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 2024-01-16.

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	5% 3 1 3 3 2 3 3 4 6% 67%	74884
	Night	6% 4 2 3 3 2 3 3 4 6% 66%	74920
Transparency	Day	20% 3 5% 17% 28% 27%	74607
	Night	21% 4 5% 16% 26% 28%	74718
Seeing	Day	6% 8% 24% 34% 21% 7%	14448
	Night	6% 3 12% 34% 36% 8%	24332

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 2 4 4 3 3 4 6% 7% 56%	1519
	11th to 20th	Day	7% 4 2 3 3 3 3 3 3 6% 63%	1666
	21st to end	Day	10% 5% 3 3 4 2 2 3 3 6% 60%	1895
February	1st to 10th	Day	8% 4 2 4 3 1 3 3 5% 5% 63%	1800
	11th to 20th	Day	8% 3 1 3 3 2 3 3 5% 7% 62%	1750
	21st to end	Day	8% 3 1 5% 4 3 3 4 4 6% 59%	1480
March	1st to 10th	Day	7% 5% 2 5% 4 3 5% 5% 5% 7% 52%	1800
	11th to 20th	Day	7% 4 1 4 4 3 3 4 6% 7% 56%	1880
	21st to end	Day	6% 3 1 3 4 2 4 5% 5% 8% 58%	2366
April	1st to 10th	Day	6% 4 2 3 3 2 3 4 5% 8% 60%	2160
	11th to 20th	Day	4 4 2 4 3 2 3 3 4 6% 66%	2129
	21st to end	Day	5% 3 2 2 3 1 2 3 4 7% 68%	2160
May	1st to 10th	Day	5% 4 2 4 3 1 3 3 4 7% 63%	2168
	11th to 20th	Day	3 4 2 3 3 2 3 4 4 8% 64%	2301
	21st to end	Day	4 4 2 3 4 2 3 3 5% 8% 64%	2530
June	1st to 10th	Day	4 4 2 4 3 2 3 3 4 9% 63%	2453
	11th to 20th	Day	3 3 1 2 2 1 3 3 5% 7% 69%	2520
	21st to end	Day	1 2 1 2 2 1 2 3 3 6% 76%	2520
July	1st to 10th	Day	1 1 1 3 3 2 3 4 4 7% 72%	2520
	11th to 20th	Day	2 2 1 2 3 1 1 3 4 5% 77%	2489
	21st to end	Day	1 1 1 2 2 1 3 3 4 6% 74%	2408
August	1st to 10th	Day	2 2 1 3 2 1 2 2 3 5% 77%	2149
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 83%	2122
	21st to end	Day	1 1 1 2 2 2 3 3 5% 81%	2376
September	1st to 10th	Day	1 2 1 2 2 1 2 3 3 5% 78%	2160
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 81%	2062
	21st to end	Day	2 2 1 1 2 1 2 2 4 4 79%	1980
October	1st to 10th	Day	4 2 1 2 2 2 3 3 4 5% 72%	1980
	11th to 20th	Day	4 2 1 2 3 2 2 3 2 4 75%	1805
	21st to end	Day	2 2 1 2 3 2 2 4 4 4 73%	1980
November	1st to 10th	Day	3 1 1 3 3 2 3 3 4 5% 71%	1677
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 6% 68%	1710
	21st to end	Day	4 3 1 4 4 2 4 3 5% 7% 63%	1710
December	1st to 10th	Day	7% 4 1 3 4 3 3 4 5% 7% 58%	1710
	11th to 20th	Day	15% 6% 3 5% 5% 2 3 4 5% 7% 45%	2155
	21st to end	Day	12% 5% 2 5% 6% 4 4 5% 5% 7% 46%	2794
January	1st to 10th	Night	9% 4 2 4 4 3 4 4 5% 7% 55%	2642
	11th to 20th	Night	6% 3 2 4 3 2 3 4 4 7% 63%	2405
	21st to end	Night	10% 5% 2 4 2 1 2 3 3 6% 62%	2456
February	1st to 10th	Night	6% 4 3 4 3 1 3 3 5% 5% 63%	2160
	11th to 20th	Night	8% 3 1 3 3 2 3 3 4 6% 63%	2098
	21st to end	Night	11% 4 2 3 4 2 3 3 4 5% 59%	1776
March	1st to 10th	Night	7% 6% 3 4 3 3 4 5% 5% 8% 53%	2160
	11th to 20th	Night	6% 5% 2 4 4 2 4 5% 5% 6% 57%	2075
	21st to end	Night	7% 5% 1 4 3 2 3 4 5% 6% 60%	1990
