Ancient Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ancient Light Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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 2 1 3 2 2 3 2 4 6% 72%	135054
	Night	4 2 1 3 2 2 3 3 5% 6% 69%	131207
Transparency	Day	15% 3 4 12% 25% 41%	129937
	Night	17% 3 4 11% 25% 40%	127327
Seeing	Day	4 3 17% 32% 30% 14%	16053
	Night	5% 1 15% 45% 30% 4	26150

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	6% 3 2 4 3 3 2 2 4 5% 67%	4098
	11th to 20th	Day	4 2 1 2 2 2 2 2 2 4 75%	4100
	21st to end	Day	7% 4 1 3 2 3 4 3 5% 7% 59%	4510
February	1st to 10th	Day	5% 3 1 3 3 2 3 3 5% 6% 66%	1855
	11th to 20th	Day	8% 3 1 4 3 2 3 3 3 5% 63%	1950
	21st to end	Day	7% 4 2 3 3 2 3 3 4 6% 63%	1709
March	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 5% 68%	4954
	11th to 20th	Day	6% 2 1 5% 3 3 4 3 4 5% 64%	5000
	21st to end	Day	6% 4 1 4 3 3 3 2 3 4 66%	5486
April	1st to 10th	Day	2 2 1 2 2 2 3 2 4 6% 74%	2200
	11th to 20th	Day	3 2 1 2 2 2 2 2 2 4 79%	2311
	21st to end	Day	2 1 1 1 2 2 2 3 5% 82%	2495
May	1st to 10th	Day	1 1 2 2 2 2 2 3 3 82%	5880
	11th to 20th	Day	1 1 2 1 2 2 2 3 3 83%	5841
	21st to end	Day	1 1 1 2 1 2 2 2 3 3 83%	6423
June	1st to 10th	Day	1 2 2 1 2 2 2 3 5% 80%	2600
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	2600
	21st to end	Day	2 2 1 2 3 2 2 3 4 4 76%	2600
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 76%	5879
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 10% 61%	5880
	21st to end	Day	3 2 1 3 3 3 4 4 7% 10% 60%	6401
August	1st to 10th	Day	2 2 1 3 2 3 3 3 6% 10% 64%	5779
	11th to 20th	Day	2 2 1 3 3 3 4 3 7% 9% 65%	5798
	21st to end	Day	2 1 1 3 2 3 3 4 5% 8% 68%	5902
September	1st to 10th	Day	4 3 1 4 3 3 3 4 5% 6% 64%	2124
	11th to 20th	Day	3 2 1 1 2 2 2 3 4 78%	2090
	21st to end	Day	3 1 1 3 2 2 3 2 4 5% 76%	2090
October	1st to 10th	Day	2 1 1 1 1 2 2 2 3 4 82%	3376
	11th to 20th	Day	2 1 1 1 1 1 1 1 2 3 86%	3150
	21st to end	Day	1 1 1 1 1 2 2 3 5% 83%	3438
November	1st to 10th	Day	2 1 1 1 1 2 2 2 4 6% 76%	1959
	11th to 20th	Day	2 2 1 3 2 3 2 2 3 5% 75%	1907
	21st to end	Day	5% 3 1 3 3 2 3 3 5% 6% 65%	1800
December	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 77%	2950
	11th to 20th	Day	4 4 2 5% 2 2 3 3 6% 8% 62%	3409
	21st to end	Day	8% 3 1 4 3 2 3 2 4 6% 65%	4510
January	1st to 10th	Night	5% 3 1 3 3 3 3 3 5% 7% 63%	5867
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 79%	5880
	21st to end	Night	12% 4 2 4 3 3 3 3 4 6% 57%	6468
February	1st to 10th	Night	6% 3 1 3 2 3 2 3 5% 6% 66%	2541
	11th to 20th	Night	8% 4 1 4 3 3 3 3 4 6% 62%	2338
	21st to end	Night	7% 3 1 4 3 3 3 3 5% 5% 63%	1920
March	1st to 10th	Night	6% 3 1 3 2 2 3 2 4 6% 68%	4940
