Santa Margarita Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Margarita Lake clear sky chart from 2005-11-06 to 2026-07-16.

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 2 2 2 2 4 6% 70%	146007
	Night	6% 4 1 4 3 3 3 2 4 6% 65%	144229
Transparency	Day	17% 4 6% 18% 29% 26%	141040
	Night	21% 4 7% 17% 25% 25%	140176
Seeing	Day	8% 9% 24% 34% 19% 5%	17798
	Night	8% 4 13% 34% 33% 8%	27368

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	11% 6% 1 4 3 3 4 3 5% 7% 55%	5022
	11th to 20th	Day	7% 4 1 3 2 2 3 3 4 6% 64%	5340
	21st to end	Day	9% 5% 2 6% 3 4 4 4 5% 8% 51%	5843
February	1st to 10th	Day	15% 6% 2 4 3 2 3 3 4 5% 53%	1979
	11th to 20th	Day	11% 7% 2 5% 4 4 3 3 5% 8% 50%	2050
	21st to end	Day	12% 6% 2 4 4 4 3 3 6% 6% 50%	1730
March	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 8% 43%	4741
	11th to 20th	Day	10% 6% 2 5% 3 3 3 2 4 5% 57%	5110
	21st to end	Day	9% 6% 2 6% 4 5% 6% 5% 6% 9% 42%	5870
April	1st to 10th	Day	9% 5% 2 5% 3 3 3 2 5% 7% 56%	2520
	11th to 20th	Day	6% 5% 2 4 3 2 4 4 5% 7% 59%	2489
	21st to end	Day	6% 4 1 3 3 3 3 3 5% 6% 63%	2520
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 77%	7058
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 6% 79%	7290
	21st to end	Day	2 2 1 3 2 2 2 2 3 6% 77%	8985
June	1st to 10th	Day	3 2 1 3 2 2 2 3 5% 8% 71%	2940
	11th to 20th	Day	1 1 1 1 1 1 2 2 4 84%	2940
	21st to end	Day	1 1 1 1 1 1 2 2 3 86%	2940
July	1st to 10th	Day	1 1 1 1 2 2 93%	6459
	11th to 20th	Day	1 1 1 1 2 1 1 2 3 86%	6001
	21st to end	Day	1 1 1 2 2 3 5% 85%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 2 4 86%	5503
	11th to 20th	Day	1 1 1 1 2 94%	5364
	21st to end	Day	1 1 1 2 1 2 3 88%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 4 85%	2400
	11th to 20th	Day	2 2 1 2 2 1 2 1 2 4 82%	2390
	21st to end	Day	2 2 1 2 2 1 2 2 4 5% 77%	2200
October	1st to 10th	Day	2 2 1 2 2 2 3 2 4 4 78%	3295
	11th to 20th	Day	2 2 1 3 2 2 2 2 2 4 78%	3261
	21st to end	Day	3 3 1 4 3 3 3 3 3 7% 67%	3558
November	1st to 10th	Day	5% 3 1 4 2 3 3 3 4 6% 66%	1966
	11th to 20th	Day	8% 4 1 3 3 3 3 3 4 5% 63%	1890
	21st to end	Day	8% 4 1 4 3 3 4 4 3 7% 58%	1890
December	1st to 10th	Day	5% 4 2 6% 3 4 4 3 5% 7% 57%	2877
	11th to 20th	Day	16% 6% 1 4 3 3 4 3 5% 6% 48%	3108
	21st to end	Day	14% 7% 2 5% 4 3 4 4 6% 11% 40%	4092
January	1st to 10th	Night	13% 8% 2 5% 3 3 3 2 4 5% 52%	7871
	11th to 20th	Night	6% 5% 2 4 2 3 3 2 4 6% 64%	7650
	21st to end	Night	11% 8% 2 6% 4 3 4 3 4 6% 48%	8371
February	1st to 10th	Night	12% 6% 2 4 3 3 3 2 5% 5% 54%	2638
	11th to 20th	Night	11% 5% 1 4 3 3 4 3 5% 7% 54%	2458
	21st to end	Night	9% 8% 2 5% 4 3 3 3 4 5% 55%	2076
March	1st to 10th	Night	10% 11% 3 6% 4 5% 4 4 5% 7% 40%	5369
	11th to 20th	Night	7% 7% 2 5% 3 3 3 3 4 6% 56%	5110
