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 2024-10-25.

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 6% 2 6% 4 4 4 4 6% 8% 52%	101050
	Night	4 7% 3 6% 4 4 4 4 6% 8% 51%	97930
Transparency	Day	26% 8% 14% 21% 19% 12%	100798
	Night	29% 8% 15% 21% 17% 10%	97771
Seeing	Day	8% 11% 26% 33% 18% 4	14995
	Night	6% 3 22% 44% 20% 3	24533

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% 6% 14% 6% 6% 6% 4 7% 8% 31%	2539
	11th to 20th	Day	3 8% 3 11% 7% 6% 5% 4 7% 8% 39%	2540
	21st to end	Day	6% 20% 5% 10% 4 3 4 3 5% 7% 32%	2990
February	1st to 10th	Day	3 8% 4 8% 5% 6% 5% 5% 8% 9% 39%	1710
	11th to 20th	Day	3 8% 3 11% 6% 6% 6% 5% 7% 9% 35%	1748
	21st to end	Day	2 6% 3 9% 6% 6% 6% 5% 7% 11% 37%	1570
March	1st to 10th	Day	2 9% 2 8% 5% 5% 5% 4 7% 9% 44%	3215
	11th to 20th	Day	3 8% 3 5% 3 4 4 3 5% 6% 57%	3490
	21st to end	Day	4 10% 4 12% 7% 7% 7% 5% 8% 9% 27%	3825
April	1st to 10th	Day	4 8% 3 6% 6% 5% 5% 4 8% 8% 41%	2199
	11th to 20th	Day	5% 7% 3 6% 5% 4 5% 4 6% 8% 47%	2424
	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	8% 6% 2 5% 3 4 4 3 5% 6% 54%	2043
	11th to 20th	Day	3 4 2 4 4 4 4 3 5% 7% 59%	1900
	21st to end	Day	4 5% 1 4 3 3 5% 5% 6% 9% 55%	1855
November	1st to 10th	Day	4 7% 3 6% 5% 4 5% 3 6% 8% 49%	1674
	11th to 20th	Day	2 5% 2 7% 4 5% 4 4 7% 9% 49%	1696
	21st to end	Day	3 8% 3 7% 3 5% 4 4 7% 7% 50%	1520
December	1st to 10th	Day	3 10% 3 8% 5% 5% 5% 5% 5% 8% 43%	1518
	11th to 20th	Day	4 11% 4 9% 6% 6% 6% 5% 6% 9% 36%	1685
	21st to end	Day	3 9% 4 10% 5% 5% 4 4 5% 7% 45%	2542
January	1st to 10th	Night	3 8% 5% 10% 5% 5% 6% 5% 6% 8% 39%	4406
	11th to 20th	Night	3 9% 4 10% 6% 6% 6% 5% 6% 7% 38%	4420
	21st to end	Night	5% 20% 6% 9% 4 3 5% 4 5% 7% 33%	4670
February	1st to 10th	Night	4 9% 3 7% 5% 5% 5% 5% 7% 8% 41%	2470
	11th to 20th	Night	4 8% 3 8% 6% 5% 5% 5% 6% 8% 42%	2318
	21st to end	Night	2 8% 3 9% 6% 6% 6% 4 7% 8% 41%	1884
March	1st to 10th	Night	2 10% 3 7% 4 4 5% 4 7% 8% 47%	3657
	11th to 20th	Night	3 4 2 4 3 3 3 4 6% 8% 61%	3490
	21st to end	Night	4 8% 3 7% 6% 5% 5% 5% 8% 11% 38%	3828
April	1st to 10th	Night	5% 8% 2 5% 4 4 4 5% 8% 9% 47%	1976
	11th to 20th	Night	4 7% 2 7% 4 5% 4 3 6% 8% 50%	1703
	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	7% 4 1 4 3 3 3 3 6% 7% 59%	2137
	11th to 20th	Night	4 4 2 4 3 3 4 3 5% 7% 61%	2280
	21st to end	Night	4 5% 2 5% 3 4 5% 4 6% 9% 55%	2602
November	1st to 10th	Night	6% 6% 2 6% 4 4 4 5% 6% 9% 48%	2405
	11th to 20th	Night	2 6% 2 5% 4 5% 5% 5% 7% 9% 49%	2470
	21st to end	Night	3 7% 2 6% 4 4 5% 4 6% 7% 51%	2653
December	1st to 10th	Night	4 8% 3 8% 5% 5% 5% 5% 7% 7% 43%	2660
	11th to 20th	Night	5% 10% 5% 8% 5% 5% 5% 4 7% 8% 36%	3506
	21st to end	Night	3 10% 4 9% 5% 5% 5% 4 6% 6% 43%	5141

