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OCA Anza Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the OCA Anza Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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%
24898
34%
74526
29%
24846
74%
24846
48%
37952

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
56%
757
30%
2457
25%
756
63%
756
35%
1331
January11th to 20thNight
59%
740
35%
2347
30%
740
68%
740
38%
1237
January21st to endNight
49%
792
35%
2376
25%
792
62%
792
33%
1360
February1st to 10thNight
58%
720
34%
2160
28%
720
67%
720
39%
1168
February11th to 20thNight
55%
700
31%
2099
27%
700
62%
700
35%
1195
February21st to endNight
49%
592
29%
1776
26%
592
58%
592
32%
1069
March1st to 10thNight
49%
720
24%
2160
23%
720
56%
720
30%
1305
March11th to 20thNight
60%
720
28%
2058
26%
720
65%
720
39%
1168
March21st to endNight
56%
792
28%
2088
20%
792
62%
792
39%
1254
April1st to 10thNight
54%
720
30%
1800
18%
720
60%
720
42%
1078
April11th to 20thNight
58%
711
34%
1779
19%
711
64%
711
50%
982
April21st to endNight
59%
720
33%
1800
17%
720
65%
720
50%
1003
May1st to 10thNight
62%
572
34%
1652
23%
572
66%
572
47%
862
May11th to 20thNight
68%
531
35%
1595
25%
531
71%
531
53%
768
May21st to endNight
65%
582
34%
1746
25%
582
70%
582
54%
820
June1st to 10thNight
79%
540
37%
1620
30%
540
82%
540
67%
681
June11th to 20thNight
78%
540
38%
1620
32%
540
81%
540
77%
639
June21st to endNight
84%
540
37%
1620
32%
540
86%
540
74%
644
July1st to 10thNight
90%
525
36%
1575
31%
525
91%
525
77%
614
July11th to 20thNight
92%
522
36%
1566
30%
522
92%
522
69%
651
July21st to endNight
88%
642
33%
1723
26%
607
88%
607
58%
834
August1st to 10thNight
90%
716
36%
1780
23%
712
91%
712
74%
840
August11th to 20thNight
93%
712
41%
1780
27%
712
93%
712
77%
815
August21st to endNight
92%
792
44%
1980
31%
792
92%
792
75%
924
September1st to 10thNight
89%
720
42%
1830
34%
720
90%
720
74%
856
September11th to 20thNight
88%
720
45%
1980
40%
720
90%
720
75%
852
September21st to endNight
84%
720
44%
1980
39%
720
88%
720
72%
868
October1st to 10thNight
75%
720
39%
1980
33%
720
80%
720
56%
981
October11th to 20thNight
77%
720
44%
1980
38%
720
82%
720
64%
914
October21st to endNight
70%
792
40%
2468
39%
792
77%
792
55%
1118
November1st to 10thNight
72%
740
36%
2405
37%
740
78%
740
51%
1099
November11th to 20thNight
69%
760
37%
2470
34%
760
78%
760
50%
1138
November21st to endNight
59%
760
33%
2470
31%
760
66%
760
35%
1319
December1st to 10thNight
57%
760
30%
2470
27%
760
64%
760
34%
1368
December11th to 20thNight
54%
760
30%
2945
28%
759
63%
759
26%
1712
December21st to endNight
53%
828
19%
4391
27%
817
62%
817
20%
2485

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
4413323346%68%
74876
Night
35%23323346%65%
74960
TransparencyDay
19%35%14%27%31%
74607
Night
22%35%13%25%32%
74752
SeeingDay
7%12%30%30%15%6%
14512
Night
6%5%15%32%32%9%
24915

