Porter
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Porter clear sky chart from 2005-11-06 to 2024-01-14.

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
41%
23953
18%
74586
13%
23905
47%
23905
22%
50389

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
18%
760
13%
2459
6%
757
27%
757
11%
1878
January11th to 20thNight
19%
736
12%
2391
6%
736
27%
736
10%
1838
January21st to endNight
21%
792
15%
2574
7%
792
31%
792
14%
1868
February1st to 10thNight
23%
720
12%
2340
6%
720
30%
720
13%
1757
February11th to 20thNight
18%
700
13%
2126
6%
700
25%
700
14%
1575
February21st to endNight
18%
592
14%
1776
6%
592
27%
592
13%
1342
March1st to 10thNight
26%
720
20%
2044
11%
720
34%
720
21%
1416
March11th to 20thNight
26%
720
17%
1980
9%
720
33%
720
19%
1426
March21st to endNight
30%
792
16%
2178
8%
792
36%
792
19%
1565
April1st to 10thNight
30%
720
18%
1980
9%
720
37%
720
23%
1340
April11th to 20thNight
33%
570
22%
1661
12%
570
42%
570
25%
1072
April21st to endNight
32%
540
20%
1620
8%
540
42%
540
22%
1089
May1st to 10thNight
40%
540
15%
1620
7%
540
45%
540
21%
1114
May11th to 20thNight
41%
531
15%
1593
9%
531
47%
531
21%
1108
May21st to endNight
61%
582
17%
1745
14%
582
64%
582
30%
1094
June1st to 10thNight
60%
540
18%
1620
16%
540
63%
540
33%
980
June11th to 20thNight
58%
540
16%
1620
13%
540
62%
540
30%
999
June21st to endNight
60%
540
17%
1620
16%
540
63%
540
32%
980
July1st to 10thNight
64%
525
16%
1575
15%
525
65%
525
30%
985
July11th to 20thNight
67%
522
18%
1566
15%
522
70%
522
35%
934
July21st to endNight
64%
591
18%
1672
15%
558
67%
558
30%
1065
August1st to 10thNight
71%
537
20%
1600
18%
534
71%
534
31%
980
August11th to 20thNight
65%
534
22%
1598
18%
534
67%
534
30%
1018
August21st to endNight
66%
612
26%
1809
21%
612
68%
612
35%
1059
September1st to 10thNight
64%
720
23%
1980
17%
720
66%
720
35%
1199
September11th to 20thNight
65%
720
28%
1980
22%
720
69%
720
39%
1172
September21st to endNight
54%
720
25%
1980
20%
720
59%
720
31%
1264
October1st to 10thNight
54%
720
25%
2052
22%
720
58%
720
34%
1229
October11th to 20thNight
44%
720
24%
2160
19%
720
53%
720
28%
1347
October21st to endNight
38%
792
24%
2376
18%
792
47%
792
23%
1574
November1st to 10thNight
48%
740
26%
2388
22%
740
57%
740
29%
1417
November11th to 20thNight
34%
760
22%
2470
14%
760
45%
760
21%
1597
November21st to endNight
35%
760
21%
2470
15%
760
43%
760
19%
1663
December1st to 10thNight
24%
760
14%
2470
9%
760
31%
760
13%
1857
December11th to 20thNight
27%
759
14%
3000
10%
759
34%
759
12%
2161
December21st to endNight
25%
826
9%
4493
8%
817
32%
817
7%
3427

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
16%8%36%6%35%5%5%7%36%
74901
Night
16%8%36%5%345%5%7%38%
74873
TransparencyDay
47%6%11%21%13%2
74624
Night
46%6%11%21%14%2
74669
SeeingDay
20%11%26%22%15%7%
13819
Night
19%9%24%29%16%2
23969

