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 2005-11-06 to 2024-01-12.

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
68%
24073
29%
74707
26%
24028
73%
24028
42%
39374

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 10thNight
51%
756
24%
2532
20%
756
59%
756
30%
1446
January11th to 20thNight
60%
724
29%
2353
25%
724
68%
724
37%
1268
January21st to endNight
56%
792
29%
2574
27%
792
64%
792
34%
1434
February1st to 10thNight
61%
720
31%
2273
30%
720
67%
720
38%
1227
February11th to 20thNight
58%
700
31%
2099
26%
700
67%
700
40%
1128
February21st to endNight
54%
592
26%
1776
23%
592
61%
592
35%
1009
March1st to 10thNight
51%
720
20%
2100
17%
720
57%
720
31%
1306
March11th to 20thNight
59%
720
23%
1980
19%
720
63%
720
35%
1209
March21st to endNight
56%
792
25%
2084
16%
792
64%
792
38%
1255
April1st to 10thNight
58%
720
26%
1800
16%
720
63%
720
43%
1066
April11th to 20thNight
57%
711
32%
1779
14%
711
64%
711
48%
989
April21st to endNight
58%
720
29%
1800
14%
720
63%
720
49%
1011
May1st to 10thNight
67%
720
25%
1800
14%
720
69%
720
50%
1011
May11th to 20thNight
69%
631
22%
1696
15%
631
71%
631
48%
932
May21st to endNight
64%
388
26%
1552
23%
388
70%
388
35%
734
June1st to 10thNight
76%
360
29%
1440
32%
360
80%
360
38%
658
June11th to 20thNight
77%
360
32%
1440
35%
360
84%
360
47%
596
June21st to endNight
87%
360
33%
1440
41%
360
88%
360
54%
550
July1st to 10thNight
89%
350
32%
1400
39%
350
89%
350
54%
529
July11th to 20thNight
89%
511
31%
1555
28%
511
89%
511
57%
688
July21st to endNight
89%
591
33%
1674
27%
558
90%
558
52%
795
August1st to 10thNight
86%
634
30%
1699
21%
631
86%
631
58%
837
August11th to 20thNight
92%
712
35%
1780
22%
712
92%
712
72%
854
August21st to endNight
90%
792
36%
1980
24%
792
90%
792
72%
952
September1st to 10thNight
88%
720
43%
1800
33%
720
88%
720
70%
874
September11th to 20thNight
85%
720
41%
1800
34%
720
86%
720
65%
915
September21st to endNight
84%
720
38%
1980
35%
720
88%
720
65%
911
October1st to 10thNight
79%
720
33%
2158
35%
720
83%
720
57%
999
October11th to 20thNight
74%
720
35%
2160
36%
720
79%
720
49%
1071
October21st to endNight
71%
792
38%
2376
37%
792
77%
792
51%
1168
November1st to 10thNight
71%
740
34%
2286
32%
740
78%
740
50%
1109
November11th to 20thNight
68%
760
34%
2470
31%
760
77%
760
47%
1177
November21st to endNight
64%
760
32%
2470
28%
760
72%
760
40%
1271
December1st to 10thNight
57%
760
29%
2568
26%
760
65%
760
34%
1384
December11th to 20thNight
52%
759
22%
3232
23%
759
61%
759
19%
2136
December21st to endNight
50%
826
16%
4801
23%
817
61%
817
17%
2875

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%313323347%64%
74708
Night
7%423323446%61%
75010
TransparencyDay
22%49%21%26%18%
74429
Night
25%410%20%23%18%
74812
SeeingDay
8%9%24%35%20%4
14425
Night
8%414%33%33%9%
24093

