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 2024-11-04.

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	5% 4 1 4 3 3 3 3 4 7% 64%	104743
	Night	7% 6% 2 5% 3 3 4 3 4 6% 57%	103044
Transparency	Day	22% 3 7% 20% 27% 22%	104501
	Night	27% 4 7% 18% 23% 21%	102877
Seeing	Day	7% 8% 23% 35% 21% 5%	15604
	Night	8% 3 12% 33% 35% 9%	25189

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	8% 3 1 5% 3 3 4 3 5% 6% 59%	2731
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 6% 8% 46%	2860
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 9% 42%	3146
February	1st to 10th	Day	14% 5% 2 4 2 3 3 2 4 6% 57%	1790
	11th to 20th	Day	9% 5% 1 6% 3 3 3 4 4 7% 54%	1850
	21st to end	Day	12% 5% 1 5% 3 4 4 3 5% 5% 53%	1570
March	1st to 10th	Day	13% 6% 3 6% 4 4 4 3 6% 8% 44%	3229
	11th to 20th	Day	6% 4 1 3 3 3 3 3 4 6% 64%	3490
	21st to end	Day	8% 7% 2 5% 4 5% 5% 5% 7% 8% 46%	4000
April	1st to 10th	Day	9% 3 1 4 3 4 3 3 6% 6% 57%	2280
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 5% 8% 59%	2249
	21st to end	Day	6% 4 1 4 2 2 3 3 4 7% 63%	2280
May	1st to 10th	Day	4 3 1 3 1 2 2 1 2 4 76%	3800
	11th to 20th	Day	1 6% 2 5% 4 3 3 3 4 7% 61%	3877
	21st to end	Day	3 4 2 4 2 3 3 2 4 7% 67%	4802
June	1st to 10th	Day	4 3 2 4 4 3 4 3 5% 9% 61%	2660
	11th to 20th	Day	1 4 1 3 2 2 3 2 4 7% 70%	2660
	21st to end	Day	1 3 1 3 3 3 3 2 4 8% 69%	2660
July	1st to 10th	Day	1 2 1 2 2 2 3 2 3 6% 76%	4420
	11th to 20th	Day	2 2 1 4 3 3 3 3 4 7% 69%	4112
	21st to end	Day	1 2 1 3 3 3 4 3 5% 8% 67%	4455
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	6% 3 1 3 2 3 3 2 4 5% 66%	1821
	11th to 20th	Day	6% 3 1 3 2 3 3 3 3 6% 67%	1710
	21st to end	Day	7% 5% 1 4 3 3 3 3 4 6% 61%	1710
December	1st to 10th	Day	9% 6% 2 5% 4 4 3 3 4 7% 55%	1628
	11th to 20th	Day	23% 6% 3 5% 3 3 4 3 4 5% 41%	1928
	21st to end	Day	16% 6% 2 5% 4 3 4 3 5% 7% 45%	2794
January	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 5% 58%	4217
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 8% 45%	4110
	21st to end	Night	18% 7% 2 5% 4 4 4 3 4 6% 43%	4521
February	1st to 10th	Night	13% 5% 2 4 3 3 2 3 5% 4 57%	2387
	11th to 20th	Night	9% 6% 1 3 3 3 4 3 4 7% 58%	2218
	21st to end	Night	11% 6% 2 5% 4 4 3 2 2 6% 55%	1884
March	1st to 10th	Night	13% 10% 3 6% 5% 4 4 4 6% 7% 40%	3629
	11th to 20th	Night	6% 4 1 4 3 3 4 3 4 7% 61%	3490
	21st to end	Night	14% 8% 3 5% 3 3 3 3 4 6% 48%	3653
April	1st to 10th	Night	7% 6% 2 4 4 3 3 2 4 7% 57%	1900
	11th to 20th	Night	6% 7% 3 3 2 3 2 3 3 6% 62%	1879
	21st to end	Night	5% 8% 2 3 2 3 3 2 4 8% 60%	1900
May	1st to 10th	Night	3 5% 1 3 2 2 2 2 3 5% 73%	3180
	11th to 20th	Night	2 15% 6% 7% 3 3 3 3 4 5% 48%	3029
	21st to end	Night	3 13% 5% 7% 2 3 3 2 4 5% 53%	2769
June	1st to 10th	Night	3 8% 3 5% 4 5% 3 3 5% 8% 54%	1520
	11th to 20th	Night	2 7% 3 3 3 4 3 3 5% 7% 60%	1520
	21st to end	Night	1 5% 2 3 3 5% 3 4 5% 9% 59%	1520
July	1st to 10th	Night	1 2 1 3 2 3 4 3 5% 6% 71%	2540
	11th to 20th	Night	1 2 3 7% 4 5% 5% 3 5% 8% 58%	2852
	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% 64%	2327
	11th to 20th	Night	4 4 1 4 2 3 4 3 5% 6% 65%	2470
	21st to end	Night	5% 4 2 4 3 3 4 3 5% 6% 60%	2470
December	1st to 10th	Night	11% 6% 2 3 3 3 3 3 4 6% 56%	2550
	11th to 20th	Night	21% 8% 2 5% 4 3 4 4 5% 5% 40%	3276
	21st to end	Night	14% 7% 3 6% 4 4 4 4 5% 6% 43%	4862

