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 2025-06-10.

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% 4 4 5% 4 7% 9% 43%	119150
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	119075
Transparency	Day	21% 42% 9% 12% 11% 5%	122976
	Night	22% 41% 8% 12% 12% 5%	123390
Seeing	Day	12% 5% 19% 27% 25% 13%	14953
	Night	12% 5% 18% 29% 27% 9%	24899

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	16% 4 2 5% 3 4 4 3 5% 7% 47%	4516
	11th to 20th	Day	14% 6% 3 5% 3 3 4 2 4 5% 50%	4520
	21st to end	Day	20% 8% 2 6% 3 3 4 4 5% 6% 37%	4972
February	1st to 10th	Day	16% 8% 3 7% 4 5% 3 3 4 6% 41%	2000
	11th to 20th	Day	16% 7% 2 5% 3 4 4 3 5% 7% 43%	1948
	21st to end	Day	16% 8% 2 7% 6% 5% 4 4 6% 7% 36%	1650
March	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 38%	4480
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 5% 7% 48%	4929
	21st to end	Day	15% 6% 3 6% 5% 4 4 3 5% 6% 42%	5875
April	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 4 6% 9% 43%	2400
	11th to 20th	Day	12% 6% 3 6% 4 4 4 4 6% 8% 43%	2374
	21st to end	Day	7% 5% 2 5% 3 4 5% 4 6% 8% 50%	2400
May	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 10% 35%	5405
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 47%	5404
	21st to end	Day	6% 4 2 5% 5% 4 4 4 8% 13% 45%	6197
June	1st to 10th	Day	8% 5% 2 7% 5% 5% 7% 6% 8% 11% 37%	2587
	11th to 20th	Day	4 4 1 5% 5% 6% 6% 6% 9% 12% 41%	2470
	21st to end	Day	8% 6% 2 6% 5% 6% 7% 7% 10% 13% 30%	2407
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 8% 7% 10% 14% 27%	3800
	11th to 20th	Day	5% 5% 2 7% 5% 6% 6% 6% 9% 11% 40%	3800
	21st to end	Day	4 4 2 8% 7% 6% 8% 7% 11% 15% 26%	4118
August	1st to 10th	Day	3 2 1 4 4 4 5% 5% 8% 13% 49%	3785
	11th to 20th	Day	5% 4 2 4 4 5% 6% 6% 8% 12% 43%	3729
	21st to end	Day	6% 6% 3 9% 6% 6% 7% 5% 8% 9% 35%	4180
September	1st to 10th	Day	10% 4 2 6% 5% 6% 6% 5% 8% 10% 37%	2280
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 9% 42%	2161
	21st to end	Day	8% 3 2 4 4 5% 5% 4 7% 8% 51%	2090
October	1st to 10th	Day	6% 2 1 4 3 3 4 5% 6% 8% 58%	2090
	11th to 20th	Day	5% 3 2 5% 3 4 4 3 5% 7% 59%	1938
	21st to end	Day	9% 4 2 4 3 4 4 3 4 6% 58%	2090
November	1st to 10th	Day	9% 4 1 4 3 2 3 4 5% 8% 56%	1933
	11th to 20th	Day	12% 4 2 4 4 3 2 3 4 5% 58%	1800
	21st to end	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	1800
December	1st to 10th	Day	16% 6% 3 6% 4 4 4 3 5% 6% 43%	2930
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 5% 8% 47%	3373
	21st to end	Day	18% 7% 3 7% 4 4 4 3 4 5% 42%	4719
January	1st to 10th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 48%	5429
	11th to 20th	Night	18% 8% 3 6% 3 2 3 2 4 6% 46%	5440
	21st to end	Night	21% 7% 2 5% 4 4 4 3 5% 7% 38%	5984
February	1st to 10th	Night	14% 8% 3 6% 5% 3 4 4 5% 6% 43%	2400
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 5% 6% 41%	2338
