Portland(ME)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland(ME) clear sky chart from 2005-11-05 to 2017-08-12.

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
27%
14665
17%
46453
8%
14633
38%
14633
18%
30777

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%
720
17%
2506
6%
716
31%
716
15%
1621
January16th to endNight
14%
768
16%
2688
4%
768
30%
768
13%
1740
February1st to 15thNight
14%
704
14%
2236
4%
701
26%
701
12%
1569
February16th to endNight
20%
631
19%
1893
7%
631
35%
631
16%
1291
March1st to 15thNight
16%
720
16%
2130
5%
720
29%
720
15%
1488
March16th to endNight
22%
768
21%
2070
7%
768
34%
768
20%
1381
April1st to 15thNight
22%
718
19%
1796
5%
718
30%
718
20%
1281
April16th to endNight
28%
544
17%
1624
5%
544
36%
544
19%
1107
May1st to 15thNight
29%
540
14%
1620
6%
540
37%
540
17%
1127
May16th to endNight
25%
424
11%
1546
8%
423
32%
423
11%
1113
June1st to 15thNight
31%
360
10%
1440
8%
360
39%
360
11%
1016
June16th to endNight
43%
360
9%
1440
11%
360
47%
360
16%
896
July1st to 15thNight
49%
340
10%
1360
16%
340
52%
340
17%
857
July16th to endNight
48%
376
14%
1418
16%
354
56%
354
19%
874
August1st to 15thNight
52%
507
15%
1556
11%
505
55%
505
23%
1006
August16th to endNight
51%
596
19%
1646
10%
596
54%
596
29%
1040
September1st to 15thNight
46%
660
20%
1730
11%
660
51%
660
31%
1111
September16th to endNight
46%
657
27%
1815
15%
657
56%
657
35%
1056
October1st to 15thNight
33%
660
21%
1859
11%
660
42%
660
23%
1228
October16th to endNight
24%
704
17%
2280
7%
704
37%
704
19%
1499
November1st to 15thNight
27%
700
20%
2277
11%
700
40%
700
20%
1457
November16th to endNight
22%
720
18%
2340
7%
720
35%
720
16%
1523
December1st to 15thNight
14%
720
15%
2495
4%
720
29%
720
14%
1667
December16th to endNight
13%
768
16%
2688
4%
768
28%
768
12%
1829

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%6%24424447%40%
47526
Night
18%7%24424446%39%
46623
Cloud CoverDay
17%6%2442445%7%40%
47536
Night
18%7%2442445%6%39%
46628
TransparencyDay
43%6%12%22%14%1
47214
Night
44%5%11%21%13%1
46453
SeeingDay
20%17%28%21%9%2
5753
Night
19%22%25%22%9%1
14665

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
16%6%34423347%41%
1432
16th to endDay
15%8%243145%6%7%39%
1536
February1st to 15thDay
17%7%25%5%335%5%6%35%
1622
16th to endDay
13%8%36%6%25%436%37%
1580
March1st to 15thDay
20%8%26%424446%36%
1820
16th to endDay
15%7%24423436%43%
2151
April1st to 15thDay
17%6%235%24446%41%
2152
16th to endDay
19%41442345%6%41%
2334
May1st to 15thDay
23%6%25%5%33346%33%
2332
16th to endDay
21%5%244245%5%8%34%
2579
June1st to 15thDay
24%6%24423347%34%
2520
16th to endDay
18%5%25%435%45%7%37%
2520
July1st to 15thDay
13%5%24426%6%5%9%38%
2520
16th to endDay
12%424435%5%6%8%40%
2654
August1st to 15thDay
12%425%425%5%6%9%41%
2295
16th to endDay
11%3135%245%6%8%47%
2201
September1st to 15thDay
13%41432445%6%49%
1900
16th to endDay
13%5%13212345%54%
1815
October1st to 15thDay
18%6%24323446%43%
1771
16th to endDay
19%5%14424445%41%
1591
November1st to 15thDay
20%5%23314347%43%
1581
16th to endDay
21%5%25%32332444%
1620
December1st to 15thDay
21%7%25%423335%38%
1464
16th to endDay
23%10%23333345%34%
1536
January1st to 15thNight
15%8%2442445%7%39%
2506
16th to endNight
16%8%3443345%7%37%
2688
February1st to 15thNight
14%9%26%5%45%5%46%34%
2235
16th to endNight
16%9%345%34445%38%
1893
March1st to 15thNight
19%9%345%23446%35%
2117
16th to endNight
17%6%34324446%44%
2070
April1st to 15thNight
20%6%335%25%345%37%
1796
16th to endNight
22%6%34424435%38%
1624
May1st to 15thNight
20%7%343246%47%33%
1620
16th to endNight
23%9%25%5%35%446%29%
1544
June1st to 15thNight
25%7%26%5%24446%30%
1440
16th to endNight
20%5%2342346%8%38%
1440
July1st to 15thNight
15%425%5%35%5%5%8%37%
1440
16th to endNight
13%5%25%5%346%7%11%36%
1514
August1st to 15thNight
12%5%25%4335%6%8%40%
1566
16th to endNight
12%424434449%45%
1646
September1st to 15thNight
14%5%24413347%48%
1730
16th to endNight
14%323212446%55%
1815
October1st to 15thNight
18%6%2432335%5%43%
1859
16th to endNight
21%5%24324445%40%
2280
November1st to 15thNight
18%6%24323446%43%
2277
16th to endNight
18%5%24424346%41%
2340
December1st to 15thNight
21%8%35%313345%37%
2495
16th to endNight
22%8%24324446%36%
2688

