Topanga
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Topanga clear sky chart from 2005-11-06 to 2026-07-31.

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 4 1 3 2 2 2 2 4 6% 69%	146430
	Night	5% 4 2 4 3 3 3 3 4 6% 63%	144152
Transparency	Day	12% 35% 4 11% 18% 19%	141519
	Night	14% 35% 5% 11% 17% 18%	140038
Seeing	Day	6% 6% 21% 34% 26% 8%	17965
	Night	6% 3 11% 35% 37% 8%	27743

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% 3 1 3 2 2 2 2 4 5% 66%	4722
	11th to 20th	Day	5% 2 1 3 2 2 2 2 4 6% 71%	5322
	21st to end	Day	8% 5% 2 4 3 3 3 3 5% 9% 55%	6355
February	1st to 10th	Day	9% 4 2 3 3 2 3 2 4 5% 61%	2100
	11th to 20th	Day	9% 4 1 3 2 3 2 3 5% 7% 60%	2050
	21st to end	Day	7% 5% 1 3 3 3 3 2 5% 7% 60%	1730
March	1st to 10th	Day	10% 4 2 5% 3 3 3 3 5% 6% 55%	4618
	11th to 20th	Day	8% 4 1 4 3 3 3 3 5% 6% 61%	4833
	21st to end	Day	8% 5% 2 4 3 4 4 4 6% 9% 52%	6101
April	1st to 10th	Day	5% 5% 1 4 3 3 3 3 5% 8% 62%	2520
	11th to 20th	Day	4 4 1 4 2 3 3 3 4 6% 66%	2489
	21st to end	Day	3 4 1 3 2 2 2 3 4 7% 68%	2520
May	1st to 10th	Day	2 5% 1 4 2 2 3 3 4 7% 67%	7114
	11th to 20th	Day	2 8% 2 5% 3 2 2 2 4 5% 64%	7596
	21st to end	Day	2 7% 2 5% 2 2 2 2 3 6% 66%	8370
June	1st to 10th	Day	4 7% 2 5% 2 2 2 2 4 8% 62%	2817
	11th to 20th	Day	3 5% 1 3 2 2 2 1 4 5% 72%	2940
	21st to end	Day	2 4 1 3 2 2 2 1 3 5% 74%	2940
July	1st to 10th	Day	1 2 1 2 1 1 1 1 2 4 84%	6459
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 5% 75%	6385
	21st to end	Day	1 2 1 2 1 1 2 1 2 4 82%	6100
August	1st to 10th	Day	1 1 2 1 1 1 1 3 4 84%	5346
	11th to 20th	Day	1 2 1 2 1 1 1 1 2 3 86%	5364
	21st to end	Day	1 1 1 1 2 2 2 3 5% 82%	5984
September	1st to 10th	Day	3 2 1 2 2 2 2 2 3 5% 79%	2400
	11th to 20th	Day	3 3 1 3 2 2 2 1 2 4 78%	2279
	21st to end	Day	2 3 1 2 2 2 2 2 3 6% 76%	2200
October	1st to 10th	Day	2 2 1 1 1 1 1 2 3 3 82%	3600
	11th to 20th	Day	2 4 1 3 1 1 2 2 4 7% 72%	3298
	21st to end	Day	2 3 1 4 3 2 3 3 5% 6% 68%	3540
November	1st to 10th	Day	3 2 1 2 2 3 3 3 3 5% 72%	1854
	11th to 20th	Day	6% 3 1 3 3 2 3 3 4 5% 68%	1890
	21st to end	Day	5% 3 1 4 3 3 4 3 5% 8% 61%	1890
December	1st to 10th	Day	4 2 1 3 3 3 4 4 6% 6% 63%	3040
	11th to 20th	Day	10% 4 1 4 2 3 4 4 6% 7% 54%	3499
	21st to end	Day	11% 4 2 5% 5% 4 4 4 7% 10% 46%	4165
January	1st to 10th	Night	10% 5% 2 4 2 2 3 2 3 5% 63%	8172
	11th to 20th	Night	5% 3 1 4 2 2 2 2 4 6% 69%	7650
	21st to end	Night	10% 5% 2 4 3 3 4 3 5% 6% 55%	7880
February	1st to 10th	Night	9% 6% 1 4 3 2 2 2 4 5% 63%	2520
	11th to 20th	Night	10% 4 2 4 3 3 3 2 4 6% 59%	2458
	21st to end	Night	10% 4 1 4 3 2 3 2 4 5% 61%	2076
March	1st to 10th	Night	7% 7% 1 4 3 3 3 3 5% 7% 56%	5496
	11th to 20th	Night	7% 5% 1 4 4 3 3 3 4 6% 59%	5369
