Iron Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Iron Mountain 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
29%
24881
16%
74706
7%
24821
37%
24821
20%
51280

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%
950
13%
2648
4%
946
27%
946
17%
1904
January11th to 20thNight
17%
930
12%
2603
4%
930
24%
930
14%
1997
January21st to endNight
15%
990
12%
2772
2%
990
23%
990
14%
2119
February1st to 10thNight
16%
900
15%
2416
3%
900
26%
900
18%
1753
February11th to 20thNight
20%
760
19%
2155
6%
756
30%
756
17%
1514
February21st to endNight
17%
592
16%
1776
6%
592
26%
592
15%
1282
March1st to 10thNight
27%
719
18%
2156
9%
718
34%
718
20%
1431
March11th to 20thNight
24%
707
20%
2124
9%
707
35%
707
20%
1385
March21st to endNight
29%
594
20%
1980
13%
594
40%
594
19%
1328
April1st to 10thNight
24%
540
13%
1800
6%
540
35%
540
16%
1277
April11th to 20thNight
29%
533
17%
1775
10%
533
39%
533
18%
1213
April21st to endNight
29%
540
15%
1620
6%
540
39%
540
20%
1114
May1st to 10thNight
30%
540
14%
1519
7%
540
36%
540
19%
1089
May11th to 20thNight
28%
531
17%
1416
7%
531
37%
531
23%
989
May21st to endNight
41%
583
12%
1552
6%
582
45%
582
30%
1010
June1st to 10thNight
43%
540
14%
1440
6%
540
48%
540
32%
907
June11th to 20thNight
44%
540
12%
1440
6%
540
47%
540
28%
953
June21st to endNight
45%
540
14%
1440
6%
540
49%
540
33%
913
July1st to 10thNight
55%
525
16%
1400
9%
525
58%
525
44%
796
July11th to 20thNight
49%
522
16%
1392
8%
522
54%
522
38%
839
July21st to endNight
54%
591
19%
1487
9%
558
59%
558
44%
850
August1st to 10thNight
51%
537
21%
1423
11%
534
57%
534
36%
885
August11th to 20thNight
54%
534
22%
1574
15%
534
62%
534
35%
939
August21st to endNight
51%
594
19%
1980
16%
594
58%
594
32%
1125
September1st to 10thNight
50%
540
22%
1800
18%
540
58%
540
32%
1027
September11th to 20thNight
44%
620
25%
1880
18%
620
53%
620
33%
1106
September21st to endNight
37%
720
18%
1980
11%
720
43%
720
27%
1306
October1st to 10thNight
35%
720
20%
2065
11%
720
42%
720
25%
1321
October11th to 20thNight
24%
720
15%
2232
7%
720
33%
720
18%
1564
October21st to endNight
26%
792
13%
2574
5%
792
34%
792
16%
1815
November1st to 10thNight
24%
740
20%
2406
10%
740
38%
740
18%
1593
November11th to 20thNight
20%
809
15%
2519
5%
809
31%
809
15%
1771
November21st to endNight
21%
950
14%
2660
5%
950
28%
950
16%
1933
December1st to 10thNight
17%
950
12%
2660
2%
950
26%
950
16%
1922
December11th to 20thNight
19%
949
13%
3225
3%
948
28%
948
12%
2421
December21st to endNight
12%
1039
8%
4817
2%
1026
22%
1026
6%
3889

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
14%9%47%6%45%5%5%7%33%
74809
Night
14%10%47%6%345%5%7%35%
75008
TransparencyDay
50%311%23%12%1
74515
Night
49%312%24%11%1
74836
SeeingDay
22%13%32%21%9%3
14865
Night
21%10%31%27%10%1
24890

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
12%10%6%9%8%447%6%7%27%
1517
11th to 20thDay
14%16%5%12%8%35%5%6%6%20%
1480
21st to endDay
