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 2024-03-18.

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	7% 4 2 4 4 2 3 4 5% 7% 59%	78820
	Night	8% 5% 2 4 4 2 4 4 5% 8% 54%	81001
Transparency	Day	25% 5% 10% 20% 23% 18%	78538
	Night	28% 5% 12% 21% 20% 15%	80804
Seeing	Day	8% 11% 28% 33% 16% 3	14445
	Night	9% 6% 19% 37% 25% 4	23911

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	16% 8% 3 6% 6% 4 5% 5% 5% 7% 36%	2667
	11th to 20th	Day	14% 10% 5% 9% 7% 3 4 3 5% 6% 33%	2860
	21st to end	Day	19% 9% 3 7% 5% 4 5% 5% 5% 7% 31%	3146
February	1st to 10th	Day	13% 7% 2 4 5% 3 4 5% 5% 7% 45%	1710
	11th to 20th	Day	11% 6% 2 6% 6% 4 4 4 6% 7% 43%	1752
	21st to end	Day	12% 4 2 5% 5% 4 4 4 5% 6% 49%	1570
March	1st to 10th	Day	12% 7% 3 7% 6% 4 5% 5% 7% 7% 37%	2038
	11th to 20th	Day	12% 6% 3 6% 6% 3 4 5% 6% 8% 41%	2059
	21st to end	Day	10% 5% 3 5% 4 3 4 5% 5% 8% 46%	2215
April	1st to 10th	Day	12% 6% 3 5% 5% 3 4 4 5% 8% 45%	2160
	11th to 20th	Day	8% 4 2 4 3 2 3 4 4 8% 58%	2129
	21st to end	Day	4 4 2 3 3 2 3 4 6% 9% 61%	2160
May	1st to 10th	Day	4 3 1 3 3 2 3 4 5% 8% 64%	2228
	11th to 20th	Day	4 2 1 2 3 2 3 3 5% 8% 67%	2374
	21st to end	Day	4 2 1 3 3 2 3 4 5% 9% 65%	2723
June	1st to 10th	Day	3 3 1 3 3 2 4 4 5% 9% 64%	2520
	11th to 20th	Day	1 1 1 1 1 2 3 5% 7% 78%	2520
	21st to end	Day	2 1 1 2 2 1 2 2 3 7% 79%	2520
July	1st to 10th	Day	1 1 1 1 2 2 4 7% 82%	2520
	11th to 20th	Day	1 1 1 2 1 2 2 3 7% 79%	2388
	21st to end	Day	1 1 1 1 1 2 2 4 6% 80%	2546
August	1st to 10th	Day	1 1 2 2 1 2 3 3 7% 77%	2328
	11th to 20th	Day	1 1 1 1 2 3 4 6% 80%	2187
	21st to end	Day	1 1 1 1 2 2 4 7% 81%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 2 3 6% 83%	2160
	11th to 20th	Day	2 1 1 2 3 2 3 3 5% 8% 70%	2160
	21st to end	Day	2 1 1 2 2 1 2 2 3 6% 78%	1914
October	1st to 10th	Day	2 2 1 3 3 2 3 3 3 6% 72%	1800
	11th to 20th	Day	4 2 1 3 3 2 3 3 5% 6% 68%	1800
	21st to end	Day	6% 3 2 3 3 2 2 3 5% 7% 64%	1980
November	1st to 10th	Day	7% 4 2 3 4 2 3 4 6% 7% 56%	1803
	11th to 20th	Day	7% 4 3 5% 5% 4 5% 5% 5% 6% 49%	1710
	21st to end	Day	10% 6% 2 4 4 2 3 4 6% 6% 54%	1555
December	1st to 10th	Day	11% 7% 3 5% 5% 3 5% 4 5% 7% 46%	1520
	11th to 20th	Day	20% 10% 4 7% 5% 3 5% 4 5% 5% 31%	1928
	21st to end	Day	15% 9% 3 7% 6% 3 5% 5% 5% 8% 34%	2794
January	1st to 10th	Night	13% 8% 3 5% 4 3 4 4 5% 8% 44%	4279
	11th to 20th	Night	15% 9% 3 9% 6% 4 4 5% 5% 6% 33%	4110
	21st to end	Night	18% 10% 4 7% 6% 4 5% 4 5% 6% 32%	4521
February	1st to 10th	Night	13% 6% 2 5% 4 3 4 4 4 5% 50%	2470
	11th to 20th	Night	10% 5% 2 5% 4 3 5% 5% 5% 9% 47%	2313
	21st to end	Night	10% 5% 2 3 5% 3 4 4 4 7% 53%	1884
March	1st to 10th	Night	14% 7% 3 5% 5% 3 4 5% 5% 7% 42%	2138
	11th to 20th	Night	12% 6% 3 5% 5% 4 4 4 5% 8% 44%	2067
	21st to end	Night	11% 6% 3 5% 4 2 3 4 5% 6% 52%	2139
