Marathon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marathon clear sky chart from 2005-11-06 to 2024-02-10.

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
28%
23025
12%
80435
8%
22930
34%
22930
13%
59092

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
12%
756
7%
4413
4%
737
18%
737
4%
3538
January11th to 20thNight
14%
758
7%
4395
4%
740
20%
740
4%
3492
January21st to endNight
12%
831
5%
4540
3%
814
20%
814
4%
3831
February1st to 10thNight
13%
759
9%
2464
4%
759
20%
759
9%
1968
February11th to 20thNight
10%
703
9%
2228
2%
700
19%
700
8%
1784
February21st to endNight
13%
592
12%
1776
6%
592
21%
592
11%
1359
March1st to 10thNight
15%
720
12%
2124
5%
720
24%
720
12%
1614
March11th to 20thNight
19%
720
13%
1980
7%
720
25%
720
14%
1506
March21st to endNight
20%
792
16%
2080
5%
792
28%
792
16%
1564
April1st to 10thNight
18%
720
13%
1800
3%
720
24%
720
16%
1372
April11th to 20thNight
23%
711
14%
1775
5%
711
28%
711
21%
1292
April21st to endNight
25%
720
15%
1800
4%
720
30%
720
20%
1310
May1st to 10thNight
31%
419
15%
1499
11%
419
37%
419
14%
1055
May11th to 20thNight
35%
354
18%
1416
15%
354
45%
354
18%
895
May21st to endNight
43%
388
16%
1551
16%
388
50%
388
19%
964
June1st to 10thNight
41%
360
16%
1440
18%
360
51%
360
19%
890
June11th to 20thNight
41%
360
14%
1440
15%
360
49%
360
19%
881
June21st to endNight
48%
360
14%
1440
16%
360
55%
360
22%
856
July1st to 10thNight
60%
350
13%
1400
16%
350
64%
350
24%
831
July11th to 20thNight
56%
348
17%
1392
22%
348
60%
348
25%
807
July21st to endNight
54%
394
16%
1486
16%
372
57%
372
21%
916
August1st to 10thNight
54%
524
16%
1588
12%
522
57%
522
28%
989
August11th to 20thNight
55%
592
14%
1656
11%
592
57%
592
25%
1143
August21st to endNight
54%
792
19%
1980
10%
792
56%
792
31%
1328
September1st to 10thNight
46%
720
16%
1800
7%
720
49%
720
29%
1232
September11th to 20thNight
50%
720
25%
1800
15%
720
54%
720
33%
1177
September21st to endNight
41%
720
19%
2031
15%
720
46%
720
24%
1397
October1st to 10thNight
35%
720
14%
2160
11%
720
41%
720
19%
1532
October11th to 20thNight
27%
720
14%
2160
7%
720
36%
720
18%
1546
October21st to endNight
19%
792
12%
2449
7%
792
27%
792
12%
1889
November1st to 10thNight
27%
740
17%
2407
11%
740
37%
740
18%
1645
November11th to 20thNight
19%
760
15%
2551
9%
760
27%
760
13%
1854
November21st to endNight
20%
760
11%
2660
7%
760
27%
760
11%
1984
December1st to 10thNight
14%
760
7%
2660
4%
760
18%
760
7%
2146
December11th to 20thNight
13%
760
7%
3247
3%
759
18%
759
6%
2557
December21st to endNight
12%
830
4%
4847
3%
817
17%
817
3%
3948

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%11%5%8%7%35%5%5%7%28%
78200
Night
17%10%5%8%7%35%5%5%6%28%
80756
TransparencyDay
55%7%11%18%9%1
77905
Night
55%7%12%17%8%1
80585
SeeingDay
25%17%29%18%9%3
13661
Night
23%16%28%23%9%1
23043

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
14%17%15%12%7%3443418%
2517
11th to 20thDay
15%17%8%17%8%5%433415%
2520
