Bel Tor Observatory
Clear Sky Chart
History

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

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	17% 7% 3 6% 4 4 4 4 5% 7% 38%	127686
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 41%	124334
Transparency	Day	43% 6% 13% 22% 14% 2	131201
	Night	42% 5% 12% 22% 15% 3	128906
Seeing	Day	22% 14% 27% 23% 11% 4	16405
	Night	20% 10% 29% 30% 10% 1	25741

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% 8% 4 8% 4 4 4 3 4 5% 43%	3677
	11th to 20th	Day	13% 11% 4 12% 5% 5% 5% 3 5% 5% 33%	3650
	21st to end	Day	16% 9% 3 8% 5% 4 4 3 4 5% 38%	4565
February	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 6% 5% 36%	1800
	11th to 20th	Day	14% 9% 3 8% 5% 5% 4 4 5% 6% 38%	1883
	21st to end	Day	19% 10% 4 6% 6% 4 4 3 4 5% 37%	1650
March	1st to 10th	Day	21% 8% 3 5% 4 4 3 3 4 5% 40%	4476
	11th to 20th	Day	13% 6% 3 7% 4 4 4 4 5% 7% 43%	4970
	21st to end	Day	17% 7% 2 6% 4 3 4 4 5% 6% 42%	5804
April	1st to 10th	Day	18% 9% 3 7% 5% 5% 4 3 6% 6% 36%	2400
	11th to 20th	Day	17% 9% 2 7% 4 4 4 3 5% 6% 39%	2374
	21st to end	Day	17% 6% 2 6% 5% 5% 4 4 6% 6% 40%	2400
May	1st to 10th	Day	28% 7% 2 6% 4 4 4 3 5% 7% 29%	5968
	11th to 20th	Day	24% 8% 3 7% 4 4 4 3 5% 6% 32%	6318
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	15% 5% 2 7% 6% 5% 5% 4 6% 8% 37%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 6% 9% 35%	2800
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	2800
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 6% 10% 38%	6360
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 8% 10% 33%	6360
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 8% 40%	6926
August	1st to 10th	Day	22% 5% 2 7% 5% 5% 5% 5% 7% 7% 32%	4238
	11th to 20th	Day	14% 6% 2 6% 4 5% 6% 4 6% 8% 40%	3888
	21st to end	Day	14% 5% 2 4 3 3 4 3 5% 9% 47%	4312
September	1st to 10th	Day	16% 5% 2 6% 4 4 4 4 5% 7% 44%	2400
	11th to 20th	Day	14% 4 1 4 3 4 4 3 5% 7% 50%	2367
	21st to end	Day	20% 7% 2 6% 5% 4 4 4 5% 5% 38%	2200
October	1st to 10th	Day	19% 5% 2 5% 4 3 4 3 4 7% 43%	2000
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 5% 7% 45%	2000
	21st to end	Day	21% 6% 2 5% 4 4 4 4 4 7% 40%	2105
November	1st to 10th	Day	13% 7% 2 5% 3 3 3 2 4 6% 52%	1851
	11th to 20th	Day	17% 7% 2 6% 4 3 4 4 5% 5% 43%	1890
	21st to end	Day	18% 7% 3 6% 3 4 4 4 6% 6% 40%	1711
December	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 6% 41%	2680
	11th to 20th	Day	23% 8% 2 5% 4 4 3 3 5% 5% 38%	3066
	21st to end	Day	18% 9% 4 10% 6% 4 4 3 5% 6% 32%	4048
January	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 5% 6% 43%	6489
	11th to 20th	Night	14% 9% 3 8% 5% 4 4 4 5% 6% 38%	6437
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 5% 40%	6633
February	1st to 10th	Night	16% 8% 2 7% 5% 4 3 3 5% 7% 39%	2601
	11th to 20th	Night	15% 8% 3 7% 5% 4 3 4 5% 6% 39%	2402
