Simi Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Simi Valley clear sky chart from 2005-11-06 to 2026-03-18.

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	4 3 1 3 2 2 3 3 4 6% 69%	139704
	Night	5% 4 1 4 2 3 3 3 4 5% 67%	139395
Transparency	Day	18% 3 4 17% 29% 29%	134791
	Night	20% 3 4 16% 27% 30%	135281
Seeing	Day	6% 8% 24% 33% 20% 7%	17393
	Night	6% 3 13% 35% 35% 8%	27260

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 4 3 3 3 2 4 5% 65%	4736
	11th to 20th	Day	5% 2 1 3 2 2 2 2 4 6% 72%	5340
	21st to end	Day	7% 4 2 4 3 3 3 3 5% 9% 56%	6233
February	1st to 10th	Day	9% 5% 1 4 2 2 3 3 5% 5% 60%	2100
	11th to 20th	Day	9% 3 1 3 3 3 3 3 6% 7% 58%	2050
	21st to end	Day	7% 3 1 5% 3 4 3 3 5% 7% 58%	1730
March	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 5% 6% 53%	4618
	11th to 20th	Day	8% 4 1 5% 3 3 3 3 4 6% 59%	4842
	21st to end	Day	8% 4 1 4 4 4 5% 4 6% 9% 50%	5959
April	1st to 10th	Day	5% 4 1 4 3 2 3 3 5% 8% 62%	2400
	11th to 20th	Day	5% 4 1 4 3 3 3 3 4 6% 64%	2369
	21st to end	Day	4 4 1 3 2 2 3 3 4 7% 66%	2400
May	1st to 10th	Day	5% 4 2 5% 3 2 3 3 4 5% 65%	5474
	11th to 20th	Day	2 4 2 5% 3 3 3 3 5% 8% 61%	5841
	21st to end	Day	2 4 1 5% 3 3 4 3 5% 8% 63%	6423
June	1st to 10th	Day	4 5% 2 4 3 3 3 3 5% 9% 60%	2727
	11th to 20th	Day	3 3 1 3 2 2 3 3 5% 7% 69%	2800
	21st to end	Day	1 2 1 2 2 1 2 2 3 6% 77%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 87%	6319
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 82%	6237
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 84%	6059
August	1st to 10th	Day	1 1 2 1 1 2 2 3 4 83%	5346
	11th to 20th	Day	1 1 1 1 1 1 2 4 88%	5364
	21st to end	Day	1 1 1 1 1 2 2 4 86%	5984
September	1st to 10th	Day	1 2 1 2 2 1 2 2 3 5% 79%	2400
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	2290
	21st to end	Day	2 2 1 1 2 1 2 2 4 4 79%	2200
October	1st to 10th	Day	2 1 1 2 1 2 2 2 3 4 82%	3600
	11th to 20th	Day	2 3 1 3 2 2 3 2 4 6% 72%	3266
	21st to end	Day	2 3 1 3 2 3 3 4 4 5% 70%	3558
November	1st to 10th	Day	3 1 1 3 3 3 3 2 4 5% 72%	1857
	11th to 20th	Day	6% 3 1 3 3 3 2 2 4 6% 67%	1890
	21st to end	Day	4 4 1 5% 4 3 3 3 5% 6% 62%	1890
December	1st to 10th	Day	4 3 1 3 3 4 4 3 5% 6% 65%	3040
	11th to 20th	Day	10% 5% 2 4 3 3 4 4 5% 6% 54%	3470
	21st to end	Day	9% 5% 2 5% 5% 4 4 4 6% 9% 49%	4092
January	1st to 10th	Night	9% 5% 2 5% 3 3 3 2 3 4 61%	8155
	11th to 20th	Night	4 3 1 4 2 2 2 3 4 6% 69%	7650
	21st to end	Night	8% 7% 2 5% 3 3 3 3 5% 6% 53%	7996
February	1st to 10th	Night	8% 5% 2 5% 2 2 3 3 5% 4 61%	2520
	11th to 20th	Night	9% 4 1 4 2 3 3 3 4 6% 60%	2458
	21st to end	Night	10% 4 2 3 3 3 3 3 4 6% 59%	2076
March	1st to 10th	Night	7% 8% 3 5% 3 3 4 3 5% 6% 53%	5496
	11th to 20th	Night	5% 6% 2 5% 4 3 4 3 4 5% 58%	5307
