Arriola Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arriola Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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 4 1 4 3 4 4 4 6% 7% 60%	157571
	Night	4 5% 2 5% 3 3 4 3 5% 7% 58%	152133
Transparency	Day	23% 4 9% 20% 27% 17%	152451
	Night	25% 4 8% 21% 26% 16%	148260
Seeing	Day	8% 13% 27% 31% 17% 5%	17703
	Night	8% 6% 21% 39% 22% 3	27928

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	4 9% 3 6% 4 4 4 3 5% 6% 52%	5318
	11th to 20th	Day	2 5% 1 4 3 3 3 2 3 5% 69%	5340
	21st to end	Day	4 8% 2 4 3 3 3 3 4 5% 61%	5844
February	1st to 10th	Day	4 9% 3 6% 4 4 3 3 5% 6% 54%	1890
	11th to 20th	Day	5% 10% 3 8% 4 4 4 3 5% 6% 48%	1845
	21st to end	Day	6% 11% 3 6% 4 3 4 3 3 5% 53%	1811
March	1st to 10th	Day	4 8% 2 4 3 4 4 4 5% 7% 54%	5064
	11th to 20th	Day	4 10% 3 9% 5% 5% 6% 5% 7% 6% 41%	5110
	21st to end	Day	5% 8% 2 7% 5% 5% 5% 5% 6% 8% 45%	5607
April	1st to 10th	Day	4 6% 2 6% 4 4 4 3 6% 6% 56%	2468
	11th to 20th	Day	3 6% 2 5% 5% 5% 5% 4 5% 7% 53%	2491
	21st to end	Day	4 4 1 5% 4 4 4 4 6% 8% 55%	2520
May	1st to 10th	Day	4 4 1 5% 4 4 5% 4 6% 8% 56%	7626
	11th to 20th	Day	3 2 1 3 3 3 4 4 5% 7% 65%	7596
	21st to end	Day	2 2 1 3 3 3 3 3 5% 7% 69%	8545
June	1st to 10th	Day	3 3 1 3 3 3 3 3 6% 8% 65%	2940
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 6% 74%	2940
	21st to end	Day	1 2 1 3 2 3 4 4 5% 7% 69%	2905
July	1st to 10th	Day	1 1 1 2 2 2 3 3 4 5% 76%	7649
	11th to 20th	Day	2 2 1 3 4 5% 6% 5% 8% 10% 54%	7650
	21st to end	Day	2 2 1 3 3 4 4 4 7% 10% 60%	8304
August	1st to 10th	Day	2 2 1 3 3 4 4 4 7% 9% 60%	7549
	11th to 20th	Day	2 2 1 4 3 4 5% 4 6% 8% 61%	7456
	21st to end	Day	3 3 1 5% 4 5% 5% 6% 9% 11% 48%	7751
September	1st to 10th	Day	5% 4 2 5% 4 4 4 3 5% 7% 58%	2520
	11th to 20th	Day	4 2 1 4 4 4 5% 4 6% 5% 59%	2412
	21st to end	Day	5% 4 2 3 3 3 2 2 4 5% 66%	2266
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 74%	3370
	11th to 20th	Day	7% 3 1 5% 4 5% 4 3 5% 6% 58%	3350
	21st to end	Day	4 3 1 3 2 3 3 3 4 6% 68%	3628
November	1st to 10th	Day	3 3 1 4 2 3 2 3 4 7% 67%	2059
	11th to 20th	Day	4 3 1 3 3 2 3 2 5% 6% 68%	1934
	21st to end	Day	4 8% 2 4 3 3 3 2 4 4 62%	1680
December	1st to 10th	Day	4 5% 2 3 2 2 2 2 4 7% 66%	2700
	11th to 20th	Day	5% 5% 2 4 3 3 4 3 5% 7% 58%	3106
	21st to end	Day	5% 8% 2 5% 4 3 3 3 4 6% 57%	4327
January	1st to 10th	Night	5% 8% 3 8% 5% 4 4 3 4 5% 49%	7605
	11th to 20th	Night	2 6% 1 3 2 2 3 2 4 6% 69%	7650
	21st to end	Night	4 9% 3 6% 3 3 3 2 3 5% 59%	8370
February	1st to 10th	Night	3 8% 3 5% 3 3 3 3 4 5% 60%	2730
	11th to 20th	Night	6% 8% 2 6% 4 4 4 3 4 6% 53%	2662
	21st to end	Night	5% 9% 3 5% 3 3 4 3 4 6% 54%	1988
March	1st to 10th	Night	5% 8% 2 5% 4 4 4 3 5% 6% 53%	5050
	11th to 20th	Night	6% 10% 3 6% 5% 4 4 4 5% 6% 47%	5110
