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Portland
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portland 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
42%
22575
13%
74768
11%
22536
45%
22536
18%
52762

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
23%
759
7%
2646
5%
756
27%
756
7%
2220
January11th to 20thNight
28%
740
9%
2590
6%
740
32%
740
9%
2067
January21st to endNight
31%
792
12%
2628
10%
792
35%
792
12%
2050
February1st to 10thNight
30%
720
7%
2340
6%
720
32%
720
10%
1908
February11th to 20thNight
28%
700
8%
2274
5%
700
33%
700
12%
1762
February21st to endNight
29%
592
10%
1851
8%
592
33%
592
11%
1487
March1st to 10thNight
33%
720
10%
2078
7%
720
36%
720
14%
1641
March11th to 20thNight
34%
720
11%
1980
8%
720
37%
720
16%
1549
March21st to endNight
31%
792
5%
2129
3%
792
31%
792
11%
1785
April1st to 10thNight
32%
720
7%
1800
3%
720
33%
720
14%
1478
April11th to 20thNight
37%
711
11%
1779
5%
711
39%
711
19%
1359
April21st to endNight
37%
546
10%
1626
6%
546
40%
546
16%
1214
May1st to 10thNight
44%
394
15%
1474
13%
394
50%
394
21%
933
May11th to 20thNight
48%
354
12%
1416
13%
354
54%
354
18%
914
May21st to endNight
43%
389
9%
1552
10%
388
49%
388
15%
1046
June1st to 10thNight
43%
360
9%
1440
11%
360
48%
360
18%
925
June11th to 20thNight
41%
360
9%
1440
12%
360
45%
360
17%
951
June21st to endNight
56%
360
17%
1440
22%
360
65%
360
33%
714
July1st to 10thNight
70%
350
17%
1400
24%
350
76%
350
39%
635
July11th to 20thNight
61%
348
24%
1392
29%
348
74%
348
45%
595
July21st to endNight
74%
394
31%
1488
38%
372
85%
372
65%
512
August1st to 10thNight
77%
501
26%
1567
23%
499
83%
499
58%
695
August11th to 20thNight
78%
534
30%
1602
23%
534
82%
534
55%
755
August21st to endNight
66%
720
32%
1908
20%
720
71%
720
52%
1020
September1st to 10thNight
65%
720
28%
1800
15%
720
69%
720
52%
1007
September11th to 20thNight
60%
720
23%
1800
15%
720
62%
720
35%
1173
September21st to endNight
61%
720
21%
2061
19%
720
63%
720
35%
1248
October1st to 10thNight
52%
720
15%
2160
15%
720
55%
720
27%
1407
October11th to 20thNight
52%
720
19%
2160
21%
720
54%
720
24%
1445
October21st to endNight
39%
792
16%
2422
14%
792
44%
792
20%
1713
November1st to 10thNight
32%
740
11%
2504
11%
740
36%
740
12%
1966
November11th to 20thNight
28%
760
8%
2660
5%
760
33%
760
10%
2114
November21st to endNight
36%
760
7%
2660
6%
760
38%
760
13%
1998
December1st to 10thNight
33%
760
10%
2660
8%
760
36%
760
12%
2019
December11th to 20thNight
24%
759
7%
3210
6%
759
27%
759
7%
2547
December21st to endNight
22%
828
3%
4831
5%
817
24%
817
4%
3910

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
21%8%36%6%34446%33%
74765
Night
23%8%36%5%34445%34%
75047
TransparencyDay
53%412%17%11%3
74468
Night
53%412%18%10%3
74865
SeeingDay
22%9%20%27%17%4
13507
Night
26%6%18%33%16%1
22589

