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 2024-11-05.

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% 59%	105053
	Night	8% 5% 2 5% 4 3 4 3 5% 7% 54%	102736
Transparency	Day	28% 5% 8% 19% 22% 17%	104814
	Night	34% 5% 9% 18% 20% 14%	102566
Seeing	Day	9% 11% 28% 32% 16% 4	15418
	Night	11% 7% 18% 33% 27% 5%	24702

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	13% 9% 3 8% 5% 4 5% 3 5% 7% 37%	2731
	11th to 20th	Day	17% 12% 4 9% 5% 5% 4 3 4 5% 32%	2860
	21st to end	Day	19% 8% 3 9% 5% 5% 5% 3 6% 7% 30%	3146
February	1st to 10th	Day	15% 6% 2 5% 5% 4 4 3 6% 6% 44%	1710
	11th to 20th	Day	13% 7% 2 6% 5% 3 5% 5% 7% 7% 41%	1722
	21st to end	Day	12% 5% 2 7% 4 4 4 4 4 6% 47%	1570
March	1st to 10th	Day	13% 8% 4 10% 7% 6% 6% 4 5% 7% 30%	3427
	11th to 20th	Day	9% 4 2 5% 3 4 3 3 6% 7% 54%	3490
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 37%	3851
April	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 7% 8% 45%	2280
	11th to 20th	Day	7% 4 2 5% 3 4 5% 3 5% 8% 55%	2249
	21st to end	Day	3 4 1 4 3 3 3 3 6% 9% 60%	2280
May	1st to 10th	Day	4 2 1 3 2 2 2 2 4 5% 73%	4000
	11th to 20th	Day	3 2 1 4 3 2 3 3 4 8% 68%	4126
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 73%	4813
June	1st to 10th	Day	4 4 2 4 3 2 3 3 5% 8% 61%	2660
	11th to 20th	Day	1 2 1 3 2 2 2 3 5% 7% 73%	2660
	21st to end	Day	2 2 1 3 3 2 3 2 4 7% 72%	2660
July	1st to 10th	Day	2 1 2 2 1 1 1 3 6% 81%	4403
	11th to 20th	Day	1 2 1 3 2 2 2 2 4 7% 73%	4110
	21st to end	Day	1 2 1 2 2 2 3 2 4 8% 73%	4455
August	1st to 10th	Day	1 2 1 4 3 3 4 4 6% 10% 62%	4091
	11th to 20th	Day	2 1 2 1 1 2 2 3 7% 78%	3878
	21st to end	Day	1 3 1 4 3 3 3 2 4 8% 68%	4180
September	1st to 10th	Day	1 3 1 3 3 2 2 2 4 7% 73%	2280
	11th to 20th	Day	3 4 2 5% 3 3 3 2 4 8% 63%	2280
	21st to end	Day	2 2 1 3 2 2 3 2 4 6% 74%	1999
October	1st to 10th	Day	2 2 1 4 3 2 3 3 4 7% 71%	3180
	11th to 20th	Day	4 4 2 6% 3 3 3 3 4 5% 63%	3161
	21st to end	Day	5% 3 1 5% 3 4 4 4 6% 8% 57%	3448
November	1st to 10th	Day	8% 4 2 4 4 4 4 4 5% 8% 54%	1884
	11th to 20th	Day	9% 8% 3 9% 4 3 3 3 5% 7% 46%	1707
	21st to end	Day	9% 7% 2 4 3 3 4 3 5% 7% 53%	1520
December	1st to 10th	Day	14% 7% 3 6% 3 4 4 3 5% 6% 44%	1520
	11th to 20th	Day	25% 11% 3 8% 5% 3 3 3 4 5% 30%	1928
	21st to end	Day	21% 8% 3 7% 4 4 3 4 5% 8% 32%	2794
January	1st to 10th	Night	14% 8% 3 7% 4 3 4 3 5% 7% 43%	4217
	11th to 20th	Night	17% 9% 4 10% 5% 4 4 4 5% 6% 33%	4110
	21st to end	Night	20% 10% 3 8% 4 4 4 3 5% 6% 32%	4521
February	1st to 10th	Night	15% 5% 2 5% 3 4 4 3 5% 6% 48%	2470
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 5% 8% 46%	2345
	21st to end	Night	12% 5% 1 4 4 4 3 3 6% 6% 53%	1884
