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 2024-03-06.

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	6% 7% 2 5% 5% 3 4 5% 6% 8% 50%	78570
	Night	7% 8% 3 5% 5% 3 4 5% 7% 8% 47%	80996
Transparency	Day	32% 4 10% 21% 21% 13%	78295
	Night	34% 4 11% 22% 18% 10%	80791
Seeing	Day	8% 8% 20% 31% 25% 8%	13804
	Night	9% 5% 15% 40% 27% 4	24448

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% 16% 5% 10% 7% 3 5% 4 8% 8% 26%	2858
	11th to 20th	Day	10% 19% 5% 10% 6% 3 4 5% 5% 7% 24%	2860
	21st to end	Day	9% 14% 4 7% 5% 3 6% 5% 9% 8% 30%	3146
February	1st to 10th	Day	10% 11% 4 7% 6% 4 5% 5% 6% 8% 36%	1710
	11th to 20th	Day	9% 11% 4 6% 5% 3 5% 4 6% 8% 38%	1744
	21st to end	Day	7% 9% 5% 7% 6% 3 4 5% 7% 9% 38%	1570
March	1st to 10th	Day	5% 11% 4 7% 6% 3 4 5% 5% 9% 43%	1984
	11th to 20th	Day	6% 7% 3 6% 5% 3 5% 6% 6% 8% 44%	1980
	21st to end	Day	8% 10% 3 7% 7% 4 6% 5% 5% 8% 36%	2178
April	1st to 10th	Day	7% 8% 3 5% 5% 3 6% 5% 6% 7% 45%	1990
	11th to 20th	Day	7% 8% 3 7% 5% 3 6% 5% 6% 7% 45%	2258
	21st to end	Day	6% 7% 3 5% 5% 3 4 4 5% 7% 51%	2340
May	1st to 10th	Day	5% 4 2 5% 5% 3 5% 6% 7% 9% 48%	2340
	11th to 20th	Day	5% 4 1 4 5% 3 4 5% 7% 9% 52%	2301
	21st to end	Day	4 4 2 4 5% 4 4 6% 9% 9% 48%	2530
June	1st to 10th	Day	2 2 2 3 3 3 5% 5% 5% 10% 61%	2340
	11th to 20th	Day	3 2 1 3 3 2 3 4 5% 7% 66%	2340
	21st to end	Day	2 2 1 2 2 2 4 4 4 7% 71%	2340
July	1st to 10th	Day	2 2 1 2 3 2 4 5% 5% 9% 65%	2340
	11th to 20th	Day	2 2 2 3 5% 3 5% 6% 6% 9% 58%	2340
	21st to end	Day	2 2 2 3 4 3 5% 5% 8% 9% 57%	2545
August	1st to 10th	Day	2 3 1 3 4 3 5% 6% 7% 9% 58%	2328
	11th to 20th	Day	2 2 1 3 4 3 4 5% 6% 6% 65%	2299
	21st to end	Day	2 2 1 3 4 3 4 4 6% 7% 64%	2548
September	1st to 10th	Day	2 1 1 2 2 2 4 3 5% 7% 71%	2024
	11th to 20th	Day	3 1 1 3 2 2 3 4 5% 7% 68%	1980
	21st to end	Day	5% 2 1 2 3 2 3 3 4 6% 70%	1980
October	1st to 10th	Day	6% 4 2 4 3 3 4 5% 5% 6% 58%	1895
	11th to 20th	Day	6% 4 2 3 3 2 3 3 4 7% 64%	1800
	21st to end	Day	5% 5% 2 3 4 2 3 4 5% 7% 59%	1980
November	1st to 10th	Day	7% 6% 2 5% 4 2 5% 4 5% 7% 52%	1684
	11th to 20th	Day	11% 5% 2 5% 4 3 4 5% 5% 8% 47%	1710
	21st to end	Day	5% 9% 3 6% 5% 3 5% 5% 6% 9% 45%	1640
December	1st to 10th	Day	9% 10% 4 6% 5% 3 4 5% 5% 9% 41%	1520
	11th to 20th	Day	8% 11% 3 7% 6% 4 5% 6% 8% 10% 30%	2002
	21st to end	Day	8% 12% 4 7% 5% 4 5% 6% 9% 10% 29%	3146
January	1st to 10th	Night	7% 17% 6% 8% 6% 3 5% 5% 8% 7% 27%	4097
	11th to 20th	Night	13% 17% 6% 9% 6% 4 5% 6% 7% 6% 21%	4110
	21st to end	Night	9% 12% 5% 8% 6% 4 5% 7% 9% 8% 27%	4521
February	1st to 10th	Night	8% 11% 4 7% 5% 3 5% 5% 8% 9% 34%	2470
	11th to 20th	Night	8% 11% 4 6% 5% 2 4 5% 8% 9% 38%	2320
	21st to end	Night	7% 10% 5% 7% 5% 2 5% 5% 7% 9% 38%	1884
March	1st to 10th	Night	7% 11% 5% 6% 5% 4 4 6% 6% 7% 38%	2092
