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 2017-04-24.

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%
14908
25%
45643
21%
14871
63%
14871
33%
26478

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
50%
696
26%
1740
11%
696
56%
696
40%
1027
May1st to 15thNight
55%
505
28%
1495
16%
505
63%
505
43%
795
May16th to endNight
56%
380
24%
1407
17%
380
66%
380
35%
680
June1st to 15thNight
57%
330
25%
1320
24%
330
66%
330
32%
645
June16th to endNight
74%
330
32%
1320
38%
330
80%
330
54%
492
July1st to 15thNight
79%
365
29%
1295
32%
365
81%
365
50%
554
July16th to endNight
73%
522
26%
1467
24%
489
73%
489
38%
786
August1st to 15thNight
71%
493
29%
1476
25%
490
72%
490
42%
764
August16th to endNight
75%
677
30%
1727
19%
677
76%
677
50%
959
September1st to 15thNight
78%
660
37%
1650
25%
660
79%
660
56%
873
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%
45964
Night
11%5%24323346%52%
45832
TransparencyDay
30%410%20%22%12%
45653
Night
33%49%19%20%12%
45644
SeeingDay
12%12%25%30%15%3
5462
Night
13%5%14%29%30%7%
14919

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%
2079
May1st to 15thDay
6%41221235%7%62%
2122
16th to endDay
7%413223347%58%
2352
June1st to 15thDay
6%413313337%61%
2310
16th to endDay
2211112225%75%
2310
July1st to 15thDay
1412222235%72%
2248
16th to endDay
15%12212236%70%
2286
August1st to 15thDay
413212136%70%
2145
16th to endDay
15%23212235%68%
2121
September1st to 15thDay
15%13211236%69%
1980
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%324313236%59%
1740
May1st to 15thNight
442432335%6%60%
1495
16th to endNight
3423413336%62%
1406
June1st to 15thNight
442431445%8%57%
1320
16th to endNight
411111225%75%
1320
July1st to 15thNight
223332336%68%
1380
16th to endNight
16%2443445%9%53%
1575
August1st to 15thNight
16%25%433338%57%
1485
16th to endNight
5%24323449%58%
1727
September1st to 15thNight
15%13313348%63%
1650
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%15%27%20%4
2079
May1st to 15thDay
22%411%27%26%8%
2122
16th to endDay
26%417%26%19%5%
2356
June1st to 15thDay
25%5%11%21%23%12%
2310
16th to endDay
14%36%16%37%21%
2310
July1st to 15thDay
16%15%14%29%32%
2104
16th to endDay
19%26%14%24%32%
2131
August1st to 15thDay
17%16%16%30%26%
2129
16th to endDay
20%16%18%30%21%
2121
September1st to 15thDay
18%16%15%30%27%
1980
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%28%22%7%
1740
May1st to 15thNight
24%48%19%29%12%
1496
16th to endNight
24%5%12%29%21%7%
1407
June1st to 15thNight
27%410%22%20%14%
1320
16th to endNight
13%2416%35%25%
1320
July1st to 15thNight
19%1410%31%32%
1295
16th to endNight
31%16%13%17%28%
1467
August1st to 15thNight
28%15%14%23%27%
1476
16th to endNight
24%26%16%26%23%
1727
September1st to 15thNight
20%1416%29%27%
1650
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
13%27%28%22%5%3
243
May1st to 15thDay
11%20%33%26%7%
228
16th to endDay
13%15%24%27%15%4
266
June1st to 15thDay
7%16%34%34%6%1
255
16th to endDay
27%32%36%16%4
255
July1st to 15thDay
3320%46%21%3
245
16th to endDay
2116%55%19%4
271
August1st to 15thDay
2425%44%18%3
255
16th to endDay
4229%41%19%3
268
September1st to 15thDay
4420%40%21%9%
255
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
10%12%21%25%23%6%
696
May1st to 15thNight
6%223%31%31%4
506
16th to endNight
5%8%22%29%28%5%
381
June1st to 15thNight
6%5%22%39%25%1
330
16th to endNight
4315%37%36%2
330
July1st to 15thNight
317%31%47%8%
365
16th to endNight
6%5%27%46%13%
522
August1st to 15thNight
48%31%44%10%
493
16th to endNight
46%33%46%7%
677
September1st to 15thNight
315%27%48%13%
660
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 2017-04-25 04:48:28. Version ac04

Copyright 2017 A. Danko


Occidental Observatory Clear Sky Chart | Charts Home