Las Brisas Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Las Brisas Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	4 5% 3 5% 5% 3 5% 5% 6% 8% 50%	74761
	Night	2 4 2 5% 4 3 4 5% 6% 8% 57%	74992
Transparency	Day	28% 8% 13% 22% 21% 8%	74471
	Night	23% 7% 14% 25% 23% 8%	74804
Seeing	Day	10% 17% 33% 25% 13% 2	14348
	Night	5% 12% 36% 33% 13% 1	24519

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	3 2 4 4 2 4 5% 5% 9% 63%	1518
	11th to 20th	Day	1 4 2 6% 4 2 3 4 5% 8% 61%	1550
	21st to end	Day	1 5% 3 5% 4 2 3 4 5% 8% 58%	1782
February	1st to 10th	Day	6% 3 6% 5% 2 3 3 5% 9% 58%	1620
	11th to 20th	Day	1 3 2 4 4 3 4 5% 7% 10% 58%	1735
	21st to end	Day	5% 4 4 5% 3 3 5% 5% 8% 57%	1480
March	1st to 10th	Day	2 5% 4 5% 5% 3 4 4 5% 9% 53%	1800
	11th to 20th	Day	2 8% 3 7% 5% 3 5% 5% 6% 8% 48%	1973
	21st to end	Day	1 7% 3 7% 5% 3 4 5% 6% 9% 48%	2300
April	1st to 10th	Day	3 6% 2 6% 6% 3 5% 6% 7% 8% 49%	2160
	11th to 20th	Day	4 9% 3 8% 7% 4 4 5% 5% 7% 44%	2131
	21st to end	Day	5% 10% 4 7% 7% 4 5% 5% 7% 8% 40%	2160
May	1st to 10th	Day	6% 9% 4 6% 7% 3 5% 6% 7% 8% 38%	2230
	11th to 20th	Day	7% 8% 3 8% 7% 4 6% 7% 7% 8% 35%	2446
	21st to end	Day	6% 5% 2 6% 6% 4 6% 6% 7% 9% 44%	2724
June	1st to 10th	Day	4 4 3 5% 5% 3 6% 6% 8% 11% 44%	2520
	11th to 20th	Day	5% 4 2 4 4 3 5% 6% 7% 9% 51%	2520
	21st to end	Day	4 4 2 5% 6% 5% 7% 7% 7% 8% 45%	2520
July	1st to 10th	Day	6% 4 2 6% 7% 3 7% 8% 8% 8% 41%	2520
	11th to 20th	Day	6% 5% 3 6% 6% 4 6% 6% 7% 10% 42%	2520
	21st to end	Day	8% 7% 4 8% 8% 5% 8% 8% 7% 8% 30%	2635
August	1st to 10th	Day	10% 6% 3 6% 7% 5% 6% 7% 7% 8% 35%	2318
	11th to 20th	Day	6% 4 2 5% 6% 4 6% 7% 8% 8% 44%	2123
	21st to end	Day	6% 5% 3 6% 6% 3 6% 5% 6% 8% 46%	2376
September	1st to 10th	Day	6% 6% 2 5% 6% 3 6% 6% 5% 5% 50%	2160
	11th to 20th	Day	6% 4 2 5% 6% 3 4 5% 5% 7% 54%	2147
	21st to end	Day	5% 4 2 4 4 2 3 4 4 7% 61%	1980
October	1st to 10th	Day	4 4 2 5% 4 2 4 6% 6% 8% 55%	1806
	11th to 20th	Day	3 3 2 3 3 2 3 3 4 6% 68%	1800
	21st to end	Day	3 6% 3 4 3 2 3 4 4 6% 62%	1945
November	1st to 10th	Day	1 4 1 3 3 2 3 3 5% 7% 67%	1673
	11th to 20th	Day	1 4 2 3 4 2 3 4 5% 8% 65%	1710
	21st to end	Day	4 3 5% 4 2 4 4 6% 8% 60%	1639
December	1st to 10th	Day	3 2 5% 4 2 3 4 5% 8% 62%	1520
	11th to 20th	Day	1 4 2 4 5% 2 4 4 7% 9% 58%	1926
	21st to end	Day	1 3 2 5% 5% 3 5% 6% 7% 9% 54%	2794
January	1st to 10th	Night	3 2 4 4 3 4 4 5% 7% 65%	2647
	11th to 20th	Night	3 1 3 4 2 3 4 5% 7% 67%	2480
	21st to end	Night	3 2 5% 4 2 4 4 5% 7% 63%	2574
February	1st to 10th	Night	4 3 5% 4 2 3 4 5% 8% 62%	2340
	11th to 20th	Night	5% 2 6% 5% 3 4 4 5% 8% 58%	2110
	21st to end	Night	5% 2 4 4 2 4 4 5% 7% 63%	1776
March	1st to 10th	Night	1 4 2 4 4 3 5% 4 5% 8% 59%	2160
	11th to 20th	Night	2 5% 3 5% 5% 3 4 4 5% 9% 54%	1984
