Luz Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Luz Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	1 1 1 2 2 2 2 2 4 5% 78%	135961
	Night	1 1 1 2 2 2 2 2 4 6% 78%	131306
Transparency	Day	10% 4 5% 14% 27% 40%	131044
	Night	10% 4 6% 17% 29% 33%	127196
Seeing	Day	4 11% 25% 32% 21% 7%	17009
	Night	2 5% 21% 38% 29% 5%	26773

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	2 3 1 4 2 2 2 2 3 5% 75%	4098
	11th to 20th	Day	1 2 1 2 1 2 2 2 4 6% 75%	4252
	21st to end	Day	3 4 2 4 3 3 4 3 4 8% 60%	5016
February	1st to 10th	Day	2 3 1 4 3 3 3 3 5% 6% 68%	2000
	11th to 20th	Day	3 2 1 2 3 3 3 3 5% 7% 69%	1950
	21st to end	Day	1 3 1 4 3 3 4 3 5% 6% 68%	1650
March	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 7% 66%	4518
	11th to 20th	Day	3 3 1 4 2 3 3 3 7% 7% 63%	4732
	21st to end	Day	1 1 1 2 2 2 3 3 6% 8% 72%	5836
April	1st to 10th	Day	2 3 1 2 2 2 2 3 5% 7% 72%	2400
	11th to 20th	Day	1 2 1 2 2 1 1 2 4 5% 81%	2369
	21st to end	Day	1 1 2 1 1 2 2 3 6% 80%	2400
May	1st to 10th	Day	1 1 1 2 1 2 2 2 3 5% 81%	5782
	11th to 20th	Day	1 1 1 1 1 2 2 3 5% 83%	5841
	21st to end	Day	1 1 1 1 1 2 3 89%	6423
June	1st to 10th	Day	1 1 1 1 2 2 3 5% 83%	2600
	11th to 20th	Day	1 1 1 1 1 3 91%	2690
	21st to end	Day	1 1 1 2 2 2 1 2 3 87%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 2 92%	6319
	11th to 20th	Day	1 1 2 2 2 2 2 3 5% 82%	5978
	21st to end	Day	1 1 1 1 1 1 2 3 4 85%	6241
August	1st to 10th	Day	1 1 1 1 1 1 1 3 4 86%	5346
	11th to 20th	Day	1 1 1 1 1 3 92%	5364
	21st to end	Day	1 1 2 1 2 2 2 3 4 81%	5984
September	1st to 10th	Day	1 2 1 2 2 2 2 2 3 6% 77%	2400
	11th to 20th	Day	1 1 1 1 1 2 2 89%	2205
	21st to end	Day	1 1 1 2 2 1 3 3 86%	2200
October	1st to 10th	Day	1 1 1 1 2 1 2 4 86%	3600
	11th to 20th	Day	1 1 1 1 1 2 4 88%	3472
	21st to end	Day	1 1 2 2 2 2 3 4 6% 78%	3262
November	1st to 10th	Day	1 1 1 2 2 2 2 3 4 5% 78%	1764
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 5% 76%	1800
	21st to end	Day	2 2 1 2 3 3 4 3 4 8% 70%	1800
December	1st to 10th	Day	1 2 1 2 2 3 3 3 6% 8% 70%	2950
	11th to 20th	Day	1 2 1 3 2 2 3 4 8% 9% 64%	3409
	21st to end	Day	3 3 2 4 3 4 5% 5% 7% 10% 55%	4510
January	1st to 10th	Night	1 2 1 2 2 2 3 3 4 6% 75%	5867
	11th to 20th	Night	1 2 1 3 2 2 3 2 3 6% 75%	5731
	21st to end	Night	3 3 1 4 3 3 4 4 6% 7% 62%	5984
February	1st to 10th	Night	2 3 1 3 2 3 3 4 5% 6% 67%	2400
	11th to 20th	Night	2 2 1 3 3 3 2 3 5% 8% 69%	2338
	21st to end	Night	2 2 1 3 3 3 4 3 4 8% 68%	1980
March	1st to 10th	Night	2 3 1 3 3 2 3 3 5% 8% 67%	5376
	11th to 20th	Night	3 3 1 4 3 3 3 4 5% 8% 63%	5266
	21st to end	Night	1 1 2 2 3 3 3 5% 8% 72%	5122
