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 2025-07-21.

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 3 4 6% 65%	124072
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	123957
Transparency	Day	21% 4 9% 19% 25% 21%	123829
	Night	24% 5% 11% 20% 21% 18%	123787
Seeing	Day	8% 11% 28% 32% 17% 3	16249
	Night	8% 6% 21% 38% 23% 4	25570

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% 6% 2 7% 4 4 4 4 6% 8% 44%	3801
	11th to 20th	Day	9% 8% 3 7% 4 3 3 2 3 5% 53%	4100
	21st to end	Day	15% 8% 3 7% 4 4 4 3 5% 6% 41%	4510
February	1st to 10th	Day	13% 7% 2 6% 5% 4 4 4 5% 6% 44%	1800
	11th to 20th	Day	10% 6% 2 6% 5% 5% 4 4 6% 7% 44%	1851
	21st to end	Day	11% 5% 2 5% 5% 4 4 4 5% 7% 49%	1650
March	1st to 10th	Day	9% 6% 2 8% 6% 6% 6% 4 7% 8% 38%	4825
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 51%	5000
	21st to end	Day	8% 5% 2 5% 5% 5% 6% 5% 7% 10% 41%	5603
April	1st to 10th	Day	10% 7% 2 5% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	7% 4 1 4 3 3 4 4 5% 7% 58%	2369
	21st to end	Day	3 4 2 3 3 3 3 3 5% 9% 63%	2400
May	1st to 10th	Day	3 1 1 2 1 1 2 1 3 4 80%	5606
	11th to 20th	Day	1 1 1 1 1 2 2 3 6% 81%	6051
	21st to end	Day	1 1 1 1 1 1 1 3 5% 83%	6902
June	1st to 10th	Day	2 2 1 2 2 2 3 3 4 8% 68%	2800
	11th to 20th	Day	1 1 1 1 1 2 2 5% 6% 80%	2800
	21st to end	Day	2 1 1 1 1 1 1 1 2 5% 84%	2800
July	1st to 10th	Day	1 1 1 2 3 91%	4560
	11th to 20th	Day	1 1 1 1 1 1 1 2 5% 86%	4337
	21st to end	Day	1 1 1 1 1 2 5% 88%	4456
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 3 3 3 3 4 6% 8% 57%	1894
	11th to 20th	Day	8% 5% 3 5% 4 4 5% 4 5% 7% 49%	1800
	21st to end	Day	11% 6% 2 5% 3 3 3 3 6% 6% 53%	1641
December	1st to 10th	Day	7% 4 2 4 3 3 4 4 5% 10% 55%	2620
	11th to 20th	Day	16% 10% 3 10% 6% 6% 6% 4 7% 8% 24%	3028
	21st to end	Day	14% 10% 3 10% 6% 5% 5% 4 6% 8% 29%	4004
January	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 6% 9% 49%	6147
	11th to 20th	Night	11% 7% 3 6% 4 3 3 3 5% 5% 52%	5880
	21st to end	Night	12% 7% 2 6% 4 4 4 3 4 6% 46%	6468
February	1st to 10th	Night	13% 6% 2 5% 5% 4 3 3 5% 5% 50%	2600
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2434
	21st to end	Night	10% 5% 2 5% 3 3 4 3 5% 7% 54%	1980
March	1st to 10th	Night	9% 7% 3 7% 5% 4 5% 4 5% 8% 42%	5055
	11th to 20th	Night	11% 4 2 5% 4 4 3 3 5% 6% 53%	5000
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 6% 9% 45%	5371
April	1st to 10th	Night	7% 5% 1 5% 5% 5% 3 3 6% 7% 53%	2000
	11th to 20th	Night	5% 4 2 3 3 3 3 3 5% 7% 61%	1979
	21st to end	Night	3 2 1 3 3 2 3 3 6% 9% 64%	2000
May	1st to 10th	Night	2 1 1 2 2 2 2 2 3 5% 78%	4381
	11th to 20th	Night	1 1 2 2 2 2 2 4 6% 78%	3850
	21st to end	Night	1 1 1 1 2 1 3 5% 84%	3972
June	1st to 10th	Night	2 2 1 3 3 2 2 3 6% 8% 69%	1600
	11th to 20th	Night	1 2 1 1 1 2 3 5% 83%	1600
	21st to end	Night	1 1 1 1 2 2 2 3 6% 82%	1600
July	1st to 10th	Night	1 1 1 3 92%	2620
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	2844
	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% 3 1 3 3 3 4 4 5% 9% 59%	2403
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 6% 8% 54%	2600
	21st to end	Night	9% 5% 1 3 3 3 3 3 5% 7% 57%	2755
December	1st to 10th	Night	7% 4 1 4 3 4 4 3 4 6% 60%	4560
	11th to 20th	Night	17% 7% 2 8% 5% 5% 5% 4 8% 9% 30%	5176
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 4 5% 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	3801
	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%	1860
	21st to end	Day	35% 5% 10% 23% 20% 7%	1650
March	1st to 10th	Day	41% 9% 9% 19% 20% 2	4825
	11th to 20th	Day	29% 7% 16% 27% 12% 8%	5000
	21st to end	Day	34% 9% 18% 26% 10% 2	5603
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	11% 3 7% 26% 34% 21%	5606
	11th to 20th	Day	8% 2 7% 30% 39% 13%	6054
	21st to end	Day	7% 3 11% 27% 45% 7%	6904
June	1st to 10th	Day	16% 5% 12% 25% 30% 12%	2800