	21st to end	Night	10% 6% 2 4 4 4 3 3 5% 6% 54%	5116
April	1st to 10th	Night	6% 6% 2 4 3 3 2 2 4 6% 62%	2100
	11th to 20th	Night	4 5% 1 4 3 2 3 3 3 5% 65%	2079
	21st to end	Night	4 5% 2 5% 2 2 3 3 4 7% 62%	2100
May	1st to 10th	Night	2 8% 3 7% 5% 4 4 2 4 6% 55%	5844
	11th to 20th	Night	1 7% 4 10% 5% 4 3 2 4 5% 56%	5296
	21st to end	Night	1 6% 2 7% 4 3 3 2 3 5% 63%	5837
June	1st to 10th	Night	2 8% 3 5% 5% 3 4 3 5% 7% 57%	1796
	11th to 20th	Night	2 6% 2 4 3 3 3 3 5% 6% 64%	1680
	21st to end	Night	2 6% 2 4 3 2 3 2 4 7% 65%	1680
July	1st to 10th	Night	2 1 4 3 3 3 3 4 5% 71%	3720
	11th to 20th	Night	1 2 2 6% 4 4 5% 5% 4 6% 61%	3781
	21st to end	Night	1 3 2 3 2 3 3 3 3 4 74%	4999
August	1st to 10th	Night	1 1 1 2 2 3 2 3 4 6% 75%	4486
	11th to 20th	Night	1 3 2 4 3 3 3 2 4 5% 71%	4506
	21st to end	Night	1 1 1 3 3 3 3 3 4 6% 73%	5015
September	1st to 10th	Night	4 3 1 3 2 2 2 2 4 5% 72%	2000
	11th to 20th	Night	3 3 1 3 2 2 2 1 3 5% 75%	2120
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 75%	2200
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 3 80%	3600
	11th to 20th	Night	3 7% 3 5% 2 2 3 3 5% 5% 61%	3855
	21st to end	Night	2 3 1 3 2 2 2 2 3 5% 75%	4269
November	1st to 10th	Night	3 3 1 3 2 2 3 3 4 5% 71%	2665
	11th to 20th	Night	4 3 1 4 2 2 2 2 4 5% 69%	2730
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 6% 62%	2730
December	1st to 10th	Night	4 3 1 4 3 4 3 3 4 6% 66%	4370
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 8% 53%	4941
	21st to end	Night	10% 5% 1 4 3 4 6% 4 7% 9% 46%	7016

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	18% 21% 6% 13% 24% 19%	4723
	11th to 20th	Day	12% 22% 5% 11% 14% 37%	5322
	21st to end	Day	21% 23% 3 13% 19% 21%	6355
February	1st to 10th	Day	12% 59% 3 6% 11% 9%	2100
	11th to 20th	Day	12% 58% 2 8% 10% 10%	2060
	21st to end	Day	9% 59% 3 10% 10% 9%	1730
March	1st to 10th	Day	20% 32% 6% 12% 15% 15%	4618
	11th to 20th	Day	17% 30% 7% 21% 16% 9%	4833
	21st to end	Day	19% 33% 10% 21% 13% 4	6101
April	1st to 10th	Day	8% 60% 4 8% 13% 6%	2520
	11th to 20th	Day	8% 60% 3 8% 13% 8%	2492
	21st to end	Day	7% 61% 5% 12% 11% 4	2520
May	1st to 10th	Day	15% 24% 6% 16% 27% 11%	7114
	11th to 20th	Day	17% 24% 5% 20% 21% 13%	7599
	21st to end	Day	15% 24% 8% 25% 22% 6%	8371
June	1st to 10th	Day	10% 59% 4 10% 12% 5%	2817
	11th to 20th	Day	7% 58% 2 7% 13% 13%	2940
	21st to end	Day	8% 57% 2 5% 14% 15%	2940
July	1st to 10th	Day	4 24% 1 4 14% 52%	6391
	11th to 20th	Day	10% 24% 2 6% 27% 30%	6310
	21st to end	Day	5% 23% 2 11% 31% 27%	5928
August	1st to 10th	Day	5% 34% 1 6% 29% 24%	3890
	11th to 20th	Day	2 35% 1 4 21% 36%	3880
	21st to end	Day	4 34% 1 6% 17% 37%	4311
September	1st to 10th	Day	6% 56% 2 6% 15% 15%	2400
	11th to 20th	Day	8% 57% 2 6% 15% 12%	2279