April	1st to 10th	Night	6% 4 2 4 4 2 3 3 4 7% 61%	1800
	11th to 20th	Night	4 4 1 4 3 2 2 3 4 6% 67%	1779
	21st to end	Night	4 3 2 2 2 2 3 2 5% 7% 67%	1800
May	1st to 10th	Night	4 4 1 4 4 2 3 3 5% 6% 62%	1787
	11th to 20th	Night	3 4 2 3 3 2 3 4 4 7% 66%	1594
	21st to end	Night	2 3 1 3 3 2 3 2 3 6% 71%	1746
June	1st to 10th	Night	1 4 2 3 4 1 3 5% 4 8% 65%	1502
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 77%	1440
	21st to end	Night	1 2 1 1 2 1 2 2 2 5% 80%	1440
July	1st to 10th	Night	1 2 1 2 2 1 2 3 4 5% 78%	1440
	11th to 20th	Night	1 1 2 3 2 2 3 3 6% 77%	1466
	21st to end	Night	2 1 1 2 3 2 3 3 5% 6% 73%	1907
August	1st to 10th	Night	2 2 1 1 2 1 3 3 4 5% 76%	1790
	11th to 20th	Night	1 2 1 2 1 1 1 3 4 5% 79%	1780
	21st to end	Night	1 1 1 1 2 1 2 2 4 5% 80%	1980
September	1st to 10th	Night	2 3 1 3 2 1 2 2 3 5% 77%	1800
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 3 81%	1898
	21st to end	Night	2 1 1 1 2 1 2 2 3 4 81%	1980
October	1st to 10th	Night	3 2 1 3 3 2 3 3 4 5% 71%	1980
	11th to 20th	Night	4 3 1 3 2 1 3 3 3 4 73%	2151
	21st to end	Night	3 3 2 3 2 1 2 3 3 4 73%	2376
November	1st to 10th	Night	3 3 2 2 3 2 3 3 5% 5% 69%	2400
	11th to 20th	Night	4 3 1 3 3 1 3 3 3 5% 71%	2470
	21st to end	Night	5% 3 1 3 4 2 4 4 4 6% 63%	2470
December	1st to 10th	Night	7% 4 2 4 3 2 4 3 5% 6% 59%	2470
	11th to 20th	Night	18% 5% 2 4 4 2 3 4 5% 8% 46%	3050
	21st to end	Night	13% 8% 2 5% 4 3 5% 4 4 6% 45%	4862

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	28% 3 6% 17% 25% 21%	1519
	11th to 20th	Day	26% 3 7% 14% 25% 26%	1666
	21st to end	Day	29% 2 5% 17% 21% 26%	1895
February	1st to 10th	Day	25% 3 3 14% 25% 30%	1800
	11th to 20th	Day	22% 2 5% 17% 28% 24%	1760
	21st to end	Day	27% 4 6% 20% 24% 20%	1480
March	1st to 10th	Day	31% 3 8% 18% 18% 22%	1800
	11th to 20th	Day	26% 3 8% 20% 23% 19%	1880
	21st to end	Day	24% 4 10% 23% 23% 15%	2366
April	1st to 10th	Day	23% 4 12% 25% 22% 14%	2160
	11th to 20th	Day	21% 3 7% 22% 30% 17%	2132
	21st to end	Day	19% 3 8% 25% 32% 13%	2160
May	1st to 10th	Day	23% 3 8% 22% 28% 16%	2168
	11th to 20th	Day	20% 4 8% 26% 30% 13%	2304
	21st to end	Day	20% 4 9% 30% 29% 8%	2530
June	1st to 10th	Day	22% 3 8% 22% 31% 14%	2453
	11th to 20th	Day	17% 2 6% 16% 32% 28%	2520
	21st to end	Day	12% 2 3 10% 35% 38%	2520
July	1st to 10th	Day	13% 4 2 10% 26% 44%	2451
	11th to 20th	Day	12% 2 4 17% 35% 30%	2414
	21st to end	Day	13% 3 6% 23% 34% 22%	2274
August	1st to 10th	Day	13% 2 1 11% 35% 37%	2135
	11th to 20th	Day	8% 2 1 13% 38% 38%	2122
	21st to end	Day	9% 2 2 10% 37% 39%	2376
September	1st to 10th	Day	10% 3 2 9% 32% 44%	2160
	11th to 20th	Day	11% 2 1 14% 31% 41%	2062
	21st to end	Day	11% 2 2 16% 36% 34%	1980