	21st to end	Night	2 5% 3 5% 6% 3 4 5% 6% 7% 55%	2055
April	1st to 10th	Night	3 6% 2 4 4 3 3 4 4 7% 60%	1800
	11th to 20th	Night	3 7% 3 6% 5% 3 5% 5% 4 8% 51%	1777
	21st to end	Night	4 8% 3 5% 5% 3 4 5% 5% 8% 51%	1800
May	1st to 10th	Night	5% 9% 3 5% 6% 4 4 4 5% 8% 48%	1728
	11th to 20th	Night	4 8% 3 6% 7% 4 5% 6% 7% 8% 42%	1445
	21st to end	Night	4 5% 2 6% 5% 3 4 5% 7% 8% 49%	1552
June	1st to 10th	Night	2 5% 2 4 4 2 5% 6% 7% 10% 53%	1440
	11th to 20th	Night	3 2 1 3 3 3 5% 6% 8% 8% 59%	1440
	21st to end	Night	3 3 2 3 4 3 6% 5% 7% 9% 53%	1440
July	1st to 10th	Night	5% 4 3 7% 5% 4 6% 7% 7% 9% 42%	1440
	11th to 20th	Night	4 5% 3 5% 4 3 5% 5% 6% 10% 51%	1440
	21st to end	Night	5% 4 2 6% 7% 5% 8% 7% 10% 12% 33%	1679
August	1st to 10th	Night	5% 4 2 6% 7% 4 6% 7% 8% 10% 40%	1619
	11th to 20th	Night	5% 5% 2 5% 5% 4 4 7% 6% 8% 48%	1778
	21st to end	Night	5% 5% 2 4 5% 4 5% 6% 7% 10% 47%	1980
September	1st to 10th	Night	5% 4 2 5% 6% 4 5% 5% 7% 10% 48%	1800
	11th to 20th	Night	4 4 2 4 4 3 3 4 6% 8% 58%	1812
	21st to end	Night	4 4 2 4 4 3 4 4 5% 6% 61%	1980
October	1st to 10th	Night	4 4 2 4 3 2 3 4 6% 7% 60%	2150
	11th to 20th	Night	2 2 1 2 3 1 3 4 5% 6% 71%	2160
	21st to end	Night	3 4 2 3 3 2 3 4 4 6% 64%	2391
November	1st to 10th	Night	1 3 2 2 2 2 4 3 5% 8% 68%	2405
	11th to 20th	Night	2 1 3 4 3 4 4 4 7% 68%	2470
	21st to end	Night	1 5% 2 4 4 2 4 4 4 8% 63%	2536
December	1st to 10th	Night	1 2 2 5% 4 2 4 4 5% 9% 62%	2660
	11th to 20th	Night	1 3 2 4 4 3 5% 6% 7% 10% 55%	3271
	21st to end	Night	1 4 3 6% 5% 3 5% 5% 6% 8% 54%	4873

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	17% 5% 17% 30% 25% 6%	1519
	11th to 20th	Day	20% 5% 16% 27% 25% 7%	1550
	21st to end	Day	23% 5% 13% 32% 20% 7%	1782
February	1st to 10th	Day	23% 5% 16% 28% 19% 9%	1620
	11th to 20th	Day	20% 4 13% 26% 29% 8%	1745
	21st to end	Day	24% 5% 10% 24% 25% 12%	1480
March	1st to 10th	Day	25% 5% 12% 27% 24% 8%	1800
	11th to 20th	Day	29% 6% 14% 25% 22% 5%	1973
	21st to end	Day	30% 7% 13% 25% 20% 6%	2300
April	1st to 10th	Day	29% 8% 14% 21% 19% 9%	2160
	11th to 20th	Day	37% 7% 11% 21% 18% 6%	2134
	21st to end	Day	37% 8% 13% 23% 17% 3	2160
May	1st to 10th	Day	39% 9% 13% 21% 14% 3	2230
	11th to 20th	Day	41% 11% 16% 18% 12% 2	2450
	21st to end	Day	31% 11% 15% 19% 16% 8%	2724
June	1st to 10th	Day	29% 11% 15% 19% 18% 8%	2520
	11th to 20th	Day	25% 9% 10% 17% 23% 16%	2520
	21st to end	Day	29% 10% 12% 20% 20% 9%	2520
July	1st to 10th	Day	34% 11% 11% 17% 19% 9%	2451
	11th to 20th	Day	32% 9% 15% 24% 13% 6%	2445
	21st to end	Day	43% 14% 17% 15% 9% 2	2488
August	1st to 10th	Day	42% 12% 15% 13% 13% 5%	2303
	11th to 20th	Day	31% 10% 13% 19% 18% 9%	2123
	21st to end	Day	32% 8% 13% 22% 20% 5%	2376
September	1st to 10th	Day	31% 7% 11% 17% 23% 11%	2160
