Portland(ME)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland(ME) clear sky chart from 2005-11-06 to 2024-02-20.

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
32%
22928
17%
80355
9%
22839
42%
22839
19%
52441

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
18%
754
21%
4347
6%
737
35%
737
11%
2472
January11th to 20thNight
17%
756
20%
4322
4%
740
29%
740
8%
2579
January21st to endNight
18%
830
12%
4845
6%
814
34%
814
9%
3207
February1st to 10thNight
18%
760
16%
2478
5%
760
32%
760
15%
1700
February11th to 20thNight
18%
741
20%
2215
5%
739
36%
739
18%
1496
February21st to endNight
22%
592
21%
1776
8%
592
40%
592
20%
1186
March1st to 10thNight
21%
720
20%
2160
8%
720
35%
720
18%
1457
March11th to 20thNight
25%
720
18%
2024
6%
720
37%
720
20%
1374
March21st to endNight
25%
792
21%
2117
6%
792
36%
792
24%
1434
April1st to 10thNight
24%
720
18%
1800
5%
720
32%
720
22%
1302
April11th to 20thNight
32%
631
18%
1695
8%
631
38%
631
25%
1159
April21st to endNight
31%
540
16%
1620
7%
540
38%
540
19%
1137
May1st to 10thNight
32%
540
13%
1620
8%
540
37%
540
20%
1157
May11th to 20thNight
35%
522
14%
1584
8%
522
41%
522
18%
1117
May21st to endNight
39%
389
13%
1551
16%
388
43%
388
16%
1033
June1st to 10thNight
38%
360
12%
1440
13%
360
45%
360
14%
986
June11th to 20thNight
47%
360
12%
1440
15%
360
52%
360
19%
920
June21st to endNight
49%
360
10%
1440
14%
360
52%
360
17%
939
July1st to 10thNight
54%
350
11%
1400
16%
350
57%
350
18%
897
July11th to 20thNight
58%
348
13%
1392
19%
348
60%
348
20%
864
July21st to endNight
59%
394
17%
1486
23%
372
61%
372
23%
882
August1st to 10thNight
60%
506
16%
1570
13%
504
62%
504
26%
991
August11th to 20thNight
56%
532
17%
1596
13%
532
59%
532
26%
1052
August21st to endNight
55%
710
21%
1898
11%
710
58%
710
31%
1246
September1st to 10thNight
52%
720
19%
1838
11%
720
56%
720
34%
1186
September11th to 20thNight
46%
720
23%
1980
14%
720
53%
720
33%
1227
September21st to endNight
47%
720
24%
1980
15%
720
54%
720
31%
1253
October1st to 10thNight
40%
720
18%
1980
11%
720
46%
720
24%
1355
October11th to 20thNight
38%
720
21%
2217
10%
720
50%
720
24%
1410
October21st to endNight
27%
792
16%
2574
9%
792
38%
792
18%
1754
November1st to 10thNight
32%
740
22%
2407
13%
740
44%
740
24%
1502
November11th to 20thNight
26%
760
23%
2470
9%
760
44%
760
21%
1564
November21st to endNight
23%
760
20%
2470
7%
760
39%
760
20%
1561
December1st to 10thNight
16%
760
17%
2621
4%
760
32%
760
15%
1794
December11th to 20thNight
21%
760
17%
3191
5%
759
34%
759
14%
1989
December21st to endNight
17%
829
12%
4811
3%
817
33%
817
9%
3259

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%7%25%5%345%5%7%41%
78246
Night
18%7%35%5%3445%7%39%
80936
TransparencyDay
43%7%12%22%14%2
77951
Night
45%6%11%22%14%2
80765
SeeingDay
20%10%31%26%10%2
13986
Night
22%10%30%27%10%1
22952

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 10thDay
16%8%35%5%34446%43%
2517
11th to 20thDay
17%6%25%5%34447%42%
