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 2024-11-02.

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% 63%	101194
	Night	1 4 2 5% 4 4 4 3 5% 7% 61%	97993
Transparency	Day	21% 7% 9% 19% 24% 20%	100940
	Night	21% 8% 11% 20% 24% 16%	97836
Seeing	Day	13% 21% 31% 22% 10% 3	15096
	Night	9% 12% 25% 31% 19% 4	24439

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 4 3 6% 5% 5% 6% 4 6% 8% 52%	2539
	11th to 20th	Day	4 3 6% 5% 7% 7% 5% 9% 11% 44%	2644
	21st to end	Day	3 9% 4 9% 7% 6% 7% 5% 6% 8% 37%	3146
February	1st to 10th	Day	2 6% 2 7% 5% 5% 5% 3 6% 8% 51%	1714
	11th to 20th	Day	1 5% 3 10% 5% 5% 4 3 6% 8% 49%	1850
	21st to end	Day	1 5% 4 8% 7% 5% 5% 5% 5% 6% 48%	1570
March	1st to 10th	Day	1 7% 5% 14% 7% 5% 5% 4 6% 8% 39%	3138
	11th to 20th	Day	1 5% 3 5% 3 4 3 4 4 8% 60%	3454
	21st to end	Day	1 10% 4 9% 5% 4 5% 5% 6% 8% 44%	4062
April	1st to 10th	Day	3 8% 3 6% 4 4 5% 4 5% 9% 49%	2280
	11th to 20th	Day	2 6% 2 6% 5% 5% 5% 3 6% 7% 53%	2249
	21st to end	Day	3 7% 2 4 3 3 4 3 5% 9% 57%	2280
May	1st to 10th	Day	3 5% 1 3 3 2 2 2 4 7% 68%	3951
	11th to 20th	Day	2 4 1 3 3 3 3 2 3 5% 69%	4299
	21st to end	Day	4 6% 2 5% 3 3 3 3 4 5% 61%	4813
June	1st to 10th	Day	3 4 2 4 3 4 4 3 5% 7% 61%	2660
	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	3 5% 2 4 4 2 3 2 4 5% 65%	1928
	11th to 20th	Day	2 3 2 3 3 2 4 2 4 6% 71%	1900
	21st to end	Day	1 3 2 3 2 4 3 2 4 6% 69%	2057
November	1st to 10th	Day	1 5% 2 5% 3 4 5% 5% 5% 7% 57%	1683
	11th to 20th	Day	2 4 1 3 4 3 3 4 8% 9% 60%	1710
	21st to end	Day	4 2 6% 4 4 5% 4 8% 9% 53%	1659
December	1st to 10th	Day	1 7% 3 5% 4 4 5% 5% 5% 10% 50%	1520
	11th to 20th	Day	1 8% 5% 10% 7% 7% 6% 5% 6% 8% 37%	1928
	21st to end	Day	1 5% 3 9% 5% 5% 6% 4 7% 12% 43%	2794
January	1st to 10th	Night	1 3 4 8% 5% 4 5% 4 7% 9% 49%	4406
	11th to 20th	Night	5% 3 8% 6% 6% 6% 6% 8% 10% 43%	4317
	21st to end	Night	2 8% 3 8% 6% 7% 7% 5% 8% 9% 37%	4521
February	1st to 10th	Night	1 5% 3 8% 5% 6% 6% 4 6% 8% 49%	2464
	11th to 20th	Night	6% 3 6% 5% 4 6% 5% 7% 9% 48%	2218
	21st to end	Night	5% 3 8% 6% 5% 6% 5% 6% 7% 49%	1884
March	1st to 10th	Night	1 7% 4 11% 5% 4 6% 5% 8% 8% 40%	3736
	11th to 20th	Night	1 5% 3 6% 3 3 5% 4 5% 8% 56%	3525
	21st to end	Night	1 9% 4 10% 6% 5% 7% 4 7% 9% 38%	3591
April	1st to 10th	Night	2 5% 3 6% 6% 5% 6% 4 6% 9% 49%	1900
	11th to 20th	Night	1 5% 2 5% 4 4 5% 4 6% 9% 55%	1879
	21st to end	Night	3 5% 2 5% 3 2 4 3 5% 8% 61%	1900
May	1st to 10th	Night	2 3 2 4 3 3 3 2 4 6% 69%	3020
	11th to 20th	Night	1 3 1 3 2 2 2 2 5% 6% 73%	2593
	21st to end	Night	3 4 1 3 2 3 3 3 4 6% 69%	2762
June	1st to 10th	Night	3 3 1 4 3 4 4 3 4 6% 65%	1520