April	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 7% 50%	1800
	11th to 20th	Night	6% 4 3 3 4 2 3 3 5% 9% 58%	1779
	21st to end	Night	3 2 1 3 3 2 3 4 6% 11% 62%	1800
May	1st to 10th	Night	3 4 2 2 3 2 3 4 6% 8% 63%	1728
	11th to 20th	Night	2 2 1 2 3 1 3 5% 6% 11% 62%	1516
	21st to end	Night	3 2 2 2 2 3 5% 6% 11% 63%	1552
June	1st to 10th	Night	3 2 1 3 3 2 4 5% 5% 9% 62%	1440
	11th to 20th	Night	1 1 2 1 1 2 3 5% 7% 77%	1440
	21st to end	Night	1 1 1 2 2 2 3 3 5% 7% 75%	1440
July	1st to 10th	Night	1 1 1 1 2 3 4 9% 78%	1440
	11th to 20th	Night	1 1 2 2 2 2 3 4 10% 74%	1568
	21st to end	Night	1 2 1 1 2 1 3 5% 7% 13% 65%	1773
August	1st to 10th	Night	2 1 2 2 2 3 5% 8% 14% 60%	1611
	11th to 20th	Night	1 1 2 3 4 8% 13% 67%	1714
	21st to end	Night	1 1 1 1 2 1 2 4 6% 12% 69%	1980
September	1st to 10th	Night	1 1 1 1 3 4 4 9% 76%	1800
	11th to 20th	Night	2 2 1 2 2 2 3 4 7% 12% 64%	1800
	21st to end	Night	1 2 1 2 2 2 2 3 5% 7% 75%	2041
October	1st to 10th	Night	3 2 1 3 2 2 3 3 4 7% 70%	2160
	11th to 20th	Night	3 2 1 3 4 2 4 4 5% 8% 62%	2160
	21st to end	Night	5% 3 1 2 3 2 3 4 5% 8% 63%	2376
November	1st to 10th	Night	5% 3 2 4 4 3 4 4 5% 9% 57%	2272
	11th to 20th	Night	8% 5% 2 4 3 2 3 5% 6% 8% 52%	2470
	21st to end	Night	8% 4 2 3 4 2 4 4 5% 7% 58%	2622
December	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 4 4 7% 48%	2660
	11th to 20th	Night	19% 9% 3 6% 6% 3 5% 5% 6% 6% 32%	3276
	21st to end	Night	14% 7% 3 7% 7% 4 6% 5% 6% 6% 36%	4862

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	43% 11% 12% 25% 8% 2	2667
	11th to 20th	Day	53% 6% 11% 14% 12% 5%	2860
	21st to end	Day	52% 7% 10% 15% 13% 3	3146
February	1st to 10th	Day	39% 5% 10% 20% 19% 7%	1710
	11th to 20th	Day	38% 6% 10% 18% 22% 6%	1761
	21st to end	Day	35% 6% 10% 23% 20% 7%	1570
March	1st to 10th	Day	44% 7% 11% 18% 16% 3	2038
	11th to 20th	Day	41% 6% 10% 21% 15% 6%	2059
	21st to end	Day	35% 7% 15% 20% 19% 4	2215
April	1st to 10th	Day	38% 6% 13% 19% 20% 5%	2160
	11th to 20th	Day	26% 5% 14% 24% 23% 7%	2132
	21st to end	Day	19% 5% 14% 29% 27% 5%	2160
May	1st to 10th	Day	18% 5% 11% 26% 30% 9%	2228
	11th to 20th	Day	16% 4 12% 28% 30% 10%	2377
	21st to end	Day	17% 6% 18% 28% 24% 7%	2724
June	1st to 10th	Day	17% 6% 12% 25% 28% 12%	2520
	11th to 20th	Day	7% 3 10% 23% 33% 23%	2520
	21st to end	Day	8% 2 6% 16% 30% 37%	2520
July	1st to 10th	Day	5% 2 6% 13% 24% 50%	2451
	11th to 20th	Day	7% 2 9% 16% 20% 46%	2316
	21st to end	Day	5% 1 6% 12% 25% 50%	2405
August	1st to 10th	Day	9% 2 7% 19% 26% 37%	2312
	11th to 20th	Day	5% 1 7% 14% 31% 41%	2187
	21st to end	Day	5% 2 7% 17% 32% 37%	2376
September	1st to 10th	Day	6% 2 5% 11% 30% 46%	2160
	11th to 20th	Day	13% 3 10% 23% 33% 19%	2160
	21st to end	Day	10% 3 7% 19% 35% 26%	1914
