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 2017-03-24.

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%
14841
24%
45316
20%
14803
60%
14803
31%
26987

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
57%
718
19%
2371
17%
716
63%
716
27%
1434
January16th to endNight
54%
768
24%
2496
24%
768
57%
768
26%
1548
February1st to 15thNight
51%
700
22%
2288
20%
700
58%
700
25%
1425
February16th to endNight
45%
632
22%
1912
17%
632
53%
632
26%
1228
March1st to 15thNight
49%
720
17%
1992
15%
720
53%
720
26%
1331
March16th to endNight
44%
740
19%
2023
13%
740
50%
740
28%
1310
April1st to 15thNight
42%
659
23%
1649
10%
659
48%
659
31%
1061
April16th to endNight
51%
660
24%
1650
10%
660
56%
660
42%
954
May1st to 15thNight
56%
544
25%
1534
15%
544
61%
544
41%
864
May16th to endNight
50%
373
21%
1400
16%
373
58%
373
30%
737
June1st to 15thNight
53%
330
22%
1320
21%
330
62%
330
26%
722
June16th to endNight
69%
330
30%
1320
36%
330
74%
330
44%
567
July1st to 15thNight
66%
361
25%
1291
26%
361
69%
361
34%
672
July16th to endNight
51%
522
18%
1467
16%
489
52%
489
21%
998
August1st to 15thNight
55%
496
22%
1479
16%
493
57%
493
26%
946
August16th to endNight
59%
700
24%
1750
14%
700
60%
700
35%
1139
September1st to 15thNight
69%
660
33%
1650
23%
660
71%
660
43%
987
September16th to endNight
66%
656
34%
1807
29%
656
71%
656
45%
993
October1st to 15thNight
62%
660
28%
1980
26%
660
69%
660
40%
1073
October16th to endNight
62%
704
30%
2112
30%
704
67%
704
37%
1196
November1st to 15thNight
62%
700
28%
2169
26%
700
67%
700
36%
1203
November16th to endNight
59%
720
24%
2448
26%
720
63%
720
31%
1365
December1st to 15thNight
52%
720
20%
2520
17%
720
58%
720
21%
1582
December16th to endNight
45%
768
21%
2688
18%
768
52%
768
22%
1652

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%
45612
Night
9%7%25%423347%47%
45505
TransparencyDay
31%48%19%22%12%
45301
Night
35%48%18%19%12%
45317
SeeingDay
10%13%31%31%10%2
5340
Night
12%6%15%30%28%7%
14849

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
15%6%34423347%44%
1566
16th to endDay
14%7%25%434446%43%
1728
February1st to 15thDay
17%7%24424336%43%
1576
16th to endDay
15%8%35%42345%8%38%
1562
March1st to 15thDay
15%6%245%2446%8%38%
1966
16th to endDay
11%6%25%6%3436%7%41%
2044
April1st to 15thDay
11%5%36%42445%7%44%
1969
16th to endDay
5%323323349%58%
1980
May1st to 15thDay
45%14313349%58%
2084
16th to endDay
6%7%3421335%7%54%
2357
June1st to 15thDay
6%5%24323348%56%
2310
16th to endDay
3413212348%65%
2310
July1st to 15thDay
17%243133411%56%
2251
16th to endDay
17%2331335%11%56%
2286
August1st to 15thDay
18%2321236%9%56%
2142
16th to endDay
17%243132410%56%
2100
September1st to 15thDay
25%23312239%62%
1980
16th to endDay
3322323236%66%
1818
October1st to 15thDay
7%423322435%59%
1650
16th to endDay
9%5%14323336%56%
1760
November1st to 15thDay
8%5%243235%47%51%
1688
16th to endDay
13%9%24423346%47%
1509
December1st to 15thDay
17%8%3432335%5%41%
1440
16th to endDay
18%7%25%323346%41%
1536
January1st to 15thNight
14%6%24324448%43%
2371
16th to endNight
16%7%24323336%46%
2496
February1st to 15thNight
15%6%34424446%43%
2276
16th to endNight
15%6%2442446%8%40%
1912
March1st to 15thNight
15%6%25%424446%41%
1992
16th to endNight
13%6%345%23346%45%
2023
April1st to 15thNight
10%5%3442345%9%46%
1649
16th to endNight
5%423423347%59%
1650
May1st to 15thNight
37%33423347%55%
1534
16th to endNight
5%10%33422337%53%
1399
June1st to 15thNight
35%37%5%2435%8%49%
1320
16th to endNight
2413422238%63%
1320
July1st to 15thNight
17%35%6%4335%11%47%
1376
16th to endNight
215%5%9%7%45%4410%30%
1575
August1st to 15thNight
217%38%4345%5%9%35%
1488
16th to endNight
211%37%5%3445%9%41%
1750
September1st to 15thNight
28%25%324449%52%
1650
16th to endNight
36%35%312238%58%
1807
October1st to 15thNight
6%6%34312337%57%
1980
16th to endNight
7%5%25%42345%6%52%
2112
November1st to 15thNight
7%6%24322337%55%
2169
16th to endNight
9%6%2442335%6%51%
2448
December1st to 15thNight
19%6%24424336%43%
2520
16th to endNight
18%6%25%323445%44%
2688

