Bellvale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellvale Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	16% 7% 3 7% 5% 4 5% 4 6% 7% 38%	127330
	Night	16% 8% 3 7% 4 4 4 3 5% 7% 40%	124005
Transparency	Day	44% 5% 11% 22% 15% 2	130844
	Night	43% 5% 12% 23% 16% 2	128578
Seeing	Day	23% 17% 30% 20% 9% 3	16363
	Night	21% 15% 31% 25% 8% 1	25565

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	12% 8% 3 8% 4 5% 5% 4 5% 6% 40%	3605
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 3 5% 5% 38%	3570
	21st to end	Day	18% 9% 4 7% 4 4 4 3 5% 5% 38%	4300
February	1st to 10th	Day	16% 8% 3 7% 6% 5% 5% 3 5% 6% 36%	1800
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 4 7% 39%	1931
	21st to end	Day	17% 10% 3 6% 4 4 5% 3 4 6% 39%	1650
March	1st to 10th	Day	21% 8% 3 7% 4 4 3 3 4 5% 39%	4476
	11th to 20th	Day	11% 6% 3 8% 5% 5% 5% 4 6% 8% 42%	4796
	21st to end	Day	17% 7% 2 7% 4 5% 4 3 5% 6% 39%	5914
April	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 36%	2400
	11th to 20th	Day	17% 8% 2 7% 5% 4 4 3 5% 7% 39%	2374
	21st to end	Day	18% 6% 3 7% 6% 4 4 3 5% 7% 38%	2425
May	1st to 10th	Day	26% 10% 3 6% 3 4 4 3 6% 7% 27%	5929
	11th to 20th	Day	20% 9% 3 7% 5% 5% 4 3 5% 6% 33%	6318
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 4 6% 9% 36%	2800
	11th to 20th	Day	15% 7% 2 7% 4 4 6% 5% 8% 8% 35%	2800
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 5% 9% 38%	2800
July	1st to 10th	Day	10% 7% 3 8% 5% 6% 5% 5% 7% 9% 34%	6360
	11th to 20th	Day	10% 6% 3 8% 6% 6% 7% 5% 8% 11% 31%	6360
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 5% 8% 40%	6926
August	1st to 10th	Day	20% 7% 2 8% 5% 5% 4 4 6% 7% 32%	4350
	11th to 20th	Day	12% 7% 3 6% 4 4 4 5% 6% 8% 40%	3850
	21st to end	Day	12% 5% 3 5% 3 3 4 4 6% 8% 47%	4312
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 3 6% 5% 43%	2400
	11th to 20th	Day	13% 4 2 5% 3 4 4 4 6% 7% 48%	2325
	21st to end	Day	16% 6% 3 7% 4 5% 4 4 5% 7% 39%	2200
October	1st to 10th	Day	19% 6% 3 6% 4 4 4 3 4 7% 42%	2068
	11th to 20th	Day	13% 7% 2 5% 4 4 5% 3 6% 7% 45%	2000
	21st to end	Day	20% 7% 2 5% 5% 3 4 4 5% 7% 39%	2043
November	1st to 10th	Day	13% 6% 2 5% 4 3 3 2 4 7% 51%	1851
	11th to 20th	Day	15% 7% 3 4 4 4 3 4 5% 7% 43%	1890
	21st to end	Day	16% 8% 2 6% 4 4 5% 4 5% 7% 38%	1764
December	1st to 10th	Day	14% 8% 3 9% 4 4 4 3 6% 6% 39%	2680
	11th to 20th	Day	19% 9% 3 5% 3 3 3 3 4 5% 43%	3066
	21st to end	Day	16% 8% 3 8% 5% 4 5% 4 5% 7% 34%	4048
January	1st to 10th	Night	13% 7% 2 8% 5% 5% 4 3 5% 7% 40%	6364
	11th to 20th	Night	12% 7% 3 8% 5% 4 4 4 5% 7% 41%	6315
	21st to end	Night	16% 10% 3 7% 4 3 4 3 5% 6% 39%	6651
February	1st to 10th	Night	16% 10% 2 6% 4 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 3 6% 6% 38%	2353
	21st to end	Night	17% 8% 3 7% 4 3 4 3 4 5% 40%	1980
March	1st to 10th	Night	18% 10% 3 6% 3 3 3 3 5% 6% 40%	5364
