Sitka
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitka 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
20%
22731
4%
75090
3%
22666
22%
22666
5%
66072

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
19%
850
8%
3026
7%
846
22%
846
6%
2579
January11th to 20thNight
11%
830
4%
2976
2%
830
14%
830
3%
2682
January21st to endNight
13%
880
4%
2992
3%
880
15%
880
4%
2696
February1st to 10thNight
20%
800
7%
2575
3%
800
24%
800
6%
2184
February11th to 20thNight
17%
784
7%
2387
4%
780
19%
780
6%
2076
February21st to endNight
21%
656
13%
1920
5%
656
27%
656
10%
1557
March1st to 10thNight
22%
720
9%
2160
7%
720
25%
720
9%
1777
March11th to 20thNight
17%
630
3%
2070
3%
630
19%
630
5%
1810
March21st to endNight
24%
594
7%
2016
7%
594
26%
594
6%
1733
April1st to 10thNight
25%
540
3%
1800
3%
540
25%
540
7%
1555
April11th to 20thNight
25%
533
4%
1523
3%
533
26%
533
8%
1303
April21st to endNight
25%
540
3%
1440
2%
540
25%
540
8%
1259
May1st to 10thNight
22%
509
0%
1409
0%
509
22%
509
6%
1258
May11th to 20thNight
41%
354
0%
1077
0%
354
41%
354
12%
857
May21st to endNight
40%
389
0%
1164
0%
388
40%
388
12%
944
June1st to 10thNight
33%
360
0%
1080
0%
360
33%
360
6%
966
June11th to 20thNight
28%
360
0%
1080
0%
360
28%
360
5%
980
June21st to endNight
26%
360
0%
1080
0%
360
26%
360
6%
963
July1st to 10thNight
31%
350
0%
1050
0%
350
31%
350
6%
928
July11th to 20thNight
28%
348
0%
1044
0%
348
28%
348
4%
965
July21st to endNight
27%
434
0%
1154
0%
410
27%
410
6%
1046
August1st to 10thNight
27%
537
0%
1328
0%
534
27%
534
6%
1208
August11th to 20thNight
23%
534
1%
1424
1%
534
23%
534
5%
1294
August21st to endNight
20%
594
2%
1668
1%
594
20%
594
4%
1527
September1st to 10thNight
21%
540
2%
1800
2%
540
21%
540
3%
1663
September11th to 20thNight
31%
540
9%
1800
9%
540
32%
540
8%
1502
September21st to endNight
13%
705
3%
2038
3%
705
14%
705
3%
1894
October1st to 10thNight
24%
720
5%
2160
4%
720
25%
720
6%
1902
October11th to 20thNight
11%
720
2%
2286
0%
720
12%
720
3%
2137
October21st to endNight
21%
857
6%
2703
3%
857
24%
857
7%
2316
November1st to 10thNight
13%
830
4%
2653
2%
830
16%
830
5%
2327
November11th to 20thNight
20%
850
4%
2854
3%
850
21%
850
5%
2504
November21st to endNight
11%
850
5%
3040
2%
850
14%
850
3%
2743
December1st to 10thNight
16%
850
5%
3040
4%
850
18%
850
4%
2694
December11th to 20thNight
17%
850
5%
3732
3%
849
20%
849
4%
3221
December21st to endNight
15%
933
3%
5541
5%
905
18%
905
2%
5022

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
36%19%5%7%5%3333413%
74545
Night
32%21%5%7%5%3333414%
75279
TransparencyDay
77%7%7%6%31
74204
Night
76%6%6%6%41
75139
SeeingDay
44%13%18%17%8%2
15629
Night
45%11%16%16%10%2
22733

