Magnuson Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Magnuson Observing Site clear sky chart from 2005-11-05 to 2017-09-22.

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
36%
16100
17%
46936
9%
16053
43%
16053
26%
30750

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
25%
900
12%
2506
6%
895
32%
895
19%
1827
January16th to endNight
22%
960
13%
2520
5%
960
28%
960
17%
1922
February1st to 15thNight
24%
879
13%
2288
3%
875
31%
875
21%
1656
February16th to endNight
28%
735
18%
1997
6%
735
37%
735
25%
1342
March1st to 15thNight
32%
579
17%
2019
11%
579
40%
579
23%
1250
March16th to endNight
31%
576
13%
2096
10%
576
37%
576
17%
1415
April1st to 15thNight
31%
539
17%
1797
12%
539
41%
539
22%
1161
April16th to endNight
28%
540
17%
1698
11%
540
39%
540
21%
1177
May1st to 15thNight
36%
540
17%
1514
10%
540
42%
540
27%
1010
May16th to endNight
41%
563
16%
1497
6%
561
47%
561
31%
985
June1st to 15thNight
48%
540
15%
1440
6%
540
52%
540
33%
916
June16th to endNight
57%
540
18%
1440
7%
540
60%
540
45%
829
July1st to 15thNight
65%
510
15%
1360
7%
510
68%
510
48%
750
July16th to endNight
69%
564
24%
1459
16%
531
71%
531
48%
784
August1st to 15thNight
65%
539
23%
1610
22%
536
69%
536
39%
898
August16th to endNight
57%
573
22%
1812
21%
573
62%
573
36%
997
September1st to 15thNight
58%
540
26%
1800
24%
540
67%
540
39%
936
September16th to endNight
45%
560
23%
1887
15%
560
56%
560
32%
1053
October1st to 15thNight
37%
660
25%
1980
15%
660
46%
660
33%
1187
October16th to endNight
34%
704
18%
2113
10%
704
42%
704
24%
1384
November1st to 15thNight
31%
799
22%
2375
10%
799
41%
799
27%
1479
November16th to endNight
24%
900
16%
2520
7%
900
32%
900
21%
1766
December1st to 15thNight
20%
900
10%
2520
2%
900
27%
900
16%
1895
December16th to endNight
19%
960
12%
2688
5%
960
26%
960
13%
2131

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%8%36%5%35%45%7%36%
47945
Night
12%8%36%5%24446%40%
47109
Cloud CoverDay
26%7%25%423446%31%
47956
Night
24%7%25%423345%35%
47113
Cloud CoverDay
14%8%36%5%3445%7%35%
47954
Night
12%8%36%5%3445%6%40%
47113
TransparencyDay
46%414%23%10%
47636
Night
43%313%24%12%1
46937
SeeingDay
17%7%27%28%14%4
6108
Night
16%8%31%30%12%1
16101

