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 2025-05-26.

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	6% 4 1 4 3 3 3 3 5% 7% 62%	118091
	Night	7% 4 2 4 3 3 3 3 5% 8% 57%	120058
Transparency	Day	21% 5% 10% 19% 24% 22%	117848
	Night	25% 5% 11% 20% 21% 18%	119890
Seeing	Day	8% 11% 28% 33% 16% 3	16035
	Night	8% 6% 20% 37% 24% 4	25458

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	13% 6% 2 6% 4 4 4 3 5% 8% 44%	3823
	11th to 20th	Day	10% 7% 4 7% 4 4 3 2 3 4 52%	4100
	21st to end	Day	15% 7% 3 7% 4 4 4 3 5% 7% 41%	4510
February	1st to 10th	Day	13% 8% 2 5% 5% 3 4 4 5% 6% 44%	1800
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	1847
	21st to end	Day	12% 5% 2 6% 4 5% 3 3 5% 6% 49%	1650
March	1st to 10th	Day	10% 8% 2 8% 6% 5% 5% 4 6% 8% 37%	4841
	11th to 20th	Day	8% 4 2 5% 5% 4 5% 4 6% 9% 50%	5000
	21st to end	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 10% 40%	5587
April	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 5% 8% 47%	2400
	11th to 20th	Day	8% 4 2 4 2 3 4 4 5% 7% 57%	2369
	21st to end	Day	4 4 1 4 3 3 3 3 6% 9% 61%	2400
May	1st to 10th	Day	5% 2 1 2 1 2 2 2 3 5% 75%	4044
	11th to 20th	Day	2 1 1 2 1 1 2 2 4 6% 79%	4343
	21st to end	Day	2 1 1 2 2 1 2 2 4 7% 76%	4886
June	1st to 10th	Day	3 3 1 3 3 3 4 3 6% 9% 64%	2660
	11th to 20th	Day	1 1 1 1 1 2 2 5% 7% 78%	2660
	21st to end	Day	2 1 1 2 1 1 2 2 3 6% 79%	2660
July	1st to 10th	Day	1 1 1 1 1 2 5% 89%	4420
	11th to 20th	Day	1 1 1 1 2 2 1 3 6% 82%	4184
	21st to end	Day	1 1 1 1 1 3 6% 85%	4455
August	1st to 10th	Day	1 1 2 2 2 3 3 5% 7% 75%	4091
	11th to 20th	Day	1 1 1 2 2 3 5% 86%	3825
	21st to end	Day	1 1 1 1 1 2 2 3 6% 82%	4180
September	1st to 10th	Day	1 1 1 2 1 1 1 2 3 5% 83%	2280
	11th to 20th	Day	2 1 1 2 2 2 4 3 5% 8% 69%	2280
	21st to end	Day	2 1 1 2 2 1 2 1 3 6% 79%	2019
October	1st to 10th	Day	1 2 1 3 2 2 3 2 4 6% 74%	3180
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 7% 72%	3161
	21st to end	Day	4 2 1 4 3 3 3 3 6% 7% 64%	3448
November	1st to 10th	Day	6% 5% 2 4 3 3 3 3 6% 7% 58%	1900
	11th to 20th	Day	8% 5% 3 6% 5% 4 5% 4 5% 7% 48%	1800
	21st to end	Day	11% 6% 2 5% 3 3 3 4 6% 6% 53%	1636
December	1st to 10th	Day	7% 4 2 3 3 3 4 3 6% 11% 54%	2620
	11th to 20th	Day	17% 9% 3 8% 6% 6% 6% 4 7% 8% 25%	3028
	21st to end	Day	15% 10% 3 9% 6% 6% 6% 4 5% 8% 29%	4004
January	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 6% 9% 49%	6114
	11th to 20th	Night	11% 7% 2 7% 3 3 3 3 5% 6% 50%	5880
	21st to end	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	6468
February	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 4 5% 49%	2600
	11th to 20th	Night	10% 6% 2 6% 4 5% 3 4 5% 9% 46%	2438
	21st to end	Night	9% 6% 1 4 4 4 3 4 4 7% 53%	1980
March	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 7% 8% 39%	5036
