Santa Rosa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Santa Rosa clear sky chart from 2005-11-06 to 2026-03-03.

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	8% 5% 2 4 3 3 3 2 4 6% 61%	139912
	Night	11% 6% 2 6% 3 3 3 3 4 5% 53%	138735
Transparency	Day	27% 3 7% 20% 24% 18%	134810
	Night	34% 4 7% 18% 21% 16%	134827
Seeing	Day	12% 9% 23% 30% 19% 6%	17012
	Night	13% 5% 13% 33% 30% 6%	26390

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	17% 7% 2 5% 3 3 4 3 5% 7% 43%	5056
	11th to 20th	Day	13% 6% 2 5% 2 3 3 2 3 6% 55%	5340
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 42%	5843
February	1st to 10th	Day	19% 6% 2 6% 4 3 4 2 3 5% 46%	1890
	11th to 20th	Day	16% 8% 3 6% 5% 3 4 4 5% 7% 40%	1885
	21st to end	Day	17% 7% 2 4 4 3 4 2 5% 7% 45%	1730
March	1st to 10th	Day	17% 11% 4 7% 4 4 4 3 4 6% 37%	4964
	11th to 20th	Day	15% 6% 2 5% 3 3 4 4 5% 7% 47%	5000
	21st to end	Day	17% 8% 2 6% 4 4 5% 4 6% 8% 35%	5519
April	1st to 10th	Day	15% 7% 2 7% 4 4 4 3 6% 7% 43%	2400
	11th to 20th	Day	10% 5% 3 5% 4 4 4 3 4 7% 51%	2369
	21st to end	Day	8% 4 1 4 3 3 3 3 6% 9% 56%	2400
May	1st to 10th	Day	4 2 1 2 2 1 2 2 3 5% 76%	5789
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 6% 74%	5958
	21st to end	Day	3 2 1 2 2 2 2 1 3 5% 78%	6902
June	1st to 10th	Day	6% 5% 2 4 3 2 3 2 5% 8% 60%	2800
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 7% 74%	2800
	21st to end	Day	2 3 1 3 2 2 2 2 3 5% 77%	2800
July	1st to 10th	Day	1 2 1 3 2 1 2 1 2 3 82%	6319
	11th to 20th	Day	1 3 3 3 2 1 2 1 3 4 77%	5893
	21st to end	Day	5% 1 3 2 2 2 1 3 5% 76%	6402
August	1st to 10th	Day	1 2 1 4 2 2 3 3 4 5% 74%	5777
	11th to 20th	Day	1 2 1 2 2 1 1 2 3 4 81%	5630
	21st to end	Day	1 3 2 5% 3 2 3 2 3 6% 71%	5984
September	1st to 10th	Day	1 3 1 3 2 2 2 2 4 6% 75%	2400
	11th to 20th	Day	4 4 2 4 3 2 4 2 4 7% 64%	2390
	21st to end	Day	4 3 1 3 2 2 2 2 3 6% 73%	2106
October	1st to 10th	Day	4 3 1 2 2 2 3 3 4 6% 71%	3280
	11th to 20th	Day	8% 4 1 4 2 2 2 2 3 5% 65%	3261
	21st to end	Day	8% 4 1 4 3 3 4 3 5% 7% 58%	3558
November	1st to 10th	Day	11% 4 1 5% 4 3 3 3 5% 7% 56%	2036
	11th to 20th	Day	14% 6% 2 6% 5% 3 4 3 4 6% 47%	1851
	21st to end	Day	11% 5% 1 4 3 3 5% 3 5% 6% 54%	1680
December	1st to 10th	Day	10% 5% 2 3 3 3 3 3 6% 10% 52%	2700
	11th to 20th	Day	28% 7% 2 6% 3 3 4 3 6% 7% 29%	3108
	21st to end	Day	26% 9% 2 7% 4 4 4 4 5% 7% 29%	4092
January	1st to 10th	Night	19% 7% 2 5% 3 3 4 3 5% 8% 41%	7852
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 6% 55%	7650
	21st to end	Night	16% 8% 3 8% 3 4 4 3 4 6% 41%	8371
February	1st to 10th	Night	19% 5% 2 5% 3 3 3 3 4 6% 47%	2730
	11th to 20th	Night	15% 9% 3 6% 4 4 3 3 5% 7% 42%	2620
	21st to end	Night	16% 6% 1 3 3 3 3 2 4 7% 51%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 4 4 3 4 6% 37%	4970
