San Onofre State Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Onofre State Beach clear sky chart from 2005-11-06 to 2026-08-15.

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 2 4 3 3 3 3 5% 9% 61%	152526
	Night	6% 7% 2 5% 3 3 3 3 5% 7% 57%	147033
Transparency	Day	14% 34% 5% 12% 18% 17%	147625
	Night	18% 35% 6% 11% 16% 15%	142912
Seeing	Day	6% 3 17% 36% 30% 8%	18255
	Night	8% 2 9% 30% 38% 12%	28293

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% 4 2 4 2 3 2 2 3 6% 62%	5318
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 7% 70%	5760
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 6% 8% 50%	6500
February	1st to 10th	Day	9% 4 2 5% 3 3 4 3 4 6% 57%	2100
	11th to 20th	Day	9% 4 1 4 4 3 3 3 5% 8% 56%	2050
	21st to end	Day	8% 5% 2 5% 3 3 3 3 5% 8% 56%	1730
March	1st to 10th	Day	7% 6% 2 6% 3 3 4 4 6% 9% 49%	4618
	11th to 20th	Day	8% 5% 2 4 3 3 3 2 6% 8% 56%	4809
	21st to end	Day	8% 5% 1 4 3 3 4 3 6% 9% 53%	6028
April	1st to 10th	Day	6% 5% 2 3 3 3 4 4 5% 8% 58%	2520
	11th to 20th	Day	4 5% 2 4 3 3 4 3 5% 6% 62%	2489
	21st to end	Day	5% 4 2 3 3 3 3 3 5% 10% 60%	2520
May	1st to 10th	Day	3 4 2 5% 3 3 3 3 6% 8% 60%	7301
	11th to 20th	Day	3 6% 2 6% 4 4 4 4 6% 9% 52%	7596
	21st to end	Day	4 5% 2 6% 4 3 3 3 5% 9% 55%	8370
June	1st to 10th	Day	6% 8% 2 5% 4 2 3 3 5% 8% 52%	2730
	11th to 20th	Day	3 5% 2 3 2 3 3 2 5% 9% 62%	2730
	21st to end	Day	2 5% 2 4 3 3 3 2 4 7% 66%	2885
July	1st to 10th	Day	1 2 1 3 2 2 3 2 6% 10% 68%	8014
	11th to 20th	Day	1 3 2 4 2 3 3 3 7% 10% 61%	7650
	21st to end	Day	1 2 1 3 2 2 2 2 6% 9% 71%	7819
August	1st to 10th	Day	1 1 1 2 1 2 2 2 4 8% 77%	5466
	11th to 20th	Day	1 1 1 2 1 2 2 2 5% 8% 77%	5413
	21st to end	Day	2 2 1 2 2 2 3 2 5% 7% 72%	5984
September	1st to 10th	Day	3 2 1 3 2 2 2 2 5% 9% 70%	2400
	11th to 20th	Day	2 4 1 3 2 3 2 2 4 7% 70%	2215
	21st to end	Day	2 4 1 4 2 2 3 2 4 7% 70%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 6% 18% 61%	3600
	11th to 20th	Day	4 4 2 4 2 2 3 2 4 10% 62%	3490
	21st to end	Day	3 3 1 3 2 2 3 3 6% 8% 66%	3439
November	1st to 10th	Day	4 3 1 2 3 2 3 3 5% 7% 66%	1854
	11th to 20th	Day	5% 3 1 3 2 3 3 2 6% 7% 64%	1890
	21st to end	Day	6% 4 1 4 3 4 4 3 6% 9% 58%	1890
December	1st to 10th	Day	4 3 1 2 2 3 3 3 6% 10% 63%	3040
	11th to 20th	Day	8% 4 1 4 4 4 5% 4 5% 7% 52%	3499
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 5% 7% 10% 39%	4609
January	1st to 10th	Night	10% 5% 2 4 2 2 3 3 4 6% 59%	7605
	11th to 20th	Night	4 4 1 3 3 2 2 3 4 6% 67%	7234
	21st to end	Night	11% 7% 2 6% 4 3 4 4 5% 6% 50%	7747
February	1st to 10th	Night	7% 6% 2 4 3 3 3 3 4 5% 58%	2520
	11th to 20th	Night	11% 6% 2 5% 4 3 3 4 5% 5% 52%	2458
	21st to end	Night	10% 6% 1 6% 4 3 3 3 4 5% 54%	2076
March	1st to 10th	Night	12% 9% 3 6% 3 3 4 3 5% 9% 44%	5496
