West Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Lake clear sky chart from 2005-11-06 to 2024-01-22.

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%
22783
13%
74918
9%
22743
40%
22743
17%
52809

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
16%
760
9%
2649
4%
757
23%
757
8%
2080
January11th to 20thNight
17%
760
7%
2656
3%
760
21%
760
7%
2165
January21st to endNight
21%
800
11%
2599
5%
800
28%
800
13%
1987
February1st to 10thNight
15%
720
7%
2340
2%
720
21%
720
9%
1857
February11th to 20thNight
18%
701
12%
2275
4%
700
28%
700
13%
1679
February21st to endNight
22%
592
13%
1813
5%
592
32%
592
16%
1341
March1st to 10thNight
26%
720
16%
2066
8%
720
34%
720
18%
1497
March11th to 20thNight
27%
720
15%
1979
7%
720
36%
720
19%
1424
March21st to endNight
30%
792
20%
2124
8%
792
38%
792
25%
1465
April1st to 10thNight
28%
720
17%
1800
5%
720
34%
720
23%
1315
April11th to 20thNight
31%
711
16%
1775
6%
711
36%
711
24%
1282
April21st to endNight
36%
612
16%
1692
8%
612
41%
612
22%
1181
May1st to 10thNight
42%
424
15%
1504
14%
424
47%
424
19%
982
May11th to 20thNight
50%
354
20%
1416
18%
354
61%
354
24%
825
May21st to endNight
54%
388
15%
1551
17%
388
60%
388
24%
889
June1st to 10thNight
49%
360
12%
1440
14%
360
56%
360
20%
902
June11th to 20thNight
53%
360
16%
1440
21%
360
60%
360
24%
825
June21st to endNight
49%
360
13%
1440
17%
360
54%
360
20%
880
July1st to 10thNight
63%
350
16%
1400
24%
350
67%
350
29%
743
July11th to 20thNight
59%
348
17%
1392
24%
348
63%
348
26%
777
July21st to endNight
62%
400
18%
1492
20%
378
66%
378
28%
815
August1st to 10thNight
63%
537
16%
1600
14%
534
64%
534
31%
979
August11th to 20thNight
62%
534
18%
1598
13%
534
65%
534
30%
1004
August21st to endNight
57%
761
19%
1949
9%
761
61%
761
35%
1240
September1st to 10thNight
53%
720
16%
1800
8%
720
56%
720
34%
1189
September11th to 20thNight
58%
720
26%
1800
16%
720
61%
720
39%
1139
September21st to endNight
51%
720
21%
2065
16%
720
55%
720
26%
1362
October1st to 10thNight
37%
720
15%
2160
11%
720
41%
720
19%
1518
October11th to 20thNight
30%
720
12%
2160
9%
720
35%
720
17%
1567
October21st to endNight
23%
792
13%
2420
7%
792
31%
792
13%
1840
November1st to 10thNight
35%
740
16%
2429
13%
740
41%
740
20%
1627
November11th to 20thNight
23%
760
15%
2660
9%
760
32%
760
15%
1846
November21st to endNight
23%
760
9%
2660
5%
760
29%
760
10%
2025
December1st to 10thNight
17%
760
7%
2660
4%
760
21%
760
8%
2142
December11th to 20thNight
16%
759
8%
3258
3%
759
22%
759
8%
2475
December21st to endNight
12%
828
4%
4856
1%
817
18%
817
4%
3945

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%8%36%5%35%5%5%7%37%
74780
Night
17%9%47%5%345%5%7%33%
75149
TransparencyDay
46%8%14%20%11%1
74486
Night
49%8%13%19%10%1
74977
SeeingDay
18%9%26%28%15%4
13414
Night
20%10%29%27%12%2
22795

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 10thDay
17%16%7%8%7%3445%6%23%
1517
11th to 20thDay
18%17%6%9%5%45%45%5%21%
1520
