Pelona Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pelona Mountain clear sky chart from 2005-11-06 to 2025-12-18.

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 4 2 5% 4 4 4 4 5% 7% 58%	135671
	Night	2 3 1 4 3 3 4 3 6% 8% 62%	132866
Transparency	Day	23% 6% 8% 14% 27% 22%	130631
	Night	18% 5% 9% 17% 30% 20%	128909
Seeing	Day	9% 13% 24% 25% 19% 9%	15902
	Night	4 8% 23% 37% 25% 3	25954

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	1 5% 3 7% 4 3 3 3 5% 6% 60%	4098
	11th to 20th	Day	1 2 2 4 2 2 3 2 4 5% 72%	4100
	21st to end	Day	2 7% 3 5% 3 2 3 3 5% 6% 60%	4510
February	1st to 10th	Day	1 4 2 4 4 3 3 2 5% 6% 66%	1800
	11th to 20th	Day	2 5% 2 5% 3 2 3 3 5% 6% 64%	1870
	21st to end	Day	2 5% 1 4 3 2 2 3 5% 6% 66%	1765
March	1st to 10th	Day	1 3 2 5% 3 3 4 4 6% 7% 63%	4954
	11th to 20th	Day	2 6% 2 6% 4 3 4 3 5% 7% 56%	5000
	21st to end	Day	1 5% 3 6% 4 4 4 3 5% 5% 59%	5486
April	1st to 10th	Day	1 3 1 4 3 4 3 3 4 7% 68%	2350
	11th to 20th	Day	2 5% 1 5% 3 2 3 2 4 5% 68%	2371
	21st to end	Day	2 3 1 4 3 3 4 3 4 6% 69%	2400
May	1st to 10th	Day	1 2 1 4 3 4 4 3 5% 7% 66%	5440
	11th to 20th	Day	2 2 1 4 3 4 4 4 5% 5% 66%	5793
	21st to end	Day	1 2 1 3 3 3 3 2 4 5% 72%	6423
June	1st to 10th	Day	3 4 1 4 4 3 4 3 5% 7% 61%	2600
	11th to 20th	Day	3 3 1 3 2 3 3 3 4 6% 69%	2600
	21st to end	Day	5% 5% 2 6% 6% 5% 5% 4 7% 7% 46%	2600
July	1st to 10th	Day	4 4 2 6% 6% 6% 5% 4 6% 7% 50%	5879
	11th to 20th	Day	4 3 2 8% 8% 8% 8% 6% 8% 9% 37%	5880
	21st to end	Day	7% 7% 3 10% 8% 8% 8% 6% 8% 8% 28%	6401
August	1st to 10th	Day	4 5% 2 8% 6% 6% 7% 6% 8% 8% 41%	5779
	11th to 20th	Day	5% 4 3 7% 7% 7% 7% 5% 7% 8% 40%	5415
	21st to end	Day	6% 5% 3 9% 7% 7% 8% 6% 8% 9% 34%	5984
September	1st to 10th	Day	7% 6% 2 7% 5% 6% 6% 4 6% 7% 44%	2400
	11th to 20th	Day	6% 4 2 7% 4 5% 5% 4 5% 6% 52%	2330
	21st to end	Day	4 3 1 5% 3 4 3 4 4 6% 62%	2136
October	1st to 10th	Day	4 3 1 4 3 3 4 2 4 5% 67%	3270
	11th to 20th	Day	3 3 1 4 3 4 3 3 3 7% 65%	3250
	21st to end	Day	1 1 1 2 1 2 1 1 3 4 83%	3548
November	1st to 10th	Day	2 3 1 3 2 2 2 2 4 7% 73%	2059
	11th to 20th	Day	1 2 1 5% 3 2 3 2 5% 6% 71%	2100
	21st to end	Day	2 5% 2 4 3 3 4 3 5% 6% 63%	2058
December	1st to 10th	Day	2 2 1 3 2 2 3 3 4 7% 72%	3040
	11th to 20th	Day	2 4 2 5% 4 3 2 2 4 6% 67%	3472
	21st to end	Day	1 3 1 3 3 4 4 4 5% 6% 68%	4510
January	1st to 10th	Night	1 5% 3 8% 4 4 5% 3 5% 6% 56%	5867
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 79%	5880
	21st to end	Night	1 5% 3 6% 4 3 4 3 5% 6% 59%	6468
February	1st to 10th	Night	1 3 1 4 2 3 3 4 6% 7% 66%	2600
	11th to 20th	Night	2 4 2 4 4 3 3 2 5% 7% 65%	2415
	21st to end	Night	1 3 1 3 2 3 4 3 5% 7% 68%	1860
March	1st to 10th	Night	1 2 1 4 3 3 3 3 5% 6% 70%	4940
	11th to 20th	Night	2 4 1 4 3 3 3 3 6% 7% 63%	5000