21st to endDay
18%18%11%10%11%4434413%
3101
February1st to 10thDay
15%16%6%13%9%46%45%5%17%
1701
11th to 20thDay
14%14%9%13%8%46%5%36%18%
1618
21st to endDay
20%13%5%11%10%34435%21%
1480
March1st to 10thDay
15%12%6%13%7%445%5%5%24%
1828
11th to 20thDay
17%14%7%8%7%34445%27%
1980
21st to endDay
20%13%5%7%5%35%35%5%29%
2276
April1st to 10thDay
18%12%5%8%6%34446%29%
2160
11th to 20thDay
18%10%47%5%35%446%34%
2134
21st to endDay
20%13%36%6%3445%6%31%
2160
May1st to 10thDay
19%10%37%7%35%45%6%31%
2474
11th to 20thDay
19%6%36%6%36%6%47%34%
2478
21st to endDay
15%6%36%5%35%5%6%8%37%
2725
June1st to 10thDay
19%7%36%5%35%5%6%8%33%
2520
11th to 20thDay
16%7%35%6%36%5%7%9%33%
2520
21st to endDay
15%6%26%7%36%5%7%9%33%
2520
July1st to 10thDay
13%5%36%6%46%6%6%10%36%
2520
11th to 20thDay
11%6%36%6%5%6%6%7%9%36%
2520
21st to endDay
13%5%37%6%46%6%6%9%34%
2744
August1st to 10thDay
13%5%25%7%46%5%7%11%34%
2337
11th to 20thDay
14%7%37%7%45%6%6%9%32%
2227
21st to endDay
16%6%36%6%46%6%7%9%33%
2376
September1st to 10thDay
17%6%36%6%3446%8%37%
2160
11th to 20thDay
15%6%35%6%34447%43%
2160
21st to endDay
19%6%35%6%45%5%5%7%35%
1925
October1st to 10thDay
19%9%47%6%36%5%5%6%31%
1800
11th to 20thDay
20%8%36%8%34447%31%
1800
21st to endDay
23%14%47%6%36%445%24%
1905
November1st to 10thDay
14%11%47%5%35%5%5%7%34%
1671
11th to 20thDay
16%13%5%10%6%45%446%27%
1625
21st to endDay
19%16%6%9%7%45%446%20%
1520
December1st to 10thDay
20%18%7%12%6%345%5%5%16%
1520
11th to 20thDay
21%14%8%13%6%2434421%
1906
21st to endDay
23%21%10%10%7%3433413%
2772
January1st to 10thNight
17%14%7%16%8%35%5%5%6%15%
4429
11th to 20thNight
13%16%8%12%11%45%445%17%
4440
21st to endNight
22%13%6%13%13%4443314%
4551
February1st to 10thNight
15%12%6%13%8%46%5%5%6%20%
2464
11th to 20thNight
14%11%7%11%10%47%6%6%6%19%
2226
21st to endNight
18%11%5%12%7%35%5%6%6%22%
1776
March1st to 10thNight
14%10%7%13%7%35%45%8%25%
2114
11th to 20thNight
17%11%5%8%8%445%5%5%28%
1979
21st to endNight
17%11%48%6%25%5%6%6%30%
2080
April1st to 10thNight
19%11%5%6%6%35%447%29%
1800
11th to 20thNight
18%10%36%6%3436%7%34%
1775
21st to endNight
19%9%35%5%4446%6%35%
1800
May1st to 10thNight
20%11%47%6%3445%5%32%
1499
11th to 20thNight
13%6%36%5%37%6%6%8%38%
1415
21st to endNight
13%6%35%6%346%5%8%40%
1550
June1st to 10thNight
16%7%35%5%335%6%8%39%
1440
11th to 20thNight
15%5%35%5%3446%10%39%
1440
21st to endNight
14%425%5%446%6%7%44%
1440
July1st to 10thNight
8%5%36%435%5%7%10%43%
1440
11th to 20thNight
12%4245%35%5%7%10%44%
1440
21st to endNight
11%6%36%5%35%5%6%8%42%
1574
August1st to 10thNight
10%5%25%5%35%5%5%9%44%
1598
11th to 20thNight
12%5%37%5%35%6%6%8%39%
1656
21st to endNight
14%5%245%35%5%5%7%43%
1980
September1st to 10thNight
16%5%25%5%445%5%7%43%
1800
11th to 20thNight
15%6%24433447%48%
1800
21st to endNight
16%6%25%6%36%5%47%39%
2031
October1st to 10thNight
19%9%36%5%3445%7%34%
2160
11th to 20thNight
20%10%36%5%3445%7%34%
2160
21st to endNight
25%14%47%6%35%445%25%
