Girdwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girdwood clear sky chart from 2005-11-05 to 2017-09-14.

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
33%
13136
6%
46449
5%
13104
35%
13104
9%
37026

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
33%
776
9%
3137
7%
773
36%
773
9%
2408
January16th to endNight
28%
800
11%
3072
8%
800
31%
800
9%
2442
February1st to 15thNight
34%
734
8%
2582
5%
730
36%
730
10%
2028
February16th to endNight
26%
596
10%
2149
6%
596
31%
596
10%
1655
March1st to 15thNight
38%
570
13%
2169
11%
570
44%
570
15%
1502
March16th to endNight
44%
585
11%
2005
12%
585
45%
585
13%
1513
April1st to 15thNight
37%
539
5%
1659
4%
539
38%
539
12%
1332
April16th to endNight
42%
540
1%
1422
1%
540
42%
540
15%
1108
May1st to 15thNight
42%
401
0%
1122
0%
401
42%
401
14%
897
May16th to endNight
51%
270
0%
991
0%
269
51%
269
16%
692
June1st to 15thNight
41%
180
0%
720
0%
180
41%
180
7%
575
June16th to endNight
41%
180
0%
720
0%
180
41%
180
8%
574
July1st to 15thNight
45%
170
0%
680
0%
170
45%
170
8%
529
July16th to endNight
40%
376
0%
1028
0%
354
40%
354
9%
862
August1st to 15thNight
26%
403
0%
1185
0%
401
26%
401
7%
1026
August16th to endNight
29%
573
1%
1528
1%
573
29%
573
9%
1314
September1st to 15thNight
22%
537
2%
1754
2%
537
22%
537
5%
1569
September16th to endNight
31%
497
6%
1795
7%
497
33%
497
8%
1450
October1st to 15thNight
29%
594
8%
2049
7%
594
30%
594
10%
1669
October16th to endNight
32%
736
7%
2425
5%
736
33%
736
8%
2019
November1st to 15thNight
28%
730
6%
2666
4%
730
30%
730
7%
2197
November16th to endNight
38%
750
12%
2976
10%
750
42%
750
11%
2246
December1st to 15thNight
33%
767
7%
3159
7%
767
35%
767
7%
2574
December16th to endNight
29%
832
6%
3456
5%
832
30%
832
6%
2845

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%14%5%9%8%45%5%5%5%19%
48338
Night
13%14%5%8%7%45%5%6%6%21%
46552
TransparencyDay
68%6%10%9%3
47942
Night
66%5%9%11%6%1
46454
SeeingDay
29%11%21%23%11%2
8455
Night
27%9%19%22%16%4
13139

