SLL Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SLL Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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
42%
24456
28%
79477
16%
24352
55%
24352
37%
42278

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
36%
940
30%
4037
11%
926
50%
926
27%
2169
January11th to 20thNight
38%
948
31%
3994
13%
930
51%
930
31%
1866
January21st to endNight
37%
1040
23%
4373
14%
1023
51%
1023
26%
2505
February1st to 10thNight
31%
796
28%
2312
11%
796
44%
796
33%
1349
February11th to 20thNight
30%
702
26%
2098
12%
700
42%
700
32%
1210
February21st to endNight
31%
592
34%
1776
14%
592
47%
592
37%
957
March1st to 10thNight
32%
684
30%
2124
12%
684
48%
684
38%
1117
March11th to 20thNight
32%
540
25%
1980
14%
540
48%
540
31%
1045
March21st to endNight
27%
594
25%
2178
12%
594
45%
594
26%
1205
April1st to 10thNight
30%
540
28%
1900
14%
540
51%
540
36%
936
April11th to 20thNight
23%
533
25%
1759
8%
533
45%
533
28%
1002
April21st to endNight
28%
540
28%
1620
13%
540
49%
540
33%
933
May1st to 10thNight
29%
540
24%
1620
12%
540
47%
540
32%
946
May11th to 20thNight
39%
531
22%
1593
17%
531
51%
531
35%
904
May21st to endNight
42%
582
22%
1745
14%
582
54%
582
36%
978
June1st to 10thNight
48%
540
23%
1620
13%
540
59%
540
42%
844
June11th to 20thNight
44%
540
24%
1620
13%
540
56%
540
43%
862
June21st to endNight
53%
540
25%
1620
17%
540
61%
540
50%
800
July1st to 10thNight
65%
525
22%
1575
20%
525
69%
525
45%
830
July11th to 20thNight
72%
522
27%
1566
24%
522
78%
522
53%
747
July21st to endNight
70%
591
27%
1673
27%
558
74%
558
45%
870
August1st to 10thNight
63%
537
25%
1601
23%
534
70%
534
40%
890
August11th to 20thNight
64%
534
28%
1639
28%
534
71%
534
41%
879
August21st to endNight
68%
594
26%
1980
27%
594
72%
594
42%
1003
September1st to 10thNight
62%
540
28%
1926
27%
540
71%
540
40%
951
September11th to 20thNight
50%
540
29%
1980
25%
540
60%
540
33%
1035
September21st to endNight
55%
540
35%
1980
31%
540
63%
540
44%
906
October1st to 10thNight
49%
540
35%
1980
26%
540
62%
540
36%
976
October11th to 20thNight
39%
655
37%
2095
17%
655
56%
655
46%
1009
October21st to endNight
41%
850
35%
2434
14%
850
54%
850
46%
1252
November1st to 10thNight
46%
925
34%
2405
12%
925
59%
925
54%
1249
November11th to 20thNight
37%
950
36%
2470
12%
950
52%
950
52%
1301
November21st to endNight
43%
950
33%
2470
15%
950
54%
950
44%
1388
December1st to 10thNight
41%
950
34%
2470
14%
950
54%
950
46%
1366
December11th to 20thNight
34%
949
35%
2872
12%
948
50%
948
37%
1580
December21st to endNight
33%
1042
22%
4392
13%
1026
48%
1026
26%
2418

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
8%5%2442445%7%56%
78686
Night
8%42442445%7%57%
80345
TransparencyDay
29%410%25%24%8%
78414
Night
28%310%25%25%9%
80140
SeeingDay
10%8%29%32%18%4
14392
Night
9%9%36%36%10%
24481

