Observatorio Ernesto Juarez Davis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatorio Ernesto Juarez Davis clear sky chart from 2005-11-06 to 2025-04-26.

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
Transparency	Day	16% 3 5% 12% 22% 43%	117921
	Night	17% 3 5% 10% 22% 42%	119227
Seeing	Day	5% 4 16% 32% 29% 15%	15602
	Night	5% 1 12% 43% 33% 6%	25818

By Month

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	13% 2 1 8% 32% 44%	4099
	11th to 20th	Day	13% 2 2 7% 24% 52%	4100
	21st to end	Day	21% 2 2 9% 25% 40%	4510
February	1st to 10th	Day	19% 2 3 7% 22% 47%	1935
	11th to 20th	Day	20% 3 3 9% 25% 40%	1965
	21st to end	Day	19% 3 2 9% 26% 42%	1815
March	1st to 10th	Day	20% 2 2 6% 20% 51%	4954
	11th to 20th	Day	16% 1 2 10% 27% 44%	5000
	21st to end	Day	19% 2 2 4 23% 50%	5486
April	1st to 10th	Day	14% 2 1 6% 20% 58%	2200
	11th to 20th	Day	11% 1 2 4 19% 64%	2294
	21st to end	Day	7% 1 1 5% 19% 67%	2341
May	1st to 10th	Day	5% 1 1 2 13% 78%	3887
	11th to 20th	Day	4 1 2 3 16% 74%	4074
	21st to end	Day	4 1 1 1 13% 80%	4477
June	1st to 10th	Day	10% 1 2 4 23% 59%	2470
	11th to 20th	Day	8% 1 1 5% 24% 60%	2470
	21st to end	Day	16% 4 6% 13% 26% 35%	2470
July	1st to 10th	Day	24% 6% 15% 19% 17% 18%	4046
	11th to 20th	Day	24% 7% 19% 29% 17% 4	4041
	21st to end	Day	31% 7% 19% 33% 9% 1	4314
August	1st to 10th	Day	21% 6% 19% 32% 19% 4	4078
	11th to 20th	Day	24% 7% 16% 32% 17% 4	3923
	21st to end	Day	19% 6% 12% 28% 24% 11%	3978
September	1st to 10th	Day	31% 6% 10% 19% 21% 12%	2090
	11th to 20th	Day	15% 4 4 16% 32% 28%	2090
	21st to end	Day	15% 2 5% 13% 30% 34%	2090
October	1st to 10th	Day	7% 2 2 8% 25% 56%	3369
	11th to 20th	Day	9% 1 1 9% 29% 50%	3150
	21st to end	Day	6% 1 2 7% 17% 67%	3438
November	1st to 10th	Day	11% 1 3 11% 24% 51%	1959
	11th to 20th	Day	15% 1 2 8% 27% 46%	2000
	21st to end	Day	23% 2 3 14% 24% 33%	1940
December	1st to 10th	Day	14% 2 2 7% 18% 57%	2950
	11th to 20th	Day	19% 1 2 7% 19% 51%	3408
	21st to end	Day	10% 2 2 9% 35% 42%	4510
January	1st to 10th	Night	13% 2 2 7% 33% 43%	5867
	11th to 20th	Night	13% 1 1 6% 22% 56%	5880
	21st to end	Night	21% 2 3 10% 27% 36%	6468
February	1st to 10th	Night	19% 2 2 8% 23% 46%	2460
	11th to 20th	Night	21% 3 5% 10% 25% 37%	2332
	21st to end	Night	21% 2 2 7% 24% 43%	1815
March	1st to 10th	Night	21% 2 2 5% 21% 49%	4940
	11th to 20th	Night	14% 3 2 12% 27% 43%	5000
	21st to end	Night	20% 2 2 7% 22% 47%	5489
April	1st to 10th	Night	12% 2 2 5% 24% 54%	2200
	11th to 20th	Night	13% 1 2 4 22% 58%	2057
	21st to end	Night	8% 1 5% 18% 67%	1955
May	1st to 10th	Night	5% 1 1 3 15% 77%	3077
	11th to 20th	Night	4 2 1 4 16% 73%	2835
	21st to end	Night	4 1 1 1 15% 79%	3110
June	1st to 10th	Night	9% 2 2 6% 23% 59%	1710
	11th to 20th	Night	10% 1 3 6% 22% 58%	1710
	21st to end	Night	23% 5% 6% 10% 26% 30%	1710
July	1st to 10th	Night	33% 5% 14% 18% 16% 14%	2815
	11th to 20th	Night	38% 7% 21% 26% 6% 2	2806
	21st to end	Night	45% 8% 21% 23% 3 1	3011
August	1st to 10th	Night	35% 6% 20% 24% 12% 3	2838
	11th to 20th	Night	33% 9% 17% 24% 14% 3	2980
	21st to end	Night	21% 7% 15% 25% 23% 9%	3676
September	1st to 10th	Night	33% 10% 13% 20% 18% 8%	2090
	11th to 20th	Night	14% 4 6% 19% 31% 25%	2090
	21st to end	Night	13% 3 5% 19% 26% 33%	2090
October	1st to 10th	Night	7% 1 3 9% 25% 55%	3604
	11th to 20th	Night	10% 1 1 11% 28% 48%	3778
	21st to end	Night	5% 1 1 5% 20% 67%	4120
November	1st to 10th	Night	11% 2 2 10% 23% 52%	2340
	11th to 20th	Night	16% 2 3 8% 27% 45%	2400
