Concord Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Concord Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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 4 3 3 3 2 4 6% 65%	105049
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	102708
Transparency	Day	21% 4 9% 19% 24% 23%	104805
	Night	24% 5% 11% 20% 20% 20%	102541
Seeing	Day	8% 11% 28% 33% 18% 3	15359
	Night	8% 6% 21% 38% 23% 4	24670

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	14% 8% 3 8% 5% 5% 5% 4 6% 6% 36%	2645
	11th to 20th	Day	13% 11% 5% 10% 6% 5% 5% 2 4 6% 34%	2860
	21st to end	Day	18% 9% 3 9% 5% 4 5% 3 6% 6% 31%	3146
February	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 45%	1710
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 3 6% 7% 45%	1756
	21st to end	Day	11% 5% 2 5% 5% 4 3 3 5% 7% 49%	1570
March	1st to 10th	Day	10% 6% 3 10% 7% 7% 7% 5% 7% 7% 31%	3354
	11th to 20th	Day	8% 4 2 5% 3 3 4 3 5% 7% 57%	3490
	21st to end	Day	10% 6% 2 6% 5% 6% 6% 5% 7% 9% 39%	3903
April	1st to 10th	Day	11% 7% 2 5% 4 4 4 4 5% 8% 47%	2280
	11th to 20th	Day	7% 5% 1 4 3 3 3 4 5% 7% 58%	2249
	21st to end	Day	3 4 1 3 3 2 3 3 5% 9% 64%	2280
May	1st to 10th	Day	5% 1 1 2 1 2 2 2 4 4 76%	3914
	11th to 20th	Day	2 1 1 1 1 1 1 2 3 6% 81%	4216
	21st to end	Day	2 1 2 1 1 2 2 4 6% 79%	4813
June	1st to 10th	Day	3 3 1 2 3 2 3 3 5% 9% 67%	2660
	11th to 20th	Day	1 1 1 1 1 2 2 5% 6% 80%	2660
	21st to end	Day	2 1 1 1 1 1 2 1 2 5% 83%	2660
July	1st to 10th	Day	1 1 1 2 4 91%	4420
	11th to 20th	Day	1 1 1 1 1 2 1 2 5% 85%	4204
	21st to end	Day	1 1 1 1 1 2 5% 88%	4455
August	1st to 10th	Day	1 1 1 1 2 2 2 5% 6% 78%	4091
	11th to 20th	Day	1 1 1 1 1 2 4 89%	3821
	21st to end	Day	1 1 1 1 1 1 1 2 5% 86%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 86%	2280
	11th to 20th	Day	2 1 1 2 2 2 2 3 5% 8% 73%	2280
	21st to end	Day	2 1 1 2 2 2 2 2 3 6% 80%	2023
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 75%	3180
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 7% 74%	3161
	21st to end	Day	3 2 1 4 3 3 3 3 6% 8% 65%	3448
November	1st to 10th	Day	6% 5% 2 4 3 3 3 4 6% 8% 56%	1829
	11th to 20th	Day	8% 5% 3 5% 4 4 5% 4 5% 7% 49%	1710
	21st to end	Day	10% 6% 2 5% 3 3 3 3 6% 6% 54%	1559
December	1st to 10th	Day	11% 7% 3 6% 4 3 4 4 5% 7% 46%	1520
	11th to 20th	Day	19% 12% 3 9% 6% 3 5% 3 5% 5% 31%	1928
	21st to end	Day	14% 9% 3 8% 5% 4 5% 3 6% 8% 34%	2794
January	1st to 10th	Night	13% 7% 2 6% 4 3 4 4 5% 7% 45%	4299
	11th to 20th	Night	15% 10% 4 9% 5% 4 4 4 6% 6% 33%	4110
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	4521
February	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 5% 50%	2470
	11th to 20th	Night	10% 5% 2 6% 4 5% 4 4 6% 8% 49%	2309
	21st to end	Night	10% 5% 2 4 3 3 4 2 5% 7% 55%	1884
March	1st to 10th	Night	10% 7% 4 7% 5% 5% 5% 4 5% 8% 39%	3506
	11th to 20th	Night	10% 4 1 4 3 3 3 3 3 5% 59%	3490
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 5% 6% 8% 43%	3749
April	1st to 10th	Night	7% 5% 1 5% 5% 5% 3 3 6% 7% 52%	1900
	11th to 20th	Night	5% 4 2 3 3 3 4 3 5% 7% 61%	1879
	21st to end	Night	3 2 1 3 3 2 2 3 6% 10% 65%	1900
May	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 76%	3057
	11th to 20th	Night	1 1 1 1 1 2 2 3 6% 81%	2680
