Cal Poly Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cal Poly Observatory clear sky chart from 2005-11-05 to 2018-08-29.

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
73%
16881
30%
50652
27%
16844
76%
16844
45%
26291

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%
776
24%
2573
25%
776
66%
776
34%
1431
January16th to endNight
62%
832
27%
2704
28%
832
65%
832
34%
1526
February1st to 15thNight
67%
758
31%
2371
32%
757
71%
757
39%
1285
February16th to endNight
59%
684
27%
2051
26%
684
64%
684
34%
1190
March1st to 15thNight
61%
780
21%
2232
20%
780
65%
780
36%
1326
March16th to endNight
64%
832
23%
2220
18%
832
68%
832
39%
1337
April1st to 15thNight
60%
779
27%
1949
17%
779
63%
779
43%
1156
April16th to endNight
63%
772
29%
1930
14%
772
68%
772
52%
1043
May1st to 15thNight
74%
779
22%
1949
14%
779
75%
779
49%
1104
May16th to endNight
71%
473
25%
1694
21%
473
75%
473
43%
797
June1st to 15thNight
82%
390
31%
1560
35%
390
85%
390
44%
667
June16th to endNight
90%
390
33%
1560
41%
390
91%
390
58%
569
July1st to 15thNight
93%
423
31%
1533
35%
423
93%
423
57%
603
July16th to endNight
91%
612
32%
1737
28%
579
91%
579
53%
815
August1st to 15thNight
90%
706
32%
1852
22%
703
91%
703
64%
883
August16th to endNight
92%
819
37%
2049
24%
819
92%
819
73%
980
September1st to 15thNight
88%
720
40%
1800
31%
720
88%
720
65%
913
September16th to endNight
88%
716
41%
1920
38%
716
90%
716
67%
887
October1st to 15thNight
76%
720
30%
2159
33%
720
80%
720
51%
1059
October16th to endNight
77%
768
38%
2304
40%
768
80%
768
48%
1162
November1st to 15thNight
75%
760
32%
2391
31%
760
80%
760
49%
1165
November16th to endNight
72%
780
32%
2535
33%
780
77%
780
43%
1275
December1st to 15thNight
64%
780
29%
2667
30%
780
69%
780
33%
1464
December16th to endNight
56%
832
27%
2912
26%
832
64%
832
29%
1654

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
6%212213347%64%
51593
Night
6%312213346%62%
50843
TransparencyDay
22%49%20%26%17%
51281
Night
23%310%20%23%17%
50656
SeeingDay
7%7%22%36%21%4
7282
Night
7%210%31%36%11%
16893

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
11%32442435%6%51%
1692
16th to endDay
12%424413336%52%
1872
February1st to 15thDay
13%423312336%54%
1817
16th to endDay
16%423414346%48%
1709
March1st to 15thDay
9%41432445%9%48%
2056
16th to endDay
11%423334448%50%
2356
April1st to 15thDay
10%5%23323337%54%
2329
16th to endDay
7%212212346%64%
2305
May1st to 15thDay
432223247%67%
2331
16th to endDay
3312212237%67%
2768
June1st to 15thDay
221112248%69%
2730
16th to endDay
111227%79%
2730
July1st to 15thDay
112212347%73%
2669
16th to endDay
11121335%8%71%
2676
August1st to 15thDay
2122247%73%
2348
16th to endDay
12111236%76%
2450
September1st to 15thDay
111111228%75%
2160
16th to endDay
112112136%76%
2036
October1st to 15thDay
5%21222223469%
1800
16th to endDay
4113323335%68%
1920
November1st to 15thDay
7%412312335%63%
1798
16th to endDay
7%213323246%60%
1755
December1st to 15thDay
11%423414436%52%
1622
16th to endDay
15%423323346%52%
1664
January1st to 15thNight
11%414424336%53%
2573
16th to endNight
11%5%23323335%54%
2704
February1st to 15thNight
10%413313346%56%
2363
16th to endNight
13%41332323455%
2051
March1st to 15thNight
10%5%1432345%7%51%
2232
16th to endNight
9%32421445%8%53%
2220
April1st to 15thNight
8%413413246%59%
1949
16th to endNight
5%311112337%66%
1930
May1st to 15thNight
3313313349%61%
1949
16th to endNight
3413222338%63%
1692
June1st to 15thNight
222213448%67%
1560
16th to endNight
111212236%76%
1560
July1st to 15thNight
1222348%72%
1617
16th to endNight
1222145%48%67%
1845
August1st to 15thNight
111111339%72%
1861
16th to endNight
1211246%76%
2049
September1st to 15thNight
122212245%73%
1800
16th to endNight
1212235%78%
1920
October1st to 15thNight
432221333469%
2159
16th to endNight
5%312223236%66%
2304
November1st to 15thNight
4322222246%65%
2391
16th to endNight
4312323446%61%
2535
December1st to 15thNight
10%41332245%6%55%
2667
16th to endNight
13%5%23323346%51%
2912

