Arroyo Grande
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arroyo Grande clear sky chart from 2005-11-06 to 2025-07-21.

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	5% 4 1 4 3 3 3 3 4 7% 64%	123671
	Night	6% 5% 2 5% 3 3 4 3 4 6% 58%	124397
Transparency	Day	21% 3 7% 20% 27% 21%	123430
	Night	27% 4 8% 18% 24% 20%	124227
Seeing	Day	8% 8% 24% 35% 21% 5%	16519
	Night	8% 3 12% 34% 34% 8%	26107

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	6% 2 1 4 3 2 3 3 4 6% 66%	3915
	11th to 20th	Day	9% 4 1 4 3 2 3 2 4 6% 62%	4100
	21st to end	Day	11% 4 1 5% 3 3 4 4 6% 8% 49%	4510
February	1st to 10th	Day	14% 5% 2 4 2 3 3 2 4 6% 56%	1885
	11th to 20th	Day	9% 5% 1 6% 3 3 3 4 4 7% 54%	1950
	21st to end	Day	12% 4 1 5% 3 4 4 3 5% 5% 53%	1650
March	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 7% 8% 45%	4648
	11th to 20th	Day	10% 4 2 4 3 3 3 2 4 6% 59%	5000
	21st to end	Day	10% 6% 2 5% 4 4 4 4 7% 9% 44%	5739
April	1st to 10th	Day	9% 3 1 4 3 3 3 3 6% 7% 58%	2400
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 5% 8% 59%	2369
	21st to end	Day	6% 4 1 4 3 3 3 3 5% 7% 62%	2400
May	1st to 10th	Day	3 3 1 2 1 2 2 1 2 4 77%	5440
	11th to 20th	Day	1 5% 2 5% 3 3 3 2 4 6% 66%	5569
	21st to end	Day	2 3 1 3 2 3 3 2 3 6% 71%	6891
June	1st to 10th	Day	4 4 2 4 4 3 4 3 5% 9% 60%	2800
	11th to 20th	Day	1 4 1 3 2 2 3 2 4 7% 71%	2800
	21st to end	Day	1 3 1 3 3 3 3 2 3 7% 70%	2800
July	1st to 10th	Day	1 2 1 2 2 2 3 2 3 6% 76%	4560
	11th to 20th	Day	2 2 1 4 3 3 4 3 4 6% 69%	4242
	21st to end	Day	1 2 1 3 3 3 4 3 5% 8% 67%	4456
August	1st to 10th	Day	1 3 1 3 3 3 3 3 5% 8% 68%	3887
	11th to 20th	Day	1 2 1 1 1 2 2 2 3 5% 80%	3724
	21st to end	Day	1 2 1 2 2 3 3 2 4 5% 75%	4180
September	1st to 10th	Day	2 3 2 2 3 3 3 4 7% 71%	2280
	11th to 20th	Day	2 3 1 3 3 3 4 3 3 7% 68%	2273
	21st to end	Day	1 3 1 3 3 3 2 2 3 7% 72%	2090
October	1st to 10th	Day	2 2 1 2 2 2 3 1 4 5% 77%	3188
	11th to 20th	Day	2 2 1 3 2 2 3 2 3 4 76%	3161
	21st to end	Day	4 2 1 4 2 3 3 2 4 6% 68%	3448
November	1st to 10th	Day	5% 3 1 3 2 3 3 2 4 5% 67%	1886
	11th to 20th	Day	6% 3 1 3 2 3 3 3 4 6% 68%	1800
	21st to end	Day	8% 5% 1 4 3 3 3 3 4 6% 60%	1800
December	1st to 10th	Day	5% 4 2 5% 4 4 4 3 4 7% 59%	2798
	11th to 20th	Day	17% 4 2 4 3 3 4 4 5% 6% 49%	3028
	21st to end	Day	14% 5% 2 5% 3 4 4 3 6% 10% 43%	4004
January	1st to 10th	Night	7% 4 2 4 3 3 3 2 4 6% 63%	6026
	11th to 20th	Night	9% 5% 1 4 3 3 3 3 4 6% 61%	5880
	21st to end	Night	14% 6% 1 4 4 4 4 3 4 6% 49%	6468
February	1st to 10th	Night	14% 5% 2 4 3 3 2 3 5% 4 56%	2512
	11th to 20th	Night	9% 6% 1 4 3 3 3 3 4 8% 57%	2338
	21st to end	Night	11% 6% 2 4 3 4 3 2 2 6% 56%	1980
March	1st to 10th	Night	12% 9% 2 6% 4 4 4 4 6% 7% 43%	5230
	11th to 20th	Night	9% 6% 1 5% 3 3 4 3 4 6% 56%	5000
