Mount Saint Helens
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Saint Helens clear sky chart from 2005-11-05 to 2017-08-12.

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
37%
14450
11%
46385
10%
14421
39%
14421
15%
34473

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
27%
720
9%
2506
8%
716
30%
716
9%
2029
January16th to endNight
28%
768
11%
2628
9%
768
32%
768
11%
2038
February1st to 15thNight
25%
697
6%
2288
5%
697
27%
697
9%
1892
February16th to endNight
24%
632
8%
1996
7%
632
27%
632
10%
1635
March1st to 15thNight
24%
720
5%
2067
4%
720
25%
720
8%
1789
March16th to endNight
20%
768
4%
2064
2%
768
20%
768
7%
1834
April1st to 15thNight
23%
719
6%
1799
3%
719
24%
719
10%
1544
April16th to endNight
36%
601
8%
1681
6%
601
36%
601
13%
1340
May1st to 15thNight
41%
359
12%
1439
15%
359
45%
359
14%
999
May16th to endNight
43%
375
7%
1496
10%
374
45%
374
11%
1118
June1st to 15thNight
40%
360
6%
1440
8%
360
41%
360
13%
1037
June16th to endNight
51%
360
11%
1332
15%
360
52%
360
22%
836
July1st to 15thNight
72%
340
14%
1345
23%
340
74%
340
37%
638
July16th to endNight
78%
376
23%
1416
37%
354
81%
354
48%
588
August1st to 15thNight
76%
460
23%
1510
25%
458
77%
458
48%
695
August16th to endNight
69%
607
26%
1657
19%
607
70%
607
47%
891
September1st to 15thNight
69%
660
25%
1650
16%
660
70%
660
49%
936
September16th to endNight
53%
656
20%
1804
17%
656
54%
656
28%
1191
October1st to 15thNight
44%
660
14%
1980
15%
660
46%
660
21%
1377
October16th to endNight
37%
704
10%
2164
11%
704
38%
704
14%
1648
November1st to 15thNight
21%
700
6%
2395
5%
700
23%
700
7%
2029
November16th to endNight
27%
720
6%
2520
6%
720
29%
720
9%
2025
December1st to 15thNight
25%
720
7%
2520
7%
720
28%
720
9%
2042
December16th to endNight
17%
768
6%
2688
5%
768
19%
768
6%
2322

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
26%15%35%42333428%
47586
Night
25%16%35%42333427%
46553
Cloud CoverDay
23%16%35%42333428%
47586
Night
22%17%35%42333427%
46557
TransparencyDay
58%310%15%9%2
47257
Night
58%310%14%9%2
46386
SeeingDay
32%11%24%21%7%2
5524
Night
32%6%15%27%16%2
14458

