Atascadero
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Atascadero clear sky chart from 2005-11-06 to 2025-08-05.

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	3 3 1 3 3 3 3 3 4 6% 68%	129113
	Night	4 4 2 5% 3 3 3 3 4 6% 63%	127924
Transparency	Day	17% 4 7% 18% 29% 26%	128827
	Night	21% 5% 8% 17% 25% 24%	127725
Seeing	Day	5% 7% 20% 34% 26% 8%	16532
	Night	6% 3 13% 37% 34% 7%	26073

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% 4 1 4 4 4 4 4 6% 7% 58%	3887
	11th to 20th	Day	6% 5% 2 4 3 3 3 2 5% 6% 60%	4100
	21st to end	Day	7% 7% 2 6% 4 4 5% 5% 6% 9% 44%	4510
February	1st to 10th	Day	12% 6% 3 4 3 3 3 3 4 5% 53%	1864
	11th to 20th	Day	7% 7% 2 6% 4 3 3 4 5% 8% 51%	1950
	21st to end	Day	9% 6% 3 5% 4 3 4 4 5% 6% 51%	1650
March	1st to 10th	Day	9% 8% 2 6% 5% 4 5% 5% 7% 9% 40%	4664
	11th to 20th	Day	5% 6% 2 6% 4 4 4 3 5% 6% 57%	5000
	21st to end	Day	5% 7% 2 7% 5% 5% 6% 5% 7% 9% 42%	5722
April	1st to 10th	Day	7% 6% 2 4 3 3 4 4 5% 8% 55%	2400
	11th to 20th	Day	5% 5% 2 5% 3 4 3 3 5% 7% 58%	2369
	21st to end	Day	5% 4 1 4 3 3 3 3 5% 7% 63%	2400
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 5% 78%	5440
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 6% 76%	5636
	21st to end	Day	1 2 1 2 1 2 2 2 3 6% 80%	6902
June	1st to 10th	Day	2 3 1 2 2 3 3 3 4 7% 71%	2800
	11th to 20th	Day	2 1 1 2 2 1 2 4 84%	2800
	21st to end	Day	1 1 2 2 1 2 1 3 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 3 91%	6319
	11th to 20th	Day	1 1 2 1 2 2 2 2 4 82%	5925
	21st to end	Day	1 2 2 2 2 2 3 5% 81%	6402
August	1st to 10th	Day	1 1 1 1 1 2 1 2 3 5% 81%	3973
	11th to 20th	Day	1 1 1 1 1 2 3 90%	3724
	21st to end	Day	1 1 1 1 1 2 1 2 4 86%	4180
September	1st to 10th	Day	1 1 1 1 1 2 1 2 5% 84%	2280
	11th to 20th	Day	2 1 1 2 2 2 1 2 3 4 81%	2279
	21st to end	Day	1 2 1 2 2 2 3 3 4 5% 76%	2085
October	1st to 10th	Day	2 1 3 2 2 3 2 4 6% 76%	3180
	11th to 20th	Day	2 2 1 3 3 3 2 2 3 5% 74%	3161
	21st to end	Day	2 3 1 5% 2 3 3 3 4 7% 67%	3448
November	1st to 10th	Day	4 4 1 4 3 2 3 2 4 6% 66%	1879
	11th to 20th	Day	5% 4 1 3 3 3 4 3 5% 6% 63%	1800
	21st to end	Day	6% 5% 2 5% 4 3 4 3 4 6% 59%	1800
December	1st to 10th	Day	4 5% 2 6% 4 4 4 3 5% 8% 54%	2752
	11th to 20th	Day	12% 7% 2 5% 4 3 5% 4 6% 8% 44%	3028
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 5% 6% 10% 37%	4004
January	1st to 10th	Night	5% 5% 2 5% 3 3 3 3 4 7% 58%	6064
	11th to 20th	Night	6% 6% 2 5% 3 3 3 3 5% 6% 58%	5880
	21st to end	Night	10% 7% 3 7% 5% 5% 4 4 4 7% 44%	6468
February	1st to 10th	Night	9% 6% 2 5% 4 4 3 2 4 5% 54%	2535
	11th to 20th	Night	7% 5% 1 4 3 3 4 3 6% 7% 57%	2338
	21st to end	Night	8% 6% 2 5% 3 4 2 3 3 6% 56%	1980
March	1st to 10th	Night	8% 11% 4 7% 5% 5% 4 4 5% 7% 41%	5227
