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 2017-09-17.

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%
15652
30%
46910
27%
15615
76%
15615
45%
24378

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
25%
2374
25%
716
67%
716
36%
1282
January16th to endNight
62%
768
27%
2496
28%
768
65%
768
33%
1418
February1st to 15thNight
65%
698
29%
2185
30%
697
69%
697
37%
1210
February16th to endNight
59%
632
25%
1895
25%
632
63%
632
32%
1134
March1st to 15thNight
64%
720
22%
2060
21%
720
67%
720
39%
1187
March16th to endNight
65%
768
23%
2049
19%
768
69%
768
39%
1235
April1st to 15thNight
61%
719
27%
1799
17%
719
64%
719
44%
1065
April16th to endNight
62%
720
29%
1800
14%
720
67%
720
51%
981
May1st to 15thNight
74%
719
21%
1799
13%
719
75%
719
49%
1019
May16th to endNight
73%
436
27%
1561
22%
436
77%
436
45%
716
June1st to 15thNight
81%
360
28%
1440
32%
360
84%
360
42%
632
June16th to endNight
92%
360
34%
1440
43%
360
93%
360
61%
511
July1st to 15thNight
93%
389
31%
1409
34%
389
93%
389
58%
551
July16th to endNight
91%
564
34%
1593
29%
531
91%
531
53%
749
August1st to 15thNight
90%
663
30%
1737
21%
660
90%
660
63%
834
August16th to endNight
92%
764
37%
1910
25%
764
92%
764
73%
911
September1st to 15thNight
88%
720
40%
1800
31%
720
88%
720
65%
913
September16th to endNight
89%
664
39%
1780
36%
664
91%
664
66%
829
October1st to 15thNight
76%
660
31%
1979
34%
660
79%
660
48%
992
October16th to endNight
77%
704
36%
2112
38%
704
80%
704
49%
1061
November1st to 15thNight
75%
700
33%
2202
33%
700
81%
700
51%
1049
November16th to endNight
72%
720
31%
2340
31%
720
77%
720
43%
1175
December1st to 15thNight
63%
720
28%
2462
29%
720
68%
720
31%
1393
December16th to endNight
55%
768
26%
2688
25%
768
63%
768
29%
1531

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%312213337%64%
47860
Night
6%312213346%62%
47101
TransparencyDay
22%49%20%26%17%
47548
Night
23%310%20%23%17%
46914
SeeingDay
7%7%22%37%20%3
6048
Night
7%210%31%36%11%
15664

