West Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Lake clear sky chart from 2005-11-05 to 2017-08-12.

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
30%
14636
13%
46515
8%
14606
37%
14606
16%
32716

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
13%
720
7%
2506
3%
716
18%
716
6%
2004
January16th to endNight
17%
768
9%
2553
3%
768
24%
768
9%
1996
February1st to 15thNight
14%
702
8%
2287
3%
701
20%
701
9%
1786
February16th to endNight
20%
631
13%
1977
5%
631
30%
631
13%
1472
March1st to 15thNight
21%
720
14%
2037
6%
720
29%
720
14%
1498
March16th to endNight
27%
768
19%
2076
7%
768
37%
768
21%
1415
April1st to 15thNight
25%
718
16%
1796
5%
718
30%
718
21%
1310
April16th to endNight
32%
648
18%
1728
5%
648
41%
648
23%
1149
May1st to 15thNight
37%
402
16%
1482
12%
402
46%
402
15%
952
May16th to endNight
43%
374
16%
1497
12%
374
55%
374
18%
880
June1st to 15thNight
38%
360
11%
1440
10%
360
48%
360
17%
906
June16th to endNight
42%
360
11%
1440
13%
360
49%
360
18%
862
July1st to 15thNight
54%
340
16%
1360
23%
340
59%
340
26%
719
July16th to endNight
54%
381
16%
1423
17%
359
64%
359
27%
760
August1st to 15thNight
55%
527
15%
1575
12%
524
58%
524
27%
977
August16th to endNight
56%
628
20%
1678
10%
628
62%
628
35%
1015
September1st to 15thNight
51%
660
18%
1650
10%
660
53%
660
31%
1098
September16th to endNight
52%
657
22%
1812
15%
657
56%
657
30%
1164
October1st to 15thNight
32%
660
16%
1980
12%
660
37%
660
17%
1411
October16th to endNight
20%
704
9%
2139
4%
704
27%
704
11%
1644
November1st to 15thNight
30%
700
17%
2351
15%
700
37%
700
18%
1584
November16th to endNight
19%
720
10%
2520
5%
720
26%
720
8%
1927
December1st to 15thNight
14%
720
6%
2520
2%
720
18%
720
8%
2006
December16th to endNight
10%
768
7%
2688
2%
768
16%
768
7%
2181

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
15%7%36%5%3445%7%36%
47462
Night
17%8%36%5%3445%6%34%
46686
Cloud CoverDay
15%7%36%5%3445%7%36%
47472
Night
17%9%36%5%34446%33%
46691
TransparencyDay
46%6%14%20%10%1
47151
Night
49%7%12%19%10%1
46515
SeeingDay
17%13%23%26%14%4
5308
Night
18%20%24%23%11%1
14639

By Month

Percentage Cloud Cover NE (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
21%14%5%7%7%33435%23%
1432
16th to endDay
13%14%7%8%7%47%5%5%420%
1668
February1st to 15thDay
14%12%6%13%8%35%446%21%
1575
16th to endDay
19%10%38%6%3345%5%27%
1495
March1st to 15thDay
17%9%47%6%44446%28%
1909
16th to endDay
16%7%345%24445%40%
2148
April1st to 15thDay
20%7%35%434446%35%
2152
16th to endDay
17%6%35%5%24446%40%
2241
May1st to 15thDay
13%5%26%5%35%5%6%7%38%
2469
16th to endDay
12%4244345%6%9%42%
2618
June1st to 15thDay
17%5%34425%5%6%9%35%
2520
16th to endDay
12%5%1442445%9%43%
2520
July1st to 15thDay
9%313425%5%410%49%
2520
16th to endDay
9%315%325%6%7%9%42%
2648
August1st to 15thDay
8%425%435%5%7%9%44%
2288
16th to endDay
11%41332446%8%48%
2160
September1st to 15thDay
14%426%424446%46%
1980
16th to endDay
12%5%245%235%5%7%45%
1814
October1st to 15thDay
18%5%346%3445%7%36%
1650
16th to endDay
25%9%35%5%33445%28%
1733
November1st to 15thDay
14%9%25%5%35%46%6%35%
1506
16th to endDay
20%12%38%6%245%46%25%
1440
December1st to 15thDay
22%15%6%7%6%245%45%18%
1440
16th to endDay
23%15%5%8%5%23446%20%
1536
January1st to 15thNight
22%15%5%9%6%34335%20%
2506
16th to endNight
16%15%5%9%6%35%5%35%22%
2553
February1st to 15thNight
16%16%6%9%7%2434423%
2285
16th to endNight
17%10%47%6%335%6%6%27%
1977
March1st to 15thNight
19%10%46%6%3433432%
2025
16th to endNight
15%7%45%424435%41%
2076
April1st to 15thNight
21%7%2342445%6%35%
1796
16th to endNight
16%5%24433446%44%
1728
May1st to 15thNight
18%6%25%42435%6%38%
1482
16th to endNight
11%4345%25%6%5%7%42%
1495
June1st to 15thNight
16%5%36%4345%6%8%37%
1440
16th to endNight
10%435%6%25%5%5%9%40%
1440
July1st to 15thNight
6%324435%45%7%51%
1440
16th to endNight
9%3233335%6%10%48%
1519
August1st to 15thNight
8%425%5%3445%7%46%
1586
16th to endNight
7%3134245%6%8%51%
1678
September1st to 15thNight
15%32341435%8%45%
1650
16th to endNight
14%43343345%7%46%
1812
October1st to 15thNight
21%6%36%5%33445%35%
1980
16th to endNight
23%8%38%6%24445%27%
2139
November1st to 15thNight
17%6%35%5%3446%5%36%
2351
16th to endNight
23%11%46%5%345%45%25%
2520
December1st to 15thNight
23%15%5%7%5%2445%5%21%
2520
16th to endNight
23%15%6%8%5%24445%19%
2688

