Welsh Mt. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Welsh Mt. Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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
45%
23842
22%
74656
14%
23794
54%
23794
27%
46170

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
28%
758
26%
2459
10%
757
47%
757
26%
1474
January11th to 20thNight
30%
729
24%
2367
12%
729
45%
729
23%
1486
January21st to endNight
28%
792
27%
2574
12%
792
47%
792
27%
1520
February1st to 10thNight
28%
720
20%
2340
10%
720
41%
720
23%
1453
February11th to 20thNight
21%
701
20%
2273
8%
699
37%
699
21%
1452
February21st to endNight
26%
592
28%
1696
11%
592
43%
592
25%
1079
March1st to 10thNight
33%
720
27%
1980
13%
720
46%
720
31%
1205
March11th to 20thNight
31%
720
21%
1980
11%
720
40%
720
24%
1307
March21st to endNight
33%
792
21%
2178
10%
792
42%
792
25%
1445
April1st to 10thNight
30%
720
21%
1850
7%
720
40%
720
30%
1187
April11th to 20thNight
40%
711
24%
1775
9%
711
48%
711
37%
1107
April21st to endNight
40%
698
23%
1778
10%
698
46%
698
31%
1164
May1st to 10thNight
42%
540
18%
1620
8%
540
48%
540
25%
1064
May11th to 20thNight
47%
531
15%
1593
11%
531
50%
531
23%
1095
May21st to endNight
60%
520
16%
1682
15%
520
62%
520
27%
1078
June1st to 10thNight
68%
360
21%
1440
24%
360
71%
360
25%
807
June11th to 20thNight
59%
360
16%
1440
17%
360
62%
360
24%
848
June21st to endNight
66%
367
20%
1447
21%
367
71%
367
28%
802
July1st to 10thNight
67%
525
14%
1575
13%
525
68%
525
28%
1018
July11th to 20thNight
74%
522
16%
1566
14%
522
75%
522
36%
923
July21st to endNight
71%
591
17%
1672
15%
558
72%
558
32%
1042
August1st to 10thNight
65%
537
18%
1600
14%
534
66%
534
27%
1052
August11th to 20thNight
62%
548
18%
1612
14%
548
64%
548
26%
1075
August21st to endNight
64%
792
17%
1980
11%
792
65%
792
30%
1362
September1st to 10thNight
64%
720
23%
1800
16%
720
66%
720
34%
1197
September11th to 20thNight
63%
720
25%
1896
21%
720
65%
720
32%
1235
September21st to endNight
49%
720
20%
2026
17%
720
53%
720
25%
1379
October1st to 10thNight
53%
720
22%
2160
21%
720
59%
720
29%
1349
October11th to 20thNight
48%
720
27%
2160
21%
720
59%
720
32%
1292
October21st to endNight
49%
792
30%
2376
23%
792
60%
792
35%
1372
November1st to 10thNight
46%
740
29%
2221
21%
740
56%
740
31%
1318
November11th to 20thNight
40%
760
27%
2441
16%
760
54%
760
30%
1400
November21st to endNight
42%
760
25%
2660
17%
760
55%
760
29%
1479
December1st to 10thNight
32%
760
23%
2660
12%
760
47%
760
21%
1653
December11th to 20thNight
38%
759
24%
3168
16%
758
52%
758
22%
1830
December21st to endNight
35%
825
16%
4611
13%
817
49%
817
15%
2621

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
14%5%25%5%35%5%6%8%43%
74606
Night
14%5%25%5%345%5%7%45%
75112
TransparencyDay
39%6%12%23%17%4
74327
Night
38%5%11%23%18%4
74910
SeeingDay
19%10%24%25%15%7%
13920
Night
17%7%24%34%16%2
23860