	11th to 20th	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	5000
	21st to end	Night	18% 11% 3 5% 3 3 3 3 5% 7% 40%	5456
April	1st to 10th	Night	12% 7% 3 5% 4 3 4 3 4 6% 50%	2000
	11th to 20th	Night	9% 7% 2 5% 4 4 3 2 3 5% 55%	1979
	21st to end	Night	7% 5% 2 4 2 3 4 3 5% 6% 60%	2000
May	1st to 10th	Night	5% 3 2 4 2 2 2 1 3 5% 72%	4178
	11th to 20th	Night	3 5% 2 7% 4 3 3 2 3 5% 63%	3946
	21st to end	Night	2 2 1 3 2 2 2 2 2 4 79%	3972
June	1st to 10th	Night	5% 6% 2 4 3 3 3 4 5% 8% 58%	1600
	11th to 20th	Night	2 3 1 4 2 2 2 3 3 6% 73%	1600
	21st to end	Night	4 4 2 4 3 2 2 2 3 4 71%	1600
July	1st to 10th	Night	1 4 3 5% 3 2 2 1 2 3 74%	3640
	11th to 20th	Night	1 11% 8% 9% 3 3 2 2 2 3 57%	4069
	21st to end	Night	1 13% 5% 7% 4 4 4 3 4 6% 50%	4472
August	1st to 10th	Night	2 3 2 9% 4 4 3 2 3 5% 63%	4035
	11th to 20th	Night	5% 2 5% 3 3 3 2 3 4 68%	4226
	21st to end	Night	2 3 2 11% 5% 3 3 2 4 5% 60%	5015
September	1st to 10th	Night	1 3 2 4 3 3 3 2 4 6% 71%	2000
	11th to 20th	Night	5% 5% 2 6% 3 3 3 3 4 7% 60%	2005
	21st to end	Night	4 4 2 5% 2 2 2 2 4 4 69%	2293
October	1st to 10th	Night	4 4 1 4 2 2 2 2 3 4 73%	3920
	11th to 20th	Night	6% 7% 3 6% 3 3 3 2 3 4 59%	3897
	21st to end	Night	10% 4 2 4 3 3 3 3 4 6% 59%	4253
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 5% 7% 54%	2481
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 6% 6% 49%	2767
	21st to end	Night	11% 4 2 5% 3 3 4 3 4 6% 55%	2940
December	1st to 10th	Night	11% 4 2 4 3 3 4 3 5% 6% 55%	4700
	11th to 20th	Night	27% 8% 2 5% 4 4 4 3 6% 7% 29%	5316
	21st to end	Night	31% 8% 2 6% 3 4 3 2 3 4 32%	7106

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	42% 6% 13% 23% 11% 5%	5057
	11th to 20th	Day	33% 3 6% 13% 23% 22%	5340
	21st to end	Day	44% 3 7% 19% 11% 16%	5843
February	1st to 10th	Day	43% 4 7% 19% 20% 8%	1890
	11th to 20th	Day	45% 4 8% 19% 17% 6%	1894
	21st to end	Day	40% 4 10% 24% 15% 7%	1730
March	1st to 10th	Day	49% 4 7% 20% 18% 2	4964
	11th to 20th	Day	37% 4 14% 27% 10% 8%	5000
	21st to end	Day	45% 6% 14% 22% 10% 1	5519
April	1st to 10th	Day	42% 4 13% 22% 16% 4	2400
	11th to 20th	Day	34% 4 13% 23% 20% 5%	2372
	21st to end	Day	26% 4 14% 32% 22% 4	2400
May	1st to 10th	Day	14% 2 8% 28% 30% 18%	5789
	11th to 20th	Day	14% 3 5% 35% 34% 9%	5961
	21st to end	Day	12% 3 8% 35% 38% 4	6904
June	1st to 10th	Day	24% 4 11% 29% 27% 6%	2800
	11th to 20th	Day	13% 3 10% 21% 37% 16%	2800
	21st to end	Day	14% 2 4 16% 36% 28%	2800
July	1st to 10th	Day	11% 2 1 7% 25% 54%	6251
	11th to 20th	Day	13% 3 2 5% 19% 58%	5823
	21st to end	Day	15% 2 3 17% 29% 34%	6217
August	1st to 10th	Day	18% 3 2 11% 28% 38%	4206
	11th to 20th	Day	11% 3 1 12% 43% 30%	4077
	21st to end	Day	18% 3 2 13% 37% 27%	4311
September	1st to 10th	Day	14% 2 3 12% 31% 38%	2400
	11th to 20th	Day	22% 3 5% 25% 33% 11%	2390