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
7%7%23424347%57%
1708
11th to 20thDay
6%6%1232346%7%60%
1722
21st to endDay
8%9%24423445%56%
1980
February1st to 10thDay
6%7%23324346%61%
1800
11th to 20thDay
5%7%3433345%6%57%
1750
21st to endDay
7%11%25%323346%52%
1480
March1st to 10thDay
6%6%2443445%9%54%
1800
11th to 20thDay
6%6%34424348%57%
1900
21st to endDay
35%24423446%61%
2263
April1st to 10thDay
6%6%23424447%59%
2160
11th to 20thDay
2412324436%70%
2129
21st to endDay
3412312338%69%
2160
May1st to 10thDay
3413313346%68%
2305
11th to 20thDay
2212222236%76%
2301
21st to endDay
2212323335%74%
2530
June1st to 10thDay
2211112236%78%
2340
11th to 20thDay
111112125%85%
2340
21st to endDay
211221222383%
2340
July1st to 10thDay
2112212335%78%
2340
11th to 20thDay
2212323447%70%
2340
21st to endDay
2213325%5%6%8%62%
2490
August1st to 10thDay
2111223335%77%
2149
11th to 20thDay
11222235%5%76%
2122
21st to endDay
211222335%476%
2376
September1st to 10thDay
3212323345%74%
2129
11th to 20thDay
111111212386%
1980
21st to endDay
211221233379%
1980
October1st to 10thDay
2323323346%70%
1980
11th to 20thDay
221231333477%
1980
21st to endDay
2312313325%76%
1883
November1st to 10thDay
4422312335%72%
1674
11th to 20thDay
3213323346%68%
1710
21st to endDay
5%5%24333447%60%
1710
December1st to 10thDay
5%6%2443346%8%55%
1710
11th to 20thDay
10%7%245%35%5%7%8%45%
2169
21st to endDay
6%8%25%7%46%6%5%5%46%
3146
January1st to 10thNight
6%6%344245%5%7%55%
2457
11th to 20thNight
47%24424446%60%
2347
21st to endNight
7%9%35%423345%55%
2376
February1st to 10thNight
5%8%23324445%59%
2160
11th to 20thNight
49%35%334435%58%
2098
21st to endNight
5%12%35%424346%51%
1776
March1st to 10thNight
5%10%45%5%35%46%8%46%
2160
11th to 20thNight
5%10%2442345%7%56%
2058
21st to endNight
38%3442446%7%53%
2088
April1st to 10thNight
47%3442445%8%54%
1800
11th to 20thNight
37%22323346%64%
1779
21st to endNight
35%3432335%6%63%
1800
May1st to 10thNight
15%2442335%6%64%
1652
11th to 20thNight
132332335%7%67%
1594
21st to endNight
232331345%6%69%
1746
June1st to 10thNight
1213211246%78%
1620
11th to 20thNight
11122123484%
1620
21st to endNight
111111113484%
1620
July1st to 10thNight
1111111235%83%
1620
11th to 20thNight
1112113446%76%
1620
21st to endNight
221333435%7%67%
1824
August1st to 10thNight
112222335%79%
1790
11th to 20thNight
111112335%81%
1780
21st to endNight
111221223382%
1980
September1st to 10thNight
1111212335%79%
1830
11th to 20thNight
121221222385%
1980
21st to endNight
221221222383%
1980
October1st to 10thNight
3323223346%69%
1980
11th to 20thNight
131221133378%
1980
21st to endNight
141332323474%
2468
November1st to 10thNight
2423323335%70%
2405
11th to 20thNight
25%33323345%68%
2470
21st to endNight
46%36%4245%5%6%56%
2470
December1st to 10thNight
6%7%25%43445%7%53%
2470
11th to 20thNight
7%9%35%5%345%6%8%45%
3041
21st to endNight
5%12%45%5%34445%48%
4521