By Month

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 10thDay
14%7%35%5%3345%5%47%
1706
11th to 20thDay
18%6%35%435%46%5%41%
1638
21st to endDay
15%7%25%434446%44%
1782
February1st to 10thDay
12%5%36%6%35%6%5%7%41%
1620
11th to 20thDay
18%6%25%4245%5%6%43%
1573
21st to endDay
18%8%35%445%447%39%
1556
March1st to 10thDay
13%5%24423446%51%
1980
11th to 20thDay
17%6%25%5%345%5%6%41%
1980
21st to endDay
17%7%35%6%345%46%39%
2178
April1st to 10thDay
10%4245%35%5%6%7%50%
2110
11th to 20thDay
13%5%15%425%5%5%6%49%
2134
21st to endDay
17%425%6%35%45%7%42%
2197
May1st to 10thDay
14%5%246%35%6%6%8%39%
2340
11th to 20thDay
17%5%25%5%36%5%6%8%37%
2301
21st to endDay
15%5%25%5%36%6%6%8%39%
2589
June1st to 10thDay
10%4245%35%5%6%10%44%
2520
11th to 20thDay
11%325%5%36%6%7%10%40%
2520
21st to endDay
8%325%5%45%5%8%11%43%
2505
July1st to 10thDay
11%5%25%6%36%6%7%10%39%
2340
11th to 20thDay
5%325%7%36%7%9%12%40%
2340
21st to endDay
10%435%7%47%7%7%10%35%
2547
August1st to 10thDay
12%5%26%6%46%6%7%10%36%
2327
11th to 20thDay
11%5%36%7%46%8%6%9%35%
2269
21st to endDay
11%425%5%45%6%7%11%41%
2376
September1st to 10thDay
12%5%2443446%7%48%
2160
11th to 20thDay
12%6%245%445%5%8%45%
2060
21st to endDay
22%6%25%5%235%5%6%39%
1932
October1st to 10thDay
14%5%2443345%7%49%
1800
11th to 20thDay
16%5%13333445%52%
1800
21st to endDay
15%5%25%424335%52%
1980
November1st to 10thDay
13%414324346%56%
1857
11th to 20thDay
14%5%24424446%51%
1730
21st to endDay
14%42442445%6%51%
1520
December1st to 10thDay
17%6%25%425%46%7%40%
1520
11th to 20thDay
16%423423345%53%
1910
21st to endDay
20%5%25%5%3446%7%39%
2909
January1st to 10thNight
15%5%35%43445%6%48%
2459
11th to 20thNight
15%7%35%5%345%5%6%43%
2367
21st to endNight
14%5%245%3446%6%48%
2574
February1st to 10thNight
14%6%25%5%346%6%7%42%
2340
11th to 20thNight
20%7%25%434445%43%
2273
21st to endNight
15%6%35%43445%7%45%
1696
March1st to 10thNight
13%41433445%6%51%
1980
11th to 20thNight
18%5%245%345%5%7%41%
1980
21st to endNight
15%6%25%5%345%5%6%43%
2178
April1st to 10thNight
14%5%25%5%3446%7%46%
1850
11th to 20thNight
12%5%35%42245%9%50%
1775
21st to endNight
16%424435%45%7%46%
1778
May1st to 10thNight
16%5%245%35%5%5%8%41%
1620
11th to 20thNight
17%7%36%435%5%5%9%36%
1592
21st to endNight
14%5%25%6%345%6%10%40%
1682
June1st to 10thNight
10%424445%6%7%10%45%
1440
11th to 20thNight
10%5%25%6%35%6%5%9%44%
1440
21st to endNight
8%325%435%5%7%11%46%
1447
July1st to 10thNight
10%435%6%45%5%7%9%42%
1620
11th to 20thNight
5%3146%35%6%8%12%46%
1620
21st to endNight
7%5%25%5%37%8%9%10%40%
1771
August1st to 10thNight
12%5%26%7%35%6%7%8%39%
1611
11th to 20thNight
13%6%36%7%25%5%6%9%39%
1612
21st to endNight
10%6%36%6%35%5%6%11%39%
1980
September1st to 10thNight
13%6%35%43446%7%47%
1800
11th to 20thNight
13%5%24445%46%7%46%
1896
21st to endNight
22%6%25%42445%6%40%
2026
October1st to 10thNight
17%4244345%5%6%47%
2160
11th to 20thNight
15%5%24324447%49%
2160
21st to endNight
15%323334445%53%
2376
November1st to 10thNight
12%7%25%5%2333454%
2221
11th to 20thNight
13%5%2442345%6%52%
2441
21st to endNight
13%4245%3445%7%50%
2660
December1st to 10thNight
17%7%25%4345%45%44%
2660
11th to 20thNight
16%6%34433346%48%
3285
21st to endNight
17%5%245%35%5%6%7%41%
4742

