Charles Nicholas Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charles Nicholas Memorial Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	3 4 1 4 3 3 4 4 6% 7% 61%	134985
	Night	2 3 1 3 2 2 3 3 5% 8% 67%	134485
Transparency	Day	21% 5% 9% 17% 25% 24%	130073
	Night	17% 4 9% 18% 27% 26%	130401
Seeing	Day	13% 16% 27% 25% 15% 4	16352
	Night	10% 15% 27% 29% 17% 3	26641

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	5% 2 3 2 2 3 4 5% 6% 68%	4188
	11th to 20th	Day	1 2 4 2 1 2 2 3 6% 78%	4190
	21st to end	Day	2 3 1 4 3 2 3 3 5% 8% 66%	4609
February	1st to 10th	Day	1 4 2 3 2 2 3 2 4 7% 71%	1983
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 6% 69%	1949
	21st to end	Day	3 1 2 3 3 2 2 4 6% 74%	1650
March	1st to 10th	Day	1 7% 2 4 2 2 3 2 4 6% 69%	4699
	11th to 20th	Day	2 7% 1 2 3 2 3 3 5% 6% 66%	5000
	21st to end	Day	1 3 1 3 3 3 3 4 6% 8% 65%	5705
April	1st to 10th	Day	1 3 1 3 3 3 2 2 4 6% 72%	2400
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 6% 71%	2371
	21st to end	Day	2 3 1 2 2 1 2 2 3 6% 77%	2400
May	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 6% 72%	5440
	11th to 20th	Day	3 2 1 2 2 3 4 3 5% 6% 70%	5405
	21st to end	Day	1 1 1 1 2 2 3 3 5% 7% 74%	6343
June	1st to 10th	Day	1 3 1 4 4 4 5% 4 8% 10% 58%	2800
	11th to 20th	Day	2 2 1 3 3 2 4 4 6% 7% 67%	2800
	21st to end	Day	8% 4 2 5% 5% 5% 7% 7% 8% 7% 42%	2777
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 7% 6% 8% 9% 37%	5879
	11th to 20th	Day	4 3 1 5% 5% 6% 7% 7% 9% 10% 42%	5880
	21st to end	Day	7% 4 2 8% 7% 8% 9% 7% 10% 10% 29%	6325
August	1st to 10th	Day	5% 4 2 6% 5% 6% 7% 6% 7% 10% 43%	5348
	11th to 20th	Day	6% 3 2 7% 6% 6% 7% 6% 8% 10% 39%	5366
	21st to end	Day	7% 4 2 6% 6% 6% 7% 6% 7% 9% 40%	5984
September	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 5% 7% 7% 43%	2400
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 3 5% 7% 50%	2388
	21st to end	Day	6% 4 1 5% 3 3 3 2 4 5% 64%	2200
October	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 4 69%	3315
	11th to 20th	Day	5% 4 1 3 2 2 3 3 4 5% 68%	3250
	21st to end	Day	1 1 2 1 1 1 1 2 3 86%	3548
November	1st to 10th	Day	2 2 1 2 2 2 2 2 2 6% 77%	2059
	11th to 20th	Day	1 2 1 4 2 2 2 2 5% 7% 72%	1897
	21st to end	Day	3 3 1 2 2 2 3 4 5% 7% 68%	1890
December	1st to 10th	Day	3 5% 1 4 2 2 3 2 4 6% 68%	3030
	11th to 20th	Day	1 2 2 5% 2 2 3 3 4 7% 69%	3213
	21st to end	Day	2 1 2 1 2 2 2 5% 5% 76%	4304
January	1st to 10th	Night	1 5% 2 5% 2 3 4 2 4 6% 67%	5997
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 77%	6010
	21st to end	Night	1 4 1 3 2 3 3 3 6% 8% 66%	6611
February	1st to 10th	Night	2 1 2 2 2 3 3 5% 8% 71%	2415
	11th to 20th	Night	2 2 1 2 2 3 2 3 5% 7% 72%	2338
	21st to end	Night	1 2 1 2 2 1 2 3 5% 6% 75%	1980
March	1st to 10th	Night	2 1 3 1 2 2 2 4 6% 76%	5167