21st to endDay
13%13%7%10%8%47%5%5%6%22%
1791
February1st to 10thDay
15%15%6%13%7%35%5%5%5%20%
1620
11th to 20thDay
16%11%5%11%8%44445%28%
1573
21st to endDay
21%12%47%6%345%5%6%27%
1442
March1st to 10thDay
16%10%48%6%45%5%5%7%30%
1887
11th to 20thDay
16%10%47%7%45%445%34%
1980
21st to endDay
19%9%45%5%34445%38%
2232
April1st to 10thDay
18%8%46%6%3445%6%35%
2160
11th to 20thDay
19%7%36%434446%41%
2134
21st to endDay
20%8%36%5%35%5%47%34%
2282
May1st to 10thDay
15%6%37%6%35%5%6%7%37%
2454
11th to 20thDay
14%5%25%4345%6%9%43%
2478
21st to endDay
12%425%5%35%5%6%9%43%
2725
June1st to 10thDay
17%5%35%5%35%5%6%8%38%
2520
11th to 20thDay
13%5%25%5%25%5%6%10%43%
2520
21st to endDay
14%6%25%43445%8%44%
2520
July1st to 10thDay
8%423435%5%5%10%50%
2520
11th to 20thDay
11%415%5%346%6%9%46%
2520
21st to endDay
10%425%435%6%7%9%45%
2735
August1st to 10thDay
10%425%435%5%7%9%46%
2327
11th to 20thDay
10%425%5%35%5%7%9%44%
2303
21st to endDay
12%5%244345%6%8%47%
2387
September1st to 10thDay
12%5%26%4345%5%7%48%
2160
11th to 20thDay
10%5%2442446%8%50%
2160
21st to endDay
15%5%25%6%35%6%5%7%41%
1891
October1st to 10thDay
18%7%45%6%4445%7%36%
1800
11th to 20thDay
21%6%36%6%445%6%8%33%
1800
21st to endDay
26%11%35%6%45%5%45%27%
1936
November1st to 10thDay
15%9%35%6%35%5%6%7%35%
1648
11th to 20thDay
16%11%38%7%45%6%6%7%28%
1520
21st to endDay
22%12%47%6%35%45%6%26%
1520
December1st to 10thDay
23%15%6%6%6%35%5%5%6%20%
1520
11th to 20thDay
19%15%5%8%6%34447%26%
1906
21st to endDay
30%13%5%8%6%34435%21%
2772
January1st to 10thNight
21%16%6%9%6%44436%22%
2649
11th to 20thNight
21%14%6%10%8%45%445%20%
2656
21st to endNight
14%15%6%10%7%45%5%45%26%
2599
February1st to 10thNight
17%16%6%10%7%25%5%5%422%
2340
11th to 20thNight
17%13%6%8%6%3445%5%28%
2273
21st to endNight
18%11%47%6%445%6%6%29%
1813
March1st to 10thNight
16%10%47%7%44445%33%
2056
11th to 20thNight
18%11%46%5%35%435%36%
1979
21st to endNight
17%7%5%6%5%34346%40%
2124
April1st to 10thNight
20%9%35%5%35%45%7%34%
1800
11th to 20thNight
18%8%35%5%4445%6%38%
1775
21st to endNight
20%8%37%5%2445%7%36%
1692
May1st to 10thNight
20%7%26%5%2447%7%37%
1504
11th to 20thNight
13%5%346%45%5%6%7%43%
1415
21st to endNight
10%435%5%36%6%6%9%44%
1550
June1st to 10thNight
15%6%26%5%35%6%6%9%36%
1440
11th to 20thNight
10%435%5%445%6%9%44%
1440
21st to endNight
11%6%37%6%36%5%47%42%
1440
July1st to 10thNight
6%424435%5%6%8%52%
1440
11th to 20thNight
11%425%4345%6%10%45%
1440
21st to endNight
11%4244345%6%9%47%
1580
August1st to 10thNight
9%5%25%5%45%5%6%8%47%
1611
11th to 20thNight
9%5%26%5%346%6%8%46%
1598
21st to endNight
9%5%25%5%35%5%6%10%47%
1949
September1st to 10thNight
12%5%245%25%45%8%46%
1800
11th to 20thNight
13%43433345%7%51%
1800
21st to endNight
16%6%345%4446%7%41%
2065
October1st to 10thNight
17%7%37%6%445%5%7%35%
2160
11th to 20thNight
21%8%46%6%345%5%6%32%
2160
21st to endNight
23%8%48%6%345%5%7%28%
2420
November1st to 10thNight
18%8%35%5%3446%6%38%
