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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 2005-11-05 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
43%
15343
12%
49159
11%
15315
46%
15315
18%
34892

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
27%
779
8%
2716
6%
776
31%
776
9%
2206
January16th to endNight
31%
832
11%
2808
9%
832
35%
832
12%
2169
February1st to 15thNight
31%
756
7%
2483
5%
756
34%
756
11%
1982
February16th to endNight
29%
684
10%
2170
6%
684
34%
684
11%
1702
March1st to 15thNight
31%
764
7%
2172
5%
764
33%
764
12%
1773
March16th to endNight
31%
768
5%
2079
4%
768
32%
768
12%
1744
April1st to 15thNight
33%
719
7%
1799
3%
719
34%
719
14%
1480
April16th to endNight
42%
603
11%
1683
6%
603
44%
603
18%
1263
May1st to 15thNight
45%
380
14%
1460
14%
380
50%
380
21%
914
May16th to endNight
46%
375
8%
1496
10%
374
50%
374
14%
1039
June1st to 15thNight
45%
360
8%
1440
9%
360
49%
360
16%
949
June16th to endNight
51%
360
12%
1440
16%
360
56%
360
24%
831
July1st to 15thNight
74%
340
17%
1360
25%
340
78%
340
46%
564
July16th to endNight
76%
376
27%
1416
36%
354
84%
354
55%
538
August1st to 15thNight
81%
514
24%
1588
22%
512
84%
512
55%
721
August16th to endNight
73%
657
31%
1803
22%
657
77%
657
55%
909
September1st to 15thNight
72%
720
27%
1800
15%
720
74%
720
57%
970
September16th to endNight
64%
716
23%
1976
19%
716
65%
716
33%
1223
October1st to 15thNight
50%
720
17%
2160
17%
720
53%
720
26%
1422
October16th to endNight
42%
768
14%
2335
14%
768
44%
768
18%
1695
November1st to 15thNight
29%
760
8%
2603
8%
760
32%
760
10%
2118
November16th to endNight
34%
780
6%
2730
5%
780
36%
780
11%
2121
December1st to 15thNight
33%
780
10%
2730
9%
780
35%
780
12%
2090
December16th to endNight
24%
832
6%
2912
5%
832
25%
832
7%
2469

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
23%8%36%5%23345%31%
49488
Night
24%7%35%423345%34%
49327
Cloud CoverDay
23%8%36%5%24345%31%
49488
Night
24%7%35%423335%34%
49331
TransparencyDay
55%411%16%9%2
49159
Night
52%412%17%9%2
49160
SeeingDay
25%7%19%26%16%4
6123
Night
27%416%32%17%1
15353

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
38%10%38%6%2333315%
1552
16th to endDay
33%9%48%6%2334420%
1765
February1st to 15thDay
32%13%5%7%6%23445%15%
1711
16th to endDay
34%11%48%6%2322319%
1590
March1st to 15thDay
35%10%48%6%34445%12%
2023
16th to endDay
36%10%37%7%34345%12%
2145
April1st to 15thDay
29%9%48%6%345%46%15%
2149
16th to endDay
25%9%47%6%35%447%21%
2274
May1st to 15thDay
21%7%36%5%23345%35%
2488
16th to endDay
26%6%36%5%3445%7%25%
2615
June1st to 15thDay
25%7%46%5%34446%26%
2520
16th to endDay
16%7%45%5%25%446%38%
2520
July1st to 15thDay
7%323313345%60%
2520
16th to endDay
8%313213236%62%
2657
August1st to 15thDay
6%41432235%7%58%
2361
16th to endDay
8%5%13313446%57%
2399
September1st to 15thDay
8%313323436%58%
2160
16th to endDay
17%7%37%433445%37%
1983
October1st to 15thDay
22%8%46%6%2444430%
1800
16th to endDay
29%9%48%5%22335%24%
1887
November1st to 15thDay
36%11%5%8%6%2334412%
1585
16th to endDay
32%10%5%7%6%3434418%
1560
December1st to 15thDay
31%13%47%432335%19%
1560
16th to endDay
40%11%47%5%2334312%
1664
January1st to 15thNight
36%9%36%43434419%
2716
16th to endNight
31%9%36%5%23345%24%
2808
February1st to 15thNight
29%10%45%5%2344422%
2471
16th to endNight
29%10%36%5%3344423%
2170
March1st to 15thNight
32%9%46%5%2335%5%20%
2172
16th to endNight
30%9%37%6%445%46%17%
2075
April1st to 15thNight
27%11%47%5%35%345%22%
1799
16th to endNight
25%8%25%5%2435%7%30%
1683
May1st to 15thNight
16%6%245%2345%6%41%
1460
16th to endNight
20%7%25%5%35%5%5%7%30%
1496
June1st to 15thNight
18%7%46%5%24335%38%
1440
16th to endNight
13%5%245%24346%47%
1440
July1st to 15thNight
422221223470%
1440
16th to endNight
31112213476%
1512
August1st to 15thNight
2211313345%70%
1596
16th to endNight
5%21232222470%
1803
September1st to 15thNight
6%21221222470%
1800
16th to endNight
12%42432445%6%48%
1976
October1st to 15thNight
19%6%24323435%43%
2160
16th to endNight
25%7%25%5%2335%5%33%
2335
November1st to 15thNight
34%8%36%5%24445%19%
2603
16th to endNight
33%7%36%42434424%
2730
December1st to 15thNight
31%10%35%423346%23%
2730
16th to endNight
37%11%47%5%2433316%
2912

