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 2024-11-04.

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% 7% 2 5% 3 3 3 3 5% 8% 54%	105009
	Night	9% 8% 3 6% 4 3 3 3 5% 6% 49%	102751
Transparency	Day	29% 5% 8% 19% 23% 16%	104770
	Night	36% 4 8% 18% 20% 14%	102580
Seeing	Day	10% 12% 29% 32% 14% 3	15429
	Night	12% 7% 16% 33% 26% 6%	24728

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% 8% 2 6% 4 4 4 3 5% 8% 39%	2735
	11th to 20th	Day	18% 9% 4 9% 5% 5% 4 3 4 6% 33%	2860
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 5% 6% 30%	3146
February	1st to 10th	Day	16% 7% 2 5% 4 4 4 3 4 6% 46%	1710
	11th to 20th	Day	13% 8% 2 5% 4 4 4 4 5% 9% 41%	1731
	21st to end	Day	11% 7% 2 5% 4 3 3 4 5% 7% 48%	1570
March	1st to 10th	Day	14% 11% 3 10% 6% 5% 5% 3 6% 7% 30%	3411
	11th to 20th	Day	10% 5% 2 4 4 3 4 3 4 7% 54%	3490
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 4 7% 9% 37%	3855
April	1st to 10th	Day	11% 7% 3 6% 3 4 4 4 5% 8% 46%	2280
	11th to 20th	Day	8% 5% 2 6% 3 4 3 4 5% 7% 54%	2249
	21st to end	Day	4 4 2 4 3 3 3 2 6% 10% 58%	2280
May	1st to 10th	Day	5% 4 1 3 2 1 2 2 3 5% 71%	3971
	11th to 20th	Day	3 4 2 4 2 2 2 2 5% 10% 63%	4126
	21st to end	Day	2 7% 2 3 2 2 2 2 4 7% 66%	4813
June	1st to 10th	Day	6% 6% 2 4 3 3 3 3 5% 9% 56%	2660
	11th to 20th	Day	2 5% 1 3 2 2 3 2 5% 8% 67%	2660
	21st to end	Day	3 6% 2 4 3 3 3 2 5% 9% 61%	2660
July	1st to 10th	Day	1 5% 1 3 1 1 2 2 3 8% 72%	4401
	11th to 20th	Day	1 5% 2 5% 2 3 3 2 5% 10% 62%	4110
	21st to end	Day	1 10% 2 4 2 2 2 2 5% 12% 56%	4455
August	1st to 10th	Day	2 9% 3 6% 4 4 4 4 7% 13% 45%	4091
	11th to 20th	Day	1 5% 2 5% 2 3 3 2 4 9% 64%	3855
	21st to end	Day	2 6% 2 5% 3 3 3 3 6% 11% 58%	4180
September	1st to 10th	Day	2 5% 2 4 3 2 2 2 4 9% 66%	2280
	11th to 20th	Day	4 7% 3 5% 4 3 3 2 5% 9% 55%	2280
	21st to end	Day	3 3 2 4 3 2 2 1 3 7% 69%	2002
October	1st to 10th	Day	3 3 1 3 2 2 3 2 5% 7% 69%	3180
	11th to 20th	Day	6% 6% 2 5% 3 2 3 2 4 6% 60%	3161
	21st to end	Day	6% 5% 2 5% 3 3 3 3 6% 7% 58%	3448
November	1st to 10th	Day	8% 6% 2 4 3 3 3 3 5% 8% 54%	1872
	11th to 20th	Day	11% 9% 2 6% 5% 3 3 3 4 7% 47%	1710
	21st to end	Day	9% 7% 1 4 4 4 3 3 5% 7% 54%	1535
December	1st to 10th	Day	14% 7% 3 5% 3 3 4 3 5% 7% 44%	1520
	11th to 20th	Day	25% 10% 3 6% 4 3 3 3 5% 6% 32%	1928
	21st to end	Day	19% 10% 3 6% 3 4 4 4 5% 8% 34%	2794
January	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 7% 44%	4214
	11th to 20th	Night	19% 8% 2 8% 5% 5% 5% 3 5% 6% 32%	4110
	21st to end	Night	22% 11% 3 8% 4 4 4 3 4 6% 30%	4521
February	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 6% 48%	2470
	11th to 20th	Night	11% 7% 3 6% 3 4 4 4 6% 8% 43%	2334
	21st to end	Night	12% 6% 1 4 4 3 4 3 5% 6% 51%	1884
March	1st to 10th	Night	15% 11% 3 8% 4 4 4 3 5% 6% 36%	3461
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	3490
	21st to end	Night	14% 10% 3 7% 5% 4 4 3 4 6% 40%	3784
April	1st to 10th	Night	9% 7% 3 7% 4 4 3 3 5% 7% 48%	1900
	11th to 20th	Night	9% 8% 2 4 3 3 3 2 4 7% 55%	1879
