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Seattle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seattle 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
42%
14392
9%
46284
9%
14364
43%
14364
14%
34690

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
32%
737
10%
2608
9%
733
34%
733
9%
2086
January16th to endNight
36%
768
8%
2655
9%
768
38%
768
11%
2092
February1st to 15thNight
32%
697
5%
2288
4%
697
33%
697
9%
1852
February16th to endNight
34%
632
9%
2014
8%
632
36%
632
11%
1594
March1st to 15thNight
29%
720
5%
2070
4%
720
30%
720
10%
1706
March16th to endNight
29%
768
5%
2064
3%
768
30%
768
9%
1742
April1st to 15thNight
36%
719
6%
1799
3%
719
37%
719
13%
1485
April16th to endNight
40%
555
7%
1635
6%
555
41%
555
14%
1257
May1st to 15thNight
47%
359
11%
1439
17%
359
49%
359
16%
966
May16th to endNight
49%
374
5%
1496
9%
374
50%
374
10%
1131
June1st to 15thNight
48%
360
4%
1350
4%
360
48%
360
13%
982
June16th to endNight
48%
360
7%
1260
8%
360
48%
360
16%
895
July1st to 15thNight
71%
340
12%
1204
16%
340
72%
340
31%
654
July16th to endNight
71%
376
18%
1416
29%
354
73%
354
35%
687
August1st to 15thNight
78%
434
19%
1483
23%
432
78%
432
36%
779
August16th to endNight
68%
589
20%
1639
16%
589
69%
589
36%
972
September1st to 15thNight
71%
660
22%
1650
14%
660
73%
660
42%
999
September16th to endNight
53%
656
19%
1807
16%
656
54%
656
23%
1265
October1st to 15thNight
46%
660
11%
1980
12%
660
47%
660
19%
1417
October16th to endNight
36%
704
7%
2178
6%
704
38%
704
12%
1715
November1st to 15thNight
31%
700
5%
2425
6%
700
32%
700
8%
1984
November16th to endNight
30%
720
6%
2520
6%
720
31%
720
9%
2026
December1st to 15thNight
33%
720
7%
2520
8%
720
34%
720
10%
2016
December16th to endNight
21%
784
4%
2784
4%
784
22%
784
6%
2388

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
18%8%36%5%3445%6%31%
47708
Night
20%8%36%6%3445%6%30%
46443
Cloud CoverDay
21%8%36%5%34446%31%
47708
Night
23%8%36%5%24446%30%
46446
TransparencyDay
54%8%13%15%6%
47370
Night
55%8%12%15%6%1
46285
SeeingDay
23%7%19%25%17%5%
5795
Night
26%6%15%26%21%3
14401

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
33%9%37%6%23435%20%
1328
16th to endDay
26%10%47%7%345%46%17%
1568
February1st to 15thDay
26%12%38%7%35%5%5%5%15%
1576
16th to endDay
22%13%47%6%35%346%21%
1460
March1st to 15thDay
29%10%5%8%6%35%446%15%
1888
16th to endDay
25%10%49%8%36%6%45%14%
2160
April1st to 15thDay
20%9%5%8%7%35%5%5%7%20%
2149
16th to endDay
18%8%47%7%36%5%5%7%25%
2322
May1st to 15thDay
15%7%36%5%35%6%6%6%32%
2511
16th to endDay
16%9%37%6%35%6%6%8%26%
2615
June1st to 15thDay
16%6%36%5%36%5%6%7%30%
2607
16th to endDay
13%7%37%5%45%5%5%7%32%
2700
July1st to 15thDay
7%41432335%6%56%
2682
16th to endDay
8%424313346%56%
2657
August1st to 15thDay
7%424313347%56%
2381
16th to endDay
9%5%24434446%49%
2210
September1st to 15thDay
8%5%24323446%54%
1980
16th to endDay
15%8%37%5%3445%7%35%
1823
October1st to 15thDay
19%9%37%5%3446%6%28%
1650
16th to endDay
27%11%47%6%3435%7%17%
1694
November1st to 15thDay
32%10%47%5%24445%17%
1433
16th to endDay
33%11%46%6%14246%17%
1440
December1st to 15thDay
25%9%5%8%7%35%5%5%5%17%
1440
16th to endDay
30%13%37%6%35%345%14%
1434
January1st to 15thNight
33%8%36%5%2344423%
2608
16th to endNight
26%9%47%6%35%446%21%
2655
February1st to 15thNight
25%11%46%6%34346%21%
2276
16th to endNight
25%9%36%6%3445%6%24%
2014
March1st to 15thNight
28%9%46%435%45%6%20%
2070
16th to endNight
22%10%5%7%7%36%5%6%6%19%
2060
April1st to 15thNight
18%9%38%8%45%6%47%24%
1799
16th to endNight
18%7%37%7%35%5%5%7%27%
1635
May1st to 15thNight
13%8%36%6%34447%36%
1439
16th to endNight
11%7%410%7%45%5%7%6%28%
1496
June1st to 15thNight
14%8%47%6%35%46%8%30%
1350
16th to endNight
11%8%48%8%4445%5%34%
1260
July1st to 15thNight
33245%2345%5%59%
1275
16th to endNight
5%323412246%62%
1512
August1st to 15thNight
5%31431335%7%60%
1491
16th to endNight
7%414425%447%54%
1639
September1st to 15thNight
6%313423445%60%
1650
16th to endNight
15%7%25%5%24448%37%
1807
October1st to 15thNight
17%7%37%5%2345%5%35%
1980
16th to endNight
25%8%26%6%345%36%25%
2178
November1st to 15thNight
31%8%35%5%34445%21%
2425
16th to endNight
28%8%46%7%34446%21%
2520
December1st to 15thNight
25%9%48%7%345%45%23%
2520
16th to endNight
30%12%48%6%3443416%
2784