	21st to end	Night	1 3 1 3 3 3 4 3 5% 7% 68%	5489
April	1st to 10th	Night	1 2 1 2 3 3 3 3 6% 8% 70%	2048
	11th to 20th	Night	2 3 1 2 3 2 3 3 4 5% 72%	1977
	21st to end	Night	1 3 1 2 2 2 3 2 4 5% 75%	2000
May	1st to 10th	Night	1 1 1 3 2 2 3 3 4 5% 75%	4540
	11th to 20th	Night	2 2 1 3 2 2 3 3 4 6% 74%	4105
	21st to end	Night	1 1 3 2 2 2 2 3 6% 78%	4474
June	1st to 10th	Night	2 4 1 4 3 3 3 3 6% 8% 64%	1800
	11th to 20th	Night	2 2 1 3 2 2 2 3 4 6% 73%	1800
	21st to end	Night	3 5% 2 6% 5% 5% 5% 6% 7% 8% 48%	1800
July	1st to 10th	Night	4 3 2 5% 4 4 6% 5% 7% 9% 50%	4100
	11th to 20th	Night	2 3 2 6% 5% 5% 7% 7% 9% 15% 39%	4100
	21st to end	Night	3 5% 2 7% 6% 7% 7% 6% 9% 13% 34%	4472
August	1st to 10th	Night	3 3 2 6% 5% 6% 6% 6% 10% 12% 42%	4033
	11th to 20th	Night	3 4 2 7% 6% 5% 7% 6% 10% 14% 36%	4432
	21st to end	Night	3 4 2 7% 6% 7% 8% 6% 11% 14% 33%	4993
September	1st to 10th	Night	5% 4 1 7% 5% 4 5% 5% 8% 9% 48%	2000
	11th to 20th	Night	4 4 1 4 3 4 4 4 5% 7% 59%	2065
	21st to end	Night	3 3 1 4 3 3 3 4 5% 6% 66%	2261
October	1st to 10th	Night	3 3 1 3 3 3 3 3 4 6% 68%	3920
	11th to 20th	Night	3 3 1 3 2 2 3 2 4 5% 72%	3898
	21st to end	Night	1 1 2 1 2 2 2 3 5% 80%	4252
November	1st to 10th	Night	1 2 1 2 2 2 3 3 3 8% 73%	2460
	11th to 20th	Night	1 2 1 3 2 3 3 3 5% 5% 73%	2520
	21st to end	Night	2 4 1 4 3 3 3 3 5% 6% 66%	2557
December	1st to 10th	Night	1 3 2 3 2 2 3 2 4 6% 71%	4370
	11th to 20th	Night	1 2 1 4 3 4 4 3 5% 8% 65%	4902
	21st to end	Night	1 3 1 4 3 3 3 3 4 6% 68%	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	21% 4 5% 16% 39% 15%	4099
	11th to 20th	Day	13% 4 6% 17% 32% 28%	4100
	21st to end	Day	21% 5% 6% 11% 30% 27%	4510
February	1st to 10th	Day	17% 3 7% 17% 27% 27%	1800
	11th to 20th	Day	20% 3 6% 20% 28% 23%	1879
	21st to end	Day	16% 3 4 14% 32% 30%	1765
March	1st to 10th	Day	17% 6% 4 9% 29% 35%	4954
	11th to 20th	Day	23% 7% 7% 12% 33% 19%	5000
	21st to end	Day	22% 6% 6% 15% 36% 16%	5486
April	1st to 10th	Day	16% 4 6% 15% 37% 23%	2350
	11th to 20th	Day	18% 3 4 12% 29% 33%	2374
	21st to end	Day	16% 3 6% 15% 36% 24%	2400
May	1st to 10th	Day	16% 4 5% 15% 33% 26%	5440
	11th to 20th	Day	16% 5% 7% 12% 31% 30%	5796
	21st to end	Day	14% 3 3 7% 22% 51%	6424
June	1st to 10th	Day	21% 5% 7% 13% 24% 30%	2600
	11th to 20th	Day	15% 5% 4 9% 22% 44%	2600
	21st to end	Day	34% 8% 10% 12% 19% 18%	2600
July	1st to 10th	Day	29% 10% 9% 13% 16% 24%	5816
	11th to 20th	Day	37% 12% 18% 17% 11% 6%	5811
	21st to end	Day	46% 13% 17% 14% 9% 2	6217
August	1st to 10th	Day	37% 13% 16% 19% 11% 3	4208
	11th to 20th	Day	36% 10% 12% 20% 18% 3	3919
	21st to end	Day	43% 12% 14% 14% 15% 3	4311
September	1st to 10th	Day	38% 8% 9% 16% 18% 12%	2400
	11th to 20th	Day	30% 7% 9% 16% 22% 16%	2330
