BarC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BarC Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	10% 5% 2 6% 4 5% 5% 4 7% 9% 43%	147328
	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	144406
Transparency	Day	23% 37% 12% 14% 10% 5%	151115
	Night	24% 36% 11% 13% 11% 5%	148696
Seeing	Day	11% 5% 18% 27% 25% 13%	16755
	Night	12% 5% 18% 29% 27% 9%	26577

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	15% 5% 2 6% 4 4 5% 3 5% 6% 43%	5856
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 5% 6% 51%	5880
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 6% 38%	6436
February	1st to 10th	Day	16% 8% 3 7% 4 4 3 3 4 6% 42%	2100
	11th to 20th	Day	15% 7% 2 5% 4 4 4 3 5% 7% 43%	2048
	21st to end	Day	16% 8% 2 7% 5% 5% 4 4 6% 7% 37%	1730
March	1st to 10th	Day	18% 8% 3 8% 4 4 4 4 5% 7% 38%	4580
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 5% 7% 48%	5038
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 6% 43%	6005
April	1st to 10th	Day	11% 6% 2 5% 5% 5% 4 4 6% 9% 42%	2520
	11th to 20th	Day	12% 6% 3 6% 4 4 4 4 6% 8% 44%	2494
	21st to end	Day	7% 6% 1 5% 3 4 5% 4 7% 8% 50%	2520
May	1st to 10th	Day	19% 6% 2 6% 4 4 5% 4 6% 9% 34%	7021
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 5% 8% 9% 46%	7031
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 9% 14% 38%	8071
June	1st to 10th	Day	8% 5% 2 7% 5% 5% 7% 6% 8% 11% 36%	2730
	11th to 20th	Day	5% 4 2 5% 5% 6% 6% 6% 9% 12% 40%	2730
	21st to end	Day	7% 5% 2 6% 5% 6% 7% 7% 10% 14% 31%	2660
July	1st to 10th	Day	4 3 1 6% 5% 6% 7% 7% 10% 15% 37%	7080
	11th to 20th	Day	4 4 2 6% 5% 6% 6% 6% 9% 11% 40%	7080
	21st to end	Day	3 3 2 7% 6% 5% 7% 7% 10% 15% 35%	7687
August	1st to 10th	Day	2 2 1 4 4 4 6% 5% 8% 14% 50%	5545
	11th to 20th	Day	4 3 2 4 4 5% 5% 5% 8% 12% 50%	5486
	21st to end	Day	5% 5% 3 9% 7% 7% 7% 6% 8% 11% 33%	6080
September	1st to 10th	Day	9% 4 2 6% 5% 6% 6% 5% 8% 10% 38%	2520
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 10% 45%	2389
	21st to end	Day	7% 3 2 4 4 5% 4 4 8% 8% 52%	2310
October	1st to 10th	Day	8% 3 1 4 3 3 4 5% 6% 8% 55%	2310
	11th to 20th	Day	5% 3 2 4 3 4 4 3 5% 7% 61%	2072
	21st to end	Day	9% 4 2 4 3 3 4 3 4 6% 59%	2200
November	1st to 10th	Day	9% 4 1 4 3 2 3 4 5% 8% 57%	2032
	11th to 20th	Day	11% 4 2 4 3 3 3 3 5% 6% 57%	1890
	21st to end	Day	11% 5% 2 6% 4 4 5% 3 5% 7% 47%	1890
December	1st to 10th	Day	16% 6% 3 6% 4 4 4 3 5% 6% 43%	3020
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 5% 8% 47%	3463
	21st to end	Day	18% 7% 3 7% 4 4 4 3 4 5% 42%	4824
January	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 6% 7% 44%	7037
	11th to 20th	Night	16% 7% 2 6% 3 3 3 3 4 7% 46%	7080
	21st to end	Night	20% 7% 3 6% 5% 4 5% 3 5% 7% 35%	7745
February	1st to 10th	Night	14% 8% 3 6% 5% 3 4 4 5% 6% 44%	2520
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 5% 6% 40%	2458
