Plum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plum clear sky chart from 2005-11-06 to 2024-01-14.

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
35%
23343
15%
74866
10%
23294
41%
23294
18%
53157

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
14%
757
11%
2483
4%
757
22%
757
9%
1990
January11th to 20thNight
14%
735
6%
2386
2%
735
20%
735
8%
1964
January21st to endNight
17%
792
9%
2574
4%
792
22%
792
10%
2060
February1st to 10thNight
20%
720
10%
2340
6%
720
25%
720
11%
1840
February11th to 20thNight
14%
702
9%
2275
4%
700
21%
700
10%
1760
February21st to endNight
15%
592
13%
1772
5%
592
24%
592
12%
1381
March1st to 10thNight
24%
720
16%
1980
8%
720
32%
720
18%
1439
March11th to 20thNight
25%
720
17%
1980
10%
720
33%
720
19%
1423
March21st to endNight
24%
792
17%
2178
8%
792
32%
792
19%
1594
April1st to 10thNight
26%
720
14%
1872
6%
720
31%
720
20%
1354
April11th to 20thNight
32%
711
16%
1775
7%
711
37%
711
26%
1251
April21st to endNight
32%
545
18%
1625
8%
545
38%
545
22%
1108
May1st to 10thNight
38%
540
14%
1620
9%
540
43%
540
19%
1161
May11th to 20thNight
39%
531
15%
1593
10%
531
44%
531
20%
1121
May21st to endNight
56%
410
17%
1573
15%
410
62%
410
26%
913
June1st to 10thNight
54%
360
16%
1440
18%
360
61%
360
23%
854
June11th to 20thNight
49%
360
16%
1440
18%
360
55%
360
21%
885
June21st to endNight
53%
360
18%
1440
19%
360
61%
360
23%
852
July1st to 10thNight
58%
385
14%
1435
16%
385
61%
385
24%
885
July11th to 20thNight
61%
522
17%
1566
15%
522
64%
522
34%
938
July21st to endNight
59%
591
16%
1672
13%
558
62%
558
26%
1086
August1st to 10thNight
64%
537
17%
1600
14%
534
65%
534
29%
992
August11th to 20thNight
62%
534
20%
1598
15%
534
65%
534
29%
1022
August21st to endNight
59%
708
20%
1896
13%
708
61%
708
32%
1233
September1st to 10thNight
58%
720
23%
1800
14%
720
62%
720
35%
1171
September11th to 20thNight
63%
720
26%
1948
20%
720
66%
720
38%
1178
September21st to endNight
44%
720
21%
1993
16%
720
49%
720
27%
1336
October1st to 10thNight
46%
720
18%
2160
17%
720
49%
720
25%
1404
October11th to 20thNight
36%
720
17%
2160
12%
720
43%
720
22%
1473
October21st to endNight
33%
792
19%
2376
13%
792
40%
792
19%
1673
November1st to 10thNight
40%
740
22%
2273
18%
740
47%
740
25%
1456
November11th to 20thNight
28%
760
16%
2617
12%
760
37%
760
16%
1824
November21st to endNight
25%
760
15%
2660
9%
760
34%
760
14%
1913
December1st to 10thNight
18%
760
7%
2660
5%
760
21%
760
8%
2121
December11th to 20thNight
18%
759
9%
3248
6%
759
24%
759
8%
2483
December21st to endNight
18%
828
6%
4858
5%
817
24%
817
4%
4019

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
17%10%47%6%35%5%5%7%31%
74617
Night
17%10%47%5%35%45%7%33%
75131
TransparencyDay
51%7%12%18%10%1
74335
Night
51%6%12%19%11%2
74932
SeeingDay
22%14%26%21%12%4
13584
Night
22%11%26%28%12%1
23358

