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Anchorage
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anchorage 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
37%
21477
6%
75148
5%
21432
38%
21432
11%
57453

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
44%
936
10%
3392
10%
936
47%
936
17%
2388
January11th to 20thNight
34%
882
9%
3082
6%
882
37%
882
13%
2322
January21st to endNight
31%
880
8%
3168
6%
880
32%
880
11%
2445
February1st to 10thNight
39%
800
10%
2718
9%
800
41%
800
16%
1939
February11th to 20thNight
26%
784
4%
2452
3%
780
27%
780
9%
2025
February21st to endNight
32%
550
12%
1964
7%
550
38%
550
13%
1436
March1st to 10thNight
40%
620
13%
2194
12%
620
43%
620
18%
1500
March11th to 20thNight
48%
620
16%
2095
16%
620
52%
620
21%
1377
March21st to endNight
44%
602
12%
1998
11%
602
47%
602
17%
1423
April1st to 10thNight
39%
540
6%
1760
6%
540
40%
540
12%
1352
April11th to 20thNight
52%
533
4%
1423
3%
533
53%
533
21%
1042
April21st to endNight
47%
540
1%
1422
1%
540
47%
540
19%
1070
May1st to 10thNight
42%
392
0%
1112
0%
392
42%
392
13%
895
May11th to 20thNight
46%
354
0%
1063
0%
354
46%
354
14%
823
May21st to endNight
49%
239
0%
925
0%
238
50%
238
12%
655
June1st to 10thNight
41%
180
0%
720
0%
180
41%
180
8%
569
June11th to 20thNight
46%
180
0%
720
0%
180
46%
180
8%
572
June21st to endNight
48%
180
0%
720
0%
180
48%
180
6%
591
July1st to 10thNight
43%
175
0%
700
0%
175
43%
175
8%
570
July11th to 20thNight
39%
266
0%
788
0%
266
39%
266
10%
660
July21st to endNight
37%
394
0%
1107
0%
372
37%
372
9%
919
August1st to 10thNight
38%
358
0%
1085
0%
356
39%
356
9%
912
August11th to 20thNight
29%
486
0%
1375
0%
486
29%
486
8%
1181
August21st to endNight
37%
594
2%
1584
2%
594
37%
594
10%
1330
September1st to 10thNight
29%
540
3%
1740
2%
540
30%
540
6%
1525
September11th to 20thNight
31%
540
5%
1800
6%
540
31%
540
7%
1523
September21st to endNight
38%
576
4%
2042
5%
576
39%
576
9%
1619
October1st to 10thNight
29%
638
7%
2168
7%
638
30%
638
9%
1757
October11th to 20thNight
38%
810
5%
2353
4%
810
39%
810
12%
1898
October21st to endNight
36%
891
9%
2798
6%
891
38%
891
12%
2177
November1st to 10thNight
35%
830
5%
2777
4%
830
36%
830
11%
2183
November11th to 20thNight
43%
850
11%
3040
9%
850
46%
850
16%
2124
November21st to endNight
31%
850
10%
3187
8%
850
36%
850
10%
2444
December1st to 10thNight
31%
908
5%
3350
6%
908
32%
908
8%
2688
December11th to 20thNight
32%
940
5%
4210
5%
939
34%
939
7%
3363
December21st to endNight
26%
1019
3%
6116
2%
1004
30%
1004
5%
4156

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
18%11%48%6%45%5%5%7%26%
74382
Night
18%14%5%8%7%45%5%5%6%23%
75443
TransparencyDay
56%12%16%11%41
74015
Night
56%11%14%12%6%1
75330
SeeingDay
19%8%18%27%22%6%
16843
Night
20%8%21%27%19%4
21496

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
9%14%6%9%7%35%5%6%12%24%
773
11th to 20thDay
17%15%5%8%5%47%5%6%9%20%
988
21st to endDay
16%17%5%10%7%46%5%5%6%18%
1188
