Lightholder Optics Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lightholder Optics Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	2 4 2 4 3 3 3 3 4 6% 65%	157801
	Night	1 4 2 5% 3 3 4 3 5% 7% 63%	151446
Transparency	Day	19% 6% 8% 19% 27% 21%	152777
	Night	20% 7% 9% 20% 25% 18%	147453
Seeing	Day	14% 21% 31% 21% 10% 3	17638
	Night	10% 12% 25% 31% 18% 4	26939

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	1 5% 3 6% 5% 4 5% 3 6% 8% 54%	4722
	11th to 20th	Day	1 2 1 4 3 4 4 3 6% 8% 64%	4956
	21st to end	Day	2 7% 3 8% 5% 5% 5% 5% 7% 9% 45%	5843
February	1st to 10th	Day	1 7% 2 7% 5% 5% 5% 4 6% 8% 51%	1896
	11th to 20th	Day	1 5% 3 10% 5% 5% 4 3 6% 8% 47%	2050
	21st to end	Day	1 5% 4 8% 6% 5% 5% 5% 6% 6% 50%	1730
March	1st to 10th	Day	1 8% 5% 12% 6% 4 4 4 5% 8% 42%	4618
	11th to 20th	Day	5% 3 6% 3 4 4 4 5% 9% 56%	5060
	21st to end	Day	1 9% 4 8% 4 4 5% 4 6% 8% 48%	5955
April	1st to 10th	Day	2 7% 3 6% 4 4 5% 4 5% 9% 50%	2520
	11th to 20th	Day	2 6% 2 6% 4 5% 5% 4 6% 7% 53%	2489
	21st to end	Day	3 8% 2 5% 3 3 4 3 5% 9% 55%	2520
May	1st to 10th	Day	5% 6% 2 4 2 2 2 2 3 6% 66%	7337
	11th to 20th	Day	1 3 1 3 2 3 3 2 3 6% 74%	8006
	21st to end	Day	3 7% 2 5% 3 3 3 2 3 5% 65%	8992
June	1st to 10th	Day	3 4 2 4 3 4 4 3 5% 7% 62%	2940
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 77%	2940
	21st to end	Day	2 2 1 2 2 2 2 2 4 4 76%	2940
July	1st to 10th	Day	1 1 2 2 1 2 1 2 3 85%	8219
	11th to 20th	Day	3 2 1 3 3 3 3 3 3 5% 71%	8220
	21st to end	Day	1 1 2 2 2 3 2 4 5% 76%	8644
August	1st to 10th	Day	1 1 1 2 2 2 2 2 3 3 82%	7444
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 80%	7001
	21st to end	Day	3 3 1 4 3 3 3 2 3 5% 69%	7750
September	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 78%	2520
	11th to 20th	Day	2 3 1 3 2 2 3 2 4 5% 72%	2489
	21st to end	Day	3 3 1 3 2 2 2 2 3 5% 75%	2310
October	1st to 10th	Day	2 4 2 4 3 2 2 3 4 6% 68%	3422
	11th to 20th	Day	1 4 2 3 2 2 2 2 3 4 76%	3292
	21st to end	Day	1 2 2 3 3 4 3 3 5% 7% 67%	3499
November	1st to 10th	Day	1 4 2 5% 3 4 5% 5% 5% 8% 58%	1854
	11th to 20th	Day	2 5% 1 3 4 3 4 4 8% 8% 58%	1890
	21st to end	Day	4 2 6% 4 4 4 4 8% 9% 55%	1833
December	1st to 10th	Day	1 4 2 4 3 4 4 4 6% 9% 60%	2700
	11th to 20th	Day	1 6% 4 8% 6% 6% 7% 5% 6% 9% 41%	3108
	21st to end	Day	1 7% 4 9% 5% 5% 6% 5% 7% 12% 40%	4092
January	1st to 10th	Night	2 4 3 7% 5% 4 5% 4 7% 10% 50%	8172
	11th to 20th	Night	3 2 4 3 3 4 4 5% 8% 63%	8013
	21st to end	Night	1 6% 3 8% 5% 5% 5% 4 7% 8% 47%	8371
February	1st to 10th	Night	1 5% 3 7% 5% 5% 6% 4 6% 8% 50%	2722
	11th to 20th	Night	5% 3 8% 5% 5% 6% 5% 7% 9% 46%	2458
	21st to end	Night	4 3 7% 5% 5% 6% 5% 6% 8% 51%	2076
March	1st to 10th	Night	1 8% 5% 10% 4 4 5% 5% 7% 8% 42%	5496
	11th to 20th	Night	5% 3 8% 4 4 5% 5% 6% 7% 52%	5159