	21st to end	Day	20% 6% 8% 19% 28% 18%	2136
October	1st to 10th	Day	20% 5% 5% 10% 29% 31%	3270
	11th to 20th	Day	20% 4 3 12% 40% 22%	3250
	21st to end	Day	7% 2 4 14% 32% 41%	3548
November	1st to 10th	Day	13% 2 4 21% 33% 26%	2059
	11th to 20th	Day	15% 4 5% 16% 34% 26%	2100
	21st to end	Day	20% 5% 7% 16% 30% 22%	2058
December	1st to 10th	Day	13% 3 5% 18% 40% 22%	3040
	11th to 20th	Day	22% 3 5% 11% 40% 20%	3471
	21st to end	Day	15% 4 7% 21% 29% 25%	4510
January	1st to 10th	Night	22% 5% 6% 18% 37% 12%	5867
	11th to 20th	Night	10% 2 6% 18% 41% 23%	5880
	21st to end	Night	21% 5% 6% 13% 29% 25%	6468
February	1st to 10th	Night	15% 3 9% 22% 28% 23%	2600
	11th to 20th	Night	19% 4 6% 21% 29% 21%	2416
	21st to end	Night	15% 4 6% 19% 28% 29%	1860
March	1st to 10th	Night	12% 4 5% 14% 40% 26%	4940
	11th to 20th	Night	16% 6% 6% 18% 35% 19%	5000
	21st to end	Night	12% 4 7% 24% 40% 13%	5489
April	1st to 10th	Night	12% 3 6% 22% 37% 19%	2048
	11th to 20th	Night	14% 4 4 15% 36% 27%	1977
	21st to end	Night	11% 4 7% 18% 38% 23%	2000
May	1st to 10th	Night	12% 4 6% 20% 40% 18%	4537
	11th to 20th	Night	11% 5% 6% 14% 36% 29%	4106
	21st to end	Night	10% 2 3 7% 24% 53%	4474
June	1st to 10th	Night	19% 5% 7% 14% 27% 28%	1800
	11th to 20th	Night	13% 4 4 10% 25% 43%	1800
	21st to end	Night	30% 7% 12% 12% 19% 21%	1800
July	1st to 10th	Night	27% 9% 13% 14% 16% 22%	4055
	11th to 20th	Night	28% 12% 24% 21% 13% 4	4046
	21st to end	Night	36% 13% 28% 12% 8% 2	4344
August	1st to 10th	Night	27% 13% 24% 24% 9% 3	2928
	11th to 20th	Night	32% 11% 17% 22% 14% 3	3204
	21st to end	Night	38% 13% 20% 16% 11% 2	3597
September	1st to 10th	Night	29% 10% 12% 19% 18% 11%	2000
	11th to 20th	Night	22% 6% 13% 19% 27% 14%	2065
	21st to end	Night	18% 6% 7% 19% 32% 17%	2261
October	1st to 10th	Night	17% 4 7% 14% 31% 27%	3920
	11th to 20th	Night	15% 4 5% 17% 38% 21%	3898
	21st to end	Night	9% 2 3 15% 37% 34%	4252
November	1st to 10th	Night	12% 3 4 20% 38% 22%	2460
	11th to 20th	Night	13% 3 6% 19% 31% 28%	2520
	21st to end	Night	19% 4 7% 18% 34% 18%	2557
December	1st to 10th	Night	15% 3 5% 23% 34% 21%	4370
	11th to 20th	Night	18% 4 6% 14% 40% 19%	4902
	21st to end	Night	16% 4 7% 21% 32% 21%	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	7% 18% 34% 26% 13% 2	300
	11th to 20th	Day	12% 17% 27% 26% 14% 4	300
	21st to end	Day	6% 20% 27% 27% 17% 4	330
February	1st to 10th	Day	8% 17% 29% 27% 15% 4	300
	11th to 20th	Day	13% 18% 32% 28% 9%	300
	21st to end	Day	16% 32% 23% 21% 7% 2	364
March	1st to 10th	Day	13% 28% 29% 21% 7% 2	500
	11th to 20th	Day	18% 24% 29% 17% 10% 2	500
	21st to end	Day	16% 29% 31% 17% 6% 2	550
April	1st to 10th	Day	11% 21% 38% 16% 11% 4	500
	11th to 20th	Day	15% 27% 34% 14% 9% 2	492
	21st to end	Day	11% 25% 37% 15% 10% 3	500