	11th to 20th	Night	5% 5% 2 5% 3 4 4 4 5% 6% 56%	5000
	21st to end	Night	7% 8% 4 7% 4 4 4 3 4 6% 48%	5253
April	1st to 10th	Night	5% 6% 2 4 3 3 3 3 4 7% 60%	2000
	11th to 20th	Night	5% 4 2 4 4 3 3 2 4 5% 64%	1979
	21st to end	Night	3 5% 2 5% 3 2 3 3 4 7% 64%	2000
May	1st to 10th	Night	1 2 1 3 3 2 2 2 3 6% 75%	4560
	11th to 20th	Night	1 2 2 7% 4 5% 3 2 3 6% 66%	4267
	21st to end	Night	1 2 1 3 4 4 3 2 3 4 75%	3972
June	1st to 10th	Night	2 2 1 4 3 4 4 3 5% 7% 66%	1600
	11th to 20th	Night	3 1 2 2 2 2 2 3 5% 77%	1600
	21st to end	Night	1 1 1 2 2 2 2 1 2 3 83%	1600
July	1st to 10th	Night	1 1 2 2 1 3 3 86%	3640
	11th to 20th	Night	1 1 3 2 2 3 3 4 5% 75%	4047
	21st to end	Night	1 4 7% 8% 5% 3 4 4 63%	4472
August	1st to 10th	Night	1 2 2 2 2 2 3 4 81%	3059
	11th to 20th	Night	1 1 1 1 1 1 2 3 88%	3126
	21st to end	Night	1 2 2 2 2 3 3 84%	3498
September	1st to 10th	Night	1 2 1 1 2 2 2 2 2 3 83%	1900
	11th to 20th	Night	1 2 1 4 2 3 2 1 3 4 77%	1900
	21st to end	Night	3 1 1 1 2 2 1 3 4 81%	2090
October	1st to 10th	Night	1 1 1 2 2 2 2 2 3 5% 80%	3800
	11th to 20th	Night	1 2 2 5% 3 3 4 3 3 4 71%	3777
	21st to end	Night	2 3 2 4 2 2 2 2 4 6% 71%	4121
November	1st to 10th	Night	4 3 1 3 3 3 3 3 4 6% 68%	2416
	11th to 20th	Night	3 4 1 4 3 3 4 3 5% 6% 65%	2600
	21st to end	Night	4 5% 2 5% 3 3 4 3 6% 6% 58%	2600
December	1st to 10th	Night	5% 4 1 5% 3 4 4 4 5% 7% 59%	4427
	11th to 20th	Night	10% 9% 3 6% 5% 4 5% 5% 7% 8% 41%	5176
	21st to end	Night	8% 8% 4 11% 6% 6% 6% 4 6% 8% 33%	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	22% 4 12% 31% 19% 11%	3887
	11th to 20th	Day	26% 4 7% 14% 20% 28%	4100
	21st to end	Day	32% 5% 10% 15% 22% 15%	4510
February	1st to 10th	Day	32% 3 7% 19% 22% 16%	1864
	11th to 20th	Day	32% 4 8% 23% 24% 9%	1960
	21st to end	Day	34% 4 10% 20% 24% 8%	1650
March	1st to 10th	Day	35% 8% 12% 16% 24% 5%	4664
	11th to 20th	Day	27% 5% 11% 31% 17% 8%	5000
	21st to end	Day	32% 9% 13% 26% 16% 3	5722
April	1st to 10th	Day	27% 5% 12% 25% 23% 8%	2400
	11th to 20th	Day	25% 5% 11% 26% 26% 7%	2372
	21st to end	Day	20% 6% 10% 28% 29% 7%	2400
May	1st to 10th	Day	11% 3 7% 28% 39% 12%	5440
	11th to 20th	Day	12% 4 6% 25% 43% 10%	5639
	21st to end	Day	9% 3 5% 27% 46% 10%	6904
June	1st to 10th	Day	14% 4 9% 24% 33% 16%	2800
	11th to 20th	Day	8% 2 4 13% 43% 29%	2800
	21st to end	Day	7% 2 3 10% 35% 44%	2800
July	1st to 10th	Day	2 1 2 4 22% 69%	6251
	11th to 20th	Day	6% 2 3 4 22% 63%	5854
	21st to end	Day	6% 2 3 14% 34% 41%	6217
August	1st to 10th	Day	8% 2 2 9% 33% 47%	3958
	11th to 20th	Day	4 1 2 7% 30% 57%	3760
	21st to end	Day	5% 1 2 10% 28% 54%	4179
September	1st to 10th	Day	6% 2 3 10% 30% 49%	2280
	11th to 20th	Day	9% 2 3 18% 40% 28%	2279