	21st to end	Day	17% 2 4 22% 32% 23%	2106
October	1st to 10th	Day	16% 2 3 12% 26% 40%	3280
	11th to 20th	Day	24% 2 5% 23% 26% 20%	3261
	21st to end	Day	27% 4 7% 24% 26% 12%	3558
November	1st to 10th	Day	29% 5% 7% 29% 19% 10%	2036
	11th to 20th	Day	39% 6% 10% 22% 16% 7%	1851
	21st to end	Day	32% 5% 10% 21% 19% 13%	1680
December	1st to 10th	Day	28% 3 7% 27% 25% 9%	2700
	11th to 20th	Day	55% 4 7% 17% 14% 4	3108
	21st to end	Day	56% 5% 11% 18% 8% 3	4092
January	1st to 10th	Night	43% 6% 17% 18% 12% 4	7852
	11th to 20th	Night	33% 3 7% 15% 21% 20%	7650
	21st to end	Night	45% 4 7% 14% 16% 15%	8371
February	1st to 10th	Night	40% 3 7% 19% 18% 12%	2730
	11th to 20th	Night	43% 4 9% 20% 18% 7%	2622
	21st to end	Night	34% 4 10% 25% 19% 7%	2076
March	1st to 10th	Night	49% 5% 9% 18% 17% 3	4970
	11th to 20th	Night	39% 4 10% 21% 18% 8%	5000
	21st to end	Night	44% 5% 14% 24% 11% 2	5456
April	1st to 10th	Night	35% 5% 13% 22% 20% 4	2000
	11th to 20th	Night	32% 4 11% 25% 17% 10%	1979
	21st to end	Night	25% 4 10% 32% 24% 6%	2000
May	1st to 10th	Night	18% 2 4 24% 32% 19%	4175
	11th to 20th	Night	23% 5% 4 27% 28% 14%	3947
	21st to end	Night	14% 3 6% 33% 39% 7%	3972
June	1st to 10th	Night	26% 5% 9% 22% 31% 7%	1600
	11th to 20th	Night	14% 3 6% 20% 37% 19%	1600
	21st to end	Night	19% 3 4 12% 32% 29%	1600
July	1st to 10th	Night	20% 2 1 6% 26% 45%	3600
	11th to 20th	Night	36% 3 2 4 14% 41%	4016
	21st to end	Night	38% 4 1 6% 22% 28%	4345
August	1st to 10th	Night	38% 3 2 11% 24% 22%	2929
	11th to 20th	Night	16% 4 2 8% 37% 32%	3051
	21st to end	Night	25% 3 3 12% 32% 25%	3608
September	1st to 10th	Night	16% 4 2 10% 30% 38%	2000
	11th to 20th	Night	28% 3 4 19% 32% 14%	2005
	21st to end	Night	21% 3 3 20% 27% 27%	2293
October	1st to 10th	Night	17% 3 3 11% 22% 44%	3920
	11th to 20th	Night	31% 2 5% 20% 20% 20%	3897
	21st to end	Night	27% 3 5% 22% 26% 17%	4253
November	1st to 10th	Night	31% 3 10% 26% 19% 11%	2481
	11th to 20th	Night	35% 5% 11% 25% 16% 8%	2767
	21st to end	Night	32% 4 8% 21% 21% 13%	2940
December	1st to 10th	Night	31% 3 7% 24% 23% 12%	4700
	11th to 20th	Night	53% 4 9% 14% 14% 5%	5316
	21st to end	Night	57% 4 10% 17% 9% 3	7106

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	29% 12% 24% 20% 11% 3	333
	11th to 20th	Day	20% 9% 22% 26% 19% 4	430
	21st to end	Day	16% 10% 22% 30% 18% 4	473
February	1st to 10th	Day	21% 11% 22% 23% 19% 4	430
	11th to 20th	Day	21% 18% 26% 22% 12% 1	447
	21st to end	Day	19% 13% 25% 29% 13% 1	446
March	1st to 10th	Day	27% 15% 26% 22% 8% 3	509
	11th to 20th	Day	26% 14% 24% 21% 11% 4	500
	21st to end	Day	19% 17% 28% 24% 11% 1	550
April	1st to 10th	Day	18% 16% 27% 25% 11% 4	500
	11th to 20th	Day	16% 16% 28% 23% 12% 5%	492
	21st to end	Day	10% 15% 29% 27% 13% 7%	500
May	1st to 10th	Day	12% 14% 32% 25% 13% 4	503
