Chula Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chula Vista Observatory clear sky chart from 2005-11-05 to 2017-09-20.

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
63%
16286
29%
47008
24%
16240
68%
16240
37%
26199

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
61%
717
29%
2306
26%
716
68%
716
35%
1192
January16th to endNight
58%
768
33%
2304
27%
768
67%
768
32%
1286
February1st to 15thNight
60%
700
31%
2108
30%
699
65%
699
34%
1149
February16th to endNight
54%
632
26%
1895
24%
632
61%
632
25%
1189
March1st to 15thNight
61%
720
27%
2160
27%
720
66%
720
35%
1193
March16th to endNight
61%
768
25%
2036
20%
768
65%
768
34%
1229
April1st to 15thNight
56%
719
25%
1799
17%
719
59%
719
36%
1118
April16th to endNight
58%
720
27%
1800
16%
720
61%
720
38%
1084
May1st to 15thNight
56%
593
24%
1673
16%
593
59%
593
30%
1037
May16th to endNight
51%
561
21%
1685
13%
561
55%
561
26%
1071
June1st to 15thNight
55%
540
19%
1620
12%
540
59%
540
27%
1033
June16th to endNight
68%
540
29%
1620
19%
540
74%
540
43%
825
July1st to 15thNight
67%
510
21%
1530
16%
510
69%
510
36%
857
July16th to endNight
65%
650
25%
1672
17%
610
71%
610
38%
986
August1st to 15thNight
73%
717
27%
1790
18%
713
73%
713
47%
1009
August16th to endNight
75%
764
35%
1910
21%
764
79%
764
50%
1038
September1st to 15thNight
75%
720
31%
1873
25%
720
77%
720
46%
1029
September16th to endNight
77%
675
39%
1869
34%
675
83%
675
57%
889
October1st to 15thNight
71%
660
33%
1815
32%
660
74%
660
45%
971
October16th to endNight
68%
704
34%
2092
30%
704
75%
704
42%
1074
November1st to 15thNight
69%
700
33%
2275
36%
700
73%
700
43%
1080
November16th to endNight
64%
720
33%
2340
32%
720
72%
720
36%
1168
December1st to 15thNight
55%
720
30%
2340
28%
720
62%
720
28%
1292
December16th to endNight
54%
768
30%
2496
26%
768
64%
768
29%
1400

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%413323347%60%
47811
Night
6%6%24323447%53%
47215
TransparencyDay
23%37%17%25%23%
47514
Night
28%37%16%21%23%
47013
SeeingDay
6%11%19%38%19%4
5953
Night
8%15%426%34%10%
16296

