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 2025-04-11.

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% 7% 53%	116836
	Night	4 7% 3 6% 4 4 4 4 6% 7% 52%	120266
Transparency	Day	26% 7% 14% 22% 19% 12%	116584
	Night	29% 7% 15% 22% 17% 9%	120107
Seeing	Day	8% 11% 26% 33% 18% 4	15506
	Night	6% 3 23% 44% 20% 3	25241

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 9% 5% 12% 6% 6% 6% 5% 7% 7% 34%	3639
	11th to 20th	Day	2 6% 3 8% 6% 5% 5% 4 7% 7% 47%	3640
	21st to end	Day	4 14% 4 8% 3 3 4 2 5% 6% 47%	4298
February	1st to 10th	Day	4 8% 4 8% 5% 6% 6% 5% 8% 9% 39%	1800
	11th to 20th	Day	3 8% 3 11% 6% 7% 6% 5% 7% 9% 34%	1843
	21st to end	Day	2 6% 3 9% 6% 6% 6% 5% 7% 11% 38%	1650
March	1st to 10th	Day	2 9% 2 8% 5% 5% 5% 4 7% 9% 44%	4621
	11th to 20th	Day	2 9% 3 6% 4 4 5% 4 6% 7% 50%	5000
	21st to end	Day	3 9% 3 10% 6% 6% 7% 5% 8% 9% 33%	5486
April	1st to 10th	Day	4 8% 3 6% 6% 5% 5% 4 8% 9% 42%	2315
	11th to 20th	Day	5% 7% 3 6% 5% 4 5% 4 6% 8% 47%	2426
	21st to end	Day	5% 9% 2 7% 4 4 5% 4 6% 9% 44%	2470
May	1st to 10th	Day	4 8% 3 7% 5% 5% 6% 6% 7% 7% 40%	4110
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 7% 8% 53%	4071
	21st to end	Day	7% 6% 2 7% 5% 4 5% 4 6% 8% 46%	4674
June	1st to 10th	Day	6% 5% 2 6% 5% 5% 4 4 8% 11% 45%	2850
	11th to 20th	Day	6% 5% 2 5% 4 4 4 4 6% 8% 52%	2850
	21st to end	Day	2 2 1 3 4 3 3 3 5% 6% 67%	2850
July	1st to 10th	Day	1 1 2 2 3 3 2 5% 6% 75%	4460
	11th to 20th	Day	2 2 1 3 3 3 4 3 5% 7% 67%	4420
	21st to end	Day	3 3 1 3 3 3 4 3 5% 7% 64%	4615
August	1st to 10th	Day	4 4 1 4 3 4 4 4 6% 8% 58%	4092
	11th to 20th	Day	2 2 1 2 2 3 3 3 6% 6% 70%	4028
	21st to end	Day	3 4 1 3 2 2 3 2 4 5% 71%	4521
September	1st to 10th	Day	3 3 1 3 2 3 3 3 4 6% 70%	2176
	11th to 20th	Day	5% 4 2 4 3 3 3 3 6% 7% 60%	2090
	21st to end	Day	5% 5% 1 3 2 2 3 3 4 5% 65%	2090
October	1st to 10th	Day	5% 4 2 4 3 3 2 2 4 4 67%	3404
	11th to 20th	Day	2 4 2 4 3 3 3 3 4 6% 65%	3150
	21st to end	Day	3 5% 2 4 3 3 4 4 6% 8% 56%	3145
November	1st to 10th	Day	4 7% 4 6% 4 4 5% 3 6% 8% 49%	1764
	11th to 20th	Day	2 6% 2 7% 4 5% 5% 4 7% 9% 48%	1785
	21st to end	Day	3 8% 3 7% 3 5% 4 4 7% 7% 49%	1600
December	1st to 10th	Day	2 6% 2 5% 3 3 3 3 4 6% 63%	2604
	11th to 20th	Day	6% 12% 4 8% 5% 5% 6% 4 6% 9% 35%	2645
	21st to end	Day	4 10% 4 10% 6% 6% 5% 4 5% 7% 40%	3654
January	1st to 10th	Night	4 8% 4 9% 5% 5% 5% 4 6% 8% 40%	6306
	11th to 20th	Night	2 6% 3 8% 5% 6% 6% 5% 6% 7% 45%	6320
	21st to end	Night	4 14% 4 7% 4 3 4 3 4 6% 47%	6669
February	1st to 10th	Night	5% 9% 3 8% 5% 5% 5% 5% 7% 8% 41%	2600
	11th to 20th	Night	4 8% 3 8% 6% 5% 5% 5% 6% 8% 42%	2443
	21st to end	Night	2 8% 3 9% 5% 6% 6% 4 7% 8% 42%	1980
March	1st to 10th	Night	2 12% 3 7% 4 4 4 4 6% 8% 47%	5271
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 6% 9% 52%	5000
