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 2025-06-10.

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 5% 3 3 4 3 5% 6% 64%	123426
	Night	1 4 2 5% 3 3 4 3 5% 7% 61%	123674
Transparency	Day	20% 6% 9% 19% 27% 18%	123173
	Night	21% 8% 10% 20% 26% 16%	123514
Seeing	Day	14% 20% 31% 22% 10% 3	15827
	Night	10% 12% 25% 31% 18% 4	25253

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 3 2 5% 4 4 5% 3 6% 8% 57%	3639
	11th to 20th	Day	3 2 4 4 5% 5% 4 6% 8% 60%	3800
	21st to end	Day	2 8% 3 8% 6% 5% 5% 4 5% 7% 46%	4510
February	1st to 10th	Day	1 6% 2 7% 5% 5% 5% 4 6% 8% 50%	1805
	11th to 20th	Day	1 5% 3 10% 5% 5% 4 3 7% 8% 48%	1950
	21st to end	Day	1 5% 4 8% 6% 5% 5% 4 5% 6% 50%	1650
March	1st to 10th	Day	1 8% 5% 12% 6% 4 4 4 5% 8% 42%	4518
	11th to 20th	Day	5% 3 6% 3 4 4 4 5% 9% 55%	4951
	21st to end	Day	1 9% 4 8% 4 4 5% 4 6% 8% 48%	5827
April	1st to 10th	Day	2 7% 3 7% 4 4 5% 4 5% 9% 49%	2400
	11th to 20th	Day	2 6% 2 6% 4 5% 5% 3 6% 7% 53%	2369
	21st to end	Day	3 8% 2 5% 3 3 4 3 5% 9% 56%	2400
May	1st to 10th	Day	3 7% 1 3 2 2 2 2 3 6% 68%	5656
	11th to 20th	Day	2 4 1 3 3 3 3 2 4 6% 69%	6160
	21st to end	Day	3 5% 1 4 2 2 3 2 4 5% 68%	6902
June	1st to 10th	Day	3 4 2 4 3 4 4 3 5% 7% 61%	2789
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 76%	2660
	21st to end	Day	2 2 1 3 2 2 3 2 5% 4 74%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 88%	4420
	11th to 20th	Day	3 3 1 4 3 3 3 3 4 5% 68%	4420
	21st to end	Day	2 1 1 2 2 2 3 2 4 5% 76%	4642
August	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 75%	4038
	11th to 20th	Day	1 1 1 1 1 1 2 1 2 3 86%	3724
	21st to end	Day	1 1 1 1 1 2 2 2 2 2 85%	4180
September	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 80%	2280
	11th to 20th	Day	2 3 1 3 2 3 3 3 4 6% 72%	2252
	21st to end	Day	3 3 1 2 2 2 2 2 3 4 76%	2090
October	1st to 10th	Day	2 4 2 4 3 2 2 3 4 6% 69%	3220
	11th to 20th	Day	1 3 2 2 2 2 2 2 3 5% 77%	3161
	21st to end	Day	1 2 2 4 3 4 3 3 5% 7% 67%	3391
November	1st to 10th	Day	1 4 2 5% 3 4 5% 5% 5% 8% 57%	1764
	11th to 20th	Day	2 4 1 3 4 3 3 4 8% 9% 60%	1800
	21st to end	Day	1 4 2 6% 4 4 5% 4 8% 9% 54%	1746
December	1st to 10th	Day	1 4 2 4 3 4 4 4 6% 9% 60%	2620
	11th to 20th	Day	1 6% 4 8% 6% 7% 7% 5% 6% 9% 41%	3028
	21st to end	Day	1 7% 4 9% 5% 5% 6% 5% 7% 12% 40%	4004
January	1st to 10th	Night	1 3 3 6% 4 3 5% 5% 7% 11% 53%	6306
	11th to 20th	Night	4 2 6% 4 4 4 4 6% 7% 58%	6165
	21st to end	Night	1 7% 3 8% 5% 5% 5% 4 7% 7% 48%	6468
February	1st to 10th	Night	1 5% 3 8% 5% 5% 6% 4 6% 8% 49%	2593
	11th to 20th	Night	6% 3 7% 5% 5% 6% 5% 7% 9% 48%	2338
	21st to end	Night	4 3 8% 6% 5% 6% 4 6% 7% 51%	1980
March	1st to 10th	Night	1 8% 5% 11% 4 4 5% 5% 7% 8% 41%	5376
	11th to 20th	Night	5% 3 9% 4 4 5% 5% 6% 7% 51%	5048