	21st to end	Night	16% 8% 2 7% 4 4 5% 4 5% 7% 39%	1980
March	1st to 10th	Night	19% 9% 3 6% 4 3 4 3 5% 6% 37%	5373
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 49%	5072
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 47%	5039
April	1st to 10th	Night	13% 7% 3 6% 4 3 5% 3 5% 6% 46%	2000
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 3 6% 6% 47%	1975
	21st to end	Night	7% 4 1 4 3 4 3 4 6% 8% 56%	2000
May	1st to 10th	Night	13% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	4490
	11th to 20th	Night	11% 5% 2 7% 4 4 5% 4 6% 9% 43%	4489
	21st to end	Night	4 4 1 6% 4 4 5% 5% 7% 14% 46%	4648
June	1st to 10th	Night	7% 5% 2 5% 5% 5% 6% 5% 8% 10% 41%	1797
	11th to 20th	Night	3 4 1 5% 4 5% 6% 6% 9% 13% 44%	1710
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 6% 10% 12% 38%	1764
July	1st to 10th	Night	4 5% 2 5% 5% 6% 6% 6% 9% 15% 36%	3160
	11th to 20th	Night	2 3 1 6% 5% 5% 5% 5% 8% 13% 47%	3160
	21st to end	Night	4 5% 2 6% 6% 6% 7% 7% 11% 16% 29%	3435
August	1st to 10th	Night	2 2 1 3 3 3 4 5% 7% 13% 57%	3144
	11th to 20th	Night	3 3 2 5% 4 5% 6% 6% 9% 15% 43%	3100
	21st to end	Night	5% 4 2 7% 6% 6% 7% 5% 8% 10% 39%	3476
September	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 6% 8% 9% 38%	1900
	11th to 20th	Night	9% 6% 3 6% 4 3 5% 5% 7% 10% 43%	1998
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 9% 52%	2090
October	1st to 10th	Night	6% 3 2 6% 3 3 4 4 4 6% 58%	2090
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 7% 61%	2236
	21st to end	Night	10% 5% 2 5% 3 4 3 3 5% 6% 56%	2508
November	1st to 10th	Night	9% 4 2 5% 3 4 3 3 5% 6% 55%	2361
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 5% 6% 54%	2600
	21st to end	Night	10% 5% 2 6% 4 5% 4 4 5% 7% 49%	2600
December	1st to 10th	Night	12% 6% 3 7% 4 4 4 3 5% 6% 44%	4250
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 6% 52%	4822
	21st to end	Night	18% 8% 3 7% 4 3 3 3 4 4 43%	6217

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	26% 28% 5% 14% 20% 7%	4517
	11th to 20th	Day	28% 27% 3 12% 24% 7%	4520
	21st to end	Day	35% 31% 5% 11% 11% 8%	4972
February	1st to 10th	Day	20% 62% 3 5% 7% 4	2000
	11th to 20th	Day	16% 62% 4 8% 8% 2	1958
	21st to end	Day	17% 63% 4 5% 7% 4	1650
March	1st to 10th	Day	35% 31% 6% 13% 9% 7%	4480
	11th to 20th	Day	24% 31% 6% 12% 16% 12%	4929
	21st to end	Day	31% 31% 9% 13% 9% 7%	5877
April	1st to 10th	Day	16% 63% 4 7% 8% 4	2400
	11th to 20th	Day	15% 63% 4 7% 8% 3	2374
	21st to end	Day	12% 63% 5% 9% 7% 4	2400
May	1st to 10th	Day	31% 33% 13% 15% 6% 2	5440
	11th to 20th	Day	21% 33% 14% 20% 11% 1	5408
	21st to end	Day	20% 32% 15% 24% 8% 1	6211
June	1st to 10th	Day	13% 64% 8% 9% 6%	2587
	11th to 20th	Day	14% 69% 7% 7% 4	2470
	21st to end	Day	15% 67% 8% 6% 3	2407
July	1st to 10th	Day	23% 45% 20% 11% 1	3740
	11th to 20th	Day	19% 44% 32% 4	3737
	21st to end	Day	26% 46% 17% 9% 2	3987
