Port Townsend
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Townsend clear sky chart from 2005-11-06 to 2024-01-12.

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
43%
22552
6%
74700
5%
22500
44%
22500
8%
62399

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
26%
848
2%
2836
2%
846
27%
846
3%
2617
January11th to 20thNight
32%
765
4%
2625
4%
765
33%
765
4%
2360
January21st to endNight
36%
792
4%
2720
4%
792
37%
792
5%
2385
February1st to 10thNight
32%
720
2%
2340
1%
720
33%
720
5%
2072
February11th to 20thNight
35%
700
5%
2274
4%
700
36%
700
7%
1938
February21st to endNight
31%
592
6%
1888
5%
592
34%
592
6%
1630
March1st to 10thNight
35%
720
5%
2099
4%
720
36%
720
7%
1812
March11th to 20thNight
34%
720
6%
1980
3%
720
35%
720
6%
1757
March21st to endNight
33%
792
3%
2120
2%
792
33%
792
6%
1900
April1st to 10thNight
39%
720
3%
1800
1%
720
40%
720
7%
1603
April11th to 20thNight
38%
655
8%
1723
4%
655
39%
655
10%
1457
April21st to endNight
40%
505
5%
1585
4%
505
41%
505
8%
1342
May1st to 10thNight
49%
360
7%
1440
11%
360
50%
360
8%
1157
May11th to 20thNight
55%
354
7%
1416
13%
354
55%
354
9%
1082
May21st to endNight
50%
389
3%
1552
6%
388
50%
388
5%
1320
June1st to 10thNight
46%
360
2%
1308
3%
360
46%
360
7%
1095
June11th to 20thNight
44%
360
2%
1260
2%
360
44%
360
6%
1067
June21st to endNight
61%
360
6%
1260
5%
360
63%
360
13%
942
July1st to 10thNight
66%
350
9%
1225
10%
350
67%
350
16%
872
July11th to 20thNight
66%
348
12%
1252
15%
348
68%
348
18%
848
July21st to endNight
77%
394
13%
1488
22%
372
79%
372
18%
963
August1st to 10thNight
72%
401
10%
1467
14%
399
73%
399
16%
1011
August11th to 20thNight
79%
534
17%
1602
17%
534
79%
534
23%
1086
August21st to endNight
66%
674
13%
1862
9%
674
67%
674
18%
1379
September1st to 10thNight
70%
720
14%
1800
8%
720
71%
720
25%
1311
September11th to 20thNight
58%
720
14%
1800
9%
720
59%
720
17%
1430
September21st to endNight
55%
720
13%
2061
11%
720
57%
720
14%
1606
October1st to 10thNight
53%
720
8%
2160
8%
720
54%
720
12%
1714
October11th to 20thNight
47%
720
7%
2160
6%
720
48%
720
11%
1728
October21st to endNight
41%
792
6%
2474
5%
792
43%
792
9%
2035
November1st to 10thNight
34%
740
4%
2581
3%
740
36%
740
5%
2274
November11th to 20thNight
32%
760
4%
2660
3%
760
34%
760
5%
2340
November21st to endNight
30%
760
2%
2660
1%
760
32%
760
4%
2350
December1st to 10thNight
35%
760
3%
2660
3%
760
35%
760
5%
2318
December11th to 20thNight
23%
797
1%
3385
1%
797
24%
797
3%
3033
December21st to endNight
20%
930
1%
5177
1%
905
22%
905
2%
4565

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
14%9%47%6%45%5%6%7%33%
74789
Night
17%10%47%6%45%5%5%7%31%
74925
TransparencyDay
34%41%11%10%41
74480
Night
35%41%9%10%41
74752
SeeingDay
17%9%20%26%21%6%
14103
Night
21%8%19%26%21%5%
22564