	11th to 20th	Night	8% 3 1 3 3 3 3 2 4 5% 65%	5000
	21st to end	Night	5% 2 1 3 2 2 2 3 5% 7% 67%	5489
April	1st to 10th	Night	3 2 1 2 2 2 3 4 4 6% 71%	2200
	11th to 20th	Night	2 2 2 2 2 2 2 3 3 78%	2036
	21st to end	Night	2 1 1 1 1 2 2 3 3 84%	1900
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 78%	4100
	11th to 20th	Night	1 1 2 1 2 2 2 3 4 82%	4074
	21st to end	Night	1 1 1 1 1 2 1 3 4 86%	4474
June	1st to 10th	Night	2 2 1 2 2 2 2 2 3 6% 77%	1800
	11th to 20th	Night	1 1 1 1 1 1 2 3 88%	1800
	21st to end	Night	1 2 1 3 2 3 3 2 4 5% 73%	1800
July	1st to 10th	Night	2 3 1 4 3 3 4 3 4 5% 69%	4100
	11th to 20th	Night	1 1 1 4 4 5% 5% 5% 8% 11% 55%	4100
	21st to end	Night	2 2 1 4 4 5% 5% 6% 10% 13% 48%	4472
August	1st to 10th	Night	1 2 1 4 4 4 5% 5% 9% 11% 54%	4033
	11th to 20th	Night	2 1 1 3 3 4 5% 5% 7% 10% 58%	4051
	21st to end	Night	2 2 1 4 3 4 5% 4 6% 10% 59%	5074
September	1st to 10th	Night	4 3 1 4 3 3 5% 4 6% 8% 58%	2123
	11th to 20th	Night	3 2 1 3 2 3 3 2 3 5% 73%	2090
	21st to end	Night	2 2 1 3 3 2 3 2 4 5% 72%	2090
October	1st to 10th	Night	2 1 1 2 2 2 2 3 4 6% 76%	3600
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 87%	3778
	21st to end	Night	3 1 2 1 1 2 2 3 5% 81%	4120
November	1st to 10th	Night	4 2 1 2 2 2 2 2 3 6% 74%	2340
	11th to 20th	Night	2 2 1 3 3 2 2 2 3 5% 77%	2490
	21st to end	Night	7% 2 1 4 2 3 3 3 4 6% 65%	2600
December	1st to 10th	Night	5% 2 1 2 2 1 2 2 3 5% 76%	4240
	11th to 20th	Night	6% 3 2 5% 3 3 3 2 4 5% 64%	4811
	21st to end	Night	7% 3 1 4 3 3 3 3 4 5% 65%	6468

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	20% 2 3 11% 36% 27%	4099
	11th to 20th	Day	15% 2 2 11% 31% 38%	4100
	21st to end	Day	23% 5% 5% 12% 27% 29%	4510
February	1st to 10th	Day	20% 2 4 14% 23% 36%	1855
	11th to 20th	Day	25% 3 4 13% 28% 28%	1960
	21st to end	Day	22% 2 3 12% 27% 33%	1709
March	1st to 10th	Day	18% 3 3 11% 35% 31%	4954
	11th to 20th	Day	21% 4 4 16% 28% 28%	5000
	21st to end	Day	22% 3 3 10% 31% 31%	5486
April	1st to 10th	Day	13% 2 1 8% 34% 42%	2200
	11th to 20th	Day	12% 1 2 6% 24% 55%	2314
	21st to end	Day	8% 1 2 8% 25% 56%	2495
May	1st to 10th	Day	9% 2 1 7% 21% 60%	5880
	11th to 20th	Day	8% 1 2 5% 20% 64%	5844
	21st to end	Day	9% 1 1 2 12% 75%	6424
June	1st to 10th	Day	9% 2 1 4 20% 63%	2600
	11th to 20th	Day	6% 1 1 1 10% 81%	2600
	21st to end	Day	13% 3 3 8% 15% 59%	2600
July	1st to 10th	Day	13% 3 3 8% 20% 53%	5816
	11th to 20th	Day	17% 5% 8% 25% 28% 16%	5811
	21st to end	Day	18% 4 10% 24% 17% 27%	6217
August	1st to 10th	Day	20% 5% 15% 26% 19% 17%	4208
	11th to 20th	Day	16% 5% 9% 22% 27% 20%	4197
	21st to end	Day	15% 5% 9% 17% 25% 29%	4256
September	1st to 10th	Day	22% 3 6% 14% 24% 31%	2124
