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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 2017-05-24.

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%
14235
9%
45702
9%
14207
43%
14207
14%
34413

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
48%
360
4%
1440
7%
360
48%
360
10%
1107
June1st to 15thNight
47%
330
4%
1238
5%
330
48%
330
13%
900
June16th to endNight
45%
330
6%
1155
6%
330
46%
330
14%
837
July1st to 15thNight
69%
310
13%
1098
16%
310
71%
310
30%
604
July16th to endNight
69%
348
16%
1304
26%
326
71%
326
33%
649
August1st to 15thNight
77%
409
19%
1392
24%
407
77%
407
34%
754
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
19%8%36%5%35%45%6%30%
46593
Night
21%8%36%6%3445%6%30%
45861
Cloud CoverDay
21%8%36%5%34446%30%
46593
Night
23%8%36%5%24446%29%
45864
TransparencyDay
55%8%13%15%6%
46255
Night
55%8%12%15%6%1
45703
SeeingDay
24%7%19%25%17%5%
5450
Night
26%6%15%26%21%3
14244

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%47%6%35%6%6%8%24%
2514
June1st to 15thDay
16%6%36%5%36%5%6%7%31%
2390
16th to endDay
13%7%37%5%46%5%5%7%31%
2475
July1st to 15thDay
7%41432335%6%55%
2459
16th to endDay
9%425%313346%55%
2461
August1st to 15thDay
8%424323347%54%
2228
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
12%7%410%7%45%6%7%7%26%
1440
June1st to 15thNight
14%8%46%5%35%5%6%8%31%
1238
16th to endNight
11%8%49%8%45%45%5%32%
1155
July1st to 15thNight
43245%2335%5%59%
1169
16th to endNight
6%32441225%6%60%
1400
August1st to 15thNight
5%32432345%7%58%
1400
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%45%5%5%7%24%
2514
June1st to 15thDay
20%8%35%445%45%6%30%
2390
16th to endDay
15%7%47%5%45%5%5%6%31%
2475
July1st to 15thDay
9%424324346%55%
2459
16th to endDay
10%5%23323246%55%
2461
August1st to 15thDay
10%414323347%54%
2228
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
16%9%38%8%46%5%5%7%26%
1440
June1st to 15thNight
18%7%38%5%35%447%31%
1238
16th to endNight
15%7%39%8%35%445%31%
1155
July1st to 15thNight
5%5%15%424326%58%
1169
16th to endNight
7%414412345%59%
1400
August1st to 15thNight
7%32331325%7%58%
1400
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
59%11%16%9%2
2518
June1st to 15thDay
55%10%18%10%5%
2390
16th to endDay
51%11%16%15%3
2475
July1st to 15thDay
32%8%15%25%16%1
2301
16th to endDay
31%6%15%27%16%2
2294
August1st to 15thDay
31%8%18%29%10%1
2211
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
56%12%16%11%2
1440
June1st to 15thNight
51%10%16%15%5%
1238
16th to endNight
53%8%15%18%31
1155
July1st to 15thNight
30%7%14%30%13%4
1098
16th to endNight
27%5%13%30%18%3
1304
August1st to 15thNight
25%9%17%30%13%2
1392
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%26%16%8%
301
June1st to 15thDay
20%8%19%27%18%5%
335
16th to endDay
17%5%17%35%20%4
420
July1st to 15thDay
8%418%33%26%8%
385
16th to endDay
11%311%30%32%10%
271
August1st to 15thDay
7%115%32%32%10%
255
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%4
361
June1st to 15thNight
20%6%15%29%24%3
330
16th to endNight
16%6%20%27%28%1
330
July1st to 15thNight
6%310%32%40%6%
310
16th to endNight
9%111%30%41%6%
348
August1st to 15thNight
9%6%26%46%10%
409
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 2017-05-25 10:27:30. Version ac04

Copyright 2017 A. Danko


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