Anstruther Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Anstruther Lake Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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
28%
22836
13%
80741
8%
22753
36%
22753
15%
58071

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
13%
749
8%
4409
3%
737
21%
737
4%
3512
January11th to 20thNight
16%
757
6%
4397
4%
740
23%
740
4%
3567
January21st to endNight
20%
827
7%
4525
7%
814
27%
814
6%
3624
February1st to 10thNight
14%
760
10%
2470
4%
760
22%
760
9%
1921
February11th to 20thNight
15%
742
13%
2405
4%
740
27%
740
12%
1777
February21st to endNight
20%
628
13%
1978
4%
628
30%
628
13%
1461
March1st to 10thNight
18%
723
17%
2033
7%
723
26%
723
16%
1463
March11th to 20thNight
21%
720
16%
1980
7%
720
30%
720
17%
1441
March21st to endNight
27%
792
20%
2156
7%
792
37%
792
22%
1501
April1st to 10thNight
22%
720
15%
1800
5%
720
28%
720
19%
1348
April11th to 20thNight
26%
708
18%
1772
6%
708
33%
708
23%
1262
April21st to endNight
28%
540
17%
1620
8%
540
35%
540
19%
1161
May1st to 10thNight
34%
446
17%
1526
10%
446
42%
446
19%
991
May11th to 20thNight
34%
354
22%
1416
15%
354
51%
354
21%
831
May21st to endNight
43%
388
16%
1551
14%
388
56%
388
26%
857
June1st to 10thNight
36%
360
12%
1440
8%
360
49%
360
20%
880
June11th to 20thNight
43%
360
14%
1440
16%
360
53%
360
21%
837
June21st to endNight
43%
360
11%
1440
13%
360
49%
360
20%
892
July1st to 10thNight
52%
350
15%
1400
20%
350
58%
350
28%
755
July11th to 20thNight
50%
348
17%
1392
21%
348
57%
348
26%
774
July21st to endNight
52%
424
16%
1515
15%
401
59%
401
30%
843
August1st to 10thNight
56%
537
16%
1600
11%
534
59%
534
32%
979
August11th to 20thNight
57%
534
19%
1598
11%
534
62%
534
31%
982
August21st to endNight
47%
709
19%
1897
8%
709
52%
709
32%
1232
September1st to 10thNight
44%
720
17%
1800
7%
720
47%
720
32%
1182
September11th to 20thNight
51%
720
24%
1818
14%
720
55%
720
36%
1149
September21st to endNight
44%
720
21%
2068
16%
720
49%
720
24%
1376
October1st to 10thNight
35%
720
16%
2160
12%
720
42%
720
20%
1503
October11th to 20thNight
22%
720
12%
2160
6%
720
31%
720
15%
1625
October21st to endNight
20%
792
13%
2394
6%
792
29%
792
12%
1860
November1st to 10thNight
30%
740
16%
2506
13%
740
37%
740
17%
1706
November11th to 20thNight
20%
760
14%
2660
7%
760
30%
760
13%
1923
November21st to endNight
20%
760
10%
2660
5%
760
26%
760
9%
2063
December1st to 10thNight
12%
760
7%
2660
2%
760
18%
760
7%
2177
December11th to 20thNight
14%
759
9%
3246
3%
758
22%
758
7%
2500
December21st to endNight
11%
829
4%
4849
1%
817
16%
817
4%
4116

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
17%11%48%6%35%5%5%6%31%
78373
Night
16%11%5%8%6%35%45%6%30%
81026
TransparencyDay
53%5%11%19%10%1
78080
Night
54%6%11%19%10%1
80853
SeeingDay
23%15%31%19%9%3
13424
Night
22%13%31%24%9%1
22869

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
16%19%6%12%7%3433424%
2517
11th to 20thDay
12%16%10%15%8%6%6%5%4316%
