Steilacoom
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Steilacoom clear sky chart from 2005-11-05 to 2017-04-26.

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
41%
14200
10%
45440
9%
14171
43%
14171
15%
33652

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
29%
732
9%
2580
8%
728
32%
728
8%
2103
January16th to endNight
36%
768
8%
2637
9%
768
37%
768
11%
2063
February1st to 15thNight
31%
698
5%
2288
4%
698
33%
698
9%
1850
February16th to endNight
30%
632
9%
2016
6%
632
35%
632
11%
1577
March1st to 15thNight
28%
720
5%
2059
3%
720
29%
720
10%
1725
March16th to endNight
25%
768
5%
2070
3%
768
26%
768
8%
1766
April1st to 15thNight
37%
719
6%
1799
3%
719
38%
719
13%
1478
April16th to endNight
42%
559
9%
1615
6%
559
43%
559
17%
1210
May1st to 15thNight
47%
329
13%
1319
17%
329
49%
329
16%
881
May16th to endNight
49%
343
4%
1368
5%
342
51%
342
11%
997
June1st to 15thNight
49%
330
6%
1251
6%
330
50%
330
17%
864
June16th to endNight
45%
330
7%
1155
7%
330
46%
330
17%
789
July1st to 15thNight
71%
310
13%
1108
16%
310
73%
310
35%
578
July16th to endNight
76%
348
18%
1304
29%
326
78%
326
39%
594
August1st to 15thNight
79%
427
19%
1411
22%
425
80%
425
39%
729
August16th to endNight
69%
588
21%
1638
16%
588
71%
588
42%
918
September1st to 15thNight
72%
660
23%
1650
14%
660
73%
660
47%
958
September16th to endNight
52%
656
19%
1809
15%
656
54%
656
26%
1215
October1st to 15thNight
46%
660
12%
1980
11%
660
49%
660
19%
1414
October16th to endNight
37%
704
8%
2166
8%
704
39%
704
13%
1677
November1st to 15thNight
29%
700
6%
2423
6%
700
32%
700
8%
1984
November16th to endNight
30%
720
6%
2520
5%
720
31%
720
9%
1989
December1st to 15thNight
35%
720
8%
2520
8%
720
36%
720
10%
1971
December16th to endNight
25%
779
5%
2754
4%
779
27%
779
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
23%8%36%5%34446%29%
46235
Night
23%7%35%5%24446%32%
45600
Cloud CoverDay
21%8%36%6%35%45%6%28%
46235
Night
22%8%35%5%34446%31%
45603
TransparencyDay
55%8%13%15%6%
45898
Night
53%8%13%15%6%1
45441
SeeingDay
22%10%20%23%17%5%
5343
Night
24%7%17%26%20%2
14207

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
37%9%36%43343418%
1352
16th to endDay
31%11%5%7%42443419%
1586
February1st to 15thDay
32%10%47%5%2445%6%15%
1576
16th to endDay
31%10%37%6%33435%20%
1457
March1st to 15thDay
35%10%38%6%4433414%
1899
16th to endDay
30%11%47%6%46%44414%
2153
April1st to 15thDay
26%9%47%6%35%46%5%18%
2149
16th to endDay
22%9%37%5%346%5%7%23%
2256
May1st to 15thDay
19%6%35%6%2445%5%33%
2301
16th to endDay
21%10%37%6%445%6%7%22%
2391
June1st to 15thDay
21%7%46%6%2445%7%27%
2376
16th to endDay
18%8%37%5%45%45%7%28%
2475
July1st to 15thDay
11%424323335%55%
2448
16th to endDay
11%413323349%52%
2461
August1st to 15thDay
8%5%2442345%7%52%
2210
16th to endDay
12%4245%2345%7%47%
2212
September1st to 15thDay
10%6%25%313445%53%
1980
16th to endDay
18%7%36%424347%35%
1821
October1st to 15thDay
23%10%46%5%335%45%28%
1650
16th to endDay
31%9%46%5%3345%6%18%
1705
November1st to 15thDay
35%8%36%4345%6%415%
1433
16th to endDay
31%11%47%5%24345%18%
1440
December1st to 15thDay
28%9%28%7%34445%20%
1440
16th to endDay
32%12%37%6%3444415%
1464
January1st to 15thNight
36%8%35%433335%21%
2580
16th to endNight
29%9%36%434446%23%
2637
February1st to 15thNight
30%9%37%5%24345%22%
2276
16th to endNight
27%8%36%5%25%44426%
2016
March1st to 15thNight
33%10%46%6%24435%18%
2059
16th to endNight
30%9%37%6%445%45%17%
2066
April1st to 15thNight
20%8%37%6%36%6%5%6%26%
1799
16th to endNight
18%8%35%5%25%5%6%7%32%
1615
May1st to 15thNight
15%7%25%4245%6%439%
1319
16th to endNight
15%5%27%5%47%5%6%8%29%
1368
June1st to 15thNight
15%5%345%3445%9%38%
1251
16th to endNight
12%6%45%5%4445%5%40%
1155
July1st to 15thNight
4213324336%63%
1180
16th to endNight
4313212236%68%
1400
August1st to 15thNight
4112223337%66%
1419
16th to endNight
8%41222225%6%60%
1638
September1st to 15thNight
6%313312345%65%
1650
16th to endNight
17%5%2442335%6%43%
1809
October1st to 15thNight
20%6%26%5%34345%37%
1980
16th to endNight
28%8%26%5%33345%27%
2166
November1st to 15thNight
36%7%25%5%34435%21%
2423
16th to endNight
27%10%37%6%24446%22%
2520
December1st to 15thNight
27%9%45%5%24436%26%
2520
16th to endNight
32%11%47%5%2434419%
2754