By Month

Percentage Cloud Cover SE (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
19%16%8%12%7%3434420%
1677
11th to 20thDay
18%21%7%12%8%3333417%
1647
21st to endDay
19%19%8%11%8%4433417%
1782
February1st to 10thDay
21%15%7%10%8%35%33419%
1620
11th to 20thDay
17%15%8%12%5%35%5%45%20%
1573
21st to endDay
22%14%5%10%6%34445%24%
1479
March1st to 10thDay
17%11%5%8%6%3445%6%31%
1980
11th to 20thDay
20%12%5%7%7%35%445%28%
1980
21st to endDay
21%11%5%8%6%3445%7%27%
2178
April1st to 10thDay
16%10%47%6%45%5%5%7%31%
2088
11th to 20thDay
17%10%46%6%35%5%5%6%32%
2142
21st to endDay
22%9%36%5%34447%33%
2340
May1st to 10thDay
21%8%36%5%46%5%7%7%29%
2340
11th to 20thDay
22%7%36%7%45%5%5%7%30%
2301
21st to endDay
13%5%25%6%36%6%7%9%37%
2699
June1st to 10thDay
14%7%35%6%47%5%7%9%34%
2520
11th to 20thDay
14%6%35%6%36%6%7%9%34%
2520
21st to endDay
14%5%36%6%45%5%6%11%36%
2520
July1st to 10thDay
11%6%36%6%36%6%5%8%39%
2478
11th to 20thDay
13%6%35%5%35%5%7%11%37%
2340
21st to endDay
13%6%37%6%46%6%6%9%34%
2547
August1st to 10thDay
12%6%26%6%46%7%8%10%33%
2327
11th to 20thDay
14%6%35%6%46%6%7%10%34%
2303
21st to endDay
14%5%25%5%45%6%7%9%38%
2440
September1st to 10thDay
17%7%245%345%5%7%42%
2160
11th to 20thDay
13%6%25%5%345%5%8%44%
2008
21st to endDay
19%8%37%5%34447%37%
1967
October1st to 10thDay
18%6%25%6%3446%8%38%
1800
11th to 20thDay
20%8%35%5%45%5%5%6%34%
1800
21st to endDay
24%10%26%5%3445%6%32%
1980
November1st to 10thDay
13%9%36%6%4445%6%41%
1800
11th to 20thDay
18%13%46%5%34446%32%
1559
21st to endDay
20%14%6%7%5%3445%427%
1520
December1st to 10thDay
22%17%7%11%7%34345%18%
1520
11th to 20thDay
21%17%8%9%5%33346%21%
1910
21st to endDay
25%22%7%8%6%3334414%
2772
January1st to 10thNight
19%17%8%9%6%45%44420%
2483
11th to 20thNight
21%16%6%11%7%3445%419%
2386
21st to endNight
19%15%7%10%7%3444422%
2574
February1st to 10thNight
21%13%6%9%7%35%446%23%
2340
11th to 20thNight
19%13%5%11%6%35%445%24%
2273
21st to endNight
21%13%5%9%6%34445%26%
1772
March1st to 10thNight
18%10%47%6%34445%34%
1980
11th to 20thNight
20%11%46%433447%34%
1980
21st to endNight
19%11%46%6%35%435%34%
2178
April1st to 10thNight
19%11%45%434445%37%
1872
11th to 20thNight
17%7%27%5%35%46%7%38%
1775
21st to endNight
17%8%37%5%35%45%7%36%
1625
May1st to 10thNight
20%7%36%6%35%5%6%6%33%
1620
11th to 20thNight
19%7%46%5%345%47%36%
1592
21st to endNight
10%6%25%435%5%6%10%45%
1572
June1st to 10thNight
11%6%25%5%45%45%9%44%
1440
11th to 20thNight
12%5%36%6%246%6%9%40%
1440
21st to endNight
12%5%245%45%5%6%10%42%
1440
July1st to 10thNight
9%6%25%6%36%5%6%9%43%
1477
11th to 20thNight
10%425%435%46%10%46%
1620
21st to endNight
11%6%35%5%45%6%7%10%38%
1771
August1st to 10thNight
7%6%25%5%46%6%8%9%43%
1611
11th to 20thNight
11%6%246%45%5%6%9%43%
1598
21st to endNight
11%6%24425%6%6%9%44%
1896
September1st to 10thNight
13%5%144345%6%7%48%
1800
11th to 20thNight
10%43342446%8%52%
1948
21st to endNight
16%7%25%6%2445%7%42%
1993
October1st to 10thNight
15%7%25%5%25%45%7%43%
2160
11th to 20thNight
19%6%35%5%45%5%5%6%38%
2160
21st to endNight
25%8%35%434347%35%
2376
November1st to 10thNight
13%9%35%5%24447%44%
2273
11th to 20thNight
19%11%45%5%35%447%32%
2617
21st to endNight
20%14%45%6%34446%30%
2660
December1st to 10thNight
23%16%7%9%6%34346%19%
2660
11th to 20thNight
20%16%6%9%7%34435%24%
3285
21st to endNight
26%18%7%8%7%34445%15%
4884