By Month

Percentage Cloud Cover NW (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
36%17%46%32222217%
1432
16th to endDay
31%19%4431222321%
1593
February1st to 15thDay
29%21%5%6%42322415%
1576
16th to endDay
29%29%46%21122216%
1477
March1st to 15thDay
34%28%46%41221211%
1890
16th to endDay
34%28%5%5%4233128%
2160
April1st to 15thDay
25%31%46%42323411%
2149
16th to endDay
28%25%45%32332316%
2276
May1st to 15thDay
24%16%35%42343426%
2511
16th to endDay
33%12%45%5%24445%16%
2615
June1st to 15thDay
34%11%35%5%24345%19%
2520
16th to endDay
25%8%36%42435%6%29%
2619
July1st to 15thDay
11%5%13323336%54%
2526
16th to endDay
9%422313336%58%
2657
August1st to 15thDay
9%5%13223346%56%
2354
16th to endDay
11%7%24313347%49%
2193
September1st to 15thDay
12%413222445%55%
1980
16th to endDay
24%7%24422345%39%
1825
October1st to 15thDay
31%8%26%413245%28%
1650
16th to endDay
38%10%3442333323%
1706
November1st to 15thDay
41%17%3443333311%
1461
16th to endDay
35%22%35%31222317%
1440
December1st to 15thDay
27%24%37%5%2312219%
1440
16th to endDay
31%27%45%31222315%
1536
January1st to 15thNight
35%18%4432222319%
2506
16th to endNight
30%17%35%41233422%
2628
February1st to 15thNight
29%20%45%42333417%
2276
16th to endNight
25%26%45%42333318%
1996
March1st to 15thNight
32%23%45%42333313%
2067
16th to endNight
29%29%5%5%42332310%
2060
April1st to 15thNight
23%28%5%6%5%2323316%
1799
16th to endNight
25%20%36%323446%18%
1681
May1st to 15thNight
21%15%25%5%3433429%
1439
16th to endNight
24%12%36%5%25%45%6%22%
1496
June1st to 15thNight
29%10%35%43434426%
1440
16th to endNight
21%8%25%5%24446%34%
1332
July1st to 15thNight
6%424313246%59%
1425
16th to endNight
4213223448%61%
1512
August1st to 15thNight
5%313313346%63%
1518
16th to endNight
8%413323446%56%
1657
September1st to 15thNight
8%424412335%60%
1650
16th to endNight
19%6%26%42344440%
1804
October1st to 15thNight
27%8%24413325%35%
1980
16th to endNight
33%10%35%31343425%
2164
November1st to 15thNight
41%14%35%42222314%
2395
16th to endNight
32%21%25%32322320%
2520
December1st to 15thNight
28%19%46%42332419%
2520
16th to endNight
32%27%45%31222214%
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
31%20%46%5%2332217%
1432
16th to endDay
27%21%45%41333322%
1593
February1st to 15thDay
26%21%5%6%5%3333415%
1576
16th to endDay
24%31%6%5%31222316%
1477
March1st to 15thDay
28%30%5%7%42222211%
1890
16th to endDay
29%31%6%6%3233228%
2160
April1st to 15thDay
23%31%46%42334412%
2149
16th to endDay
24%26%46%42333317%
2276
May1st to 15thDay
21%17%45%5%2344426%
2511
16th to endDay
30%13%36%5%24445%17%
2615
June1st to 15thDay
30%12%46%42435%5%20%
2520
16th to endDay
22%10%36%5%34446%30%
2619
July1st to 15thDay
10%5%13323335%56%
2526
16th to endDay
9%413322336%59%
2657
August1st to 15thDay
8%5%13323445%57%
2354
16th to endDay
11%6%24322337%51%
2193
September1st to 15thDay
10%6%12323346%55%
1980
16th to endDay
23%7%34422335%39%
1825
October1st to 15thDay
30%8%35%43234429%
1650
16th to endDay
36%11%25%5%1333323%
1706
November1st to 15thDay
38%18%35%5%3323411%
1461
16th to endDay
30%24%35%32223317%
1440
December1st to 15thDay
24%25%47%43322220%
1440
16th to endDay
27%27%46%31322314%
1536
January1st to 15thNight
31%20%46%32222419%
2506
16th to endNight
26%19%35%5%2333421%
2628
February1st to 15thNight
26%20%46%5%2333417%
2276
16th to endNight
21%28%5%5%42233318%
1996
March1st to 15thNight
27%25%5%6%43332313%
2067
16th to endNight
25%30%5%6%42333310%
2064
April1st to 15thNight
21%28%5%7%5%2233316%
1799
16th to endNight
21%23%45%433445%18%
1681
May1st to 15thNight
18%16%26%5%3444428%
1439
16th to endNight
21%12%38%6%34446%22%
1496
June1st to 15thNight
25%12%5%5%5%3434426%
1440
16th to endNight
19%7%36%5%33446%34%
1332
July1st to 15thNight
6%324413346%59%
1425
16th to endNight
421432335%8%60%
1512
August1st to 15thNight
4312422338%62%
1518
16th to endNight
8%313432446%56%
1657
September1st to 15thNight
7%5%2442224360%
1650
16th to endNight
18%6%26%5%23446%39%
1804
October1st to 15thNight
25%8%35%323335%34%
1980