	21st to end	Night	15% 8% 2 6% 4 4 5% 4 5% 7% 40%	2076
March	1st to 10th	Night	19% 9% 3 6% 4 3 4 3 5% 6% 37%	5493
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 49%	5184
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	5151
April	1st to 10th	Night	12% 7% 3 6% 4 3 5% 3 5% 6% 46%	2100
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 3 6% 6% 48%	2075
	21st to end	Night	7% 4 1 4 3 4 3 4 6% 8% 55%	2100
May	1st to 10th	Night	18% 7% 3 7% 4 4 4 3 5% 8% 37%	5832
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 6% 10% 43%	5829
	21st to end	Night	5% 4 2 7% 5% 5% 6% 5% 8% 14% 38%	6057
June	1st to 10th	Night	7% 5% 2 5% 5% 5% 6% 5% 8% 10% 40%	1890
	11th to 20th	Night	3 4 1 5% 4 5% 6% 6% 9% 13% 43%	1890
	21st to end	Night	6% 4 2 6% 5% 5% 5% 6% 10% 12% 39%	1950
July	1st to 10th	Night	3 3 1 5% 5% 5% 7% 5% 9% 16% 42%	5878
	11th to 20th	Night	2 3 1 6% 5% 5% 6% 5% 9% 13% 45%	5877
	21st to end	Night	3 3 1 5% 5% 6% 7% 7% 11% 16% 34%	6389
August	1st to 10th	Night	2 2 1 4 3 3 4 4 8% 14% 55%	4604
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 8% 14% 51%	4560
	21st to end	Night	4 4 2 7% 6% 6% 7% 6% 9% 11% 38%	5046
September	1st to 10th	Night	8% 4 2 7% 5% 6% 6% 6% 8% 9% 39%	2100
	11th to 20th	Night	8% 6% 2 5% 3 3 4 5% 6% 10% 47%	2208
	21st to end	Night	6% 4 2 5% 4 4 4 4 6% 9% 53%	2310
October	1st to 10th	Night	8% 4 2 6% 3 4 4 3 4 7% 55%	2310
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 7% 62%	2389
	21st to end	Night	10% 4 2 5% 3 4 3 3 5% 7% 55%	2640
November	1st to 10th	Night	9% 4 2 5% 3 4 3 3 5% 6% 56%	2482
	11th to 20th	Night	11% 4 2 5% 3 4 3 3 5% 5% 55%	2730
	21st to end	Night	10% 5% 1 6% 4 4 4 4 5% 7% 49%	2730
December	1st to 10th	Night	13% 6% 3 7% 4 4 4 3 5% 6% 44%	4380
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 52%	4952
	21st to end	Night	18% 8% 3 7% 4 3 3 3 4 4 43%	6354

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	31% 25% 7% 13% 18% 5%	5857
	11th to 20th	Day	27% 21% 3 15% 24% 9%	5880
	21st to end	Day	38% 25% 5% 10% 12% 10%	6436
February	1st to 10th	Day	19% 59% 3 5% 8% 5%	2100
	11th to 20th	Day	17% 60% 5% 9% 8% 2	2058
	21st to end	Day	17% 60% 4 6% 8% 4	1730
March	1st to 10th	Day	35% 30% 6% 13% 9% 7%	4580
	11th to 20th	Day	24% 30% 6% 12% 16% 11%	5038
	21st to end	Day	31% 30% 9% 13% 9% 7%	6007
April	1st to 10th	Day	18% 60% 4 7% 8% 3	2520
	11th to 20th	Day	15% 60% 4 8% 9% 3	2494
	21st to end	Day	13% 60% 6% 10% 7% 3	2520
May	1st to 10th	Day	37% 26% 13% 13% 8% 3	7056
	11th to 20th	Day	22% 28% 16% 20% 12% 2	7035
	21st to end	Day	27% 30% 17% 19% 6% 1	8085
June	1st to 10th	Day	16% 62% 8% 9% 6%	2730
	11th to 20th	Day	16% 63% 9% 8% 4	2730
	21st to end	Day	16% 62% 11% 9% 3	2660
July	1st to 10th	Day	22% 32% 28% 17% 2	7020
	11th to 20th	Day	25% 31% 35% 9%	7017
	21st to end	Day	24% 31% 22% 22% 1	7517
August	1st to 10th	Day	13% 32% 18% 21% 16% 1	5533