	21st to end	Night	4 4 2 4 3 3 3 3 5% 8% 59%	1900
May	1st to 10th	Night	4 5% 2 3 3 2 2 2 3 5% 69%	2999
	11th to 20th	Night	2 8% 4 10% 4 3 3 2 4 6% 54%	2770
	21st to end	Night	3 7% 2 4 2 3 2 3 3 5% 65%	2762
June	1st to 10th	Night	4 7% 3 7% 4 4 4 3 5% 10% 49%	1520
	11th to 20th	Night	2 5% 2 5% 4 3 2 3 4 8% 62%	1520
	21st to end	Night	3 8% 3 7% 4 3 3 2 3 8% 57%	1520
July	1st to 10th	Night	1 7% 3 4 4 3 2 2 4 7% 65%	2555
	11th to 20th	Night	1 9% 4 9% 4 4 3 3 4 7% 51%	2860
	21st to end	Night	1 21% 6% 9% 5% 3 4 3 5% 6% 37%	3109
August	1st to 10th	Night	3 19% 7% 12% 5% 5% 5% 4 5% 7% 28%	2848
	11th to 20th	Night	1 9% 3 8% 3 4 3 3 4 6% 56%	2990
	21st to end	Night	2 9% 4 7% 4 4 4 4 6% 6% 51%	3498
September	1st to 10th	Night	1 6% 3 5% 3 3 3 3 5% 8% 60%	1900
	11th to 20th	Night	4 11% 3 7% 3 3 4 3 5% 8% 49%	1900
	21st to end	Night	3 6% 2 6% 3 2 3 2 4 8% 61%	2171
October	1st to 10th	Night	3 3 2 4 2 3 2 3 4 6% 67%	3800
	11th to 20th	Night	5% 10% 3 6% 3 3 3 4 4 6% 55%	3777
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 6% 58%	4121
November	1st to 10th	Night	7% 7% 2 6% 4 3 3 3 4 8% 54%	2275
	11th to 20th	Night	9% 8% 3 4 3 3 4 3 5% 7% 50%	2470
	21st to end	Night	9% 4 2 4 4 3 3 3 5% 6% 56%	2641
December	1st to 10th	Night	15% 6% 3 6% 3 3 3 3 5% 6% 46%	2660
	11th to 20th	Night	29% 9% 2 5% 3 3 3 3 5% 5% 32%	3276
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 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	53% 6% 11% 16% 13% 2	3411
	11th to 20th	Day	31% 5% 13% 30% 11% 11%	3490
	21st to end	Day	41% 6% 17% 24% 10% 2	3855
April	1st to 10th	Day	38% 5% 14% 21% 20% 4	2280
	11th to 20th	Day	30% 4 16% 25% 19% 5%	2252
	21st to end	Day	22% 5% 15% 31% 23% 4	2280
May	1st to 10th	Day	19% 2 10% 31% 24% 14%	3971
	11th to 20th	Day	17% 7% 5% 28% 34% 9%	4129
	21st to end	Day	21% 4 10% 31% 31% 3	4814
June	1st to 10th	Day	26% 4 12% 27% 23% 7%	2660
	11th to 20th	Day	17% 4 8% 21% 36% 14%	2660
	21st to end	Day	21% 7% 3 12% 30% 26%	2660
July	1st to 10th	Day	13% 4 1 6% 14% 62%	4332
	11th to 20th	Day	19% 5% 3 5% 19% 48%	4041
	21st to end	Day	23% 4 1 8% 30% 34%	4315
August	1st to 10th	Day	31% 5% 2 11% 29% 22%	4076
	11th to 20th	Day	19% 3 1 10% 43% 24%	3894
	21st to end	Day	22% 6% 3 13% 33% 24%	4179
September	1st to 10th	Day	18% 5% 1 9% 30% 37%	2280
	11th to 20th	Day	28% 4 4 23% 28% 13%	2280
	21st to end	Day	18% 3 3 22% 32% 21%	2002
October	1st to 10th	Day	17% 3 3 12% 30% 36%	3180
	11th to 20th	Day	28% 3 5% 22% 24% 18%	3161
	21st to end	Day	27% 2 7% 27% 24% 13%	3448
November	1st to 10th	Day	28% 4 11% 28% 18% 11%	1872
	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	49% 5% 14% 16% 13% 3	3461
	11th to 20th	Night	33% 3 9% 25% 19% 10%	3490
	21st to end	Night	46% 5% 15% 21% 10% 3	3784
April	1st to 10th	Night	36% 4 14% 23% 19% 4	1900
	11th to 20th	Night	31% 4 12% 26% 19% 8%	1879
	21st to end	Night	23% 5% 12% 31% 23% 6%	1900
May	1st to 10th	Night	22% 3 8% 27% 27% 13%	2999
	11th to 20th	Night	31% 5% 7% 20% 26% 11%	2771
	21st to end	Night	24% 3 12% 30% 29% 2	2762
