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 2005-11-06 to 2024-03-23.

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
35%
22759
11%
80718
10%
22653
38%
22653
13%
61332

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
19%
758
4%
4396
5%
736
23%
736
4%
3886
January11th to 20thNight
24%
760
11%
4349
6%
740
29%
740
5%
3367
January21st to endNight
25%
836
7%
4703
10%
814
29%
814
6%
3983
February1st to 10thNight
21%
760
6%
2470
5%
760
23%
760
8%
2077
February11th to 20thNight
24%
740
8%
2404
6%
740
28%
740
10%
1942
February21st to endNight
20%
628
8%
1955
5%
628
24%
628
8%
1651
March1st to 10thNight
25%
760
8%
2226
7%
760
26%
760
10%
1839
March11th to 20thNight
25%
760
10%
2090
7%
760
28%
760
12%
1727
March21st to endNight
21%
800
4%
2130
3%
800
21%
800
6%
1905
April1st to 10thNight
22%
720
6%
1800
2%
720
23%
720
9%
1575
April11th to 20thNight
28%
711
10%
1779
5%
711
30%
711
13%
1466
April21st to endNight
32%
545
7%
1625
4%
545
33%
545
10%
1341
May1st to 10thNight
35%
360
12%
1440
13%
360
39%
360
12%
1032
May11th to 20thNight
43%
354
10%
1416
14%
354
45%
354
12%
1044
May21st to endNight
41%
389
7%
1552
10%
388
44%
388
11%
1155
June1st to 10thNight
40%
360
6%
1440
8%
360
41%
360
12%
1047
June11th to 20thNight
38%
360
5%
1436
8%
360
39%
360
12%
1066
June21st to endNight
58%
360
15%
1260
18%
360
61%
360
26%
755
July1st to 10thNight
64%
350
14%
1354
21%
350
65%
350
30%
725
July11th to 20thNight
66%
348
20%
1392
32%
348
69%
348
36%
665
July21st to endNight
76%
394
26%
1488
40%
372
79%
372
47%
622
August1st to 10thNight
72%
442
22%
1508
27%
440
73%
440
38%
753
August11th to 20thNight
76%
534
28%
1602
25%
534
77%
534
45%
822
August21st to endNight
63%
725
27%
1913
18%
725
64%
725
41%
1117
September1st to 10thNight
61%
720
24%
1800
14%
720
62%
720
41%
1101
September11th to 20thNight
54%
720
21%
1800
14%
720
55%
720
28%
1251
September21st to endNight
51%
720
18%
2051
15%
720
53%
720
26%
1369
October1st to 10thNight
46%
720
14%
2160
14%
720
49%
720
23%
1468
October11th to 20thNight
42%
720
16%
2160
16%
720
45%
720
19%
1546
October21st to endNight
34%
792
14%
2460
12%
792
38%
792
17%
1786
November1st to 10thNight
23%
740
9%
2504
8%
740
25%
740
8%
2070
November11th to 20thNight
22%
760
7%
2660
5%
760
26%
760
8%
2209
November21st to endNight
26%
760
7%
2660
6%
760
28%
760
10%
2098
December1st to 10thNight
26%
760
8%
2660
8%
760
28%
760
9%
2128
December11th to 20thNight
21%
759
8%
3227
5%
759
24%
759
5%
2631
December21st to endNight
13%
834
4%
4848
5%
817
16%
817
3%
4113

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
23%16%46%5%23345%29%
78818
Night
23%18%5%7%5%3333426%
81010
TransparencyDay
57%5%10%15%10%2
78545
Night
60%5%9%14%9%2
80863
SeeingDay
29%14%24%22%9%2
13847
Night
32%9%17%26%15%2
22768

