West Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Mountain clear sky chart from 2005-11-06 to 2025-04-26.

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	5% 7% 2 5% 4 4 4 4 6% 7% 52%	117731
	Night	6% 8% 3 6% 4 4 4 4 6% 7% 49%	119828
Transparency	Day	29% 5% 9% 21% 21% 15%	117496
	Night	31% 5% 11% 23% 19% 11%	119651
Seeing	Day	8% 8% 20% 31% 24% 8%	15335
	Night	9% 5% 15% 40% 27% 4	25971

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	8% 16% 4 10% 5% 4 4 3 8% 8% 30%	4098
	11th to 20th	Day	7% 16% 4 11% 6% 5% 4 4 6% 6% 30%	4100
	21st to end	Day	6% 13% 3 6% 4 4 5% 5% 8% 7% 40%	4510
February	1st to 10th	Day	10% 11% 3 8% 5% 5% 5% 4 6% 8% 36%	1800
	11th to 20th	Day	9% 12% 4 7% 5% 4 5% 3 6% 8% 36%	1839
	21st to end	Day	6% 10% 4 8% 5% 4 4 4 7% 9% 39%	1650
March	1st to 10th	Day	4 12% 4 8% 5% 4 5% 4 6% 7% 42%	4842
	11th to 20th	Day	5% 12% 3 7% 5% 5% 5% 5% 7% 8% 41%	5000
	21st to end	Day	6% 11% 4 9% 6% 6% 6% 5% 6% 7% 32%	5486
April	1st to 10th	Day	7% 8% 3 5% 4 5% 5% 4 6% 7% 46%	2212
	11th to 20th	Day	7% 8% 2 7% 5% 5% 5% 3 6% 6% 46%	2513
	21st to end	Day	6% 7% 2 6% 5% 4 4 4 5% 7% 51%	2537
May	1st to 10th	Day	8% 8% 2 5% 5% 5% 5% 5% 6% 8% 43%	4110
	11th to 20th	Day	4 3 1 4 3 3 4 3 7% 9% 59%	4071
	21st to end	Day	5% 4 2 4 4 4 4 4 7% 8% 54%	4477
June	1st to 10th	Day	2 3 1 4 3 4 4 4 6% 9% 60%	2470
	11th to 20th	Day	3 3 1 4 3 3 4 3 5% 7% 66%	2470
	21st to end	Day	1 2 1 3 2 2 4 3 4 6% 71%	2470
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 5% 79%	4110
	11th to 20th	Day	1 2 1 4 4 4 5% 6% 7% 10% 57%	4110
	21st to end	Day	2 2 1 3 3 3 4 3 6% 7% 67%	4454
August	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 9% 65%	4092
	11th to 20th	Day	1 1 1 3 3 3 4 4 6% 6% 68%	4028
	21st to end	Day	2 2 1 2 2 2 3 3 5% 6% 72%	4469
September	1st to 10th	Day	2 2 1 2 2 2 4 3 5% 7% 71%	2136
	11th to 20th	Day	3 2 1 3 2 3 4 2 5% 7% 68%	2090
	21st to end	Day	4 2 1 3 2 2 3 2 4 6% 71%	2090
October	1st to 10th	Day	4 3 1 3 2 3 3 3 4 5% 72%	3335
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 5% 8% 59%	3150
	21st to end	Day	7% 5% 1 4 3 3 3 2 4 5% 63%	3438
November	1st to 10th	Day	7% 6% 3 5% 4 3 4 4 6% 7% 52%	1774
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 8% 47%	1800
	21st to end	Day	6% 10% 3 7% 4 4 4 4 6% 8% 45%	1726
December	1st to 10th	Day	5% 7% 2 6% 3 3 4 4 6% 10% 52%	2620
	11th to 20th	Day	6% 10% 3 6% 4 4 4 4 7% 10% 40%	3144
	21st to end	Day	11% 14% 4 9% 4 4 6% 4 8% 8% 28%	4510
January	1st to 10th	Night	8% 17% 5% 8% 5% 4 4 4 7% 7% 31%	5867
	11th to 20th	Night	10% 14% 6% 12% 5% 5% 6% 5% 7% 6% 25%	5880
	21st to end	Night	6% 11% 4 7% 4 4 6% 5% 7% 6% 39%	6468
February	1st to 10th	Night	8% 12% 4 7% 4 4 5% 4 8% 8% 34%	2600
	11th to 20th	Night	8% 11% 3 7% 5% 3 3 4 8% 9% 37%	2445
	21st to end	Night	6% 11% 4 8% 4 4 4 4 7% 8% 39%	1980