	11th to 20th	Night	6% 7% 2 5% 5% 3 5% 5% 6% 9% 49%	1980
	21st to end	Night	7% 9% 4 7% 6% 3 5% 5% 6% 9% 42%	2178
April	1st to 10th	Night	10% 8% 2 4 5% 3 3 4 4 6% 51%	1969
	11th to 20th	Night	6% 6% 3 6% 4 2 5% 5% 6% 8% 49%	1647
	21st to end	Night	8% 6% 2 5% 4 3 3 4 5% 7% 52%	1620
May	1st to 10th	Night	6% 5% 2 5% 5% 3 5% 5% 7% 9% 47%	1620
	11th to 20th	Night	5% 5% 1 4 4 3 4 4 6% 8% 55%	1594
	21st to end	Night	5% 4 2 4 6% 3 5% 6% 6% 9% 51%	1746
June	1st to 10th	Night	3 2 2 3 3 3 5% 5% 5% 8% 61%	1620
	11th to 20th	Night	3 3 1 3 3 2 3 3 4 6% 69%	1620
	21st to end	Night	2 2 1 2 3 2 4 4 5% 7% 69%	1620
July	1st to 10th	Night	2 1 1 3 4 2 5% 6% 6% 9% 60%	1620
	11th to 20th	Night	3 3 2 4 4 3 6% 6% 9% 8% 53%	1620
	21st to end	Night	2 3 1 4 6% 3 6% 6% 7% 10% 52%	1773
August	1st to 10th	Night	2 3 1 3 4 3 4 5% 6% 8% 60%	1611
	11th to 20th	Night	2 2 1 4 4 3 5% 4 5% 7% 61%	1602
	21st to end	Night	1 2 1 3 4 2 4 5% 6% 8% 63%	1807
September	1st to 10th	Night	2 1 1 2 2 2 3 4 5% 5% 72%	1933
	11th to 20th	Night	3 3 1 3 3 2 3 3 5% 7% 67%	1980
	21st to end	Night	5% 2 2 3 2 2 2 3 3 5% 70%	1980
October	1st to 10th	Night	6% 3 2 3 4 2 4 4 4 6% 62%	2063
	11th to 20th	Night	6% 4 1 3 3 2 3 3 4 5% 65%	2160
	21st to end	Night	5% 6% 2 3 3 2 3 4 4 6% 62%	2376
November	1st to 10th	Night	6% 6% 2 4 4 2 3 3 5% 8% 56%	2391
	11th to 20th	Night	8% 6% 2 4 3 3 4 4 6% 8% 54%	2470
	21st to end	Night	6% 9% 5% 6% 4 3 4 4 7% 6% 47%	2513
December	1st to 10th	Night	9% 11% 3 5% 4 3 4 4 6% 6% 44%	2660
	11th to 20th	Night	12% 12% 4 5% 5% 3 5% 6% 8% 10% 31%	3208
	21st to end	Night	9% 11% 5% 9% 6% 4 6% 7% 12% 8% 22%	4521

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	55% 4 15% 18% 7%	2859
	11th to 20th	Day	56% 6% 18% 14% 6% 1	2860
	21st to end	Day	48% 4 21% 18% 8% 2	3146
February	1st to 10th	Day	45% 5% 16% 22% 10% 1	1710
	11th to 20th	Day	46% 4 13% 24% 12% 1	1753
	21st to end	Day	42% 5% 12% 27% 14% 1	1570
March	1st to 10th	Day	40% 4 12% 28% 14% 2	1984
	11th to 20th	Day	37% 4 13% 28% 15% 4	1980
	21st to end	Day	45% 6% 13% 22% 11% 3	2178
April	1st to 10th	Day	39% 4 11% 22% 17% 6%	1990
	11th to 20th	Day	40% 4 10% 20% 20% 6%	2261
	21st to end	Day	34% 3 9% 26% 24% 4	2340
May	1st to 10th	Day	31% 5% 9% 25% 24% 6%	2340
	11th to 20th	Day	28% 4 8% 26% 27% 8%	2304
	21st to end	Day	29% 6% 11% 25% 23% 7%	2530
June	1st to 10th	Day	20% 4 6% 20% 33% 17%	2340
	11th to 20th	Day	19% 3 5% 15% 31% 27%	2340
	21st to end	Day	14% 4 1 10% 29% 42%	2340
July	1st to 10th	Day	16% 5% 3 12% 24% 41%	2276
	11th to 20th	Day	22% 5% 5% 14% 24% 30%	2271
	21st to end	Day	22% 4 9% 18% 21% 27%	2404
August	1st to 10th	Day	22% 4 6% 19% 22% 27%	2313
	11th to 20th	Day	20% 4 3 12% 27% 33%	2299
	21st to end	Day	19% 3 4 17% 32% 25%	2548
September	1st to 10th	Day	15% 3 5% 14% 28% 36%	2024
	11th to 20th	Day	17% 3 6% 20% 26% 27%	1980
