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 2024-11-28.

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% 5% 4 4 4 5% 7% 38%	101591
	Night	18% 7% 2 6% 4 4 4 4 5% 6% 40%	97957
Transparency	Day	43% 6% 13% 22% 14% 2	105124
	Night	42% 6% 13% 23% 15% 2	102513
Seeing	Day	21% 14% 27% 23% 12% 4	14974
	Night	20% 9% 29% 30% 11% 1	24208

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	14% 9% 3 6% 4 4 4 3 4 5% 43%	2517
	11th to 20th	Day	14% 13% 5% 9% 5% 5% 5% 4 6% 5% 29%	2520
	21st to end	Day	20% 11% 4 8% 5% 4 4 4 4 5% 32%	3124
February	1st to 10th	Day	15% 8% 2 8% 5% 5% 5% 4 6% 5% 36%	1710
	11th to 20th	Day	14% 9% 2 7% 5% 5% 4 4 5% 6% 38%	1786
	21st to end	Day	20% 10% 4 6% 6% 4 4 3 4 5% 36%	1570
March	1st to 10th	Day	26% 10% 3 6% 4 3 3 3 4 5% 34%	3116
	11th to 20th	Day	10% 7% 2 7% 5% 5% 4 4 6% 7% 43%	3470
	21st to end	Day	20% 8% 2 6% 3 3 3 3 4 5% 42%	4056
April	1st to 10th	Day	17% 9% 3 7% 5% 5% 4 3 6% 6% 36%	2280
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 5% 6% 40%	2254
	21st to end	Day	17% 6% 2 6% 5% 5% 4 4 6% 6% 39%	2280
May	1st to 10th	Day	27% 7% 3 6% 4 4 4 3 5% 7% 30%	4160
	11th to 20th	Day	24% 9% 3 8% 5% 4 5% 4 5% 6% 28%	4398
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 9% 43%	4837
June	1st to 10th	Day	15% 5% 2 7% 6% 5% 5% 4 6% 8% 37%	2660
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 4 6% 9% 35%	2660
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	2660
July	1st to 10th	Day	8% 5% 3 7% 6% 6% 6% 5% 6% 10% 37%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	4814
August	1st to 10th	Day	22% 5% 2 7% 5% 5% 5% 5% 7% 7% 31%	4108
	11th to 20th	Day	14% 6% 2 6% 3 5% 6% 4 6% 8% 40%	3767
	21st to end	Day	15% 5% 2 4 4 3 4 3 5% 9% 47%	4180
September	1st to 10th	Day	17% 5% 2 6% 4 4 4 4 5% 7% 43%	2280
	11th to 20th	Day	14% 4 1 4 3 4 4 3 5% 7% 49%	2249
	21st to end	Day	20% 6% 2 6% 4 4 4 4 5% 5% 39%	2090
October	1st to 10th	Day	20% 6% 3 6% 4 3 4 3 4 8% 41%	1900
	11th to 20th	Day	16% 6% 2 4 4 4 4 3 4 6% 46%	1900
	21st to end	Day	21% 6% 2 5% 4 4 4 3 4 7% 39%	2000
November	1st to 10th	Day	14% 7% 2 5% 3 3 3 2 4 6% 53%	1761
	11th to 20th	Day	18% 6% 2 6% 4 3 4 4 5% 5% 44%	1800
	21st to end	Day	18% 6% 3 6% 3 4 4 4 6% 6% 40%	1606
December	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 3 4 6% 35%	1520
	11th to 20th	Day	21% 8% 2 5% 4 4 3 3 5% 6% 37%	1906
	21st to end	Day	19% 11% 5% 10% 6% 5% 4 4 5% 6% 26%	2772
January	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	4429
	11th to 20th	Night	16% 9% 3 8% 5% 5% 4 4 5% 6% 35%	4436
	21st to end	Night	23% 11% 3 6% 4 3 4 3 4 5% 34%	4532
February	1st to 10th	Night	16% 8% 2 7% 5% 3 3 3 5% 7% 39%	2470
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 4 5% 6% 39%	2279
	21st to end	Night	19% 7% 2 6% 5% 4 4 3 5% 5% 40%	1884
