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 2005-11-05 to 2017-10-06.

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
31%
14871
17%
47073
8%
14839
41%
14839
21%
30995

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
19%
720
19%
2506
6%
716
35%
716
17%
1602
January16th to endNight
15%
768
18%
2688
4%
768
34%
768
15%
1730
February1st to 15thNight
17%
704
15%
2243
5%
701
30%
701
16%
1541
February16th to endNight
23%
631
20%
1893
7%
631
38%
631
19%
1281
March1st to 15thNight
18%
720
17%
2130
5%
720
32%
720
18%
1482
March16th to endNight
27%
768
22%
2069
7%
768
37%
768
23%
1370
April1st to 15thNight
25%
718
18%
1796
5%
718
33%
718
23%
1276
April16th to endNight
31%
544
17%
1624
7%
544
37%
544
21%
1107
May1st to 15thNight
32%
540
14%
1620
8%
540
38%
540
19%
1125
May16th to endNight
32%
417
10%
1539
9%
416
37%
416
13%
1112
June1st to 15thNight
36%
360
10%
1440
9%
360
42%
360
13%
1003
June16th to endNight
51%
360
9%
1440
12%
360
54%
360
20%
899
July1st to 15thNight
55%
340
11%
1360
16%
340
58%
340
20%
857
July16th to endNight
55%
376
13%
1418
17%
354
58%
354
20%
879
August1st to 15thNight
54%
513
14%
1584
11%
511
57%
511
26%
1024
August16th to endNight
56%
651
21%
1797
13%
651
59%
651
32%
1123
September1st to 15thNight
46%
720
19%
1884
10%
720
51%
720
34%
1208
September16th to endNight
47%
717
26%
1980
16%
717
55%
717
37%
1167
October1st to 15thNight
38%
684
21%
1923
11%
684
48%
684
27%
1259
October16th to endNight
27%
704
17%
2282
7%
704
39%
704
20%
1487
November1st to 15thNight
32%
700
21%
2277
11%
700
46%
700
23%
1440
November16th to endNight
27%
720
20%
2340
9%
720
39%
720
19%
1511
December1st to 15thNight
18%
728
17%
2552
4%
728
33%
728
16%
1704
December16th to endNight
17%
768
18%
2688
5%
768
32%
768
15%
1808

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%6%2442445%6%39%
48101
Night
18%7%24424446%39%
47243
Cloud CoverDay
17%6%2442445%6%40%
48111
Night
18%7%24424446%39%
47248
TransparencyDay
43%5%12%22%14%1
47789
Night
44%5%11%22%14%1
47073
SeeingDay
21%10%31%22%10%3
5964
Night
21%10%30%26%9%1
14871

