San Francisco
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco clear sky chart from 2005-11-06 to 2024-03-28.

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	9% 7% 2 5% 4 2 3 4 5% 8% 51%	78935
	Night	11% 8% 3 6% 5% 3 4 4 5% 7% 46%	81096
Transparency	Day	32% 5% 9% 19% 22% 13%	78656
	Night	38% 4 9% 19% 18% 11%	80897
Seeing	Day	11% 12% 29% 31% 14% 3	14545
	Night	13% 7% 16% 32% 26% 6%	23996

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	16% 7% 3 5% 5% 3 4 4 5% 8% 39%	2735
	11th to 20th	Day	18% 9% 4 8% 6% 4 4 4 4 6% 33%	2860
	21st to end	Day	20% 11% 3 6% 6% 3 4 5% 5% 7% 30%	3146
February	1st to 10th	Day	16% 6% 2 4 4 3 4 4 4 6% 46%	1710
	11th to 20th	Day	13% 7% 3 5% 4 3 4 5% 5% 9% 42%	1731
	21st to end	Day	12% 6% 3 5% 4 2 3 4 5% 7% 48%	1570
March	1st to 10th	Day	15% 9% 2 6% 5% 3 4 4 6% 8% 38%	2069
	11th to 20th	Day	14% 7% 3 5% 6% 3 5% 4 5% 8% 39%	2090
	21st to end	Day	11% 5% 2 4 4 4 4 4 6% 9% 47%	2274
April	1st to 10th	Day	12% 6% 3 6% 4 2 5% 4 5% 8% 45%	2160
	11th to 20th	Day	8% 4 2 5% 3 2 3 5% 5% 7% 55%	2129
	21st to end	Day	5% 4 2 3 4 2 3 3 6% 11% 58%	2160
May	1st to 10th	Day	5% 6% 2 4 4 2 3 3 4 8% 59%	2253
	11th to 20th	Day	6% 5% 2 4 3 2 3 3 5% 9% 58%	2330
	21st to end	Day	5% 8% 3 5% 4 2 3 4 5% 8% 54%	2723
June	1st to 10th	Day	6% 6% 2 4 3 2 3 4 4 9% 56%	2520
	11th to 20th	Day	2 5% 1 3 3 1 3 3 5% 8% 67%	2520
	21st to end	Day	3 6% 2 4 3 2 3 3 4 9% 61%	2520
July	1st to 10th	Day	1 8% 2 5% 3 1 2 4 4 11% 58%	2514
	11th to 20th	Day	2 7% 3 4 3 2 4 4 5% 12% 54%	2340
	21st to end	Day	1 7% 2 4 3 2 3 3 5% 12% 56%	2546
August	1st to 10th	Day	2 10% 2 4 4 2 4 5% 6% 11% 50%	2328
	11th to 20th	Day	2 7% 2 5% 3 2 3 3 5% 10% 58%	2205
	21st to end	Day	2 8% 3 5% 4 2 3 3 5% 10% 55%	2376
September	1st to 10th	Day	2 4 2 4 3 1 3 3 4 8% 66%	2160
	11th to 20th	Day	4 7% 3 4 5% 2 3 3 5% 9% 56%	2160
	21st to end	Day	3 3 2 3 3 2 3 1 4 6% 70%	1898
October	1st to 10th	Day	5% 4 2 3 3 2 2 4 4 6% 65%	1800
	11th to 20th	Day	8% 4 3 4 5% 2 3 4 4 6% 58%	1800
	21st to end	Day	8% 5% 2 5% 3 2 3 3 4 7% 59%	1980
November	1st to 10th	Day	8% 5% 2 4 3 2 3 4 5% 8% 54%	1841
	11th to 20th	Day	11% 8% 2 5% 5% 3 3 4 4 7% 47%	1710
	21st to end	Day	10% 6% 2 3 4 3 3 3 5% 7% 54%	1535
December	1st to 10th	Day	15% 6% 3 5% 4 3 4 4 5% 7% 44%	1520
	11th to 20th	Day	25% 9% 3 6% 5% 2 3 4 5% 6% 32%	1928
	21st to end	Day	19% 10% 3 5% 4 3 4 4 4 8% 34%	2794
January	1st to 10th	Night	14% 7% 3 5% 5% 2 4 4 5% 7% 44%	4214
	11th to 20th	Night	19% 8% 3 7% 6% 4 5% 4 5% 6% 32%	4110
	21st to end	Night	22% 11% 3 7% 5% 3 4 4 4 6% 30%	4521
February	1st to 10th	Night	16% 6% 2 4 4 2 4 4 4 6% 48%	2470
	11th to 20th	Night	11% 6% 4 5% 4 3 5% 4 6% 8% 44%	2334
	21st to end	Night	12% 6% 2 4 5% 2 4 4 5% 7% 51%	1884
March	1st to 10th	Night	17% 7% 3 6% 5% 3 4 4 5% 5% 39%	2109