	21st to end	Night	14% 8% 3 6% 3 3 3 2 4 6% 49%	5236
April	1st to 10th	Night	6% 6% 2 4 4 3 4 2 4 7% 58%	2000
	11th to 20th	Night	6% 8% 3 3 3 3 2 3 4 6% 61%	1979
	21st to end	Night	5% 8% 2 3 3 3 3 2 4 8% 59%	2000
May	1st to 10th	Night	2 6% 3 4 2 2 2 2 3 5% 69%	4560
	11th to 20th	Night	2 13% 5% 6% 3 3 3 3 3 5% 54%	4353
	21st to end	Night	2 9% 5% 9% 3 4 3 2 4 4 56%	3979
June	1st to 10th	Night	2 8% 3 6% 4 5% 3 4 5% 8% 53%	1600
	11th to 20th	Night	2 6% 3 3 3 4 3 3 5% 7% 62%	1600
	21st to end	Night	1 5% 2 4 3 5% 3 4 5% 9% 60%	1600
July	1st to 10th	Night	2 1 3 2 3 4 3 5% 6% 71%	2620
	11th to 20th	Night	1 2 3 7% 4 6% 5% 3 4 8% 57%	2942
	21st to end	Night	1 2 5% 8% 4 6% 8% 5% 6% 8% 46%	3109
August	1st to 10th	Night	1 3 2 5% 4 5% 7% 4 6% 7% 55%	3057
	11th to 20th	Night	2 1 3 2 3 5% 4 5% 7% 68%	3126
	21st to end	Night	4 2 6% 3 4 5% 3 4 6% 63%	3498
September	1st to 10th	Night	2 4 2 4 2 2 2 3 3 6% 69%	1900
	11th to 20th	Night	3 6% 1 4 3 3 4 3 4 7% 63%	1905
	21st to end	Night	2 3 1 3 2 4 3 2 3 4 73%	2090
October	1st to 10th	Night	2 2 1 2 2 2 3 2 3 5% 75%	3788
	11th to 20th	Night	2 3 3 5% 3 5% 4 3 3 5% 65%	3777
	21st to end	Night	4 4 2 4 2 2 2 2 3 6% 69%	4121
November	1st to 10th	Night	4 4 1 4 3 3 3 2 4 6% 65%	2410
	11th to 20th	Night	3 4 1 4 2 3 4 2 5% 6% 66%	2600
	21st to end	Night	6% 5% 2 4 3 4 4 3 5% 6% 59%	2600
December	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 6% 61%	4385
	11th to 20th	Night	15% 5% 1 4 4 4 4 4 5% 7% 46%	5176
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	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	21% 2 9% 34% 23% 12%	3915
	11th to 20th	Day	26% 2 9% 14% 21% 28%	4100
	21st to end	Day	33% 2 9% 18% 22% 16%	4510
February	1st to 10th	Day	32% 3 6% 20% 22% 17%	1885
	11th to 20th	Day	30% 3 10% 24% 24% 9%	1960
	21st to end	Day	32% 3 10% 21% 25% 8%	1650
March	1st to 10th	Day	35% 3 14% 19% 23% 5%	4648
	11th to 20th	Day	29% 2 12% 32% 17% 9%	5000
	21st to end	Day	35% 4 14% 28% 16% 3	5739
April	1st to 10th	Day	27% 4 12% 29% 21% 8%	2400
	11th to 20th	Day	24% 4 14% 28% 23% 8%	2372
	21st to end	Day	23% 4 11% 28% 28% 7%	2400
May	1st to 10th	Day	14% 2 7% 32% 36% 10%	5440
	11th to 20th	Day	18% 5% 5% 29% 33% 10%	5572
	21st to end	Day	15% 4 6% 29% 39% 6%	6893
June	1st to 10th	Day	21% 4 11% 26% 28% 10%	2800
	11th to 20th	Day	15% 3 6% 15% 41% 20%	2800
	21st to end	Day	15% 3 4 10% 34% 34%	2800
July	1st to 10th	Day	9% 3 2 8% 19% 60%	4491
	11th to 20th	Day	15% 3 4 10% 22% 46%	4172
	21st to end	Day	13% 3 6% 14% 28% 35%	4316
August	1st to 10th	Day	15% 3 4 13% 33% 32%	3872
	11th to 20th	Day	9% 2 2 7% 34% 45%	3760
	21st to end	Day	11% 4 3 14% 31% 38%	4179
September	1st to 10th	Day	14% 3 3 10% 32% 37%	2280
	11th to 20th	Day	17% 4 3 18% 38% 19%	2273
	21st to end	Day	15% 2 3 17% 37% 26%	2090
