Santa Monica Canyon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Monica Canyon Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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 3 2 4 6% 67%	140243
	Night	5% 5% 2 4 3 3 3 3 5% 7% 61%	139491
Transparency	Day	12% 37% 4 11% 18% 18%	135333
	Night	14% 37% 5% 11% 16% 16%	135374
Seeing	Day	6% 7% 22% 34% 24% 7%	17557
	Night	6% 3 11% 35% 36% 8%	27459

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% 65%	4722
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 6% 70%	5316
	21st to end	Day	8% 5% 2 5% 3 3 3 3 5% 9% 54%	6416
February	1st to 10th	Day	9% 4 2 3 4 2 2 2 5% 5% 61%	2100
	11th to 20th	Day	9% 3 1 3 3 3 2 3 6% 7% 59%	2050
	21st to end	Day	6% 4 2 5% 3 2 3 3 5% 7% 60%	1730
March	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 5% 7% 54%	4618
	11th to 20th	Day	8% 4 1 3 3 3 3 3 5% 7% 60%	4840
	21st to end	Day	8% 5% 2 4 4 4 4 3 6% 8% 52%	6101
April	1st to 10th	Day	4 4 2 3 3 3 4 3 6% 8% 60%	2520
	11th to 20th	Day	4 4 2 3 3 3 3 2 5% 7% 65%	2442
	21st to end	Day	2 4 2 3 2 2 3 2 5% 8% 67%	2400
May	1st to 10th	Day	2 5% 2 5% 2 3 2 2 4 7% 65%	5517
	11th to 20th	Day	3 9% 3 5% 3 3 2 3 5% 5% 60%	5841
	21st to end	Day	2 6% 3 6% 2 2 3 2 4 7% 63%	6423
June	1st to 10th	Day	4 7% 1 5% 2 2 2 2 5% 7% 61%	2663
	11th to 20th	Day	3 5% 2 3 2 2 2 2 4 8% 69%	2800
	21st to end	Day	2 4 2 3 2 2 3 2 4 6% 71%	2800
July	1st to 10th	Day	1 2 1 2 1 1 2 2 3 5% 80%	6319
	11th to 20th	Day	1 3 2 3 2 2 3 2 4 5% 73%	6280
	21st to end	Day	1 2 1 3 1 1 2 2 4 5% 79%	5950
August	1st to 10th	Day	1 1 1 1 1 1 1 1 3 6% 81%	5346
	11th to 20th	Day	1 2 1 2 1 1 1 1 3 5% 83%	5364
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 79%	5984
September	1st to 10th	Day	2 2 1 2 1 2 2 2 4 6% 76%	2400
	11th to 20th	Day	2 3 1 3 2 2 2 2 3 6% 75%	2272
	21st to end	Day	2 3 1 3 2 3 2 2 4 6% 74%	2200
October	1st to 10th	Day	2 2 1 2 1 2 2 2 3 5% 79%	3600
	11th to 20th	Day	2 3 2 3 1 2 2 2 4 8% 71%	3308
	21st to end	Day	2 3 2 3 2 3 3 3 5% 6% 68%	3533
November	1st to 10th	Day	2 3 1 3 2 3 2 3 5% 6% 70%	1854
	11th to 20th	Day	6% 3 1 3 3 2 3 2 5% 6% 67%	1890
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 9% 61%	1890
December	1st to 10th	Day	4 2 1 4 3 3 4 4 7% 6% 62%	3040
	11th to 20th	Day	10% 4 1 4 3 3 5% 4 5% 8% 53%	3499
	21st to end	Day	10% 5% 2 5% 4 5% 4 4 7% 9% 45%	4215
January	1st to 10th	Night	9% 5% 2 4 2 3 3 2 3 5% 62%	8172
	11th to 20th	Night	4 4 2 4 2 2 2 2 4 6% 68%	7670
	21st to end	Night	9% 5% 2 5% 3 4 4 4 5% 6% 53%	7822
February	1st to 10th	Night	8% 6% 2 4 3 2 3 3 3 5% 61%	2520
	11th to 20th	Night	9% 5% 2 5% 3 2 3 3 4 6% 58%	2458
	21st to end	Night	9% 4 2 4 3 3 3 3 4 6% 60%	2076
March	1st to 10th	Night	7% 6% 2 5% 3 3 4 3 5% 8% 55%	5496
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 6% 57%	5361