2449
November1st to 10thNight
12%13%47%6%3445%6%36%
2407
11th to 20thNight
19%10%5%7%5%45%5%5%7%28%
2551
21st to endNight
15%14%6%7%6%35%5%6%7%24%
2660
December1st to 10thNight
20%14%7%11%7%45%45%5%18%
2660
11th to 20thNight
19%13%7%11%7%46%5%45%19%
3288
21st to endNight
26%14%9%8%6%3445%5%16%
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
68%411%12%4
2518
11th to 20thDay
73%49%9%41
2520
21st to endDay
74%5%9%8%3
3101
February1st to 10thDay
70%5%9%12%4
1701
11th to 20thDay
68%5%7%14%5%
1627
21st to endDay
65%6%10%13%5%1
1480
March1st to 10thDay
61%5%12%14%6%1
1831
11th to 20thDay
59%5%9%15%10%2
1980
21st to endDay
57%5%8%16%12%2
2276
April1st to 10thDay
55%6%7%21%9%2
2160
11th to 20thDay
51%6%10%18%13%2
2134
21st to endDay
53%6%7%19%11%3
2160
May1st to 10thDay
54%6%9%19%10%2
2474
11th to 20thDay
48%8%10%22%11%2
2482
21st to endDay
43%8%16%20%12%1
2725
June1st to 10thDay
48%8%11%20%13%1
2520
11th to 20thDay
45%8%14%21%10%1
2520
21st to endDay
46%9%13%22%11%1
2520
July1st to 10thDay
43%8%12%24%11%1
2450
11th to 20thDay
40%9%17%24%9%
2445
21st to endDay
45%8%13%21%12%1
2591
August1st to 10thDay
42%10%15%22%10%1
2323
11th to 20thDay
47%10%14%22%8%
2227
21st to endDay
46%8%11%23%10%1
2376
September1st to 10thDay
45%6%13%24%12%1
2160
11th to 20thDay
42%5%11%23%17%1
2160
21st to endDay
48%6%11%22%12%1
1925
October1st to 10thDay
52%7%10%19%11%1
1800
11th to 20thDay
53%7%11%21%8%
1800
21st to endDay
62%6%12%12%7%1
1905
November1st to 10thDay
48%7%14%18%11%2
1671
11th to 20thDay
58%5%13%15%7%2
1625
21st to endDay
63%6%12%14%4
1520
December1st to 10thDay
68%7%11%10%3
1520
11th to 20thDay
67%6%13%10%4
1906
21st to endDay
76%47%10%3
2772
January1st to 10thNight
68%7%11%9%4
4429
11th to 20thNight
68%7%9%12%4
4440
21st to endNight
75%47%10%3
4551
February1st to 10thNight
62%8%11%12%6%
2464
11th to 20thNight
64%7%10%13%5%1
2228
21st to endNight
59%6%12%14%7%1
1776
March1st to 10thNight
59%6%12%17%6%1
2124
11th to 20thNight
56%6%10%18%8%1
1980
21st to endNight
54%6%9%17%12%2
2080
April1st to 10thNight
55%6%10%18%9%2
1800
11th to 20thNight
49%5%12%17%15%1
1775
21st to endNight
49%6%11%19%12%3
1800
May1st to 10thNight
54%5%8%18%12%2
1499
11th to 20thNight
41%10%12%21%14%2
1416
21st to endNight
40%9%14%23%12%2
1551
June1st to 10thNight
42%7%12%25%14%1
1440
11th to 20thNight
39%9%15%24%11%2
1440
21st to endNight
37%9%14%27%13%1
1440
July1st to 10thNight
33%11%15%23%15%2
1400
11th to 20thNight
35%8%15%28%13%1
1392
21st to endNight
39%9%14%21%15%2
1486
August1st to 10thNight
35%10%15%28%11%1
1588
11th to 20thNight
42%9%17%24%8%
1656
21st to endNight
38%7%15%25%14%1
1980
September1st to 10thNight
40%8%14%24%14%2
1800
11th to 20thNight
36%6%11%27%18%1
1800
21st to endNight
44%7%12%22%14%1
2031
October1st to 10thNight
50%7%11%21%11%1
2160
11th to 20thNight
50%7%14%19%9%1
2160
21st to endNight
61%5%12%14%7%1
2449
November1st to 10thNight
48%6%12%18%14%2
2407
11th to 20thNight
53%7%12%17%8%2
2551