	11th to 20th	Day	27% 6% 11% 19% 26% 12%	2147
	21st to end	Day	22% 6% 8% 24% 31% 9%	1980
October	1st to 10th	Day	23% 9% 12% 21% 25% 10%	1806
	11th to 20th	Day	17% 4 8% 23% 34% 14%	1800
	21st to end	Day	22% 5% 10% 26% 26% 11%	1945
November	1st to 10th	Day	15% 4 13% 22% 33% 14%	1673
	11th to 20th	Day	15% 5% 10% 33% 28% 9%	1710
	21st to end	Day	20% 5% 11% 24% 27% 13%	1639
December	1st to 10th	Day	19% 6% 17% 27% 24% 8%	1520
	11th to 20th	Day	19% 5% 16% 26% 26% 7%	1924
	21st to end	Day	20% 7% 16% 24% 24% 10%	2794
January	1st to 10th	Night	18% 5% 15% 30% 27% 6%	2647
	11th to 20th	Night	16% 5% 15% 28% 30% 7%	2480
	21st to end	Night	19% 5% 12% 30% 27% 7%	2574
February	1st to 10th	Night	21% 4 14% 32% 22% 8%	2340
	11th to 20th	Night	25% 5% 12% 27% 24% 7%	2110
	21st to end	Night	20% 5% 10% 27% 29% 10%	1776
March	1st to 10th	Night	20% 6% 12% 26% 29% 7%	2160
	11th to 20th	Night	26% 7% 13% 29% 22% 4	1984
	21st to end	Night	27% 6% 12% 26% 25% 4	2055
April	1st to 10th	Night	24% 5% 14% 27% 24% 7%	1800
	11th to 20th	Night	31% 6% 12% 27% 19% 5%	1777
	21st to end	Night	31% 7% 16% 26% 17% 4	1800
May	1st to 10th	Night	33% 8% 14% 26% 15% 5%	1728
	11th to 20th	Night	37% 11% 16% 21% 13% 2	1446
	21st to end	Night	30% 10% 16% 22% 16% 7%	1552
June	1st to 10th	Night	23% 9% 22% 23% 13% 11%	1440
	11th to 20th	Night	17% 10% 15% 20% 22% 17%	1440
	21st to end	Night	24% 10% 14% 22% 20% 11%	1440
July	1st to 10th	Night	32% 13% 14% 19% 15% 7%	1400
	11th to 20th	Night	25% 10% 17% 29% 14% 5%	1392
	21st to end	Night	35% 17% 22% 18% 7% 1	1586
August	1st to 10th	Night	35% 13% 18% 17% 12% 5%	1610
	11th to 20th	Night	30% 10% 15% 24% 16% 5%	1778
	21st to end	Night	27% 11% 17% 26% 16% 4	1980
September	1st to 10th	Night	30% 8% 13% 22% 20% 8%	1800
	11th to 20th	Night	22% 6% 14% 22% 23% 13%	1812
	21st to end	Night	23% 6% 10% 25% 28% 8%	1980
October	1st to 10th	Night	22% 8% 13% 22% 25% 10%	2150
	11th to 20th	Night	13% 5% 11% 23% 35% 13%	2160
	21st to end	Night	20% 5% 10% 28% 28% 10%	2391
November	1st to 10th	Night	14% 5% 13% 26% 29% 13%	2405
	11th to 20th	Night	16% 5% 10% 31% 29% 10%	2470
	21st to end	Night	20% 5% 11% 24% 29% 12%	2536
December	1st to 10th	Night	18% 6% 16% 27% 25% 8%	2660
	11th to 20th	Night	19% 5% 17% 25% 28% 6%	3272
	21st to end	Night	22% 7% 15% 23% 24% 9%	4873

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	10% 22% 34% 24% 10%	270
	11th to 20th	Day	11% 23% 41% 22% 2 1	246
	21st to end	Day	4 24% 48% 20% 4	264
February	1st to 10th	Day	11% 37% 37% 14% 1 1	240
	11th to 20th	Day	11% 33% 41% 11% 4	240
	21st to end	Day	17% 30% 38% 12% 3	198
March	1st to 10th	Day	15% 23% 39% 18% 5%	240
	11th to 20th	Day	13% 27% 37% 18% 5%	413
	21st to end	Day	17% 33% 35% 12% 2	462
April	1st to 10th	Day	15% 33% 35% 15% 3 1	420
	11th to 20th	Day	20% 33% 31% 13% 3	412
