Southern Maine Astronomers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Southern Maine Astronomers Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
30%
22827
11%
74697
5%
22783
37%
22783
11%
57909

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%
760
11%
2649
3%
757
29%
757
9%
2055
January11th to 20thNight
16%
744
8%
2602
2%
744
24%
744
6%
2140
January21st to endNight
15%
792
13%
2772
4%
792
28%
792
9%
2064
February1st to 10thNight
17%
720
9%
2358
3%
720
25%
720
9%
1847
February11th to 20thNight
16%
703
13%
2100
2%
700
27%
700
9%
1659
February21st to endNight
22%
592
16%
1776
5%
592
36%
592
11%
1371
March1st to 10thNight
17%
720
13%
2160
3%
720
29%
720
10%
1702
March11th to 20thNight
23%
720
11%
2026
4%
720
29%
720
11%
1615
March21st to endNight
24%
792
15%
2115
5%
792
31%
792
13%
1641
April1st to 10thNight
24%
720
12%
1800
3%
720
29%
720
13%
1453
April11th to 20thNight
29%
631
13%
1695
5%
631
34%
631
14%
1331
April21st to endNight
28%
540
11%
1620
4%
540
33%
540
10%
1286
May1st to 10thNight
28%
540
10%
1620
6%
540
31%
540
10%
1304
May11th to 20thNight
30%
511
10%
1573
4%
511
37%
511
9%
1261
May21st to endNight
36%
389
9%
1551
11%
388
40%
388
9%
1192
June1st to 10thNight
35%
360
8%
1440
8%
360
40%
360
7%
1148
June11th to 20thNight
46%
360
9%
1440
10%
360
50%
360
10%
1106
June21st to endNight
45%
360
6%
1440
8%
360
48%
360
8%
1134
July1st to 10thNight
52%
350
5%
1400
7%
350
54%
350
8%
1117
July11th to 20thNight
51%
348
9%
1392
13%
348
54%
348
11%
1044
July21st to endNight
58%
394
11%
1486
13%
372
60%
372
12%
1099
August1st to 10thNight
56%
499
8%
1563
6%
497
58%
497
13%
1214
August11th to 20thNight
52%
532
12%
1596
8%
532
53%
532
15%
1223
August21st to endNight
51%
710
12%
1898
6%
710
54%
710
16%
1476
September1st to 10thNight
47%
720
13%
1836
5%
720
51%
720
19%
1412
September11th to 20thNight
43%
720
17%
1980
9%
720
49%
720
18%
1461
September21st to endNight
43%
720
14%
1980
9%
720
48%
720
16%
1503
October1st to 10thNight
39%
720
12%
1980
6%
720
44%
720
14%
1561
October11th to 20thNight
31%
720
12%
2220
5%
720
39%
720
11%
1735
October21st to endNight
25%
792
11%
2574
5%
792
33%
792
10%
1996
November1st to 10thNight
30%
740
13%
2407
7%
740
37%
740
12%
1837
November11th to 20thNight
26%
760
16%
2470
8%
760
37%
760
12%
1880
November21st to endNight
23%
760
12%
2470
5%
760
31%
760
10%
1900
December1st to 10thNight
17%
792
10%
2697
2%
792
27%
792
8%
2161
December11th to 20thNight
18%
767
13%
3198
2%
766
29%
766
7%
2346
December21st to endNight
16%
829
8%
4813
3%
817
24%
817
4%
3635

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
18%7%35%5%3445%7%40%
74718
Night
18%7%35%5%3445%6%40%
75044
TransparencyDay
29%37%8%15%9%2
74423
Night
31%37%8%15%9%1
74873
SeeingDay
21%13%33%22%9%2
13871
Night
22%12%31%26%9%1
22834

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%9%36%5%33436%42%
1517
11th to 20thDay
19%9%35%435%446%39%
1486
21st to endDay
13%10%36%435%5%5%5%41%
