Arco
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arco clear sky chart from 2005-11-06 to 2026-08-15.

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	3 6% 2 6% 4 4 4 4 6% 8% 54%	151831
	Night	4 6% 2 6% 4 4 4 4 6% 8% 53%	147392
Transparency	Day	26% 6% 13% 22% 21% 12%	146536
	Night	28% 7% 14% 22% 19% 10%	143693
Seeing	Day	8% 11% 27% 33% 17% 4	17360
	Night	6% 3 23% 44% 20% 3	26854

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 10% 5% 11% 6% 5% 6% 4 7% 8% 35%	4722
	11th to 20th	Day	1 4 2 6% 5% 4 4 4 6% 8% 55%	4740
	21st to end	Day	3 12% 4 8% 4 3 4 3 6% 7% 46%	5563
February	1st to 10th	Day	3 8% 4 8% 5% 6% 6% 5% 8% 9% 40%	1890
	11th to 20th	Day	4 8% 4 10% 6% 7% 6% 5% 7% 9% 35%	1938
	21st to end	Day	2 6% 3 8% 6% 6% 6% 5% 7% 11% 39%	1730
March	1st to 10th	Day	2 9% 3 8% 5% 5% 5% 4 7% 9% 44%	4723
	11th to 20th	Day	2 9% 3 6% 4 4 5% 4 6% 7% 50%	5110
	21st to end	Day	3 9% 3 9% 6% 6% 7% 5% 8% 10% 34%	5607
April	1st to 10th	Day	4 8% 3 6% 6% 5% 5% 4 8% 9% 43%	2431
	11th to 20th	Day	5% 7% 3 6% 5% 4 5% 4 6% 8% 48%	2683
	21st to end	Day	6% 9% 3 7% 5% 5% 5% 4 6% 9% 43%	2730
May	1st to 10th	Day	3 5% 2 5% 4 4 4 4 6% 7% 54%	7626
	11th to 20th	Day	6% 7% 3 6% 4 4 4 4 6% 7% 48%	7596
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 7% 8% 48%	8728
June	1st to 10th	Day	5% 5% 2 6% 5% 5% 4 4 7% 10% 46%	3150
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 6% 8% 54%	3150
	21st to end	Day	3 3 1 3 4 3 3 3 5% 6% 66%	3150
July	1st to 10th	Day	2 1 1 2 2 3 4 3 5% 6% 70%	8287
	11th to 20th	Day	1 2 1 3 3 3 3 3 6% 9% 66%	8220
	21st to end	Day	3 3 1 4 3 3 4 3 5% 7% 64%	8591
August	1st to 10th	Day	3 3 1 4 3 3 4 4 6% 8% 61%	5909
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 6% 73%	5851
	21st to end	Day	5% 4 1 4 3 3 3 3 4 5% 65%	6468
September	1st to 10th	Day	3 2 1 3 2 3 3 3 4 6% 70%	2290
	11th to 20th	Day	5% 4 2 4 3 3 3 3 6% 7% 61%	2200
	21st to end	Day	5% 5% 1 4 3 2 3 3 4 5% 65%	2200
October	1st to 10th	Day	5% 4 2 4 3 3 2 2 4 4 67%	3511
	11th to 20th	Day	3 4 2 4 3 3 3 3 4 6% 64%	3250
	21st to end	Day	3 5% 2 4 3 3 4 4 6% 8% 56%	3246
November	1st to 10th	Day	4 7% 3 6% 5% 4 5% 3 6% 8% 50%	1854
	11th to 20th	Day	2 6% 2 7% 4 5% 5% 5% 7% 9% 48%	1874
	21st to end	Day	2 7% 3 7% 3 5% 4 4 8% 8% 50%	1680
December	1st to 10th	Day	2 6% 2 5% 3 3 4 4 4 6% 63%	2684
	11th to 20th	Day	6% 12% 4 8% 5% 5% 5% 4 6% 9% 36%	2715
	21st to end	Day	5% 10% 4 10% 5% 6% 5% 3 5% 7% 40%	3734
January	1st to 10th	Night	5% 10% 5% 9% 5% 5% 5% 5% 6% 8% 38%	8172
	11th to 20th	Night	2 5% 2 6% 4 5% 5% 4 5% 8% 53%	8220
	21st to end	Night	3 11% 4 7% 4 3 4 3 5% 7% 49%	8623
February	1st to 10th	Night	4 9% 3 7% 5% 5% 5% 5% 7% 8% 41%	2730
	11th to 20th	Night	4 8% 3 8% 6% 5% 5% 5% 6% 8% 41%	2568
	21st to end	Night	2 8% 3 9% 5% 6% 6% 4 7% 8% 43%	2076
March	1st to 10th	Night	2 12% 3 7% 4 4 4 4 6% 8% 47%	5389
