Anaheim
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anaheim clear sky chart from 2005-11-06 to 2025-09-04.

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 4 3 2 3 3 4 6% 66%	134973
	Night	5% 6% 2 5% 3 3 3 3 4 7% 61%	131225
Transparency	Day	20% 3 7% 17% 27% 25%	130065
	Night	24% 5% 9% 17% 23% 22%	127110
Seeing	Day	6% 7% 24% 37% 19% 7%	16860
	Night	7% 3 11% 35% 35% 9%	26701

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% 3 1 4 2 2 3 2 3 6% 70%	3685
	11th to 20th	Day	6% 4 2 4 2 2 3 2 4 6% 65%	4324
	21st to end	Day	10% 5% 2 5% 3 3 4 3 6% 9% 50%	5016
February	1st to 10th	Day	10% 4 2 4 3 2 4 3 4 7% 58%	2000
	11th to 20th	Day	9% 3 1 4 3 3 3 2 5% 7% 59%	1950
	21st to end	Day	7% 5% 2 4 3 3 3 4 4 7% 57%	1650
March	1st to 10th	Day	8% 5% 2 6% 3 3 4 5% 6% 8% 49%	4518
	11th to 20th	Day	8% 5% 2 5% 4 3 3 3 5% 8% 54%	4663
	21st to end	Day	9% 5% 2 4 4 3 4 4 6% 8% 54%	5969
April	1st to 10th	Day	5% 5% 1 3 3 2 3 3 6% 8% 61%	2400
	11th to 20th	Day	4 4 2 4 4 2 3 3 4 5% 65%	2369
	21st to end	Day	4 4 1 4 3 2 2 2 4 7% 66%	2400
May	1st to 10th	Day	3 4 2 6% 3 3 3 2 3 6% 66%	5608
	11th to 20th	Day	3 6% 2 6% 3 3 4 3 5% 6% 59%	5841
	21st to end	Day	3 5% 2 5% 3 3 3 3 4 7% 62%	6423
June	1st to 10th	Day	5% 8% 2 5% 3 3 2 2 4 7% 60%	2600
	11th to 20th	Day	3 5% 2 3 2 2 2 2 3 7% 70%	2732
	21st to end	Day	1 4 2 3 3 2 3 2 3 5% 72%	2800
July	1st to 10th	Day	1 2 1 2 1 1 1 2 3 5% 81%	6319
	11th to 20th	Day	1 3 2 4 2 2 2 2 4 6% 71%	6087
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 78%	5996
August	1st to 10th	Day	1 1 1 2 1 1 1 1 3 6% 82%	5346
	11th to 20th	Day	1 2 1 2 1 1 1 2 2 4 83%	5364
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 78%	5981
September	1st to 10th	Day	3 2 3 2 1 2 1 4 5% 77%	2317
	11th to 20th	Day	2 3 1 3 1 2 2 2 3 5% 75%	2125
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 76%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 78%	3490
	11th to 20th	Day	3 5% 2 4 2 2 2 2 3 7% 68%	3316
	21st to end	Day	4 2 1 3 2 2 3 3 6% 6% 67%	3361
November	1st to 10th	Day	3 3 1 3 2 2 2 2 5% 7% 70%	1764
	11th to 20th	Day	5% 2 1 2 3 3 2 2 4 6% 71%	1800
	21st to end	Day	5% 4 1 4 4 3 4 3 4 9% 59%	1800
December	1st to 10th	Day	4 3 1 4 2 3 3 3 6% 8% 62%	2950
	11th to 20th	Day	8% 5% 2 4 3 3 5% 5% 7% 9% 51%	3409
	21st to end	Day	9% 6% 3 6% 4 4 4 4 6% 9% 44%	4510
January	1st to 10th	Night	5% 3 2 3 2 2 3 2 4 5% 69%	6220
	11th to 20th	Night	6% 5% 2 4 3 3 3 2 4 6% 64%	5622
	21st to end	Night	12% 6% 2 4 3 3 4 4 4 5% 52%	5984
February	1st to 10th	Night	8% 6% 2 5% 4 3 3 2 4 5% 59%	2400
	11th to 20th	Night	9% 5% 1 4 3 3 3 2 5% 5% 60%	2338
	21st to end	Night	9% 5% 2 4 3 4 5% 2 4 5% 57%	1980
March	1st to 10th	Night	8% 7% 2 6% 3 3 3 2 5% 8% 51%	5376
	11th to 20th	Night	7% 7% 2 5% 3 4 4 3 5% 7% 53%	5307
