San Francisco State University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Francisco State University Observatory clear sky chart from 2005-11-05 to

2017-09-12
.

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
47%
15377
24%
46869
17%
15339
55%
15339
27%
27927

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
47%
718
18%
2366
15%
716
55%
716
22%
1438
January16th to endNight
44%
768
24%
2496
19%
768
53%
768
22%
1546
February1st to 15thNight
42%
700
22%
2288
19%
700
50%
700
21%
1423
February16th to endNight
38%
632
21%
1912
13%
632
50%
632
22%
1228
March1st to 15thNight
41%
720
17%
1989
13%
720
46%
720
23%
1335
March16th to endNight
38%
768
20%
2100
11%
768
46%
768
25%
1348
April1st to 15thNight
38%
719
22%
1799
10%
719
44%
719
29%
1158
April16th to endNight
42%
720
25%
1800
10%
720
48%
720
34%
1060
May1st to 15thNight
48%
593
24%
1673
12%
593
55%
593
34%
939
May16th to endNight
38%
414
21%
1537
13%
414
49%
414
23%
836
June1st to 15thNight
47%
360
21%
1440
18%
360
58%
360
24%
766
June16th to endNight
59%
360
31%
1440
30%
360
72%
360
35%
623
July1st to 15thNight
56%
404
23%
1424
22%
404
62%
404
29%
743
July16th to endNight
40%
564
20%
1593
13%
531
49%
531
18%
1054
August1st to 15thNight
47%
539
22%
1612
15%
536
50%
536
21%
1043
August16th to endNight
53%
764
25%
1910
13%
764
57%
764
32%
1249
September1st to 15thNight
62%
706
32%
1765
21%
706
65%
706
40%
1058
September16th to endNight
60%
656
33%
1810
24%
656
69%
656
41%
996
October1st to 15thNight
59%
660
28%
1980
26%
660
65%
660
37%
1071
October16th to endNight
56%
704
30%
2112
24%
704
67%
704
32%
1203
November1st to 15thNight
55%
700
27%
2164
25%
700
60%
700
33%
1203
November16th to endNight
48%
720
24%
2451
22%
720
55%
720
25%
1369
December1st to 15thNight
41%
720
20%
2520
15%
720
49%
720
17%
1585
December16th to endNight
38%
768
20%
2688
15%
768
48%
768
19%
1653

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%2431335%9%49%
47787
Night
9%7%25%423347%47%
47058
TransparencyDay
31%48%19%22%13%
47477
Night
35%49%18%19%12%
46870
SeeingDay
9%20%27%28%10%2
6026
Night
10%19%13%25%24%6%
15385