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
69%5%11%11%4
1678
11th to 20thDay
73%411%11%2
1647
21st to endDay
71%5%10%10%3
1782
February1st to 10thDay
70%39%12%5%1
1620
11th to 20thDay
66%5%11%13%5%
1582
21st to endDay
62%5%11%14%7%1
1479
March1st to 10thDay
53%6%13%20%8%
1980
11th to 20thDay
57%6%11%17%8%2
1980
21st to endDay
57%6%10%16%10%2
2178
April1st to 10thDay
51%6%12%18%11%2
2088
11th to 20thDay
50%7%11%15%14%2
2142
21st to endDay
52%6%10%20%9%2
2340
May1st to 10thDay
51%8%12%19%9%2
2340
11th to 20thDay
53%8%11%15%11%2
2305
21st to endDay
39%11%19%21%8%2
2699
June1st to 10thDay
44%9%14%21%10%1
2520
11th to 20thDay
44%10%14%20%11%1
2520
21st to endDay
42%8%12%25%11%1
2520
July1st to 10thDay
42%9%12%20%14%3
2410
11th to 20thDay
39%8%16%23%13%1
2271
21st to endDay
44%8%14%20%13%1
2405
August1st to 10thDay
42%11%16%23%8%
2314
11th to 20thDay
41%10%15%23%10%1
2303
21st to endDay
39%9%14%22%12%3
2440
September1st to 10thDay
40%8%12%20%17%4
2160
11th to 20thDay
37%7%13%27%14%1
2008
21st to endDay
46%5%11%22%14%1
1967
October1st to 10thDay
43%7%14%20%15%1
1800
11th to 20thDay
49%7%12%22%10%
1800
21st to endDay
53%5%13%18%10%1
1980
November1st to 10thDay
43%6%12%20%16%3
1800
11th to 20thDay
52%6%11%18%10%2
1559
21st to endDay
59%6%12%17%5%1
1520
December1st to 10thDay
70%410%11%4
1520
11th to 20thDay
65%5%11%15%3
1906
21st to endDay
73%5%9%10%3
2772
January1st to 10thNight
67%5%9%14%5%1
2483
11th to 20thNight
69%5%11%12%3
2386
21st to endNight
66%5%10%14%5%1
2574
February1st to 10thNight
64%5%10%15%6%
2340
11th to 20thNight
62%5%11%15%7%
2275
21st to endNight
62%5%9%15%9%1
1772
March1st to 10thNight
51%6%11%20%12%1
1980
11th to 20thNight
50%6%12%21%9%2
1980
21st to endNight
53%5%8%17%13%3
2178
April1st to 10thNight
50%5%10%19%14%2
1872
11th to 20thNight
45%6%12%18%15%3
1775
21st to endNight
46%6%10%23%11%4
1625
May1st to 10thNight
50%7%10%17%12%3
1620
11th to 20thNight
48%7%11%17%14%3
1593
21st to endNight
34%9%16%26%13%1
1573
June1st to 10thNight
36%9%13%25%15%2
1440
11th to 20thNight
39%8%14%23%13%1
1440
21st to endNight
36%9%13%24%16%2
1440
July1st to 10thNight
38%9%13%20%15%4
1435
11th to 20thNight
31%8%17%24%19%2
1566
21st to endNight
38%10%16%21%13%3
1672
August1st to 10thNight
34%10%18%23%15%
1600
11th to 20thNight
36%9%15%24%14%2
1598
21st to endNight
36%8%14%24%14%4
1896
September1st to 10thNight
33%9%12%23%19%4
1800
11th to 20thNight
30%8%14%26%22%1
1948
21st to endNight
42%6%12%21%17%1
1993
October1st to 10thNight
40%8%12%24%14%2
2160
11th to 20thNight
45%7%11%23%13%1
2160
21st to endNight
50%5%11%20%12%1
2376
November1st to 10thNight
41%5%11%22%17%4
2273
11th to 20thNight
52%5%11%19%11%2
2617
21st to endNight
55%5%11%21%7%1
2660
December1st to 10thNight
66%6%11%12%4
2660
11th to 20thNight
64%411%17%3
3288
21st to endNight
71%7%10%10%2
4884