October	1st to 10th	Day	12% 2 5% 14% 24% 43%	3188
	11th to 20th	Day	13% 2 6% 21% 35% 22%	3161
	21st to end	Day	19% 2 4 13% 35% 27%	3448
November	1st to 10th	Day	20% 2 6% 20% 30% 22%	1886
	11th to 20th	Day	20% 2 8% 22% 28% 21%	1800
	21st to end	Day	26% 4 7% 16% 26% 22%	1800
December	1st to 10th	Day	27% 3 5% 14% 30% 21%	2798
	11th to 20th	Day	36% 3 10% 19% 22% 10%	3028
	21st to end	Day	36% 4 13% 28% 13% 6%	4004
January	1st to 10th	Night	25% 2 8% 27% 26% 12%	6026
	11th to 20th	Night	28% 2 7% 16% 20% 27%	5880
	21st to end	Night	37% 2 8% 15% 20% 17%	6468
February	1st to 10th	Night	33% 3 6% 18% 25% 15%	2512
	11th to 20th	Night	29% 2 7% 24% 25% 13%	2339
	21st to end	Night	31% 3 7% 22% 26% 10%	1980
March	1st to 10th	Night	40% 2 12% 18% 22% 6%	5230
	11th to 20th	Night	30% 4 12% 28% 19% 8%	5000
	21st to end	Night	39% 2 12% 26% 17% 3	5236
April	1st to 10th	Night	27% 5% 13% 27% 19% 10%	2000
	11th to 20th	Night	26% 5% 9% 26% 25% 9%	1979
	21st to end	Night	25% 5% 8% 27% 27% 8%	2000
May	1st to 10th	Night	20% 3 9% 24% 30% 14%	4557
	11th to 20th	Night	29% 7% 6% 20% 28% 10%	4355
	21st to end	Night	27% 8% 7% 15% 33% 9%	3979
June	1st to 10th	Night	29% 4 11% 19% 27% 10%	1600
	11th to 20th	Night	22% 5% 4 11% 34% 24%	1600
	21st to end	Night	21% 6% 4 6% 29% 34%	1600
July	1st to 10th	Night	11% 5% 2 9% 14% 59%	2580
	11th to 20th	Night	21% 5% 3 13% 14% 43%	2889
	21st to end	Night	24% 6% 7% 14% 25% 23%	3011
August	1st to 10th	Night	21% 5% 10% 11% 22% 33%	3048
	11th to 20th	Night	12% 4 4 8% 23% 48%	3156
	21st to end	Night	17% 5% 4 12% 24% 38%	3498
September	1st to 10th	Night	18% 4 3 8% 23% 44%	1900
	11th to 20th	Night	21% 5% 4 12% 31% 27%	1905
	21st to end	Night	15% 6% 2 14% 35% 27%	2090
October	1st to 10th	Night	12% 3 4 13% 23% 45%	3788
	11th to 20th	Night	19% 4 8% 17% 27% 25%	3777
	21st to end	Night	19% 2 4 11% 34% 30%	4121
November	1st to 10th	Night	21% 3 6% 18% 27% 24%	2410
	11th to 20th	Night	20% 3 9% 19% 28% 22%	2600
	21st to end	Night	27% 3 9% 18% 22% 22%	2600
December	1st to 10th	Night	25% 3 5% 14% 31% 22%	4385
	11th to 20th	Night	37% 3 11% 17% 21% 11%	5176
	21st to end	Night	43% 4 11% 26% 11% 5%	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	21% 9% 31% 25% 12% 2	320
	11th to 20th	Day	12% 10% 33% 27% 16% 2	400
	21st to end	Day	11% 12% 30% 30% 15% 2	440
February	1st to 10th	Day	15% 13% 27% 26% 16% 3	443
	11th to 20th	Day	14% 12% 30% 30% 13% 2	495
	21st to end	Day	9% 14% 34% 31% 10% 2	414
March	1st to 10th	Day	18% 14% 32% 28% 7%	500
	11th to 20th	Day	17% 11% 31% 30% 8% 2	500
	21st to end	Day	14% 16% 34% 26% 7% 2	550
April	1st to 10th	Day	11% 14% 32% 27% 12% 4	500
	11th to 20th	Day	11% 13% 29% 32% 11% 3	492
	21st to end	Day	6% 13% 30% 32% 14% 5%	500
May	1st to 10th	Day	8% 10% 29% 33% 17% 3	503