	21st to end	Night	12% 9% 3 5% 3 4 3 3 5% 7% 46%	5346
April	1st to 10th	Night	6% 6% 2 4 3 3 2 3 5% 6% 59%	2100
	11th to 20th	Night	5% 5% 1 4 3 3 3 2 3 6% 64%	2079
	21st to end	Night	5% 6% 2 5% 3 2 3 3 4 5% 63%	2100
May	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 76%	5919
	11th to 20th	Night	1 1 1 6% 3 3 2 2 3 7% 72%	5598
	21st to end	Night	1 3 2 5% 2 2 2 2 3 6% 73%	5183
June	1st to 10th	Night	1 2 1 3 3 2 3 4 4 7% 70%	1680
	11th to 20th	Night	1 2 1 2 1 1 2 2 4 6% 79%	1680
	21st to end	Night	1 1 2 2 1 1 1 2 4 84%	1680
July	1st to 10th	Night	1 1 1 1 1 1 4 91%	3720
	11th to 20th	Night	1 1 1 1 1 2 1 2 6% 84%	4069
	21st to end	Night	1 3 3 2 2 2 4 8% 74%	4472
August	1st to 10th	Night	1 1 1 1 1 1 3 3 86%	4322
	11th to 20th	Night	1 1 1 1 1 3 92%	4506
	21st to end	Night	1 1 1 2 2 2 3 87%	5015
September	1st to 10th	Night	1 2 1 2 2 1 2 2 3 3 83%	2000
	11th to 20th	Night	2 3 1 3 2 1 2 2 2 4 79%	2010
	21st to end	Night	1 2 1 2 1 2 1 2 3 3 81%	2200
October	1st to 10th	Night	2 2 1 2 2 1 2 2 3 4 80%	3901
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 4 78%	3897
	21st to end	Night	3 3 1 3 2 2 2 2 3 5% 72%	4253
November	1st to 10th	Night	4 4 1 3 2 3 3 2 4 6% 68%	2549
	11th to 20th	Night	4 4 2 4 2 3 3 3 4 6% 64%	2730
	21st to end	Night	6% 5% 2 4 3 4 4 4 5% 6% 58%	2730
December	1st to 10th	Night	6% 4 2 4 3 3 5% 3 5% 6% 60%	4525
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 7% 45%	5316
	21st to end	Night	11% 7% 3 8% 5% 5% 6% 4 7% 9% 34%	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	28% 5% 10% 28% 18% 12%	5023
	11th to 20th	Day	21% 3 6% 11% 19% 39%	5340
	21st to end	Day	30% 5% 7% 15% 27% 16%	5843
February	1st to 10th	Day	33% 3 6% 18% 22% 18%	1979
	11th to 20th	Day	33% 4 8% 22% 24% 9%	2060
	21st to end	Day	33% 4 9% 21% 25% 8%	1730
March	1st to 10th	Day	36% 6% 12% 16% 25% 6%	4741
	11th to 20th	Day	29% 4 10% 31% 17% 9%	5110
	21st to end	Day	32% 8% 14% 27% 16% 3	5870
April	1st to 10th	Day	28% 4 11% 25% 23% 8%	2520
	11th to 20th	Day	24% 5% 11% 27% 26% 7%	2492
	21st to end	Day	21% 6% 10% 27% 28% 8%	2520
May	1st to 10th	Day	12% 2 7% 28% 37% 14%	7058
	11th to 20th	Day	7% 6% 5% 23% 45% 14%	7293
	21st to end	Day	11% 4 6% 27% 43% 9%	8987
June	1st to 10th	Day	13% 5% 8% 25% 32% 16%	2940
	11th to 20th	Day	7% 3 5% 14% 41% 30%	2940
	21st to end	Day	7% 2 2 11% 34% 44%	2940
July	1st to 10th	Day	3 1 1 5% 23% 68%	6391
	11th to 20th	Day	7% 2 2 5% 21% 63%	5930
	21st to end	Day	5% 2 2 15% 36% 40%	6217
August	1st to 10th	Day	8% 2 1 10% 35% 44%	3998
	11th to 20th	Day	3 1 1 7% 33% 55%	3880
	21st to end	Day	4 2 1 11% 29% 52%	4311
September	1st to 10th	Day	7% 2 2 12% 32% 45%	2400
	11th to 20th	Day	9% 3 2 20% 41% 25%	2390
	21st to end	Day	11% 3 3 18% 35% 31%	2200