	21st to end	Night	9% 9% 3 5% 3 3 3 3 4 7% 52%	5006
April	1st to 10th	Night	7% 6% 3 5% 3 2 3 2 4 8% 57%	2000
	11th to 20th	Night	5% 6% 2 4 2 2 2 2 4 6% 63%	1979
	21st to end	Night	5% 7% 2 5% 4 2 3 3 5% 7% 57%	2000
May	1st to 10th	Night	4 12% 3 5% 3 2 2 2 4 5% 58%	4381
	11th to 20th	Night	2 11% 4 8% 3 3 4 2 4 7% 51%	4074
	21st to end	Night	2 7% 2 5% 3 2 3 2 4 6% 63%	4474
June	1st to 10th	Night	3 11% 3 7% 4 2 3 3 5% 7% 51%	1800
	11th to 20th	Night	3 7% 2 4 3 2 2 3 4 8% 62%	1659
	21st to end	Night	2 7% 2 5% 4 3 3 3 5% 7% 59%	1600
July	1st to 10th	Night	1 4 2 6% 3 3 3 2 4 8% 66%	3640
	11th to 20th	Night	1 4 3 5% 3 3 4 3 5% 9% 60%	3865
	21st to end	Night	1 4 1 5% 3 4 4 3 4 6% 65%	4878
August	1st to 10th	Night	1 2 1 3 2 2 3 2 4 8% 71%	4486
	11th to 20th	Night	1 6% 3 5% 2 2 3 2 4 7% 66%	4506
	21st to end	Night	1 3 2 3 2 2 3 2 4 7% 71%	5015
September	1st to 10th	Night	4 4 2 4 2 3 3 2 5% 6% 67%	1930
	11th to 20th	Night	2 4 1 2 2 2 2 1 3 5% 75%	2052
	21st to end	Night	2 3 2 3 2 2 2 2 3 5% 73%	2090
October	1st to 10th	Night	2 2 1 3 2 3 2 2 4 8% 73%	3490
	11th to 20th	Night	4 12% 1 4 3 2 3 2 3 7% 59%	3617
	21st to end	Night	2 3 1 3 2 2 2 2 3 6% 73%	4202
November	1st to 10th	Night	3 3 1 3 3 2 2 2 4 7% 69%	2535
	11th to 20th	Night	3 4 2 3 2 2 2 2 4 6% 70%	2600
	21st to end	Night	5% 5% 2 5% 4 3 2 4 6% 7% 58%	2600
December	1st to 10th	Night	5% 3 1 5% 3 3 4 3 4 7% 62%	4240
	11th to 20th	Night	9% 5% 2 4 4 4 4 4 5% 7% 51%	4811
	21st to end	Night	8% 5% 2 5% 5% 4 4 4 7% 11% 43%	6468

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	18% 2 5% 20% 29% 26%	3685
	11th to 20th	Day	22% 4 10% 15% 18% 31%	4324
	21st to end	Day	30% 4 5% 16% 23% 22%	5016
February	1st to 10th	Day	27% 4 7% 13% 24% 24%	2000
	11th to 20th	Day	23% 5% 7% 18% 24% 22%	1960
	21st to end	Day	26% 5% 10% 17% 22% 20%	1650
March	1st to 10th	Day	29% 7% 9% 18% 21% 16%	4518
	11th to 20th	Day	26% 5% 13% 26% 18% 11%	4663
	21st to end	Day	27% 5% 12% 30% 17% 8%	5969
April	1st to 10th	Day	21% 6% 12% 25% 23% 12%	2400
	11th to 20th	Day	21% 5% 8% 22% 27% 17%	2372
	21st to end	Day	19% 4 10% 26% 30% 11%	2400
May	1st to 10th	Day	21% 4 9% 21% 33% 13%	5608
	11th to 20th	Day	24% 4 9% 21% 35% 7%	5844
	21st to end	Day	21% 4 11% 25% 31% 8%	6424
June	1st to 10th	Day	25% 5% 10% 20% 28% 12%	2600
	11th to 20th	Day	18% 3 8% 13% 32% 27%	2732
	21st to end	Day	16% 2 7% 9% 26% 40%	2800
July	1st to 10th	Day	9% 2 3 6% 21% 59%	6251
	11th to 20th	Day	16% 3 5% 12% 41% 24%	6013
	21st to end	Day	10% 2 5% 19% 34% 30%	5824
August	1st to 10th	Day	10% 1 3 18% 41% 27%	3890
	11th to 20th	Day	7% 1 3 15% 28% 46%	3880
	21st to end	Day	10% 2 4 9% 30% 44%	4311
September	1st to 10th	Day	12% 2 7% 10% 34% 36%	2317
	11th to 20th	Day	13% 2 7% 12% 33% 33%	2125
	21st to end	Day	11% 2 6% 16% 32% 32%	2090
