Langley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Langley clear sky chart from 2005-11-06 to 2024-02-05.

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
35%
22633
9%
80659
7%
22505
38%
22505
12%
61659

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
15%
844
5%
4715
5%
816
19%
816
3%
4091
January11th to 20thNight
26%
837
15%
4577
5%
812
29%
812
6%
3160
January21st to endNight
23%
836
5%
4848
7%
814
26%
814
4%
4286
February1st to 10thNight
18%
738
8%
2397
3%
738
24%
738
8%
1988
February11th to 20thNight
23%
700
8%
2274
5%
700
28%
700
9%
1855
February21st to endNight
25%
592
12%
1897
6%
592
32%
592
11%
1481
March1st to 10thNight
28%
720
7%
2113
6%
720
30%
720
11%
1701
March11th to 20thNight
25%
720
8%
1980
5%
720
27%
720
11%
1638
March21st to endNight
27%
792
4%
2111
1%
792
28%
792
9%
1811
April1st to 10thNight
30%
720
6%
1800
3%
720
31%
720
13%
1497
April11th to 20thNight
29%
637
10%
1705
5%
637
32%
637
14%
1350
April21st to endNight
32%
478
6%
1558
4%
478
34%
478
11%
1229
May1st to 10thNight
47%
360
10%
1440
14%
360
49%
360
16%
969
May11th to 20thNight
57%
354
9%
1416
17%
354
58%
354
15%
956
May21st to endNight
55%
388
5%
1530
8%
388
56%
388
16%
1050
June1st to 10thNight
46%
360
3%
1260
2%
360
47%
360
15%
924
June11th to 20thNight
43%
360
2%
1260
1%
360
43%
360
12%
966
June21st to endNight
59%
360
6%
1260
3%
360
60%
360
22%
807
July1st to 10thNight
65%
350
7%
1225
7%
350
65%
350
27%
732
July11th to 20thNight
70%
348
14%
1218
19%
348
71%
348
33%
662
July21st to endNight
80%
394
18%
1463
29%
372
82%
372
41%
665
August1st to 10thNight
75%
362
17%
1428
25%
360
75%
360
35%
694
August11th to 20thNight
79%
534
22%
1602
22%
534
79%
534
45%
826
August21st to endNight
63%
663
18%
1851
15%
663
64%
663
31%
1162
September1st to 10thNight
67%
720
17%
1800
10%
720
68%
720
36%
1161
September11th to 20thNight
53%
720
17%
1800
10%
720
54%
720
27%
1269
September21st to endNight
46%
720
15%
2058
12%
720
48%
720
21%
1469
October1st to 10thNight
45%
720
9%
2160
9%
720
46%
720
19%
1540
October11th to 20thNight
35%
720
9%
2160
8%
720
37%
720
15%
1653
October21st to endNight
29%
792
10%
2513
7%
792
33%
792
12%
1954
November1st to 10thNight
21%
740
5%
2590
4%
740
24%
740
7%
2158
November11th to 20thNight
21%
760
7%
2660
4%
760
24%
760
7%
2181
November21st to endNight
18%
760
5%
2660
2%
760
22%
760
7%
2196
December1st to 10thNight
27%
760
11%
2660
7%
760
32%
760
10%
2081
December11th to 20thNight
13%
841
3%
3496
1%
840
16%
840
4%
3072
December21st to endNight
12%
933
4%
5174
2%
905
17%
905
3%
4425

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
26%8%36%5%34446%31%
77826
Night
31%9%36%5%34445%29%
80973
TransparencyDay
57%8%13%14%7%1
77537
Night
59%7%12%13%7%2
80842
SeeingDay
27%10%21%25%14%3
14207
Night
33%7%17%23%17%2
22650

