Vista
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vista 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	4 4 2 4 3 3 3 3 4 7% 63%	157570
	Night	6% 8% 2 5% 3 3 4 3 5% 6% 55%	152310
Transparency	Day	20% 4 7% 18% 26% 24%	152670
	Night	27% 5% 8% 17% 22% 21%	148188
Seeing	Day	7% 5% 22% 38% 21% 8%	18516
	Night	9% 3 9% 33% 36% 10%	28863

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	9% 5% 2 4 3 3 3 3 3 5% 62%	5318
	11th to 20th	Day	4 3 1 3 1 2 3 2 3 6% 72%	5716
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	6500
February	1st to 10th	Day	8% 4 2 5% 4 3 4 2 4 5% 58%	2100
	11th to 20th	Day	9% 5% 2 4 3 3 4 4 6% 8% 53%	2050
	21st to end	Day	8% 5% 1 5% 4 3 4 4 5% 7% 53%	1730
March	1st to 10th	Day	8% 5% 2 6% 4 4 4 4 7% 9% 48%	4618
	11th to 20th	Day	10% 4 2 4 3 3 3 3 5% 8% 55%	4840
	21st to end	Day	9% 5% 2 4 3 3 3 3 6% 9% 52%	5955
April	1st to 10th	Day	6% 5% 2 4 4 3 3 3 5% 9% 56%	2520
	11th to 20th	Day	4 4 2 5% 3 3 4 3 4 7% 62%	2489
	21st to end	Day	5% 4 1 4 3 2 3 3 5% 8% 61%	2520
May	1st to 10th	Day	4 5% 2 5% 3 3 3 2 4 6% 62%	7358
	11th to 20th	Day	3 6% 3 7% 4 4 4 3 5% 7% 55%	7596
	21st to end	Day	3 6% 2 5% 3 4 4 3 5% 7% 58%	8370
June	1st to 10th	Day	5% 9% 2 5% 3 3 3 3 5% 7% 56%	2730
	11th to 20th	Day	3 6% 2 4 2 3 3 2 4 7% 64%	2730
	21st to end	Day	2 6% 2 4 3 2 3 2 4 5% 66%	2730
July	1st to 10th	Day	1 3 1 3 2 2 3 2 4 5% 73%	7649
	11th to 20th	Day	1 4 2 5% 3 3 4 3 5% 8% 63%	7650
	21st to end	Day	1 2 1 3 2 2 3 2 4 6% 74%	7894
August	1st to 10th	Day	1 1 2 1 1 2 2 3 5% 80%	6986
	11th to 20th	Day	1 2 1 3 1 2 2 2 4 7% 76%	7001
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 73%	7750
September	1st to 10th	Day	3 3 1 3 2 2 3 3 4 6% 69%	2515
	11th to 20th	Day	3 3 2 3 2 3 3 2 3 6% 70%	2310
	21st to end	Day	2 5% 1 4 3 2 3 2 3 6% 69%	2310
October	1st to 10th	Day	2 3 1 3 1 2 2 2 4 5% 75%	3710
	11th to 20th	Day	3 4 2 4 3 2 3 2 4 6% 67%	3657
	21st to end	Day	3 3 1 3 2 2 3 2 4 7% 69%	3486
November	1st to 10th	Day	3 3 1 2 2 3 3 4 4 7% 68%	1854
	11th to 20th	Day	4 3 2 3 3 3 3 3 5% 6% 65%	1890
	21st to end	Day	5% 4 1 4 3 2 4 4 6% 8% 58%	1890
December	1st to 10th	Day	4 2 1 2 2 2 3 3 5% 8% 68%	3040
	11th to 20th	Day	7% 4 1 4 4 5% 5% 4 6% 8% 52%	3499
	21st to end	Day	8% 5% 2 5% 5% 6% 5% 5% 7% 9% 44%	4609
January	1st to 10th	Night	11% 5% 2 5% 3 3 3 2 4 5% 57%	7605
	11th to 20th	Night	5% 5% 2 3 3 2 2 2 4 6% 66%	7278
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 48%	7747
February	1st to 10th	Night	9% 6% 2 5% 3 3 4 3 5% 5% 57%	2520
	11th to 20th	Night	11% 8% 2 5% 4 3 4 3 3 5% 52%	2458
	21st to end	Night	10% 6% 2 6% 4 3 4 3 4 6% 52%	2076
March	1st to 10th	Night	16% 9% 3 6% 3 3 3 3 5% 7% 41%	5496
	11th to 20th	Night	11% 8% 2 6% 4 4 4 4 4 5% 48%	5366
	21st to end	Night	11% 13% 3 6% 4 3 4 3 4 5% 44%	5256