	11th to 20th	Night	15% 7% 3 5% 4 3 4 4 6% 7% 43%	2090
	21st to end	Night	13% 7% 3 4 5% 2 4 4 3 6% 50%	2236
April	1st to 10th	Night	10% 6% 4 6% 5% 3 3 3 5% 8% 48%	1800
	11th to 20th	Night	10% 7% 1 3 3 2 3 3 4 7% 56%	1779
	21st to end	Night	4 4 2 4 4 2 3 4 5% 9% 60%	1800
May	1st to 10th	Night	4 8% 3 4 5% 3 3 3 4 6% 56%	1703
	11th to 20th	Night	3 7% 3 6% 5% 3 4 3 5% 9% 52%	1558
	21st to end	Night	6% 12% 3 5% 4 3 3 3 4 6% 51%	1552
June	1st to 10th	Night	5% 7% 3 6% 4 3 5% 4 5% 9% 49%	1440
	11th to 20th	Night	2 5% 3 5% 5% 2 2 3 4 8% 61%	1440
	21st to end	Night	3 8% 3 7% 5% 2 3 2 3 7% 57%	1440
July	1st to 10th	Night	1 10% 5% 4 5% 3 3 3 4 11% 51%	1441
	11th to 20th	Night	2 10% 4 8% 6% 3 4 4 5% 11% 43%	1620
	21st to end	Night	2 17% 7% 10% 7% 3 5% 4 5% 7% 32%	1773
August	1st to 10th	Night	3 18% 5% 7% 6% 3 4 4 4 9% 37%	1611
	11th to 20th	Night	2 12% 5% 9% 5% 3 5% 5% 6% 9% 38%	1696
	21st to end	Night	2 12% 4 7% 6% 3 5% 5% 5% 8% 44%	1980
September	1st to 10th	Night	1 6% 3 4 4 2 3 4 4 8% 61%	1800
	11th to 20th	Night	4 12% 2 6% 4 2 4 3 4 9% 49%	1800
	21st to end	Night	3 5% 3 5% 4 2 2 3 4 8% 62%	2056
October	1st to 10th	Night	5% 5% 2 6% 3 2 3 3 3 6% 62%	2160
	11th to 20th	Night	6% 7% 3 5% 4 2 3 4 5% 7% 55%	2160
	21st to end	Night	7% 5% 3 6% 4 2 3 4 5% 6% 56%	2376
November	1st to 10th	Night	7% 6% 3 5% 4 2 3 3 4 8% 54%	2234
	11th to 20th	Night	9% 7% 3 4 4 3 3 4 5% 7% 51%	2470
	21st to end	Night	9% 4 2 4 4 2 3 4 5% 6% 56%	2641
December	1st to 10th	Night	15% 6% 3 5% 4 2 3 4 4 6% 46%	2660
	11th to 20th	Night	29% 8% 3 4 4 2 4 4 4 6% 32%	3276
	21st to end	Night	19% 7% 2 6% 5% 3 5% 5% 6% 6% 36%	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	43% 5% 14% 28% 8% 3	2735
	11th to 20th	Day	54% 4 12% 13% 10% 7%	2860
	21st to end	Day	54% 5% 9% 16% 11% 4	3146
February	1st to 10th	Day	41% 3 8% 19% 21% 8%	1710
	11th to 20th	Day	40% 5% 9% 20% 21% 6%	1740
	21st to end	Day	36% 4 10% 26% 18% 6%	1570
March	1st to 10th	Day	45% 5% 11% 21% 15% 3	2069
	11th to 20th	Day	43% 5% 10% 22% 15% 5%	2090
	21st to end	Day	35% 6% 16% 24% 16% 4	2274
April	1st to 10th	Day	39% 5% 14% 19% 20% 4	2160
	11th to 20th	Day	29% 4 16% 25% 20% 6%	2132
	21st to end	Day	23% 4 15% 30% 23% 4	2160
May	1st to 10th	Day	26% 4 9% 28% 28% 6%	2253
	11th to 20th	Day	26% 5% 9% 30% 22% 8%	2333
	21st to end	Day	30% 6% 13% 25% 22% 3	2724
June	1st to 10th	Day	27% 4 12% 26% 23% 8%	2520
	11th to 20th	Day	17% 5% 9% 21% 35% 14%	2520
	21st to end	Day	21% 7% 3 13% 31% 24%	2520
July	1st to 10th	Day	21% 6% 2 10% 24% 36%	2445
	11th to 20th	Day	24% 9% 5% 9% 22% 32%	2271
	21st to end	Day	22% 8% 2 10% 26% 33%	2405
August	1st to 10th	Day	28% 9% 2 12% 27% 22%	2312
	11th to 20th	Day	23% 6% 1 11% 33% 27%	2205
	21st to end	Day	25% 9% 2 12% 28% 24%	2376
September	1st to 10th	Day	19% 5% 1 9% 30% 36%	2160
	11th to 20th	Day	27% 4 4 23% 28% 14%	2160