	11th to 20th	Day	11% 2 3 8% 29% 46%	2090
	21st to end	Day	13% 2 3 8% 22% 51%	2090
October	1st to 10th	Day	9% 2 3 8% 15% 64%	3376
	11th to 20th	Day	7% 2 1 5% 25% 60%	3150
	21st to end	Day	7% 1 1 6% 23% 61%	3438
November	1st to 10th	Day	11% 2 3 11% 34% 40%	1959
	11th to 20th	Day	15% 2 2 9% 28% 44%	1907
	21st to end	Day	20% 3 4 13% 24% 36%	1800
December	1st to 10th	Day	14% 2 2 11% 40% 31%	2950
	11th to 20th	Day	21% 2 4 12% 28% 33%	3408
	21st to end	Day	22% 3 4 19% 33% 19%	4510
January	1st to 10th	Night	19% 4 3 10% 40% 25%	5867
	11th to 20th	Night	12% 2 2 11% 36% 37%	5880
	21st to end	Night	28% 4 5% 9% 28% 26%	6468
February	1st to 10th	Night	20% 2 5% 12% 24% 37%	2541
	11th to 20th	Night	24% 2 4 16% 27% 27%	2339
	21st to end	Night	23% 3 3 11% 25% 34%	1920
March	1st to 10th	Night	18% 2 3 11% 34% 32%	4940
	11th to 20th	Night	21% 3 3 16% 31% 26%	5000
	21st to end	Night	16% 3 3 11% 37% 30%	5489
April	1st to 10th	Night	14% 3 2 9% 28% 43%	2200
	11th to 20th	Night	12% 2 2 7% 23% 55%	2036
	21st to end	Night	7% 2 2 7% 23% 60%	1900
May	1st to 10th	Night	11% 2 2 7% 25% 53%	4097
	11th to 20th	Night	9% 2 2 5% 20% 63%	4075
	21st to end	Night	6% 1 1 3 10% 79%	4474
June	1st to 10th	Night	11% 2 2 4 18% 64%	1800
	11th to 20th	Night	5% 1 2 3 12% 78%	1800
	21st to end	Night	14% 4 3 6% 12% 61%	1800
July	1st to 10th	Night	18% 3 4 7% 16% 52%	4055
	11th to 20th	Night	21% 5% 9% 23% 27% 15%	4046
	21st to end	Night	23% 5% 12% 22% 11% 26%	4344
August	1st to 10th	Night	27% 5% 18% 23% 11% 17%	2928
	11th to 20th	Night	21% 5% 9% 22% 22% 21%	2928
	21st to end	Night	20% 4 9% 15% 19% 33%	3650
September	1st to 10th	Night	23% 5% 7% 17% 18% 29%	2123
	11th to 20th	Night	14% 3 4 8% 24% 47%	2090
	21st to end	Night	14% 3 3 8% 21% 50%	2090
October	1st to 10th	Night	10% 3 3 7% 14% 63%	3600
	11th to 20th	Night	6% 1 1 7% 21% 64%	3778
	21st to end	Night	9% 1 1 6% 19% 63%	4120
November	1st to 10th	Night	13% 2 2 10% 29% 45%	2340
	11th to 20th	Night	13% 2 2 11% 25% 46%	2490
	21st to end	Night	21% 2 3 12% 26% 35%	2600
December	1st to 10th	Night	13% 2 3 14% 37% 32%	4240
	11th to 20th	Night	24% 2 3 10% 24% 37%	4811
	21st to end	Night	22% 3 4 18% 31% 22%	6468

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	11% 2 27% 40% 16% 4	300
	11th to 20th	Day	12% 2 26% 35% 20% 5%	300
	21st to end	Day	3 6% 22% 42% 23% 4	330
February	1st to 10th	Day	6% 4 23% 39% 22% 6%	355
	11th to 20th	Day	11% 7% 26% 38% 17% 2	495
	21st to end	Day	9% 7% 29% 38% 15% 2	414
March	1st to 10th	Day	7% 8% 31% 34% 16% 4	500
	11th to 20th	Day	6% 6% 27% 35% 17% 8%	500
	21st to end	Day	6% 7% 25% 36% 21% 5%	550
April	1st to 10th	Day	3 7% 24% 33% 24% 9%	500
	11th to 20th	Day	3 8% 26% 36% 20% 7%	492