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
28%25%5%7%42423317%
1139
11th to 20thDay
38%24%45%33323412%
1111
21st to endDay
35%24%5%6%5%3443211%
1360
February1st to 10thDay
27%24%5%7%5%2323220%
1385
11th to 20thDay
30%25%5%6%43323316%
1462
21st to endDay
16%24%6%6%423336%25%
1336
March1st to 10thDay
19%25%6%7%424435%22%
1800
11th to 20thDay
19%31%6%9%6%3433313%
1886
21st to endDay
19%27%6%9%7%3443314%
2340
April1st to 10thDay
23%31%7%6%7%3233214%
2160
11th to 20thDay
23%21%6%9%7%3443417%
2384
21st to endDay
27%23%6%9%6%3342314%
2520
May1st to 10thDay
35%20%5%7%6%3433311%
2551
11th to 20thDay
27%16%47%6%4444420%
2815
21st to endDay
28%17%5%8%7%34445%15%
3111
June1st to 10thDay
33%15%6%8%7%34445%11%
2880
11th to 20thDay
40%15%5%8%5%3434410%
2880
21st to endDay
44%15%5%7%5%3423310%
2880
July1st to 10thDay
39%14%47%5%33345%14%
2880
11th to 20thDay
47%14%5%8%5%343336%
2880
21st to endDay
48%13%46%42333311%
3092
August1st to 10thDay
43%12%48%5%2444410%
2603
11th to 20thDay
51%12%36%4333338%
2476
21st to endDay
53%12%45%5%233229%
2686
September1st to 10thDay
52%11%46%4233348%
2160
11th to 20thDay
43%12%35%423335%17%
2160
21st to endDay
53%13%37%4232327%
1918
October1st to 10thDay
45%14%45%42434311%
1800
11th to 20thDay
50%16%46%4233328%
1674
21st to endDay
40%17%25%423335%15%
1652
November1st to 10thDay
36%19%37%42324316%
1424
11th to 20thDay
37%21%45%32223416%
1325
21st to endDay
35%27%5%6%5%2222311%
1140
December1st to 10thDay
28%24%5%7%43234415%
1140
11th to 20thDay
23%26%5%9%6%3333316%
1445
21st to endDay
31%31%6%8%4222228%
2090
January1st to 10thNight
28%23%6%7%42432317%
3026
11th to 20thNight
39%22%5%5%42333312%
2976
21st to endNight
35%24%6%6%5%2332311%
2992
February1st to 10thNight
28%20%5%7%5%3432419%
2575
11th to 20thNight
28%28%6%6%41323316%
2384
21st to endNight
18%26%6%7%5%2322425%
1920
March1st to 10thNight
21%24%6%6%43333423%
2160
11th to 20thNight
18%29%7%8%6%35%43413%
2068
21st to endNight
18%25%6%9%6%3444416%
2016
April1st to 10thNight
23%28%6%6%43433417%
1800
11th to 20thNight
20%21%7%9%7%3443418%
1523
21st to endNight
26%22%5%9%6%4433415%
1440
May1st to 10thNight
34%19%5%7%5%2344413%
1409
11th to 20thNight
21%12%49%7%336%6%6%23%
1077
21st to endNight
23%14%47%7%5%5%45%6%19%
1164
June1st to 10thNight
28%15%5%8%8%46%445%14%
1080
11th to 20thNight
34%14%5%9%6%45%44410%
1080
21st to endNight
36%16%5%8%6%3435%5%10%
1080
July1st to 10thNight
31%14%6%9%5%45%36%413%
1080
11th to 20thNight
38%16%6%8%6%25%245%9%
1080
21st to endNight
40%14%5%8%5%2442413%
1222
August1st to 10thNight
39%14%47%5%2434414%
1335
11th to 20thNight
48%13%35%6%2323313%
1424
21st to endNight
47%11%47%6%2332311%
1668
September1st to 10thNight
50%12%46%5%333239%
1800
11th to 20thNight
38%11%36%5%3343419%
1800
21st to endNight
49%13%47%5%332339%
2038
October1st to 10thNight
41%11%37%6%3434314%
2160
11th to 20thNight
46%17%36%5%334329%
2286
21st to endNight
38%17%46%42323418%
2703
November1st to 10thNight
37%18%46%43333316%
2653
11th to 20thNight
31%22%46%5%3433415%
2854
21st to endNight
34%25%5%6%42333311%
3040
December1st to 10thNight
31%25%5%6%42333314%
3040
11th to 20thNight
26%26%5%7%5%3333316%
3760
21st to endNight
30%29%7%7%42333310%
5566