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
17%6%34323448%40%
1432
16th to endDay
16%8%243135%6%7%40%
1536
February1st to 15thDay
18%7%26%4235%5%7%36%
1622
16th to endDay
13%9%35%6%34446%38%
1580
March1st to 15thDay
20%8%25%5%33346%35%
1830
16th to endDay
15%7%24423346%44%
2151
April1st to 15thDay
18%5%2342445%6%41%
2152
16th to endDay
18%42442345%7%41%
2334
May1st to 15thDay
23%6%35%5%23446%33%
2332
16th to endDay
21%5%25%425%45%7%34%
2579
June1st to 15thDay
25%6%2433435%7%33%
2520
16th to endDay
18%5%245%345%5%7%37%
2520
July1st to 15thDay
12%6%24426%5%6%9%38%
2520
16th to endDay
12%5%145%35%5%6%9%39%
2654
August1st to 15thDay
11%424425%5%6%9%42%
2295
16th to endDay
11%3134245%6%7%47%
2201
September1st to 15thDay
13%425%32345%6%49%
1900
16th to endDay
14%5%13322336%54%
1815
October1st to 15thDay
19%6%24323346%43%
1771
16th to endDay
19%6%234145%45%42%
1591
November1st to 15thDay
20%7%13323346%43%
1581
16th to endDay
20%6%14324335%44%
1620
December1st to 15thDay
21%6%16%424335%39%
1464
16th to endDay
25%10%24423345%33%
1536
January1st to 15thNight
16%7%25%4245%5%6%39%
2506
16th to endNight
17%8%3443335%7%38%
2688
February1st to 15thNight
15%9%35%5%36%445%35%
2228
16th to endNight
16%9%25%5%2435%5%38%
1893
March1st to 15thNight
20%8%25%5%24436%35%
2130
16th to endNight
17%6%23324346%43%
2069
April1st to 15thNight
20%7%245%34446%37%
1796
16th to endNight
21%7%24423346%37%
1624
May1st to 15thNight
21%7%244245%5%7%33%
1620
16th to endNight
24%9%346%34446%29%
1544
June1st to 15thNight
26%7%25%42445%6%30%
1440
16th to endNight
20%5%2432345%8%38%
1440
July1st to 15thNight
14%5%246%36%46%7%36%
1440
16th to endNight
13%5%245%35%5%7%9%37%
1514
August1st to 15thNight
11%5%245%345%6%7%41%
1566
16th to endNight
13%5%2442446%8%44%
1646
September1st to 15thNight
15%4235%13337%48%
1730
16th to endNight
13%412312446%54%
1815
October1st to 15thNight
18%6%2342335%5%42%
1859
16th to endNight
21%6%14424446%40%
2280
November1st to 15thNight
18%6%24424345%44%
2277
16th to endNight
18%5%335%24346%40%
2340
December1st to 15thNight
22%8%34423345%37%
2495
16th to endNight
22%8%25%323346%35%
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%5%14%23%12%2
1433
16th to endDay
41%7%14%24%11%
1536
February1st to 15thDay
46%7%14%19%11%
1625
16th to endDay
46%5%11%19%13%2
1580
March1st to 15thDay
50%412%17%12%2
1830
16th to endDay
41%48%20%18%5%
2151
April1st to 15thDay
42%6%8%20%17%3
2152
16th to endDay
42%7%8%21%16%3
2334
May1st to 15thDay
51%6%7%16%16%1
2332
16th to endDay
47%7%13%18%11%1
2583
June1st to 15thDay
50%5%13%20%9%1
2520
16th to endDay
45%7%10%22%12%1
2520
July1st to 15thDay
39%8%13%25%12%
2371
16th to endDay
39%8%15%22%12%1
2487
August1st to 15thDay
37%9%16%25%10%
2281
16th to endDay
32%8%14%26%15%1
2201
September1st to 15thDay
34%6%11%26%20%
1900
16th to endDay
32%49%27%22%2
1815
October1st to 15thDay
42%5%9%24%15%2
1771
16th to endDay
42%6%12%24%13%1
1591
November1st to 15thDay
42%414%22%14%1
1581
16th to endDay
44%413%18%17%1
1620
December1st to 15thDay
48%5%13%18%13%