April	1st to 10th	Night	1 2 1 3 3 2 3 3 5% 8% 70%	2000
	11th to 20th	Night	1 2 2 1 2 2 2 4 7% 76%	1979
	21st to end	Night	1 1 1 1 1 1 2 4 6% 81%	2000
May	1st to 10th	Night	1 1 1 1 2 2 2 3 4 83%	4198
	11th to 20th	Night	1 1 1 1 2 3 4 86%	4074
	21st to end	Night	1 1 1 1 1 2 3 89%	4474
June	1st to 10th	Night	1 1 1 1 1 2 3 5% 85%	1800
	11th to 20th	Night	1 1 1 2 3 92%	1701
	21st to end	Night	1 1 1 1 1 1 1 3 90%	1600
July	1st to 10th	Night	1 1 1 95%	3640
	11th to 20th	Night	1 1 1 1 1 2 3 87%	3962
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	4622
August	1st to 10th	Night	1 1 1 1 1 1 2 3 89%	4486
	11th to 20th	Night	1 1 2 94%	4506
	21st to end	Night	1 1 1 1 2 2 3 4 85%	5015
September	1st to 10th	Night	1 1 1 1 2 2 2 3 5% 83%	2000
	11th to 20th	Night	1 1 1 1 1 2 2 91%	2193
	21st to end	Night	1 1 1 1 1 1 2 4 88%	2200
October	1st to 10th	Night	1 1 1 1 1 2 1 2 3 88%	3600
	11th to 20th	Night	1 1 1 1 1 3 5% 88%	3639
	21st to end	Night	1 1 1 1 1 2 2 2 3 4 83%	4299
November	1st to 10th	Night	1 2 1 2 2 2 4 6% 79%	2535
	11th to 20th	Night	1 1 2 2 2 2 3 4 6% 77%	2600
	21st to end	Night	2 1 1 2 2 4 3 3 6% 8% 68%	2600
December	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 7% 73%	4240
	11th to 20th	Night	1 1 1 2 2 3 4 4 7% 9% 65%	4811
	21st to end	Night	2 3 1 4 3 4 4 4 6% 11% 58%	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	14% 3 6% 26% 36% 14%	4099
	11th to 20th	Day	13% 4 11% 16% 24% 33%	4252
	21st to end	Day	22% 7% 8% 15% 26% 23%	5016
February	1st to 10th	Day	19% 5% 6% 17% 27% 26%	2000
	11th to 20th	Day	16% 6% 6% 17% 33% 22%	1960
	21st to end	Day	19% 6% 8% 19% 27% 21%	1650
March	1st to 10th	Day	18% 7% 6% 20% 28% 21%	4518
	11th to 20th	Day	17% 7% 9% 25% 29% 13%	4732
	21st to end	Day	11% 6% 11% 32% 27% 13%	5836
April	1st to 10th	Day	15% 5% 9% 20% 31% 20%	2400
	11th to 20th	Day	10% 4 5% 20% 35% 26%	2372
	21st to end	Day	9% 3 7% 20% 40% 20%	2400
May	1st to 10th	Day	7% 4 7% 17% 33% 32%	5782
	11th to 20th	Day	6% 3 4 14% 39% 34%	5844
	21st to end	Day	5% 2 3 10% 40% 41%	6424
June	1st to 10th	Day	7% 3 4 13% 32% 40%	2600
	11th to 20th	Day	2 2 3 8% 23% 62%	2690
	21st to end	Day	7% 2 2 4 17% 69%	2800
July	1st to 10th	Day	3 1 1 3 10% 82%	6251
	11th to 20th	Day	7% 3 3 6% 15% 66%	5906
	21st to end	Day	5% 2 3 7% 21% 61%	6060
August	1st to 10th	Day	8% 2 3 7% 27% 53%	3890
	11th to 20th	Day	4 2 2 8% 20% 65%	3880
	21st to end	Day	6% 2 2 4 17% 68%	4311
September	1st to 10th	Day	10% 4 6% 7% 21% 53%	2400
	11th to 20th	Day	4 2 3 7% 29% 56%	2205
	21st to end	Day	7% 2 3 10% 29% 49%	2200
October	1st to 10th	Day	6% 3 3 8% 18% 63%	3600
	11th to 20th	Day	4 2 3 9% 32% 51%	3472