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
29%45%16%25%21%
1709
11th to 20thDay
24%5%6%12%29%23%
1722
21st to endDay
32%36%13%21%25%
1980
February1st to 10thDay
28%3412%24%29%
1800
11th to 20thDay
28%4414%26%24%
1760
21st to endDay
34%46%13%23%20%
1480
March1st to 10thDay
29%46%17%24%20%
1800
11th to 20thDay
27%37%15%29%19%
1900
21st to endDay
25%48%22%28%14%
2263
April1st to 10thDay
27%37%21%30%13%
2160
11th to 20thDay
17%35%20%32%23%
2132
21st to endDay
17%37%18%38%17%
2160
May1st to 10thDay
20%38%15%31%23%
2305
11th to 20thDay
13%2418%39%23%
2304
21st to endDay
15%36%20%38%18%
2530
June1st to 10thDay
11%2416%35%32%
2340
11th to 20thDay
6%138%29%52%
2340
21st to endDay
10%135%18%64%
2340
July1st to 10thDay
10%238%20%56%
2276
11th to 20thDay
14%47%19%25%32%
2271
21st to endDay
18%7%9%19%26%21%
2352
August1st to 10thDay
11%3411%25%46%
2135
11th to 20thDay
10%4415%25%42%
2122
21st to endDay
12%35%10%32%38%
2376
September1st to 10thDay
14%45%8%24%45%
2129
11th to 20thDay
7%226%31%52%
1980
21st to endDay
11%3310%29%44%
1980
October1st to 10thDay
17%3414%26%36%
1980
11th to 20thDay
14%2410%26%44%
1980
21st to endDay
14%239%25%47%
1883
November1st to 10thDay
18%2313%30%34%
1674
11th to 20thDay
18%36%10%28%36%
1710
21st to endDay
25%36%11%25%31%
1710
December1st to 10thDay
28%46%15%22%25%
1710
11th to 20thDay
36%6%8%12%22%16%
2168
21st to endDay
40%45%17%21%14%
3146
January1st to 10thNight
29%5%415%24%23%
2457
11th to 20thNight
27%45%13%26%25%
2347
21st to endNight
32%46%11%21%27%
2376
February1st to 10thNight
28%4412%22%31%
2160
11th to 20thNight
32%4410%24%26%
2099
21st to endNight
37%3412%22%23%
1776
March1st to 10thNight
36%5%5%15%19%20%
2160
11th to 20thNight
29%46%14%28%19%
2058
21st to endNight
30%5%7%19%24%15%
2088
April1st to 10thNight
29%47%18%26%17%
1800
11th to 20thNight
22%46%17%27%25%
1779
21st to endNight
22%46%16%33%19%
1800
May1st to 10thNight
22%45%15%27%28%
1652
11th to 20thNight
18%36%16%31%26%
1595
21st to endNight
16%38%17%36%19%
1746
June1st to 10thNight
10%2413%33%37%
1620
11th to 20thNight
7%238%24%57%
1620
21st to endNight
8%135%18%65%
1620
July1st to 10thNight
7%237%20%62%
1575
11th to 20thNight
9%47%19%27%34%
1566
21st to endNight
16%49%20%27%24%
1723
August1st to 10thNight
9%2411%28%45%
1780
11th to 20thNight
8%3317%26%44%
1780
21st to endNight
9%239%30%46%
1980
September1st to 10thNight
9%3410%23%51%
1830
11th to 20thNight
9%226%27%53%
1980
21st to endNight
10%2310%30%45%
1980
October1st to 10thNight
18%3414%27%35%
1980
11th to 20thNight
13%2310%27%45%
1980
21st to endNight
17%228%21%48%
2468
November1st to 10thNight
19%3410%29%36%
2405
11th to 20thNight
20%2411%25%38%
2470
21st to endNight
29%4412%20%30%
2470
December1st to 10thNight
31%5%5%12%20%29%
2470
11th to 20thNight
36%5%6%13%21%18%
3041
21st to endNight
39%46%15%23%13%
4521