	21st to end	Day	10% 2 3 17% 37% 30%	2085
October	1st to 10th	Day	11% 3 4 12% 23% 47%	3180
	11th to 20th	Day	13% 3 6% 19% 38% 22%	3161
	21st to end	Day	18% 3 4 13% 34% 28%	3448
November	1st to 10th	Day	20% 3 7% 20% 30% 20%	1879
	11th to 20th	Day	21% 4 10% 20% 26% 19%	1800
	21st to end	Day	28% 4 6% 17% 25% 21%	1800
December	1st to 10th	Day	28% 4 5% 14% 28% 21%	2752
	11th to 20th	Day	35% 6% 11% 17% 21% 10%	3028
	21st to end	Day	35% 8% 16% 22% 13% 5%	4004
January	1st to 10th	Night	25% 5% 10% 27% 23% 10%	6064
	11th to 20th	Night	25% 5% 6% 18% 20% 25%	5880
	21st to end	Night	35% 5% 9% 15% 18% 17%	6468
February	1st to 10th	Night	32% 4 8% 17% 23% 16%	2535
	11th to 20th	Night	27% 4 8% 23% 25% 12%	2339
	21st to end	Night	31% 5% 8% 21% 27% 9%	1980
March	1st to 10th	Night	38% 8% 10% 18% 21% 6%	5227
	11th to 20th	Night	27% 7% 12% 26% 20% 8%	5000
	21st to end	Night	34% 7% 14% 24% 18% 4	5253
April	1st to 10th	Night	25% 4 12% 27% 20% 11%	2000
	11th to 20th	Night	22% 4 9% 27% 28% 10%	1979
	21st to end	Night	22% 5% 8% 26% 31% 9%	2000
May	1st to 10th	Night	10% 4 10% 27% 32% 16%	4557
	11th to 20th	Night	16% 9% 5% 22% 33% 15%	4268
	21st to end	Night	9% 10% 5% 21% 44% 11%	3972
June	1st to 10th	Night	14% 6% 12% 19% 33% 16%	1600
	11th to 20th	Night	10% 4 4 13% 38% 33%	1600
	21st to end	Night	8% 2 2 6% 33% 49%	1600
July	1st to 10th	Night	3 1 2 3 20% 71%	3600
	11th to 20th	Night	6% 2 4 4 19% 65%	3995
	21st to end	Night	8% 8% 10% 9% 28% 37%	4345
August	1st to 10th	Night	8% 2 2 8% 29% 51%	3050
	11th to 20th	Night	4 1 1 4 23% 66%	3156
	21st to end	Night	5% 2 3 9% 27% 55%	3498
September	1st to 10th	Night	9% 2 3 8% 25% 54%	1900
	11th to 20th	Night	12% 3 3 14% 33% 35%	1900
	21st to end	Night	9% 1 3 14% 39% 33%	2090
October	1st to 10th	Night	10% 2 3 11% 23% 51%	3800
	11th to 20th	Night	14% 3 7% 18% 32% 25%	3777
	21st to end	Night	16% 2 4 13% 35% 30%	4121
November	1st to 10th	Night	18% 3 7% 20% 29% 24%	2416
	11th to 20th	Night	19% 4 10% 20% 26% 22%	2600
	21st to end	Night	26% 4 9% 16% 23% 21%	2600
December	1st to 10th	Night	25% 4 6% 13% 31% 20%	4427
	11th to 20th	Night	37% 7% 11% 16% 18% 11%	5176
	21st to end	Night	43% 9% 13% 19% 13% 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	17% 10% 29% 31% 11% 2	305
	11th to 20th	Day	10% 10% 25% 37% 15% 2	400
	21st to end	Day	9% 10% 26% 31% 21% 3	440
February	1st to 10th	Day	13% 12% 25% 27% 18% 4	433
	11th to 20th	Day	11% 12% 27% 31% 17% 3	495
	21st to end	Day	8% 12% 28% 36% 13% 3	414
March	1st to 10th	Day	12% 16% 25% 34% 13%	500
	11th to 20th	Day	12% 12% 29% 30% 16% 1	500
	21st to end	Day	9% 16% 28% 30% 15% 1	550
April	1st to 10th	Day	11% 9% 26% 30% 20% 4	500
	11th to 20th	Day	6% 13% 25% 34% 17% 4	492
	21st to end	Day	4 7% 28% 33% 21% 7%	500
