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San Francisco
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco clear sky chart from 2005-11-05 to 2018-02-22.

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 TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(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

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
55%
16019
24%
48945
20%
15981
60%
15981
31%
29067

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
56%
778
18%
2569
17%
776
61%
776
25%
1585
January16th to endNight
54%
832
23%
2704
23%
832
58%
832
26%
1685
February1st to 15thNight
53%
760
25%
2483
22%
760
61%
760
27%
1493
February16th to endNight
46%
660
23%
1997
18%
660
54%
660
27%
1263
March1st to 15thNight
49%
720
17%
1992
15%
720
53%
720
26%
1331
March16th to endNight
44%
768
20%
2099
13%
768
50%
768
29%
1346
April1st to 15thNight
42%
719
22%
1799
10%
719
47%
719
31%
1160
April16th to endNight
49%
720
25%
1800
11%
720
55%
720
41%
1059
May1st to 15thNight
56%
593
24%
1673
14%
593
61%
593
41%
935
May16th to endNight
49%
408
21%
1531
17%
408
57%
408
28%
830
June1st to 15thNight
53%
360
21%
1440
20%
360
62%
360
27%
773
June16th to endNight
69%
360
31%
1440
38%
360
74%
360
43%
625
July1st to 15thNight
64%
396
23%
1416
24%
396
68%
396
32%
751
July16th to endNight
51%
564
20%
1593
17%
531
53%
531
22%
1069
August1st to 15thNight
53%
541
21%
1614
16%
538
54%
538
24%
1051
August16th to endNight
59%
764
24%
1910
15%
764
60%
764
34%
1260
September1st to 15thNight
68%
720
32%
1800
21%
720
70%
720
43%
1083
September16th to endNight
67%
716
36%
1972
31%
716
72%
716
47%
1056
October1st to 15thNight
64%
720
28%
2160
26%
720
71%
720
43%
1133
October16th to endNight
62%
768
31%
2304
32%
768
67%
768
37%
1306
November1st to 15thNight
62%
760
26%
2355
25%
760
67%
760
35%
1326
November16th to endNight
59%
780
24%
2652
26%
780
62%
780
30%
1500
December1st to 15thNight
54%
780
21%
2730
19%
780
60%
780
24%
1660
December16th to endNight
46%
832
22%
2912
18%
832
53%
832
22%
1787

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 ForecastData
Points
Cloud CoverDay
8%6%24323348%52%
49300
Night
9%7%35%423347%48%
49134
TransparencyDay
31%48%19%22%13%
48989
Night
35%48%18%19%12%
48946
SeeingDay
10%12%29%31%12%2
6527
Night
12%6%15%30%28%7%
16027

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
16%6%34423347%42%
1697
16th to endDay
14%7%25%433446%42%
1872
February1st to 15thDay
16%6%23324346%45%
1711
16th to endDay
14%7%3442345%7%40%
1623
March1st to 15thDay
15%6%245%2446%8%38%
1966
16th to endDay
11%6%245%3436%7%42%
2124
April1st to 15thDay
12%5%36%42445%7%44%
2149
16th to endDay
6%423323249%57%
2160
May1st to 15thDay
45%14312348%60%
2274
16th to endDay
6%7%34213347%54%
2578
June1st to 15thDay
6%5%23323347%56%
2520
16th to endDay
2413212348%65%
2520
July1st to 15thDay
17%242133410%58%
2455
16th to endDay
17%233123411%57%
2468
August1st to 15thDay
29%2331236%10%53%
2337
16th to endDay
17%243132410%56%
2292
September1st to 15thDay
25%23312239%61%
2160
16th to endDay
3322323136%67%
1983
October1st to 15thDay
7%423322335%61%
1800
16th to endDay
8%5%14323336%57%
1920
November1st to 15thDay
9%6%24323447%51%
1832
16th to endDay
13%9%24423346%46%
1635
December1st to 15thDay
16%7%34323345%43%
1560
16th to endDay
17%8%25%324346%41%
1664
January1st to 15thNight
15%7%24424448%41%
2569
16th to endNight
16%6%24323346%45%
2704
February1st to 15thNight
14%5%34424446%46%
2471
16th to endNight
14%6%2442446%7%41%
1997
March1st to 15thNight
15%6%25%424446%41%
1992
16th to endNight
13%6%345%23346%46%
2099
April1st to 15thNight
11%5%3442335%8%46%
1799
16th to endNight
6%423423347%58%
1800
May1st to 15thNight
36%34423347%57%
1673
16th to endNight
5%10%34422337%51%
1530
June1st to 15thNight
45%36%5%2435%8%50%
1440
16th to endNight
25%14422238%62%
1440
July1st to 15thNight
16%46%6%4335%11%45%
1501
16th to endNight
115%5%9%6%45%45%10%30%
1701
August1st to 15thNight
317%48%5%34449%33%
1623
16th to endNight
212%48%5%345%6%9%39%
1910
September1st to 15thNight
27%26%424449%51%
1800
16th to endNight
35%25%312248%60%
1972
October1st to 15thNight
5%6%34212336%60%
2160
16th to endNight
7%5%25%42345%6%52%
2304
November1st to 15thNight
7%6%24323337%54%
2355
16th to endNight
10%6%2442235%6%50%
2652
December1st to 15thNight
17%6%24324346%45%
2730
16th to endNight
17%6%25%323446%44%
2912