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
9%10%6%9%8%5%7%5%5%8%22%
1432
16th to endDay
10%13%6%11%8%46%5%6%7%19%
1702
February1st to 15thDay
8%13%6%9%8%47%5%5%5%24%
1575
16th to endDay
13%9%37%5%38%45%8%29%
1474
March1st to 15thDay
15%10%5%7%6%335%46%31%
1939
16th to endDay
18%9%47%43435%6%32%
2127
April1st to 15thDay
18%10%46%5%2435%6%31%
2151
16th to endDay
23%11%35%424446%27%
2259
May1st to 15thDay
20%8%36%5%25%45%7%29%
2436
16th to endDay
16%6%26%4245%5%7%38%
2617
June1st to 15thDay
16%5%35%5%2445%7%40%
2520
16th to endDay
10%5%25%4345%6%8%43%
2520
July1st to 15thDay
6%3245%35%5%6%8%48%
2520
16th to endDay
7%414435%5%7%10%43%
2607
August1st to 15thDay
9%5%25%5%35%6%6%8%42%
2340
16th to endDay
13%425%4345%5%7%43%
2390
September1st to 15thDay
11%5%145%2445%7%47%
2160
16th to endDay
14%6%25%42435%7%42%
1871
October1st to 15thDay
16%7%24323336%44%
1650
16th to endDay
17%10%25%423446%38%
1757
November1st to 15thDay
11%8%36%424446%43%
1482
16th to endDay
14%11%47%5%24346%35%
1440
December1st to 15thDay
17%10%5%8%7%5%7%5%45%22%
1440
16th to endDay
18%15%6%10%6%45%345%18%
1536
January1st to 15thNight
10%8%48%7%37%5%5%7%29%
2506
16th to endNight
11%10%5%11%7%45%5%5%7%25%
2520
February1st to 15thNight
9%12%5%9%7%33446%32%
2285
16th to endNight
9%8%37%6%445%5%6%38%
1997
March1st to 15thNight
14%9%35%323347%41%
2007
16th to endNight
15%10%36%5%34446%33%
2096
April1st to 15thNight
17%8%34424346%39%
1797
16th to endNight
23%9%25%423345%35%
1698
May1st to 15thNight
17%6%24324436%43%
1514
16th to endNight
13%6%345%2346%8%41%
1496
June1st to 15thNight
10%325%6%3446%9%44%
1440
16th to endNight
8%21332336%9%54%
1440
July1st to 15thNight
421343346%8%58%
1440
16th to endNight
43133235%310%58%
1558
August1st to 15thNight
5%3245%2445%8%51%
1620
16th to endNight
9%423413346%56%
1812
September1st to 15thNight
10%31331235%5%58%
1800
16th to endNight
10%6%23324347%50%
1887
October1st to 15thNight
14%6%23413346%49%
1980
16th to endNight
16%8%25%423445%40%
2113
November1st to 15thNight
11%7%25%5%24346%44%
2375
16th to endNight
13%11%46%6%23345%36%
2520
December1st to 15thNight
13%11%5%8%6%45%5%5%6%27%
2520
16th to endNight
18%13%5%10%7%34445%22%
2688