2520
21st to endDay
17%11%46%4245%5%6%37%
2772
February1st to 10thDay
18%8%35%5%345%6%7%37%
1698
11th to 20thDay
13%7%36%7%345%5%7%39%
1838
21st to endDay
15%11%37%5%25%45%5%38%
1480
March1st to 10thDay
17%8%26%5%25%5%5%7%38%
1797
11th to 20thDay
16%7%25%5%33447%44%
1934
21st to endDay
17%8%35%433446%43%
2236
April1st to 10thDay
19%8%35%5%345%46%38%
2160
11th to 20thDay
17%6%245%35%45%7%41%
2213
21st to endDay
22%7%25%5%345%5%6%35%
2340
May1st to 10thDay
22%7%35%5%3445%7%36%
2340
11th to 20thDay
18%7%35%5%3445%8%38%
2325
21st to endDay
18%6%25%5%35%5%6%8%37%
2725
June1st to 10thDay
22%7%36%5%3446%7%34%
2520
11th to 20thDay
15%6%35%5%3445%7%42%
2520
21st to endDay
21%6%35%6%36%5%5%7%33%
2520
July1st to 10thDay
16%6%245%35%6%5%10%39%
2520
11th to 20thDay
11%7%25%5%35%6%7%10%38%
2520
21st to endDay
13%4245%45%6%6%9%42%
2744
August1st to 10thDay
11%5%25%5%36%5%7%10%42%
2340
11th to 20thDay
12%5%35%5%35%5%7%10%41%
2304
21st to endDay
12%4235%346%6%8%47%
2454
September1st to 10thDay
13%4144345%5%7%48%
2120
11th to 20thDay
12%6%24323345%56%
1980
21st to endDay
15%6%2343445%6%47%
1980
October1st to 10thDay
19%8%25%43345%6%42%
1980
11th to 20thDay
17%6%25%5%3445%7%42%
1740
21st to endDay
23%6%26%3245%4440%
1782
November1st to 10thDay
16%6%24424347%48%
1671
11th to 20thDay
20%42332345%7%46%
1710
21st to endDay
22%6%25%323435%43%
1710
December1st to 10thDay
22%9%35%5%24445%39%
1558
11th to 20thDay
23%7%23434345%43%
1906
21st to endDay
25%9%35%5%345%5%5%31%
2772
January1st to 10thNight
17%8%3442435%6%43%
4429
11th to 20thNight
14%8%38%6%345%5%6%39%
4440
21st to endNight
20%11%36%5%33446%34%
4884
February1st to 10thNight
17%9%36%5%46%447%36%
2478
11th to 20thNight
14%10%35%5%45%45%5%40%
2213
21st to endNight
15%8%36%5%35%45%6%40%
1776
March1st to 10thNight
15%10%35%5%345%5%6%39%
2150
11th to 20thNight
18%7%35%5%345%5%7%38%
2024
21st to endNight
19%7%35%324446%43%
2117
April1st to 10thNight
22%8%345%34445%37%
1800
11th to 20thNight
19%6%25%5%25%5%5%6%40%
1695
21st to endNight
23%7%346%3445%5%35%
1620
May1st to 10thNight
22%8%45%5%345%5%7%32%
1620
11th to 20thNight
18%7%36%5%47%6%6%8%31%
1583
21st to endNight
21%10%35%43435%6%35%
1550
June1st to 10thNight
22%7%37%6%3445%6%33%
1440
11th to 20thNight
18%7%35%5%2445%8%39%
1440
21st to endNight
23%6%34425%45%9%35%
1440
July1st to 10thNight
17%6%25%6%36%5%5%8%38%
1440
11th to 20thNight
16%6%36%5%336%7%9%36%
1440
21st to endNight
14%4245%35%6%6%10%40%
1574
August1st to 10thNight
12%5%26%5%346%7%9%42%
1580
11th to 20thNight
14%6%26%4445%6%9%38%
1596
21st to endNight
14%5%25%4345%48%45%
1898
September1st to 10thNight
13%5%245%2445%7%48%
1838
11th to 20thNight
15%6%24423447%49%
1980
21st to endNight
15%5%343235%5%7%48%
1980
October1st to 10thNight
19%7%35%5%3445%6%40%
1980
11th to 20thNight
16%5%34435%5%6%7%41%
2217
21st to endNight
25%6%25%42445%5%37%
2574
November1st to 10thNight
17%6%24434446%45%
2407
11th to 20thNight
19%6%23434446%45%
2470
21st to endNight