February1st to 10thDay
11%16%5%8%5%25%5%48%30%
1241
11th to 20thDay
16%21%6%9%7%45%5%4418%
1399
21st to endDay
13%14%48%8%3445%6%30%
1292
March1st to 10thDay
13%16%47%5%335%5%5%36%
1766
11th to 20thDay
7%9%48%5%36%6%5%7%40%
1862
21st to endDay
8%15%5%9%6%35%45%7%33%
2358
April1st to 10thDay
10%15%6%9%7%35%5%5%7%28%
2200
11th to 20thDay
8%10%47%6%46%7%6%6%36%
2486
21st to endDay
12%11%48%6%46%6%6%8%30%
2538
May1st to 10thDay
16%12%5%9%8%46%5%5%6%24%
2847
11th to 20thDay
15%10%47%7%45%5%5%6%32%
2833
21st to endDay
17%9%36%5%35%45%8%33%
3346
June1st to 10thDay
15%10%47%6%45%6%7%9%27%
3240
11th to 20thDay
16%7%37%7%5%6%6%7%8%28%
3240
21st to endDay
18%8%48%6%46%7%6%7%27%
3240
July1st to 10thDay
19%8%48%7%46%6%6%7%26%
3240
11th to 20thDay
20%8%47%6%46%6%7%8%25%
3146
21st to endDay
26%8%46%6%445%5%6%26%
3147
August1st to 10thDay
26%9%37%6%35%45%6%24%
2846
11th to 20thDay
30%10%47%5%35%45%7%21%
2525
21st to endDay
29%9%36%6%45%5%5%6%22%
2772
September1st to 10thDay
29%11%38%7%445%5%6%18%
2219
11th to 20thDay
33%10%47%6%3345%5%19%
2160
21st to endDay
25%10%48%6%34445%26%
1915
October1st to 10thDay
28%12%5%7%6%44445%21%
1792
11th to 20thDay
22%12%37%7%45%5%5%5%25%
1603
21st to endDay
16%13%48%5%246%7%7%27%
1558
November1st to 10thDay
15%16%6%10%7%246%46%23%
1298
11th to 20thDay
13%11%36%6%46%7%6%6%30%
1140
21st to endDay
16%16%48%8%46%5%6%7%19%
993
December1st to 10thDay
20%16%6%10%7%3345%7%18%
827
11th to 20thDay
9%22%10%9%7%5%5%45%7%16%
967
21st to endDay
7%15%5%10%9%6%10%9%8%10%11%
1397
January1st to 10thNight
10%12%6%8%7%45%5%7%9%27%
3392
11th to 20thNight
15%17%5%8%6%35%5%5%8%22%
3082
21st to endNight
17%16%6%9%7%35%5%7%6%20%
3168
February1st to 10thNight
12%16%5%9%6%35%5%5%6%30%
2718
11th to 20thNight
17%21%7%9%7%34445%17%
2448
21st to endNight
12%15%5%9%7%45%5%45%30%
1964
March1st to 10thNight
11%15%5%8%6%4445%5%35%
2194
11th to 20thNight
8%11%5%8%5%35%45%8%38%
2095
21st to endNight
10%15%5%8%5%3445%6%36%
1998
April1st to 10thNight
11%20%6%9%7%45%44426%
1760
11th to 20thNight
12%12%46%6%5%6%5%6%6%32%
1423
21st to endNight
11%11%5%9%7%46%5%5%7%30%
1422
May1st to 10thNight
17%12%5%7%8%5%6%6%7%5%22%
1112
11th to 20thNight
18%9%48%7%46%5%5%7%28%
1062
21st to endNight
18%8%35%6%45%6%7%8%30%
925
June1st to 10thNight
19%11%6%9%8%5%46%7%5%22%
720
11th to 20thNight
19%10%49%5%5%6%8%6%8%21%
720
21st to endNight
21%7%5%10%7%5%5%5%7%9%20%
720
July1st to 10thNight
24%12%37%8%46%5%6%6%22%
720
11th to 20thNight
28%8%48%6%46%5%6%6%20%
813
21st to endNight
30%9%47%6%345%45%22%
1172
August1st to 10thNight
28%9%5%6%7%345%6%7%20%
1091
11th to 20thNight
34%10%38%6%35%5%4418%
1375
21st to endNight
33%9%45%5%35%446%21%
1584
September1st to 10thNight
36%10%46%6%45%435%16%
1740
11th to 20thNight
35%11%38%6%33445%19%
1800
21st to endNight
28%11%46%5%45%5%5%424%
2042
October1st to 10thNight
32%13%47%5%3444422%
2168
11th to 20thNight
24%12%38%6%345%5%5%24%
2353
21st to endNight
18%15%5%9%6%45%5%5%6%25%
2798
November1st to 10thNight
