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 2026-08-30.

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 5% 3 4 4 4 6% 7% 55%	152132
	Night	5% 7% 2 6% 4 4 4 4 6% 7% 52%	147696
Transparency	Day	27% 5% 9% 20% 22% 17%	146880
	Night	29% 5% 11% 23% 20% 13%	143954
Seeing	Day	8% 8% 20% 31% 25% 8%	17165
	Night	9% 5% 16% 40% 26% 3	27682

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	10% 17% 4 10% 5% 4 4 3 8% 7% 30%	5318
	11th to 20th	Day	5% 12% 3 9% 4 4 4 4 5% 6% 44%	5340
	21st to end	Day	5% 11% 3 6% 4 4 5% 5% 7% 7% 44%	5844
February	1st to 10th	Day	9% 11% 3 8% 5% 5% 4 4 6% 8% 37%	1890
	11th to 20th	Day	9% 12% 4 7% 5% 4 5% 4 6% 8% 35%	1933
	21st to end	Day	7% 10% 4 8% 5% 4 4 4 6% 8% 40%	1730
March	1st to 10th	Day	4 12% 4 8% 5% 4 5% 4 6% 7% 43%	4949
	11th to 20th	Day	5% 12% 3 7% 5% 5% 5% 5% 6% 8% 41%	5110
	21st to end	Day	6% 11% 4 9% 6% 6% 6% 4 6% 7% 33%	5607
April	1st to 10th	Day	7% 8% 3 5% 4 5% 5% 4 6% 7% 46%	2323
	11th to 20th	Day	6% 8% 2 7% 5% 5% 5% 3 6% 7% 46%	2640
	21st to end	Day	6% 7% 2 6% 4 4 4 4 5% 7% 51%	2730
May	1st to 10th	Day	6% 6% 2 4 4 4 4 4 6% 7% 53%	7626
	11th to 20th	Day	6% 4 1 5% 4 4 4 4 6% 8% 55%	7596
	21st to end	Day	3 4 2 4 3 4 4 4 7% 8% 58%	8370
June	1st to 10th	Day	2 3 1 3 3 3 4 4 6% 9% 61%	2730
	11th to 20th	Day	3 2 1 3 3 3 3 3 4 6% 68%	2730
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 72%	2730
July	1st to 10th	Day	1 1 1 2 2 2 3 3 4 5% 75%	7649
	11th to 20th	Day	1 2 1 4 3 4 5% 5% 7% 10% 58%	7650
	21st to end	Day	2 1 1 3 2 3 4 3 5% 7% 70%	8304
August	1st to 10th	Day	1 1 1 2 2 3 4 3 5% 8% 70%	5909
	11th to 20th	Day	1 1 1 3 3 3 4 3 5% 5% 72%	5928
	21st to end	Day	2 2 1 4 3 4 4 4 6% 8% 61%	6508
September	1st to 10th	Day	2 2 1 2 2 3 4 3 5% 7% 70%	2248
	11th to 20th	Day	3 2 1 3 2 3 4 3 5% 7% 68%	2200
	21st to end	Day	4 2 1 3 2 3 3 2 4 6% 71%	2200
October	1st to 10th	Day	4 3 1 3 2 3 3 3 4 5% 72%	3440
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 5% 7% 59%	3250
	21st to end	Day	6% 5% 1 4 3 3 3 2 4 5% 63%	3548
November	1st to 10th	Day	7% 6% 3 5% 3 3 4 3 5% 7% 54%	1864
	11th to 20th	Day	10% 6% 2 5% 4 5% 4 4 5% 8% 47%	1890
	21st to end	Day	6% 9% 3 7% 4 4 4 4 6% 8% 45%	1812
December	1st to 10th	Day	5% 7% 2 6% 3 3 4 4 6% 9% 52%	2700
	11th to 20th	Day	7% 10% 3 6% 4 4 4 4 7% 10% 40%	3227
	21st to end	Day	11% 14% 4 9% 4 4 5% 4 8% 8% 28%	4609
January	1st to 10th	Night	9% 16% 5% 8% 5% 4 4 4 7% 7% 30%	7605
	11th to 20th	Night	8% 11% 4 9% 4 4 4 4 6% 5% 42%	7650
	21st to end	Night	5% 9% 4 6% 4 4 6% 5% 7% 7% 44%	8370
February	1st to 10th	Night	8% 11% 4 7% 4 4 5% 4 8% 8% 35%	2730
	11th to 20th	Night	8% 11% 3 7% 5% 3 4 4 8% 9% 37%	2570
	21st to end	Night	7% 11% 4 7% 4 4 4 4 7% 8% 41%	2076
March	1st to 10th	Night	6% 15% 5% 7% 4 4 4 4 7% 6% 38%	5165
