Saddleback Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saddleback Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	7% 8% 2 3 2 2 2 2 3 5% 63%	135631
	Night	8% 9% 2 3 2 2 3 2 4 5% 58%	132140
Transparency	Day	26% 3 6% 18% 26% 21%	130564
	Night	31% 3 7% 18% 24% 18%	128191
Seeing	Day	17% 18% 31% 21% 10% 3	16419
	Night	17% 10% 18% 31% 22% 3	25671

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	12% 17% 2 4 3 3 3 3 4 6% 43%	3639
	11th to 20th	Day	8% 13% 2 5% 3 3 3 2 2 4 54%	3848
	21st to end	Day	13% 13% 3 5% 3 3 3 2 3 5% 48%	4510
February	1st to 10th	Day	17% 14% 3 4 2 3 2 3 4 6% 43%	1800
	11th to 20th	Day	12% 17% 2 5% 3 3 2 3 4 5% 43%	1880
	21st to end	Day	10% 19% 3 5% 2 3 3 2 4 5% 44%	1650
March	1st to 10th	Day	10% 27% 7% 6% 3 3 3 2 4 4 31%	4598
	11th to 20th	Day	11% 19% 3 4 2 2 2 3 3 5% 46%	5000
	21st to end	Day	13% 26% 3 4 3 3 3 3 4 5% 32%	5739
April	1st to 10th	Day	12% 16% 2 4 3 3 4 3 5% 6% 40%	2400
	11th to 20th	Day	9% 13% 2 4 4 3 3 3 5% 6% 48%	2369
	21st to end	Day	10% 10% 1 3 3 3 3 3 6% 7% 52%	2400
May	1st to 10th	Day	5% 6% 2 3 2 2 2 2 3 5% 68%	5713
	11th to 20th	Day	5% 5% 1 3 2 2 2 2 3 4 69%	6277
	21st to end	Day	5% 3 1 2 2 2 2 2 3 4 74%	6902
June	1st to 10th	Day	7% 4 2 4 3 3 3 3 5% 6% 59%	2800
	11th to 20th	Day	3 3 1 2 2 2 2 2 3 5% 75%	2800
	21st to end	Day	3 1 1 2 1 1 1 1 3 4 82%	2800
July	1st to 10th	Day	1 1 1 2 2 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6320
	21st to end	Day	2 1 1 2 1 2 2 2 3 4 81%	6586
August	1st to 10th	Day	2 1 1 2 1 1 2 2 3 3 82%	5777
	11th to 20th	Day	1 1 1 1 1 2 3 90%	5455
	21st to end	Day	2 1 1 2 1 2 2 2 3 3 81%	5984
September	1st to 10th	Day	2 1 1 1 2 2 1 2 3 84%	2400
	11th to 20th	Day	5% 3 1 3 2 2 3 2 3 5% 72%	2400
	21st to end	Day	5% 3 1 2 1 2 2 2 3 4 75%	2165
October	1st to 10th	Day	5% 3 1 3 1 1 2 3 4 5% 72%	3280
	11th to 20th	Day	9% 4 1 3 2 2 3 2 3 6% 66%	3261
	21st to end	Day	6% 7% 1 5% 3 4 3 2 4 6% 58%	3545
November	1st to 10th	Day	12% 9% 1 3 3 2 3 3 4 6% 52%	1854
	11th to 20th	Day	13% 7% 2 4 4 4 4 3 5% 7% 47%	1863
	21st to end	Day	11% 11% 2 4 3 3 4 3 5% 5% 50%	1645
December	1st to 10th	Day	10% 5% 1 4 3 2 4 4 5% 6% 57%	2620
	11th to 20th	Day	21% 15% 3 5% 3 3 4 3 5% 7% 31%	3028
	21st to end	Day	21% 13% 2 4 3 4 4 3 5% 8% 33%	4004
January	1st to 10th	Night	8% 14% 4 5% 3 3 4 4 6% 7% 42%	6306
	11th to 20th	Night	10% 12% 2 4 2 3 3 3 4 5% 53%	6117
	21st to end	Night	9% 15% 3 5% 3 4 4 3 4 5% 45%	6468
February	1st to 10th	Night	16% 10% 3 4 3 2 3 3 5% 6% 46%	2600
	11th to 20th	Night	10% 15% 2 4 3 4 4 4 5% 6% 43%	2407
	21st to end	Night	13% 14% 3 5% 2 2 3 3 4 5% 44%	1980
March	1st to 10th	Night	10% 25% 8% 6% 3 3 3 2 4 5% 32%	5293
	11th to 20th	Night	12% 22% 3 3 2 2 2 2 3 4 45%	5000