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
16%12%29%32%10%1
270
11th to 20thDay
14%12%24%35%13%2
312
21st to endDay
14%18%31%24%11%2
462
February1st to 10thDay
12%14%34%24%12%5%
420
11th to 20thDay
12%24%32%22%10%1
415
21st to endDay
17%22%33%18%9%1
346
March1st to 10thDay
14%24%32%21%7%2
420
11th to 20thDay
14%19%33%21%10%3
420
21st to endDay
10%27%34%18%10%1
462
April1st to 10thDay
11%19%34%21%12%3
420
11th to 20thDay
6%25%40%17%9%3
412
21st to endDay
9%19%32%23%11%6%
420
May1st to 10thDay
9%18%35%24%11%4
423
11th to 20thDay
6%15%37%27%12%4
448
21st to endDay
6%16%34%25%14%4
485
June1st to 10thDay
29%34%30%18%7%
450
11th to 20thDay
8%36%31%16%8%
450
21st to endDay
28%31%33%18%8%
450
July1st to 10thDay
2227%41%18%11%
446
11th to 20thDay
1121%43%21%14%
444
21st to endDay
2118%43%22%14%
487
August1st to 10thDay
2227%39%17%13%
447
11th to 20thDay
2222%38%21%15%
437
21st to endDay
1120%45%17%15%
495
September1st to 10thDay
3329%40%17%8%
450
11th to 20thDay
36%34%34%17%6%
450
21st to endDay
212%25%36%18%7%
450
October1st to 10thDay
5%11%31%32%17%4
450
11th to 20thDay
410%34%31%19%3
450
21st to endDay
49%30%29%22%6%
374
November1st to 10thDay
10%9%22%33%23%4
270
11th to 20thDay
313%25%36%18%5%
270
21st to endDay
10%16%28%29%12%6%
270
December1st to 10thDay
9%12%31%31%14%3
270
11th to 20thDay
6%13%29%33%17%3
270
21st to endDay
14%20%27%27%11%2
297
January1st to 10thNight
11%7%21%32%22%8%
760
11th to 20thNight
10%6%19%37%22%7%
740
21st to endNight
13%9%19%33%22%3
792
February1st to 10thNight
10%8%20%34%22%6%
720
11th to 20thNight
10%10%19%34%21%6%
704
21st to endNight
15%9%21%31%20%4
592
March1st to 10thNight
12%11%23%34%17%3
720
11th to 20thNight
11%8%15%34%23%8%
720
21st to endNight
8%9%21%33%24%4
792
April1st to 10thNight
8%8%25%32%23%4
720
11th to 20thNight
6%11%22%31%24%5%
711
21st to endNight
6%8%23%28%26%8%
720
May1st to 10thNight
5%8%22%35%26%5%
572
11th to 20thNight
26%19%34%30%8%
531
21st to endNight
47%20%34%30%5%
584
June1st to 10thNight
1412%40%37%5%
540
11th to 20thNight
1119%38%37%5%
540
21st to endNight
2210%37%41%8%
540
July1st to 10thNight
15%32%51%11%
525
11th to 20thNight
1330%55%11%
522
21st to endNight
2223%57%15%
642
August1st to 10thNight
25%25%51%17%
716
11th to 20thNight
1218%56%22%
712
21st to endNight
2218%56%22%
792
September1st to 10thNight
21423%55%15%
720
11th to 20thNight
27%31%47%13%
720
21st to endNight
2110%34%39%13%
720
October1st to 10thNight
4412%39%33%7%
720
11th to 20thNight
4313%37%33%11%
720
21st to endNight
4416%31%32%13%
792
November1st to 10thNight
5%414%39%28%10%
740
11th to 20thNight
5%217%35%31%10%
760
21st to endNight
8%9%17%34%25%8%
760
December1st to 10thNight
10%7%19%34%23%6%
760
11th to 20thNight
13%7%18%37%21%4
760
21st to endNight
13%8%20%35%18%7%
836
Page generated at 2024-01-12 22:38:12. Version ac07

Copyright 2024 Attilla Danko


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