Thorn Garden Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thorn Garden 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
43%
22620
8%
74663
6%
22571
46%
22571
10%
58603

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
23%
847
4%
2836
3%
846
25%
846
5%
2471
January11th to 20thNight
33%
798
5%
2704
4%
798
34%
798
5%
2322
January21st to endNight
37%
792
6%
2688
5%
792
40%
792
8%
2194
February1st to 10thNight
27%
720
5%
2340
3%
720
30%
720
7%
1957
February11th to 20thNight
33%
700
7%
2274
5%
700
37%
700
8%
1849
February21st to endNight
30%
592
7%
1915
5%
592
33%
592
5%
1659
March1st to 10thNight
38%
720
8%
2066
5%
720
40%
720
9%
1693
March11th to 20thNight
35%
720
8%
1980
4%
720
38%
720
10%
1616
March21st to endNight
35%
792
6%
2144
3%
792
37%
792
8%
1847
April1st to 10thNight
36%
720
5%
1800
1%
720
39%
720
10%
1520
April11th to 20thNight
38%
619
11%
1687
6%
619
41%
619
15%
1329
April21st to endNight
38%
536
8%
1616
5%
536
40%
536
11%
1315
May1st to 10thNight
53%
360
9%
1440
10%
360
57%
360
12%
1054
May11th to 20thNight
56%
354
10%
1416
14%
354
60%
354
11%
1026
May21st to endNight
53%
389
5%
1546
7%
388
57%
388
8%
1217
June1st to 10thNight
49%
360
5%
1260
5%
360
51%
360
10%
973
June11th to 20thNight
48%
360
4%
1260
2%
360
50%
360
13%
938
June21st to endNight
60%
360
8%
1260
6%
360
62%
360
18%
850
July1st to 10thNight
69%
350
10%
1225
11%
350
70%
350
20%
814
July11th to 20thNight
68%
348
14%
1218
15%
348
73%
348
21%
786
July21st to endNight
76%
394
17%
1471
23%
372
84%
372
22%
888
August1st to 10thNight
74%
439
14%
1505
15%
437
78%
437
21%
978
August11th to 20thNight
83%
534
18%
1602
16%
534
85%
534
27%
1043
August21st to endNight
66%
648
18%
1836
11%
648
69%
648
23%
1279
September1st to 10thNight
70%
720
17%
1800
10%
720
71%
720
28%
1256
September11th to 20thNight
63%
720
16%
1826
11%
720
64%
720
21%
1368
September21st to endNight
56%
720
14%
2047
11%
720
59%
720
18%
1511
October1st to 10thNight
55%
720
9%
2160
9%
720
57%
720
14%
1666
October11th to 20thNight
48%
720
10%
2160
8%
720
52%
720
13%
1670
October21st to endNight
40%
792
8%
2471
6%
792
44%
792
11%
1961
November1st to 10thNight
34%
740
6%
2590
5%
740
36%
740
6%
2213
November11th to 20thNight
31%
760
5%
2660
3%
760
34%
760
6%
2257
November21st to endNight
31%
760
4%
2660
2%
760
35%
760
6%
2249
December1st to 10thNight
34%
760
5%
2660
4%
760
36%
760
7%
2216
December11th to 20thNight
26%
828
4%
3425
2%
828
28%
828
4%
2767
December21st to endNight
21%
928
3%
5115
2%
905
24%
905
3%
3851

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%8%48%8%46%6%7%8%33%
74751
Night
7%8%48%7%5%7%6%7%8%34%
74983
TransparencyDay
32%38%11%12%5%1
74442
Night
31%38%11%13%6%1
74810
SeeingDay
12%11%25%29%19%3
14240
Night
14%11%23%29%18%4
22635