	21st to end	Night	4 8% 3 6% 4 5% 5% 4 6% 7% 48%	5610
April	1st to 10th	Night	7% 6% 2 5% 4 4 5% 5% 5% 6% 53%	2151
	11th to 20th	Night	5% 5% 2 6% 4 4 4 4 5% 6% 55%	2077
	21st to end	Night	4 5% 1 4 4 4 4 3 4 5% 60%	2096
May	1st to 10th	Night	4 3 2 5% 3 4 4 4 6% 8% 59%	5321
	11th to 20th	Night	4 3 2 4 4 3 4 3 5% 7% 61%	5296
	21st to end	Night	3 2 1 3 3 2 3 3 4 7% 70%	5657
June	1st to 10th	Night	2 2 1 3 3 2 3 3 5% 7% 68%	1680
	11th to 20th	Night	3 3 1 3 2 3 2 3 4 6% 71%	1680
	21st to end	Night	1 2 1 3 3 3 4 4 5% 7% 66%	1707
July	1st to 10th	Night	2 2 1 4 3 3 4 3 5% 6% 68%	5340
	11th to 20th	Night	2 3 2 6% 4 5% 6% 6% 9% 10% 48%	5340
	21st to end	Night	3 3 2 4 4 5% 6% 5% 8% 12% 49%	5805
August	1st to 10th	Night	1 2 1 4 4 5% 5% 5% 7% 10% 55%	5273
	11th to 20th	Night	2 2 1 4 4 4 5% 4 7% 10% 55%	5399
	21st to end	Night	4 3 2 6% 5% 6% 6% 6% 9% 13% 40%	6465
September	1st to 10th	Night	5% 3 2 6% 5% 5% 4 4 7% 7% 54%	2100
	11th to 20th	Night	4 3 2 5% 3 3 4 3 4 7% 61%	2203
	21st to end	Night	6% 3 1 4 3 3 3 2 3 6% 67%	2352
October	1st to 10th	Night	4 2 1 2 2 2 2 2 3 4 75%	4040
	11th to 20th	Night	7% 3 2 6% 4 5% 5% 3 5% 5% 56%	4018
	21st to end	Night	5% 3 1 4 3 2 3 2 3 5% 69%	4348
November	1st to 10th	Night	4 5% 1 2 2 2 3 3 4 6% 68%	2460
	11th to 20th	Night	5% 3 1 3 2 2 3 2 4 5% 70%	2657
	21st to end	Night	5% 7% 2 4 3 2 3 2 5% 5% 62%	2940
December	1st to 10th	Night	4 4 1 2 2 2 2 2 3 7% 70%	4710
	11th to 20th	Night	4 7% 2 4 3 3 3 3 5% 7% 58%	5321
	21st to end	Night	6% 8% 2 5% 3 3 3 2 3 5% 60%	6922

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	33% 4 14% 30% 18% 2	5321
	11th to 20th	Day	20% 2 9% 23% 31% 14%	5340
	21st to end	Day	27% 3 7% 21% 34% 8%	5844
February	1st to 10th	Day	31% 4 12% 22% 23% 8%	1890
	11th to 20th	Day	38% 3 8% 22% 23% 6%	1854
	21st to end	Day	35% 3 7% 17% 30% 8%	1811
March	1st to 10th	Day	27% 3 7% 18% 36% 8%	5064
	11th to 20th	Day	39% 4 9% 22% 20% 6%	5110
	21st to end	Day	34% 4 9% 25% 25% 3	5607
April	1st to 10th	Day	28% 4 6% 19% 29% 14%	2468
	11th to 20th	Day	28% 4 6% 19% 28% 14%	2494
	21st to end	Day	24% 4 7% 21% 32% 12%	2520
May	1st to 10th	Day	24% 5% 8% 19% 28% 15%	7626
	11th to 20th	Day	17% 4 5% 15% 40% 19%	7599
	21st to end	Day	14% 3 4 15% 41% 23%	8546
June	1st to 10th	Day	16% 3 6% 16% 31% 28%	2940
	11th to 20th	Day	14% 3 3 9% 24% 47%	2940
	21st to end	Day	14% 4 5% 12% 24% 41%	2905
July	1st to 10th	Day	11% 3 6% 6% 14% 60%	7586
	11th to 20th	Day	21% 6% 12% 23% 23% 16%	7581
	21st to end	Day	16% 6% 13% 27% 19% 18%	8120
August	1st to 10th	Day	22% 7% 14% 17% 19% 22%	5978
	11th to 20th	Day	21% 7% 13% 24% 21% 14%	5879
	21st to end	Day	24% 8% 12% 26% 20% 10%	6078
September	1st to 10th	Day	25% 5% 8% 16% 25% 22%	2520
	11th to 20th	Day	22% 6% 9% 16% 22% 25%	2412
	21st to end	Day	21% 3 5% 15% 34% 22%	2266
