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 2024-11-05.

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%
24934
29%
97365
26%
24780
73%
24780
36%
47686

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%
752
30%
4117
21%
736
59%
736
22%
1908
January11th to 20thNight
58%
752
26%
4027
25%
740
67%
740
20%
2324
January21st to endNight
54%
830
18%
4360
26%
814
62%
814
18%
2667
February1st to 10thNight
58%
760
30%
2400
29%
760
64%
760
36%
1323
February11th to 20thNight
57%
740
30%
2219
26%
740
66%
740
38%
1210
February21st to endNight
53%
628
25%
1884
22%
628
59%
628
34%
1075
March1st to 10thNight
50%
755
22%
3611
17%
744
57%
744
19%
2208
March11th to 20thNight
58%
748
26%
3457
19%
740
64%
740
29%
1478
March21st to endNight
55%
829
15%
3587
15%
814
63%
814
23%
2102
April1st to 10thNight
57%
760
28%
1900
16%
760
62%
760
43%
1124
April11th to 20thNight
56%
751
31%
1879
14%
751
63%
751
48%
1053
April21st to endNight
57%
760
29%
1900
13%
760
62%
760
49%
1067
May1st to 10thNight
66%
755
38%
2965
14%
749
69%
749
47%
1124
May11th to 20thNight
68%
653
18%
2981
14%
647
70%
647
29%
1550
May21st to endNight
64%
400
21%
2725
23%
399
70%
399
23%
1139
June1st to 10thNight
75%
380
28%
1520
31%
380
79%
380
37%
699
June11th to 20thNight
77%
380
32%
1520
34%
380
83%
380
47%
625
June21st to endNight
88%
380
32%
1520
41%
380
89%
380
55%
577
July1st to 10thNight
89%
360
44%
2072
41%
360
89%
360
53%
550
July11th to 20thNight
90%
534
27%
2264
27%
532
90%
532
41%
1008
July21st to endNight
89%
613
27%
2461
27%
580
90%
580
37%
1147
August1st to 10thNight
87%
658
39%
2414
22%
652
87%
652
53%
964
August11th to 20thNight
92%
732
33%
2486
22%
731
92%
731
70%
911
August21st to endNight
89%
818
34%
2927
24%
814
90%
814
59%
1194
September1st to 10thNight
88%
760
45%
1900
34%
760
88%
760
70%
918
September11th to 20thNight
83%
760
39%
1900
32%
760
84%
760
62%
985
September21st to endNight
84%
760
38%
2090
35%
760
88%
760
64%
968
October1st to 10thNight
80%
740
47%
3318
36%
740
84%
740
54%
1100
October11th to 20thNight
74%
742
27%
3268
35%
740
79%
740
43%
1251
October21st to endNight
71%
819
42%
3807
38%
813
78%
813
42%
1460
November1st to 10thNight
71%
760
35%
2345
32%
760
79%
760
50%
1134
November11th to 20thNight
68%
760
34%
2470
31%
760
77%
760
47%
1177
November21st to endNight
63%
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
51%
759
22%
3232
23%
759
61%
759
19%
2136
December21st to endNight
50%
826
16%
4801
23%
817
60%
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%313223346%66%
104777
Night
7%5%24333346%62%
103036
TransparencyDay
20%5%9%20%25%22%
104535
Night
24%5%9%19%22%21%
102869
SeeingDay
8%9%24%35%20%5%
15590
Night
8%414%33%33%9%
25157