March	1st to 10th	Night	13% 8% 3 7% 5% 4 4 4 5% 6% 39%	3445
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 4 6% 58%	3490
	21st to end	Night	11% 7% 3 7% 4 4 4 4 5% 7% 43%	3801
April	1st to 10th	Night	7% 6% 2 5% 4 5% 3 4 5% 8% 51%	1900
	11th to 20th	Night	7% 4 2 6% 3 3 3 3 5% 6% 59%	1879
	21st to end	Night	4 3 1 3 3 3 4 3 6% 8% 61%	1900
May	1st to 10th	Night	3 3 1 3 2 2 2 2 4 5% 73%	2970
	11th to 20th	Night	1 2 1 7% 6% 5% 4 4 5% 7% 58%	2770
	21st to end	Night	2 2 1 2 2 2 3 2 3 6% 75%	2762
June	1st to 10th	Night	3 3 2 4 3 3 5% 4 7% 11% 56%	1520
	11th to 20th	Night	1 1 1 3 3 2 3 3 6% 9% 68%	1520
	21st to end	Night	1 2 1 4 3 3 3 3 4 9% 66%	1520
July	1st to 10th	Night	1 1 2 2 2 3 2 4 7% 76%	2540
	11th to 20th	Night	1 2 1 4 3 3 4 4 6% 10% 63%	2859
	21st to end	Night	3 1 4 3 3 5% 5% 7% 13% 53%	3109
August	1st to 10th	Night	3 4 2 6% 4 4 5% 5% 7% 11% 51%	2848
	11th to 20th	Night	2 1 4 3 3 3 3 5% 7% 69%	2963
	21st to end	Night	1 3 2 4 2 3 3 3 5% 9% 64%	3498
September	1st to 10th	Night	1 2 1 3 3 3 3 3 5% 9% 67%	1900
	11th to 20th	Night	2 3 2 7% 3 3 4 4 6% 9% 58%	1900
	21st to end	Night	2 3 1 5% 3 3 4 3 4 8% 65%	2176
October	1st to 10th	Night	2 2 1 3 3 2 3 2 5% 7% 69%	3800
	11th to 20th	Night	3 4 2 4 3 3 3 3 4 6% 64%	3777
	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 2 4 9% 56%	2293
	11th to 20th	Night	9% 5% 2 6% 3 3 4 3 5% 7% 52%	2470
	21st to end	Night	7% 4 2 5% 4 4 4 3 5% 7% 56%	2660
December	1st to 10th	Night	15% 6% 2 7% 4 3 4 3 5% 6% 45%	2660
	11th to 20th	Night	28% 8% 2 6% 3 4 3 3 5% 6% 31%	3276
	21st to end	Night	18% 8% 3 7% 5% 5% 5% 4 5% 6% 34%	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	44% 5% 14% 27% 7% 3	2731
	11th to 20th	Day	53% 5% 12% 13% 10% 7%	2860
	21st to end	Day	54% 6% 9% 16% 11% 4	3146
February	1st to 10th	Day	40% 5% 7% 20% 21% 7%	1710
	11th to 20th	Day	39% 6% 10% 19% 20% 6%	1731
	21st to end	Day	36% 6% 10% 26% 17% 6%	1570
March	1st to 10th	Day	52% 7% 10% 16% 12% 2	3427
	11th to 20th	Day	30% 6% 12% 30% 10% 11%	3490
	21st to end	Day	40% 8% 17% 23% 10% 2	3851
April	1st to 10th	Day	37% 6% 14% 20% 19% 4	2280
	11th to 20th	Day	29% 6% 16% 25% 19% 5%	2252
	21st to end	Day	22% 5% 15% 30% 24% 4	2280
May	1st to 10th	Day	17% 4 10% 30% 25% 14%	4000
	11th to 20th	Day	15% 9% 5% 28% 34% 9%	4129
	21st to end	Day	17% 7% 9% 32% 32% 3	4814
June	1st to 10th	Day	25% 5% 11% 27% 24% 7%	2660
	11th to 20th	Day	16% 5% 9% 20% 35% 15%	2660
	21st to end	Day	18% 7% 5% 12% 31% 27%	2660
July	1st to 10th	Day	12% 3 3 5% 14% 62%	4334
	11th to 20th	Day	17% 5% 5% 6% 16% 51%	4041
	21st to end	Day	19% 4 3 8% 29% 37%	4315
August	1st to 10th	Day	27% 5% 4 10% 25% 29%	4076