October	1st to 10th	Day	11% 3 4 14% 23% 46%	3295
	11th to 20th	Day	13% 3 4 19% 39% 23%	3261
	21st to end	Day	19% 3 4 14% 33% 28%	3558
November	1st to 10th	Day	19% 4 6% 20% 29% 21%	1966
	11th to 20th	Day	23% 4 7% 21% 25% 20%	1890
	21st to end	Day	27% 4 6% 16% 25% 22%	1890
December	1st to 10th	Day	27% 3 6% 13% 28% 23%	2877
	11th to 20th	Day	36% 3 9% 17% 22% 11%	3108
	21st to end	Day	36% 6% 13% 27% 12% 6%	4092
January	1st to 10th	Night	32% 5% 11% 22% 20% 11%	7871
	11th to 20th	Night	22% 3 6% 13% 16% 39%	7650
	21st to end	Night	36% 4 7% 14% 23% 17%	8371
February	1st to 10th	Night	31% 4 7% 17% 24% 17%	2638
	11th to 20th	Night	28% 5% 7% 22% 24% 13%	2459
	21st to end	Night	32% 4 7% 22% 25% 10%	2076
March	1st to 10th	Night	39% 7% 10% 17% 22% 6%	5369
	11th to 20th	Night	28% 5% 11% 26% 21% 9%	5110
	21st to end	Night	35% 5% 14% 23% 18% 4	5346
April	1st to 10th	Night	26% 4 12% 26% 21% 10%	2100
	11th to 20th	Night	23% 4 9% 28% 27% 10%	2079
	21st to end	Night	22% 5% 9% 26% 30% 8%	2100
May	1st to 10th	Night	13% 3 10% 24% 36% 14%	5917
	11th to 20th	Night	11% 10% 5% 20% 36% 18%	5599
	21st to end	Night	12% 8% 5% 21% 39% 14%	5184
June	1st to 10th	Night	13% 7% 10% 20% 32% 18%	1680
	11th to 20th	Night	10% 3 3 15% 34% 33%	1680
	21st to end	Night	8% 3 2 9% 31% 47%	1680
July	1st to 10th	Night	3 2 4 19% 71%	3680
	11th to 20th	Night	6% 2 2 5% 18% 68%	4017
	21st to end	Night	11% 2 2 14% 32% 39%	4345
August	1st to 10th	Night	8% 2 1 9% 30% 50%	3144
	11th to 20th	Night	3 2 4 25% 65%	3256
	21st to end	Night	5% 2 2 8% 27% 56%	3608
September	1st to 10th	Night	8% 4 2 8% 26% 52%	2000
	11th to 20th	Night	11% 4 3 13% 35% 33%	2010
	21st to end	Night	11% 2 3 16% 36% 33%	2200
October	1st to 10th	Night	10% 2 3 13% 21% 52%	3901
	11th to 20th	Night	13% 3 3 20% 33% 28%	3897
	21st to end	Night	15% 3 4 11% 34% 33%	4253
November	1st to 10th	Night	18% 3 5% 20% 27% 26%	2549
	11th to 20th	Night	21% 3 9% 18% 26% 22%	2730
	21st to end	Night	26% 4 8% 17% 23% 22%	2730
December	1st to 10th	Night	25% 3 5% 13% 30% 24%	4525
	11th to 20th	Night	37% 5% 10% 16% 19% 13%	5316
	21st to end	Night	41% 8% 12% 21% 13% 5%	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	24% 16% 31% 20% 9% 1	325
	11th to 20th	Day	14% 13% 31% 28% 13% 1	430
	21st to end	Day	13% 14% 31% 26% 15% 1	473
February	1st to 10th	Day	19% 13% 29% 25% 12% 2	477
	11th to 20th	Day	19% 16% 32% 23% 10% 1	535
	21st to end	Day	11% 18% 35% 26% 8% 2	446
March	1st to 10th	Day	18% 18% 35% 24% 6%	540
	11th to 20th	Day	17% 15% 31% 27% 9% 1	540
	21st to end	Day	16% 18% 30% 26% 8% 1	594
April	1st to 10th	Day	14% 13% 32% 28% 11% 2	540
	11th to 20th	Day	12% 13% 32% 28% 13% 2	532
	21st to end	Day	7% 13% 35% 28% 12% 4	540
May	1st to 10th	Day	10% 10% 33% 30% 13% 5%	543
