Plymouth Digital Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plymouth Digital Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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
38%
16045
16%
47545
10%
16003
43%
16003
21%
32945

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
16%
900
10%
2506
4%
895
21%
895
13%
1967
January16th to endNight
20%
847
9%
2575
4%
847
25%
847
10%
2038
February1st to 15thNight
18%
701
9%
2258
3%
701
24%
701
11%
1731
February16th to endNight
24%
631
14%
1893
6%
631
32%
631
16%
1395
March1st to 15thNight
27%
720
14%
2076
8%
720
33%
720
17%
1516
March16th to endNight
32%
768
18%
2112
11%
768
39%
768
22%
1481
April1st to 15thNight
30%
590
14%
1815
7%
590
37%
590
19%
1250
April16th to endNight
36%
540
20%
1620
8%
540
46%
540
24%
1082
May1st to 15thNight
37%
540
14%
1620
8%
540
43%
540
21%
1124
May16th to endNight
49%
562
15%
1684
11%
561
55%
561
29%
1064
June1st to 15thNight
56%
540
15%
1445
11%
540
60%
540
32%
925
June16th to endNight
59%
540
17%
1440
12%
540
64%
540
35%
915
July1st to 15thNight
64%
510
17%
1360
14%
510
67%
510
39%
814
July16th to endNight
68%
564
19%
1509
17%
531
71%
531
39%
879
August1st to 15thNight
66%
539
20%
1609
16%
536
69%
536
35%
957
August16th to endNight
67%
573
23%
1740
20%
573
70%
573
34%
1033
September1st to 15thNight
59%
669
25%
1929
19%
669
63%
669
35%
1131
September16th to endNight
60%
717
26%
1980
20%
717
64%
717
34%
1221
October1st to 15thNight
47%
720
19%
2068
16%
720
51%
720
27%
1355
October16th to endNight
27%
768
17%
2402
11%
768
35%
768
18%
1663
November1st to 15thNight
36%
700
20%
2276
16%
700
43%
700
23%
1475
November16th to endNight
25%
720
14%
2372
9%
720
32%
720
14%
1717
December1st to 15thNight
20%
726
8%
2526
4%
726
25%
726
11%
1936
December16th to endNight
15%
960
8%
2730
2%
960
20%
960
9%
2276

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
14%8%36%6%35%45%7%33%
48192
Night
15%8%36%5%3445%6%36%
47724
Cloud CoverDay
14%8%36%6%35%45%7%33%
48202
Night
15%8%36%5%3445%6%36%
47728
TransparencyDay
49%6%11%20%11%1
47889
Night
47%5%11%21%12%1
47545
SeeingDay
16%7%26%25%16%7%
5996
Night
19%7%27%30%13%1
16049

By Month

Percentage Cloud Cover NE (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%14%5%10%7%5%4446%16%
1432
16th to endDay
15%16%9%8%8%45%45%5%15%
1647
February1st to 15thDay
14%15%6%9%7%5%5%5%45%20%
1604
16th to endDay
15%14%5%11%7%35%5%5%6%21%
1580
March1st to 15thDay
16%12%5%8%5%34445%28%
1873
16th to endDay
17%10%48%5%34446%29%
2112
April1st to 15thDay
19%10%45%5%35%346%31%
2143
16th to endDay
17%7%36%5%23446%37%
2340
May1st to 15thDay
19%8%35%6%34446%31%
2331
16th to endDay
13%6%26%5%24448%39%
2431
June1st to 15thDay
10%5%25%6%45%5%6%8%39%
2513
16th to endDay
10%5%26%5%36%7%8%10%34%
2520
July1st to 15thDay
7%425%5%35%5%6%8%44%
2520
16th to endDay
9%425%6%45%6%7%10%38%
2554
August1st to 15thDay
7%427%6%46%5%7%8%38%
2340
16th to endDay
11%425%6%26%6%7%8%38%
2459
September1st to 15thDay
12%5%346%35%46%8%40%
2019
16th to endDay
12%5%25%4345%5%9%42%
1980
October1st to 15thDay
16%5%26%5%3435%7%38%
1890
16th to endDay
21%8%26%5%35%35%5%32%
1819
November1st to 15thDay
14%8%25%6%25%35%6%38%
1582
16th to endDay
18%12%37%435%5%45%27%
1584
December1st to 15thDay
17%12%7%10%7%35%35%6%21%
1431
16th to endDay
22%15%8%10%6%23436%14%
1488
January1st to 15thNight
21%13%5%7%6%34445%23%
2506
16th to endNight
14%13%7%10%8%44445%21%
2575
February1st to 15thNight
16%13%5%9%6%35%435%26%
2256
16th to endNight
16%10%5%8%6%34345%30%
1893
March1st to 15thNight
18%9%37%6%34446%32%
2064
16th to endNight
15%7%26%5%35%5%5%7%36%
2112
April1st to 15thNight
18%8%35%5%3445%6%34%
1815
16th to endNight
15%7%244246%5%6%39%
1620
May1st to 15thNight
17%6%36%43445%7%34%
1620
16th to endNight
12%5%245%3345%7%45%
1682
June1st to 15thNight
10%325%425%46%8%45%
1445
16th to endNight
8%425%3345%6%9%44%
1440
July1st to 15thNight
7%31432445%8%53%
1440
16th to endNight
4324425%45%9%52%
1613
August1st to 15thNight
44146%35%5%5%8%49%
1620
16th to endNight
10%323424347%51%
1740
September1st to 15thNight
11%5%2332445%7%51%
1929
16th to endNight
11%5%2443345%7%48%
1980
October1st to 15thNight
15%6%25%4135%46%42%
2068
16th to endNight
21%6%35%43445%5%36%
2402
November1st to 15thNight
14%7%344345%47%40%
2276
16th to endNight
22%11%45%42435%6%29%
2372
December1st to 15thNight
21%12%6%8%5%25%345%23%
2526
16th to endNight
23%14%6%10%6%25%446%16%
2730