	21st to end	Day	8% 3 4 9% 30% 46%	3262
November	1st to 10th	Day	10% 4 6% 15% 33% 31%	1764
	11th to 20th	Day	12% 3 7% 14% 29% 35%	1800
	21st to end	Day	16% 5% 8% 15% 28% 28%	1800
December	1st to 10th	Day	14% 4 6% 14% 33% 28%	2950
	11th to 20th	Day	15% 8% 10% 19% 36% 11%	3408
	21st to end	Day	24% 7% 11% 24% 25% 10%	4510
January	1st to 10th	Night	13% 5% 7% 22% 36% 18%	5867
	11th to 20th	Night	14% 4 7% 20% 24% 31%	5731
	21st to end	Night	20% 8% 8% 15% 28% 21%	5984
February	1st to 10th	Night	19% 7% 8% 18% 23% 25%	2400
	11th to 20th	Night	16% 6% 9% 20% 28% 22%	2339
	21st to end	Night	18% 9% 9% 18% 24% 23%	1980
March	1st to 10th	Night	17% 8% 8% 27% 23% 18%	5376
	11th to 20th	Night	15% 9% 10% 33% 23% 10%	5266
	21st to end	Night	12% 7% 12% 34% 26% 9%	5122
April	1st to 10th	Night	15% 6% 11% 25% 25% 18%	2000
	11th to 20th	Night	11% 5% 7% 24% 30% 23%	1979
	21st to end	Night	7% 4 9% 26% 38% 17%	2000
May	1st to 10th	Night	8% 3 8% 22% 33% 26%	4195
	11th to 20th	Night	5% 2 5% 18% 47% 23%	4075
	21st to end	Night	4 2 3 18% 48% 25%	4474
June	1st to 10th	Night	5% 3 6% 17% 34% 35%	1800
	11th to 20th	Night	3 2 2 11% 30% 53%	1701
	21st to end	Night	4 1 3 5% 22% 66%	1600
July	1st to 10th	Night	2 1 1 3 14% 80%	3600
	11th to 20th	Night	5% 2 3 8% 17% 65%	3911
	21st to end	Night	5% 1 3 8% 27% 55%	4491
August	1st to 10th	Night	5% 2 3 10% 33% 48%	3256
	11th to 20th	Night	3 2 2 9% 27% 57%	3256
	21st to end	Night	4 1 2 5% 22% 66%	3608
September	1st to 10th	Night	8% 3 5% 6% 27% 52%	2000
	11th to 20th	Night	3 1 3 8% 34% 51%	2193
	21st to end	Night	4 2 3 12% 30% 49%	2200
October	1st to 10th	Night	5% 2 3 7% 20% 62%	3600
	11th to 20th	Night	4 2 3 9% 36% 46%	3639
	21st to end	Night	7% 4 3 7% 33% 46%	4299
November	1st to 10th	Night	9% 5% 5% 15% 34% 31%	2535
	11th to 20th	Night	10% 4 6% 15% 30% 35%	2600
	21st to end	Night	17% 6% 9% 18% 23% 28%	2600
December	1st to 10th	Night	13% 4 5% 12% 34% 32%	4240
	11th to 20th	Night	15% 7% 11% 22% 32% 12%	4811
	21st to end	Night	19% 7% 11% 28% 27% 8%	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	5% 13% 28% 35% 15% 4	300
	11th to 20th	Day	4 14% 24% 37% 18% 2	361
	21st to end	Day	5% 21% 25% 31% 15% 3	550
February	1st to 10th	Day	7% 21% 24% 28% 15% 3	500
	11th to 20th	Day	8% 19% 30% 29% 11% 2	495
	21st to end	Day	6% 19% 32% 31% 11% 1	414
March	1st to 10th	Day	10% 23% 32% 23% 11% 2	500
	11th to 20th	Day	7% 21% 32% 27% 12% 2	500
	21st to end	Day	7% 26% 29% 25% 10% 2	550
April	1st to 10th	Day	6% 17% 31% 27% 15% 5%	500
	11th to 20th	Day	3 22% 33% 24% 13% 4	492
	21st to end	Day	5% 20% 26% 26% 16% 7%	500
May	1st to 10th	Day	3 17% 29% 30% 17% 5%	503
	11th to 20th	Day	3 14% 34% 28% 16% 6%	528
