CBA Concord Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CBA Concord Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	5% 3 1 3 3 3 3 3 4 7% 66%	157592
	Night	6% 4 1 4 3 3 3 3 5% 9% 58%	151347
Transparency	Day	19% 4 9% 19% 26% 24%	152523
	Night	23% 5% 10% 20% 22% 20%	147417
Seeing	Day	8% 11% 29% 33% 16% 3	17854
	Night	8% 6% 21% 37% 23% 4	27133

By Month

Percentage Cloud Cover SW (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	12% 7% 3 6% 4 4 4 4 6% 8% 41%	4962
	11th to 20th	Day	7% 5% 3 6% 4 3 3 2 4 6% 57%	5340
	21st to end	Day	12% 7% 3 8% 5% 4 4 4 6% 7% 40%	5843
February	1st to 10th	Day	13% 8% 2 5% 4 3 4 4 5% 6% 45%	1890
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	1942
	21st to end	Day	12% 5% 2 6% 5% 5% 4 3 6% 7% 47%	1730
March	1st to 10th	Day	9% 8% 2 8% 6% 5% 5% 4 6% 8% 38%	4948
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 51%	5110
	21st to end	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 10% 40%	5710
April	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 5% 8% 47%	2520
	11th to 20th	Day	8% 4 2 4 3 3 4 4 5% 7% 56%	2489
	21st to end	Day	4 5% 1 4 3 3 3 3 5% 9% 61%	2520
May	1st to 10th	Day	4 3 1 3 2 2 2 2 3 5% 73%	7281
	11th to 20th	Day	1 1 1 1 1 1 1 3 6% 82%	7837
	21st to end	Day	1 1 2 1 1 2 2 3 8% 78%	8992
June	1st to 10th	Day	3 3 1 3 2 3 3 3 5% 9% 66%	2940
	11th to 20th	Day	1 1 1 1 1 2 2 5% 7% 79%	2940
	21st to end	Day	1 1 1 2 1 1 2 2 3 6% 81%	2940
July	1st to 10th	Day	1 1 1 1 1 2 5% 88%	8219
	11th to 20th	Day	2 1 1 2 2 2 2 1 2 5% 79%	7789
	21st to end	Day	1 1 1 1 2 3 6% 84%	8304
August	1st to 10th	Day	1 1 1 1 2 2 3 5% 85%	7547
	11th to 20th	Day	1 1 1 2 2 4 7% 80%	7193
	21st to end	Day	2 1 1 2 1 2 2 2 4 8% 76%	7750
September	1st to 10th	Day	1 1 1 2 1 1 2 2 4 6% 81%	2520
	11th to 20th	Day	2 1 1 2 2 2 3 3 5% 8% 69%	2520
	21st to end	Day	2 2 1 2 2 1 2 1 3 6% 77%	2212
October	1st to 10th	Day	1 2 1 3 2 2 3 2 4 6% 74%	3280
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 7% 72%	3261
	21st to end	Day	4 2 1 4 3 3 3 3 6% 7% 63%	3558
November	1st to 10th	Day	6% 5% 2 4 3 3 4 3 6% 7% 58%	1997
	11th to 20th	Day	8% 5% 3 6% 5% 5% 5% 4 5% 7% 46%	1890
	21st to end	Day	10% 5% 2 5% 3 3 3 4 6% 6% 53%	1718
December	1st to 10th	Day	7% 4 2 3 3 3 4 3 6% 11% 55%	2700
	11th to 20th	Day	17% 9% 3 8% 6% 6% 6% 4 7% 8% 25%	3108
	21st to end	Day	15% 10% 3 9% 6% 6% 6% 4 5% 8% 29%	4092
January	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 6% 8% 45%	7928
	11th to 20th	Night	8% 5% 2 6% 3 3 3 3 5% 7% 56%	7650
	21st to end	Night	11% 8% 3 7% 4 4 4 3 6% 8% 42%	8371
February	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 4 5% 50%	2730
	11th to 20th	Night	11% 6% 2 6% 4 5% 4 4 5% 9% 45%	2563
	21st to end	Night	9% 6% 1 4 4 4 4 4 5% 8% 52%	2076
March	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 7% 8% 40%	5149
