BarCla Hill Private Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BarCla Hill Private Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	11% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	126336
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 45%	124656
Transparency	Day	38% 6% 13% 19% 17% 8%	130110
	Night	36% 6% 13% 19% 18% 9%	128957
Seeing	Day	14% 9% 22% 25% 20% 10%	15831
	Night	13% 6% 20% 29% 24% 7%	25546

By Month

Percentage Cloud Cover SE (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	17% 5% 2 5% 4 3 4 3 5% 6% 47%	4516
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 4 5% 51%	4520
	21st to end	Day	23% 6% 3 6% 4 3 4 3 5% 6% 36%	4972
February	1st to 10th	Day	18% 7% 3 6% 5% 4 4 3 5% 5% 40%	2000
	11th to 20th	Day	18% 6% 2 5% 3 4 3 3 5% 7% 43%	1948
	21st to end	Day	17% 7% 3 8% 4 5% 4 3 6% 7% 38%	1650
March	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 5% 6% 37%	4480
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	4952
	21st to end	Day	15% 7% 3 7% 4 4 5% 4 5% 5% 42%	5870
April	1st to 10th	Day	12% 6% 2 6% 6% 5% 5% 5% 6% 8% 40%	2400
	11th to 20th	Day	14% 6% 3 6% 4 3 4 5% 7% 7% 41%	2374
	21st to end	Day	9% 5% 1 5% 5% 4 5% 4 7% 9% 47%	2400
May	1st to 10th	Day	16% 6% 3 6% 4 5% 5% 4 7% 9% 35%	5405
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 7% 9% 45%	5404
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 8% 12% 42%	6201
June	1st to 10th	Day	10% 4 3 8% 6% 6% 8% 6% 8% 10% 31%	2600
	11th to 20th	Day	5% 5% 2 7% 7% 7% 8% 6% 8% 12% 33%	2600
	21st to end	Day	7% 6% 3 8% 8% 7% 7% 8% 8% 11% 27%	2626
July	1st to 10th	Day	4 4 2 9% 7% 8% 8% 7% 9% 13% 28%	5695
	11th to 20th	Day	5% 4 2 7% 7% 8% 8% 7% 9% 11% 31%	5440
	21st to end	Day	5% 5% 2 8% 7% 8% 8% 8% 10% 14% 25%	5922
August	1st to 10th	Day	4 3 2 6% 5% 5% 6% 6% 9% 10% 45%	3905
	11th to 20th	Day	6% 4 2 6% 6% 7% 8% 6% 9% 11% 35%	3849
	21st to end	Day	7% 6% 3 11% 7% 7% 7% 6% 7% 8% 31%	4312
September	1st to 10th	Day	11% 7% 3 8% 7% 5% 6% 5% 7% 8% 34%	2400
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 4 7% 8% 39%	2282
	21st to end	Day	8% 4 2 6% 4 5% 5% 3 7% 7% 49%	2200
October	1st to 10th	Day	7% 4 2 5% 4 5% 4 5% 5% 6% 54%	2200
	11th to 20th	Day	7% 4 1 5% 3 4 4 3 5% 6% 58%	2025
	21st to end	Day	9% 4 2 5% 4 3 3 2 5% 7% 57%	2200
November	1st to 10th	Day	9% 5% 2 5% 2 3 4 3 4 7% 56%	2033
	11th to 20th	Day	11% 4 1 4 3 2 3 3 4 6% 57%	1890
	21st to end	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	1890
December	1st to 10th	Day	18% 6% 2 5% 4 4 4 3 4 6% 44%	3020
	11th to 20th	Day	11% 7% 2 6% 4 3 4 4 6% 6% 48%	3436
	21st to end	Day	20% 7% 2 5% 3 4 3 3 4 5% 44%	4719
January	1st to 10th	Night	17% 6% 3 5% 3 3 4 3 4 5% 48%	5429
	11th to 20th	Night	19% 8% 2 5% 3 3 2 2 3 5% 48%	5440
	21st to end	Night	23% 7% 3 6% 4 3 3 3 4 6% 38%	5984
February	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	2400
