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 2025-10-19.

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% 57%	134877
	Night	4 6% 2 5% 3 4 4 3 5% 7% 57%	132305
Transparency	Day	22% 7% 13% 23% 24% 11%	129883
	Night	23% 7% 13% 23% 25% 10%	128303
Seeing	Day	9% 17% 29% 28% 14% 3	16568
	Night	7% 9% 32% 35% 15% 1	26582

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% 4 6% 3 3 4 3 4 6% 57%	3776
	11th to 20th	Day	1 8% 4 8% 3 3 3 2 4 6% 57%	4100
	21st to end	Day	3 7% 3 5% 3 4 3 3 4 6% 58%	4510
February	1st to 10th	Day	2 9% 4 6% 3 4 4 2 4 6% 57%	1835
	11th to 20th	Day	3 9% 3 7% 3 3 3 3 4 7% 55%	1949
	21st to end	Day	3 11% 3 6% 3 3 3 3 5% 7% 54%	1650
March	1st to 10th	Day	2 9% 3 4 3 3 3 2 4 6% 60%	4610
	11th to 20th	Day	2 11% 2 6% 5% 4 5% 3 7% 8% 47%	5000
	21st to end	Day	2 8% 2 5% 4 4 4 3 6% 7% 55%	5739
April	1st to 10th	Day	3 7% 2 4 3 3 4 4 5% 6% 59%	2400
	11th to 20th	Day	3 7% 2 5% 3 4 4 4 5% 6% 58%	2371
	21st to end	Day	4 6% 2 4 4 4 3 3 5% 8% 57%	2400
May	1st to 10th	Day	5% 5% 1 4 3 4 4 4 7% 8% 53%	5440
	11th to 20th	Day	4 4 2 4 3 4 4 4 5% 6% 61%	5906
	21st to end	Day	2 2 1 2 2 3 4 4 6% 9% 66%	6925
June	1st to 10th	Day	3 2 1 3 3 4 4 4 7% 10% 59%	2800
	11th to 20th	Day	2 2 1 3 2 3 3 4 6% 7% 68%	2800
	21st to end	Day	4 3 1 4 4 4 5% 5% 8% 7% 55%	2800
July	1st to 10th	Day	2 3 1 3 3 4 4 4 7% 7% 61%	6339
	11th to 20th	Day	1 2 1 3 4 6% 8% 7% 11% 12% 47%	6118
	21st to end	Day	3 3 2 5% 5% 6% 7% 6% 9% 11% 42%	6401
August	1st to 10th	Day	3 3 1 5% 5% 6% 7% 6% 9% 11% 44%	5616
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 7% 11% 56%	5366
	21st to end	Day	4 3 1 5% 4 5% 7% 6% 10% 12% 43%	5984
September	1st to 10th	Day	6% 4 1 5% 4 5% 5% 5% 7% 7% 51%	2400
	11th to 20th	Day	5% 3 2 5% 3 4 4 3 7% 6% 57%	2397
	21st to end	Day	4 3 1 3 2 3 4 3 5% 7% 65%	2195
October	1st to 10th	Day	4 2 1 3 2 2 3 2 3 6% 71%	3270
	11th to 20th	Day	5% 4 1 5% 3 3 5% 4 5% 5% 59%	3230
	21st to end	Day	4 3 1 2 2 2 2 2 3 4 74%	3438
November	1st to 10th	Day	3 5% 1 3 2 2 2 3 5% 7% 68%	1820
	11th to 20th	Day	3 3 1 2 2 2 3 2 4 6% 71%	1800
	21st to end	Day	4 8% 1 5% 4 3 3 2 4 5% 61%	1800
December	1st to 10th	Day	3 4 1 2 1 2 1 2 2 6% 75%	2662
	11th to 20th	Day	2 5% 2 3 3 3 4 3 6% 9% 61%	3026
	21st to end	Day	2 7% 2 4 3 3 3 2 4 6% 63%	4004
January	1st to 10th	Night	1 7% 5% 8% 4 4 3 3 4 6% 56%	6218
	11th to 20th	Night	2 11% 5% 8% 4 3 4 3 4 4 53%	5880
	21st to end	Night	3 9% 3 6% 3 4 4 2 4 6% 55%	6468
February	1st to 10th	Night	3 10% 3 5% 3 3 3 2 3 7% 58%	2562
	11th to 20th	Night	4 10% 3 6% 3 3 4 3 4 6% 54%	2338
	21st to end	Night	5% 13% 2 6% 2 2 2 2 4 5% 56%	1980
March	1st to 10th	Night	2 12% 3 5% 4 3 3 2 4 4 58%	5254
	11th to 20th	Night	4 12% 3 6% 4 4 4 2 4 6% 52%	5000
	21st to end	Night	6% 11% 2 4 3 3 3 3 4 7% 54%	5222