19%15%5%8%5%45%45%6%23%
2777
11th to 20thNight
12%12%47%6%35%7%6%8%30%
3040
21st to endNight
15%17%6%9%6%345%5%8%22%
3187
December1st to 10thNight
22%16%5%8%7%45%5%47%16%
3350
11th to 20thNight
12%20%7%12%8%45%5%5%6%17%
4240
21st to endNight
9%15%6%11%10%5%8%9%6%8%14%
6270

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
47%10%15%11%14%3
773
11th to 20thDay
55%12%14%10%6%3
988
21st to endDay
62%10%13%8%6%2
1188
February1st to 10thDay
51%8%13%15%9%4
1241
11th to 20thDay
67%10%10%9%41
1406
21st to endDay
51%10%10%17%10%1
1292
March1st to 10thDay
49%9%12%13%12%5%
1766
11th to 20thDay
42%10%16%16%15%2
1862
21st to endDay
51%11%14%12%10%2
2358
April1st to 10thDay
57%10%14%12%6%1
2200
11th to 20thDay
46%12%17%18%7%
2486
21st to endDay
53%9%14%16%7%1
2538
May1st to 10thDay
60%10%15%12%3
2847
11th to 20thDay
52%9%16%17%5%
2836
21st to endDay
50%10%18%19%4
3347
June1st to 10thDay
52%17%18%11%1
3240
11th to 20thDay
53%16%19%9%3
3240
21st to endDay
55%17%20%7%
3240
July1st to 10thDay
56%16%18%9%1
3151
11th to 20thDay
56%19%21%4
3048
21st to endDay
59%17%17%7%
2972
August1st to 10thDay
62%15%16%6%
2827
11th to 20thDay
65%12%14%6%11
2525
21st to endDay
63%14%14%7%2
2772
September1st to 10thDay
68%13%9%8%2
2219
11th to 20thDay
68%10%14%8%1
2160
21st to endDay
63%10%14%12%2
1915
October1st to 10thDay
64%11%13%10%2
1792
11th to 20thDay
59%12%14%12%3
1603
21st to endDay
53%14%15%10%8%1
1558
November1st to 10thDay
58%10%13%11%7%1
1298
11th to 20thDay
48%11%20%15%6%
1140
21st to endDay
60%10%12%10%6%2
993
December1st to 10thDay
64%10%14%8%41
827
11th to 20thDay
64%10%14%9%3
970
21st to endDay
45%15%19%10%10%2
1397
January1st to 10thNight
46%12%15%11%13%3
3392
11th to 20thNight
55%10%13%12%7%3
3082
21st to endNight
58%9%12%11%7%2
3168
February1st to 10thNight
50%9%12%16%8%5%
2718
11th to 20thNight
66%9%10%10%5%1
2452
21st to endNight
54%9%10%16%10%2
1964
March1st to 10thNight
48%8%13%14%12%6%
2194
11th to 20thNight
43%8%11%18%19%2
2097
21st to endNight
49%10%14%14%11%3
1998
April1st to 10thNight
59%8%13%15%5%1
1760
11th to 20thNight
48%9%17%18%8%
1423
21st to endNight
51%9%14%16%10%
1422
May1st to 10thNight
58%11%14%15%2
1112
11th to 20thNight
54%12%11%17%5%
1063
21st to endNight
49%13%17%18%3
925
June1st to 10thNight
60%13%17%11%1
720
11th to 20thNight
56%16%15%9%3
720
21st to endNight
60%16%18%6%
720
July1st to 10thNight
58%17%15%10%
700
11th to 20thNight
61%14%20%5%
788
21st to endNight
64%11%18%7%
1107
August1st to 10thNight
62%15%18%5%
1085
11th to 20thNight
67%11%13%8%1
1375
21st to endNight
64%10%14%9%2
1584
September1st to 10thNight
69%13%8%8%2
1740
11th to 20thNight
67%11%11%9%2
1800
21st to endNight
62%10%14%12%2
2042
October1st to 10thNight
65%8%11%12%3
2168
11th to 20thNight
60%11%15%9%5%
2353
21st to endNight
58%9%12%12%8%
2798
November1st to 10thNight
59%11%12%11%7%1
2777
11th to 20thNight
48%10%18%17%8%
3040
21st to endNight
58%10%14%11%6%1
3187
December1st to 10thNight
63%10%13%8%5%
3350
11th to 20thNight