October	1st to 10th	Day	15% 3 5% 20% 24% 34%	1980
	11th to 20th	Day	17% 2 3 14% 28% 37%	1805
	21st to end	Day	15% 3 4 12% 23% 43%	1980
November	1st to 10th	Day	16% 3 3 18% 25% 35%	1677
	11th to 20th	Day	18% 3 5% 13% 22% 39%	1710
	21st to end	Day	22% 3 6% 13% 23% 33%	1710
December	1st to 10th	Day	26% 4 5% 16% 23% 26%	1710
	11th to 20th	Day	40% 4 5% 15% 22% 15%	2154
	21st to end	Day	38% 4 5% 24% 16% 12%	2794
January	1st to 10th	Night	30% 3 4 14% 28% 21%	2642
	11th to 20th	Night	22% 3 6% 17% 22% 29%	2405
	21st to end	Night	26% 3 6% 14% 21% 30%	2456
February	1st to 10th	Night	24% 2 4 14% 25% 30%	2160
	11th to 20th	Night	24% 3 3 16% 26% 27%	2099
	21st to end	Night	28% 3 6% 15% 25% 23%	1776
March	1st to 10th	Night	29% 5% 6% 20% 21% 19%	2160
	11th to 20th	Night	26% 5% 7% 17% 26% 19%	2075
	21st to end	Night	25% 4 8% 23% 24% 16%	1990
April	1st to 10th	Night	25% 3 9% 25% 21% 15%	1800
	11th to 20th	Night	20% 5% 6% 24% 27% 19%	1779
	21st to end	Night	17% 5% 7% 24% 32% 15%	1800
May	1st to 10th	Night	23% 5% 8% 21% 26% 16%	1787
	11th to 20th	Night	21% 5% 7% 23% 30% 14%	1595
	21st to end	Night	17% 4 10% 31% 28% 11%	1746
June	1st to 10th	Night	19% 5% 11% 20% 29% 15%	1502
	11th to 20th	Night	12% 5% 5% 18% 35% 26%	1440
	21st to end	Night	12% 4 3 11% 32% 39%	1440
July	1st to 10th	Night	10% 4 3 11% 25% 48%	1400
	11th to 20th	Night	12% 3 5% 18% 27% 36%	1418
	21st to end	Night	13% 4 5% 20% 35% 23%	1801
August	1st to 10th	Night	12% 3 3 9% 33% 39%	1780
	11th to 20th	Night	9% 3 4 8% 39% 38%	1780
	21st to end	Night	8% 4 2 8% 37% 41%	1980
September	1st to 10th	Night	13% 3 3 7% 29% 45%	1800
	11th to 20th	Night	11% 3 1 11% 32% 42%	1898
	21st to end	Night	9% 3 2 15% 32% 39%	1980
October	1st to 10th	Night	17% 4 3 16% 25% 35%	1980
	11th to 20th	Night	17% 3 3 14% 24% 38%	2151
	21st to end	Night	16% 3 3 10% 21% 47%	2376
November	1st to 10th	Night	17% 3 4 14% 27% 35%	2400
	11th to 20th	Night	18% 2 6% 12% 22% 40%	2470
	21st to end	Night	22% 3 5% 17% 20% 33%	2470
December	1st to 10th	Night	26% 4 4 15% 21% 31%	2470
	11th to 20th	Night	39% 3 6% 13% 22% 17%	3050
	21st to end	Night	41% 4 4 21% 18% 12%	4862

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% 11% 28% 30% 13%	180
	11th to 20th	Day	13% 13% 36% 27% 12%	356
	21st to end	Day	14% 18% 35% 22% 10% 1	425
February	1st to 10th	Day	11% 11% 35% 28% 11% 4	420
	11th to 20th	Day	10% 15% 35% 32% 7% 2	415
	21st to end	Day	11% 16% 37% 27% 8% 1	346
March	1st to 10th	Day	12% 19% 33% 26% 9% 1	420
	11th to 20th	Day	12% 14% 33% 28% 11% 3	420
	21st to end	Day	11% 14% 35% 28% 8% 2	462
April	1st to 10th	Day	9% 10% 31% 31% 15% 3	420
	11th to 20th	Day	6% 17% 32% 29% 11% 4	412
	21st to end	Day	5% 11% 33% 30% 13% 8%	420
May	1st to 10th	Day	8% 10% 27% 37% 14% 4	423