	21st to end	Night	19% 7% 2 6% 5% 4 4 3 6% 5% 40%	1980
March	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 5% 4 41%	5364
	11th to 20th	Night	13% 7% 3 5% 3 4 4 3 4 6% 48%	4988
	21st to end	Night	16% 9% 3 7% 4 3 4 3 4 4 42%	5128
April	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 6% 6% 36%	2000
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 5% 7% 39%	1975
	21st to end	Night	20% 6% 2 5% 3 3 4 3 6% 6% 42%	2000
May	1st to 10th	Night	31% 8% 2 5% 4 4 4 4 5% 6% 28%	4011
	11th to 20th	Night	25% 9% 3 6% 3 3 4 3 5% 5% 33%	3575
	21st to end	Night	23% 6% 2 6% 4 4 3 3 5% 7% 37%	3926
June	1st to 10th	Night	17% 5% 2 5% 4 4 4 4 5% 7% 43%	1600
	11th to 20th	Night	14% 5% 2 4 4 4 4 5% 7% 9% 41%	1600
	21st to end	Night	13% 4 1 5% 3 3 5% 5% 7% 9% 44%	1600
July	1st to 10th	Night	12% 6% 2 7% 4 6% 6% 4 6% 9% 38%	3600
	11th to 20th	Night	15% 7% 2 7% 6% 5% 5% 5% 7% 9% 32%	3600
	21st to end	Night	14% 5% 2 4 3 4 4 3 6% 7% 48%	3926
August	1st to 10th	Night	22% 6% 2 6% 4 5% 4 4 6% 8% 34%	2907
	11th to 20th	Night	16% 7% 2 7% 4 4 3 3 5% 6% 42%	3126
	21st to end	Night	14% 4 2 4 3 3 4 3 5% 8% 50%	3586
September	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 7% 52%	2031
	21st to end	Night	19% 7% 2 6% 3 3 3 3 4 6% 43%	2200
October	1st to 10th	Night	22% 4 2 5% 4 3 4 3 3 6% 44%	2396
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 5% 5% 48%	2400
	21st to end	Night	21% 7% 2 4 3 2 3 3 4 6% 45%	2735
November	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 6% 51%	2667
	11th to 20th	Night	17% 7% 3 5% 3 3 3 4 4 6% 45%	2730
	21st to end	Night	15% 6% 3 6% 4 4 4 3 5% 8% 43%	2903
December	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 4 5% 44%	4720
	11th to 20th	Night	24% 8% 2 4 4 3 3 3 4 5% 39%	5348
	21st to end	Night	21% 11% 3 7% 4 4 3 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	42% 5% 11% 25% 16% 2	3678
	11th to 20th	Day	51% 5% 11% 18% 11% 4	3680
	21st to end	Day	48% 5% 11% 22% 13% 1	4565
February	1st to 10th	Day	47% 6% 14% 21% 12% 1	1800
	11th to 20th	Day	46% 5% 10% 22% 14% 2	1892
	21st to end	Day	50% 4 11% 21% 11% 3	1650
March	1st to 10th	Day	46% 4 11% 21% 12% 6%	4479
	11th to 20th	Day	39% 6% 15% 27% 9% 4	4970
	21st to end	Day	42% 5% 12% 13% 22% 5%	5804
April	1st to 10th	Day	48% 5% 10% 19% 13% 4	2400
	11th to 20th	Day	45% 5% 11% 21% 15% 3	2374
	21st to end	Day	42% 6% 11% 21% 15% 4	2400
May	1st to 10th	Day	53% 7% 14% 16% 9% 1	5979
	11th to 20th	Day	52% 6% 9% 20% 11% 1	6322
	21st to end	Day	44% 7% 12% 22% 13% 1	6949
June	1st to 10th	Day	43% 8% 14% 21% 14% 1	2800
	11th to 20th	Day	44% 8% 14% 21% 11% 2	2800
	21st to end	Day	40% 8% 14% 25% 13% 1	2800
July	1st to 10th	Day	40% 10% 13% 21% 15% 1	6290
	11th to 20th	Day	42% 9% 17% 21% 10% 1	6285
	21st to end	Day	40% 6% 11% 21% 20% 1	6728
August	1st to 10th	Day	49% 7% 14% 19% 10%	4224
