Sitka
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sitka clear sky chart from 2005-11-05 to 2017-10-06.

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
22%
14344
4%
46914
3%
14308
23%
14308
5%
41308

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
18%
750
7%
2865
6%
746
20%
746
5%
2472
January16th to endNight
13%
800
5%
2955
4%
800
14%
800
3%
2671
February1st to 15thNight
20%
734
4%
2500
3%
730
22%
730
6%
2150
February16th to endNight
22%
658
10%
2067
5%
658
27%
658
9%
1703
March1st to 15thNight
20%
720
6%
2160
5%
720
22%
720
8%
1848
March16th to endNight
22%
576
4%
2004
6%
576
22%
576
4%
1774
April1st to 15thNight
26%
539
2%
1745
3%
539
26%
539
6%
1523
April16th to endNight
26%
540
3%
1440
2%
540
26%
540
8%
1250
May1st to 15thNight
28%
458
0%
1308
0%
458
28%
458
10%
1107
May16th to endNight
44%
375
0%
1122
0%
374
44%
374
12%
905
June1st to 15thNight
31%
360
0%
1080
0%
360
31%
360
6%
964
June16th to endNight
29%
360
0%
1080
0%
360
29%
360
4%
980
July1st to 15thNight
29%
340
0%
1020
0%
340
29%
340
4%
942
July16th to endNight
24%
403
0%
1087
0%
379
24%
379
4%
1004
August1st to 15thNight
31%
538
1%
1369
1%
535
32%
535
9%
1198
August16th to endNight
19%
573
1%
1585
1%
573
19%
573
3%
1482
September1st to 15thNight
24%
540
4%
1800
4%
540
24%
540
5%
1612
September16th to endNight
17%
647
3%
1960
3%
647
18%
647
3%
1813
October1st to 15thNight
21%
684
5%
2074
3%
684
23%
684
5%
1863
October16th to endNight
21%
719
4%
2368
3%
719
22%
719
5%
2114
November1st to 15thNight
15%
730
2%
2550
1%
730
16%
730
3%
2307
November16th to endNight
18%
750
4%
2823
3%
750
20%
750
5%
2457
December1st to 15thNight
20%
750
6%
2880
5%
750
22%
750
5%
2466
December16th to endNight
20%
800
5%
3072
4%
800
21%
800
4%
2703

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
38%17%46%5%2323311%
48346
Night
34%18%46%42333314%
47037
TransparencyDay
78%6%6%5%2
47974
Night
76%6%6%6%3
46914
SeeingDay
47%11%17%16%6%1
7338
Night
46%8%15%15%11%2
14345

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
36%23%45%32211314%
1074
16th to endDay
40%20%3442332211%
1266
February1st to 15thDay
29%24%46%42212216%
1364
16th to endDay
19%21%5%6%423435%22%
1406
March1st to 15thDay
22%26%5%7%5%2432415%
1800
16th to endDay
18%26%6%9%8%3433311%
2217
April1st to 15thDay
27%29%5%7%6%1221211%
2202
16th to endDay
26%20%5%8%6%3443312%
2520
May1st to 15thDay
36%18%45%5%2323314%
2640
16th to endDay
27%14%5%7%6%34445%16%
2991
June1st to 15thDay
39%13%5%7%5%2434411%
2880
16th to endDay
44%13%5%7%5%242237%
2880
July1st to 15thDay
49%12%36%5%232327%
2880
16th to endDay
51%13%46%4222126%
3005
August1st to 15thDay
46%9%36%32334312%
2583
16th to endDay
57%13%45%4222115%
2616
September1st to 15thDay
52%10%36%4122339%
2160
16th to endDay
53%11%36%3222228%
1998
October1st to 15thDay
47%12%46%42323210%
1683
16th to endDay
48%14%245%1222311%
1504
November1st to 15thDay
38%19%36%42212314%
1308
16th to endDay
33%22%45%42222415%
1137
December1st to 15thDay
28%20%46%43223418%
1080
16th to endDay
31%21%47%42243313%
1152
January1st to 15thNight
37%21%45%31221216%
2865
16th to endNight
38%22%45%42222310%
2955
February1st to 15thNight
30%20%46%42432416%
2488
16th to endNight
20%25%5%6%5%2322422%
2067
March1st to 15thNight
23%26%5%6%42333316%
2160
16th to endNight
18%25%5%9%7%35%43411%
1999
April1st to 15thNight
26%26%45%42332316%
1745
16th to endNight
24%18%49%6%33435%15%
1440
May1st to 15thNight
30%17%5%6%32333418%
1307
16th to endNight
22%12%36%7%45%46%7%20%
1122
June1st to 15thNight
32%12%48%7%3443413%
1080
16th to endNight
37%11%5%8%5%45%345%8%
1080
July1st to 15thNight
36%13%6%8%5%35%35%48%
1080
16th to endNight
43%13%48%4132239%
1161
August1st to 15thNight
39%12%36%41324317%
1376
16th to endNight
51%10%36%6%233227%
1585
September1st to 15thNight
48%10%35%42232411%
1800
16th to endNight
49%11%36%4222328%
1960
October1st to 15thNight
43%10%37%5%2433212%
2074
16th to endNight
45%14%25%31232214%
2368
November1st to 15thNight
37%20%46%42322312%
2550
16th to endNight
31%21%36%42434314%
2823
December1st to 15thNight
34%21%35%32232316%
2880
16th to endNight
29%22%5%6%5%2333313%
3072