	11th to 20th	Night	2 10% 2 3 2 2 2 2 3 5% 67%	5000
	21st to end	Night	1 1 2 2 2 2 3 5% 8% 74%	5244
April	1st to 10th	Night	2 2 1 2 2 3 3 3 5% 7% 71%	2000
	11th to 20th	Night	2 2 1 1 1 2 2 2 4 7% 75%	1977
	21st to end	Night	1 3 1 1 2 2 3 2 3 5% 77%	2000
May	1st to 10th	Night	3 4 1 2 2 2 3 2 5% 6% 71%	4540
	11th to 20th	Night	2 2 1 2 2 3 3 2 3 5% 74%	4510
	21st to end	Night	2 1 1 1 2 2 2 3 4 81%	4530
June	1st to 10th	Night	2 4 1 4 4 4 4 3 7% 9% 59%	1600
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 6% 72%	1600
	21st to end	Night	6% 4 2 4 4 5% 6% 4 6% 8% 49%	1614
July	1st to 10th	Night	9% 6% 1 4 3 3 4 4 7% 9% 50%	4100
	11th to 20th	Night	2 3 1 4 3 4 6% 5% 8% 12% 51%	4100
	21st to end	Night	6% 5% 1 4 3 4 5% 5% 10% 16% 40%	4548
August	1st to 10th	Night	3 2 1 3 3 4 4 4 8% 14% 54%	4464
	11th to 20th	Night	4 3 1 3 3 4 4 5% 8% 15% 51%	4483
	21st to end	Night	4 3 1 4 3 4 4 5% 7% 13% 52%	4993
September	1st to 10th	Night	8% 4 1 5% 3 3 4 4 7% 9% 51%	2000
	11th to 20th	Night	7% 3 1 3 2 2 3 3 7% 7% 60%	2010
	21st to end	Night	5% 3 1 3 2 2 3 3 4 7% 67%	2200
October	1st to 10th	Night	3 2 2 2 2 2 2 3 5% 77%	3869
	11th to 20th	Night	3 4 1 3 2 2 3 3 4 4 72%	3898
	21st to end	Night	2 1 1 2 1 1 2 1 3 4 83%	4252
November	1st to 10th	Night	3 3 1 2 2 2 2 2 3 5% 75%	2460
	11th to 20th	Night	1 2 1 2 2 2 3 3 5% 7% 73%	2699
	21st to end	Night	2 5% 1 4 3 3 3 3 4 6% 67%	2730
December	1st to 10th	Night	2 6% 2 3 2 2 3 2 4 6% 68%	4380
	11th to 20th	Night	1 2 1 2 2 3 4 4 5% 7% 70%	5208
	21st to end	Night	1 3 1 3 2 2 2 3 4 7% 74%	6958

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	17% 3 4 18% 30% 28%	4189
	11th to 20th	Day	11% 2 5% 16% 37% 29%	4190
	21st to end	Day	17% 1 9% 14% 30% 28%	4609
February	1st to 10th	Day	15% 4 5% 17% 29% 29%	1983
	11th to 20th	Day	18% 4 5% 19% 27% 27%	1959
	21st to end	Day	14% 1 6% 12% 29% 38%	1650
March	1st to 10th	Day	18% 3 4 8% 19% 48%	4699
	11th to 20th	Day	16% 7% 5% 10% 30% 33%	5000
	21st to end	Day	17% 3 6% 17% 30% 26%	5705
April	1st to 10th	Day	15% 3 5% 17% 35% 26%	2400
	11th to 20th	Day	16% 2 4 14% 30% 34%	2374
	21st to end	Day	12% 2 5% 17% 34% 30%	2400
May	1st to 10th	Day	14% 3 5% 17% 26% 36%	5440
	11th to 20th	Day	14% 6% 4 14% 23% 38%	5408
	21st to end	Day	9% 4 7% 11% 17% 51%	6344
June	1st to 10th	Day	19% 6% 11% 20% 24% 20%	2800
	11th to 20th	Day	15% 4 6% 16% 24% 36%	2800
	21st to end	Day	33% 8% 12% 16% 18% 13%	2777
July	1st to 10th	Day	38% 10% 14% 18% 13% 7%	5816
	11th to 20th	Day	30% 11% 18% 21% 17% 4	5811
	21st to end	Day	41% 11% 20% 18% 9% 1	6142
August	1st to 10th	Day	30% 8% 12% 32% 15% 3	3892
	11th to 20th	Day	33% 7% 14% 21% 23% 2	3882
	21st to end	Day	36% 10% 18% 25% 10% 1	4311
September	1st to 10th	Day	36% 6% 13% 17% 18% 10%	2400
	11th to 20th	Day	34% 5% 8% 18% 26% 9%	2388
