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 2024-04-25.

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 2 1 2 2 2 3 3 4 6% 74%	84355
	Night	1 2 1 2 2 2 3 3 4 6% 74%	84870
Transparency	Day	13% 5% 7% 17% 27% 31%	84078
	Night	13% 5% 8% 19% 28% 27%	84669
Seeing	Day	3 12% 26% 33% 20% 6%	14924
	Night	2 5% 21% 38% 29% 5%	24838

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 2 1 4 3 2 3 3 4 7% 70%	2858
	11th to 20th	Day	2 3 1 3 3 2 3 4 6% 9% 64%	2956
	21st to end	Day	4 4 2 4 4 3 4 5% 5% 8% 58%	3498
February	1st to 10th	Day	2 3 1 4 3 2 3 3 4 7% 68%	1900
	11th to 20th	Day	3 1 1 2 3 2 3 4 4 8% 70%	1850
	21st to end	Day	1 3 1 4 3 3 4 3 5% 6% 67%	1570
March	1st to 10th	Day	2 2 1 2 3 2 3 4 6% 9% 65%	3138
	11th to 20th	Day	3 3 1 3 3 2 3 3 5% 8% 66%	3300
	21st to end	Day	2 1 1 2 2 2 3 4 6% 10% 67%	4071
April	1st to 10th	Day	2 3 1 2 2 2 3 3 4 8% 71%	2280
	11th to 20th	Day	1 1 1 1 2 1 1 2 3 5% 82%	2249
	21st to end	Day	1 1 2 1 1 2 2 3 6% 80%	2210
May	1st to 10th	Day	1 1 1 1 1 1 3 3 6% 80%	2294
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 6% 83%	2301
	21st to end	Day	1 1 2 2 1 2 2 2 5% 81%	2530
June	1st to 10th	Day	1 1 1 1 2 3 3 6% 82%	2340
	11th to 20th	Day	1 1 1 1 2 4 91%	2420
	21st to end	Day	1 1 1 1 2 1 2 2 2 3 86%	2520
July	1st to 10th	Day	1 1 2 1 1 2 2 3 5% 82%	2520
	11th to 20th	Day	1 1 2 2 2 3 3 4 6% 77%	2386
	21st to end	Day	1 1 1 2 2 2 3 4 5% 8% 70%	2486
August	1st to 10th	Day	2 1 1 1 1 2 2 3 4 82%	2149
	11th to 20th	Day	1 1 1 1 1 2 3 3 5% 81%	2122
	21st to end	Day	1 1 1 1 2 2 2 3 4 4 80%	2376
September	1st to 10th	Day	1 1 1 2 2 1 3 3 3 6% 77%	2160
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	1985
	21st to end	Day	1 1 1 1 2 2 3 4 85%	1980
October	1st to 10th	Day	1 1 1 2 2 1 3 3 3 5% 79%	1980
	11th to 20th	Day	1 1 1 2 1 2 2 2 4 84%	1932
	21st to end	Day	1 1 1 1 1 1 2 3 5% 84%	1875
November	1st to 10th	Day	1 1 1 1 2 2 3 3 3 5% 78%	1674
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 5% 76%	1710
	21st to end	Day	2 1 1 2 3 2 4 3 3 7% 70%	1710
December	1st to 10th	Day	3 3 1 2 3 3 4 4 6% 9% 64%	1710
	11th to 20th	Day	2 2 1 3 4 2 5% 5% 7% 10% 58%	2169
	21st to end	Day	4 4 2 4 5% 3 5% 6% 6% 8% 52%	3146
January	1st to 10th	Night	2 2 1 3 3 2 3 4 4 7% 69%	4097
	11th to 20th	Night	2 3 1 4 3 2 4 4 5% 9% 64%	4013
	21st to end	Night	4 4 1 4 3 2 4 4 5% 7% 61%	4180
February	1st to 10th	Night	2 3 1 3 3 2 3 4 5% 5% 68%	2280
	11th to 20th	Night	2 2 1 3 3 2 3 3 5% 7% 70%	2218
	21st to end	Night	2 2 1 2 3 2 4 4 5% 8% 67%	1884
March	1st to 10th	Night	2 2 1 2 2 2 4 4 6% 8% 67%	3736
	11th to 20th	Night	3 3 2 3 3 2 4 4 5% 9% 62%	3678
	21st to end	Night	1 1 1 2 2 2 3 4 4 8% 71%	3565