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 10thDay
14%5%25%435%35%6%47%
1628
11th to 20thDay
9%424422346%59%
1630
21st to endDay
12%5%244245%47%51%
1782
February1st to 10thDay
15%423422345%56%
1684
11th to 20thDay
11%42442335%7%54%
1750
21st to endDay
13%5%23424446%52%
1480
March1st to 10thDay
13%6%26%4345%5%8%43%
1857
11th to 20thDay
12%6%345%3445%8%46%
1980
21st to endDay
9%5%23334448%54%
2272
April1st to 10thDay
10%5%24323447%56%
2160
11th to 20thDay
7%5%2332345%7%60%
2129
21st to endDay
7%323223446%64%
2160
May1st to 10thDay
5%412212347%67%
2160
11th to 20thDay
3213323347%69%
2193
21st to endDay
4423223347%66%
2723
June1st to 10thDay
431222335%9%66%
2520
11th to 20thDay
1212212236%77%
2520
21st to endDay
1112212337%78%
2520
July1st to 10thDay
112212348%75%
2520
11th to 20thDay
1112323345%75%
2347
21st to endDay
111222346%9%69%
2546
August1st to 10thDay
1312313348%71%
2228
11th to 20thDay
1112212236%79%
2122
21st to endDay
1212212346%76%
2376
September1st to 10thDay
121212337%79%
2160
11th to 20thDay
2212112227%78%
2159
21st to endDay
2212222246%76%
1979
October1st to 10thDay
3211223345%73%
1800
11th to 20thDay
4223322336%71%
1800
21st to endDay
4223323336%69%
1980
November1st to 10thDay
6%413313346%66%
1791
11th to 20thDay
6%313323345%66%
1710
21st to endDay
8%324424446%60%
1710
December1st to 10thDay
10%5%24424447%53%
1610
11th to 20thDay
25%6%35%324446%38%
1928
21st to endDay
16%8%345%3445%7%43%
2794
January1st to 10thNight
13%6%24434346%50%
2532
11th to 20thNight
9%424324346%58%
2353
21st to endNight
12%6%33423336%55%
2574
February1st to 10thNight
12%424413445%58%
2273
11th to 20thNight
9%5%13324347%58%
2098
21st to endNight
12%6%24424235%57%
1776
March1st to 10thNight
12%7%35%42446%7%47%
2100
11th to 20thNight
11%5%25%4345%5%7%49%
1980
21st to endNight
11%5%23314447%57%
2084
April1st to 10thNight
8%5%23423348%58%
1800
11th to 20thNight
5%5%23322335%66%
1779
21st to endNight
5%6%22223447%64%
1800
May1st to 10thNight
5%5%13314348%63%
1800
11th to 20thNight
441432345%9%62%
1694
21st to endNight
35%35%432437%61%
1552
June1st to 10thNight
34143245%48%63%
1440
11th to 20thNight
1313223447%70%
1440
21st to endNight
1313312336%75%
1440
July1st to 10thNight
112431345%8%69%
1440
11th to 20thNight
1213213447%72%
1608
21st to endNight
1212325%6%6%7%65%
1773
August1st to 10thNight
121332345%8%69%
1708
11th to 20thNight
1112212447%75%
1780
21st to endNight
1112322346%76%
1980
September1st to 10thNight
221221333477%
1800
11th to 20thNight
2313212445%75%
1800
21st to endNight
1212213225%79%
1980
October1st to 10thNight
3312222435%71%
2158
11th to 20thNight
4323323335%69%
2160
21st to endNight
4423223335%69%
2376
November1st to 10thNight
5%32232335%7%66%
2286
11th to 20thNight
4423323447%65%
2470
21st to endNight
5%5%233345%5%5%60%
2470
December1st to 10thNight
12%5%2432345%6%56%
2568
11th to 20thNight
23%6%2443445%5%39%
3276
21st to endNight
15%8%35%5%345%5%6%41%
4862