Percentage Cloud Cover SW (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
23%8%48%8%45%448%19%
1432
16th to endDay
24%12%49%7%45%45%5%15%
1702
February1st to 15thDay
22%12%48%7%4444422%
1575
16th to endDay
25%9%27%5%35%446%25%
1474
March1st to 15thDay
27%8%36%5%23445%27%
1950
16th to endDay
30%8%35%42335%5%27%
2127
April1st to 15thDay
31%8%35%5%2335%5%25%
2151
16th to endDay
36%10%24323345%21%
2259
May1st to 15thDay
33%7%26%424346%25%
2436
16th to endDay
31%5%15%323445%31%
2617
June1st to 15thDay
30%424423445%32%
2520
16th to endDay
25%414423446%36%
2520
July1st to 15thDay
22%3145%2445%6%39%
2520
16th to endDay
23%3144245%5%8%35%
2607
August1st to 15thDay
16%4244346%5%7%39%
2340
16th to endDay
19%414333447%41%
2390
September1st to 15thDay
17%414424447%44%
2160
16th to endDay
20%6%2442335%6%39%
1871
October1st to 15thDay
24%6%14323336%41%
1650
16th to endDay
24%8%15%422346%35%
1757
November1st to 15thDay
24%6%25%31344438%
1482
16th to endDay
28%9%25%422335%32%
1440
December1st to 15thDay
31%9%38%6%445%3418%
1440
16th to endDay
31%12%5%9%6%3333316%
1536
January1st to 15thNight
23%7%37%6%36%446%26%
2506
16th to endNight
25%10%49%5%34446%20%
2520
February1st to 15thNight
23%11%48%6%3233428%
2278
16th to endNight
22%7%25%5%34445%33%
1997
March1st to 15thNight
26%8%34322246%36%
2019
16th to endNight
28%8%26%5%23345%28%
2096
April1st to 15thNight
31%7%2342334433%
1797
16th to endNight
36%8%1431324429%
1698
May1st to 15thNight
31%5%1332223437%
1513
16th to endNight
29%5%2342235%7%34%
1496
June1st to 15thNight
25%315%5%2435%8%35%
1440
16th to endNight
23%21232235%7%45%
1440
July1st to 15thNight
20%21332335%7%47%
1440
16th to endNight
20%213213428%49%
1558
August1st to 15thNight
13%31442345%7%48%
1620
16th to endNight
16%412312345%53%
1812
September1st to 15thNight
16%313312345%54%
1800
16th to endNight
17%5%13323246%48%
1887
October1st to 15thNight
22%5%13413245%45%
1980
16th to endNight
23%7%25%423435%38%
2113
November1st to 15thNight
24%6%25%412335%40%
2375
16th to endNight
26%9%36%5%2233433%
2520
December1st to 15thNight
28%10%47%6%33445%22%
2520
16th to endNight
32%11%39%6%3323419%
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
9%10%7%9%8%5%7%5%5%8%23%
1432
16th to endDay
10%14%6%12%8%46%46%7%18%
1702
February1st to 15thDay
8%12%7%9%8%47%5%6%5%23%
1575
16th to endDay
13%9%37%6%36%45%8%29%
1474
March1st to 15thDay
14%10%5%7%6%34446%31%
1950
16th to endDay
19%9%37%43445%6%32%
2125
April1st to 15thDay
17%9%47%5%3435%6%31%
2151
16th to endDay
23%12%36%42445%5%27%
2259
May1st to 15thDay
20%8%36%435%45%7%29%
2436
16th to endDay
16%6%25%5%2446%7%38%
2617
June1st to 15thDay
15%5%25%5%2435%7%41%
2520
16th to endDay
10%425%5%245%6%8%43%
2520
July1st to 15thDay
6%3244345%6%9%48%
2520
16th to endDay
7%325%435%6%7%10%43%
2607
August1st to 15thDay
9%5%25%436%6%6%8%41%
2340
16th to endDay
12%5%25%4345%5%8%43%
2390
September1st to 15thDay
11%42442445%7%47%
2160
16th to endDay
14%6%25%42435%7%41%
1871
October1st to 15thDay
15%8%24323346%44%
1650
16th to endDay
18%10%25%5%23347%37%
1757
November1st to 15thDay
11%8%36%323447%42%
1482
16th to endDay
13%11%5%7%5%23447%34%
1440
December1st to 15thDay
16%10%48%8%5%7%5%45%22%
1440
16th to endDay
18%15%6%10%7%45%24419%
1536
January1st to 15thNight
10%8%5%8%7%36%5%5%8%28%
2506
16th to endNight
11%10%6%11%7%45%5%5%6%26%
2520
February1st to 15thNight
9%12%5%9%7%34446%33%
2278
16th to endNight
9%8%37%6%35%5%47%38%
1997
March1st to 15thNight
14%9%35%423347%41%
2019
16th to endNight
15%9%36%5%3445%6%33%
2096
April1st to 15thNight
17%9%24434347%39%
1797
16th to endNight
23%9%15%424245%35%
1698
May1st to 15thNight
18%5%24324346%43%
1513
16th to endNight
13%6%25%42445%8%41%
1496
June1st to 15thNight
10%425%5%335%6%8%44%
1440
16th to endNight
8%31342345%8%55%
1440
July1st to 15thNight
421343336%8%58%
1440
16th to endNight
5%313314449%59%
1558
August1st to 15thNight
5%225%5%2436%8%51%
1620
16th to endNight
10%423323346%56%
1812
September1st to 15thNight
9%413323346%57%
1800
16th to endNight
10%5%23324347%51%
1887
October1st to 15thNight
15%5%23313346%49%
1980
16th to endNight