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
76%5%48%5%2
1139
11th to 20thDay
80%5%35%6%1
1111
21st to endDay
83%437%31
1360
February1st to 10thDay
73%36%8%7%3
1385
11th to 20thDay
76%6%46%8%1
1470
21st to endDay
64%6%6%14%7%3
1336
March1st to 10thDay
68%6%7%8%7%3
1800
11th to 20thDay
78%5%5%6%5%1
1886
21st to endDay
75%7%6%9%4
2340
April1st to 10thDay
78%7%5%7%4
2160
11th to 20thDay
72%7%6%10%5%
2384
21st to endDay
76%6%6%7%4
2520
May1st to 10thDay
80%6%5%6%3
2551
11th to 20thDay
68%6%9%12%41
2818
21st to endDay
71%8%11%8%2
3112
June1st to 10thDay
76%9%8%42
2880
11th to 20thDay
78%8%8%5%1
2880
21st to endDay
81%6%8%31
2880
July1st to 10thDay
72%9%12%5%1
2801
11th to 20thDay
85%7%6%2
2793
21st to endDay
79%7%9%4
2920
August1st to 10thDay
79%8%8%41
2585
11th to 20thDay
83%6%7%41
2476
21st to endDay
83%6%5%5%1
2686
September1st to 10thDay
81%7%6%6%
2160
11th to 20thDay
72%6%7%11%5%
2160
21st to endDay
85%5%45%1
1918
October1st to 10thDay
78%7%5%5%41
1800
11th to 20thDay
85%6%442
1674
21st to endDay
73%6%7%8%5%
1652
November1st to 10thDay
75%6%6%9%3
1424
11th to 20thDay
76%5%6%8%5%1
1325
21st to endDay
82%6%3431
1140
December1st to 10thDay
75%8%8%5%41
1140
11th to 20thDay
75%7%5%6%6%
1448
21st to endDay
84%6%3322
2090
January1st to 10thNight
76%449%5%2
3026
11th to 20thNight
81%5%346%1
2976
21st to endNight
82%446%4
2992
February1st to 10thNight
73%5%5%7%8%3
2575
11th to 20thNight
77%346%9%1
2387
21st to endNight
67%46%12%9%2
1920
March1st to 10thNight
68%5%5%8%10%3
2160
11th to 20thNight
76%6%7%6%41
2070
21st to endNight
73%7%7%10%31
2016
April1st to 10thNight
74%6%9%6%5%1
1800
11th to 20thNight
70%7%7%7%8%
1523
21st to endNight
75%6%8%8%4
1440
May1st to 10thNight
77%6%6%6%4
1409
11th to 20thNight
61%8%11%13%6%1
1077
21st to endNight
64%7%16%9%4
1164
June1st to 10thNight
74%9%9%5%21
1080
11th to 20thNight
78%8%8%5%1
1080
21st to endNight
78%6%11%41
1080
July1st to 10thNight
72%10%11%5%1
1050
11th to 20thNight
78%11%9%2
1044
21st to endNight
78%7%9%41
1154
August1st to 10thNight
77%8%10%41
1328
11th to 20thNight
80%6%8%5%1
1424
21st to endNight
80%7%6%6%1
1668
September1st to 10thNight
82%7%5%5%
1800
11th to 20thNight
69%7%8%10%6%1
1800
21st to endNight
83%6%35%2
2038
October1st to 10thNight
76%6%7%7%31
2160
11th to 20thNight
84%6%432
2286
21st to endNight
73%6%6%9%6%
2703
November1st to 10thNight
75%7%6%9%3
2653
11th to 20thNight
76%7%6%6%41
2854
21st to endNight
80%6%5%431
3040
December1st to 10thNight
77%8%7%5%4
3040
11th to 20thNight
76%6%5%6%8%1
3756
21st to endNight
82%5%3442
5566