	21st to end	Night	8% 4 9% 5% 5% 6% 5% 7% 9% 41%	5261
April	1st to 10th	Night	2 5% 3 7% 6% 5% 5% 4 6% 9% 50%	2100
	11th to 20th	Night	1 6% 3 5% 4 4 5% 4 6% 9% 54%	2079
	21st to end	Night	3 6% 2 5% 3 2 4 3 5% 8% 60%	2100
May	1st to 10th	Night	3 5% 2 4 3 3 3 2 3 5% 68%	5621
	11th to 20th	Night	1 3 1 3 2 2 2 2 4 5% 75%	4845
	21st to end	Night	2 5% 2 4 2 3 3 2 4 6% 69%	5181
June	1st to 10th	Night	3 3 3 3 4 4 3 4 7% 66%	1680
	11th to 20th	Night	1 1 1 2 2 2 2 1 4 4 81%	1680
	21st to end	Night	2 2 1 2 2 2 2 2 3 3 81%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	4740
	11th to 20th	Night	3 3 1 5% 3 3 2 2 3 4 73%	4740
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 77%	5442
August	1st to 10th	Night	2 1 1 2 1 2 2 1 3 4 82%	5368
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 5% 81%	5886
	21st to end	Night	3 2 1 3 2 3 2 3 3 5% 73%	6499
September	1st to 10th	Night	1 2 1 1 1 1 1 3 4 84%	2100
	11th to 20th	Night	2 2 1 2 1 2 3 1 3 5% 78%	2129
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 75%	2310
October	1st to 10th	Night	2 3 1 2 2 2 2 2 3 4 77%	3995
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 4 78%	3933
	21st to end	Night	1 2 2 4 2 3 3 3 5% 7% 68%	4310
November	1st to 10th	Night	1 4 3 4 3 3 4 4 6% 8% 60%	2665
	11th to 20th	Night	2 3 2 3 3 4 4 5% 7% 10% 57%	2730
	21st to end	Night	2 3 2 5% 3 4 4 3 5% 9% 59%	2783
December	1st to 10th	Night	1 2 1 4 3 4 4 4 5% 8% 64%	4700
	11th to 20th	Night	1 5% 2 6% 4 6% 7% 5% 8% 13% 41%	5316
	21st to end	Night	1 5% 3 9% 6% 7% 7% 5% 8% 10% 40%	7106

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	24% 11% 16% 24% 21% 2	4723
	11th to 20th	Day	18% 8% 7% 16% 22% 28%	4956
	21st to end	Day	31% 10% 9% 17% 22% 12%	5843
February	1st to 10th	Day	31% 8% 15% 19% 19% 9%	1896
	11th to 20th	Day	33% 7% 12% 24% 19% 5%	2060
	21st to end	Day	33% 8% 12% 23% 17% 7%	1730
March	1st to 10th	Day	37% 10% 12% 16% 19% 6%	4618
	11th to 20th	Day	23% 10% 14% 33% 17% 3	5060
	21st to end	Day	31% 13% 15% 24% 14% 3	5955
April	1st to 10th	Day	31% 9% 15% 22% 19% 4	2520
	11th to 20th	Day	29% 7% 13% 27% 18% 5%	2492
	21st to end	Day	26% 7% 16% 26% 21% 3	2520
May	1st to 10th	Day	20% 5% 7% 26% 35% 6%	7337
	11th to 20th	Day	13% 3 7% 24% 44% 9%	8009
	21st to end	Day	20% 5% 7% 26% 37% 5%	8994
June	1st to 10th	Day	21% 6% 10% 23% 30% 9%	2940
	11th to 20th	Day	11% 3 8% 21% 36% 21%	2940
	21st to end	Day	11% 4 5% 15% 35% 31%	2940
July	1st to 10th	Day	6% 2 1 6% 21% 63%	8151
	11th to 20th	Day	14% 4 4 8% 16% 54%	8145
	21st to end	Day	8% 4 5% 13% 25% 44%	8450
August	1st to 10th	Day	10% 3 3 10% 27% 47%	5895
	11th to 20th	Day	9% 4 3 18% 24% 41%	5517
	21st to end	Day	13% 2 4 21% 35% 25%	6077
September	1st to 10th	Day	10% 3 4 11% 35% 37%	2520
	11th to 20th	Day	15% 3 6% 20% 36% 20%	2489
	21st to end	Day	15% 2 4 20% 38% 20%	2310