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
17%6%35%424446%41%
1432
16th to endDay
15%8%35%325%5%45%40%
1536
February1st to 15thDay
17%7%26%4445%46%34%
1618
16th to endDay
14%9%36%5%3444437%
1580
March1st to 15thDay
19%8%35%6%24345%35%
1820
16th to endDay
16%7%35%423345%43%
2154
April1st to 15thDay
19%7%25%333447%39%
2152
16th to endDay
20%6%1343435%5%40%
2334
May1st to 15thDay
24%7%25%6%23346%31%
2332
16th to endDay
20%6%25%4245%6%7%34%
2584
June1st to 15thDay
26%6%2442435%5%33%
2520
16th to endDay
20%5%25%5%25%5%5%7%34%
2520
July1st to 15thDay
13%5%35%5%35%5%6%8%37%
2520
16th to endDay
14%425%5%35%5%7%8%37%
2654
August1st to 15thDay
14%4245%35%5%6%8%38%
2352
16th to endDay
11%4145%2445%8%46%
2402
September1st to 15thDay
14%41443345%6%48%
2076
16th to endDay
13%414213345%54%
1980
October1st to 15thDay
19%7%25%423235%42%
1834
16th to endDay
19%6%24324446%40%
1588
November1st to 15thDay
20%5%2432345%6%43%
1581
16th to endDay
22%5%24422335%43%
1620
December1st to 15thDay
22%7%24422445%37%
1376
16th to endDay
23%10%24433345%33%
1536
January1st to 15thNight
15%8%2442445%6%40%
2506
16th to endNight
16%9%35%32445%5%38%
2688
February1st to 15thNight
14%9%26%6%46%5%45%34%
2242
16th to endNight
17%9%25%5%25%346%38%
1893
March1st to 15thNight
19%9%35%5%2445%5%35%
2117
16th to endNight
17%6%25%313345%44%
2069
April1st to 15thNight
19%8%345%24445%37%
1796
16th to endNight
21%6%34423436%38%
1624
May1st to 15thNight
20%7%25%435%5%5%8%32%
1620
16th to endNight
21%8%36%5%3445%7%29%
1537
June1st to 15thNight
26%6%25%5%34346%31%
1440
16th to endNight
20%42432435%8%38%
1440
July1st to 15thNight
14%5%246%25%5%6%8%38%
1440
16th to endNight
13%5%245%345%7%9%38%
1514
August1st to 15thNight
13%415%435%46%8%41%
1594
16th to endNight
13%4245%24448%46%
1797
September1st to 15thNight
15%5%14323446%48%
1884
16th to endNight
13%413313346%53%
1980
October1st to 15thNight
17%5%24423446%43%
1923
16th to endNight
21%6%24433445%39%
2282
November1st to 15thNight
18%6%23324446%43%
2277
16th to endNight
17%6%25%334346%41%
2340
December1st to 15thNight
22%8%34423335%37%
2552
16th to endNight
22%9%2443434436%
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
16%7%34423346%42%
1432
16th to endDay
15%8%25%3245%5%6%40%
1536
February1st to 15thDay
17%8%25%425%5%47%35%
1618
16th to endDay
14%8%47%5%35%446%37%
1580
March1st to 15thDay
20%8%26%424446%35%
1830
16th to endDay
15%7%35%423435%43%
2154
April1st to 15thDay
18%6%2443435%6%40%
2152
16th to endDay
19%5%2443445%6%41%
2334
May1st to 15thDay
23%6%35%5%24347%31%
2332
16th to endDay
21%5%25%4345%5%7%34%
2584
June1st to 15thDay
25%6%2442335%6%33%
2520
16th to endDay
19%425%435%5%47%36%
2520
July1st to 15thDay
12%5%25%436%6%6%9%36%
2520
16th to endDay
13%5%145%35%5%6%8%39%
2654
August1st to 15thDay
13%5%24435%6%5%9%40%
2352
16th to endDay
11%31442446%8%47%
2402
September1st to 15thDay
13%41432335%6%48%
2076
16th to endDay
13%5%13213345%54%
1980
October1st to 15thDay
18%5%25%424335%43%
1834
16th to endDay
19%5%24324445%41%
1588
November1st to 15thDay
20%5%23323446%43%
1581
16th to endDay
22%5%2431443443%
1620
December1st to 15thDay
22%7%35%432345%39%
1376
16th to endDay
23%11%24423345%34%
1536
January1st to 15thNight
15%8%24434447%40%
2506
16th to endNight
16%9%3442445%5%39%
2688
February1st to 15thNight
14%9%26%5%46%45%5%35%
2235
16th to endNight
17%8%35%5%24445%39%
1893
March1st to 15thNight
20%8%35%42435%6%35%
2130
16th to endNight
17%6%34323345%44%
2068
April1st to 15thNight
19%7%24434436%37%
1796
16th to endNight
21%6%25%423435%38%
1624
May1st to 15thNight
21%7%345%245%5%7%33%
1620
16th to endNight
22%8%45%5%345%47%27%
1537
June1st to 15thNight
25%6%25%5%2435%6%30%
1440
16th to endNight
20%6%225%1445%8%38%
1440
July1st to 15thNight
15%425%5%35%6%6%7%37%
1440
16th to endNight
13%425%5%345%7%10%36%
1514
August1st to 15thNight
13%5%25%4346%5%8%40%
1594
16th to endNight
12%5%14435%35%8%45%
1797
September1st to 15thNight
14%5%14423346%48%
1884
16th to endNight
13%5%1331335%6%53%
1980
October1st to 15thNight
17%6%2432335%5%44%
1923
16th to endNight
21%5%23425%445%40%
2282
