University of Califoria San Diego
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Califoria San Diego clear sky chart from 2005-11-05 to 2017-08-12.

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
61%
16139
27%
46605
22%
16092
66%
16092
32%
27235

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
62%
717
29%
2303
26%
716
68%
716
34%
1190
January16th to endNight
57%
768
32%
2304
26%
768
65%
768
31%
1312
February1st to 15thNight
60%
700
32%
2106
31%
698
65%
698
34%
1162
February16th to endNight
53%
632
25%
1895
22%
632
60%
632
24%
1207
March1st to 15thNight
59%
720
26%
2160
25%
720
65%
720
31%
1226
March16th to endNight
58%
768
23%
2030
19%
768
62%
768
30%
1282
April1st to 15thNight
56%
719
24%
1799
16%
719
59%
719
34%
1126
April16th to endNight
57%
720
26%
1800
14%
720
60%
720
33%
1140
May1st to 15thNight
54%
599
21%
1680
14%
599
57%
599
25%
1142
May16th to endNight
50%
561
18%
1685
11%
561
53%
561
19%
1179
June1st to 15thNight
51%
540
16%
1620
10%
540
55%
540
19%
1152
June16th to endNight
65%
540
21%
1620
13%
540
69%
540
30%
989
July1st to 15thNight
63%
510
19%
1530
15%
510
65%
510
30%
927
July16th to endNight
62%
656
23%
1678
16%
616
68%
616
32%
1062
August1st to 15thNight
66%
701
24%
1750
16%
697
67%
697
36%
1093
August16th to endNight
74%
700
32%
1750
18%
700
78%
700
46%
972
September1st to 15thNight
72%
660
32%
1710
25%
660
74%
660
43%
959
September16th to endNight
72%
656
36%
1815
31%
656
77%
656
45%
949
October1st to 15thNight
71%
660
32%
1815
31%
660
72%
660
42%
992
October16th to endNight
68%
704
33%
2104
30%
704
74%
704
37%
1143
November1st to 15thNight
69%
700
32%
2275
35%
700
72%
700
40%
1105
November16th to endNight
62%
720
32%
2340
30%
720
70%
720
32%
1244
December1st to 15thNight
54%
720
29%
2340
27%
720
60%
720
26%
1320
December16th to endNight
53%
768
30%
2496
26%
768
63%
768
29%
1362

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
5%423323348%58%
47352
Night
7%6%24423348%51%
46813
TransparencyDay
25%47%16%24%21%
47056
Night
31%46%14%20%21%
46611
SeeingDay
6%11%18%36%22%4
5806
Night
8%16%422%35%13%
16149

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
7%414324337%57%
1632
16th to endDay
9%424323347%55%
1920
February1st to 15thDay
7%314323447%57%
1751
16th to endDay
10%435%5%245%48%46%
1579
March1st to 15thDay
44244233410%55%
1800
16th to endDay
5%424424449%55%
2191
April1st to 15thDay
6%424314448%54%
2149
16th to endDay
4322312348%62%
2160
May1st to 15thDay
6%6%24413447%55%
2268
16th to endDay
5%6%13312449%57%
2429
June1st to 15thDay
6%8%34313348%52%
2340
16th to endDay
15%24312237%65%
2340
July1st to 15thDay
36%2432445%10%52%
2340
16th to endDay
34143235%49%58%
2372
August1st to 15thDay
2413212337%66%
2114
16th to endDay
31332335%8%64%
2100
September1st to 15thDay
1313413348%63%
1920
16th to endDay
2324213347%65%
1815
October1st to 15thDay
4423323446%59%
1815
16th to endDay
5%312313237%65%
1765
November1st to 15thDay
4313323347%61%
1584
16th to endDay
5%413312448%60%
1620
December1st to 15thDay
6%32442445%8%53%
1620
16th to endDay
11%313334347%53%
1728
January1st to 15thNight
6%424323347%57%
2303
16th to endNight
9%5%24323236%55%
2304
February1st to 15thNight
7%424424346%56%
2101
16th to endNight
11%7%26%434445%45%
1895
March1st to 15thNight
7%6%34314448%50%
2160
16th to endNight
7%7%26%425%35%7%47%
2030
April1st to 15thNight
11%7%34322247%50%
1799
16th to endNight
8%8%34323346%51%
1800
May1st to 15thNight
9%11%45%5%23349%39%
1679
16th to endNight
12%12%45%423347%38%
1684
June1st to 15thNight
8%15%45%43436%10%31%
1620
16th to endNight
210%26%425%46%12%43%
1620
July1st to 15thNight
5%9%37%32446%10%41%
1620
16th to endNight
5%7%35%4345%6%9%44%
1793
August1st to 15thNight
39%45%42345%10%46%
1760
16th to endNight
241432445%10%55%
1750
September1st to 15thNight
35%14323448%57%
1710
16th to endNight
35%24323247%61%
1815
October1st to 15thNight
3423423348%58%
1815
16th to endNight
7%6%12413336%58%
2104
November1st to 15thNight
45%12223236%63%
2275
16th to endNight
5%424424446%56%
2340
December1st to 15thNight
9%4245%2345%7%51%
2340
16th to endNight
10%323423336%55%
2496