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	45% 15% 16% 18% 6% 1	2539
	11th to 20th	Day	38% 14% 19% 19% 8% 2	2540
	21st to end	Day	52% 10% 16% 16% 5% 1	2990
February	1st to 10th	Day	35% 16% 21% 21% 6% 1	1710
	11th to 20th	Day	40% 12% 22% 18% 8% 1	1757
	21st to end	Day	33% 15% 19% 21% 9% 2	1570
March	1st to 10th	Day	30% 12% 20% 27% 11% 1	3215
	11th to 20th	Day	25% 9% 17% 35% 13% 1	3490
	21st to end	Day	42% 16% 19% 17% 5% 1	3825
April	1st to 10th	Day	33% 12% 20% 21% 12% 1	2199
	11th to 20th	Day	32% 9% 15% 26% 17% 2	2427
	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	29% 6% 13% 24% 21% 6%	2043
	11th to 20th	Day	22% 7% 16% 33% 17% 5%	1900
	21st to end	Day	22% 9% 22% 27% 18% 2	1855
November	1st to 10th	Day	34% 7% 19% 22% 16% 1	1674
	11th to 20th	Day	29% 13% 23% 25% 8% 2	1696
	21st to end	Day	32% 8% 25% 24% 11% 2	1520
December	1st to 10th	Day	41% 12% 18% 20% 8% 2	1518
	11th to 20th	Day	43% 13% 21% 19% 5%	1684
	21st to end	Day	37% 10% 20% 17% 14% 2	2542
January	1st to 10th	Night	41% 11% 17% 20% 10% 1	4406
	11th to 20th	Night	43% 13% 17% 16% 8% 3	4420
	21st to end	Night	48% 13% 16% 15% 6% 1	4670
February	1st to 10th	Night	39% 12% 21% 21% 7% 2	2470
	11th to 20th	Night	36% 11% 19% 23% 9% 2	2319
	21st to end	Night	36% 10% 20% 22% 9% 3	1884
March	1st to 10th	Night	30% 12% 19% 23% 14% 2	3657
	11th to 20th	Night	20% 8% 19% 32% 20% 2	3490
	21st to end	Night	33% 11% 20% 29% 5% 1	3828
April	1st to 10th	Night	30% 10% 18% 24% 16% 3	1976
	11th to 20th	Night	32% 6% 16% 28% 17% 2	1703
	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	24% 5% 13% 25% 24% 9%	2137
	11th to 20th	Night	22% 5% 13% 33% 22% 5%	2280
	21st to end	Night	23% 8% 18% 27% 21% 2	2602
November	1st to 10th	Night	30% 8% 18% 24% 17% 3	2405
	11th to 20th	Night	29% 10% 21% 26% 11% 3	2470
	21st to end	Night	30% 8% 21% 26% 14% 2	2653
December	1st to 10th	Night	37% 13% 21% 18% 10% 2	2660
	11th to 20th	Night	43% 11% 19% 21% 5% 1	3507
	21st to end	Night	38% 11% 19% 17% 13% 2	5141

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% 6% 14% 40% 24% 7%	180
	11th to 20th	Day	8% 2 12% 44% 22% 11%	180
	21st to end	Day	6% 3 15% 45% 22% 9%	254
February	1st to 10th	Day	7% 6% 31% 40% 14% 2	270
	11th to 20th	Day	6% 6% 29% 39% 16% 5%	353
	21st to end	Day	5% 9% 29% 35% 19% 3	382
March	1st to 10th	Day	7% 7% 30% 39% 16% 1	460
	11th to 20th	Day	7% 10% 25% 30% 24% 4	460
	21st to end	Day	9% 13% 27% 35% 15% 2	506
April	1st to 10th	Day	8% 13% 24% 34% 19% 1	460
	11th to 20th	Day	8% 18% 33% 27% 13% 2	452
	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% 12% 26% 35% 16% 2	311
November	1st to 10th	Day	11% 12% 22% 36% 14% 5%	270
	11th to 20th	Day	2 7% 28% 38% 18% 7%	270
	21st to end	Day	8% 4 24% 41% 20% 4	270
December	1st to 10th	Day	6% 4 21% 43% 20% 7%	268
	11th to 20th	Day	11% 3 14% 47% 22% 3	180
	21st to end	Day	11% 4 13% 44% 22% 7%	198
January	1st to 10th	Night	7% 2 20% 45% 21% 4	760
	11th to 20th	Night	7% 3 21% 43% 21% 6%	760
	21st to end	Night	8% 1 21% 47% 17% 6%	836
February	1st to 10th	Night	8% 2 31% 42% 16% 2	760
	11th to 20th	Night	8% 3 29% 41% 17% 3	744
	21st to end	Night	4 4 29% 42% 18% 3	628
March	1st to 10th	Night	6% 3 25% 48% 15% 2	760
	11th to 20th	Night	5% 6% 25% 44% 17% 3	760
	21st to end	Night	8% 5% 25% 45% 15% 2	836
April	1st to 10th	Night	7% 5% 27% 44% 15% 2	760
	11th to 20th	Night	6% 4 34% 41% 13% 2	575
	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 25% 43% 19% 5%	812
November	1st to 10th	Night	8% 3 20% 43% 23% 3	740
	11th to 20th	Night	5% 2 30% 44% 16% 3	760
	21st to end	Night	6% 3 24% 44% 19% 4	943
December	1st to 10th	Night	7% 2 24% 47% 16% 2	950
	11th to 20th	Night	7% 3 24% 48% 15% 3	950
	21st to end	Night	7% 5% 25% 47% 13% 3	998

Page generated at 2024-10-22 18:21:01. Version ac07

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