	21st to end	Day	22% 3 7% 23% 31% 13%	2200
October	1st to 10th	Day	20% 2 7% 11% 31% 29%	3315
	11th to 20th	Day	18% 3 9% 18% 29% 23%	3250
	21st to end	Day	8% 1 2 14% 40% 34%	3548
November	1st to 10th	Day	12% 2 4 21% 35% 26%	2059
	11th to 20th	Day	14% 1 7% 21% 31% 26%	1897
	21st to end	Day	16% 3 8% 17% 28% 27%	1890
December	1st to 10th	Day	20% 2 5% 18% 31% 25%	3030
	11th to 20th	Day	16% 2 6% 14% 35% 27%	3211
	21st to end	Day	11% 2 6% 14% 36% 31%	4304
January	1st to 10th	Night	20% 2 5% 14% 35% 23%	5997
	11th to 20th	Night	12% 2 6% 12% 34% 34%	6010
	21st to end	Night	17% 2 8% 16% 28% 28%	6611
February	1st to 10th	Night	13% 3 8% 18% 26% 32%	2415
	11th to 20th	Night	14% 4 7% 18% 31% 27%	2338
	21st to end	Night	11% 1 9% 15% 28% 36%	1980
March	1st to 10th	Night	11% 2 4 10% 28% 44%	5167
	11th to 20th	Night	19% 5% 4 10% 30% 31%	5000
	21st to end	Night	11% 2 6% 21% 35% 25%	5244
April	1st to 10th	Night	12% 3 6% 18% 36% 26%	2000
	11th to 20th	Night	11% 2 7% 17% 31% 32%	1977
	21st to end	Night	11% 4 5% 18% 34% 29%	2000
May	1st to 10th	Night	14% 4 4 17% 21% 39%	4537
	11th to 20th	Night	14% 2 6% 17% 28% 33%	4511
	21st to end	Night	9% 2 5% 10% 15% 58%	4531
June	1st to 10th	Night	20% 6% 10% 19% 23% 22%	1600
	11th to 20th	Night	14% 2 5% 17% 22% 39%	1600
	21st to end	Night	31% 5% 14% 18% 14% 16%	1614
July	1st to 10th	Night	30% 7% 18% 27% 14% 5%	4055
	11th to 20th	Night	22% 8% 22% 30% 16% 3	4046
	21st to end	Night	28% 9% 29% 23% 11% 1	4419
August	1st to 10th	Night	21% 6% 19% 34% 17% 3	3244
	11th to 20th	Night	20% 5% 17% 29% 26% 2	3243
	21st to end	Night	26% 8% 22% 27% 15% 1	3597
September	1st to 10th	Night	29% 5% 16% 18% 20% 12%	2000
	11th to 20th	Night	23% 6% 13% 20% 27% 12%	2010
	21st to end	Night	19% 3 10% 22% 32% 13%	2200
October	1st to 10th	Night	13% 2 6% 11% 34% 33%	3869
	11th to 20th	Night	17% 3 8% 14% 32% 25%	3898
	21st to end	Night	9% 2 3 14% 37% 35%	4252
November	1st to 10th	Night	14% 2 4 19% 32% 30%	2460
	11th to 20th	Night	11% 3 5% 20% 32% 28%	2699
	21st to end	Night	19% 3 7% 18% 29% 24%	2730
December	1st to 10th	Night	19% 2 7% 18% 32% 22%	4380
	11th to 20th	Night	15% 1 7% 16% 29% 32%	5209
	21st to end	Night	13% 2 5% 18% 31% 32%	6958

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	18% 26% 35% 16% 4 1	330
	11th to 20th	Day	20% 22% 32% 18% 7% 1	330
	21st to end	Day	13% 26% 29% 22% 9% 1	363
February	1st to 10th	Day	19% 25% 28% 21% 6% 1	300
	11th to 20th	Day	24% 24% 33% 15% 3 1	300
	21st to end	Day	30% 29% 26% 10% 4 1	249
March	1st to 10th	Day	23% 28% 34% 12% 4	382
	11th to 20th	Day	26% 25% 31% 12% 6% 1	500
	21st to end	Day	24% 30% 33% 9% 3 1	550
April	1st to 10th	Day	16% 31% 36% 13% 3 1	500
	11th to 20th	Day	18% 34% 31% 14% 3	492
	21st to end	Day	16% 31% 34% 15% 3	500
May	1st to 10th	Day	8% 30% 40% 19% 3	503