2429
11th to 20thNight
18%9%46%6%35%5%5%7%32%
2660
21st to endNight
23%11%46%5%445%5%7%26%
2660
December1st to 10thNight
21%17%5%9%6%35%45%6%19%
2660
11th to 20thNight
21%14%6%8%5%35%445%26%
3288
21st to endNight
31%15%5%7%5%245%5%5%18%
4884

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
61%7%12%15%5%
1518
11th to 20thDay
65%7%10%11%7%1
1520
21st to endDay
61%7%11%12%7%1
1791
February1st to 10thDay
64%9%11%12%31
1620
11th to 20thDay
58%6%12%14%9%1
1582
21st to endDay
58%6%12%16%8%1
1442
March1st to 10thDay
53%7%15%15%8%2
1890
11th to 20thDay
52%6%11%19%11%2
1980
21st to endDay
50%6%8%19%15%4
2232
April1st to 10thDay
49%6%11%19%11%3
2160
11th to 20thDay
45%5%11%23%15%2
2134
21st to endDay
50%7%11%17%12%3
2282
May1st to 10thDay
46%7%11%19%14%3
2454
11th to 20thDay
38%8%12%25%15%2
2482
21st to endDay
37%9%17%22%13%3
2725
June1st to 10thDay
43%8%14%20%14%1
2520
11th to 20thDay
37%7%15%26%14%2
2520
21st to endDay
38%8%17%23%14%1
2520
July1st to 10thDay
30%8%14%29%17%1
2450
11th to 20thDay
33%8%19%29%11%
2445
21st to endDay
35%9%18%26%12%1
2582
August1st to 10thDay
33%9%19%26%11%1
2314
11th to 20thDay
33%10%20%25%13%
2303
21st to endDay
33%8%16%30%11%1
2387
September1st to 10thDay
35%8%17%28%12%1
2160
11th to 20thDay
32%6%15%30%16%1
2160
21st to endDay
40%8%15%27%9%
1891
October1st to 10thDay
47%7%14%19%12%1
1800
11th to 20thDay
48%9%19%17%7%1
1800
21st to endDay
58%7%11%17%7%
1936
November1st to 10thDay
46%8%17%19%10%1
1648
11th to 20thDay
52%10%15%15%8%1
1520
21st to endDay
58%9%13%14%6%
1520
December1st to 10thDay
64%8%15%10%3
1520
11th to 20thDay
60%9%15%12%3
1906
21st to endDay
66%9%11%12%3
2772
January1st to 10thNight
63%7%11%15%4
2649
11th to 20thNight
66%9%10%10%5%1
2656
21st to endNight
59%7%10%15%8%1
2599
February1st to 10thNight
63%7%12%14%31
2340
11th to 20thNight
56%6%12%17%8%1
2275
21st to endNight
52%8%14%18%7%
1813
March1st to 10thNight
52%6%12%18%10%1
2066
11th to 20thNight
49%7%11%20%12%1
1980
21st to endNight
45%6%10%19%16%4
2124
April1st to 10thNight
49%6%10%21%12%3
1800
11th to 20thNight
47%7%10%17%18%1
1775
21st to endNight
47%7%12%16%15%3
1692
May1st to 10thNight
46%6%9%18%17%3
1504
11th to 20thNight
38%6%13%23%17%3
1416
21st to endNight
34%9%15%20%19%3
1551
June1st to 10thNight
42%8%13%22%13%2
1440
11th to 20thNight
36%7%13%20%21%3
1440
21st to endNight
40%8%13%22%15%2
1440
July1st to 10thNight
29%8%12%25%24%2
1400
11th to 20thNight
33%8%13%30%16%1
1392
21st to endNight
31%8%14%32%13%2
1492
August1st to 10thNight
33%9%17%28%12%1
1600
11th to 20thNight
34%10%17%25%14%1
1598
21st to endNight
31%10%15%30%13%1
1949
September1st to 10thNight
35%8%17%26%12%2
1800
11th to 20thNight
31%8%14%30%17%1
1800
21st to endNight
40%8%12%27%12%1
2065
October1st to 10thNight
48%8%11%20%12%1
2160
11th to 20thNight
51%8%15%19%7%
2160
21st to endNight
55%9%12%13%10%1
2420
November1st to 10thNight
45%7%16%20%11%2
2429
11th to 20thNight
49%9%14%18%9%1
2660
21st to endNight
56%10%16%14%4