By Month

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 10thDay
40%12%48%6%35%43311%
1519
11th to 20thDay
37%11%37%7%2433419%
1480
21st to endDay
27%12%49%7%3434422%
1725
February1st to 10thDay
31%12%5%8%6%35%5%5%6%15%
1620
11th to 20thDay
30%14%5%8%6%4434418%
1575
21st to endDay
32%11%5%7%7%33335%19%
1404
March1st to 10thDay
29%11%5%8%7%3446%6%16%
1880
11th to 20thDay
30%10%46%7%45%346%19%
1980
21st to endDay
31%12%48%8%45%6%5%5%14%
2227
April1st to 10thDay
29%11%48%7%4445%6%17%
2160
11th to 20thDay
26%11%48%6%35%445%24%
2129
21st to endDay
22%9%47%7%46%6%5%7%24%
2330
May1st to 10thDay
17%8%47%7%3445%5%35%
2484
11th to 20thDay
20%8%37%6%45%46%6%30%
2477
21st to endDay
25%7%36%6%5%6%5%5%7%26%
2723
June1st to 10thDay
23%7%46%5%35%5%6%6%30%
2520
11th to 20thDay
21%8%47%7%46%6%5%6%27%
2520
21st to endDay
9%6%35%434446%51%
2520
July1st to 10thDay
8%424434446%57%
2520
11th to 20thDay
8%423423346%61%
2520
21st to endDay
5%313312235%71%
2741
August1st to 10thDay
5%423423446%62%
2361
11th to 20thDay
6%31331335%6%65%
2298
21st to endDay
8%4244245%5%8%54%
2448
September1st to 10thDay
8%423333447%60%
2160
11th to 20thDay
13%8%35%5%3445%7%43%
2160
21st to endDay
15%6%35%5%34446%43%
1896
October1st to 10thDay
21%8%35%6%35%45%436%
1800
11th to 20thDay
22%9%37%7%35%45%7%30%
1800
21st to endDay
28%9%47%6%3323429%
1932
November1st to 10thDay
31%11%5%9%6%35%45%5%15%
1574
11th to 20thDay
31%11%5%9%6%23445%20%
1520
21st to endDay
30%10%5%7%7%34445%22%
1520
December1st to 10thDay
29%13%48%5%23435%23%
1520
11th to 20thDay
32%13%5%6%7%4345%6%16%
1928
21st to endDay
32%14%5%10%9%5%5%4449%
2794
January1st to 10thNight
36%12%37%5%45%43417%
2646
11th to 20thNight
35%9%37%6%34435%20%
2590
21st to endNight
28%12%48%6%33345%25%
2628
February1st to 10thNight
29%12%48%6%35%5%46%20%
2340
11th to 20thNight
31%11%36%6%3434425%
2272
21st to endNight
29%12%47%6%345%45%20%
1851
March1st to 10thNight
27%11%47%6%34445%24%
2078
11th to 20thNight
27%9%37%6%36%45%6%24%
1980
21st to endNight
28%11%49%7%45%5%46%18%
2129
April1st to 10thNight
27%12%48%5%35%35%5%22%
1800
11th to 20thNight
22%10%46%6%3445%7%31%
1779
21st to endNight
27%9%35%5%345%47%29%
1626
May1st to 10thNight
16%7%36%5%34446%41%
1474
11th to 20thNight
19%6%247%446%6%7%34%
1416
21st to endNight
17%7%36%7%45%6%5%8%34%
1552
June1st to 10thNight
18%7%36%6%44337%38%
1440
11th to 20thNight
17%9%48%6%3435%5%38%
1440
21st to endNight
8%424334345%60%
1440
July1st to 10thNight
7%323333235%67%
1440
11th to 20thNight
6%313223335%70%
1440
21st to endNight
21222122481%
1576
August1st to 10thNight
4212322346%72%
1575
11th to 20thNight
3213333335%72%
1602
21st to endNight
6%413323345%66%
1908
September1st to 10thNight
6%323323436%67%
1800
11th to 20thNight
12%5%36%5%35%35%5%49%
1800
21st to endNight
12%5%245%34446%51%
2061
October1st to 10thNight
18%6%34433446%45%
2160
11th to 20thNight
19%6%26%4435%5%7%39%
2160
21st to endNight
24%8%36%5%33445%36%
2422
November1st to 10thNight
31%9%47%6%3445%6%22%
2504
11th to 20thNight
35%9%37%5%34445%21%
2660
21st to endNight
28%7%47%5%45%445%27%
2660
December1st to 10thNight
31%9%45%5%34446%25%
2660
11th to 20thNight
31%11%48%6%34446%20%
3276
21st to endNight
31%13%5%9%7%45%44415%
4862