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	27% 2 9% 37% 20% 5%	2731
	11th to 20th	Day	37% 3 13% 19% 17% 11%	2860
	21st to end	Day	40% 2 10% 24% 14% 9%	3146
February	1st to 10th	Day	32% 2 6% 20% 22% 18%	1790
	11th to 20th	Day	30% 3 9% 24% 24% 10%	1860
	21st to end	Day	33% 3 11% 21% 25% 8%	1570
March	1st to 10th	Day	39% 3 13% 22% 17% 6%	3229
	11th to 20th	Day	23% 2 13% 33% 16% 12%	3490
	21st to end	Day	35% 3 14% 29% 14% 4	4000
April	1st to 10th	Day	28% 4 12% 28% 20% 8%	2280
	11th to 20th	Day	24% 3 14% 27% 23% 8%	2252
	21st to end	Day	22% 3 11% 28% 29% 7%	2280
May	1st to 10th	Day	16% 2 6% 29% 35% 12%	3800
	11th to 20th	Day	21% 6% 4 27% 32% 9%	3880
	21st to end	Day	19% 5% 6% 31% 35% 4	4803
June	1st to 10th	Day	21% 4 11% 26% 28% 10%	2660
	11th to 20th	Day	16% 3 6% 15% 41% 19%	2660
	21st to end	Day	16% 3 4 10% 34% 33%	2660
July	1st to 10th	Day	9% 3 2 7% 18% 61%	4351
	11th to 20th	Day	15% 3 4 10% 21% 47%	4042
	21st to end	Day	13% 3 6% 14% 28% 35%	4315
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	21% 2 6% 20% 28% 22%	1821
	11th to 20th	Day	21% 2 8% 20% 27% 23%	1710
	21st to end	Day	25% 4 7% 16% 27% 23%	1710
December	1st to 10th	Day	31% 4 7% 18% 23% 17%	1628
	11th to 20th	Day	46% 3 8% 18% 17% 9%	1928
	21st to end	Day	40% 3 10% 26% 15% 6%	2794
January	1st to 10th	Night	30% 2 9% 29% 25% 6%	4217
	11th to 20th	Night	39% 3 9% 23% 14% 12%	4110
	21st to end	Night	44% 3 9% 19% 16% 10%	4521
February	1st to 10th	Night	32% 3 6% 18% 26% 16%	2387
	11th to 20th	Night	28% 2 7% 22% 26% 14%	2219
	21st to end	Night	32% 3 8% 23% 24% 9%	1884
March	1st to 10th	Night	43% 3 11% 23% 15% 6%	3629
	11th to 20th	Night	25% 3 11% 28% 23% 10%	3490
	21st to end	Night	38% 3 15% 23% 16% 4	3653
April	1st to 10th	Night	28% 5% 13% 26% 18% 10%	1900
	11th to 20th	Night	25% 5% 9% 27% 25% 9%	1879
	21st to end	Night	24% 5% 8% 27% 28% 8%	1900
May	1st to 10th	Night	17% 3 6% 27% 28% 19%	3180
	11th to 20th	Night	35% 8% 4 19% 24% 9%	3031
	21st to end	Night	31% 9% 7% 18% 31% 4	2769
June	1st to 10th	Night	28% 3 11% 20% 27% 11%	1520
	11th to 20th	Night	23% 5% 4 12% 34% 22%	1520
	21st to end	Night	22% 6% 3 6% 29% 34%	1520
July	1st to 10th	Night	11% 5% 2 9% 14% 60%	2500
	11th to 20th	Night	21% 5% 3 13% 14% 44%	2799
	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	22% 3 7% 19% 26% 23%	2327
	11th to 20th	Night	21% 3 10% 18% 26% 23%	2470
	21st to end	Night	25% 3 9% 18% 22% 23%	2470
December	1st to 10th	Night	30% 4 7% 16% 24% 19%	2550
	11th to 20th	Night	46% 3 8% 15% 18% 9%	3276
	21st to end	Night	44% 2 9% 23% 15% 7%	4862

