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 2017-12-20.

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%
15150
13%
48081
8%
15120
37%
15120
16%
33740

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
25%
768
9%
1996
February1st to 15thNight
15%
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
28%
768
19%
2076
7%
768
38%
768
21%
1415
April1st to 15thNight
25%
718
16%
1796
5%
718
31%
718
21%
1310
April16th to endNight
32%
648
18%
1728
5%
648
41%
648
23%
1149
May1st to 15thNight
38%
402
16%
1482
12%
402
46%
402
15%
952
May16th to endNight
42%
374
16%
1497
12%
374
54%
374
18%
880
June1st to 15thNight
38%
360
11%
1440
11%
360
48%
360
17%
906
June16th to endNight
44%
360
11%
1440
13%
360
51%
360
18%
862
July1st to 15thNight
55%
340
16%
1360
23%
340
60%
340
26%
719
July16th to endNight
53%
381
16%
1423
18%
359
64%
359
27%
760
August1st to 15thNight
56%
539
15%
1609
12%
536
59%
536
26%
1000
August16th to endNight
58%
686
22%
1832
11%
686
64%
686
36%
1096
September1st to 15thNight
51%
720
18%
1800
10%
720
53%
720
32%
1188
September16th to endNight
54%
717
24%
1977
17%
717
58%
717
31%
1246
October1st to 15thNight
32%
720
15%
2160
12%
720
37%
720
18%
1533
October16th to endNight
21%
768
11%
2334
6%
768
28%
768
13%
1757
November1st to 15thNight
31%
760
17%
2552
14%
760
38%
760
18%
1726
November16th to endNight
19%
780
11%
2730
5%
780
27%
780
9%
2073
December1st to 15thNight
14%
780
6%
2730
2%
780
18%
780
7%
2179
December16th to endNight
11%
788
7%
2755
2%
788
17%
788
7%
2233

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%35%5%3445%7%36%
48764
Night
17%8%36%5%3445%6%34%
48252
Cloud CoverDay
15%7%36%5%3445%7%36%
48774
Night
17%9%36%5%34446%34%
48257
TransparencyDay
46%6%14%20%10%1
48453
Night
49%6%12%19%10%1
48081
SeeingDay
17%13%23%26%14%4
5736
Night
18%20%24%23%11%1
15153

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%8%44%
2340
16th to endDay
11%31332446%7%48%
2357
September1st to 15thDay
13%425%424446%47%
2160
16th to endDay
11%4245%235%5%7%48%
1979
October1st to 15thDay
18%5%346%3445%7%36%
1800
16th to endDay
24%8%35%5%3445%5%29%
1890
November1st to 15thDay
15%9%25%5%45%46%6%34%
1635
16th to endDay
20%11%38%6%345%46%26%
1560
December1st to 15thDay
21%15%6%7%6%345%45%18%
1560
16th to endDay
23%15%5%7%5%24446%20%
1568
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%426%5%3445%8%46%
1620
16th to endNight
6%3134245%6%9%52%
1832
September1st to 15thNight
15%32341435%8%46%
1800
16th to endNight
14%43333235%7%47%
1977
October1st to 15thNight
20%6%36%5%33445%35%
2160
16th to endNight
22%7%37%6%24446%29%
2334
November1st to 15thNight
18%6%35%43446%5%36%
2552
16th to endNight
23%10%36%5%345%45%26%
2730
December1st to 15thNight
22%15%5%7%5%2445%5%20%
2730
16th to endNight
23%15%6%8%5%24445%19%
2755

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%35%46%9%44%
2340
16th to endDay
10%4134245%5%8%47%
2357
September1st to 15thDay
13%5%2442445%7%46%
2160
16th to endDay
12%42442345%7%46%
1979
October1st to 15thDay
18%6%26%5%345%47%36%
1800
16th to endDay
24%8%26%6%3445%5%28%
1890
November1st to 15thDay
14%10%36%5%35%45%6%33%
1635
16th to endDay
20%10%48%6%2445%6%25%
1560
December1st to 15thDay
21%16%48%7%34446%17%
1560
16th to endDay
23%14%6%8%5%24445%19%
1568
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%
1620
16th to endNight
7%3144345%6%7%52%
1832
September1st to 15thNight
14%42441435%8%46%
1800
16th to endNight
13%424423447%48%
1977
October1st to 15thNight
20%7%36%6%34345%35%
2160
16th to endNight
22%8%37%6%34447%27%
2334
November1st to 15thNight
19%6%35%42445%6%36%
2552
16th to endNight
22%11%36%5%345%5%5%25%
2730
December1st to 15thNight
22%16%46%6%345%46%19%
2730
16th to endNight
23%14%6%8%5%24445%19%
2755

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
2327
16th to endDay
31%7%17%30%12%
2357
September1st to 15thDay
37%5%15%26%13%
2160
16th to endDay
35%7%14%30%11%
1979
October1st to 15thDay
46%6%14%21%10%
1800
16th to endDay
55%6%15%17%4
1890
November1st to 15thDay
48%8%16%17%9%
1635
16th to endDay
57%8%14%13%6%
1560
December1st to 15thDay
65%7%14%10%1
1560
16th to endDay
65%6%12%11%3
1568
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%12%
1609
16th to endNight
27%8%15%33%14%1
1832
September1st to 15thNight
35%7%15%26%13%1
1800
16th to endNight
35%6%11%29%16%
1977
October1st to 15thNight
50%6%10%21%10%
2160
16th to endNight
55%8%14%15%6%
2334
November1st to 15thNight
46%7%15%18%11%1
2552
16th to endNight
57%9%13%13%4
2730
December1st to 15thNight
62%8%14%11%2
2730
16th to endNight
65%7%12%10%3
2755

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%15%36%23%7%
313
16th to endDay
12%7%16%26%25%12%
332
September1st to 15thDay
12%6%17%25%26%11%
315
16th to endDay
13%8%17%24%21%14%
314
October1st to 15thDay
20%11%22%25%16%4
315
16th to endDay
24%17%26%22%8%
328
November1st to 15thDay
19%14%26%28%11%1
199
16th to endDay
21%15%41%13%7%1
90
December1st to 15thDay
20%25%24%21%7%1
90
16th to endDay
34%30%27%6%
72
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%10%13%35%24%5%
539
16th to endNight
7%10%17%31%29%3
686
September1st to 15thNight
13%11%18%25%26%4
720
16th to endNight
14%11%16%25%22%9%
717
October1st to 15thNight
20%16%23%21%14%2
720
16th to endNight
23%19%29%21%6%
768
November1st to 15thNight
17%20%27%21%12%1
760
16th to endNight
24%27%24%16%7%
780
December1st to 15thNight
27%27%26%15%2
780
16th to endNight
24%26%30%14%2
788
Page generated at 2017-12-21 07:04:53. Version ac04

Copyright 2017 A. Danko


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