16%8%25%434346%40%
2113
November1st to 15thNight
11%7%35%424346%45%
2375
16th to endNight
13%12%46%6%23345%36%
2520
December1st to 15thNight
13%11%5%8%7%45%45%6%27%
2520
16th to endNight
18%13%5%10%7%34345%22%
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
58%411%15%9%
1433
16th to endDay
62%314%14%4
1702
February1st to 15thDay
59%312%17%6%
1576
16th to endDay
51%5%11%22%7%1
1474
March1st to 15thDay
52%414%20%7%
1950
16th to endDay
53%415%21%5%
2127
April1st to 15thDay
53%411%20%9%
2151
16th to endDay
57%413%17%6%1
2259
May1st to 15thDay
53%414%21%5%
2436
16th to endDay
45%414%22%11%1
2621
June1st to 15thDay
42%414%26%12%
2520
16th to endDay
37%6%15%27%13%1
2520
July1st to 15thDay
32%5%16%31%13%1
2371
16th to endDay
32%6%16%28%14%1
2444
August1st to 15thDay
36%6%14%29%11%1
2326
16th to endDay
38%5%13%28%13%1
2390
September1st to 15thDay
34%413%33%13%
2160
16th to endDay
42%313%29%10%
1871
October1st to 15thDay
40%213%25%15%1
1650
16th to endDay
46%315%25%9%
1757
November1st to 15thDay
41%413%23%15%1
1482
16th to endDay
49%320%18%7%
1440
December1st to 15thDay
61%414%12%5%
1440
16th to endDay
68%38%14%4
1536
January1st to 15thNight
52%5%13%20%9%
2506
16th to endNight
57%313%17%7%
2520
February1st to 15thNight
51%313%22%8%
2288
16th to endNight
44%213%25%11%1
1997
March1st to 15thNight
42%315%26%10%1
2019
16th to endNight
50%415%22%6%
2096
April1st to 15thNight
46%29%25%13%1
1797
16th to endNight
50%310%19%13%2
1698
May1st to 15thNight
42%5%12%25%12%1
1514
16th to endNight
39%5%14%25%14%1
1498
June1st to 15thNight
36%5%14%30%12%1
1440
16th to endNight
25%5%16%29%20%2
1440
July1st to 15thNight
24%417%33%17%3
1360
16th to endNight
21%5%14%34%19%3
1459
August1st to 15thNight
28%413%31%18%2
1610
16th to endNight
29%415%28%18%3
1812
September1st to 15thNight
27%311%32%24%
1800
16th to endNight
33%314%30%17%1
1887
October1st to 15thNight
35%311%29%17%2
1980
16th to endNight
44%313%23%13%
2113
November1st to 15thNight
40%312%23%17%2
2375
16th to endNight
49%314%23%7%1
2520
December1st to 15thNight
56%414%17%6%
2520
16th to endNight
64%310%14%6%
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
25%7%34%25%5%1
90
16th to endDay
18%12%36%25%5%2
96
February1st to 15thDay
7%20%45%24%2
90
16th to endDay
25%9%27%27%8%1
81
March1st to 15thDay
26%8%32%28%21
90
16th to endDay
33%11%25%23%6%
111
April1st to 15thDay
24%10%29%25%9%
270
16th to endDay
31%11%25%20%8%1
369
May1st to 15thDay
20%11%34%22%8%2
468
16th to endDay
21%9%26%25%13%2
517
June1st to 15thDay
16%5%25%31%16%4
495
16th to endDay
11%7%29%30%17%2
495
July1st to 15thDay
7%226%33%20%9%
475
16th to endDay
9%323%34%22%6%
515
August1st to 15thDay
10%319%36%24%6%
493
16th to endDay
15%320%31%19%9%
430
September1st to 15thDay
11%5%26%27%20%8%
315
16th to endDay
17%7%31%26%11%4
156
October1st to 15thDay
8%10%34%25%16%4
90
16th to endDay
28%12%30%20%8%
96
November1st to 15thDay
5%7%27%46%7%4
90
16th to endDay
34%12%27%21%4
90
December1st to 15thDay
23%14%34%22%5%
90
16th to endDay
31%17%33%15%2
96
January1st to 15thNight
15%11%39%25%7%1
900
16th to endNight
17%11%38%25%6%
960
February1st to 15thNight
16%11%37%26%7%1
880
16th to endNight
14%13%37%24%9%
735
March1st to 15thNight
18%8%34%27%9%
579
16th to endNight
19%8%32%27%10%1
576
April1st to 15thNight
22%10%32%29%5%
539
16th to endNight
28%6%30%25%7%1
540
May1st to 15thNight
21%6%33%29%7%1
540
16th to endNight
17%6%28%33%14%
563
June1st to 15thNight
13%426%39%15%
540
16th to endNight
9%227%46%14%
540
July1st to 15thNight
4222%38%30%1
510
16th to endNight
4217%48%25%2
564
August1st to 15thNight
6%217%41%28%3
539
16th to endNight
10%221%38%22%3
573
September1st to 15thNight
12%222%35%22%5%
540
16th to endNight
12%7%29%31%17%1
560
October1st to 15thNight
18%6%35%25%14%1
660
16th to endNight
20%9%29%31%8%
704
November1st to 15thNight
16%11%36%25%8%1
799
16th to endNight
18%14%36%21%9%
900
December1st to 15thNight
19%10%39%25%5%
900
16th to endNight
25%9%33%24%6%
960
Page generated at 2017-09-23 05:43:42. Version ac04

Copyright 2017 A. Danko


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