Cedar Glen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cedar Glen Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	2 3 1 3 2 2 3 2 4 5% 72%	141409
	Night	2 4 1 4 3 2 3 2 4 5% 70%	138689
Transparency	Day	16% 4 5% 14% 28% 34%	136497
	Night	17% 4 5% 14% 26% 34%	134576
Seeing	Day	8% 15% 34% 27% 11% 4	17706
	Night	6% 7% 21% 34% 26% 7%	27687

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	5% 7% 2 4 2 3 3 2 4 6% 62%	5318
	11th to 20th	Day	3 3 1 2 1 1 2 2 3 6% 75%	5538
	21st to end	Day	5% 8% 3 5% 4 3 3 3 4 7% 55%	6500
February	1st to 10th	Day	5% 7% 2 5% 3 2 3 2 5% 5% 59%	2100
	11th to 20th	Day	5% 7% 3 5% 4 3 3 2 5% 6% 57%	2050
	21st to end	Day	5% 8% 2 6% 4 3 3 2 4 6% 56%	1730
March	1st to 10th	Day	6% 9% 3 6% 3 3 3 3 5% 7% 52%	4618
	11th to 20th	Day	3 8% 3 6% 3 3 3 3 5% 7% 56%	4857
	21st to end	Day	3 5% 2 5% 3 3 4 4 5% 8% 59%	5938
April	1st to 10th	Day	5% 5% 2 4 2 3 3 3 6% 6% 61%	2520
	11th to 20th	Day	3 4 1 4 2 2 3 2 4 5% 68%	2489
	21st to end	Day	2 4 1 3 2 3 3 3 5% 7% 67%	2520
May	1st to 10th	Day	2 3 1 4 2 2 3 2 4 5% 72%	5796
	11th to 20th	Day	1 2 1 3 2 2 2 2 4 5% 76%	5841
	21st to end	Day	1 1 1 1 1 1 2 1 3 5% 82%	6423
June	1st to 10th	Day	1 2 1 2 2 2 2 2 5% 6% 76%	2600
	11th to 20th	Day	1 1 1 1 1 2 4 87%	2616
	21st to end	Day	1 2 1 2 1 1 2 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	6280
	11th to 20th	Day	1 1 2 1 2 2 2 3 5% 81%	5880
	21st to end	Day	1 1 1 2 2 2 1 3 4 83%	6239
August	1st to 10th	Day	1 1 2 1 1 2 1 3 4 83%	5346
	11th to 20th	Day	1 1 1 1 1 2 3 91%	5364
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 79%	5984
September	1st to 10th	Day	2 2 1 3 2 2 2 2 4 6% 74%	2397
	11th to 20th	Day	1 1 1 1 2 1 2 3 4 85%	2200
	21st to end	Day	1 1 2 2 2 3 2 3 4 81%	2200
October	1st to 10th	Day	1 2 1 2 2 2 2 2 5% 5% 77%	3600
	11th to 20th	Day	1 1 1 1 1 1 1 1 3 5% 83%	3520
	21st to end	Day	1 2 1 3 1 2 3 3 4 5% 74%	3363
November	1st to 10th	Day	2 4 1 3 3 3 2 3 4 5% 71%	1854
	11th to 20th	Day	4 4 1 4 2 3 3 2 4 6% 66%	1890
	21st to end	Day	4 5% 2 4 4 3 3 2 5% 7% 61%	1890
December	1st to 10th	Day	3 3 1 3 3 3 4 4 6% 7% 64%	3040
	11th to 20th	Day	4 5% 2 5% 2 4 6% 4 6% 8% 53%	3499
	21st to end	Day	5% 7% 3 5% 4 5% 4 4 8% 9% 47%	4609
January	1st to 10th	Night	5% 8% 2 6% 3 3 3 2 3 5% 61%	7605
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 6% 75%	7446
	21st to end	Night	5% 7% 3 6% 4 4 4 3 4 5% 55%	7747
February	1st to 10th	Night	4 9% 2 6% 3 3 3 3 3 6% 57%	2520
	11th to 20th	Night	4 7% 3 5% 4 4 3 3 4 6% 57%	2458
	21st to end	Night	4 8% 3 6% 4 4 4 3 5% 6% 54%	2076
March	1st to 10th	Night	4 13% 2 9% 4 4 4 3 4 6% 47%	5496
	11th to 20th	Night	2 7% 3 8% 5% 4 4 3 5% 6% 54%	5363
	21st to end	Night	1 7% 2 7% 4 4 4 3 5% 7% 56%	5262