October	1st to 10th	Day	11% 2 3 12% 23% 49%	3490
	11th to 20th	Day	20% 2 4 15% 33% 26%	3316
	21st to end	Day	17% 3 4 10% 31% 35%	3361
November	1st to 10th	Day	16% 3 8% 17% 23% 33%	1764
	11th to 20th	Day	17% 4 7% 16% 19% 37%	1800
	21st to end	Day	23% 4 10% 13% 21% 30%	1800
December	1st to 10th	Day	20% 3 5% 20% 20% 31%	2950
	11th to 20th	Day	29% 5% 9% 16% 26% 17%	3408
	21st to end	Day	37% 4 12% 22% 16% 8%	4510
January	1st to 10th	Night	19% 3 5% 15% 31% 26%	6220
	11th to 20th	Night	23% 5% 9% 18% 17% 28%	5622
	21st to end	Night	32% 5% 6% 14% 21% 22%	5984
February	1st to 10th	Night	29% 4 8% 14% 20% 26%	2400
	11th to 20th	Night	27% 4 9% 17% 21% 23%	2339
	21st to end	Night	31% 4 7% 14% 21% 22%	1980
March	1st to 10th	Night	30% 7% 12% 18% 22% 12%	5376
	11th to 20th	Night	30% 5% 12% 25% 17% 11%	5307
	21st to end	Night	31% 7% 15% 21% 19% 6%	5006
April	1st to 10th	Night	26% 6% 12% 22% 20% 13%	2000
	11th to 20th	Night	25% 5% 10% 20% 24% 16%	1979
	21st to end	Night	25% 6% 16% 21% 22% 10%	2000
May	1st to 10th	Night	29% 6% 11% 19% 28% 8%	4378
	11th to 20th	Night	33% 5% 14% 21% 23% 5%	4075
	21st to end	Night	24% 5% 15% 25% 21% 9%	4474
June	1st to 10th	Night	32% 7% 15% 15% 20% 10%	1800
	11th to 20th	Night	21% 5% 12% 14% 28% 20%	1659
	21st to end	Night	25% 3 12% 11% 21% 29%	1600
July	1st to 10th	Night	18% 4 5% 11% 16% 46%	3600
	11th to 20th	Night	20% 6% 7% 15% 33% 19%	3817
	21st to end	Night	20% 5% 5% 22% 27% 21%	4739
August	1st to 10th	Night	14% 2 6% 22% 34% 21%	3256
	11th to 20th	Night	13% 2 6% 16% 31% 32%	3256
	21st to end	Night	14% 3 7% 9% 30% 37%	3608
September	1st to 10th	Night	20% 2 9% 9% 29% 30%	1930
	11th to 20th	Night	15% 3 10% 10% 29% 33%	2052
	21st to end	Night	15% 3 7% 13% 33% 29%	2090
October	1st to 10th	Night	14% 3 4 14% 23% 42%	3490
	11th to 20th	Night	28% 4 5% 12% 25% 27%	3617
	21st to end	Night	16% 3 4 10% 31% 37%	4202
November	1st to 10th	Night	18% 3 9% 15% 20% 35%	2535
	11th to 20th	Night	18% 3 7% 15% 19% 37%	2600
	21st to end	Night	26% 5% 8% 15% 17% 29%	2600
December	1st to 10th	Night	23% 3 6% 13% 22% 31%	4240
	11th to 20th	Night	32% 4 8% 13% 22% 21%	4811
	21st to end	Night	33% 6% 15% 26% 12% 9%	6468

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	12% 8% 23% 33% 21% 3	210
	11th to 20th	Day	12% 10% 26% 33% 16% 4	422
	21st to end	Day	9% 12% 31% 30% 16% 3	550
February	1st to 10th	Day	13% 10% 28% 30% 14% 4	500
	11th to 20th	Day	11% 11% 32% 32% 11% 3	495
	21st to end	Day	10% 14% 30% 34% 10% 1	414
March	1st to 10th	Day	12% 15% 35% 27% 9% 2	500
	11th to 20th	Day	14% 11% 34% 28% 12% 2	500
	21st to end	Day	10% 15% 36% 27% 9% 3	550
April	1st to 10th	Day	8% 12% 32% 28% 16% 4	500
	11th to 20th	Day	6% 13% 39% 29% 9% 4	492
	21st to end	Day	5% 13% 30% 33% 12% 6%	500