October	1st to 10th	Day	15% 2 5% 12% 31% 34%	3370
	11th to 20th	Day	24% 4 5% 19% 35% 13%	3350
	21st to end	Day	18% 2 6% 23% 43% 8%	3628
November	1st to 10th	Day	18% 2 7% 26% 38% 9%	2059
	11th to 20th	Day	18% 3 7% 29% 36% 8%	1934
	21st to end	Day	27% 3 7% 25% 27% 10%	1680
December	1st to 10th	Day	20% 2 9% 36% 28% 5%	2700
	11th to 20th	Day	27% 4 8% 37% 23% 1	3104
	21st to end	Day	30% 4 12% 24% 22% 7%	4327
January	1st to 10th	Night	36% 3 11% 28% 19% 3	7605
	11th to 20th	Night	19% 2 10% 27% 30% 12%	7650
	21st to end	Night	29% 2 6% 23% 28% 12%	8370
February	1st to 10th	Night	26% 3 11% 25% 25% 9%	2730
	11th to 20th	Night	32% 3 8% 23% 26% 7%	2663
	21st to end	Night	31% 3 7% 20% 29% 10%	1988
March	1st to 10th	Night	30% 4 5% 17% 34% 11%	5050
	11th to 20th	Night	37% 5% 7% 20% 24% 7%	5110
	21st to end	Night	32% 5% 8% 28% 24% 4	5610
April	1st to 10th	Night	30% 4 6% 19% 27% 13%	2151
	11th to 20th	Night	29% 4 5% 15% 30% 17%	2077
	21st to end	Night	26% 4 4 18% 35% 13%	2096
May	1st to 10th	Night	22% 5% 8% 22% 32% 12%	5319
	11th to 20th	Night	21% 4 5% 16% 35% 20%	5297
	21st to end	Night	16% 3 4 14% 39% 25%	5658
June	1st to 10th	Night	16% 4 5% 14% 29% 33%	1680
	11th to 20th	Night	15% 3 4 10% 20% 48%	1680
	21st to end	Night	16% 5% 5% 12% 21% 41%	1707
July	1st to 10th	Night	17% 4 6% 7% 11% 56%	5295
	11th to 20th	Night	26% 7% 12% 21% 16% 18%	5286
	21st to end	Night	25% 6% 15% 23% 14% 16%	5677
August	1st to 10th	Night	25% 7% 13% 11% 14% 29%	4168
	11th to 20th	Night	25% 7% 16% 21% 14% 16%	4252
	21st to end	Night	27% 7% 14% 26% 16% 11%	5069
September	1st to 10th	Night	27% 6% 9% 14% 21% 24%	2100
	11th to 20th	Night	24% 5% 7% 17% 20% 27%	2203
	21st to end	Night	21% 4 5% 13% 31% 27%	2352
October	1st to 10th	Night	15% 3 5% 11% 27% 40%	4040
	11th to 20th	Night	28% 4 6% 12% 35% 15%	4018
	21st to end	Night	20% 3 4 20% 42% 11%	4348
November	1st to 10th	Night	20% 2 5% 21% 38% 14%	2460
	11th to 20th	Night	18% 3 7% 25% 38% 10%	2657
	21st to end	Night	25% 3 7% 21% 32% 12%	2940
December	1st to 10th	Night	17% 2 6% 41% 24% 9%	4710
	11th to 20th	Night	26% 3 5% 34% 29% 2	5322
	21st to end	Night	29% 3 11% 26% 24% 8%	6922

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	16% 13% 29% 29% 11% 2	330
	11th to 20th	Day	13% 14% 29% 29% 14% 2	330
	21st to end	Day	10% 13% 29% 33% 12% 3	363
February	1st to 10th	Day	9% 17% 32% 30% 10% 2	330
	11th to 20th	Day	15% 19% 33% 25% 8%	330
	21st to end	Day	13% 20% 37% 21% 9% 1	357
March	1st to 10th	Day	14% 21% 32% 22% 10% 1	540
	11th to 20th	Day	13% 22% 32% 23% 9% 1	540
	21st to end	Day	14% 26% 36% 16% 7% 1	594
April	1st to 10th	Day	11% 21% 34% 21% 10% 2	540
	11th to 20th	Day	16% 24% 34% 18% 7% 2	532
	21st to end	Day	11% 26% 32% 21% 9% 1	540
May	1st to 10th	Day	10% 20% 37% 23% 8% 1	543
