Vallecito Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vallecito Observatory clear sky chart from 2005-11-06 to 2024-11-06.

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 5% 2 4 3 4 4 4 6% 8% 56%	104945
	Night	4 7% 2 5% 3 4 4 3 5% 7% 56%	102859
Transparency	Day	23% 8% 13% 23% 23% 10%	104698
	Night	24% 8% 13% 22% 24% 9%	102696
Seeing	Day	9% 17% 30% 27% 15% 3	15268
	Night	7% 9% 32% 35% 16% 1	25325

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	1 8% 5% 6% 3 3 4 3 5% 8% 54%	2634
	11th to 20th	Day	2 12% 5% 9% 3 3 3 2 3 6% 53%	2860
	21st to end	Day	4 9% 4 6% 4 5% 3 4 4 6% 51%	3146
February	1st to 10th	Day	2 9% 4 6% 3 4 4 2 4 6% 56%	1743
	11th to 20th	Day	2 9% 3 7% 3 3 3 3 4 8% 55%	1849
	21st to end	Day	3 11% 3 6% 3 3 3 3 5% 8% 52%	1570
March	1st to 10th	Day	2 8% 3 4 3 3 3 2 4 6% 63%	3212
	11th to 20th	Day	3 10% 2 6% 5% 5% 5% 3 7% 8% 46%	3490
	21st to end	Day	2 10% 3 6% 4 4 5% 3 5% 5% 53%	4000
April	1st to 10th	Day	3 6% 2 4 3 3 4 4 5% 6% 59%	2280
	11th to 20th	Day	3 7% 2 5% 3 4 4 4 5% 6% 58%	2251
	21st to end	Day	4 6% 2 4 4 4 3 3 5% 8% 56%	2280
May	1st to 10th	Day	5% 4 1 3 3 3 3 4 5% 6% 62%	3800
	11th to 20th	Day	4 5% 2 5% 4 5% 5% 5% 7% 7% 51%	4116
	21st to end	Day	2 2 1 2 2 2 4 3 5% 8% 68%	4825
June	1st to 10th	Day	3 2 1 3 3 4 4 4 7% 10% 60%	2660
	11th to 20th	Day	2 2 1 3 2 3 3 4 6% 7% 67%	2660
	21st to end	Day	4 3 1 4 4 5% 5% 5% 8% 7% 55%	2660
July	1st to 10th	Day	3 3 1 4 4 4 5% 5% 7% 7% 56%	4430
	11th to 20th	Day	2 2 1 3 5% 6% 8% 7% 11% 11% 44%	4278
	21st to end	Day	4 4 2 5% 5% 6% 7% 6% 10% 11% 39%	4454
August	1st to 10th	Day	4 4 2 6% 6% 7% 8% 7% 10% 11% 36%	3977
	11th to 20th	Day	2 2 1 3 3 4 5% 4 6% 11% 58%	3726
	21st to end	Day	4 4 1 5% 4 5% 6% 6% 10% 12% 42%	4180
September	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 7% 7% 51%	2280
	11th to 20th	Day	5% 4 2 5% 4 4 4 3 7% 6% 57%	2277
	21st to end	Day	4 2 1 3 2 3 4 3 5% 7% 66%	2085
October	1st to 10th	Day	4 2 1 3 3 2 3 2 3 6% 71%	3170
	11th to 20th	Day	5% 4 1 5% 3 3 5% 4 5% 5% 59%	3150
	21st to end	Day	4 3 1 2 2 2 2 2 3 4 74%	3438
November	1st to 10th	Day	3 4 1 3 2 2 2 3 5% 7% 70%	1775
	11th to 20th	Day	3 3 1 2 2 2 3 2 4 6% 71%	1710
	21st to end	Day	3 9% 1 5% 4 3 3 2 4 5% 61%	1710
December	1st to 10th	Day	5% 8% 2 4 3 3 2 2 4 6% 62%	1549
	11th to 20th	Day	3 8% 3 5% 4 4 4 4 7% 9% 51%	1926
	21st to end	Day	3 9% 3 5% 3 3 2 3 5% 6% 59%	2794
January	1st to 10th	Night	2 9% 6% 9% 4 4 3 2 4 5% 52%	4350
	11th to 20th	Night	3 15% 6% 7% 3 3 3 2 4 5% 48%	4110
	21st to end	Night	4 11% 3 6% 3 4 3 2 4 7% 51%	4521
February	1st to 10th	Night	3 10% 3 6% 3 3 3 2 4 7% 57%	2434
	11th to 20th	Night	3 10% 3 6% 3 3 4 3 4 6% 54%	2218
	21st to end	Night	5% 13% 2 6% 2 2 2 2 4 6% 55%	1884
March	1st to 10th	Night	2 10% 3 5% 4 4 3 2 4 4 60%	3653
	11th to 20th	Night	4 12% 3 7% 3 4 4 3 5% 6% 50%	3490