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
43%13%36%5%3444411%
2219
11th to 20thDay
30%13%45%5%23235%26%
2229
21st to endDay
56%11%35%42323310%
2805
February1st to 10thDay
36%11%47%6%35%336%17%
1657
11th to 20thDay
33%9%46%5%45%436%22%
1575
21st to endDay
26%14%48%7%43445%22%
1358
March1st to 10thDay
31%12%47%5%35%346%20%
1844
11th to 20thDay
34%12%46%5%35%335%20%
1980
21st to endDay
31%10%38%6%35%5%45%20%
2244
April1st to 10thDay
30%10%36%6%445%5%6%21%
2160
11th to 20thDay
24%9%48%7%4445%6%25%
2201
21st to endDay
24%10%47%6%35%5%5%6%26%
2402
May1st to 10thDay
19%8%36%6%45%5%46%34%
2520
11th to 20thDay
18%8%36%5%45%5%5%9%32%
2477
21st to endDay
21%9%47%6%34446%33%
2742
June1st to 10thDay
23%7%36%6%3445%6%31%
2700
11th to 20thDay
24%7%38%6%35%5%6%7%26%
2700
21st to endDay
15%6%36%5%445%5%7%39%
2700
July1st to 10thDay
14%42442445%6%50%
2700
11th to 20thDay
11%5%2432445%6%54%
2700
21st to endDay
9%22332335%5%64%
2765
August1st to 10thDay
11%5%24323345%57%
2501
11th to 20thDay
10%4243235%5%6%57%
2298
21st to endDay
15%6%245%34447%47%
2504
September1st to 10thDay
16%7%35%433346%47%
2160
11th to 20thDay
19%7%36%5%33346%40%
2160
21st to endDay
25%7%35%424346%36%
1897
October1st to 10thDay
26%7%36%434347%34%
1800
11th to 20thDay
31%9%47%5%2445%6%23%
1800
21st to endDay
37%8%35%5%3435%5%23%
1838
November1st to 10thDay
41%8%36%6%3445%5%17%
1488
11th to 20thDay
37%11%45%43443422%
1520
21st to endDay
39%11%46%42345%417%
1520
December1st to 10thDay
35%11%45%43343424%
1520
11th to 20thDay
37%14%46%6%35%34413%
1700
21st to endDay
40%13%48%6%2433412%
2442
January1st to 10thNight
42%13%37%6%3433314%
4726
11th to 20thNight
34%8%2432345%8%26%
4667
21st to endNight
53%11%36%42222313%
4848
February1st to 10thNight
38%10%35%433345%21%
2397
11th to 20thNight
32%12%46%5%3334425%
2272
21st to endNight
29%12%45%434446%26%
1897
March1st to 10thNight
31%11%35%5%35%445%24%
2113
11th to 20thNight
36%8%36%5%33445%22%
1979
21st to endNight
35%10%37%6%24435%19%
2111
April1st to 10thNight
31%10%36%6%3445%5%23%
1800
11th to 20thNight
24%9%48%6%445%5%5%28%
1705
21st to endNight
27%9%46%6%35%346%25%
1558
May1st to 10thNight
19%7%35%5%3445%6%40%
1440
11th to 20thNight
16%6%35%5%27%6%7%7%36%
1416
21st to endNight
18%7%26%5%45%46%7%36%
1530
June1st to 10thNight
23%7%36%6%345%5%7%33%
1260
11th to 20thNight
19%8%48%6%36%46%7%28%
1260
21st to endNight
15%5%36%5%35%46%8%43%
1260
July1st to 10thNight
11%425%6%23346%54%
1260
11th to 20thNight
9%324424346%60%
1260
21st to endNight
5%21232345%468%
1549
August1st to 10thNight
9%314322445%63%
1436
11th to 20thNight
7%313223348%63%
1602
21st to endNight
10%425%5%3347%8%47%
1851
September1st to 10thNight
11%3143345%5%6%56%
1800
11th to 20thNight
20%6%25%5%24346%43%
1800
21st to endNight
22%5%36%5%33446%39%
2058
October1st to 10thNight
22%7%26%6%3443439%
2160
11th to 20thNight
31%7%35%5%24446%30%
2160
21st to endNight
35%6%35%434345%29%
2513
November1st to 10thNight
41%9%25%6%34335%18%
2590
11th to 20thNight
38%9%35%5%2443424%
2660
21st to endNight
38%8%47%6%4433320%
2660
December1st to 10thNight
33%8%35%5%34445%25%
2660
11th to 20thNight
44%13%47%5%2333313%
3501
21st to endNight
41%13%47%5%2333316%
5214