2552
21st to endDay
18%20%8%11%7%3433419%
3124
February1st to 10thDay
13%16%8%12%9%35%44421%
1710
11th to 20thDay
13%14%7%12%8%35%44426%
1663
21st to endDay
17%14%6%9%8%45%445%24%
1475
March1st to 10thDay
15%12%5%9%8%45%45%6%29%
1929
11th to 20thDay
13%16%5%9%7%4445%5%29%
1980
21st to endDay
19%11%46%4335%45%36%
2200
April1st to 10thDay
21%12%37%6%44446%29%
2160
11th to 20thDay
22%9%35%44335%5%37%
2152
21st to endDay
22%10%37%6%34346%32%
2340
May1st to 10thDay
19%8%36%6%45%46%6%34%
2432
11th to 20thDay
16%9%36%5%3445%7%39%
2478
21st to endDay
13%6%26%6%35%5%6%7%40%
2725
June1st to 10thDay
19%8%36%6%45%45%8%33%
2520
11th to 20thDay
15%7%35%5%46%5%6%10%35%
2520
21st to endDay
17%7%35%5%45%5%6%9%34%
2520
July1st to 10thDay
12%5%26%6%35%5%6%8%43%
2520
11th to 20thDay
11%5%35%6%46%6%6%9%38%
2520
21st to endDay
11%5%36%6%46%8%8%9%34%
2710
August1st to 10thDay
12%7%26%6%46%7%7%9%36%
2327
11th to 20thDay
11%7%37%6%45%5%7%8%37%
2303
21st to endDay
13%7%26%6%35%6%6%10%36%
2440
September1st to 10thDay
14%6%35%6%346%5%9%39%
2160
11th to 20thDay
13%5%26%5%35%5%47%45%
2138
21st to endDay
17%6%36%6%5%45%6%7%37%
1891
October1st to 10thDay
21%9%47%5%3445%7%32%
1800
11th to 20thDay
25%10%38%6%45%45%6%25%
1800
21st to endDay
27%13%47%6%4345%5%22%
1961
November1st to 10thDay
17%10%47%6%35%5%5%7%32%
1568
11th to 20thDay
17%13%5%8%5%3446%7%27%
1520
21st to endDay
22%17%6%8%6%45%6%45%18%
1520
December1st to 10thDay
19%21%8%10%7%34345%15%
1520
11th to 20thDay
18%17%6%10%5%3344425%
1906
21st to endDay
26%20%6%10%5%2333417%
2772
January1st to 10thNight
18%19%8%12%7%3444417%
4429
11th to 20thNight
13%14%7%14%12%5%6%5%45%15%
4405
21st to endNight
18%14%9%10%8%44445%18%
4532
February1st to 10thNight
13%15%6%10%8%47%5%6%6%20%
2470
11th to 20thNight
13%13%6%10%7%45%45%5%28%
2403
21st to endNight
16%14%48%6%45%5%5%6%27%
1978
March1st to 10thNight
15%10%5%7%7%45%45%6%33%
2023
11th to 20thNight
14%13%6%8%6%24445%33%
1979
21st to endNight
15%7%37%6%45%45%6%38%
2156
April1st to 10thNight
20%10%5%6%6%35%5%5%6%31%
1800
11th to 20thNight
16%9%47%5%34447%37%
1772
21st to endNight
18%10%26%6%445%6%5%33%
1620
May1st to 10thNight
18%7%26%6%3445%7%38%
1526
11th to 20thNight
14%5%25%5%35%46%8%44%
1415
21st to endNight
11%5%24435%5%49%48%
1550
June1st to 10thNight
15%7%25%5%36%46%9%37%
1440
11th to 20thNight
13%5%25%5%45%5%5%8%44%
1440
21st to endNight
13%7%36%5%35%46%8%40%
1440
July1st to 10thNight
8%426%5%35%5%5%7%51%
1440
11th to 20thNight
11%6%25%5%35%37%9%45%
1440
21st to endNight
9%5%244346%6%9%47%
1604
August1st to 10thNight
10%425%5%345%6%9%48%
1611
11th to 20thNight
9%5%25%6%345%6%9%45%
1598
21st to endNight
10%6%35%6%35%5%5%8%44%
1897
September1st to 10thNight
11%5%35%5%345%7%9%42%
1800
11th to 20thNight
13%41442446%6%50%
1818
21st to endNight
19%6%25%5%35%445%43%
2068
October1st to 10thNight
18%7%37%5%35%45%7%36%
2160
11th to 20thNight
22%10%47%6%445%5%6%28%
2160
21st to endNight
22%13%47%6%345%45%26%