	21st to end	Night	8% 15% 3 5% 3 2 3 2 3 5% 52%	3645
April	1st to 10th	Night	5% 6% 2 4 3 3 3 3 4 7% 60%	1900
	11th to 20th	Night	4 7% 2 4 3 3 5% 3 5% 6% 57%	1877
	21st to end	Night	5% 7% 2 5% 3 3 3 3 5% 5% 58%	1900
May	1st to 10th	Night	6% 5% 1 4 3 3 3 3 5% 5% 62%	3170
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 7% 8% 50%	2785
	21st to end	Night	4 3 1 3 3 3 4 3 5% 7% 65%	2762
June	1st to 10th	Night	2 2 1 3 3 4 4 5% 8% 8% 61%	1520
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 7% 68%	1520
	21st to end	Night	4 3 1 4 4 3 4 4 8% 10% 53%	1520
July	1st to 10th	Night	3 3 2 6% 6% 5% 6% 5% 7% 8% 50%	2540
	11th to 20th	Night	2 2 1 4 4 6% 6% 7% 10% 13% 45%	2687
	21st to end	Night	5% 4 2 6% 4 5% 7% 6% 9% 11% 42%	3109
August	1st to 10th	Night	4 3 2 6% 5% 7% 9% 7% 11% 12% 33%	2960
	11th to 20th	Night	3 3 1 5% 4 4 6% 6% 9% 14% 45%	3113
	21st to end	Night	3 3 2 7% 6% 7% 9% 7% 11% 12% 34%	3487
September	1st to 10th	Night	6% 4 1 4 3 4 6% 5% 7% 8% 52%	1900
	11th to 20th	Night	5% 2 1 4 3 3 4 3 6% 8% 62%	1901
	21st to end	Night	5% 3 1 4 3 3 3 2 4 6% 66%	2090
October	1st to 10th	Night	4 3 1 2 2 2 2 2 4 5% 72%	3800
	11th to 20th	Night	6% 3 1 3 3 3 3 3 4 7% 64%	3778
	21st to end	Night	4 3 1 2 2 2 2 2 3 5% 74%	4120
November	1st to 10th	Night	3 2 1 3 2 2 2 2 4 6% 74%	2405
	11th to 20th	Night	3 4 1 2 1 2 2 3 4 5% 74%	2470
	21st to end	Night	5% 8% 2 4 2 3 3 3 5% 6% 60%	2470
December	1st to 10th	Night	6% 8% 2 4 2 2 3 2 3 6% 61%	2626
	11th to 20th	Night	4 8% 2 6% 3 3 4 3 6% 8% 52%	3271
	21st to end	Night	3 10% 4 6% 4 4 3 2 4 5% 54%	4873

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	24% 6% 13% 32% 22% 3	2635
	11th to 20th	Day	24% 11% 19% 30% 14% 2	2860
	21st to end	Day	30% 9% 12% 25% 21% 4	3146
February	1st to 10th	Day	28% 6% 14% 23% 22% 6%	1743
	11th to 20th	Day	26% 5% 12% 25% 26% 6%	1859
	21st to end	Day	29% 6% 8% 18% 28% 11%	1570
March	1st to 10th	Day	19% 7% 9% 22% 34% 9%	3212
	11th to 20th	Day	29% 11% 15% 25% 17% 4	3490
	21st to end	Day	26% 9% 12% 30% 18% 6%	4000
April	1st to 10th	Day	23% 5% 11% 23% 25% 12%	2280
	11th to 20th	Day	24% 7% 9% 21% 29% 12%	2254
	21st to end	Day	23% 6% 11% 24% 27% 8%	2280
May	1st to 10th	Day	17% 8% 13% 16% 28% 18%	3800
	11th to 20th	Day	24% 11% 13% 23% 20% 9%	4120
	21st to end	Day	12% 6% 9% 16% 33% 24%	4825
June	1st to 10th	Day	17% 8% 14% 22% 22% 16%	2660
	11th to 20th	Day	13% 6% 10% 17% 25% 28%	2660
	21st to end	Day	25% 7% 13% 20% 20% 16%	2660
July	1st to 10th	Day	23% 9% 12% 15% 19% 21%	4361
	11th to 20th	Day	25% 12% 20% 32% 8% 4	4203
	21st to end	Day	32% 12% 20% 26% 7% 3	4314
August	1st to 10th	Day	36% 13% 21% 19% 7% 4	3963
	11th to 20th	Day	19% 8% 18% 26% 17% 11%	3762
	21st to end	Day	29% 11% 22% 23% 12% 2	4179
September	1st to 10th	Day	28% 9% 14% 20% 20% 8%	2280
