This page made possible by the sponsorship of: Rob & Theresa Britschgi, Jason Carlson.

Seattle
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seattle clear sky chart from 2005-11-06 to 2024-01-16.

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%
22487
10%
74571
10%
22444
43%
22444
14%
55805

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
25%
835
7%
2806
6%
831
26%
831
7%
2398
January11th to 20thNight
33%
740
8%
2590
8%
740
34%
740
9%
2069
January21st to endNight
33%
792
8%
2722
9%
792
35%
792
10%
2165
February1st to 10thNight
28%
720
7%
2340
5%
720
30%
720
9%
1928
February11th to 20thNight
32%
700
7%
2274
7%
700
34%
700
10%
1808
February21st to endNight
32%
592
9%
1866
8%
592
34%
592
11%
1492
March1st to 10thNight
35%
720
9%
2116
7%
720
37%
720
13%
1652
March11th to 20thNight
34%
720
11%
1980
8%
720
36%
720
14%
1570
March21st to endNight
29%
792
5%
2106
3%
792
30%
792
9%
1784
April1st to 10thNight
35%
720
6%
1800
3%
720
36%
720
13%
1471
April11th to 20thNight
36%
685
10%
1753
5%
685
39%
685
16%
1379
April21st to endNight
37%
500
8%
1580
6%
500
38%
500
13%
1199
May1st to 10thNight
45%
360
10%
1440
15%
360
46%
360
15%
976
May11th to 20thNight
52%
354
10%
1416
15%
354
54%
354
15%
971
May21st to endNight
48%
388
6%
1552
10%
388
48%
388
11%
1159
June1st to 10thNight
48%
360
5%
1396
7%
360
48%
360
14%
968
June11th to 20thNight
43%
360
4%
1260
5%
360
43%
360
12%
958
June21st to endNight
58%
360
10%
1260
10%
360
60%
360
23%
791
July1st to 10thNight
65%
350
11%
1225
14%
350
66%
350
28%
711
July11th to 20thNight
68%
348
17%
1328
26%
348
70%
348
32%
691
July21st to endNight
76%
394
21%
1488
35%
372
79%
372
41%
662
August1st to 10thNight
73%
397
20%
1463
26%
395
74%
395
34%
756
August11th to 20thNight
78%
534
25%
1602
23%
534
79%
534
43%
858
August21st to endNight
64%
699
22%
1887
17%
699
65%
699
34%
1171
September1st to 10thNight
65%
720
20%
1800
12%
720
67%
720
39%
1124
September11th to 20thNight
58%
720
19%
1800
13%
720
59%
720
27%
1276
September21st to endNight
52%
720
18%
2055
16%
720
53%
720
24%
1417
October1st to 10thNight
48%
720
10%
2160
11%
720
49%
720
20%
1529
October11th to 20thNight
44%
720
12%
2160
13%
720
45%
720
16%
1593
October21st to endNight
37%
792
11%
2484
9%
792
40%
792
15%
1856
November1st to 10thNight
31%
740
7%
2548
7%
740
33%
740
9%
2043
November11th to 20thNight
28%
760
8%
2660
7%
760
31%
760
9%
2128
November21st to endNight
31%
760
6%
2660
6%
760
32%
760
9%
2128
December1st to 10thNight
35%
760
10%
2660
10%
760
36%
760
11%
2093
December11th to 20thNight
23%
759
4%
3250
4%
759
23%
759
5%
2756
December21st to endNight
20%
896
3%
5084
4%
881
22%
881
3%
4275

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%9%47%6%35%5%5%7%32%
74987
Night
20%9%47%6%345%5%6%30%
74807
TransparencyDay
53%9%14%16%7%1
74681
Night
55%9%13%16%7%1
74631
SeeingDay
22%9%19%25%19%6%
13964
Night
26%8%16%27%20%3
22505