	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	3 5% 2 3 2 2 2 2 3 4 71%	2250
	11th to 20th	Night	2 2 2 3 2 2 3 2 3 5% 74%	2280
	21st to end	Night	1 2 1 3 2 2 3 2 4 7% 72%	2541
November	1st to 10th	Night	1 4 3 4 3 3 5% 4 7% 8% 58%	2430
	11th to 20th	Night	1 3 2 3 3 4 4 5% 7% 10% 60%	2470
	21st to end	Night	2 3 2 5% 3 4 5% 3 5% 9% 58%	2517
December	1st to 10th	Night	2 3 2 6% 5% 5% 4 4 7% 9% 53%	2660
	11th to 20th	Night	2 9% 3 7% 5% 6% 7% 5% 7% 10% 40%	3276
	21st to end	Night	5% 3 10% 6% 7% 7% 5% 7% 9% 40%	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	25% 15% 18% 23% 17% 2	2539
	11th to 20th	Day	31% 15% 12% 22% 14% 6%	2644
	21st to end	Day	41% 11% 12% 19% 15% 3	3146
February	1st to 10th	Day	31% 8% 15% 20% 19% 7%	1714
	11th to 20th	Day	32% 7% 12% 24% 19% 5%	1860
	21st to end	Day	35% 8% 12% 23% 17% 5%	1570
March	1st to 10th	Day	39% 14% 15% 17% 10% 5%	3138
	11th to 20th	Day	21% 11% 12% 34% 18% 3	3454
	21st to end	Day	33% 15% 17% 22% 11% 2	4062
April	1st to 10th	Day	32% 8% 16% 22% 18% 4	2280
	11th to 20th	Day	29% 7% 14% 27% 18% 5%	2252
	21st to end	Day	25% 7% 17% 27% 21% 3	2280
May	1st to 10th	Day	17% 5% 9% 32% 29% 8%	3951
	11th to 20th	Day	17% 4 8% 23% 39% 9%	4302
	21st to end	Day	21% 6% 10% 29% 30% 3	4814
June	1st to 10th	Day	22% 6% 11% 23% 30% 8%	2660
	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	22% 3 6% 16% 32% 21%	1928
	11th to 20th	Day	16% 3 7% 24% 31% 19%	1900
	21st to end	Day	17% 5% 10% 22% 31% 15%	2057
November	1st to 10th	Day	26% 8% 11% 21% 25% 11%	1683
	11th to 20th	Day	19% 7% 16% 21% 27% 9%	1710
	21st to end	Day	25% 7% 10% 25% 23% 10%	1659
December	1st to 10th	Day	29% 9% 13% 21% 22% 7%	1520
	11th to 20th	Day	44% 11% 10% 16% 13% 6%	1928
	21st to end	Day	31% 10% 17% 21% 18% 4	2794
January	1st to 10th	Night	26% 12% 21% 24% 15% 2	4406
	11th to 20th	Night	30% 17% 17% 17% 14% 6%	4317
	21st to end	Night	37% 13% 13% 19% 14% 4	4521
February	1st to 10th	Night	32% 9% 11% 21% 18% 9%	2464
	11th to 20th	Night	28% 9% 14% 23% 20% 6%	2219
	21st to end	Night	33% 8% 13% 23% 19% 5%	1884
March	1st to 10th	Night	33% 15% 15% 20% 14% 3	3736
	11th to 20th	Night	21% 17% 8% 36% 15% 3	3525
	21st to end	Night	34% 16% 17% 21% 10% 2	3591
April	1st to 10th	Night	30% 10% 17% 22% 15% 5%	1900
	11th to 20th	Night	24% 8% 17% 27% 16% 7%	1879
	21st to end	Night	22% 7% 16% 32% 19% 3	1900
May	1st to 10th	Night	15% 5% 12% 32% 30% 5%	3020
	11th to 20th	Night	13% 5% 8% 23% 40% 11%	2593
	21st to end	Night	16% 5% 13% 32% 32% 2	2762
June	1st to 10th	Night	20% 6% 11% 24% 31% 8%	1520
	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	19% 3 7% 19% 32% 20%	2250
