Third Stone Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Third Stone Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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
39%
23581
12%
80765
8%
23527
44%
23527
18%
54718

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
34%
934
13%
4675
7%
926
39%
926
11%
3174
January11th to 20thNight
31%
940
19%
4618
7%
930
38%
930
11%
3034
January21st to endNight
31%
895
11%
4930
9%
887
39%
887
10%
3075
February1st to 10thNight
36%
760
14%
2660
11%
760
43%
760
15%
1919
February11th to 20thNight
32%
741
12%
2438
9%
740
39%
740
15%
1755
February21st to endNight
44%
610
15%
1912
13%
610
50%
610
21%
1338
March1st to 10thNight
38%
720
15%
2160
10%
720
44%
720
17%
1590
March11th to 20thNight
38%
720
15%
1999
8%
720
43%
720
21%
1432
March21st to endNight
37%
792
11%
2150
4%
792
42%
792
18%
1615
April1st to 10thNight
36%
644
13%
1724
5%
644
41%
644
20%
1266
April11th to 20thNight
36%
533
14%
1601
10%
533
40%
533
18%
1150
April21st to endNight
40%
540
12%
1620
10%
540
44%
540
22%
1082
May1st to 10thNight
38%
414
8%
1386
7%
414
42%
414
15%
1012
May11th to 20thNight
47%
354
9%
1239
7%
354
50%
354
21%
814
May21st to endNight
44%
388
2%
1358
1%
388
44%
388
16%
950
June1st to 10thNight
43%
360
0%
1260
0%
360
43%
360
14%
915
June11th to 20thNight
40%
360
0%
1260
0%
360
40%
360
12%
957
June21st to endNight
53%
360
0%
1260
0%
360
53%
360
21%
821
July1st to 10thNight
60%
350
0%
1225
0%
350
60%
350
25%
755
July11th to 20thNight
53%
348
1%
1218
0%
348
53%
348
21%
806
July21st to endNight
62%
394
6%
1302
7%
372
63%
372
27%
760
August1st to 10thNight
53%
358
7%
1398
11%
356
55%
356
18%
897
August11th to 20thNight
57%
478
11%
1546
12%
478
60%
478
25%
992
August21st to endNight
55%
594
15%
1782
12%
594
58%
594
27%
1147
September1st to 10thNight
51%
595
16%
1688
12%
595
53%
595
27%
1106
September11th to 20thNight
51%
720
18%
1980
12%
720
54%
720
29%
1279
September21st to endNight
48%
720
20%
1980
14%
720
52%
720
30%
1288
October1st to 10thNight
45%
720
17%
2078
11%
720
50%
720
25%
1411
October11th to 20thNight
39%
720
18%
2301
11%
720
46%
720
22%
1513
October21st to endNight
36%
792
16%
2574
9%
792
46%
792
19%
1790
November1st to 10thNight
31%
899
15%
2564
9%
899
36%
899
18%
1890
November11th to 20thNight
32%
950
15%
2778
9%
950
38%
950
18%
2009
November21st to endNight
31%
950
11%
2850
5%
950
37%
950
19%
2016
December1st to 10thNight
33%
950
15%
2850
9%
950
38%
950
18%
2063
December11th to 20thNight
32%
949
13%
3475
7%
949
38%
949
16%
2440
December21st to endNight
31%
1029
11%
4926
7%
1026
38%
1026
15%
2657

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
10%9%47%6%45%6%6%9%34%
77816
Night
8%10%48%7%45%6%6%8%33%
81437
TransparencyDay
46%8%16%22%8%1
77518
Night
47%7%14%21%9%2
81315
SeeingDay
13%7%26%30%19%5%
14364
Night
13%6%29%35%15%2
23642

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
411%5%10%10%5%6%5%5%8%30%
2219
11th to 20thDay
5%8%5%9%7%37%8%9%9%31%
2199
21st to endDay