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
79%228%6%
1074
16th to endDay
81%5%18%3
1266
February1st to 15thDay
76%447%5%1
1364
16th to endDay
65%7%6%11%7%1
1406
March1st to 15thDay
75%5%5%6%42
1800
16th to endDay
78%6%45%3
2217
April1st to 15thDay
81%5%45%2
2202
16th to endDay
75%8%7%7%1
2520
May1st to 15thDay
75%45%8%4
2640
16th to endDay
71%6%11%8%1
2995
June1st to 15thDay
76%7%9%41
2880
16th to endDay
82%7%7%2
2880
July1st to 15thDay
81%7%8%2
2711
16th to endDay
85%5%6%1
2816
August1st to 15thDay
76%7%9%5%1
2565
16th to endDay
88%433
2616
September1st to 15thDay
80%5%5%6%
2160
16th to endDay
83%5%45%1
1998
October1st to 15thDay
80%7%434
1683
16th to endDay
80%5%5%6%2
1504
November1st to 15thDay
78%5%5%6%2
1308
16th to endDay
76%6%47%21
1137
December1st to 15thDay
71%7%8%6%6%
1080
16th to endDay
77%7%45%4
1152
January1st to 15thNight
77%338%6%
2865
16th to endNight
82%426%3
2955
February1st to 15thNight
75%447%6%1
2500
16th to endNight
69%45%10%8%1
2067
March1st to 15thNight
73%5%5%6%7%2
2160
16th to endNight
77%6%6%6%2
2004
April1st to 15thNight
75%6%8%6%4
1745
16th to endNight
72%6%10%8%1
1440
May1st to 15thNight
69%6%7%10%5%
1308
16th to endNight
62%7%17%10%2
1122
June1st to 15thNight
74%8%10%41
1080
16th to endNight
79%7%11%2
1080
July1st to 15thNight
77%10%8%3
1020
16th to endNight
80%7%9%2
1087
August1st to 15thNight
73%6%13%5%1
1369
16th to endNight
83%6%5%4
1585
September1st to 15thNight
78%6%6%7%1
1800
16th to endNight
81%6%35%1
1960
October1st to 15thNight
79%5%5%6%3
2074
16th to endNight
78%5%5%7%3
2368
November1st to 15thNight
80%6%5%41
2550
16th to endNight
75%7%6%6%31
2823
December1st to 15thNight
74%6%6%6%6%
2880
16th to endNight
76%6%45%4
3072

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
33%6%23%15%18%3
60
16th to endDay
47%14%17%15%4
178
February1st to 15thDay
42%17%19%14%41
236
16th to endDay
31%20%23%15%6%2
211
March1st to 15thDay
35%17%26%15%5%1
240
16th to endDay
34%18%23%14%6%1
274
April1st to 15thDay
45%12%16%14%10%
302
16th to endDay
39%14%19%15%9%1
450
May1st to 15thDay
46%10%17%19%41
468
16th to endDay
34%8%22%26%7%
518
June1st to 15thDay
44%10%16%20%6%
495
16th to endDay
45%11%16%19%5%1
495
July1st to 15thDay
51%8%15%16%7%1
475
16th to endDay
56%11%13%14%4
515
August1st to 15thDay
52%5%12%20%7%1
494
16th to endDay
61%9%13%10%4
508
September1st to 15thDay
53%6%16%14%6%1
315
16th to endDay
55%11%15%11%5%
302
October1st to 15thDay
47%10%19%11%9%1
242
16th to endDay
51%11%15%15%6%
240
November1st to 15thDay
61%13%16%43
136
16th to endDay
51%313%18%8%5%
60
December1st to 15thDay
41%6%11%16%18%5%
60
16th to endDay
57%6%21%7%6%
64
January1st to 15thNight
49%10%18%12%7%1
750
16th to endNight
52%13%16%10%6%1
800
February1st to 15thNight
46%11%16%15%7%1
734
16th to endNight
36%12%21%17%10%1
658
March1st to 15thNight
40%12%20%16%7%1
720
16th to endNight
35%8%18%18%13%5%
576
April1st to 15thNight
42%9%14%15%13%4
539
16th to endNight
34%10%20%14%14%5%
540
May1st to 15thNight
44%5%13%17%17%3
459
16th to endNight
30%5%10%24%26%3
375
June1st to 15thNight
39%5%11%17%23%2
360
16th to endNight
40%5%15%18%18%2
360
July1st to 15thNight
44%411%21%15%2
340
16th to endNight
48%310%19%15%1
403
August1st to 15thNight
49%210%13%17%6%
538
16th to endNight
56%29%16%11%2
573
September1st to 15thNight
55%47%17%11%3
540
16th to endNight
57%7%11%11%10%2
647
October1st to 15thNight
50%7%11%17%10%1
684
16th to endNight
54%5%12%16%8%2
719
November1st to 15thNight
53%13%14%11%6%
730
16th to endNight
45%12%19%16%6%
750
December1st to 15thNight
49%10%15%14%8%1
750
16th to endNight
44%13%16%17%7%
800
Page generated at 2017-10-07 10:07:51. Version ac04

Copyright 2017 A. Danko


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