March	1st to 10th	Night	6% 15% 5% 7% 4 4 4 4 7% 6% 38%	5052
	11th to 20th	Night	6% 11% 4 8% 5% 4 5% 4 6% 8% 41%	5000
	21st to end	Night	6% 11% 3 8% 5% 4 4 3 5% 7% 43%	5489
April	1st to 10th	Night	9% 8% 2 4 4 3 3 3 4 6% 52%	2187
	11th to 20th	Night	6% 6% 2 6% 4 3 5% 4 6% 7% 50%	1831
	21st to end	Night	8% 7% 2 6% 4 3 4 3 6% 7% 50%	1759
May	1st to 10th	Night	6% 8% 2 7% 4 4 4 4 6% 9% 45%	2860
	11th to 20th	Night	3 4 1 5% 3 4 3 3 6% 7% 62%	2834
	21st to end	Night	5% 4 1 4 4 3 4 4 5% 7% 58%	3110
June	1st to 10th	Night	3 3 1 3 3 4 5% 3 5% 8% 60%	1710
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 5% 69%	1710
	21st to end	Night	2 2 1 3 3 3 4 3 5% 7% 69%	1710
July	1st to 10th	Night	3 1 1 2 2 2 3 3 4 5% 73%	2860
	11th to 20th	Night	2 2 1 4 4 5% 7% 6% 10% 9% 48%	2860
	21st to end	Night	2 3 1 5% 4 5% 5% 4 5% 8% 58%	3109
August	1st to 10th	Night	2 2 1 4 4 4 5% 4 6% 8% 61%	2847
	11th to 20th	Night	2 3 1 4 4 4 5% 4 5% 7% 62%	2811
	21st to end	Night	1 2 1 3 3 3 3 3 4 6% 72%	3197
September	1st to 10th	Night	2 1 1 2 2 3 2 3 5% 5% 71%	2041
	11th to 20th	Night	3 3 1 3 3 3 3 2 5% 7% 68%	2090
	21st to end	Night	5% 3 1 3 2 3 2 2 4 5% 71%	2090
October	1st to 10th	Night	3 2 1 3 2 2 3 3 3 5% 73%	3638
	11th to 20th	Night	7% 4 1 4 2 3 3 2 4 6% 62%	3778
	21st to end	Night	5% 6% 2 4 2 3 3 3 5% 7% 61%	4120
November	1st to 10th	Night	6% 6% 2 5% 3 3 3 3 5% 8% 56%	2521
	11th to 20th	Night	8% 6% 2 4 3 4 3 3 6% 7% 54%	2600
	21st to end	Night	6% 11% 4 6% 3 3 3 3 7% 6% 47%	2646
December	1st to 10th	Night	5% 7% 2 4 3 5% 4 3 6% 7% 53%	4570
	11th to 20th	Night	10% 11% 3 5% 3 3 4 3 6% 8% 44%	5090
	21st to end	Night	11% 11% 4 10% 5% 6% 6% 5% 10% 8% 25%	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	49% 6% 19% 18% 8%	4099
	11th to 20th	Day	47% 7% 16% 19% 8% 1	4100
	21st to end	Day	39% 4 18% 22% 16% 1	4510
February	1st to 10th	Day	45% 5% 15% 21% 11% 2	1800
	11th to 20th	Day	46% 4 13% 24% 12% 1	1848
	21st to end	Day	40% 5% 12% 26% 15% 2	1650
March	1st to 10th	Day	38% 5% 10% 26% 17% 4	4842
	11th to 20th	Day	36% 7% 11% 29% 15% 2	5000
	21st to end	Day	47% 6% 15% 21% 10% 1	5486
April	1st to 10th	Day	38% 4 11% 24% 18% 6%	2212
	11th to 20th	Day	38% 5% 9% 21% 21% 6%	2516
	21st to end	Day	33% 3 9% 26% 23% 4	2537
May	1st to 10th	Day	35% 8% 8% 22% 22% 6%	4110
	11th to 20th	Day	21% 5% 5% 30% 32% 6%	4074
	21st to end	Day	25% 6% 8% 25% 29% 8%	4477
June	1st to 10th	Day	20% 4 6% 20% 33% 16%	2470
	11th to 20th	Day	18% 3 5% 15% 31% 27%	2470
	21st to end	Day	14% 4 1 10% 28% 43%	2470
July	1st to 10th	Day	10% 3 2 7% 14% 65%	4046
	11th to 20th	Day	18% 5% 5% 12% 26% 33%	4041
	21st to end	Day	15% 3 6% 13% 21% 41%	4314
August	1st to 10th	Day	16% 4 5% 18% 30% 27%	4078
	11th to 20th	Day	16% 5% 4 14% 31% 30%	4067
	21st to end	Day	13% 3 4 14% 29% 38%	4468
September	1st to 10th	Day	14% 4 5% 14% 29% 35%	2136