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%31432435%6%53%
1561
16th to endDay
13%5%24313335%52%
1728
February1st to 15thDay
14%423312326%53%
1673
16th to endDay
17%5%23414346%46%
1579
March1st to 15thDay
8%41443445%9%49%
1898
16th to endDay
10%423434448%50%
2175
April1st to 15thDay
11%5%23223337%54%
2149
16th to endDay
7%212222446%63%
2160
May1st to 15thDay
422213247%68%
2151
16th to endDay
3311212237%68%
2550
June1st to 15thDay
221112325%8%67%
2520
16th to endDay
111227%80%
2520
July1st to 15thDay
112212337%73%
2464
16th to endDay
111121325%8%71%
2468
August1st to 15thDay
21122247%72%
2211
16th to endDay
12111235%77%
2292
September1st to 15thDay
111111228%75%
2160
16th to endDay
2212112136%74%
1881
October1st to 15thDay
6%212222245%67%
1650
16th to endDay
4112323335%68%
1760
November1st to 15thDay
7%41231223465%
1657
16th to endDay
8%213323246%61%
1620
December1st to 15thDay
12%4245%25%436%50%
1497
16th to endDay
16%423323335%52%
1536
January1st to 15thNight
10%413323335%56%
2374
16th to endNight
11%5%2332333454%
2496
February1st to 15thNight
11%413313445%55%
2177
16th to endNight
14%5%1342323452%
1895
March1st to 15thNight
9%42322345%7%54%
2060
16th to endNight
8%32421445%8%53%
2049
April1st to 15thNight
8%413413236%59%
1799
16th to endNight
5%312122337%66%
1800
May1st to 15thNight
331331335%9%61%
1799
16th to endNight
3412212338%65%
1559
June1st to 15thNight
223213448%65%
1440
16th to endNight
112236%78%
1440
July1st to 15thNight
1222349%72%
1493
16th to endNight
1221145%47%68%
1701
August1st to 15thNight
211211339%71%
1746
16th to endNight
1211246%77%
1910
September1st to 15thNight
122212245%73%
1800
16th to endNight
1213235%78%
1780
October1st to 15thNight
5%32321333467%
1979
16th to endNight
4312213236%66%
2112
November1st to 15thNight
4322212246%67%
2202
16th to endNight
4413323445%62%
2340
December1st to 15thNight
11%423423446%53%
2462
16th to endNight
14%5%23223346%51%
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
29%5%8%21%22%12%
1561
16th to endDay
33%410%14%18%18%
1728
February1st to 15thDay
32%36%21%22%12%
1673
16th to endDay
37%6%10%23%18%3
1579
March1st to 15thDay
30%5%14%20%18%10%
1898
16th to endDay
30%5%14%23%19%6%
2175
April1st to 15thDay
30%5%12%25%20%5%
2149
16th to endDay
21%6%12%25%24%9%
2160
May1st to 15thDay
19%5%10%25%27%11%
2151
16th to endDay
17%413%29%28%7%
2554
June1st to 15thDay
17%411%23%27%15%
2520
16th to endDay
10%17%15%34%30%
2520
July1st to 15thDay
14%15%17%26%34%
2319
16th to endDay
16%28%16%28%26%
2313
August1st to 15thDay
17%17%16%30%26%
2195
16th to endDay
12%17%18%34%24%
2292
September1st to 15thDay
14%17%19%28%27%
2160
16th to endDay
12%37%18%33%24%
1881
October1st to 15thDay
19%48%21%25%20%
1650
16th to endDay
19%47%18%28%21%
1760
November1st to 15thDay
23%38%18%30%15%
1657
16th to endDay
24%47%19%28%16%
1620
December1st to 15thDay
34%5%9%16%21%11%
1497
16th to endDay
34%48%22%19%10%
1536
January1st to 15thNight
30%49%19%23%13%
2374
16th to endNight
32%410%11%20%20%
2496
February1st to 15thNight
29%49%20%24%10%
2188
16th to endNight
35%48%24%21%6%
1895
March1st to 15thNight
28%6%13%23%18%10%
2060
16th to endNight
28%7%15%19%20%8%
2049
April1st to 15thNight
27%412%28%19%7%
1799
16th to endNight
20%412%29%23%10%
1800
May1st to 15thNight
22%5%15%19%24%12%
1800
16th to endNight
21%216%24%24%8%
1561
June1st to 15thNight
20%411%23%23%16%
1440
16th to endNight
11%29%14%30%31%
1440
July1st to 15thNight
14%17%20%20%35%
1409
16th to endNight
19%29%20%22%25%
1593
August1st to 15thNight
16%11%22%24%23%
1737
16th to endNight
11%110%19%29%28%
1910
September1st to 15thNight
15%27%18%23%33%
1800
16th to endNight
11%38%17%32%26%
1780
October1st to 15thNight
22%28%21%23%21%
1979
16th to endNight
21%38%17%27%21%
2112
November1st to 15thNight
20%210%21%26%18%
2202
16th to endNight
22%5%10%19%24%17%
2340
December1st to 15thNight
32%6%7%17%22%14%
2462
16th to endNight
33%5%10%19%19%11%
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
21%7%25%24%19%1
189
16th to endDay
14%11%27%26%19%1
256
February1st to 15thDay
16%7%28%27%16%3
253
16th to endDay
18%13%31%25%9%2
238
March1st to 15thDay
8%14%37%30%10%
270
16th to endDay
10%16%28%34%10%
288
April1st to 15thDay
13%15%33%26%9%1
269
16th to endDay
8%18%35%28%8%1
270
May1st to 15thDay
5%10%30%42%8%2
288
16th to endDay
6%10%27%35%16%2
330
June1st to 15thDay
36%21%46%19%2
315
16th to endDay
118%46%25%7%
315
July1st to 15thDay
210%47%32%6%
305
16th to endDay
16%38%40%11%
327
August1st to 15thDay
313%47%31%3
315
16th to endDay
26%45%35%8%
332
September1st to 15thDay
5%214%41%30%6%
315
16th to endDay
1418%41%30%3
260
October1st to 15thDay
5%7%30%40%13%3
255
16th to endDay
5%6%27%40%18%1
272
November1st to 15thDay
6%8%12%33%31%7%
163
16th to endDay
11%11%30%32%12%3
90
December1st to 15thDay
15%8%28%39%7%
69
16th to endDay
18%9%26%31%14%
64
January1st to 15thNight
12%311%34%29%8%
720
16th to endNight
14%212%36%24%9%
768
February1st to 15thNight
13%211%34%29%7%
704
16th to endNight
16%5%12%36%23%5%
632
March1st to 15thNight
9%316%37%24%8%
720
16th to endNight
10%314%35%30%5%
768
April1st to 15thNight
9%5%15%38%27%4
719
16th to endNight
7%320%27%31%9%
720
May1st to 15thNight
5%29%35%36%10%
720
16th to endNight
6%113%32%35%10%
437
June1st to 15thNight
418%32%43%10%
360
16th to endNight
1326%53%15%
360
July1st to 15thNight
1111%60%25%
389
16th to endNight
111%53%31%
564
August1st to 15thNight
1223%54%17%
663
16th to endNight
119%59%19%
764
September1st to 15thNight
3221%53%19%
720
16th to endNight
15%27%48%17%
664
October1st to 15thNight
7%18%30%39%12%
660
16th to endNight
6%17%33%36%13%
704
November1st to 15thNight
6%11%38%33%9%
700
16th to endNight
5%310%36%32%10%
720
December1st to 15thNight
12%312%35%30%5%
720
16th to endNight
15%5%16%36%20%4
768
Page generated at 2017-09-18 05:38:25. Version ac04

Copyright 2017 A. Danko


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