	21st to end	Night	8% 4 9% 5% 5% 6% 5% 7% 9% 41%	5148
April	1st to 10th	Night	2 5% 3 7% 6% 5% 6% 4 6% 9% 49%	2000
	11th to 20th	Night	1 5% 3 5% 4 4 5% 4 6% 9% 55%	1979
	21st to end	Night	3 5% 2 5% 3 2 4 3 5% 8% 61%	2000
May	1st to 10th	Night	2 4 1 3 2 2 3 2 3 5% 72%	4330
	11th to 20th	Night	1 4 1 4 2 2 2 2 4 6% 71%	3721
	21st to end	Night	2 3 1 3 2 2 2 2 4 5% 75%	3972
June	1st to 10th	Night	3 3 1 4 3 4 4 3 4 7% 66%	1595
	11th to 20th	Night	1 1 1 2 2 2 2 1 4 4 80%	1520
	21st to end	Night	2 3 1 2 2 2 2 2 3 3 79%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 2 90%	2540
	11th to 20th	Night	3 3 2 5% 2 2 2 2 3 3 74%	2540
	21st to end	Night	1 2 1 2 2 1 1 2 3 4 81%	2910
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 6% 73%	2901
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 3 87%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	3498
September	1st to 10th	Night	1 2 1 1 1 1 1 2 3 86%	1900
	11th to 20th	Night	2 2 1 2 1 2 3 1 3 5% 79%	1926
	21st to end	Night	2 2 1 3 2 2 2 2 3 4 76%	2090
October	1st to 10th	Night	2 3 1 2 2 2 3 2 3 4 77%	3758
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 4 78%	3777
	21st to end	Night	1 3 2 4 2 3 3 3 5% 7% 68%	4176
November	1st to 10th	Night	1 4 3 4 3 4 4 4 7% 8% 58%	2535
	11th to 20th	Night	1 3 2 3 3 4 4 4 7% 11% 59%	2600
	21st to end	Night	2 3 2 5% 4 4 5% 3 5% 9% 59%	2650
December	1st to 10th	Night	1 2 1 4 4 4 4 4 6% 8% 63%	4560
	11th to 20th	Night	1 6% 3 6% 4 6% 7% 5% 9% 13% 40%	5176
	21st to end	Night	1 5% 3 9% 6% 7% 7% 5% 8% 10% 40%	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	21% 12% 20% 22% 22% 2	3639
	11th to 20th	Day	21% 11% 8% 18% 27% 15%	3800
	21st to end	Day	33% 12% 9% 14% 20% 12%	4510
February	1st to 10th	Day	31% 8% 15% 19% 19% 9%	1805
	11th to 20th	Day	32% 7% 12% 24% 19% 5%	1960
	21st to end	Day	34% 8% 11% 23% 17% 7%	1650
March	1st to 10th	Day	38% 10% 12% 15% 19% 6%	4518
	11th to 20th	Day	23% 10% 14% 33% 17% 3	4951
	21st to end	Day	31% 13% 15% 24% 14% 3	5827
April	1st to 10th	Day	32% 8% 16% 22% 19% 4	2400
	11th to 20th	Day	29% 7% 13% 27% 19% 6%	2372
	21st to end	Day	26% 7% 16% 27% 21% 3	2400
May	1st to 10th	Day	18% 5% 7% 30% 35% 6%	5656
	11th to 20th	Day	16% 4 9% 26% 39% 7%	6163
	21st to end	Day	17% 5% 7% 24% 41% 6%	6904
June	1st to 10th	Day	22% 6% 11% 24% 30% 7%	2789
	11th to 20th	Day	12% 3 9% 23% 36% 18%	2660
	21st to end	Day	12% 4 5% 15% 32% 32%	2660
July	1st to 10th	Day	5% 2 2 6% 15% 71%	4351
	11th to 20th	Day	15% 5% 4 10% 17% 50%	4345
	21st to end	Day	9% 4 5% 11% 25% 47%	4495
August	1st to 10th	Day	13% 3 3 8% 28% 45%	4023
	11th to 20th	Day	6% 2 2 6% 24% 60%	3760
	21st to end	Day	7% 2 2 12% 41% 36%	4179
September	1st to 10th	Day	9% 3 4 9% 35% 40%	2280
	11th to 20th	Day	14% 4 6% 21% 35% 19%	2252
	21st to end	Day	15% 2 4 19% 38% 21%	2090