By Month

Percentage Cloud Cover SE (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%11%5%10%8%45%446%25%
1706
11th to 20thDay
20%15%6%10%7%34445%23%
1647
21st to endDay
19%14%5%8%6%35%45%426%
1782
February1st to 10thDay
23%12%6%8%6%23345%27%
1620
11th to 20thDay
21%13%5%8%435%447%27%
1716
21st to endDay
22%13%47%5%24445%31%
1480
March1st to 10thDay
18%9%37%6%3445%7%36%
1916
11th to 20thDay
21%8%47%6%3445%5%31%
1980
21st to endDay
21%9%47%5%345%46%31%
2178
April1st to 10thDay
14%9%46%5%35%46%7%37%
1980
11th to 20thDay
17%8%36%42445%7%40%
2261
21st to endDay
20%8%35%5%35%5%6%6%35%
2340
May1st to 10thDay
18%7%37%5%45%5%6%7%32%
2340
11th to 20thDay
20%7%36%7%45%5%5%7%32%
2301
21st to endDay
11%425%6%45%6%6%10%40%
2531
June1st to 10thDay
12%5%35%436%6%7%9%41%
2340
11th to 20thDay
8%5%36%7%46%6%7%9%39%
2340
21st to endDay
11%5%245%46%7%7%10%39%
2340
July1st to 10thDay
12%5%36%6%45%6%7%9%38%
2340
11th to 20thDay
10%5%25%7%46%7%7%9%39%
2340
21st to endDay
13%5%25%5%35%5%7%9%39%
2547
August1st to 10thDay
10%5%25%6%37%7%8%11%37%
2327
11th to 20thDay
10%6%36%5%47%5%7%10%36%
2303
21st to endDay
9%426%5%445%7%9%44%
2525
September1st to 10thDay
15%4245%35%5%6%6%46%
1980
11th to 20thDay
11%425%5%345%5%7%48%
1980
21st to endDay
17%6%25%6%3445%5%45%
1980
October1st to 10thDay
13%5%245%245%6%7%47%
1908
11th to 20thDay
16%6%26%5%34446%44%
1800
21st to endDay
21%6%26%434347%39%
1980
November1st to 10thDay
12%5%345%25%45%6%51%
1686
11th to 20thDay
17%10%35%5%34346%39%
1710
21st to endDay
17%8%46%5%35%5%46%37%
1710
December1st to 10thDay
24%13%6%7%6%34445%25%
1710
11th to 20thDay
20%14%5%8%5%3445%6%27%
2153
21st to endDay
21%19%5%9%7%45%5%5%5%18%
3124
January1st to 10thNight
18%13%6%9%7%45%5%45%26%
2459
11th to 20thNight
22%13%6%8%6%3445%426%
2391
21st to endNight
19%13%6%8%5%3435%5%29%
2574
February1st to 10thNight
23%12%47%6%34436%27%
2340
11th to 20thNight
22%10%47%6%35%445%29%
2125
21st to endNight
23%14%5%6%5%33445%28%
1776
March1st to 10thNight
17%10%36%6%3345%6%38%
2044
11th to 20thNight
20%9%45%6%345%5%7%33%
1980
21st to endNight
20%9%36%5%35%445%35%
2178
April1st to 10thNight
15%8%36%5%4446%7%39%
1980
11th to 20thNight
15%7%35%6%2445%8%41%
1661
21st to endNight
17%6%37%5%44447%38%
1620
May1st to 10thNight
17%7%36%5%35%5%5%8%37%
1620
11th to 20thNight
16%6%47%6%35%46%7%37%
1592
21st to endNight
9%425%5%46%6%7%10%43%
1744
June1st to 10thNight
9%4145%25%7%6%9%47%
1620
11th to 20thNight
8%5%26%5%35%6%7%11%44%
1620
21st to endNight
10%425%425%7%7%10%45%
1620
July1st to 10thNight
10%5%25%5%45%5%7%9%44%
1620
11th to 20thNight
8%5%25%426%5%5%9%50%
1620
21st to endNight
10%4246%35%6%7%9%43%
1771
August1st to 10thNight
8%426%5%36%7%6%9%45%
1611
11th to 20thNight
10%5%25%7%35%5%7%8%43%
1598
21st to endNight
9%425%5%245%7%8%50%
1809
September1st to 10thNight
13%3245%335%5%6%51%
1980
11th to 20thNight
10%424434446%54%
1980
21st to endNight
15%5%24435%447%47%
1980
October1st to 10thNight
12%5%25%5%2436%6%51%
2052
11th to 20thNight
15%6%246%2445%8%44%
2160
21st to endNight
22%6%245%33435%42%
2376
November1st to 10thNight
12%6%25%434446%50%
2388
11th to 20thNight
17%7%35%6%3445%7%39%
2470
21st to endNight
18%10%35%5%24446%38%
2470
December1st to 10thNight
22%12%5%7%5%34435%28%
2470
11th to 20thNight
17%12%7%8%6%34446%30%
3042
21st to endNight
19%16%48%7%45%5%46%21%
4532