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
80%37%5%41
1519
11th to 20thDay
71%47%9%6%3
1480
21st to endDay
67%5%9%11%8%
1725
February1st to 10thDay
71%5%9%10%5%
1620
11th to 20thDay
72%48%11%41
1584
21st to endDay
70%49%9%6%2
1404
March1st to 10thDay
68%412%11%5%
1880
11th to 20thDay
68%49%13%6%1
1980
21st to endDay
73%5%12%10%1
2227
April1st to 10thDay
69%412%12%3
2160
11th to 20thDay
64%5%9%13%7%2
2132
21st to endDay
60%5%14%12%8%1
2330
May1st to 10thDay
52%414%20%9%1
2484
11th to 20thDay
54%5%15%18%7%1
2480
21st to endDay
58%6%16%17%3
2724
June1st to 10thDay
53%5%18%17%6%
2520
11th to 20thDay
56%6%19%13%5%
2520
21st to endDay
35%419%28%10%4
2520
July1st to 10thDay
30%414%26%20%5%
2451
11th to 20thDay
26%410%27%27%6%
2445
21st to endDay
17%210%32%32%6%
2589
August1st to 10thDay
24%414%33%20%5%
2344
11th to 20thDay
21%415%31%21%8%
2298
21st to endDay
30%313%30%21%4
2448
September1st to 10thDay
26%313%27%19%12%
2160
11th to 20thDay
41%5%14%19%14%8%
2160
21st to endDay
42%5%14%18%15%6%
1896
October1st to 10thDay
52%48%19%13%4
1800
11th to 20thDay
55%5%12%14%11%2
1800
21st to endDay
62%37%13%14%1
1932
November1st to 10thDay
72%49%11%4
1574
11th to 20thDay
68%411%9%6%2
1520
21st to endDay
66%5%6%12%11%1
1520
December1st to 10thDay
66%49%12%8%2
1520
11th to 20thDay
70%412%12%2
1928
21st to endDay
81%37%6%3
2794
January1st to 10thNight
72%5%9%8%5%1
2646
11th to 20thNight
68%49%10%7%2
2590
21st to endNight
64%49%13%9%1
2628
February1st to 10thNight
67%6%11%11%5%
2340
11th to 20thNight
64%413%12%6%2
2274
21st to endNight
68%310%11%6%2
1851
March1st to 10thNight
63%412%15%6%
2078
11th to 20thNight
59%6%14%14%6%1
1980
21st to endNight
68%6%14%11%1
2129
April1st to 10thNight
66%413%13%4
1800
11th to 20thNight
55%6%14%15%7%3
1779
21st to endNight
57%5%13%15%6%3
1626
May1st to 10thNight
45%416%24%8%2
1474
11th to 20thNight
47%6%19%19%6%4
1416
21st to endNight
49%7%18%20%5%
1552
June1st to 10thNight
49%316%25%6%
1440
11th to 20thNight
50%5%19%22%5%
1440
21st to endNight
29%418%32%12%5%
1440
July1st to 10thNight
24%414%31%22%7%
1400
11th to 20thNight
19%413%28%30%6%
1392
21st to endNight
10%210%38%31%10%
1488
August1st to 10thNight
16%217%38%21%5%
1567
11th to 20thNight
18%316%37%18%8%
1602
21st to endNight
22%413%38%19%5%
1908
September1st to 10thNight
22%413%30%20%12%
1800
11th to 20thNight
38%412%24%12%10%
1800
21st to endNight
36%317%20%17%7%
2061
October1st to 10thNight
42%5%10%24%16%3
2160
11th to 20thNight
45%5%12%20%14%3
2160
21st to endNight
52%410%16%15%3
2422
November1st to 10thNight
64%5%9%16%6%
2504
11th to 20thNight
67%410%10%7%2
2660
21st to endNight
60%410%11%13%2
2660
December1st to 10thNight
62%5%12%12%10%1
2660
11th to 20thNight
68%412%11%5%
3276
21st to endNight
74%312%9%2
4862

