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 2017-03-21.

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
37%
15308
15%
45411
10%
15266
42%
15266
21%
31603

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
33%
728
18%
2002
11%
728
39%
728
22%
1392
April1st to 15thNight
29%
541
15%
1664
7%
541
38%
541
19%
1138
April16th to endNight
37%
495
19%
1485
8%
495
47%
495
24%
992
May1st to 15thNight
37%
495
15%
1485
8%
495
44%
495
21%
1032
May16th to endNight
51%
514
15%
1540
11%
513
56%
513
29%
964
June1st to 15thNight
53%
495
16%
1325
11%
495
57%
495
30%
866
June16th to endNight
60%
495
15%
1320
11%
495
64%
495
34%
845
July1st to 15thNight
64%
465
18%
1240
15%
465
67%
465
39%
738
July16th to endNight
67%
522
17%
1389
15%
489
69%
489
38%
820
August1st to 15thNight
65%
494
21%
1474
16%
491
69%
491
35%
878
August16th to endNight
68%
525
21%
1594
18%
525
70%
525
34%
944
September1st to 15thNight
58%
613
26%
1768
20%
613
62%
613
35%
1041
September16th to endNight
59%
657
24%
1815
18%
657
63%
657
33%
1131
October1st to 15thNight
47%
660
21%
1895
17%
660
52%
660
28%
1226
October16th to endNight
25%
704
15%
2203
9%
704
34%
704
17%
1545
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%37%6%35%45%7%32%
45434
Night
15%8%36%5%3445%6%35%
45590
Cloud CoverDay
14%8%36%6%35%45%7%32%
45444
Night
15%8%46%5%3445%6%35%
45594
TransparencyDay
49%6%11%19%10%1
45131
Night
48%5%12%20%11%1
45411
SeeingDay
17%7%25%25%16%7%
5063
Night
19%8%27%29%12%1
15312

