This page made possible by the sponsorship of: Theresa and Robert Britschgi,
.

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 2018-03-11.

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%
15242
9%
48951
9%
15214
43%
15214
14%
36704

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%
805
9%
2826
9%
801
34%
801
9%
2274
January16th to endNight
33%
832
8%
2876
8%
832
34%
832
10%
2299
February1st to 15thNight
31%
757
6%
2483
5%
757
33%
757
9%
2021
February16th to endNight
33%
684
9%
2180
8%
684
36%
684
11%
1728
March1st to 15thNight
30%
764
6%
2199
4%
764
31%
764
10%
1797
March16th to endNight
29%
768
6%
2064
3%
768
30%
768
9%
1742
April1st to 15thNight
36%
719
6%
1799
3%
719
37%
719
13%
1485
April16th to endNight
39%
555
7%
1635
6%
555
40%
555
14%
1257
May1st to 15thNight
47%
359
11%
1439
17%
359
48%
359
16%
966
May16th to endNight
50%
374
5%
1496
9%
374
50%
374
10%
1131
June1st to 15thNight
49%
360
4%
1350
4%
360
49%
360
13%
982
June16th to endNight
48%
360
7%
1260
8%
360
49%
360
16%
895
July1st to 15thNight
71%
340
12%
1204
16%
340
72%
340
31%
654
July16th to endNight
72%
376
18%
1416
29%
354
75%
354
35%
687
August1st to 15thNight
78%
446
18%
1519
23%
444
78%
444
36%
803
August16th to endNight
71%
643
21%
1789
18%
643
72%
643
38%
1047
September1st to 15thNight
73%
720
21%
1800
14%
720
74%
720
43%
1087
September16th to endNight
54%
716
19%
1971
16%
716
55%
716
23%
1382
October1st to 15thNight
48%
720
10%
2160
12%
720
48%
720
20%
1526
October16th to endNight
38%
768
9%
2376
8%
768
39%
768
13%
1844
November1st to 15thNight
30%
760
5%
2633
6%
760
32%
760
8%
2162
November16th to endNight
29%
780
6%
2730
6%
780
30%
780
8%
2215
December1st to 15thNight
37%
780
8%
2730
10%
780
37%
780
11%
2140
December16th to endNight
22%
856
5%
3016
4%
856
24%
856
6%
2580

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%3445%6%31%
49693
Night
20%8%36%6%3445%6%30%
49110
Cloud CoverDay
21%8%36%5%34446%31%
49693
Night
23%8%36%5%24445%30%
49113
TransparencyDay
54%8%13%15%6%
49355
Night
55%8%12%15%6%1
48952
SeeingDay
24%7%19%24%18%5%
6430
Night
26%6%15%25%21%3
15251

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
34%9%37%6%2343419%
1439
16th to endDay
28%10%47%7%345%46%16%
1699
February1st to 15thDay
26%12%48%7%3445%5%15%
1711
16th to endDay
21%13%47%6%35%346%21%
1580
March1st to 15thDay
28%10%5%8%6%35%446%16%
1995
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%424323347%56%
2430
16th to endDay
8%5%2443445%6%50%
2412
September1st to 15thDay
7%5%24423446%53%
2160
16th to endDay
15%8%37%5%3445%7%35%
1988
October1st to 15thDay
19%8%36%5%345%6%6%29%
1800
16th to endDay
27%10%47%5%3435%7%19%
1848
November1st to 15thDay
33%11%47%5%2444417%
1555
16th to endDay
34%10%46%6%14346%16%
1560
December1st to 15thDay
24%9%5%8%7%35%5%5%5%19%
1560
16th to endDay
31%13%37%6%35%345%14%
1553
January1st to 15thNight
33%8%36%5%2344422%
2826
16th to endNight
27%10%47%6%35%446%19%
2876
February1st to 15thNight
27%11%46%6%34346%21%
2471
16th to endNight
24%10%36%6%3445%6%23%
2180
March1st to 15thNight
28%8%46%435%5%6%6%21%
2199
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%59%
1527
16th to endNight
6%414424446%55%
1789
September1st to 15thNight
6%313323445%60%
1800
16th to endNight
15%7%25%5%34447%38%
1971
October1st to 15thNight
16%8%37%5%2345%5%37%
2160
16th to endNight
24%8%26%6%345%45%27%
2376
November1st to 15thNight
31%8%35%5%3445%5%21%
2633
16th to endNight
28%8%46%6%35%446%20%
2730
December1st to 15thNight
23%8%47%7%345%45%24%
2730
16th to endNight
31%12%47%5%3443417%
3016