May	1st to 10th	Day	8% 22% 34% 23% 10% 3	503
	11th to 20th	Day	9% 15% 33% 23% 15% 5%	528
	21st to end	Day	8% 16% 38% 23% 13% 2	573
June	1st to 10th	Day	6% 8% 32% 30% 20% 5%	530
	11th to 20th	Day	4 11% 25% 32% 21% 8%	530
	21st to end	Day	7% 7% 23% 31% 22% 11%	530
July	1st to 10th	Day	7% 2 13% 37% 29% 13%	526
	11th to 20th	Day	9% 1 7% 26% 35% 22%	524
	21st to end	Day	8% 3 8% 22% 32% 26%	575
August	1st to 10th	Day	9% 1 11% 23% 34% 23%	527
	11th to 20th	Day	10% 3 9% 25% 33% 20%	517
	21st to end	Day	7% 2 8% 30% 31% 22%	583
September	1st to 10th	Day	10% 4 14% 28% 29% 15%	530
	11th to 20th	Day	7% 4 21% 33% 24% 12%	530
	21st to end	Day	4 8% 22% 30% 25% 10%	466
October	1st to 10th	Day	9% 9% 21% 24% 23% 14%	330
	11th to 20th	Day	11% 9% 22% 28% 18% 12%	330
	21st to end	Day	7% 11% 21% 25% 25% 10%	363
November	1st to 10th	Day	6% 13% 27% 30% 14% 10%	330
	11th to 20th	Day	5% 12% 19% 34% 23% 8%	330
	21st to end	Day	8% 16% 30% 24% 16% 5%	330
December	1st to 10th	Day	8% 15% 26% 28% 20% 3	330
	11th to 20th	Day	7% 17% 25% 28% 19% 4	321
	21st to end	Day	11% 18% 27% 30% 11% 3	330
January	1st to 10th	Night	4 10% 26% 38% 17% 4	800
	11th to 20th	Night	3 9% 30% 38% 17% 2	800
	21st to end	Night	3 11% 30% 36% 17% 3	880
February	1st to 10th	Night	3 13% 30% 34% 17% 3	800
	11th to 20th	Night	6% 13% 36% 34% 11% 1	784
	21st to end	Night	5% 17% 38% 32% 8%	660
March	1st to 10th	Night	4 17% 34% 35% 10% 1	800
	11th to 20th	Night	5% 14% 29% 35% 16% 2	800
	21st to end	Night	4 14% 34% 35% 12% 1	880
April	1st to 10th	Night	2 13% 37% 36% 10%	648
	11th to 20th	Night	3 18% 36% 32% 10% 1	593
	21st to end	Night	3 14% 39% 34% 11% 1	600
May	1st to 10th	Night	3 8% 34% 43% 11% 1	600
	11th to 20th	Night	3 7% 30% 42% 17% 1	591
	21st to end	Night	2 6% 32% 44% 16% 1	650
June	1st to 10th	Night	3 4 20% 52% 21% 1	600
	11th to 20th	Night	3 2 23% 51% 21% 1	600
	21st to end	Night	4 3 14% 39% 38% 2	600
July	1st to 10th	Night	6% 1 4 37% 45% 6%	585
	11th to 20th	Night	4 4 24% 60% 8%	582
	21st to end	Night	5% 2 4 20% 58% 11%	657
August	1st to 10th	Night	4 1 2 18% 61% 13%	597
	11th to 20th	Night	5% 1 4 27% 53% 11%	594
	21st to end	Night	5% 1 5% 23% 56% 10%	660
September	1st to 10th	Night	6% 1 5% 36% 46% 6%	600
	11th to 20th	Night	4 1 6% 43% 41% 4	665
	21st to end	Night	3 3 12% 42% 37% 4	800
October	1st to 10th	Night	3 3 20% 45% 26% 3	800
	11th to 20th	Night	4 3 19% 47% 24% 3	800
	21st to end	Night	4 6% 17% 39% 29% 5%	880
November	1st to 10th	Night	3 5% 20% 43% 24% 5%	820
	11th to 20th	Night	2 7% 19% 43% 25% 4	840
	21st to end	Night	7% 10% 23% 35% 21% 4	840
December	1st to 10th	Night	5% 10% 23% 41% 20% 2	840
	11th to 20th	Night	3 10% 30% 38% 18% 1	828
	21st to end	Night	4 13% 27% 40% 13% 2	880

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