	21st to end	Night	12% 25% 4 5% 3 3 4 3 4 5% 30%	5236
April	1st to 10th	Night	11% 14% 2 6% 4 3 3 3 5% 5% 44%	2000
	11th to 20th	Night	10% 11% 2 4 3 3 3 3 5% 7% 49%	1979
	21st to end	Night	9% 9% 1 3 3 2 2 3 4 7% 57%	2000
May	1st to 10th	Night	4 8% 2 3 2 2 2 2 3 4 67%	4255
	11th to 20th	Night	4 4 2 3 2 3 3 2 3 4 69%	3616
	21st to end	Night	4 3 1 3 2 2 2 2 2 3 76%	3972
June	1st to 10th	Night	6% 5% 1 3 3 3 2 3 4 7% 62%	1600
	11th to 20th	Night	3 3 1 2 2 2 2 2 4 5% 75%	1600
	21st to end	Night	3 2 1 2 2 2 2 2 3 3 79%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 91%	3640
	11th to 20th	Night	2 1 2 1 1 1 1 2 2 87%	3640
	21st to end	Night	2 1 1 2 2 2 2 3 4 4 77%	4276
August	1st to 10th	Night	3 2 1 2 1 2 2 2 3 3 79%	4035
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	4408
	21st to end	Night	2 2 1 2 2 2 2 2 2 3 80%	5015
September	1st to 10th	Night	3 1 2 1 1 2 2 2 4 82%	2000
	11th to 20th	Night	4 2 1 3 2 2 2 2 3 5% 75%	2000
	21st to end	Night	4 2 1 2 2 2 2 2 3 5% 76%	2230
October	1st to 10th	Night	4 3 2 4 2 1 2 1 2 3 77%	3920
	11th to 20th	Night	4 4 1 2 2 2 2 2 4 4 73%	3897
	21st to end	Night	10% 5% 1 3 2 2 3 3 5% 6% 61%	4253
November	1st to 10th	Night	12% 9% 2 4 2 2 3 3 5% 6% 52%	2665
	11th to 20th	Night	11% 7% 2 3 3 3 4 3 6% 8% 50%	2691
	21st to end	Night	11% 9% 2 5% 2 3 3 2 4 7% 53%	2753
December	1st to 10th	Night	10% 6% 1 3 2 2 3 2 4 6% 61%	4560
	11th to 20th	Night	18% 18% 2 4 3 4 4 4 6% 8% 28%	5176
	21st to end	Night	22% 15% 3 5% 4 4 4 3 4 6% 31%	6952

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	43% 6% 11% 18% 17% 4	3639
	11th to 20th	Day	38% 3 5% 15% 24% 15%	3848
	21st to end	Day	41% 3 6% 14% 20% 16%	4510
February	1st to 10th	Day	43% 3 9% 19% 17% 9%	1800
	11th to 20th	Day	44% 4 8% 20% 20% 5%	1889
	21st to end	Day	44% 3 7% 23% 15% 7%	1650
March	1st to 10th	Day	57% 4 5% 12% 17% 5%	4598
	11th to 20th	Day	43% 3 9% 27% 15% 3	5000
	21st to end	Day	55% 5% 9% 20% 10% 1	5739
April	1st to 10th	Day	44% 4 12% 21% 16% 3	2400
	11th to 20th	Day	37% 5% 12% 24% 17% 6%	2372
	21st to end	Day	32% 4 13% 29% 19% 3	2400
May	1st to 10th	Day	21% 2 9% 24% 34% 10%	5713
	11th to 20th	Day	20% 3 9% 24% 38% 8%	6280
	21st to end	Day	16% 3 6% 32% 38% 6%	6904
June	1st to 10th	Day	26% 4 9% 25% 28% 8%	2800
	11th to 20th	Day	14% 3 7% 22% 36% 18%	2800
	21st to end	Day	10% 1 3 16% 39% 30%	2800
July	1st to 10th	Day	3 1 1 7% 20% 68%	6251
	11th to 20th	Day	6% 2 6% 16% 70%	6245
	21st to end	Day	10% 2 2 15% 30% 41%	6396
August	1st to 10th	Day	13% 1 2 10% 23% 50%	4206
	11th to 20th	Day	5% 1 1 8% 43% 42%	3948
	21st to end	Day	8% 1 2 10% 44% 35%	4311
September	1st to 10th	Day	8% 1 1 12% 33% 44%	2400
	11th to 20th	Day	17% 2 4 22% 35% 20%	2400
	21st to end	Day	15% 3 4 20% 33% 24%	2165