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 10thDay
36%6%8%23%21%7%
1628
11th to 20thDay
27%49%21%21%19%
1630
21st to endDay
32%7%11%16%18%16%
1782
February1st to 10thDay
31%35%19%23%19%
1684
11th to 20thDay
28%6%9%22%24%11%
1760
21st to endDay
33%6%10%20%24%8%
1480
March1st to 10thDay
37%6%12%17%19%9%
1857
11th to 20thDay
35%7%11%20%18%9%
1980
21st to endDay
27%7%13%26%19%7%
2272
April1st to 10thDay
28%6%12%27%19%9%
2160
11th to 20thDay
24%5%13%25%24%8%
2132
21st to endDay
21%6%12%26%27%8%
2160
May1st to 10thDay
20%5%11%25%30%9%
2160
11th to 20thDay
17%5%11%29%28%10%
2196
21st to endDay
21%6%14%30%26%4
2724
June1st to 10thDay
18%6%10%27%27%12%
2520
11th to 20thDay
12%311%18%35%21%
2520
21st to endDay
13%28%18%31%29%
2520
July1st to 10thDay
12%27%18%28%33%
2451
11th to 20thDay
15%29%17%31%27%
2277
21st to endDay
15%410%19%29%22%
2405
August1st to 10thDay
18%28%17%25%31%
2212
11th to 20thDay
11%19%17%34%27%
2122
21st to endDay
13%29%18%32%27%
2376
September1st to 10thDay
11%28%15%29%36%
2160
11th to 20thDay
13%28%20%36%21%
2159
21st to endDay
11%48%18%35%25%
1979
October1st to 10thDay
14%5%9%20%24%29%
1800
11th to 20thDay
17%47%18%28%26%
1800
21st to endDay
19%46%15%28%28%
1980
November1st to 10thDay
21%48%18%29%20%
1791
11th to 20thDay
21%5%8%19%27%21%
1710
21st to endDay
25%5%5%17%25%22%
1710
December1st to 10thDay
31%6%8%17%23%16%
1610
11th to 20thDay
47%5%8%16%16%9%
1928
21st to endDay
39%7%9%25%14%6%
2794
January1st to 10thNight
34%5%9%20%24%7%
2532
11th to 20thNight
28%5%8%19%19%20%
2353
21st to endNight
32%411%16%19%18%
2574
February1st to 10thNight
30%37%18%25%17%
2273
11th to 20thNight
26%5%8%22%25%14%
2099
21st to endNight
31%5%8%22%24%10%
1776
March1st to 10thNight
36%6%11%22%16%9%
2100
11th to 20thNight
32%7%13%17%22%9%
1980
21st to endNight
28%7%12%23%22%7%
2084
April1st to 10thNight
27%5%14%26%18%11%
1800
11th to 20thNight
23%413%29%23%9%
1779
21st to endNight
23%412%27%27%8%
1800
May1st to 10thNight
22%5%14%23%25%11%
1800
11th to 20thNight
23%5%16%23%23%10%
1696
21st to endNight
27%317%23%24%6%
1552
June1st to 10thNight
22%6%13%23%24%13%
1440
11th to 20thNight
18%39%19%29%22%
1440
21st to endNight
16%211%16%28%28%
1440
July1st to 10thNight
16%28%20%21%34%
1400
11th to 20thNight
16%311%22%22%26%
1555
21st to endNight
20%311%22%25%20%
1674
August1st to 10thNight
19%111%22%20%26%
1699
11th to 20thNight
11%212%20%28%28%
1780
21st to endNight
12%111%19%26%31%
1980
September1st to 10thNight
13%27%15%24%40%
1800
11th to 20thNight
15%210%18%28%27%
1800
21st to endNight
12%48%16%34%26%
1980
October1st to 10thNight
16%39%20%22%29%
2158
11th to 20thNight
21%410%16%24%26%
2160
21st to endNight
21%26%16%27%27%
2376
November1st to 10thNight
20%49%20%25%23%
2286
11th to 20thNight
20%410%18%26%22%
2470
21st to endNight
25%5%9%17%22%23%
2470
December1st to 10thNight
29%5%7%18%22%19%
2568
11th to 20thNight
46%6%7%15%17%9%
3276
21st to endNight
40%8%10%21%14%7%
4862