October	1st to 10th	Day	17% 3 4 12% 39% 26%	3220
	11th to 20th	Day	13% 2 5% 18% 49% 12%	3161
	21st to end	Day	17% 4 8% 26% 32% 13%	3391
November	1st to 10th	Day	25% 7% 11% 22% 25% 10%	1764
	11th to 20th	Day	19% 7% 17% 22% 26% 9%	1800
	21st to end	Day	25% 6% 11% 24% 22% 11%	1746
December	1st to 10th	Day	20% 6% 11% 21% 28% 15%	2620
	11th to 20th	Day	38% 12% 13% 14% 15% 8%	3028
	21st to end	Day	33% 12% 16% 19% 17% 4	4004
January	1st to 10th	Night	21% 11% 20% 23% 21% 3	6306
	11th to 20th	Night	22% 12% 12% 16% 25% 14%	6165
	21st to end	Night	31% 10% 9% 15% 22% 13%	6468
February	1st to 10th	Night	32% 9% 11% 20% 17% 11%	2593
	11th to 20th	Night	29% 9% 13% 23% 20% 6%	2339
	21st to end	Night	31% 7% 13% 23% 18% 7%	1980
March	1st to 10th	Night	34% 11% 11% 17% 21% 6%	5376
	11th to 20th	Night	25% 16% 8% 36% 12% 3	5048
	21st to end	Night	32% 15% 19% 23% 10% 2	5148
April	1st to 10th	Night	30% 10% 17% 22% 17% 5%	2000
	11th to 20th	Night	24% 8% 17% 27% 17% 7%	1979
	21st to end	Night	22% 7% 15% 32% 20% 4	2000
May	1st to 10th	Night	15% 5% 9% 33% 35% 4	4327
	11th to 20th	Night	15% 4 9% 28% 36% 8%	3721
	21st to end	Night	13% 4 9% 25% 43% 6%	3972
June	1st to 10th	Night	19% 6% 11% 26% 30% 8%	1595
	11th to 20th	Night	9% 3 7% 26% 35% 19%	1520
	21st to end	Night	11% 2 4 16% 35% 32%	1520
July	1st to 10th	Night	4 1 2 6% 17% 70%	2500
	11th to 20th	Night	17% 2 3 8% 16% 55%	2492
	21st to end	Night	9% 2 3 11% 26% 50%	2819
August	1st to 10th	Night	13% 3 3 10% 32% 40%	2892
	11th to 20th	Night	6% 1 2 8% 24% 58%	3156
	21st to end	Night	5% 1 2 13% 42% 37%	3498
September	1st to 10th	Night	5% 1 3 10% 39% 41%	1900
	11th to 20th	Night	9% 3 8% 22% 37% 21%	1926
	21st to end	Night	13% 2 5% 21% 35% 23%	2090
October	1st to 10th	Night	13% 2 4 13% 39% 28%	3758
	11th to 20th	Night	11% 3 5% 17% 47% 17%	3777
	21st to end	Night	16% 4 8% 23% 34% 14%	4176
November	1st to 10th	Night	21% 8% 13% 21% 25% 12%	2535
	11th to 20th	Night	19% 7% 13% 26% 26% 9%	2600
	21st to end	Night	23% 8% 10% 24% 24% 11%	2650
December	1st to 10th	Night	20% 7% 10% 20% 25% 18%	4560
	11th to 20th	Night	31% 14% 14% 16% 19% 6%	5176
	21st to end	Night	33% 14% 14% 17% 16% 6%	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	32% 23% 22% 17% 8% 1	200
	11th to 20th	Day	27% 18% 23% 20% 9% 2	269
	21st to end	Day	19% 23% 26% 20% 10% 3	440
February	1st to 10th	Day	26% 24% 25% 17% 6% 2	403
	11th to 20th	Day	25% 24% 26% 19% 5%	495
	21st to end	Day	22% 22% 33% 19% 3	414
March	1st to 10th	Day	28% 29% 26% 12% 5% 1	500
	11th to 20th	Day	26% 27% 22% 15% 9% 1	500
	21st to end	Day	24% 30% 26% 15% 5%	550
April	1st to 10th	Day	18% 28% 30% 15% 7% 1	500
	11th to 20th	Day	21% 29% 28% 14% 7% 1	492
	21st to end	Day	15% 23% 32% 19% 10% 2	500
May	1st to 10th	Day	20% 26% 28% 18% 7% 2	503