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
60%5%9%17%9%
1707
11th to 20thDay
65%5%10%16%4
1647
21st to endDay
61%5%9%16%8%2
1782
February1st to 10thDay
61%410%17%7%1
1620
11th to 20thDay
59%410%18%8%1
1726
21st to endDay
57%37%21%10%2
1480
March1st to 10thDay
49%39%27%10%1
1916
11th to 20thDay
55%5%8%18%13%2
1980
21st to endDay
53%6%8%20%10%2
2178
April1st to 10thDay
46%5%11%22%13%3
1980
11th to 20thDay
44%5%9%20%17%4
2261
21st to endDay
48%6%12%19%11%3
2340
May1st to 10thDay
50%6%12%20%10%2
2340
11th to 20thDay
52%6%11%19%10%2
2305
21st to endDay
38%8%16%22%14%1
2531
June1st to 10thDay
38%8%13%24%15%2
2340
11th to 20thDay
39%10%13%21%15%2
2340
21st to endDay
37%10%15%21%15%2
2340
July1st to 10thDay
40%10%13%20%16%1
2275
11th to 20thDay
38%9%14%21%15%2
2271
21st to endDay
38%9%14%23%13%3
2405
August1st to 10thDay
38%10%15%25%11%1
2314
11th to 20thDay
41%8%14%26%11%2
2303
21st to endDay
35%8%16%21%16%4
2525
September1st to 10thDay
37%7%10%21%19%6%
1980
11th to 20thDay
34%6%12%26%21%2
1980
21st to endDay
41%6%9%23%20%1
1980
October1st to 10thDay
35%6%12%25%20%2
1908
11th to 20thDay
41%5%9%26%18%2
1800
21st to endDay
46%410%24%15%1
1980
November1st to 10thDay
35%5%9%30%18%3
1686
11th to 20thDay
48%37%21%17%3
1710
21st to endDay
48%5%9%22%14%1
1710
December1st to 10thDay
62%49%19%5%1
1710
11th to 20thDay
58%6%9%18%9%
2150
21st to endDay
67%6%11%12%4
3124
January1st to 10thNight
61%5%8%15%9%1
2459
11th to 20thNight
61%5%10%16%6%1
2391
21st to endNight
56%410%20%7%2
2574
February1st to 10thNight
60%410%19%7%1
2340
11th to 20thNight
58%48%18%11%1
2126
21st to endNight
59%5%9%17%8%3
1776
March1st to 10thNight
48%48%23%16%2
2044
11th to 20thNight
51%5%11%20%12%2
1980
21st to endNight
52%5%9%19%13%2
2178
April1st to 10thNight
44%6%9%22%17%3
1980
11th to 20thNight
41%5%10%20%16%7%
1661
21st to endNight
46%5%10%20%16%2
1620
May1st to 10thNight
46%7%13%18%13%3
1620
11th to 20thNight
46%7%13%18%13%3
1593
21st to endNight
34%9%16%24%15%1
1745
June1st to 10thNight
32%8%12%27%18%4
1620
11th to 20thNight
33%10%16%19%20%3
1620
21st to endNight
31%9%16%23%18%3
1620
July1st to 10thNight
35%8%13%24%17%3
1575
11th to 20thNight
31%7%16%23%19%3
1566
21st to endNight
34%10%14%25%15%2
1672
August1st to 10thNight
33%9%15%27%14%2
1600
11th to 20thNight
35%9%13%24%18%1
1598
21st to endNight
31%7%11%27%20%5%
1809
September1st to 10thNight
32%6%13%23%18%7%
1980
11th to 20thNight
30%6%11%26%24%3
1980
21st to endNight
37%6%11%20%24%1
1980
October1st to 10thNight
33%5%11%26%21%3
2052
11th to 20thNight
39%5%10%24%20%2
2160
21st to endNight
45%48%24%16%3
2376
November1st to 10thNight
37%49%23%23%5%
2388
11th to 20thNight
45%49%20%18%4
2470
21st to endNight
49%5%9%23%13%1
2470
December1st to 10thNight
59%5%9%20%7%
2470
11th to 20thNight
57%5%10%20%8%1
3044
21st to endNight
64%6%10%15%6%
4532