April	1st to 10th	Night	1 2 1 3 3 1 2 3 4 9% 70%	1900
	11th to 20th	Night	1 1 1 2 1 1 2 3 4 7% 77%	1879
	21st to end	Night	1 1 1 2 1 2 2 4 7% 80%	1850
May	1st to 10th	Night	1 1 1 1 2 2 4 7% 81%	1662
	11th to 20th	Night	1 2 1 2 2 3 5% 83%	1594
	21st to end	Night	1 1 1 1 1 1 2 3 5% 85%	1746
June	1st to 10th	Night	1 1 1 1 1 3 3 5% 85%	1620
	11th to 20th	Night	1 1 2 3 91%	1531
	21st to end	Night	1 1 1 1 1 2 1 3 89%	1440
July	1st to 10th	Night	1 1 1 1 1 2 3 89%	1440
	11th to 20th	Night	1 1 1 2 2 2 3 3 3 5% 78%	1566
	21st to end	Night	2 2 1 2 2 2 2 2 4 6% 75%	1828
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 4 83%	1790
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 87%	1780
	21st to end	Night	1 1 1 1 1 2 3 2 4 85%	1980
September	1st to 10th	Night	1 1 1 1 1 2 2 2 4 83%	1800
	11th to 20th	Night	1 1 1 1 1 2 2 91%	1973
	21st to end	Night	1 1 1 1 1 1 2 4 88%	1980
October	1st to 10th	Night	1 1 1 2 1 1 3 2 3 6% 80%	1980
	11th to 20th	Night	1 1 1 1 2 1 1 2 3 4 85%	2024
	21st to end	Night	1 1 1 1 1 2 2 3 4 83%	2479
November	1st to 10th	Night	1 2 1 1 2 3 3 6% 79%	2405
	11th to 20th	Night	1 1 1 2 2 1 2 3 4 6% 77%	2470
	21st to end	Night	2 1 1 2 3 3 3 3 6% 8% 68%	2470
December	1st to 10th	Night	2 3 2 3 2 2 3 4 5% 9% 65%	2470
	11th to 20th	Night	2 2 1 3 3 2 4 5% 7% 11% 60%	3041
	21st to end	Night	3 3 1 5% 5% 3 5% 5% 6% 8% 56%	4521

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	16% 4 8% 25% 36% 11%	2859
	11th to 20th	Day	18% 6% 15% 23% 27% 11%	2956
	21st to end	Day	25% 8% 10% 19% 26% 12%	3498
February	1st to 10th	Day	19% 5% 6% 17% 27% 26%	1900
	11th to 20th	Day	16% 6% 7% 17% 32% 22%	1860
	21st to end	Day	20% 7% 8% 20% 28% 18%	1570
March	1st to 10th	Day	17% 8% 9% 24% 25% 16%	3138
	11th to 20th	Day	17% 7% 10% 29% 22% 15%	3300
	21st to end	Day	14% 7% 12% 32% 26% 9%	4071
April	1st to 10th	Day	16% 5% 9% 21% 31% 19%	2280
	11th to 20th	Day	10% 4 5% 21% 35% 25%	2252
	21st to end	Day	9% 3 8% 20% 38% 21%	2210
May	1st to 10th	Day	8% 5% 8% 20% 33% 26%	2294
	11th to 20th	Day	8% 3 6% 18% 38% 28%	2304
	21st to end	Day	10% 3 6% 18% 41% 23%	2530
June	1st to 10th	Day	7% 3 4 13% 31% 41%	2340
	11th to 20th	Day	2 2 3 9% 24% 60%	2420
	21st to end	Day	8% 2 2 4 16% 68%	2520
July	1st to 10th	Day	7% 3 3 7% 15% 65%	2451
	11th to 20th	Day	10% 4 6% 13% 24% 43%	2314
	21st to end	Day	13% 6% 7% 16% 25% 34%	2348
August	1st to 10th	Day	8% 2 4 8% 16% 62%	2135
	11th to 20th	Day	7% 4 4 11% 22% 53%	2122
	21st to end	Day	9% 3 4 7% 23% 53%	2376
September	1st to 10th	Day	10% 4 6% 7% 21% 52%	2160
	11th to 20th	Day	4 2 3 7% 29% 56%	1985
	21st to end	Day	7% 2 3 10% 30% 47%	1980
October	1st to 10th	Day	10% 4 4 14% 26% 41%	1980