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
39%11%37%6%2333316%
1552
16th to endDay
33%10%38%6%2433421%
1765
February1st to 15thDay
32%13%5%6%5%2444415%
1711
16th to endDay
34%11%5%7%6%3222319%
1590
March1st to 15thDay
35%11%47%6%34345%13%
2023
16th to endDay
36%9%47%7%45%34412%
2145
April1st to 15thDay
28%10%39%6%44346%15%
2149
16th to endDay
25%10%47%6%35%5%46%21%
2274
May1st to 15thDay
20%7%46%6%33245%35%
2488
16th to endDay
26%7%36%5%45%447%25%
2615
June1st to 15thDay
24%7%46%5%34446%25%
2520
16th to endDay
15%7%36%425%45%5%38%
2520
July1st to 15thDay
7%323323346%60%
2520
16th to endDay
8%313313236%62%
2657
August1st to 15thDay
5%413423346%58%
2361
16th to endDay
8%414313346%57%
2399
September1st to 15thDay
8%413323346%57%
2160
16th to endDay
17%7%35%5%24346%37%
1983
October1st to 15thDay
23%8%46%6%3434331%
1800
16th to endDay
29%9%47%5%23235%25%
1887
November1st to 15thDay
36%11%48%5%23345%12%
1585
16th to endDay
33%10%5%7%7%2433418%
1560
December1st to 15thDay
32%14%47%5%2233420%
1560
16th to endDay
40%12%46%5%3323412%
1664
January1st to 15thNight
35%10%35%5%2443419%
2716
16th to endNight
31%9%36%5%2333425%
2808
February1st to 15thNight
30%9%36%5%24445%22%
2471
16th to endNight
30%9%36%5%3433422%
2170
March1st to 15thNight
33%10%37%5%2344420%
2172
16th to endNight
30%10%37%6%35%446%17%
2079
April1st to 15thNight
26%12%47%5%24345%22%
1799
16th to endNight
25%8%345%24447%30%
1683
May1st to 15thNight
17%5%244235%45%41%
1460
16th to endNight
20%6%35%6%35%5%5%7%29%
1496
June1st to 15thNight
18%7%36%5%3422438%
1440
16th to endNight
12%6%26%424245%47%
1440
July1st to 15thNight
4213221235%69%
1440
16th to endNight
31111123475%
1512
August1st to 15thNight
222322346%70%
1596
16th to endNight
5%212312225%69%
1803
September1st to 15thNight
6%21221222470%
1800
16th to endNight
13%424424446%48%
1976
October1st to 15thNight
19%6%24323435%43%
2160
16th to endNight
25%7%25%432445%32%
2335
November1st to 15thNight
35%9%36%5%2435%5%18%
2603
16th to endNight
34%7%35%434335%24%
2730
December1st to 15thNight
32%9%35%413436%24%
2730
16th to endNight
38%11%47%42433316%
2912

