San Rafael
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Rafael 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	6% 4 2 5% 3 3 3 3 5% 7% 60%	135142
	Night	7% 5% 2 5% 4 3 4 3 5% 8% 54%	131997
Transparency	Day	27% 5% 8% 20% 24% 16%	130052
	Night	33% 5% 8% 19% 21% 13%	128076
Seeing	Day	9% 11% 27% 33% 17% 4	16664
	Night	11% 6% 17% 33% 27% 5%	25933

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	11% 7% 3 6% 4 4 5% 4 6% 7% 43%	3915
	11th to 20th	Day	12% 9% 3 7% 4 4 3 2 4 5% 48%	4100
	21st to end	Day	15% 7% 3 9% 5% 4 4 3 5% 6% 39%	4510
February	1st to 10th	Day	16% 6% 2 5% 5% 4 4 3 6% 6% 44%	1800
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 5% 7% 8% 40%	1815
	21st to end	Day	12% 5% 3 6% 4 4 4 4 5% 7% 47%	1650
March	1st to 10th	Day	11% 8% 3 8% 6% 5% 6% 4 6% 8% 36%	4924
	11th to 20th	Day	10% 4 2 6% 4 4 4 3 6% 8% 50%	5000
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 5% 8% 10% 37%	5525
April	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 8% 45%	2400
	11th to 20th	Day	7% 4 1 5% 3 4 5% 3 5% 8% 55%	2369
	21st to end	Day	3 4 1 5% 3 4 3 3 6% 9% 59%	2400
May	1st to 10th	Day	3 2 1 2 2 1 2 2 3 5% 77%	5718
	11th to 20th	Day	2 1 1 3 2 2 2 2 4 8% 73%	5922
	21st to end	Day	1 2 1 2 2 1 2 1 3 7% 78%	6902
June	1st to 10th	Day	4 4 2 4 3 2 3 3 5% 8% 62%	2800
	11th to 20th	Day	1 2 1 3 2 2 2 3 5% 7% 73%	2800
	21st to end	Day	2 2 1 3 3 2 2 2 4 7% 73%	2800
July	1st to 10th	Day	1 1 2 2 2 2 1 3 6% 80%	6302
	11th to 20th	Day	2 1 3 2 3 2 2 4 8% 71%	5880
	21st to end	Day	2 1 3 3 3 3 2 4 8% 70%	6402
August	1st to 10th	Day	1 2 1 3 2 2 3 3 5% 8% 71%	5777
	11th to 20th	Day	1 1 2 1 2 2 1 3 6% 81%	5583
	21st to end	Day	1 2 1 3 3 3 3 2 4 9% 69%	5984
September	1st to 10th	Day	1 2 1 3 3 2 2 2 4 8% 72%	2400
	11th to 20th	Day	2 4 2 5% 3 3 3 2 4 8% 63%	2400
	21st to end	Day	3 2 1 3 2 2 3 2 4 6% 73%	2104
October	1st to 10th	Day	2 2 1 4 3 2 3 3 4 7% 70%	3280
	11th to 20th	Day	4 4 2 5% 3 3 3 3 4 5% 63%	3242
	21st to end	Day	5% 3 1 5% 3 4 4 4 6% 8% 57%	3448
November	1st to 10th	Day	7% 4 2 4 4 4 4 4 5% 8% 55%	1942
	11th to 20th	Day	9% 8% 3 9% 4 3 3 3 5% 7% 46%	1796
	21st to end	Day	11% 6% 2 4 3 3 4 3 5% 7% 52%	1600
December	1st to 10th	Day	9% 4 2 5% 3 4 4 3 6% 10% 51%	2620
	11th to 20th	Day	20% 9% 3 11% 6% 5% 5% 4 6% 6% 25%	3028
	21st to end	Day	22% 8% 3 8% 5% 5% 4 4 6% 8% 27%	4004
January	1st to 10th	Night	10% 6% 3 6% 4 3 4 4 6% 8% 47%	6026
	11th to 20th	Night	12% 6% 3 8% 5% 4 4 3 4 6% 46%	5880
	21st to end	Night	14% 8% 3 7% 4 4 4 3 5% 6% 43%	6468
February	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 47%	2600
	11th to 20th	Night	11% 6% 3 6% 4 3 4 4 5% 8% 45%	2472
	21st to end	Night	11% 5% 1 5% 4 4 3 3 6% 6% 53%	1980
March	1st to 10th	Night	11% 7% 2 7% 5% 4 5% 4 5% 7% 42%	4955
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 53%	5000