April	1st to 10th	Night	3 7% 1 5% 4 3 4 3 5% 6% 58%	2100
	11th to 20th	Night	2 4 1 5% 4 3 3 3 4 6% 65%	2079
	21st to end	Night	2 4 2 4 4 4 4 3 4 5% 66%	2100
May	1st to 10th	Night	2 4 1 4 3 2 2 2 3 4 73%	4207
	11th to 20th	Night	1 2 1 2 2 2 2 2 4 5% 77%	4074
	21st to end	Night	1 1 1 2 3 3 2 3 5% 79%	4474
June	1st to 10th	Night	1 1 2 2 3 3 1 4 5% 78%	1800
	11th to 20th	Night	1 2 1 1 2 1 3 4 85%	1772
	21st to end	Night	1 1 1 1 1 2 1 3 3 87%	1600
July	1st to 10th	Night	1 1 1 2 94%	3658
	11th to 20th	Night	1 1 1 1 1 2 1 2 3 86%	4100
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 87%	4624
August	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	4486
	11th to 20th	Night	1 1 1 1 2 3 91%	4506
	21st to end	Night	1 2 1 2 1 1 2 1 3 3 84%	5015
September	1st to 10th	Night	2 1 1 2 2 2 2 2 2 4 81%	2000
	11th to 20th	Night	1 1 1 1 1 1 2 1 2 3 86%	2200
	21st to end	Night	1 1 2 1 2 2 2 3 3 83%	2200
October	1st to 10th	Night	1 2 1 2 1 1 1 2 2 3 84%	3600
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 4 81%	3633
	21st to end	Night	1 3 2 3 3 2 2 2 3 4 76%	4441
November	1st to 10th	Night	2 4 1 4 3 3 3 2 4 6% 70%	2665
	11th to 20th	Night	2 4 2 4 3 3 3 3 4 5% 66%	2730
	21st to end	Night	3 5% 2 5% 4 3 4 3 5% 6% 58%	2730
December	1st to 10th	Night	3 4 1 4 3 2 4 3 4 6% 66%	4370
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 6% 8% 53%	4941
	21st to end	Night	3 5% 3 7% 4 4 4 4 7% 11% 48%	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	23% 4 6% 17% 30% 20%	5321
	11th to 20th	Day	13% 2 7% 12% 21% 44%	5538
	21st to end	Day	28% 4 5% 11% 24% 27%	6500
February	1st to 10th	Day	26% 3 5% 14% 26% 26%	2100
	11th to 20th	Day	28% 4 4 16% 28% 20%	2060
	21st to end	Day	30% 4 6% 16% 24% 20%	1730
March	1st to 10th	Day	27% 8% 6% 16% 25% 18%	4618
	11th to 20th	Day	26% 5% 9% 24% 21% 14%	4857
	21st to end	Day	20% 10% 9% 28% 24% 9%	5938
April	1st to 10th	Day	24% 5% 7% 22% 28% 14%	2520
	11th to 20th	Day	20% 5% 5% 18% 30% 21%	2492
	21st to end	Day	18% 5% 6% 20% 37% 13%	2520
May	1st to 10th	Day	15% 6% 6% 19% 37% 17%	5796
	11th to 20th	Day	9% 5% 7% 18% 45% 16%	5844
	21st to end	Day	9% 6% 4 15% 42% 25%	6424
June	1st to 10th	Day	13% 3 5% 17% 36% 26%	2600
	11th to 20th	Day	5% 4 4 10% 28% 50%	2616
	21st to end	Day	9% 2 2 5% 22% 61%	2800
July	1st to 10th	Day	4 1 2 6% 11% 76%	6212
	11th to 20th	Day	8% 2 4 9% 25% 52%	5811
	21st to end	Day	7% 2 4 15% 17% 56%	6058
August	1st to 10th	Day	9% 2 4 14% 32% 38%	3890
	11th to 20th	Day	5% 1 3 12% 22% 56%	3880
	21st to end	Day	7% 1 3 5% 24% 59%	4311
September	1st to 10th	Day	11% 3 6% 8% 31% 42%	2397
	11th to 20th	Day	6% 1 3 8% 32% 49%	2200
	21st to end	Day	9% 2 4 12% 31% 43%	2200
October	1st to 10th	Day	11% 2 6% 9% 22% 50%	3600