	11th to 20th	Night	20% 7% 3 5% 4 3 3 3 4 6% 41%	2338
	21st to end	Night	18% 8% 2 8% 4 3 4 4 5% 6% 37%	1980
March	1st to 10th	Night	21% 10% 3 6% 3 3 3 3 5% 6% 36%	5373
	11th to 20th	Night	15% 7% 1 5% 3 4 4 3 5% 6% 48%	5036
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 6% 46%	5059
April	1st to 10th	Night	16% 8% 2 5% 5% 3 3 3 5% 6% 44%	2000
	11th to 20th	Night	13% 5% 2 5% 5% 3 5% 3 5% 7% 46%	1975
	21st to end	Night	7% 4 2 5% 4 3 4 3 5% 8% 56%	2000
May	1st to 10th	Night	16% 7% 2 7% 4 4 4 3 5% 7% 38%	4490
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 6% 9% 43%	4489
	21st to end	Night	6% 4 2 6% 4 4 5% 4 7% 14% 44%	4648
June	1st to 10th	Night	7% 5% 2 6% 5% 4 6% 6% 7% 11% 41%	1800
	11th to 20th	Night	3 3 1 5% 5% 4 6% 6% 8% 12% 47%	1800
	21st to end	Night	6% 5% 2 6% 4 5% 6% 6% 8% 11% 39%	1740
July	1st to 10th	Night	3 4 2 6% 5% 6% 7% 6% 8% 13% 39%	4227
	11th to 20th	Night	2 3 2 6% 5% 6% 5% 5% 9% 12% 46%	4517
	21st to end	Night	3 4 2 7% 5% 6% 7% 6% 11% 15% 33%	4929
August	1st to 10th	Night	1 2 1 3 3 4 4 5% 7% 12% 58%	3244
	11th to 20th	Night	3 3 2 5% 5% 6% 5% 6% 10% 13% 43%	3200
	21st to end	Night	5% 4 2 7% 5% 6% 8% 6% 9% 11% 39%	3586
September	1st to 10th	Night	8% 5% 3 7% 5% 5% 5% 5% 7% 10% 39%	2000
	11th to 20th	Night	8% 6% 2 6% 4 5% 4 4 6% 9% 46%	2095
	21st to end	Night	6% 5% 2 5% 5% 4 4 4 5% 7% 51%	2200
October	1st to 10th	Night	7% 4 1 5% 4 3 4 3 4 7% 57%	2200
	11th to 20th	Night	6% 4 2 3 3 2 3 3 4 7% 64%	2370
	21st to end	Night	10% 4 2 5% 3 3 3 3 4 6% 57%	2640
November	1st to 10th	Night	10% 5% 1 5% 4 4 3 2 5% 5% 56%	2481
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 56%	2730
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 44%	4380
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 4 6% 50%	4914
	21st to end	Night	21% 8% 2 6% 3 3 3 2 4 5% 43%	6232

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	38% 3 5% 19% 26% 10%	4517
	11th to 20th	Day	36% 3 6% 16% 30% 10%	4520
	21st to end	Day	49% 4 7% 13% 16% 11%	4972
February	1st to 10th	Day	46% 4 7% 12% 20% 10%	2000
	11th to 20th	Day	41% 4 8% 16% 19% 11%	1958
	21st to end	Day	46% 4 9% 16% 16% 9%	1650
March	1st to 10th	Day	47% 4 7% 17% 15% 10%	4480
	11th to 20th	Day	36% 5% 7% 15% 21% 15%	4952
	21st to end	Day	42% 5% 12% 18% 15% 9%	5872
April	1st to 10th	Day	38% 7% 11% 18% 16% 10%	2400
	11th to 20th	Day	39% 6% 8% 18% 20% 9%	2374
	21st to end	Day	31% 6% 11% 23% 22% 7%	2400
May	1st to 10th	Day	43% 7% 15% 21% 11% 3	5440
	11th to 20th	Day	34% 4 17% 26% 17% 1	5408
	21st to end	Day	32% 7% 17% 31% 12% 2	6215
June	1st to 10th	Day	42% 10% 16% 22% 10% 1	2600
	11th to 20th	Day	37% 11% 16% 24% 10% 2	2600
	21st to end	Day	43% 12% 20% 16% 9%	2626
July	1st to 10th	Day	42% 8% 27% 19% 3	5635
	11th to 20th	Day	42% 9% 36% 11% 1	5377
	21st to end	Day	44% 10% 24% 19% 2	5752
August	1st to 10th	Day	30% 8% 14% 25% 22% 1	3893