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
18%14%5%11%9%45%5%6%5%18%
1329
11th to 20thDay
17%13%7%10%8%5%5%6%6%7%17%
1354
21st to endDay
21%13%6%11%9%5%5%445%15%
1634
February1st to 10thDay
20%12%48%8%48%6%5%6%18%
1620
11th to 20thDay
18%13%5%7%7%45%7%6%8%20%
1575
21st to endDay
17%11%5%8%8%5%6%6%5%6%23%
1368
March1st to 10thDay
16%12%37%7%47%6%7%8%21%
1858
11th to 20thDay
18%10%5%7%7%5%6%6%5%6%26%
1980
21st to endDay
14%12%5%9%8%5%7%6%6%8%20%
2236
April1st to 10thDay
16%10%49%8%46%7%7%7%23%
2160
11th to 20thDay
16%9%47%8%46%7%6%6%27%
2186
21st to endDay
16%9%37%7%5%5%5%7%7%28%
2373
May1st to 10thDay
11%8%37%6%46%5%6%8%37%
2520
11th to 20thDay
15%7%26%6%46%6%6%8%34%
2477
21st to endDay
18%9%47%6%346%5%6%31%
2723
June1st to 10thDay
19%7%36%5%45%45%7%34%
2648
11th to 20thDay
15%7%37%8%5%6%5%6%7%29%
2700
21st to endDay
10%6%35%5%36%5%7%9%41%
2700
July1st to 10thDay
9%5%25%5%35%45%6%53%
2700
11th to 20thDay
10%5%25%43445%7%53%
2664
21st to endDay
7%41332335%6%62%
2741
August1st to 10thDay
8%5%2432345%8%56%
2460
11th to 20thDay
6%4245%3345%8%56%
2298
21st to endDay
9%5%25%4245%6%9%49%
2494
September1st to 10thDay
9%5%25%5%3445%8%51%
2160
11th to 20thDay
10%8%47%6%35%46%6%41%
2160
21st to endDay
12%8%37%6%45%5%5%7%38%
1897
October1st to 10thDay
15%7%47%7%446%6%7%32%
1800
11th to 20thDay
16%10%5%8%8%5%5%46%7%27%
1800
21st to endDay
19%12%38%7%45%6%6%7%23%
1881
November1st to 10thDay
21%12%5%8%7%45%7%6%6%18%
1493
11th to 20thDay
19%10%6%8%6%5%6%5%5%7%23%
1520
21st to endDay
21%11%49%6%46%6%6%8%19%
1520
December1st to 10thDay
16%13%5%8%7%5%7%5%5%7%22%
1520
11th to 20thDay
17%13%6%13%10%5%6%6%6%5%13%
1798
21st to endDay
18%17%5%10%9%46%6%5%6%14%
2442
January1st to 10thNight
20%14%6%9%7%5%6%5%6%5%19%
2836
11th to 20thNight
20%13%5%8%7%5%5%6%5%6%20%
2625
21st to endNight
19%12%48%8%46%6%6%8%20%
2720
February1st to 10thNight
20%14%48%7%46%5%5%5%22%
2340
11th to 20thNight
18%12%5%8%7%45%5%46%26%
2272
21st to endNight
18%12%5%8%7%35%5%6%8%24%
1888
March1st to 10thNight
16%11%48%8%5%6%5%6%6%26%
2099
11th to 20thNight
17%13%5%6%8%46%6%5%7%23%
1979
21st to endNight
18%13%6%8%7%46%6%6%7%19%
2120
April1st to 10thNight
18%11%47%6%47%5%7%9%23%
1800
11th to 20thNight
18%8%47%6%46%46%7%29%
1723
21st to endNight
18%8%48%7%5%6%5%5%7%27%
1585
May1st to 10thNight
15%6%37%5%4445%7%40%
1440
11th to 20thNight
15%6%35%6%45%5%7%8%36%
1416
21st to endNight
19%8%37%6%35%5%6%7%30%
1552
June1st to 10thNight
19%7%37%6%36%5%5%6%33%
1308
11th to 20thNight
18%9%48%7%46%46%7%29%
1260
21st to endNight
11%5%246%35%5%7%8%45%
1260
July1st to 10thNight
11%424333346%55%
1260
11th to 20thNight
9%5%23323336%61%
1294
21st to endNight
7%223323445%66%
1576
August1st to 10thNight
9%314434445%59%
1475
11th to 20thNight
6%31432335%6%64%
1602
21st to endNight
8%4235%3446%7%53%
1862
September1st to 10thNight
9%4145%3345%7%56%
1800
11th to 20thNight
13%7%36%5%245%5%6%42%
1800
21st to endNight
13%6%26%5%34347%46%
2061
October1st to 10thNight
13%7%47%6%345%5%6%40%
2160
11th to 20thNight
17%8%36%6%445%5%8%35%
2160
21st to endNight
18%9%46%6%335%5%7%34%
2474
November1st to 10thNight
22%10%47%7%45%5%5%6%24%
2581
11th to 20thNight
18%11%6%8%7%5%6%5%5%6%24%
2660
21st to endNight
20%10%5%9%8%35%5%5%6%23%
2660
December1st to 10thNight
18%14%5%9%7%35%45%6%25%
2660
11th to 20thNight
17%15%6%11%8%46%5%6%7%15%
3403
21st to endNight
21%16%5%9%7%45%45%6%18%
5214