	11th to 20th	Day	14% 3 5% 10% 42% 27%	3917
	21st to end	Day	20% 7% 6% 13% 30% 25%	4179
September	1st to 10th	Day	17% 4 3 9% 30% 37%	2280
	11th to 20th	Day	25% 5% 5% 22% 29% 13%	2280
	21st to end	Day	16% 5% 4 22% 32% 21%	1999
October	1st to 10th	Day	16% 3 4 11% 28% 37%	3180
	11th to 20th	Day	26% 4 6% 22% 25% 17%	3161
	21st to end	Day	26% 5% 8% 25% 24% 12%	3448
November	1st to 10th	Day	28% 5% 11% 28% 18% 11%	1884
	11th to 20th	Day	39% 4 9% 22% 20% 7%	1707
	21st to end	Day	30% 5% 12% 20% 23% 10%	1520
December	1st to 10th	Day	40% 6% 10% 19% 17% 8%	1520
	11th to 20th	Day	55% 5% 9% 13% 15% 4	1928
	21st to end	Day	48% 5% 14% 20% 9% 3	2794
January	1st to 10th	Night	41% 5% 18% 25% 7% 3	4217
	11th to 20th	Night	53% 6% 12% 12% 9% 7%	4110
	21st to end	Night	55% 6% 9% 14% 11% 5%	4521
February	1st to 10th	Night	38% 4 8% 21% 19% 10%	2470
	11th to 20th	Night	38% 6% 10% 19% 20% 8%	2347
	21st to end	Night	33% 6% 10% 30% 17% 4	1884
March	1st to 10th	Night	48% 5% 14% 17% 13% 3	3445
	11th to 20th	Night	31% 6% 9% 26% 19% 10%	3490
	21st to end	Night	44% 6% 14% 22% 11% 3	3801
April	1st to 10th	Night	35% 6% 13% 23% 19% 5%	1900
	11th to 20th	Night	30% 5% 12% 26% 19% 8%	1879
	21st to end	Night	22% 6% 11% 31% 24% 7%	1900
May	1st to 10th	Night	20% 4 8% 27% 28% 13%	2970
	11th to 20th	Night	25% 11% 6% 20% 27% 11%	2771
	21st to end	Night	19% 7% 8% 35% 29% 2	2762
June	1st to 10th	Night	31% 6% 9% 22% 25% 7%	1520
	11th to 20th	Night	22% 4 7% 18% 34% 16%	1520
	21st to end	Night	23% 7% 3 12% 28% 27%	1520
July	1st to 10th	Night	18% 5% 3 4 14% 57%	2500
	11th to 20th	Night	28% 4 4 8% 12% 45%	2806
	21st to end	Night	38% 4 2 6% 19% 30%	3011
August	1st to 10th	Night	44% 4 2 13% 20% 17%	2839
	11th to 20th	Night	24% 4 3 7% 37% 25%	2990
	21st to end	Night	25% 7% 5% 11% 30% 22%	3498
September	1st to 10th	Night	21% 4 4 10% 27% 34%	1900
	11th to 20th	Night	31% 4 5% 20% 25% 14%	1900
	21st to end	Night	23% 4 4 20% 28% 21%	2176
October	1st to 10th	Night	18% 5% 5% 10% 23% 40%	3800
	11th to 20th	Night	27% 3 7% 21% 21% 20%	3777
	21st to end	Night	27% 5% 6% 22% 26% 16%	4121
November	1st to 10th	Night	30% 4 10% 25% 18% 12%	2293
	11th to 20th	Night	32% 5% 11% 26% 18% 8%	2470
	21st to end	Night	28% 6% 11% 21% 24% 11%	2660
December	1st to 10th	Night	40% 5% 8% 19% 17% 10%	2660
	11th to 20th	Night	53% 5% 9% 15% 15% 3	3276
	21st to end	Night	48% 6% 16% 17% 10% 3	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	28% 11% 30% 18% 13% 1	294
	11th to 20th	Day	19% 12% 22% 34% 13% 1	370
	21st to end	Day	14% 9% 31% 30% 15% 1	407
February	1st to 10th	Day	19% 12% 22% 28% 17% 2	370
	11th to 20th	Day	18% 19% 26% 26% 11% 1	392
	21st to end	Day	13% 15% 30% 26% 13% 2	382