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
35%11%48%7%3445%416%
1352
11th to 20thDay
26%11%49%8%345%46%19%
1480
21st to endDay
26%12%5%8%7%46%5%5%7%15%
1633
February1st to 10thDay
28%13%5%8%8%35%5%6%6%15%
1620
11th to 20thDay
24%13%49%6%45%5%46%19%
1575
21st to endDay
22%13%5%7%8%46%5%5%5%21%
1390
March1st to 10thDay
22%10%5%10%8%36%5%5%7%20%
1842
11th to 20thDay
26%12%48%7%45%5%47%19%
1980
21st to endDay
22%10%49%8%5%7%6%5%6%17%
2249
April1st to 10thDay
21%11%5%8%7%46%6%6%6%20%
2160
11th to 20thDay
19%9%47%7%35%5%6%8%26%
2153
21st to endDay
16%9%5%8%7%46%6%6%8%27%
2379
May1st to 10thDay
13%7%38%7%46%6%6%6%34%
2520
11th to 20thDay
15%7%46%6%45%6%7%9%32%
2477
21st to endDay
18%10%38%6%45%5%5%7%29%
2723
June1st to 10thDay
18%7%37%5%35%5%7%8%31%
2560
11th to 20thDay
17%9%48%7%47%6%6%8%25%
2700
21st to endDay
11%5%36%6%446%6%7%42%
2700
July1st to 10thDay
7%6%25%4345%5%7%52%
2700
11th to 20thDay
8%5%244235%5%7%54%
2586
21st to endDay
6%324313346%64%
2741
August1st to 10thDay
7%5%35%433447%57%
2464
11th to 20thDay
8%42443345%8%57%
2298
21st to endDay
10%7%35%5%3445%7%48%
2468
September1st to 10thDay
9%6%245%3445%8%51%
2160
11th to 20thDay
12%7%37%6%45%45%7%40%
2160
21st to endDay
15%7%37%6%245%5%8%37%
1903
October1st to 10thDay
17%9%37%5%35%5%8%7%31%
1800
11th to 20thDay
21%9%47%6%5%5%5%6%7%27%
1800
21st to endDay
26%11%48%6%3445%8%22%
1872
November1st to 10thDay
29%11%48%8%445%5%6%17%
1528
11th to 20thDay
28%10%5%8%7%25%345%22%
1520
21st to endDay
30%11%46%6%35%35%7%19%
1520
December1st to 10thDay
26%9%5%8%7%45%45%7%22%
1520
11th to 20thDay
29%13%49%9%46%5%5%5%12%
1925
21st to endDay
25%14%5%10%9%46%5%45%12%
2529
January1st to 10thNight
35%10%47%7%345%4417%
2806
11th to 20thNight
29%11%47%5%35%446%21%
2590
21st to endNight
25%11%49%7%35%5%5%7%21%
2722
February1st to 10thNight
28%11%47%7%45%446%20%
2340
11th to 20thNight
22%12%47%7%4445%6%25%
2272
21st to endNight
25%12%47%6%35%5%6%7%21%
1866
March1st to 10thNight
23%9%37%5%46%5%6%7%25%
2116
11th to 20thNight
23%10%47%5%345%6%6%27%
1979
21st to endNight
21%12%5%8%7%36%6%5%6%20%
2106
April1st to 10thNight
20%10%38%7%45%6%5%8%23%
1800
11th to 20thNight
17%9%47%7%35%6%5%6%30%
1753
21st to endNight
19%10%48%7%445%5%6%27%
1580
May1st to 10thNight
15%9%36%7%445%48%36%
1440
11th to 20thNight
14%6%37%7%45%5%6%7%35%
1416
21st to endNight
13%8%410%8%45%5%6%6%31%
1552
June1st to 10thNight
14%8%5%7%6%46%5%6%8%33%
1396
11th to 20thNight
16%9%5%8%8%445%5%8%28%
1260
21st to endNight
8%5%37%7%45%45%7%46%
1260
July1st to 10thNight
6%5%26%6%2445%6%54%
1260
11th to 20thNight
6%424423446%61%
1372
21st to endNight
5%313423346%66%
1576
August1st to 10thNight
6%424424447%58%
1471
11th to 20thNight
6%31332325%8%64%
1602
21st to endNight
9%5%26%5%2445%6%51%
1887
September1st to 10thNight
9%42442445%6%57%
1800
11th to 20thNight
13%7%36%5%345%5%5%43%
1800
21st to endNight
14%8%36%6%34447%40%
2055
October1st to 10thNight
15%8%38%6%345%5%6%37%
2160
11th to 20thNight
21%9%37%6%34445%35%
2160
21st to endNight
24%9%36%5%345%5%7%30%
2484
November1st to 10thNight
30%9%37%6%45%5%5%5%22%
2548
11th to 20thNight
27%10%47%7%35%45%6%23%
2660
21st to endNight
28%8%47%7%45%5%6%7%20%
2660
December1st to 10thNight
26%10%5%8%6%34445%24%
2660
11th to 20thNight
25%13%5%10%9%45%5%5%5%15%
3280
21st to endNight
26%15%5%10%8%445%45%14%
5118