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
46%10%18%14%11%2
180
11th to 20thDay
42%9%22%16%8%4
170
21st to endDay
36%9%17%21%13%4
317
February1st to 10thDay
39%11%16%22%11%2
340
11th to 20thDay
33%11%26%20%7%2
335
21st to endDay
38%15%18%20%8%1
312
March1st to 10thDay
33%13%22%23%8%2
420
11th to 20thDay
30%11%25%18%13%2
420
21st to endDay
31%15%21%20%11%2
462
April1st to 10thDay
30%14%20%22%8%5%
420
11th to 20thDay
30%13%26%19%8%4
412
21st to endDay
22%12%29%23%12%2
420
May1st to 10thDay
18%14%20%28%15%4
423
11th to 20thDay
29%9%16%26%17%3
448
21st to endDay
21%8%19%27%20%5%
485
June1st to 10thDay
23%8%20%27%19%3
450
11th to 20thDay
20%9%25%30%13%3
450
21st to endDay
7%6%21%38%23%5%
450
July1st to 10thDay
9%417%38%27%5%
446
11th to 20thDay
5%6%22%38%23%6%
444
21st to endDay
6%215%41%29%7%
487
August1st to 10thDay
6%313%40%30%9%
447
11th to 20thDay
4213%37%32%11%
437
21st to endDay
8%5%22%33%24%8%
495
September1st to 10thDay
9%6%20%33%26%6%
450
11th to 20thDay
15%6%15%30%27%7%
450
21st to endDay
14%7%22%32%21%4
450
October1st to 10thDay
21%8%19%26%20%6%
450
11th to 20thDay
24%7%17%27%20%5%
450
21st to endDay
32%10%21%19%13%5%
447
November1st to 10thDay
27%13%26%20%11%3
222
11th to 20thDay
29%13%25%17%12%4
180
21st to endDay
31%10%20%23%14%2
180
December1st to 10thDay
36%9%23%18%11%3
180
11th to 20thDay
43%11%20%18%6%1
180
21st to endDay
41%14%13%21%11%1
198
January1st to 10thNight
41%9%19%21%9%1
760
11th to 20thNight
38%8%16%24%11%1
740
21st to endNight
33%6%16%31%12%2
792
February1st to 10thNight
33%8%17%28%13%1
720
11th to 20thNight
36%7%20%23%13%1
704
21st to endNight
36%7%20%28%9%
592
March1st to 10thNight
31%9%18%28%13%1
720
11th to 20thNight
30%7%20%31%11%1
720
21st to endNight
30%9%19%28%13%
792
April1st to 10thNight
32%8%18%27%15%1
720
11th to 20thNight
26%7%20%31%14%2
711
21st to endNight
29%8%19%30%14%1
546
May1st to 10thNight
18%5%26%37%13%1
394
11th to 20thNight
21%418%37%19%1
354
21st to endNight
19%6%21%36%18%1
389
June1st to 10thNight
20%323%38%16%
360
11th to 20thNight
18%7%20%44%10%1
360
21st to endNight
8%226%49%14%
360
July1st to 10thNight
7%315%55%19%1
350
11th to 20thNight
7%224%49%17%
348
21st to endNight
3120%58%19%
394
August1st to 10thNight
5%112%47%32%3
501
11th to 20thNight
429%42%39%4
534
21st to endNight
8%318%44%26%2
720
September1st to 10thNight
9%219%43%25%2
720
11th to 20thNight
13%411%37%32%4
720
21st to endNight
13%315%43%23%3
720
October1st to 10thNight
20%416%33%23%3
720
11th to 20thNight
21%6%13%33%24%2
720
21st to endNight
27%7%19%30%14%2
792
November1st to 10thNight
33%7%19%29%12%1
740
11th to 20thNight
36%9%19%25%11%1
760
21st to endNight
29%7%19%31%13%
760
December1st to 10thNight
32%10%19%26%11%1
760
11th to 20thNight
41%9%17%24%9%1
760
21st to endNight
42%9%19%22%8%1
836
Page generated at 2024-01-12 22:38:09. Version ac07

Copyright 2024 Attilla Danko


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