May	1st to 10th	Day	6% 8% 27% 31% 22% 6%	503
	11th to 20th	Day	2 9% 22% 38% 22% 7%	528
	21st to end	Day	3 10% 22% 33% 23% 9%	573
June	1st to 10th	Day	3 4 18% 34% 28% 12%	530
	11th to 20th	Day	1 4 22% 36% 26% 11%	530
	21st to end	Day	1 2 15% 36% 30% 16%	530
July	1st to 10th	Day	10% 40% 33% 17%	526
	11th to 20th	Day	2 1 4 37% 38% 19%	524
	21st to end	Day	8% 33% 38% 20%	575
August	1st to 10th	Day	1 1 13% 39% 32% 14%	504
	11th to 20th	Day	1 1 9% 35% 36% 18%	477
	21st to end	Day	2 1 8% 31% 42% 16%	539
September	1st to 10th	Day	1 2 11% 32% 40% 15%	490
	11th to 20th	Day	2 3 13% 36% 37% 9%	490
	21st to end	Day	1 4 13% 34% 38% 10%	490
October	1st to 10th	Day	1 5% 17% 39% 32% 6%	490
	11th to 20th	Day	3 4 22% 34% 30% 7%	490
	21st to end	Day	3 6% 16% 37% 32% 5%	539
November	1st to 10th	Day	4 6% 22% 32% 30% 6%	415
	11th to 20th	Day	6% 7% 21% 39% 24% 4	300
	21st to end	Day	8% 6% 21% 38% 23% 3	300
December	1st to 10th	Day	8% 10% 23% 37% 17% 4	240
	11th to 20th	Day	8% 8% 25% 33% 23% 5%	200
	21st to end	Day	13% 15% 27% 32% 10% 2	220
January	1st to 10th	Night	12% 6% 21% 39% 19% 4	800
	11th to 20th	Night	9% 2 19% 46% 21% 3	800
	21st to end	Night	12% 3 18% 42% 21% 3	880
February	1st to 10th	Night	13% 5% 14% 40% 24% 4	800
	11th to 20th	Night	9% 4 19% 42% 21% 4	784
	21st to end	Night	10% 5% 20% 45% 17% 3	660
March	1st to 10th	Night	9% 8% 19% 42% 20% 2	800
	11th to 20th	Night	8% 5% 19% 39% 25% 5%	800
	21st to end	Night	9% 4 22% 39% 24% 3	880
April	1st to 10th	Night	7% 4 19% 41% 26% 2	800
	11th to 20th	Night	6% 6% 20% 37% 27% 5%	791
	21st to end	Night	4 3 23% 34% 29% 7%	800
May	1st to 10th	Night	4 2 17% 40% 31% 6%	800
	11th to 20th	Night	3 2 14% 38% 37% 6%	681
	21st to end	Night	3 1 17% 40% 33% 6%	433
June	1st to 10th	Night	2 1 9% 39% 44% 4	400
	11th to 20th	Night	1 1 11% 37% 45% 7%	400
	21st to end	Night	1 1 3 33% 54% 10%	400
July	1st to 10th	Night	1 1 23% 62% 13%	390
	11th to 20th	Night	1 1 9% 64% 25%	557
	21st to end	Night	1 1 1 12% 60% 26%	657
August	1st to 10th	Night	1 3 25% 53% 17%	676
	11th to 20th	Night	3 25% 57% 15%	752
	21st to end	Night	1 2 20% 61% 16%	836
September	1st to 10th	Night	1 1 3 23% 58% 14%	760
	11th to 20th	Night	2 1 6% 34% 45% 11%	760
	21st to end	Night	1 1 8% 31% 49% 11%	760
October	1st to 10th	Night	4 1 8% 39% 44% 5%	760
	11th to 20th	Night	3 3 9% 39% 41% 6%	760
	21st to end	Night	4 2 9% 44% 33% 9%	836
November	1st to 10th	Night	4 3 13% 42% 34% 4	780
	11th to 20th	Night	4 2 15% 47% 27% 4	800
	21st to end	Night	5% 3 18% 45% 26% 3	800
December	1st to 10th	Night	10% 4 16% 45% 23% 3	800
	11th to 20th	Night	13% 2 18% 45% 20% 2	800
	21st to end	Night	10% 5% 25% 42% 16% 2	880

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