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
74%37%8%5%
1552
16th to endDay
67%47%9%7%2
1765
February1st to 15thDay
71%6%7%9%4
1711
16th to endDay
72%37%10%5%
1590
March1st to 15thDay
73%310%8%3
2023
16th to endDay
75%49%8%2
2145
April1st to 15thDay
70%5%11%10%1
2149
16th to endDay
63%6%11%11%5%1
2274
May1st to 15thDay
52%313%20%9%1
2488
16th to endDay
59%6%15%14%3
2619
June1st to 15thDay
58%5%17%13%5%
2520
16th to endDay
47%5%17%21%6%2
2520
July1st to 15thDay
26%414%27%21%5%
2371
16th to endDay
24%212%26%27%5%
2490
August1st to 15thDay
26%414%31%17%5%
2344
16th to endDay
28%314%29%19%4
2399
September1st to 15thDay
27%313%22%20%12%
2160
16th to endDay
47%5%14%16%12%3
1983
October1st to 15thDay
56%49%18%8%3
1800
16th to endDay
63%49%11%10%1
1887
November1st to 15thDay
75%39%8%2
1585
16th to endDay
70%47%9%7%
1560
December1st to 15thDay
68%37%12%6%1
1560
16th to endDay
77%46%8%3
1664
January1st to 15thNight
68%5%9%9%6%1
2716
16th to endNight
63%38%11%9%2
2808
February1st to 15thNight
64%6%12%10%5%
2483
16th to endNight
65%310%12%6%1
2170
March1st to 15thNight
66%411%13%4
2172
16th to endNight
67%6%13%10%2
2079
April1st to 15thNight
65%413%11%4
1799
16th to endNight
54%6%14%17%5%1
1683
May1st to 15thNight
43%5%16%22%8%2
1461
16th to endNight
51%7%18%16%41
1496
June1st to 15thNight
51%315%22%6%
1440
16th to endNight
39%419%26%7%2
1440
July1st to 15thNight
18%315%32%23%6%
1360
16th to endNight
13%311%33%29%8%
1416
August1st to 15thNight
16%318%37%17%6%
1588
16th to endNight
18%315%37%17%6%
1803
September1st to 15thNight
19%314%28%20%14%
1800
16th to endNight
36%317%22%14%5%
1976
October1st to 15thNight
44%410%22%14%3
2160
16th to endNight
53%5%11%14%13%2
2335
November1st to 15thNight
68%49%13%3
2603
16th to endNight
63%410%9%10%1
2730
December1st to 15thNight
62%410%13%8%
2730
16th to endNight
73%5%8%9%3
2912

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
38%417%21%14%3
90
16th to endDay
43%7%19%13%12%4
197
February1st to 15thDay
41%7%19%20%8%2
281
16th to endDay
41%12%19%21%4
262
March1st to 15thDay
44%14%16%16%6%
312
16th to endDay
39%12%24%15%6%1
288
April1st to 15thDay
27%14%22%22%7%6%
269
16th to endDay
29%12%26%21%7%2
270
May1st to 15thDay
24%14%23%23%12%1
288
16th to endDay
27%6%13%23%22%6%
330
June1st to 15thDay
24%7%22%26%16%2
315
16th to endDay
15%6%22%34%18%2
315
July1st to 15thDay
7%317%39%25%6%
305
16th to endDay
9%317%37%25%6%
327
August1st to 15thDay
6%313%36%29%10%
315
16th to endDay
6%417%31%29%10%
332
September1st to 15thDay
10%5%17%32%27%6%
315
16th to endDay
17%6%19%31%22%2
315
October1st to 15thDay
25%7%22%26%14%2
315
16th to endDay
35%5%16%21%15%4
303
November1st to 15thDay
35%7%28%18%6%2
103
16th to endDay
41%6%24%16%11%
90
December1st to 15thDay
45%6%16%18%12%
90
16th to endDay
47%6%12%20%12%
96
January1st to 15thNight
39%6%19%22%11%1
780
16th to endNight
34%5%15%27%14%2
832
February1st to 15thNight
32%6%19%26%14%1
764
16th to endNight
36%6%20%25%10%
684
March1st to 15thNight
37%7%14%26%11%1
764
16th to endNight
34%6%19%26%12%
768
April1st to 15thNight
31%7%17%26%16%1
719
16th to endNight
28%5%15%32%16%1
603
May1st to 15thNight
20%224%37%14%
381
16th to endNight
23%416%35%19%
375
June1st to 15thNight
20%318%43%14%
360
16th to endNight
14%222%46%13%
360
July1st to 15thNight
5%113%55%23%
340
16th to endNight
414%60%19%
376
August1st to 15thNight
37%45%38%4
514
16th to endNight
7%111%43%32%2
657
September1st to 15thNight
8%113%40%32%3
720
16th to endNight
15%310%43%26%2
716
October1st to 15thNight
21%417%32%21%2
720
16th to endNight
29%315%29%18%2
768
November1st to 15thNight
38%5%17%27%10%
760
16th to endNight
36%7%16%27%12%
780
December1st to 15thNight
36%7%17%27%10%
780
16th to endNight
44%6%16%22%10%1
832
Page generated at 2018-03-12 10:28:01. Version ac04

Copyright 2018 A. Danko


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