21st to endNight
57%8%15%16%4
2660
December1st to 10thNight
66%7%13%12%3
2660
11th to 20thNight
66%6%11%13%4
3288
21st to endNight
70%7%10%12%2
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
31%22%34%9%21
180
11th to 20thDay
39%27%27%321
180
21st to endDay
27%29%32%9%3
292
February1st to 10thDay
31%31%31%6%1
267
11th to 20thDay
35%32%28%4
285
21st to endDay
37%24%27%10%1
346
March1st to 10thDay
33%22%29%12%4
420
11th to 20thDay
31%25%28%13%4
420
21st to endDay
34%24%27%9%5%1
462
April1st to 10thDay
34%20%28%13%41
420
11th to 20thDay
28%24%25%11%8%3
412
21st to endDay
33%19%27%15%5%2
420
May1st to 10thDay
30%17%31%12%5%4
423
11th to 20thDay
23%16%32%17%10%2
447
21st to endDay
19%13%30%22%13%4
485
June1st to 10thDay
22%14%30%21%10%2
450
11th to 20thDay
18%15%28%24%12%3
450
21st to endDay
18%14%35%24%7%2
450
July1st to 10thDay
11%10%28%29%17%5%
445
11th to 20thDay
13%7%29%30%16%5%
444
21st to endDay
14%7%36%27%12%3
488
August1st to 10thDay
14%7%28%25%18%7%
446
11th to 20thDay
16%9%25%26%16%8%
439
21st to endDay
14%11%28%25%13%8%
495
September1st to 10thDay
20%12%22%24%17%7%
450
11th to 20thDay
16%9%26%23%17%9%
450
21st to endDay
23%13%26%19%14%5%
450
October1st to 10thDay
26%12%26%16%14%6%
450
11th to 20thDay
25%18%28%20%8%1
450
21st to endDay
35%18%26%17%5%
457
November1st to 10thDay
24%18%26%21%9%1
365
11th to 20thDay
31%23%25%16%5%
285
21st to endDay
32%24%32%9%2
180
December1st to 10thDay
34%23%29%8%41
180
11th to 20thDay
37%22%29%10%11
180
21st to endDay
35%23%33%8%11
198
January1st to 10thNight
26%25%32%14%2
760
11th to 20thNight
28%21%30%18%3
760
21st to endNight
20%23%36%17%3
836
February1st to 10thNight
27%20%34%17%1
759
11th to 20thNight
25%25%35%14%1
703
21st to endNight
30%21%31%15%3
592
March1st to 10thNight
24%24%32%18%3
720
11th to 20thNight
27%20%31%18%4
720
21st to endNight
23%23%30%16%7%1
792
April1st to 10thNight
28%20%29%19%3
720
11th to 20thNight
26%18%29%19%7%
711
21st to endNight
26%16%30%23%5%
720
May1st to 10thNight
27%11%26%27%10%
419
11th to 20thNight
16%11%35%30%8%1
354
21st to endNight
16%7%28%34%15%1
389
June1st to 10thNight
19%8%26%36%11%1
360
11th to 20thNight
18%9%26%32%15%
360
21st to endNight
12%6%28%42%12%
360
July1st to 10thNight
8%422%44%20%2
350
11th to 20thNight
11%323%39%22%1
348
21st to endNight
13%422%42%18%2
394
August1st to 10thNight
11%5%23%35%22%3
524
11th to 20thNight
14%5%17%34%27%4
592
21st to endNight
17%6%17%35%24%1
792
September1st to 10thNight
19%6%23%33%17%3
720
11th to 20thNight
16%8%20%28%23%4
720
21st to endNight
18%10%26%27%17%2
720
October1st to 10thNight
25%11%23%24%15%2
720
11th to 20thNight
26%13%28%25%7%1
720
21st to endNight
33%15%28%20%4
792
November1st to 10thNight
21%16%29%26%8%
740
11th to 20thNight
27%17%31%17%7%1
760
21st to endNight
24%21%30%20%5%
760
December1st to 10thNight
29%24%29%15%4
760
11th to 20thNight
26%25%29%16%3
760
21st to endNight
30%22%32%15%2
836
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