August	1st to 10th	Day	9% 39% 11% 22% 18% 1	3773
	11th to 20th	Day	12% 43% 15% 22% 7%	3765
	21st to end	Day	27% 43% 9% 11% 9% 1	4179
September	1st to 10th	Day	16% 63% 8% 6% 4 2	2280
	11th to 20th	Day	9% 62% 10% 9% 8% 2	2161
	21st to end	Day	11% 63% 6% 13% 6% 2	2090
October	1st to 10th	Day	16% 38% 5% 8% 11% 22%	3480
	11th to 20th	Day	5% 37% 2 13% 18% 24%	3204
	21st to end	Day	9% 38% 3 6% 29% 15%	3428
November	1st to 10th	Day	16% 56% 5% 12% 8% 3	1933
	11th to 20th	Day	17% 57% 3 8% 11% 5%	1800
	21st to end	Day	17% 57% 4 9% 8% 6%	1800
December	1st to 10th	Day	30% 36% 5% 8% 17% 4	2930
	11th to 20th	Day	20% 33% 8% 13% 17% 9%	3370
	21st to end	Day	33% 28% 5% 11% 19% 4	4719
January	1st to 10th	Night	29% 27% 4 13% 19% 7%	5429
	11th to 20th	Night	32% 27% 3 11% 23% 4	5440
	21st to end	Night	34% 30% 5% 11% 13% 7%	5984
February	1st to 10th	Night	17% 62% 3 7% 8% 3	2400
	11th to 20th	Night	18% 62% 2 8% 8% 3	2339
	21st to end	Night	18% 62% 4 6% 7% 4	1980
March	1st to 10th	Night	37% 30% 7% 9% 11% 5%	5373
	11th to 20th	Night	25% 31% 6% 10% 14% 13%	5072
	21st to end	Night	28% 31% 8% 15% 12% 7%	5039
April	1st to 10th	Night	16% 62% 4 5% 8% 5%	2000
	11th to 20th	Night	14% 64% 3 8% 8% 3	1975
	21st to end	Night	13% 62% 4 10% 8% 3	2000
May	1st to 10th	Night	33% 32% 12% 13% 9% 2	4520
	11th to 20th	Night	28% 31% 12% 20% 9%	4490
	21st to end	Night	19% 32% 19% 23% 7% 1	4656
June	1st to 10th	Night	13% 64% 8% 10% 6% 1	1797
	11th to 20th	Night	12% 67% 7% 9% 4	1710
	21st to end	Night	13% 67% 10% 5% 4	1764
July	1st to 10th	Night	18% 43% 26% 11% 2	3110
	11th to 20th	Night	15% 42% 36% 5% 1	3100
	21st to end	Night	24% 48% 14% 12% 2	3325
August	1st to 10th	Night	9% 39% 12% 22% 16% 1	3132
	11th to 20th	Night	13% 42% 18% 22% 5%	3130
	21st to end	Night	22% 44% 11% 14% 7% 2	3476
September	1st to 10th	Night	15% 62% 9% 7% 5% 2	1900
	11th to 20th	Night	11% 61% 9% 8% 8% 3	1998
	21st to end	Night	12% 61% 7% 13% 5% 2	2090
October	1st to 10th	Night	20% 39% 6% 8% 13% 15%	3480
	11th to 20th	Night	6% 37% 3 13% 19% 23%	3709
	21st to end	Night	9% 36% 3 5% 31% 16%	4120
November	1st to 10th	Night	19% 55% 5% 9% 8% 3	2361
	11th to 20th	Night	18% 57% 3 7% 11% 5%	2600
	21st to end	Night	15% 57% 3 10% 10% 5%	2600
December	1st to 10th	Night	29% 36% 4 9% 15% 7%	4250
	11th to 20th	Night	20% 33% 6% 17% 15% 9%	4824
	21st to end	Night	35% 28% 4 11% 18% 5%	6217

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	18% 10% 37% 26% 8% 1	300
	11th to 20th	Day	16% 12% 35% 24% 12% 1	300
	21st to end	Day	19% 10% 38% 27% 6%	330
February	1st to 10th	Day	20% 13% 29% 26% 10% 1	300
	11th to 20th	Day	23% 10% 38% 18% 10% 1	300
	21st to end	Day	14% 14% 47% 21% 4	249
March	1st to 10th	Day	15% 13% 35% 31% 6%	300
	11th to 20th	Day	15% 13% 39% 23% 9% 1	300
	21st to end	Day	16% 9% 30% 30% 13% 2	510
April	1st to 10th	Day	16% 8% 33% 30% 12% 2	500
	11th to 20th	Day	16% 11% 30% 27% 13% 3	492