	11th to 20th	Day	7% 2 5% 11% 30% 45%	1932
	21st to end	Day	6% 3 5% 11% 29% 47%	1875
November	1st to 10th	Day	11% 4 6% 16% 33% 31%	1674
	11th to 20th	Day	13% 4 7% 14% 28% 34%	1710
	21st to end	Day	17% 5% 8% 15% 28% 28%	1710
December	1st to 10th	Day	20% 5% 9% 19% 24% 23%	1710
	11th to 20th	Day	21% 10% 14% 17% 25% 12%	2168
	21st to end	Day	29% 8% 8% 23% 21% 11%	3146
January	1st to 10th	Night	16% 7% 9% 18% 37% 13%	4097
	11th to 20th	Night	19% 6% 11% 28% 24% 12%	4013
	21st to end	Night	23% 7% 11% 20% 27% 12%	4180
February	1st to 10th	Night	19% 7% 8% 17% 23% 26%	2280
	11th to 20th	Night	16% 6% 9% 18% 28% 23%	2219
	21st to end	Night	18% 10% 9% 19% 24% 20%	1884
March	1st to 10th	Night	18% 9% 10% 29% 20% 15%	3736
	11th to 20th	Night	17% 8% 10% 30% 22% 12%	3678
	21st to end	Night	13% 8% 13% 32% 26% 7%	3565
April	1st to 10th	Night	15% 6% 11% 26% 24% 18%	1900
	11th to 20th	Night	11% 5% 7% 25% 30% 23%	1879
	21st to end	Night	7% 5% 9% 25% 37% 18%	1850
May	1st to 10th	Night	6% 5% 9% 27% 29% 24%	1662
	11th to 20th	Night	8% 3 8% 21% 41% 20%	1595
	21st to end	Night	6% 3 8% 25% 41% 17%	1746
June	1st to 10th	Night	5% 3 6% 18% 32% 36%	1620
	11th to 20th	Night	3 2 2 12% 29% 51%	1531
	21st to end	Night	5% 1 3 5% 23% 64%	1440
July	1st to 10th	Night	4 2 3 6% 19% 66%	1400
	11th to 20th	Night	11% 3 5% 13% 23% 44%	1515
	21st to end	Night	13% 3 7% 15% 29% 32%	1726
August	1st to 10th	Night	7% 3 4 8% 23% 55%	1780
	11th to 20th	Night	5% 2 4 11% 27% 51%	1780
	21st to end	Night	6% 1 4 8% 30% 50%	1980
September	1st to 10th	Night	8% 3 5% 6% 26% 53%	1800
	11th to 20th	Night	3 1 3 8% 32% 52%	1973
	21st to end	Night	5% 2 3 12% 32% 47%	1980
October	1st to 10th	Night	10% 3 5% 13% 30% 40%	1980
	11th to 20th	Night	6% 4 4 10% 34% 41%	2024
	21st to end	Night	7% 4 5% 9% 27% 47%	2479
November	1st to 10th	Night	10% 5% 6% 15% 34% 31%	2405
	11th to 20th	Night	10% 4 6% 15% 29% 35%	2470
	21st to end	Night	17% 6% 8% 18% 23% 28%	2470
December	1st to 10th	Night	19% 6% 8% 19% 25% 24%	2470
	11th to 20th	Night	20% 10% 12% 19% 25% 13%	3041
	21st to end	Night	24% 9% 9% 24% 23% 10%	4521

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	6% 13% 29% 36% 15% 2	270
	11th to 20th	Day	5% 16% 23% 36% 18% 2	327
	21st to end	Day	5% 22% 26% 31% 14% 2	506
February	1st to 10th	Day	6% 22% 25% 28% 16% 4	460
	11th to 20th	Day	8% 19% 30% 29% 12% 2	455
	21st to end	Day	6% 20% 32% 31% 10% 1	382
March	1st to 10th	Day	8% 23% 34% 23% 11% 2	460
	11th to 20th	Day	5% 21% 33% 28% 12% 2	460
	21st to end	Day	7% 26% 30% 26% 9% 1	506
April	1st to 10th	Day	6% 17% 32% 28% 14% 3	460
	11th to 20th	Day	3 23% 33% 25% 13% 4	452
	21st to end	Day	3 21% 27% 26% 16% 6%	437
