Port Townsend
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Townsend clear sky chart from 2005-11-05 to 2018-04-23.

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
36%
15419
9%
49444
7%
15388
39%
15388
13%
36950

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
29%
825
8%
2911
8%
821
31%
821
8%
2369
January16th to endNight
28%
832
6%
2875
6%
832
30%
832
8%
2339
February1st to 15thNight
27%
763
7%
2483
5%
760
30%
760
8%
2017
February16th to endNight
30%
684
9%
2196
8%
684
33%
684
11%
1704
March1st to 15thNight
23%
780
6%
2231
4%
780
25%
780
9%
1825
March16th to endNight
26%
832
5%
2246
3%
832
27%
832
10%
1873
April1st to 15thNight
31%
779
5%
1949
3%
779
32%
779
12%
1636
April16th to endNight
33%
557
8%
1673
5%
557
36%
557
13%
1281
May1st to 15thNight
40%
359
12%
1439
15%
359
45%
359
14%
935
May16th to endNight
38%
374
6%
1496
8%
374
41%
374
10%
1064
June1st to 15thNight
40%
360
3%
1291
3%
360
40%
360
13%
937
June16th to endNight
42%
360
6%
1260
3%
360
45%
360
13%
895
July1st to 15thNight
60%
340
12%
1190
12%
340
62%
340
31%
642
July16th to endNight
63%
376
16%
1380
22%
354
71%
354
32%
677
August1st to 15thNight
69%
449
16%
1522
19%
447
71%
447
33%
798
August16th to endNight
62%
627
20%
1773
14%
627
65%
627
35%
1017
September1st to 15thNight
67%
720
21%
1800
12%
720
69%
720
40%
1081
September16th to endNight
45%
716
18%
1977
12%
716
49%
716
20%
1388
October1st to 15thNight
44%
720
10%
2160
10%
720
45%
720
19%
1517
October16th to endNight
36%
768
9%
2370
7%
768
39%
768
13%
1810
November1st to 15thNight
26%
760
6%
2654
4%
760
29%
760
8%
2172
November16th to endNight
25%
780
5%
2730
3%
780
28%
780
8%
2184
December1st to 15thNight
29%
780
7%
2730
7%
780
31%
780
9%
2161
December16th to endNight
18%
878
5%
3108
3%
878
21%
878
6%
2628

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%8%37%6%35%5%5%7%31%
50099
Night
16%9%46%6%35%5%5%6%30%
49601
Cloud CoverDay
16%8%37%6%35%5%5%7%31%
50099
Night
18%9%46%6%3445%6%30%
49604
TransparencyDay
51%9%15%15%6%
49759
Night
53%9%14%15%6%1
49445
SeeingDay
21%14%17%24%17%4
6698
Night
23%18%13%21%18%3
15422

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
18%12%6%10%9%446%5%5%17%
1358
16th to endDay
20%12%5%10%8%5%6%45%5%15%
1697
February1st to 15thDay
18%11%5%8%8%37%6%6%7%17%
1711
16th to endDay
16%10%47%8%5%5%5%5%7%22%
1564
March1st to 15thDay
18%11%47%7%47%6%6%6%19%
2057
16th to endDay
15%11%5%9%7%5%7%6%6%6%19%
2330
April1st to 15thDay
17%9%38%7%46%7%6%7%22%
2329
16th to endDay
15%9%36%7%46%6%7%8%24%
2399
May1st to 15thDay
11%7%25%5%47%5%6%7%35%
2511
16th to endDay
18%8%37%6%35%6%47%29%
2615
June1st to 15thDay
16%6%36%6%45%45%6%32%
2666
16th to endDay
13%7%36%7%36%5%6%8%31%
2700
July1st to 15thDay
7%5%24424345%55%
2700
16th to endDay
9%41332345%6%56%
2693
August1st to 15thDay
6%5%15%423348%54%
2427
16th to endDay
8%4244235%5%8%49%
2429
September1st to 15thDay
8%42442435%7%52%
2160
16th to endDay
12%8%47%5%35%46%7%34%
1983
October1st to 15thDay
13%9%47%7%445%6%7%28%
1800
16th to endDay
18%11%49%7%45%5%6%6%19%
1854
November1st to 15thDay
21%12%5%7%6%45%6%46%18%
1531
16th to endDay
20%11%38%6%5%7%6%6%7%15%
1560
December1st to 15thDay
15%12%5%9%8%47%5%5%6%19%
1560
16th to endDay
18%12%5%8%7%46%6%5%6%18%
1465
January1st to 15thNight
20%12%5%7%6%35%5%5%5%21%
2911
16th to endNight
19%10%47%7%5%6%6%6%7%18%
2875
February1st to 15thNight
19%11%47%7%46%5%5%6%22%
2471
16th to endNight
16%10%47%7%35%5%5%7%25%
2196
March1st to 15thNight
18%11%47%7%5%6%5%5%6%21%
2231
16th to endNight
16%14%5%7%7%36%5%5%7%19%
2242
April1st to 15thNight
17%9%46%7%46%6%6%8%21%
1949
16th to endNight
17%8%36%6%46%45%7%27%
1673
May1st to 15thNight
14%5%26%5%4435%7%39%
1439
16th to endNight
17%7%36%6%35%6%6%7%29%
1496
June1st to 15thNight
20%7%37%5%35%446%32%
1291
16th to endNight
13%6%26%7%46%5%6%7%33%
1260
July1st to 15thNight
8%423321336%59%
1260
16th to endNight
7%313213336%62%
1475
August1st to 15thNight
6%313424346%60%
1530
16th to endNight
6%413423346%57%
1773
September1st to 15thNight
6%413423346%60%
1800
16th to endNight
14%6%26%5%2445%6%40%
1977
October1st to 15thNight
13%7%37%6%33446%40%
2160
16th to endNight
17%8%36%6%346%5%8%29%
2370
November1st to 15thNight
24%9%5%7%6%45%5%46%21%
2654
16th to endNight
16%10%5%9%8%35%45%6%22%
2730
December1st to 15thNight
15%13%5%8%7%35%5%5%5%23%
2730
16th to endNight
20%14%5%8%8%45%45%5%18%
3108