October	1st to 10th	Day	17% 4 4 12% 38% 26%	3422
	11th to 20th	Day	14% 2 5% 18% 48% 12%	3292
	21st to end	Day	17% 4 8% 26% 32% 13%	3499
November	1st to 10th	Day	24% 7% 11% 23% 25% 10%	1854
	11th to 20th	Day	21% 8% 16% 21% 25% 8%	1890
	21st to end	Day	24% 6% 10% 25% 23% 11%	1833
December	1st to 10th	Day	19% 6% 10% 21% 29% 14%	2700
	11th to 20th	Day	37% 12% 13% 14% 15% 9%	3108
	21st to end	Day	33% 12% 16% 19% 17% 4	4092
January	1st to 10th	Night	25% 11% 18% 23% 20% 2	8172
	11th to 20th	Night	18% 9% 9% 13% 22% 29%	8013
	21st to end	Night	31% 9% 8% 16% 23% 13%	8371
February	1st to 10th	Night	31% 8% 11% 20% 17% 12%	2722
	11th to 20th	Night	30% 9% 13% 23% 19% 6%	2459
	21st to end	Night	31% 8% 13% 22% 19% 7%	2076
March	1st to 10th	Night	33% 11% 11% 17% 21% 6%	5496
	11th to 20th	Night	24% 16% 8% 36% 12% 3	5159
	21st to end	Night	32% 15% 18% 23% 10% 3	5261
April	1st to 10th	Night	29% 10% 16% 22% 17% 6%	2100
	11th to 20th	Night	25% 8% 16% 27% 17% 7%	2079
	21st to end	Night	22% 7% 15% 32% 20% 3	2100
May	1st to 10th	Night	19% 5% 9% 27% 34% 7%	5619
	11th to 20th	Night	12% 4 7% 26% 40% 11%	4845
	21st to end	Night	16% 5% 9% 28% 36% 5%	5182
June	1st to 10th	Night	18% 6% 11% 25% 30% 10%	1680
	11th to 20th	Night	9% 3 6% 24% 36% 22%	1680
	21st to end	Night	11% 2 4 16% 37% 31%	1680
July	1st to 10th	Night	4 1 2 7% 23% 63%	4700
	11th to 20th	Night	17% 3 3 7% 17% 54%	4692
	21st to end	Night	11% 3 4 14% 22% 47%	5324
August	1st to 10th	Night	11% 2 2 11% 29% 45%	4238
	11th to 20th	Night	7% 2 4 18% 28% 40%	4636
	21st to end	Night	12% 3 3 22% 33% 27%	5092
September	1st to 10th	Night	6% 2 4 11% 39% 38%	2100
	11th to 20th	Night	10% 3 7% 21% 38% 21%	2129
	21st to end	Night	14% 2 5% 21% 35% 22%	2310
October	1st to 10th	Night	13% 2 4 13% 39% 28%	3995
	11th to 20th	Night	12% 3 5% 17% 46% 17%	3933
	21st to end	Night	16% 4 8% 24% 35% 14%	4310
November	1st to 10th	Night	20% 8% 13% 22% 25% 12%	2665
	11th to 20th	Night	20% 7% 13% 26% 25% 9%	2730
	21st to end	Night	22% 7% 10% 24% 25% 11%	2783
December	1st to 10th	Night	19% 7% 10% 21% 25% 18%	4700
	11th to 20th	Night	30% 14% 13% 16% 19% 7%	5316
	21st to end	Night	33% 15% 14% 17% 16% 6%	7106

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	34% 23% 20% 15% 7% 1	220
	11th to 20th	Day	25% 17% 25% 20% 10% 2	293
	21st to end	Day	19% 22% 25% 21% 11% 3	473
February	1st to 10th	Day	25% 24% 25% 18% 6% 2	434
	11th to 20th	Day	27% 24% 25% 18% 4	535
	21st to end	Day	24% 22% 33% 18% 3	446
March	1st to 10th	Day	28% 29% 25% 12% 5% 1	540
	11th to 20th	Day	24% 26% 24% 16% 9% 1	540
	21st to end	Day	23% 30% 26% 15% 5%	594
April	1st to 10th	Day	18% 29% 30% 15% 7% 1	540
	11th to 20th	Day	21% 29% 28% 14% 6% 2	532
	21st to end	Day	16% 23% 31% 18% 10% 2	540
May	1st to 10th	Day	19% 25% 28% 18% 7% 2	543
	11th to 20th	Day	19% 23% 29% 18% 9% 3	568