October	1st to 10th	Day	15% 4 8% 16% 30% 27%	1800
	11th to 20th	Day	18% 5% 10% 20% 23% 25%	1800
	21st to end	Day	21% 4 10% 20% 26% 20%	1980
November	1st to 10th	Day	26% 5% 11% 25% 21% 12%	1803
	11th to 20th	Day	33% 7% 10% 23% 22% 6%	1710
	21st to end	Day	30% 4 10% 20% 26% 9%	1555
December	1st to 10th	Day	37% 7% 11% 20% 17% 8%	1520
	11th to 20th	Day	55% 6% 8% 13% 14% 3	1928
	21st to end	Day	46% 8% 14% 18% 11% 2	2794
January	1st to 10th	Night	36% 8% 20% 23% 10% 3	4279
	11th to 20th	Night	50% 8% 13% 12% 10% 7%	4110
	21st to end	Night	51% 7% 11% 13% 12% 5%	4521
February	1st to 10th	Night	36% 5% 9% 20% 20% 10%	2470
	11th to 20th	Night	34% 5% 12% 19% 21% 9%	2315
	21st to end	Night	31% 5% 12% 28% 19% 5%	1884
March	1st to 10th	Night	41% 7% 14% 17% 17% 5%	2138
	11th to 20th	Night	38% 6% 7% 23% 19% 7%	2067
	21st to end	Night	33% 6% 14% 21% 19% 7%	2139
April	1st to 10th	Night	33% 5% 16% 19% 20% 7%	1800
	11th to 20th	Night	24% 6% 13% 26% 19% 11%	1779
	21st to end	Night	17% 6% 14% 31% 25% 7%	1800
May	1st to 10th	Night	18% 6% 13% 22% 30% 12%	1728
	11th to 20th	Night	15% 4 19% 27% 25% 10%	1516
	21st to end	Night	13% 7% 24% 26% 23% 7%	1552
June	1st to 10th	Night	17% 7% 14% 24% 26% 12%	1440
	11th to 20th	Night	8% 4 11% 20% 32% 25%	1440
	21st to end	Night	8% 1 7% 24% 26% 33%	1440
July	1st to 10th	Night	5% 2 6% 20% 23% 45%	1400
	11th to 20th	Night	8% 2 7% 22% 19% 42%	1517
	21st to end	Night	9% 2 7% 26% 20% 37%	1674
August	1st to 10th	Night	12% 1 11% 24% 23% 29%	1602
	11th to 20th	Night	6% 2 9% 22% 23% 37%	1714
	21st to end	Night	7% 2 9% 25% 25% 32%	1980
September	1st to 10th	Night	5% 3 6% 16% 25% 45%	1800
	11th to 20th	Night	15% 2 11% 27% 25% 20%	1800
	21st to end	Night	10% 3 8% 22% 28% 29%	2041
October	1st to 10th	Night	14% 5% 10% 20% 23% 28%	2160
	11th to 20th	Night	18% 4 10% 20% 21% 26%	2160
	21st to end	Night	19% 3 9% 22% 24% 23%	2376
November	1st to 10th	Night	24% 5% 12% 26% 17% 15%	2272
	11th to 20th	Night	29% 5% 11% 25% 22% 8%	2470
	21st to end	Night	27% 5% 11% 19% 26% 12%	2622
December	1st to 10th	Night	36% 6% 11% 19% 17% 12%	2660
	11th to 20th	Night	51% 6% 10% 13% 16% 3	3276
	21st to end	Night	46% 10% 14% 15% 12% 4	4862

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	25% 13% 30% 17% 13% 2	256
	11th to 20th	Day	17% 13% 21% 32% 15% 3	370
	21st to end	Day	14% 11% 26% 29% 18% 2	407
February	1st to 10th	Day	19% 12% 21% 28% 18% 2	370
	11th to 20th	Day	17% 18% 26% 22% 14% 1	407
	21st to end	Day	13% 15% 28% 30% 12% 2	382
March	1st to 10th	Day	19% 19% 29% 22% 9% 2	460
	11th to 20th	Day	19% 18% 26% 20% 14% 3	449
	21st to end	Day	14% 20% 31% 24% 11% 1	462
April	1st to 10th	Day	15% 18% 33% 20% 12% 3	420
	11th to 20th	Day	13% 20% 34% 22% 7% 4	412
	21st to end	Day	7% 21% 28% 28% 12% 5%	420
May	1st to 10th	Day	8% 17% 32% 31% 10% 1	423