	21st to end	Night	4 7% 3 7% 5% 5% 5% 5% 7% 12% 40%	5489
April	1st to 10th	Night	5% 8% 2 5% 4 4 4 5% 7% 9% 47%	2080
	11th to 20th	Night	4 7% 2 7% 4 5% 4 3 6% 8% 50%	1707
	21st to end	Night	5% 6% 3 6% 5% 4 5% 4 6% 7% 49%	1710
May	1st to 10th	Night	6% 7% 3 6% 4 4 4 4 5% 7% 50%	2860
	11th to 20th	Night	4 4 2 6% 4 4 4 4 6% 8% 56%	2834
	21st to end	Night	6% 6% 2 6% 5% 4 4 4 6% 8% 48%	2901
June	1st to 10th	Night	5% 4 1 5% 5% 4 5% 6% 7% 10% 48%	1330
	11th to 20th	Night	6% 3 2 5% 5% 4 4 4 6% 8% 54%	1330
	21st to end	Night	2 2 1 3 3 3 3 3 5% 6% 68%	1330
July	1st to 10th	Night	1 1 2 2 2 2 3 4 5% 77%	2495
	11th to 20th	Night	1 1 1 2 2 3 3 4 6% 9% 69%	2540
	21st to end	Night	2 3 1 4 3 3 4 3 5% 7% 64%	2940
August	1st to 10th	Night	2 3 1 4 3 4 5% 4 6% 8% 59%	2847
	11th to 20th	Night	2 2 1 3 2 2 3 4 4 6% 70%	2811
	21st to end	Night	3 2 1 3 2 3 3 2 4 6% 70%	3146
September	1st to 10th	Night	3 3 1 3 2 3 3 2 4 6% 71%	2000
	11th to 20th	Night	5% 4 1 3 3 3 4 3 5% 7% 64%	2090
	21st to end	Night	6% 3 2 3 3 3 3 2 4 5% 67%	2090
October	1st to 10th	Night	4 3 1 3 2 2 2 2 4 5% 72%	3573
	11th to 20th	Night	6% 6% 2 4 3 3 4 3 4 5% 59%	3778
	21st to end	Night	4 5% 2 5% 3 4 5% 4 6% 8% 53%	4409
November	1st to 10th	Night	5% 5% 2 6% 4 5% 4 5% 6% 9% 49%	2535
	11th to 20th	Night	3 7% 2 5% 5% 5% 5% 5% 7% 9% 48%	2600
	21st to end	Night	3 7% 2 6% 4 4 5% 4 6% 8% 51%	2793
December	1st to 10th	Night	3 5% 2 6% 4 3 3 3 5% 6% 60%	4570
	11th to 20th	Night	5% 12% 5% 9% 5% 5% 5% 5% 7% 8% 34%	5546
	21st to end	Night	5% 11% 4 10% 6% 5% 5% 4 6% 7% 38%	7343

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	44% 15% 14% 20% 6%	3639
	11th to 20th	Day	33% 14% 19% 22% 10% 2	3640
	21st to end	Day	39% 9% 15% 24% 7% 5%	4298
February	1st to 10th	Day	35% 15% 20% 20% 7% 2	1800
	11th to 20th	Day	40% 12% 21% 19% 7% 1	1852
	21st to end	Day	33% 14% 18% 22% 10% 2	1650
March	1st to 10th	Day	32% 10% 19% 26% 10% 4	4621
	11th to 20th	Day	28% 8% 14% 35% 14% 1	5000
	21st to end	Day	36% 13% 21% 22% 7%	5486
April	1st to 10th	Day	32% 12% 20% 22% 13% 1	2315
	11th to 20th	Day	32% 9% 15% 26% 17% 2	2429
	21st to end	Day	32% 8% 16% 28% 16% 1	2470
May	1st to 10th	Day	30% 9% 15% 26% 17% 3	4110
	11th to 20th	Day	21% 7% 12% 33% 18% 9%	4074
	21st to end	Day	27% 10% 13% 32% 15% 3	4674
June	1st to 10th	Day	29% 8% 16% 26% 19% 2	2850
	11th to 20th	Day	25% 6% 12% 20% 23% 14%	2850
	21st to end	Day	14% 5% 8% 20% 35% 19%	2850
July	1st to 10th	Day	9% 3 5% 10% 24% 49%	4391
	11th to 20th	Day	14% 1 5% 10% 21% 48%	4345
	21st to end	Day	16% 3 6% 10% 30% 34%	4469
August	1st to 10th	Day	20% 3 8% 13% 30% 25%	4078
	11th to 20th	Day	12% 3 6% 19% 37% 23%	4067
	21st to end	Day	15% 3 6% 18% 28% 30%	4520
September	1st to 10th	Day	16% 3 7% 17% 31% 26%	2176
	11th to 20th	Day	21% 6% 10% 24% 27% 12%	2090