16%6%35%43345%7%43%
2470
December1st to 10thNight
21%9%36%5%345%5%5%36%
2621
11th to 20thNight
21%6%2442445%6%41%
3288
21st to endNight
24%10%36%5%24445%33%
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
43%412%28%12%1
2518
11th to 20thDay
44%7%12%24%11%2
2520
21st to endDay
48%6%11%23%11%1
2772
February1st to 10thDay
46%7%14%18%12%3
1698
11th to 20thDay
43%7%14%19%14%4
1847
21st to endDay
48%5%10%20%14%3
1480
March1st to 10thDay
47%413%18%14%5%
1800
11th to 20thDay
41%5%10%21%17%5%
1934
21st to endDay
44%5%8%19%18%6%
2236
April1st to 10thDay
46%6%9%19%17%4
2160
11th to 20thDay
41%7%11%23%15%3
2213
21st to endDay
48%6%9%18%16%3
2340
May1st to 10thDay
48%6%9%20%16%2
2340
11th to 20thDay
44%6%10%19%17%3
2329
21st to endDay
44%8%14%20%12%3
2725
June1st to 10thDay
49%8%12%21%10%1
2520
11th to 20thDay
42%6%11%21%17%3
2520
21st to endDay
47%8%12%21%10%1
2520
July1st to 10thDay
40%8%12%25%15%1
2450
11th to 20thDay
40%8%14%26%12%1
2445
21st to endDay
35%10%16%26%12%2
2591
August1st to 10thDay
35%11%16%27%10%1
2326
11th to 20thDay
38%9%15%24%12%1
2304
21st to endDay
33%8%13%25%19%2
2454
September1st to 10thDay
33%7%15%26%18%1
2120
11th to 20thDay
32%5%10%27%23%2
1980
21st to endDay
39%6%9%26%18%2
1980
October1st to 10thDay
43%6%10%22%16%3
1980
11th to 20thDay
41%6%12%29%11%1
1740
21st to endDay
47%5%11%20%14%2
1782
November1st to 10thDay
38%5%13%25%17%2
1671
11th to 20thDay
39%412%22%19%3
1710
21st to endDay
45%417%20%14%1
1710
December1st to 10thDay
48%6%12%19%14%1
1558
11th to 20thDay
46%411%18%16%5%
1906
21st to endDay
54%7%11%17%9%2
2772
January1st to 10thNight
43%415%26%10%3
4429
11th to 20thNight
46%6%14%20%12%3
4440
21st to endNight
53%411%20%10%1
4884
February1st to 10thNight
49%6%13%17%13%3
2478
11th to 20thNight
47%410%21%15%3
2215
21st to endNight
45%5%11%22%13%4
1776
March1st to 10thNight
46%6%8%21%16%4
2160
11th to 20thNight
46%411%18%16%5%
2025
21st to endNight
44%49%18%20%5%
2117
April1st to 10thNight
49%5%8%18%16%5%
1800
11th to 20thNight
43%6%8%23%19%2
1695
21st to endNight
50%6%10%17%14%3
1620
May1st to 10thNight
51%6%9%19%13%2
1620
11th to 20thNight
48%8%10%18%12%4
1584
21st to endNight
51%5%12%18%13%1
1551
June1st to 10thNight
51%7%9%20%12%1
1440
11th to 20thNight
45%7%12%20%15%2
1440
21st to endNight
45%8%13%22%10%1
1440
July1st to 10thNight
44%8%14%20%13%1
1400
11th to 20thNight
41%8%13%27%11%
1392
21st to endNight
37%9%14%28%12%1
1486
August1st to 10thNight
37%9%14%27%12%1
1570
11th to 20thNight
40%9%15%22%14%
1596
21st to endNight
37%8%9%26%19%1
1898
September1st to 10thNight
36%6%11%27%18%1
1838
11th to 20thNight
36%5%9%28%21%2
1980
21st to endNight
37%6%9%29%17%2
1980
October1st to 10thNight
44%6%10%23%15%2
1980
11th to 20thNight
40%7%12%28%13%1
2217
21st to endNight
48%6%10%21%14%2
2574
November1st to 10thNight
41%5%11%24%16%2
2407
11th to 20thNight
43%410%22%18%3
2470
21st to endNight
42%5%16%22%14%2
2470
December1st to 10thNight
51%5%12%20%11%1
2621
11th to 20thNight
