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 2024-03-08.

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 4 3 4 4 5% 7% 59%	78635
	Night	2 4 2 5% 4 3 5% 5% 5% 8% 58%	80957
Transparency	Day	24% 7% 10% 20% 24% 16%	78344
	Night	23% 8% 12% 20% 23% 13%	80769
Seeing	Day	13% 20% 31% 22% 10% 3	14144
	Night	9% 12% 24% 31% 19% 4	23651

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 5% 6% 4 6% 5% 6% 9% 52%	2539
	11th to 20th	Day	4 3 5% 6% 4 7% 7% 8% 11% 44%	2644
	21st to end	Day	3 9% 4 8% 8% 4 7% 7% 6% 8% 37%	3146
February	1st to 10th	Day	2 6% 2 6% 6% 4 5% 4 6% 8% 51%	1714
	11th to 20th	Day	1 4 4 9% 5% 3 5% 4 6% 8% 49%	1850
	21st to end	Day	1 5% 4 8% 7% 4 6% 6% 5% 6% 48%	1570
March	1st to 10th	Day	2 7% 5% 11% 7% 3 6% 6% 6% 8% 39%	1871
	11th to 20th	Day	1 7% 5% 6% 6% 5% 5% 6% 6% 9% 44%	1957
	21st to end	Day	1 6% 4 8% 6% 3 5% 6% 7% 9% 45%	2312
April	1st to 10th	Day	3 7% 3 6% 4 3 5% 6% 5% 9% 48%	2160
	11th to 20th	Day	2 5% 3 5% 5% 4 5% 5% 6% 7% 53%	2129
	21st to end	Day	3 6% 3 4 3 2 4 4 4 8% 57%	2160
May	1st to 10th	Day	5% 7% 2 4 4 2 3 3 5% 8% 57%	2246
	11th to 20th	Day	4 6% 2 4 4 3 5% 4 4 7% 56%	2425
	21st to end	Day	7% 9% 3 6% 4 2 5% 4 4 6% 50%	2723
June	1st to 10th	Day	4 4 2 4 4 3 4 4 5% 7% 61%	2520
	11th to 20th	Day	2 2 1 2 2 2 2 3 3 5% 76%	2520
	21st to end	Day	2 2 1 2 3 2 3 3 4 5% 73%	2520
July	1st to 10th	Day	1 2 1 1 2 1 3 2 4 4 80%	2520
	11th to 20th	Day	2 2 1 2 2 2 3 3 3 5% 76%	2520
	21st to end	Day	3 2 1 2 3 2 4 3 4 5% 71%	2655
August	1st to 10th	Day	2 3 1 2 3 2 3 3 4 4 73%	2301
	11th to 20th	Day	2 2 1 1 2 1 2 3 4 5% 77%	2122
	21st to end	Day	1 2 1 1 1 1 2 2 2 3 84%	2376
September	1st to 10th	Day	1 1 1 2 2 2 2 3 3 4 80%	2160
	11th to 20th	Day	2 2 1 2 3 2 3 3 4 5% 72%	2134
	21st to end	Day	3 3 1 2 2 2 2 2 3 4 75%	1980
October	1st to 10th	Day	3 5% 2 4 4 2 3 3 3 6% 65%	1827
	11th to 20th	Day	2 3 2 3 3 2 3 3 4 6% 71%	1800
	21st to end	Day	1 3 2 3 2 3 3 3 4 6% 69%	1949
November	1st to 10th	Day	1 4 2 5% 3 3 5% 6% 4 8% 58%	1674
	11th to 20th	Day	2 4 1 3 4 2 4 4 8% 9% 60%	1710
	21st to end	Day	4 2 5% 5% 3 5% 5% 8% 10% 54%	1659
December	1st to 10th	Day	1 6% 3 5% 5% 3 5% 6% 5% 10% 51%	1520
	11th to 20th	Day	2 8% 5% 9% 8% 5% 7% 6% 6% 8% 37%	1928
	21st to end	Day	1 5% 3 8% 5% 4 6% 6% 6% 12% 43%	2794
January	1st to 10th	Night	1 3 4 7% 7% 3 5% 5% 6% 10% 49%	4406
	11th to 20th	Night	5% 3 7% 7% 4 6% 7% 8% 10% 43%	4317
	21st to end	Night	2 8% 3 7% 7% 5% 7% 7% 8% 9% 37%	4521
February	1st to 10th	Night	1 4 3 7% 6% 4 7% 5% 6% 8% 49%	2464
	11th to 20th	Night	1 5% 3 5% 6% 3 6% 7% 7% 9% 48%	2218
	21st to end	Night	4 3 6% 7% 4 7% 6% 6% 8% 49%	1884
March	1st to 10th	Night	1 6% 3 8% 5% 3 6% 7% 7% 8% 45%	2249
	11th to 20th	Night	1 4 4 5% 6% 3 7% 7% 7% 10% 46%	2002
	21st to end	Night	5% 3 6% 4 3 6% 6% 6% 10% 50%	2044
April	1st to 10th	Night	2 5% 3 6% 7% 3 6% 6% 6% 9% 48%	1800
	11th to 20th	Night	1 4 3 4 4 2 6% 5% 6% 10% 55%	1779
	21st to end	Night	3 4 2 5% 3 2 4 3 5% 8% 61%	1800
May	1st to 10th	Night	3 4 2 4 4 2 5% 3 4 7% 61%	1710
	11th to 20th	Night	2 5% 2 4 4 2 3 3 7% 9% 59%	1465