	21st to end	Day	17% 3 3 20% 35% 22%	1980
October	1st to 10th	Day	28% 4 7% 19% 32% 10%	1895
	11th to 20th	Day	23% 3 7% 27% 32% 9%	1800
	21st to end	Day	26% 2 10% 31% 25% 5%	1980
November	1st to 10th	Day	32% 3 10% 28% 22% 5%	1684
	11th to 20th	Day	35% 5% 10% 29% 19% 2	1710
	21st to end	Day	35% 4 14% 29% 16% 3	1640
December	1st to 10th	Day	38% 4 18% 26% 10% 3	1520
	11th to 20th	Day	46% 4 15% 25% 9%	2000
	21st to end	Day	43% 7% 17% 18% 15% 1	3146
January	1st to 10th	Night	51% 4 14% 21% 8%	4097
	11th to 20th	Night	57% 6% 15% 15% 6% 1	4110
	21st to end	Night	45% 5% 24% 17% 7% 2	4521
February	1st to 10th	Night	42% 7% 17% 24% 9% 1	2470
	11th to 20th	Night	42% 4 12% 25% 15% 2	2321
	21st to end	Night	41% 5% 10% 28% 14% 2	1884
March	1st to 10th	Night	41% 5% 11% 29% 13% 1	2092
	11th to 20th	Night	31% 5% 11% 31% 18% 4	1980
	21st to end	Night	38% 6% 12% 28% 13% 4	2178
April	1st to 10th	Night	35% 4 11% 24% 18% 8%	1969
	11th to 20th	Night	32% 5% 10% 26% 23% 5%	1647
	21st to end	Night	31% 5% 10% 27% 22% 4	1620
May	1st to 10th	Night	33% 4 9% 25% 24% 6%	1620
	11th to 20th	Night	27% 4 7% 25% 26% 10%	1595
	21st to end	Night	28% 5% 12% 28% 22% 5%	1746
June	1st to 10th	Night	21% 4 6% 23% 31% 15%	1620
	11th to 20th	Night	18% 2 5% 17% 30% 27%	1620
	21st to end	Night	14% 4 3 11% 25% 43%	1620
July	1st to 10th	Night	19% 7% 4 12% 19% 40%	1575
	11th to 20th	Night	25% 5% 7% 14% 20% 29%	1566
	21st to end	Night	26% 5% 10% 15% 19% 24%	1674
August	1st to 10th	Night	22% 4 10% 19% 20% 27%	1602
	11th to 20th	Night	22% 3 5% 15% 22% 32%	1602
	21st to end	Night	19% 4 7% 15% 27% 28%	1807
September	1st to 10th	Night	15% 3 5% 14% 28% 36%	1933
	11th to 20th	Night	18% 3 5% 20% 25% 29%	1980
	21st to end	Night	18% 2 3 16% 32% 28%	1980
October	1st to 10th	Night	24% 3 8% 21% 31% 13%	2063
	11th to 20th	Night	23% 3 6% 25% 33% 10%	2160
	21st to end	Night	24% 4 9% 31% 27% 6%	2376
November	1st to 10th	Night	28% 3 9% 29% 24% 7%	2391
	11th to 20th	Night	28% 5% 12% 32% 20% 3	2470
	21st to end	Night	35% 4 9% 29% 18% 4	2513
December	1st to 10th	Night	38% 4 15% 29% 11% 3	2660
	11th to 20th	Night	44% 5% 14% 25% 11%	3208
	21st to end	Night	49% 5% 14% 18% 13%	4521

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	24% 7% 11% 30% 22% 5%	270
	11th to 20th	Day	15% 9% 17% 33% 22% 3	270
	21st to end	Day	14% 6% 16% 33% 24% 5%	297
February	1st to 10th	Day	16% 11% 20% 35% 16% 3	270
	11th to 20th	Day	14% 9% 23% 33% 18% 2	349
	21st to end	Day	10% 11% 27% 35% 15% 1	382
March	1st to 10th	Day	12% 16% 22% 32% 15% 3	441
	11th to 20th	Day	9% 9% 29% 32% 18% 3	420
	21st to end	Day	16% 15% 29% 24% 14% 2	462
April	1st to 10th	Day	12% 14% 24% 26% 19% 5%	420
	11th to 20th	Day	12% 19% 25% 26% 14% 4	412
	21st to end	Day	11% 18% 24% 26% 17% 4	420
May	1st to 10th	Day	7% 11% 29% 33% 16% 4	423