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 15thDay
18%14%6%10%6%45%446%17%
1432
16th to endDay
16%16%9%8%7%35%446%15%
1647
February1st to 15thDay
15%14%6%9%7%46%5%45%20%
1604
16th to endDay
15%13%5%11%7%345%5%6%21%
1580
March1st to 15thDay
17%11%5%8%6%34446%27%
1884
16th to endDay
17%10%47%5%34446%29%
2111
April1st to 15thDay
19%10%45%434446%31%
2143
16th to endDay
17%7%36%5%23447%37%
2340
May1st to 15thDay
19%8%35%7%3445%6%31%
2331
16th to endDay
13%6%25%5%2445%7%40%
2431
June1st to 15thDay
10%5%25%6%45%5%6%8%39%
2513
16th to endDay
10%5%25%6%36%6%8%10%34%
2520
July1st to 15thDay
8%4245%35%5%6%7%45%
2520
16th to endDay
8%5%25%6%36%5%7%9%39%
2554
August1st to 15thDay
8%5%25%7%46%5%6%8%39%
2340
16th to endDay
11%425%5%35%5%7%8%39%
2459
September1st to 15thDay
12%5%25%5%35%5%6%7%40%
2019
16th to endDay
12%425%4345%5%9%42%
1980
October1st to 15thDay
15%6%26%6%2445%7%38%
1890
16th to endDay
21%9%26%5%3435%5%32%
1819
November1st to 15thDay
14%8%35%5%3445%6%38%
1582
16th to endDay
18%12%37%4445%45%27%
1584
December1st to 15thDay
17%12%6%10%7%35%45%6%21%
1431
16th to endDay
22%16%8%10%6%23346%14%
1488
January1st to 15thNight
21%13%5%7%6%3345%5%23%
2506
16th to endNight
13%13%8%11%7%44445%21%
2575
February1st to 15thNight
15%12%5%9%7%35%445%26%
2249
16th to endNight
16%10%5%8%6%35%345%30%
1893
March1st to 15thNight
18%9%37%5%34346%32%
2076
16th to endNight
15%6%36%5%345%5%7%35%
2112
April1st to 15thNight
18%8%35%5%2445%6%34%
1815
16th to endNight
15%7%25%4245%5%6%39%
1620
May1st to 15thNight
17%6%36%5%3445%8%35%
1619
16th to endNight
12%5%245%2335%7%44%
1682
June1st to 15thNight
10%4244245%6%8%45%
1445
16th to endNight
8%4244345%7%9%44%
1440
July1st to 15thNight
8%224424447%54%
1440
16th to endNight
4324425%5%5%9%52%
1613
August1st to 15thNight
5%4136%35%5%5%9%49%
1620
16th to endNight
10%41442335%6%52%
1740
September1st to 15thNight
10%5%2332346%6%51%
1929
16th to endNight
11%42442345%7%49%
1980
October1st to 15thNight
15%6%35%4245%46%42%
2068
16th to endNight
20%6%35%5%24445%36%
2402
November1st to 15thNight
14%7%345%25%5%5%7%40%
2276
16th to endNight
22%11%45%33345%6%29%
2372
December1st to 15thNight
21%12%6%7%5%35%346%23%
2526
16th to endNight
22%14%6%10%6%35%446%16%
2730