	11th to 20th	Day	39% 7% 19% 24% 11% 1	3885
	21st to end	Day	47% 11% 13% 15% 12% 2	4311
September	1st to 10th	Day	42% 11% 15% 18% 11% 4	2400
	11th to 20th	Day	39% 10% 12% 17% 16% 6%	2282
	21st to end	Day	32% 7% 11% 23% 18% 10%	2200
October	1st to 10th	Day	29% 3 6% 13% 20% 29%	3590
	11th to 20th	Day	16% 3 4 18% 28% 31%	3278
	21st to end	Day	20% 3 5% 13% 38% 22%	3538
November	1st to 10th	Day	29% 4 8% 20% 26% 13%	2033
	11th to 20th	Day	29% 4 7% 18% 29% 13%	1890
	21st to end	Day	37% 4 9% 19% 20% 12%	1890
December	1st to 10th	Day	43% 3 7% 16% 24% 7%	3020
	11th to 20th	Day	35% 4 8% 21% 21% 10%	3433
	21st to end	Day	44% 3 6% 16% 26% 6%	4719
January	1st to 10th	Night	39% 3 5% 17% 26% 9%	5429
	11th to 20th	Night	41% 2 4 16% 29% 9%	5440
	21st to end	Night	47% 3 6% 13% 20% 10%	5984
February	1st to 10th	Night	42% 4 7% 17% 20% 9%	2400
	11th to 20th	Night	45% 4 5% 15% 19% 12%	2339
	21st to end	Night	45% 7% 9% 14% 13% 12%	1980
March	1st to 10th	Night	48% 4 7% 13% 19% 8%	5373
	11th to 20th	Night	37% 5% 9% 14% 20% 15%	5036
	21st to end	Night	39% 4 11% 20% 16% 9%	5059
April	1st to 10th	Night	41% 5% 10% 16% 17% 11%	2000
	11th to 20th	Night	36% 5% 9% 21% 19% 9%	1975
	21st to end	Night	27% 5% 11% 25% 24% 9%	2000
May	1st to 10th	Night	44% 6% 15% 19% 14% 3	4520
	11th to 20th	Night	38% 5% 16% 26% 13% 1	4490
	21st to end	Night	29% 6% 23% 29% 11% 1	4656
June	1st to 10th	Night	33% 10% 20% 24% 11% 1	1800
	11th to 20th	Night	26% 11% 26% 25% 11% 1	1800
	21st to end	Night	34% 11% 26% 20% 9% 1	1740
July	1st to 10th	Night	34% 9% 33% 19% 4	4181
	11th to 20th	Night	29% 12% 45% 13% 1	4460
	21st to end	Night	37% 16% 21% 25% 2	4785
August	1st to 10th	Night	19% 7% 22% 30% 21% 1	3232
	11th to 20th	Night	28% 10% 27% 27% 8%	3230
	21st to end	Night	37% 13% 19% 19% 9% 3	3586
September	1st to 10th	Night	37% 10% 18% 17% 12% 5%	2000
	11th to 20th	Night	32% 9% 18% 17% 16% 7%	2095
	21st to end	Night	30% 6% 13% 24% 17% 10%	2200
October	1st to 10th	Night	32% 3 7% 13% 22% 23%	3590
	11th to 20th	Night	15% 3 5% 19% 27% 30%	3856
	21st to end	Night	20% 2 5% 11% 38% 24%	4252
November	1st to 10th	Night	31% 3 7% 18% 26% 15%	2481
	11th to 20th	Night	31% 4 6% 17% 28% 15%	2730
	21st to end	Night	36% 4 7% 20% 20% 13%	2730
December	1st to 10th	Night	43% 3 7% 16% 23% 9%	4380
	11th to 20th	Night	35% 4 9% 21% 21% 11%	4916
	21st to end	Night	45% 3 6% 17% 22% 7%	6232

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% 15% 40% 15% 6% 1	300
	11th to 20th	Day	24% 19% 32% 20% 5% 1	300
	21st to end	Day	23% 23% 37% 14% 3	330
February	1st to 10th	Day	27% 20% 34% 13% 5% 1	300
	11th to 20th	Day	31% 21% 30% 14% 5%	300
	21st to end	Day	17% 33% 37% 11% 2	249
March	1st to 10th	Day	18% 24% 38% 16% 3	300
	11th to 20th	Day	25% 23% 31% 17% 4	300
	21st to end	Day	19% 19% 31% 21% 9% 1	505
April	1st to 10th	Day	20% 15% 33% 22% 9% 1	500
	11th to 20th	Day	17% 17% 33% 19% 11% 2	492