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
41%310%23%18%5%
1707
11th to 20thDay
45%49%21%18%3
1638
21st to endDay
40%48%24%18%5%
1782
February1st to 10thDay
42%5%11%24%15%3
1620
11th to 20thDay
42%5%8%20%21%4
1582
21st to endDay
46%411%15%18%6%
1556
March1st to 10thDay
35%49%22%23%6%
1980
11th to 20thDay
42%6%10%17%18%7%
1980
21st to endDay
46%5%9%19%15%5%
2178
April1st to 10thDay
33%5%11%24%19%9%
2110
11th to 20thDay
35%412%22%20%6%
2134
21st to endDay
42%5%11%22%15%5%
2197
May1st to 10thDay
41%6%15%20%14%3
2340
11th to 20thDay
44%8%15%22%10%2
2305
21st to endDay
41%7%15%25%12%
2589
June1st to 10thDay
34%7%16%27%14%1
2520
11th to 20thDay
37%8%17%21%15%2
2520
21st to endDay
33%8%17%27%14%1
2505
July1st to 10thDay
39%8%14%21%16%3
2275
11th to 20thDay
32%9%18%25%15%2
2271
21st to endDay
40%9%14%23%13%1
2405
August1st to 10thDay
41%8%15%26%9%1
2314
11th to 20thDay
42%9%14%22%11%1
2269
21st to endDay
36%7%18%24%13%2
2376
September1st to 10thDay
36%6%14%19%21%4
2160
11th to 20thDay
36%6%8%27%19%3
2060
21st to endDay
46%5%8%20%18%2
1932
October1st to 10thDay
36%5%8%29%20%4
1800
11th to 20thDay
34%39%26%23%4
1800
21st to endDay
35%38%26%23%6%
1980
November1st to 10thDay
31%49%23%28%5%
1857
11th to 20thDay
35%49%22%22%8%
1730
21st to endDay
34%5%8%24%25%4
1520
December1st to 10thDay
43%410%23%15%4
1520
11th to 20thDay
35%38%27%18%9%
1906
21st to endDay
44%410%27%14%2
2909
January1st to 10thNight
38%5%9%22%20%7%
2459
11th to 20thNight
42%5%10%22%17%5%
2367
21st to endNight
38%39%25%22%4
2574
February1st to 10thNight
40%6%11%23%17%3
2340
11th to 20thNight
44%48%19%18%8%
2275
21st to endNight
40%49%23%18%6%
1696
March1st to 10thNight
33%5%9%25%22%6%
1980
11th to 20thNight
41%5%8%21%20%4
1980
21st to endNight
41%6%9%21%17%6%
2178
April1st to 10thNight
37%5%12%18%22%7%
1850
11th to 20thNight
33%412%22%23%6%
1775
21st to endNight
38%6%10%25%16%5%
1778
May1st to 10thNight
41%7%13%22%13%5%
1620
11th to 20thNight
44%8%12%22%12%2
1593
21st to endNight
39%7%16%26%11%1
1683
June1st to 10thNight
33%8%13%30%15%1
1440
11th to 20thNight
38%7%18%21%15%2
1440
21st to endNight
32%8%16%23%18%3
1447
July1st to 10thNight
38%8%16%18%16%4
1575
11th to 20thNight
29%9%22%26%12%2
1566
21st to endNight
34%8%18%26%14%
1672
August1st to 10thNight
39%9%17%23%11%1
1600
11th to 20thNight
43%6%15%23%13%1
1612
21st to endNight
39%8%17%22%10%3
1980
September1st to 10thNight
37%7%13%21%17%5%
1800
11th to 20thNight
37%7%11%25%17%3
1896
21st to endNight
45%5%9%21%17%3
2026
October1st to 10thNight
38%6%9%26%19%3
2160
11th to 20thNight
36%47%26%23%5%
2160
21st to endNight
33%48%24%25%6%
2376
November1st to 10thNight
36%46%21%25%7%
2221
11th to 20thNight
35%38%22%24%8%
2441
21st to endNight
34%49%25%22%6%
2660
December1st to 10thNight
44%48%24%17%4
2660
11th to 20thNight
39%48%21%21%7%
3288
21st to endNight
40%6%10%25%16%2
4742