November1st to 15thNight
18%6%2432445%5%43%
2277
16th to endNight
18%5%34434345%42%
2340
December1st to 15thNight
21%8%35%323345%38%
2552
16th to endNight
21%9%24424445%37%
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
43%412%24%14%2
1433
16th to endDay
42%314%25%13%1
1536
February1st to 15thDay
48%413%21%11%1
1621
16th to endDay
48%311%20%13%3
1580
March1st to 15thDay
49%312%17%13%3
1830
16th to endDay
43%48%20%18%5%
2154
April1st to 15thDay
44%48%20%17%3
2152
16th to endDay
43%5%10%21%15%3
2334
May1st to 15thDay
52%48%16%15%1
2332
16th to endDay
47%7%12%19%11%1
2588
June1st to 15thDay
50%5%14%19%9%
2520
16th to endDay
46%6%11%22%11%1
2520
July1st to 15thDay
40%7%14%25%12%
2371
16th to endDay
40%8%15%22%11%1
2487
August1st to 15thDay
39%8%17%23%10%
2338
16th to endDay
33%6%15%26%16%1
2402
September1st to 15thDay
35%5%11%26%18%1
2076
16th to endDay
32%5%9%27%23%2
1980
October1st to 15thDay
43%39%24%16%2
1834
16th to endDay
43%411%24%13%1
1588
November1st to 15thDay
41%5%14%23%14%1
1581
16th to endDay
43%413%19%16%1
1620
December1st to 15thDay
47%413%19%13%
1376
16th to endDay
52%411%19%11%
1536
January1st to 15thNight
43%6%11%23%13%1
2506
16th to endNight
45%410%21%15%2
2688
February1st to 15thNight
50%411%19%12%1
2243
16th to endNight
47%410%21%13%2
1893
March1st to 15thNight
50%39%21%13%2
2130
16th to endNight
41%49%18%19%5%
2069
April1st to 15thNight
47%37%20%17%3
1796
16th to endNight
47%410%19%13%4
1624
May1st to 15thNight
48%6%10%18%13%2
1620
16th to endNight
55%6%10%15%9%1
1539
June1st to 15thNight
54%5%10%18%9%1
1440
16th to endNight
42%6%14%21%12%1
1440
July1st to 15thNight
40%9%14%23%11%
1360
16th to endNight
40%8%14%24%10%
1418
August1st to 15thNight
39%7%15%25%11%
1584
16th to endNight
36%5%11%28%17%
1797
September1st to 15thNight
36%5%10%29%16%1
1884
16th to endNight
31%410%31%20%1
1980
October1st to 15thNight
41%410%25%15%1
1923
16th to endNight
45%411%25%12%1
2282
November1st to 15thNight
42%412%23%15%2
2277
16th to endNight
44%412%21%14%1
2340
December1st to 15thNight
49%311%22%12%
2552
16th to endNight
50%411%21%12%
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
18%20%33%21%6%
60
16th to endDay
18%25%31%21%3
64
February1st to 15thDay
25%10%41%18%4
224
16th to endDay
20%12%39%20%5%
239
March1st to 15thDay
28%16%35%14%4
270
16th to endDay
21%18%34%14%8%3
288
April1st to 15thDay
27%11%33%18%7%2
270
16th to endDay
23%13%34%17%7%3
270
May1st to 15thDay
33%8%31%15%7%3
288
16th to endDay
26%8%34%22%6%1
331
June1st to 15thDay
23%11%34%21%6%1
315
16th to endDay
21%7%34%24%10%1
315
July1st to 15thDay
16%333%27%13%5%
305
16th to endDay
13%332%31%14%5%
326
August1st to 15thDay
15%6%26%27%20%3
313
16th to endDay
15%6%21%28%17%11%
332
September1st to 15thDay
15%6%29%24%17%6%
315
16th to endDay
15%7%29%21%12%13%
314
October1st to 15thDay
20%10%20%28%14%5%
270
16th to endDay
25%12%30%21%10%
240
November1st to 15thDay
24%12%28%22%9%2
235
16th to endDay
27%13%27%22%7%1
240
December1st to 15thDay
25%13%36%19%5%
76
16th to endDay
35%20%28%15%
64
January1st to 15thNight
20%17%37%20%4
720
16th to endNight
22%17%37%19%2
768
February1st to 15thNight
20%16%39%21%3
704
16th to endNight
21%13%35%23%5%
631
March1st to 15thNight
25%13%37%19%3
720
16th to endNight
21%13%33%23%7%1
768
April1st to 15thNight
24%10%35%24%5%
718
16th to endNight
24%11%27%27%7%
544
May1st to 15thNight
25%8%28%27%9%1
540
16th to endNight
26%8%24%28%11%1
417
June1st to 15thNight
27%6%20%32%12%
360
16th to endNight
22%20%40%15%
360
July1st to 15thNight
15%218%40%20%1
340
16th to endNight
17%17%41%22%1
376
August1st to 15thNight
14%218%42%20%1
513
16th to endNight
15%119%37%22%3
651
September1st to 15thNight
17%226%31%19%2
720
16th to endNight
15%5%25%27%21%4
717
October1st to 15thNight
21%7%25%27%15%2
684
16th to endNight
24%11%32%25%6%
704
November1st to 15thNight
22%8%32%26%7%1
700
16th to endNight
21%13%30%23%10%
720
December1st to 15thNight
26%13%36%17%4
728
16th to endNight
25%14%37%18%2
768
Page generated at 2017-10-07 14:07:43. Version ac04

Copyright 2017 A. Danko


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