	11th to 20th	Day	19% 22% 29% 19% 9% 3	528
	21st to end	Day	16% 18% 31% 18% 13% 5%	573
June	1st to 10th	Day	9% 20% 33% 23% 10% 5%	527
	11th to 20th	Day	11% 23% 34% 21% 8% 3	490
	21st to end	Day	4 18% 41% 23% 9% 5%	490
July	1st to 10th	Day	2 13% 45% 24% 12% 3	486
	11th to 20th	Day	2 11% 43% 26% 13% 4	484
	21st to end	Day	3 11% 40% 28% 13% 4	531
August	1st to 10th	Day	4 16% 36% 27% 13% 3	487
	11th to 20th	Day	4 17% 39% 24% 11% 5%	477
	21st to end	Day	4 11% 43% 27% 13% 2	539
September	1st to 10th	Day	4 14% 40% 27% 13% 2	490
	11th to 20th	Day	8% 20% 32% 28% 10% 2	490
	21st to end	Day	8% 16% 36% 31% 7% 1	490
October	1st to 10th	Day	8% 22% 26% 28% 14% 1	490
	11th to 20th	Day	10% 18% 27% 29% 13% 3	490
	21st to end	Day	13% 22% 25% 24% 14% 3	506
November	1st to 10th	Day	17% 19% 23% 18% 20% 3	300
	11th to 20th	Day	16% 16% 26% 23% 14% 5%	300
	21st to end	Day	14% 23% 24% 23% 12% 4	273
December	1st to 10th	Day	18% 20% 22% 20% 14% 8%	200
	11th to 20th	Day	21% 23% 27% 16% 10% 5%	200
	21st to end	Day	29% 16% 25% 19% 10% 1	220
January	1st to 10th	Night	16% 14% 28% 22% 14% 7%	800
	11th to 20th	Night	13% 15% 28% 31% 10% 4	800
	21st to end	Night	10% 14% 32% 25% 15% 4	880
February	1st to 10th	Night	15% 13% 30% 25% 14% 3	800
	11th to 20th	Night	14% 16% 27% 24% 14% 4	784
	21st to end	Night	15% 17% 25% 29% 11% 2	660
March	1st to 10th	Night	16% 23% 26% 22% 11% 3	800
	11th to 20th	Night	15% 19% 23% 21% 18% 3	800
	21st to end	Night	14% 21% 28% 24% 11% 1	880
April	1st to 10th	Night	10% 17% 30% 29% 13% 2	800
	11th to 20th	Night	12% 17% 29% 25% 14% 3	791
	21st to end	Night	10% 13% 29% 28% 17% 4	800
May	1st to 10th	Night	9% 18% 25% 27% 16% 4	700
	11th to 20th	Night	8% 11% 24% 38% 18% 2	447
	21st to end	Night	9% 8% 23% 33% 25% 2	433
June	1st to 10th	Night	6% 8% 24% 38% 22% 3	399
	11th to 20th	Night	3 10% 24% 37% 25% 1	380
	21st to end	Night	4 5% 24% 40% 24% 3	380
July	1st to 10th	Night	2 3 20% 45% 28% 2	370
	11th to 20th	Night	2 2 27% 40% 28% 1	368
	21st to end	Night	4 3 18% 35% 34% 6%	503
August	1st to 10th	Night	4 3 18% 42% 30% 3	594
	11th to 20th	Night	3 5% 20% 41% 25% 5%	752
	21st to end	Night	4 4 20% 43% 26% 3	836
September	1st to 10th	Night	3 5% 21% 40% 28% 4	760
	11th to 20th	Night	4 8% 21% 34% 25% 7%	760
	21st to end	Night	4 9% 21% 33% 26% 8%	760
October	1st to 10th	Night	7% 9% 21% 36% 20% 6%	760
	11th to 20th	Night	8% 8% 18% 37% 24% 5%	760
	21st to end	Night	8% 11% 23% 26% 22% 11%	836
November	1st to 10th	Night	11% 13% 20% 28% 22% 6%	780
	11th to 20th	Night	10% 14% 28% 29% 15% 5%	800
	21st to end	Night	13% 14% 24% 27% 18% 4	800
December	1st to 10th	Night	13% 13% 27% 30% 13% 5%	800
	11th to 20th	Night	14% 16% 28% 27% 11% 5%	800
	21st to end	Night	16% 19% 30% 24% 10% 2	880

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