	11th to 20th	Night	4 6% 2 5% 4 4 4 4 6% 9% 52%	5110
	21st to end	Night	4 7% 3 7% 5% 5% 5% 5% 7% 12% 40%	5610
April	1st to 10th	Night	5% 8% 2 5% 4 4 4 4 7% 9% 47%	2184
	11th to 20th	Night	4 7% 2 7% 4 5% 4 3 6% 8% 50%	1884
	21st to end	Night	4 7% 3 7% 5% 4 5% 4 6% 8% 48%	1890
May	1st to 10th	Night	4 5% 2 5% 3 3 4 3 5% 7% 59%	5321
	11th to 20th	Night	5% 6% 2 6% 4 4 4 4 6% 8% 50%	5296
	21st to end	Night	5% 5% 2 5% 4 4 5% 5% 7% 8% 51%	5432
June	1st to 10th	Night	5% 4 1 5% 5% 4 6% 6% 7% 9% 48%	1470
	11th to 20th	Night	5% 3 2 5% 4 4 4 4 6% 7% 56%	1470
	21st to end	Night	3 3 2 3 3 3 3 3 5% 6% 66%	1470
July	1st to 10th	Night	1 1 1 3 3 3 4 4 6% 6% 70%	4653
	11th to 20th	Night	2 2 1 3 2 3 3 4 6% 8% 67%	4740
	21st to end	Night	2 3 1 4 3 4 4 3 5% 8% 63%	5486
August	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 8% 63%	4123
	11th to 20th	Night	2 1 1 2 2 2 3 3 4 6% 73%	4087
	21st to end	Night	6% 3 2 4 3 3 3 3 5% 7% 62%	4509
September	1st to 10th	Night	3 3 1 3 2 3 4 2 4 6% 71%	2106
	11th to 20th	Night	5% 3 1 3 3 3 4 3 5% 7% 64%	2200
	21st to end	Night	6% 3 2 3 3 3 3 3 4 5% 67%	2200
October	1st to 10th	Night	4 3 1 4 2 2 2 2 4 5% 71%	3686
	11th to 20th	Night	6% 6% 2 5% 3 3 4 3 4 5% 58%	3898
	21st to end	Night	4 5% 2 5% 3 4 5% 4 6% 8% 53%	4550
November	1st to 10th	Night	5% 5% 2 5% 4 5% 4 5% 6% 9% 49%	2665
	11th to 20th	Night	3 7% 2 5% 5% 5% 5% 5% 7% 9% 47%	2730
	21st to end	Night	3 6% 2 6% 5% 4 5% 4 6% 8% 51%	2933
December	1st to 10th	Night	3 5% 2 6% 4 3 4 3 5% 6% 60%	4710
	11th to 20th	Night	5% 11% 5% 9% 5% 5% 5% 5% 7% 8% 34%	5696
	21st to end	Night	5% 10% 4 10% 6% 5% 5% 4 6% 7% 38%	7505

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	43% 14% 19% 18% 7%	4723
	11th to 20th	Day	26% 12% 22% 25% 12% 2	4740
	21st to end	Day	38% 9% 16% 24% 9% 4	5563
February	1st to 10th	Day	35% 15% 21% 21% 7% 2	1890
	11th to 20th	Day	40% 12% 21% 19% 8% 1	1947
	21st to end	Day	33% 14% 18% 22% 11% 2	1730
March	1st to 10th	Day	33% 9% 19% 25% 10% 4	4723
	11th to 20th	Day	28% 8% 15% 34% 14% 1	5110
	21st to end	Day	35% 13% 21% 23% 7% 1	5607
April	1st to 10th	Day	32% 11% 19% 24% 13% 1	2431
	11th to 20th	Day	32% 8% 14% 26% 17% 2	2686
	21st to end	Day	33% 8% 16% 27% 16% 1	2730
May	1st to 10th	Day	22% 6% 12% 28% 26% 6%	7626
	11th to 20th	Day	29% 5% 10% 23% 25% 9%	7599
	21st to end	Day	26% 6% 11% 30% 24% 3	8729
June	1st to 10th	Day	28% 7% 15% 25% 21% 3	3150
	11th to 20th	Day	24% 5% 11% 19% 26% 14%	3150
	21st to end	Day	16% 4 7% 19% 35% 19%	3150
July	1st to 10th	Day	12% 2 5% 10% 27% 44%	8219
	11th to 20th	Day	14% 1 5% 13% 28% 39%	8145
	21st to end	Day	17% 2 6% 12% 33% 30%	8400
August	1st to 10th	Day	20% 3 8% 13% 30% 26%	4338
	11th to 20th	Day	13% 3 6% 19% 36% 23%	4250
	21st to end	Day	16% 3 6% 18% 28% 29%	4663
September	1st to 10th	Day	16% 3 7% 17% 31% 25%	2290
