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 2024-01-12.

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	7% 4 2 4 4 2 3 4 5% 8% 56%	74670
	Night	9% 5% 2 5% 4 3 4 4 5% 8% 52%	75052
Transparency	Day	19% 37% 7% 12% 15% 10%	74391
	Night	22% 37% 7% 12% 13% 9%	74854
Seeing	Day	9% 12% 28% 32% 15% 3	14263
	Night	12% 7% 18% 33% 26% 5%	23665

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	18% 8% 4 6% 5% 3 4 4 6% 6% 34%	1633
	11th to 20th	Day	12% 6% 3 5% 5% 4 4 5% 4 6% 46%	1630
	21st to end	Day	12% 6% 2 6% 6% 4 5% 4 6% 7% 40%	1782
February	1st to 10th	Day	15% 6% 2 4 5% 3 4 4 5% 6% 45%	1620
	11th to 20th	Day	12% 6% 2 4 5% 2 5% 6% 7% 8% 43%	1629
	21st to end	Day	13% 5% 3 6% 4 3 4 4 4 6% 49%	1480
March	1st to 10th	Day	14% 6% 3 5% 6% 3 5% 5% 5% 9% 38%	1965
	11th to 20th	Day	14% 7% 3 5% 6% 4 5% 4 6% 8% 38%	1980
	21st to end	Day	11% 4 2 4 5% 3 4 5% 6% 9% 47%	2192
April	1st to 10th	Day	11% 5% 3 5% 5% 3 5% 5% 7% 8% 44%	2160
	11th to 20th	Day	7% 4 2 4 3 2 5% 4 4 8% 57%	2129
	21st to end	Day	3 3 2 4 3 3 3 4 6% 10% 59%	2160
May	1st to 10th	Day	4 4 2 4 3 2 3 3 4 7% 63%	2269
	11th to 20th	Day	5% 3 2 3 3 1 3 4 4 9% 64%	2330
	21st to end	Day	3 4 3 4 4 2 3 4 5% 8% 59%	2723
June	1st to 10th	Day	5% 4 2 3 4 2 3 4 5% 8% 61%	2520
	11th to 20th	Day	1 2 1 3 2 1 2 3 4 8% 73%	2520
	21st to end	Day	2 2 1 3 3 2 3 3 4 7% 72%	2520
July	1st to 10th	Day	1 3 1 3 3 2 3 2 4 9% 69%	2516
	11th to 20th	Day	1 3 1 4 3 2 3 3 5% 9% 66%	2340
	21st to end	Day	1 4 2 2 3 2 2 3 4 9% 69%	2546
August	1st to 10th	Day	2 3 2 5% 4 2 3 4 4 9% 64%	2328
	11th to 20th	Day	1 3 2 4 3 2 2 3 4 8% 68%	2215
	21st to end	Day	1 5% 2 5% 3 2 3 3 3 9% 64%	2376
September	1st to 10th	Day	1 2 2 3 3 2 2 2 3 7% 73%	2160
	11th to 20th	Day	3 4 2 4 4 2 3 3 4 8% 63%	2160
	21st to end	Day	2 2 1 2 2 2 3 2 4 6% 74%	1895
October	1st to 10th	Day	3 3 2 4 4 2 3 3 3 6% 66%	1800
	11th to 20th	Day	6% 5% 2 4 3 3 3 4 5% 6% 59%	1800
	21st to end	Day	8% 5% 2 4 3 2 4 3 5% 8% 58%	1980
November	1st to 10th	Day	8% 3 2 4 5% 3 4 5% 5% 8% 54%	1843
	11th to 20th	Day	9% 7% 4 7% 5% 2 4 4 5% 7% 46%	1707
	21st to end	Day	9% 6% 2 4 3 3 4 4 5% 7% 53%	1520
December	1st to 10th	Day	14% 6% 4 5% 4 3 4 4 5% 6% 44%	1520
	11th to 20th	Day	25% 10% 4 8% 5% 2 3 4 4 5% 30%	1928
	21st to end	Day	21% 8% 3 6% 5% 3 4 4 5% 8% 32%	2794
January	1st to 10th	Night	16% 7% 3 5% 5% 3 5% 5% 5% 8% 39%	2529
	11th to 20th	Night	12% 6% 3 6% 5% 2 4 5% 5% 7% 44%	2358
	21st to end	Night	14% 6% 2 4 5% 2 4 5% 5% 7% 47%	2574
February	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 49%	2340
	11th to 20th	Night	10% 5% 3 5% 4 3 4 5% 5% 8% 48%	2218
	21st to end	Night	12% 4 2 4 5% 3 3 4 6% 6% 53%	1776
March	1st to 10th	Night	15% 7% 3 5% 5% 4 5% 4 4 6% 41%	1993
	11th to 20th	Night	12% 7% 2 5% 5% 4 4 5% 6% 8% 43%	1980