October	1st to 10th	Day	16% 2 4 14% 34% 30%	3280
	11th to 20th	Day	21% 2 6% 17% 41% 13%	3261
	21st to end	Day	29% 2 6% 23% 27% 13%	3545
November	1st to 10th	Day	35% 3 8% 22% 20% 12%	1854
	11th to 20th	Day	37% 4 10% 20% 22% 7%	1863
	21st to end	Day	36% 3 6% 21% 21% 12%	1645
December	1st to 10th	Day	29% 3 7% 20% 27% 14%	2620
	11th to 20th	Day	54% 3 7% 16% 15% 4	3028
	21st to end	Day	50% 5% 11% 19% 13% 3	4004
January	1st to 10th	Night	41% 5% 13% 19% 18% 4	6306
	11th to 20th	Night	35% 4 7% 13% 28% 13%	6117
	21st to end	Night	43% 4 6% 11% 17% 20%	6468
February	1st to 10th	Night	41% 4 7% 19% 20% 9%	2600
	11th to 20th	Night	42% 4 8% 20% 20% 6%	2409
	21st to end	Night	42% 4 8% 22% 17% 7%	1980
March	1st to 10th	Night	54% 3 6% 17% 14% 6%	5293
	11th to 20th	Night	45% 3 7% 22% 21% 3	5000
	21st to end	Night	55% 7% 10% 18% 9% 1	5236
April	1st to 10th	Night	41% 6% 11% 22% 16% 3	2000
	11th to 20th	Night	35% 5% 12% 23% 18% 6%	1979
	21st to end	Night	29% 4 14% 33% 18% 3	2000
May	1st to 10th	Night	22% 4 9% 26% 31% 8%	4252
	11th to 20th	Night	20% 3 8% 26% 35% 8%	3616
	21st to end	Night	16% 3 8% 30% 37% 7%	3972
June	1st to 10th	Night	23% 6% 11% 25% 25% 9%	1600
	11th to 20th	Night	13% 3 8% 23% 36% 18%	1600
	21st to end	Night	14% 2 3 16% 34% 32%	1600
July	1st to 10th	Night	4 1 1 6% 21% 66%	3600
	11th to 20th	Night	8% 1 1 6% 16% 68%	3592
	21st to end	Night	12% 1 3 15% 26% 43%	4154
August	1st to 10th	Night	16% 1 2 8% 23% 50%	2929
	11th to 20th	Night	6% 1 1 10% 39% 42%	3183
	21st to end	Night	8% 1 2 13% 36% 39%	3608
September	1st to 10th	Night	9% 2 2 10% 32% 46%	2000
	11th to 20th	Night	15% 2 5% 20% 37% 21%	2000
	21st to end	Night	14% 3 4 21% 34% 25%	2230
October	1st to 10th	Night	17% 1 4 11% 30% 37%	3920
	11th to 20th	Night	16% 2 5% 22% 42% 13%	3897
	21st to end	Night	24% 2 7% 24% 27% 15%	4253
November	1st to 10th	Night	34% 5% 7% 21% 20% 13%	2665
	11th to 20th	Night	32% 5% 11% 22% 21% 9%	2691
	21st to end	Night	34% 3 7% 22% 22% 12%	2753
December	1st to 10th	Night	26% 3 7% 20% 25% 20%	4560
	11th to 20th	Night	54% 3 6% 13% 20% 3	5176
	21st to end	Night	55% 5% 9% 16% 12% 4	6952

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	46% 13% 24% 14% 4 1	200
	11th to 20th	Day	30% 18% 26% 22% 5%	291
	21st to end	Day	28% 17% 24% 20% 10% 1	440
February	1st to 10th	Day	32% 19% 26% 17% 6% 1	400
	11th to 20th	Day	31% 24% 27% 13% 4	460
	21st to end	Day	26% 22% 30% 17% 4	414
March	1st to 10th	Day	40% 26% 19% 10% 4 1	500
	11th to 20th	Day	34% 21% 21% 15% 8% 1	500
	21st to end	Day	33% 26% 24% 12% 4 1	550
April	1st to 10th	Day	27% 25% 28% 13% 7% 1	500
	11th to 20th	Day	27% 23% 27% 17% 5% 1	492
	21st to end	Day	18% 24% 28% 18% 10% 2	500
May	1st to 10th	Day	18% 27% 30% 17% 5% 2	503
	11th to 20th	Day	20% 20% 30% 20% 7% 4	528