	21st to end	Day	20% 4 9% 28% 28% 11%	2090
October	1st to 10th	Day	22% 4 8% 18% 36% 13%	3404
	11th to 20th	Day	20% 5% 11% 35% 20% 10%	3150
	21st to end	Day	23% 7% 16% 30% 23% 2	3145
November	1st to 10th	Day	34% 7% 19% 23% 16% 1	1764
	11th to 20th	Day	29% 13% 23% 25% 8% 2	1785
	21st to end	Day	32% 8% 25% 23% 11% 2	1600
December	1st to 10th	Day	24% 7% 22% 32% 13% 1	2604
	11th to 20th	Day	45% 10% 23% 17% 5%	2644
	21st to end	Day	41% 9% 18% 17% 13% 1	3654
January	1st to 10th	Night	41% 11% 14% 23% 10%	6306
	11th to 20th	Night	37% 12% 15% 21% 10% 5%	6320
	21st to end	Night	38% 11% 16% 21% 10% 4	6669
February	1st to 10th	Night	39% 11% 20% 20% 7% 2	2600
	11th to 20th	Night	37% 11% 18% 23% 10% 2	2444
	21st to end	Night	35% 10% 20% 22% 10% 3	1980
March	1st to 10th	Night	32% 9% 15% 24% 12% 7%	5271
	11th to 20th	Night	27% 6% 16% 30% 19% 2	5000
	21st to end	Night	31% 9% 22% 29% 8% 1	5489
April	1st to 10th	Night	29% 9% 17% 24% 17% 3	2080
	11th to 20th	Night	32% 6% 15% 28% 17% 2	1707
	21st to end	Night	29% 7% 15% 28% 19% 1	1710
May	1st to 10th	Night	29% 7% 11% 28% 18% 6%	2860
	11th to 20th	Night	19% 8% 12% 27% 29% 6%	2835
	21st to end	Night	27% 7% 15% 27% 22% 3	2901
June	1st to 10th	Night	26% 11% 15% 23% 21% 3	1330
	11th to 20th	Night	25% 6% 10% 18% 26% 15%	1330
	21st to end	Night	15% 4 8% 20% 28% 25%	1330
July	1st to 10th	Night	10% 2 5% 10% 21% 52%	2455
	11th to 20th	Night	11% 1 5% 11% 21% 51%	2492
	21st to end	Night	17% 3 6% 12% 29% 33%	2848
August	1st to 10th	Night	20% 3 8% 12% 28% 29%	2838
	11th to 20th	Night	15% 3 6% 17% 33% 27%	2838
	21st to end	Night	15% 3 6% 15% 27% 34%	3146
September	1st to 10th	Night	15% 5% 6% 12% 34% 29%	2000
	11th to 20th	Night	19% 4 10% 24% 25% 17%	2090
	21st to end	Night	20% 4 8% 22% 32% 13%	2090
October	1st to 10th	Night	16% 3 8% 16% 36% 20%	3573
	11th to 20th	Night	28% 4 9% 28% 23% 10%	3778
	21st to end	Night	26% 6% 15% 30% 21% 2	4409
November	1st to 10th	Night	30% 8% 17% 24% 18% 3	2535
	11th to 20th	Night	30% 9% 21% 25% 12% 3	2600
	21st to end	Night	30% 8% 21% 26% 13% 2	2793
December	1st to 10th	Night	25% 8% 24% 31% 10% 1	4570
	11th to 20th	Night	46% 10% 20% 19% 3	5547
	21st to end	Night	43% 10% 17% 16% 13% 1	7343

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	8% 6% 18% 39% 23% 6%	200
	11th to 20th	Day	7% 2 15% 44% 22% 11%	200
	21st to end	Day	6% 3 17% 44% 22% 8%	283
February	1st to 10th	Day	9% 7% 29% 39% 13% 2	300
	11th to 20th	Day	6% 5% 29% 39% 15% 4	388
	21st to end	Day	5% 10% 29% 35% 19% 3	414
March	1st to 10th	Day	7% 8% 30% 38% 16% 2	500
	11th to 20th	Day	8% 12% 25% 29% 22% 3	500
	21st to end	Day	9% 14% 27% 34% 14% 2	550
April	1st to 10th	Day	8% 13% 25% 34% 18% 2	500
	11th to 20th	Day	8% 18% 33% 27% 13% 2	453
	21st to end	Day	10% 15% 31% 27% 13% 4	460