	21st to end	Night	12% 5% 2 5% 4 2 4 4 4 6% 53%	2163
April	1st to 10th	Night	7% 6% 2 6% 4 3 4 5% 5% 8% 51%	1800
	11th to 20th	Night	7% 3 2 5% 3 2 4 4 5% 6% 59%	1779
	21st to end	Night	4 3 1 3 3 2 4 4 6% 9% 62%	1800
May	1st to 10th	Night	4 3 2 3 4 3 3 5% 6% 8% 60%	1688
	11th to 20th	Night	2 3 2 5% 3 2 4 5% 5% 9% 60%	1558
	21st to end	Night	4 2 2 4 4 3 5% 4 5% 10% 58%	1552
June	1st to 10th	Night	3 3 2 4 4 3 5% 5% 7% 11% 55%	1440
	11th to 20th	Night	1 1 1 2 3 2 3 3 6% 10% 67%	1440
	21st to end	Night	1 2 1 3 4 3 4 3 5% 8% 66%	1440
July	1st to 10th	Night	2 2 3 4 2 4 5% 6% 11% 61%	1440
	11th to 20th	Night	1 3 2 4 5% 3 5% 6% 6% 11% 54%	1619
	21st to end	Night	1 5% 3 6% 6% 3 6% 7% 8% 12% 43%	1773
August	1st to 10th	Night	2 5% 3 7% 6% 3 4 6% 6% 11% 47%	1611
	11th to 20th	Night	3 2 6% 5% 3 5% 6% 7% 12% 49%	1683
	21st to end	Night	1 4 2 5% 4 3 4 4 6% 9% 57%	1980
September	1st to 10th	Night	1 2 1 3 3 2 4 3 5% 9% 67%	1800
	11th to 20th	Night	2 3 2 5% 4 2 3 5% 6% 9% 58%	1800
	21st to end	Night	2 3 2 4 4 2 4 3 3 8% 66%	2060
October	1st to 10th	Night	4 4 1 4 4 2 3 3 4 7% 64%	2160
	11th to 20th	Night	4 4 2 4 4 3 4 4 4 7% 59%	2160
	21st to end	Night	7% 3 2 4 4 2 3 4 5% 7% 59%	2376
November	1st to 10th	Night	7% 4 3 4 4 3 4 3 4 9% 56%	2234
	11th to 20th	Night	9% 5% 2 5% 4 2 4 4 5% 8% 52%	2470
	21st to end	Night	8% 3 2 4 4 3 4 3 5% 7% 56%	2660
December	1st to 10th	Night	16% 6% 2 6% 5% 3 3 4 5% 6% 45%	2660
	11th to 20th	Night	28% 8% 3 5% 4 3 3 4 5% 7% 31%	3276
	21st to end	Night	18% 8% 3 6% 6% 3 5% 5% 5% 6% 35%	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	29% 41% 9% 11% 7% 2	1633
	11th to 20th	Day	19% 42% 5% 11% 14% 8%	1630
	21st to end	Day	28% 38% 7% 8% 14% 5%	1782
February	1st to 10th	Day	23% 36% 5% 14% 15% 7%	1620
	11th to 20th	Day	24% 39% 7% 14% 13% 4	1638
	21st to end	Day	26% 37% 6% 15% 12% 4	1480
March	1st to 10th	Day	25% 39% 9% 14% 10% 3	1965
	11th to 20th	Day	30% 38% 8% 14% 7% 2	1980
	21st to end	Day	24% 40% 9% 13% 11% 2	2192
April	1st to 10th	Day	23% 38% 11% 11% 13% 3	2160
	11th to 20th	Day	19% 38% 10% 15% 14% 4	2132
	21st to end	Day	14% 36% 10% 21% 16% 3	2160
May	1st to 10th	Day	14% 37% 7% 16% 21% 6%	2269
	11th to 20th	Day	12% 38% 6% 22% 16% 6%	2333
	21st to end	Day	15% 39% 10% 17% 17% 2	2724
June	1st to 10th	Day	17% 38% 9% 17% 16% 3	2520
	11th to 20th	Day	10% 37% 8% 11% 22% 12%	2520
	21st to end	Day	12% 35% 6% 10% 20% 17%	2520
July	1st to 10th	Day	10% 36% 4 7% 15% 28%	2447
	11th to 20th	Day	14% 37% 5% 6% 12% 26%	2271
	21st to end	Day	14% 36% 5% 5% 14% 26%	2405
August	1st to 10th	Day	14% 36% 6% 9% 17% 18%	2312
	11th to 20th	Day	12% 34% 4 6% 20% 23%	2215
	21st to end	Day	14% 35% 5% 7% 18% 21%	2376
September	1st to 10th	Day	10% 35% 3 5% 18% 29%	2160
	11th to 20th	Day	16% 36% 6% 13% 19% 9%	2160
	21st to end	Day	9% 35% 4 15% 21% 16%	1895