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
9%425%34435%6%55%
2713
11th to 20thDay
14%7%25%43446%8%44%
2860
21st to endDay
14%6%26%446%46%8%38%
3146
February1st to 10thDay
15%424322345%55%
1777
11th to 20thDay
11%5%25%43325%7%53%
1850
21st to endDay
13%5%25%43435%6%51%
1570
March1st to 10thDay
14%7%26%43436%8%41%
3247
11th to 20thDay
7%414333346%62%
3490
21st to endDay
10%7%25%5%45%46%8%43%
3996
April1st to 10thDay
9%5%25%333347%57%
2280
11th to 20thDay
7%6%1432335%7%59%
2249
21st to endDay
7%313333346%64%
2280
May1st to 10thDay
5%31211212477%
3800
11th to 20thDay
2425%34435%8%60%
3898
21st to endDay
2313222246%72%
4813
June1st to 10thDay
431333335%9%65%
2660
11th to 20thDay
1212222236%77%
2660
21st to endDay
1212222236%79%
2660
July1st to 10thDay
11111225%85%
4420
11th to 20thDay
1113223245%77%
4117
21st to endDay
12122246%79%
4455
August1st to 10thDay
1212122247%77%
3913
11th to 20thDay
11111112388%
3724
21st to endDay
11111123485%
4180
September1st to 10thDay
111112237%79%
2280
11th to 20thDay
2212112237%77%
2278
21st to endDay
2212222246%76%
2088
October1st to 10thDay
221122324477%
3180
11th to 20thDay
221322223477%
3161
21st to endDay
3224233346%68%
3448
November1st to 10thDay
6%413323346%66%
1832
11th to 20thDay
6%313333345%65%
1710
21st to endDay
8%424433346%60%
1710
December1st to 10thDay
10%6%25%434346%53%
1610
11th to 20thDay
25%7%25%33335%6%38%
1928
21st to endDay
16%8%25%44335%6%43%
2794
January1st to 10thNight
11%6%25%343346%56%
4220
11th to 20thNight
13%8%35%33435%6%45%
4110
21st to endNight
17%8%25%444347%42%
4521
February1st to 10thNight
13%5%24332345%56%
2400
11th to 20thNight
10%5%1433335%7%57%
2218
21st to endNight
11%7%25%333235%56%
1884
March1st to 10thNight
16%9%36%44446%6%39%
3624
11th to 20thNight
6%41433335%6%62%
3490
21st to endNight
15%8%2433335%7%47%
3657
April1st to 10thNight
8%6%2433325%7%58%
1900
11th to 20thNight
5%6%24332235%65%
1879
21st to endNight
5%7%22222346%64%
1900
May1st to 10thNight
3312212235%76%
3180
11th to 20thNight
27%5%10%43445%6%51%
3004
21st to endNight
27%38%443435%57%
2762
June1st to 10thNight
2415%33445%7%62%
1520
11th to 20thNight
131422335%7%70%
1520
21st to endNight
1213212337%75%
1520
July1st to 10thNight
13112235%82%
2540
11th to 20thNight
11122236%7%6%70%
2848
21st to endNight
1143345%6%6%66%
3109
August1st to 10thNight
112223246%76%
3029
11th to 20thNight
11111235%84%
3126
21st to endNight
11221235%6%78%
3498
September1st to 10thNight
221222224478%
1900
11th to 20thNight
2314113335%73%
1900
21st to endNight
131222223478%
2090
October1st to 10thNight
2212222335%78%
3798
11th to 20thNight
2212233346%73%
3777
21st to endNight
4413222246%70%
4121
November1st to 10thNight
5%42233235%6%66%
2345
11th to 20thNight
442323335%7%64%
2470
21st to endNight
5%5%2333445%5%60%
2470
December1st to 10thNight
12%5%2423335%6%56%
2568
11th to 20thNight
23%6%2443435%5%39%
3276
21st to endNight
15%8%26%44445%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
26%7%10%35%18%4
2713
11th to 20thDay
36%7%11%19%16%11%
2860
21st to endDay
39%8%11%20%14%9%
3146
February1st to 10thDay
32%36%19%22%18%
1777
11th to 20thDay
29%6%9%22%23%10%
1860
21st to endDay
33%6%10%21%23%7%
1570
March1st to 10thDay
35%10%14%20%17%5%
3247
11th to 20thDay
22%7%11%32%16%12%
3490
21st to endDay
31%11%16%24%13%4
3996
April1st to 10thDay
27%6%12%27%20%8%
2280
11th to 20thDay
24%6%13%25%24%8%
2252
21st to endDay
21%6%13%26%26%8%
2280
May1st to 10thDay
15%37%28%35%12%
3800
11th to 20thDay
18%414%28%26%10%
3901
21st to endDay
13%7%14%29%33%5%
4814
June1st to 10thDay
17%6%12%27%26%11%
2660
11th to 20thDay
12%311%18%36%21%
2660
21st to endDay
12%27%17%30%32%
2660
July1st to 10thDay
7%1410%16%62%
4351
11th to 20thDay
12%35%10%21%49%
4047
21st to endDay
9%36%15%28%39%
4315
August1st to 10thDay
12%1417%32%32%
3898
11th to 20thDay
6%15%10%33%46%
3760
21st to endDay
8%15%16%30%40%
4179
September1st to 10thDay
11%27%14%30%37%
2280
11th to 20thDay
14%37%21%36%20%
2278
21st to endDay
11%47%17%34%26%
2088