1584
February1st to 10thDay
19%8%36%5%345%5%6%35%
1602
11th to 20thDay
13%9%36%6%45%46%6%38%
1748
21st to endDay
16%11%46%5%25%5%45%37%
1480
March1st to 10thDay
16%7%36%7%35%5%5%6%38%
1797
11th to 20thDay
16%7%35%5%2435%6%43%
1933
21st to endDay
17%8%45%5%24445%42%
2240
April1st to 10thDay
21%9%36%434446%37%
2160
11th to 20thDay
17%6%25%5%35%5%5%7%40%
2213
21st to endDay
23%8%35%5%35%446%34%
2340
May1st to 10thDay
24%7%35%6%24447%34%
2340
11th to 20thDay
18%7%25%5%3445%7%38%
2336
21st to endDay
17%6%25%435%5%6%8%37%
2725
June1st to 10thDay
22%7%35%5%35%46%7%33%
2520
11th to 20thDay
18%6%25%5%3445%7%41%
2520
21st to endDay
22%6%25%6%345%5%8%33%
2520
July1st to 10thDay
15%6%35%5%35%46%8%39%
2520
11th to 20thDay
13%6%35%6%45%6%6%9%37%
2520
21st to endDay
14%425%5%36%5%7%8%40%
2744
August1st to 10thDay
13%5%25%5%35%6%7%9%41%
2346
11th to 20thDay
13%5%25%6%35%5%6%9%40%
2304
21st to endDay
12%4246%345%6%8%45%
2455
September1st to 10thDay
13%425%43446%7%49%
2124
11th to 20thDay
13%5%15%323335%55%
1980
21st to endDay
16%5%1433445%7%48%
1980
October1st to 10thDay
20%8%35%5%3435%6%40%
1980
11th to 20thDay
17%7%25%5%33447%42%
1736
21st to endDay
24%7%25%424445%39%
1782
November1st to 10thDay
18%6%13424346%48%
1671
11th to 20thDay
20%42442346%6%45%
1710
21st to endDay
23%6%35%323436%43%
1710
December1st to 10thDay
23%8%345%33446%38%
1455
11th to 20thDay
23%7%34433445%42%
1868
21st to endDay
25%10%35%5%3445%5%30%
2772
January1st to 10thNight
16%9%35%4245%47%41%
2649
11th to 20thNight
18%11%36%5%45%45%6%34%
2602
21st to endNight
13%9%35%435%46%6%41%
2772
February1st to 10thNight
17%10%26%5%35%45%6%35%
2358
11th to 20thNight
15%9%36%6%35%45%5%40%
2098
21st to endNight
16%9%35%5%25%45%7%39%
1776
March1st to 10thNight
15%11%35%5%25%45%5%40%
2150
11th to 20thNight
17%7%36%5%35%45%7%38%
2025
21st to endNight
18%9%25%423445%43%
2115
April1st to 10thNight
21%9%35%5%34446%37%
1800
11th to 20thNight
19%6%35%425%5%5%7%40%
1695
21st to endNight
23%7%36%434446%34%
1620
May1st to 10thNight
21%8%35%5%35%5%5%7%32%
1620
11th to 20thNight
18%8%36%5%45%5%6%8%32%
1572
21st to endNight
19%9%35%6%3345%7%35%
1550
June1st to 10thNight
23%6%37%7%3435%6%34%
1440
11th to 20thNight
19%6%25%43445%7%40%
1440
21st to endNight
23%6%245%35%45%8%35%
1440
July1st to 10thNight
16%6%246%35%5%6%8%40%
1440
11th to 20thNight
15%6%36%5%345%6%9%37%
1440
21st to endNight
14%4245%45%6%6%9%42%
1574
August1st to 10thNight
12%425%5%35%5%7%9%42%
1573
11th to 20thNight
14%7%26%5%35%5%7%8%39%
1596
21st to endNight
15%5%245%34447%46%
1898
September1st to 10thNight
13%5%245%345%5%6%49%
1836
11th to 20thNight
16%5%24333447%50%
1980
21st to endNight
15%5%2442435%6%49%
1980
October1st to 10thNight
19%7%25%5%35%45%5%40%
1980
11th to 20thNight
17%6%35%5%35%5%46%41%
2220
21st to endNight
25%7%35%434445%37%
2574
November1st to 10thNight
17%6%25%434445%45%
2407
11th to 20thNight
19%6%24424447%45%
2470
21st to endNight
16%7%25%43445%7%43%