	21st to end	Day	3 12% 29% 32% 18% 7%	573
June	1st to 10th	Day	2 8% 29% 30% 22% 10%	530
	11th to 20th	Day	8% 26% 34% 24% 8%	530
	21st to end	Day	2 4 27% 36% 22% 10%	530
July	1st to 10th	Day	1 3 21% 37% 25% 13%	526
	11th to 20th	Day	1 15% 40% 27% 17%	524
	21st to end	Day	1 2 15% 37% 27% 18%	575
August	1st to 10th	Day	2 2 21% 37% 23% 15%	527
	11th to 20th	Day	1 2 18% 33% 28% 18%	517
	21st to end	Day	2 2 18% 36% 27% 15%	583
September	1st to 10th	Day	2 2 21% 35% 28% 12%	530
	11th to 20th	Day	2 5% 25% 36% 26% 6%	530
	21st to end	Day	1 8% 24% 31% 28% 8%	530
October	1st to 10th	Day	2 10% 26% 31% 25% 7%	530
	11th to 20th	Day	2 8% 22% 36% 25% 7%	523
	21st to end	Day	2 7% 24% 33% 28% 7%	428
November	1st to 10th	Day	2 10% 23% 30% 30% 6%	300
	11th to 20th	Day	2 7% 23% 37% 24% 6%	300
	21st to end	Day	3 11% 24% 35% 25% 3	300
December	1st to 10th	Day	4 7% 23% 37% 25% 5%	300
	11th to 20th	Day	4 8% 23% 38% 24% 3	300
	21st to end	Day	8% 16% 25% 33% 15% 2	330
January	1st to 10th	Night	5% 7% 26% 37% 20% 5%	800
	11th to 20th	Night	3 6% 26% 42% 21% 4	800
	21st to end	Night	6% 8% 27% 39% 19% 2	880
February	1st to 10th	Night	4 10% 26% 36% 21% 4	800
	11th to 20th	Night	3 9% 29% 35% 19% 4	784
	21st to end	Night	4 10% 33% 36% 15% 3	660
March	1st to 10th	Night	4 12% 30% 36% 17% 2	800
	11th to 20th	Night	3 10% 27% 34% 21% 5%	800
	21st to end	Night	2 12% 32% 34% 18% 2	880
April	1st to 10th	Night	3 10% 29% 35% 21% 3	800
	11th to 20th	Night	2 10% 31% 33% 22% 2	791
	21st to end	Night	1 9% 28% 34% 23% 5%	800
May	1st to 10th	Night	1 7% 30% 43% 18% 2	646
	11th to 20th	Night	1 7% 27% 39% 24% 2	591
	21st to end	Night	1 6% 28% 41% 22% 2	650
June	1st to 10th	Night	3 21% 49% 25% 1	600
	11th to 20th	Night	1 18% 51% 28% 2	501
	21st to end	Night	1 1 8% 51% 37% 3	400
July	1st to 10th	Night	2 39% 53% 6%	390
	11th to 20th	Night	1 7% 42% 47% 3	525
	21st to end	Night	1 1 7% 41% 45% 5%	719
August	1st to 10th	Night	2 8% 37% 43% 9%	796
	11th to 20th	Night	1 6% 33% 49% 11%	792
	21st to end	Night	1 6% 32% 51% 10%	880
September	1st to 10th	Night	2 1 8% 30% 48% 11%	800
	11th to 20th	Night	1 1 10% 40% 40% 9%	800
	21st to end	Night	2 14% 37% 39% 8%	800
October	1st to 10th	Night	2 2 19% 40% 33% 4	800
	11th to 20th	Night	1 4 16% 39% 33% 8%	792
	21st to end	Night	1 3 20% 36% 29% 11%	836
November	1st to 10th	Night	1 5% 17% 39% 31% 7%	780
	11th to 20th	Night	1 2 18% 43% 28% 7%	800
	21st to end	Night	2 4 23% 41% 24% 6%	800
December	1st to 10th	Night	3 6% 25% 39% 24% 4	800
	11th to 20th	Night	4 7% 26% 40% 21% 3	800
	21st to end	Night	5% 8% 30% 36% 18% 4	880

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