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
23%11%6%10%7%4444417%
1358
16th to endDay
23%13%6%10%8%345%45%14%
1697
February1st to 15thDay
21%12%48%8%36%6%5%5%17%
1711
16th to endDay
17%12%47%8%45%5%5%6%22%
1564
March1st to 15thDay
21%11%38%6%45%5%5%7%18%
2057
16th to endDay
18%10%5%9%7%5%5%5%6%6%19%
2330
April1st to 15thDay
19%9%49%7%35%6%5%8%21%
2329
16th to endDay
17%9%37%6%45%6%7%8%24%
2399
May1st to 15thDay
13%7%35%5%36%5%6%6%35%
2511
16th to endDay
18%8%37%5%25%5%5%7%29%
2615
June1st to 15thDay
17%7%26%5%35%45%7%33%
2666
16th to endDay
13%7%27%5%36%5%6%7%33%
2700
July1st to 15thDay
6%5%14423445%56%
2700
16th to endDay
9%41331335%6%57%
2693
August1st to 15thDay
7%32442335%7%54%
2427
16th to endDay
8%4144345%5%9%49%
2429
September1st to 15thDay
8%6%13423247%53%
2160
16th to endDay
14%8%36%6%35%5%5%7%33%
1983
October1st to 15thDay
15%9%48%6%35%46%8%27%
1800
16th to endDay
21%13%49%7%345%5%6%18%
1854
November1st to 15thDay
25%12%37%6%35%5%5%6%17%
1531
16th to endDay
23%11%49%8%35%448%16%
1560
December1st to 15thDay
19%12%48%7%46%5%46%19%
1560
16th to endDay
21%12%5%8%7%36%5%5%5%17%
1465
January1st to 15thNight
25%11%47%6%34445%21%
2910
16th to endNight
22%10%47%7%36%5%5%7%18%
2875
February1st to 15thNight
23%10%47%7%35%446%21%
2471
16th to endNight
19%10%48%6%4445%6%25%
2196
March1st to 15thNight
21%9%5%7%7%45%5%46%20%
2231
16th to endNight
19%15%47%7%35%5%5%6%19%
2246
April1st to 15thNight
20%9%37%6%5%6%5%6%8%21%
1949
16th to endNight
19%8%37%7%35%46%6%27%
1673
May1st to 15thNight
15%425%6%3445%6%39%
1439
16th to endNight
18%7%35%6%45%5%5%7%29%
1496
June1st to 15thNight
22%7%35%5%34446%31%
1291
16th to endNight
14%7%36%7%45%5%5%7%32%
1260
July1st to 15thNight
8%424322235%60%
1260
16th to endNight
7%323213346%61%
1475
August1st to 15thNight
6%313323445%60%
1530
16th to endNight
7%413413446%56%
1773
September1st to 15thNight
6%313413346%59%
1800
16th to endNight
14%7%36%4335%5%7%39%
1977
October1st to 15thNight
13%8%36%6%34345%39%
2160
16th to endNight
20%9%36%5%35%5%6%7%27%
2370
November1st to 15thNight
27%10%5%6%6%35%445%21%
2654
16th to endNight
20%11%5%8%7%3445%6%21%
2730
December1st to 15thNight
19%12%5%8%6%25%5%5%6%22%
2730
16th to endNight
23%14%5%8%7%34445%18%
3108