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
12%14%5%7%6%45%46%9%23%
798
16th to endDay
9%15%411%8%6%5%6%7%7%16%
1152
February1st to 15thDay
8%16%5%10%8%5%6%45%6%23%
1282
16th to endDay
6%14%7%12%9%36%5%6%6%20%
1325
March1st to 15thDay
6%14%47%6%46%5%5%6%32%
1791
16th to endDay
6%15%5%9%9%6%7%6%6%6%21%
2213
April1st to 15thDay
8%18%6%11%9%46%45%6%18%
2286
16th to endDay
5%14%5%11%9%47%5%5%6%22%
2538
May1st to 15thDay
9%14%6%11%8%45%45%5%23%
2829
16th to endDay
12%10%45%5%35%5%5%6%33%
3120
June1st to 15thDay
14%12%5%9%8%47%5%6%6%17%
3240
16th to endDay
14%13%5%9%9%46%6%6%6%18%
3240
July1st to 15thDay
15%12%5%11%9%47%5%5%5%17%
3238
16th to endDay
25%11%48%7%35%5%5%5%16%
3069
August1st to 15thDay
28%12%410%8%4445%5%11%
2768
16th to endDay
30%14%48%7%34445%12%
2674
September1st to 15thDay
33%15%5%9%6%3443311%
2179
16th to endDay
29%15%47%6%3433318%
1832
October1st to 15thDay
24%14%39%7%45%43318%
1581
16th to endDay
15%14%5%10%7%45%5%7%6%16%
1445
November1st to 15thDay
9%18%7%10%6%46%5%6%6%19%
1189
16th to endDay
11%17%38%6%46%5%7%7%20%
983
December1st to 15thDay
17%15%5%8%7%35%5%5%5%18%
798
16th to endDay
7%16%7%8%11%5%7%7%8%7%12%
768
January1st to 15thNight
8%13%5%10%7%45%5%6%6%24%
3137
16th to endNight
9%15%6%9%8%46%6%7%7%18%
3072
February1st to 15thNight
7%14%5%9%8%46%6%5%6%24%
2570
16th to endNight
9%15%6%8%8%46%6%6%7%19%
2149
March1st to 15thNight
6%13%47%7%45%5%5%7%31%
2169
16th to endNight
6%15%5%8%6%45%5%5%7%28%
2005
April1st to 15thNight
8%21%5%9%6%35%5%6%423%
1659
16th to endNight
5%12%6%7%6%47%7%7%9%25%
1422
May1st to 15thNight
10%11%47%8%47%6%5%5%26%
1121
16th to endNight
8%10%5%7%5%35%7%7%8%30%
990
June1st to 15thNight
14%13%5%8%5%35%5%9%9%19%
720
16th to endNight
15%12%6%8%7%35%5%5%7%20%
720
July1st to 15thNight
16%9%49%7%46%6%5%7%23%
720
16th to endNight
23%11%45%6%45%5%6%6%20%
1099
August1st to 15thNight
27%12%48%6%33347%18%
1191
16th to endNight
26%11%37%6%35%445%19%
1528
September1st to 15thNight
29%12%5%10%7%36%5%4412%
1754
16th to endNight
28%12%48%6%3345%320%
1795
October1st to 15thNight
22%14%49%8%25%23323%
2049
16th to endNight
14%16%5%9%8%45%5%4420%
2425
November1st to 15thNight
11%17%49%7%47%6%5%6%19%
2666
16th to endNight
11%14%47%7%35%6%8%7%23%
2976
December1st to 15thNight
15%15%5%8%7%46%46%6%18%
3159
16th to endNight
9%17%6%11%7%5%8%6%6%5%14%
3456

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
63%36%12%8%5%
798
16th to endDay
70%37%10%7%1
1152
February1st to 15thDay
66%46%13%6%3
1282
16th to endDay
63%7%6%11%9%1
1325
March1st to 15thDay
52%6%5%13%13%7%
1791
16th to endDay
64%7%8%10%8%1
2213
April1st to 15thDay
68%7%8%10%3
2286
16th to endDay
63%7%11%14%3
2538
May1st to 15thDay
65%6%10%13%5%
2829
16th to endDay
51%7%17%19%4
3124
June1st to 15thDay
69%7%11%10%
3240
16th to endDay
66%9%15%7%1
3240
July1st to 15thDay
68%10%16%4
3049
16th to endDay
69%9%16%5%
2877
August1st to 15thDay
77%6%11%4
2749
16th to endDay
75%6%8%5%2
2674
September1st to 15thDay
82%45%5%1
2179
16th to endDay
72%6%8%10%2
1832
October1st to 15thDay
73%5%8%10%2
1581
16th to endDay
73%5%7%6%5%1
1445
November1st to 15thDay
69%6%8%10%4
1189
16th to endDay
66%28%15%6%1
983
December1st to 15thDay
73%37%7%6%1
798
16th to endDay
73%5%6%10%3
768
January1st to 15thNight
64%46%10%8%5%
3137
16th to endNight
68%36%12%7%1
3072
February1st to 15thNight
64%47%11%9%2
2582
16th to endNight
62%5%8%12%9%1
2149
March1st to 15thNight
54%37%10%16%8%
2169
16th to endNight
59%5%8%13%11%1
2010
April1st to 15thNight
65%5%10%14%4
1659
16th to endNight
58%6%13%16%4
1422
May1st to 15thNight
60%7%12%14%4
1122
16th to endNight
51%6%18%18%4
991
June1st to 15thNight
64%8%14%10%1
720
16th to endNight
63%9%17%8%1
720
July1st to 15thNight
60%10%21%7%
680
16th to endNight
65%10%18%5%
1028
August1st to 15thNight
72%7%12%7%
1185
16th to endNight
71%5%12%7%2
1528
September1st to 15thNight
79%5%7%6%
1754
16th to endNight
69%411%10%3
1795
October1st to 15thNight
68%48%14%3
2049
16th to endNight
71%48%8%5%1
2425
November1st to 15thNight
69%6%8%11%4
2666
16th to endNight
63%38%14%8%1
2976
December1st to 15thNight
71%37%8%7%
3159
16th to endNight
72%47%9%5%1
3456