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
46%44%45%1
1329
11th to 20thDay
42%46%6%411
1354
21st to endDay
44%42%7%5%2
1634
February1st to 10thDay
43%42%8%5%2
1620
11th to 20thDay
39%42%9%6%4
1584
21st to endDay
41%43%7%6%3
1368
March1st to 10thDay
42%41%10%5%2
1858
11th to 20thDay
40%42%8%8%2
1980
21st to endDay
43%41%9%8%
2236
April1st to 10thDay
39%42%12%6%1
2160
11th to 20thDay
38%40%7%9%5%1
2186
21st to endDay
36%42%10%8%41
2373
May1st to 10thDay
32%41%13%11%3
2520
11th to 20thDay
30%41%13%13%5%
2480
21st to endDay
38%40%12%9%2
2724
June1st to 10thDay
36%39%13%7%31
2648
11th to 20thDay
37%41%14%7%1
2700
21st to endDay
26%41%16%11%5%1
2700
July1st to 10thDay
19%41%13%18%7%2
2626
11th to 20thDay
20%40%12%20%8%1
2583
21st to endDay
15%39%12%22%10%2
2589
August1st to 10thDay
21%40%15%18%6%
2443
11th to 20thDay
17%38%15%18%8%3
2298
21st to endDay
22%40%14%18%6%1
2494
September1st to 10thDay
17%39%13%18%10%3
2160
11th to 20thDay
26%39%11%13%10%1
2160
21st to endDay
30%42%10%11%5%1
1897
October1st to 10thDay
33%40%10%12%5%
1800
11th to 20thDay
35%41%12%9%21
1800
21st to endDay
40%41%7%7%5%
1881
November1st to 10thDay
41%44%7%43
1493
11th to 20thDay
39%44%8%5%3
1520
21st to endDay
41%44%6%6%2
1520
December1st to 10thDay
39%46%6%6%3
1520
11th to 20thDay
51%37%7%5%1
1800
21st to endDay
54%31%10%41
2442
January1st to 10thNight
44%44%6%5%1
2836
11th to 20thNight
40%46%6%5%21
2625
21st to endNight
41%41%7%7%31
2720
February1st to 10thNight
42%41%8%7%2
2340
11th to 20thNight
37%42%9%7%5%
2274
21st to endNight
40%40%9%8%3
1888
March1st to 10thNight
38%41%10%9%2
2099
11th to 20thNight
41%41%8%7%2
1980
21st to endNight
42%41%9%7%1
2120
April1st to 10thNight
40%42%9%6%2
1800
11th to 20thNight
35%41%10%8%6%1
1723
21st to endNight
36%41%10%8%5%1
1585
May1st to 10thNight
32%42%10%12%5%
1440
11th to 20thNight
28%41%14%11%5%1
1416
21st to endNight
39%41%8%10%2
1552
June1st to 10thNight
37%40%11%7%32
1308
11th to 20thNight
38%41%14%7%1
1260
21st to endNight
24%41%16%12%5%3
1260
July1st to 10thNight
20%39%11%19%7%3
1225
11th to 20thNight
15%40%12%23%9%1
1252
21st to endNight
13%40%10%18%15%4
1488
August1st to 10thNight
18%39%13%19%8%2
1467
11th to 20thNight
14%38%13%18%14%3
1602
21st to endNight
20%40%13%18%8%1
1862
September1st to 10thNight
15%39%10%21%10%4
1800
11th to 20thNight
25%40%8%12%12%3
1800
21st to endNight
26%40%10%17%7%1
2061
October1st to 10thNight
30%39%9%14%7%
2160
11th to 20thNight
29%41%13%13%4
2160
21st to endNight
32%41%10%11%5%
2474
November1st to 10thNight
37%46%8%6%31
2581
11th to 20thNight
39%44%9%6%2
2660
21st to endNight
40%44%8%6%2
2660
December1st to 10thNight
38%44%9%6%3
2660
11th to 20thNight
47%40%8%41
3400
21st to endNight
52%34%7%6%1
5214