April	1st to 10th	Night	5% 6% 2 4 3 3 3 3 5% 7% 59%	2000
	11th to 20th	Night	4 7% 2 5% 3 4 5% 3 5% 6% 57%	1977
	21st to end	Night	4 7% 2 5% 3 3 3 3 5% 5% 59%	2000
May	1st to 10th	Night	5% 5% 2 5% 3 4 4 4 6% 7% 54%	4540
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 5% 7% 58%	4005
	21st to end	Night	3 3 1 3 3 3 4 3 6% 8% 63%	3972
June	1st to 10th	Night	2 2 1 3 3 4 4 5% 8% 8% 60%	1600
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 7% 68%	1600
	21st to end	Night	4 4 1 4 4 3 5% 4 8% 10% 53%	1600
July	1st to 10th	Night	3 3 2 5% 5% 5% 6% 5% 7% 7% 54%	3640
	11th to 20th	Night	1 2 1 4 4 5% 7% 7% 10% 13% 46%	3857
	21st to end	Night	4 3 2 6% 5% 6% 7% 6% 11% 12% 39%	4472
August	1st to 10th	Night	3 3 2 5% 4 6% 7% 6% 9% 11% 44%	4194
	11th to 20th	Night	2 3 1 4 4 5% 6% 6% 9% 14% 46%	4483
	21st to end	Night	5% 4 2 8% 6% 6% 8% 6% 9% 11% 35%	4993
September	1st to 10th	Night	6% 4 1 4 4 4 6% 5% 7% 8% 52%	2000
	11th to 20th	Night	5% 2 1 4 2 3 4 3 6% 8% 62%	2001
	21st to end	Night	5% 3 1 4 3 3 4 3 5% 7% 65%	2200
October	1st to 10th	Night	4 3 1 2 2 2 2 3 4 5% 72%	3920
	11th to 20th	Night	6% 3 1 4 3 3 3 3 4 7% 63%	3874
	21st to end	Night	4 3 1 2 2 2 2 2 3 5% 74%	4120
November	1st to 10th	Night	3 3 1 3 1 2 2 2 4 5% 73%	2457
	11th to 20th	Night	3 4 1 2 1 2 2 3 4 5% 74%	2600
	21st to end	Night	5% 7% 2 4 2 3 3 3 5% 6% 60%	2600
December	1st to 10th	Night	3 5% 1 3 2 2 2 2 2 4 75%	4523
	11th to 20th	Night	2 5% 1 4 2 2 2 2 4 7% 67%	5181
	21st to end	Night	3 8% 3 5% 3 4 4 3 5% 6% 57%	6974

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% 4 10% 31% 26% 4	3777
	11th to 20th	Day	22% 8% 15% 21% 23% 11%	4100
	21st to end	Day	25% 6% 10% 19% 30% 10%	4510
February	1st to 10th	Day	28% 6% 14% 23% 22% 8%	1835
	11th to 20th	Day	27% 5% 12% 24% 26% 6%	1959
	21st to end	Day	28% 6% 8% 17% 28% 13%	1650
March	1st to 10th	Day	21% 5% 9% 17% 34% 14%	4610
	11th to 20th	Day	31% 8% 12% 21% 24% 4	5000
	21st to end	Day	22% 6% 11% 37% 19% 5%	5739
April	1st to 10th	Day	23% 5% 11% 23% 26% 12%	2400
	11th to 20th	Day	24% 6% 9% 20% 29% 12%	2374
	21st to end	Day	22% 6% 11% 24% 28% 9%	2400
May	1st to 10th	Day	22% 9% 14% 20% 22% 13%	5440
	11th to 20th	Day	20% 8% 11% 18% 32% 12%	5910
	21st to end	Day	12% 5% 12% 19% 31% 21%	6926
June	1st to 10th	Day	17% 8% 14% 22% 22% 16%	2800
	11th to 20th	Day	13% 6% 10% 17% 25% 29%	2800
	21st to end	Day	24% 7% 13% 19% 20% 17%	2800
July	1st to 10th	Day	20% 9% 12% 14% 20% 25%	6271
	11th to 20th	Day	21% 11% 26% 30% 7% 5%	6043
	21st to end	Day	30% 12% 21% 24% 10% 2	6217
August	1st to 10th	Day	35% 12% 20% 20% 7% 5%	4089
	11th to 20th	Day	20% 8% 18% 26% 17% 11%	3882
	21st to end	Day	29% 11% 23% 23% 12% 2	4311
September	1st to 10th	Day	28% 9% 15% 20% 20% 8%	2400
	11th to 20th	Day	25% 7% 11% 18% 27% 12%	2397