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 4 6% 50%	5409
	21st to end	Night	10% 12% 3 6% 4 3 3 3 4 6% 48%	5172
April	1st to 10th	Night	8% 8% 3 7% 4 3 3 2 5% 8% 49%	2100
	11th to 20th	Night	7% 8% 2 5% 2 3 3 2 5% 6% 57%	2079
	21st to end	Night	8% 10% 3 5% 4 3 4 3 5% 6% 50%	2100
May	1st to 10th	Night	7% 14% 3 7% 4 3 2 2 4 5% 49%	5647
	11th to 20th	Night	6% 16% 3 7% 4 3 3 3 6% 7% 42%	5296
	21st to end	Night	8% 14% 3 6% 3 2 3 3 6% 6% 45%	5837
June	1st to 10th	Night	7% 18% 5% 9% 5% 4 3 3 5% 7% 34%	1890
	11th to 20th	Night	5% 11% 2 5% 4 3 3 3 7% 8% 49%	1890
	21st to end	Night	4 9% 3 5% 4 4 3 4 5% 8% 50%	1718
July	1st to 10th	Night	1 5% 2 3 2 3 3 3 7% 7% 65%	4915
	11th to 20th	Night	1 5% 2 5% 4 3 3 3 6% 9% 59%	5340
	21st to end	Night	2 5% 1 3 3 2 2 2 5% 7% 67%	6298
August	1st to 10th	Night	1 3 1 2 2 2 2 2 6% 6% 73%	4586
	11th to 20th	Night	1 6% 1 3 2 2 3 2 6% 8% 66%	4546
	21st to end	Night	2 4 1 4 2 2 2 2 5% 8% 67%	5015
September	1st to 10th	Night	3 6% 2 4 4 4 3 2 5% 6% 62%	2000
	11th to 20th	Night	5% 5% 2 3 2 2 2 2 3 5% 68%	2185
	21st to end	Night	3 6% 2 4 3 2 2 2 4 5% 69%	2200
October	1st to 10th	Night	2 3 1 3 2 2 3 2 5% 6% 71%	3600
	11th to 20th	Night	5% 10% 2 4 3 3 2 3 4 6% 57%	3663
	21st to end	Night	3 4 1 3 2 2 3 2 4 5% 70%	4364
November	1st to 10th	Night	4 4 2 4 3 3 3 2 4 6% 65%	2665
	11th to 20th	Night	4 5% 2 4 3 3 3 2 3 5% 65%	2730
	21st to end	Night	6% 5% 2 5% 4 3 4 4 6% 7% 53%	2730
December	1st to 10th	Night	5% 2 1 3 3 3 3 3 7% 7% 62%	4370
	11th to 20th	Night	8% 6% 2 5% 3 4 5% 4 6% 7% 50%	4941
	21st to end	Night	9% 6% 3 6% 4 4 4 5% 7% 10% 42%	6611

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	20% 21% 5% 14% 20% 19%	5321
	11th to 20th	Day	12% 21% 8% 10% 14% 34%	5760
	21st to end	Day	25% 23% 4 11% 18% 19%	6500
February	1st to 10th	Day	13% 59% 4 6% 8% 10%	2100
	11th to 20th	Day	12% 58% 3 7% 11% 9%	2060
	21st to end	Day	11% 59% 3 8% 12% 7%	1730
March	1st to 10th	Day	20% 31% 8% 14% 17% 8%	4618
	11th to 20th	Day	18% 29% 10% 21% 14% 8%	4809
	21st to end	Day	20% 31% 10% 23% 12% 5%	6028
April	1st to 10th	Day	9% 60% 5% 9% 12% 6%	2520
	11th to 20th	Day	9% 59% 3 8% 13% 8%	2492
	21st to end	Day	9% 59% 6% 14% 9% 3	2520
May	1st to 10th	Day	16% 25% 6% 18% 28% 8%	7301
	11th to 20th	Day	21% 24% 8% 15% 23% 9%	7599
	21st to end	Day	18% 25% 8% 22% 22% 7%	8371
June	1st to 10th	Day	13% 58% 4 9% 12% 4	2730
	11th to 20th	Day	9% 58% 3 6% 14% 11%	2730
	21st to end	Day	10% 57% 3 5% 12% 14%	2885
July	1st to 10th	Day	8% 19% 3 7% 16% 48%	7949
	11th to 20th	Day	13% 19% 7% 13% 34% 14%	7581
	21st to end	Day	7% 19% 4 19% 25% 26%	7646
August	1st to 10th	Day	7% 34% 3 13% 29% 15%	4010
	11th to 20th	Day	4 35% 3 8% 16% 34%	3929
	21st to end	Day	6% 35% 3 5% 20% 31%	4311
September	1st to 10th	Day	6% 57% 3 5% 17% 12%	2400