2470
December1st to 10thNight
21%9%36%435%445%36%
2697
11th to 20thNight
21%8%345%34345%40%
3303
21st to endNight
24%10%36%5%34345%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
29%39%7%16%9%
1518
11th to 20thDay
30%41%9%12%6%1
1486
21st to endDay
28%36%8%17%10%2
1584
February1st to 10thDay
31%36%7%15%9%2
1602
11th to 20thDay
29%38%10%14%7%3
1757
21st to endDay
35%35%7%14%8%2
1480
March1st to 10thDay
31%36%9%12%8%3
1800
11th to 20thDay
29%36%8%16%8%2
1933
21st to endDay
27%37%5%14%12%4
2240
April1st to 10thDay
31%36%6%12%11%2
2160
11th to 20thDay
29%37%8%15%10%2
2213
21st to endDay
32%36%7%12%9%4
2340
May1st to 10thDay
33%37%5%13%11%1
2340
11th to 20thDay
29%37%7%12%11%3
2340
21st to endDay
27%39%9%15%8%2
2725
June1st to 10thDay
35%37%9%13%5%
2520
11th to 20thDay
30%37%9%13%9%2
2520
21st to endDay
35%38%9%12%5%1
2520
July1st to 10thDay
30%39%10%15%6%
2450
11th to 20thDay
27%40%11%15%6%
2445
21st to endDay
24%40%11%17%7%1
2591
August1st to 10thDay
27%39%12%16%7%
2332
11th to 20thDay
27%37%10%18%8%1
2304
21st to endDay
24%39%9%16%11%2
2455
September1st to 10thDay
23%37%10%18%12%1
2124
11th to 20thDay
21%36%7%19%16%2
1980
21st to endDay
26%39%7%15%12%1
1980
October1st to 10thDay
28%37%8%15%11%1
1980
11th to 20thDay
30%36%9%17%7%1
1736
21st to endDay
29%37%7%14%10%2
1782
November1st to 10thDay
24%40%7%16%12%1
1671
11th to 20thDay
23%40%8%15%12%1
1710
21st to endDay
27%40%11%15%6%
1710
December1st to 10thDay
32%39%8%12%7%2
1455
11th to 20thDay
31%33%7%15%10%5%
1868
21st to endDay
44%25%10%14%5%2
2772
January1st to 10thNight
28%40%8%16%7%
2649
11th to 20thNight
33%42%7%11%5%1
2602
21st to endNight
27%37%7%16%11%2
2772
February1st to 10thNight
32%36%7%13%10%1
2358
11th to 20thNight
31%37%6%15%8%2
2100
21st to endNight
29%36%7%16%9%2
1776
March1st to 10thNight
32%36%5%15%10%1
2160
11th to 20thNight
32%37%9%14%7%1
2026
21st to endNight
28%36%6%14%13%3
2115
April1st to 10thNight
32%36%6%14%9%3
1800
11th to 20thNight
29%36%6%16%12%1
1695
21st to endNight
33%36%7%12%10%2
1620
May1st to 10thNight
33%37%6%13%8%2
1620
11th to 20thNight
33%39%7%11%7%4
1573
21st to endNight
32%37%8%13%9%1
1551
June1st to 10thNight
36%38%8%12%6%1
1440
11th to 20thNight
31%38%9%11%10%1
1440
21st to endNight
33%38%11%12%5%
1440
July1st to 10thNight
33%40%10%12%41
1400
11th to 20thNight
27%40%10%16%7%
1392
21st to endNight
24%40%11%16%7%1
1486
August1st to 10thNight
29%38%10%17%5%
1563
11th to 20thNight
28%37%10%15%10%
1596
21st to endNight
28%37%8%17%10%1
1898
September1st to 10thNight
25%36%8%18%12%1
1836
11th to 20thNight
22%37%7%20%13%1
1980
21st to endNight
26%36%7%17%11%2
1980
October1st to 10thNight
29%38%8%15%10%
1980
11th to 20thNight
30%37%8%17%7%1
2220
21st to endNight
31%37%7%14%10%1
2574
November1st to 10thNight
25%41%8%15%10%1
2407
11th to 20thNight
26%40%7%16%10%2
2470
21st to endNight
28%39%9%15%9%1
2470
December1st to 10thNight
32%40%8%13%6%1
2697
11th to 20thNight
31%33%9%14%13%1
3303