	21st to end	Day	10% 12% 30% 28% 16% 3	500
May	1st to 10th	Day	14% 13% 29% 25% 16% 4	503
	11th to 20th	Day	12% 8% 25% 29% 20% 6%	527
	21st to end	Day	7% 4 22% 36% 22% 9%	573
June	1st to 10th	Day	10% 4 14% 34% 22% 16%	530
	11th to 20th	Day	8% 3 14% 32% 28% 15%	530
	21st to end	Day	10% 2 12% 30% 28% 18%	618
July	1st to 10th	Day	9% 1 6% 30% 31% 22%	637
	11th to 20th	Day	9% 2 4 19% 38% 29%	524
	21st to end	Day	7% 2 5% 27% 38% 21%	576
August	1st to 10th	Day	6% 2 5% 25% 36% 26%	526
	11th to 20th	Day	8% 2 4 20% 37% 28%	519
	21st to end	Day	10% 3 10% 23% 33% 22%	583
September	1st to 10th	Day	11% 2 6% 29% 34% 17%	530
	11th to 20th	Day	11% 3 12% 26% 35% 13%	530
	21st to end	Day	8% 5% 12% 35% 29% 11%	530
October	1st to 10th	Day	8% 5% 24% 35% 22% 6%	530
	11th to 20th	Day	7% 8% 26% 33% 21% 5%	355
	21st to end	Day	16% 10% 26% 34% 11% 3	363
November	1st to 10th	Day	16% 8% 26% 33% 15% 1	330
	11th to 20th	Day	16% 10% 33% 26% 12% 2	330
	21st to end	Day	15% 16% 37% 22% 8% 2	330
December	1st to 10th	Day	21% 12% 35% 22% 9% 1	330
	11th to 20th	Day	19% 15% 38% 23% 5%	321
	21st to end	Day	27% 15% 29% 18% 8% 1	330
January	1st to 10th	Night	20% 12% 33% 29% 6%	800
	11th to 20th	Night	18% 12% 29% 33% 8%	800
	21st to end	Night	22% 13% 33% 28% 5%	880
February	1st to 10th	Night	18% 12% 34% 29% 5%	800
	11th to 20th	Night	25% 14% 28% 27% 7%	783
	21st to end	Night	19% 17% 34% 26% 4	660
March	1st to 10th	Night	19% 13% 32% 30% 6%	800
	11th to 20th	Night	15% 10% 31% 31% 13% 1	630
	21st to end	Night	15% 9% 30% 33% 13%	660
April	1st to 10th	Night	20% 12% 26% 28% 14% 1	600
	11th to 20th	Night	15% 8% 30% 30% 17% 1	593
	21st to end	Night	9% 6% 25% 39% 21% 1	600
May	1st to 10th	Night	12% 4 21% 40% 22% 2	600
	11th to 20th	Night	8% 3 18% 40% 27% 4	591
	21st to end	Night	6% 2 12% 36% 37% 7%	650
June	1st to 10th	Night	10% 2 9% 32% 39% 9%	600
	11th to 20th	Night	4 2 6% 30% 44% 15%	600
	21st to end	Night	8% 2 6% 24% 44% 18%	600
July	1st to 10th	Night	7% 2 3 17% 48% 23%	585
	11th to 20th	Night	3 1 2 12% 49% 33%	582
	21st to end	Night	5% 1 4 18% 46% 26%	657
August	1st to 10th	Night	3 2 15% 50% 30%	597
	11th to 20th	Night	6% 1 3 10% 51% 30%	594
	21st to end	Night	9% 2 5% 17% 45% 22%	660
September	1st to 10th	Night	10% 2 6% 21% 43% 19%	600
	11th to 20th	Night	10% 2 8% 28% 42% 10%	695
	21st to end	Night	8% 2 9% 34% 39% 7%	800
October	1st to 10th	Night	10% 1 17% 37% 31% 5%	800
	11th to 20th	Night	9% 4 20% 35% 27% 5%	800
	21st to end	Night	13% 5% 20% 37% 22% 2	880
November	1st to 10th	Night	13% 6% 17% 40% 21% 3	820
	11th to 20th	Night	15% 7% 26% 35% 16% 2	840
	21st to end	Night	13% 8% 30% 33% 14% 1	840
December	1st to 10th	Night	19% 9% 29% 30% 13% 1	840
	11th to 20th	Night	19% 7% 29% 33% 11% 1	829
	21st to end	Night	27% 9% 30% 27% 7%	880

Page generated at 2025-12-15 00:13:50. Version ac07

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