	21st to end	Day	18% 5% 11% 24% 32% 11%	2195
October	1st to 10th	Day	16% 4 7% 16% 37% 21%	3270
	11th to 20th	Day	23% 5% 9% 20% 32% 11%	3230
	21st to end	Day	14% 2 7% 24% 42% 11%	3438
November	1st to 10th	Day	16% 4 12% 24% 34% 11%	1820
	11th to 20th	Day	14% 3 12% 31% 32% 8%	1800
	21st to end	Day	26% 4 8% 22% 30% 10%	1800
December	1st to 10th	Day	13% 4 11% 32% 34% 5%	2662
	11th to 20th	Day	19% 5% 11% 31% 30% 4	3024
	21st to end	Day	20% 4 13% 29% 29% 5%	4004
January	1st to 10th	Night	26% 5% 10% 30% 22% 6%	6218
	11th to 20th	Night	31% 7% 11% 20% 23% 9%	5880
	21st to end	Night	28% 6% 10% 21% 29% 6%	6468
February	1st to 10th	Night	28% 4 14% 23% 23% 8%	2562
	11th to 20th	Night	31% 5% 11% 24% 23% 5%	2338
	21st to end	Night	29% 3 8% 17% 32% 10%	1980
March	1st to 10th	Night	25% 4 8% 16% 36% 11%	5254
	11th to 20th	Night	30% 6% 11% 20% 28% 5%	5000
	21st to end	Night	25% 7% 11% 35% 18% 4	5222
April	1st to 10th	Night	23% 4 11% 26% 25% 11%	2000
	11th to 20th	Night	23% 7% 11% 19% 26% 12%	1977
	21st to end	Night	24% 7% 10% 24% 26% 9%	2000
May	1st to 10th	Night	21% 10% 14% 22% 21% 12%	4537
	11th to 20th	Night	19% 10% 13% 19% 21% 16%	4006
	21st to end	Night	14% 6% 15% 16% 25% 24%	3972
June	1st to 10th	Night	16% 11% 17% 19% 21% 17%	1600
	11th to 20th	Night	15% 5% 9% 15% 26% 31%	1600
	21st to end	Night	23% 9% 14% 18% 20% 16%	1600
July	1st to 10th	Night	25% 10% 13% 13% 15% 24%	3600
	11th to 20th	Night	23% 12% 29% 22% 7% 7%	3809
	21st to end	Night	30% 13% 24% 21% 10% 1	4344
August	1st to 10th	Night	34% 15% 23% 15% 8% 5%	3045
	11th to 20th	Night	25% 11% 22% 18% 16% 8%	3243
	21st to end	Night	35% 13% 21% 18% 10% 3	3597
September	1st to 10th	Night	27% 9% 15% 22% 18% 9%	2000
	11th to 20th	Night	19% 7% 15% 18% 26% 15%	2001
	21st to end	Night	19% 5% 11% 22% 32% 11%	2200
October	1st to 10th	Night	15% 4 8% 15% 41% 17%	3920
	11th to 20th	Night	20% 6% 8% 19% 34% 13%	3874
	21st to end	Night	14% 3 7% 25% 41% 11%	4120
November	1st to 10th	Night	14% 3 10% 26% 37% 11%	2457
	11th to 20th	Night	12% 3 10% 32% 33% 10%	2600
	21st to end	Night	24% 4 10% 22% 30% 10%	2600
December	1st to 10th	Night	16% 3 8% 37% 32% 5%	4523
	11th to 20th	Night	18% 4 9% 30% 34% 5%	5182
	21st to end	Night	25% 5% 14% 27% 26% 3	6974

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	8% 30% 30% 23% 9%	300
	11th to 20th	Day	14% 22% 31% 27% 5% 1	300
	21st to end	Day	5% 16% 42% 24% 11% 2	330
February	1st to 10th	Day	10% 30% 33% 21% 6%	300
	11th to 20th	Day	10% 35% 32% 19% 4	300
	21st to end	Day	13% 34% 32% 18% 2	249
March	1st to 10th	Day	12% 28% 33% 20% 6% 1	345
	11th to 20th	Day	15% 28% 33% 19% 5% 1	500
	21st to end	Day	16% 39% 29% 14% 2	550
April	1st to 10th	Day	14% 32% 34% 15% 4 1	500
	11th to 20th	Day	16% 40% 30% 11% 3 1	492