	11th to 20th	Day	17% 3 6% 20% 27% 27%	2090
	21st to end	Day	16% 3 3 20% 35% 23%	2090
October	1st to 10th	Day	18% 3 4 13% 37% 25%	3335
	11th to 20th	Day	24% 3 7% 23% 30% 12%	3150
	21st to end	Day	24% 2 9% 29% 29% 7%	3438
November	1st to 10th	Day	31% 3 10% 28% 23% 5%	1774
	11th to 20th	Day	34% 5% 10% 30% 20% 2	1800
	21st to end	Day	35% 4 13% 29% 16% 3	1726
December	1st to 10th	Day	24% 5% 16% 45% 8% 2	2620
	11th to 20th	Day	36% 4 18% 31% 11%	3142
	21st to end	Day	46% 7% 15% 21% 11%	4510
January	1st to 10th	Night	48% 5% 18% 21% 8%	5867
	11th to 20th	Night	49% 8% 16% 20% 7% 1	5880
	21st to end	Night	36% 4 21% 27% 11% 1	6468
February	1st to 10th	Night	42% 7% 16% 24% 9% 2	2600
	11th to 20th	Night	42% 4 13% 24% 15% 2	2446
	21st to end	Night	40% 5% 10% 28% 15% 2	1980
March	1st to 10th	Night	39% 7% 10% 26% 17% 1	5052
	11th to 20th	Night	36% 6% 9% 30% 16% 2	5000
	21st to end	Night	38% 5% 15% 25% 15% 2	5489
April	1st to 10th	Night	33% 4 11% 26% 18% 8%	2187
	11th to 20th	Night	31% 5% 10% 25% 24% 6%	1831
	21st to end	Night	31% 6% 11% 28% 21% 4	1759
May	1st to 10th	Night	34% 6% 6% 24% 24% 6%	2860
	11th to 20th	Night	22% 4 5% 30% 32% 7%	2835
	21st to end	Night	24% 4 8% 30% 29% 5%	3110
June	1st to 10th	Night	21% 5% 7% 23% 30% 15%	1710
	11th to 20th	Night	18% 2 5% 17% 31% 27%	1710
	21st to end	Night	14% 4 3 11% 25% 43%	1710
July	1st to 10th	Night	13% 4 2 7% 11% 62%	2815
	11th to 20th	Night	24% 6% 6% 12% 24% 28%	2806
	21st to end	Night	24% 4 6% 12% 20% 34%	3011
August	1st to 10th	Night	18% 5% 8% 19% 24% 25%	2838
	11th to 20th	Night	21% 5% 6% 15% 23% 30%	2838
	21st to end	Night	14% 3 5% 12% 30% 36%	3197
September	1st to 10th	Night	15% 3 5% 14% 28% 35%	2041
	11th to 20th	Night	18% 3 5% 20% 26% 28%	2090
	21st to end	Night	18% 2 3 16% 32% 28%	2090
October	1st to 10th	Night	14% 2 5% 14% 34% 29%	3638
	11th to 20th	Night	24% 3 7% 23% 31% 12%	3778
	21st to end	Night	23% 4 7% 33% 28% 6%	4120
November	1st to 10th	Night	28% 3 9% 29% 24% 6%	2521
	11th to 20th	Night	28% 5% 12% 32% 20% 3	2600
	21st to end	Night	35% 4 9% 29% 19% 4	2646
December	1st to 10th	Night	25% 5% 19% 38% 11% 2	4570
	11th to 20th	Night	36% 5% 22% 29% 9%	5090
	21st to end	Night	46% 7% 17% 20% 10%	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	22% 7% 12% 33% 21% 5%	300
	11th to 20th	Day	14% 8% 19% 33% 22% 5%	300
	21st to end	Day	13% 6% 18% 35% 23% 5%	330
February	1st to 10th	Day	17% 12% 21% 34% 14% 2	300
	11th to 20th	Day	14% 10% 24% 32% 17% 2	384
	21st to end	Day	10% 11% 26% 36% 16% 2	414
March	1st to 10th	Day	12% 14% 21% 32% 17% 4	500
	11th to 20th	Day	9% 10% 31% 30% 17% 3	500
	21st to end	Day	15% 15% 28% 25% 14% 3	550
April	1st to 10th	Day	11% 13% 25% 27% 18% 6%	500
	11th to 20th	Day	11% 18% 23% 28% 15% 4	492
	21st to end	Day	10% 16% 24% 29% 17% 4	481
May	1st to 10th	Day	8% 12% 28% 33% 16% 4	463