	21st to end	Day	18% 3 3 23% 31% 21%	1898
October	1st to 10th	Day	21% 4 6% 18% 31% 21%	1800
	11th to 20th	Day	28% 5% 7% 18% 23% 20%	1800
	21st to end	Day	28% 4 8% 20% 22% 17%	1980
November	1st to 10th	Day	29% 4 11% 27% 18% 11%	1841
	11th to 20th	Day	38% 5% 8% 22% 20% 6%	1710
	21st to end	Day	31% 4 10% 21% 24% 10%	1535
December	1st to 10th	Day	40% 5% 11% 19% 17% 8%	1520
	11th to 20th	Day	55% 5% 9% 14% 15% 4	1928
	21st to end	Day	49% 5% 14% 21% 10% 3	2794
January	1st to 10th	Night	41% 5% 18% 26% 7% 3	4214
	11th to 20th	Night	54% 4 13% 12% 9% 7%	4110
	21st to end	Night	57% 4 9% 14% 11% 5%	4521
February	1st to 10th	Night	38% 3 9% 21% 18% 10%	2470
	11th to 20th	Night	38% 5% 10% 19% 21% 7%	2336
	21st to end	Night	33% 4 11% 30% 17% 4	1884
March	1st to 10th	Night	46% 5% 11% 18% 16% 4	2109
	11th to 20th	Night	41% 5% 8% 23% 18% 6%	2090
	21st to end	Night	38% 4 14% 23% 17% 4	2236
April	1st to 10th	Night	37% 4 14% 21% 19% 5%	1800
	11th to 20th	Night	31% 4 13% 26% 19% 8%	1779
	21st to end	Night	24% 5% 12% 30% 23% 7%	1800
May	1st to 10th	Night	30% 4 7% 24% 24% 10%	1703
	11th to 20th	Night	32% 4 10% 23% 24% 7%	1559
	21st to end	Night	36% 4 13% 23% 21% 4	1552
June	1st to 10th	Night	33% 6% 8% 22% 24% 7%	1440
	11th to 20th	Night	25% 2 7% 18% 33% 15%	1440
	21st to end	Night	29% 3 3 13% 27% 24%	1440
July	1st to 10th	Night	30% 5% 1 8% 24% 32%	1401
	11th to 20th	Night	35% 5% 4 10% 16% 28%	1566
	21st to end	Night	49% 6% 1 5% 15% 25%	1674
August	1st to 10th	Night	44% 4 1 10% 23% 17%	1602
	11th to 20th	Night	39% 6% 2 9% 20% 23%	1696
	21st to end	Night	36% 6% 2 11% 24% 21%	1980
September	1st to 10th	Night	23% 5% 3 10% 26% 34%	1800
	11th to 20th	Night	33% 3 5% 21% 23% 14%	1800
	21st to end	Night	23% 4 3 20% 28% 21%	2056
October	1st to 10th	Night	25% 4 7% 16% 25% 24%	2160
	11th to 20th	Night	29% 4 6% 19% 20% 22%	2160
	21st to end	Night	30% 4 6% 21% 22% 19%	2376
November	1st to 10th	Night	31% 5% 9% 25% 18% 12%	2234
	11th to 20th	Night	33% 5% 10% 27% 18% 8%	2470
	21st to end	Night	29% 5% 10% 22% 24% 11%	2641
December	1st to 10th	Night	40% 4 9% 19% 18% 10%	2660
	11th to 20th	Night	54% 4 9% 15% 15% 3	3276
	21st to end	Night	48% 6% 15% 17% 10% 4	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% 29% 20% 12% 1	298
	11th to 20th	Day	20% 11% 22% 34% 12% 1	370
	21st to end	Day	15% 9% 30% 31% 14% 1	407
February	1st to 10th	Day	18% 12% 27% 25% 16% 1	370
	11th to 20th	Day	19% 15% 30% 25% 10% 1	396
	21st to end	Day	13% 15% 31% 27% 12% 2	382
March	1st to 10th	Day	22% 17% 31% 20% 9% 1	460
	11th to 20th	Day	20% 15% 27% 22% 12% 4	460
	21st to end	Day	14% 20% 33% 25% 7% 1	491
April	1st to 10th	Day	18% 18% 32% 22% 9% 2	420
	11th to 20th	Day	14% 21% 33% 21% 8% 3	412
	21st to end	Day	6% 25% 30% 24% 10% 4	420
May	1st to 10th	Day	11% 19% 34% 24% 9% 2	423