12%14%7%10%7%45%6%6%6%23%
1584
February1st to 10thDay
9%11%5%10%6%46%6%6%6%30%
1540
11th to 20thDay
10%13%49%7%35%5%46%33%
1690
21st to endDay
13%13%6%10%6%45%5%5%8%27%
1480
March1st to 10thDay
13%10%5%8%6%45%5%6%7%31%
1798
11th to 20thDay
11%12%49%6%34446%37%
1849
21st to endDay
16%12%5%9%6%5%6%445%28%
2376
April1st to 10thDay
17%14%5%8%6%3445%6%28%
2160
11th to 20thDay
20%11%47%6%35%5%45%31%
2134
21st to endDay
18%10%37%5%35%6%5%7%32%
2340
May1st to 10thDay
17%9%48%7%445%5%7%32%
2438
11th to 20thDay
16%8%36%6%35%5%6%6%36%
2479
21st to endDay
12%6%36%6%46%6%5%9%37%
2725
June1st to 10thDay
13%5%35%6%445%6%8%41%
2520
11th to 20thDay
15%6%36%5%36%5%6%9%36%
2520
21st to endDay
13%7%27%6%46%6%6%9%36%
2520
July1st to 10thDay
8%4246%36%5%7%9%45%
2520
11th to 20thDay
9%5%26%6%46%5%6%8%44%
2520
21st to endDay
8%5%25%5%46%6%7%9%43%
2743
August1st to 10thDay
9%6%35%6%5%6%6%7%8%40%
2507
11th to 20thDay
11%6%35%5%45%5%6%7%42%
2324
21st to endDay
15%5%25%5%35%5%47%44%
2376
September1st to 10thDay
13%5%36%6%4445%7%42%
2160
11th to 20thDay
13%6%36%5%35%5%6%8%39%
2061
21st to endDay
15%8%36%6%46%6%5%9%32%
1980
October1st to 10thDay
17%8%46%6%345%6%8%34%
1891
11th to 20thDay
22%12%37%6%45%5%5%7%24%
1728
21st to endDay
22%14%47%6%44446%26%
1782
November1st to 10thDay
15%10%5%6%6%45%5%6%7%32%
1672
11th to 20thDay
15%12%7%10%7%45%5%5%6%25%
1658
21st to endDay
15%12%6%10%6%45%6%5%7%24%
1520
December1st to 10thDay
11%12%6%10%8%46%5%5%7%26%
1520
11th to 20thDay
16%13%6%10%8%46%5%46%21%
1914
21st to endDay
24%22%7%8%5%3344417%
2783
January1st to 10thNight
13%12%49%5%45%5%6%7%30%
2648
11th to 20thNight
14%13%6%10%9%5%5%5%47%23%
2603
21st to endNight
11%13%6%10%9%5%5%5%6%7%22%
2772
February1st to 10thNight
10%12%6%10%7%36%45%7%30%
2416
11th to 20thNight
10%11%48%8%5%5%5%5%6%33%
2155
21st to endNight
14%15%6%8%6%445%5%6%27%
1776
March1st to 10thNight
12%11%47%5%35%5%5%6%38%
2149
11th to 20thNight
13%8%36%6%35%5%5%6%39%
2124
21st to endNight
14%9%37%7%3445%6%37%
1980
April1st to 10thNight
19%11%46%6%345%5%7%30%
1800
11th to 20thNight
22%8%36%5%24445%36%
1775
21st to endNight
16%8%46%6%45%5%5%7%35%
1620
May1st to 10thNight
17%9%36%7%35%5%5%7%32%
1519
11th to 20thNight
19%8%37%5%245%6%7%35%
1416
21st to endNight
12%425%6%345%7%9%41%
1551
June1st to 10thNight
11%6%35%43445%8%46%
1440
11th to 20thNight
13%7%35%5%45%5%6%7%39%
1440
21st to endNight
13%6%24435%5%6%10%42%
1440
July1st to 10thNight
8%3243345%6%12%51%
1440
11th to 20thNight
10%415%5%345%6%8%49%
1440
21st to endNight
7%32443446%9%53%
1575
August1st to 10thNight
9%426%6%3446%7%48%
1432
11th to 20thNight
9%5%15%5%345%6%7%50%
1574
21st to endNight
12%5%35%434446%51%
1980
September1st to 10thNight
10%6%344245%5%7%51%
1800
11th to 20thNight
12%6%25%43445%7%49%
1880
21st to endNight
16%7%25%5%345%5%7%41%
1980
October1st to 10thNight
15%7%25%5%345%5%6%42%
2065
11th to 20thNight
18%11%47%6%44446%32%
2232
21st to endNight
20%11%46%6%345%5%8%27%
2574