	11th to 20th	Day	9% 57% 3 5% 17% 10%	2215
	21st to end	Day	8% 57% 4 5% 17% 10%	2200
October	1st to 10th	Day	4 34% 1 5% 13% 42%	3600
	11th to 20th	Day	12% 36% 3 7% 22% 19%	3490
	21st to end	Day	11% 35% 3 5% 25% 21%	3439
November	1st to 10th	Day	5% 53% 3 6% 12% 22%	1854
	11th to 20th	Day	12% 54% 4 7% 9% 13%	1890
	21st to end	Day	12% 54% 5% 9% 8% 12%	1890
December	1st to 10th	Day	8% 35% 3 18% 16% 21%	3040
	11th to 20th	Day	20% 32% 5% 11% 21% 12%	3498
	21st to end	Day	28% 30% 11% 17% 11% 4	4609
January	1st to 10th	Night	23% 22% 5% 12% 21% 17%	7605
	11th to 20th	Night	14% 23% 7% 11% 13% 33%	7234
	21st to end	Night	29% 23% 4 8% 18% 19%	7747
February	1st to 10th	Night	12% 60% 5% 7% 9% 8%	2520
	11th to 20th	Night	17% 59% 3 6% 8% 9%	2459
	21st to end	Night	13% 59% 2 7% 12% 7%	2076
March	1st to 10th	Night	25% 32% 10% 11% 14% 7%	5496
	11th to 20th	Night	24% 30% 8% 20% 11% 7%	5409
	21st to end	Night	27% 32% 10% 12% 14% 4	5172
April	1st to 10th	Night	13% 60% 4 7% 10% 6%	2100
	11th to 20th	Night	10% 60% 4 8% 12% 6%	2079
	21st to end	Night	15% 59% 4 11% 7% 4	2100
May	1st to 10th	Night	26% 24% 7% 17% 21% 5%	5645
	11th to 20th	Night	28% 26% 8% 13% 17% 7%	5297
	21st to end	Night	29% 26% 7% 16% 16% 6%	5838
June	1st to 10th	Night	22% 59% 3 4 8% 3	1890
	11th to 20th	Night	14% 58% 4 6% 10% 8%	1890
	21st to end	Night	17% 57% 2 4 9% 10%	1718
July	1st to 10th	Night	10% 19% 4 7% 15% 45%	4873
	11th to 20th	Night	15% 19% 8% 13% 32% 12%	5286
	21st to end	Night	10% 19% 5% 20% 24% 22%	6159
August	1st to 10th	Night	8% 35% 2 12% 30% 13%	3356
	11th to 20th	Night	6% 35% 4 10% 15% 30%	3296
	21st to end	Night	9% 35% 4 4 19% 30%	3608
September	1st to 10th	Night	13% 58% 2 5% 14% 9%	2000
	11th to 20th	Night	10% 57% 3 4 16% 10%	2185
	21st to end	Night	8% 57% 3 6% 17% 10%	2200
October	1st to 10th	Night	5% 35% 4 6% 14% 37%	3600
	11th to 20th	Night	18% 36% 5% 5% 17% 18%	3663
	21st to end	Night	11% 35% 3 7% 23% 21%	4364
November	1st to 10th	Night	6% 53% 3 6% 11% 22%	2665
	11th to 20th	Night	13% 54% 4 6% 8% 14%	2730
	21st to end	Night	15% 55% 4 8% 8% 11%	2730
December	1st to 10th	Night	10% 34% 4 14% 18% 20%	4370
	11th to 20th	Night	21% 32% 6% 7% 17% 16%	4941
	21st to end	Night	24% 30% 12% 20% 8% 4	6611

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	13% 5% 18% 37% 22% 4	330
	11th to 20th	Day	10% 3 19% 36% 27% 5%	477
	21st to end	Day	8% 6% 23% 37% 23% 4	594
February	1st to 10th	Day	11% 6% 23% 31% 23% 6%	540
	11th to 20th	Day	12% 7% 25% 36% 19% 2	535
	21st to end	Day	9% 7% 26% 39% 17% 2	446
March	1st to 10th	Day	11% 7% 30% 34% 14% 3	540
	11th to 20th	Day	11% 4 22% 36% 22% 5%	540
	21st to end	Day	7% 7% 29% 37% 17% 3	594
April	1st to 10th	Day	7% 6% 23% 36% 22% 5%	540
	11th to 20th	Day	5% 7% 28% 38% 18% 4	532
	21st to end	Day	5% 6% 26% 33% 25% 6%	540