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
26%18%23%18%11%3
180
11th to 20thDay
21%15%29%19%13%2
162
21st to endDay
27%15%25%19%13%2
226
February1st to 10thDay
31%14%26%18%11%1
340
11th to 20thDay
27%14%25%24%9%1
335
21st to endDay
21%17%24%23%14%1
296
March1st to 10thDay
24%12%26%24%12%2
420
11th to 20thDay
20%16%22%21%17%4
420
21st to endDay
26%11%27%23%11%2
462
April1st to 10thDay
20%16%23%25%13%4
420
11th to 20thDay
18%14%24%25%13%6%
412
21st to endDay
19%12%25%24%18%2
420
May1st to 10thDay
12%10%21%31%22%3
423
11th to 20thDay
17%6%18%27%23%9%
448
21st to endDay
15%8%18%29%20%9%
485
June1st to 10thDay
17%7%23%29%20%4
578
11th to 20thDay
15%8%24%34%17%3
630
21st to endDay
11%320%32%27%7%
630
July1st to 10thDay
11%216%32%29%10%
621
11th to 20thDay
8%417%30%31%10%
582
21st to endDay
5%38%30%39%15%
487
August1st to 10thDay
8%39%26%38%16%
447
11th to 20thDay
6%18%31%38%16%
437
21st to endDay
9%5%14%33%29%10%
495
September1st to 10thDay
12%313%39%27%6%
450
11th to 20thDay
15%5%14%29%31%6%
450
21st to endDay
15%8%20%28%23%5%
450
October1st to 10thDay
19%6%21%22%25%8%
450
11th to 20thDay
21%12%22%20%17%8%
450
21st to endDay
26%15%21%20%12%5%
396
November1st to 10thDay
31%16%22%17%13%
183
11th to 20thDay
26%10%27%13%16%7%
180
21st to endDay
25%15%25%18%7%10%
180
December1st to 10thDay
26%14%27%10%16%8%
180
11th to 20thDay
31%19%27%14%7%1
180
21st to endDay
27%16%30%20%6%2
198
January1st to 10thNight
30%14%22%19%12%2
850
11th to 20thNight
28%12%19%20%19%2
765
21st to endNight
27%7%20%23%19%4
792
February1st to 10thNight
26%9%23%27%12%3
720
11th to 20thNight
23%9%23%26%16%2
704
21st to endNight
25%13%22%23%14%4
592
March1st to 10thNight
24%13%21%23%17%3
720
11th to 20thNight
22%13%23%24%15%3
720
21st to endNight
25%11%22%28%12%3
792
April1st to 10thNight
24%10%18%28%17%3
720
11th to 20thNight
19%7%24%30%15%4
655
21st to endNight
22%7%23%30%14%4
505
May1st to 10thNight
14%7%18%34%23%4
360
11th to 20thNight
16%6%14%27%32%6%
354
21st to endNight
20%6%14%28%28%5%
389
June1st to 10thNight
21%5%20%30%22%2
360
11th to 20thNight
18%6%21%34%18%4
360
21st to endNight
14%216%33%31%4
360
July1st to 10thNight
12%312%31%37%4
350
11th to 20thNight
9%316%28%39%5%
348
21st to endNight
6%310%31%43%7%
394
August1st to 10thNight
10%28%24%45%11%
401
11th to 20thNight
9%15%24%46%15%
534
21st to endNight
11%313%29%38%6%
674
September1st to 10thNight
10%210%34%35%9%
720
11th to 20thNight
16%312%23%32%15%
720
21st to endNight
16%416%28%29%7%
720
October1st to 10thNight
18%5%14%24%28%10%
720
11th to 20thNight
21%8%18%23%22%8%
720
21st to endNight
22%10%21%26%17%4
792
November1st to 10thNight
27%10%22%26%14%1
740
11th to 20thNight
25%16%19%23%15%3
760
21st to endNight
25%13%22%22%13%6%
760
December1st to 10thNight
26%9%21%24%16%4
760
11th to 20thNight
30%11%27%21%10%1
798
21st to endNight
32%15%24%19%9%1
935
Page generated at 2024-01-08 19:31:24. Version ac07

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