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
29%19%35%10%6%1
270
11th to 20thDay
30%20%36%11%2
249
21st to endDay
29%25%28%14%3
264
February1st to 10thDay
36%21%27%12%4
240
11th to 20thDay
44%20%30%41
240
21st to endDay
32%26%34%7%1
198
March1st to 10thDay
26%19%34%17%4
356
11th to 20thDay
25%18%35%15%6%1
420
21st to endDay
31%13%31%21%4
462
April1st to 10thDay
21%16%29%22%10%1
420
11th to 20thDay
23%14%30%20%9%3
412
21st to endDay
26%14%32%17%8%2
420
May1st to 10thDay
24%15%31%17%10%4
423
11th to 20thDay
18%11%28%24%13%6%
447
21st to endDay
13%7%23%26%23%8%
485
June1st to 10thDay
14%5%23%28%22%8%
450
11th to 20thDay
11%7%24%29%20%9%
450
21st to endDay
13%6%27%24%20%9%
450
July1st to 10thDay
14%316%24%20%23%
445
11th to 20thDay
8%421%29%25%13%
444
21st to endDay
15%218%26%24%15%
488
August1st to 10thDay
12%316%29%28%13%
446
11th to 20thDay
13%5%14%29%24%14%
439
21st to endDay
9%5%16%32%23%14%
495
September1st to 10thDay
15%414%25%26%15%
450
11th to 20thDay
10%314%28%30%15%
450
21st to endDay
19%8%18%26%22%7%
450
October1st to 10thDay
14%8%23%28%19%8%
450
11th to 20thDay
17%10%30%26%13%4
450
21st to endDay
25%12%30%22%10%1
495
November1st to 10thDay
18%14%32%25%8%2
284
11th to 20thDay
26%16%27%22%7%2
270
21st to endDay
19%17%40%19%5%1
270
December1st to 10thDay
29%19%31%15%6%1
270
11th to 20thDay
25%20%33%17%41
270
21st to endDay
30%18%33%13%6%
297
January1st to 10thNight
26%18%31%20%5%
760
11th to 20thNight
32%15%30%21%3
736
21st to endNight
27%14%32%24%3
792
February1st to 10thNight
31%11%30%27%1
720
11th to 20thNight
30%17%31%20%2
703
21st to endNight
31%14%33%20%2
592
March1st to 10thNight
25%14%28%27%5%
720
11th to 20thNight
25%14%30%21%8%1
720
21st to endNight
23%14%30%26%7%
792
April1st to 10thNight
21%16%29%26%8%
720
11th to 20thNight
18%11%32%25%13%1
570
21st to endNight
19%9%32%31%9%1
540
May1st to 10thNight
19%8%28%33%12%1
540
11th to 20thNight
17%8%26%30%18%2
531
21st to endNight
9%419%33%31%4
584
June1st to 10thNight
9%319%42%25%2
540
11th to 20thNight
9%319%39%27%3
540
21st to endNight
11%319%36%29%3
540
July1st to 10thNight
11%312%32%34%8%
525
11th to 20thNight
8%314%33%37%5%
522
21st to endNight
10%213%33%36%6%
591
August1st to 10thNight
8%47%40%37%3
537
11th to 20thNight
10%211%35%33%8%
534
21st to endNight
10%113%34%35%7%
612
September1st to 10thNight
15%211%34%33%5%
720
11th to 20thNight
12%312%32%35%6%
720
21st to endNight
17%5%19%32%24%3
720
October1st to 10thNight
13%5%20%34%26%3
720
11th to 20thNight
17%8%27%32%16%1
720
21st to endNight
24%10%23%29%13%1
792
November1st to 10thNight
15%9%24%33%18%1
740
11th to 20thNight
21%15%26%28%10%1
760
21st to endNight
23%10%25%32%8%1
760
December1st to 10thNight
29%14%27%23%6%
760
11th to 20thNight
23%13%29%28%6%
760
21st to endNight
28%14%25%29%3
836
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