	11th to 20th	Night	12% 7% 3 7% 5% 5% 5% 3 5% 7% 42%	5146
	21st to end	Night	17% 9% 4 7% 5% 4 4 4 4 5% 38%	5018
April	1st to 10th	Night	18% 8% 2 8% 5% 4 5% 4 4 8% 34%	2000
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 7% 38%	1975
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 6% 40%	1975
May	1st to 10th	Night	28% 9% 3 7% 4 4 3 3 6% 7% 27%	4040
	11th to 20th	Night	21% 8% 3 8% 4 4 5% 3 5% 5% 34%	3575
	21st to end	Night	20% 7% 2 7% 4 4 4 3 6% 7% 35%	3926
June	1st to 10th	Night	16% 6% 2 5% 6% 4 4 4 6% 8% 40%	1600
	11th to 20th	Night	13% 6% 2 7% 5% 4 4 4 5% 10% 39%	1600
	21st to end	Night	11% 5% 2 6% 4 4 6% 3 6% 9% 44%	1600
July	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 7% 9% 40%	3600
	11th to 20th	Night	12% 6% 2 9% 5% 5% 6% 4 7% 9% 34%	3600
	21st to end	Night	12% 7% 2 5% 3 4 4 4 6% 8% 46%	3926
August	1st to 10th	Night	20% 7% 2 7% 5% 4 4 4 6% 8% 33%	2750
	11th to 20th	Night	14% 8% 3 7% 4 5% 5% 4 4 6% 41%	3195
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 52%	3586
September	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 5% 6% 47%	2000
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 7% 53%	2075
	21st to end	Night	16% 6% 2 6% 3 3 4 3 4 7% 45%	2200
October	1st to 10th	Night	18% 6% 2 6% 4 3 3 3 4 7% 43%	2330
	11th to 20th	Night	15% 6% 2 6% 3 3 3 4 4 6% 48%	2400
	21st to end	Night	20% 7% 2 4 4 3 3 3 5% 7% 42%	2795
November	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 6% 7% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 4 4 3 2 5% 6% 44%	2730
	21st to end	Night	15% 7% 2 6% 4 4 5% 3 5% 7% 42%	2851
December	1st to 10th	Night	14% 8% 3 8% 4 4 3 3 5% 6% 43%	4720
	11th to 20th	Night	22% 9% 3 5% 3 3 4 3 4 5% 40%	5348
	21st to end	Night	18% 11% 3 8% 4 4 4 3 4 5% 35%	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	40% 5% 11% 29% 14% 2	3606
	11th to 20th	Day	47% 4 9% 21% 13% 5%	3600
	21st to end	Day	48% 4 9% 23% 15% 1	4300
February	1st to 10th	Day	48% 5% 11% 21% 14% 1	1800
	11th to 20th	Day	45% 5% 8% 23% 16% 3	1940
	21st to end	Day	47% 4 12% 22% 14% 3	1650
March	1st to 10th	Day	47% 4 11% 19% 13% 8%	4479
	11th to 20th	Day	39% 6% 13% 28% 10% 4	4796
	21st to end	Day	45% 4 9% 13% 23% 5%	5914
April	1st to 10th	Day	47% 5% 8% 20% 15% 4	2400
	11th to 20th	Day	45% 5% 10% 21% 17% 3	2374
	21st to end	Day	46% 5% 9% 22% 15% 4	2425
May	1st to 10th	Day	56% 5% 13% 15% 11% 1	5940
	11th to 20th	Day	51% 5% 10% 21% 12% 1	6322
	21st to end	Day	47% 6% 12% 22% 13% 1	6949
June	1st to 10th	Day	44% 7% 12% 21% 15% 1	2800
	11th to 20th	Day	44% 7% 12% 25% 11% 2	2800
	21st to end	Day	42% 6% 12% 24% 15% 1	2800
July	1st to 10th	Day	45% 7% 11% 21% 14% 1	6290
	11th to 20th	Day	43% 8% 15% 22% 11%	6285
	21st to end	Day	42% 6% 10% 22% 19% 1	6728
August	1st to 10th	Day	50% 6% 12% 21% 9% 1	4336
	11th to 20th	Day	39% 6% 15% 29% 11% 1	3886