	11th to 20th	Night	6% 10% 3 8% 5% 4 5% 4 6% 8% 42%	5110
	21st to end	Night	6% 11% 3 8% 5% 4 4 3 5% 7% 43%	5610
April	1st to 10th	Night	9% 8% 2 4 4 3 3 4 4 6% 52%	2296
	11th to 20th	Night	5% 6% 2 6% 4 3 5% 4 6% 7% 50%	1924
	21st to end	Night	7% 7% 2 6% 4 3 4 3 6% 7% 50%	1890
May	1st to 10th	Night	5% 5% 2 5% 4 4 4 4 6% 7% 54%	5321
	11th to 20th	Night	4 4 1 4 3 4 3 3 6% 7% 60%	5296
	21st to end	Night	4 4 1 4 3 3 4 4 6% 8% 60%	5837
June	1st to 10th	Night	3 3 1 3 3 4 5% 3 5% 8% 62%	1890
	11th to 20th	Night	3 3 1 3 3 2 2 2 4 5% 70%	1890
	21st to end	Night	2 2 1 3 3 3 4 3 5% 7% 68%	1890
July	1st to 10th	Night	2 2 1 4 3 4 4 3 4 5% 69%	5340
	11th to 20th	Night	1 2 1 4 5% 6% 7% 5% 9% 9% 49%	5340
	21st to end	Night	1 2 1 4 4 4 4 4 6% 8% 63%	5805
August	1st to 10th	Night	1 2 1 3 3 3 4 3 5% 7% 68%	4123
	11th to 20th	Night	2 2 1 4 3 4 4 3 5% 6% 65%	4141
	21st to end	Night	2 3 2 6% 4 4 5% 4 5% 6% 59%	4667
September	1st to 10th	Night	2 1 1 3 3 3 2 3 6% 6% 70%	2149
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 67%	2200
	21st to end	Night	5% 3 1 3 2 3 2 2 4 5% 70%	2200
October	1st to 10th	Night	3 2 1 3 2 2 3 3 3 5% 73%	3753
	11th to 20th	Night	8% 5% 1 4 2 3 3 2 4 6% 62%	3898
	21st to end	Night	6% 6% 2 4 2 3 3 3 4 7% 61%	4252
November	1st to 10th	Night	6% 6% 2 5% 3 3 3 3 5% 8% 57%	2650
	11th to 20th	Night	7% 6% 2 4 3 4 4 4 6% 7% 53%	2730
	21st to end	Night	6% 11% 4 6% 3 4 4 3 7% 6% 47%	2779
December	1st to 10th	Night	6% 8% 2 4 3 5% 4 3 6% 7% 53%	4710
	11th to 20th	Night	9% 11% 3 5% 3 3 4 3 6% 8% 44%	5228
	21st to end	Night	11% 11% 4 10% 5% 6% 6% 5% 9% 8% 26%	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	51% 6% 20% 17% 6%	5321
	11th to 20th	Day	36% 6% 17% 28% 12% 2	5340
	21st to end	Day	34% 5% 19% 26% 15% 1	5844
February	1st to 10th	Day	44% 5% 15% 23% 12% 1	1890
	11th to 20th	Day	46% 5% 13% 23% 12% 1	1942
	21st to end	Day	40% 5% 12% 28% 14% 2	1730
March	1st to 10th	Day	38% 5% 10% 26% 17% 3	4949
	11th to 20th	Day	36% 7% 11% 29% 15% 3	5110
	21st to end	Day	47% 6% 15% 21% 10% 1	5607
April	1st to 10th	Day	37% 4 11% 24% 19% 6%	2323
	11th to 20th	Day	37% 5% 10% 21% 22% 6%	2643
	21st to end	Day	32% 4 9% 26% 24% 4	2730
May	1st to 10th	Day	26% 7% 8% 21% 28% 9%	7626
	11th to 20th	Day	23% 7% 5% 22% 30% 12%	7599
	21st to end	Day	20% 6% 6% 22% 35% 10%	8371
June	1st to 10th	Day	19% 4 6% 20% 34% 17%	2730
	11th to 20th	Day	16% 3 5% 14% 30% 32%	2730
	21st to end	Day	13% 4 1 10% 29% 42%	2730
July	1st to 10th	Day	11% 3 2 5% 15% 64%	7586
	11th to 20th	Day	17% 6% 8% 14% 20% 35%	7581
	21st to end	Day	13% 4 5% 12% 22% 44%	8120
August	1st to 10th	Day	16% 4 5% 17% 30% 28%	4338
	11th to 20th	Day	16% 5% 4 14% 31% 30%	4327
	21st to end	Day	14% 3 4 15% 28% 36%	4727
September	1st to 10th	Day	14% 4 5% 14% 28% 34%	2248
	11th to 20th	Day	17% 3 6% 20% 28% 26%	2200