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
31%5%8%21%21%11%
1692
16th to endDay
32%5%10%15%18%17%
1872
February1st to 15thDay
31%36%20%23%14%
1817
16th to endDay
36%5%10%23%19%4
1709
March1st to 15thDay
31%5%14%20%17%10%
2056
16th to endDay
31%5%13%23%19%6%
2356
April1st to 15thDay
30%5%12%25%20%6%
2329
16th to endDay
20%5%12%25%26%9%
2305
May1st to 15thDay
20%410%26%27%10%
2331
16th to endDay
18%413%29%27%6%
2772
June1st to 15thDay
16%410%23%30%15%
2730
16th to endDay
11%16%16%34%29%
2730
July1st to 15thDay
14%15%17%27%32%
2524
16th to endDay
15%310%17%28%24%
2521
August1st to 15thDay
17%17%16%29%28%
2332
16th to endDay
13%18%19%35%23%
2450
September1st to 15thDay
14%17%19%28%27%
2160
16th to endDay
11%37%18%34%23%
2036
October1st to 15thDay
18%47%20%24%25%
1800
16th to endDay
19%47%18%26%23%
1920
November1st to 15thDay
24%48%18%29%14%
1798
16th to endDay
24%47%19%28%15%
1755
December1st to 15thDay
32%5%8%15%21%16%
1622
16th to endDay
33%48%21%21%11%
1664
January1st to 15thNight
32%48%19%22%12%
2573
16th to endNight
32%411%12%20%19%
2704
February1st to 15thNight
28%38%19%26%13%
2374
16th to endNight
33%48%25%21%7%
2051
March1st to 15thNight
31%6%13%22%17%9%
2232
16th to endNight
29%6%15%19%21%7%
2220
April1st to 15thNight
27%412%27%19%7%
1949
16th to endNight
20%312%29%23%9%
1930
May1st to 15thNight
22%5%14%20%25%11%
1950
16th to endNight
23%317%24%23%8%
1694
June1st to 15thNight
19%410%23%25%16%
1560
16th to endNight
13%29%14%30%30%
1560
July1st to 15thNight
14%17%20%20%34%
1533
16th to endNight
18%39%21%22%23%
1737
August1st to 15thNight
16%10%22%23%26%
1852
16th to endNight
11%110%20%28%27%
2049
September1st to 15thNight
15%27%18%23%33%
1800
16th to endNight
11%38%16%33%26%
1920
October1st to 15thNight
20%27%19%23%25%
2159
16th to endNight
22%37%17%25%22%
2304
November1st to 15thNight
21%310%21%26%17%
2391
16th to endNight
22%5%10%19%25%16%
2535
December1st to 15thNight
30%5%7%16%21%18%
2667
16th to endNight
33%5%9%18%20%12%
2912

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%27%24%18%1
230
16th to endDay
12%11%27%27%19%1
304
February1st to 15thDay
13%7%28%27%18%4
307
16th to endDay
16%15%30%24%11%2
290
March1st to 15thDay
12%14%36%28%8%
330
16th to endDay
13%14%28%30%11%1
352
April1st to 15thDay
13%14%31%27%10%1
329
16th to endDay
7%17%32%29%11%2
318
May1st to 15thDay
6%10%29%40%10%2
348
16th to endDay
7%9%29%34%15%2
394
June1st to 15thDay
35%19%45%21%4
375
16th to endDay
118%44%27%7%
375
July1st to 15thDay
210%45%33%6%
365
16th to endDay
15%40%41%10%
391
August1st to 15thDay
313%44%33%5%
359
16th to endDay
27%44%35%8%
385
September1st to 15thDay
5%214%41%30%6%
315
16th to endDay
15%19%42%27%3
315
October1st to 15thDay
48%30%39%13%3
315
16th to endDay
5%6%25%38%21%2
336
November1st to 15thDay
6%8%16%33%28%5%
214
16th to endDay
7%11%25%32%20%3
135
December1st to 15thDay
10%10%22%35%17%3
104
16th to endDay
13%8%18%34%18%6%
96
January1st to 15thNight
13%311%33%29%7%
780
16th to endNight
13%213%36%25%8%
832
February1st to 15thNight
12%210%34%31%8%
764
16th to endNight
15%5%13%36%23%4
684
March1st to 15thNight
11%316%37%23%7%
780
16th to endNight
11%314%35%30%4
832
April1st to 15thNight
8%5%16%37%27%4
779
16th to endNight
7%419%27%30%10%
772
May1st to 15thNight
5%210%35%36%9%
780
16th to endNight
5%114%33%34%9%
474
June1st to 15thNight
317%31%43%12%
390
16th to endNight
1426%51%15%
390
July1st to 15thNight
1111%58%26%
423
16th to endNight
110%55%31%
612
August1st to 15thNight
1222%54%18%
706
16th to endNight
118%58%19%
819
September1st to 15thNight
3221%53%19%
720
16th to endNight
15%27%48%16%
716
October1st to 15thNight
7%19%31%38%12%
720
16th to endNight
7%18%32%37%12%
768
November1st to 15thNight
6%11%40%32%8%
760
16th to endNight
6%310%37%32%9%
780
December1st to 15thNight
11%312%34%31%5%
780
16th to endNight
14%416%36%22%5%
832
Page generated at 2018-08-30 02:34:19. Version ac04

Copyright 2018 A. Danko


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