	11th to 20th	Day	12% 22% 36% 21% 7% 2	528
	21st to end	Day	7% 17% 43% 23% 8% 1	692
June	1st to 10th	Day	3 9% 33% 34% 18% 3	730
	11th to 20th	Day	3 8% 31% 39% 15% 4	730
	21st to end	Day	9% 4 19% 34% 28% 6%	707
July	1st to 10th	Day	8% 2 14% 36% 32% 10%	526
	11th to 20th	Day	4 2 8% 38% 36% 12%	524
	21st to end	Day	5% 1 7% 32% 40% 15%	575
August	1st to 10th	Day	5% 2 8% 31% 38% 15%	527
	11th to 20th	Day	9% 2 11% 34% 33% 11%	517
	21st to end	Day	9% 3 9% 36% 32% 11%	583
September	1st to 10th	Day	11% 3 18% 38% 23% 6%	530
	11th to 20th	Day	6% 5% 20% 43% 21% 5%	518
	21st to end	Day	6% 7% 31% 32% 20% 4	330
October	1st to 10th	Day	11% 11% 26% 32% 15% 5%	330
	11th to 20th	Day	14% 11% 34% 21% 17% 4	330
	21st to end	Day	12% 17% 31% 24% 10% 6%	363
November	1st to 10th	Day	16% 22% 32% 18% 11% 2	330
	11th to 20th	Day	15% 16% 32% 20% 14% 3	330
	21st to end	Day	17% 25% 31% 15% 10% 1	330
December	1st to 10th	Day	19% 23% 30% 21% 6% 1	330
	11th to 20th	Day	17% 25% 29% 23% 5% 1	330
	21st to end	Day	22% 26% 32% 16% 4	363
January	1st to 10th	Night	10% 23% 32% 22% 10% 3	840
	11th to 20th	Night	10% 22% 32% 24% 9% 3	840
	21st to end	Night	13% 22% 29% 25% 9% 2	924
February	1st to 10th	Night	10% 24% 35% 20% 8% 3	800
	11th to 20th	Night	14% 21% 41% 19% 4 1	784
	21st to end	Night	17% 29% 36% 15% 3 1	660
March	1st to 10th	Night	15% 27% 30% 22% 6% 1	800
	11th to 20th	Night	15% 20% 30% 25% 9% 1	800
	21st to end	Night	12% 25% 37% 19% 6%	777
April	1st to 10th	Night	12% 23% 43% 18% 5%	600
	11th to 20th	Night	12% 23% 37% 24% 5%	593
	21st to end	Night	9% 24% 35% 27% 6%	600
May	1st to 10th	Night	5% 19% 43% 29% 4	600
	11th to 20th	Night	9% 14% 31% 34% 10% 2	591
	21st to end	Night	6% 12% 38% 33% 10%	650
June	1st to 10th	Night	3 5% 30% 45% 17% 1	600
	11th to 20th	Night	4 5% 31% 42% 17% 2	600
	21st to end	Night	6% 3 18% 43% 28% 2	600
July	1st to 10th	Night	8% 2 7% 40% 41% 3	585
	11th to 20th	Night	4 1 5% 36% 45% 8%	582
	21st to end	Night	6% 2 4 29% 49% 10%	657
August	1st to 10th	Night	6% 1 3 31% 47% 12%	597
	11th to 20th	Night	5% 2 5% 33% 43% 13%	594
	21st to end	Night	5% 1 7% 30% 50% 9%	660
September	1st to 10th	Night	7% 3 8% 38% 37% 7%	600
	11th to 20th	Night	9% 1 12% 44% 30% 4	600
	21st to end	Night	7% 4 15% 45% 26% 3	600
October	1st to 10th	Night	7% 8% 23% 40% 19% 3	762
	11th to 20th	Night	9% 10% 28% 35% 16% 3	800
	21st to end	Night	9% 13% 28% 29% 16% 6%	880
November	1st to 10th	Night	9% 14% 30% 29% 14% 5%	820
	11th to 20th	Night	7% 15% 29% 34% 13% 3	1019
	21st to end	Night	12% 20% 30% 23% 12% 3	1050
December	1st to 10th	Night	14% 20% 29% 27% 9% 2	1050
	11th to 20th	Night	12% 24% 31% 24% 8% 1	1050
	21st to end	Night	15% 24% 32% 23% 5% 2	1076

Page generated at 2026-01-28 23:21:45. Version ac07

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