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 5% 8% 51%	5000
	21st to end	Night	11% 7% 2 6% 4 4 4 4 6% 7% 43%	5386
April	1st to 10th	Night	7% 5% 2 6% 4 5% 3 3 6% 8% 51%	2000
	11th to 20th	Night	5% 5% 2 4 3 3 3 3 5% 9% 57%	1979
	21st to end	Night	3 3 1 4 3 3 3 3 6% 10% 62%	2000
May	1st to 10th	Night	2 3 1 2 2 2 2 2 4 5% 74%	3148
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 9% 74%	2771
	21st to end	Night	1 1 1 1 2 2 3 4 8% 75%	2805
June	1st to 10th	Night	3 2 1 4 3 3 3 4 5% 10% 62%	1520
	11th to 20th	Night	1 1 2 1 2 2 3 4 7% 78%	1520
	21st to end	Night	1 1 1 2 2 2 3 2 5% 7% 76%	1520
July	1st to 10th	Night	1 1 1 1 3 6% 86%	2540
	11th to 20th	Night	1 1 2 1 2 1 1 3 11% 77%	2780
	21st to end	Night	1 1 1 1 1 2 3 6% 17% 68%	3109
August	1st to 10th	Night	2 1 2 2 2 3 3 7% 12% 67%	2848
	11th to 20th	Night	1 1 1 2 2 5% 12% 76%	3021
	21st to end	Night	2 1 1 1 1 1 2 2 5% 10% 75%	3498
September	1st to 10th	Night	1 1 1 2 3 3 4 9% 77%	1900
	11th to 20th	Night	2 2 1 2 2 2 4 4 7% 12% 63%	1900
	21st to end	Night	1 2 1 2 1 2 2 2 5% 7% 76%	2156
October	1st to 10th	Night	2 1 1 3 2 2 2 2 4 7% 74%	3800
	11th to 20th	Night	2 3 1 3 2 2 3 3 6% 13% 61%	3777
	21st to end	Night	3 3 1 3 3 3 4 4 5% 7% 64%	4121
November	1st to 10th	Night	5% 4 1 4 4 3 4 3 6% 9% 57%	2395
	11th to 20th	Night	8% 5% 2 5% 3 3 4 4 6% 8% 52%	2600
	21st to end	Night	9% 5% 1 4 4 3 4 3 5% 7% 56%	2760
December	1st to 10th	Night	7% 4 2 4 3 3 4 4 4 7% 59%	4560
	11th to 20th	Night	16% 8% 3 7% 6% 5% 5% 4 8% 8% 30%	5176
	21st to end	Night	16% 8% 3 9% 5% 5% 5% 4 6% 6% 33%	6952

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% 10% 17% 20% 15% 4	3823
	11th to 20th	Day	37% 5% 8% 15% 23% 13%	4100
	21st to end	Day	43% 6% 8% 13% 16% 14%	4510
February	1st to 10th	Day	40% 5% 9% 19% 19% 8%	1800
	11th to 20th	Day	38% 7% 11% 18% 21% 6%	1856
	21st to end	Day	35% 5% 10% 23% 20% 7%	1650
March	1st to 10th	Day	41% 9% 9% 19% 20% 2	4841
	11th to 20th	Day	29% 7% 16% 27% 12% 8%	5000
	21st to end	Day	34% 9% 18% 26% 10% 2	5587
April	1st to 10th	Day	36% 6% 13% 20% 21% 5%	2400
	11th to 20th	Day	26% 6% 14% 24% 23% 8%	2372
	21st to end	Day	19% 6% 13% 30% 27% 5%	2400
May	1st to 10th	Day	13% 3 9% 30% 27% 18%	4044
	11th to 20th	Day	9% 2 8% 25% 38% 18%	4346
	21st to end	Day	10% 4 14% 30% 34% 8%	4887
June	1st to 10th	Day	17% 6% 12% 24% 29% 12%	2660
	11th to 20th	Day	7% 3 10% 22% 35% 23%	2660
	21st to end	Day	8% 2 6% 15% 30% 39%	2660
July	1st to 10th	Day	3 1 3 7% 14% 72%	4351
	11th to 20th	Day	6% 1 6% 10% 13% 65%	4112
	21st to end	Day	3 1 3 8% 26% 59%	4315
August	1st to 10th	Day	8% 2 5% 12% 23% 50%	4076
	11th to 20th	Day	3 1 4 11% 38% 43%	3863
	21st to end	Day	5% 2 6% 13% 34% 40%	4179
September	1st to 10th	Day	6% 2 5% 11% 30% 47%	2280
	11th to 20th	Day	13% 3 10% 22% 33% 19%	2280