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
35%9%35%6%2324420%
1328
16th to endDay
29%11%36%6%2445%5%19%
1568
February1st to 15thDay
29%11%38%7%35%36%416%
1576
16th to endDay
25%13%47%5%24346%21%
1460
March1st to 15thDay
32%12%5%7%424445%15%
1888
16th to endDay
28%10%5%8%8%35%5%45%14%
2160
April1st to 15thDay
23%11%48%6%35%5%5%6%19%
2149
16th to endDay
20%8%46%7%35%5%6%7%23%
2322
May1st to 15thDay
17%7%46%435%5%5%7%31%
2511
16th to endDay
21%8%36%6%35%5%5%7%26%
2615
June1st to 15thDay
20%8%35%5%46%45%6%29%
2607
16th to endDay
15%7%47%5%45%5%5%6%32%
2700
July1st to 15thDay
8%424323346%56%
2682
16th to endDay
9%5%23223246%56%
2657
August1st to 15thDay
9%413323337%56%
2381
16th to endDay
11%5%15%424446%48%
2210
September1st to 15thDay
9%5%24423336%54%
1980
16th to endDay
18%7%35%5%2435%7%34%
1823
October1st to 15thDay
21%9%36%5%3445%6%29%
1650
16th to endDay
31%9%36%5%335%47%18%
1694
November1st to 15thDay
39%8%26%5%2345%418%
1433
16th to endDay
36%10%46%5%23336%18%
1440
December1st to 15thDay
26%12%5%7%6%34446%17%
1440
16th to endDay
32%14%46%5%34445%16%
1434
January1st to 15thNight
35%8%36%42334323%
2607
16th to endNight
29%9%35%42445%6%21%
2655
February1st to 15thNight
29%10%36%5%345%46%21%
2276
16th to endNight
27%11%36%5%34446%24%
2014
March1st to 15thNight
31%9%45%5%34446%21%
2070
16th to endNight
25%10%48%6%35%46%6%18%
2064
April1st to 15thNight
20%8%48%8%45%5%47%22%
1799
16th to endNight
21%7%37%6%45%5%5%7%26%
1635
May1st to 15thNight
15%8%37%42445%5%36%
1439
16th to endNight
15%9%38%8%36%5%47%28%
1496
June1st to 15thNight
17%7%39%5%35%447%30%
1350
16th to endNight
15%7%38%7%35%445%33%
1260
July1st to 15thNight
5%5%15%424335%58%
1275
16th to endNight
7%414312245%61%
1512
August1st to 15thNight
7%323313247%60%
1491
16th to endNight
9%415%324346%54%
1639
September1st to 15thNight
7%324323345%59%
1650
16th to endNight
17%6%35%5%23347%38%
1807
October1st to 15thNight
19%8%36%6%24445%35%
1980
16th to endNight
28%7%36%6%34446%24%
2178
November1st to 15thNight
34%8%35%5%345%4420%
2425
16th to endNight
30%8%37%6%24336%21%
2520
December1st to 15thNight
27%10%48%6%24335%23%
2520
16th to endNight
33%13%47%6%2334416%
2784