	21st to end	Night	11% 7% 2 6% 4 4 4 4 5% 8% 45%	5449
April	1st to 10th	Night	7% 6% 2 5% 4 5% 3 4 5% 8% 52%	2000
	11th to 20th	Night	7% 4 2 6% 3 3 4 3 5% 6% 58%	1979
	21st to end	Night	4 3 1 3 3 3 4 3 6% 8% 61%	2000
May	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 74%	4252
	11th to 20th	Night	1 1 1 5% 5% 4 4 3 5% 7% 63%	3982
	21st to end	Night	2 1 1 2 2 2 2 2 3 5% 79%	3972
June	1st to 10th	Night	3 3 2 4 3 3 4 4 7% 11% 56%	1600
	11th to 20th	Night	1 1 1 3 3 2 3 3 5% 9% 70%	1600
	21st to end	Night	1 2 1 3 3 3 3 3 4 9% 67%	1600
July	1st to 10th	Night	2 1 3 3 2 3 3 4 7% 72%	3640
	11th to 20th	Night	1 3 1 4 3 4 5% 4 7% 12% 55%	4099
	21st to end	Night	3 1 5% 4 4 5% 6% 8% 13% 51%	4472
August	1st to 10th	Night	2 3 1 4 3 3 3 3 5% 9% 63%	4035
	11th to 20th	Night	2 1 3 2 3 3 3 5% 7% 72%	4273
	21st to end	Night	1 2 1 3 2 3 3 3 5% 10% 65%	5015
September	1st to 10th	Night	1 2 1 3 3 3 3 3 5% 9% 67%	2000
	11th to 20th	Night	2 4 2 6% 3 3 4 4 6% 9% 58%	2000
	21st to end	Night	2 3 1 5% 3 3 4 3 4 8% 65%	2291
October	1st to 10th	Night	2 2 1 3 3 2 3 3 5% 7% 69%	3920
	11th to 20th	Night	3 4 2 5% 3 3 3 3 4 6% 64%	3873
	21st to end	Night	5% 3 1 5% 4 3 4 3 5% 7% 60%	4121
November	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 9% 56%	2355
	11th to 20th	Night	9% 5% 2 6% 3 4 4 3 5% 7% 52%	2600
	21st to end	Night	8% 4 2 5% 4 4 4 3 5% 7% 54%	2800
December	1st to 10th	Night	9% 4 1 6% 4 4 5% 5% 6% 8% 48%	4560
	11th to 20th	Night	20% 9% 3 7% 5% 4 5% 5% 6% 8% 27%	5176
	21st to end	Night	20% 9% 3 10% 6% 4 4 3 5% 5% 30%	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	37% 6% 17% 21% 14% 5%	3915
	11th to 20th	Day	40% 4 9% 13% 19% 15%	4100
	21st to end	Day	46% 5% 7% 13% 13% 17%	4510
February	1st to 10th	Day	41% 5% 7% 19% 21% 8%	1800
	11th to 20th	Day	39% 6% 10% 19% 19% 6%	1824
	21st to end	Day	36% 6% 9% 25% 17% 7%	1650
March	1st to 10th	Day	45% 6% 8% 21% 18% 2	4924
	11th to 20th	Day	32% 5% 16% 29% 10% 8%	5000
	21st to end	Day	39% 8% 17% 25% 9% 1	5525
April	1st to 10th	Day	36% 6% 14% 21% 19% 3	2400
	11th to 20th	Day	29% 6% 16% 24% 19% 6%	2372
	21st to end	Day	22% 5% 15% 30% 23% 4	2400
May	1st to 10th	Day	14% 3 8% 29% 31% 16%	5718
	11th to 20th	Day	12% 7% 6% 33% 36% 7%	5925
	21st to end	Day	12% 5% 7% 32% 40% 3	6904
June	1st to 10th	Day	25% 6% 10% 28% 25% 7%	2800
	11th to 20th	Day	15% 4 9% 20% 36% 16%	2800
	21st to end	Day	18% 6% 5% 13% 31% 27%	2800
July	1st to 10th	Day	13% 4 3 7% 21% 52%	6234
	11th to 20th	Day	19% 6% 4 4 18% 48%	5811
	21st to end	Day	25% 4 6% 13% 25% 27%	6217
August	1st to 10th	Day	26% 5% 4 10% 26% 28%	4206
	11th to 20th	Day	14% 3 5% 10% 41% 27%	4042
	21st to end	Day	19% 7% 6% 13% 31% 24%	4311
September	1st to 10th	Day	18% 4 4 9% 30% 35%	2400
	11th to 20th	Day	25% 6% 5% 22% 29% 13%	2400