	11th to 20th	Day	4 12% 28% 36% 16% 4	568
	21st to end	Day	7% 10% 23% 39% 17% 4	617
June	1st to 10th	Day	3 6% 20% 42% 22% 6%	570
	11th to 20th	Day	2 5% 26% 38% 24% 5%	570
	21st to end	Day	3 20% 40% 26% 11%	570
July	1st to 10th	Day	1 1 13% 46% 28% 11%	566
	11th to 20th	Day	2 6% 44% 34% 13%	545
	21st to end	Day	1 9% 44% 33% 14%	575
August	1st to 10th	Day	2 1 14% 42% 29% 11%	527
	11th to 20th	Day	2 1 9% 43% 33% 13%	517
	21st to end	Day	1 1 10% 44% 33% 11%	583
September	1st to 10th	Day	2 2 14% 45% 28% 8%	530
	11th to 20th	Day	4 5% 22% 42% 21% 5%	530
	21st to end	Day	2 8% 22% 39% 26% 4	530
October	1st to 10th	Day	1 9% 23% 42% 20% 4	530
	11th to 20th	Day	3 8% 27% 36% 21% 4	530
	21st to end	Day	5% 7% 27% 37% 22% 4	583
November	1st to 10th	Day	7% 9% 27% 31% 22% 4	442
	11th to 20th	Day	15% 12% 28% 29% 14% 3	330
	21st to end	Day	10% 10% 28% 30% 18% 4	330
December	1st to 10th	Day	10% 13% 29% 32% 14% 2	278
	11th to 20th	Day	13% 14% 23% 29% 18% 3	220
	21st to end	Day	21% 20% 29% 19% 8% 3	242
January	1st to 10th	Night	16% 7% 20% 32% 18% 6%	840
	11th to 20th	Night	10% 4 19% 42% 20% 5%	840
	21st to end	Night	15% 6% 19% 38% 19% 4	924
February	1st to 10th	Night	16% 7% 15% 37% 20% 5%	840
	11th to 20th	Night	13% 8% 19% 34% 20% 5%	824
	21st to end	Night	13% 7% 21% 42% 15% 3	692
March	1st to 10th	Night	14% 6% 20% 40% 17% 3	840
	11th to 20th	Night	11% 5% 19% 36% 23% 6%	840
	21st to end	Night	13% 8% 18% 35% 24% 3	924
April	1st to 10th	Night	9% 6% 19% 38% 25% 3	840
	11th to 20th	Night	8% 7% 19% 33% 27% 6%	831
	21st to end	Night	6% 6% 22% 33% 26% 8%	840
May	1st to 10th	Night	5% 4 14% 37% 32% 8%	840
	11th to 20th	Night	4 4 12% 37% 36% 8%	730
	21st to end	Night	3 4 10% 40% 34% 9%	457
June	1st to 10th	Night	3 2 6% 37% 44% 8%	420
	11th to 20th	Night	1 2 10% 33% 45% 9%	420
	21st to end	Night	1 1 3 31% 51% 13%	420
July	1st to 10th	Night	1 20% 64% 14%	410
	11th to 20th	Night	2 1 1 10% 58% 27%	562
	21st to end	Night	1 1 13% 58% 27%	657
August	1st to 10th	Night	1 1 2 25% 55% 17%	721
	11th to 20th	Night	1 1 22% 61% 14%	792
	21st to end	Night	2 22% 60% 15%	880
September	1st to 10th	Night	2 1 3 25% 56% 13%	800
	11th to 20th	Night	3 1 6% 36% 43% 11%	800
	21st to end	Night	3 2 7% 34% 45% 9%	800
October	1st to 10th	Night	4 1 8% 40% 41% 6%	800
	11th to 20th	Night	5% 3 10% 37% 36% 8%	800
	21st to end	Night	5% 2 16% 36% 31% 9%	880
November	1st to 10th	Night	5% 5% 14% 37% 31% 7%	820
	11th to 20th	Night	6% 5% 19% 38% 26% 6%	840
	21st to end	Night	7% 7% 19% 41% 22% 5%	840
December	1st to 10th	Night	12% 6% 19% 36% 23% 4	840
	11th to 20th	Night	14% 4 20% 37% 21% 4	840
	21st to end	Night	16% 7% 22% 36% 16% 3	924

Page generated at 2026-07-13 00:14:53. Version ac07

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