	21st to end	Day	16% 3 3 21% 34% 23%	2200
October	1st to 10th	Day	18% 3 4 13% 37% 25%	3440
	11th to 20th	Day	24% 3 7% 24% 31% 12%	3250
	21st to end	Day	24% 2 9% 29% 30% 6%	3548
November	1st to 10th	Day	30% 3 10% 28% 23% 5%	1864
	11th to 20th	Day	35% 5% 10% 30% 19% 2	1890
	21st to end	Day	34% 4 14% 29% 16% 3	1812
December	1st to 10th	Day	24% 5% 17% 45% 8% 2	2700
	11th to 20th	Day	35% 4 18% 31% 11%	3225
	21st to end	Day	45% 7% 15% 21% 11%	4609
January	1st to 10th	Night	47% 7% 22% 18% 6%	7605
	11th to 20th	Night	38% 6% 18% 28% 9% 1	7650
	21st to end	Night	31% 4 21% 29% 13% 1	8370
February	1st to 10th	Night	41% 6% 16% 25% 9% 2	2730
	11th to 20th	Night	42% 5% 13% 24% 15% 2	2571
	21st to end	Night	39% 4 10% 30% 14% 2	2076
March	1st to 10th	Night	39% 7% 10% 26% 17% 1	5165
	11th to 20th	Night	36% 6% 10% 30% 16% 3	5110
	21st to end	Night	37% 5% 14% 26% 16% 2	5610
April	1st to 10th	Night	32% 4 11% 26% 19% 7%	2296
	11th to 20th	Night	31% 4 10% 25% 24% 6%	1924
	21st to end	Night	31% 6% 11% 27% 22% 4	1890
May	1st to 10th	Night	26% 6% 7% 23% 31% 7%	5319
	11th to 20th	Night	20% 5% 5% 25% 32% 13%	5297
	21st to end	Night	20% 5% 6% 25% 39% 6%	5838
June	1st to 10th	Night	20% 5% 6% 21% 32% 16%	1890
	11th to 20th	Night	17% 2 4 16% 30% 31%	1890
	21st to end	Night	15% 4 3 11% 27% 41%	1890
July	1st to 10th	Night	16% 4 3 6% 14% 58%	5295
	11th to 20th	Night	23% 7% 9% 11% 18% 32%	5286
	21st to end	Night	17% 4 7% 12% 20% 39%	5677
August	1st to 10th	Night	18% 5% 7% 18% 24% 27%	3018
	11th to 20th	Night	21% 5% 6% 15% 23% 29%	3018
	21st to end	Night	16% 3 5% 13% 29% 34%	3379
September	1st to 10th	Night	15% 3 6% 15% 28% 34%	2149
	11th to 20th	Night	18% 3 5% 19% 27% 28%	2200
	21st to end	Night	18% 3 4 17% 32% 27%	2200
October	1st to 10th	Night	14% 2 5% 15% 34% 29%	3753
	11th to 20th	Night	24% 3 7% 23% 31% 12%	3898
	21st to end	Night	23% 4 7% 33% 29% 6%	4252
November	1st to 10th	Night	27% 3 9% 30% 24% 6%	2650
	11th to 20th	Night	28% 5% 12% 32% 19% 3	2730
	21st to end	Night	34% 5% 10% 30% 18% 3	2779
December	1st to 10th	Night	25% 5% 20% 37% 11% 2	4710
	11th to 20th	Night	35% 5% 21% 29% 9%	5228
	21st to end	Night	46% 8% 17% 20% 10%	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	23% 7% 12% 34% 19% 5%	330
	11th to 20th	Day	13% 7% 18% 34% 23% 5%	330
	21st to end	Day	12% 6% 17% 37% 23% 4	363
February	1st to 10th	Day	15% 12% 20% 35% 15% 4	330
	11th to 20th	Day	14% 10% 25% 33% 17% 1	418
	21st to end	Day	9% 11% 26% 36% 16% 2	446
March	1st to 10th	Day	11% 14% 22% 32% 16% 4	540
	11th to 20th	Day	8% 10% 31% 32% 16% 3	540
	21st to end	Day	14% 14% 28% 25% 15% 3	594
April	1st to 10th	Day	11% 13% 24% 27% 19% 7%	540
	11th to 20th	Day	12% 18% 23% 28% 15% 4	532
	21st to end	Day	10% 15% 25% 29% 17% 4	540
May	1st to 10th	Day	7% 11% 28% 31% 17% 5%	543