	21st to end	Night	5% 6% 3 4 3 3 4 4 4 8% 57%	1552
June	1st to 10th	Night	4 3 1 3 3 2 5% 4 3 6% 66%	1440
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 4 80%	1440
	21st to end	Night	2 3 1 1 2 2 2 3 3 4 79%	1440
July	1st to 10th	Night	1 1 1 1 1 1 2 3 3 4 83%	1440
	11th to 20th	Night	2 2 1 3 2 1 2 3 3 3 78%	1440
	21st to end	Night	3 3 2 3 3 2 2 3 3 4 74%	1658
August	1st to 10th	Night	4 2 1 3 2 2 3 3 3 6% 72%	1636
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 4 78%	1780
	21st to end	Night	2 2 1 1 1 1 1 2 2 4 83%	1980
September	1st to 10th	Night	1 2 1 1 1 1 2 2 4 86%	1800
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 5% 80%	1824
	21st to end	Night	2 3 1 2 3 1 2 3 4 5% 75%	1980
October	1st to 10th	Night	3 5% 2 3 3 2 3 3 3 4 70%	2131
	11th to 20th	Night	2 2 2 2 2 2 3 3 3 6% 73%	2160
	21st to end	Night	1 2 1 3 2 2 2 3 4 8% 73%	2407
November	1st to 10th	Night	1 4 3 4 3 2 4 5% 6% 9% 58%	2405
	11th to 20th	Night	1 3 1 3 3 2 5% 5% 6% 10% 60%	2470
	21st to end	Night	2 3 2 4 3 3 5% 4 5% 9% 59%	2517
December	1st to 10th	Night	2 3 2 5% 5% 3 5% 5% 6% 10% 53%	2660
	11th to 20th	Night	2 8% 3 6% 6% 4 7% 7% 6% 10% 40%	3276
	21st to end	Night	5% 4 8% 8% 5% 8% 7% 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	42% 9% 13% 19% 14% 3	1871
	11th to 20th	Day	36% 11% 12% 17% 18% 6%	1957
	21st to end	Day	35% 12% 18% 22% 10% 3	2312
April	1st to 10th	Day	33% 7% 17% 20% 19% 4	2160
	11th to 20th	Day	30% 7% 14% 25% 19% 6%	2132
	21st to end	Day	26% 7% 17% 26% 20% 4	2160
May	1st to 10th	Day	26% 4 11% 32% 23% 4	2246
	11th to 20th	Day	29% 5% 11% 27% 24% 5%	2428
	21st to end	Day	35% 5% 11% 25% 19% 5%	2724
June	1st to 10th	Day	23% 6% 10% 23% 30% 8%	2520
	11th to 20th	Day	12% 3 9% 23% 35% 18%	2520
	21st to end	Day	13% 4 5% 15% 33% 30%	2520
July	1st to 10th	Day	8% 3 3 11% 27% 48%	2451
	11th to 20th	Day	11% 3 5% 14% 24% 43%	2445
	21st to end	Day	15% 3 6% 11% 24% 41%	2507
August	1st to 10th	Day	14% 3 4 11% 27% 40%	2285
	11th to 20th	Day	10% 4 3 10% 34% 38%	2122
	21st to end	Day	9% 2 2 13% 36% 38%	2376
September	1st to 10th	Day	9% 3 4 10% 34% 40%	2160
	11th to 20th	Day	14% 4 6% 21% 35% 20%	2134
	21st to end	Day	16% 2 4 20% 38% 19%	1980
October	1st to 10th	Day	23% 3 6% 17% 31% 20%	1827
	11th to 20th	Day	16% 3 8% 25% 29% 19%	1800
	21st to end	Day	17% 5% 10% 22% 31% 15%	1949
November	1st to 10th	Day	25% 7% 11% 21% 25% 11%	1674
	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% 12% 16% 18% 16% 4	2249
	11th to 20th	Night	29% 15% 11% 20% 20% 5%	2002
	21st to end	Night	29% 10% 20% 25% 13% 3	2044
April	1st to 10th	Night	32% 10% 17% 21% 16% 5%	1800
	11th to 20th	Night	25% 8% 18% 25% 17% 7%	1779
	21st to end	Night	22% 8% 16% 32% 19% 3	1800
May	1st to 10th	Night	23% 7% 12% 29% 25% 4	1710
	11th to 20th	Night	21% 6% 12% 32% 24% 5%	1465
	21st to end	Night	26% 6% 16% 27% 21% 4	1552
June	1st to 10th	Night	20% 5% 11% 24% 31% 8%	1440
	11th to 20th	Night	10% 3 7% 26% 34% 20%	1440
	21st to end	Night	12% 2 4 17% 35% 30%	1440
July	1st to 10th	Night	8% 2 3 11% 30% 47%	1400