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
74%7%5%10%31
1352
11th to 20thDay
64%11%11%8%5%1
1480
21st to endDay
67%8%12%10%3
1633
February1st to 10thDay
70%9%9%8%41
1620
11th to 20thDay
65%9%10%10%6%
1584
21st to endDay
67%7%11%10%5%
1390
March1st to 10thDay
64%9%12%13%2
1842
11th to 20thDay
67%7%10%13%31
1980
21st to endDay
69%10%10%10%
2249
April1st to 10thDay
64%10%16%10%1
2160
11th to 20thDay
58%9%14%13%5%1
2153
21st to endDay
58%10%14%12%5%1
2379
May1st to 10thDay
51%10%16%16%6%
2520
11th to 20thDay
50%10%19%15%5%
2480
21st to endDay
58%9%16%14%3
2724
June1st to 10thDay
52%10%19%13%6%
2560
11th to 20thDay
59%11%16%11%3
2700
21st to endDay
41%11%19%20%8%2
2700
July1st to 10thDay
34%10%15%26%13%2
2626
11th to 20thDay
31%7%15%30%16%1
2507
21st to endDay
22%7%14%36%18%3
2589
August1st to 10thDay
30%9%16%29%14%2
2447
11th to 20thDay
26%8%21%29%13%3
2298
21st to endDay
37%9%13%30%10%1
2468
September1st to 10thDay
34%7%16%26%13%4
2160
11th to 20thDay
45%8%13%17%15%2
2160
21st to endDay
46%9%17%17%9%2
1903
October1st to 10thDay
49%11%14%20%6%
1800
11th to 20thDay
55%11%13%15%5%1
1800
21st to endDay
61%8%13%11%7%
1872
November1st to 10thDay
66%8%12%10%3
1528
11th to 20thDay
65%8%10%9%6%2
1520
21st to endDay
64%10%7%10%8%1
1520
December1st to 10thDay
64%8%8%12%8%1
1520
11th to 20thDay
69%11%10%8%21
1928
21st to endDay
70%11%11%6%2
2529
January1st to 10thNight
71%8%7%10%31
2806
11th to 20thNight
64%8%12%10%5%1
2590
21st to endNight
62%9%11%13%5%1
2722
February1st to 10thNight
67%8%11%9%5%
2340
11th to 20thNight
61%10%12%10%7%1
2274
21st to endNight
63%8%12%12%5%
1866
March1st to 10thNight
59%8%14%15%4
2116
11th to 20thNight
57%10%11%17%41
1980
21st to endNight
65%11%13%11%1
2106
April1st to 10thNight
60%11%15%13%2
1800
11th to 20thNight
54%11%14%15%6%1
1753
21st to endNight
58%7%15%13%6%1
1580
May1st to 10thNight
47%10%18%17%8%1
1440
11th to 20thNight
48%10%19%16%6%1
1416
21st to endNight
54%12%16%16%3
1552
June1st to 10thNight
50%9%18%18%5%
1396
11th to 20thNight
56%11%17%14%3
1260
21st to endNight
38%9%22%20%8%3
1260
July1st to 10thNight
33%8%16%25%12%5%
1225
11th to 20thNight
29%6%15%32%16%2
1328
21st to endNight
19%6%13%34%22%5%
1488
August1st to 10thNight
26%8%14%29%19%3
1463
11th to 20thNight
21%9%18%32%16%4
1602
21st to endNight
34%7%15%31%11%1
1887
September1st to 10thNight
30%8%16%27%15%4
1800
11th to 20thNight
43%9%10%21%14%4
1800
21st to endNight
45%7%12%23%11%2
2055
October1st to 10thNight
49%8%13%23%7%1
2160
11th to 20thNight
52%8%14%18%7%
2160
21st to endNight
55%8%13%15%9%
2484
November1st to 10thNight
64%9%13%11%3
2548
11th to 20thNight
64%8%11%9%5%2
2660
21st to endNight
62%10%10%10%8%1
2660
December1st to 10thNight
64%6%10%12%7%1
2660
11th to 20thNight
67%12%11%7%3
3276
21st to endNight
69%11%9%8%2
5118