March	1st to 10th	Night	22% 11% 3 6% 4 3 3 3 5% 4 36%	3724
	11th to 20th	Night	11% 8% 3 5% 4 4 4 4 4 6% 46%	3488
	21st to end	Night	16% 10% 3 7% 4 3 3 2 4 4 43%	3576
April	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 6% 6% 36%	1900
	11th to 20th	Night	18% 6% 2 6% 4 4 4 4 5% 7% 40%	1875
	21st to end	Night	20% 6% 2 5% 3 3 3 3 7% 6% 42%	1900
May	1st to 10th	Night	28% 9% 2 6% 4 4 3 3 5% 6% 31%	2805
	11th to 20th	Night	26% 9% 3 7% 4 3 4 3 5% 5% 31%	2495
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 9% 41%	2738
June	1st to 10th	Night	16% 5% 2 5% 5% 4 4 4 5% 7% 43%	1520
	11th to 20th	Night	13% 5% 2 4 4 4 4 5% 7% 10% 42%	1520
	21st to end	Night	13% 4 1 4 3 3 5% 5% 7% 9% 44%	1520
July	1st to 10th	Night	11% 5% 2 6% 5% 6% 7% 5% 6% 9% 38%	2520
	11th to 20th	Night	11% 5% 2 6% 5% 4 5% 6% 8% 10% 38%	2520
	21st to end	Night	17% 6% 2 5% 4 5% 4 4 7% 7% 39%	2739
August	1st to 10th	Night	22% 6% 2 6% 4 5% 4 4 6% 8% 33%	2817
	11th to 20th	Night	17% 7% 2 7% 4 4 3 3 5% 6% 42%	3028
	21st to end	Night	14% 4 2 4 3 3 4 3 5% 8% 50%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 7% 52%	1929
	21st to end	Night	19% 6% 2 6% 3 3 3 3 4 6% 44%	2090
October	1st to 10th	Night	23% 5% 2 5% 4 3 4 3 3 6% 43%	2276
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 5% 5% 48%	2280
	21st to end	Night	21% 7% 2 4 3 2 3 3 5% 6% 44%	2598
November	1st to 10th	Night	13% 5% 2 4 3 4 4 4 5% 5% 51%	2537
	11th to 20th	Night	18% 7% 2 4 3 3 3 4 4 6% 45%	2600
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 8% 43%	2724
December	1st to 10th	Night	19% 8% 3 7% 4 5% 5% 3 5% 6% 36%	2660
	11th to 20th	Night	19% 9% 2 5% 4 4 4 4 5% 6% 38%	3288
	21st to end	Night	21% 10% 3 7% 4 4 4 3 4 5% 33%	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	42% 4 14% 28% 10% 2	2518
	11th to 20th	Day	53% 6% 13% 18% 9% 2	2520
	21st to end	Day	53% 5% 13% 20% 9% 1	3124
February	1st to 10th	Day	47% 5% 14% 21% 12% 1	1710
	11th to 20th	Day	45% 5% 10% 23% 14% 2	1795
	21st to end	Day	51% 4 10% 21% 11% 3	1570
March	1st to 10th	Day	52% 4 11% 24% 7% 2	3119
	11th to 20th	Day	37% 6% 15% 28% 12% 2	3470
	21st to end	Day	45% 4 10% 16% 21% 4	4056
April	1st to 10th	Day	47% 6% 10% 20% 13% 4	2280
	11th to 20th	Day	45% 5% 11% 22% 14% 3	2254
	21st to end	Day	43% 6% 11% 20% 15% 4	2280
May	1st to 10th	Day	52% 6% 12% 18% 10% 2	4174
	11th to 20th	Day	55% 7% 11% 17% 8% 1	4402
	21st to end	Day	35% 7% 14% 26% 17% 2	4837
June	1st to 10th	Day	43% 8% 14% 21% 13% 1	2660
	11th to 20th	Day	43% 8% 14% 22% 10% 2	2660
	21st to end	Day	40% 8% 14% 25% 13%	2660
July	1st to 10th	Day	40% 9% 14% 21% 14% 2	4370
	11th to 20th	Day	38% 8% 18% 23% 13% 1	4365
	21st to end	Day	48% 7% 11% 17% 15% 1	4661
August	1st to 10th	Day	50% 7% 15% 19% 10%	4094
	11th to 20th	Day	40% 7% 13% 29% 11% 1	3804
	21st to end	Day	35% 5% 12% 31% 15% 2	4180