	11th to 20th	Night	11% 2 5% 12% 26% 44%	1392
	21st to end	Night	15% 2 4 11% 25% 42%	1566
August	1st to 10th	Night	15% 4 4 14% 26% 37%	1627
	11th to 20th	Night	11% 2 4 13% 32% 37%	1780
	21st to end	Night	8% 1 2 12% 36% 40%	1980
September	1st to 10th	Night	6% 1 4 10% 37% 42%	1800
	11th to 20th	Night	8% 3 8% 23% 37% 21%	1824
	21st to end	Night	14% 2 6% 22% 35% 22%	1980
October	1st to 10th	Night	19% 3 7% 20% 30% 19%	2131
	11th to 20th	Night	13% 4 9% 22% 33% 19%	2160
	21st to end	Night	13% 4 11% 23% 35% 14%	2407
November	1st to 10th	Night	21% 8% 13% 20% 26% 12%	2405
	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% 30% 24% 13% 5%	449
	11th to 20th	Day	23% 28% 23% 16% 10% 1	420
	21st to end	Day	22% 32% 26% 16% 5%	462
April	1st to 10th	Day	19% 28% 31% 15% 7% 1	420
	11th to 20th	Day	22% 30% 28% 13% 5% 1	412
	21st to end	Day	15% 23% 31% 19% 10% 2	420
May	1st to 10th	Day	19% 24% 29% 19% 7% 2	423
	11th to 20th	Day	19% 23% 29% 18% 8% 3	448
	21st to end	Day	18% 17% 29% 18% 13% 4	485
June	1st to 10th	Day	9% 22% 33% 22% 9% 4	450
	11th to 20th	Day	11% 23% 34% 22% 8% 3	450
	21st to end	Day	4 16% 42% 24% 9% 5%	450
July	1st to 10th	Day	2 14% 46% 24% 11% 3	446
	11th to 20th	Day	2 11% 43% 27% 13% 5%	444
	21st to end	Day	3 10% 41% 28% 13% 5%	487
August	1st to 10th	Day	4 17% 36% 28% 13% 3	447
	11th to 20th	Day	3 13% 40% 26% 12% 5%	437
	21st to end	Day	3 11% 44% 27% 13% 2	495
September	1st to 10th	Day	4 12% 40% 28% 14% 2	450
	11th to 20th	Day	8% 19% 33% 29% 10% 2	450
	21st to end	Day	8% 16% 35% 32% 6% 2	450
October	1st to 10th	Day	8% 23% 26% 27% 15% 1	450
	11th to 20th	Day	10% 16% 27% 31% 14% 3	450
	21st to end	Day	11% 20% 25% 26% 15% 3	464
November	1st to 10th	Day	16% 18% 22% 19% 21% 3	270
	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	750
	11th to 20th	Night	12% 19% 24% 21% 20% 4	720
	21st to end	Night	12% 22% 27% 26% 12% 2	792
April	1st to 10th	Night	10% 15% 30% 29% 14% 2	720
	11th to 20th	Night	12% 18% 28% 24% 14% 4	711
	21st to end	Night	9% 13% 28% 27% 18% 4	720
May	1st to 10th	Night	8% 17% 24% 29% 17% 4	630
	11th to 20th	Night	7% 11% 23% 36% 19% 3	403
	21st to end	Night	10% 8% 22% 32% 27% 2	389
June	1st to 10th	Night	6% 8% 22% 38% 22% 3	360
	11th to 20th	Night	3 9% 24% 36% 26% 1	360
	21st to end	Night	4 4 24% 41% 25% 3	360
July	1st to 10th	Night	2 3 21% 44% 29% 2	350
	11th to 20th	Night	1 2 27% 41% 28% 1	348
	21st to end	Night	4 2 17% 35% 35% 7%	476
August	1st to 10th	Night	4 2 17% 42% 31% 3	562
	11th to 20th	Night	3 3 20% 43% 26% 5%	712
	21st to end	Night	3 4 20% 44% 26% 3	792
September	1st to 10th	Night	2 4 19% 41% 29% 4	720
	11th to 20th	Night	4 7% 20% 35% 26% 7%	720
	21st to end	Night	4 9% 21% 32% 26% 8%	720
October	1st to 10th	Night	8% 9% 21% 36% 20% 7%	720
	11th to 20th	Night	8% 7% 18% 38% 24% 5%	720
	21st to end	Night	7% 11% 21% 27% 23% 11%	792
November	1st to 10th	Night	11% 12% 19% 29% 23% 6%	740
	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 2024-03-05 04:24:05. Version ac07

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