November1st to 10thNight
14%10%46%6%35%5%5%7%35%
2406
11th to 20thNight
16%11%48%6%45%6%5%6%29%
2519
21st to endNight
15%12%5%9%6%45%6%5%6%28%
2660
December1st to 10thNight
12%14%5%8%7%345%6%7%28%
2660
11th to 20thNight
14%11%7%10%8%45%5%5%5%27%
3283
21st to endNight
24%19%7%8%5%3433419%
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
54%5%10%19%10%1
1518
11th to 20thDay
64%39%16%7%1
1480
21st to endDay
61%311%16%8%1
1584
February1st to 10thDay
52%410%20%13%1
1540
11th to 20thDay
51%310%24%10%1
1699
21st to endDay
56%39%19%12%1
1480
March1st to 10thDay
52%311%18%14%2
1800
11th to 20thDay
51%311%23%10%3
1849
21st to endDay
60%210%18%10%1
2376
April1st to 10thDay
57%310%20%10%1
2160
11th to 20thDay
55%37%21%12%2
2134
21st to endDay
53%29%20%13%2
2340
May1st to 10thDay
53%39%21%13%2
2438
11th to 20thDay
48%38%23%15%2
2483
21st to endDay
45%410%25%13%2
2725
June1st to 10thDay
40%311%28%15%2
2520
11th to 20thDay
45%413%22%16%1
2520
21st to endDay
44%414%25%13%1
2520
July1st to 10thDay
35%412%32%17%1
2451
11th to 20thDay
37%312%28%17%3
2445
21st to endDay
36%414%26%18%1
2591
August1st to 10thDay
40%413%29%12%2
2492
11th to 20thDay
41%313%29%13%1
2324
21st to endDay
41%313%26%15%1
2376
September1st to 10thDay
42%312%29%14%1
2160
11th to 20thDay
42%318%29%7%
2061
21st to endDay
48%312%29%8%
1980
October1st to 10thDay
49%312%27%9%
1891
11th to 20thDay
58%311%20%7%1
1728
21st to endDay
60%311%19%6%
1782
November1st to 10thDay
49%411%24%12%1
1672
11th to 20thDay
60%212%16%9%
1658
21st to endDay
58%5%12%19%41
1520
December1st to 10thDay
56%310%23%7%
1520
11th to 20thDay
62%310%16%8%1
1915
21st to endDay
72%26%15%6%
2783
January1st to 10thNight
52%313%20%10%1
2648
11th to 20thNight
61%310%20%6%
2603
21st to endNight
59%48%19%10%1
2772
February1st to 10thNight
54%38%21%12%2
2416
11th to 20thNight
50%410%22%11%3
2156
21st to endNight
56%211%20%10%1
1776
March1st to 10thNight
47%312%24%13%2
2160
11th to 20thNight
45%311%26%14%1
2125
21st to endNight
50%210%22%14%2
1980
April1st to 10thNight
54%39%17%16%1
1800
11th to 20thNight
50%49%23%12%3
1775
21st to endNight
49%410%22%14%2
1620
May1st to 10thNight
51%49%22%11%2
1519
11th to 20thNight
48%37%26%12%2
1417
21st to endNight
40%417%23%14%2
1552
June1st to 10thNight
38%212%31%14%2
1440
11th to 20thNight
43%312%25%15%2
1440
21st to endNight
38%415%29%12%2
1440
July1st to 10thNight
28%315%33%21%1
1400
11th to 20thNight
32%416%31%13%4
1392
21st to endNight
28%418%32%16%2
1487
August1st to 10thNight
35%415%29%14%3
1423
11th to 20thNight
34%414%30%15%2
1574
21st to endNight
34%414%28%18%2
1980
September1st to 10thNight
34%413%33%15%1
1800
11th to 20thNight
36%216%37%9%
1880
21st to endNight
43%315%30%8%
1980
October1st to 10thNight
42%313%30%10%1
2065
11th to 20thNight
53%212%23%9%1
2232
21st to endNight
54%412%22%9%
2574
November1st to 10thNight
48%311%25%13%1
2406
11th to 20thNight
53%412%20%11%1
2519
21st to endNight
54%313%22%6%1
2660
