Stellar Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stellar Vista Observatory clear sky chart from 2005-11-06 to 2025-02-25.

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	3 3 1 3 3 3 4 3 5% 7% 65%	113385
	Night	3 4 1 3 3 3 3 3 5% 7% 66%	114191
Transparency	Day	11% 43% 4 11% 17% 15%	113152
	Night	11% 42% 4 12% 17% 14%	114014
Seeing	Day	6% 10% 27% 33% 18% 6%	15286
	Night	5% 4 20% 41% 26% 4	25548

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 5% 2 5% 3 4 5% 3 4 7% 61%	4098
	11th to 20th	Day	2 3 1 3 2 2 3 2 4 6% 72%	4100
	21st to end	Day	6% 8% 2 4 3 3 3 3 4 5% 58%	4510
February	1st to 10th	Day	4 5% 2 4 3 2 3 3 5% 7% 62%	1820
	11th to 20th	Day	4 6% 2 4 2 3 4 4 5% 7% 59%	1950
	21st to end	Day	5% 7% 1 5% 4 5% 3 4 4 5% 57%	1611
March	1st to 10th	Day	4 6% 2 4 3 3 3 3 5% 8% 57%	3352
	11th to 20th	Day	3 7% 2 5% 4 5% 4 5% 6% 9% 49%	3490
	21st to end	Day	4 6% 2 4 4 4 4 4 7% 10% 51%	3825
April	1st to 10th	Day	3 4 2 4 4 3 4 4 6% 6% 61%	2090
	11th to 20th	Day	2 4 1 3 2 3 4 3 4 7% 67%	2306
	21st to end	Day	3 3 1 4 3 3 3 3 5% 6% 66%	2470
May	1st to 10th	Day	4 2 1 4 3 3 3 2 4 5% 69%	4110
	11th to 20th	Day	1 2 1 3 3 4 5% 4 6% 8% 64%	4071
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 76%	4477
June	1st to 10th	Day	1 1 1 2 2 2 3 3 4 7% 76%	2470
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 78%	2470
	21st to end	Day	1 1 1 2 2 2 3 3 4 6% 75%	2470
July	1st to 10th	Day	1 2 1 2 2 2 3 3 3 5% 77%	4110
	11th to 20th	Day	2 2 1 4 4 5% 5% 5% 8% 9% 56%	4110
	21st to end	Day	2 3 2 4 5% 5% 5% 4 6% 8% 55%	4454
August	1st to 10th	Day	2 2 1 4 4 4 6% 5% 8% 11% 52%	4092
	11th to 20th	Day	2 2 1 4 3 4 5% 4 6% 6% 65%	4028
	21st to end	Day	3 2 1 3 2 3 4 4 5% 6% 66%	4319
September	1st to 10th	Day	3 2 1 4 3 4 3 3 6% 7% 66%	2090
	11th to 20th	Day	3 1 1 3 2 2 2 2 3 5% 75%	2090
	21st to end	Day	3 1 1 2 2 2 2 2 3 7% 73%	2090
October	1st to 10th	Day	3 1 1 2 2 2 2 2 3 4 79%	3434
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 6% 74%	3150
	21st to end	Day	2 1 1 2 2 3 3 2 3 4 78%	3438
November	1st to 10th	Day	3 3 1 2 2 2 2 3 4 5% 73%	1821
	11th to 20th	Day	1 2 1 3 2 2 3 2 3 5% 76%	1800
	21st to end	Day	5% 4 1 4 2 2 3 3 4 7% 65%	1800
December	1st to 10th	Day	2 3 1 3 2 3 3 3 6% 7% 67%	2950
	11th to 20th	Day	4 4 1 4 3 3 3 4 7% 9% 58%	3409
	21st to end	Day	4 4 2 4 3 4 5% 4 6% 7% 56%	4510
January	1st to 10th	Night	3 5% 2 4 3 3 3 3 4 6% 63%	5867
	11th to 20th	Night	2 4 1 3 2 2 3 3 4 5% 71%	5880
	21st to end	Night	5% 8% 2 4 3 4 4 4 5% 6% 57%	6468
February	1st to 10th	Night	4 5% 1 4 3 3 3 3 4 7% 63%	2575
	11th to 20th	Night	4 5% 2 3 3 3 3 4 5% 6% 61%	2338
	21st to end	Night	5% 7% 2 5% 3 3 3 3 4 7% 58%	1937
March	1st to 10th	Night	6% 6% 1 3 3 3 3 3 5% 9% 58%	3520