	11th to 20th	Day	40% 7% 13% 29% 11% 1	3925
	21st to end	Day	35% 5% 12% 31% 15% 2	4312
September	1st to 10th	Day	41% 6% 11% 21% 19% 2	2400
	11th to 20th	Day	34% 4 12% 23% 24% 2	2367
	21st to end	Day	47% 5% 10% 18% 18% 2	2200
October	1st to 10th	Day	39% 5% 9% 20% 22% 4	3260
	11th to 20th	Day	31% 5% 10% 28% 22% 4	3240
	21st to end	Day	29% 4 15% 26% 22% 3	3382
November	1st to 10th	Day	36% 4 13% 24% 20% 3	1851
	11th to 20th	Day	40% 5% 12% 24% 16% 3	1890
	21st to end	Day	43% 5% 14% 26% 10% 2	1711
December	1st to 10th	Day	45% 4 15% 25% 9% 1	2680
	11th to 20th	Day	47% 4 16% 21% 11% 1	3066
	21st to end	Day	50% 6% 13% 18% 13% 1	4048
January	1st to 10th	Night	40% 5% 13% 22% 18% 3	6489
	11th to 20th	Night	43% 6% 15% 19% 12% 5%	6482
	21st to end	Night	45% 4 12% 18% 17% 3	6633
February	1st to 10th	Night	45% 6% 13% 24% 11% 2	2601
	11th to 20th	Night	45% 6% 10% 20% 15% 4	2404
	21st to end	Night	45% 5% 9% 23% 15% 3	1980
March	1st to 10th	Night	45% 4 12% 21% 11% 7%	5374
	11th to 20th	Night	38% 4 16% 25% 12% 4	4989
	21st to end	Night	45% 5% 7% 15% 23% 5%	5128
April	1st to 10th	Night	48% 6% 10% 20% 13% 3	2000
	11th to 20th	Night	43% 7% 10% 19% 19% 1	1975
	21st to end	Night	40% 5% 11% 22% 17% 4	2000
May	1st to 10th	Night	56% 7% 11% 17% 9% 2	4017
	11th to 20th	Night	53% 7% 11% 15% 13% 2	3576
	21st to end	Night	45% 7% 13% 22% 11% 2	3927
June	1st to 10th	Night	41% 6% 10% 24% 18% 1	1600
	11th to 20th	Night	37% 9% 16% 24% 13% 1	1600
	21st to end	Night	33% 9% 15% 28% 14% 1	1600
July	1st to 10th	Night	43% 9% 14% 18% 16% 2	3560
	11th to 20th	Night	45% 9% 14% 20% 11% 1	3552
	21st to end	Night	35% 6% 14% 24% 21% 1	3809
August	1st to 10th	Night	47% 8% 16% 18% 10% 1	2897
	11th to 20th	Night	43% 6% 14% 24% 13% 1	3155
	21st to end	Night	33% 6% 10% 33% 17% 1	3586
September	1st to 10th	Night	39% 5% 11% 24% 19% 2	2000
	11th to 20th	Night	33% 6% 10% 22% 26% 3	2031
	21st to end	Night	42% 6% 10% 21% 20% 2	2200
October	1st to 10th	Night	41% 5% 10% 22% 20% 2	3916
	11th to 20th	Night	29% 4 10% 28% 25% 4	3899
	21st to end	Night	30% 4 14% 30% 21% 2	4408
November	1st to 10th	Night	34% 6% 13% 24% 20% 3	2667
	11th to 20th	Night	40% 5% 11% 23% 18% 3	2730
	21st to end	Night	41% 5% 13% 27% 13% 1	2903
December	1st to 10th	Night	43% 4 14% 30% 9% 1	4720
	11th to 20th	Night	47% 4 12% 21% 14% 2	5348
	21st to end	Night	51% 4 12% 18% 13% 2	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	28% 26% 26% 17% 3	220
	11th to 20th	Day	32% 25% 28% 12% 2	220
	21st to end	Day	23% 28% 31% 12% 4 1	363
February	1st to 10th	Day	25% 27% 35% 10% 2	300
	11th to 20th	Day	28% 21% 37% 13% 2	431
	21st to end	Day	27% 20% 31% 19% 3	414
March	1st to 10th	Day	26% 22% 27% 21% 4	500
	11th to 20th	Day	24% 21% 34% 18% 4 1	500
	21st to end	Day	28% 21% 27% 17% 5% 2	550
April	1st to 10th	Day	26% 22% 28% 17% 6% 1	500
	11th to 20th	Day	26% 20% 28% 16% 10% 1	492