	11th to 20th	Day	8% 2 3 8% 36% 45%	3520
	21st to end	Day	14% 3 2 7% 32% 41%	3363
November	1st to 10th	Day	16% 2 4 13% 32% 32%	1854
	11th to 20th	Day	21% 2 5% 12% 23% 36%	1890
	21st to end	Day	24% 4 5% 13% 24% 30%	1890
December	1st to 10th	Day	18% 3 5% 13% 28% 32%	3040
	11th to 20th	Day	26% 6% 7% 16% 28% 17%	3498
	21st to end	Day	30% 7% 9% 19% 22% 14%	4609
January	1st to 10th	Night	24% 5% 5% 14% 27% 25%	7605
	11th to 20th	Night	13% 3 5% 14% 18% 47%	7446
	21st to end	Night	29% 5% 5% 10% 24% 27%	7747
February	1st to 10th	Night	29% 3 4 13% 21% 29%	2520
	11th to 20th	Night	28% 4 5% 15% 24% 23%	2459
	21st to end	Night	32% 4 4 16% 21% 24%	2076
March	1st to 10th	Night	30% 12% 5% 15% 18% 20%	5496
	11th to 20th	Night	27% 5% 9% 24% 23% 11%	5363
	21st to end	Night	23% 9% 11% 21% 26% 10%	5262
April	1st to 10th	Night	25% 5% 8% 21% 23% 17%	2100
	11th to 20th	Night	19% 7% 6% 19% 28% 22%	2079
	21st to end	Night	20% 6% 5% 20% 34% 15%	2100
May	1st to 10th	Night	15% 6% 6% 24% 30% 19%	4204
	11th to 20th	Night	9% 4 6% 21% 43% 16%	4075
	21st to end	Night	8% 7% 4 16% 44% 21%	4474
June	1st to 10th	Night	9% 4 5% 18% 36% 28%	1800
	11th to 20th	Night	6% 3 4 11% 27% 49%	1772
	21st to end	Night	5% 2 3 6% 21% 63%	1600
July	1st to 10th	Night	2 1 5% 13% 79%	3618
	11th to 20th	Night	5% 1 3 10% 22% 58%	4046
	21st to end	Night	6% 1 3 16% 19% 55%	4493
August	1st to 10th	Night	6% 1 3 18% 33% 39%	3256
	11th to 20th	Night	4 1 3 12% 26% 54%	3256
	21st to end	Night	5% 1 1 6% 22% 65%	3608
September	1st to 10th	Night	8% 4 4 7% 29% 48%	2000
	11th to 20th	Night	5% 2 3 9% 33% 48%	2200
	21st to end	Night	7% 3 4 11% 30% 45%	2200
October	1st to 10th	Night	9% 2 3 8% 22% 57%	3600
	11th to 20th	Night	8% 2 2 7% 31% 49%	3633
	21st to end	Night	13% 3 2 6% 27% 48%	4441
November	1st to 10th	Night	18% 2 4 11% 30% 35%	2665
	11th to 20th	Night	20% 3 4 12% 26% 36%	2730
	21st to end	Night	25% 5% 5% 14% 20% 31%	2730
December	1st to 10th	Night	19% 3 3 11% 29% 35%	4370
	11th to 20th	Night	26% 6% 6% 17% 28% 18%	4941
	21st to end	Night	26% 7% 8% 23% 21% 14%	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	21% 16% 30% 25% 7% 1	330
	11th to 20th	Day	16% 14% 30% 30% 9% 1	398
	21st to end	Day	15% 24% 29% 24% 7% 2	594
February	1st to 10th	Day	16% 22% 31% 20% 8% 2	540
	11th to 20th	Day	19% 21% 34% 20% 5% 1	535
	21st to end	Day	14% 26% 33% 22% 4	446
March	1st to 10th	Day	20% 26% 33% 15% 6% 1	540
	11th to 20th	Day	15% 23% 32% 23% 6% 1	540
	21st to end	Day	15% 26% 35% 18% 6%	594
April	1st to 10th	Day	13% 19% 36% 21% 9% 2	540
	11th to 20th	Day	8% 30% 36% 16% 8% 2	532
	21st to end	Day	11% 24% 31% 21% 9% 4	540
May	1st to 10th	Day	8% 21% 37% 23% 6% 4	508
	11th to 20th	Day	6% 17% 40% 24% 9% 5%	528