	21st to end	Night	10% 5% 2 5% 3 3 3 3 5% 6% 55%	5016
April	1st to 10th	Night	6% 5% 2 4 3 3 2 2 4 7% 62%	2000
	11th to 20th	Night	4 5% 1 4 3 2 2 3 4 6% 66%	1979
	21st to end	Night	4 5% 1 2 3 3 2 2 5% 7% 66%	2000
May	1st to 10th	Night	3 5% 2 6% 4 4 4 3 4 5% 61%	4519
	11th to 20th	Night	1 3 1 4 4 3 4 4 5% 7% 63%	4074
	21st to end	Night	1 2 1 4 3 3 3 3 5% 5% 71%	4474
June	1st to 10th	Night	1 4 1 4 4 2 4 4 4 7% 65%	1668
	11th to 20th	Night	1 2 2 2 2 2 2 3 6% 77%	1600
	21st to end	Night	1 2 1 2 2 2 2 2 3 5% 80%	1600
July	1st to 10th	Night	1 1 1 1 1 1 3 4 87%	3640
	11th to 20th	Night	1 1 1 1 2 1 4 5% 82%	3722
	21st to end	Night	1 1 2 2 2 2 2 4 4 80%	4827
August	1st to 10th	Night	1 1 1 1 2 2 2 3 5% 81%	4486
	11th to 20th	Night	1 1 1 1 2 2 5% 3 84%	4506
	21st to end	Night	1 1 1 1 1 2 2 4 4 84%	5015
September	1st to 10th	Night	2 2 1 3 1 1 2 2 3 4 78%	2000
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 3 80%	2110
	21st to end	Night	1 1 1 3 2 1 2 2 3 4 80%	2200
October	1st to 10th	Night	2 1 1 2 2 1 2 2 3 3 82%	3600
	11th to 20th	Night	2 3 1 4 2 2 3 3 6% 6% 67%	3888
	21st to end	Night	2 3 1 3 2 2 2 2 3 4 76%	4253
November	1st to 10th	Night	3 3 1 2 2 2 3 2 5% 5% 71%	2660
	11th to 20th	Night	5% 3 1 4 2 2 3 2 3 5% 69%	2730
	21st to end	Night	5% 3 1 4 3 3 3 3 5% 6% 62%	2730
December	1st to 10th	Night	4 3 1 3 3 3 3 3 4 5% 68%	4370
	11th to 20th	Night	11% 4 1 3 3 3 4 4 6% 8% 52%	4964
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 6% 9% 44%	7106

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	23% 2 6% 16% 30% 22%	4737
	11th to 20th	Day	16% 2 6% 14% 20% 43%	5340
	21st to end	Day	27% 3 3 16% 26% 25%	6233
February	1st to 10th	Day	27% 3 3 14% 25% 27%	2100
	11th to 20th	Day	25% 2 5% 19% 27% 22%	2060
	21st to end	Day	26% 4 5% 21% 23% 21%	1730
March	1st to 10th	Day	30% 3 7% 18% 21% 21%	4618
	11th to 20th	Day	26% 4 6% 29% 22% 13%	4842
	21st to end	Day	29% 4 11% 29% 19% 8%	5959
April	1st to 10th	Day	22% 4 11% 24% 25% 14%	2400
	11th to 20th	Day	22% 3 6% 23% 30% 16%	2372
	21st to end	Day	19% 3 8% 26% 32% 12%	2400
May	1st to 10th	Day	22% 3 7% 18% 28% 21%	5474
	11th to 20th	Day	22% 3 5% 27% 32% 10%	5844
	21st to end	Day	19% 4 5% 31% 32% 9%	6424
June	1st to 10th	Day	22% 3 8% 24% 30% 13%	2727
	11th to 20th	Day	16% 3 6% 16% 32% 29%	2800
	21st to end	Day	12% 2 3 10% 33% 41%	2800
July	1st to 10th	Day	5% 2 1 5% 20% 67%	6251
	11th to 20th	Day	7% 2 2 10% 38% 41%	6162
	21st to end	Day	7% 2 2 16% 40% 34%	5885
August	1st to 10th	Day	9% 1 1 8% 44% 37%	3890
	11th to 20th	Day	5% 1 9% 34% 51%	3880
	21st to end	Day	5% 2 1 9% 32% 51%	4311
September	1st to 10th	Day	9% 3 2 9% 31% 46%	2400
	11th to 20th	Day	12% 2 1 13% 33% 38%	2290
	21st to end	Day	11% 2 2 15% 34% 35%	2200