September	1st to 10th	Day	41% 6% 11% 21% 18% 2	2280
	11th to 20th	Day	34% 4 12% 22% 25% 3	2249
	21st to end	Day	46% 5% 9% 18% 19% 2	2090
October	1st to 10th	Day	40% 5% 9% 19% 22% 4	3160
	11th to 20th	Day	30% 5% 10% 28% 23% 4	3140
	21st to end	Day	29% 4 15% 26% 22% 3	3277
November	1st to 10th	Day	36% 4 13% 24% 20% 3	1761
	11th to 20th	Day	40% 4 12% 24% 17% 2	1800
	21st to end	Day	43% 5% 15% 26% 10% 2	1606
December	1st to 10th	Day	51% 5% 14% 18% 10% 1	1520
	11th to 20th	Day	46% 6% 18% 21% 8% 1	1906
	21st to end	Day	56% 7% 13% 17% 7%	2772
January	1st to 10th	Night	42% 5% 14% 27% 10% 3	4429
	11th to 20th	Night	47% 6% 15% 22% 9% 2	4436
	21st to end	Night	51% 5% 12% 20% 11% 1	4532
February	1st to 10th	Night	44% 6% 13% 24% 11% 2	2470
	11th to 20th	Night	45% 6% 10% 20% 15% 4	2281
	21st to end	Night	45% 5% 9% 24% 14% 3	1884
March	1st to 10th	Night	50% 4 11% 22% 11% 2	3734
	11th to 20th	Night	37% 5% 15% 27% 14% 2	3489
	21st to end	Night	44% 5% 7% 17% 22% 6%	3576
April	1st to 10th	Night	46% 6% 10% 21% 13% 3	1900
	11th to 20th	Night	43% 7% 10% 20% 19% 1	1875
	21st to end	Night	40% 5% 12% 22% 17% 4	1900
May	1st to 10th	Night	53% 7% 9% 21% 9% 1	2811
	11th to 20th	Night	54% 8% 13% 14% 10% 1	2496
	21st to end	Night	38% 8% 15% 23% 13% 2	2739
June	1st to 10th	Night	40% 6% 10% 25% 18% 1	1520
	11th to 20th	Night	36% 9% 16% 25% 13% 1	1520
	21st to end	Night	33% 9% 15% 28% 14% 1	1520
July	1st to 10th	Night	41% 10% 15% 17% 15% 2	2480
	11th to 20th	Night	38% 10% 15% 24% 12% 1	2472
	21st to end	Night	42% 7% 15% 16% 18% 1	2651
August	1st to 10th	Night	48% 9% 16% 17% 9% 1	2807
	11th to 20th	Night	43% 6% 14% 23% 14% 1	3057
	21st to end	Night	33% 6% 11% 32% 17% 1	3476
September	1st to 10th	Night	39% 5% 11% 24% 19% 2	1900
	11th to 20th	Night	33% 6% 10% 22% 26% 3	1929
	21st to end	Night	41% 6% 9% 21% 21% 1	2090
October	1st to 10th	Night	42% 5% 10% 22% 19% 1	3796
	11th to 20th	Night	29% 4 9% 28% 25% 4	3779
	21st to end	Night	30% 4 14% 30% 21% 2	4271
November	1st to 10th	Night	34% 6% 13% 24% 20% 3	2537
	11th to 20th	Night	41% 4 11% 23% 18% 3	2600
	21st to end	Night	40% 5% 14% 27% 12% 1	2724
December	1st to 10th	Night	48% 6% 13% 21% 10% 1	2660
	11th to 20th	Night	45% 5% 15% 20% 13% 2	3288
	21st to end	Night	51% 5% 15% 21% 8%	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	26% 23% 28% 19% 4	180
	11th to 20th	Day	28% 26% 28% 14% 3 1	180
	21st to end	Day	21% 25% 33% 14% 5% 1	297
February	1st to 10th	Day	24% 26% 36% 11% 3	270
	11th to 20th	Day	25% 20% 39% 14% 2	394
	21st to end	Day	28% 20% 30% 19% 3	382
March	1st to 10th	Day	25% 23% 26% 22% 4	460
	11th to 20th	Day	23% 21% 33% 18% 3 1	460
	21st to end	Day	28% 20% 27% 17% 6% 2	506
April	1st to 10th	Day	23% 22% 29% 18% 6% 1	460
	11th to 20th	Day	24% 19% 29% 17% 10% 2	452
	21st to end	Day	25% 16% 33% 18% 6% 2	460