May	1st to 10th	Day	3 17% 30% 29% 16% 5%	423
	11th to 20th	Day	2 15% 33% 30% 15% 6%	448
	21st to end	Day	4 13% 28% 32% 17% 7%	485
June	1st to 10th	Day	2 9% 29% 30% 22% 9%	450
	11th to 20th	Day	8% 26% 34% 24% 7%	450
	21st to end	Day	1 4 28% 37% 21% 10%	450
July	1st to 10th	Day	1 2 22% 40% 25% 11%	446
	11th to 20th	Day	1 15% 41% 28% 15%	444
	21st to end	Day	1 1 13% 40% 28% 17%	487
August	1st to 10th	Day	3 1 21% 37% 23% 14%	447
	11th to 20th	Day	1 2 16% 35% 30% 17%	437
	21st to end	Day	1 1 18% 39% 28% 14%	495
September	1st to 10th	Day	2 1 21% 38% 30% 8%	450
	11th to 20th	Day	1 4 26% 37% 26% 5%	450
	21st to end	Day	1 8% 25% 32% 28% 7%	450
October	1st to 10th	Day	2 10% 27% 33% 24% 4	450
	11th to 20th	Day	1 8% 22% 38% 26% 4	450
	21st to end	Day	3 7% 24% 34% 27% 5%	390
November	1st to 10th	Day	2 10% 21% 31% 31% 4	270
	11th to 20th	Day	2 7% 23% 38% 23% 7%	270
	21st to end	Day	3 11% 24% 35% 24% 3	270
December	1st to 10th	Day	4 7% 25% 38% 22% 3	270
	11th to 20th	Day	4 9% 25% 38% 22% 2	270
	21st to end	Day	9% 16% 27% 32% 13% 2	297
January	1st to 10th	Night	5% 7% 27% 36% 20% 6%	760
	11th to 20th	Night	3 6% 25% 41% 21% 4	760
	21st to end	Night	6% 8% 26% 39% 19% 2	836
February	1st to 10th	Night	4 9% 25% 36% 21% 4	760
	11th to 20th	Night	3 9% 28% 35% 20% 4	744
	21st to end	Night	4 10% 33% 35% 15% 3	628
March	1st to 10th	Night	3 12% 30% 36% 17% 2	760
	11th to 20th	Night	3 9% 27% 34% 22% 5%	760
	21st to end	Night	2 12% 32% 35% 18% 2	836
April	1st to 10th	Night	3 10% 28% 35% 21% 3	760
	11th to 20th	Night	2 10% 31% 34% 22% 2	751
	21st to end	Night	1 8% 29% 34% 23% 5%	740
May	1st to 10th	Night	6% 31% 42% 19% 2	582
	11th to 20th	Night	6% 26% 40% 26% 2	531
	21st to end	Night	1 7% 28% 40% 22% 2	584
June	1st to 10th	Night	3 20% 49% 26% 1	540
	11th to 20th	Night	1 18% 50% 29% 2	451
	21st to end	Night	1 1 8% 48% 39% 4	360
July	1st to 10th	Night	1 37% 54% 7%	350
	11th to 20th	Night	1 6% 42% 47% 3	471
	21st to end	Night	1 5% 41% 47% 5%	646
August	1st to 10th	Night	2 7% 37% 43% 10%	716
	11th to 20th	Night	1 5% 32% 50% 12%	712
	21st to end	Night	1 4 32% 52% 11%	792
September	1st to 10th	Night	2 7% 31% 48% 12%	720
	11th to 20th	Night	8% 39% 43% 9%	720
	21st to end	Night	2 13% 36% 41% 9%	720
October	1st to 10th	Night	2 3 19% 40% 32% 5%	720
	11th to 20th	Night	1 3 15% 38% 35% 8%	720
	21st to end	Night	1 3 20% 35% 29% 11%	792
November	1st to 10th	Night	1 5% 16% 39% 31% 8%	740
	11th to 20th	Night	1 2 18% 43% 29% 8%	760
	21st to end	Night	2 4 22% 40% 26% 6%	760
December	1st to 10th	Night	3 6% 26% 38% 24% 4	760
	11th to 20th	Night	4 7% 26% 39% 21% 3	760
	21st to end	Night	6% 8% 30% 35% 18% 4	836

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