	21st to end	Day	34% 4 11% 32% 17% 2	4312
September	1st to 10th	Day	41% 5% 11% 24% 18% 2	2400
	11th to 20th	Day	34% 5% 9% 25% 25% 2	2325
	21st to end	Day	45% 5% 10% 21% 17% 3	2200
October	1st to 10th	Day	40% 5% 7% 20% 24% 5%	3367
	11th to 20th	Day	30% 4 10% 27% 25% 4	3240
	21st to end	Day	29% 4 16% 26% 22% 4	3281
November	1st to 10th	Day	36% 4 12% 26% 19% 3	1851
	11th to 20th	Day	41% 4 10% 24% 19% 3	1890
	21st to end	Day	42% 6% 12% 27% 11% 2	1764
December	1st to 10th	Day	45% 4 14% 26% 10% 1	2680
	11th to 20th	Day	44% 4 14% 22% 14% 2	3066
	21st to end	Day	48% 6% 9% 21% 14% 2	4048
January	1st to 10th	Night	42% 3 12% 24% 16% 3	6364
	11th to 20th	Night	42% 4 14% 21% 14% 4	6360
	21st to end	Night	45% 4 10% 20% 17% 4	6651
February	1st to 10th	Night	45% 4 14% 24% 12% 2	2600
	11th to 20th	Night	45% 5% 9% 19% 17% 4	2355
	21st to end	Night	46% 4 7% 24% 16% 4	1980
March	1st to 10th	Night	45% 4 11% 20% 14% 6%	5374
	11th to 20th	Night	39% 7% 15% 24% 11% 4	5147
	21st to end	Night	47% 5% 7% 15% 22% 4	5018
April	1st to 10th	Night	49% 5% 9% 20% 15% 3	2000
	11th to 20th	Night	46% 5% 9% 20% 19% 1	1975
	21st to end	Night	43% 6% 8% 22% 18% 4	1975
May	1st to 10th	Night	56% 5% 13% 15% 10% 1	4046
	11th to 20th	Night	51% 6% 12% 16% 14% 1	3576
	21st to end	Night	47% 6% 14% 23% 9% 1	3927
June	1st to 10th	Night	41% 6% 11% 24% 19% 1	1600
	11th to 20th	Night	40% 7% 16% 28% 10% 1	1600
	21st to end	Night	35% 7% 15% 27% 15% 1	1600
July	1st to 10th	Night	41% 7% 15% 20% 15% 2	3560
	11th to 20th	Night	45% 8% 13% 22% 11% 1	3552
	21st to end	Night	35% 5% 13% 25% 20% 1	3809
August	1st to 10th	Night	49% 6% 16% 18% 9% 1	2740
	11th to 20th	Night	44% 6% 13% 28% 9%	3225
	21st to end	Night	32% 4 12% 34% 17% 1	3586
September	1st to 10th	Night	39% 5% 11% 26% 19% 1	2000
	11th to 20th	Night	32% 4 11% 26% 25% 2	2075
	21st to end	Night	41% 4 9% 22% 22% 1	2200
October	1st to 10th	Night	41% 4 10% 23% 20% 2	3811
	11th to 20th	Night	27% 4 10% 28% 27% 4	3899
	21st to end	Night	32% 3 14% 31% 18% 1	4507
November	1st to 10th	Night	36% 5% 12% 23% 21% 2	2667
	11th to 20th	Night	41% 4 9% 23% 19% 4	2730
	21st to end	Night	40% 5% 13% 28% 12% 1	2851
December	1st to 10th	Night	43% 3 13% 31% 10% 1	4720
	11th to 20th	Night	47% 4 11% 20% 15% 3	5348
	21st to end	Night	50% 4 10% 18% 14% 3	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	32% 27% 25% 14% 2	202
	11th to 20th	Day	37% 23% 30% 10% 1 1	200
	21st to end	Day	27% 29% 30% 11% 2 2	293
February	1st to 10th	Day	32% 27% 31% 8% 1	300
	11th to 20th	Day	32% 28% 30% 9% 1	479
	21st to end	Day	30% 24% 34% 10% 3	414
March	1st to 10th	Day	31% 23% 29% 15% 3	500
	11th to 20th	Day	26% 26% 31% 12% 4 1	500
	21st to end	Day	31% 22% 30% 11% 4 2	550
April	1st to 10th	Day	34% 20% 28% 12% 5% 1	500
	11th to 20th	Day	27% 24% 30% 11% 6% 1	492
	21st to end	Day	29% 17% 33% 14% 6% 1	500