	21st to end	Night	24% 2 3 12% 25% 34%	2455
December	1st to 10th	Night	15% 2 2 9% 21% 51%	4240
	11th to 20th	Night	17% 2 2 7% 22% 50%	4811
	21st to end	Night	14% 2 2 8% 30% 44%	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% 3 17% 42% 28% 4	300
	11th to 20th	Day	6% 2 13% 40% 32% 7%	300
	21st to end	Day	5% 5% 23% 37% 25% 5%	330
February	1st to 10th	Day	6% 5% 20% 33% 29% 7%	435
	11th to 20th	Day	9% 8% 22% 38% 20% 3	495
	21st to end	Day	7% 7% 32% 34% 15% 4	414
March	1st to 10th	Day	9% 7% 28% 34% 17% 4	500
	11th to 20th	Day	7% 7% 27% 30% 19% 10%	500
	21st to end	Day	6% 8% 30% 33% 18% 6%	550
April	1st to 10th	Day	4 8% 30% 28% 22% 8%	500
	11th to 20th	Day	3 7% 32% 35% 16% 8%	492
	21st to end	Day	1 11% 27% 31% 22% 7%	481
May	1st to 10th	Day	1 9% 30% 35% 18% 8%	463
	11th to 20th	Day	1 5% 28% 38% 22% 5%	488
	21st to end	Day	1 5% 23% 43% 24% 5%	529
June	1st to 10th	Day	4 2 16% 43% 27% 8%	490
	11th to 20th	Day	1 1 16% 43% 27% 12%	490
	21st to end	Day	3 1 9% 38% 32% 16%	490
July	1st to 10th	Day	5% 1 6% 29% 34% 25%	486
	11th to 20th	Day	3 1 4 25% 37% 30%	484
	21st to end	Day	6% 1 5% 25% 34% 29%	531
August	1st to 10th	Day	3 6% 22% 35% 34%	487
	11th to 20th	Day	6% 3 22% 36% 34%	477
	21st to end	Day	4 3 19% 38% 36%	539
September	1st to 10th	Day	8% 1 3 24% 36% 27%	490
	11th to 20th	Day	4 5% 26% 39% 26%	490
	21st to end	Day	4 8% 29% 36% 23%	490
October	1st to 10th	Day	3 10% 28% 40% 19%	421
	11th to 20th	Day	6% 2 8% 24% 34% 27%	300
	21st to end	Day	4 1 10% 22% 42% 22%	330
November	1st to 10th	Day	3 2 11% 31% 38% 15%	300
	11th to 20th	Day	4 1 14% 29% 32% 20%	300
	21st to end	Day	9% 4 16% 36% 29% 7%	300
December	1st to 10th	Day	7% 1 10% 37% 34% 10%	300
	11th to 20th	Day	6% 1 11% 43% 33% 7%	300
	21st to end	Day	6% 3 16% 40% 27% 8%	330
January	1st to 10th	Night	4 13% 52% 30% 2	800
	11th to 20th	Night	5% 2 12% 52% 27% 2	800
	21st to end	Night	8% 1 19% 50% 22% 2	880
February	1st to 10th	Night	5% 1 15% 49% 27% 2	800
	11th to 20th	Night	6% 1 19% 52% 20% 1	784
	21st to end	Night	6% 1 22% 56% 13% 2	660
March	1st to 10th	Night	7% 2 21% 51% 17% 2	800
	11th to 20th	Night	5% 1 19% 50% 22% 3	800
	21st to end	Night	3 2 19% 51% 23% 2	880
April	1st to 10th	Night	3 1 19% 53% 23% 2	800
	11th to 20th	Night	2 1 23% 52% 20% 1	673
	21st to end	Night	1 2 19% 57% 20% 1	587
May	1st to 10th	Night	2 1 19% 55% 23% 1	570
	11th to 20th	Night	1 15% 52% 32% 1	561
	21st to end	Night	1 1 13% 52% 32% 1	617
June	1st to 10th	Night	2 10% 47% 38% 3	570
	11th to 20th	Night	3 9% 45% 39% 4	570
	21st to end	Night	4 1 8% 36% 43% 8%	570
July	1st to 10th	Night	5% 10% 30% 42% 12%	555
	11th to 20th	Night	5% 2 10% 29% 45% 10%	552
	21st to end	Night	9% 1 8% 22% 41% 18%	624
August	1st to 10th	Night	6% 1 7% 23% 46% 18%	567
	11th to 20th	Night	6% 1 4 19% 46% 25%	564
	21st to end	Night	7% 1 5% 18% 48% 21%	738
September	1st to 10th	Night	10% 1 3 18% 49% 20%	760
	11th to 20th	Night	4 1 3 24% 55% 13%	760
	21st to end	Night	4 4 28% 55% 8%	760
October	1st to 10th	Night	3 5% 38% 47% 7%	760
	11th to 20th	Night	5% 6% 40% 42% 7%	760
	21st to end	Night	2 8% 43% 40% 7%	836
November	1st to 10th	Night	2 7% 42% 42% 7%	780
	11th to 20th	Night	5% 1 8% 47% 34% 7%	800
	21st to end	Night	8% 2 12% 49% 26% 4	800
December	1st to 10th	Night	6% 2 8% 51% 29% 4	800
	11th to 20th	Night	5% 1 12% 56% 25% 2	800
	21st to end	Night	5% 1 15% 50% 26% 3	880

Page generated at 2025-04-23 06:25:01. Version ac07

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