	21st to end	Night	1 1 1 1 2 2 1 3 6% 81%	2762
June	1st to 10th	Night	3 2 1 3 3 2 3 3 5% 8% 68%	1520
	11th to 20th	Night	1 2 1 1 1 2 3 5% 82%	1520
	21st to end	Night	1 1 1 1 2 2 2 3 6% 82%	1520
July	1st to 10th	Night	1 1 1 1 3 92%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 85%	2757
	21st to end	Night	1 1 1 1 1 2 2 4 6% 82%	3109
August	1st to 10th	Night	2 2 1 1 1 1 1 2 5% 10% 74%	2848
	11th to 20th	Night	1 1 1 1 1 3 7% 85%	3025
	21st to end	Night	2 1 1 1 1 1 1 2 5% 84%	3498
September	1st to 10th	Night	1 1 1 1 2 2 3 6% 83%	1900
	11th to 20th	Night	2 2 1 2 1 2 2 3 5% 10% 72%	1900
	21st to end	Night	1 1 1 1 1 2 2 1 4 6% 80%	2152
October	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 77%	3800
	11th to 20th	Night	2 2 1 3 3 2 2 3 4 8% 71%	3777
	21st to end	Night	3 2 1 3 3 3 4 3 5% 6% 66%	4121
November	1st to 10th	Night	5% 4 1 3 3 3 4 4 5% 9% 58%	2320
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 6% 8% 54%	2470
	21st to end	Night	8% 5% 1 3 4 3 3 3 5% 7% 58%	2617
December	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2660
	11th to 20th	Night	19% 8% 3 7% 5% 5% 4 4 7% 7% 32%	3276
	21st to end	Night	14% 8% 3 7% 5% 5% 6% 4 5% 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	2645
	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%	1765
	21st to end	Day	35% 6% 10% 23% 20% 7%	1570
March	1st to 10th	Day	48% 11% 10% 14% 15% 2	3354
	11th to 20th	Day	27% 5% 13% 27% 16% 12%	3490
	21st to end	Day	39% 8% 17% 21% 12% 3	3903
April	1st to 10th	Day	37% 6% 13% 20% 20% 5%	2280
	11th to 20th	Day	26% 6% 14% 24% 23% 7%	2252
	21st to end	Day	18% 6% 14% 30% 27% 5%	2280
May	1st to 10th	Day	13% 3 9% 30% 26% 18%	3914
	11th to 20th	Day	9% 2 9% 24% 38% 18%	4219
	21st to end	Day	10% 4 14% 30% 34% 8%	4814
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%	4131
	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%	3859
	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%	2023
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	26% 5% 12% 25% 20% 12%	1829
	11th to 20th	Day	33% 7% 10% 23% 22% 6%	1710
	21st to end	Day	30% 4 10% 20% 26% 9%	1559
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	4299
	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%	2311
	21st to end	Night	31% 5% 12% 28% 19% 5%	1884
March	1st to 10th	Night	43% 11% 14% 13% 15% 3	3506
	11th to 20th	Night	29% 5% 13% 23% 19% 12%	3490
	21st to end	Night	38% 9% 17% 21% 11% 4	3749
April	1st to 10th	Night	31% 5% 16% 20% 20% 7%	1900
	11th to 20th	Night	24% 6% 14% 27% 18% 10%	1879
	21st to end	Night	17% 6% 14% 31% 25% 7%	1900
May	1st to 10th	Night	12% 6% 12% 25% 29% 16%	3057
	11th to 20th	Night	8% 2 16% 29% 24% 20%	2680
	21st to end	Night	8% 5% 17% 33% 29% 9%	2762
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% 58%	2707
	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%	3053
	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%	2152
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% 17% 15%	2320
	11th to 20th	Night	29% 5% 11% 25% 22% 8%	2470
	21st to end	Night	27% 5% 11% 19% 26% 12%	2617
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	24% 13% 28% 21% 14% 1	247
	11th to 20th	Day	17% 10% 22% 33% 15% 3	370