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 10thDay
25%13%29%19%13%1
289
11th to 20thDay
15%12%28%28%17%1
344
21st to endDay
12%14%30%27%15%2
374
February1st to 10thDay
14%11%29%28%15%4
369
11th to 20thDay
14%14%31%26%12%2
415
21st to endDay
11%19%32%28%9%2
346
March1st to 10thDay
17%16%35%25%6%
420
11th to 20thDay
16%16%28%29%9%1
420
21st to endDay
14%20%31%27%7%1
462
April1st to 10thDay
13%13%35%26%12%2
420
11th to 20thDay
11%17%31%29%10%2
412
21st to endDay
6%15%34%27%14%4
420
May1st to 10thDay
9%11%32%33%11%3
423
11th to 20thDay
411%29%38%14%3
448
21st to endDay
10%12%26%32%16%3
485
June1st to 10thDay
6%6%21%42%20%5%
450
11th to 20thDay
36%25%40%22%5%
450
21st to endDay
2320%42%26%7%
450
July1st to 10thDay
1114%47%31%6%
446
11th to 20thDay
27%45%34%11%
444
21st to endDay
117%46%35%10%
487
August1st to 10thDay
3116%47%27%6%
447
11th to 20thDay
3110%40%37%9%
437
21st to endDay
2211%47%31%8%
495
September1st to 10thDay
3314%42%29%8%
450
11th to 20thDay
35%23%41%24%5%
450
21st to endDay
17%18%40%29%5%
450
October1st to 10thDay
38%26%41%18%4
450
11th to 20thDay
5%7%26%38%20%4
450
21st to endDay
5%6%26%39%20%4
495
November1st to 10thDay
6%10%26%32%23%4
387
11th to 20thDay
8%12%25%33%19%3
270
21st to endDay
10%7%29%29%21%5%
270
December1st to 10thDay
15%13%28%31%12%2
222
11th to 20thDay
14%12%26%27%18%3
180
21st to endDay
20%16%27%23%10%4
198
January1st to 10thNight
16%7%18%32%21%6%
760
11th to 20thNight
12%318%37%23%6%
724
21st to endNight
14%418%39%19%6%
792
February1st to 10thNight
14%5%14%36%25%6%
720
11th to 20thNight
11%8%18%35%24%5%
704
21st to endNight
16%7%18%37%19%3
592
March1st to 10thNight
15%5%20%37%19%4
720
11th to 20thNight
13%416%37%23%7%
720
21st to endNight
13%7%18%34%24%3
792
April1st to 10thNight
9%6%18%38%25%3
720
11th to 20thNight
8%6%21%33%26%6%
711
21st to endNight
7%5%24%31%24%10%
720
May1st to 10thNight
8%415%35%30%9%
720
11th to 20thNight
6%414%34%35%7%
631
21st to endNight
5%316%38%30%8%
389
June1st to 10thNight
4211%35%40%9%
360
11th to 20thNight
3210%30%46%9%
360
21st to endNight
215%29%47%16%
360
July1st to 10thNight
1419%61%15%
350
11th to 20thNight
2213%58%26%
511
21st to endNight
1217%54%25%
591
August1st to 10thNight
15%30%49%14%
634
11th to 20thNight
11223%60%14%
712
21st to endNight
11323%57%16%
792
September1st to 10thNight
31324%54%15%
720
11th to 20thNight
316%33%45%12%
720
21st to endNight
319%30%46%12%
720
October1st to 10thNight
5%19%36%40%8%
720
11th to 20thNight
6%211%35%37%9%
720
21st to endNight
7%213%35%32%11%
792
November1st to 10thNight
6%414%37%31%8%
740
11th to 20thNight
6%316%38%29%7%
760
21st to endNight
7%5%17%42%25%5%
760
December1st to 10thNight
13%6%17%35%24%4
760
11th to 20thNight
17%419%37%19%4
760
21st to endNight
14%7%22%34%19%3
836
Page generated at 2024-01-08 19:31:29. Version ac07

Copyright 2024 Attilla Danko


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