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	22% 9% 30% 25% 12% 2	294
	11th to 20th	Day	13% 10% 32% 28% 15% 2	370
	21st to end	Day	11% 12% 31% 29% 15% 2	407
February	1st to 10th	Day	14% 12% 26% 27% 17% 3	408
	11th to 20th	Day	13% 11% 29% 30% 14% 3	455
	21st to end	Day	9% 14% 34% 30% 10% 2	382
March	1st to 10th	Day	18% 14% 32% 29% 8%	460
	11th to 20th	Day	15% 12% 31% 31% 9% 2	460
	21st to end	Day	14% 17% 35% 26% 7% 2	506
April	1st to 10th	Day	12% 14% 33% 26% 12% 3	460
	11th to 20th	Day	12% 13% 28% 33% 11% 3	452
	21st to end	Day	6% 13% 31% 31% 15% 5%	460
May	1st to 10th	Day	9% 10% 29% 33% 17% 3	463
	11th to 20th	Day	4 10% 27% 40% 16% 3	488
	21st to end	Day	9% 12% 24% 34% 17% 4	529
June	1st to 10th	Day	6% 5% 20% 38% 26% 6%	490
	11th to 20th	Day	4 6% 25% 36% 24% 6%	490
	21st to end	Day	4 3 19% 40% 25% 10%	490
July	1st to 10th	Day	2 15% 41% 32% 10%	486
	11th to 20th	Day	2 1 10% 41% 33% 12%	484
	21st to end	Day	2 1 7% 41% 37% 13%	531
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	6% 7% 23% 35% 25% 5%	399
	11th to 20th	Day	8% 6% 23% 36% 21% 6%	270
	21st to end	Day	9% 6% 25% 30% 25% 5%	270
December	1st to 10th	Day	13% 8% 27% 36% 16% 1	230
	11th to 20th	Day	12% 11% 20% 31% 21% 5%	180
	21st to end	Day	17% 13% 24% 29% 13% 5%	198
January	1st to 10th	Night	14% 4 18% 35% 23% 6%	760
	11th to 20th	Night	10% 3 16% 42% 23% 6%	760
	21st to end	Night	15% 3 17% 41% 18% 5%	836
February	1st to 10th	Night	15% 5% 13% 38% 24% 6%	760
	11th to 20th	Night	11% 5% 18% 38% 24% 4	744
	21st to end	Night	14% 4 17% 43% 19% 2	628
March	1st to 10th	Night	15% 4 18% 40% 20% 3	760
	11th to 20th	Night	12% 3 14% 40% 25% 6%	760
	21st to end	Night	13% 5% 17% 37% 24% 3	836
April	1st to 10th	Night	8% 4 16% 42% 26% 4	760
	11th to 20th	Night	8% 6% 16% 36% 29% 5%	751
	21st to end	Night	8% 5% 16% 35% 29% 8%	760
May	1st to 10th	Night	8% 3 13% 37% 32% 7%	760
	11th to 20th	Night	6% 5% 10% 35% 38% 7%	678
	21st to end	Night	6% 4 13% 35% 33% 9%	418
June	1st to 10th	Night	4 3 8% 31% 47% 7%	380
	11th to 20th	Night	3 3 7% 29% 51% 7%	380
	21st to end	Night	3 2 2 27% 55% 12%	380
July	1st to 10th	Night	1 2 3 15% 65% 14%	370
	11th to 20th	Night	3 3 8% 58% 27%	545
	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 14% 38% 33% 7%	754
	11th to 20th	Night	6% 1 15% 42% 28% 7%	760
	21st to end	Night	6% 4 16% 43% 26% 5%	760
December	1st to 10th	Night	13% 4 16% 38% 25% 4	760
	11th to 20th	Night	16% 3 17% 39% 22% 4	760
	21st to end	Night	13% 5% 22% 39% 19% 3	836

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