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
34%14%23%16%11%1
180
11th to 20thDay
49%15%16%15%6%
171
21st to endDay
48%14%18%14%5%
348
February1st to 10thDay
41%16%22%15%41
340
11th to 20thDay
44%17%19%13%5%1
335
21st to endDay
34%19%26%14%5%2
280
March1st to 10thDay
34%15%24%16%10%2
340
11th to 20thDay
35%21%24%16%41
340
21st to endDay
36%19%20%14%10%1
446
April1st to 10thDay
37%18%22%15%8%
420
11th to 20thDay
32%12%21%19%14%3
548
21st to endDay
43%14%19%14%8%2
600
May1st to 10thDay
45%12%23%16%41
603
11th to 20thDay
33%9%20%23%13%3
625
21st to endDay
35%10%21%22%10%2
680
June1st to 10thDay
38%15%17%18%10%2
630
11th to 20thDay
42%11%16%22%7%1
630
21st to endDay
47%9%14%18%8%3
630
July1st to 10thDay
35%8%18%24%11%3
621
11th to 20thDay
47%11%16%18%6%1
618
21st to endDay
48%8%12%19%10%2
684
August1st to 10thDay
43%9%19%20%8%1
626
11th to 20thDay
54%10%15%14%6%1
615
21st to endDay
53%10%14%15%7%1
671
September1st to 10thDay
49%10%16%14%8%2
450
11th to 20thDay
40%11%19%18%10%2
450
21st to endDay
58%17%13%9%21
412
October1st to 10thDay
49%11%17%15%7%2
360
11th to 20thDay
58%15%18%6%3
360
21st to endDay
44%15%20%15%6%
396
November1st to 10thDay
51%15%18%11%5%
302
11th to 20thDay
53%8%17%14%6%2
180
21st to endDay
58%16%13%11%2
180
December1st to 10thDay
49%19%12%12%7%2
180
11th to 20thDay
42%17%22%11%7%1
180
21st to endDay
46%14%23%8%6%4
198
January1st to 10thNight
41%15%21%15%8%1
850
11th to 20thNight
52%16%17%10%5%1
830
21st to endNight
50%15%15%12%6%1
880
February1st to 10thNight
43%14%18%17%7%1
800
11th to 20thNight
45%14%17%15%8%
784
21st to endNight
37%15%23%17%8%1
656
March1st to 10thNight
37%13%21%16%11%2
720
11th to 20thNight
37%16%22%17%7%1
630
21st to endNight
34%10%20%18%13%5%
594
April1st to 10thNight
39%13%15%16%13%4
540
11th to 20thNight
31%12%21%19%14%3
533
21st to endNight
39%11%16%16%12%4
540
May1st to 10thNight
48%6%15%15%14%3
509
11th to 20thNight
28%6%14%26%22%4
354
21st to endNight
30%8%12%24%24%2
389
June1st to 10thNight
38%6%12%17%24%4
360
11th to 20thNight
38%5%14%24%18%1
360
21st to endNight
41%7%15%16%19%3
360
July1st to 10thNight
36%6%11%23%23%1
350
11th to 20thNight
46%410%20%16%4
348
21st to endNight
43%411%22%18%2
434
August1st to 10thNight
46%5%12%17%15%4
537
11th to 20thNight
53%6%9%14%15%4
534
21st to endNight
52%5%11%17%12%2
594
September1st to 10thNight
54%8%9%16%11%2
540
11th to 20thNight
43%8%11%18%16%4
540
21st to endNight
57%10%13%12%6%2
705
October1st to 10thNight
48%7%12%20%12%1
720
11th to 20thNight
58%10%16%11%32
720
21st to endNight
46%10%17%18%9%1
857
November1st to 10thNight
48%15%19%12%5%
830
11th to 20thNight
45%14%18%15%8%1
850
21st to endNight
52%14%17%12%4
850
December1st to 10thNight
47%14%17%13%7%1
850
11th to 20thNight
44%16%19%14%7%1
850
21st to endNight
47%15%17%14%5%2
935
Page generated at 2024-01-13 15:28:31. Version ac07

Copyright 2024 Attilla Danko


Sitka Clear Sky Chart |Charts Home