October	1st to 10th	Day	16% 36% 6% 10% 21% 11%	1800
	11th to 20th	Day	17% 36% 6% 13% 15% 12%	1800
	21st to end	Day	18% 37% 5% 13% 15% 11%	1980
November	1st to 10th	Day	21% 42% 7% 13% 10% 7%	1843
	11th to 20th	Day	24% 41% 7% 12% 11% 5%	1707
	21st to end	Day	18% 39% 8% 13% 14% 8%	1520
December	1st to 10th	Day	22% 40% 7% 13% 13% 4	1520
	11th to 20th	Day	42% 33% 7% 8% 8% 2	1928
	21st to end	Day	41% 28% 14% 12% 5% 1	2794
January	1st to 10th	Night	27% 41% 8% 14% 6% 3	2529
	11th to 20th	Night	19% 42% 7% 12% 11% 9%	2358
	21st to end	Night	25% 38% 8% 8% 13% 7%	2574
February	1st to 10th	Night	21% 36% 7% 14% 15% 8%	2340
	11th to 20th	Night	24% 38% 8% 12% 15% 4	2220
	21st to end	Night	24% 37% 7% 16% 12% 3	1776
March	1st to 10th	Night	26% 37% 9% 14% 11% 3	1993
	11th to 20th	Night	28% 37% 7% 15% 9% 2	1980
	21st to end	Night	24% 38% 9% 13% 11% 4	2163
April	1st to 10th	Night	22% 38% 10% 14% 13% 3	1800
	11th to 20th	Night	18% 36% 9% 16% 15% 6%	1779
	21st to end	Night	14% 36% 7% 21% 17% 5%	1800
May	1st to 10th	Night	16% 35% 6% 14% 21% 8%	1688
	11th to 20th	Night	16% 38% 6% 16% 19% 5%	1559
	21st to end	Night	17% 38% 8% 17% 17% 3	1552
June	1st to 10th	Night	20% 38% 8% 14% 18% 3	1440
	11th to 20th	Night	11% 38% 5% 10% 22% 13%	1440
	21st to end	Night	15% 35% 7% 10% 17% 16%	1440
July	1st to 10th	Night	13% 36% 5% 4 17% 26%	1400
	11th to 20th	Night	21% 36% 5% 6% 8% 24%	1566
	21st to end	Night	31% 36% 5% 2 7% 18%	1674
August	1st to 10th	Night	25% 34% 6% 7% 12% 16%	1602
	11th to 20th	Night	22% 34% 7% 6% 13% 19%	1683
	21st to end	Night	18% 35% 5% 5% 16% 21%	1980
September	1st to 10th	Night	13% 35% 4 7% 14% 28%	1800
	11th to 20th	Night	21% 36% 6% 11% 16% 10%	1800
	21st to end	Night	14% 35% 4 13% 19% 15%	2060
October	1st to 10th	Night	17% 36% 8% 12% 15% 12%	2160
	11th to 20th	Night	18% 36% 5% 12% 15% 14%	2160
	21st to end	Night	18% 37% 5% 13% 16% 12%	2376
November	1st to 10th	Night	21% 42% 7% 13% 9% 7%	2234
	11th to 20th	Night	20% 41% 7% 15% 11% 6%	2470
	21st to end	Night	15% 40% 8% 13% 16% 8%	2660
December	1st to 10th	Night	22% 40% 5% 12% 14% 7%	2660
	11th to 20th	Night	41% 33% 7% 9% 9% 1	3276
	21st to end	Night	41% 30% 12% 11% 5% 1	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	27% 11% 31% 18% 13% 1	294
	11th to 20th	Day	19% 12% 22% 33% 13% 1	344
	21st to end	Day	13% 9% 30% 30% 16% 1	374
February	1st to 10th	Day	19% 11% 23% 28% 18% 2	340
	11th to 20th	Day	16% 18% 27% 26% 12% 1	359
	21st to end	Day	14% 16% 30% 26% 13% 1	346
March	1st to 10th	Day	19% 19% 31% 20% 10% 1	420
	11th to 20th	Day	18% 17% 27% 22% 11% 4	420
	21st to end	Day	12% 21% 32% 25% 8% 1	462
April	1st to 10th	Day	15% 18% 32% 24% 9% 2	420
	11th to 20th	Day	13% 21% 33% 20% 8% 3	412
	21st to end	Day	5% 24% 30% 25% 11% 5%	420
May	1st to 10th	Day	9% 19% 34% 26% 11% 1	423