	11th to 20th	Day	12% 32% 24% 25% 6%	5522
	21st to end	Day	31% 33% 12% 15% 8%	6079
September	1st to 10th	Day	16% 58% 10% 9% 5% 2	2520
	11th to 20th	Day	9% 57% 11% 13% 9% 2	2389
	21st to end	Day	11% 57% 8% 16% 6% 2	2310
October	1st to 10th	Day	18% 36% 5% 9% 11% 21%	3700
	11th to 20th	Day	5% 35% 2 13% 19% 25%	3338
	21st to end	Day	10% 37% 3 7% 29% 15%	3538
November	1st to 10th	Day	16% 54% 5% 12% 9% 5%	2032
	11th to 20th	Day	16% 55% 4 9% 11% 5%	1890
	21st to end	Day	17% 55% 5% 9% 8% 6%	1890
December	1st to 10th	Day	31% 35% 5% 8% 17% 4	3020
	11th to 20th	Day	20% 33% 8% 14% 18% 9%	3460
	21st to end	Day	33% 28% 5% 11% 19% 4	4824
January	1st to 10th	Night	32% 23% 9% 12% 17% 7%	7038
	11th to 20th	Night	32% 22% 3 14% 22% 8%	7080
	21st to end	Night	38% 24% 5% 12% 14% 7%	7745
February	1st to 10th	Night	17% 59% 3 7% 10% 4	2520
	11th to 20th	Night	19% 59% 3 8% 8% 3	2459
	21st to end	Night	18% 59% 5% 6% 8% 4	2076
March	1st to 10th	Night	38% 29% 7% 9% 11% 5%	5493
	11th to 20th	Night	25% 31% 6% 11% 14% 13%	5184
	21st to end	Night	28% 30% 8% 15% 12% 7%	5151
April	1st to 10th	Night	18% 60% 4 5% 8% 5%	2100
	11th to 20th	Night	15% 61% 3 10% 9% 3	2075
	21st to end	Night	14% 60% 5% 10% 9% 3	2100
May	1st to 10th	Night	38% 25% 11% 12% 10% 3	5862
	11th to 20th	Night	28% 27% 12% 22% 10% 1	5830
	21st to end	Night	28% 29% 19% 18% 5%	6068
June	1st to 10th	Night	15% 61% 8% 10% 5% 1	1890
	11th to 20th	Night	14% 62% 11% 9% 4	1890
	21st to end	Night	14% 62% 12% 7% 4	1950
July	1st to 10th	Night	20% 29% 32% 17% 3	5830
	11th to 20th	Night	22% 29% 39% 9%	5820
	21st to end	Night	25% 32% 20% 22% 1	6245
August	1st to 10th	Night	12% 31% 20% 22% 14% 2	4592
	11th to 20th	Night	13% 31% 26% 26% 4	4590
	21st to end	Night	26% 34% 16% 17% 6% 1	5046
September	1st to 10th	Night	15% 57% 12% 9% 5% 2	2100
	11th to 20th	Night	11% 56% 10% 11% 9% 2	2208
	21st to end	Night	12% 56% 8% 16% 5% 2	2310
October	1st to 10th	Night	22% 37% 6% 9% 12% 14%	3700
	11th to 20th	Night	6% 35% 3 13% 19% 24%	3862
	21st to end	Night	10% 35% 3 5% 31% 15%	4252
November	1st to 10th	Night	19% 53% 5% 10% 9% 4	2482
	11th to 20th	Night	17% 54% 3 9% 12% 6%	2730
	21st to end	Night	15% 55% 4 10% 10% 5%	2730
December	1st to 10th	Night	30% 35% 4 9% 15% 7%	4380
	11th to 20th	Night	21% 32% 6% 17% 15% 10%	4954
	21st to end	Night	35% 27% 4 11% 17% 5%	6354

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	17% 9% 39% 25% 8% 1	330
	11th to 20th	Day	14% 12% 37% 24% 12% 1	330
	21st to end	Day	20% 10% 36% 28% 6%	363
February	1st to 10th	Day	18% 12% 32% 27% 9% 1	330
	11th to 20th	Day	22% 11% 38% 19% 10% 1	330
	21st to end	Day	13% 15% 46% 22% 4	273
March	1st to 10th	Day	15% 12% 35% 32% 6%	330
	11th to 20th	Day	14% 13% 39% 24% 9% 1	330
	21st to end	Day	16% 9% 29% 32% 14% 1	552
April	1st to 10th	Day	15% 9% 32% 31% 12% 1	540
	11th to 20th	Day	15% 11% 30% 29% 13% 2	532