	11th to 20th	Night	5% 6% 2 5% 3 4 4 3 6% 9% 52%	3490
	21st to end	Night	7% 6% 2 5% 3 3 4 3 5% 7% 55%	3828
April	1st to 10th	Night	4 4 1 4 3 3 3 4 4 6% 66%	2090
	11th to 20th	Night	2 3 1 3 2 3 3 3 6% 6% 68%	1817
	21st to end	Night	4 3 1 3 2 3 3 3 5% 7% 67%	1710
May	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 6% 71%	2860
	11th to 20th	Night	1 3 1 3 2 2 3 3 5% 6% 72%	2834
	21st to end	Night	2 1 1 3 2 2 4 3 4 6% 72%	3110
June	1st to 10th	Night	2 2 1 2 1 2 2 3 4 5% 77%	1710
	11th to 20th	Night	1 1 2 2 2 2 2 3 5% 79%	1710
	21st to end	Night	1 1 1 3 2 2 2 3 3 5% 78%	1710
July	1st to 10th	Night	1 1 2 2 2 2 2 4 4 80%	2860
	11th to 20th	Night	2 4 1 5% 4 5% 6% 5% 8% 9% 51%	2860
	21st to end	Night	2 2 1 4 3 3 4 5% 7% 9% 59%	3109
August	1st to 10th	Night	1 2 1 6% 4 5% 5% 5% 9% 11% 52%	2847
	11th to 20th	Night	1 2 1 3 2 2 4 4 6% 7% 68%	2811
	21st to end	Night	2 1 1 2 2 2 3 3 5% 7% 72%	3348
September	1st to 10th	Night	3 2 1 3 3 2 3 4 6% 7% 65%	2090
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 74%	2090
	21st to end	Night	2 2 1 2 2 2 2 2 4 4 77%	2090
October	1st to 10th	Night	2 2 1 3 2 2 2 2 3 4 76%	3542
	11th to 20th	Night	6% 3 1 3 3 3 3 3 4 6% 66%	3778
	21st to end	Night	2 2 1 2 2 2 2 2 5% 6% 75%	4120
November	1st to 10th	Night	2 3 1 2 2 2 3 3 4 7% 72%	2473
	11th to 20th	Night	2 2 1 2 2 2 3 3 3 6% 75%	2600
	21st to end	Night	4 4 1 3 3 2 2 2 4 6% 69%	2600
December	1st to 10th	Night	3 3 1 2 2 3 3 2 5% 7% 69%	4240
	11th to 20th	Night	5% 4 1 3 2 3 3 3 6% 10% 61%	4811
	21st to end	Night	5% 5% 2 4 3 3 3 3 5% 7% 60%	6468

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	19% 28% 8% 23% 16% 7%	4099
	11th to 20th	Day	12% 26% 10% 13% 17% 21%	4100
	21st to end	Day	22% 28% 3 14% 25% 7%	4510
February	1st to 10th	Day	11% 62% 4 11% 8% 5%	1820
	11th to 20th	Day	11% 60% 3 10% 12% 4	1960
	21st to end	Day	12% 63% 2 5% 13% 5%	1611
March	1st to 10th	Day	13% 44% 2 15% 18% 8%	3352
	11th to 20th	Day	16% 43% 7% 20% 11% 3	3490
	21st to end	Day	15% 43% 6% 18% 15% 3	3825
April	1st to 10th	Day	9% 65% 3 9% 9% 5%	2090
	11th to 20th	Day	7% 65% 2 7% 11% 7%	2309
	21st to end	Day	9% 65% 2 8% 12% 4	2470
May	1st to 10th	Day	11% 41% 1 7% 21% 17%	4110
	11th to 20th	Day	8% 43% 3 10% 25% 12%	4074
	21st to end	Day	6% 40% 2 5% 25% 23%	4477
June	1st to 10th	Day	2 64% 1 4 12% 17%	2470
	11th to 20th	Day	3 64% 2 3 10% 19%	2470
	21st to end	Day	4 62% 1 2 10% 20%	2470
July	1st to 10th	Day	5% 35% 2 2 4 52%	4046
	11th to 20th	Day	12% 37% 4 8% 17% 22%	4041
	21st to end	Day	11% 36% 5% 13% 7% 28%	4314
August	1st to 10th	Day	15% 38% 6% 17% 15% 8%	4078
	11th to 20th	Day	12% 38% 6% 8% 17% 18%	4067
	21st to end	Day	11% 37% 3 9% 16% 24%	4318
September	1st to 10th	Day	6% 59% 3 6% 12% 14%	2090
	11th to 20th	Day	5% 59% 2 5% 11% 18%	2090