May	1st to 10th	Day	35% 18% 24% 15% 6% 1	503
	11th to 20th	Day	23% 16% 35% 17% 8% 1	527
	21st to end	Day	22% 11% 27% 26% 12% 2	573
June	1st to 10th	Day	18% 14% 33% 23% 10% 2	530
	11th to 20th	Day	18% 14% 30% 26% 10% 1	530
	21st to end	Day	15% 14% 31% 28% 11% 2	530
July	1st to 10th	Day	15% 10% 30% 28% 13% 5%	525
	11th to 20th	Day	11% 7% 30% 32% 16% 4	524
	21st to end	Day	13% 9% 33% 32% 11% 2	576
August	1st to 10th	Day	14% 7% 29% 31% 14% 7%	526
	11th to 20th	Day	14% 8% 25% 33% 14% 6%	519
	21st to end	Day	13% 11% 28% 26% 15% 6%	583
September	1st to 10th	Day	18% 13% 25% 23% 15% 7%	530
	11th to 20th	Day	14% 8% 30% 23% 14% 11%	530
	21st to end	Day	21% 12% 29% 23% 10% 5%	530
October	1st to 10th	Day	25% 11% 25% 19% 14% 5%	530
	11th to 20th	Day	20% 19% 29% 20% 10% 2	530
	21st to end	Day	25% 15% 33% 17% 9% 1	496
November	1st to 10th	Day	20% 19% 30% 20% 8% 2	425
	11th to 20th	Day	27% 23% 28% 17% 4 1	430
	21st to end	Day	29% 24% 27% 14% 4 1	304
December	1st to 10th	Day	28% 28% 29% 9% 5%	220
	11th to 20th	Day	38% 22% 28% 10% 2	220
	21st to end	Day	27% 24% 33% 11% 4 1	242
January	1st to 10th	Night	21% 23% 37% 16% 3	805
	11th to 20th	Night	25% 19% 33% 20% 4	800
	21st to end	Night	20% 22% 37% 19% 2	880
February	1st to 10th	Night	24% 19% 37% 19% 1	800
	11th to 20th	Night	26% 23% 36% 13% 1	783
	21st to end	Night	28% 21% 33% 15% 3	660
March	1st to 10th	Night	25% 22% 32% 17% 4	800
	11th to 20th	Night	24% 22% 32% 18% 3 1	800
	21st to end	Night	23% 21% 34% 17% 5%	880
April	1st to 10th	Night	24% 22% 34% 17% 4	800
	11th to 20th	Night	24% 18% 32% 21% 5%	791
	21st to end	Night	23% 16% 33% 22% 5%	775
May	1st to 10th	Night	27% 16% 30% 23% 4	599
	11th to 20th	Night	22% 13% 34% 25% 6% 1	394
	21st to end	Night	16% 9% 27% 37% 10% 1	433
June	1st to 10th	Night	18% 7% 29% 36% 10%	400
	11th to 20th	Night	15% 7% 33% 33% 13% 1	400
	21st to end	Night	12% 6% 31% 40% 11%	400
July	1st to 10th	Night	14% 6% 20% 41% 18% 1	390
	11th to 20th	Night	13% 3 26% 43% 15% 1	388
	21st to end	Night	11% 5% 29% 38% 17% 1	438
August	1st to 10th	Night	11% 2 24% 43% 18% 2	497
	11th to 20th	Night	14% 5% 23% 36% 22% 1	788
	21st to end	Night	15% 5% 20% 38% 20% 2	880
September	1st to 10th	Night	16% 6% 24% 36% 16% 2	800
	11th to 20th	Night	16% 5% 24% 33% 19% 4	800
	21st to end	Night	20% 6% 26% 31% 16% 2	800
October	1st to 10th	Night	22% 10% 27% 25% 15% 2	800
	11th to 20th	Night	18% 14% 32% 28% 8%	800
	21st to end	Night	24% 13% 35% 21% 7%	880
November	1st to 10th	Night	17% 13% 36% 24% 9% 1	820
	11th to 20th	Night	24% 18% 30% 21% 7%	840
	21st to end	Night	22% 20% 33% 20% 6%	840
December	1st to 10th	Night	26% 18% 32% 18% 5%	840
	11th to 20th	Night	26% 18% 34% 19% 3	840
	21st to end	Night	27% 18% 36% 17% 2	924

Page generated at 2025-12-30 01:14:11. Version ac07

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