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
29%26%27%12%42
256
11th to 20thDay
32%27%29%8%21
249
21st to endDay
27%32%28%8%31
264
February1st to 10thDay
35%22%35%8%1
240
11th to 20thDay
34%30%29%7%
240
21st to endDay
36%24%31%8%2
240
March1st to 10thDay
27%20%33%15%4
420
11th to 20thDay
32%20%30%12%5%1
420
21st to endDay
30%19%29%16%5%1
462
April1st to 10thDay
27%20%25%17%8%1
420
11th to 20thDay
21%21%28%16%9%4
412
21st to endDay
30%17%27%16%7%2
420
May1st to 10thDay
30%15%27%15%9%3
423
11th to 20thDay
24%12%28%21%11%4
447
21st to endDay
16%8%24%26%18%7%
485
June1st to 10thDay
16%9%28%27%16%3
450
11th to 20thDay
18%12%25%25%16%4
450
21st to endDay
14%9%29%30%15%3
450
July1st to 10thDay
13%7%19%28%20%13%
445
11th to 20thDay
11%7%27%28%18%9%
444
21st to endDay
15%5%23%30%19%7%
488
August1st to 10thDay
14%5%19%29%23%9%
446
11th to 20thDay
16%5%19%26%23%11%
439
21st to endDay
12%6%24%26%21%11%
495
September1st to 10thDay
16%8%20%25%20%10%
450
11th to 20thDay
15%417%30%23%11%
450
21st to endDay
23%11%23%22%14%6%
450
October1st to 10thDay
18%9%26%23%20%4
450
11th to 20thDay
23%16%27%24%9%1
450
21st to endDay
31%16%28%17%8%1
495
November1st to 10thDay
21%15%28%22%11%3
398
11th to 20thDay
29%23%26%14%6%2
198
21st to endDay
23%24%39%9%31
180
December1st to 10thDay
31%26%27%10%5%1
180
11th to 20thDay
32%21%32%11%4
180
21st to endDay
31%25%26%14%4
198
January1st to 10thNight
28%22%29%17%4
760
11th to 20thNight
32%16%31%19%2
735
21st to endNight
28%18%30%21%3
792
February1st to 10thNight
30%16%28%23%2
720
11th to 20thNight
28%19%36%16%1
703
21st to endNight
29%15%35%19%2
592
March1st to 10thNight
23%17%32%24%4
720
11th to 20thNight
27%16%29%21%7%1
720
21st to endNight
24%14%33%22%5%1
792
April1st to 10thNight
25%17%30%23%6%
720
11th to 20thNight
21%11%33%24%10%1
711
21st to endNight
19%11%33%30%7%
545
May1st to 10thNight
21%11%22%36%9%1
540
11th to 20thNight
21%8%28%33%10%
531
21st to endNight
11%323%36%25%3
412
June1st to 10thNight
13%423%42%18%1
360
11th to 20thNight
14%424%38%19%1
360
21st to endNight
12%327%38%19%1
360
July1st to 10thNight
9%320%42%21%5%
385
11th to 20thNight
10%321%33%28%5%
522
21st to endNight
11%416%39%28%2
591
August1st to 10thNight
8%315%43%29%2
537
11th to 20thNight
13%413%38%30%4
534
21st to endNight
11%420%33%28%5%
708
September1st to 10thNight
14%418%34%26%4
720
11th to 20thNight
11%416%38%27%4
720
21st to endNight
18%8%22%28%21%3
720
October1st to 10thNight
18%7%22%30%21%1
720
11th to 20thNight
23%9%28%29%10%1
720
21st to endNight
28%13%23%28%7%1
792
November1st to 10thNight
17%11%26%32%12%
740
11th to 20thNight
26%15%27%24%6%2
760
21st to endNight
28%14%29%23%6%1
760
December1st to 10thNight
32%17%28%19%5%
760
11th to 20thNight
28%18%29%21%4
760
21st to endNight
30%16%30%22%2
836
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