	21st to end	Day	7% 14% 39% 26% 8% 5%	573
June	1st to 10th	Day	2 14% 37% 28% 14% 6%	530
	11th to 20th	Day	1 12% 42% 28% 11% 7%	530
	21st to end	Day	4 9% 36% 31% 12% 7%	530
July	1st to 10th	Day	2 5% 38% 34% 13% 8%	526
	11th to 20th	Day	1 1 33% 38% 16% 11%	524
	21st to end	Day	1 3 32% 36% 16% 11%	575
August	1st to 10th	Day	2 5% 41% 30% 14% 9%	527
	11th to 20th	Day	1 4 31% 35% 16% 11%	517
	21st to end	Day	3 4 34% 35% 16% 8%	583
September	1st to 10th	Day	3 6% 35% 34% 16% 5%	530
	11th to 20th	Day	4 10% 40% 31% 14% 2	530
	21st to end	Day	2 16% 32% 33% 15% 2	530
October	1st to 10th	Day	4 14% 36% 35% 11% 1	530
	11th to 20th	Day	6% 13% 36% 32% 11% 2	530
	21st to end	Day	5% 14% 33% 31% 15% 2	463
November	1st to 10th	Day	7% 15% 28% 35% 14% 2	330
	11th to 20th	Day	10% 19% 32% 24% 11% 4	330
	21st to end	Day	9% 17% 33% 27% 11% 3	330
December	1st to 10th	Day	10% 19% 32% 27% 12% 1	330
	11th to 20th	Day	7% 18% 28% 30% 14% 2	330
	21st to end	Day	17% 24% 30% 19% 9% 2	363
January	1st to 10th	Night	13% 12% 21% 31% 17% 6%	840
	11th to 20th	Night	10% 9% 26% 33% 17% 5%	840
	21st to end	Night	11% 13% 27% 31% 15% 4	924
February	1st to 10th	Night	10% 13% 25% 30% 17% 5%	840
	11th to 20th	Night	10% 12% 27% 30% 16% 5%	824
	21st to end	Night	10% 14% 30% 28% 14% 3	692
March	1st to 10th	Night	12% 14% 28% 30% 15% 3	840
	11th to 20th	Night	9% 10% 26% 30% 19% 6%	840
	21st to end	Night	7% 11% 27% 34% 18% 3	924
April	1st to 10th	Night	7% 10% 23% 37% 19% 4	840
	11th to 20th	Night	5% 11% 28% 34% 19% 4	831
	21st to end	Night	4 8% 25% 34% 22% 6%	840
May	1st to 10th	Night	4 6% 27% 39% 21% 3	649
	11th to 20th	Night	4 5% 23% 35% 27% 6%	591
	21st to end	Night	3 7% 23% 38% 24% 6%	650
June	1st to 10th	Night	1 4 20% 39% 32% 5%	600
	11th to 20th	Night	1 2 21% 42% 31% 3	572
	21st to end	Night	1 1 12% 37% 42% 9%	400
July	1st to 10th	Night	1 6% 33% 48% 12%	395
	11th to 20th	Night	2 10% 38% 42% 7%	582
	21st to end	Night	1 9% 37% 43% 9%	721
August	1st to 10th	Night	1 1 8% 36% 41% 13%	796
	11th to 20th	Night	2 1 5% 31% 46% 15%	792
	21st to end	Night	2 1 7% 31% 43% 17%	880
September	1st to 10th	Night	2 2 9% 31% 42% 15%	800
	11th to 20th	Night	1 2 14% 36% 35% 12%	800
	21st to end	Night	1 3 17% 36% 33% 10%	800
October	1st to 10th	Night	3 5% 17% 45% 25% 6%	800
	11th to 20th	Night	4 4 20% 36% 29% 9%	800
	21st to end	Night	3 9% 21% 34% 23% 11%	880
November	1st to 10th	Night	5% 6% 18% 37% 26% 8%	820
	11th to 20th	Night	6% 6% 24% 32% 24% 8%	840
	21st to end	Night	6% 9% 24% 34% 21% 6%	840
December	1st to 10th	Night	7% 11% 27% 30% 20% 5%	840
	11th to 20th	Night	10% 9% 25% 34% 18% 4	840
	21st to end	Night	11% 14% 27% 28% 16% 4	924

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