	11th to 20th	Night	13% 4 8% 21% 35% 19%	2280
	21st to end	Night	13% 4 10% 23% 35% 14%	2541
November	1st to 10th	Night	21% 8% 13% 20% 26% 12%	2430
	11th to 20th	Night	19% 7% 13% 25% 27% 9%	2470
	21st to end	Night	23% 8% 10% 24% 25% 10%	2517
December	1st to 10th	Night	26% 11% 13% 20% 21% 9%	2660
	11th to 20th	Night	37% 14% 11% 16% 16% 6%	3276
	21st to end	Night	33% 14% 15% 15% 18% 5%	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	31% 23% 21% 17% 8% 1	180
	11th to 20th	Day	26% 19% 21% 21% 10% 2	245
	21st to end	Day	18% 22% 27% 20% 11% 2	407
February	1st to 10th	Day	23% 25% 26% 18% 6% 2	372
	11th to 20th	Day	23% 25% 26% 21% 5%	455
	21st to end	Day	23% 23% 32% 19% 3 1	382
March	1st to 10th	Day	27% 29% 26% 13% 5%	460
	11th to 20th	Day	23% 28% 23% 16% 10% 1	460
	21st to end	Day	22% 32% 27% 15% 4	506
April	1st to 10th	Day	18% 28% 31% 15% 8% 1	460
	11th to 20th	Day	21% 29% 28% 14% 6% 1	452
	21st to end	Day	15% 23% 31% 19% 10% 2	460
May	1st to 10th	Day	20% 26% 28% 18% 7% 2	463
	11th to 20th	Day	17% 22% 29% 20% 9% 3	488
	21st to end	Day	17% 16% 31% 19% 13% 5%	529
June	1st to 10th	Day	9% 21% 33% 23% 10% 5%	490
	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	16% 18% 22% 19% 21% 3	273
	11th to 20th	Day	12% 17% 26% 25% 16% 5%	270
	21st to end	Day	10% 24% 26% 24% 13% 4	246
December	1st to 10th	Day	19% 22% 22% 19% 11% 7%	180
	11th to 20th	Day	18% 25% 27% 15% 11% 5%	180
	21st to end	Day	26% 16% 25% 21% 11% 2	198
January	1st to 10th	Night	15% 13% 28% 23% 14% 7%	760
	11th to 20th	Night	13% 14% 28% 31% 10% 4	760
	21st to end	Night	10% 14% 31% 25% 15% 5%	836
February	1st to 10th	Night	13% 13% 30% 26% 15% 3	760
	11th to 20th	Night	13% 17% 26% 25% 15% 5%	744
	21st to end	Night	16% 17% 24% 30% 11% 2	628
March	1st to 10th	Night	16% 22% 25% 23% 11% 3	760
	11th to 20th	Night	12% 19% 24% 22% 19% 4	760
	21st to end	Night	12% 21% 29% 25% 11% 1	836
April	1st to 10th	Night	10% 16% 30% 29% 13% 2	760
	11th to 20th	Night	12% 18% 28% 25% 14% 3	751
	21st to end	Night	9% 13% 29% 28% 17% 4	760
May	1st to 10th	Night	9% 18% 25% 28% 16% 4	665
	11th to 20th	Night	7% 11% 23% 38% 19% 3	425
	21st to end	Night	9% 8% 23% 33% 26% 2	411
June	1st to 10th	Night	6% 8% 24% 38% 21% 3	380
	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% 12% 20% 28% 22% 6%	748
	11th to 20th	Night	8% 13% 28% 30% 16% 5%	760
	21st to end	Night	11% 14% 24% 28% 19% 4	760
December	1st to 10th	Night	13% 13% 27% 30% 12% 5%	760
	11th to 20th	Night	12% 17% 28% 27% 12% 5%	760
	21st to end	Night	14% 19% 30% 25% 10% 2	836

Page generated at 2024-10-30 10:19:24. Version ac07

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