63%10%13%10%4
4236
21st to endNight
49%13%19%10%7%2
6270

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
5%9%22%29%28%8%
104
11th to 20thDay
15%9%24%30%15%7%
271
21st to endDay
19%12%21%28%13%8%
286
February1st to 10thDay
14%7%18%30%18%13%
260
11th to 20thDay
21%12%21%30%13%4
335
21st to endDay
19%9%27%27%10%9%
280
March1st to 10thDay
18%8%21%28%19%7%
340
11th to 20thDay
8%6%20%33%25%8%
402
21st to endDay
11%9%23%27%23%7%
572
April1st to 10thDay
12%9%24%31%19%5%
528
11th to 20thDay
10%9%13%29%31%8%
590
21st to endDay
15%7%20%31%24%5%
600
May1st to 10thDay
17%12%23%25%21%2
603
11th to 20thDay
17%8%19%30%20%6%
625
21st to endDay
20%8%18%30%20%4
765
June1st to 10thDay
13%10%17%24%28%9%
810
11th to 20thDay
13%5%17%31%27%6%
810
21st to endDay
16%7%15%26%27%8%
810
July1st to 10thDay
17%6%14%27%30%5%
796
11th to 20thDay
22%415%29%24%6%
792
21st to endDay
24%6%15%23%24%9%
694
August1st to 10thDay
25%6%15%27%21%5%
626
11th to 20thDay
27%9%19%24%17%4
615
21st to endDay
30%8%13%24%20%6%
693
September1st to 10thDay
28%11%17%21%20%3
555
11th to 20thDay
31%8%16%21%19%5%
540
21st to endDay
29%6%18%16%24%6%
370
October1st to 10thDay
31%8%21%22%13%4
360
11th to 20thDay
22%9%18%28%17%7%
360
21st to endDay
20%9%19%24%20%7%
396
November1st to 10thDay
21%9%26%20%19%6%
363
11th to 20thDay
16%5%25%24%24%5%
280
21st to endDay
16%8%29%25%16%7%
133
December1st to 10thDay
20%11%16%26%22%6%
90
11th to 20thDay
17%7%26%36%10%6%
90
21st to endDay
14%12%32%25%13%3
99
January1st to 10thNight
10%7%25%33%21%4
940
11th to 20thNight
17%10%25%28%15%4
882
21st to endNight
19%9%23%28%16%4
880
February1st to 10thNight
15%8%20%34%18%4
800
11th to 20thNight
22%12%26%25%14%1
784
21st to endNight
16%7%27%32%13%5%
550
March1st to 10thNight
14%8%24%31%19%3
620
11th to 20thNight
9%6%22%34%24%5%
620
21st to endNight
13%7%21%25%24%9%
602
April1st to 10thNight
17%9%18%31%21%4
540
11th to 20thNight
12%7%14%29%30%8%
533
21st to endNight
14%5%19%30%28%4
540
May1st to 10thNight
17%6%19%30%24%3
392
11th to 20thNight
16%8%14%28%31%3
354
21st to endNight
20%414%34%25%3
239
June1st to 10thNight
24%7%14%24%28%3
180
11th to 20thNight
19%6%16%30%27%3
180
21st to endNight
21%314%28%31%3
180
July1st to 10thNight
23%7%12%18%35%3
175
11th to 20thNight
32%49%23%27%6%
266
21st to endNight
30%5%15%19%22%9%
394
August1st to 10thNight
32%6%15%25%18%4
358
11th to 20thNight
35%7%18%21%15%4
486
21st to endNight
33%7%12%20%21%6%
594
September1st to 10thNight
35%9%16%21%14%4
540
11th to 20thNight
35%8%12%17%21%6%
540
21st to endNight
30%8%15%21%20%6%
576
October1st to 10thNight
33%8%17%22%16%3
638
11th to 20thNight
21%8%20%25%19%6%
810
21st to endNight
20%8%20%28%20%4
891
November1st to 10thNight
22%9%23%26%17%3
830
11th to 20thNight
15%7%24%32%19%3
850
21st to endNight
19%10%25%24%17%5%
850
December1st to 10thNight
22%11%21%26%16%4
908
11th to 20thNight
14%9%26%31%15%4
940
21st to endNight
18%10%29%24%15%5%
1034
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