April	1st to 10th	Night	9% 10% 3 6% 3 3 3 2 5% 7% 48%	2100
	11th to 20th	Night	7% 11% 2 6% 4 3 3 3 4 6% 52%	2079
	21st to end	Night	8% 10% 4 6% 3 3 3 3 5% 7% 48%	2100
May	1st to 10th	Night	8% 14% 4 5% 2 3 3 2 3 5% 50%	5585
	11th to 20th	Night	7% 19% 4 7% 4 3 3 2 4 6% 41%	5296
	21st to end	Night	9% 14% 3 6% 3 3 4 2 4 5% 46%	5837
June	1st to 10th	Night	9% 18% 4 9% 5% 4 4 3 5% 6% 34%	1890
	11th to 20th	Night	4 13% 3 5% 2 3 4 3 5% 10% 47%	1890
	21st to end	Night	5% 14% 3 6% 3 3 4 4 6% 8% 45%	1890
July	1st to 10th	Night	1 6% 2 4 3 3 4 3 5% 6% 62%	5340
	11th to 20th	Night	1 7% 3 6% 3 4 4 4 6% 8% 54%	5340
	21st to end	Night	2 5% 2 5% 3 3 4 3 4 6% 63%	6219
August	1st to 10th	Night	1 3 1 2 3 2 5% 3 5% 7% 68%	5866
	11th to 20th	Night	2 8% 2 4 2 2 3 2 4 8% 61%	5886
	21st to end	Night	2 4 2 3 2 3 4 3 5% 6% 66%	6499
September	1st to 10th	Night	4 7% 2 5% 4 3 4 3 4 6% 59%	2105
	11th to 20th	Night	5% 7% 2 3 2 3 3 3 4 7% 63%	2310
	21st to end	Night	3 7% 2 4 2 3 4 3 4 5% 61%	2310
October	1st to 10th	Night	2 3 1 3 3 3 4 3 4 7% 67%	3710
	11th to 20th	Night	6% 13% 2 5% 3 2 4 2 4 5% 55%	3716
	21st to end	Night	3 5% 1 3 2 3 3 3 4 6% 67%	4493
November	1st to 10th	Night	4 5% 2 5% 3 3 3 2 4 6% 63%	2665
	11th to 20th	Night	4 6% 2 4 3 3 3 3 4 5% 62%	2730
	21st to end	Night	6% 5% 3 4 4 4 4 4 6% 7% 53%	2730
December	1st to 10th	Night	5% 3 1 4 4 3 4 3 5% 5% 63%	4370
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 7% 50%	4941
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	6611

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	25% 4 7% 15% 23% 26%	5321
	11th to 20th	Day	15% 3 7% 15% 19% 41%	5716
	21st to end	Day	30% 5% 6% 11% 22% 25%	6500
February	1st to 10th	Day	28% 4 6% 14% 21% 26%	2100
	11th to 20th	Day	28% 5% 8% 15% 26% 17%	2060
	21st to end	Day	29% 5% 8% 17% 23% 18%	1730
March	1st to 10th	Day	28% 6% 10% 21% 22% 13%	4618
	11th to 20th	Day	26% 5% 10% 26% 21% 11%	4840
	21st to end	Day	27% 6% 12% 30% 15% 9%	5955
April	1st to 10th	Day	26% 5% 11% 24% 23% 12%	2520
	11th to 20th	Day	22% 5% 7% 21% 27% 18%	2492
	21st to end	Day	21% 5% 11% 26% 28% 10%	2520
May	1st to 10th	Day	22% 5% 8% 22% 33% 11%	7358
	11th to 20th	Day	23% 7% 7% 19% 31% 12%	7599
	21st to end	Day	21% 6% 8% 25% 31% 9%	8371
June	1st to 10th	Day	27% 6% 6% 21% 27% 12%	2730
	11th to 20th	Day	20% 4 5% 12% 32% 27%	2730
	21st to end	Day	20% 3 5% 8% 24% 40%	2730
July	1st to 10th	Day	11% 2 4 8% 22% 52%	7586
	11th to 20th	Day	17% 4 7% 19% 37% 17%	7581
	21st to end	Day	10% 3 6% 24% 29% 27%	7720
August	1st to 10th	Day	8% 2 5% 25% 37% 21%	5530
	11th to 20th	Day	11% 2 7% 22% 28% 30%	5517
	21st to end	Day	12% 3 4 15% 32% 35%	6077
September	1st to 10th	Day	16% 4 6% 10% 32% 32%	2515
	11th to 20th	Day	15% 3 5% 11% 34% 32%	2310
	21st to end	Day	17% 3 5% 12% 32% 30%	2310