	21st to end	Day	9% 3 7% 18% 36% 27%	2019
October	1st to 10th	Day	12% 3 5% 11% 27% 42%	3180
	11th to 20th	Day	13% 4 8% 24% 31% 20%	3161
	21st to end	Day	18% 4 10% 22% 32% 15%	3448
November	1st to 10th	Day	25% 5% 11% 26% 21% 12%	1900
	11th to 20th	Day	34% 7% 10% 23% 21% 5%	1800
	21st to end	Day	31% 5% 10% 20% 25% 9%	1636
December	1st to 10th	Day	25% 5% 9% 30% 26% 5%	2620
	11th to 20th	Day	50% 11% 10% 15% 13% 2	3028
	21st to end	Day	50% 9% 14% 16% 9% 2	4004
January	1st to 10th	Night	30% 9% 21% 22% 15% 3	6114
	11th to 20th	Night	35% 6% 10% 15% 23% 11%	5880
	21st to end	Night	39% 6% 9% 12% 19% 14%	6468
February	1st to 10th	Night	37% 5% 8% 19% 20% 10%	2600
	11th to 20th	Night	35% 5% 12% 20% 20% 8%	2440
	21st to end	Night	30% 5% 12% 27% 19% 7%	1980
March	1st to 10th	Night	39% 10% 11% 19% 18% 3	5036
	11th to 20th	Night	31% 7% 15% 23% 15% 9%	5000
	21st to end	Night	35% 9% 17% 24% 12% 3	5386
April	1st to 10th	Night	31% 6% 16% 21% 21% 7%	2000
	11th to 20th	Night	24% 6% 14% 27% 18% 11%	1979
	21st to end	Night	17% 6% 14% 31% 25% 7%	2000
May	1st to 10th	Night	12% 6% 12% 26% 29% 16%	3148
	11th to 20th	Night	8% 3 16% 29% 25% 19%	2771
	21st to end	Night	8% 5% 16% 33% 29% 9%	2805
June	1st to 10th	Night	16% 7% 15% 24% 26% 13%	1520
	11th to 20th	Night	8% 4 10% 20% 33% 25%	1520
	21st to end	Night	8% 1 7% 23% 26% 35%	1520
July	1st to 10th	Night	3 1 3 11% 14% 68%	2500
	11th to 20th	Night	7% 1 4 13% 17% 57%	2729
	21st to end	Night	5% 1 4 15% 28% 46%	3011
August	1st to 10th	Night	11% 1 7% 14% 20% 48%	2839
	11th to 20th	Night	3 1 6% 17% 30% 43%	3049
	21st to end	Night	7% 2 6% 20% 28% 38%	3498
September	1st to 10th	Night	5% 3 6% 16% 24% 47%	1900
	11th to 20th	Night	15% 3 11% 27% 26% 19%	1900
	21st to end	Night	9% 3 8% 21% 29% 30%	2156
October	1st to 10th	Night	11% 3 6% 12% 17% 50%	3800
	11th to 20th	Night	14% 5% 13% 24% 21% 24%	3777
	21st to end	Night	18% 4 8% 22% 26% 21%	4121
November	1st to 10th	Night	24% 5% 12% 26% 18% 15%	2395
	11th to 20th	Night	29% 5% 12% 25% 21% 8%	2600
	21st to end	Night	28% 5% 11% 19% 26% 12%	2760
December	1st to 10th	Night	26% 5% 8% 25% 26% 11%	4560
	11th to 20th	Night	48% 7% 11% 15% 16% 2	5176
	21st to end	Night	50% 8% 15% 15% 9% 3	6952

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	24% 13% 31% 18% 13% 2	280
	11th to 20th	Day	16% 14% 21% 32% 15% 3	400
	21st to end	Day	14% 10% 26% 29% 18% 2	440
February	1st to 10th	Day	20% 13% 21% 28% 17% 2	400
	11th to 20th	Day	17% 19% 26% 22% 14% 1	442
	21st to end	Day	13% 14% 28% 32% 11% 2	414
March	1st to 10th	Day	19% 19% 29% 22% 9% 2	500
	11th to 20th	Day	20% 17% 25% 21% 14% 3	500
	21st to end	Day	15% 20% 31% 22% 10% 1	550
April	1st to 10th	Day	13% 17% 34% 21% 12% 3	500
	11th to 20th	Day	13% 18% 33% 23% 9% 4	492
	21st to end	Day	6% 19% 30% 27% 13% 4	500
May	1st to 10th	Day	9% 16% 34% 31% 10% 1	503