Percentage Cloud Cover SE (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
20%16%5%7%7%34345%22%
1432
16th to endDay
13%15%6%8%7%47%6%3420%
1668
February1st to 15thDay
14%13%5%14%8%34446%20%
1575
16th to endDay
19%11%48%7%34445%26%
1495
March1st to 15thDay
17%10%38%7%44446%27%
1920
16th to endDay
16%7%35%5%2444440%
2147
April1st to 15thDay
20%7%36%425%35%6%36%
2152
16th to endDay
17%7%35%5%24445%40%
2241
May1st to 15thDay
13%5%36%5%345%6%7%38%
2469
16th to endDay
11%5%245%345%5%8%42%
2618
June1st to 15thDay
17%6%245%2446%8%35%
2520
16th to endDay
12%5%2442446%8%43%
2520
July1st to 15thDay
9%32443445%9%48%
2520
16th to endDay
9%424435%5%6%10%42%
2648
August1st to 15thDay
8%5%245%3446%9%44%
2288
16th to endDay
11%4134245%5%8%47%
2160
September1st to 15thDay
14%5%25%5%24446%45%
1980
16th to endDay
13%425%42345%8%44%
1814
October1st to 15thDay
17%6%25%5%345%47%36%
1650
16th to endDay
25%9%25%6%3445%5%27%
1733
November1st to 15thDay
13%9%36%5%35%46%7%34%
1506
16th to endDay
20%11%48%6%2445%6%25%
1440
December1st to 15thDay
22%16%48%7%34446%18%
1440
16th to endDay
22%14%6%8%6%24445%19%
1536
January1st to 15thNight
22%15%5%10%6%24335%20%
2506
16th to endNight
16%14%5%9%7%35%445%22%
2553
February1st to 15thNight
16%15%6%10%7%24445%22%
2278
16th to endNight
17%10%5%6%7%345%5%6%26%
1977
March1st to 15thNight
20%10%37%5%34335%32%
2037
16th to endNight
17%6%46%423345%41%
2076
April1st to 15thNight
21%7%3443435%6%35%
1796
16th to endNight
17%424434345%44%
1728
May1st to 15thNight
18%5%24435%446%39%
1482
16th to endNight
11%5%236%35%5%5%7%43%
1495
June1st to 15thNight
17%5%25%5%3446%8%36%
1440
16th to endNight
11%5%36%435%5%5%9%40%
1440
July1st to 15thNight
5%3245%35%45%7%50%
1440
16th to endNight
9%4233345%5%10%48%
1519
August1st to 15thNight
8%5%25%5%335%6%8%45%
1586
16th to endNight
7%314435%5%6%7%51%
1678
September1st to 15thNight
15%42441435%8%45%
1650
16th to endNight
14%424423447%46%
1812
October1st to 15thNight
20%6%36%6%3435%5%35%
1980
16th to endNight
22%9%47%6%34447%26%
2139
November1st to 15thNight
18%6%35%435%45%6%36%
2351
16th to endNight
23%11%36%5%345%5%5%24%
2520
December1st to 15thNight
23%16%46%5%245%46%20%
2520
16th to endNight
23%14%6%8%5%24345%19%
2688