	21st to end	Day	17% 4 4 22% 32% 21%	2104
October	1st to 10th	Day	16% 3 4 12% 28% 36%	3280
	11th to 20th	Day	26% 4 5% 22% 25% 17%	3242
	21st to end	Day	26% 5% 8% 25% 24% 12%	3448
November	1st to 10th	Day	28% 5% 10% 28% 19% 11%	1942
	11th to 20th	Day	38% 4 10% 22% 19% 6%	1796
	21st to end	Day	31% 5% 12% 20% 22% 10%	1600
December	1st to 10th	Day	28% 5% 7% 29% 24% 5%	2620
	11th to 20th	Day	53% 5% 10% 15% 15% 2	3028
	21st to end	Day	53% 5% 13% 19% 8% 2	4004
January	1st to 10th	Night	34% 7% 21% 23% 12% 4	6026
	11th to 20th	Night	41% 5% 9% 16% 17% 11%	5880
	21st to end	Night	43% 6% 7% 11% 19% 13%	6468
February	1st to 10th	Night	39% 4 8% 20% 19% 10%	2600
	11th to 20th	Night	38% 6% 10% 19% 19% 7%	2474
	21st to end	Night	32% 6% 10% 29% 18% 5%	1980
March	1st to 10th	Night	43% 6% 11% 20% 17% 3	4955
	11th to 20th	Night	34% 6% 12% 25% 15% 8%	5000
	21st to end	Night	41% 7% 14% 25% 12% 2	5449
April	1st to 10th	Night	34% 6% 12% 24% 19% 5%	2000
	11th to 20th	Night	31% 5% 12% 25% 19% 8%	1979
	21st to end	Night	22% 6% 11% 31% 23% 6%	2000
May	1st to 10th	Night	19% 3 6% 26% 32% 14%	4249
	11th to 20th	Night	21% 8% 6% 29% 28% 8%	3983
	21st to end	Night	15% 5% 6% 31% 39% 3	3972
June	1st to 10th	Night	31% 6% 9% 23% 25% 7%	1600
	11th to 20th	Night	20% 4 6% 18% 35% 17%	1600
	21st to end	Night	23% 6% 3 12% 29% 27%	1600
July	1st to 10th	Night	24% 5% 2 6% 19% 43%	3600
	11th to 20th	Night	38% 5% 3 8% 11% 36%	4046
	21st to end	Night	44% 3 3 10% 19% 22%	4345
August	1st to 10th	Night	43% 4 2 13% 22% 16%	2929
	11th to 20th	Night	24% 4 3 8% 36% 25%	3085
	21st to end	Night	25% 7% 5% 12% 30% 22%	3608
September	1st to 10th	Night	22% 4 4 10% 26% 33%	2000
	11th to 20th	Night	31% 5% 5% 21% 24% 14%	2000
	21st to end	Night	24% 4 4 20% 27% 21%	2291
October	1st to 10th	Night	18% 5% 5% 10% 23% 39%	3920
	11th to 20th	Night	27% 3 7% 22% 21% 20%	3873
	21st to end	Night	27% 5% 6% 22% 26% 16%	4121
November	1st to 10th	Night	30% 4 10% 26% 19% 12%	2355
	11th to 20th	Night	32% 5% 11% 26% 18% 8%	2600
	21st to end	Night	30% 6% 10% 21% 23% 11%	2800
December	1st to 10th	Night	29% 7% 6% 28% 21% 9%	4560
	11th to 20th	Night	52% 6% 9% 17% 14% 3	5176
	21st to end	Night	54% 5% 13% 18% 8% 2	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	26% 10% 29% 19% 15% 1	320
	11th to 20th	Day	18% 11% 22% 34% 14% 2	400
	21st to end	Day	13% 9% 30% 30% 18% 1	440
February	1st to 10th	Day	21% 12% 21% 28% 17% 2	400
	11th to 20th	Day	17% 18% 26% 26% 12% 1	425
	21st to end	Day	13% 14% 29% 28% 14% 2	414
March	1st to 10th	Day	19% 18% 30% 22% 10% 1	500
	11th to 20th	Day	17% 17% 27% 23% 13% 4	500
	21st to end	Day	14% 20% 31% 23% 10% 1	550
April	1st to 10th	Day	14% 17% 31% 26% 11% 3	500
	11th to 20th	Day	13% 19% 31% 23% 10% 3	492
	21st to end	Day	5% 22% 31% 25% 12% 5%	500
May	1st to 10th	Day	8% 17% 33% 28% 12% 2	503