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 8% 51%	5110
	21st to end	Night	11% 7% 2 6% 4 4 4 4 6% 8% 44%	5505
April	1st to 10th	Night	6% 5% 2 6% 4 5% 3 4 6% 8% 51%	2100
	11th to 20th	Night	5% 5% 2 5% 4 3 4 3 5% 9% 56%	2079
	21st to end	Night	3 3 1 3 3 3 3 3 6% 10% 61%	2100
May	1st to 10th	Night	3 3 1 3 2 3 3 2 4 10% 66%	5681
	11th to 20th	Night	1 1 1 1 2 2 2 4 8% 76%	5022
	21st to end	Night	1 1 1 1 2 2 2 4 10% 75%	5181
June	1st to 10th	Night	3 2 1 4 3 3 3 4 6% 10% 63%	1680
	11th to 20th	Night	1 1 2 1 2 2 3 4 7% 78%	1680
	21st to end	Night	1 1 1 2 1 2 2 2 5% 7% 77%	1680
July	1st to 10th	Night	1 1 1 1 1 2 11% 82%	4740
	11th to 20th	Night	2 1 1 3 1 2 2 1 3 12% 72%	5176
	21st to end	Night	1 1 1 1 1 2 2 5% 18% 67%	5806
August	1st to 10th	Night	1 1 1 1 1 2 1 4 11% 77%	5275
	11th to 20th	Night	1 1 1 2 3 3 6% 12% 71%	5669
	21st to end	Night	1 1 1 2 2 2 2 3 5% 11% 70%	6499
September	1st to 10th	Night	1 1 1 2 3 3 4 10% 75%	2100
	11th to 20th	Night	2 2 1 2 2 2 4 4 7% 12% 63%	2100
	21st to end	Night	1 2 1 3 2 2 2 2 4 8% 74%	2359
October	1st to 10th	Night	2 1 1 3 2 2 2 2 4 6% 74%	3920
	11th to 20th	Night	3 3 1 3 2 2 3 3 6% 13% 61%	3897
	21st to end	Night	3 3 1 3 3 3 4 4 5% 7% 63%	4253
November	1st to 10th	Night	5% 4 1 4 4 3 3 3 6% 9% 58%	2518
	11th to 20th	Night	9% 5% 2 5% 3 3 4 4 7% 8% 51%	2730
	21st to end	Night	8% 5% 1 4 4 3 4 3 5% 8% 56%	2898
December	1st to 10th	Night	7% 4 2 4 3 3 4 4 4 7% 59%	4700
	11th to 20th	Night	16% 8% 3 7% 6% 5% 5% 4 8% 8% 30%	5316
	21st to end	Night	17% 8% 3 9% 5% 5% 5% 4 6% 6% 33%	7106

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	35% 11% 16% 22% 13% 3	4963
	11th to 20th	Day	30% 4 7% 16% 27% 17%	5340
	21st to end	Day	41% 6% 8% 18% 15% 12%	5843
February	1st to 10th	Day	39% 5% 9% 20% 20% 8%	1890
	11th to 20th	Day	39% 7% 11% 18% 20% 5%	1951
	21st to end	Day	36% 5% 11% 23% 19% 7%	1730
March	1st to 10th	Day	41% 9% 9% 19% 20% 2	4948
	11th to 20th	Day	28% 7% 16% 27% 13% 9%	5110
	21st to end	Day	34% 9% 18% 26% 11% 2	5710
April	1st to 10th	Day	36% 5% 13% 20% 21% 5%	2520
	11th to 20th	Day	26% 6% 14% 23% 22% 8%	2492
	21st to end	Day	20% 6% 13% 30% 26% 5%	2520
May	1st to 10th	Day	15% 3 6% 24% 35% 17%	7281
	11th to 20th	Day	6% 2 6% 26% 43% 18%	7840
	21st to end	Day	8% 3 10% 27% 43% 9%	8994
June	1st to 10th	Day	15% 5% 12% 24% 31% 13%	2940
	11th to 20th	Day	6% 3 9% 21% 35% 26%	2940
	21st to end	Day	7% 2 6% 15% 31% 40%	2940
July	1st to 10th	Day	3 1 2 7% 24% 63%	8151
	11th to 20th	Day	11% 1 4 6% 9% 69%	7717
	21st to end	Day	4 2 5% 13% 26% 51%	8119
August	1st to 10th	Day	6% 1 4 9% 26% 54%	5976
	11th to 20th	Day	5% 2 7% 20% 34% 31%	5675
	21st to end	Day	11% 2 6% 13% 37% 31%	6077
September	1st to 10th	Day	6% 2 6% 12% 30% 43%	2520
	11th to 20th	Day	13% 3 10% 21% 35% 18%	2520