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 10thDay
27%34%7%8%5%221229%
2539
11th to 20thDay
27%21%47%5%2333421%
2516
21st to endDay
32%27%5%7%42322313%
2954
February1st to 10thDay
27%22%6%7%5%2423416%
1710
11th to 20thDay
24%27%5%7%43333318%
1665
21st to endDay
25%30%7%7%42223415%
1498
March1st to 10thDay
23%26%7%8%5%2433414%
1952
11th to 20thDay
27%25%5%7%5%3233417%
2090
21st to endDay
28%26%8%8%6%343338%
2270
April1st to 10thDay
27%28%6%8%5%2334412%
2160
11th to 20thDay
24%25%6%8%42333419%
2132
21st to endDay
26%24%46%5%3333418%
2333
May1st to 10thDay
22%18%47%5%23436%27%
2520
11th to 20thDay
23%15%48%6%36%5%45%21%
2477
21st to endDay
32%13%46%6%2445%5%20%
2723
June1st to 10thDay
32%11%36%5%3445%5%23%
2520
11th to 20thDay
30%12%38%6%35%5%5%5%18%
2520
21st to endDay
17%7%25%5%345%5%7%40%
2691
July1st to 10thDay
11%7%345%33446%51%
2554
11th to 20thDay
11%5%24323446%53%
2520
21st to endDay
7%323212346%66%
2741
August1st to 10thDay
7%6%24323448%55%
2419
11th to 20thDay
10%5%2332235%7%60%
2298
21st to endDay
11%7%25%425%46%7%47%
2442
September1st to 10thDay
11%5%2432445%5%55%
2160
11th to 20thDay
17%7%35%5%35%445%42%
2160
21st to endDay
21%8%36%424336%40%
1905
October1st to 10thDay
25%11%35%42334436%
1800
11th to 20thDay
27%12%25%5%23346%30%
1800
21st to endDay
31%13%36%5%2323329%
1895
November1st to 10thDay
34%18%5%7%5%3433415%
1575
11th to 20thDay
36%19%46%41233417%
1520
21st to endDay
29%20%5%7%42333321%
1520
December1st to 10thDay
23%21%6%7%5%2432224%
1520
11th to 20thDay
25%22%6%7%5%33345%18%
1925
21st to endDay
23%25%5%9%6%3344415%
2794
January1st to 10thNight
31%30%6%6%5%3322210%
4406
11th to 20thNight
27%18%7%7%5%23335%19%
4382
21st to endNight
32%24%5%7%5%2233414%
4704
February1st to 10thNight
26%22%7%7%43434416%
2470
11th to 20thNight
23%24%5%6%42333422%
2402
21st to endNight
22%29%7%7%5%2233316%
1955
March1st to 10thNight
24%22%7%8%42443418%
2226
11th to 20thNight
24%23%5%7%5%2334420%
2089
21st to endNight
24%28%6%8%6%3433312%
2130
April1st to 10thNight
21%27%6%9%6%3423316%
1800
11th to 20thNight
22%23%5%8%43334421%
1779
21st to endNight
25%20%48%4335%46%17%
1625
May1st to 10thNight
21%18%36%5%3433429%
1440
11th to 20thNight
20%15%47%6%35%5%45%25%
1416
21st to endNight
21%15%47%5%445%5%6%24%
1552
June1st to 10thNight
27%12%37%434336%27%
1440
11th to 20thNight
25%14%47%6%2445%6%23%
1436
21st to endNight
14%7%36%43435%6%45%
1260
July1st to 10thNight
7%8%36%435%35%5%52%
1394
11th to 20thNight
7%5%345%2335%6%58%
1440
21st to endNight
3325%423346%64%
1576
August1st to 10thNight
6%6%23423447%59%
1516
11th to 20thNight
5%424423345%63%
1602
21st to endNight
9%7%25%424445%53%
1913
September1st to 10thNight
10%5%35%5%24435%56%
1800
11th to 20thNight
16%9%37%5%34345%42%
1800
21st to endNight
17%10%36%5%23445%41%
2051
October1st to 10thNight
23%9%36%424335%39%
2160
11th to 20thNight
24%11%36%434335%33%
2160
21st to endNight
27%13%35%42333433%
2460
November1st to 10thNight
35%15%47%5%2333319%
2504
11th to 20thNight
33%18%46%5%2333319%
2660
21st to endNight
28%19%5%7%5%2333323%
2660
December1st to 10thNight
25%18%5%7%5%34335%21%
2660
11th to 20thNight
25%21%5%7%5%34445%17%
3280
21st to endNight
24%25%7%9%6%3433314%
4862