	21st to end	Night	9% 6% 2 5% 3 4 4 3 5% 7% 53%	5116
April	1st to 10th	Night	6% 6% 1 5% 3 3 3 3 4 6% 59%	2100
	11th to 20th	Night	4 5% 2 4 3 3 3 2 4 6% 64%	2039
	21st to end	Night	4 6% 3 5% 3 3 4 3 5% 7% 59%	2000
May	1st to 10th	Night	3 10% 3 6% 3 3 3 3 4 5% 56%	4478
	11th to 20th	Night	2 9% 5% 7% 3 3 3 3 5% 7% 54%	4074
	21st to end	Night	1 7% 3 5% 3 3 3 2 5% 6% 61%	4474
June	1st to 10th	Night	2 9% 3 5% 3 4 4 4 5% 8% 52%	1730
	11th to 20th	Night	2 7% 2 4 2 2 3 4 5% 8% 60%	1600
	21st to end	Night	1 7% 2 5% 3 3 3 3 8% 9% 57%	1600
July	1st to 10th	Night	1 2 2 6% 4 3 3 2 5% 8% 65%	3640
	11th to 20th	Night	1 3 3 7% 4 4 5% 3 6% 7% 56%	3677
	21st to end	Night	1 3 2 4 3 3 3 2 4 6% 67%	4943
August	1st to 10th	Night	1 2 1 3 2 3 3 3 5% 8% 70%	4486
	11th to 20th	Night	1 5% 2 5% 3 3 2 2 5% 7% 66%	4506
	21st to end	Night	1 2 1 4 3 3 3 4 5% 7% 67%	5015
September	1st to 10th	Night	3 4 2 3 2 3 3 2 4 7% 67%	2000
	11th to 20th	Night	3 3 2 3 2 2 3 2 5% 5% 70%	2126
	21st to end	Night	1 3 1 4 2 2 3 3 5% 6% 70%	2200
October	1st to 10th	Night	2 2 1 2 2 2 3 2 7% 8% 68%	3600
	11th to 20th	Night	2 9% 3 5% 3 2 3 3 6% 7% 57%	3833
	21st to end	Night	2 3 2 3 2 2 2 2 4 6% 72%	4274
November	1st to 10th	Night	2 3 1 3 2 3 3 2 5% 6% 69%	2665
	11th to 20th	Night	4 3 2 3 3 2 3 2 4 6% 67%	2730
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 8% 60%	2730
December	1st to 10th	Night	4 3 2 5% 3 3 4 3 4 6% 64%	4370
	11th to 20th	Night	10% 4 1 4 3 3 4 4 6% 8% 52%	4941
	21st to end	Night	10% 5% 2 5% 4 5% 6% 5% 8% 8% 44%	6969

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	17% 21% 5% 13% 24% 19%	4723
	11th to 20th	Day	13% 22% 7% 10% 13% 36%	5316
	21st to end	Day	22% 23% 4 13% 18% 20%	6416
February	1st to 10th	Day	12% 59% 4 6% 11% 9%	2100
	11th to 20th	Day	12% 58% 3 8% 10% 11%	2060
	21st to end	Day	9% 59% 4 10% 11% 9%	1730
March	1st to 10th	Day	19% 32% 7% 14% 16% 12%	4618
	11th to 20th	Day	16% 30% 9% 20% 14% 10%	4840
	21st to end	Day	20% 32% 10% 21% 13% 4	6101
April	1st to 10th	Day	8% 60% 4 9% 12% 6%	2520
	11th to 20th	Day	8% 61% 3 8% 13% 8%	2445
	21st to end	Day	6% 63% 5% 12% 10% 4	2400
May	1st to 10th	Day	15% 31% 7% 15% 24% 8%	5517
	11th to 20th	Day	20% 31% 7% 14% 22% 6%	5844
	21st to end	Day	18% 31% 7% 20% 18% 6%	6424
June	1st to 10th	Day	10% 62% 3 10% 11% 4	2663
	11th to 20th	Day	7% 61% 2 7% 12% 11%	2800
	21st to end	Day	8% 59% 1 5% 12% 14%	2800
July	1st to 10th	Day	5% 25% 1 4 16% 49%	6251
	11th to 20th	Day	11% 26% 3 6% 28% 27%	6205
	21st to end	Day	6% 24% 2 11% 29% 27%	5779
August	1st to 10th	Day	5% 35% 1 7% 30% 22%	3890
	11th to 20th	Day	3 35% 1 5% 21% 35%	3880
	21st to end	Day	5% 35% 2 5% 18% 34%	4311
September	1st to 10th	Day	6% 56% 2 5% 15% 15%	2400
	11th to 20th	Day	8% 57% 2 6% 15% 12%	2272