21st to endNight
44%25%7%15%7%
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
28%19%26%20%6%
180
11th to 20thDay
26%20%28%20%5%1
172
21st to endDay
16%21%39%18%6%
176
February1st to 10thDay
30%14%37%16%2
312
11th to 20thDay
21%21%40%15%3
416
21st to endDay
25%13%41%17%5%
346
March1st to 10thDay
25%15%39%18%4
420
11th to 20thDay
21%22%38%14%41
420
21st to endDay
23%20%33%15%7%2
462
April1st to 10thDay
27%20%31%16%5%1
420
11th to 20thDay
21%19%34%18%6%2
412
21st to endDay
30%15%31%17%42
420
May1st to 10thDay
27%18%32%15%6%2
423
11th to 20thDay
20%15%39%19%5%1
447
21st to endDay
21%12%33%25%7%2
485
June1st to 10thDay
21%13%34%25%6%2
450
11th to 20thDay
14%14%38%23%9%2
450
21st to endDay
22%8%34%24%10%2
450
July1st to 10thDay
20%5%32%27%12%4
445
11th to 20thDay
14%5%32%33%11%5%
444
21st to endDay
13%5%34%31%14%4
488
August1st to 10thDay
13%7%29%32%16%3
446
11th to 20thDay
16%6%28%30%14%7%
439
21st to endDay
13%7%30%28%15%7%
495
September1st to 10thDay
15%7%29%28%16%4
450
11th to 20thDay
16%10%34%21%12%6%
450
21st to endDay
16%11%32%23%12%6%
450
October1st to 10thDay
22%14%20%28%12%4
450
11th to 20thDay
21%12%31%23%10%3
369
21st to endDay
30%13%26%21%9%
396
November1st to 10thDay
20%16%28%25%9%2
365
11th to 20thDay
25%16%31%22%5%1
370
21st to endDay
26%14%29%22%7%1
370
December1st to 10thDay
25%16%34%17%8%
205
11th to 20thDay
24%22%34%17%3
180
21st to endDay
27%21%30%15%7%
198
January1st to 10thNight
22%18%36%20%4
760
11th to 20thNight
25%18%34%19%4
744
21st to endNight
19%19%39%20%3
792
February1st to 10thNight
24%16%37%20%3
720
11th to 20thNight
21%18%39%19%2
703
21st to endNight
21%16%35%24%4
592
March1st to 10thNight
22%16%40%20%2
720
11th to 20thNight
22%14%35%21%8%
720
21st to endNight
24%14%33%24%5%1
792
April1st to 10thNight
28%13%31%23%5%
720
11th to 20thNight
21%12%34%24%9%
631
21st to endNight
27%11%27%29%5%1
540
May1st to 10thNight
26%8%31%26%7%1
540
11th to 20thNight
21%11%30%29%8%1
511
21st to endNight
25%8%22%29%14%1
389
June1st to 10thNight
23%9%22%32%12%2
360
11th to 20thNight
21%424%34%17%1
360
21st to endNight
24%221%38%14%1
360
July1st to 10thNight
17%418%41%18%1
350
11th to 20thNight
18%318%38%22%2
348
21st to endNight
16%315%44%20%2
394
August1st to 10thNight
13%219%45%18%2
499
11th to 20thNight
17%420%37%21%2
532
21st to endNight
16%420%38%20%3
710
September1st to 10thNight
15%227%34%19%2
720
11th to 20thNight
17%7%27%28%18%3
720
21st to endNight
18%7%26%30%17%3
720
October1st to 10thNight
23%8%23%28%16%2
720
11th to 20thNight
20%10%32%26%11%2
720
21st to endNight
28%12%29%25%6%
792
November1st to 10thNight
20%11%33%27%7%2
740
11th to 20thNight
23%13%33%23%7%1
760
21st to endNight
21%18%33%22%6%
760
December1st to 10thNight
25%15%39%16%5%1
792
11th to 20thNight
23%19%34%19%3
767
21st to endNight
25%16%37%20%3
836
Page generated at 2024-01-13 19:28:34. Version ac07

Copyright 2024 Attilla Danko


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