	21st to end	Day	25% 17% 32% 18% 6% 2	500
May	1st to 10th	Day	30% 20% 22% 16% 9% 3	503
	11th to 20th	Day	24% 13% 29% 21% 9% 3	527
	21st to end	Day	21% 9% 25% 24% 18% 3	573
June	1st to 10th	Day	19% 14% 25% 25% 14% 3	530
	11th to 20th	Day	22% 10% 26% 27% 13% 3	530
	21st to end	Day	16% 10% 28% 29% 15% 2	530
July	1st to 10th	Day	12% 4 25% 29% 22% 8%	525
	11th to 20th	Day	11% 5% 23% 31% 23% 6%	524
	21st to end	Day	15% 8% 27% 31% 16% 4	576
August	1st to 10th	Day	15% 5% 20% 32% 20% 8%	526
	11th to 20th	Day	14% 5% 19% 34% 20% 8%	519
	21st to end	Day	13% 7% 26% 27% 18% 9%	583
September	1st to 10th	Day	20% 8% 21% 26% 17% 9%	530
	11th to 20th	Day	14% 5% 24% 28% 17% 12%	530
	21st to end	Day	24% 8% 23% 26% 14% 5%	530
October	1st to 10th	Day	23% 8% 23% 22% 17% 7%	530
	11th to 20th	Day	21% 15% 28% 24% 10% 3	530
	21st to end	Day	23% 14% 31% 22% 9% 1	531
November	1st to 10th	Day	20% 14% 28% 25% 10% 3	425
	11th to 20th	Day	23% 19% 29% 23% 6%	430
	21st to end	Day	25% 22% 31% 17% 4	251
December	1st to 10th	Day	25% 25% 31% 13% 5% 1	220
	11th to 20th	Day	34% 20% 25% 17% 4	220
	21st to end	Day	29% 17% 33% 16% 5% 1	242
January	1st to 10th	Night	21% 14% 40% 22% 3	840
	11th to 20th	Night	25% 16% 32% 25% 2	840
	21st to end	Night	18% 15% 42% 24% 2	924
February	1st to 10th	Night	22% 11% 39% 27% 1	801
	11th to 20th	Night	22% 18% 40% 19% 1	783
	21st to end	Night	23% 15% 37% 22% 3	660
March	1st to 10th	Night	22% 14% 36% 24% 4	800
	11th to 20th	Night	20% 15% 36% 24% 5%	800
	21st to end	Night	20% 14% 38% 22% 6%	880
April	1st to 10th	Night	22% 14% 38% 22% 4	800
	11th to 20th	Night	21% 13% 34% 24% 8%	791
	21st to end	Night	24% 10% 29% 31% 7%	800
May	1st to 10th	Night	27% 9% 28% 28% 8% 1	583
	11th to 20th	Night	20% 9% 32% 33% 7%	394
	21st to end	Night	19% 4 21% 39% 17%	433
June	1st to 10th	Night	18% 5% 21% 37% 19% 1	400
	11th to 20th	Night	15% 4 26% 39% 17%	400
	21st to end	Night	15% 3 18% 49% 17%	400
July	1st to 10th	Night	14% 3 19% 41% 22% 2	390
	11th to 20th	Night	11% 5% 13% 47% 22% 2	388
	21st to end	Night	13% 4 16% 45% 21% 1	438
August	1st to 10th	Night	14% 2 11% 45% 27% 2	589
	11th to 20th	Night	15% 3 15% 38% 28% 2	743
	21st to end	Night	16% 3 16% 38% 25% 1	880
September	1st to 10th	Night	18% 4 20% 35% 21% 2	800
	11th to 20th	Night	14% 5% 17% 37% 24% 5%	800
	21st to end	Night	21% 4 21% 33% 19% 1	800
October	1st to 10th	Night	24% 5% 23% 27% 20% 2	800
	11th to 20th	Night	20% 9% 27% 34% 10%	800
	21st to end	Night	25% 7% 32% 28% 8%	880
November	1st to 10th	Night	18% 8% 31% 33% 10% 1	820
	11th to 20th	Night	22% 14% 29% 27% 7% 1	840
	21st to end	Night	19% 13% 33% 28% 7%	840
December	1st to 10th	Night	23% 11% 34% 25% 5%	840
	11th to 20th	Night	24% 13% 32% 27% 4	840
	21st to end	Night	24% 13% 36% 25% 2	924

Page generated at 2026-01-29 01:21:45. Version ac07

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