	11th to 20th	Day	7% 14% 36% 29% 13% 1	528
	21st to end	Day	4 13% 34% 34% 12% 2	550
June	1st to 10th	Day	5% 13% 35% 32% 13% 3	490
	11th to 20th	Day	3 11% 36% 39% 10% 1	490
	21st to end	Day	7% 37% 40% 14% 1	490
July	1st to 10th	Day	5% 32% 46% 16% 1	486
	11th to 20th	Day	2 27% 51% 17% 3	484
	21st to end	Day	1 1 30% 49% 15% 4	531
August	1st to 10th	Day	2 6% 29% 45% 15% 3	487
	11th to 20th	Day	1 4 32% 41% 17% 5%	477
	21st to end	Day	1 4 25% 46% 21% 4	539
September	1st to 10th	Day	1 4 22% 44% 22% 7%	490
	11th to 20th	Day	3 10% 27% 38% 15% 6%	490
	21st to end	Day	2 9% 24% 35% 26% 4	490
October	1st to 10th	Day	3 7% 28% 31% 24% 6%	490
	11th to 20th	Day	6% 10% 21% 30% 25% 8%	490
	21st to end	Day	8% 9% 24% 29% 24% 5%	539
November	1st to 10th	Day	8% 13% 20% 25% 28% 6%	436
	11th to 20th	Day	7% 11% 19% 34% 23% 7%	300
	21st to end	Day	12% 8% 29% 31% 19% 2	217
December	1st to 10th	Day	13% 15% 23% 28% 19% 4	200
	11th to 20th	Day	17% 15% 23% 25% 20% 2	200
	21st to end	Day	21% 13% 28% 25% 12% 1	220
January	1st to 10th	Night	17% 6% 16% 34% 22% 6%	800
	11th to 20th	Night	13% 7% 17% 36% 23% 5%	800
	21st to end	Night	16% 6% 15% 36% 23% 4	880
February	1st to 10th	Night	16% 9% 15% 35% 22% 4	800
	11th to 20th	Night	12% 10% 19% 34% 20% 5%	784
	21st to end	Night	12% 9% 21% 36% 20% 3	660
March	1st to 10th	Night	18% 10% 22% 29% 20% 2	800
	11th to 20th	Night	15% 10% 20% 32% 20% 3	800
	21st to end	Night	13% 10% 24% 36% 15% 2	880
April	1st to 10th	Night	10% 9% 26% 37% 17% 2	800
	11th to 20th	Night	8% 13% 28% 31% 18% 3	791
	21st to end	Night	4 9% 31% 35% 19% 3	800
May	1st to 10th	Night	5% 7% 28% 40% 18% 2	720
	11th to 20th	Night	3 6% 24% 42% 23% 1	504
	21st to end	Night	3 9% 32% 38% 16% 2	422
June	1st to 10th	Night	4 5% 29% 45% 16% 2	380
	11th to 20th	Night	1 5% 25% 54% 15% 1	380
	21st to end	Night	4 24% 46% 24% 1	380
July	1st to 10th	Night	1 2 21% 50% 26%	370
	11th to 20th	Night	1 1 18% 49% 27% 4	501
	21st to end	Night	1 2 18% 43% 31% 4	624
August	1st to 10th	Night	3 4 23% 38% 28% 4	567
	11th to 20th	Night	1 3 22% 45% 27% 3	683
	21st to end	Night	1 1 17% 47% 30% 4	836
September	1st to 10th	Night	1 2 16% 42% 33% 6%	760
	11th to 20th	Night	4 4 18% 42% 28% 4	760
	21st to end	Night	2 4 16% 37% 34% 7%	760
October	1st to 10th	Night	4 5% 15% 38% 32% 6%	760
	11th to 20th	Night	5% 6% 17% 34% 32% 6%	760
	21st to end	Night	6% 5% 15% 33% 31% 10%	836
November	1st to 10th	Night	6% 5% 17% 31% 33% 7%	780
	11th to 20th	Night	10% 4 17% 35% 28% 6%	800
	21st to end	Night	11% 6% 15% 36% 26% 6%	800
December	1st to 10th	Night	12% 8% 16% 36% 23% 5%	800
	11th to 20th	Night	20% 8% 13% 35% 21% 3	800
	21st to end	Night	18% 10% 22% 30% 19% 1	880

Page generated at 2025-05-23 06:21:00. Version ac07

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