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 2026-08-30.

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 6% 13% 21% 42%	147131
	Night	19% 3 5% 11% 21% 41%	143705
Seeing	Day	4 4 16% 32% 29% 15%	17446
	Night	5% 1 12% 43% 33% 6%	27525

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	17% 2 3 13% 28% 37%	5321
	11th to 20th	Day	11% 2 2 6% 21% 59%	5340
	21st to end	Day	27% 2 2 10% 22% 38%	5844
February	1st to 10th	Day	19% 2 3 8% 23% 45%	2032
	11th to 20th	Day	21% 3 3 9% 25% 39%	2065
	21st to end	Day	18% 3 2 8% 25% 44%	1903
March	1st to 10th	Day	19% 2 2 6% 20% 51%	5064
	11th to 20th	Day	16% 1 2 10% 26% 45%	5110
	21st to end	Day	18% 2 2 5% 23% 49%	5607
April	1st to 10th	Day	15% 2 1 6% 20% 56%	2310
	11th to 20th	Day	11% 1 2 4 19% 63%	2410
	21st to end	Day	8% 1 1 5% 20% 65%	2520
May	1st to 10th	Day	9% 1 3 14% 73%	7221
	11th to 20th	Day	9% 1 1 4 16% 68%	7599
	21st to end	Day	7% 1 1 1 11% 79%	8371
June	1st to 10th	Day	10% 1 2 4 23% 60%	2730
	11th to 20th	Day	8% 1 1 5% 24% 60%	2730
	21st to end	Day	15% 4 5% 13% 28% 35%	2730
July	1st to 10th	Day	20% 5% 14% 24% 22% 16%	7586
	11th to 20th	Day	28% 5% 22% 27% 15% 3	7581
	21st to end	Day	23% 4 16% 31% 14% 12%	8120
August	1st to 10th	Day	21% 6% 19% 31% 19% 4	4338
	11th to 20th	Day	25% 7% 16% 33% 17% 4	4174
	21st to end	Day	19% 6% 12% 28% 24% 11%	4209
September	1st to 10th	Day	31% 6% 10% 20% 21% 12%	2200
	11th to 20th	Day	16% 4 5% 17% 32% 26%	2200
	21st to end	Day	15% 2 5% 14% 30% 34%	2200
October	1st to 10th	Day	7% 2 2 9% 24% 56%	3475
	11th to 20th	Day	10% 1 1 9% 29% 49%	3250
	21st to end	Day	6% 1 2 7% 18% 67%	3548
November	1st to 10th	Day	10% 1 2 11% 23% 53%	2059
	11th to 20th	Day	15% 1 2 9% 28% 44%	2100
	21st to end	Day	22% 3 3 14% 25% 33%	2037
December	1st to 10th	Day	14% 2 2 8% 18% 56%	3040
	11th to 20th	Day	18% 1 2 7% 19% 51%	3498
	21st to end	Day	11% 2 2 9% 35% 41%	4609
January	1st to 10th	Night	18% 2 3 13% 29% 34%	7605
	11th to 20th	Night	12% 1 1 5% 21% 59%	7650
	21st to end	Night	29% 2 3 10% 24% 33%	8370
February	1st to 10th	Night	21% 2 2 8% 23% 45%	2583
	11th to 20th	Night	22% 2 4 10% 24% 36%	2451
	21st to end	Night	20% 2 2 7% 24% 45%	1903
March	1st to 10th	Night	21% 2 2 5% 21% 50%	5050
	11th to 20th	Night	14% 3 2 11% 27% 44%	5110
	21st to end	Night	20% 2 2 7% 22% 47%	5610
April	1st to 10th	Night	13% 2 2 6% 25% 52%	2310
	11th to 20th	Night	13% 1 2 4 22% 57%	2161
	21st to end	Night	10% 1 5% 19% 65%	2100
May	1st to 10th	Night	7% 1 1 4 18% 70%	5718
	11th to 20th	Night	11% 1 1 4 16% 67%	5297
	21st to end	Night	7% 1 1 14% 76%	5838
June	1st to 10th	Night	9% 1 2 6% 23% 60%	1890
	11th to 20th	Night	10% 1 3 7% 22% 58%	1890
	21st to end	Night	22% 4 5% 10% 28% 31%	1890
July	1st to 10th	Night	32% 4 14% 19% 19% 12%	5295
	11th to 20th	Night	44% 4 19% 23% 9% 1	5286
	21st to end	Night	36% 5% 20% 22% 7% 10%	5677
August	1st to 10th	Night	36% 6% 20% 25% 12% 3	3018
	11th to 20th	Night	34% 8% 17% 24% 14% 3	3169
	21st to end	Night	23% 6% 15% 26% 22% 8%	3886
September	1st to 10th	Night	33% 9% 13% 20% 18% 8%	2200
	11th to 20th	Night	16% 4 6% 20% 30% 24%	2200
	21st to end	Night	14% 3 5% 20% 26% 33%	2200
October	1st to 10th	Night	7% 1 3 9% 25% 55%	3718
	11th to 20th	Night	11% 1 1 11% 28% 47%	3898
	21st to end	Night	5% 1 1 5% 20% 67%	4252
November	1st to 10th	Night	10% 2 2 9% 23% 53%	2460
	11th to 20th	Night	17% 2 3 7% 28% 44%	2520
	21st to end	Night	24% 2 3 12% 27% 33%	2578
December	1st to 10th	Night	15% 2 2 9% 21% 51%	4370
	11th to 20th	Night	17% 2 2 7% 22% 51%	4941
	21st to end	Night	15% 2 2 8% 29% 44%	6611