	11th to 20th	Day	25% 7% 11% 18% 26% 13%	2277
	21st to end	Day	17% 5% 11% 24% 32% 11%	2085
October	1st to 10th	Day	16% 4 7% 16% 37% 21%	3170
	11th to 20th	Day	23% 5% 8% 21% 32% 11%	3150
	21st to end	Day	14% 2 7% 24% 42% 11%	3438
November	1st to 10th	Day	14% 4 12% 24% 35% 11%	1775
	11th to 20th	Day	15% 3 12% 32% 32% 7%	1710
	21st to end	Day	26% 4 8% 22% 29% 10%	1710
December	1st to 10th	Day	22% 7% 17% 26% 21% 8%	1549
	11th to 20th	Day	26% 8% 15% 19% 26% 6%	1924
	21st to end	Day	22% 5% 12% 24% 29% 8%	2794
January	1st to 10th	Night	29% 7% 13% 25% 22% 4	4350
	11th to 20th	Night	33% 11% 13% 24% 16% 3	4110
	21st to end	Night	30% 8% 12% 27% 20% 3	4521
February	1st to 10th	Night	29% 5% 14% 23% 23% 7%	2434
	11th to 20th	Night	30% 5% 11% 25% 24% 5%	2218
	21st to end	Night	30% 3 9% 18% 31% 9%	1884
March	1st to 10th	Night	21% 6% 10% 21% 36% 7%	3653
	11th to 20th	Night	30% 9% 12% 25% 20% 4	3490
	21st to end	Night	28% 10% 11% 29% 20% 3	3645
April	1st to 10th	Night	23% 4 11% 26% 25% 11%	1900
	11th to 20th	Night	23% 7% 12% 20% 26% 12%	1877
	21st to end	Night	25% 7% 10% 24% 26% 8%	1900
May	1st to 10th	Night	15% 10% 12% 16% 29% 17%	3170
	11th to 20th	Night	24% 14% 17% 21% 15% 10%	2786
	21st to end	Night	12% 7% 13% 16% 30% 22%	2762
June	1st to 10th	Night	16% 11% 16% 19% 20% 17%	1520
	11th to 20th	Night	15% 5% 8% 15% 26% 30%	1520
	21st to end	Night	23% 9% 15% 18% 20% 15%	1520
July	1st to 10th	Night	28% 10% 12% 15% 12% 23%	2500
	11th to 20th	Night	23% 13% 26% 26% 7% 5%	2639
	21st to end	Night	29% 13% 23% 25% 9% 1	3011
August	1st to 10th	Night	35% 15% 23% 15% 7% 5%	2951
	11th to 20th	Night	25% 11% 22% 17% 16% 9%	3143
	21st to end	Night	35% 13% 21% 18% 10% 3	3487
September	1st to 10th	Night	26% 9% 16% 22% 18% 9%	1900
	11th to 20th	Night	19% 8% 15% 18% 26% 15%	1901
	21st to end	Night	19% 5% 11% 22% 31% 11%	2090
October	1st to 10th	Night	14% 4 8% 15% 41% 17%	3800
	11th to 20th	Night	19% 6% 8% 19% 35% 13%	3778
	21st to end	Night	14% 3 7% 25% 41% 11%	4120
November	1st to 10th	Night	13% 3 9% 26% 37% 11%	2405
	11th to 20th	Night	13% 3 10% 33% 32% 9%	2470
	21st to end	Night	24% 4 10% 22% 29% 11%	2470
December	1st to 10th	Night	25% 5% 12% 29% 24% 5%	2626
	11th to 20th	Night	27% 6% 14% 22% 25% 5%	3272
	21st to end	Night	29% 6% 13% 23% 25% 4	4873

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	9% 31% 28% 22% 10%	270
	11th to 20th	Day	14% 22% 31% 26% 5% 1	270
	21st to end	Day	5% 16% 41% 23% 12% 2	297
February	1st to 10th	Day	10% 29% 34% 21% 6%	270
	11th to 20th	Day	7% 36% 34% 18% 4	270
	21st to end	Day	15% 37% 31% 16% 2	225
March	1st to 10th	Day	11% 30% 34% 19% 5% 1	313
	11th to 20th	Day	15% 27% 32% 20% 5% 1	460
	21st to end	Day	18% 39% 28% 13% 1	506
April	1st to 10th	Day	14% 32% 35% 14% 4 1	460
	11th to 20th	Day	16% 40% 30% 11% 3 1	452
	21st to end	Day	17% 33% 36% 13% 2	460