	11th to 20th	Day	5% 8% 27% 35% 20% 5%	448
	21st to end	Day	7% 9% 26% 35% 18% 6%	485
June	1st to 10th	Day	6% 5% 22% 31% 28% 8%	450
	11th to 20th	Day	4 4 15% 39% 29% 9%	450
	21st to end	Day	2 2 13% 35% 35% 13%	450
July	1st to 10th	Day	1 7% 38% 40% 15%	446
	11th to 20th	Day	2 1 5% 32% 36% 23%	444
	21st to end	Day	1 1 4 35% 37% 21%	487
August	1st to 10th	Day	2 1 9% 43% 32% 14%	447
	11th to 20th	Day	2 1 4 39% 37% 17%	437
	21st to end	Day	1 1 10% 42% 32% 14%	495
September	1st to 10th	Day	2 1 16% 41% 28% 12%	450
	11th to 20th	Day	3 2 20% 43% 25% 8%	450
	21st to end	Day	1 5% 21% 40% 24% 10%	450
October	1st to 10th	Day	3 6% 26% 37% 25% 4	450
	11th to 20th	Day	3 5% 28% 38% 21% 4	450
	21st to end	Day	4 8% 29% 35% 19% 5%	495
November	1st to 10th	Day	4 6% 22% 35% 26% 8%	273
	11th to 20th	Day	5% 10% 26% 40% 16% 4	270
	21st to end	Day	7% 11% 28% 35% 16% 3	270
December	1st to 10th	Day	11% 7% 29% 31% 19% 3	270
	11th to 20th	Day	7% 13% 27% 34% 19% 1	264
	21st to end	Day	15% 14% 28% 30% 12% 1	198
January	1st to 10th	Night	10% 6% 21% 41% 18% 4	760
	11th to 20th	Night	8% 5% 26% 41% 17% 3	740
	21st to end	Night	14% 7% 23% 37% 16% 2	792
February	1st to 10th	Night	9% 6% 19% 38% 24% 4	720
	11th to 20th	Night	9% 4 19% 42% 21% 6%	704
	21st to end	Night	12% 7% 20% 39% 20% 2	592
March	1st to 10th	Night	10% 4 19% 44% 21% 2	720
	11th to 20th	Night	7% 5% 14% 40% 29% 6%	720
	21st to end	Night	8% 5% 15% 43% 25% 3	792
April	1st to 10th	Night	7% 3 12% 46% 28% 4	720
	11th to 20th	Night	5% 4 16% 42% 31% 3	711
	21st to end	Night	5% 4 11% 40% 32% 9%	720
May	1st to 10th	Night	6% 2 9% 42% 35% 5%	707
	11th to 20th	Night	5% 2 10% 36% 43% 5%	531
	21st to end	Night	2 2 11% 36% 41% 7%	584
June	1st to 10th	Night	2 1 5% 36% 47% 9%	422
	11th to 20th	Night	1 1 2 28% 58% 10%	360
	21st to end	Night	1 1 26% 61% 10%	360
July	1st to 10th	Night	2 9% 73% 15%	350
	11th to 20th	Night	1 1 8% 68% 21%	374
	21st to end	Night	2 1 1 8% 65% 23%	725
August	1st to 10th	Night	3 1 1 19% 61% 16%	716
	11th to 20th	Night	1 1 11% 65% 21%	712
	21st to end	Night	1 1 16% 61% 21%	792
September	1st to 10th	Night	3 1 4 21% 55% 18%	720
	11th to 20th	Night	3 1 3 28% 52% 14%	720
	21st to end	Night	2 5% 33% 48% 13%	720
October	1st to 10th	Night	4 1 7% 42% 41% 5%	720
	11th to 20th	Night	5% 3 8% 40% 38% 7%	720
	21st to end	Night	5% 3 15% 36% 29% 11%	792
November	1st to 10th	Night	5% 2 13% 43% 29% 8%	740
	11th to 20th	Night	6% 3 16% 42% 27% 6%	760
	21st to end	Night	7% 5% 21% 39% 23% 5%	760
December	1st to 10th	Night	10% 6% 21% 37% 21% 5%	760
	11th to 20th	Night	14% 5% 19% 40% 19% 3	760
	21st to end	Night	12% 7% 19% 40% 19% 3	836

Page generated at 2024-01-12 22:38:14. Version ac07

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