	21st to end	Day	9% 6% 24% 36% 21% 4	540
May	1st to 10th	Day	15% 5% 25% 33% 18% 4	543
	11th to 20th	Day	10% 4 17% 36% 23% 10%	567
	21st to end	Day	7% 3 14% 34% 32% 10%	617
June	1st to 10th	Day	8% 3 11% 31% 31% 16%	570
	11th to 20th	Day	8% 2 10% 28% 34% 18%	570
	21st to end	Day	8% 1 5% 26% 37% 23%	570
July	1st to 10th	Day	7% 2 3 18% 38% 32%	565
	11th to 20th	Day	9% 2 4 12% 37% 36%	564
	21st to end	Day	6% 1 4 20% 41% 27%	620
August	1st to 10th	Day	4 2 2 16% 43% 33%	566
	11th to 20th	Day	7% 1 2 16% 37% 36%	559
	21st to end	Day	7% 3 6% 20% 38% 26%	627
September	1st to 10th	Day	9% 2 4 22% 40% 23%	570
	11th to 20th	Day	10% 3 7% 22% 38% 20%	570
	21st to end	Day	7% 2 8% 30% 36% 16%	570
October	1st to 10th	Day	9% 3 15% 35% 30% 9%	570
	11th to 20th	Day	6% 2 18% 33% 29% 13%	381
	21st to end	Day	14% 6% 19% 34% 21% 7%	363
November	1st to 10th	Day	13% 4 19% 34% 25% 4	330
	11th to 20th	Day	16% 5% 24% 32% 18% 5%	330
	21st to end	Day	13% 5% 34% 28% 16% 3	330
December	1st to 10th	Day	19% 6% 29% 32% 12% 2	330
	11th to 20th	Day	18% 7% 30% 31% 12% 1	330
	21st to end	Day	21% 10% 28% 23% 16% 3	363
January	1st to 10th	Night	18% 8% 32% 33% 9%	840
	11th to 20th	Night	15% 8% 30% 35% 11% 1	840
	21st to end	Night	22% 8% 31% 34% 6%	924
February	1st to 10th	Night	16% 7% 31% 35% 10%	840
	11th to 20th	Night	23% 9% 29% 28% 11%	823
	21st to end	Night	18% 10% 33% 32% 7%	692
March	1st to 10th	Night	17% 9% 29% 33% 11% 1	840
	11th to 20th	Night	14% 5% 29% 32% 18% 2	672
	21st to end	Night	14% 5% 25% 37% 19% 1	693
April	1st to 10th	Night	14% 7% 26% 35% 16% 1	630
	11th to 20th	Night	13% 5% 28% 30% 22% 2	623
	21st to end	Night	9% 4 20% 39% 26% 2	630
May	1st to 10th	Night	12% 3 16% 37% 28% 3	630
	11th to 20th	Night	7% 3 14% 38% 32% 6%	621
	21st to end	Night	5% 3 8% 32% 41% 11%	683
June	1st to 10th	Night	9% 2 6% 27% 44% 13%	630
	11th to 20th	Night	3 2 5% 22% 51% 17%	630
	21st to end	Night	6% 1 4 20% 45% 23%	630
July	1st to 10th	Night	6% 2 2 12% 51% 27%	615
	11th to 20th	Night	3 1 2 7% 47% 41%	612
	21st to end	Night	5% 2 3 13% 50% 28%	690
August	1st to 10th	Night	3 1 2 10% 47% 37%	627
	11th to 20th	Night	5% 3 9% 47% 36%	624
	21st to end	Night	8% 2 4 15% 44% 26%	693
September	1st to 10th	Night	9% 1 5% 17% 43% 25%	630
	11th to 20th	Night	9% 2 6% 22% 47% 14%	738
	21st to end	Night	8% 1 8% 30% 43% 10%	840
October	1st to 10th	Night	9% 3 13% 36% 34% 6%	840
	11th to 20th	Night	7% 4 13% 35% 32% 8%	813
	21st to end	Night	12% 4 17% 37% 28% 3	880
November	1st to 10th	Night	10% 5% 15% 39% 26% 4	820
	11th to 20th	Night	13% 5% 23% 35% 22% 2	840
	21st to end	Night	11% 6% 26% 35% 19% 2	840
December	1st to 10th	Night	16% 7% 27% 30% 18% 2	840
	11th to 20th	Night	16% 7% 26% 34% 15% 2	840
	21st to end	Night	24% 8% 26% 30% 11% 1	924

Page generated at 2026-10-11 04:06:32. Version ac07

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