	11th to 20th	Day	7% 19% 31% 28% 13% 1	528
	21st to end	Day	6% 15% 34% 27% 16% 3	573
June	1st to 10th	Day	7% 12% 32% 34% 12% 3	530
	11th to 20th	Day	4 11% 34% 36% 13% 2	530
	21st to end	Day	1 8% 34% 40% 13% 4	530
July	1st to 10th	Day	2 5% 31% 45% 15% 2	526
	11th to 20th	Day	2 3 22% 49% 22% 2	524
	21st to end	Day	3 1 25% 51% 16% 3	575
August	1st to 10th	Day	3 5% 27% 46% 15% 4	527
	11th to 20th	Day	4 3 28% 43% 18% 5%	517
	21st to end	Day	3 3 23% 43% 24% 5%	583
September	1st to 10th	Day	3 4 23% 42% 21% 8%	530
	11th to 20th	Day	5% 8% 25% 38% 18% 6%	530
	21st to end	Day	5% 8% 23% 32% 26% 6%	530
October	1st to 10th	Day	4 8% 25% 31% 23% 10%	530
	11th to 20th	Day	8% 7% 22% 27% 28% 8%	523
	21st to end	Day	11% 6% 24% 30% 23% 7%	539
November	1st to 10th	Day	9% 9% 23% 25% 27% 6%	478
	11th to 20th	Day	12% 8% 17% 33% 25% 6%	297
	21st to end	Day	13% 8% 24% 38% 15% 3	200
December	1st to 10th	Day	19% 10% 18% 30% 19% 6%	200
	11th to 20th	Day	17% 16% 23% 29% 15% 1	200
	21st to end	Day	23% 11% 25% 29% 11%	220
January	1st to 10th	Night	20% 7% 15% 30% 23% 5%	800
	11th to 20th	Night	16% 7% 15% 32% 25% 5%	800
	21st to end	Night	18% 7% 13% 34% 24% 4	880
February	1st to 10th	Night	19% 6% 15% 33% 23% 3	800
	11th to 20th	Night	14% 11% 17% 33% 20% 4	784
	21st to end	Night	14% 11% 21% 31% 21% 3	660
March	1st to 10th	Night	20% 10% 20% 28% 19% 3	800
	11th to 20th	Night	18% 9% 19% 30% 22% 3	800
	21st to end	Night	15% 11% 26% 29% 17% 2	880
April	1st to 10th	Night	11% 10% 28% 32% 17% 3	800
	11th to 20th	Night	12% 14% 22% 30% 18% 4	791
	21st to end	Night	5% 12% 27% 31% 22% 4	800
May	1st to 10th	Night	5% 7% 27% 33% 24% 3	674
	11th to 20th	Night	5% 3 24% 34% 30% 3	552
	21st to end	Night	6% 10% 24% 36% 21% 3	433
June	1st to 10th	Night	5% 7% 23% 39% 25% 1	400
	11th to 20th	Night	1 4 21% 51% 21% 3	400
	21st to end	Night	4 4 18% 39% 32% 3	400
July	1st to 10th	Night	3 2 14% 47% 31% 3	390
	11th to 20th	Night	7% 1 9% 40% 36% 7%	582
	21st to end	Night	10% 3 12% 35% 34% 5%	657
August	1st to 10th	Night	11% 3 15% 33% 33% 5%	597
	11th to 20th	Night	5% 2 17% 40% 30% 6%	685
	21st to end	Night	7% 1 13% 38% 36% 6%	880
September	1st to 10th	Night	4 1 13% 37% 37% 9%	800
	11th to 20th	Night	7% 3 17% 35% 32% 6%	800
	21st to end	Night	4 4 15% 31% 37% 9%	800
October	1st to 10th	Night	7% 4 14% 32% 34% 10%	800
	11th to 20th	Night	8% 5% 14% 30% 34% 9%	792
	21st to end	Night	10% 5% 12% 28% 34% 11%	836
November	1st to 10th	Night	9% 6% 15% 27% 34% 9%	780
	11th to 20th	Night	12% 4 15% 34% 31% 6%	800
	21st to end	Night	12% 6% 13% 40% 25% 5%	800
December	1st to 10th	Night	18% 6% 14% 32% 24% 6%	800
	11th to 20th	Night	24% 7% 15% 30% 21% 3	800
	21st to end	Night	21% 11% 18% 30% 19% 1	880

Page generated at 2025-10-15 22:50:25. Version ac07

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