	11th to 20th	Day	8% 8% 25% 30% 22% 7%	488
	21st to end	Day	8% 9% 22% 27% 27% 8%	529
June	1st to 10th	Day	2 9% 20% 34% 27% 7%	490
	11th to 20th	Day	4 9% 26% 33% 22% 6%	490
	21st to end	Day	3 6% 25% 34% 26% 7%	490
July	1st to 10th	Day	1 2 21% 35% 31% 10%	486
	11th to 20th	Day	4 2 16% 36% 34% 9%	484
	21st to end	Day	2 1 10% 34% 41% 11%	531
August	1st to 10th	Day	3 2 17% 33% 32% 12%	487
	11th to 20th	Day	2 3 18% 31% 35% 12%	477
	21st to end	Day	2 3 18% 31% 35% 12%	539
September	1st to 10th	Day	3 4 15% 30% 30% 18%	490
	11th to 20th	Day	6% 6% 21% 27% 29% 12%	490
	21st to end	Day	5% 7% 16% 24% 34% 14%	490
October	1st to 10th	Day	6% 7% 19% 24% 27% 18%	400
	11th to 20th	Day	7% 7% 16% 23% 31% 17%	300
	21st to end	Day	10% 5% 18% 33% 26% 8%	330
November	1st to 10th	Day	12% 13% 15% 28% 21% 11%	300
	11th to 20th	Day	10% 6% 14% 36% 23% 11%	300
	21st to end	Day	12% 6% 12% 35% 27% 8%	300
December	1st to 10th	Day	12% 7% 13% 35% 21% 13%	300
	11th to 20th	Day	14% 5% 12% 39% 25% 4	300
	21st to end	Day	20% 9% 14% 35% 20% 3	330
January	1st to 10th	Night	16% 5% 13% 38% 22% 6%	800
	11th to 20th	Night	13% 7% 14% 36% 26% 5%	800
	21st to end	Night	14% 6% 14% 39% 24% 4	880
February	1st to 10th	Night	14% 7% 19% 42% 15% 4	800
	11th to 20th	Night	13% 4 19% 43% 18% 2	784
	21st to end	Night	11% 6% 20% 45% 17% 1	660
March	1st to 10th	Night	13% 10% 16% 42% 17% 2	800
	11th to 20th	Night	9% 6% 18% 44% 23% 1	800
	21st to end	Night	10% 9% 22% 41% 15% 2	880
April	1st to 10th	Night	12% 10% 19% 39% 18% 2	787
	11th to 20th	Night	8% 8% 25% 41% 17%	593
	21st to end	Night	9% 10% 18% 40% 22% 2	587
May	1st to 10th	Night	9% 6% 21% 41% 22% 1	570
	11th to 20th	Night	6% 5% 17% 40% 29% 3	561
	21st to end	Night	6% 6% 16% 36% 30% 4	617
June	1st to 10th	Night	4 7% 16% 41% 30% 2	570
	11th to 20th	Night	5% 6% 22% 40% 25% 1	570
	21st to end	Night	2 2 17% 43% 34% 2	570
July	1st to 10th	Night	2 1 15% 35% 43% 5%	555
	11th to 20th	Night	5% 1 8% 42% 41% 3	552
	21st to end	Night	3 2 6% 35% 51% 2	624
August	1st to 10th	Night	4 1 10% 43% 40% 2	567
	11th to 20th	Night	2 2 12% 44% 38% 2	564
	21st to end	Night	2 2 12% 40% 42% 3	627
September	1st to 10th	Night	3 2 10% 39% 40% 7%	711
	11th to 20th	Night	5% 3 14% 35% 36% 7%	760
	21st to end	Night	7% 3 11% 33% 35% 10%	760
October	1st to 10th	Night	7% 3 16% 41% 28% 5%	760
	11th to 20th	Night	7% 4 13% 41% 31% 4	760
	21st to end	Night	8% 4 16% 40% 29% 4	836
November	1st to 10th	Night	8% 8% 16% 37% 27% 5%	780
	11th to 20th	Night	10% 5% 16% 42% 22% 5%	800
	21st to end	Night	11% 5% 15% 37% 25% 6%	846
December	1st to 10th	Night	14% 5% 13% 44% 21% 4	1000
	11th to 20th	Night	18% 6% 14% 40% 21% 2	960
	21st to end	Night	20% 6% 14% 41% 16% 3	880

Page generated at 2025-04-23 07:25:06. Version ac07

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