	21st to end	Day	16% 18% 30% 19% 12% 5%	617
June	1st to 10th	Day	8% 22% 33% 23% 11% 4	570
	11th to 20th	Day	10% 23% 35% 20% 8% 3	570
	21st to end	Day	5% 18% 41% 22% 9% 5%	570
July	1st to 10th	Day	3 15% 44% 23% 11% 3	566
	11th to 20th	Day	4 11% 43% 25% 13% 4	564
	21st to end	Day	4 12% 39% 27% 13% 5%	619
August	1st to 10th	Day	4 16% 38% 26% 13% 3	567
	11th to 20th	Day	4 17% 38% 25% 11% 5%	557
	21st to end	Day	5% 13% 42% 25% 12% 3	627
September	1st to 10th	Day	6% 16% 39% 26% 12% 2	570
	11th to 20th	Day	9% 21% 32% 26% 10% 2	570
	21st to end	Day	9% 17% 35% 29% 7% 2	570
October	1st to 10th	Day	10% 21% 26% 28% 13% 2	570
	11th to 20th	Day	13% 18% 26% 28% 13% 3	543
	21st to end	Day	14% 21% 24% 25% 14% 3	548
November	1st to 10th	Day	16% 18% 24% 19% 19% 3	330
	11th to 20th	Day	18% 17% 25% 22% 13% 4	330
	21st to end	Day	14% 22% 24% 24% 12% 4	300
December	1st to 10th	Day	17% 19% 23% 20% 14% 7%	220
	11th to 20th	Day	21% 22% 25% 15% 10% 5%	220
	21st to end	Day	31% 16% 24% 18% 10% 1	242
January	1st to 10th	Night	17% 15% 27% 21% 13% 7%	840
	11th to 20th	Night	12% 14% 28% 31% 10% 4	840
	21st to end	Night	10% 14% 32% 26% 14% 4	924
February	1st to 10th	Night	15% 13% 29% 27% 14% 2	840
	11th to 20th	Night	15% 16% 27% 24% 14% 4	824
	21st to end	Night	17% 18% 25% 28% 10% 2	692
March	1st to 10th	Night	16% 23% 26% 22% 10% 3	840
	11th to 20th	Night	14% 18% 24% 22% 18% 3	840
	21st to end	Night	13% 21% 30% 24% 11% 1	924
April	1st to 10th	Night	11% 16% 30% 29% 13% 2	840
	11th to 20th	Night	13% 18% 28% 25% 14% 3	831
	21st to end	Night	10% 13% 28% 29% 16% 4	840
May	1st to 10th	Night	9% 18% 25% 29% 16% 3	735
	11th to 20th	Night	8% 12% 24% 37% 17% 2	470
	21st to end	Night	10% 8% 22% 34% 24% 2	455
June	1st to 10th	Night	6% 8% 24% 39% 21% 3	420
	11th to 20th	Night	3 10% 25% 37% 24% 1	420
	21st to end	Night	5% 6% 25% 40% 22% 2	420
July	1st to 10th	Night	2 3 21% 45% 27% 2	410
	11th to 20th	Night	2 2 29% 39% 27% 1	408
	21st to end	Night	4 4 18% 36% 33% 6%	557
August	1st to 10th	Night	4 2 19% 43% 29% 3	657
	11th to 20th	Night	3 6% 24% 39% 23% 5%	832
	21st to end	Night	4 5% 21% 42% 25% 3	924
September	1st to 10th	Night	5% 7% 20% 38% 26% 4	840
	11th to 20th	Night	5% 8% 21% 35% 24% 6%	840
	21st to end	Night	4 10% 21% 34% 24% 7%	840
October	1st to 10th	Night	8% 9% 21% 36% 20% 6%	840
	11th to 20th	Night	9% 9% 18% 36% 23% 5%	812
	21st to end	Night	9% 11% 22% 27% 21% 10%	880
November	1st to 10th	Night	12% 12% 20% 28% 22% 6%	820
	11th to 20th	Night	11% 14% 28% 28% 14% 5%	840
	21st to end	Night	12% 14% 25% 28% 18% 4	840
December	1st to 10th	Night	12% 13% 28% 30% 13% 5%	840
	11th to 20th	Night	14% 16% 27% 27% 12% 5%	840
	21st to end	Night	18% 18% 29% 23% 9% 2	924

Page generated at 2026-10-11 02:06:26. Version ac07

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