March	1st to 10th	Day	20% 19% 31% 20% 9% 1	460
	11th to 20th	Day	17% 17% 27% 23% 12% 4	460
	21st to end	Day	14% 21% 32% 24% 8% 1	506
April	1st to 10th	Day	15% 18% 31% 24% 10% 2	460
	11th to 20th	Day	13% 21% 32% 21% 9% 4	452
	21st to end	Day	5% 23% 31% 25% 10% 5%	460
May	1st to 10th	Day	8% 19% 34% 27% 11% 1	463
	11th to 20th	Day	8% 19% 32% 28% 12% 1	488
	21st to end	Day	6% 17% 35% 26% 14% 3	529
June	1st to 10th	Day	7% 13% 34% 30% 12% 3	490
	11th to 20th	Day	4 12% 34% 36% 13% 2	490
	21st to end	Day	1 8% 36% 41% 12% 3	490
July	1st to 10th	Day	2 5% 31% 44% 15% 2	486
	11th to 20th	Day	1 3 24% 49% 20% 3	484
	21st to end	Day	3 1 26% 51% 15% 3	531
August	1st to 10th	Day	4 5% 29% 46% 13% 3	487
	11th to 20th	Day	4 3 30% 43% 16% 5%	477
	21st to end	Day	3 3 25% 43% 22% 5%	539
September	1st to 10th	Day	3 4 23% 42% 20% 8%	490
	11th to 20th	Day	5% 9% 26% 38% 17% 6%	490
	21st to end	Day	3 8% 23% 33% 26% 6%	490
October	1st to 10th	Day	4 8% 24% 31% 23% 10%	490
	11th to 20th	Day	8% 8% 22% 26% 27% 8%	490
	21st to end	Day	11% 6% 24% 30% 23% 7%	539
November	1st to 10th	Day	10% 10% 23% 26% 26% 5%	456
	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%	760
	21st to end	Night	18% 7% 14% 34% 24% 3	836
February	1st to 10th	Night	18% 6% 16% 33% 24% 3	760
	11th to 20th	Night	14% 12% 18% 32% 20% 5%	744
	21st to end	Night	14% 11% 22% 30% 20% 3	628
March	1st to 10th	Night	20% 11% 19% 28% 19% 3	760
	11th to 20th	Night	18% 9% 18% 30% 22% 3	760
	21st to end	Night	15% 11% 27% 29% 16% 2	836
April	1st to 10th	Night	11% 10% 28% 32% 17% 3	760
	11th to 20th	Night	12% 15% 23% 28% 18% 4	751
	21st to end	Night	5% 12% 28% 30% 21% 4	760
May	1st to 10th	Night	5% 7% 28% 33% 23% 3	641
	11th to 20th	Night	6% 3 24% 34% 30% 3	524
	21st to end	Night	6% 10% 25% 36% 20% 3	411
June	1st to 10th	Night	5% 7% 24% 38% 24% 1	380
	11th to 20th	Night	1 4 22% 50% 20% 3	380
	21st to end	Night	4 4 18% 40% 32% 2	380
July	1st to 10th	Night	4 2 14% 46% 32% 3	370
	11th to 20th	Night	7% 1 10% 41% 34% 8%	552
	21st to end	Night	10% 3 12% 35% 34% 5%	624
August	1st to 10th	Night	11% 3 16% 33% 32% 4	567
	11th to 20th	Night	5% 2 18% 39% 30% 6%	650
	21st to end	Night	7% 1 13% 38% 35% 6%	836
September	1st to 10th	Night	3 1 12% 37% 37% 9%	760
	11th to 20th	Night	7% 3 18% 35% 32% 6%	760
	21st to end	Night	4 4 15% 31% 38% 9%	760
October	1st to 10th	Night	6% 4 14% 31% 35% 9%	760
	11th to 20th	Night	8% 5% 14% 29% 34% 9%	760
	21st to end	Night	10% 5% 12% 28% 34% 11%	836
November	1st to 10th	Night	10% 6% 15% 27% 34% 9%	760
	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 2024-11-02 23:11:06. Version ac07

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