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%414322337%57%
1629
16th to endDay
9%424323346%55%
1920
February1st to 15thDay
6%31441445%5%57%
1751
16th to endDay
10%5%35%5%25%45%7%44%
1579
March1st to 15thDay
5%42441335%9%55%
1800
16th to endDay
5%32432445%8%55%
2183
April1st to 15thDay
6%434423446%54%
2149
16th to endDay
5%412212338%64%
2160
May1st to 15thDay
6%5%13323347%58%
2274
16th to endDay
45%13313337%61%
2429
June1st to 15thDay
46%14313347%58%
2340
16th to endDay
1322213236%70%
2340
July1st to 15thDay
25%1332435%8%57%
2340
16th to endDay
241232347%9%57%
2378
August1st to 15thDay
1312222336%68%
2160
16th to endDay
12232335%6%68%
2292
September1st to 15thDay
1212324347%66%
2085
16th to endDay
222113245%72%
1860
October1st to 15thDay
4323423346%60%
1815
16th to endDay
5%413111236%66%
1775
November1st to 15thDay
421332335%7%62%
1584
16th to endDay
6%41332235%6%61%
1620
December1st to 15thDay
7%3235%2345%8%52%
1620
16th to endDay
11%413324347%53%
1728
January1st to 15thNight
7%313324447%57%
2306
16th to endNight
10%424313346%55%
2304
February1st to 15thNight
7%424323446%56%
2101
16th to endNight
11%6%36%424445%45%
1895
March1st to 15thNight
7%5%2432345%8%52%
2160
16th to endNight
6%8%334235%6%8%48%
2036
April1st to 15thNight
9%7%25%312348%50%
1799
16th to endNight
7%7%25%313347%54%
1800
May1st to 15thNight
7%11%35%334449%42%
1673
16th to endNight
9%12%35%424446%41%
1684
June1st to 15thNight
415%45%43445%9%37%
1620
16th to endNight
16%34324448%55%
1620
July1st to 15thNight
310%144245%6%9%46%
1620
16th to endNight
38%245%2445%10%46%
1787
August1st to 15thNight
28%3333435%11%50%
1800
16th to endNight
25%23323348%60%
1910
September1st to 15thNight
15%23423436%61%
1873
16th to endNight
3412124247%65%
1869
October1st to 15thNight
3413334347%59%
1815
16th to endNight
6%413324346%60%
2092
November1st to 15thNight
4323212345%64%
2275
16th to endNight
442432345%7%57%
2340
December1st to 15thNight
9%6%24323346%53%
2340
16th to endNight
11%42432335%5%54%
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
27%36%13%21%27%
1630
16th to endDay
30%38%14%14%28%
1920
February1st to 15thDay
26%27%14%22%26%
1751
16th to endDay
37%47%14%22%13%
1579
March1st to 15thDay
25%38%19%22%21%
1800
16th to endDay
26%48%25%20%14%
2183
April1st to 15thDay
29%28%22%26%9%
2149
16th to endDay
19%210%26%22%19%
2160
May1st to 15thDay
24%37%22%25%16%
2274
16th to endDay
23%38%23%32%9%
2432
June1st to 15thDay
25%27%17%28%19%
2340
16th to endDay
14%138%30%40%
2340
July1st to 15thDay
22%35%13%28%26%
2201
16th to endDay
19%5%12%24%27%10%
2230
August1st to 15thDay
16%2418%34%23%
2146
16th to endDay
15%2417%33%26%
2292
September1st to 15thDay
16%3413%34%27%
2085
16th to endDay
13%2413%36%30%
1860
October1st to 15thDay
23%35%17%26%23%
1815
16th to endDay
19%25%12%23%36%
1775
November1st to 15thDay
21%27%15%17%34%
1584
16th to endDay
22%36%15%19%32%
1620
December1st to 15thDay
29%311%13%17%24%
1620
16th to endDay
31%49%15%21%18%
1728
January1st to 15thNight
25%37%12%21%29%
2306
16th to endNight
29%38%12%13%31%
2304
February1st to 15thNight
27%37%14%19%26%
2112
16th to endNight
39%45%13%20%16%
1895
March1st to 15thNight
30%28%18%18%22%
2160
16th to endNight
31%410%18%20%14%
2036
April1st to 15thNight
33%28%23%21%10%
1799
16th to endNight
29%37%20%19%18%
1800
May1st to 15thNight
36%38%20%15%16%
1674
16th to endNight
40%28%20%16%11%
1685
June1st to 15thNight
39%28%15%21%12%
1620
16th to endNight
22%1311%27%34%
1620
July1st to 15thNight
29%25%13%25%23%
1530
16th to endNight
30%49%23%25%7%
1672
August1st to 15thNight
24%35%22%28%15%
1790
16th to endNight
21%25%13%30%25%
1910
September1st to 15thNight
22%3312%30%27%
1873
16th to endNight
16%1311%36%30%
1869
October1st to 15thNight
23%37%18%23%23%
1815
16th to endNight
24%2411%21%35%
2092
November1st to 15thNight
21%27%15%16%36%
2275
16th to endNight
24%26%16%17%31%
2340
December1st to 15thNight
32%37%14%18%23%
2340
16th to endNight
31%310%14%17%21%
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%317%38%19%
108
16th to endDay
14%16%27%32%7%1
288
February1st to 15thDay
8%15%26%33%15%1
266
16th to endDay
9%18%34%27%8%1
238
March1st to 15thDay
5%17%26%34%13%2
270
16th to endDay
419%26%33%11%4
288
April1st to 15thDay
8%20%28%30%9%2
269
16th to endDay
419%31%32%7%4
270
May1st to 15thDay
14%16%23%33%9%2
288
16th to endDay
11%16%23%35%10%1
330
June1st to 15thDay
5%10%20%40%19%3
315
16th to endDay
19%8%44%29%6%
315
July1st to 15thDay
5%6%7%40%32%6%
305
16th to endDay
210%438%33%11%
327
August1st to 15thDay
25%8%46%29%7%
315
16th to endDay
34346%32%8%
332
September1st to 15thDay
45%12%53%20%2
315
16th to endDay
36%15%51%18%4
272
October1st to 15thDay
38%31%39%14%1
255
16th to endDay
9%6%22%35%22%4
221
November1st to 15thDay
28%10%30%35%13%
90
16th to endDay
6%8%17%34%25%6%
90
December1st to 15thDay
5%311%41%33%5%
90
16th to endDay
14%6%17%40%18%2
96
January1st to 15thNight
7%17%7%33%24%9%
720
16th to endNight
12%18%6%31%25%7%
768
February1st to 15thNight
7%17%6%32%28%7%
704
16th to endNight
12%17%6%35%23%4
632
March1st to 15thNight
9%17%5%35%26%5%
720
16th to endNight
7%17%6%30%32%6%
768
April1st to 15thNight
9%18%7%30%27%5%
719
16th to endNight
8%18%6%31%26%8%
720
May1st to 15thNight
9%17%6%29%29%7%
594
16th to endNight
11%18%8%28%29%3
563
June1st to 15thNight
8%17%5%34%30%3
540
16th to endNight
316%321%47%6%
540
July1st to 15thNight
6%11%216%50%12%
510
16th to endNight
5%16%110%47%18%
650
August1st to 15thNight
49%12%52%19%
717
16th to endNight
48%19%48%28%
764
September1st to 15thNight
48%214%51%18%
720
16th to endNight
6%8%221%44%16%
675
October1st to 15thNight
6%9%430%40%8%
660
16th to endNight
10%9%425%33%17%
704
November1st to 15thNight
5%15%331%32%12%
700
16th to endNight
6%17%329%29%12%
720
December1st to 15thNight
12%18%6%30%24%7%
720
16th to endNight
13%19%8%29%22%7%
768
Page generated at 2017-09-21 04:18:12. Version ac04

Copyright 2017 A. Danko


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