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 2018-03-21.

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
34%
14048
7%
49220
6%
14016
36%
14016
10%
39120

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
32%
864
9%
3399
8%
861
36%
861
9%
2631
January16th to endNight
30%
880
11%
3328
9%
880
32%
880
10%
2617
February1st to 15thNight
36%
809
9%
2801
6%
805
38%
805
12%
2165
February16th to endNight
27%
656
10%
2326
6%
656
32%
656
11%
1773
March1st to 15thNight
38%
630
13%
2350
11%
630
43%
630
15%
1648
March16th to endNight
43%
608
11%
2073
12%
608
45%
608
13%
1564
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%
540
2%
1764
2%
540
22%
540
5%
1579
September16th to endNight
30%
545
6%
1959
6%
545
32%
545
8%
1587
October1st to 15thNight
30%
660
7%
2235
7%
660
31%
660
10%
1821
October16th to endNight
33%
816
8%
2646
6%
816
34%
816
9%
2206
November1st to 15thNight
31%
805
8%
2894
6%
805
33%
805
9%
2319
November16th to endNight
38%
825
12%
3224
11%
825
43%
825
11%
2441
December1st to 15thNight
31%
850
6%
3422
6%
850
33%
850
7%
2821
December16th to endNight
31%
928
7%
3744
6%
928
33%
928
7%
3039

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%
49699
Night
13%14%5%8%7%45%5%6%6%22%
49323
TransparencyDay
67%6%10%9%3
49303
Night
65%5%9%11%6%1
49225
SeeingDay
29%11%21%23%11%2
8890
Night
27%9%19%23%16%4
14051

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%8%7%45%46%9%23%
866
16th to endDay
8%15%411%8%6%6%6%7%7%17%
1248
February1st to 15thDay
8%16%49%8%45%45%6%25%
1393
16th to endDay
6%14%7%12%9%45%5%6%6%22%
1434
March1st to 15thDay
7%14%5%7%7%46%5%5%6%30%
1940
16th to endDay
6%15%5%10%9%6%7%6%6%6%21%
2272
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%
2194
16th to endDay
29%15%47%6%3443317%
1998
October1st to 15thDay
24%14%39%8%35%43318%
1725
16th to endDay
15%15%5%10%8%45%5%6%6%15%
1576
November1st to 15thDay
8%17%7%10%5%46%5%6%7%21%
1290
16th to endDay
10%15%38%7%46%5%7%9%20%
1065
December1st to 15thDay
18%15%5%8%7%35%5%5%5%18%
864
16th to endDay
7%16%7%8%11%47%6%8%7%13%
832
January1st to 15thNight
8%14%5%10%7%45%5%6%6%24%
3399
16th to endNight
9%15%6%9%8%46%6%7%7%19%
3328
February1st to 15thNight
7%14%5%9%8%45%6%5%6%26%
2789
16th to endNight
9%15%6%8%7%46%6%6%8%20%
2326
March1st to 15thNight
7%13%47%7%45%5%5%7%31%
2350
16th to endNight
6%15%5%8%6%45%5%5%7%28%
2073
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
28%12%410%7%36%5%4412%
1764
16th to endNight
28%12%48%6%3345%419%
1959
October1st to 15thNight
22%13%49%8%2423323%
2235
16th to endNight
14%16%5%9%8%45%5%45%20%
2646
November1st to 15thNight
11%16%49%6%46%5%5%7%21%
2894
16th to endNight
10%14%48%7%35%6%8%7%23%
3224
December1st to 15thNight
15%16%6%8%7%45%46%6%18%
3422
16th to endNight
8%16%6%10%7%48%6%6%6%16%
3744

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%46%12%8%5%
866
16th to endDay
69%37%10%7%1
1248
February1st to 15thDay
64%45%13%7%5%
1393
16th to endDay
61%7%6%12%10%1
1434
March1st to 15thDay
54%6%5%13%13%7%
1940
16th to endDay
64%6%7%10%8%1
2272
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
2194
16th to endDay
73%5%8%9%2
1998
October1st to 15thDay
72%5%8%10%2
1725
16th to endDay
73%47%7%5%1
1576
November1st to 15thDay
66%6%7%12%5%1
1290
16th to endDay
65%27%15%7%1
1065
December1st to 15thDay
74%37%7%5%1
864
16th to endDay
71%5%7%10%4
832
January1st to 15thNight
64%45%10%8%5%
3399
16th to endNight
67%35%12%8%2
3328
February1st to 15thNight
63%46%10%9%5%
2801
16th to endNight
61%5%8%12%10%1
2326
March1st to 15thNight
54%36%10%16%7%
2350
16th to endNight
59%5%8%13%11%1
2078
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%47%6%
1764
16th to endNight
69%312%10%3
1959
October1st to 15thNight
67%5%8%14%3
2235
16th to endNight
71%48%8%6%1
2646
November1st to 15thNight
66%5%8%12%6%
2894
16th to endNight
63%37%13%9%1
3224
December1st to 15thNight
72%37%7%7%
3422
16th to endNight
70%47%10%5%
3744

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%20%19%16%3
117
16th to endDay
29%8%26%18%12%4
256
February1st to 15thDay
21%10%27%19%14%6%
256
16th to endDay
26%11%30%22%7%2
250
March1st to 15thDay
16%13%29%26%11%2
285
16th to endDay
18%13%24%22%16%3
437
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
457
16th to endDay
43%13%15%12%12%2
324
October1st to 15thDay
38%12%15%20%11%2
270
16th to endDay
30%11%20%20%12%4
288
November1st to 15thDay
23%12%26%19%12%5%
269
16th to endDay
18%8%24%29%15%3
135
December1st to 15thDay
17%8%20%20%24%8%
45
16th to endDay
25%10%25%27%10%2
48
January1st to 15thNight
24%12%22%24%13%3
865
16th to endNight
27%9%22%22%13%4
880
February1st to 15thNight
22%10%24%25%13%4
809
16th to endNight
24%10%29%22%10%1
656
March1st to 15thNight
17%9%26%28%15%3
630
16th to endNight
19%8%19%25%19%7%
609
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
540
16th to endNight
40%7%13%17%16%3
545
October1st to 15thNight
38%5%17%20%15%2
660
16th to endNight
30%8%15%25%14%4
816
November1st to 15thNight
26%9%23%25%11%2
805
16th to endNight
22%7%22%26%15%5%
825
December1st to 15thNight
29%10%18%20%14%6%
850
16th to endNight
23%12%20%23%16%3
928
Page generated at 2018-03-22 07:34:24. Version ac04

Copyright 2018 A. Danko


Girdwood Clear Sky Chart |Charts Home