October1st to 10thDay
12%35%14%25%42%
3180
11th to 20thDay
13%25%21%37%21%
3161
21st to endDay
20%2314%34%27%
3448
November1st to 10thDay
21%48%18%29%20%
1832
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
28%6%11%26%23%5%
4220
11th to 20thNight
36%8%9%23%12%11%
4110
21st to endNight
42%6%10%17%15%10%
4521
February1st to 10thNight
31%38%17%24%16%
2400
11th to 20thNight
27%5%9%22%24%14%
2219
21st to endNight
31%5%8%23%23%9%
1884
March1st to 10thNight
39%10%11%21%15%5%
3624
11th to 20thNight
22%7%12%26%22%10%
3490
21st to endNight
34%9%14%23%16%4
3657
April1st to 10thNight
27%5%13%26%18%10%
1900
11th to 20thNight
24%412%29%23%9%
1879
21st to endNight
23%412%27%27%8%
1900
May1st to 10thNight
13%5%10%26%27%19%
3180
11th to 20thNight
32%418%20%17%9%
3006
21st to endNight
25%6%15%21%28%4
2762
June1st to 10thNight
22%6%13%23%24%12%
1520
11th to 20thNight
18%39%19%29%22%
1520
21st to endNight
15%210%15%27%31%
1520
July1st to 10thNight
9%15%11%12%62%
2500
11th to 20thNight
12%10%6%12%13%47%
2795
21st to endNight
17%7%6%13%29%29%
3011
August1st to 10thNight
14%16%18%28%32%
3020
11th to 20thNight
7%26%11%23%51%
3156
21st to endNight
10%46%14%25%41%
3498
September1st to 10thNight
13%17%14%24%42%
1900
11th to 20thNight
17%29%18%28%26%
1900
21st to endNight
12%47%16%34%27%
2090
October1st to 10thNight
11%25%12%24%46%
3798
11th to 20thNight
16%25%20%33%23%
3777
21st to endNight
19%1413%34%29%
4121
November1st to 10thNight
19%49%20%25%22%
2345
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%13%28%28%16%1
370
21st to endDay
12%15%29%27%14%2
407
February1st to 10thDay
16%12%28%27%13%3
402
11th to 20thDay
15%15%30%26%13%2
455
21st to endDay
10%19%32%28%8%2
382
March1st to 10thDay
17%17%34%25%6%
460
11th to 20thDay
15%16%30%28%10%1
460
21st to endDay
15%21%32%26%7%1
506
April1st to 10thDay
13%13%35%25%12%2
460
11th to 20thDay
11%17%30%29%11%2
452
21st to endDay
6%14%34%28%13%3
460
May1st to 10thDay
10%11%33%33%11%3
463
11th to 20thDay
5%10%28%39%15%3
488
21st to endDay
10%11%26%33%16%4
529
June1st to 10thDay
5%6%20%44%20%5%
490
11th to 20thDay
36%27%38%22%4
490
21st to endDay
1319%42%27%8%
490
July1st to 10thDay
1114%46%32%7%
486
11th to 20thDay
28%45%33%12%
484
21st to endDay
118%44%35%11%
531
August1st to 10thDay
3116%46%28%7%
487
11th to 20thDay
3112%40%35%10%
477
21st to endDay
2212%46%30%8%
539
September1st to 10thDay
3313%42%30%9%
490
11th to 20thDay
36%23%41%22%4
490
21st to endDay
17%18%41%28%5%
490
October1st to 10thDay
37%24%41%19%5%
490
11th to 20thDay
5%7%25%39%20%5%
490
21st to endDay
5%6%26%38%21%4
539
November1st to 10thDay
6%10%27%32%22%4
404
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%8%18%31%21%6%
760
11th to 20thNight
12%419%37%22%6%
760
21st to endNight
15%5%18%39%18%5%
836
February1st to 10thNight
16%5%15%34%24%6%
760
11th to 20thNight
12%8%18%35%23%4
744
21st to endNight
15%6%20%37%18%3
628
March1st to 10thNight
15%6%21%37%18%4
760
11th to 20thNight
12%5%17%38%22%7%
760
21st to endNight
13%8%18%34%23%3
836
April1st to 10thNight
9%6%19%38%25%3
760
11th to 20thNight
8%6%21%33%26%6%
751
21st to endNight
7%5%24%31%24%9%
760
May1st to 10thNight
7%417%36%28%8%
760
11th to 20thNight
5%414%35%35%7%
666
21st to endNight
5%216%38%30%9%
411
June1st to 10thNight
4211%36%39%9%
380
11th to 20thNight
3210%31%45%9%
380
21st to endNight
21429%48%15%
380
July1st to 10thNight
1418%61%16%
370
11th to 20thNight
2113%59%25%
541
21st to endNight
1218%54%25%
624
August1st to 10thNight
15%29%50%15%
670
11th to 20thNight
11224%59%13%
752
21st to endNight
11324%56%16%
836
September1st to 10thNight
31424%54%14%
760
11th to 20thNight
416%34%43%12%
760
21st to endNight
318%30%47%11%
760
October1st to 10thNight
5%18%35%42%8%
760
11th to 20thNight
5%211%35%37%9%
760
21st to endNight
7%214%36%31%10%
836
November1st to 10thNight
6%414%38%31%8%
760
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 2024-11-02 23:11:09. Version ac07

Copyright © 2024 Attilla Danko.


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