 4 22% 37% 25% 3	567
	11th to 20th	Night	9% 6% 20% 37% 25% 4	564
	21st to end	Night	11% 5% 26% 35% 22% 2	796
September	1st to 10th	Night	13% 6% 27% 31% 20% 3	760
	11th to 20th	Night	13% 5% 21% 28% 27% 7%	760
	21st to end	Night	20% 7% 22% 29% 19% 4	760
October	1st to 10th	Night	19% 11% 29% 22% 17% 3	760
	11th to 20th	Night	23% 13% 31% 24% 8% 1	760
	21st to end	Night	26% 14% 31% 24% 5%	836
November	1st to 10th	Night	23% 14% 26% 28% 9% 1	780
	11th to 20th	Night	22% 18% 31% 23% 6% 1	800
	21st to end	Night	29% 17% 29% 20% 6%	800
December	1st to 10th	Night	29% 18% 30% 19% 4	800
	11th to 20th	Night	26% 17% 36% 18% 3	800
	21st to end	Night	33% 16% 29% 19% 3	880

Page generated at 2025-02-18 03:26:30. Version ac07

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