44%5%15%18%17%1
3288
21st to endNight
53%5%11%20%10%1
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
26%23%27%21%31
180
11th to 20thDay
26%19%29%21%5%1
180
21st to endDay
18%18%39%18%8%
198
February1st to 10thDay
26%16%40%16%2
348
11th to 20thDay
21%19%41%17%3
452
21st to endDay
23%16%38%19%4
346
March1st to 10thDay
23%16%36%22%3
420
11th to 20thDay
20%18%38%18%6%1
420
21st to endDay
21%16%37%17%7%2
462
April1st to 10thDay
24%18%33%18%5%2
420
11th to 20thDay
20%13%35%22%8%1
412
21st to endDay
29%11%32%19%7%1
420
May1st to 10thDay
27%10%34%21%6%2
423
11th to 20thDay
18%9%37%27%8%1
447
21st to endDay
20%7%32%28%11%1
485
June1st to 10thDay
22%431%30%11%2
450
11th to 20thDay
16%8%36%28%9%2
450
21st to endDay
21%5%27%34%11%2
450
July1st to 10thDay
18%425%35%16%3
445
11th to 20thDay
16%420%40%16%5%
444
21st to endDay
11%326%37%20%3
488
August1st to 10thDay
12%226%38%19%2
446
11th to 20thDay
16%321%35%20%5%
439
21st to endDay
14%323%38%18%5%
495
September1st to 10thDay
16%325%32%19%4
450
11th to 20thDay
16%7%30%25%15%6%
450
21st to endDay
17%9%25%32%13%5%
450
October1st to 10thDay
22%10%22%27%14%4
450
11th to 20thDay
19%11%29%27%12%2
369
21st to endDay
29%9%30%24%8%
396
November1st to 10thDay
18%13%31%26%10%2
365
11th to 20thDay
24%15%32%24%5%1
370
21st to endDay
25%16%29%22%7%1
370
December1st to 10thDay
22%16%38%17%7%
218
11th to 20thDay
27%19%33%18%3
180
21st to endDay
26%22%32%13%7%1
198
January1st to 10thNight
22%17%37%20%4
760
11th to 20thNight
26%17%35%18%4
760
21st to endNight
19%19%37%21%4
836
February1st to 10thNight
24%17%35%21%3
760
11th to 20thNight
20%17%39%20%3
742
21st to endNight
20%15%37%22%6%
592
March1st to 10thNight
23%14%38%23%2
720
11th to 20thNight
23%12%36%20%8%1
720
21st to endNight
23%13%35%23%6%1
792
April1st to 10thNight
27%10%32%26%4
720
11th to 20thNight
22%8%34%28%8%
631
21st to endNight
26%6%30%30%8%1
540
May1st to 10thNight
25%5%31%27%11%1
540
11th to 20thNight
21%5%32%30%11%1
522
21st to endNight
28%322%31%15%2
389
June1st to 10thNight
24%5%22%34%13%2
360
11th to 20thNight
22%419%35%18%2
360
21st to endNight
24%217%41%15%1
360
July1st to 10thNight
19%215%43%20%2
350
11th to 20thNight
16%212%44%24%2
348
21st to endNight
15%116%46%21%2
394
August1st to 10thNight
14%216%44%22%2
506
11th to 20thNight
14%313%43%24%3
532
21st to endNight
16%319%37%24%3
710
September1st to 10thNight
13%222%38%23%2
720
11th to 20thNight
18%425%28%19%5%
720
21st to endNight
19%424%32%18%3
720
October1st to 10thNight
23%8%22%28%18%2
720
11th to 20thNight
18%8%29%30%13%2
720
21st to endNight
27%9%29%27%7%1
792
November1st to 10thNight
19%9%34%29%7%1
740
11th to 20thNight
22%14%35%21%8%1
760
21st to endNight
21%16%35%22%6%
760
December1st to 10thNight
25%12%42%15%5%1
760
11th to 20thNight
24%17%34%21%3
760
21st to endNight
27%13%37%20%3
836
Page generated at 2024-02-17 06:28:40. Version ac07

Copyright 2024 Attilla Danko


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