	11th to 20th	Day	8% 8% 26% 30% 21% 7%	448
	21st to end	Day	8% 7% 22% 27% 27% 8%	485
June	1st to 10th	Day	2 9% 22% 33% 27% 7%	450
	11th to 20th	Day	4 10% 25% 32% 22% 6%	450
	21st to end	Day	3 6% 24% 34% 26% 7%	450
July	1st to 10th	Day	1 2 19% 36% 32% 10%	446
	11th to 20th	Day	4 2 17% 37% 33% 6%	444
	21st to end	Day	2 1 9% 35% 41% 11%	487
August	1st to 10th	Day	3 3 17% 34% 33% 10%	447
	11th to 20th	Day	2 3 16% 30% 36% 13%	437
	21st to end	Day	2 2 17% 31% 36% 13%	495
September	1st to 10th	Day	3 4 16% 30% 29% 18%	450
	11th to 20th	Day	6% 5% 20% 27% 29% 13%	450
	21st to end	Day	6% 7% 16% 25% 33% 14%	450
October	1st to 10th	Day	6% 7% 20% 25% 26% 15%	365
	11th to 20th	Day	6% 7% 16% 24% 30% 17%	270
	21st to end	Day	9% 4 20% 35% 26% 7%	297
November	1st to 10th	Day	13% 13% 16% 26% 21% 11%	270
	11th to 20th	Day	10% 6% 14% 34% 24% 12%	270
	21st to end	Day	10% 6% 13% 33% 29% 9%	270
December	1st to 10th	Day	13% 7% 13% 37% 20% 10%	270
	11th to 20th	Day	15% 5% 11% 39% 26% 3	270
	21st to end	Day	20% 8% 14% 34% 21% 3	297
January	1st to 10th	Night	15% 5% 12% 39% 23% 6%	760
	11th to 20th	Night	13% 7% 14% 35% 26% 4	760
	21st to end	Night	14% 6% 13% 38% 25% 4	836
February	1st to 10th	Night	13% 6% 19% 43% 16% 4	760
	11th to 20th	Night	13% 4 19% 43% 19% 2	744
	21st to end	Night	11% 6% 20% 44% 17% 1	628
March	1st to 10th	Night	13% 11% 16% 42% 17% 2	742
	11th to 20th	Night	8% 5% 17% 44% 23% 2	720
	21st to end	Night	10% 9% 22% 42% 15% 2	792
April	1st to 10th	Night	13% 10% 18% 38% 18% 2	709
	11th to 20th	Night	9% 8% 24% 40% 18%	533
	21st to end	Night	9% 10% 17% 39% 23% 2	540
May	1st to 10th	Night	9% 5% 21% 41% 22% 1	540
	11th to 20th	Night	6% 4 17% 39% 30% 3	531
	21st to end	Night	7% 6% 17% 36% 30% 4	584
June	1st to 10th	Night	4 7% 17% 40% 29% 2	540
	11th to 20th	Night	4 7% 22% 40% 26% 1	540
	21st to end	Night	3 3 17% 41% 35% 2	540
July	1st to 10th	Night	2 1 13% 35% 44% 5%	525
	11th to 20th	Night	5% 2 8% 42% 41% 3	522
	21st to end	Night	3 1 5% 35% 53% 3	591
August	1st to 10th	Night	4 1 10% 42% 40% 2	537
	11th to 20th	Night	2 1 10% 43% 40% 3	534
	21st to end	Night	2 2 10% 39% 43% 3	594
September	1st to 10th	Night	3 1 10% 38% 40% 7%	673
	11th to 20th	Night	5% 3 13% 34% 38% 8%	720
	21st to end	Night	7% 3 11% 33% 35% 11%	720
October	1st to 10th	Night	7% 3 16% 41% 27% 6%	720
	11th to 20th	Night	7% 4 13% 39% 32% 5%	720
	21st to end	Night	7% 4 15% 40% 29% 5%	792
November	1st to 10th	Night	9% 7% 15% 36% 28% 5%	740
	11th to 20th	Night	10% 5% 15% 42% 23% 5%	760
	21st to end	Night	11% 5% 15% 36% 26% 6%	803
December	1st to 10th	Night	14% 4 14% 44% 21% 3	950
	11th to 20th	Night	18% 6% 13% 40% 21% 2	912
	21st to end	Night	20% 6% 14% 41% 17% 3	836

Page generated at 2024-03-03 04:28:58. Version ac07

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