	11th to 20th	Day	7% 3 9% 22% 35% 24%	2800
	21st to end	Day	7% 2 6% 15% 31% 39%	2800
July	1st to 10th	Day	3 1 3 7% 15% 71%	4491
	11th to 20th	Day	6% 1 5% 9% 13% 66%	4264
	21st to end	Day	3 1 3 8% 26% 59%	4316
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	25% 5% 11% 26% 21% 12%	1894
	11th to 20th	Day	34% 7% 10% 23% 21% 5%	1800
	21st to end	Day	31% 5% 10% 20% 25% 9%	1641
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	6147
	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%	2436
	21st to end	Night	30% 5% 12% 27% 19% 7%	1980
March	1st to 10th	Night	39% 10% 11% 18% 18% 4	5055
	11th to 20th	Night	31% 7% 15% 23% 15% 9%	5000
	21st to end	Night	35% 9% 17% 24% 13% 3	5371
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	11% 5% 9% 24% 30% 21%	4378
	11th to 20th	Night	9% 3 15% 32% 26% 14%	3850
	21st to end	Night	6% 4 13% 28% 38% 11%	3972
June	1st to 10th	Night	15% 7% 14% 24% 27% 13%	1600
	11th to 20th	Night	7% 3 10% 19% 33% 27%	1600
	21st to end	Night	8% 1 7% 22% 27% 36%	1600
July	1st to 10th	Night	3 1 3 11% 15% 67%	2580
	11th to 20th	Night	6% 1 4 13% 17% 58%	2794
	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% 18% 15%	2403
	11th to 20th	Night	29% 5% 12% 25% 21% 8%	2600
	21st to end	Night	28% 5% 11% 19% 26% 12%	2755
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	23% 13% 28% 21% 14% 1	271
	11th to 20th	Day	16% 12% 21% 34% 15% 3	400
	21st to end	Day	14% 10% 23% 28% 23% 3	440
February	1st to 10th	Day	19% 15% 20% 27% 17% 3	400
	11th to 20th	Day	16% 19% 25% 23% 15% 2	444
	21st to end	Day	14% 14% 28% 31% 12% 2	414
March	1st to 10th	Day	17% 18% 30% 22% 10% 2	500
	11th to 20th	Day	19% 16% 26% 21% 14% 4	500
	21st to end	Day	14% 20% 31% 23% 10% 1	550
April	1st to 10th	Day	13% 17% 32% 23% 13% 3	500
	11th to 20th	Day	12% 17% 34% 22% 10% 4	492
	21st to end	Day	5% 21% 29% 27% 14% 4	500
May	1st to 10th	Day	8% 16% 36% 28% 12% 1	503
	11th to 20th	Day	8% 13% 36% 27% 16% 1	528
	21st to end	Day	5% 12% 31% 37% 13% 2	573
June	1st to 10th	Day	4 13% 36% 31% 14% 3	530
	11th to 20th	Day	3 12% 35% 38% 11% 1	530
	21st to end	Day	1 7% 33% 41% 15% 2	530
July	1st to 10th	Day	4 33% 47% 16%	526
	11th to 20th	Day	2 27% 50% 19% 2	524
	21st to end	Day	1 1 30% 46% 18% 4	532
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% 13% 20% 25% 28% 7%	430
	11th to 20th	Day	7% 10% 21% 31% 24% 7%	300
	21st to end	Day	12% 8% 25% 32% 21% 1	220
December	1st to 10th	Day	13% 13% 24% 26% 20% 5%	200
	11th to 20th	Day	18% 12% 24% 25% 21% 2	200
	21st to end	Day	19% 15% 27% 23% 15% 1	220
January	1st to 10th	Night	16% 6% 19% 33% 21% 6%	800
	11th to 20th	Night	13% 6% 17% 36% 23% 6%	800
	21st to end	Night	16% 6% 15% 36% 24% 4	880
February	1st to 10th	Night	16% 9% 17% 34% 21% 4	800
	11th to 20th	Night	12% 9% 21% 33% 19% 5%	784
	21st to end	Night	12% 10% 22% 37% 18% 2	660
March	1st to 10th	Night	17% 9% 22% 29% 20% 3	800
	11th to 20th	Night	14% 11% 22% 32% 18% 3	800
	21st to end	Night	11% 11% 27% 34% 15% 2	880
April	1st to 10th	Night	8% 9% 28% 36% 17% 2	800
	11th to 20th	Night	7% 13% 31% 31% 16% 3	791
	21st to end	Night	4 9% 31% 34% 19% 2	800
May	1st to 10th	Night	4 7% 30% 40% 17% 2	725
	11th to 20th	Night	3 5% 26% 45% 20% 1	498
	21st to end	Night	3 7% 34% 41% 15%	433
June	1st to 10th	Night	3 4 32% 48% 12% 1	400
	11th to 20th	Night	1 5% 30% 50% 14% 1	400
	21st to end	Night	4 28% 48% 20% 1	400
July	1st to 10th	Night	1 1 27% 52% 19%	390
	11th to 20th	Night	1 1 18% 53% 23% 3	519
	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	7% 5% 17% 32% 32% 7%	780
	11th to 20th	Night	9% 3 19% 37% 27% 6%	800
	21st to end	Night	11% 4 17% 35% 27% 7%	800
December	1st to 10th	Night	12% 7% 17% 35% 25% 5%	800
	11th to 20th	Night	19% 7% 13% 36% 22% 3	800
	21st to end	Night	18% 8% 21% 32% 19% 1	880

Page generated at 2025-07-17 22:10:00. Version ac07

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