Cabrillo College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabrillo College Observatory clear sky chart from 2005-11-05 to 2017-09-18.

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
66%
15471
26%
46927
23%
15435
69%
15435
36%
26714

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
63%
716
21%
2368
21%
716
67%
716
33%
1346
January16th to endNight
62%
768
25%
2496
27%
768
64%
768
30%
1475
February1st to 15thNight
58%
698
23%
2275
23%
698
63%
698
27%
1407
February16th to endNight
54%
632
23%
1895
19%
632
60%
632
29%
1177
March1st to 15thNight
57%
720
19%
2007
17%
720
60%
720
30%
1289
March16th to endNight
55%
768
21%
2085
16%
768
59%
768
33%
1317
April1st to 15thNight
53%
719
23%
1799
14%
719
56%
719
34%
1163
April16th to endNight
60%
720
25%
1800
11%
720
64%
720
45%
1048
May1st to 15thNight
69%
641
23%
1721
16%
641
70%
641
44%
960
May16th to endNight
66%
408
23%
1531
21%
408
72%
408
33%
772
June1st to 15thNight
71%
360
24%
1440
27%
360
75%
360
32%
733
June16th to endNight
83%
360
31%
1440
40%
360
85%
360
46%
610
July1st to 15thNight
83%
400
25%
1420
30%
400
83%
400
42%
670
July16th to endNight
71%
564
27%
1593
26%
531
72%
531
33%
924
August1st to 15thNight
66%
574
24%
1647
19%
571
66%
571
31%
999
August16th to endNight
76%
764
30%
1910
19%
764
76%
764
44%
1139
September1st to 15thNight
81%
720
34%
1800
26%
720
82%
720
52%
1008
September16th to endNight
81%
667
39%
1822
36%
667
82%
667
57%
911
October1st to 15thNight
72%
660
29%
1980
32%
660
75%
660
44%
1051
October16th to endNight
71%
704
33%
2112
35%
704
75%
704
40%
1167
November1st to 15thNight
71%
700
30%
2175
30%
700
74%
700
42%
1144
November16th to endNight
69%
720
26%
2403
27%
720
73%
720
35%
1298
December1st to 15thNight
58%
720
23%
2520
22%
720
65%
720
27%
1505
December16th to endNight
54%
768
22%
2688
21%
768
60%
768
26%
1601

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%423423347%54%
47864
Night
9%5%2442335%7%51%
47116
TransparencyDay
29%48%20%23%13%
47553
Night
32%39%19%21%13%
46928
SeeingDay
9%5%18%33%26%7%
6052
Night
10%28%30%35%11%
15483