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% 3 18% 42% 28% 4	330
	11th to 20th	Day	5% 2 12% 42% 32% 7%	330
	21st to end	Day	6% 5% 23% 37% 24% 4	363
February	1st to 10th	Day	6% 5% 21% 33% 28% 6%	472
	11th to 20th	Day	9% 9% 22% 39% 19% 3	535
	21st to end	Day	7% 7% 31% 34% 17% 5%	446
March	1st to 10th	Day	9% 7% 28% 33% 18% 5%	540
	11th to 20th	Day	7% 7% 25% 29% 20% 11%	540
	21st to end	Day	5% 7% 28% 33% 19% 7%	594
April	1st to 10th	Day	4 8% 31% 29% 22% 7%	540
	11th to 20th	Day	3 8% 32% 35% 16% 7%	532
	21st to end	Day	1 12% 28% 31% 21% 7%	540
May	1st to 10th	Day	1 10% 30% 34% 17% 7%	543
	11th to 20th	Day	2 7% 29% 37% 20% 5%	568
	21st to end	Day	1 6% 24% 41% 22% 5%	617
June	1st to 10th	Day	3 2 19% 41% 26% 7%	570
	11th to 20th	Day	1 1 16% 43% 26% 12%	570
	21st to end	Day	3 2 9% 39% 31% 17%	570
July	1st to 10th	Day	5% 1 8% 29% 34% 23%	566
	11th to 20th	Day	3 1 6% 26% 37% 27%	564
	21st to end	Day	5% 1 5% 26% 35% 28%	619
August	1st to 10th	Day	3 7% 24% 35% 32%	567
	11th to 20th	Day	5% 3 22% 37% 33%	557
	21st to end	Day	4 4 19% 38% 35%	620
September	1st to 10th	Day	8% 2 4 24% 36% 27%	530
	11th to 20th	Day	4 5% 27% 38% 26%	530
	21st to end	Day	4 9% 29% 35% 24%	530
October	1st to 10th	Day	3 9% 27% 40% 20%	457
	11th to 20th	Day	6% 2 8% 23% 35% 25%	330
	21st to end	Day	3 1 10% 21% 43% 21%	363
November	1st to 10th	Day	2 2 10% 30% 38% 17%	330
	11th to 20th	Day	4 2 14% 29% 31% 20%	330
	21st to end	Day	9% 3 17% 36% 28% 6%	330
December	1st to 10th	Day	7% 2 11% 36% 35% 9%	330
	11th to 20th	Day	5% 1 11% 42% 34% 8%	330
	21st to end	Day	7% 2 17% 40% 26% 8%	363
January	1st to 10th	Night	4 13% 52% 29% 2	840
	11th to 20th	Night	4 2 12% 53% 26% 2	840
	21st to end	Night	8% 2 18% 50% 21% 2	924
February	1st to 10th	Night	5% 2 15% 50% 26% 2	840
	11th to 20th	Night	7% 1 20% 52% 20% 1	824
	21st to end	Night	6% 1 21% 56% 15% 2	692
March	1st to 10th	Night	6% 2 21% 50% 18% 2	840
	11th to 20th	Night	5% 1 18% 51% 23% 3	840
	21st to end	Night	3 2 19% 51% 24% 2	924
April	1st to 10th	Night	3 1 19% 54% 22% 2	840
	11th to 20th	Night	3 1 23% 52% 20% 1	707
	21st to end	Night	1 2 20% 57% 19% 1	630
May	1st to 10th	Night	1 1 21% 54% 21% 1	630
	11th to 20th	Night	1 1 16% 53% 29% 1	621
	21st to end	Night	1 1 14% 52% 30% 1	683
June	1st to 10th	Night	2 12% 48% 36% 3	630
	11th to 20th	Night	2 1 11% 44% 38% 3	630
	21st to end	Night	3 1 9% 37% 41% 8%	630
July	1st to 10th	Night	5% 13% 32% 39% 11%	615
	11th to 20th	Night	5% 2 13% 28% 43% 9%	612
	21st to end	Night	9% 2 9% 23% 41% 17%	690
August	1st to 10th	Night	5% 1 9% 24% 44% 16%	627
	11th to 20th	Night	6% 1 6% 20% 45% 23%	624
	21st to end	Night	6% 1 6% 19% 48% 20%	808
September	1st to 10th	Night	10% 1 4 18% 49% 19%	800
	11th to 20th	Night	4 1 4 24% 54% 13%	800
	21st to end	Night	4 5% 29% 54% 8%	800
October	1st to 10th	Night	3 4 38% 48% 7%	800
	11th to 20th	Night	6% 1 6% 39% 43% 6%	800
	21st to end	Night	2 8% 43% 40% 7%	880
November	1st to 10th	Night	2 7% 42% 43% 6%	820
	11th to 20th	Night	5% 1 8% 47% 34% 6%	840
	21st to end	Night	8% 2 13% 49% 25% 4	840
December	1st to 10th	Night	6% 2 8% 52% 28% 4	840
	11th to 20th	Night	5% 1 11% 56% 26% 2	840
	21st to end	Night	5% 1 15% 50% 26% 3	924

Page generated at 2026-08-27 01:07:13. Version ac07

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