	21st to end	Day	3 6% 25% 32% 23% 10%	500
May	1st to 10th	Day	1 6% 24% 32% 27% 9%	503
	11th to 20th	Day	2 4 22% 33% 28% 11%	528
	21st to end	Day	1 4 20% 33% 33% 9%	573
June	1st to 10th	Day	2 2 12% 33% 37% 14%	530
	11th to 20th	Day	2 2 16% 31% 34% 16%	530
	21st to end	Day	3 1 8% 35% 37% 17%	530
July	1st to 10th	Day	2 4 25% 44% 25%	526
	11th to 20th	Day	4 2 21% 43% 30%	524
	21st to end	Day	5% 1 16% 43% 34%	575
August	1st to 10th	Day	3 1 2 18% 42% 34%	527
	11th to 20th	Day	3 2 16% 45% 33%	517
	21st to end	Day	2 2 16% 42% 38%	583
September	1st to 10th	Day	5% 1 7% 29% 39% 19%	503
	11th to 20th	Day	4 1 10% 29% 39% 18%	490
	21st to end	Day	4 2 13% 34% 28% 19%	490
October	1st to 10th	Day	3 2 15% 36% 31% 12%	428
	11th to 20th	Day	3 3 19% 28% 33% 14%	300
	21st to end	Day	2 2 18% 31% 30% 15%	330
November	1st to 10th	Day	3 3 22% 36% 27% 9%	300
	11th to 20th	Day	4 2 21% 43% 23% 6%	300
	21st to end	Day	6% 2 26% 37% 22% 6%	300
December	1st to 10th	Day	8% 20% 41% 29% 3	300
	11th to 20th	Day	5% 3 27% 41% 20% 4	300
	21st to end	Day	12% 4 25% 37% 19% 5%	330
January	1st to 10th	Night	8% 2 23% 48% 17% 1	800
	11th to 20th	Night	7% 2 21% 48% 19% 3	800
	21st to end	Night	11% 3 20% 49% 16% 2	880
February	1st to 10th	Night	7% 2 21% 47% 21% 3	800
	11th to 20th	Night	8% 2 23% 50% 16% 1	784
	21st to end	Night	8% 2 26% 49% 14% 1	660
March	1st to 10th	Night	7% 3 27% 48% 15% 1	800
	11th to 20th	Night	7% 2 22% 41% 23% 6%	800
	21st to end	Night	5% 2 20% 47% 24% 2	880
April	1st to 10th	Night	3 2 18% 53% 22% 2	800
	11th to 20th	Night	3 2 23% 48% 22% 2	652
	21st to end	Night	2 2 20% 53% 20% 4	600
May	1st to 10th	Night	3 2 18% 52% 24% 2	600
	11th to 20th	Night	2 2 16% 52% 26% 2	591
	21st to end	Night	2 1 14% 52% 27% 3	650
June	1st to 10th	Night	2 1 9% 51% 34% 2	600
	11th to 20th	Night	1 10% 51% 35% 2	600
	21st to end	Night	3 1 10% 35% 47% 5%	600
July	1st to 10th	Night	3 1 5% 36% 52% 4	585
	11th to 20th	Night	3 3 25% 62% 7%	582
	21st to end	Night	5% 3 20% 60% 12%	657
August	1st to 10th	Night	2 2 21% 65% 11%	597
	11th to 20th	Night	4 1 3 25% 58% 8%	594
	21st to end	Night	4 1 3 26% 59% 8%	730
September	1st to 10th	Night	5% 1 7% 33% 48% 7%	772
	11th to 20th	Night	3 1 10% 41% 41% 4	760
	21st to end	Night	3 1 10% 45% 37% 5%	760
October	1st to 10th	Night	4 1 14% 49% 30% 2	760
	11th to 20th	Night	2 1 13% 50% 30% 3	760
	21st to end	Night	3 1 15% 47% 27% 7%	836
November	1st to 10th	Night	4 2 16% 51% 23% 4	780
	11th to 20th	Night	2 1 20% 51% 23% 3	800
	21st to end	Night	8% 2 22% 46% 20% 3	800
December	1st to 10th	Night	8% 2 17% 50% 22% 2	800
	11th to 20th	Night	10% 2 19% 48% 19% 2	800
	21st to end	Night	9% 1 19% 49% 18% 3	880

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