	21st to end	Day	7% 59% 2 6% 13% 13%	2090
October	1st to 10th	Day	4 36% 1 4 12% 43%	3434
	11th to 20th	Day	10% 36% 2 11% 27% 13%	3150
	21st to end	Day	9% 37% 2 10% 26% 16%	3438
November	1st to 10th	Day	4 55% 2 7% 21% 12%	1821
	11th to 20th	Day	6% 56% 2 9% 18% 9%	1800
	21st to end	Day	11% 56% 3 9% 15% 5%	1800
December	1st to 10th	Day	9% 35% 3 17% 20% 16%	2950
	11th to 20th	Day	13% 32% 5% 22% 26% 2	3408
	21st to end	Day	19% 27% 7% 20% 20% 5%	4510
January	1st to 10th	Night	17% 27% 4 25% 20% 6%	5867
	11th to 20th	Night	12% 26% 10% 17% 15% 20%	5880
	21st to end	Night	21% 28% 4 14% 21% 12%	6468
February	1st to 10th	Night	11% 62% 4 11% 8% 4	2575
	11th to 20th	Night	10% 60% 4 8% 12% 6%	2339
	21st to end	Night	10% 62% 2 7% 15% 4	1937
March	1st to 10th	Night	13% 43% 4 18% 15% 6%	3520
	11th to 20th	Night	16% 44% 8% 19% 11% 4	3490
	21st to end	Night	19% 41% 3 17% 17% 3	3828
April	1st to 10th	Night	7% 64% 2 9% 12% 5%	2090
	11th to 20th	Night	5% 65% 3 8% 13% 6%	1817
	21st to end	Night	8% 65% 2 8% 15% 3	1710
May	1st to 10th	Night	10% 40% 3 7% 25% 15%	2860
	11th to 20th	Night	7% 41% 3 10% 24% 15%	2835
	21st to end	Night	7% 40% 2 7% 22% 21%	3110
June	1st to 10th	Night	3 64% 1 4 11% 17%	1710
	11th to 20th	Night	3 64% 2 4 8% 18%	1710
	21st to end	Night	4 62% 1 3 7% 22%	1710
July	1st to 10th	Night	4 35% 2 2 3 54%	2815
	11th to 20th	Night	16% 37% 5% 10% 15% 17%	2806
	21st to end	Night	9% 36% 7% 14% 8% 26%	3011
August	1st to 10th	Night	18% 39% 6% 15% 12% 10%	2838
	11th to 20th	Night	9% 38% 8% 11% 14% 20%	2838
	21st to end	Night	7% 36% 4 8% 16% 29%	3348
September	1st to 10th	Night	7% 60% 3 6% 10% 13%	2090
	11th to 20th	Night	5% 59% 2 5% 12% 17%	2090
	21st to end	Night	7% 59% 2 6% 12% 14%	2090
October	1st to 10th	Night	7% 36% 1 3 10% 43%	3542
	11th to 20th	Night	16% 37% 2 7% 26% 13%	3778
	21st to end	Night	8% 36% 3 10% 25% 18%	4120
November	1st to 10th	Night	4 55% 2 9% 18% 11%	2473
	11th to 20th	Night	6% 56% 2 7% 21% 9%	2600
	21st to end	Night	10% 56% 2 10% 15% 8%	2600
December	1st to 10th	Night	10% 35% 3 15% 23% 15%	4240
	11th to 20th	Night	12% 32% 4 26% 24% 3	4811
	21st to end	Night	17% 27% 9% 20% 21% 6%	6468

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	13% 7% 35% 36% 8% 1	300
	11th to 20th	Day	8% 11% 37% 30% 14% 2	300
	21st to end	Day	10% 6% 26% 38% 16% 3	330
February	1st to 10th	Day	9% 12% 32% 29% 15% 3	320
	11th to 20th	Day	11% 13% 38% 27% 9% 1	495
	21st to end	Day	7% 20% 37% 24% 11% 1	399
March	1st to 10th	Day	12% 16% 33% 26% 11% 2	460
	11th to 20th	Day	7% 18% 31% 30% 12% 3	460
	21st to end	Day	11% 21% 35% 26% 6% 1	506
April	1st to 10th	Day	8% 18% 33% 23% 14% 4	460
	11th to 20th	Day	10% 22% 32% 24% 11% 2	452
	21st to end	Day	8% 19% 28% 29% 13% 3	460