	21st to end	Day	10% 3 7% 18% 36% 25%	2212
October	1st to 10th	Day	12% 3 6% 11% 27% 41%	3280
	11th to 20th	Day	13% 4 8% 24% 31% 19%	3261
	21st to end	Day	18% 4 10% 22% 32% 14%	3558
November	1st to 10th	Day	25% 5% 11% 26% 22% 11%	1997
	11th to 20th	Day	35% 6% 11% 23% 20% 5%	1890
	21st to end	Day	30% 5% 10% 21% 25% 9%	1718
December	1st to 10th	Day	24% 5% 9% 30% 26% 5%	2700
	11th to 20th	Day	49% 11% 10% 14% 13% 3	3108
	21st to end	Day	50% 9% 14% 16% 9% 2	4092
January	1st to 10th	Night	35% 9% 19% 21% 14% 3	7928
	11th to 20th	Night	28% 5% 8% 14% 28% 16%	7650
	21st to end	Night	38% 6% 9% 17% 19% 12%	8371
February	1st to 10th	Night	36% 5% 8% 19% 21% 11%	2730
	11th to 20th	Night	36% 5% 12% 20% 19% 8%	2565
	21st to end	Night	31% 5% 13% 27% 19% 6%	2076
March	1st to 10th	Night	38% 10% 11% 19% 19% 4	5149
	11th to 20th	Night	30% 6% 15% 23% 16% 9%	5110
	21st to end	Night	35% 9% 17% 23% 13% 3	5505
April	1st to 10th	Night	30% 6% 16% 21% 21% 7%	2100
	11th to 20th	Night	25% 7% 14% 26% 18% 10%	2079
	21st to end	Night	18% 6% 14% 31% 25% 7%	2100
May	1st to 10th	Night	16% 4 10% 24% 29% 17%	5679
	11th to 20th	Night	8% 2 12% 27% 31% 20%	5022
	21st to end	Night	7% 4 13% 26% 37% 12%	5182
June	1st to 10th	Night	15% 6% 14% 24% 27% 14%	1680
	11th to 20th	Night	7% 3 10% 19% 33% 29%	1680
	21st to end	Night	7% 1 7% 22% 26% 37%	1680
July	1st to 10th	Night	3 1 2 10% 26% 58%	4700
	11th to 20th	Night	11% 1 3 7% 17% 60%	5125
	21st to end	Night	5% 2 7% 16% 27% 42%	5679
August	1st to 10th	Night	7% 1 5% 10% 20% 57%	4169
	11th to 20th	Night	8% 2 10% 19% 29% 33%	4469
	21st to end	Night	10% 2 6% 19% 33% 30%	5092
September	1st to 10th	Night	6% 3 7% 17% 24% 44%	2100
	11th to 20th	Night	14% 3 11% 26% 27% 19%	2100
	21st to end	Night	11% 3 8% 21% 29% 29%	2359
October	1st to 10th	Night	12% 3 6% 13% 17% 49%	3920
	11th to 20th	Night	14% 5% 13% 24% 21% 23%	3897
	21st to end	Night	19% 4 8% 22% 27% 21%	4253
November	1st to 10th	Night	23% 5% 12% 26% 19% 14%	2518
	11th to 20th	Night	30% 5% 12% 25% 20% 7%	2730
	21st to end	Night	27% 5% 11% 20% 25% 11%	2898
December	1st to 10th	Night	25% 5% 8% 25% 26% 10%	4700
	11th to 20th	Night	48% 7% 11% 15% 16% 3	5316
	21st to end	Night	50% 8% 15% 15% 9% 3	7106

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% 14% 32% 18% 12% 2	304
	11th to 20th	Day	14% 13% 21% 35% 15% 2	430
	21st to end	Day	13% 11% 27% 29% 17% 2	473
February	1st to 10th	Day	19% 12% 22% 29% 17% 2	430
	11th to 20th	Day	18% 20% 26% 22% 13% 1	477
	21st to end	Day	13% 14% 29% 31% 11% 2	446
March	1st to 10th	Day	18% 19% 29% 23% 9% 2	540
	11th to 20th	Day	19% 16% 25% 22% 16% 3	540
	21st to end	Day	14% 19% 31% 24% 11% 1	594
April	1st to 10th	Day	13% 17% 33% 23% 12% 3	540
	11th to 20th	Day	12% 18% 33% 24% 9% 4	532
	21st to end	Day	7% 19% 31% 26% 13% 4	540