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
36%7%46%6%2434418%
1352
16th to endDay
28%11%47%5%345%46%17%
1586
February1st to 15thDay
31%10%5%8%5%35%346%15%
1576
16th to endDay
28%10%47%6%345%45%19%
1457
March1st to 15thDay
34%9%5%7%7%35%34413%
1899
16th to endDay
28%11%38%7%5%6%45%5%12%
2153
April1st to 15thDay
23%10%5%8%6%35%5%6%6%17%
2149
16th to endDay
19%8%47%6%46%5%6%7%22%
2256
May1st to 15thDay
16%8%46%5%26%5%5%7%32%
2301
16th to endDay
19%8%5%8%7%46%5%6%7%21%
2391
June1st to 15thDay
17%8%37%6%35%5%5%8%26%
2376
16th to endDay
16%7%36%7%35%6%6%7%28%
2475
July1st to 15thDay
10%424323345%54%
2448
16th to endDay
10%414324447%52%
2461
August1st to 15thDay
7%425%423448%51%
2210
16th to endDay
9%5%25%5%2445%7%48%
2212
September1st to 15thDay
9%5%25%424345%52%
1980
16th to endDay
17%8%36%6%235%47%35%
1821
October1st to 15thDay
22%8%47%5%24447%28%
1650
16th to endDay
31%9%46%5%3445%6%17%
1705
November1st to 15thDay
33%8%37%6%345%45%15%
1433
16th to endDay
31%10%47%6%24345%18%
1440
December1st to 15thDay
27%9%47%8%35%45%419%
1440
16th to endDay
31%13%5%7%6%3444414%
1464
January1st to 15thNight
37%8%35%42333421%
2579
16th to endNight
28%9%36%425%446%22%
2637
February1st to 15thNight
29%10%47%6%24346%20%
2276
16th to endNight
25%9%46%5%35%445%26%
2016
March1st to 15thNight
32%9%46%5%34445%18%
2059
16th to endNight
28%10%47%6%35%5%45%16%
2070
April1st to 15thNight
19%9%37%6%46%5%5%7%24%
1799
16th to endNight
17%7%26%6%35%46%9%29%
1615
May1st to 15thNight
15%8%25%6%35%45%5%37%
1319
16th to endNight
13%7%37%6%5%7%5%8%6%29%
1368
June1st to 15thNight
12%6%36%6%246%49%37%
1251
16th to endNight
11%7%36%6%45%5%5%6%37%
1155
July1st to 15thNight
4313323436%62%
1180
16th to endNight
4313213336%66%
1400
August1st to 15thNight
3312213448%63%
1419
16th to endNight
7%413313446%59%
1638
September1st to 15thNight
6%313213345%64%
1650
16th to endNight
15%7%34423348%42%
1809
October1st to 15thNight
20%6%36%6%34445%35%
1980
16th to endNight
27%8%25%5%33446%26%
2166
November1st to 15thNight
34%7%35%5%4444421%
2423
16th to endNight
27%9%37%6%34445%22%
2520
December1st to 15thNight
26%8%47%6%34445%24%
2520
16th to endNight
32%11%48%5%2343418%
2754