	21st to end	Day	14% 10% 23% 28% 23% 3	407
February	1st to 10th	Day	17% 14% 20% 28% 18% 3	370
	11th to 20th	Day	16% 19% 25% 23% 16% 2	409
	21st to end	Day	14% 14% 27% 29% 13% 2	382
March	1st to 10th	Day	17% 18% 30% 22% 10% 2	460
	11th to 20th	Day	17% 17% 26% 21% 15% 4	460
	21st to end	Day	13% 20% 32% 24% 10% 1	506
April	1st to 10th	Day	13% 17% 31% 22% 13% 3	460
	11th to 20th	Day	13% 18% 32% 22% 10% 5%	452
	21st to end	Day	5% 22% 28% 27% 13% 5%	460
May	1st to 10th	Day	8% 16% 35% 27% 12% 1	463
	11th to 20th	Day	6% 14% 36% 27% 16% 1	488
	21st to end	Day	5% 12% 29% 38% 13% 2	529
June	1st to 10th	Day	4 13% 35% 30% 14% 3	490
	11th to 20th	Day	2 12% 34% 39% 11% 1	490
	21st to end	Day	1 7% 34% 42% 15% 2	490
July	1st to 10th	Day	4 31% 48% 17%	486
	11th to 20th	Day	2 27% 50% 18% 2	484
	21st to end	Day	1 1 31% 46% 18% 4	531
August	1st to 10th	Day	2 5% 30% 43% 17% 3	487
	11th to 20th	Day	1 4 31% 41% 17% 5%	477
	21st to end	Day	1 3 24% 46% 23% 4	539
September	1st to 10th	Day	1 4 22% 44% 23% 6%	490
	11th to 20th	Day	3 9% 28% 37% 17% 6%	490
	21st to end	Day	2 7% 25% 34% 26% 6%	490
October	1st to 10th	Day	3 7% 27% 31% 26% 6%	490
	11th to 20th	Day	5% 10% 21% 27% 28% 8%	490
	21st to end	Day	8% 9% 22% 31% 24% 6%	539
November	1st to 10th	Day	8% 12% 20% 25% 29% 7%	407
	11th to 20th	Day	6% 10% 19% 32% 26% 8%	270
	21st to end	Day	10% 8% 25% 33% 24% 1	198
December	1st to 10th	Day	14% 14% 22% 24% 19% 6%	180
	11th to 20th	Day	18% 11% 23% 24% 22% 2	180
	21st to end	Day	19% 12% 28% 24% 16% 2	198
January	1st to 10th	Night	16% 5% 18% 33% 22% 6%	760
	11th to 20th	Night	14% 6% 16% 36% 23% 6%	760
	21st to end	Night	16% 6% 15% 35% 23% 4	836
February	1st to 10th	Night	16% 7% 17% 34% 22% 4	760
	11th to 20th	Night	12% 10% 20% 33% 20% 5%	744
	21st to end	Night	12% 10% 22% 34% 19% 2	628
March	1st to 10th	Night	18% 9% 21% 30% 20% 3	760
	11th to 20th	Night	13% 11% 21% 33% 19% 3	760
	21st to end	Night	11% 11% 28% 35% 14% 2	836
April	1st to 10th	Night	8% 9% 27% 36% 17% 2	760
	11th to 20th	Night	7% 13% 30% 31% 17% 3	751
	21st to end	Night	4 9% 30% 35% 19% 2	760
May	1st to 10th	Night	4 7% 29% 40% 17% 2	689
	11th to 20th	Night	3 5% 24% 46% 21% 1	473
	21st to end	Night	3 7% 32% 41% 16%	411
June	1st to 10th	Night	3 4 31% 48% 12% 1	380
	11th to 20th	Night	1 4 27% 52% 15% 1	380
	21st to end	Night	3 26% 49% 21% 1	380
July	1st to 10th	Night	1 24% 54% 21%	370
	11th to 20th	Night	1 1 18% 52% 24% 3	492
	21st to end	Night	1 1 20% 45% 30% 2	624
August	1st to 10th	Night	3 1 25% 41% 26% 3	567
	11th to 20th	Night	2 24% 46% 26% 2	687
	21st to end	Night	1 1 18% 48% 29% 3	836
September	1st to 10th	Night	1 1 19% 43% 31% 5%	760
	11th to 20th	Night	2 4 22% 43% 26% 4	760
	21st to end	Night	1 3 17% 41% 32% 6%	760
October	1st to 10th	Night	3 4 19% 37% 32% 6%	760
	11th to 20th	Night	4 5% 17% 38% 30% 6%	760
	21st to end	Night	6% 5% 15% 35% 30% 9%	836
November	1st to 10th	Night	6% 5% 17% 33% 31% 8%	754
	11th to 20th	Night	9% 2 17% 38% 29% 6%	760
	21st to end	Night	10% 4 16% 35% 28% 7%	760
December	1st to 10th	Night	12% 7% 17% 35% 24% 5%	760
	11th to 20th	Night	19% 7% 13% 35% 22% 3	760
	21st to end	Night	17% 8% 20% 32% 20% 2	836

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