	21st to end	Day	5% 57% 3 8% 16% 12%	2200
October	1st to 10th	Day	3 34% 1 5% 11% 45%	3600
	11th to 20th	Day	8% 36% 2 10% 26% 17%	3298
	21st to end	Day	12% 35% 2 6% 23% 23%	3540
November	1st to 10th	Day	3 53% 3 7% 13% 22%	1854
	11th to 20th	Day	12% 55% 3 6% 8% 16%	1890
	21st to end	Day	11% 53% 5% 7% 10% 13%	1890
December	1st to 10th	Day	10% 35% 2 9% 18% 25%	3040
	11th to 20th	Day	19% 31% 4 9% 18% 18%	3498
	21st to end	Day	23% 29% 8% 16% 18% 6%	4165
January	1st to 10th	Night	22% 21% 5% 8% 25% 19%	8172
	11th to 20th	Night	14% 22% 4 12% 11% 37%	7650
	21st to end	Night	24% 24% 3 9% 18% 21%	7880
February	1st to 10th	Night	13% 59% 3 7% 9% 9%	2520
	11th to 20th	Night	12% 57% 4 7% 10% 11%	2459
	21st to end	Night	10% 59% 3 8% 10% 10%	2076
March	1st to 10th	Night	17% 33% 7% 11% 19% 13%	5496
	11th to 20th	Night	18% 30% 9% 21% 11% 11%	5369
	21st to end	Night	19% 30% 12% 19% 16% 3	5116
April	1st to 10th	Night	10% 59% 3 10% 12% 6%	2100
	11th to 20th	Night	8% 59% 4 9% 13% 7%	2079
	21st to end	Night	8% 59% 6% 12% 11% 5%	2100
May	1st to 10th	Night	22% 25% 7% 14% 22% 9%	5842
	11th to 20th	Night	16% 27% 7% 19% 21% 11%	5297
	21st to end	Night	16% 25% 7% 25% 21% 6%	5838
June	1st to 10th	Night	12% 58% 7% 9% 10% 5%	1796
	11th to 20th	Night	7% 58% 2 8% 13% 12%	1680
	21st to end	Night	11% 56% 2 6% 12% 12%	1680
July	1st to 10th	Night	9% 24% 6% 17% 44%	3680
	11th to 20th	Night	14% 24% 3 10% 22% 27%	3733
	21st to end	Night	10% 23% 2 14% 26% 25%	4859
August	1st to 10th	Night	6% 34% 2 10% 26% 23%	3256
	11th to 20th	Night	6% 34% 1 7% 23% 29%	3256
	21st to end	Night	8% 34% 1 4 24% 28%	3608
September	1st to 10th	Night	8% 56% 2 5% 15% 14%	2000
	11th to 20th	Night	7% 57% 2 6% 15% 12%	2120
	21st to end	Night	5% 56% 3 6% 16% 13%	2200
October	1st to 10th	Night	4 34% 1 5% 9% 47%	3600
	11th to 20th	Night	16% 36% 3 8% 19% 18%	3855
	21st to end	Night	9% 35% 2 6% 20% 29%	4269
November	1st to 10th	Night	4 52% 3 8% 13% 21%	2665
	11th to 20th	Night	11% 54% 3 6% 10% 15%	2730
	21st to end	Night	11% 55% 5% 8% 9% 13%	2730
December	1st to 10th	Night	11% 34% 3 10% 15% 27%	4370
	11th to 20th	Night	19% 31% 5% 10% 14% 21%	4941
	21st to end	Night	23% 30% 9% 19% 14% 5%	7016

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	18% 8% 25% 33% 15% 1	220
	11th to 20th	Day	9% 10% 31% 32% 16% 1	425
	21st to end	Day	10% 12% 31% 33% 14% 1	567
February	1st to 10th	Day	12% 10% 30% 30% 15% 4	540
	11th to 20th	Day	12% 13% 30% 33% 10% 2	535
	21st to end	Day	7% 12% 36% 33% 10% 1	446
March	1st to 10th	Day	12% 15% 31% 26% 14% 1	540
	11th to 20th	Day	9% 14% 29% 30% 16% 3	540
	21st to end	Day	8% 13% 32% 30% 13% 3	594
April	1st to 10th	Day	8% 9% 27% 33% 17% 6%	540
	11th to 20th	Day	5% 10% 35% 32% 14% 5%	532
	21st to end	Day	4 10% 31% 31% 17% 7%	540