By Month

Percentage Cloud Cover NW (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
9%13%6%11%9%5%7%7%9%9%15%
1329
11th to 20thDay
7%12%48%10%5%9%8%10%9%17%
1297
21st to endDay
10%11%49%11%6%10%9%7%8%16%
1665
February1st to 10thDay
11%11%410%9%5%8%7%9%9%17%
1620
11th to 20thDay
10%12%5%11%9%7%8%7%6%7%19%
1575
21st to endDay
8%14%5%9%11%6%10%7%7%7%17%
1340
March1st to 10thDay
5%12%5%9%9%6%8%8%7%9%22%
1891
11th to 20thDay
7%11%5%10%10%5%8%7%8%8%20%
1980
21st to endDay
8%13%5%12%11%6%8%7%6%7%18%
2211
April1st to 10thDay
9%11%5%10%9%5%9%7%8%8%19%
2160
11th to 20thDay
9%11%5%10%9%5%7%7%6%7%23%
2220
21st to endDay
9%9%5%9%9%6%7%6%7%8%25%
2344
May1st to 10thDay
9%7%37%8%5%7%7%7%8%33%
2520
11th to 20thDay
9%7%38%9%46%6%7%8%33%
2477
21st to endDay
12%9%47%7%45%6%6%7%32%
2726
June1st to 10thDay
10%8%47%8%5%6%7%7%8%31%
2700
11th to 20thDay
8%7%39%8%6%7%7%7%9%30%
2700
21st to endDay
7%5%36%6%36%6%7%7%43%
2700
July1st to 10thDay
6%5%25%5%25%5%5%8%53%
2700
11th to 20thDay
5%5%244245%47%57%
2700
21st to endDay
221332345%7%68%
2755
August1st to 10thDay
442443345%8%60%
2423
11th to 20thDay
43245%345%6%8%58%
2298
21st to endDay
44245%346%6%10%50%
2519
September1st to 10thDay
6%425%5%34448%54%
2160
11th to 20thDay
6%6%38%7%35%5%6%6%44%
2133
21st to endDay
7%6%26%6%46%5%7%9%42%
1912
October1st to 10thDay
9%7%27%7%5%5%6%8%9%35%
1800
11th to 20thDay
9%7%48%7%5%7%6%9%9%30%
1800
21st to endDay
10%9%48%8%36%6%8%10%29%
1882
November1st to 10thDay
10%12%5%11%9%6%9%8%8%6%17%
1488
11th to 20thDay
9%10%410%9%5%8%7%7%8%22%
1520
21st to endDay
10%11%5%9%7%39%7%10%8%21%
1520
December1st to 10thDay
8%11%5%9%8%5%8%7%6%8%24%
1520
11th to 20thDay
6%13%5%11%11%5%8%8%7%8%16%
1724
21st to endDay
10%13%5%11%11%6%7%9%7%7%14%
2442
January1st to 10thNight
9%11%5%10%9%6%8%8%8%7%19%
2836
11th to 20thNight
7%9%5%9%11%6%8%9%8%8%20%
2704
21st to endNight
10%9%48%8%5%8%7%9%10%22%
2688
February1st to 10thNight
11%11%5%9%9%5%7%6%8%8%22%
2340
11th to 20thNight
8%10%5%10%8%5%7%6%8%8%25%
2272
21st to endNight
10%13%5%10%8%5%9%7%6%7%20%
1915
March1st to 10thNight
6%8%37%7%6%9%8%10%10%26%
2066
11th to 20thNight
6%8%47%7%5%8%9%8%10%28%
1979
21st to endNight
7%10%410%9%6%9%8%8%8%21%
2144
April1st to 10thNight
8%9%410%9%5%6%8%8%8%27%
1800
11th to 20thNight
7%9%47%7%47%6%7%10%32%
1687
21st to endNight
8%7%38%10%5%6%6%6%8%33%
1616
May1st to 10thNight
5%5%36%447%5%6%8%45%
1440
11th to 20thNight
6%6%36%7%36%6%6%10%40%
1416
21st to endNight
8%5%36%7%36%5%7%9%40%
1546
June1st to 10thNight
9%5%37%8%45%5%5%7%43%
1260
11th to 20thNight
5%7%25%6%35%5%6%10%45%
1260
21st to endNight
5%3235%236%6%10%55%
1260
July1st to 10thNight
441342345%7%63%
1260
11th to 20thNight
2324423437%66%
1260
21st to endNight
2111213345%77%
1558
August1st to 10thNight
231342235%7%68%
1513
11th to 20thNight
321232335%8%69%
1602
21st to endNight
43133345%6%8%60%
1836
September1st to 10thNight
431442445%7%62%
1800
11th to 20thNight
8%5%25%5%45%5%5%7%49%
1826
21st to endNight
7%426%6%45%5%6%8%47%
2047
October1st to 10thNight
7%7%38%7%45%6%6%6%41%
2160
11th to 20thNight
8%7%37%8%46%6%7%9%35%
2160
21st to endNight
11%7%38%8%46%6%7%8%34%
2471
November1st to 10thNight
10%10%49%8%6%8%7%7%8%22%
2590
11th to 20thNight
8%9%49%9%6%7%7%8%9%24%
2660
21st to endNight
8%9%410%9%6%8%7%8%7%23%
2660
December1st to 10thNight
6%11%5%10%9%5%9%6%7%7%24%
2660
11th to 20thNight
7%11%5%11%9%6%9%8%7%8%18%
3477
21st to endNight
8%12%5%11%10%6%8%7%7%7%19%
5214