5%11%5%9%8%445%6%9%36%
2698
February1st to 10thDay
9%13%5%10%6%45%6%7%8%27%
1520
11th to 20thDay
6%15%5%11%8%47%5%5%7%26%
1629
21st to endDay
411%7%10%9%5%8%6%6%7%28%
1435
March1st to 10thDay
6%12%6%11%10%5%6%6%6%8%24%
1800
11th to 20thDay
8%11%5%8%6%346%7%10%33%
1959
21st to endDay
11%12%6%8%7%36%5%7%9%25%
2204
April1st to 10thDay
9%12%5%9%8%45%6%6%8%28%
2236
11th to 20thDay
12%14%5%8%7%45%5%6%8%26%
2310
21st to endDay
13%10%47%7%5%5%5%6%8%31%
2340
May1st to 10thDay
11%12%5%7%6%46%7%6%8%28%
2571
11th to 20thDay
8%6%37%6%45%6%6%10%38%
2654
21st to endDay
18%8%36%5%45%6%6%8%32%
2917
June1st to 10thDay
16%7%27%6%46%6%7%9%31%
2700
11th to 20thDay
19%8%47%7%5%6%5%6%8%25%
2700
21st to endDay
10%5%26%6%36%6%9%10%37%
2700
July1st to 10thDay
11%6%26%5%35%6%7%10%38%
2700
11th to 20thDay
12%5%25%6%35%6%6%10%39%
2700
21st to endDay
8%426%5%35%5%6%10%46%
2938
August1st to 10thDay
10%6%246%46%6%7%9%39%
2531
11th to 20thDay
9%5%25%5%446%6%11%44%
2353
21st to endDay
11%6%35%435%6%7%9%43%
2574
September1st to 10thDay
12%5%36%6%35%5%6%8%41%
2269
11th to 20thDay
16%7%35%5%2336%9%42%
1980
21st to endDay
9%8%35%5%35%5%6%8%43%
1980
October1st to 10thDay
12%10%47%5%25%446%40%
1882
11th to 20thDay
12%7%47%6%446%5%7%39%
1656
21st to endDay
9%11%5%8%7%46%6%6%8%31%
1782
November1st to 10thDay
7%16%6%8%8%46%6%5%8%26%
1495
11th to 20thDay
6%12%6%10%8%36%46%7%32%
1401
21st to endDay
6%8%411%7%5%6%6%6%8%32%
1330
December1st to 10thDay
7%13%5%10%8%6%7%45%7%27%
1330
11th to 20thDay
5%11%5%11%8%5%7%6%7%8%27%
1682
21st to endDay
48%38%7%345%5%10%42%
2442
January1st to 10thNight
413%5%12%9%46%6%6%8%28%
4726
11th to 20thNight
48%5%10%11%46%7%6%7%32%
4699
21st to endNight
6%13%48%8%36%6%6%8%31%
4987
February1st to 10thNight
6%12%5%10%9%5%6%6%6%7%28%
2660
11th to 20thNight
6%13%6%10%9%45%6%6%8%27%
2437
21st to endNight
6%11%5%9%8%5%7%6%6%7%31%
1912
March1st to 10thNight
5%12%5%9%9%6%6%6%7%8%28%
2160
11th to 20thNight
6%9%5%9%8%47%7%7%9%29%
1999
21st to endNight
7%13%7%10%6%5%5%7%6%9%25%
2150
April1st to 10thNight
7%12%48%8%5%6%5%7%8%29%
1724
11th to 20thNight
12%10%49%8%45%6%6%8%28%
1601
21st to endNight
13%9%5%7%5%35%5%6%9%35%
1620
May1st to 10thNight
14%11%6%8%7%45%6%6%8%26%
1386
11th to 20thNight
11%7%36%7%36%6%6%8%37%
1239
21st to endNight
19%8%36%6%45%5%7%7%30%
1358
June1st to 10thNight
18%7%47%5%46%6%7%8%27%
1260
11th to 20thNight
18%8%5%7%8%46%6%7%7%22%
1260
21st to endNight
9%5%36%7%5%6%7%8%8%36%
1260
July1st to 10thNight
11%6%25%5%35%5%6%11%41%
1260
11th to 20thNight
8%6%36%7%35%7%8%9%38%
1260
21st to endNight
9%4345%45%6%7%9%44%
1379
August1st to 10thNight
11%6%26%6%47%6%7%8%37%
1405
11th to 20thNight
10%6%35%5%35%7%7%10%39%
1546
21st to endNight
10%5%26%5%35%5%7%9%42%
1782
September1st to 10thNight
11%6%36%6%45%5%5%7%42%
1688
11th to 20thNight
14%7%26%5%3445%7%44%
1980
21st to endNight
9%7%36%7%345%6%9%42%
1980
October1st to 10thNight
10%9%47%6%45%45%8%39%
2078
11th to 20thNight
10%8%48%6%35%46%8%38%
2301
21st to endNight
8%10%5%7%7%5%6%6%6%9%32%
2574