	21st to end	Day	6% 57% 3 8% 16% 11%	2200
October	1st to 10th	Day	4 35% 1 6% 11% 44%	3600
	11th to 20th	Day	8% 36% 3 11% 26% 17%	3308
	21st to end	Day	12% 36% 2 6% 21% 24%	3533
November	1st to 10th	Day	4 53% 2 7% 13% 21%	1854
	11th to 20th	Day	12% 54% 4 6% 8% 16%	1890
	21st to end	Day	11% 54% 5% 7% 10% 13%	1890
December	1st to 10th	Day	11% 35% 4 11% 15% 24%	3040
	11th to 20th	Day	19% 32% 4 10% 20% 15%	3498
	21st to end	Day	25% 29% 10% 16% 16% 5%	4215
January	1st to 10th	Night	21% 23% 5% 10% 23% 19%	8172
	11th to 20th	Night	15% 23% 5% 11% 12% 34%	7670
	21st to end	Night	24% 26% 5% 10% 16% 20%	7822
February	1st to 10th	Night	13% 60% 3 7% 9% 9%	2520
	11th to 20th	Night	13% 58% 3 6% 9% 11%	2459
	21st to end	Night	10% 59% 3 8% 11% 10%	2076
March	1st to 10th	Night	17% 33% 7% 12% 20% 10%	5496
	11th to 20th	Night	18% 31% 9% 21% 11% 11%	5361
	21st to end	Night	19% 33% 12% 17% 15% 3	5116
April	1st to 10th	Night	11% 59% 3 10% 11% 6%	2100
	11th to 20th	Night	8% 61% 3 9% 13% 7%	2039
	21st to end	Night	8% 61% 6% 11% 9% 5%	2000
May	1st to 10th	Night	20% 32% 7% 13% 22% 6%	4475
	11th to 20th	Night	15% 34% 9% 17% 21% 5%	4075
	21st to end	Night	13% 33% 10% 21% 19% 5%	4474
June	1st to 10th	Night	11% 62% 6% 8% 8% 4	1730
	11th to 20th	Night	7% 61% 3 7% 11% 10%	1600
	21st to end	Night	10% 60% 3 5% 12% 10%	1600
July	1st to 10th	Night	12% 25% 3 8% 13% 39%	3600
	11th to 20th	Night	18% 27% 5% 8% 21% 22%	3629
	21st to end	Night	11% 26% 3 14% 26% 20%	4802
August	1st to 10th	Night	8% 35% 3 12% 26% 17%	3256
	11th to 20th	Night	7% 35% 2 7% 23% 25%	3256
	21st to end	Night	9% 37% 2 4 26% 23%	3608
September	1st to 10th	Night	9% 57% 2 5% 16% 13%	2000
	11th to 20th	Night	9% 57% 3 6% 14% 12%	2126
	21st to end	Night	6% 57% 3 7% 16% 11%	2200
October	1st to 10th	Night	5% 35% 2 7% 15% 36%	3600
	11th to 20th	Night	18% 38% 4 8% 18% 15%	3833
	21st to end	Night	9% 35% 2 7% 20% 27%	4274
November	1st to 10th	Night	4 53% 3 7% 11% 21%	2665
	11th to 20th	Night	11% 55% 3 7% 9% 15%	2730
	21st to end	Night	11% 55% 5% 9% 8% 12%	2730
December	1st to 10th	Night	13% 35% 3 9% 17% 23%	4370
	11th to 20th	Night	19% 33% 6% 9% 15% 19%	4941
	21st to end	Night	24% 32% 10% 18% 10% 5%	6969

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	17% 7% 26% 31% 15% 2	220
	11th to 20th	Day	10% 9% 33% 30% 16% 2	419
	21st to end	Day	10% 12% 32% 30% 14% 2	578
February	1st to 10th	Day	11% 9% 32% 29% 15% 3	540
	11th to 20th	Day	12% 14% 32% 31% 9% 2	535
	21st to end	Day	7% 14% 34% 34% 11% 1	446
March	1st to 10th	Day	12% 17% 32% 25% 13% 1	540
	11th to 20th	Day	10% 13% 31% 30% 14% 3	540
	21st to end	Day	9% 13% 34% 29% 12% 3	594
April	1st to 10th	Day	7% 11% 29% 31% 17% 5%	540
	11th to 20th	Day	5% 13% 35% 31% 11% 4	517
	21st to end	Day	3 12% 30% 30% 16% 8%	500