	11th to 20th	Day	8% 9% 25% 30% 22% 6%	568
	21st to end	Day	7% 9% 23% 28% 26% 8%	617
June	1st to 10th	Day	2 10% 20% 34% 26% 7%	570
	11th to 20th	Day	4 9% 27% 33% 21% 5%	570
	21st to end	Day	3 7% 27% 33% 24% 6%	570
July	1st to 10th	Day	1 2 21% 34% 32% 9%	566
	11th to 20th	Day	4 2 16% 34% 34% 10%	564
	21st to end	Day	3 1 11% 34% 39% 12%	619
August	1st to 10th	Day	3 3 19% 32% 32% 11%	567
	11th to 20th	Day	2 2 18% 30% 36% 12%	557
	21st to end	Day	3 3 17% 31% 35% 11%	620
September	1st to 10th	Day	3 4 14% 30% 30% 19%	530
	11th to 20th	Day	5% 6% 21% 28% 28% 12%	530
	21st to end	Day	5% 7% 15% 25% 34% 15%	530
October	1st to 10th	Day	5% 7% 19% 25% 28% 17%	435
	11th to 20th	Day	6% 7% 17% 24% 31% 15%	330
	21st to end	Day	10% 5% 17% 32% 27% 10%	363
November	1st to 10th	Day	12% 12% 15% 28% 22% 12%	330
	11th to 20th	Day	10% 6% 14% 36% 24% 11%	330
	21st to end	Day	11% 6% 12% 35% 27% 8%	330
December	1st to 10th	Day	12% 6% 13% 38% 20% 12%	330
	11th to 20th	Day	15% 5% 11% 38% 26% 5%	330
	21st to end	Day	20% 10% 13% 34% 20% 3	363
January	1st to 10th	Night	16% 5% 14% 38% 22% 5%	840
	11th to 20th	Night	12% 7% 14% 36% 26% 5%	840
	21st to end	Night	13% 6% 13% 41% 23% 4	924
February	1st to 10th	Night	13% 7% 18% 42% 16% 4	840
	11th to 20th	Night	13% 5% 20% 43% 17% 2	824
	21st to end	Night	11% 6% 20% 46% 17% 1	692
March	1st to 10th	Night	13% 11% 16% 42% 17% 2	840
	11th to 20th	Night	8% 6% 18% 44% 23% 1	840
	21st to end	Night	10% 9% 22% 42% 15% 2	924
April	1st to 10th	Night	12% 10% 19% 39% 19% 2	826
	11th to 20th	Night	8% 8% 26% 41% 16%	623
	21st to end	Night	9% 10% 18% 40% 21% 2	630
May	1st to 10th	Night	8% 6% 21% 42% 22% 1	630
	11th to 20th	Night	7% 5% 18% 41% 27% 3	621
	21st to end	Night	6% 6% 16% 39% 29% 4	683
June	1st to 10th	Night	4 7% 17% 42% 28% 2	630
	11th to 20th	Night	5% 7% 22% 42% 23% 1	630
	21st to end	Night	3 3 18% 42% 32% 1	630
July	1st to 10th	Night	2 1 14% 38% 40% 4	615
	11th to 20th	Night	5% 1 8% 43% 39% 3	612
	21st to end	Night	3 2 7% 37% 49% 2	690
August	1st to 10th	Night	4 1 12% 43% 38% 2	627
	11th to 20th	Night	3 1 12% 44% 38% 2	624
	21st to end	Night	2 2 11% 41% 41% 3	687
September	1st to 10th	Night	3 1 10% 39% 39% 7%	749
	11th to 20th	Night	5% 3 14% 36% 36% 7%	800
	21st to end	Night	6% 3 12% 34% 35% 10%	800
October	1st to 10th	Night	7% 3 16% 42% 27% 5%	800
	11th to 20th	Night	7% 4 14% 40% 30% 4	800
	21st to end	Night	8% 4 16% 40% 29% 4	880
November	1st to 10th	Night	8% 7% 15% 38% 27% 5%	820
	11th to 20th	Night	10% 5% 17% 42% 22% 4	840
	21st to end	Night	11% 5% 15% 38% 25% 6%	889
December	1st to 10th	Night	14% 5% 14% 43% 21% 3	1050
	11th to 20th	Night	17% 6% 14% 40% 22% 2	1008
	21st to end	Night	19% 7% 14% 40% 16% 3	924

Page generated at 2026-08-27 02:07:13. Version ac07

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