	11th to 20th	Day	12% 12% 29% 26% 16% 5%	528
	21st to end	Day	9% 10% 26% 31% 18% 7%	573
June	1st to 10th	Day	10% 10% 25% 33% 16% 6%	530
	11th to 20th	Day	4 12% 30% 36% 15% 4	530
	21st to end	Day	2 5% 26% 36% 24% 6%	530
July	1st to 10th	Day	4 4 23% 44% 19% 6%	526
	11th to 20th	Day	1 2 17% 42% 27% 10%	524
	21st to end	Day	2 2 12% 48% 27% 10%	575
August	1st to 10th	Day	2 3 20% 41% 24% 10%	527
	11th to 20th	Day	3 4 22% 37% 26% 8%	517
	21st to end	Day	2 3 18% 39% 29% 9%	583
September	1st to 10th	Day	2 4 21% 38% 27% 9%	530
	11th to 20th	Day	7% 6% 22% 33% 24% 9%	530
	21st to end	Day	5% 7% 22% 33% 24% 8%	530
October	1st to 10th	Day	9% 7% 24% 26% 24% 10%	530
	11th to 20th	Day	9% 8% 18% 27% 25% 12%	530
	21st to end	Day	12% 8% 20% 30% 23% 7%	583
November	1st to 10th	Day	13% 8% 18% 26% 27% 8%	512
	11th to 20th	Day	13% 8% 18% 27% 23% 11%	309
	21st to end	Day	12% 8% 24% 25% 20% 10%	220
December	1st to 10th	Day	18% 10% 17% 20% 23% 11%	220
	11th to 20th	Day	25% 11% 18% 19% 17% 10%	220
	21st to end	Day	25% 11% 24% 21% 14% 4	242
January	1st to 10th	Night	23% 6% 15% 32% 21% 4	840
	11th to 20th	Night	19% 5% 15% 37% 20% 4	840
	21st to end	Night	20% 4 14% 34% 23% 4	924
February	1st to 10th	Night	20% 5% 14% 37% 22% 2	840
	11th to 20th	Night	19% 7% 18% 34% 19% 4	824
	21st to end	Night	19% 9% 16% 37% 17% 2	692
March	1st to 10th	Night	23% 8% 18% 28% 19% 3	808
	11th to 20th	Night	21% 7% 15% 31% 23% 3	800
	21st to end	Night	19% 7% 21% 32% 18% 2	880
April	1st to 10th	Night	15% 8% 20% 35% 20% 3	800
	11th to 20th	Night	12% 12% 19% 31% 22% 5%	791
	21st to end	Night	8% 10% 22% 27% 26% 6%	800
May	1st to 10th	Night	9% 5% 22% 31% 29% 5%	639
	11th to 20th	Night	7% 3 17% 32% 35% 6%	542
	21st to end	Night	6% 7% 18% 30% 33% 6%	433
June	1st to 10th	Night	9% 4 18% 33% 33% 3	400
	11th to 20th	Night	4 5% 15% 45% 29% 3	400
	21st to end	Night	5% 2 13% 36% 40% 5%	400
July	1st to 10th	Night	2 1 8% 36% 48% 5%	390
	11th to 20th	Night	3 2 3 29% 51% 13%	565
	21st to end	Night	3 1 2 31% 52% 11%	657
August	1st to 10th	Night	4 1 6% 32% 46% 12%	597
	11th to 20th	Night	3 1 7% 35% 46% 9%	664
	21st to end	Night	3 1 6% 32% 48% 9%	880
September	1st to 10th	Night	2 1 8% 31% 49% 9%	800
	11th to 20th	Night	8% 2 8% 34% 39% 10%	800
	21st to end	Night	6% 4 9% 34% 37% 11%	800
October	1st to 10th	Night	7% 3 13% 32% 35% 9%	800
	11th to 20th	Night	9% 3 11% 31% 37% 9%	800
	21st to end	Night	11% 3 10% 32% 33% 11%	880
November	1st to 10th	Night	13% 4 12% 30% 32% 9%	820
	11th to 20th	Night	15% 4 12% 35% 28% 5%	840
	21st to end	Night	13% 4 14% 38% 26% 6%	840
December	1st to 10th	Night	20% 5% 12% 34% 23% 6%	840
	11th to 20th	Night	27% 5% 13% 31% 20% 4	840
	21st to end	Night	26% 5% 19% 32% 16% 2	924

Page generated at 2026-02-27 22:40:26. Version ac07

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