May	1st to 10th	Day	8% 16% 34% 27% 11% 2	463
	11th to 20th	Day	6% 12% 25% 32% 19% 6%	488
	21st to end	Day	10% 13% 27% 27% 16% 6%	529
June	1st to 10th	Day	3 7% 27% 38% 18% 8%	490
	11th to 20th	Day	4 12% 28% 34% 16% 7%	490
	21st to end	Day	1 6% 28% 38% 21% 7%	490
July	1st to 10th	Day	1 3 22% 38% 26% 10%	486
	11th to 20th	Day	2 1 14% 38% 29% 15%	484
	21st to end	Day	3 2 11% 40% 30% 14%	531
August	1st to 10th	Day	4 3 17% 38% 25% 13%	487
	11th to 20th	Day	3 3 19% 39% 23% 12%	477
	21st to end	Day	3 4 18% 35% 25% 14%	539
September	1st to 10th	Day	4 3 18% 34% 30% 13%	490
	11th to 20th	Day	4 7% 25% 36% 22% 7%	490
	21st to end	Day	4 7% 23% 32% 27% 7%	490
October	1st to 10th	Day	6% 9% 24% 32% 20% 10%	460
	11th to 20th	Day	3 10% 23% 33% 23% 8%	300
	21st to end	Day	5% 9% 26% 33% 24% 3	330
November	1st to 10th	Day	5% 10% 21% 36% 21% 5%	300
	11th to 20th	Day	5% 7% 28% 39% 15% 6%	300
	21st to end	Day	9% 8% 30% 35% 14% 4	300
December	1st to 10th	Day	5% 8% 30% 37% 17% 3	300
	11th to 20th	Day	5% 7% 34% 38% 14% 2	300
	21st to end	Day	15% 11% 29% 33% 12%	330
January	1st to 10th	Night	9% 5% 27% 40% 17% 4	800
	11th to 20th	Night	6% 4 24% 44% 19% 4	800
	21st to end	Night	9% 3 23% 41% 19% 4	880
February	1st to 10th	Night	6% 5% 30% 40% 17% 3	800
	11th to 20th	Night	6% 5% 29% 43% 15% 2	784
	21st to end	Night	9% 5% 30% 40% 15% 1	646
March	1st to 10th	Night	9% 10% 23% 41% 16% 2	760
	11th to 20th	Night	7% 6% 23% 38% 23% 3	760
	21st to end	Night	6% 7% 28% 41% 17% 2	836
April	1st to 10th	Night	5% 8% 30% 39% 16% 2	760
	11th to 20th	Night	3 7% 32% 40% 16% 1	629
	21st to end	Night	5% 10% 26% 40% 16% 2	570
May	1st to 10th	Night	5% 5% 30% 45% 14% 2	570
	11th to 20th	Night	3 5% 22% 40% 28% 2	561
	21st to end	Night	4 7% 24% 41% 22% 3	617
June	1st to 10th	Night	2 4 20% 45% 28% 2	570
	11th to 20th	Night	1 5% 21% 46% 25% 2	570
	21st to end	Night	1 2 15% 47% 34% 1	570
July	1st to 10th	Night	2 1 9% 42% 43% 3	555
	11th to 20th	Night	4 1 6% 38% 47% 5%	552
	21st to end	Night	5% 1 5% 32% 53% 5%	624
August	1st to 10th	Night	3 1 5% 38% 49% 3	567
	11th to 20th	Night	2 1 6% 41% 47% 3	564
	21st to end	Night	4 1 6% 35% 47% 8%	707
September	1st to 10th	Night	4 1 7% 34% 49% 6%	760
	11th to 20th	Night	3 1 10% 43% 36% 7%	760
	21st to end	Night	3 2 13% 37% 38% 7%	760
October	1st to 10th	Night	5% 3 16% 46% 27% 3	760
	11th to 20th	Night	3 3 18% 43% 26% 7%	760
	21st to end	Night	3 2 22% 39% 28% 6%	836
November	1st to 10th	Night	3 2 17% 48% 24% 6%	780
	11th to 20th	Night	2 2 22% 47% 22% 5%	800
	21st to end	Night	7% 3 23% 42% 22% 3	800
December	1st to 10th	Night	8% 3 22% 43% 19% 3	800
	11th to 20th	Night	9% 3 26% 42% 19% 2	800
	21st to end	Night	13% 4 26% 40% 16% 1	880

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