October	1st to 10th	Day	9% 2 3 12% 19% 56%	3600
	11th to 20th	Day	16% 2 4 12% 34% 31%	3266
	21st to end	Day	16% 3 2 9% 33% 36%	3558
November	1st to 10th	Day	15% 3 3 17% 27% 35%	1857
	11th to 20th	Day	20% 3 5% 13% 22% 37%	1890
	21st to end	Day	23% 3 6% 13% 22% 32%	1890
December	1st to 10th	Day	20% 3 3 10% 27% 36%	3040
	11th to 20th	Day	31% 4 5% 15% 26% 20%	3469
	21st to end	Day	33% 4 7% 22% 25% 10%	4092
January	1st to 10th	Night	28% 2 4 12% 31% 22%	8155
	11th to 20th	Night	18% 2 4 16% 17% 44%	7650
	21st to end	Night	30% 4 3 12% 24% 27%	7996
February	1st to 10th	Night	27% 3 4 14% 24% 28%	2520
	11th to 20th	Night	27% 3 4 17% 25% 24%	2459
	21st to end	Night	27% 3 5% 17% 23% 24%	2076
March	1st to 10th	Night	29% 5% 7% 18% 25% 18%	5496
	11th to 20th	Night	27% 4 7% 27% 20% 15%	5307
	21st to end	Night	29% 4 9% 27% 24% 7%	5016
April	1st to 10th	Night	24% 3 9% 25% 24% 15%	2000
	11th to 20th	Night	20% 4 5% 23% 28% 18%	1979
	21st to end	Night	18% 5% 7% 25% 32% 13%	2000
May	1st to 10th	Night	21% 7% 9% 17% 27% 18%	4516
	11th to 20th	Night	19% 7% 6% 26% 30% 12%	4075
	21st to end	Night	12% 6% 5% 33% 33% 11%	4474
June	1st to 10th	Night	19% 5% 11% 22% 28% 14%	1668
	11th to 20th	Night	11% 5% 4 19% 34% 27%	1600
	21st to end	Night	11% 3 2 11% 32% 41%	1600
July	1st to 10th	Night	4 2 1 6% 20% 67%	3600
	11th to 20th	Night	6% 2 2 11% 35% 43%	3674
	21st to end	Night	8% 3 2 17% 36% 34%	4689
August	1st to 10th	Night	8% 2 2 10% 39% 39%	3256
	11th to 20th	Night	5% 2 2 7% 35% 48%	3256
	21st to end	Night	5% 2 1 8% 34% 50%	3608
September	1st to 10th	Night	12% 3 3 6% 29% 46%	2000
	11th to 20th	Night	12% 3 1 12% 33% 39%	2110
	21st to end	Night	10% 3 2 15% 32% 39%	2200
October	1st to 10th	Night	10% 2 2 10% 17% 59%	3600
	11th to 20th	Night	15% 5% 3 16% 27% 35%	3888
	21st to end	Night	13% 3 2 8% 29% 45%	4253
November	1st to 10th	Night	16% 3 4 14% 28% 35%	2660
	11th to 20th	Night	19% 3 5% 12% 23% 38%	2730
	21st to end	Night	23% 3 5% 17% 20% 32%	2730
December	1st to 10th	Night	19% 3 3 12% 23% 41%	4370
	11th to 20th	Night	30% 3 7% 15% 22% 23%	4964
	21st to end	Night	36% 4 6% 23% 22% 9%	7106

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% 13% 29% 29% 11%	221
	11th to 20th	Day	14% 13% 37% 25% 10%	430
	21st to end	Day	13% 16% 37% 24% 9% 1	544
February	1st to 10th	Day	13% 14% 35% 26% 9% 3	540
	11th to 20th	Day	13% 15% 34% 28% 7% 2	535
	21st to end	Day	10% 16% 37% 27% 8% 1	446
March	1st to 10th	Day	14% 20% 31% 25% 9% 1	540
	11th to 20th	Day	12% 15% 32% 28% 11% 2	529
	21st to end	Day	12% 15% 36% 26% 9% 2	550
April	1st to 10th	Day	10% 11% 30% 30% 15% 4	500
	11th to 20th	Day	8% 15% 32% 29% 11% 5%	492
	21st to end	Day	5% 11% 33% 30% 14% 8%	500
May	1st to 10th	Day	9% 9% 28% 36% 14% 4	503