By Month

Percentage Cloud Cover SW (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
7%7%26%5%33445%53%
2847
11th to 20thDay
5%5%244345%5%7%56%
2850
21st to endDay
12%9%35%323346%52%
3135
February1st to 10thDay
9%9%44323346%55%
1868
11th to 20thDay
5%5%345%34345%58%
1749
21st to endDay
7%5%24424346%58%
1480
March1st to 10thDay
9%6%3443345%6%53%
1836
11th to 20thDay
12%6%34323346%54%
1980
21st to endDay
11%6%24423447%51%
2178
April1st to 10thDay
8%423423447%59%
2056
11th to 20thDay
12%5%25%5%3445%7%49%
2149
21st to endDay
13%5%24424447%52%
2340
May1st to 10thDay
13%5%35%32435%6%52%
2340
11th to 20thDay
10%41442445%7%54%
2302
21st to endDay
9%424435%5%6%9%48%
2531
June1st to 10thDay
6%3144245%7%9%55%
2340
11th to 20thDay
5%314425%5%5%7%58%
2340
21st to endDay
6%3234245%6%7%58%
2340
July1st to 10thDay
7%3145%35%5%6%9%52%
2340
11th to 20thDay
5%214335%5%5%8%59%
2340
21st to endDay
5%214435%6%6%9%55%
2546
August1st to 10thDay
6%2235%45%5%6%9%54%
2328
11th to 20thDay
7%425%4345%5%8%52%
2258
21st to endDay
5%32443445%7%59%
2376
September1st to 10thDay
10%425%423346%58%
2033
11th to 20thDay
9%4233335%6%7%56%
1980
21st to endDay
7%314422446%62%
1980
October1st to 10thDay
11%323423335%60%
1980
11th to 20thDay
9%31221223470%
1846
21st to endDay
10%5%2431233464%
1919
November1st to 10thDay
5%313223345%67%
1673
11th to 20thDay
6%323213337%67%
1710
21st to endDay
10%5%23223335%62%
1710
December1st to 10thDay
7%424423447%59%
1710
11th to 20thDay
8%5%24423445%58%
2161
21st to endDay
8%8%37%7%4445%6%46%
3135
January1st to 10thNight
9%7%36%323345%54%
4108
11th to 20thNight
5%424434447%59%
4120
21st to endNight
14%8%3432334452%
4532
February1st to 10thNight
10%7%36%423436%54%
2312
11th to 20thNight
9%5%35%42445%7%53%
2098
21st to endNight
7%5%24313446%59%
1776
March1st to 10thNight
6%5%2442445%7%58%
2124
11th to 20thNight
13%5%2332335%6%55%
1980
21st to endNight
11%5%2443445%7%50%
2178
April1st to 10thNight
8%313434447%60%
1900
11th to 20thNight
12%425%43446%7%51%
1759
21st to endNight
12%323425%447%53%
1620
May1st to 10thNight
12%4234245%6%6%51%
1620
11th to 20thNight
10%3243246%5%9%52%
1593
21st to endNight
9%3144345%6%11%50%
1745
June1st to 10thNight
5%414325%46%10%57%
1620
11th to 20thNight
5%2243245%7%10%57%
1620
21st to endNight
6%22322436%9%61%
1620
July1st to 10thNight
6%22442447%11%55%
1620
11th to 20thNight
32133245%6%12%59%
1620
21st to endNight
5%3243235%5%12%55%
1772
August1st to 10thNight
5%3145%36%6%6%10%50%
1611
11th to 20thNight
6%4245%345%5%8%55%
1639
21st to endNight
43134346%6%11%55%
1980
September1st to 10thNight
9%313323446%62%
1926
11th to 20thNight
10%4244235%5%7%55%
1980
21st to endNight
6%21432345%7%64%
1980
October1st to 10thNight
11%313323345%62%
1980
11th to 20thNight
9%21222333469%
2095
21st to endNight
8%313322335%67%
2434
November1st to 10thNight
7%212213346%68%
2405
11th to 20thNight
5%413323336%67%
2470
21st to endNight
9%423323335%62%
2470
December1st to 10thNight
7%424422447%60%
2470
11th to 20thNight
8%5%24333347%59%
3036
21st to endNight
8%8%245%35%45%6%50%
4532