	11th to 20th	Day	20% 5% 10% 24% 28% 12%	2200
	21st to end	Day	21% 4 9% 27% 29% 11%	2200
October	1st to 10th	Day	22% 4 8% 18% 35% 13%	3511
	11th to 20th	Day	21% 5% 11% 34% 19% 9%	3250
	21st to end	Day	23% 7% 16% 31% 22% 2	3246
November	1st to 10th	Day	33% 7% 19% 24% 16% 1	1854
	11th to 20th	Day	30% 13% 23% 25% 8% 2	1874
	21st to end	Day	31% 8% 25% 24% 10% 2	1680
December	1st to 10th	Day	25% 8% 22% 32% 13% 1	2684
	11th to 20th	Day	44% 10% 23% 17% 5% 1	2714
	21st to end	Day	41% 9% 18% 17% 13% 1	3734
January	1st to 10th	Night	44% 10% 15% 21% 9%	8172
	11th to 20th	Night	29% 9% 21% 25% 11% 4	8220
	21st to end	Night	34% 9% 18% 24% 11% 3	8623
February	1st to 10th	Night	39% 11% 20% 20% 8% 2	2730
	11th to 20th	Night	38% 11% 18% 22% 9% 1	2569
	21st to end	Night	35% 10% 20% 22% 10% 3	2076
March	1st to 10th	Night	32% 9% 15% 24% 12% 7%	5389
	11th to 20th	Night	27% 6% 17% 29% 19% 2	5110
	21st to end	Night	31% 9% 22% 29% 8% 1	5610
April	1st to 10th	Night	30% 9% 17% 25% 16% 3	2184
	11th to 20th	Night	31% 6% 15% 27% 19% 2	1884
	21st to end	Night	30% 7% 15% 27% 19% 1	1890
May	1st to 10th	Night	22% 5% 9% 28% 27% 9%	5319
	11th to 20th	Night	27% 5% 10% 19% 29% 8%	5297
	21st to end	Night	25% 5% 12% 26% 27% 5%	5433
June	1st to 10th	Night	26% 10% 14% 23% 22% 4	1470
	11th to 20th	Night	24% 6% 9% 18% 27% 16%	1470
	21st to end	Night	17% 3 7% 19% 29% 24%	1470
July	1st to 10th	Night	14% 2 5% 9% 26% 45%	4613
	11th to 20th	Night	14% 1 5% 13% 26% 41%	4692
	21st to end	Night	19% 2 7% 11% 29% 32%	5365
August	1st to 10th	Night	19% 3 8% 12% 29% 30%	3018
	11th to 20th	Night	15% 3 6% 17% 33% 27%	2964
	21st to end	Night	16% 3 6% 15% 26% 34%	3245
September	1st to 10th	Night	15% 4 7% 13% 34% 27%	2106
	11th to 20th	Night	19% 4 10% 25% 26% 17%	2200
	21st to end	Night	20% 4 9% 22% 33% 13%	2200
October	1st to 10th	Night	17% 3 8% 17% 36% 19%	3686
	11th to 20th	Night	28% 4 9% 28% 22% 10%	3898
	21st to end	Night	26% 6% 14% 31% 21% 2	4550
November	1st to 10th	Night	30% 7% 17% 25% 18% 3	2665
	11th to 20th	Night	31% 9% 21% 25% 11% 3	2730
	21st to end	Night	30% 8% 22% 26% 14% 2	2933
December	1st to 10th	Night	25% 8% 24% 31% 10% 1	4710
	11th to 20th	Night	46% 10% 21% 19% 4 1	5697
	21st to end	Night	43% 10% 17% 17% 12% 1	7505

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	9% 7% 19% 37% 22% 5%	220
	11th to 20th	Day	6% 2 16% 44% 22% 10%	220
	21st to end	Day	5% 3 18% 45% 22% 8%	311
February	1st to 10th	Day	9% 8% 29% 38% 14% 3	330
	11th to 20th	Day	6% 7% 29% 38% 15% 4	423
	21st to end	Day	5% 11% 30% 35% 17% 3	446
March	1st to 10th	Day	8% 8% 31% 37% 15% 1	540
	11th to 20th	Day	9% 13% 26% 29% 21% 3	540
	21st to end	Day	10% 15% 27% 34% 13% 2	594
April	1st to 10th	Day	8% 14% 25% 33% 18% 2	540
	11th to 20th	Day	9% 20% 31% 26% 12% 2	532
	21st to end	Day	10% 16% 31% 26% 13% 4	540