May	1st to 10th	Day	11% 13% 30% 31% 13% 2	463
	11th to 20th	Day	13% 13% 25% 28% 16% 4	488
	21st to end	Day	14% 12% 22% 24% 20% 7%	576
June	1st to 10th	Day	8% 13% 21% 31% 22% 6%	680
	11th to 20th	Day	13% 16% 28% 26% 14% 4	680
	21st to end	Day	4 10% 26% 36% 20% 4	680
July	1st to 10th	Day	4 13% 27% 35% 18% 4	513
	11th to 20th	Day	3 14% 36% 32% 13% 3	484
	21st to end	Day	4 10% 31% 33% 19% 3	531
August	1st to 10th	Day	7% 11% 30% 31% 19% 3	487
	11th to 20th	Day	4 10% 23% 35% 22% 6%	477
	21st to end	Day	4 12% 30% 31% 20% 4	539
September	1st to 10th	Day	4 11% 29% 31% 18% 6%	490
	11th to 20th	Day	8% 12% 28% 28% 19% 5%	490
	21st to end	Day	9% 11% 25% 31% 19% 6%	490
October	1st to 10th	Day	10% 7% 26% 32% 20% 4	443
	11th to 20th	Day	6% 11% 22% 27% 24% 10%	300
	21st to end	Day	8% 13% 26% 35% 16% 2	330
November	1st to 10th	Day	11% 12% 22% 36% 14% 5%	300
	11th to 20th	Day	2 8% 31% 36% 16% 7%	300
	21st to end	Day	8% 4 25% 41% 19% 4	300
December	1st to 10th	Day	6% 4 20% 41% 21% 8%	297
	11th to 20th	Day	13% 4 15% 45% 22% 3	200
	21st to end	Day	10% 5% 16% 43% 20% 6%	220
January	1st to 10th	Night	7% 2 21% 46% 21% 4	800
	11th to 20th	Night	6% 3 22% 43% 21% 5%	800
	21st to end	Night	8% 1 22% 47% 17% 5%	880
February	1st to 10th	Night	8% 2 32% 41% 16% 2	800
	11th to 20th	Night	8% 3 28% 42% 17% 3	784
	21st to end	Night	4 4 30% 42% 18% 3	660
March	1st to 10th	Night	6% 3 25% 47% 16% 2	800
	11th to 20th	Night	6% 6% 26% 43% 16% 3	800
	21st to end	Night	8% 5% 25% 45% 14% 2	880
April	1st to 10th	Night	7% 5% 27% 43% 16% 2	800
	11th to 20th	Night	6% 4 34% 41% 13% 2	577
	21st to end	Night	6% 4 27% 43% 17% 2	570
May	1st to 10th	Night	6% 7% 29% 39% 17% 2	570
	11th to 20th	Night	8% 4 24% 35% 25% 3	561
	21st to end	Night	12% 5% 20% 36% 25% 3	551
June	1st to 10th	Night	5% 4 18% 45% 27% 2	380
	11th to 20th	Night	7% 3 25% 44% 19% 2	380
	21st to end	Night	3 1 15% 55% 23% 3	380
July	1st to 10th	Night	4 1 15% 50% 29% 2	370
	11th to 20th	Night	1 1 11% 64% 23%	368
	21st to end	Night	3 1 14% 48% 32% 3	520
August	1st to 10th	Night	3 3 17% 50% 25% 2	567
	11th to 20th	Night	2 1 15% 50% 29% 2	564
	21st to end	Night	3 2 19% 45% 27% 4	627
September	1st to 10th	Night	5% 1 13% 46% 29% 6%	753
	11th to 20th	Night	6% 2 16% 41% 28% 8%	760
	21st to end	Night	8% 2 17% 36% 29% 8%	760
October	1st to 10th	Night	9% 3 16% 44% 24% 4	760
	11th to 20th	Night	6% 4 20% 41% 24% 5%	760
	21st to end	Night	6% 3 26% 43% 19% 4	836
November	1st to 10th	Night	7% 3 21% 43% 22% 3	780
	11th to 20th	Night	5% 2 31% 44% 16% 3	800
	21st to end	Night	6% 3 25% 44% 18% 4	993
December	1st to 10th	Night	7% 3 24% 47% 18% 3	1000
	11th to 20th	Night	7% 3 24% 48% 14% 3	1000
	21st to end	Night	8% 4 26% 47% 12% 2	1050

Page generated at 2025-04-08 07:17:26. Version ac07

Copyright © 2025 Attilla Danko.