16th to endNight
30%12%36%41343424%
2164
November1st to 15thNight
38%16%36%5%2222314%
2395
16th to endNight
29%23%3442322320%
2520
December1st to 15thNight
25%21%46%42432419%
2520
16th to endNight
28%29%5%5%41222214%
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
72%26%9%7%1
1432
16th to endDay
67%27%9%8%5%
1593
February1st to 15thDay
73%38%10%3
1576
16th to endDay
77%16%9%4
1477
March1st to 15thDay
81%16%7%2
1890
16th to endDay
83%35%6%1
2160
April1st to 15thDay
75%310%9%1
2149
16th to endDay
71%49%9%4
2276
May1st to 15thDay
58%312%18%7%
2511
16th to endDay
65%6%12%12%2
2619
June1st to 15thDay
63%415%10%5%
2520
16th to endDay
51%415%18%8%1
2619
July1st to 15thDay
28%413%25%21%5%
2377
16th to endDay
23%412%24%27%6%
2490
August1st to 15thDay
26%412%30%18%6%
2337
16th to endDay
32%312%31%16%4
2193
September1st to 15thDay
28%311%24%22%9%
1980
16th to endDay
46%312%17%14%5%
1825
October1st to 15thDay
56%29%18%11%1
1650
16th to endDay
65%27%14%9%
1706
November1st to 15thDay
77%36%9%21
1461
16th to endDay
72%25%9%8%1
1440
December1st to 15thDay
73%25%11%6%
1440
16th to endDay
75%28%8%3
1536
January1st to 15thNight
69%37%9%8%1
2506
16th to endNight
66%27%10%7%5%
2628
February1st to 15thNight
70%49%9%4
2288
16th to endNight
70%28%11%5%
1996
March1st to 15thNight
75%37%10%2
2067
16th to endNight
78%38%7%1
2064
April1st to 15thNight
72%49%10%3
1799
16th to endNight
63%5%12%14%31
1681
May1st to 15thNight
55%312%18%8%
1440
16th to endNight
55%10%16%13%4
1496
June1st to 15thNight
57%414%16%6%
1440
16th to endNight
44%320%21%7%2
1332
July1st to 15thNight
24%5%14%25%23%6%
1345
16th to endNight
17%412%27%27%9%
1416
August1st to 15thNight
18%5%17%29%20%7%
1510
16th to endNight
23%415%33%18%4
1657
September1st to 15thNight
24%314%24%23%10%
1650
16th to endNight
42%414%17%13%7%
1804
October1st to 15thNight
50%39%18%14%3
1980
16th to endNight
61%37%15%10%1
2164
November1st to 15thNight
74%37%10%31
2395
16th to endNight
69%25%8%11%2
2520
December1st to 15thNight
69%37%12%6%
2520
16th to endNight
77%36%7%5%
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
33%10%18%28%6%3
60
16th to endDay
47%11%15%17%6%1
121
February1st to 15thDay
48%11%20%13%6%
236
16th to endDay
47%15%22%12%3
221
March1st to 15thDay
63%10%15%8%1
270
16th to endDay
48%18%20%10%2
288
April1st to 15thDay
42%15%23%11%5%1
269
16th to endDay
45%17%18%12%32
270
May1st to 15thDay
35%15%22%18%5%1
288
16th to endDay
31%13%20%22%9%2
330
June1st to 15thDay
31%15%22%18%8%3
315
16th to endDay
20%12%33%21%11%
414
July1st to 15thDay
15%432%35%9%2
311
16th to endDay
7%8%35%31%13%3
327
August1st to 15thDay
12%5%24%38%13%6%
300
16th to endDay
13%10%26%29%13%5%
268
September1st to 15thDay
17%9%27%31%10%2
255
16th to endDay
24%10%27%28%7%1
255
October1st to 15thDay
35%12%23%20%7%
255
16th to endDay
49%5%17%20%33
218
November1st to 15thDay
47%5%27%15%2
69
16th to endDay
48%320%16%11%
60
December1st to 15thDay
51%21%21%5%
60
16th to endDay
31%7%26%25%9%
64
January1st to 15thNight
42%9%14%23%9%
720
16th to endNight
37%6%19%22%11%1
768
February1st to 15thNight
37%6%20%24%8%1
704
16th to endNight
38%9%18%23%9%
632
March1st to 15thNight
45%10%13%21%9%
720
16th to endNight
43%11%16%18%8%
768
April1st to 15thNight
36%10%17%20%13%1
719
16th to endNight
31%9%12%27%15%2
601
May1st to 15thNight
26%5%16%29%19%2
360
16th to endNight
27%5%13%27%23%2
375
June1st to 15thNight
27%414%31%20%1
360
16th to endNight
20%315%36%22%1
360
July1st to 15thNight
7%11%40%37%2
340
16th to endNight
428%47%32%3
376
August1st to 15thNight
5%9%33%43%7%
460
16th to endNight
10%211%34%35%6%
607
September1st to 15thNight
10%29%36%34%6%
660
16th to endNight
21%214%33%24%4
656
October1st to 15thNight
28%415%28%19%3
660
16th to endNight
38%311%29%13%3
704
November1st to 15thNight
47%6%15%23%5%
700
16th to endNight
44%7%15%22%9%
720
December1st to 15thNight
39%6%20%27%41
720
16th to endNight
45%11%18%16%6%
768
Page generated at 2017-08-13 11:16:37. Version ac04

Copyright 2017 A. Danko


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