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
21%23%30%20%4
270
11th to 20thDay
29%21%33%15%2
246
21st to endDay
28%23%32%12%5%
264
February1st to 10thDay
23%24%38%11%3
240
11th to 20thDay
37%24%30%8%1
240
21st to endDay
32%19%27%18%4
328
March1st to 10thDay
24%16%30%25%4
420
11th to 20thDay
21%20%31%21%5%1
420
21st to endDay
25%14%33%22%5%2
462
April1st to 10thDay
22%17%28%22%9%2
420
11th to 20thDay
20%11%30%23%11%5%
412
21st to endDay
23%13%29%23%11%2
420
May1st to 10thDay
22%13%32%21%9%3
423
11th to 20thDay
23%8%25%27%12%6%
447
21st to endDay
18%5%20%30%20%7%
485
June1st to 10thDay
12%424%28%23%8%
450
11th to 20thDay
13%8%21%31%16%12%
450
21st to endDay
10%425%32%22%7%
450
July1st to 10thDay
12%217%26%28%14%
445
11th to 20thDay
5%216%32%30%14%
444
21st to endDay
11%318%29%23%15%
488
August1st to 10thDay
13%5%16%26%27%13%
446
11th to 20thDay
11%218%27%26%16%
439
21st to endDay
11%213%28%26%19%
495
September1st to 10thDay
15%313%26%24%19%
450
11th to 20thDay
15%5%11%29%29%10%
450
21st to endDay
29%6%17%26%18%5%
450
October1st to 10thDay
20%5%19%30%18%8%
450
11th to 20thDay
16%11%24%30%17%3
450
21st to endDay
17%10%28%31%11%3
495
November1st to 10thDay
19%8%25%30%15%3
455
11th to 20thDay
20%12%29%23%15%2
338
21st to endDay
16%15%32%26%8%2
180
December1st to 10thDay
20%17%36%22%31
180
11th to 20thDay
23%15%33%21%8%1
180
21st to endDay
27%14%30%21%7%1
238
January1st to 10thNight
18%11%37%31%4
760
11th to 20thNight
21%10%33%33%3
729
21st to endNight
17%10%40%29%4
792
February1st to 10thNight
18%10%39%30%3
720
11th to 20thNight
25%13%36%24%1
703
21st to endNight
20%15%34%27%3
592
March1st to 10thNight
16%13%34%32%5%
720
11th to 20thNight
23%11%31%28%7%1
720
21st to endNight
20%10%31%32%6%1
792
April1st to 10thNight
16%14%37%27%7%
720
11th to 20thNight
15%9%30%33%11%1
711
21st to endNight
19%5%30%36%9%1
698
May1st to 10thNight
18%6%26%36%13%1
540
11th to 20thNight
20%421%35%19%2
531
21st to endNight
16%111%38%30%3
522
June1st to 10thNight
9%312%46%28%2
360
11th to 20thNight
12%214%40%30%2
360
21st to endNight
9%114%48%25%2
367
July1st to 10thNight
12%110%33%37%7%
525
11th to 20thNight
5%18%34%46%7%
522
21st to endNight
8%210%39%37%4
591
August1st to 10thNight
13%36%38%35%5%
537
11th to 20thNight
16%18%31%39%4
548
21st to endNight
13%210%35%35%5%
792
September1st to 10thNight
15%211%32%33%8%
720
11th to 20thNight
14%312%32%33%7%
720
21st to endNight
25%316%31%23%2
720
October1st to 10thNight
20%416%31%23%6%
720
11th to 20thNight
18%6%23%34%18%2
720
21st to endNight
17%6%24%38%15%1
792
November1st to 10thNight
16%5%25%39%12%3
740
11th to 20thNight
16%8%29%35%11%1
760
21st to endNight
15%7%30%38%9%1
760
December1st to 10thNight
20%10%31%32%7%1
760
11th to 20thNight
16%8%31%39%6%
760
21st to endNight
20%8%29%39%3
836
Page generated at 2024-01-10 01:25:05. Version ac07

Copyright 2024 Attilla Danko


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