May	1st to 10th	Day	6% 11% 29% 35% 15% 4	503
	11th to 20th	Day	6% 7% 26% 35% 22% 4	528
	21st to end	Day	6% 5% 28% 37% 19% 5%	573
June	1st to 10th	Day	7% 3 17% 38% 29% 7%	530
	11th to 20th	Day	4 3 17% 39% 28% 8%	530
	21st to end	Day	4 2 13% 39% 31% 11%	530
July	1st to 10th	Day	3 1 6% 38% 40% 13%	526
	11th to 20th	Day	2 3 31% 45% 19%	524
	21st to end	Day	2 4 34% 42% 18%	575
August	1st to 10th	Day	2 1 7% 38% 36% 16%	527
	11th to 20th	Day	3 5% 33% 44% 15%	517
	21st to end	Day	1 1 9% 39% 37% 14%	583
September	1st to 10th	Day	4 1 12% 42% 29% 14%	530
	11th to 20th	Day	3 3 18% 41% 27% 8%	530
	21st to end	Day	2 3 21% 38% 28% 8%	530
October	1st to 10th	Day	2 5% 19% 41% 25% 8%	530
	11th to 20th	Day	3 5% 24% 39% 23% 5%	530
	21st to end	Day	4 6% 24% 33% 25% 7%	570
November	1st to 10th	Day	4 5% 18% 31% 32% 9%	330
	11th to 20th	Day	8% 7% 21% 37% 22% 5%	330
	21st to end	Day	7% 6% 23% 31% 29% 5%	330
December	1st to 10th	Day	7% 5% 22% 35% 26% 4	330
	11th to 20th	Day	6% 6% 17% 37% 28% 6%	330
	21st to end	Day	17% 12% 23% 34% 13% 1	272
January	1st to 10th	Night	12% 5% 14% 43% 21% 5%	840
	11th to 20th	Night	7% 3 19% 44% 23% 4	840
	21st to end	Night	11% 4 19% 41% 22% 2	924
February	1st to 10th	Night	11% 4 15% 40% 25% 6%	840
	11th to 20th	Night	10% 5% 18% 39% 22% 6%	824
	21st to end	Night	10% 5% 20% 42% 21% 3	692
March	1st to 10th	Night	9% 6% 18% 43% 21% 3	840
	11th to 20th	Night	8% 5% 15% 39% 28% 6%	840
	21st to end	Night	8% 5% 17% 43% 24% 4	924
April	1st to 10th	Night	8% 4 15% 41% 28% 3	840
	11th to 20th	Night	6% 4 16% 41% 30% 3	815
	21st to end	Night	5% 6% 15% 38% 30% 7%	800
May	1st to 10th	Night	7% 2 15% 39% 32% 5%	770
	11th to 20th	Night	4 2 16% 38% 36% 4	591
	21st to end	Night	4 1 16% 40% 36% 4	650
June	1st to 10th	Night	3 2 11% 37% 42% 5%	530
	11th to 20th	Night	3 2 3 33% 54% 6%	400
	21st to end	Night	5% 3 27% 59% 7%	400
July	1st to 10th	Night	2 2 14% 71% 11%	390
	11th to 20th	Night	1 1 10% 71% 17%	399
	21st to end	Night	3 1 1 17% 60% 19%	858
August	1st to 10th	Night	4 1 21% 59% 15%	796
	11th to 20th	Night	4 1 16% 62% 18%	792
	21st to end	Night	2 1 1 19% 59% 18%	880
September	1st to 10th	Night	5% 1 3 23% 52% 16%	800
	11th to 20th	Night	4 1 3 30% 50% 12%	800
	21st to end	Night	2 1 6% 32% 49% 10%	800
October	1st to 10th	Night	4 1 7% 40% 42% 6%	800
	11th to 20th	Night	6% 2 10% 35% 39% 10%	800
	21st to end	Night	4 2 11% 35% 35% 12%	880
November	1st to 10th	Night	2 4 11% 38% 35% 10%	820
	11th to 20th	Night	7% 3 10% 39% 32% 9%	840
	21st to end	Night	7% 2 14% 42% 28% 6%	840
December	1st to 10th	Night	7% 4 17% 41% 26% 6%	840
	11th to 20th	Night	11% 5% 13% 43% 24% 4	840
	21st to end	Night	11% 5% 18% 43% 18% 5%	924

Page generated at 2026-04-13 23:44:23. Version ac07

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