May	1st to 10th	Day	8% 17% 32% 31% 11% 1	543
	11th to 20th	Day	8% 14% 36% 29% 13% 1	568
	21st to end	Day	4 13% 35% 34% 12% 2	617
June	1st to 10th	Day	4 12% 36% 33% 12% 2	570
	11th to 20th	Day	3 11% 38% 37% 10% 1	570
	21st to end	Day	2 8% 36% 40% 13% 2	570
July	1st to 10th	Day	1 6% 34% 44% 14% 1	566
	11th to 20th	Day	1 3 29% 49% 16% 2	564
	21st to end	Day	1 2 33% 46% 14% 3	619
August	1st to 10th	Day	2 5% 28% 45% 16% 3	567
	11th to 20th	Day	1 3 32% 43% 17% 4	557
	21st to end	Day	1 4 25% 45% 21% 3	627
September	1st to 10th	Day	2 5% 24% 43% 21% 6%	570
	11th to 20th	Day	4 10% 29% 37% 15% 6%	570
	21st to end	Day	3 8% 25% 34% 25% 4	563
October	1st to 10th	Day	3 8% 28% 30% 25% 6%	530
	11th to 20th	Day	6% 11% 21% 31% 24% 8%	530
	21st to end	Day	8% 9% 23% 30% 24% 5%	583
November	1st to 10th	Day	8% 12% 20% 26% 28% 6%	473
	11th to 20th	Day	8% 12% 20% 33% 21% 6%	330
	21st to end	Day	10% 7% 29% 33% 18% 2	239
December	1st to 10th	Day	11% 14% 24% 30% 18% 4	220
	11th to 20th	Day	17% 14% 22% 25% 20% 1	220
	21st to end	Day	21% 14% 29% 24% 12% 1	242
January	1st to 10th	Night	17% 7% 16% 33% 21% 5%	840
	11th to 20th	Night	13% 6% 16% 37% 24% 4	840
	21st to end	Night	15% 6% 15% 38% 22% 4	924
February	1st to 10th	Night	16% 9% 15% 36% 21% 3	840
	11th to 20th	Night	13% 11% 20% 33% 19% 4	824
	21st to end	Night	11% 9% 21% 37% 19% 2	692
March	1st to 10th	Night	17% 9% 23% 29% 19% 2	840
	11th to 20th	Night	15% 10% 20% 34% 19% 3	840
	21st to end	Night	12% 10% 25% 36% 15% 2	924
April	1st to 10th	Night	9% 9% 26% 37% 17% 2	840
	11th to 20th	Night	8% 13% 28% 32% 17% 3	831
	21st to end	Night	4 9% 31% 35% 18% 3	840
May	1st to 10th	Night	5% 8% 29% 40% 17% 2	756
	11th to 20th	Night	3 6% 25% 42% 23% 1	530
	21st to end	Night	3 9% 33% 38% 15% 2	455
June	1st to 10th	Night	3 5% 33% 44% 14% 1	420
	11th to 20th	Night	1 7% 27% 50% 14% 1	420
	21st to end	Night	5% 28% 44% 22% 1	420
July	1st to 10th	Night	1 2 27% 46% 24%	410
	11th to 20th	Night	1 1 22% 48% 25% 3	555
	21st to end	Night	1 2 23% 42% 28% 3	690
August	1st to 10th	Night	2 4 24% 39% 27% 4	627
	11th to 20th	Night	1 3 26% 44% 24% 3	755
	21st to end	Night	1 1 20% 47% 28% 4	924
September	1st to 10th	Night	1 3 18% 42% 31% 6%	840
	11th to 20th	Night	4 5% 21% 40% 27% 3	840
	21st to end	Night	2 4 19% 36% 32% 6%	832
October	1st to 10th	Night	4 5% 16% 38% 31% 6%	800
	11th to 20th	Night	5% 6% 18% 34% 31% 6%	800
	21st to end	Night	6% 5% 15% 33% 30% 10%	880
November	1st to 10th	Night	6% 5% 17% 31% 33% 7%	820
	11th to 20th	Night	11% 4 18% 35% 27% 6%	840
	21st to end	Night	10% 6% 15% 37% 26% 6%	840
December	1st to 10th	Night	12% 8% 15% 36% 24% 5%	840
	11th to 20th	Night	20% 8% 13% 35% 22% 3	840
	21st to end	Night	19% 10% 21% 30% 18% 1	924

Page generated at 2026-09-26 01:18:40. Version ac07

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