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
77%8%7%6%21
2219
11th to 20thDay
62%7%7%13%7%4
2250
21st to endDay
81%46%6%2
2805
February1st to 10thDay
72%6%8%8%42
1657
11th to 20thDay
67%7%9%10%7%1
1584
21st to endDay
65%8%9%10%5%2
1358
March1st to 10thDay
67%8%11%9%41
1844
11th to 20thDay
71%6%7%10%41
1980
21st to endDay
70%7%10%11%2
2244
April1st to 10thDay
67%9%13%8%2
2160
11th to 20thDay
64%8%10%10%7%1
2201
21st to endDay
62%9%12%10%7%1
2402
May1st to 10thDay
54%9%15%15%7%1
2520
11th to 20thDay
52%11%16%16%6%
2480
21st to endDay
57%9%15%16%3
2743
June1st to 10thDay
57%8%17%12%5%2
2700
11th to 20thDay
62%10%16%9%3
2700
21st to endDay
49%8%17%16%8%2
2700
July1st to 10thDay
37%9%16%23%12%3
2626
11th to 20thDay
34%7%20%28%9%2
2619
21st to endDay
23%8%19%32%14%3
2611
August1st to 10thDay
31%10%17%28%11%2
2484
11th to 20thDay
29%9%20%26%14%2
2298
21st to endDay
39%9%19%25%6%1
2504
September1st to 10thDay
41%8%15%22%11%4
2160
11th to 20thDay
48%8%11%17%14%2
2160
21st to endDay
50%9%15%15%10%1
1897
October1st to 10thDay
51%8%15%16%10%
1800
11th to 20thDay
61%8%12%14%41
1800
21st to endDay
63%7%11%10%8%1
1838
November1st to 10thDay
71%7%10%8%31
1488
11th to 20thDay
69%6%8%8%6%2
1520
21st to endDay
69%6%8%9%7%1
1520
December1st to 10thDay
65%8%6%11%9%1
1520
11th to 20thDay
72%7%11%6%32
1703
21st to endDay
78%8%6%6%21
2442
January1st to 10thNight
76%7%7%7%21
4726
11th to 20thNight
56%7%10%15%9%4
4708
21st to endNight
80%45%8%31
4848
February1st to 10thNight
68%7%10%9%42
2397
11th to 20thNight
66%7%8%10%7%2
2274
21st to endNight
61%7%10%14%6%2
1897
March1st to 10thNight
62%8%13%12%5%
2113
11th to 20thNight
66%6%12%9%6%1
1980
21st to endNight
69%8%13%8%2
2111
April1st to 10thNight
63%8%14%12%2
1800
11th to 20thNight
59%8%11%13%7%2
1705
21st to endNight
61%9%13%10%7%1
1558
May1st to 10thNight
47%9%19%17%7%1
1440
11th to 20thNight
45%12%20%16%7%
1416
21st to endNight
48%9%20%17%5%
1530
June1st to 10thNight
54%8%18%13%5%2
1260
11th to 20thNight
56%10%18%12%3
1260
21st to endNight
41%9%21%18%8%3
1260
July1st to 10thNight
36%7%14%25%14%4
1225
11th to 20thNight
29%7%20%29%13%1
1218
21st to endNight
19%8%14%34%20%5%
1463
August1st to 10thNight
25%6%19%29%18%3
1428
11th to 20thNight
20%8%24%29%14%6%
1602
21st to endNight
34%9%19%27%10%1
1851
September1st to 10thNight
32%10%16%24%14%4
1800
11th to 20thNight
45%6%13%16%16%4
1800
21st to endNight
48%8%15%19%9%2
2058
October1st to 10thNight
49%8%14%18%9%1
2160
11th to 20thNight
57%7%15%14%6%1
2160
21st to endNight
60%7%10%14%8%
2513
November1st to 10thNight
71%6%10%9%31
2590
11th to 20thNight
68%7%8%9%6%2
2660
21st to endNight
69%7%9%7%7%1
2660
December1st to 10thNight
63%6%10%11%8%1
2660
11th to 20thNight
77%6%7%5%31
3497
21st to endNight
75%6%7%8%31
5214