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
39%43%9%7%2
1329
11th to 20thDay
35%44%10%6%32
1297
21st to endDay
39%38%8%10%41
1665
February1st to 10thDay
40%39%9%8%3
1620
11th to 20thDay
40%38%9%8%5%1
1584
21st to endDay
43%40%8%7%2
1340
March1st to 10thDay
38%39%10%9%31
1891
11th to 20thDay
41%39%8%8%31
1980
21st to endDay
46%38%7%7%2
2211
April1st to 10thDay
42%39%9%8%1
2160
11th to 20thDay
40%39%5%9%6%
2220
21st to endDay
40%38%8%11%3
2344
May1st to 10thDay
35%40%9%12%3
2520
11th to 20thDay
30%39%12%13%6%
2480
21st to endDay
40%38%9%10%3
2727
June1st to 10thDay
38%39%11%7%41
2700
11th to 20thDay
38%39%13%9%1
2700
21st to endDay
26%39%15%12%7%1
2700
July1st to 10thDay
20%39%12%18%8%2
2626
11th to 20thDay
18%38%14%21%9%1
2619
21st to endDay
14%38%10%23%11%3
2602
August1st to 10thDay
17%37%16%22%7%
2406
11th to 20thDay
19%37%12%20%10%2
2298
21st to endDay
20%39%13%22%6%1
2519
September1st to 10thDay
17%37%13%18%12%3
2160
11th to 20thDay
25%36%11%15%11%2
2133
21st to endDay
25%39%12%14%8%2
1912
October1st to 10thDay
28%37%12%16%6%1
1800
11th to 20thDay
31%39%13%13%41
1800
21st to endDay
31%39%13%12%6%
1882
November1st to 10thDay
42%42%6%6%41
1488
11th to 20thDay
36%43%9%8%3
1520
21st to endDay
37%42%8%9%3
1520
December1st to 10thDay
34%42%12%9%4
1520
11th to 20thDay
42%33%12%11%1
1727
21st to endDay
47%27%13%10%2
2442
January1st to 10thNight
39%42%9%8%2
2836
11th to 20thNight
36%45%9%5%22
2704
21st to endNight
35%38%9%9%7%2
2688
February1st to 10thNight
39%38%10%10%4
2340
11th to 20thNight
34%39%10%10%6%1
2274
21st to endNight
41%39%7%10%3
1915
March1st to 10thNight
33%39%11%12%4
2066
11th to 20thNight
33%39%14%10%5%
1980
21st to endNight
39%39%10%10%2
2144
April1st to 10thNight
36%39%11%10%3
1800
11th to 20thNight
29%37%11%14%8%1
1687
21st to endNight
34%38%9%13%5%
1616
May1st to 10thNight
27%37%14%16%6%
1440
11th to 20thNight
25%38%14%15%7%1
1416
21st to endNight
34%38%12%12%4
1546
June1st to 10thNight
30%38%12%13%5%1
1260
11th to 20thNight
26%38%20%13%3
1260
21st to endNight
17%37%19%16%7%3
1260
July1st to 10thNight
14%39%14%21%8%4
1225
11th to 20thNight
14%37%13%23%10%3
1218
21st to endNight
8%38%8%25%15%5%
1471
August1st to 10thNight
14%37%14%25%9%2
1505
11th to 20thNight
11%36%15%20%13%5%
1602
21st to endNight
16%37%12%24%10%1
1836
September1st to 10thNight
12%38%11%20%14%5%
1800
11th to 20thNight
21%37%10%15%12%4
1826
21st to endNight
24%36%12%16%9%3
2047
October1st to 10thNight
28%38%9%14%11%1
2160
11th to 20thNight
29%37%11%16%7%
2160
21st to endNight
31%39%11%12%8%
2471
November1st to 10thNight
37%43%6%10%4
2590
11th to 20thNight
35%44%11%8%21
2660
21st to endNight
36%42%9%9%4
2660
December1st to 10thNight
35%42%11%8%4
2660
11th to 20thNight
40%34%14%10%1
3473
21st to endNight
44%27%13%13%3
5214