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%7%11%3
1358
16th to endDay
69%8%11%8%2
1697
February1st to 15thDay
66%10%10%7%41
1711
16th to endDay
60%10%12%11%4
1564
March1st to 15thDay
64%9%14%10%1
2057
16th to endDay
65%9%13%11%
2330
April1st to 15thDay
61%11%14%11%1
2329
16th to endDay
55%11%15%13%3
2399
May1st to 15thDay
47%10%16%17%7%
2511
16th to endDay
52%10%20%13%3
2619
June1st to 15thDay
50%8%22%12%5%1
2666
16th to endDay
47%10%21%16%3
2700
July1st to 15thDay
29%8%16%29%13%3
2541
16th to endDay
27%7%17%32%13%1
2525
August1st to 15thDay
29%8%21%27%11%2
2410
16th to endDay
31%9%20%27%9%2
2429
September1st to 15thDay
30%7%18%22%19%2
2160
16th to endDay
48%9%15%15%8%1
1983
October1st to 15thDay
53%8%15%16%6%
1800
16th to endDay
64%9%12%9%4
1854
November1st to 15thDay
67%9%11%7%21
1531
16th to endDay
67%8%10%9%3
1560
December1st to 15thDay
63%10%8%11%5%
1560
16th to endDay
68%8%11%7%3
1465
January1st to 15thNight
63%10%10%11%4
2911
16th to endNight
64%9%10%10%41
2875
February1st to 15thNight
63%9%10%9%41
2483
16th to endNight
58%8%14%13%5%
2196
March1st to 15thNight
63%8%12%12%2
2231
16th to endNight
63%9%13%11%1
2246
April1st to 15thNight
61%11%13%11%2
1949
16th to endNight
55%9%15%14%4
1673
May1st to 15thNight
42%10%16%19%9%1
1440
16th to endNight
53%8%20%13%4
1496
June1st to 15thNight
54%7%20%11%31
1291
16th to endNight
49%10%18%17%31
1260
July1st to 15thNight
28%6%15%31%13%4
1190
16th to endNight
24%6%17%30%17%3
1380
August1st to 15thNight
23%10%19%29%13%4
1522
16th to endNight
27%7%22%27%12%3
1773
September1st to 15thNight
26%7%16%24%20%5%
1800
16th to endNight
43%10%12%22%9%1
1977
October1st to 15thNight
46%8%15%20%8%
2160
16th to endNight
54%9%16%13%5%
2370
November1st to 15thNight
64%9%12%9%22
2654
16th to endNight
63%8%12%9%5%
2730
December1st to 15thNight
61%9%12%11%5%
2730
16th to endNight
68%10%9%7%3
3108

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
24%6%15%18%27%6%
90
16th to endDay
46%17%15%13%6%
132
February1st to 15thDay
34%18%16%17%11%
281
16th to endDay
28%20%21%16%9%2
256
March1st to 15thDay
35%19%20%16%7%1
330
16th to endDay
32%18%21%15%8%3
352
April1st to 15thDay
25%21%20%18%10%3
329
16th to endDay
21%19%19%25%10%3
288
May1st to 15thDay
19%16%17%32%13%1
288
16th to endDay
16%13%13%25%20%10%
330
June1st to 15thDay
14%16%18%28%19%3
461
16th to endDay
14%16%19%29%15%4
495
July1st to 15thDay
7%11%17%31%26%6%
475
16th to endDay
7%12%13%24%30%11%
362
August1st to 15thDay
7%6%9%33%27%14%
315
16th to endDay
8%6%13%33%28%8%
332
September1st to 15thDay
14%5%17%28%29%4
315
16th to endDay
18%10%17%27%21%3
315
October1st to 15thDay
26%12%22%19%16%3
315
16th to endDay
36%12%14%18%14%2
270
November1st to 15thDay
54%7%23%9%4
91
16th to endDay
41%12%20%11%5%10%
90
December1st to 15thDay
30%10%21%16%15%6%
90
16th to endDay
39%11%27%12%7%2
96
January1st to 15thNight
29%21%11%17%16%3
825
16th to endNight
29%21%13%16%15%2
832
February1st to 15thNight
28%21%14%18%13%2
764
16th to endNight
25%20%15%22%12%2
684
March1st to 15thNight
29%26%14%15%11%2
780
16th to endNight
28%23%15%21%8%2
832
April1st to 15thNight
23%20%17%21%14%2
779
16th to endNight
22%20%15%24%14%2
557
May1st to 15thNight
18%19%14%23%21%3
360
16th to endNight
21%18%11%21%22%3
375
June1st to 15thNight
21%18%13%26%16%2
360
16th to endNight
20%18%10%29%19%1
360
July1st to 15thNight
10%13%9%32%32%2
340
16th to endNight
6%17%7%25%39%4
376
August1st to 15thNight
9%10%5%20%43%10%
449
16th to endNight
10%10%10%26%37%5%
627
September1st to 15thNight
9%10%7%28%33%10%
720
16th to endNight
20%11%14%23%23%6%
716
October1st to 15thNight
19%13%16%24%21%4
720
16th to endNight
28%14%14%21%17%3
768
November1st to 15thNight
32%18%17%22%7%
760
16th to endNight
30%22%15%15%13%2
780
December1st to 15thNight
28%22%11%20%13%2
780
16th to endNight
33%21%18%16%9%
878
Page generated at 2018-04-24 07:03:01. Version ac04

Copyright 2018 A. Danko


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