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
40%8%16%14%16%6%
180
11th to 20thDay
32%10%18%19%15%6%
170
21st to endDay
40%13%16%18%9%5%
225
February1st to 10thDay
41%9%18%19%10%3
340
11th to 20thDay
40%12%21%17%8%2
335
21st to endDay
32%14%22%20%10%2
306
March1st to 10thDay
32%14%23%21%8%2
420
11th to 20thDay
29%13%20%20%13%4
420
21st to endDay
33%10%26%18%11%2
462
April1st to 10thDay
31%13%21%19%10%4
420
11th to 20thDay
25%15%25%19%10%6%
412
21st to endDay
21%11%26%27%13%1
420
May1st to 10thDay
15%10%23%32%16%4
423
11th to 20thDay
21%5%22%26%18%8%
448
21st to endDay
20%6%22%27%18%8%
485
June1st to 10thDay
23%8%20%26%18%6%
490
11th to 20thDay
23%9%19%30%17%3
630
21st to endDay
11%5%19%35%23%7%
630
July1st to 10thDay
11%318%31%27%10%
621
11th to 20thDay
9%413%34%32%8%
506
21st to endDay
7%210%29%40%11%
487
August1st to 10thDay
8%414%28%33%13%
447
11th to 20thDay
7%211%30%37%14%
437
21st to endDay
14%6%19%31%24%7%
495
September1st to 10thDay
12%6%23%36%19%4
450
11th to 20thDay
15%7%18%28%24%7%
450
21st to endDay
18%12%18%28%20%4
450
October1st to 10thDay
22%10%19%23%19%8%
450
11th to 20thDay
28%10%15%25%17%6%
450
21st to endDay
31%13%17%21%14%4
387
November1st to 10thDay
33%14%19%21%11%3
200
11th to 20thDay
36%13%17%14%14%7%
180
21st to endDay
34%11%17%18%8%12%
180
December1st to 10thDay
37%11%16%14%14%8%
180
11th to 20thDay
36%13%23%17%9%1
180
21st to endDay
36%12%15%21%13%3
198
January1st to 10thNight
42%11%14%16%15%2
835
11th to 20thNight
36%10%13%21%17%2
740
21st to endNight
32%8%15%27%15%2
792
February1st to 10thNight
35%9%18%25%10%2
720
11th to 20thNight
30%10%16%27%15%1
704
21st to endNight
35%9%17%24%11%3
592
March1st to 10thNight
30%12%16%25%15%2
720
11th to 20thNight
28%11%20%25%13%2
720
21st to endNight
30%10%19%28%11%2
792
April1st to 10thNight
27%8%20%28%14%3
720
11th to 20thNight
23%11%19%28%17%3
685
21st to endNight
26%7%19%32%14%2
500
May1st to 10thNight
19%9%18%33%21%1
360
11th to 20thNight
18%5%15%31%26%5%
354
21st to endNight
20%315%32%25%5%
389
June1st to 10thNight
18%5%19%33%23%3
360
11th to 20thNight
20%10%21%27%20%2
360
21st to endNight
11%5%16%34%31%3
360
July1st to 10thNight
10%411%31%39%5%
350
11th to 20thNight
9%213%38%33%5%
348
21st to endNight
6%310%33%45%4
394
August1st to 10thNight
9%111%28%43%10%
397
11th to 20thNight
9%17%27%46%10%
534
21st to endNight
13%313%31%36%4
699
September1st to 10thNight
11%315%35%31%5%
720
11th to 20thNight
17%411%23%37%7%
720
21st to endNight
19%6%14%31%25%4
720
October1st to 10thNight
21%416%28%25%5%
720
11th to 20thNight
26%8%14%26%22%4
720
21st to endNight
31%8%19%24%16%2
792
November1st to 10thNight
35%9%17%28%10%
740
11th to 20thNight
34%10%20%21%14%2
760
21st to endNight
34%9%19%21%15%3
760
December1st to 10thNight
33%5%18%26%16%2
760
11th to 20thNight
38%11%19%21%11%1
760
21st to endNight
41%12%18%20%9%1
908
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