	11th to 20th	Day	6% 16% 37% 28% 13% 1	448
	21st to end	Day	5% 14% 33% 34% 12% 2	485
June	1st to 10th	Day	5% 13% 34% 32% 14% 3	450
	11th to 20th	Day	3 11% 35% 40% 10% 1	450
	21st to end	Day	7% 36% 40% 15% 1	450
July	1st to 10th	Day	5% 33% 46% 14% 1	446
	11th to 20th	Day	2 27% 51% 18% 3	444
	21st to end	Day	1 1 29% 49% 16% 4	487
August	1st to 10th	Day	2 6% 29% 45% 15% 3	447
	11th to 20th	Day	3 29% 43% 19% 5%	437
	21st to end	Day	1 3 25% 47% 21% 3	495
September	1st to 10th	Day	2 4 20% 46% 22% 7%	450
	11th to 20th	Day	3 8% 26% 40% 15% 7%	450
	21st to end	Day	2 9% 24% 36% 25% 4	450
October	1st to 10th	Day	3 8% 29% 30% 24% 6%	450
	11th to 20th	Day	6% 10% 20% 31% 25% 8%	450
	21st to end	Day	9% 8% 23% 29% 25% 5%	495
November	1st to 10th	Day	8% 12% 19% 25% 30% 6%	399
	11th to 20th	Day	5% 10% 17% 36% 25% 7%	270
	21st to end	Day	9% 8% 28% 33% 20% 2	196
December	1st to 10th	Day	14% 17% 21% 27% 17% 4	180
	11th to 20th	Day	17% 14% 22% 25% 21% 2	180
	21st to end	Day	21% 10% 28% 26% 14% 2	198
January	1st to 10th	Night	18% 5% 16% 33% 23% 6%	760
	11th to 20th	Night	14% 7% 15% 36% 23% 5%	760
	21st to end	Night	17% 6% 15% 35% 23% 4	836
February	1st to 10th	Night	16% 8% 15% 35% 23% 4	760
	11th to 20th	Night	12% 10% 19% 34% 21% 5%	744
	21st to end	Night	12% 10% 21% 34% 21% 3	628
March	1st to 10th	Night	18% 9% 21% 30% 20% 3	760
	11th to 20th	Night	15% 10% 19% 32% 20% 3	752
	21st to end	Night	13% 10% 24% 36% 16% 2	792
April	1st to 10th	Night	10% 9% 24% 37% 17% 2	720
	11th to 20th	Night	8% 13% 27% 30% 18% 3	711
	21st to end	Night	4 8% 30% 35% 19% 3	720
May	1st to 10th	Night	5% 7% 25% 41% 20% 2	648
	11th to 20th	Night	3 5% 23% 42% 26% 2	454
	21st to end	Night	4 9% 31% 38% 17% 2	389
June	1st to 10th	Night	4 5% 28% 46% 16% 2	360
	11th to 20th	Night	1 4 23% 56% 16% 1	360
	21st to end	Night	4 22% 48% 25% 1	360
July	1st to 10th	Night	1 2 20% 49% 28%	350
	11th to 20th	Night	1 1 16% 50% 29% 4	473
	21st to end	Night	2 2 16% 44% 33% 4	591
August	1st to 10th	Night	3 4 21% 38% 30% 4	537
	11th to 20th	Night	1 2 20% 47% 28% 3	646
	21st to end	Night	1 1 16% 47% 31% 4	792
September	1st to 10th	Night	1 2 14% 42% 34% 7%	720
	11th to 20th	Night	3 4 16% 43% 30% 4	720
	21st to end	Night	2 4 15% 37% 35% 7%	720
October	1st to 10th	Night	4 5% 15% 38% 32% 6%	720
	11th to 20th	Night	5% 6% 16% 34% 33% 6%	720
	21st to end	Night	6% 5% 13% 33% 31% 11%	792
November	1st to 10th	Night	6% 5% 16% 31% 33% 8%	740
	11th to 20th	Night	10% 4 15% 36% 29% 7%	760
	21st to end	Night	10% 6% 15% 36% 27% 6%	760
December	1st to 10th	Night	13% 8% 15% 36% 23% 5%	760
	11th to 20th	Night	20% 8% 13% 35% 21% 3	760
	21st to end	Night	17% 10% 21% 31% 20% 1	836

Page generated at 2024-03-15 11:01:41. Version ac07

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