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
32%7%20%17%18%3
96
16th to endDay
32%8%28%16%9%4
224
February1st to 15thDay
23%12%26%18%12%6%
219
16th to endDay
28%13%27%20%8%2
211
March1st to 15thDay
14%14%30%26%12%2
240
16th to endDay
18%12%24%23%16%3
417
April1st to 15thDay
21%18%24%23%10%1
433
16th to endDay
22%14%21%28%10%2
450
May1st to 15thDay
26%11%22%26%10%1
468
16th to endDay
22%7%32%27%8%1
542
June1st to 15thDay
26%9%22%29%11%1
675
16th to endDay
26%9%17%30%13%2
675
July1st to 15thDay
26%9%18%30%12%2
645
16th to endDay
35%10%16%25%10%1
548
August1st to 15thDay
41%11%16%20%7%2
494
16th to endDay
43%9%21%18%6%
523
September1st to 15thDay
48%13%14%15%6%1
452
16th to endDay
43%13%13%13%13%2
275
October1st to 15thDay
38%12%14%19%12%2
225
16th to endDay
30%12%21%20%11%4
240
November1st to 15thDay
27%14%25%15%11%4
228
16th to endDay
21%8%26%23%15%4
113
December1st to 15thDay
36%13%26%36%13%
30
16th to endDay
31%9%21%21%12%3
32
January1st to 15thNight
24%10%22%23%14%3
777
16th to endNight
29%9%22%21%12%4
800
February1st to 15thNight
22%11%24%24%13%3
734
16th to endNight
26%10%28%21%11%1
596
March1st to 15thNight
17%8%26%27%15%3
570
16th to endNight
18%8%19%25%19%8%
586
April1st to 15thNight
24%9%17%21%22%4
539
16th to endNight
17%12%18%25%22%3
540
May1st to 15thNight
18%12%18%23%23%4
402
16th to endNight
17%5%12%28%31%4
270
June1st to 15thNight
23%8%13%31%19%3
180
16th to endNight
27%7%10%27%24%3
180
July1st to 15thNight
23%6%11%22%32%2
170
16th to endNight
33%7%9%26%19%4
376
August1st to 15thNight
44%8%14%16%13%1
403
16th to endNight
36%7%15%21%13%4
573
September1st to 15thNight
41%10%15%16%13%2
537
16th to endNight
41%6%12%17%17%4
497
October1st to 15thNight
38%6%17%19%15%2
594
16th to endNight
31%8%15%25%14%4
736
November1st to 15thNight
28%10%23%22%11%3
730
16th to endNight
24%7%22%25%15%5%
750
December1st to 15thNight
28%8%18%22%14%7%
767
16th to endNight
24%13%20%21%15%4
832
Page generated at 2017-09-15 06:29:59. Version ac04

Copyright 2017 A. Danko


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