October	1st to 10th	Day	12% 3 4 11% 17% 53%	3710
	11th to 20th	Day	17% 3 5% 12% 24% 38%	3657
	21st to end	Day	15% 3 3 11% 32% 36%	3486
November	1st to 10th	Day	16% 4 7% 18% 20% 34%	1854
	11th to 20th	Day	20% 3 7% 14% 20% 35%	1890
	21st to end	Day	22% 5% 9% 15% 19% 30%	1890
December	1st to 10th	Day	15% 4 7% 18% 23% 34%	3040
	11th to 20th	Day	29% 5% 8% 15% 25% 18%	3498
	21st to end	Day	33% 6% 10% 24% 18% 8%	4609
January	1st to 10th	Night	29% 5% 6% 14% 24% 22%	7605
	11th to 20th	Night	20% 4 7% 13% 16% 40%	7278
	21st to end	Night	35% 4 6% 10% 19% 25%	7747
February	1st to 10th	Night	29% 4 7% 15% 21% 24%	2520
	11th to 20th	Night	36% 4 6% 14% 21% 20%	2459
	21st to end	Night	33% 4 5% 15% 23% 19%	2076
March	1st to 10th	Night	38% 8% 10% 14% 17% 13%	5496
	11th to 20th	Night	34% 7% 9% 24% 17% 9%	5366
	21st to end	Night	38% 8% 12% 17% 17% 8%	5256
April	1st to 10th	Night	34% 6% 8% 20% 19% 13%	2100
	11th to 20th	Night	32% 7% 8% 17% 21% 15%	2079
	21st to end	Night	34% 7% 9% 21% 19% 10%	2100
May	1st to 10th	Night	34% 6% 8% 22% 22% 8%	5583
	11th to 20th	Night	39% 7% 7% 18% 22% 7%	5297
	21st to end	Night	36% 7% 9% 21% 20% 8%	5838
June	1st to 10th	Night	45% 10% 9% 9% 18% 9%	1890
	11th to 20th	Night	29% 7% 7% 11% 26% 20%	1890
	21st to end	Night	33% 6% 6% 9% 19% 27%	1890
July	1st to 10th	Night	16% 3 6% 10% 21% 43%	5295
	11th to 20th	Night	21% 4 11% 19% 32% 12%	5286
	21st to end	Night	18% 4 7% 27% 27% 18%	6082
August	1st to 10th	Night	12% 3 8% 27% 35% 14%	4636
	11th to 20th	Night	17% 4 7% 26% 22% 23%	4636
	21st to end	Night	16% 3 5% 17% 31% 28%	5092
September	1st to 10th	Night	26% 5% 5% 11% 25% 29%	2105
	11th to 20th	Night	21% 5% 5% 10% 33% 27%	2310
	21st to end	Night	23% 4 6% 15% 31% 22%	2310
October	1st to 10th	Night	14% 3 6% 12% 22% 43%	3710
	11th to 20th	Night	30% 3 5% 10% 20% 32%	3716
	21st to end	Night	19% 3 4 12% 30% 33%	4493
November	1st to 10th	Night	21% 5% 7% 15% 18% 33%	2665
	11th to 20th	Night	24% 3 6% 13% 18% 36%	2730
	21st to end	Night	29% 4 8% 15% 15% 29%	2730
December	1st to 10th	Night	21% 4 7% 16% 22% 31%	4370
	11th to 20th	Night	31% 4 8% 13% 22% 21%	4941
	21st to end	Night	32% 8% 14% 23% 13% 9%	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	15% 6% 15% 40% 20% 4	330
	11th to 20th	Day	11% 3 22% 37% 22% 4	459
	21st to end	Day	9% 6% 27% 37% 16% 5%	594
February	1st to 10th	Day	11% 7% 29% 32% 15% 6%	540
	11th to 20th	Day	13% 10% 28% 33% 14% 2	535
	21st to end	Day	9% 10% 34% 32% 12% 3	446
March	1st to 10th	Day	11% 12% 33% 30% 11% 4	540
	11th to 20th	Day	12% 6% 27% 36% 14% 4	540
	21st to end	Day	8% 11% 34% 31% 13% 4	594
April	1st to 10th	Day	9% 10% 31% 28% 15% 6%	540
	11th to 20th	Day	5% 10% 34% 33% 13% 5%	532
	21st to end	Day	6% 10% 30% 32% 15% 7%	540
May	1st to 10th	Day	9% 7% 30% 34% 16% 5%	543