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
62%5%11%15%3
1433
16th to endDay
64%6%10%11%6%
1668
February1st to 15thDay
64%6%11%12%3
1577
16th to endDay
57%5%12%14%8%
1495
March1st to 15thDay
56%5%14%16%5%1
1920
16th to endDay
47%48%21%14%2
2148
April1st to 15thDay
49%5%10%20%11%2
2152
16th to endDay
45%410%20%15%2
2241
May1st to 15thDay
43%7%12%20%13%2
2469
16th to endDay
37%8%15%23%12%1
2622
June1st to 15thDay
45%7%14%21%10%
2520
16th to endDay
37%6%17%22%14%1
2520
July1st to 15thDay
31%7%14%30%15%
2371
16th to endDay
36%8%19%25%9%
2481
August1st to 15thDay
35%8%19%23%12%1
2275
16th to endDay
32%7%17%30%11%
2160
September1st to 15thDay
38%6%15%27%12%
1980
16th to endDay
37%7%14%29%10%
1814
October1st to 15thDay
45%6%15%20%11%
1650
16th to endDay
56%6%14%16%4
1733
November1st to 15thDay
46%8%16%17%9%
1506
16th to endDay
58%8%13%13%6%
1440
December1st to 15thDay
66%8%14%9%2
1440
16th to endDay
66%6%11%12%3
1536
January1st to 15thNight
65%6%11%11%3
2506
16th to endNight
61%7%10%13%6%
2553
February1st to 15thNight
63%5%12%15%3
2287
16th to endNight
55%6%13%17%7%
1977
March1st to 15thNight
55%411%19%8%
2037
16th to endNight
44%411%19%16%2
2076
April1st to 15thNight
48%5%9%19%14%2
1796
16th to endNight
40%6%12%19%19%2
1728
May1st to 15thNight
45%5%10%19%15%3
1482
16th to endNight
37%7%13%22%16%2
1497
June1st to 15thNight
43%6%14%21%13%
1440
16th to endNight
40%6%15%19%16%1
1440
July1st to 15thNight
30%6%11%27%22%1
1360
16th to endNight
29%7%13%30%15%2
1423
August1st to 15thNight
33%9%16%26%13%
1575
16th to endNight
28%8%14%32%14%1
1678
September1st to 15thNight
36%7%15%27%12%
1650
16th to endNight
37%6%11%30%14%
1812
October1st to 15thNight
51%5%11%20%10%
1980
16th to endNight
57%8%13%13%6%
2139
November1st to 15thNight
45%7%15%18%11%1
2351
16th to endNight
58%9%13%13%4
2520
December1st to 15thNight
62%9%14%11%2
2520
16th to endNight
65%8%12%10%4
2688

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
16%30%31%11%6%3
60
16th to endDay
24%18%32%20%3
86
February1st to 15thDay
22%22%35%18%1
90
16th to endDay
31%15%27%20%3
154
March1st to 15thDay
22%20%26%24%5%
270
16th to endDay
25%15%28%20%8%1
288
April1st to 15thDay
20%19%32%17%8%1
270
16th to endDay
15%15%31%21%10%4
270
May1st to 15thDay
20%14%21%29%9%5%
288
16th to endDay
13%11%25%31%12%4
331
June1st to 15thDay
19%12%20%32%13%2
315
16th to endDay
15%11%24%27%16%3
315
July1st to 15thDay
9%8%15%35%24%6%
305
16th to endDay
11%12%21%30%19%4
326
August1st to 15thDay
10%7%14%35%22%8%
297
16th to endDay
13%7%14%26%26%11%
268
September1st to 15thDay
12%8%16%25%26%10%
255
16th to endDay
16%10%18%27%17%9%
254
October1st to 15thDay
18%12%22%24%17%3
255
16th to endDay
27%18%25%22%5%
267
November1st to 15thDay
15%16%26%28%12%1
160
16th to endDay
25%11%33%16%11%1
60
December1st to 15thDay
20%26%23%18%11%
60
16th to endDay
35%31%26%6%
64
January1st to 15thNight
23%31%24%15%5%
720
16th to endNight
20%24%30%19%4
768
February1st to 15thNight
22%25%31%17%3
704
16th to endNight
19%22%30%21%5%
631
March1st to 15thNight
23%24%24%24%3
720
16th to endNight
18%21%27%23%7%1
768
April1st to 15thNight
20%22%28%22%6%
718
16th to endNight
15%21%24%23%13%
648
May1st to 15thNight
20%19%16%26%15%1
402
16th to endNight
10%18%22%34%12%1
375
June1st to 15thNight
14%16%22%30%14%1
360
16th to endNight
12%18%17%35%15%1
360
July1st to 15thNight
5%13%17%35%24%3
340
16th to endNight
7%17%16%34%22%2
381
August1st to 15thNight
10%11%13%34%24%5%
527
16th to endNight
7%10%17%30%28%3
628
September1st to 15thNight
13%12%18%25%26%4
660
16th to endNight
15%12%16%26%22%6%
657
October1st to 15thNight
21%17%23%21%15%1
660
16th to endNight
23%20%28%21%5%
704
November1st to 15thNight
16%21%27%21%12%1
700
16th to endNight
24%28%23%16%6%
720
December1st to 15thNight
27%28%25%15%2
720
16th to endNight
24%26%30%14%2
768
Page generated at 2017-08-13 14:16:47. Version ac04

Copyright 2017 A. Danko


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