2394
November1st to 10thNight
18%9%46%6%3445%7%35%
2506
11th to 20thNight
19%12%46%6%35%5%5%6%29%
2660
21st to endNight
20%12%49%7%45%46%6%22%
2660
December1st to 10thNight
21%16%7%10%7%34445%18%
2660
11th to 20thNight
17%18%6%11%6%33345%24%
3288
21st to endNight
27%19%6%12%5%3433414%
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
66%5%11%11%7%1
2518
11th to 20thDay
71%5%9%8%6%1
2552
21st to endDay
72%47%10%6%1
3124
February1st to 10thDay
67%48%14%41
1710
11th to 20thDay
61%5%8%13%11%2
1672
21st to endDay
62%49%14%10%1
1475
March1st to 10thDay
57%414%14%9%2
1932
11th to 20thDay
57%410%16%10%2
1980
21st to endDay
52%47%18%15%4
2200
April1st to 10thDay
57%48%19%11%3
2160
11th to 20thDay
50%48%22%15%1
2152
21st to endDay
55%48%19%12%3
2340
May1st to 10thDay
50%5%9%19%14%3
2432
11th to 20thDay
46%48%25%15%1
2482
21st to endDay
42%7%13%21%14%3
2725
June1st to 10thDay
50%5%12%19%14%1
2520
11th to 20thDay
44%6%11%24%13%2
2520
21st to endDay
46%6%13%22%12%1
2520
July1st to 10thDay
38%6%12%28%15%2
2450
11th to 20thDay
40%8%14%27%10%1
2445
21st to endDay
43%8%13%26%10%1
2558
August1st to 10thDay
42%8%14%24%11%1
2314
11th to 20thDay
43%8%15%24%10%1
2303
21st to endDay
42%6%13%26%11%1
2440
September1st to 10thDay
42%6%13%27%11%1
2160
11th to 20thDay
39%5%13%29%13%1
2138
21st to endDay
46%6%14%24%10%
1891
October1st to 10thDay
52%5%11%20%11%1
1800
11th to 20thDay
60%6%13%14%6%1
1800
21st to endDay
64%410%15%6%1
1961
November1st to 10thDay
52%5%15%20%8%1
1568
11th to 20thDay
57%6%13%14%8%2
1520
21st to endDay
67%6%11%10%5%1
1520
December1st to 10thDay
73%5%9%9%4
1520
11th to 20thDay
64%7%8%12%9%
1906
21st to endDay
73%37%12%4
2772
January1st to 10thNight
71%49%12%4
4429
11th to 20thNight
70%7%9%9%5%1
4405
21st to endNight
69%5%9%10%6%
4532
February1st to 10thNight
63%6%10%15%6%1
2470
11th to 20thNight
59%49%19%9%
2405
21st to endNight
57%5%11%19%9%1
1978
March1st to 10thNight
52%5%12%17%12%2
2033
11th to 20thNight
53%5%10%20%11%2
1980
21st to endNight
47%5%9%21%15%3
2156
April1st to 10thNight
53%5%8%21%12%2
1800
11th to 20thNight
48%49%22%16%2
1772
21st to endNight
50%6%9%17%16%2
1620
May1st to 10thNight
46%5%9%20%16%4
1526
11th to 20thNight
39%6%9%25%18%3
1416
21st to endNight
34%6%15%24%18%4
1551
June1st to 10thNight
43%5%13%24%14%1
1440
11th to 20thNight
38%7%11%24%19%2
1440
21st to endNight
42%6%15%23%12%1
1440
July1st to 10thNight
32%6%12%27%20%2
1400
11th to 20thNight
35%6%13%32%14%
1392
21st to endNight
32%6%17%31%13%1
1515
August1st to 10thNight
33%8%18%28%13%1
1600
11th to 20thNight
34%8%17%27%14%1
1598
21st to endNight
37%7%13%30%12%1
1897
September1st to 10thNight
36%8%16%25%14%1
1800
11th to 20thNight
35%5%13%34%12%1
1818
21st to endNight
44%5%11%29%11%
2068
October1st to 10thNight
48%6%11%21%13%
2160
11th to 20thNight
56%6%11%20%6%
2160
21st to endNight
61%6%10%13%10%
2394
November1st to 10thNight
49%6%13%23%9%
2506
11th to 20thNight
55%6%13%17%8%2
2660
21st to endNight
61%7%13%14%4