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
70%6%7%9%41
1352
16th to endDay
65%7%11%10%31
1586
February1st to 15thDay
68%9%10%7%3
1576
16th to endDay
66%8%9%10%4
1457
March1st to 15thDay
73%7%10%7%1
1899
16th to endDay
72%8%8%9%
2153
April1st to 15thDay
64%10%13%10%1
2149
16th to endDay
59%10%14%12%2
2256
May1st to 15thDay
50%8%16%16%7%
2301
16th to endDay
60%10%17%10%1
2395
June1st to 15thDay
55%10%19%9%4
2376
16th to endDay
52%11%17%14%3
2475
July1st to 15thDay
31%8%14%27%16%2
2291
16th to endDay
28%8%16%26%16%2
2294
August1st to 15thDay
30%9%18%29%10%1
2193
16th to endDay
35%6%14%31%9%2
2212
September1st to 15thDay
32%6%15%24%16%4
1980
16th to endDay
47%8%16%18%7%1
1821
October1st to 15thDay
55%8%13%16%6%
1650
16th to endDay
66%6%13%11%2
1705
November1st to 15thDay
69%8%9%8%21
1433
16th to endDay
66%8%9%11%4
1440
December1st to 15thDay
65%8%10%10%4
1440
16th to endDay
73%6%7%7%4
1464
January1st to 15thNight
65%7%9%11%41
2580
16th to endNight
61%7%11%11%6%2
2637
February1st to 15thNight
63%8%13%8%5%
2288
16th to endNight
59%8%12%13%5%
2016
March1st to 15thNight
67%7%10%12%1
2059
16th to endNight
67%9%10%10%1
2070
April1st to 15thNight
57%13%13%14%1
1799
16th to endNight
50%11%17%16%4
1615
May1st to 15thNight
46%9%14%18%10%
1320
16th to endNight
50%13%20%12%2
1368
June1st to 15thNight
45%11%20%17%5%
1251
16th to endNight
45%10%19%19%31
1155
July1st to 15thNight
25%8%20%28%12%5%
1108
16th to endNight
19%8%16%32%18%4
1304
August1st to 15thNight
19%11%19%31%15%2
1411
16th to endNight
25%7%20%32%12%2
1638
September1st to 15thNight
22%7%17%27%18%6%
1650
16th to endNight
42%6%14%22%11%2
1809
October1st to 15thNight
49%7%12%21%7%1
1980
16th to endNight
58%8%14%13%5%
2166
November1st to 15thNight
66%8%11%10%22
2423
16th to endNight
62%8%12%9%7%
2520
December1st to 15thNight
61%7%12%13%4
2520
16th to endNight
70%7%9%8%3
2754

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
21%6%20%10%26%15%
60
16th to endDay
35%11%16%15%19%
114
February1st to 15thDay
38%12%14%20%10%2
236
16th to endDay
34%17%17%17%10%2
217
March1st to 15thDay
42%17%20%15%31
270
16th to endDay
31%18%27%14%5%2
288
April1st to 15thDay
25%13%27%21%7%4
269
16th to endDay
23%13%27%20%11%4
254
May1st to 15thDay
22%11%21%25%15%3
228
16th to endDay
19%7%24%22%18%7%
266
June1st to 15thDay
14%13%23%23%18%7%
321
16th to endDay
14%9%23%32%16%3
420
July1st to 15thDay
9%315%31%30%9%
375
16th to endDay
8%212%33%32%10%
271
August1st to 15thDay
6%413%32%33%10%
255
16th to endDay
11%6%15%28%26%11%
268
September1st to 15thDay
13%6%21%25%25%8%
255
16th to endDay
17%7%18%30%19%5%
255
October1st to 15thDay
29%8%19%20%18%2
255
16th to endDay
34%8%20%18%11%5%
217
November1st to 15thDay
43%15%24%12%4
65
16th to endDay
30%10%25%11%11%11%
60
December1st to 15thDay
38%20%30%11%
60
16th to endDay
28%10%25%17%12%6%
64
January1st to 15thNight
38%11%13%16%17%2
732
16th to endNight
30%7%15%24%19%2
768
February1st to 15thNight
33%8%16%22%16%2
704
16th to endNight
28%12%21%22%12%2
632
March1st to 15thNight
33%13%18%21%11%1
720
16th to endNight
32%10%21%25%9%1
768
April1st to 15thNight
21%9%24%26%17%1
719
16th to endNight
19%6%21%33%15%2
559
May1st to 15thNight
16%7%19%33%20%2
330
16th to endNight
13%6%20%30%26%2
343
June1st to 15thNight
11%6%26%29%25%
330
16th to endNight
14%6%23%31%23%
330
July1st to 15thNight
6%214%35%36%4
310
16th to endNight
4113%40%35%3
348
August1st to 15thNight
6%9%33%43%6%
427
16th to endNight
9%114%29%40%4
588
September1st to 15thNight
8%211%30%39%7%
660
16th to endNight
17%415%30%27%4
656
October1st to 15thNight
24%416%27%23%4
660
16th to endNight
31%5%16%24%18%2
704
November1st to 15thNight
35%9%19%25%10%
700
16th to endNight
30%11%18%20%15%2
720
December1st to 15thNight
30%8%17%26%15%1
720
16th to endNight
38%10%18%19%12%1
779
Page generated at 2017-04-27 14:48:46. Version ac04

Copyright 2017 A. Danko


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