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 2017-12-20.

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
62%
16662
28%
48129
23%
16615
66%
16615
32%
28097

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%
717
24%
1790
16%
713
66%
713
36%
1129
August16th to endNight
73%
764
32%
1910
19%
764
77%
764
44%
1083
September1st to 15thNight
72%
720
30%
1866
24%
720
74%
720
42%
1060
September16th to endNight
73%
716
38%
1980
32%
716
78%
716
46%
1029
October1st to 15thNight
72%
720
31%
1980
29%
720
73%
720
43%
1082
October16th to endNight
68%
768
35%
2295
32%
768
74%
768
37%
1248
November1st to 15thNight
70%
760
30%
2470
34%
760
73%
760
39%
1227
November16th to endNight
63%
780
33%
2535
32%
780
71%
780
32%
1354
December1st to 15thNight
56%
780
29%
2535
28%
780
62%
780
28%
1404
December16th to endNight
54%
787
31%
2558
27%
787
63%
787
30%
1385

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%
48695
Night
6%6%24423348%51%
48337
TransparencyDay
24%47%16%24%22%
48399
Night
31%46%14%20%21%
48135
SeeingDay
7%10%17%36%22%5%
6272
Night
8%15%422%35%13%
16672

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%
2160
16th to endDay
131332335%8%64%
2292
September1st to 15thDay
1313413348%63%
2094
16th to endDay
2313212347%66%
1980
October1st to 15thDay
4423323446%60%
1980
16th to endDay
4412312236%65%
1925
November1st to 15thDay
4313424447%59%
1719
16th to endDay
5%413312447%60%
1755
December1st to 15thDay
6%324424448%55%
1755
16th to endDay
10%313334347%54%
1764
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%46%42345%10%45%
1800
16th to endNight
35%1432445%10%53%
1910
September1st to 15thNight
341442345%8%57%
1866
16th to endNight
35%24323247%62%
1980
October1st to 15thNight
342342345%8%57%
1980
16th to endNight
7%6%13413336%58%
2295
November1st to 15thNight
45%13324346%61%
2470
16th to endNight
4424434446%56%
2535
December1st to 15thNight
8%4245%2345%7%52%
2535
16th to endNight
10%323413336%56%
2558

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%14%32%24%
2146
16th to endDay
17%46%13%34%23%
2292
September1st to 15thDay
19%46%10%32%25%
2094
16th to endDay
17%36%10%34%27%
1980
October1st to 15thDay
24%35%16%25%24%
1980
16th to endDay
21%35%12%21%35%
1925
November1st to 15thDay
24%36%17%16%30%
1719
16th to endDay
23%37%14%19%31%
1755
December1st to 15thDay
27%49%14%17%26%
1755
16th to endDay
30%49%16%22%17%
1764
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
31%6%416%26%14%
1790
16th to endNight
23%5%6%12%30%23%
1910
September1st to 15thNight
24%5%411%30%23%
1866
16th to endNight
21%3211%34%26%
1980
October1st to 15thNight
25%46%17%24%22%
1980
16th to endNight
28%3311%20%33%
2295
November1st to 15thNight
23%37%17%16%31%
2470
16th to endNight
28%45%14%17%29%
2535
December1st to 15thNight
30%47%14%17%24%
2535
16th to endNight
30%39%16%19%20%
2558

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
25%6%41%37%6%
315
16th to endDay
34438%40%7%
332
September1st to 15thDay
45%13%46%28%2
315
16th to endDay
5%6%15%42%24%5%
315
October1st to 15thDay
48%24%40%17%4
315
16th to endDay
7%5%17%34%25%9%
287
November1st to 15thDay
5%317%33%28%11%
135
16th to endDay
11%7%15%25%28%11%
135
December1st to 15thDay
3214%40%27%11%
135
16th to endDay
12%5%13%37%23%7%
108
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%
717
16th to endNight
6%8%16%41%35%
764
September1st to 15thNight
5%9%111%49%21%
720
16th to endNight
6%9%219%45%17%
716
October1st to 15thNight
5%9%326%44%10%
720
16th to endNight
10%10%220%37%18%
768
November1st to 15thNight
5%14%327%35%12%
760
16th to endNight
7%16%325%31%15%
780
December1st to 15thNight
9%17%6%29%30%7%
780
16th to endNight
11%19%8%27%24%8%
787
Page generated at 2017-12-21 04:04:49. Version ac04

Copyright 2017 A. Danko


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