2660
December1st to 10thNight
63%9%13%12%3
2660
11th to 20thNight
59%9%13%14%5%1
3288
21st to endNight
68%8%12%10%2
4884

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
22%16%34%17%9%2
180
11th to 20thDay
26%20%29%17%7%1
180
21st to endDay
17%16%34%25%6%2
267
February1st to 10thDay
27%20%37%13%3
240
11th to 20thDay
21%14%39%23%3
240
21st to endDay
31%18%31%18%3
308
March1st to 10thDay
20%13%35%27%4
420
11th to 20thDay
21%15%34%24%6%
420
21st to endDay
28%10%32%19%10%1
462
April1st to 10thDay
19%14%32%26%9%1
420
11th to 20thDay
23%9%32%20%12%4
412
21st to endDay
23%9%32%27%7%2
420
May1st to 10thDay
22%6%26%30%13%4
423
11th to 20thDay
16%5%23%35%15%5%
447
21st to endDay
13%323%35%18%8%
485
June1st to 10thDay
19%5%23%36%15%2
450
11th to 20thDay
12%427%32%20%5%
450
21st to endDay
16%5%22%34%20%4
450
July1st to 10thDay
8%417%34%31%6%
445
11th to 20thDay
14%420%30%27%5%
444
21st to endDay
9%423%42%18%4
488
August1st to 10thDay
11%217%37%24%8%
446
11th to 20thDay
9%318%35%27%8%
439
21st to endDay
11%5%23%33%21%8%
495
September1st to 10thDay
11%5%22%32%22%8%
450
11th to 20thDay
9%420%30%23%13%
450
21st to endDay
14%8%22%29%22%5%
450
October1st to 10thDay
22%8%24%21%18%6%
450
11th to 20thDay
19%16%30%22%12%1
450
21st to endDay
29%12%26%25%8%
473
November1st to 10thDay
21%15%24%29%9%1
342
11th to 20thDay
19%22%32%20%6%1
180
21st to endDay
22%14%42%16%5%1
180
December1st to 10thDay
22%21%29%21%7%1
180
11th to 20thDay
23%28%29%16%31
180
21st to endDay
33%17%32%16%2
198
January1st to 10thNight
28%16%31%19%6%
760
11th to 20thNight
28%15%31%20%6%
760
21st to endNight
22%11%36%26%6%
800
February1st to 10thNight
23%16%39%19%3
720
11th to 20thNight
23%12%39%21%5%
703
21st to endNight
23%13%38%23%4
592
March1st to 10thNight
19%10%35%30%4
720
11th to 20thNight
23%12%32%27%5%1
720
21st to endNight
21%7%35%27%9%1
792
April1st to 10thNight
23%9%34%28%6%
720
11th to 20thNight
20%10%33%25%11%1
711
21st to endNight
22%8%27%28%14%1
612
May1st to 10thNight
23%422%32%16%2
424
11th to 20thNight
15%326%41%14%2
354
21st to endNight
10%326%38%19%3
389
June1st to 10thNight
15%124%39%18%2
360
11th to 20thNight
11%324%39%21%3
360
21st to endNight
13%423%41%18%1
360
July1st to 10thNight
7%318%41%28%2
350
11th to 20thNight
11%319%36%27%4
348
21st to endNight
11%217%48%21%3
400
August1st to 10thNight
10%214%42%27%5%
537
11th to 20thNight
9%417%34%32%5%
534
21st to endNight
9%421%36%26%2
761
September1st to 10thNight
13%324%32%23%4
720
11th to 20thNight
13%5%17%31%25%8%
720
21st to endNight
17%6%21%30%21%5%
720
October1st to 10thNight
18%10%28%22%18%3
720
11th to 20thNight
22%12%30%26%9%1
720
21st to endNight
26%13%31%25%5%
792
November1st to 10thNight
21%12%27%27%12%1
740
11th to 20thNight
21%17%33%21%7%1
760
21st to endNight
28%14%30%22%6%
760
December1st to 10thNight
29%18%31%19%4
760
11th to 20thNight
25%16%37%18%31
760
21st to endNight
32%15%34%16%3
836
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