	11th to 20th	Day	5% 8% 29% 34% 20% 5%	528
	21st to end	Day	6% 8% 25% 36% 18% 6%	573
June	1st to 10th	Day	5% 5% 22% 32% 28% 8%	530
	11th to 20th	Day	3 4 17% 39% 27% 9%	530
	21st to end	Day	2 2 15% 35% 34% 13%	530
July	1st to 10th	Day	1 8% 38% 38% 15%	526
	11th to 20th	Day	2 1 6% 33% 35% 23%	524
	21st to end	Day	1 1 6% 36% 35% 21%	575
August	1st to 10th	Day	2 1 9% 42% 32% 15%	527
	11th to 20th	Day	2 1 6% 39% 35% 16%	517
	21st to end	Day	1 1 12% 42% 31% 14%	583
September	1st to 10th	Day	2 1 16% 40% 29% 12%	530
	11th to 20th	Day	4 3 21% 42% 22% 8%	530
	21st to end	Day	1 5% 23% 38% 23% 9%	530
October	1st to 10th	Day	2 6% 25% 36% 26% 5%	530
	11th to 20th	Day	3 7% 29% 36% 20% 5%	530
	21st to end	Day	4 8% 30% 33% 18% 6%	583
November	1st to 10th	Day	5% 6% 21% 34% 25% 8%	333
	11th to 20th	Day	9% 10% 27% 36% 15% 3	330
	21st to end	Day	8% 12% 28% 34% 15% 3	330
December	1st to 10th	Day	10% 8% 32% 30% 18% 2	330
	11th to 20th	Day	6% 11% 28% 35% 19% 1	322
	21st to end	Day	17% 15% 27% 27% 13% 1	242
January	1st to 10th	Night	11% 7% 23% 39% 17% 4	840
	11th to 20th	Night	8% 5% 27% 40% 17% 2	840
	21st to end	Night	14% 7% 26% 37% 15% 2	924
February	1st to 10th	Night	11% 6% 21% 38% 21% 3	840
	11th to 20th	Night	10% 5% 19% 42% 19% 5%	824
	21st to end	Night	11% 7% 22% 40% 18% 2	692
March	1st to 10th	Night	10% 5% 20% 44% 19% 2	840
	11th to 20th	Night	7% 5% 15% 41% 26% 5%	828
	21st to end	Night	9% 5% 16% 44% 24% 3	880
April	1st to 10th	Night	7% 4 14% 45% 28% 4	800
	11th to 20th	Night	5% 4 15% 42% 30% 3	791
	21st to end	Night	5% 4 11% 40% 32% 9%	800
May	1st to 10th	Night	6% 3 9% 42% 35% 5%	785
	11th to 20th	Night	4 2 11% 37% 41% 5%	591
	21st to end	Night	2 2 11% 37% 41% 6%	650
June	1st to 10th	Night	2 1 5% 35% 48% 9%	468
	11th to 20th	Night	1 1 2 31% 56% 9%	400
	21st to end	Night	1 1 28% 61% 10%	400
July	1st to 10th	Night	2 1 13% 71% 14%	390
	11th to 20th	Night	1 9% 68% 21%	418
	21st to end	Night	2 1 11% 63% 22%	807
August	1st to 10th	Night	2 1 1 19% 62% 15%	796
	11th to 20th	Night	1 1 13% 65% 20%	792
	21st to end	Night	1 1 19% 59% 19%	880
September	1st to 10th	Night	3 1 4 24% 53% 16%	800
	11th to 20th	Night	3 2 4 29% 50% 13%	800
	21st to end	Night	2 1 5% 36% 45% 12%	800
October	1st to 10th	Night	4 1 7% 43% 41% 5%	800
	11th to 20th	Night	5% 2 10% 39% 37% 7%	800
	21st to end	Night	5% 2 15% 38% 29% 10%	880
November	1st to 10th	Night	5% 2 14% 43% 29% 7%	820
	11th to 20th	Night	7% 3 17% 43% 25% 6%	840
	21st to end	Night	7% 6% 22% 39% 22% 5%	840
December	1st to 10th	Night	9% 6% 22% 38% 21% 4	840
	11th to 20th	Night	13% 5% 20% 42% 18% 3	840
	21st to end	Night	12% 7% 20% 40% 18% 3	924

Page generated at 2026-03-14 23:23:51. Version ac07

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