	21st to end	Day	14% 19% 29% 24% 9% 4	573
June	1st to 10th	Day	11% 17% 34% 23% 11% 5%	530
	11th to 20th	Day	9% 21% 39% 19% 9% 3	530
	21st to end	Day	4 15% 36% 27% 12% 5%	530
July	1st to 10th	Day	2 12% 45% 25% 11% 5%	526
	11th to 20th	Day	2 9% 44% 27% 14% 4	524
	21st to end	Day	2 6% 43% 26% 16% 6%	575
August	1st to 10th	Day	4 9% 41% 25% 15% 5%	527
	11th to 20th	Day	3 11% 36% 29% 15% 6%	517
	21st to end	Day	3 11% 40% 27% 16% 2	583
September	1st to 10th	Day	4 13% 39% 26% 16% 2	530
	11th to 20th	Day	10% 15% 33% 28% 13% 2	530
	21st to end	Day	8% 16% 38% 26% 10% 2	530
October	1st to 10th	Day	8% 21% 28% 28% 13% 2	530
	11th to 20th	Day	13% 16% 27% 28% 14% 2	530
	21st to end	Day	18% 17% 25% 25% 13% 2	582
November	1st to 10th	Day	21% 18% 20% 23% 15% 3	330
	11th to 20th	Day	21% 13% 27% 22% 15% 1	321
	21st to end	Day	21% 18% 26% 20% 12% 4	223
December	1st to 10th	Day	26% 17% 23% 15% 15% 5%	200
	11th to 20th	Day	29% 23% 16% 19% 12% 2	200
	21st to end	Day	30% 21% 21% 17% 10% 1	220
January	1st to 10th	Night	28% 12% 23% 24% 11% 4	800
	11th to 20th	Night	23% 14% 27% 26% 9% 1	800
	21st to end	Night	26% 15% 20% 24% 14% 1	880
February	1st to 10th	Night	26% 15% 21% 27% 10% 1	800
	11th to 20th	Night	25% 12% 21% 27% 14% 2	784
	21st to end	Night	25% 13% 23% 30% 7% 2	660
March	1st to 10th	Night	31% 14% 19% 22% 12% 2	800
	11th to 20th	Night	27% 13% 16% 24% 18% 2	800
	21st to end	Night	29% 14% 19% 24% 12% 1	880
April	1st to 10th	Night	22% 13% 21% 29% 15% 1	800
	11th to 20th	Night	19% 11% 22% 30% 14% 3	791
	21st to end	Night	14% 9% 20% 34% 19% 4	800
May	1st to 10th	Night	14% 14% 22% 27% 19% 3	677
	11th to 20th	Night	13% 6% 16% 33% 28% 4	394
	21st to end	Night	13% 6% 16% 31% 30% 4	433
June	1st to 10th	Night	9% 5% 15% 32% 35% 5%	400
	11th to 20th	Night	5% 6% 18% 38% 31% 3	400
	21st to end	Night	4 3 13% 40% 37% 4	400
July	1st to 10th	Night	2 1 8% 42% 45% 2	390
	11th to 20th	Night	1 1 7% 46% 41% 4	388
	21st to end	Night	2 1 5% 35% 50% 8%	565
August	1st to 10th	Night	4 1 6% 39% 44% 7%	597
	11th to 20th	Night	2 1 9% 42% 38% 8%	744
	21st to end	Night	2 2 7% 49% 37% 4	880
September	1st to 10th	Night	3 3 13% 42% 36% 4	800
	11th to 20th	Night	5% 4 14% 36% 32% 9%	800
	21st to end	Night	6% 6% 23% 32% 25% 8%	800
October	1st to 10th	Night	10% 8% 20% 34% 26% 4	800
	11th to 20th	Night	11% 8% 20% 31% 26% 4	800
	21st to end	Night	13% 11% 22% 28% 22% 5%	880
November	1st to 10th	Night	19% 10% 16% 30% 21% 3	820
	11th to 20th	Night	17% 14% 21% 32% 14% 2	828
	21st to end	Night	18% 12% 26% 25% 17% 2	800
December	1st to 10th	Night	24% 14% 21% 30% 11% 1	800
	11th to 20th	Night	31% 12% 20% 26% 10% 1	800
	21st to end	Night	29% 16% 21% 26% 7% 1	880

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