June	1st to 10th	Night	31% 6% 9% 22% 25% 7%	1520
	11th to 20th	Night	24% 2 6% 18% 34% 15%	1520
	21st to end	Night	28% 3 3 12% 27% 27%	1520
July	1st to 10th	Night	20% 4 1 4 14% 57%	2515
	11th to 20th	Night	32% 3 2 6% 12% 44%	2806
	21st to end	Night	45% 3 4 20% 28%	3011
August	1st to 10th	Night	51% 3 1 9% 22% 14%	2839
	11th to 20th	Night	28% 4 1 7% 39% 21%	3017
	21st to end	Night	30% 4 3 12% 30% 22%	3498
September	1st to 10th	Night	23% 4 2 10% 26% 34%	1900
	11th to 20th	Night	33% 3 5% 21% 24% 13%	1900
	21st to end	Night	24% 4 3 20% 28% 22%	2171
October	1st to 10th	Night	19% 2 4 10% 25% 40%	3800
	11th to 20th	Night	30% 2 6% 21% 21% 20%	3777
	21st to end	Night	28% 2 5% 23% 26% 16%	4121
November	1st to 10th	Night	31% 5% 10% 26% 18% 11%	2275
	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% 32% 25% 7% 1	506
April	1st to 10th	Day	17% 18% 32% 22% 9% 2	460
	11th to 20th	Day	14% 21% 31% 23% 8% 3	452
	21st to end	Day	6% 24% 32% 24% 10% 5%	460
May	1st to 10th	Day	11% 19% 35% 24% 10% 2	463
	11th to 20th	Day	9% 20% 31% 29% 9% 2	488
	21st to end	Day	9% 18% 31% 27% 11% 3	529
June	1st to 10th	Day	9% 12% 37% 28% 11% 3	490
	11th to 20th	Day	5% 13% 37% 33% 11% 2	490
	21st to end	Day	4 11% 35% 38% 11% 2	490
July	1st to 10th	Day	5% 7% 31% 43% 13% 1	486
	11th to 20th	Day	2 5% 26% 50% 15% 2	484
	21st to end	Day	4 3 27% 48% 13% 3	531
August	1st to 10th	Day	6% 7% 31% 44% 9% 2	487
	11th to 20th	Day	5% 3 32% 41% 16% 4	477
	21st to end	Day	6% 4 24% 44% 17% 5%	539
September	1st to 10th	Day	4 5% 27% 39% 20% 6%	490
	11th to 20th	Day	6% 11% 27% 37% 14% 5%	490
	21st to end	Day	4 9% 25% 34% 21% 6%	490
October	1st to 10th	Day	5% 9% 27% 30% 19% 10%	490
	11th to 20th	Day	8% 10% 24% 27% 23% 8%	490
	21st to end	Day	11% 7% 25% 29% 22% 6%	539
November	1st to 10th	Day	11% 10% 24% 27% 24% 5%	450
	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	836
April	1st to 10th	Night	13% 9% 27% 32% 16% 3	760
	11th to 20th	Night	14% 14% 21% 29% 17% 5%	751
	21st to end	Night	6% 11% 28% 29% 21% 4	760
May	1st to 10th	Night	8% 8% 25% 32% 23% 4	657
	11th to 20th	Night	7% 5% 22% 33% 30% 4	524
	21st to end	Night	13% 10% 20% 34% 20% 4	411
June	1st to 10th	Night	7% 7% 23% 36% 24% 2	380
	11th to 20th	Night	3 7% 19% 48% 20% 3	380
	21st to end	Night	6% 3 18% 39% 29% 4	380
July	1st to 10th	Night	4 3 13% 46% 29% 4	372
	11th to 20th	Night	8% 3 7% 39% 33% 8%	552
	21st to end	Night	10% 6% 9% 32% 34% 7%	624
August	1st to 10th	Night	14% 4 13% 34% 29% 6%	567
	11th to 20th	Night	9% 4 16% 36% 29% 7%	664
	21st to end	Night	7% 3 11% 38% 33% 7%	836
September	1st to 10th	Night	4 2 13% 35% 36% 10%	760
	11th to 20th	Night	8% 5% 17% 34% 30% 7%	760
	21st to end	Night	4 4 14% 31% 35% 11%	760
October	1st to 10th	Night	8% 4 13% 33% 33% 9%	760
	11th to 20th	Night	8% 6% 12% 29% 35% 9%	760
	21st to end	Night	10% 4 12% 29% 33% 12%	836
November	1st to 10th	Night	9% 6% 15% 28% 34% 9%	754
	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 2024-11-01 19:47:48. Version ac07

Copyright © 2024 Attilla Danko.