May	1st to 10th	Day	4 4 27% 36% 23% 5%	543
	11th to 20th	Day	7% 3 21% 39% 25% 4	568
	21st to end	Day	7% 4 22% 39% 24% 4	617
June	1st to 10th	Day	5% 1 16% 35% 37% 6%	570
	11th to 20th	Day	4 2 16% 37% 33% 8%	570
	21st to end	Day	2 2 11% 37% 39% 9%	570
July	1st to 10th	Day	2 1 6% 34% 44% 13%	566
	11th to 20th	Day	2 2 29% 49% 18%	564
	21st to end	Day	2 3 29% 48% 17%	619
August	1st to 10th	Day	2 3 33% 47% 14%	567
	11th to 20th	Day	2 6% 27% 48% 16%	534
	21st to end	Day	2 1 5% 31% 47% 14%	583
September	1st to 10th	Day	2 2 7% 42% 35% 12%	530
	11th to 20th	Day	4 1 15% 39% 33% 7%	530
	21st to end	Day	2 2 18% 36% 32% 9%	530
October	1st to 10th	Day	3 2 16% 39% 31% 8%	530
	11th to 20th	Day	5% 4 15% 39% 30% 7%	530
	21st to end	Day	4 3 16% 36% 29% 11%	513
November	1st to 10th	Day	3 4 15% 32% 34% 13%	330
	11th to 20th	Day	6% 3 14% 40% 30% 8%	330
	21st to end	Day	6% 3 20% 35% 27% 9%	330
December	1st to 10th	Day	7% 3 13% 42% 29% 6%	330
	11th to 20th	Day	6% 2 15% 34% 32% 10%	330
	21st to end	Day	16% 7% 21% 33% 19% 4	363
January	1st to 10th	Night	11% 3 14% 38% 25% 9%	840
	11th to 20th	Night	8% 2 11% 38% 32% 8%	840
	21st to end	Night	11% 2 14% 39% 30% 4	924
February	1st to 10th	Night	9% 2 13% 34% 32% 9%	840
	11th to 20th	Night	13% 3 14% 37% 26% 7%	824
	21st to end	Night	10% 4 13% 41% 27% 4	692
March	1st to 10th	Night	11% 3 16% 39% 25% 5%	840
	11th to 20th	Night	11% 4 12% 33% 31% 10%	840
	21st to end	Night	10% 3 11% 36% 33% 6%	924
April	1st to 10th	Night	11% 4 12% 36% 32% 5%	840
	11th to 20th	Night	9% 4 12% 37% 30% 7%	831
	21st to end	Night	9% 4 11% 33% 35% 7%	840
May	1st to 10th	Night	11% 4 10% 33% 34% 7%	748
	11th to 20th	Night	12% 4 13% 29% 34% 9%	621
	21st to end	Night	10% 5% 11% 31% 37% 7%	683
June	1st to 10th	Night	10% 3 12% 33% 34% 9%	630
	11th to 20th	Night	8% 1 7% 29% 47% 8%	630
	21st to end	Night	8% 1 6% 23% 52% 11%	458
July	1st to 10th	Night	6% 1 3 15% 52% 23%	466
	11th to 20th	Night	3 3 13% 54% 26%	612
	21st to end	Night	4 2 13% 52% 28%	862
August	1st to 10th	Night	4 1 2 15% 51% 27%	836
	11th to 20th	Night	4 1 3 11% 50% 30%	808
	21st to end	Night	4 1 2 12% 53% 29%	880
September	1st to 10th	Night	5% 1 5% 13% 52% 23%	800
	11th to 20th	Night	6% 2 5% 20% 52% 16%	800
	21st to end	Night	4 1 4 28% 47% 16%	800
October	1st to 10th	Night	4 2 5% 35% 46% 9%	800
	11th to 20th	Night	8% 2 6% 32% 38% 14%	800
	21st to end	Night	5% 2 8% 30% 39% 15%	880
November	1st to 10th	Night	5% 2 8% 33% 39% 13%	820
	11th to 20th	Night	7% 1 7% 35% 36% 13%	840
	21st to end	Night	8% 1 10% 39% 32% 9%	840
December	1st to 10th	Night	9% 2 9% 41% 32% 7%	840
	11th to 20th	Night	12% 2 12% 36% 31% 7%	840
	21st to end	Night	12% 5% 14% 35% 26% 8%	924

Page generated at 2026-08-12 00:37:15. Version ac07

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