	11th to 20th	Day	8% 7% 27% 37% 18% 4	568
	21st to end	Day	7% 8% 28% 36% 18% 4	617
June	1st to 10th	Day	6% 3 22% 38% 23% 7%	570
	11th to 20th	Day	5% 3 21% 43% 20% 8%	570
	21st to end	Day	5% 2 16% 44% 24% 9%	570
July	1st to 10th	Day	4 2 8% 45% 29% 12%	566
	11th to 20th	Day	4 6% 44% 30% 16%	564
	21st to end	Day	3 1 7% 44% 30% 15%	619
August	1st to 10th	Day	3 1 6% 47% 29% 15%	567
	11th to 20th	Day	4 1 11% 43% 27% 14%	557
	21st to end	Day	5% 1 12% 40% 30% 12%	627
September	1st to 10th	Day	6% 1 14% 47% 24% 8%	570
	11th to 20th	Day	5% 4 20% 43% 21% 7%	570
	21st to end	Day	5% 5% 22% 39% 22% 8%	570
October	1st to 10th	Day	5% 5% 22% 38% 23% 8%	570
	11th to 20th	Day	5% 5% 26% 35% 23% 6%	570
	21st to end	Day	5% 5% 19% 34% 26% 11%	525
November	1st to 10th	Day	5% 3 16% 34% 31% 10%	330
	11th to 20th	Day	6% 4 19% 37% 26% 8%	330
	21st to end	Day	6% 4 22% 39% 21% 8%	330
December	1st to 10th	Day	8% 2 20% 39% 24% 8%	330
	11th to 20th	Day	6% 2 16% 38% 28% 10%	330
	21st to end	Day	15% 6% 20% 35% 20% 4	363
January	1st to 10th	Night	11% 3 13% 41% 25% 7%	840
	11th to 20th	Night	8% 3 10% 45% 28% 7%	840
	21st to end	Night	13% 2 14% 43% 23% 4	924
February	1st to 10th	Night	11% 4 11% 40% 28% 7%	840
	11th to 20th	Night	13% 5% 13% 39% 25% 5%	824
	21st to end	Night	12% 4 15% 41% 24% 4	692
March	1st to 10th	Night	12% 4 15% 42% 23% 3	840
	11th to 20th	Night	13% 3 10% 37% 30% 7%	840
	21st to end	Night	11% 4 10% 41% 29% 5%	924
April	1st to 10th	Night	12% 5% 10% 39% 29% 4	840
	11th to 20th	Night	10% 5% 13% 37% 29% 7%	831
	21st to end	Night	12% 4 10% 36% 31% 8%	840
May	1st to 10th	Night	11% 6% 10% 34% 33% 6%	725
	11th to 20th	Night	12% 5% 12% 31% 34% 6%	621
	21st to end	Night	13% 6% 10% 32% 35% 4	683
June	1st to 10th	Night	13% 4 10% 33% 34% 6%	630
	11th to 20th	Night	8% 3 5% 35% 44% 6%	630
	21st to end	Night	9% 3 7% 30% 46% 6%	630
July	1st to 10th	Night	8% 1 3 20% 53% 15%	615
	11th to 20th	Night	4 1 3 13% 58% 20%	612
	21st to end	Night	6% 1 2 12% 55% 23%	834
August	1st to 10th	Night	5% 2 3 16% 51% 23%	836
	11th to 20th	Night	7% 3 2 12% 53% 23%	832
	21st to end	Night	6% 2 2 12% 54% 24%	924
September	1st to 10th	Night	8% 2 3 17% 54% 17%	840
	11th to 20th	Night	8% 2 5% 22% 52% 11%	840
	21st to end	Night	4 3 5% 30% 44% 13%	840
October	1st to 10th	Night	4 1 5% 36% 45% 8%	840
	11th to 20th	Night	9% 4 6% 36% 37% 8%	840
	21st to end	Night	7% 2 8% 34% 35% 13%	912
November	1st to 10th	Night	5% 3 8% 38% 37% 10%	820
	11th to 20th	Night	6% 3 8% 37% 34% 12%	840
	21st to end	Night	7% 3 11% 42% 29% 10%	840
December	1st to 10th	Night	9% 2 12% 42% 30% 6%	840
	11th to 20th	Night	13% 3 12% 41% 26% 5%	840
	21st to end	Night	11% 4 17% 38% 22% 8%	924

Page generated at 2026-10-26 01:37:47. Version ac07

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