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
83%45%5%31
2539
11th to 20thDay
67%48%12%5%3
2540
21st to endDay
78%35%7%6%1
2954
February1st to 10thDay
73%47%10%5%
1710
11th to 20thDay
73%46%11%41
1674
21st to endDay
76%36%8%6%1
1498
March1st to 10thDay
73%48%11%41
1952
11th to 20thDay
71%36%15%31
2090
21st to endDay
81%6%6%6%
2270
April1st to 10thDay
76%5%8%10%1
2160
11th to 20thDay
70%39%10%6%1
2132
21st to endDay
70%5%8%10%6%1
2333
May1st to 10thDay
58%6%13%16%7%
2520
11th to 20thDay
59%9%12%14%5%
2480
21st to endDay
63%7%13%14%3
2724
June1st to 10thDay
60%7%14%13%6%
2520
11th to 20thDay
64%8%14%10%4
2520
21st to endDay
40%7%15%24%11%2
2691
July1st to 10thDay
33%6%13%25%18%5%
2485
11th to 20thDay
29%5%10%26%25%6%
2445
21st to endDay
18%5%10%29%33%6%
2589
August1st to 10thDay
25%7%13%28%22%5%
2402
11th to 20thDay
23%5%14%32%19%7%
2298
21st to endDay
32%6%13%29%17%4
2442
September1st to 10thDay
29%5%11%24%21%10%
2160
11th to 20thDay
43%6%11%16%17%7%
2160
21st to endDay
45%5%12%17%15%5%
1905
October1st to 10thDay
52%47%20%14%3
1800
11th to 20thDay
55%410%15%14%2
1800
21st to endDay
62%46%15%13%1
1895
November1st to 10thDay
74%46%10%41
1575
11th to 20thDay
72%38%10%6%2
1520
21st to endDay
70%3310%12%2
1520
December1st to 10thDay
67%45%11%10%2
1520
11th to 20thDay
70%6%10%10%4
1928
21st to endDay
71%6%10%9%31
2794
January1st to 10thNight
81%35%6%31
4406
11th to 20thNight
66%5%8%14%43
4420
21st to endNight
75%47%7%5%2
4704
February1st to 10thNight
71%6%8%9%5%
2470
11th to 20thNight
67%5%9%11%5%2
2404
21st to endNight
73%47%9%6%1
1955
March1st to 10thNight
69%5%9%12%5%
2226
11th to 20thNight
68%48%14%41
2090
21st to endNight
77%6%7%8%1
2130
April1st to 10thNight
73%6%8%10%3
1800
11th to 20thNight
65%5%9%13%6%1
1779
21st to endNight
65%9%9%11%5%1
1625
May1st to 10thNight
57%5%12%16%9%
1440
11th to 20thNight
58%6%13%15%6%1
1416
21st to endNight
56%9%15%16%5%
1552
June1st to 10thNight
59%5%13%16%6%1
1440
11th to 20thNight
57%8%16%14%5%
1436
21st to endNight
38%6%17%26%10%3
1260
July1st to 10thNight
31%6%13%24%21%5%
1354
11th to 20thNight
24%6%13%23%26%7%
1392
21st to endNight
17%5%10%28%30%9%
1488
August1st to 10thNight
23%8%15%27%21%7%
1508
11th to 20thNight
21%6%13%31%21%7%
1602
21st to endNight
29%5%13%30%19%4
1913
September1st to 10thNight
30%5%11%24%21%9%
1800
11th to 20thNight
43%6%8%18%15%9%
1800
21st to endNight
43%6%14%15%15%6%
2051
October1st to 10thNight
48%49%20%16%4
2160
11th to 20thNight
53%5%9%15%16%3
2160
21st to endNight
56%48%14%16%2
2460
November1st to 10thNight
70%5%7%11%6%1
2504
11th to 20thNight
72%47%8%6%2
2660
21st to endNight
67%35%8%14%3
2660
December1st to 10thNight
67%48%10%9%1
2660
11th to 20thNight
69%6%9%10%6%
3276
21st to endNight
74%7%8%7%3
4862

