Occidental Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Occidental Observatory clear sky chart from 2005-11-05 to 2017-09-15.

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
59%
15317
25%
46904
21%
15280
63%
15280
33%
27108

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
57%
716
17%
2369
15%
716
61%
716
25%
1487
January16th to endNight
56%
768
25%
2496
26%
768
59%
768
28%
1533
February1st to 15thNight
51%
699
21%
2288
20%
698
56%
698
23%
1470
February16th to endNight
46%
632
20%
1933
15%
632
54%
632
24%
1248
March1st to 15thNight
49%
720
14%
1980
12%
720
52%
720
23%
1377
March16th to endNight
44%
768
21%
2112
14%
768
50%
768
29%
1367
April1st to 15thNight
41%
719
23%
1799
10%
719
48%
719
29%
1169
April16th to endNight
49%
720
26%
1800
11%
720
56%
720
39%
1065
May1st to 15thNight
55%
551
27%
1631
16%
551
63%
551
43%
860
May16th to endNight
57%
415
25%
1538
20%
415
66%
415
35%
745
June1st to 15thNight
57%
360
24%
1440
23%
360
66%
360
33%
694
June16th to endNight
75%
360
33%
1440
41%
360
80%
360
54%
541
July1st to 15thNight
80%
401
27%
1421
31%
401
82%
401
51%
603
July16th to endNight
74%
564
28%
1593
27%
531
74%
531
39%
840
August1st to 15thNight
70%
538
28%
1611
23%
535
71%
535
41%
835
August16th to endNight
74%
739
32%
1885
21%
739
76%
739
49%
1055
September1st to 15thNight
77%
719
35%
1799
24%
719
78%
719
54%
967
September16th to endNight
73%
656
36%
1820
31%
656
78%
656
52%
932
October1st to 15thNight
65%
660
28%
1980
27%
660
71%
660
40%
1065
October16th to endNight
66%
704
31%
2112
31%
704
70%
704
37%
1204
November1st to 15thNight
64%
700
25%
2160
23%
700
68%
700
35%
1245
November16th to endNight
58%
720
22%
2489
24%
720
62%
720
27%
1454
December1st to 15thNight
50%
720
19%
2520
18%
720
56%
720
20%
1646
December16th to endNight
46%
768
21%
2688
18%
768
52%
768
22%
1706

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
9%5%23323346%56%
47824
Night
11%5%24323346%52%
47093
TransparencyDay
30%49%20%22%13%
47513
Night
33%49%19%21%12%
46905
SeeingDay
12%12%24%30%16%4
6030
Night
13%5%14%29%30%7%
15328