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
16%11%48%5%34446%30%
1992
April1st to 15thDay
19%11%45%5%34436%31%
1963
16th to endDay
17%7%36%423446%38%
2145
May1st to 15thDay
19%8%36%6%45%446%29%
2136
16th to endDay
13%6%16%424448%41%
2223
June1st to 15thDay
11%5%25%6%45%5%6%8%37%
2303
16th to endDay
10%5%26%5%36%6%8%10%34%
2310
July1st to 15thDay
8%425%5%35%5%6%8%44%
2310
16th to endDay
9%4246%45%6%7%10%37%
2366
August1st to 15thDay
8%427%6%46%5%7%8%38%
2145
16th to endDay
11%425%6%26%6%7%8%38%
2253
September1st to 15thDay
12%5%345%25%45%7%40%
1851
16th to endDay
12%5%25%4345%5%9%40%
1815
October1st to 15thDay
15%5%26%5%3435%7%38%
1733
16th to endDay
22%9%16%5%35%35%5%31%
1668
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
14%7%26%5%35%5%5%7%36%
2002
April1st to 15thNight
17%8%35%5%3446%6%34%
1664
16th to endNight
16%7%244245%5%5%39%
1485
May1st to 15thNight
17%6%36%43446%7%34%
1485
16th to endNight
12%5%245%3345%7%46%
1538
June1st to 15thNight
11%426%425%46%8%43%
1325
16th to endNight
8%5%25%425%5%6%10%43%
1320
July1st to 15thNight
7%324324448%53%
1320
16th to endNight
4324425%5%6%9%51%
1493
August1st to 15thNight
5%4135%35%5%5%8%49%
1485
16th to endNight
10%32342435%7%51%
1594
September1st to 15thNight
11%5%2332335%7%51%
1768
16th to endNight
11%5%2443345%7%47%
1815
October1st to 15thNight
15%5%35%4135%46%43%
1895
16th to endNight
21%7%35%43345%5%34%
2203
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
16%10%48%5%34446%30%
1991
April1st to 15thDay
19%10%46%434436%30%
1963
16th to endDay
18%6%36%5%23447%38%
2145
May1st to 15thDay
19%8%35%7%3445%6%30%
2136
16th to endDay
13%6%25%5%2445%7%41%
2223
June1st to 15thDay
11%5%25%6%45%5%6%8%37%
2303
16th to endDay
10%5%25%6%36%6%8%10%34%
2310
July1st to 15thDay
8%4245%35%5%6%7%44%
2310
16th to endDay
9%5%25%6%46%5%7%10%38%
2366
August1st to 15thDay
8%5%35%7%46%5%6%8%39%
2145
16th to endDay
11%425%5%35%5%7%8%39%
2253
September1st to 15thDay
13%5%25%5%3445%7%41%
1851
16th to endDay
13%5%25%4345%5%9%40%
1815
October1st to 15thDay
15%6%25%5%3445%7%39%
1733
16th to endDay
21%9%26%5%3435%5%31%
1668
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
14%6%36%5%35%5%5%7%36%
2002
April1st to 15thNight
17%8%35%5%2445%6%34%
1664
16th to endNight
16%7%244245%5%6%39%
1485
May1st to 15thNight
17%5%36%43445%8%34%
1484
16th to endNight
12%5%245%2335%7%45%
1538
June1st to 15thNight
11%425%4245%6%8%43%
1325
16th to endNight
8%4244345%7%9%43%
1320
July1st to 15thNight
7%224424447%54%
1320
16th to endNight
4324425%5%5%10%51%
1493
August1st to 15thNight
5%4135%35%5%6%9%49%
1485
16th to endNight
10%41342335%6%52%
1594
September1st to 15thNight
11%5%2332336%6%51%
1768
16th to endNight
11%5%2442345%6%48%
1815
October1st to 15thNight
15%5%35%3245%46%43%
1895
16th to endNight
21%7%35%5%2445%5%34%
2203
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%5%11%20%7%1
1992
April1st to 15thDay
54%5%9%18%11%
1963
16th to endDay
46%412%18%15%2
2145
May1st to 15thDay
54%5%11%18%9%1
2136
16th to endDay
42%5%11%24%13%2
2227
June1st to 15thDay
41%7%12%22%13%1
2303
16th to endDay
41%7%14%19%16%1
2310
July1st to 15thDay
35%7%10%23%19%3
2161
16th to endDay
38%8%14%21%14%2
2207
August1st to 15thDay
40%7%12%24%13%1
2132
16th to endDay
39%8%11%22%15%2
2253
September1st to 15thDay
40%6%13%23%14%1
1851
16th to endDay
38%6%11%30%13%
1815
October1st to 15thDay
43%5%12%23%12%1
1733
16th to endDay
53%5%13%18%7%
1668
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%12%23%10%1
2002
April1st to 15thNight
48%5%11%19%14%
1664
16th to endNight
44%5%10%18%18%3
1485
May1st to 15thNight
47%7%11%20%12%1
1485
16th to endNight
38%5%13%23%15%3
1540
June1st to 15thNight
38%7%12%25%15%1
1325
16th to endNight
34%7%15%23%17%1
1320
July1st to 15thNight
29%6%10%28%22%2
1240
16th to endNight
27%8%15%24%19%4
1389
August1st to 15thNight
30%7%12%29%16%2
1474
16th to endNight
32%6%11%28%18%3
1594
September1st to 15thNight
32%5%10%29%20%1
1768
16th to endNight
35%5%10%29%17%
1815
October1st to 15thNight
40%5%11%23%17%
1895
16th to endNight
50%5%12%23%7%
2203
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
19%10%28%28%10%2
245
April1st to 15thDay
23%9%30%22%13%
210
16th to endDay
23%6%30%17%12%10%
210
May1st to 15thDay
23%7%26%24%10%7%
228
16th to endDay
16%7%19%26%19%9%
267
June1st to 15thDay
14%6%30%31%13%4
413
16th to endDay
13%6%29%28%14%7%
420
July1st to 15thDay
11%5%22%30%21%9%
400
16th to endDay
10%321%30%23%11%
352
August1st to 15thDay
11%115%30%22%18%
253
16th to endDay
15%213%21%29%18%
268
September1st to 15thDay
12%319%20%27%17%
255
16th to endDay
18%314%22%30%10%
254
October1st to 15thDay
18%5%20%27%20%7%
255
16th to endDay
23%10%32%25%7%
180
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
16%9%32%29%9%1
728
April1st to 15thNight
20%13%31%28%5%
541
16th to endNight
21%7%30%30%9%1
495
May1st to 15thNight
19%6%28%33%10%1
495
16th to endNight
15%222%36%19%2
515
June1st to 15thNight
12%223%47%13%
495
16th to endNight
9%220%45%21%1
495
July1st to 15thNight
9%216%40%27%3
465
16th to endNight
4118%48%23%3
522
August1st to 15thNight
7%117%41%27%3
494
16th to endNight
12%111%34%35%4
525
September1st to 15thNight
13%419%28%29%5%
613
16th to endNight
13%416%33%27%4
657
October1st to 15thNight
16%5%25%30%19%3
660
16th to endNight
24%9%34%25%5%
704
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 2017-03-22 02:42:24. Version ac04

Copyright 2017 A. Danko


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