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
28%36%14%20%26%
1633
16th to endDay
30%37%15%13%28%
1920
February1st to 15thDay
26%37%14%21%25%
1751
16th to endDay
35%39%16%24%9%
1579
March1st to 15thDay
25%48%19%20%20%
1800
16th to endDay
27%410%25%19%12%
2191
April1st to 15thDay
28%48%26%23%8%
2149
16th to endDay
21%49%22%23%18%
2160
May1st to 15thDay
29%36%21%23%15%
2268
16th to endDay
25%5%8%24%28%8%
2432
June1st to 15thDay
30%5%5%14%27%15%
2340
16th to endDay
20%36%6%24%38%
2340
July1st to 15thDay
26%5%8%9%24%25%
2201
16th to endDay
22%6%12%20%26%12%
2225
August1st to 15thDay
19%45%15%32%24%
2100
16th to endDay
17%47%13%34%22%
2100
September1st to 15thDay
19%46%9%32%26%
1920
16th to endDay
18%36%10%33%28%
1815
October1st to 15thDay
25%45%17%24%23%
1815
16th to endDay
21%35%12%22%33%
1765
November1st to 15thDay
22%36%17%17%32%
1584
16th to endDay
22%37%15%19%30%
1620
December1st to 15thDay
29%410%14%18%23%
1620
16th to endDay
30%49%16%21%17%
1728
January1st to 15thNight
26%37%13%21%27%
2303
16th to endNight
31%37%11%14%30%
2304
February1st to 15thNight
29%36%14%20%24%
2112
16th to endNight
40%4413%23%13%
1895
March1st to 15thNight
31%47%19%17%20%
2160
16th to endNight
34%5%9%17%20%12%
2030
April1st to 15thNight
34%37%23%21%10%
1799
16th to endNight
33%47%17%20%16%
1800
May1st to 15thNight
42%5%5%15%14%15%
1680
16th to endNight
46%7%416%15%10%
1685
June1st to 15thNight
46%7%7%9%18%11%
1620
16th to endNight
31%6%37%20%28%
1620
July1st to 15thNight
34%46%11%21%21%
1530
16th to endNight
34%6%7%20%23%7%
1678
August1st to 15thNight
30%6%416%26%14%
1750
16th to endNight
21%46%12%30%23%
1750
September1st to 15thNight
24%4410%31%24%
1710
16th to endNight
22%3211%32%27%
1815
October1st to 15thNight
25%46%18%24%20%
1815
16th to endNight
28%3311%20%32%
2104
November1st to 15thNight
22%36%17%16%34%
2275
16th to endNight
27%45%15%16%30%
2340
December1st to 15thNight
32%5%7%15%17%22%
2340
16th to endNight
30%310%16%18%20%
2496

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
20%7%16%31%22%1
111
16th to endDay
14%16%20%33%11%2
288
February1st to 15thDay
10%16%20%33%16%3
266
16th to endDay
10%15%29%29%13%1
238
March1st to 15thDay
6%15%21%36%15%4
270
16th to endDay
417%25%34%14%3
288
April1st to 15thDay
417%31%30%11%4
269
16th to endDay
418%30%30%10%5%
270
May1st to 15thDay
10%14%25%32%13%2
288
16th to endDay
9%14%25%36%12%1
330
June1st to 15thDay
8%11%22%37%18%2
315
16th to endDay
49%7%43%28%6%
315
July1st to 15thDay
6%6%9%34%37%6%
305
16th to endDay
49%131%41%10%
327
August1st to 15thDay
26%6%40%38%6%
300
16th to endDay
25%338%41%8%
268
September1st to 15thDay
5%5%11%47%28%1
255
16th to endDay
46%14%43%25%5%
255
October1st to 15thDay
310%26%40%16%2
255
16th to endDay
7%7%17%36%25%5%
227
November1st to 15thDay
25%14%27%32%17%
90
16th to endDay
8%8%17%30%28%5%
90
December1st to 15thDay
4314%38%27%11%
90
16th to endDay
14%6%13%38%21%5%
96
January1st to 15thNight
7%17%430%29%10%
720
16th to endNight
12%17%5%27%29%6%
768
February1st to 15thNight
9%17%5%28%29%9%
704
16th to endNight
11%17%7%31%25%5%
632
March1st to 15thNight
8%17%6%31%30%5%
720
16th to endNight
8%19%425%33%8%
768
April1st to 15thNight
11%18%6%28%29%5%
719
16th to endNight
10%18%6%24%31%8%
720
May1st to 15thNight
11%19%5%22%33%8%
600
16th to endNight
13%20%8%20%32%5%
563
June1st to 15thNight
11%20%6%24%32%4
540
16th to endNight
5%18%218%46%8%
540
July1st to 15thNight
8%14%29%44%20%
510
16th to endNight
6%17%15%42%27%
656
August1st to 15thNight
6%10%29%43%26%
701
16th to endNight
5%9%16%42%35%
700
September1st to 15thNight
5%10%18%50%23%
660
16th to endNight
6%10%117%44%18%
656
October1st to 15thNight
5%10%326%42%11%
660
16th to endNight
10%11%220%37%17%
704
November1st to 15thNight
5%15%425%35%13%
700
16th to endNight
7%18%324%31%14%
720
December1st to 15thNight
10%18%6%27%29%7%
720
16th to endNight
11%19%8%27%24%8%
768
Page generated at 2017-08-13 11:16:22. Version ac04

Copyright 2017 A. Danko


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