2660
December1st to 10thNight
68%6%10%11%5%
2660
11th to 20thNight
65%410%14%7%
3288
21st to endNight
76%46%10%4
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
23%24%31%16%6%1
180
11th to 20thDay
26%21%34%14%41
189
21st to endDay
26%19%35%13%5%1
297
February1st to 10thDay
29%21%37%9%3
270
11th to 20thDay
28%26%40%5%1
270
21st to endDay
34%18%36%10%1
287
March1st to 10thDay
24%18%39%15%4
420
11th to 20thDay
26%19%33%18%31
420
21st to endDay
32%18%30%13%7%1
462
April1st to 10thDay
26%22%32%13%7%
420
11th to 20thDay
32%20%21%17%8%3
412
21st to endDay
29%17%33%15%5%1
420
May1st to 10thDay
27%14%30%18%6%5%
423
11th to 20thDay
20%15%35%18%9%3
447
21st to endDay
21%11%35%19%9%4
485
June1st to 10thDay
23%13%34%21%8%1
450
11th to 20thDay
18%12%31%24%11%3
450
21st to endDay
20%9%36%23%10%2
450
July1st to 10thDay
13%8%30%32%13%4
445
11th to 20thDay
15%10%30%25%14%5%
444
21st to endDay
13%11%32%29%10%3
488
August1st to 10thDay
13%7%30%29%13%7%
446
11th to 20thDay
16%5%24%32%16%7%
439
21st to endDay
16%8%32%23%14%7%
495
September1st to 10thDay
15%11%31%23%14%7%
450
11th to 20thDay
12%11%27%22%19%8%
450
21st to endDay
20%12%27%24%11%6%
450
October1st to 10thDay
26%15%23%17%15%4
450
11th to 20thDay
26%23%28%16%6%1
450
21st to endDay
34%18%27%16%5%
485
November1st to 10thDay
27%17%30%17%8%1
262
11th to 20thDay
30%23%32%9%42
180
21st to endDay
33%17%35%13%21
180
December1st to 10thDay
31%22%28%18%2
180
11th to 20thDay
35%22%27%13%21
180
21st to endDay
32%23%31%10%31
198
January1st to 10thNight
28%18%31%18%5%
760
11th to 20thNight
29%15%31%19%5%
760
21st to endNight
20%14%35%24%6%1
836
February1st to 10thNight
22%20%36%19%2
760
11th to 20thNight
20%19%40%17%4
743
21st to endNight
24%21%31%20%4
628
March1st to 10thNight
23%19%35%19%3
723
11th to 20thNight
24%17%34%20%5%1
720
21st to endNight
20%13%34%24%8%1
792
April1st to 10thNight
25%14%33%22%5%
720
11th to 20thNight
23%17%31%20%8%1
708
21st to endNight
23%11%32%23%11%
540
May1st to 10thNight
24%7%29%26%13%2
446
11th to 20thNight
17%9%35%30%9%
354
21st to endNight
12%8%33%34%12%1
389
June1st to 10thNight
17%6%35%30%11%
360
11th to 20thNight
15%9%28%36%11%1
360
21st to endNight
15%7%28%37%13%
360
July1st to 10thNight
6%6%28%42%17%1
350
11th to 20thNight
12%427%37%18%1
348
21st to endNight
11%329%41%16%
424
August1st to 10thNight
12%322%36%25%2
537
11th to 20thNight
10%421%37%25%3
534
21st to endNight
13%5%29%31%20%2
709
September1st to 10thNight
12%7%32%29%18%3
720
11th to 20thNight
14%426%31%21%4
720
21st to endNight
21%10%21%28%18%3
720
October1st to 10thNight
21%12%27%23%15%2
720
11th to 20thNight
28%14%30%20%7%1
720
21st to endNight
30%15%30%20%4
792
November1st to 10thNight
24%13%28%27%8%
740
11th to 20thNight
27%14%34%17%6%2
760
21st to endNight
29%15%29%21%5%1
760
December1st to 10thNight
32%17%32%15%4
760
11th to 20thNight
25%18%35%17%41
760
21st to endNight
34%14%31%17%3
836
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