May	1st to 10th	Day	5% 9% 27% 38% 16% 5%	543
	11th to 20th	Day	5% 5% 25% 38% 23% 4	568
	21st to end	Day	5% 6% 24% 38% 22% 6%	617
June	1st to 10th	Day	5% 2 18% 34% 32% 7%	570
	11th to 20th	Day	4 1 15% 37% 32% 11%	570
	21st to end	Day	4 1 12% 35% 36% 12%	570
July	1st to 10th	Day	2 1 5% 34% 41% 17%	566
	11th to 20th	Day	1 5% 25% 47% 22%	564
	21st to end	Day	1 3 30% 44% 22%	616
August	1st to 10th	Day	2 1 5% 35% 41% 16%	527
	11th to 20th	Day	2 1 4 31% 46% 17%	517
	21st to end	Day	1 7% 38% 39% 15%	583
September	1st to 10th	Day	3 1 9% 38% 34% 15%	530
	11th to 20th	Day	4 2 17% 40% 30% 8%	530
	21st to end	Day	2 2 18% 36% 32% 9%	530
October	1st to 10th	Day	2 4 18% 38% 30% 8%	530
	11th to 20th	Day	3 4 22% 40% 25% 6%	530
	21st to end	Day	4 6% 22% 36% 26% 7%	577
November	1st to 10th	Day	4 6% 16% 32% 33% 9%	330
	11th to 20th	Day	8% 7% 23% 38% 19% 5%	330
	21st to end	Day	7% 7% 24% 33% 25% 4	330
December	1st to 10th	Day	8% 4 23% 37% 25% 3	330
	11th to 20th	Day	5% 6% 21% 35% 28% 5%	330
	21st to end	Day	16% 14% 22% 32% 16% 1	258
January	1st to 10th	Night	12% 5% 16% 42% 19% 5%	840
	11th to 20th	Night	8% 3 20% 44% 21% 4	840
	21st to end	Night	13% 4 21% 40% 21% 2	924
February	1st to 10th	Night	10% 4 17% 40% 24% 5%	840
	11th to 20th	Night	10% 4 18% 39% 23% 6%	824
	21st to end	Night	11% 4 22% 41% 19% 3	692
March	1st to 10th	Night	9% 6% 18% 43% 21% 3	840
	11th to 20th	Night	8% 3 16% 39% 29% 5%	840
	21st to end	Night	8% 4 17% 43% 25% 4	924
April	1st to 10th	Night	8% 3 14% 43% 28% 4	840
	11th to 20th	Night	6% 3 16% 42% 31% 3	831
	21st to end	Night	5% 5% 14% 39% 30% 7%	840
May	1st to 10th	Night	7% 2 10% 41% 33% 6%	816
	11th to 20th	Night	5% 1 14% 37% 38% 5%	621
	21st to end	Night	4 1 14% 37% 40% 5%	683
June	1st to 10th	Night	2 1 9% 35% 48% 6%	536
	11th to 20th	Night	3 1 3 32% 54% 8%	420
	21st to end	Night	4 2 27% 58% 8%	420
July	1st to 10th	Night	2 1 11% 72% 13%	410
	11th to 20th	Night	2 9% 68% 21%	430
	21st to end	Night	2 2 13% 64% 20%	880
August	1st to 10th	Night	4 1 18% 61% 16%	796
	11th to 20th	Night	3 1 14% 63% 20%	792
	21st to end	Night	2 1 1 18% 59% 19%	880
September	1st to 10th	Night	4 1 3 23% 51% 18%	800
	11th to 20th	Night	4 3 29% 52% 12%	800
	21st to end	Night	2 5% 32% 49% 12%	800
October	1st to 10th	Night	4 1 7% 39% 44% 6%	800
	11th to 20th	Night	6% 2 9% 33% 41% 9%	800
	21st to end	Night	4 2 11% 37% 35% 12%	880
November	1st to 10th	Night	2 4 11% 40% 34% 9%	820
	11th to 20th	Night	6% 3 11% 41% 32% 8%	840
	21st to end	Night	7% 2 16% 43% 27% 6%	840
December	1st to 10th	Night	8% 4 17% 40% 26% 5%	840
	11th to 20th	Night	12% 4 16% 42% 23% 4	840
	21st to end	Night	12% 5% 21% 41% 18% 4	924

Page generated at 2026-07-28 00:25:20. Version ac07

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