	21st to end	Day	9% 6% 22% 37% 21% 4	500
May	1st to 10th	Day	13% 4 25% 34% 19% 4	503
	11th to 20th	Day	10% 4 17% 35% 24% 10%	527
	21st to end	Day	7% 3 12% 34% 33% 11%	573
June	1st to 10th	Day	8% 2 10% 31% 32% 17%	526
	11th to 20th	Day	8% 2 10% 27% 36% 19%	490
	21st to end	Day	9% 2 6% 27% 34% 23%	490
July	1st to 10th	Day	9% 2 3 19% 36% 32%	485
	11th to 20th	Day	10% 2 4 12% 37% 36%	484
	21st to end	Day	7% 1 4 19% 41% 29%	532
August	1st to 10th	Day	5% 1 2 14% 43% 34%	486
	11th to 20th	Day	8% 1 3 16% 35% 37%	479
	21st to end	Day	8% 3 6% 18% 38% 27%	539
September	1st to 10th	Day	11% 2 4 18% 40% 25%	490
	11th to 20th	Day	12% 3 8% 21% 38% 19%	490
	21st to end	Day	8% 2 9% 30% 34% 17%	490
October	1st to 10th	Day	6% 2 14% 36% 31% 10%	490
	11th to 20th	Day	6% 2 18% 33% 28% 12%	338
	21st to end	Day	14% 5% 18% 34% 22% 7%	330
November	1st to 10th	Day	14% 2 19% 34% 26% 4	300
	11th to 20th	Day	18% 5% 25% 31% 16% 5%	300
	21st to end	Day	14% 5% 31% 29% 17% 3	300
December	1st to 10th	Day	17% 6% 28% 33% 13% 3	300
	11th to 20th	Day	20% 8% 30% 31% 11% 1	300
	21st to end	Day	22% 9% 29% 23% 15% 2	330
January	1st to 10th	Night	18% 8% 32% 32% 9%	800
	11th to 20th	Night	15% 8% 30% 35% 11% 1	800
	21st to end	Night	22% 8% 31% 34% 6%	880
February	1st to 10th	Night	17% 7% 31% 34% 10%	800
	11th to 20th	Night	23% 9% 30% 27% 11%	783
	21st to end	Night	18% 10% 33% 32% 7%	660
March	1st to 10th	Night	16% 9% 30% 33% 11% 1	800
	11th to 20th	Night	14% 5% 29% 33% 18% 1	640
	21st to end	Night	14% 5% 25% 37% 18% 1	660
April	1st to 10th	Night	14% 7% 27% 35% 16% 1	600
	11th to 20th	Night	13% 5% 30% 29% 21% 2	593
	21st to end	Night	9% 4 20% 38% 27% 2	600
May	1st to 10th	Night	11% 3 16% 38% 29% 4	600
	11th to 20th	Night	6% 3 14% 38% 32% 6%	591
	21st to end	Night	5% 3 8% 32% 41% 11%	650
June	1st to 10th	Night	9% 1 5% 27% 44% 13%	599
	11th to 20th	Night	3 2 5% 22% 51% 18%	570
	21st to end	Night	7% 2 4 20% 44% 24%	570
July	1st to 10th	Night	7% 2 2 12% 49% 28%	555
	11th to 20th	Night	3 1 2 7% 46% 41%	552
	21st to end	Night	5% 1 2 13% 49% 29%	624
August	1st to 10th	Night	3 1 2 10% 47% 37%	567
	11th to 20th	Night	5% 3 9% 46% 36%	564
	21st to end	Night	9% 2 4 15% 44% 27%	627
September	1st to 10th	Night	10% 1 5% 16% 42% 26%	570
	11th to 20th	Night	10% 2 6% 23% 45% 13%	668
	21st to end	Night	9% 1 8% 30% 42% 10%	760
October	1st to 10th	Night	8% 2 12% 37% 35% 6%	760
	11th to 20th	Night	8% 5% 13% 36% 31% 8%	760
	21st to end	Night	12% 3 16% 36% 29% 4	836
November	1st to 10th	Night	11% 5% 15% 39% 25% 4	780
	11th to 20th	Night	14% 5% 24% 35% 21% 2	800
	21st to end	Night	12% 6% 25% 35% 20% 2	800
December	1st to 10th	Night	15% 7% 27% 31% 18% 2	800
	11th to 20th	Night	16% 7% 26% 34% 15% 2	800
	21st to end	Night	25% 7% 27% 30% 11%	880

Page generated at 2025-06-07 09:28:13. Version ac07

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