	21st to end	Day	18% 33% 30% 15% 4 1	420
May	1st to 10th	Day	14% 25% 36% 19% 5% 1	493
	11th to 20th	Day	11% 16% 33% 27% 10% 2	625
	21st to end	Day	10% 18% 29% 28% 13% 2	679
June	1st to 10th	Day	5% 14% 29% 28% 20% 3	630
	11th to 20th	Day	8% 12% 35% 28% 14% 3	630
	21st to end	Day	3 10% 36% 28% 18% 4	630
July	1st to 10th	Day	6% 5% 22% 34% 27% 5%	621
	11th to 20th	Day	7% 3 20% 37% 27% 7%	618
	21st to end	Day	10% 3 20% 33% 28% 6%	577
August	1st to 10th	Day	13% 5% 27% 36% 16% 4	447
	11th to 20th	Day	8% 6% 29% 35% 17% 6%	437
	21st to end	Day	9% 6% 33% 32% 17% 4	495
September	1st to 10th	Day	8% 10% 30% 32% 16% 4	450
	11th to 20th	Day	7% 11% 38% 29% 13% 2	437
	21st to end	Day	7% 14% 22% 32% 22% 4	270
October	1st to 10th	Day	9% 12% 30% 24% 20% 6%	270
	11th to 20th	Day	8% 20% 33% 23% 15% 2	270
	21st to end	Day	10% 22% 35% 22% 10% 1	297
November	1st to 10th	Day	11% 17% 39% 21% 11% 2	270
	11th to 20th	Day	9% 18% 39% 24% 8% 1	270
	21st to end	Day	7% 19% 44% 21% 8% 1	270
December	1st to 10th	Day	10% 24% 39% 20% 7%	270
	11th to 20th	Day	7% 23% 49% 15% 6%	270
	21st to end	Day	11% 29% 37% 17% 5% 1	297
January	1st to 10th	Night	4 19% 39% 28% 9% 1	950
	11th to 20th	Night	4 22% 42% 23% 8% 1	834
	21st to end	Night	2 13% 50% 29% 5% 1	792
February	1st to 10th	Night	4 17% 55% 21% 3	720
	11th to 20th	Night	4 18% 49% 25% 3	704
	21st to end	Night	4 21% 50% 24% 2	592
March	1st to 10th	Night	3 18% 47% 28% 5%	720
	11th to 20th	Night	5% 13% 40% 35% 8%	720
	21st to end	Night	5% 17% 46% 27% 4	669
April	1st to 10th	Night	7% 19% 41% 29% 3	540
	11th to 20th	Night	8% 19% 38% 29% 6% 1	533
	21st to end	Night	10% 15% 38% 29% 7%	540
May	1st to 10th	Night	10% 9% 35% 37% 8% 1	540
	11th to 20th	Night	9% 9% 29% 39% 13% 1	531
	21st to end	Night	6% 10% 34% 33% 15% 1	584
June	1st to 10th	Night	5% 4 26% 44% 19% 3	540
	11th to 20th	Night	3 6% 29% 46% 14% 1	540
	21st to end	Night	4 3 21% 44% 26% 2	540
July	1st to 10th	Night	5% 2 14% 44% 32% 2	525
	11th to 20th	Night	6% 1 12% 41% 36% 3	522
	21st to end	Night	6% 1 9% 39% 40% 4	591
August	1st to 10th	Night	8% 3 11% 42% 32% 3	537
	11th to 20th	Night	6% 2 14% 44% 30% 3	534
	21st to end	Night	6% 3 15% 40% 33% 3	594
September	1st to 10th	Night	6% 2 20% 42% 26% 5%	540
	11th to 20th	Night	6% 3 21% 45% 22% 3	540
	21st to end	Night	4 8% 28% 39% 19% 2	540
October	1st to 10th	Night	5% 8% 35% 37% 14% 2	710
	11th to 20th	Night	3 6% 39% 35% 14% 3	720
	21st to end	Night	7% 10% 37% 32% 13% 2	807
November	1st to 10th	Night	3 12% 41% 33% 10% 1	925
	11th to 20th	Night	4 16% 42% 29% 7% 1	950
	21st to end	Night	5% 15% 40% 32% 8%	950
December	1st to 10th	Night	3 20% 41% 30% 6%	950
	11th to 20th	Night	4 17% 48% 24% 6%	950
	21st to end	Night	6% 22% 44% 23% 5%	1045

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