	11th to 20th	Day	10% 22% 33% 25% 8% 2	448
	21st to end	Day	9% 20% 31% 27% 10% 3	485
June	1st to 10th	Day	10% 12% 36% 28% 11% 3	450
	11th to 20th	Day	6% 13% 36% 32% 10% 2	450
	21st to end	Day	4 12% 34% 38% 11% 2	450
July	1st to 10th	Day	6% 7% 33% 42% 12% 1	446
	11th to 20th	Day	2 5% 27% 50% 14% 2	444
	21st to end	Day	4 3 26% 49% 13% 3	487
August	1st to 10th	Day	6% 7% 31% 44% 9% 2	447
	11th to 20th	Day	5% 2 30% 42% 17% 4	437
	21st to end	Day	6% 4 23% 46% 17% 4	495
September	1st to 10th	Day	4 5% 25% 40% 20% 6%	450
	11th to 20th	Day	6% 10% 27% 38% 14% 6%	450
	21st to end	Day	5% 9% 25% 34% 21% 6%	450
October	1st to 10th	Day	5% 10% 27% 30% 20% 9%	450
	11th to 20th	Day	7% 10% 24% 27% 24% 8%	450
	21st to end	Day	12% 6% 24% 28% 22% 7%	495
November	1st to 10th	Day	11% 10% 23% 27% 24% 5%	437
	11th to 20th	Day	11% 8% 16% 33% 26% 6%	270
	21st to end	Day	11% 8% 26% 35% 18% 2	187
December	1st to 10th	Day	19% 11% 19% 28% 17% 4	180
	11th to 20th	Day	17% 15% 23% 30% 14% 1	180
	21st to end	Day	21% 12% 26% 30% 11%	198
January	1st to 10th	Night	20% 7% 15% 31% 21% 5%	760
	11th to 20th	Night	17% 8% 13% 33% 24% 6%	760
	21st to end	Night	19% 7% 14% 34% 23% 3	836
February	1st to 10th	Night	19% 7% 14% 33% 24% 3	760
	11th to 20th	Night	14% 11% 18% 33% 19% 5%	744
	21st to end	Night	14% 10% 22% 30% 20% 3	628
March	1st to 10th	Night	20% 12% 18% 28% 19% 3	760
	11th to 20th	Night	19% 9% 17% 30% 22% 4	760
	21st to end	Night	15% 11% 26% 28% 17% 2	820
April	1st to 10th	Night	13% 9% 27% 32% 16% 3	720
	11th to 20th	Night	14% 14% 22% 28% 16% 5%	711
	21st to end	Night	6% 10% 28% 29% 22% 4	720
May	1st to 10th	Night	9% 8% 25% 32% 23% 4	623
	11th to 20th	Night	8% 4 22% 33% 29% 4	496
	21st to end	Night	13% 11% 20% 33% 19% 4	389
June	1st to 10th	Night	8% 6% 23% 36% 25% 2	360
	11th to 20th	Night	3 7% 19% 48% 20% 3	360
	21st to end	Night	6% 4 19% 37% 30% 4	360
July	1st to 10th	Night	4 4 14% 44% 29% 5%	351
	11th to 20th	Night	8% 3 7% 39% 33% 8%	522
	21st to end	Night	10% 7% 9% 32% 36% 7%	591
August	1st to 10th	Night	15% 4 13% 32% 30% 6%	537
	11th to 20th	Night	9% 4 14% 35% 30% 7%	628
	21st to end	Night	8% 4 11% 38% 33% 7%	792
September	1st to 10th	Night	4 2 13% 34% 36% 11%	720
	11th to 20th	Night	8% 5% 16% 34% 31% 7%	720
	21st to end	Night	4 4 14% 32% 34% 11%	720
October	1st to 10th	Night	8% 4 14% 32% 33% 9%	720
	11th to 20th	Night	8% 6% 12% 29% 36% 9%	720
	21st to end	Night	10% 4 12% 28% 33% 13%	792
November	1st to 10th	Night	9% 6% 14% 28% 34% 9%	740
	11th to 20th	Night	12% 4 13% 33% 32% 7%	760
	21st to end	Night	11% 6% 12% 38% 27% 6%	760
December	1st to 10th	Night	19% 6% 14% 33% 23% 6%	760
	11th to 20th	Night	26% 7% 13% 29% 20% 4	760
	21st to end	Night	22% 10% 19% 30% 20% 1	836

Page generated at 2024-03-24 20:37:02. Version ac07

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