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
18%5%24422345%45%
1567
16th to endDay
15%7%245%24335%43%
1728
February1st to 15thDay
18%5%25%433345%43%
1576
16th to endDay
19%7%24424448%38%
1538
March1st to 15thDay
18%6%35%5%3445%8%35%
1980
16th to endDay
17%5%25%423447%41%
2112
April1st to 15thDay
13%6%345%24447%42%
2149
16th to endDay
8%313223349%57%
2160
May1st to 15thDay
6%31211235%7%63%
2316
16th to endDay
6%5%24222347%58%
2572
June1st to 15thDay
6%413313237%62%
2520
16th to endDay
2211112225%75%
2520
July1st to 15thDay
1412212235%73%
2452
16th to endDay
15%12212225%71%
2468
August1st to 15thDay
15%13212236%68%
2340
16th to endDay
16%23212235%67%
2315
September1st to 15thDay
25%13211236%68%
2150
16th to endDay
4212111336%69%
1807
October1st to 15thDay
9%312223345%61%
1650
16th to endDay
11%41431235%6%55%
1760
November1st to 15thDay
9%41443346%6%52%
1698
16th to endDay
15%6%3342433448%
1470
December1st to 15thDay
21%7%33422435%40%
1440
16th to endDay
22%6%24323345%39%
1536
January1st to 15thNight
17%5%2342445%7%42%
2369
16th to endNight
19%5%13313346%46%
2496
February1st to 15thNight
18%5%34423446%42%
2276
16th to endNight
17%6%2432345%7%42%
1933
March1st to 15thNight
17%5%35%425%5%5%7%38%
1980
16th to endNight
16%5%24323345%46%
2112
April1st to 15thNight
11%5%2442445%6%49%
1799
16th to endNight
7%324413236%60%
1800
May1st to 15thNight
442332325%6%61%
1631
16th to endNight
4423413336%61%
1537
June1st to 15thNight
442431435%7%58%
1440
16th to endNight
311111225%75%
1440
July1st to 15thNight
1213332336%67%
1506
16th to endNight
16%2443445%9%54%
1701
August1st to 15thNight
16%26%423437%57%
1620
16th to endNight
6%24322449%57%
1885
September1st to 15thNight
15%14323348%63%
1799
16th to endNight
35%13212236%67%
1820
October1st to 15thNight
7%423324246%59%
1980
16th to endNight
10%324324346%53%
2112
November1st to 15thNight
9%42332335%8%52%
2160
16th to endNight
14%424323346%50%
2489
December1st to 15thNight
24%424424345%40%
2520
16th to endNight
21%5%14323236%44%
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
42%411%18%13%9%
1567
16th to endDay
44%5%7%13%17%12%
1728
February1st to 15thDay
43%5%9%19%18%3
1576
16th to endDay
45%5%10%23%12%3
1538
March1st to 15thDay
48%6%11%15%13%4
1980
16th to endDay
43%6%13%19%13%3
2112
April1st to 15thDay
42%414%20%15%1
2149
16th to endDay
26%6%14%26%20%4
2160
May1st to 15thDay
21%410%29%25%7%
2316
16th to endDay
26%416%26%20%5%
2576
June1st to 15thDay
24%5%11%23%23%11%
2520
16th to endDay
14%36%16%36%22%
2520
July1st to 15thDay
16%15%13%29%33%
2308
16th to endDay
18%25%13%23%35%
2313
August1st to 15thDay
19%26%18%29%24%
2324
16th to endDay
20%16%18%30%21%
2315
September1st to 15thDay
19%16%17%28%26%
2150
16th to endDay
18%27%25%27%18%
1807
October1st to 15thDay
25%39%19%26%14%
1650
16th to endDay
30%49%23%22%8%
1760
November1st to 15thDay
31%5%13%29%12%7%
1698
16th to endDay
40%5%10%19%17%6%
1470
December1st to 15thDay
46%6%9%14%19%3
1440
16th to endDay
47%5%9%17%14%6%
1536
January1st to 15thNight
40%6%12%18%13%8%
2369
16th to endNight
39%38%16%17%14%
2496
February1st to 15thNight
43%5%10%17%16%7%
2288
16th to endNight
40%6%12%21%15%3
1933
March1st to 15thNight
45%6%12%15%15%4
1980
16th to endNight
39%411%22%16%5%
2112
April1st to 15thNight
35%6%14%20%18%3
1799
16th to endNight
27%49%27%22%8%
1800
May1st to 15thNight
24%48%21%29%11%
1632
16th to endNight
24%5%12%29%21%7%
1538
June1st to 15thNight
26%49%23%22%13%
1440
16th to endNight
14%25%16%34%27%
1440
July1st to 15thNight
19%1310%31%33%
1421
16th to endNight
30%16%12%18%30%
1593
August1st to 15thNight
28%15%15%22%26%
1611
16th to endNight
25%26%15%25%23%
1885
September1st to 15thNight
22%1415%28%27%
1799
16th to endNight
21%2422%28%21%
1820
October1st to 15thNight
27%38%19%25%16%
1980
16th to endNight
31%47%21%23%12%
2112
November1st to 15thNight
31%5%14%27%14%7%
2160
16th to endNight
36%48%22%19%7%
2489
December1st to 15thNight
46%47%16%18%5%
2520
16th to endNight
42%5%9%16%18%6%
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
28%10%23%17%16%2
196
16th to endDay
17%14%23%26%15%2
256
February1st to 15thDay
23%10%23%25%15%1
236
16th to endDay
21%16%25%23%10%1
231
March1st to 15thDay
26%18%24%17%9%3
270
16th to endDay
18%19%37%17%42
288
April1st to 15thDay
16%28%26%20%5%2
269
16th to endDay
14%27%28%21%5%2
270
May1st to 15thDay
10%21%30%26%9%1
288
16th to endDay
12%13%23%28%17%4
330
June1st to 15thDay
8%16%30%33%8%2
315
16th to endDay
35%29%37%19%5%
315
July1st to 15thDay
3217%46%25%4
305
16th to endDay
2115%53%21%5%
327
August1st to 15thDay
4322%45%19%4
315
16th to endDay
4124%39%24%6%
332
September1st to 15thDay
5%421%39%20%8%
312
16th to endDay
5%9%18%31%27%7%
255
October1st to 15thDay
9%12%25%28%16%7%
255
16th to endDay
16%11%21%26%22%1
272
November1st to 15thDay
9%12%19%23%27%7%
204
16th to endDay
10%15%36%16%16%3
65
December1st to 15thDay
38%13%20%16%10%1
60
16th to endDay
18%18%10%25%20%6%
64
January1st to 15thNight
19%410%32%26%6%
720
16th to endNight
22%5%10%29%26%5%
768
February1st to 15thNight
20%5%12%30%26%3
704
16th to endNight
20%6%20%28%18%4
632
March1st to 15thNight
19%9%14%26%25%4
720
16th to endNight
17%8%22%28%18%3
768
April1st to 15thNight
13%11%25%27%18%3
719
16th to endNight
9%12%22%25%23%5%
720
May1st to 15thNight
6%323%30%30%4
552
16th to endNight
6%7%21%29%30%5%
416
June1st to 15thNight
6%5%22%37%25%1
360
16th to endNight
4214%37%38%2
360
July1st to 15thNight
37%31%49%7%
401
16th to endNight
5%5%29%45%13%
564
August1st to 15thNight
47%32%43%10%
538
16th to endNight
46%34%46%7%
739
September1st to 15thNight
316%27%48%12%
719
16th to endNight
6%210%28%37%14%
656
October1st to 15thNight
10%312%29%33%11%
660
16th to endNight
12%310%24%36%12%
704
November1st to 15thNight
11%314%29%32%9%
700
16th to endNight
14%6%13%29%27%8%
720
December1st to 15thNight
26%412%26%23%6%
720
16th to endNight
24%7%16%29%19%2
768
Page generated at 2017-09-15 21:58:32. Version ac04

Copyright 2017 A. Danko


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