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
34%29%25%27%3
2848
11th to 20thDay
30%214%23%23%9%
2850
21st to endDay
36%28%25%25%4
3135
February1st to 10thDay
34%210%25%21%7%
1868
11th to 20thDay
31%39%26%22%9%
1759
21st to endDay
30%26%26%24%12%
1480
March1st to 10thDay
33%38%23%21%12%
1836
11th to 20thDay
33%310%21%25%8%
1980
21st to endDay
35%410%26%19%6%
2178
April1st to 10thDay
26%39%25%24%12%
2056
11th to 20thDay
36%48%25%19%8%
2152
21st to endDay
34%310%27%22%4
2340
May1st to 10thDay
35%411%20%21%9%
2340
11th to 20thDay
30%413%31%20%1
2306
21st to endDay
33%5%14%27%15%4
2531
June1st to 10thDay
25%5%19%27%16%7%
2340
11th to 20thDay
25%412%26%25%8%
2340
21st to endDay
24%6%10%24%25%11%
2340
July1st to 10thDay
29%5%12%27%22%5%
2276
11th to 20thDay
21%6%10%26%29%8%
2271
21st to endDay
25%6%13%29%25%2
2405
August1st to 10thDay
25%6%14%33%16%6%
2314
11th to 20thDay
30%5%13%24%23%5%
2258
21st to endDay
25%5%11%25%24%10%
2376
September1st to 10thDay
29%49%23%23%12%
2033
11th to 20thDay
28%5%9%22%30%6%
1980
21st to endDay
24%39%24%29%11%
1980
October1st to 10thDay
28%39%21%28%10%
1980
11th to 20thDay
20%26%23%32%17%
1846
21st to endDay
27%37%23%28%12%
1919
November1st to 10thDay
21%37%24%31%14%
1673
11th to 20thDay
20%27%29%30%11%
1710
21st to endDay
27%25%25%29%11%
1710
December1st to 10thDay
27%310%30%25%5%
1710
11th to 20thDay
30%28%25%27%8%
2159
21st to endDay
42%18%22%22%5%
3135
January1st to 10thNight
35%28%30%21%5%
4108
11th to 20thNight
27%214%24%26%7%
4120
21st to endNight
38%28%21%28%5%
4532
February1st to 10thNight
35%38%26%21%8%
2312
11th to 20thNight
33%49%23%23%9%
2098
21st to endNight
28%35%24%25%15%
1776
March1st to 10thNight
26%47%24%26%12%
2124
11th to 20thNight
31%49%23%23%10%
1980
21st to endNight
34%5%11%25%19%6%
2178
April1st to 10thNight
25%310%23%25%14%
1900
11th to 20thNight
33%411%25%18%9%
1759
21st to endNight
32%410%28%22%4
1620
May1st to 10thNight
33%5%12%20%22%8%
1620
11th to 20thNight
29%5%16%26%18%6%
1594
21st to endNight
28%6%17%30%15%4
1746
June1st to 10thNight
25%316%32%17%6%
1620
11th to 20thNight
23%7%12%27%24%8%
1620
21st to endNight
22%212%29%24%11%
1620
July1st to 10thNight
24%5%15%30%21%5%
1575
11th to 20thNight
17%5%12%28%29%9%
1566
21st to endNight
24%412%27%29%4
1673
August1st to 10thNight
27%6%13%28%20%6%
1601
11th to 20thNight
27%412%28%22%7%
1639
21st to endNight
23%6%12%25%24%9%
1980
September1st to 10thNight
24%5%9%25%26%11%
1926
11th to 20thNight
30%5%9%19%28%9%
1980
21st to endNight
21%49%23%30%12%
1980
October1st to 10thNight
26%48%20%30%12%
1980
11th to 20thNight
22%25%19%34%18%
2095
21st to endNight
22%35%25%29%16%
2434
November1st to 10thNight
19%29%24%28%17%
2405
11th to 20thNight
20%36%28%30%12%
2470
21st to endNight
27%26%24%28%13%
2470
December1st to 10thNight
25%310%28%27%7%
2470
11th to 20thNight
28%26%28%27%8%
3037
21st to endNight
36%210%23%22%7%
4532