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
40%412%18%15%8%
1566
16th to endDay
42%5%10%12%18%11%
1728
February1st to 15thDay
44%39%18%20%3
1576
16th to endDay
43%49%25%12%3
1562
March1st to 15thDay
43%5%12%19%14%4
1966
16th to endDay
41%6%14%20%14%3
2044
April1st to 15thDay
38%315%20%18%2
1969
16th to endDay
24%315%28%21%5%
1980
May1st to 15thDay
25%39%27%25%8%
2084
16th to endDay
29%413%26%19%5%
2361
June1st to 15thDay
28%411%20%22%12%
2310
16th to endDay
18%3418%35%20%
2310
July1st to 15thDay
23%4210%23%35%
2107
16th to endDay
24%6%39%26%29%
2131
August1st to 15thDay
24%5%110%31%25%
2126
16th to endDay
25%5%213%33%19%
2100
September1st to 15thDay
23%3112%30%27%
1980
16th to endDay
21%2224%29%18%
1818
October1st to 15thDay
27%36%20%26%14%
1650
16th to endDay
30%39%24%20%11%
1760
November1st to 15thDay
32%412%29%13%7%
1688
16th to endDay
39%49%18%22%5%
1509
December1st to 15thDay
45%410%17%17%4
1440
16th to endDay
44%5%11%18%15%5%
1536
January1st to 15thNight
39%412%20%15%8%
2371
16th to endNight
41%39%14%16%14%
2496
February1st to 15thNight
41%310%19%17%7%
2288
16th to endNight
40%5%12%24%14%2
1912
March1st to 15thNight
42%5%11%20%14%5%
1992
16th to endNight
39%412%19%18%4
2023
April1st to 15thNight
35%415%21%19%3
1649
16th to endNight
25%412%26%22%8%
1650
May1st to 15thNight
28%48%20%25%11%
1535
16th to endNight
32%212%24%20%7%
1400
June1st to 15thNight
33%6%9%18%18%13%
1320
16th to endNight
21%2314%34%23%
1320
July1st to 15thNight
28%5%29%21%32%
1291
16th to endNight
47%6%35%13%23%
1467
August1st to 15thNight
42%418%22%19%
1479
16th to endNight
36%4211%24%19%
1750
September1st to 15thNight
28%4310%24%28%
1650
16th to endNight
25%3322%25%19%
1807
October1st to 15thNight
27%37%20%23%17%
1980
16th to endNight
30%47%22%21%13%
2112
November1st to 15thNight
30%411%30%16%7%
2169
16th to endNight
33%39%23%20%8%
2448
December1st to 15thNight
44%39%17%19%6%
2520
16th to endNight
40%5%12%17%17%7%
2688

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
22%11%23%25%15%1
195
16th to endDay
15%11%27%30%14%
256
February1st to 15thDay
19%10%28%27%12%1
236
16th to endDay
18%16%29%25%9%1
235
March1st to 15thDay
22%15%27%18%11%3
270
16th to endDay
15%16%37%21%7%
260
April1st to 15thDay
16%25%35%17%3
209
16th to endDay
7%31%30%23%5%1
210
May1st to 15thDay
9%23%37%25%3
228
16th to endDay
12%24%27%27%6%2
266
June1st to 15thDay
7%16%44%25%5%
255
16th to endDay
19%38%37%12%
255
July1st to 15thDay
45%32%48%8%
245
16th to endDay
2329%50%9%4
271
August1st to 15thDay
4434%44%10%1
255
16th to endDay
2135%44%11%3
268
September1st to 15thDay
36%27%43%12%6%
255
16th to endDay
311%26%32%19%6%
255
October1st to 15thDay
8%10%27%35%12%5%
255
16th to endDay
13%10%30%29%13%2
272
November1st to 15thDay
10%10%22%24%24%7%
194
16th to endDay
12%14%35%29%7%1
71
December1st to 15thDay
25%21%26%13%11%1
60
16th to endDay
12%26%14%37%7%1
64
January1st to 15thNight
17%5%10%31%27%6%
720
16th to endNight
19%5%12%29%26%6%
768
February1st to 15thNight
17%6%14%30%26%4
704
16th to endNight
17%7%22%28%19%5%
632
March1st to 15thNight
17%10%14%27%25%5%
720
16th to endNight
15%10%22%27%20%3
740
April1st to 15thNight
12%13%23%31%15%4
659
16th to endNight
7%11%23%26%25%5%
660
May1st to 15thNight
6%422%32%28%5%
545
16th to endNight
11%8%22%31%21%5%
374
June1st to 15thNight
5%6%21%40%23%3
330
16th to endNight
4315%41%30%5%
330
July1st to 15thNight
429%37%36%8%
361
16th to endNight
12%47%25%39%11%
522
August1st to 15thNight
13%311%27%36%8%
496
16th to endNight
8%412%32%34%8%
700
September1st to 15thNight
6%18%27%41%13%
660
16th to endNight
5%312%29%36%12%
656
October1st to 15thNight
9%5%11%31%33%9%
660
16th to endNight
10%313%24%33%14%
704
November1st to 15thNight
9%314%29%35%7%
700
16th to endNight
12%6%11%31%29%8%
720
December1st to 15thNight
21%7%11%30%22%6%
720
16th to endNight
21%9%16%30%20%1
768
Page generated at 2017-03-25 09:39:51. Version ac04

Copyright 2017 A. Danko


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