November1st to 10thNight
10%15%6%9%6%35%5%6%7%29%
2564
11th to 20thNight
7%12%5%9%8%46%5%6%8%29%
2778
21st to endNight
6%10%5%9%8%45%7%6%8%31%
2850
December1st to 10thNight
7%11%6%10%7%45%5%5%8%31%
2850
11th to 20thNight
410%6%12%10%45%5%7%9%28%
3510
21st to endNight
48%47%6%35%6%7%8%42%
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
51%5%12%20%10%1
2219
11th to 20thDay
45%8%10%15%19%4
2220
21st to endDay
45%6%14%27%6%1
2698
February1st to 10thDay
53%5%13%16%11%1
1520
11th to 20thDay
55%7%13%16%8%1
1638
21st to endDay
54%6%12%19%10%
1435
March1st to 10thDay
57%7%14%16%5%
1800
11th to 20thDay
45%7%14%22%12%1
1960
21st to endDay
54%9%16%15%6%1
2204
April1st to 10thDay
53%8%13%16%9%1
2236
11th to 20thDay
54%5%11%20%9%
2310
21st to endDay
50%6%13%22%6%2
2340
May1st to 10thDay
52%8%14%19%7%
2571
11th to 20thDay
40%7%14%28%10%1
2658
21st to endDay
49%8%12%21%9%
2918
June1st to 10thDay
48%8%15%23%7%
2700
11th to 20thDay
55%9%14%17%41
2700
21st to endDay
38%11%23%23%5%1
2700
July1st to 10thDay
39%11%20%24%6%1
2626
11th to 20thDay
39%10%25%23%3
2619
21st to endDay
33%9%23%28%5%
2775
August1st to 10thDay
38%11%23%23%5%
2513
11th to 20thDay
33%9%26%24%8%
2353
21st to endDay
36%7%20%28%9%1
2574
September1st to 10thDay
39%7%19%24%8%3
2269
11th to 20thDay
40%7%16%29%8%1
1980
21st to endDay
38%7%17%26%10%2
1980
October1st to 10thDay
45%6%16%23%10%1
1882
11th to 20thDay
42%7%16%23%10%1
1656
21st to endDay
49%8%11%23%10%1
1782
November1st to 10thDay
56%6%13%18%6%1
1495
11th to 20thDay
51%6%14%20%8%
1401
21st to endDay
48%8%16%19%6%2
1330
December1st to 10thDay
57%6%11%16%8%2
1330
11th to 20thDay
50%11%15%17%6%1
1684
21st to endDay
38%5%10%24%18%4
2442
January1st to 10thNight
52%7%13%18%8%2
4726
11th to 20thNight
51%5%11%14%16%3
4740
21st to endNight
48%6%14%22%8%1
4987
February1st to 10thNight
54%6%10%19%10%1
2660
11th to 20thNight
54%6%12%16%11%1
2438
21st to endNight
50%5%12%21%11%1
1912
March1st to 10thNight
52%8%11%21%7%1
2160
11th to 20thNight
50%6%13%19%11%1
1999
21st to endNight
52%8%13%17%8%1
2150
April1st to 10thNight
50%7%13%19%10%1
1724
11th to 20thNight
52%6%11%20%8%2
1601
21st to endNight
45%6%15%23%10%1
1620
May1st to 10thNight
54%7%11%19%8%1
1386
11th to 20thNight
44%8%11%23%14%
1239
21st to endNight
51%7%12%19%9%1
1358
June1st to 10thNight
52%10%12%19%7%
1260
11th to 20thNight
57%9%12%17%41
1260
21st to endNight
39%12%19%23%6%1
1260
July1st to 10thNight
36%10%21%26%6%1
1225
11th to 20thNight
40%12%21%22%5%
1218
21st to endNight
33%9%23%28%7%
1302
August1st to 10thNight
41%11%20%23%5%
1398
11th to 20thNight
36%11%20%25%7%1
1546
21st to endNight
38%8%16%26%10%1
1782
September1st to 10thNight
41%7%15%24%11%2
1688
11th to 20thNight
39%6%14%28%9%3
1980
21st to endNight
39%6%13%26%14%3
1980
October1st to 10thNight
44%7%15%22%12%1
2078
11th to 20thNight
44%6%14%25%10%1
2301
21st to endNight
47%9%14%21%8%1
2574
November1st to 10thNight
54%6%12%20%7%1
2564
11th to 20thNight
51%7%12%19%9%1
2778
21st to endNight
49%7%17%19%7%2