	11th to 20th	Day	8% 21% 33% 25% 11% 1	448
	21st to end	Day	7% 17% 36% 25% 12% 3	485
June	1st to 10th	Day	8% 14% 34% 30% 12% 4	450
	11th to 20th	Day	4 13% 35% 35% 11% 2	450
	21st to end	Day	2 8% 37% 40% 11% 3	450
July	1st to 10th	Day	2 5% 34% 46% 12% 1	446
	11th to 20th	Day	1 3 25% 50% 18% 2	444
	21st to end	Day	3 1 27% 51% 15% 3	487
August	1st to 10th	Day	4 6% 30% 47% 11% 3	447
	11th to 20th	Day	4 3 30% 43% 16% 5%	437
	21st to end	Day	3 3 25% 45% 20% 4	495
September	1st to 10th	Day	3 4 24% 41% 20% 8%	450
	11th to 20th	Day	5% 9% 26% 38% 16% 6%	450
	21st to end	Day	4 8% 25% 33% 25% 5%	450
October	1st to 10th	Day	4 9% 26% 30% 22% 8%	450
	11th to 20th	Day	7% 8% 22% 27% 28% 7%	450
	21st to end	Day	12% 6% 24% 28% 23% 7%	495
November	1st to 10th	Day	10% 10% 22% 25% 26% 5%	439
	11th to 20th	Day	11% 8% 16% 32% 26% 7%	268
	21st to end	Day	11% 8% 25% 37% 16% 3	180
December	1st to 10th	Day	21% 11% 19% 28% 17% 4	180
	11th to 20th	Day	17% 16% 23% 29% 14% 1	180
	21st to end	Day	21% 12% 26% 29% 12%	198
January	1st to 10th	Night	20% 8% 15% 29% 22% 5%	760
	11th to 20th	Night	17% 7% 15% 32% 24% 5%	726
	21st to end	Night	18% 7% 14% 34% 24% 4	792
February	1st to 10th	Night	18% 6% 15% 33% 25% 3	720
	11th to 20th	Night	13% 11% 18% 33% 20% 5%	704
	21st to end	Night	15% 11% 22% 29% 21% 3	592
March	1st to 10th	Night	20% 10% 19% 28% 20% 3	720
	11th to 20th	Night	18% 9% 18% 30% 22% 4	720
	21st to end	Night	15% 10% 27% 29% 17% 2	792
April	1st to 10th	Night	12% 10% 27% 32% 16% 3	720
	11th to 20th	Night	12% 15% 24% 27% 18% 5%	711
	21st to end	Night	5% 12% 28% 30% 21% 4	720
May	1st to 10th	Night	6% 7% 27% 32% 24% 4	608
	11th to 20th	Night	6% 4 24% 33% 30% 3	496
	21st to end	Night	7% 11% 25% 35% 20% 3	389
June	1st to 10th	Night	5% 7% 24% 38% 25% 1	360
	11th to 20th	Night	1 4 23% 50% 19% 3	360
	21st to end	Night	4 4 19% 39% 31% 2	360
July	1st to 10th	Night	4 2 15% 46% 30% 3	350
	11th to 20th	Night	7% 1 9% 42% 33% 8%	522
	21st to end	Night	11% 3 13% 34% 34% 5%	591
August	1st to 10th	Night	12% 3 17% 33% 30% 5%	537
	11th to 20th	Night	6% 2 18% 38% 30% 6%	615
	21st to end	Night	7% 1 14% 38% 34% 6%	792
September	1st to 10th	Night	4 1 13% 36% 37% 10%	720
	11th to 20th	Night	7% 3 18% 34% 32% 6%	720
	21st to end	Night	4 4 16% 31% 37% 9%	720
October	1st to 10th	Night	7% 4 15% 32% 34% 8%	720
	11th to 20th	Night	8% 5% 15% 29% 34% 9%	720
	21st to end	Night	10% 5% 12% 27% 34% 11%	792
November	1st to 10th	Night	9% 6% 15% 27% 34% 9%	740
	11th to 20th	Night	12% 4 14% 32% 32% 6%	760
	21st to end	Night	11% 6% 13% 39% 26% 5%	760
December	1st to 10th	Night	19% 6% 14% 32% 24% 5%	760
	11th to 20th	Night	24% 8% 15% 29% 20% 4	760
	21st to end	Night	21% 11% 18% 29% 20% 1	836

Page generated at 2024-01-09 04:14:45. Version ac07

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