May	1st to 10th	Day	6% 13% 34% 30% 13% 5%	503
	11th to 20th	Day	6% 9% 33% 36% 13% 4	528
	21st to end	Day	7% 9% 33% 33% 13% 4	573
June	1st to 10th	Day	5% 4 25% 41% 21% 5%	530
	11th to 20th	Day	4 6% 25% 40% 18% 7%	530
	21st to end	Day	2 3 21% 45% 21% 9%	530
July	1st to 10th	Day	3 1 11% 48% 27% 9%	526
	11th to 20th	Day	2 1 6% 47% 28% 16%	524
	21st to end	Day	2 1 10% 46% 27% 14%	575
August	1st to 10th	Day	2 1 13% 47% 24% 12%	527
	11th to 20th	Day	2 1 9% 49% 26% 13%	517
	21st to end	Day	2 1 14% 45% 26% 12%	583
September	1st to 10th	Day	2 1 20% 46% 18% 12%	503
	11th to 20th	Day	3 5% 24% 44% 17% 7%	490
	21st to end	Day	2 5% 23% 40% 21% 8%	490
October	1st to 10th	Day	4 7% 22% 38% 23% 6%	490
	11th to 20th	Day	6% 6% 25% 38% 19% 6%	490
	21st to end	Day	3 7% 24% 34% 23% 8%	488
November	1st to 10th	Day	6% 5% 18% 33% 30% 8%	300
	11th to 20th	Day	4 5% 20% 36% 26% 8%	300
	21st to end	Day	6% 5% 26% 33% 22% 8%	300
December	1st to 10th	Day	8% 7% 17% 38% 25% 5%	300
	11th to 20th	Day	4 6% 20% 38% 24% 8%	300
	21st to end	Day	12% 10% 24% 36% 15% 3	330
January	1st to 10th	Night	10% 4 13% 42% 23% 7%	800
	11th to 20th	Night	8% 4 15% 40% 28% 5%	800
	21st to end	Night	13% 4 18% 35% 26% 4	880
February	1st to 10th	Night	11% 4 16% 38% 26% 6%	800
	11th to 20th	Night	10% 3 15% 41% 25% 6%	784
	21st to end	Night	10% 5% 17% 42% 22% 3	660
March	1st to 10th	Night	9% 3 19% 42% 23% 4	800
	11th to 20th	Night	10% 3 16% 39% 25% 7%	800
	21st to end	Night	9% 4 16% 40% 28% 4	880
April	1st to 10th	Night	9% 4 15% 41% 28% 4	800
	11th to 20th	Night	7% 4 17% 39% 28% 5%	791
	21st to end	Night	8% 4 14% 37% 32% 6%	800
May	1st to 10th	Night	6% 4 16% 38% 32% 4	725
	11th to 20th	Night	6% 3 15% 35% 36% 5%	591
	21st to end	Night	5% 3 17% 41% 31% 4	650
June	1st to 10th	Night	6% 3 14% 37% 35% 4	600
	11th to 20th	Night	3 2 8% 34% 46% 6%	459
	21st to end	Night	4 1 6% 28% 53% 9%	400
July	1st to 10th	Night	4 3 13% 66% 14%	390
	11th to 20th	Night	2 2 20% 59% 16%	483
	21st to end	Night	3 1 3 24% 51% 19%	832
August	1st to 10th	Night	4 1 3 26% 50% 16%	796
	11th to 20th	Night	4 1 3 20% 52% 20%	792
	21st to end	Night	3 2 22% 51% 22%	880
September	1st to 10th	Night	4 1 5% 23% 50% 17%	772
	11th to 20th	Night	4 1 6% 29% 49% 12%	760
	21st to end	Night	3 1 5% 33% 46% 13%	760
October	1st to 10th	Night	3 1 7% 38% 44% 7%	760
	11th to 20th	Night	6% 2 9% 34% 39% 11%	760
	21st to end	Night	3 3 10% 36% 36% 12%	836
November	1st to 10th	Night	5% 2 9% 37% 35% 12%	780
	11th to 20th	Night	4 1 9% 41% 34% 11%	800
	21st to end	Night	6% 3 13% 43% 29% 7%	800
December	1st to 10th	Night	8% 4 12% 43% 28% 6%	800
	11th to 20th	Night	13% 3 16% 38% 26% 5%	800
	21st to end	Night	10% 4 18% 40% 23% 6%	880

Page generated at 2025-08-31 22:51:09. Version ac07

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