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
68%8%6%11%5%
1328
16th to endDay
65%7%13%8%31
1568
February1st to 15thDay
69%9%8%7%4
1576
16th to endDay
65%6%10%12%4
1460
March1st to 15thDay
71%7%10%8%1
1888
16th to endDay
73%7%8%9%
2160
April1st to 15thDay
64%9%14%9%1
2149
16th to endDay
60%10%15%11%2
2322
May1st to 15thDay
52%8%14%17%6%
2511
16th to endDay
58%10%16%11%2
2619
June1st to 15thDay
55%10%17%10%5%
2607
16th to endDay
50%11%16%16%4
2700
July1st to 15thDay
31%7%15%27%16%2
2524
16th to endDay
29%6%15%29%16%2
2490
August1st to 15thDay
29%8%17%29%12%2
2364
16th to endDay
35%9%13%29%9%2
2210
September1st to 15thDay
32%6%14%25%17%3
1980
16th to endDay
48%8%16%16%8%2
1823
October1st to 15thDay
52%10%13%16%6%
1650
16th to endDay
65%8%13%10%2
1694
November1st to 15thDay
68%8%10%8%21
1433
16th to endDay
69%8%6%9%6%
1440
December1st to 15thDay
68%8%7%11%4
1440
16th to endDay
72%6%8%8%3
1434
January1st to 15thNight
64%7%9%12%5%1
2608
16th to endNight
61%8%11%11%5%1
2655
February1st to 15thNight
63%9%11%8%5%
2288
16th to endNight
61%7%11%13%5%
2014
March1st to 15thNight
64%8%10%13%1
2070
16th to endNight
64%10%11%12%1
2064
April1st to 15thNight
61%9%13%13%1
1799
16th to endNight
56%8%16%13%3
1635
May1st to 15thNight
48%7%16%17%9%1
1440
16th to endNight
55%12%16%12%3
1496
June1st to 15thNight
52%10%16%15%5%
1350
16th to endNight
51%8%16%18%41
1260
July1st to 15thNight
29%7%14%30%14%4
1204
16th to endNight
25%5%14%32%18%3
1416
August1st to 15thNight
23%9%16%30%16%3
1483
16th to endNight
30%7%17%31%11%2
1639
September1st to 15thNight
26%7%15%26%17%5%
1650
16th to endNight
46%6%11%22%10%2
1807
October1st to 15thNight
50%6%13%19%7%
1980
16th to endNight
60%8%13%13%4
2178
November1st to 15thNight
65%8%12%9%12
2425
16th to endNight
63%8%10%9%7%
2520
December1st to 15thNight
64%7%11%12%4
2520
16th to endNight
73%6%9%7%3
2784

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
23%10%18%28%20%
60
16th to endDay
40%10%19%11%12%5%
96
February1st to 15thDay
40%7%17%20%11%2
236
16th to endDay
37%11%21%19%8%2
218
March1st to 15thDay
43%17%20%14%31
270
16th to endDay
39%11%26%14%6%1
288
April1st to 15thDay
28%14%22%18%11%4
269
16th to endDay
24%8%29%22%10%3
270
May1st to 15thDay
20%11%23%28%13%2
288
16th to endDay
20%421%24%17%10%
330
June1st to 15thDay
21%7%19%27%18%4
402
16th to endDay
16%5%18%34%19%4
495
July1st to 15thDay
8%316%33%30%8%
458
16th to endDay
11%311%31%33%10%
327
August1st to 15thDay
6%113%28%33%16%
300
16th to endDay
11%418%30%27%7%
268
September1st to 15thDay
14%221%32%21%7%
255
16th to endDay
20%9%16%28%19%5%
255
October1st to 15thDay
28%7%20%24%16%2
255
16th to endDay
33%9%15%24%13%3
206
November1st to 15thDay
46%10%24%15%3
65
16th to endDay
43%6%13%11%10%15%
60
December1st to 15thDay
46%328%18%3
60
16th to endDay
35%9%17%20%10%6%
64
January1st to 15thNight
38%9%13%17%18%2
737
16th to endNight
33%7%13%24%17%3
768
February1st to 15thNight
32%8%15%27%13%2
704
16th to endNight
32%9%16%25%13%2
632
March1st to 15thNight
35%12%18%19%12%2
720
16th to endNight
30%10%19%27%9%2
768
April1st to 15thNight
24%7%20%29%15%2
719
16th to endNight
24%6%18%30%18%1
555
May1st to 15thNight
18%8%16%31%24%1
360
16th to endNight
18%314%31%27%5%
375
June1st to 15thNight
20%6%15%30%24%2
360
16th to endNight
16%5%18%29%27%2
360
July1st to 15thNight
6%310%32%41%5%
340
16th to endNight
8%111%31%41%6%
376
August1st to 15thNight
9%6%25%46%11%
434
16th to endNight
11%29%27%41%6%
589
September1st to 15thNight
8%210%28%41%8%
660
16th to endNight
20%412%30%27%6%
656
October1st to 15thNight
23%316%28%23%4
660
16th to endNight
31%7%16%24%17%3
704
November1st to 15thNight
35%7%17%28%9%
700
16th to endNight
33%10%18%19%14%3
720
December1st to 15thNight
33%6%17%26%15%1
720
16th to endNight
40%9%18%19%10%
784
Page generated at 2017-08-13 11:16:32. Version ac04

Copyright 2017 A. Danko


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