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
11%9%38%30%11%1
270
11th to 20thDay
14%14%38%25%9%
270
21st to endDay
6%15%43%26%10%1
297
February1st to 10thDay
13%13%41%27%6%1
267
11th to 20thDay
12%10%50%21%6%1
240
21st to endDay
12%19%45%19%5%
198
March1st to 10thDay
6%17%48%23%5%
240
11th to 20thDay
20%15%38%20%5%3
240
21st to endDay
17%18%35%25%5%
264
April1st to 10thDay
13%18%40%23%7%
316
11th to 20thDay
14%18%45%21%2
430
21st to endDay
17%15%37%20%10%1
600
May1st to 10thDay
12%12%40%28%7%1
603
11th to 20thDay
15%7%31%31%14%2
625
21st to endDay
13%8%32%32%13%3
679
June1st to 10thDay
6%5%24%38%19%7%
630
11th to 20thDay
5%5%27%40%20%2
630
21st to endDay
5%327%39%21%4
630
July1st to 10thDay
7%212%37%31%10%
621
11th to 20thDay
3112%41%34%9%
618
21st to endDay
6%110%42%33%9%
684
August1st to 10thDay
7%114%41%33%5%
626
11th to 20thDay
9%113%40%32%5%
572
21st to endDay
6%212%39%33%8%
495
September1st to 10thDay
9%214%33%34%8%
323
11th to 20thDay
6%221%39%26%7%
270
21st to endDay
9%226%34%24%5%
270
October1st to 10thDay
10%5%31%31%19%4
270
11th to 20thDay
10%7%34%33%15%1
270
21st to endDay
18%9%35%23%11%3
297
November1st to 10thDay
7%7%40%29%14%3
270
11th to 20thDay
9%10%40%25%13%3
270
21st to endDay
17%12%33%24%12%3
270
December1st to 10thDay
8%8%41%31%11%1
270
11th to 20thDay
10%11%35%33%10%1
270
21st to endDay
14%17%37%26%5%
297
January1st to 10thNight
7%11%41%33%8%
950
11th to 20thNight
11%12%35%34%7%
950
21st to endNight
9%10%40%34%7%
1045
February1st to 10thNight
13%10%40%31%5%
796
11th to 20thNight
11%13%43%29%5%
704
21st to endNight
11%14%42%29%4
592
March1st to 10thNight
8%13%45%28%6%
684
11th to 20thNight
14%10%41%30%5%
540
21st to endNight
13%14%44%24%5%
594
April1st to 10thNight
10%13%45%26%6%
540
11th to 20thNight
14%17%44%21%4
533
21st to endNight
14%12%43%23%7%
540
May1st to 10thNight
11%12%44%28%4
540
11th to 20thNight
11%8%37%33%10%1
531
21st to endNight
9%9%36%33%12%1
584
June1st to 10thNight
7%6%34%37%15%1
540
11th to 20thNight
6%6%41%39%9%
540
21st to endNight
6%333%47%11%
540
July1st to 10thNight
6%120%47%25%2
525
11th to 20thNight
4117%54%23%1
522
21st to endNight
6%116%54%21%2
591
August1st to 10thNight
5%121%48%22%1
537
11th to 20thNight
7%219%52%20%
534
21st to endNight
5%219%46%27%2
594
September1st to 10thNight
9%222%46%19%1
540
11th to 20thNight
11%328%45%13%1
540
21st to endNight
6%431%41%16%1
540
October1st to 10thNight
12%6%31%40%11%
540
11th to 20thNight
8%9%42%35%6%
655
21st to endNight
10%10%37%29%14%
850
November1st to 10thNight
8%7%35%38%11%1
925
11th to 20thNight
7%11%43%31%8%
950
21st to endNight
11%10%32%37%9%
950
December1st to 10thNight
8%9%38%35%9%1
950
11th to 20thNight
13%10%40%33%5%
950
21st to endNight
11%11%41%32%4
1045
Page generated at 2024-02-07 05:11:56. Version ac07

Copyright 2024 Attilla Danko


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