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%22%22%21%10%2
180
11th to 20thDay
21%15%30%17%15%1
164
21st to endDay
20%15%28%21%15%1
257
February1st to 10thDay
21%20%33%20%7%
340
11th to 20thDay
22%18%27%23%10%1
335
21st to endDay
15%24%27%24%10%
284
March1st to 10thDay
15%17%29%26%12%1
420
11th to 20thDay
13%19%26%26%14%1
420
21st to endDay
15%17%28%29%10%1
462
April1st to 10thDay
15%18%28%25%13%2
420
11th to 20thDay
14%16%25%26%15%3
412
21st to endDay
15%15%29%29%11%1
420
May1st to 10thDay
9%11%27%33%19%1
423
11th to 20thDay
12%8%20%36%20%4
448
21st to endDay
11%7%28%30%18%5%
488
June1st to 10thDay
13%8%32%29%15%3
630
11th to 20thDay
8%11%38%29%12%2
630
21st to endDay
6%7%29%32%22%4
630
July1st to 10thDay
8%5%21%38%26%2
621
11th to 20thDay
47%26%35%25%4
618
21st to endDay
1320%34%32%10%
501
August1st to 10thDay
6%415%30%35%10%
447
11th to 20thDay
3214%32%40%9%
437
21st to endDay
5%6%21%36%26%6%
495
September1st to 10thDay
9%421%40%24%2
450
11th to 20thDay
10%7%17%35%27%4
450
21st to endDay
12%8%22%33%23%3
450
October1st to 10thDay
14%9%22%26%23%6%
450
11th to 20thDay
16%16%20%25%18%4
450
21st to endDay
17%19%28%22%11%3
410
November1st to 10thDay
27%14%27%23%9%
180
11th to 20thDay
21%13%23%21%19%3
180
21st to endDay
19%17%22%26%13%4
180
December1st to 10thDay
23%11%24%22%17%2
180
11th to 20thDay
19%20%26%23%11%1
180
21st to endDay
25%15%30%25%41
198
January1st to 10thNight
22%21%27%18%10%2
850
11th to 20thNight
19%18%21%24%17%1
798
21st to endNight
18%13%24%25%17%3
792
February1st to 10thNight
20%16%27%27%9%2
720
11th to 20thNight
17%15%23%28%15%1
704
21st to endNight
19%17%24%25%12%3
592
March1st to 10thNight
13%15%27%27%15%3
720
11th to 20thNight
13%16%29%25%14%3
720
21st to endNight
14%13%28%30%13%3
792
April1st to 10thNight
17%11%27%27%17%2
720
11th to 20thNight
11%12%29%31%15%2
619
21st to endNight
10%10%32%33%11%4
536
May1st to 10thNight
8%7%26%35%21%3
360
11th to 20thNight
8%6%19%38%25%4
354
21st to endNight
10%5%21%41%21%2
389
June1st to 10thNight
12%5%23%37%21%1
360
11th to 20thNight
8%5%30%39%16%2
360
21st to endNight
6%326%33%27%5%
360
July1st to 10thNight
6%217%38%33%3
350
11th to 20thNight
3420%43%29%3
348
21st to endNight
2119%38%32%7%
394
August1st to 10thNight
3413%31%40%9%
439
11th to 20thNight
417%36%40%12%
534
21st to endNight
6%320%33%31%6%
648
September1st to 10thNight
6%315%37%30%9%
720
11th to 20thNight
10%414%31%28%13%
720
21st to endNight
11%5%19%31%26%8%
720
October1st to 10thNight
12%7%16%29%28%8%
720
11th to 20thNight
13%11%18%27%22%8%
720
21st to endNight
18%12%22%28%15%4
792
November1st to 10thNight
21%14%26%27%12%1
740
11th to 20thNight
17%19%26%22%14%3
760
21st to endNight
18%18%27%22%11%4
760
December1st to 10thNight
19%13%26%24%15%3
760
11th to 20thNight
19%18%28%26%8%1
829
21st to endNight
21%19%31%22%6%1
935
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Copyright 2024 Attilla Danko


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