May	1st to 10th	Day	30% 20% 22% 16% 9% 3	463
	11th to 20th	Day	23% 13% 29% 22% 10% 3	487
	21st to end	Day	19% 8% 26% 25% 19% 4	529
June	1st to 10th	Day	18% 13% 26% 26% 14% 3	490
	11th to 20th	Day	20% 9% 26% 28% 14% 3	490
	21st to end	Day	16% 10% 29% 27% 15% 3	490
July	1st to 10th	Day	13% 4 24% 28% 22% 8%	485
	11th to 20th	Day	11% 5% 23% 32% 24% 6%	484
	21st to end	Day	16% 7% 27% 30% 16% 4	532
August	1st to 10th	Day	16% 5% 20% 32% 20% 7%	486
	11th to 20th	Day	14% 4 20% 34% 20% 8%	479
	21st to end	Day	14% 6% 24% 28% 18% 10%	539
September	1st to 10th	Day	21% 7% 21% 25% 17% 9%	490
	11th to 20th	Day	14% 5% 24% 27% 17% 13%	490
	21st to end	Day	25% 9% 23% 24% 14% 5%	490
October	1st to 10th	Day	25% 8% 23% 22% 16% 6%	490
	11th to 20th	Day	19% 13% 29% 26% 10% 3	490
	21st to end	Day	23% 14% 30% 22% 9% 1	492
November	1st to 10th	Day	20% 13% 27% 26% 11% 3	395
	11th to 20th	Day	23% 18% 30% 24% 6% 1	400
	21st to end	Day	24% 18% 33% 19% 4	224
December	1st to 10th	Day	24% 24% 31% 13% 6% 1	180
	11th to 20th	Day	32% 19% 27% 18% 4	180
	21st to end	Day	27% 18% 33% 16% 5% 2	198
January	1st to 10th	Night	20% 13% 41% 23% 3	760
	11th to 20th	Night	24% 15% 33% 26% 3	760
	21st to end	Night	17% 14% 42% 25% 2	836
February	1st to 10th	Night	21% 11% 38% 28% 2	760
	11th to 20th	Night	21% 18% 42% 19% 1	743
	21st to end	Night	23% 15% 36% 22% 3	628
March	1st to 10th	Night	22% 13% 37% 25% 4	760
	11th to 20th	Night	20% 15% 36% 23% 5%	760
	21st to end	Night	20% 14% 38% 22% 6%	836
April	1st to 10th	Night	21% 14% 38% 23% 4	760
	11th to 20th	Night	21% 12% 34% 25% 8%	751
	21st to end	Night	24% 9% 29% 30% 7%	760
May	1st to 10th	Night	26% 9% 28% 28% 8% 1	554
	11th to 20th	Night	19% 8% 33% 33% 7%	374
	21st to end	Night	18% 4 19% 41% 18%	411
June	1st to 10th	Night	18% 4 21% 36% 20% 1	380
	11th to 20th	Night	14% 3 26% 40% 17%	380
	21st to end	Night	14% 2 17% 50% 16%	380
July	1st to 10th	Night	13% 2 19% 42% 22% 2	370
	11th to 20th	Night	11% 5% 13% 47% 22% 2	368
	21st to end	Night	14% 4 16% 44% 20% 1	416
August	1st to 10th	Night	14% 2 11% 44% 26% 3	559
	11th to 20th	Night	15% 3 15% 38% 27% 2	705
	21st to end	Night	17% 3 15% 38% 26% 1	836
September	1st to 10th	Night	19% 3 20% 35% 21% 2	760
	11th to 20th	Night	14% 5% 18% 35% 24% 5%	760
	21st to end	Night	22% 4 21% 32% 19% 1	760
October	1st to 10th	Night	24% 5% 23% 27% 19% 2	760
	11th to 20th	Night	19% 8% 28% 35% 10%	760
	21st to end	Night	25% 7% 32% 27% 9%	836
November	1st to 10th	Night	17% 8% 32% 33% 10% 1	780
	11th to 20th	Night	22% 13% 30% 27% 8% 1	800
	21st to end	Night	18% 12% 34% 28% 7%	789
December	1st to 10th	Night	24% 11% 34% 25% 6% 1	760
	11th to 20th	Night	23% 12% 33% 29% 4	760
	21st to end	Night	23% 13% 37% 26% 2	836

Page generated at 2024-11-25 00:22:20. Version ac07

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