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
14%4135%2245%8%47%
1568
16th to endDay
16%5%24324345%45%
1728
February1st to 15thDay
17%424324446%44%
1588
16th to endDay
18%5%25%5%33446%39%
1579
March1st to 15thDay
16%4245%345%6%9%37%
1950
16th to endDay
12%5%25%5%45%5%46%41%
2138
April1st to 15thDay
13%5%34425%448%44%
2149
16th to endDay
8%314323347%55%
2160
May1st to 15thDay
5%31331345%9%58%
2226
16th to endDay
5%6%25%5%24347%52%
2581
June1st to 15thDay
45%13425%45%8%54%
2520
16th to endDay
1212323248%67%
2520
July1st to 15thDay
231342345%6%63%
2452
16th to endDay
2212323348%65%
2468
August1st to 15thDay
15%24423237%62%
2304
16th to endDay
313323338%65%
2292
September1st to 15thDay
1313312247%67%
2160
16th to endDay
331221324469%
1858
October1st to 15thDay
5%213324447%61%
1650
16th to endDay
8%413323347%57%
1760
November1st to 15thDay
8%323324447%53%
1683
16th to endDay
13%324413345%52%
1554
December1st to 15thDay
17%5%25%4235%45%43%
1440
16th to endDay
16%6%24423446%42%
1536
January1st to 15thNight
13%41432445%8%48%
2368
16th to endNight
15%424323447%48%
2496
February1st to 15thNight
16%5%1443445%6%43%
2263
16th to endNight
16%6%2432335%7%43%
1895
March1st to 15thNight
15%5%2442335%7%43%
2007
16th to endNight
14%5%34423448%45%
2085
April1st to 15thNight
12%5%24324447%46%
1799
16th to endNight
8%312323348%58%
1800
May1st to 15thNight
45%23324348%56%
1721
16th to endNight
47%1432335%9%53%
1530
June1st to 15thNight
25%245%3435%8%54%
1440
16th to endNight
2223313248%64%
1440
July1st to 15thNight
14144235%5%8%58%
1505
16th to endNight
16%36%7%45%47%8%44%
1701
August1st to 15thNight
19%38%5%2445%7%45%
1656
16th to endNight
7%26%42346%8%52%
1910
September1st to 15thNight
2213435%35%7%60%
1800
16th to endNight
3412322237%66%
1822
October1st to 15thNight
6%5%23312346%60%
1980
16th to endNight
6%423423446%56%
2112
November1st to 15thNight
8%41331335%7%57%
2175
16th to endNight
7%32342445%6%53%
2403
December1st to 15thNight
16%6%24413345%47%
2520
16th to endNight
18%413323345%47%
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
34%5%12%21%16%10%
1568
16th to endDay
38%6%8%13%20%12%
1728
February1st to 15thDay
39%49%19%20%6%
1588
16th to endDay
43%6%8%24%12%4
1579
March1st to 15thDay
40%7%10%20%14%5%
1950
16th to endDay
39%8%14%18%16%2
2138
April1st to 15thDay
35%6%12%23%19%2
2149
16th to endDay
28%414%24%19%7%
2160
May1st to 15thDay
23%311%29%24%6%
2226
16th to endDay
31%413%24%20%5%
2585
June1st to 15thDay
27%6%10%23%22%10%
2520
16th to endDay
19%16%20%32%19%
2520
July1st to 15thDay
22%1311%26%33%
2308
16th to endDay
21%1316%29%28%
2313
August1st to 15thDay
24%1216%30%24%
2288
16th to endDay
21%1319%32%22%
2292
September1st to 15thDay
20%1416%31%26%
2160
16th to endDay
18%2425%30%18%
1858
October1st to 15thDay
21%49%21%24%17%
1650
16th to endDay
26%49%24%21%13%
1760
November1st to 15thDay
29%49%26%23%5%
1683
16th to endDay
31%5%11%17%27%6%
1554
December1st to 15thDay
41%410%17%18%6%
1440
16th to endDay
39%5%13%16%18%6%
1536
January1st to 15thNight
33%413%21%17%9%
2368
16th to endNight
35%5%10%14%17%16%
2496
February1st to 15thNight
40%39%18%18%8%
2275
16th to endNight
38%412%26%14%3
1895
March1st to 15thNight
39%412%20%16%6%
2007
16th to endNight
37%413%20%18%5%
2085
April1st to 15thNight
35%5%12%24%17%3
1799
16th to endNight
26%413%27%20%8%
1800
May1st to 15thNight
27%310%23%25%10%
1722
16th to endNight
29%313%25%22%5%
1531
June1st to 15thNight
28%5%9%20%20%14%
1440
16th to endNight
20%25%15%30%25%
1440
July1st to 15thNight
24%229%25%35%
1420
16th to endNight
36%2412%20%24%
1593
August1st to 15thNight
39%1213%21%21%
1647
16th to endNight
31%1212%26%25%
1910
September1st to 15thNight
25%1412%29%27%
1800
16th to endNight
19%1224%34%16%
1822
October1st to 15thNight
26%28%21%22%18%
1980
16th to endNight
30%26%23%21%16%
2112
November1st to 15thNight
26%311%25%25%7%
2175
16th to endNight
29%49%22%23%10%
2403
December1st to 15thNight
39%310%18%20%8%
2520
16th to endNight
38%413%17%17%8%
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
20%5%20%27%19%7%
197
16th to endDay
19%221%24%23%7%
256
February1st to 15thDay
16%7%22%24%20%8%
239
16th to endDay
16%11%22%26%18%4
238
March1st to 15thDay
18%9%22%22%20%5%
270
16th to endDay
12%9%27%32%15%3
288
April1st to 15thDay
13%15%27%27%12%3
269
16th to endDay
7%13%30%29%17%2
270
May1st to 15thDay
8%11%23%36%16%3
288
16th to endDay
7%7%18%34%24%6%
330
June1st to 15thDay
45%27%38%20%3
315
16th to endDay
113%43%31%8%
315
July1st to 15thDay
36%39%39%10%
305
16th to endDay
16%43%34%12%
327
August1st to 15thDay
410%41%35%7%
315
16th to endDay
38%39%35%13%
332
September1st to 15thDay
414%36%33%10%
315
16th to endDay
5%412%29%36%11%
264
October1st to 15thDay
5%5%20%32%27%7%
255
16th to endDay
12%320%28%27%7%
272
November1st to 15thDay
10%114%23%33%16%
189
16th to endDay
8%6%12%45%25%1
79
December1st to 15thDay
26%8%20%26%16%1
60
16th to endDay
15%417%20%34%7%
64
January1st to 15thNight
14%39%31%31%8%
720
16th to endNight
17%28%32%29%9%
768
February1st to 15thNight
17%5%11%28%30%7%
704
16th to endNight
19%412%36%20%6%
632
March1st to 15thNight
15%5%11%30%31%6%
720
16th to endNight
15%414%33%27%4
768
April1st to 15thNight
14%6%15%34%25%4
719
16th to endNight
9%417%32%28%7%
720
May1st to 15thNight
6%211%34%35%8%
642
16th to endNight
6%411%30%35%11%
409
June1st to 15thNight
5%18%37%40%6%
360
16th to endNight
2434%49%8%
360
July1st to 15thNight
4118%57%19%
400
16th to endNight
6%218%49%23%
564
August1st to 15thNight
7%2224%49%14%
574
16th to endNight
5%1123%50%16%
764
September1st to 15thNight
4223%48%21%
720
16th to endNight
41423%47%17%
667
October1st to 15thNight
7%36%29%37%16%
660
16th to endNight
8%16%28%35%19%
704
November1st to 15thNight
10%9%31%37%11%
700
16th to endNight
9%28%36%33%10%
720
December1st to 15thNight
17%39%34%27%7%
720
16th to endNight
19%5%13%35%21%4
768
Page generated at 2017-09-19 04:32:26. Version ac04

Copyright 2017 A. Danko


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