	21st to end	Day	16% 32% 36% 13% 2	500
May	1st to 10th	Day	13% 29% 39% 15% 4 1	503
	11th to 20th	Day	10% 19% 34% 25% 9% 2	670
	21st to end	Day	9% 18% 33% 30% 10% 1	789
June	1st to 10th	Day	3 9% 32% 35% 20% 2	730
	11th to 20th	Day	4 12% 33% 32% 17% 2	730
	21st to end	Day	5% 9% 30% 31% 21% 4	730
July	1st to 10th	Day	6% 3 20% 36% 31% 5%	721
	11th to 20th	Day	3 2 15% 40% 31% 9%	626
	21st to end	Day	8% 1 13% 37% 34% 7%	575
August	1st to 10th	Day	6% 3 23% 38% 25% 5%	527
	11th to 20th	Day	4 3 22% 40% 25% 5%	517
	21st to end	Day	6% 3 21% 42% 24% 5%	583
September	1st to 10th	Day	6% 4 25% 35% 25% 5%	530
	11th to 20th	Day	5% 7% 34% 35% 16% 3	527
	21st to end	Day	4 11% 26% 32% 21% 6%	330
October	1st to 10th	Day	8% 17% 22% 29% 19% 6%	330
	11th to 20th	Day	9% 17% 32% 26% 13% 4	324
	21st to end	Day	11% 19% 27% 28% 12% 2	330
November	1st to 10th	Day	11% 21% 31% 27% 8% 1	300
	11th to 20th	Day	7% 16% 34% 28% 14% 1	300
	21st to end	Day	11% 22% 31% 25% 10% 2	300
December	1st to 10th	Day	11% 22% 31% 23% 11% 2	300
	11th to 20th	Day	7% 20% 40% 27% 6% 1	300
	21st to end	Day	15% 26% 35% 18% 4 1	330
January	1st to 10th	Night	7% 13% 36% 32% 11% 1	956
	11th to 20th	Night	9% 10% 38% 32% 9% 2	800
	21st to end	Night	9% 7% 42% 30% 11% 1	880
February	1st to 10th	Night	9% 11% 44% 30% 7%	800
	11th to 20th	Night	10% 12% 49% 24% 5% 1	784
	21st to end	Night	13% 15% 47% 23% 1	660
March	1st to 10th	Night	11% 15% 40% 28% 6%	800
	11th to 20th	Night	9% 11% 35% 35% 10%	800
	21st to end	Night	11% 15% 43% 26% 5%	768
April	1st to 10th	Night	10% 19% 42% 25% 3	600
	11th to 20th	Night	8% 21% 38% 28% 5%	593
	21st to end	Night	8% 15% 43% 29% 5%	600
May	1st to 10th	Night	7% 10% 41% 34% 8%	600
	11th to 20th	Night	7% 11% 33% 36% 12% 1	591
	21st to end	Night	6% 10% 36% 39% 8%	650
June	1st to 10th	Night	3 4 28% 48% 18%	600
	11th to 20th	Night	2 5% 33% 47% 13% 1	600
	21st to end	Night	3 3 22% 44% 26% 2	600
July	1st to 10th	Night	4 1 14% 47% 33% 3	585
	11th to 20th	Night	2 8% 41% 45% 4	582
	21st to end	Night	4 1 6% 34% 48% 6%	657
August	1st to 10th	Night	3 2 7% 37% 46% 5%	597
	11th to 20th	Night	3 2 11% 46% 35% 3	594
	21st to end	Night	3 2 10% 44% 37% 3	660
September	1st to 10th	Night	5% 2 15% 47% 29% 2	600
	11th to 20th	Night	5% 2 19% 47% 26% 1	600
	21st to end	Night	5% 5% 22% 48% 18% 1	600
October	1st to 10th	Night	7% 5% 30% 40% 17% 2	800
	11th to 20th	Night	6% 6% 36% 36% 13% 2	792
	21st to end	Night	7% 8% 31% 35% 15% 4	836
November	1st to 10th	Night	6% 9% 31% 39% 13% 1	897
	11th to 20th	Night	5% 10% 37% 35% 12% 2	1000
	21st to end	Night	9% 10% 35% 32% 13% 1	1000
December	1st to 10th	Night	9% 13% 38% 29% 10% 1	1000
	11th to 20th	Night	11% 11% 38% 30% 9%	1000
	21st to end	Night	10% 13% 43% 28% 6% 1	1100

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