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 2018-04-23.

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%
16190
25%
49581
21%
16153
63%
16153
33%
28644

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
55%
776
17%
2567
15%
776
60%
776
23%
1644
January16th to endNight
56%
832
24%
2704
24%
832
59%
832
27%
1676
February1st to 15thNight
53%
759
24%
2483
22%
758
59%
758
25%
1537
February16th to endNight
47%
684
22%
2093
16%
684
55%
684
25%
1314
March1st to 15thNight
49%
780
14%
2145
12%
780
52%
780
22%
1515
March16th to endNight
44%
832
20%
2288
14%
832
50%
832
29%
1483
April1st to 15thNight
40%
779
23%
1949
10%
779
47%
779
29%
1276
April16th to endNight
50%
744
27%
1860
11%
744
57%
744
40%
1097
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%
720
35%
1800
24%
720
78%
720
54%
968
September16th to endNight
73%
716
39%
1985
32%
716
78%
716
53%
998
October1st to 15thNight
67%
720
28%
2160
27%
720
73%
720
43%
1126
October16th to endNight
67%
768
32%
2304
34%
768
70%
768
38%
1299
November1st to 15thNight
63%
760
23%
2345
22%
760
66%
760
33%
1376
November16th to endNight
57%
780
22%
2697
24%
780
61%
780
27%
1595
December1st to 15thNight
53%
780
20%
2730
20%
780
58%
780
22%
1722
December16th to endNight
46%
832
22%
2912
19%
832
53%
832
22%
1845

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%
49935
Night
11%5%24323346%52%
49770
TransparencyDay
30%49%20%22%13%
49624
Night
33%49%19%20%13%
49582
SeeingDay
12%12%23%30%16%4
6751
Night
13%5%14%29%30%7%
16201

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
19%5%25%422345%42%
1699
16th to endDay
15%7%245%25%345%42%
1872
February1st to 15thDay
16%5%24423345%46%
1711
16th to endDay
18%6%24414448%40%
1664
March1st to 15thDay
18%6%35%5%3445%7%34%
2145
16th to endDay
17%425%423447%41%
2288
April1st to 15thDay
14%6%35%5%24447%42%
2329
16th to endDay
8%313223349%57%
2214
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%
2160
16th to endDay
4212111336%70%
1971
October1st to 15thDay
8%312223345%63%
1800
16th to endDay
10%414212346%56%
1920
November1st to 15thDay
10%41443336%6%50%
1843
16th to endDay
15%6%34434335%46%
1592
December1st to 15thDay
20%7%33422435%43%
1560
16th to endDay
20%7%25%423345%39%
1664
January1st to 15thNight
17%5%2342445%7%40%
2567
16th to endNight
18%5%13314346%45%
2704
February1st to 15thNight
16%5%34423446%45%
2471
16th to endNight
16%5%2432345%7%45%
2093
March1st to 15thNight
17%5%35%435%5%5%6%37%
2145
16th to endNight
16%5%24323345%46%
2288
April1st to 15thNight
11%5%25%424446%48%
1949
16th to endNight
7%324423336%60%
1860
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%
1800
16th to endNight
3412212226%68%
1985
October1st to 15thNight
6%423213236%62%
2160
16th to endNight
9%324323347%54%
2304
November1st to 15thNight
10%5%2342335%8%50%
2345
16th to endNight
15%5%24323346%48%
2697
December1st to 15thNight
22%424323345%42%
2730
16th to endNight
20%5%14323346%43%
2912

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
44%411%17%13%8%
1699
16th to endDay
44%5%8%13%16%11%
1872
February1st to 15thDay
40%5%8%19%20%5%
1711
16th to endDay
43%5%10%24%13%3
1664
March1st to 15thDay
49%6%11%16%12%4
2145
16th to endDay
43%6%13%20%14%2
2288
April1st to 15thDay
42%414%21%14%1
2329
16th to endDay
26%6%14%26%21%4
2214
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%
2160
16th to endDay
17%37%26%28%17%
1971
October1st to 15thDay
23%38%19%26%18%
1800
16th to endDay
29%410%22%22%10%
1920
November1st to 15thDay
33%6%13%28%12%6%
1843
16th to endDay
41%5%10%19%16%6%
1592
December1st to 15thDay
43%5%9%14%18%7%
1560
16th to endDay
47%48%17%14%6%
1664
January1st to 15thNight
41%6%12%18%13%8%
2567
16th to endNight
40%49%15%17%13%
2704
February1st to 15thNight
40%49%17%18%9%
2483
16th to endNight
38%6%13%22%16%3
2093
March1st to 15thNight
46%6%12%15%14%4
2145
16th to endNight
40%411%22%17%4
2288
April1st to 15thNight
36%6%14%20%18%3
1949
16th to endNight
27%49%27%23%7%
1860
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%
1800
16th to endNight
20%2423%28%20%
1985
October1st to 15thNight
25%27%17%24%21%
2160
16th to endNight
30%37%21%22%14%
2304
November1st to 15thNight
32%5%13%27%13%6%
2345
16th to endNight
37%48%22%19%7%
2697
December1st to 15thNight
43%47%16%18%9%
2730
16th to endNight
43%5%9%17%18%6%
2912

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
30%10%21%18%17%2
238
16th to endDay
18%13%22%27%15%3
304
February1st to 15thDay
20%9%22%25%17%3
281
16th to endDay
20%16%25%24%11%1
279
March1st to 15thDay
26%18%24%17%9%3
330
16th to endDay
18%19%33%19%6%2
352
April1st to 15thDay
17%27%25%20%6%2
329
16th to endDay
13%25%28%22%5%4
288
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%
315
16th to endDay
49%20%31%26%6%
315
October1st to 15thDay
7%12%25%28%18%7%
315
16th to endDay
15%9%18%26%23%6%
336
November1st to 15thDay
13%11%19%23%25%6%
259
16th to endDay
12%12%31%24%14%4
97
December1st to 15thDay
26%11%16%15%16%13%
90
16th to endDay
15%15%9%27%23%8%
96
January1st to 15thNight
19%410%31%26%6%
780
16th to endNight
21%411%30%25%5%
832
February1st to 15thNight
18%6%12%30%28%3
764
16th to endNight
18%8%20%28%19%4
684
March1st to 15thNight
20%8%13%26%25%4
780
16th to endNight
18%7%22%28%19%3
832
April1st to 15thNight
13%11%25%27%18%3
779
16th to endNight
9%12%22%25%24%5%
744
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%
720
16th to endNight
5%310%28%37%14%
716
October1st to 15thNight
9%312%30%33%10%
720
16th to endNight
11%39%23%37%13%
768
November1st to 15thNight
12%313%29%31%8%
760
16th to endNight
15%6%12%28%27%8%
780
December1st to 15thNight
24%412%26%24%7%
780
16th to endNight
23%6%16%29%21%2
832
Page generated at 2018-04-24 07:03:00. Version ac04

Copyright 2018 A. Danko


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