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 2017-04-07.

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%
14160
11%
45361
10%
14131
39%
14131
15%
33855

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
24%
687
6%
1719
3%
687
25%
687
10%
1469
April16th to endNight
37%
551
9%
1541
7%
551
37%
551
14%
1219
May1st to 15thNight
41%
329
13%
1319
16%
329
45%
329
14%
910
May16th to endNight
41%
343
5%
1368
7%
342
42%
342
10%
1051
June1st to 15thNight
42%
330
7%
1320
9%
330
43%
330
14%
935
June16th to endNight
49%
330
9%
1221
12%
330
50%
330
21%
780
July1st to 15thNight
70%
310
15%
1226
23%
310
73%
310
36%
594
July16th to endNight
77%
348
21%
1304
33%
326
80%
326
45%
558
August1st to 15thNight
75%
432
24%
1416
25%
430
76%
430
47%
661
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%42333427%
45872
Night
25%16%35%42333427%
45529
Cloud CoverDay
23%16%35%42333428%
45872
Night
23%17%35%42333427%
45533
TransparencyDay
58%310%15%9%2
45543
Night
59%310%14%9%2
45362
SeeingDay
33%11%24%20%7%1
5016
Night
32%6%15%27%16%2
14168

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%30%46%42324412%
2044
16th to endDay
26%26%45%32332317%
2086
May1st to 15thDay
24%16%35%5%2343426%
2301
16th to endDay
35%12%45%5%2444414%
2391
June1st to 15thDay
34%10%35%424345%20%
2310
16th to endDay
26%9%36%42435%6%27%
2401
July1st to 15thDay
12%5%13323336%52%
2316
16th to endDay
10%422313337%56%
2461
August1st to 15thDay
9%5%13324347%54%
2203
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%27%5%6%5%2433316%
1719
16th to endNight
23%20%45%323446%19%
1541
May1st to 15thNight
20%16%25%5%3433429%
1319
16th to endNight
25%13%36%5%25%5%5%6%19%
1368
June1st to 15thNight
29%10%35%32434428%
1320
16th to endNight
22%8%35%5%24446%32%
1221
July1st to 15thNight
7%424323246%58%
1306
16th to endNight
5%21332345%8%59%
1400
August1st to 15thNight
5%313213347%62%
1424
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%30%46%42334413%
2044
16th to endDay
23%26%37%42333318%
2086
May1st to 15thDay
21%16%45%5%2344427%
2301
16th to endDay
32%14%47%423445%15%
2391
June1st to 15thDay
31%11%35%42435%5%21%
2310
16th to endDay
23%11%36%5%34446%28%
2401
July1st to 15thDay
11%6%23423335%54%
2316
16th to endDay
9%413322336%57%
2461
August1st to 15thDay
9%5%13323345%55%
2203
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%27%47%5%2233316%
1719
16th to endNight
21%23%45%423445%19%
1541
May1st to 15thNight
18%16%25%5%3444429%
1319
16th to endNight
22%13%48%7%345%5%7%18%
1368
June1st to 15thNight
26%12%445%3424428%
1320
16th to endNight
20%7%46%5%44436%32%
1221
July1st to 15thNight
7%324413346%57%
1306
16th to endNight
5%21432335%8%58%
1400
August1st to 15thNight
4413423338%60%
1424
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
2044
16th to endDay
69%410%9%5%
2086
May1st to 15thDay
58%311%18%7%
2301
16th to endDay
67%6%12%11%1
2395
June1st to 15thDay
62%416%10%6%
2310
16th to endDay
53%415%17%6%1
2401
July1st to 15thDay
30%414%24%20%5%
2167
16th to endDay
25%413%22%28%6%
2294
August1st to 15thDay
27%5%13%32%17%4
2186
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
71%5%9%10%3
1719
16th to endNight
62%5%12%14%31
1541
May1st to 15thNight
55%312%18%9%
1320
16th to endNight
58%10%16%10%3
1368
June1st to 15thNight
56%414%17%7%
1320
16th to endNight
46%321%20%5%1
1221
July1st to 15thNight
25%5%15%24%22%6%
1226
16th to endNight
18%413%25%27%10%
1304
August1st to 15thNight
18%5%18%31%19%5%
1416
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
39%17%23%11%6%2
234
16th to endDay
41%17%18%14%43
210
May1st to 15thDay
35%16%21%18%6%1
228
16th to endDay
35%12%21%21%7%1
266
June1st to 15thDay
31%14%23%18%9%3
255
16th to endDay
21%13%34%19%9%
346
July1st to 15thDay
17%5%33%34%6%2
251
16th to endDay
9%9%36%28%12%3
271
August1st to 15thDay
14%6%27%39%9%2
255
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
35%11%17%20%13%1
687
16th to endNight
31%9%12%27%15%2
551
May1st to 15thNight
27%5%15%29%20%2
330
16th to endNight
28%413%28%22%1
343
June1st to 15thNight
28%413%30%21%1
330
16th to endNight
20%416%36%21%
330
July1st to 15thNight
7%12%38%37%2
310
16th to endNight
428%48%31%3
348
August1st to 15thNight
6%9%35%43%4
432
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 2017-04-08 02:35:13. Version ac04

Copyright 2017 A. Danko


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