1464
16th to endDay
53%5%11%18%10%
1536
January1st to 15thNight
44%6%14%21%12%1
2506
16th to endNight
46%413%20%12%1
2688
February1st to 15thNight
50%5%12%17%12%1
2236
16th to endNight
47%5%11%21%11%2
1893
March1st to 15thNight
50%5%8%20%13%2
2130
16th to endNight
41%410%18%18%5%
2070
April1st to 15thNight
46%5%5%19%18%3
1796
16th to endNight
47%49%19%14%4
1624
May1st to 15thNight
47%6%10%17%14%3
1620
16th to endNight
56%5%10%16%9%1
1546
June1st to 15thNight
54%7%9%19%8%1
1440
16th to endNight
42%6%14%21%12%1
1440
July1st to 15thNight
41%8%15%23%11%
1360
16th to endNight
40%8%14%26%10%
1418
August1st to 15thNight
38%8%13%25%12%
1556
16th to endNight
36%6%11%28%15%
1646
September1st to 15thNight
37%5%10%29%17%
1730
16th to endNight
30%5%8%31%23%1
1815
October1st to 15thNight
42%411%23%15%2
1859
16th to endNight
44%5%10%25%13%
2280
November1st to 15thNight
42%412%22%15%2
2277
16th to endNight
44%414%19%14%1
2340
December1st to 15thNight
48%414%21%10%
2495
16th to endNight
50%5%12%20%11%
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
13%26%30%28%1
60
16th to endDay
18%21%35%20%3
64
February1st to 15thDay
22%21%37%14%3
228
16th to endDay
19%21%33%19%6%
239
March1st to 15thDay
23%28%30%14%2
270
16th to endDay
18%26%33%12%6%2
288
April1st to 15thDay
22%22%29%17%5%1
270
16th to endDay
20%22%32%14%7%2
270
May1st to 15thDay
28%20%27%13%7%2
288
16th to endDay
24%17%31%19%6%1
331
June1st to 15thDay
20%19%30%21%6%
315
16th to endDay
19%15%29%24%9%1
315
July1st to 15thDay
15%8%30%28%13%3
305
16th to endDay
12%11%25%30%14%3
326
August1st to 15thDay
13%10%23%27%21%3
297
16th to endDay
13%9%17%29%19%9%
268
September1st to 15thDay
16%8%22%26%18%8%
255
16th to endDay
18%11%24%24%8%12%
254
October1st to 15thDay
22%15%19%25%12%4
247
16th to endDay
23%15%33%20%7%
240
November1st to 15thDay
21%20%25%20%9%2
235
16th to endDay
22%24%25%19%7%1
240
December1st to 15thDay
21%19%39%15%4
84
16th to endDay
39%18%29%10%1
64
January1st to 15thNight
15%31%32%16%3
720
16th to endNight
19%30%30%16%2
768
February1st to 15thNight
18%29%31%17%2
704
16th to endNight
19%27%28%19%4
631
March1st to 15thNight
23%26%30%16%2
720
16th to endNight
18%28%27%18%6%
768
April1st to 15thNight
19%24%29%21%5%
718
16th to endNight
19%23%23%24%7%
544
May1st to 15thNight
20%20%24%23%8%1
540
16th to endNight
24%20%21%23%9%1
424
June1st to 15thNight
23%18%16%28%11%
360
16th to endNight
20%16%13%34%13%
360
July1st to 15thNight
14%13%14%36%18%2
340
16th to endNight
14%17%13%37%17%
376
August1st to 15thNight
12%10%15%37%21%1
507
16th to endNight
13%10%17%33%20%3
596
September1st to 15thNight
14%11%21%29%20%2
660
16th to endNight
15%12%22%25%19%4
657
October1st to 15thNight
20%16%23%23%14%2
660
16th to endNight
23%17%30%21%6%
704
November1st to 15thNight
19%20%28%22%6%1
700
16th to endNight
17%28%25%19%7%
720
December1st to 15thNight
23%27%30%13%4
720
16th to endNight
24%28%30%15%1
768
Page generated at 2017-08-13 14:16:38. Version ac04

Copyright 2017 A. Danko


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