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
47%15%22%8%6%1
180
11th to 20thDay
40%22%19%12%7%
180
21st to endDay
40%19%16%16%9%1
293
February1st to 10thDay
43%16%19%15%6%1
370
11th to 20thDay
45%17%22%14%2
365
21st to endDay
41%21%20%16%2
348
March1st to 10thDay
40%17%27%14%3
460
11th to 20thDay
37%17%23%15%6%1
460
21st to endDay
39%21%21%14%31
471
April1st to 10thDay
42%16%23%12%5%1
420
11th to 20thDay
43%20%18%12%5%2
412
21st to endDay
38%19%23%13%6%1
420
May1st to 10thDay
29%17%24%22%7%1
423
11th to 20thDay
34%12%25%19%8%2
448
21st to endDay
34%13%22%20%9%1
485
June1st to 10thDay
31%13%21%23%9%2
450
11th to 20thDay
32%18%27%16%6%1
450
21st to endDay
15%13%31%28%11%2
621
July1st to 10thDay
14%8%33%33%9%2
480
11th to 20thDay
12%10%34%29%12%3
444
21st to endDay
7%6%30%37%15%5%
487
August1st to 10thDay
9%10%26%37%14%5%
447
11th to 20thDay
8%7%26%37%17%6%
437
21st to endDay
12%15%28%29%11%4
495
September1st to 10thDay
15%8%33%32%10%3
450
11th to 20thDay
19%11%27%29%12%1
450
21st to endDay
24%13%23%29%10%
450
October1st to 10thDay
29%10%26%21%14%
450
11th to 20thDay
34%10%19%22%12%2
450
21st to endDay
35%16%20%17%9%3
410
November1st to 10thDay
40%17%19%17%5%
223
11th to 20thDay
43%16%17%13%9%2
180
21st to endDay
36%16%19%16%13%
180
December1st to 10thDay
44%12%14%18%10%1
180
11th to 20thDay
43%12%24%16%4
180
21st to endDay
43%11%27%14%5%
198
January1st to 10thNight
47%13%16%16%8%
760
11th to 20thNight
39%12%19%19%10%1
760
21st to endNight
36%11%21%23%8%1
836
February1st to 10thNight
38%10%23%22%6%1
760
11th to 20thNight
38%9%19%26%8%
744
21st to endNight
39%15%19%22%4
628
March1st to 10thNight
34%14%18%22%10%1
760
11th to 20thNight
38%10%19%23%8%1
760
21st to endNight
39%13%18%20%8%1
800
April1st to 10thNight
38%11%19%20%11%1
720
11th to 20thNight
33%12%19%22%13%2
711
21st to endNight
34%9%15%30%11%1
545
May1st to 10thNight
29%10%16%25%19%1
360
11th to 20thNight
27%6%12%31%22%2
354
21st to endNight
29%7%14%25%23%2
389
June1st to 10thNight
33%6%12%28%19%2
360
11th to 20thNight
31%6%18%29%17%
360
21st to endNight
17%414%35%28%2
360
July1st to 10thNight
12%315%36%33%2
350
11th to 20thNight
9%215%46%25%3
348
21st to endNight
3312%42%36%5%
394
August1st to 10thNight
8%49%31%41%6%
442
11th to 20thNight
7%28%32%43%9%
534
21st to endNight
14%412%39%27%4
725
September1st to 10thNight
14%415%40%23%4
720
11th to 20thNight
18%412%30%30%5%
720
21st to endNight
22%5%15%34%21%3
720
October1st to 10thNight
26%5%16%31%20%3
720
11th to 20thNight
30%7%15%27%18%3
720
21st to endNight
35%8%18%24%11%3
792
November1st to 10thNight
44%8%19%23%5%
740
11th to 20thNight
44%11%18%20%6%1
760
21st to endNight
38%11%20%21%8%1
760
December1st to 10thNight
35%11%22%23%8%1
760
11th to 20thNight
44%10%20%21%5%1
760
21st to endNight
47%15%19%14%5%
836
Page generated at 2024-03-19 20:31:09. Version ac07

Copyright © 2024 Attilla Danko.


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