2850
December1st to 10thNight
52%6%10%20%10%2
2850
11th to 20thNight
51%6%16%20%6%
3507
21st to endNight
37%6%13%24%15%5%
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
48%29%33%19%6%
180
11th to 20thDay
8%8%32%31%16%6%
180
21st to endDay
9%11%29%25%21%6%
198
February1st to 10thDay
14%8%31%31%11%5%
180
11th to 20thDay
14%7%32%30%13%4
254
21st to endDay
5%7%31%38%12%7%
277
March1st to 10thDay
12%11%33%24%18%3
420
11th to 20thDay
10%8%35%29%13%6%
420
21st to endDay
15%8%31%27%16%3
462
April1st to 10thDay
11%14%34%26%11%4
420
11th to 20thDay
18%12%22%29%14%5%
412
21st to endDay
16%10%28%31%12%3
420
May1st to 10thDay
14%10%24%31%17%5%
531
11th to 20thDay
10%6%22%36%22%4
625
21st to endDay
17%5%21%29%21%7%
679
June1st to 10thDay
18%6%24%33%17%3
630
11th to 20thDay
21%7%22%26%20%4
630
21st to endDay
12%321%29%26%10%
630
July1st to 10thDay
14%418%31%25%9%
621
11th to 20thDay
15%5%19%32%24%5%
618
21st to endDay
7%419%31%29%10%
684
August1st to 10thDay
12%5%19%30%23%11%
471
11th to 20thDay
10%423%31%25%7%
437
21st to endDay
13%6%18%30%24%9%
495
September1st to 10thDay
15%8%28%24%22%3
450
11th to 20thDay
18%8%27%25%18%4
450
21st to endDay
12%13%29%26%15%5%
450
October1st to 10thDay
18%8%24%28%16%7%
352
11th to 20thDay
11%10%32%29%14%4
270
21st to endDay
13%13%33%28%11%1
297
November1st to 10thDay
19%6%32%30%10%4
270
11th to 20thDay
12%8%34%34%10%2
213
21st to endDay
7%9%35%29%14%6%
180
December1st to 10thDay
9%7%34%27%17%6%
180
11th to 20thDay
10%9%37%28%14%2
180
21st to endDay
11%5%25%35%18%6%
198
January1st to 10thNight
12%8%33%31%14%1
950
11th to 20thNight
12%8%34%33%11%2
950
21st to endNight
14%10%32%31%12%2
909
February1st to 10thNight
10%10%28%32%18%2
760
11th to 20thNight
12%6%34%32%15%1
744
21st to endNight
9%428%37%18%4
610
March1st to 10thNight
10%5%30%37%14%3
720
11th to 20thNight
11%434%35%14%3
720
21st to endNight
14%5%29%35%15%2
792
April1st to 10thNight
13%8%30%32%17%1
644
11th to 20thNight
17%7%27%39%10%1
533
21st to endNight
16%8%24%36%14%1
540
May1st to 10thNight
18%7%25%36%14%1
414
11th to 20thNight
13%5%26%35%20%2
354
21st to endNight
24%5%20%33%16%3
389
June1st to 10thNight
21%8%18%36%14%3
360
11th to 20thNight
21%7%20%35%16%1
360
21st to endNight
11%323%39%22%2
360
July1st to 10thNight
14%215%45%23%1
350
11th to 20thNight
13%320%41%22%1
348
21st to endNight
9%418%38%28%2
394
August1st to 10thNight
11%420%31%30%4
358
11th to 20thNight
13%417%38%25%2
478
21st to endNight
11%421%40%23%1
594
September1st to 10thNight
14%322%39%19%3
595
11th to 20thNight
16%423%38%18%1
720
21st to endNight
11%428%41%15%2
720
October1st to 10thNight
15%428%37%14%2
720
11th to 20thNight
14%5%32%36%11%2
720
21st to endNight
13%8%33%34%12%
792
November1st to 10thNight
16%7%35%31%10%1
899
11th to 20thNight
14%7%36%30%11%2
950
21st to endNight
11%5%39%33%11%2
950
December1st to 10thNight
14%7%32%32%12%3
950
11th to 20thNight
11%7%37%32%12%2
950
21st to endNight
11%436%31%16%2
1045
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