	11th to 20th	Day	8% 17% 31% 26% 14% 3	568
	21st to end	Day	8% 16% 31% 26% 15% 4	675
June	1st to 10th	Day	4 8% 23% 38% 22% 5%	780
	11th to 20th	Day	3 9% 27% 36% 19% 5%	780
	21st to end	Day	3 8% 28% 38% 18% 5%	745
July	1st to 10th	Day	2 3 19% 43% 27% 6%	566
	11th to 20th	Day	4 2 12% 39% 31% 13%	564
	21st to end	Day	5% 1 16% 39% 30% 10%	619
August	1st to 10th	Day	5% 3 18% 41% 27% 6%	567
	11th to 20th	Day	4 3 20% 38% 26% 9%	557
	21st to end	Day	3 3 20% 41% 25% 8%	627
September	1st to 10th	Day	6% 3 22% 38% 26% 6%	570
	11th to 20th	Day	4 8% 28% 36% 19% 5%	570
	21st to end	Day	7% 12% 22% 32% 21% 6%	526
October	1st to 10th	Day	9% 13% 22% 21% 22% 13%	360
	11th to 20th	Day	11% 13% 26% 27% 19% 5%	360
	21st to end	Day	10% 15% 24% 29% 18% 4	387
November	1st to 10th	Day	8% 15% 26% 30% 16% 4	330
	11th to 20th	Day	8% 10% 29% 30% 18% 5%	330
	21st to end	Day	10% 15% 27% 31% 15% 3	330
December	1st to 10th	Day	9% 12% 27% 32% 16% 4	330
	11th to 20th	Day	9% 12% 28% 26% 19% 5%	330
	21st to end	Day	17% 14% 25% 32% 11% 1	363
January	1st to 10th	Night	10% 5% 24% 40% 18% 4	840
	11th to 20th	Night	8% 6% 27% 37% 20% 3	840
	21st to end	Night	10% 6% 29% 36% 15% 3	924
February	1st to 10th	Night	8% 6% 32% 38% 14% 2	840
	11th to 20th	Night	9% 8% 33% 37% 11% 2	824
	21st to end	Night	12% 8% 32% 37% 11% 1	692
March	1st to 10th	Night	10% 8% 29% 36% 15% 2	840
	11th to 20th	Night	8% 8% 25% 39% 18% 2	840
	21st to end	Night	10% 10% 26% 38% 14% 1	924
April	1st to 10th	Night	11% 11% 27% 41% 9% 1	681
	11th to 20th	Night	9% 16% 26% 34% 13% 1	623
	21st to end	Night	7% 12% 28% 40% 12% 1	630
May	1st to 10th	Night	8% 8% 25% 43% 15% 1	630
	11th to 20th	Night	9% 6% 23% 40% 20% 3	621
	21st to end	Night	8% 9% 23% 37% 21% 3	683
June	1st to 10th	Night	2 5% 18% 44% 28% 2	630
	11th to 20th	Night	4 5% 18% 45% 24% 4	630
	21st to end	Night	2 2 20% 43% 30% 3	630
July	1st to 10th	Night	4 1 13% 42% 37% 3	615
	11th to 20th	Night	3 1 7% 34% 48% 7%	612
	21st to end	Night	5% 2 7% 32% 47% 8%	690
August	1st to 10th	Night	3 8% 35% 44% 9%	627
	11th to 20th	Night	5% 2 7% 39% 41% 6%	624
	21st to end	Night	4 2 9% 37% 41% 7%	693
September	1st to 10th	Night	5% 1 11% 42% 36% 5%	630
	11th to 20th	Night	4 2 16% 43% 29% 5%	733
	21st to end	Night	7% 4 15% 39% 29% 6%	840
October	1st to 10th	Night	8% 6% 19% 39% 25% 2	840
	11th to 20th	Night	5% 5% 20% 45% 21% 4	840
	21st to end	Night	6% 7% 21% 38% 23% 5%	912
November	1st to 10th	Night	6% 4 19% 45% 21% 5%	820
	11th to 20th	Night	6% 5% 22% 42% 20% 4	914
	21st to end	Night	10% 9% 22% 36% 18% 4	1050
December	1st to 10th	Night	11% 6% 22% 42% 18% 2	1050
	11th to 20th	Night	14% 6% 26% 35% 18% 2	1050
	21st to end	Night	12% 10% 24% 39% 13% 2	1066

Page generated at 2026-10-26 02:37:52. Version ac07

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