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
40%17%19%12%8%4
180
11th to 20thDay
47%13%14%16%8%2
180
21st to endDay
45%14%16%13%11%1
209
February1st to 10thDay
44%12%19%17%7%1
353
11th to 20thDay
38%14%21%19%8%1
335
21st to endDay
34%16%26%15%8%1
292
March1st to 10thDay
34%15%23%20%7%1
420
11th to 20thDay
37%12%18%19%12%1
420
21st to endDay
39%14%18%20%8%1
462
April1st to 10thDay
34%13%23%19%10%1
420
11th to 20thDay
28%13%25%21%10%3
412
21st to endDay
26%14%28%19%12%1
420
May1st to 10thDay
21%10%25%31%11%2
423
11th to 20thDay
23%7%21%29%15%5%
448
21st to endDay
22%12%27%24%12%4
504
June1st to 10thDay
23%11%26%27%11%2
630
11th to 20thDay
24%12%27%27%8%1
630
21st to endDay
19%9%23%29%18%3
630
July1st to 10thDay
15%6%26%32%17%3
621
11th to 20thDay
10%8%24%38%16%4
618
21st to endDay
9%5%19%37%23%7%
511
August1st to 10thDay
14%6%17%33%21%9%
447
11th to 20thDay
9%317%38%25%8%
437
21st to endDay
14%10%23%30%17%6%
495
September1st to 10thDay
19%8%24%30%15%4
450
11th to 20thDay
23%9%18%28%20%3
450
21st to endDay
24%10%20%23%17%5%
450
October1st to 10thDay
30%7%19%21%18%5%
450
11th to 20thDay
38%8%16%18%16%4
450
21st to endDay
39%12%19%16%11%3
362
November1st to 10thDay
39%16%16%18%11%
180
11th to 20thDay
47%8%14%11%14%6%
180
21st to endDay
46%8%22%7%9%8%
180
December1st to 10thDay
44%14%13%12%11%5%
180
11th to 20thDay
51%10%16%15%7%1
180
21st to endDay
49%10%17%19%5%1
198
January1st to 10thNight
49%13%16%14%7%1
850
11th to 20thNight
46%9%15%22%8%
841
21st to endNight
43%11%16%21%9%
836
February1st to 10thNight
43%11%22%18%6%
738
11th to 20thNight
38%10%21%23%8%
704
21st to endNight
35%10%23%22%9%1
592
March1st to 10thNight
35%10%21%20%13%1
720
11th to 20thNight
41%9%19%22%9%1
720
21st to endNight
41%8%18%22%10%2
792
April1st to 10thNight
35%8%19%24%12%2
720
11th to 20thNight
28%9%25%25%12%2
637
21st to endNight
33%6%21%27%13%1
478
May1st to 10thNight
20%7%17%31%24%2
360
11th to 20thNight
20%49%33%31%4
354
21st to endNight
21%412%33%27%2
389
June1st to 10thNight
27%414%29%24%2
360
11th to 20thNight
22%7%19%29%20%3
360
21st to endNight
15%415%29%33%3
360
July1st to 10thNight
13%311%31%39%4
350
11th to 20thNight
11%49%32%37%6%
348
21st to endNight
5%19%27%50%8%
394
August1st to 10thNight
13%16%22%47%10%
362
11th to 20thNight
7%26%28%46%10%
534
21st to endNight
13%312%29%38%4
663
September1st to 10thNight
14%211%38%31%4
720
11th to 20thNight
22%414%22%33%6%
720
21st to endNight
27%5%14%29%24%2
720
October1st to 10thNight
26%5%15%27%24%3
720
11th to 20thNight
37%6%15%23%16%2
720
21st to endNight
38%8%20%23%9%2
792
November1st to 10thNight
46%9%20%22%4
740
11th to 20thNight
45%8%22%16%9%1
760
21st to endNight
43%11%18%16%10%1
760
December1st to 10thNight
37%8%21%23%10%
760
11th to 20thNight
51%11%20%14%3
841
21st to endNight
49%12%22%14%3
935
Page generated at 2024-02-02 02:20:33. Version ac07

Copyright 2024 Attilla Danko


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