December1st to 10thNight
53%313%22%7%
2660
11th to 20thNight
59%410%20%7%
3281
21st to endNight
70%26%16%5%
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
22%13%33%22%8%1
270
11th to 20thDay
31%14%30%20%41
255
21st to endDay
26%13%37%19%5%
264
February1st to 10thDay
22%24%38%13%2
240
11th to 20thDay
25%28%33%11%3
240
21st to endDay
28%22%42%7%11
198
March1st to 10thDay
20%23%41%13%3
240
11th to 20thDay
26%15%37%16%5%
262
21st to endDay
28%19%27%17%8%1
462
April1st to 10thDay
31%14%30%19%6%
420
11th to 20thDay
32%15%25%21%7%1
412
21st to endDay
34%13%25%18%8%2
420
May1st to 10thDay
27%16%31%14%9%2
521
11th to 20thDay
25%15%33%16%7%3
625
21st to endDay
20%13%32%22%8%5%
679
June1st to 10thDay
17%13%33%23%9%4
630
11th to 20thDay
20%10%37%19%11%3
630
21st to endDay
17%10%31%27%10%4
630
July1st to 10thDay
10%8%33%29%15%5%
621
11th to 20thDay
12%9%36%23%15%6%
618
21st to endDay
10%8%35%27%12%8%
684
August1st to 10thDay
12%8%29%30%15%6%
626
11th to 20thDay
16%6%22%30%17%9%
498
21st to endDay
22%8%25%23%18%4
495
September1st to 10thDay
20%7%26%29%12%6%
450
11th to 20thDay
14%7%32%31%11%4
450
21st to endDay
23%13%25%24%12%4
450
October1st to 10thDay
27%11%27%20%12%2
361
11th to 20thDay
30%20%33%13%4
270
21st to endDay
34%16%36%11%2
297
November1st to 10thDay
26%15%35%17%7%1
270
11th to 20thDay
25%24%34%11%41
270
21st to endDay
27%19%36%11%6%1
270
December1st to 10thDay
23%16%39%14%8%
270
11th to 20thDay
31%16%34%16%3
270
21st to endDay
37%16%31%11%31
297
January1st to 10thNight
19%15%38%22%5%
950
11th to 20thNight
23%14%35%20%7%1
930
21st to endNight
19%11%40%21%8%1
990
February1st to 10thNight
19%14%39%21%6%1
900
11th to 20thNight
22%16%34%20%7%
760
21st to endNight
24%15%35%21%4
592
March1st to 10thNight
20%13%33%26%8%1
720
11th to 20thNight
20%16%36%22%6%1
707
21st to endNight
20%10%30%29%9%1
594
April1st to 10thNight
25%12%31%24%7%1
540
11th to 20thNight
28%9%29%26%7%1
533
21st to endNight
23%10%32%27%7%1
540
May1st to 10thNight
23%10%30%28%8%1
540
11th to 20thNight
24%8%33%27%7%2
531
21st to endNight
15%6%29%36%13%1
584
June1st to 10thNight
16%8%26%39%12%
540
11th to 20thNight
19%6%25%39%10%1
540
21st to endNight
16%427%36%15%1
540
July1st to 10thNight
11%425%40%18%1
525
11th to 20thNight
13%327%37%20%
522
21st to endNight
10%228%40%18%2
591
August1st to 10thNight
12%225%40%19%2
537
11th to 20thNight
12%322%34%26%3
534
21st to endNight
14%424%38%20%1
594
September1st to 10thNight
13%6%25%31%21%4
540
11th to 20thNight
14%6%30%33%15%2
620
21st to endNight
20%9%27%29%13%1
720
October1st to 10thNight
22%9%28%26%14%1
720
11th to 20thNight
25%13%30%24%8%
720
21st to endNight
30%10%29%24%7%1
792
November1st to 10thNight
21%12%35%24%8%1
740
11th to 20thNight
26%15%32%20%7%
809
21st to endNight
24%17%30%24%6%
950
December1st to 10thNight
23%15%36%20%6%1
950
11th to 20thNight
22%13%32%24%7%2
950
21st to endNight
31%13%31%19%5%
1045
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