May	1st to 10th	Day	9% 13% 31% 31% 13% 3	543
	11th to 20th	Day	15% 15% 25% 27% 15% 4	568
	21st to end	Day	14% 13% 23% 24% 20% 7%	669
June	1st to 10th	Day	9% 14% 22% 30% 21% 5%	780
	11th to 20th	Day	12% 17% 28% 26% 13% 3	780
	21st to end	Day	6% 11% 27% 35% 19% 3	780
July	1st to 10th	Day	4 14% 28% 33% 17% 3	595
	11th to 20th	Day	3 14% 35% 32% 13% 3	564
	21st to end	Day	4 10% 32% 33% 18% 3	619
August	1st to 10th	Day	6% 12% 31% 30% 17% 3	567
	11th to 20th	Day	4 11% 24% 35% 21% 6%	534
	21st to end	Day	5% 11% 30% 30% 20% 4	583
September	1st to 10th	Day	4 10% 28% 32% 19% 7%	530
	11th to 20th	Day	7% 12% 28% 28% 19% 5%	530
	21st to end	Day	8% 11% 25% 32% 18% 6%	530
October	1st to 10th	Day	11% 7% 27% 32% 19% 5%	480
	11th to 20th	Day	7% 12% 22% 27% 23% 9%	330
	21st to end	Day	9% 12% 26% 35% 15% 2	363
November	1st to 10th	Day	11% 11% 23% 36% 14% 5%	330
	11th to 20th	Day	4 8% 30% 37% 15% 6%	330
	21st to end	Day	7% 4 26% 40% 18% 4	330
December	1st to 10th	Day	5% 4 21% 43% 20% 7%	327
	11th to 20th	Day	13% 5% 15% 44% 20% 3	220
	21st to end	Day	11% 8% 16% 40% 20% 5%	242
January	1st to 10th	Night	7% 3 21% 45% 20% 4	840
	11th to 20th	Night	6% 3 22% 44% 20% 5%	840
	21st to end	Night	7% 1 22% 48% 17% 5%	924
February	1st to 10th	Night	8% 2 32% 40% 16% 2	840
	11th to 20th	Night	8% 3 29% 41% 16% 3	824
	21st to end	Night	4 4 31% 41% 17% 3	692
March	1st to 10th	Night	6% 3 26% 47% 15% 2	840
	11th to 20th	Night	6% 7% 28% 42% 16% 2	840
	21st to end	Night	8% 5% 27% 45% 14% 2	924
April	1st to 10th	Night	7% 5% 28% 43% 15% 2	840
	11th to 20th	Night	6% 4 36% 40% 13% 2	636
	21st to end	Night	7% 5% 28% 43% 16% 2	630
May	1st to 10th	Night	5% 7% 29% 40% 17% 2	630
	11th to 20th	Night	8% 6% 25% 35% 23% 3	621
	21st to end	Night	11% 5% 21% 36% 24% 3	609
June	1st to 10th	Night	5% 4 19% 44% 26% 2	420
	11th to 20th	Night	6% 4 25% 45% 17% 2	420
	21st to end	Night	4 2 16% 54% 21% 3	420
July	1st to 10th	Night	4 1 14% 53% 26% 1	410
	11th to 20th	Night	1 1 12% 63% 23%	408
	21st to end	Night	3 1 14% 50% 30% 3	575
August	1st to 10th	Night	3 3 18% 51% 24% 2	627
	11th to 20th	Night	2 1 17% 50% 28% 2	606
	21st to end	Night	4 2 18% 45% 27% 3	660
September	1st to 10th	Night	5% 1 13% 46% 30% 5%	793
	11th to 20th	Night	6% 2 15% 42% 28% 8%	800
	21st to end	Night	7% 2 17% 37% 30% 8%	800
October	1st to 10th	Night	8% 3 17% 44% 23% 4	800
	11th to 20th	Night	6% 4 21% 41% 23% 5%	800
	21st to end	Night	6% 3 26% 43% 19% 4	880
November	1st to 10th	Night	7% 3 22% 43% 22% 3	820
	11th to 20th	Night	5% 2 31% 44% 16% 3	840
	21st to end	Night	5% 3 25% 44% 19% 4	1043
December	1st to 10th	Night	6% 3 26% 46% 17% 2	1050
	11th to 20th	Night	7% 4 25% 48% 14% 3	1050
	21st to end	Night	8% 5% 27% 46% 12% 2	1102

Page generated at 2026-08-12 01:37:16. Version ac07

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