	11th to 20th	Day	5% 11% 27% 39% 16% 3	528
	21st to end	Day	8% 12% 25% 34% 17% 4	573
June	1st to 10th	Day	5% 5% 21% 38% 25% 6%	530
	11th to 20th	Day	3 6% 25% 36% 23% 6%	530
	21st to end	Day	4 3 19% 40% 24% 11%	530
July	1st to 10th	Day	2 16% 41% 31% 10%	526
	11th to 20th	Day	2 1 10% 41% 33% 13%	524
	21st to end	Day	2 1 7% 41% 36% 13%	532
August	1st to 10th	Day	3 1 18% 43% 27% 8%	487
	11th to 20th	Day	4 1 10% 41% 32% 13%	477
	21st to end	Day	4 2 13% 40% 33% 8%	539
September	1st to 10th	Day	3 2 14% 37% 35% 9%	490
	11th to 20th	Day	4 6% 22% 38% 24% 6%	490
	21st to end	Day	1 5% 20% 37% 30% 7%	490
October	1st to 10th	Day	2 7% 23% 36% 25% 6%	490
	11th to 20th	Day	3 7% 23% 41% 21% 4	490
	21st to end	Day	5% 6% 21% 40% 23% 5%	539
November	1st to 10th	Day	5% 7% 24% 34% 25% 5%	422
	11th to 20th	Day	8% 7% 24% 37% 19% 5%	300
	21st to end	Day	11% 6% 26% 30% 23% 4	300
December	1st to 10th	Day	11% 7% 25% 36% 17% 3	255
	11th to 20th	Day	12% 11% 21% 31% 22% 5%	200
	21st to end	Day	17% 14% 24% 29% 13% 4	220
January	1st to 10th	Night	14% 4 20% 36% 21% 6%	800
	11th to 20th	Night	10% 3 18% 42% 23% 5%	800
	21st to end	Night	15% 4 17% 41% 19% 5%	880
February	1st to 10th	Night	16% 5% 14% 37% 22% 6%	800
	11th to 20th	Night	12% 5% 19% 38% 23% 4	784
	21st to end	Night	14% 4 16% 44% 19% 2	660
March	1st to 10th	Night	15% 4 19% 40% 20% 3	800
	11th to 20th	Night	13% 3 15% 40% 24% 5%	800
	21st to end	Night	13% 5% 18% 38% 23% 3	880
April	1st to 10th	Night	8% 4 17% 42% 26% 4	800
	11th to 20th	Night	8% 6% 16% 36% 29% 5%	791
	21st to end	Night	8% 5% 17% 34% 29% 8%	800
May	1st to 10th	Night	8% 3 12% 37% 32% 7%	800
	11th to 20th	Night	6% 5% 10% 36% 37% 6%	714
	21st to end	Night	6% 4 13% 36% 33% 8%	440
June	1st to 10th	Night	4 3 8% 31% 47% 8%	400
	11th to 20th	Night	3 3 7% 31% 50% 7%	400
	21st to end	Night	3 2 3 27% 54% 13%	400
July	1st to 10th	Night	1 2 3 16% 65% 14%	390
	11th to 20th	Night	3 4 8% 56% 29%	575
	21st to end	Night	1 1 2 10% 58% 27%	624
August	1st to 10th	Night	3 1 4 22% 53% 17%	685
	11th to 20th	Night	1 1 3 19% 59% 17%	752
	21st to end	Night	2 1 3 17% 58% 19%	836
September	1st to 10th	Night	4 1 4 20% 56% 16%	760
	11th to 20th	Night	4 3 5% 32% 44% 12%	760
	21st to end	Night	3 1 7% 28% 51% 10%	760
October	1st to 10th	Night	5% 1 9% 35% 43% 8%	760
	11th to 20th	Night	5% 2 10% 35% 40% 8%	760
	21st to end	Night	7% 2 11% 38% 34% 9%	836
November	1st to 10th	Night	6% 2 15% 38% 33% 7%	780
	11th to 20th	Night	6% 1 17% 43% 27% 7%	800
	21st to end	Night	7% 4 17% 42% 25% 5%	800
December	1st to 10th	Night	13% 4 16% 39% 25% 4	800
	11th to 20th	Night	15% 2 17% 41% 21% 4	800
	21st to end	Night	13% 5% 22% 40% 18% 3	880

Page generated at 2025-07-17 22:10:02. Version ac07

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