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
42%412%17%15%7%
1697
16th to endDay
42%5%10%12%17%10%
1872
February1st to 15thDay
41%38%18%22%5%
1711
16th to endDay
42%49%26%13%3
1623
March1st to 15thDay
43%5%12%19%14%4
1966
16th to endDay
40%5%15%20%14%3
2124
April1st to 15thDay
39%316%19%18%2
2149
16th to endDay
26%315%26%21%6%
2160
May1st to 15thDay
24%38%29%26%7%
2274
16th to endDay
30%412%26%20%5%
2582
June1st to 15thDay
27%411%21%22%12%
2520
16th to endDay
18%4417%33%21%
2520
July1st to 15thDay
22%429%23%37%
2311
16th to endDay
23%5%38%26%32%
2313
August1st to 15thDay
26%6%110%30%23%
2321
16th to endDay
25%5%213%32%20%
2292
September1st to 15thDay
23%3213%29%26%
2160
16th to endDay
20%2325%30%17%
1983
October1st to 15thDay
25%36%19%25%19%
1800
16th to endDay
29%39%23%21%12%
1920
November1st to 15thDay
32%412%29%13%6%
1832
16th to endDay
39%49%19%22%5%
1635
December1st to 15thDay
43%49%16%18%7%
1560
16th to endDay
44%411%17%16%4
1664
January1st to 15thNight
40%313%19%14%7%
2569
16th to endNight
40%410%14%16%13%
2704
February1st to 15thNight
39%39%18%19%9%
2483
16th to endNight
39%5%12%24%15%3
1997
March1st to 15thNight
42%5%11%20%14%5%
1992
16th to endNight
39%413%19%18%4
2099
April1st to 15thNight
36%415%21%18%3
1799
16th to endNight
26%5%12%26%21%8%
1800
May1st to 15thNight
28%38%22%25%11%
1674
16th to endNight
34%311%23%20%6%
1531
June1st to 15thNight
32%5%8%20%20%12%
1440
16th to endNight
22%2314%33%23%
1440
July1st to 15thNight
30%5%28%20%33%
1416
16th to endNight
46%6%35%13%25%
1593
August1st to 15thNight
44%418%22%19%
1614
16th to endNight
37%5%210%23%20%
1910
September1st to 15thNight
29%4310%24%27%
1800
16th to endNight
23%3423%25%19%
1972
October1st to 15thNight
25%37%19%22%22%
2160
16th to endNight
30%47%21%20%15%
2304
November1st to 15thNight
30%411%29%16%6%
2355
16th to endNight
34%39%23%21%7%
2652
December1st to 15thNight
41%38%16%19%9%
2730
16th to endNight
40%5%11%18%17%6%
2912

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
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
25%9%22%25%14%1
236
16th to endDay
15%10%27%30%15%
304
February1st to 15thDay
16%9%27%28%15%1
281
16th to endDay
17%17%29%24%10%1
260
March1st to 15thDay
22%15%27%18%11%3
270
16th to endDay
14%18%37%21%6%
288
April1st to 15thDay
19%23%31%20%5%
269
16th to endDay
9%30%29%21%6%2
270
May1st to 15thDay
8%23%35%25%6%
288
16th to endDay
11%22%26%29%7%3
330
June1st to 15thDay
8%16%42%25%6%
315
16th to endDay
27%35%39%12%1
315
July1st to 15thDay
3431%47%11%1
305
16th to endDay
2428%47%11%4
327
August1st to 15thDay
7%5%33%42%9%2
315
16th to endDay
4130%44%13%5%
332
September1st to 15thDay
5%6%27%40%14%5%
315
16th to endDay
411%26%32%18%6%
315
October1st to 15thDay
6%10%28%33%15%5%
315
16th to endDay
12%9%27%28%17%5%
336
November1st to 15thDay
12%9%22%26%22%6%
248
16th to endDay
14%11%32%28%10%1
107
December1st to 15thDay
16%17%20%22%22%1
90
16th to endDay
11%21%14%35%15%1
96
January1st to 15thNight
18%6%10%30%27%6%
780
16th to endNight
18%5%12%29%26%6%
832
February1st to 15thNight
15%6%14%30%28%4
764
16th to endNight
16%8%22%28%19%5%
660
March1st to 15thNight
17%10%14%27%25%5%
720
16th to endNight
15%10%23%27%20%2
768
April1st to 15thNight
13%12%23%31%14%3
719
16th to endNight
7%11%25%27%23%5%
720
May1st to 15thNight
6%422%33%28%5%
594
16th to endNight
11%7%22%31%21%5%
409
June1st to 15thNight
5%6%21%39%22%3
360
16th to endNight
4315%42%30%4
360
July1st to 15thNight
429%38%36%8%
396
16th to endNight
12%38%25%39%11%
564
August1st to 15thNight
15%310%27%35%7%
541
16th to endNight
9%311%33%33%8%
764
September1st to 15thNight
6%19%28%40%12%
720
16th to endNight
4312%30%35%12%
716
October1st to 15thNight
8%5%12%32%32%8%
720
16th to endNight
10%313%24%34%14%
768
November1st to 15thNight
9%314%30%35%6%
760
16th to endNight
13%6%11%31%28%7%
780
December1st to 15thNight
20%6%11%29%24%7%
780
16th to endNight
21%8%16%29%21%1
832
Page generated at 2018-02-23 08:21:38. Version ac04

Copyright 2018 A. Danko


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