May	1st to 10th	Day	13% 31% 40% 13% 3	463
	11th to 20th	Day	10% 18% 34% 26% 10% 2	623
	21st to end	Day	9% 18% 33% 29% 10% 1	734
June	1st to 10th	Day	3 9% 32% 34% 21% 1	680
	11th to 20th	Day	4 13% 34% 31% 17% 2	680
	21st to end	Day	5% 9% 31% 31% 20% 4	680
July	1st to 10th	Day	6% 3 20% 35% 31% 5%	671
	11th to 20th	Day	3 2 16% 39% 31% 9%	581
	21st to end	Day	8% 1 14% 36% 35% 6%	531
August	1st to 10th	Day	7% 3 21% 38% 26% 6%	487
	11th to 20th	Day	4 3 23% 41% 23% 5%	477
	21st to end	Day	7% 3 21% 41% 23% 5%	539
September	1st to 10th	Day	7% 4 24% 33% 26% 6%	490
	11th to 20th	Day	5% 7% 34% 35% 16% 3	487
	21st to end	Day	4 11% 26% 33% 20% 6%	300
October	1st to 10th	Day	7% 18% 22% 28% 19% 6%	300
	11th to 20th	Day	9% 16% 31% 27% 13% 4	300
	21st to end	Day	11% 19% 27% 28% 12% 2	330
November	1st to 10th	Day	10% 22% 32% 26% 8% 1	285
	11th to 20th	Day	7% 16% 35% 25% 16% 1	270
	21st to end	Day	10% 23% 33% 22% 10% 2	270
December	1st to 10th	Day	12% 24% 33% 20% 10% 2	270
	11th to 20th	Day	7% 22% 41% 22% 6% 1	270
	21st to end	Day	16% 27% 36% 16% 4 1	297
January	1st to 10th	Night	8% 13% 34% 33% 12% 1	909
	11th to 20th	Night	10% 10% 37% 31% 10% 2	760
	21st to end	Night	9% 7% 41% 30% 12% 1	836
February	1st to 10th	Night	9% 11% 43% 30% 7%	760
	11th to 20th	Night	9% 12% 49% 25% 6% 1	744
	21st to end	Night	13% 16% 47% 23% 1	628
March	1st to 10th	Night	10% 15% 41% 28% 6%	760
	11th to 20th	Night	9% 11% 33% 36% 11%	760
	21st to end	Night	11% 16% 42% 26% 5%	730
April	1st to 10th	Night	10% 19% 43% 25% 3	570
	11th to 20th	Night	8% 21% 37% 29% 5%	563
	21st to end	Night	8% 15% 43% 29% 6%	570
May	1st to 10th	Night	7% 11% 41% 33% 8%	570
	11th to 20th	Night	8% 10% 32% 37% 13% 1	561
	21st to end	Night	6% 11% 35% 39% 9%	617
June	1st to 10th	Night	2 4 27% 48% 19%	570
	11th to 20th	Night	2 5% 33% 47% 13% 1	570
	21st to end	Night	3 3 22% 44% 27% 2	570
July	1st to 10th	Night	4 1 13% 46% 34% 3	555
	11th to 20th	Night	2 8% 39% 46% 5%	552
	21st to end	Night	4 1 6% 33% 49% 7%	624
August	1st to 10th	Night	4 2 7% 35% 48% 5%	567
	11th to 20th	Night	3 2 11% 44% 37% 3	564
	21st to end	Night	3 1 10% 44% 39% 4	627
September	1st to 10th	Night	5% 1 14% 47% 30% 2	570
	11th to 20th	Night	5% 2 17% 47% 27% 1	570
	21st to end	Night	5% 5% 21% 48% 19% 1	570
October	1st to 10th	Night	7% 5% 29% 40% 18% 2	760
	11th to 20th	Night	5% 6% 36% 36% 14% 2	760
	21st to end	Night	7% 8% 31% 35% 15% 4	836
November	1st to 10th	Night	5% 9% 31% 40% 13% 1	877
	11th to 20th	Night	5% 11% 35% 35% 13% 2	950
	21st to end	Night	9% 10% 34% 32% 13% 1	950
December	1st to 10th	Night	10% 13% 38% 28% 10% 1	950
	11th to 20th	Night	11% 11% 37% 30% 9%	950
	21st to end	Night	10% 13% 43% 28% 6% 1	1045

Page generated at 2024-11-03 11:19:05. Version ac07

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