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
37%9%35%6%2324419%
1439
16th to endDay
31%11%36%6%24445%18%
1699
February1st to 15thDay
29%11%38%7%35%35%416%
1711
16th to endDay
24%14%5%6%5%24335%22%
1580
March1st to 15thDay
31%11%5%7%424445%16%
1995
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%413423337%56%
2430
16th to endDay
10%5%15%424446%50%
2412
September1st to 15thDay
9%5%24433336%53%
2160
16th to endDay
18%7%35%5%2445%7%34%
1988
October1st to 15thDay
20%8%36%43446%7%29%
1800
16th to endDay
31%9%36%5%33447%20%
1848
November1st to 15thDay
39%8%26%5%2345%417%
1555
16th to endDay
36%10%46%5%23345%17%
1560
December1st to 15thDay
26%11%47%6%34446%19%
1560
16th to endDay
33%13%46%5%3443416%
1553
January1st to 15thNight
35%8%36%42335%323%
2825
16th to endNight
30%10%46%5%24446%20%
2876
February1st to 15thNight
30%10%36%5%345%46%20%
2471
16th to endNight
27%11%36%5%34446%23%
2180
March1st to 15thNight
30%9%45%5%24446%22%
2199
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%59%
1527
16th to endNight
8%414323346%55%
1789
September1st to 15thNight
7%324323345%59%
1800
16th to endNight
17%6%36%5%24447%38%
1971
October1st to 15thNight
18%7%36%6%24345%36%
2160
16th to endNight
27%7%36%5%33446%26%
2376
November1st to 15thNight
34%9%35%5%345%4420%
2633
16th to endNight
31%8%37%6%25%346%20%
2730
December1st to 15thNight
26%9%47%6%24445%25%
2730
16th to endNight
34%13%46%5%2334317%
3016

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
69%7%6%10%4
1439
16th to endDay
66%6%12%8%31
1699
February1st to 15thDay
69%8%8%8%4
1711
16th to endDay
65%6%9%13%4
1580
March1st to 15thDay
70%7%10%10%1
1995
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
30%8%17%28%12%2
2413
16th to endDay
34%9%14%29%9%2
2412
September1st to 15thDay
33%6%13%23%18%3
2160
16th to endDay
49%8%16%14%8%2
1988
October1st to 15thDay
52%10%13%16%6%
1800
16th to endDay
64%7%13%10%4
1848
November1st to 15thDay
69%8%9%8%21
1555
16th to endDay
69%8%6%8%5%
1560
December1st to 15thDay
66%8%7%11%5%
1560
16th to endDay
72%6%8%8%4
1553
January1st to 15thNight
64%7%9%12%41
2826
16th to endNight
63%7%10%10%5%1
2876
February1st to 15thNight
64%9%11%8%6%
2483
16th to endNight
61%7%11%13%5%
2180
March1st to 15thNight
63%8%11%14%2
2199
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
24%9%16%30%16%3
1519
16th to endNight
29%6%18%31%11%2
1789
September1st to 15thNight
26%7%14%26%19%5%
1800
16th to endNight
46%7%12%21%10%2
1971
October1st to 15thNight
49%6%14%20%7%
2160
16th to endNight
58%8%13%13%5%
2376
November1st to 15thNight
66%8%11%9%12
2633
16th to endNight
64%8%10%8%6%
2730
December1st to 15thNight
61%7%12%12%5%
2730
16th to endNight
72%6%8%7%3
3016

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%8%12%30%15%
90
16th to endDay
47%15%16%8%9%3
131
February1st to 15thDay
41%7%17%20%10%2
281
16th to endDay
36%12%23%17%7%1
260
March1st to 15thDay
41%14%20%17%41
312
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
7%113%28%33%15%
315
16th to endDay
10%315%28%32%9%
332
September1st to 15thDay
12%221%34%21%7%
315
16th to endDay
20%8%16%28%20%4
315
October1st to 15thDay
26%7%20%23%17%4
315
16th to endDay
33%9%14%20%18%4
264
November1st to 15thDay
51%9%20%12%42
97
16th to endDay
46%5%17%12%7%10%
90
December1st to 15thDay
37%322%14%13%8%
90
16th to endDay
39%6%16%20%12%4
96
January1st to 15thNight
39%8%12%17%19%2
805
16th to endNight
34%9%13%23%15%3
832
February1st to 15thNight
34%8%15%26%13%2
764
16th to endNight
33%8%16%25%13%2
684
March1st to 15thNight
35%11%17%19%13%2
764
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%
446
16th to endNight
11%29%26%44%7%
643
September1st to 15thNight
7%310%29%41%8%
720
16th to endNight
19%312%29%27%5%
716
October1st to 15thNight
22%415%28%24%4
720
16th to endNight
30%7%16%23%19%2
768
November1st to 15thNight
36%7%16%28%9%
760
16th to endNight
34%10%19%19%13%3
780
December1st to 15thNight
31%6%16%26%17%2
780
16th to endNight
41%8%18%20%10%
856
Page generated at 2018-03-12 10:28:13. Version ac04

Copyright 2018 A. Danko


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