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
67%5%11%11%3
1433
16th to endDay
68%5%10%9%4
1647
February1st to 15thDay
65%6%8%14%5%
1606
16th to endDay
61%6%10%15%5%
1580
March1st to 15thDay
57%5%13%16%6%1
1884
16th to endDay
54%411%20%7%1
2112
April1st to 15thDay
53%5%10%18%11%
2143
16th to endDay
46%412%17%15%2
2340
May1st to 15thDay
52%5%10%19%9%1
2331
16th to endDay
43%5%11%24%12%2
2435
June1st to 15thDay
40%7%12%23%14%1
2513
16th to endDay
41%7%14%19%15%1
2520
July1st to 15thDay
35%7%10%24%19%2
2371
16th to endDay
37%8%13%22%15%2
2395
August1st to 15thDay
40%7%12%25%12%1
2327
16th to endDay
39%8%10%23%15%1
2459
September1st to 15thDay
40%6%13%25%13%
2019
16th to endDay
37%6%11%30%14%
1980
October1st to 15thDay
43%5%11%23%12%1
1890
16th to endDay
52%5%13%18%8%
1819
November1st to 15thDay
46%6%10%22%13%1
1582
16th to endDay
57%5%12%19%5%
1584
December1st to 15thDay
63%413%13%4
1431
16th to endDay
72%5%11%8%2
1488
January1st to 15thNight
62%411%15%5%
2506
16th to endNight
64%412%13%4
2575
February1st to 15thNight
60%5%11%14%6%
2258
16th to endNight
56%411%19%7%
1893
March1st to 15thNight
52%6%10%18%10%1
2076
16th to endNight
47%5%11%23%10%1
2112
April1st to 15thNight
48%5%11%19%14%
1815
16th to endNight
43%6%10%18%17%3
1620
May1st to 15thNight
47%6%10%20%12%2
1620
16th to endNight
38%5%13%23%14%3
1684
June1st to 15thNight
35%7%12%27%16%1
1445
16th to endNight
33%7%14%25%17%1
1440
July1st to 15thNight
29%6%10%29%21%2
1360
16th to endNight
26%8%15%24%21%3
1509
August1st to 15thNight
30%7%12%30%15%2
1609
16th to endNight
32%6%11%27%18%3
1740
September1st to 15thNight
31%5%10%30%20%1
1929
16th to endNight
34%5%10%30%18%
1980
October1st to 15thNight
41%5%11%23%17%
2068
16th to endNight
49%5%11%23%8%
2402
November1st to 15thNight
42%6%11%24%13%1
2276
16th to endNight
55%413%19%6%
2372
December1st to 15thNight
62%413%13%5%
2526
16th to endNight
68%5%10%10%3
2730

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
17%14%41%21%41
90
16th to endDay
27%12%42%14%3
96
February1st to 15thDay
23%21%44%8%2
90
16th to endDay
23%14%39%20%1
81
March1st to 15thDay
29%10%31%21%6%
174
16th to endDay
22%10%28%27%9%2
288
April1st to 15thDay
24%10%30%23%11%
270
16th to endDay
22%10%29%17%11%7%
270
May1st to 15thDay
23%7%26%24%11%6%
288
16th to endDay
17%9%22%25%17%8%
331
June1st to 15thDay
12%5%30%32%14%4
488
16th to endDay
13%7%32%27%13%6%
495
July1st to 15thDay
11%5%23%29%21%8%
475
16th to endDay
9%321%29%23%11%
414
August1st to 15thDay
10%116%28%22%19%
313
16th to endDay
15%313%22%28%17%
332
September1st to 15thDay
10%320%21%26%17%
315
16th to endDay
15%314%19%30%15%
314
October1st to 15thDay
19%6%20%26%19%7%
315
16th to endDay
22%10%30%28%6%1
235
November1st to 15thDay
7%15%26%31%15%3
90
16th to endDay
22%19%33%17%7%
84
December1st to 15thDay
20%21%38%15%5%
60
16th to endDay
27%26%37%9%
88
January1st to 15thNight
28%13%34%18%5%
900
16th to endNight
19%12%35%26%5%
847
February1st to 15thNight
24%12%36%23%3
704
16th to endNight
21%11%32%30%3
631
March1st to 15thNight
23%11%30%27%6%
720
16th to endNight
17%10%31%29%9%1
768
April1st to 15thNight
21%12%31%29%5%
590
16th to endNight
20%7%32%30%8%1
540
May1st to 15thNight
20%5%28%33%10%1
540
16th to endNight
15%224%36%18%2
563
June1st to 15thNight
11%222%48%15%
540
16th to endNight
8%321%45%19%1
540
July1st to 15thNight
10%216%40%27%2
510
16th to endNight
4117%47%25%3
564
August1st to 15thNight
7%117%42%27%4
539
16th to endNight
11%112%34%35%3
573
September1st to 15thNight
12%418%29%29%5%
669
16th to endNight
13%316%31%29%5%
717
October1st to 15thNight
17%5%24%29%18%3
720
16th to endNight
24%9%34%25%5%
768
November1st to 15thNight
16%9%31%26%13%2
700
16th to endNight
28%12%28%21%8%
720
December1st to 15thNight
27%11%33%22%4
726
16th to endNight
30%10%35%20%3
960
Page generated at 2017-11-01 17:23:38. Version ac04

Copyright 2017 A. Danko


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