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%
1570
16th to endDay
14%6%2442435%6%42%
1728
February1st to 15thDay
17%7%24424336%42%
1576
16th to endDay
15%7%36%32445%8%38%
1559
March1st to 15thDay
15%6%35%5%25%35%8%38%
1970
16th to endDay
13%5%25%5%2445%7%42%
2123
April1st to 15thDay
12%6%35%32445%7%44%
2149
16th to endDay
6%42431335%9%56%
2160
May1st to 15thDay
5%5%2321235%10%57%
2275
16th to endDay
6%9%2321235%9%50%
2572
June1st to 15thDay
7%5%2431235%9%54%
2520
16th to endDay
341221236%10%60%
2520
July1st to 15thDay
27%2421237%14%51%
2448
16th to endDay
27%2331346%18%47%
2468
August1st to 15thDay
38%343135%8%13%44%
2337
16th to endDay
28%25%31338%13%47%
2292
September1st to 15thDay
27%2331235%12%54%
2114
16th to endDay
44132123310%62%
1818
October1st to 15thDay
8%5%23323237%58%
1650
16th to endDay
9%5%14312247%54%
1760
November1st to 15thDay
9%5%24423347%51%
1693
16th to endDay
14%8%15%412335%47%
1509
December1st to 15thDay
18%8%44313435%41%
1440
16th to endDay
19%6%34423347%40%
1536
January1st to 15thNight
15%5%2442435%7%43%
2366
16th to endNight
16%7%24323345%46%
2496
February1st to 15thNight
15%6%35%323446%43%
2276
16th to endNight
15%6%244235%5%7%40%
1912
March1st to 15thNight
15%6%34424446%41%
1989
16th to endNight
13%7%34324346%46%
2100
April1st to 15thNight
12%5%24325%45%7%46%
1799
16th to endNight
6%414323347%58%
1800
May1st to 15thNight
36%35%423247%57%
1673
16th to endNight
5%12%25%313337%51%
1536
June1st to 15thNight
46%37%4233310%49%
1440
16th to endNight
25%24222248%62%
1440
July1st to 15thNight
17%27%5%2345%12%45%
1509
16th to endNight
215%6%9%6%2446%11%30%
1701
August1st to 15thNight
417%38%4244410%35%
1621
16th to endNight
212%37%5%3447%9%39%
1910
September1st to 15thNight
27%35%5%1446%9%49%
1765
16th to endNight
35%26%412338%58%
1810
October1st to 15thNight
6%6%14312347%57%
1980
16th to endNight
7%5%25%423447%53%
2112
November1st to 15thNight
7%5%25%313237%55%
2164
16th to endNight
10%6%24323346%51%
2451
December1st to 15thNight
19%6%24424445%43%
2520
16th to endNight
18%5%24313345%45%
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%19%15%8%
1570
16th to endDay
42%5%9%12%18%11%
1728
February1st to 15thDay
43%39%18%20%4
1576
16th to endDay
44%49%25%12%3
1559
March1st to 15thDay
43%5%12%19%13%4
1970
16th to endDay
40%6%15%20%15%2
2123
April1st to 15thDay
38%416%20%17%2
2149
16th to endDay
25%415%26%21%6%
2160
May1st to 15thDay
24%39%28%26%7%
2275
16th to endDay
30%413%25%20%5%
2576
June1st to 15thDay
27%412%21%22%12%
2520
16th to endDay
18%3418%33%21%
2520
July1st to 15thDay
22%4210%23%36%
2305
16th to endDay
22%6%49%25%32%
2313
August1st to 15thDay
26%6%211%29%23%
2321
16th to endDay
25%5%214%31%20%
2292
September1st to 15thDay
23%3212%29%27%
2114
16th to endDay
20%3324%28%18%
1818
October1st to 15thDay
27%37%20%27%14%
1650
16th to endDay
30%39%24%20%11%
1760
November1st to 15thDay
32%412%28%13%7%
1693
16th to endDay
39%49%17%22%5%
1509
December1st to 15thDay
45%410%17%17%4
1440
16th to endDay
44%5%11%17%16%4
1536
January1st to 15thNight
39%412%19%15%8%
2366
16th to endNight
41%39%14%16%14%
2496
February1st to 15thNight
41%310%19%17%7%
2288
16th to endNight
40%5%12%25%13%2
1912
March1st to 15thNight
42%5%11%20%14%5%
1989
16th to endNight
39%413%19%18%4
2100
April1st to 15thNight
36%415%21%18%3
1799
16th to endNight
26%5%12%26%21%9%
1800
May1st to 15thNight
27%5%8%22%25%10%
1674
16th to endNight
33%312%23%20%6%
1537
June1st to 15thNight
30%6%9%19%20%12%
1440
16th to endNight
21%3314%33%23%
1440
July1st to 15thNight
26%7%38%22%31%
1424
16th to endNight
43%7%35%13%25%
1593
August1st to 15thNight
42%6%28%20%19%
1612
16th to endNight
35%6%310%23%20%
1910
September1st to 15thNight
27%5%410%24%27%
1765
16th to endNight
24%4322%25%19%
1810
October1st to 15thNight
27%38%20%23%16%
1980
16th to endNight
31%47%22%21%13%
2112
November1st to 15thNight
30%412%29%16%7%
2164
16th to endNight
33%310%23%20%8%
2451
December1st to 15thNight
43%48%17%19%6%
2520
16th to endNight
41%412%17%16%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
19%22%20%21%14%1
199
16th to endDay
14%22%22%25%14%
256
February1st to 15thDay
16%24%23%26%8%
236
16th to endDay
16%25%26%22%8%1
235
March1st to 15thDay
20%26%21%17%10%2
270
16th to endDay
11%29%33%18%6%
288
April1st to 15thDay
13%34%28%17%5%
269
16th to endDay
7%36%30%17%6%2
270
May1st to 15thDay
6%32%32%22%5%
288
16th to endDay
12%28%23%24%7%2
330
June1st to 15thDay
6%25%37%24%5%
315
16th to endDay
117%30%37%11%1
315
July1st to 15thDay
410%27%45%11%
305
16th to endDay
212%25%42%11%4
327
August1st to 15thDay
6%9%30%40%11%1
315
16th to endDay
37%29%39%15%5%
332
September1st to 15thDay
5%10%27%36%14%6%
300
16th to endDay
118%25%29%18%6%
255
October1st to 15thDay
6%16%25%31%14%5%
255
16th to endDay
12%17%28%27%11%2
272
November1st to 15thDay
9%16%21%22%23%7%
199
16th to endDay
9%15%36%26%9%1
71
December1st to 15thDay
31%15%26%13%13%
60
16th to endDay
9%26%17%35%9%1
64
January1st to 15thNight
15%19%8%25%24%7%
720
16th to endNight
18%21%8%22%22%6%
768
February1st to 15thNight
14%22%11%25%21%4
704
16th to endNight
14%21%18%24%16%4
632
March1st to 15thNight
15%24%11%21%22%4
720
16th to endNight
13%23%19%23%18%2
768
April1st to 15thNight
9%26%21%24%14%3
719
16th to endNight
6%25%20%23%18%4
720
May1st to 15thNight
5%20%19%27%23%3
594
16th to endNight
11%22%19%24%16%5%
415
June1st to 15thNight
5%21%16%32%20%3
360
16th to endNight
419%10%36%26%3
360
July1st to 15thNight
412%9%31%32%8%
404
16th to endNight
9%18%8%20%35%8%
564
August1st to 15thNight
12%12%12%23%32%6%
539
16th to endNight
6%10%13%30%31%6%
764
September1st to 15thNight
5%9%9%25%39%10%
706
16th to endNight
5%11%11%26%31%12%
656
October1st to 15thNight
7%13%11%28%29%9%
660
16th to endNight
10%12%11%23%30%12%
704
November1st to 15thNight
7%16%11%24%31%7%
700
16th to endNight
10%22%10%25%26%5%
720
December1st to 15thNight
18%23%11%25%17%4
720
16th to endNight
17%24%14%23%18%1
768
Page generated at 2017-09-13 04:39:07. Version ac04

Copyright 2017 A. Danko


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