Joey's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joey's Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
58%
24616
20%
74540
16%
24563
63%
24563
28%
47530

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
44%
758
23%
2459
16%
757
54%
757
28%
1466
January11th to 20thNight
51%
744
25%
2417
20%
744
61%
744
30%
1417
January21st to endNight
39%
792
23%
2532
15%
792
50%
792
26%
1559
February1st to 10thNight
45%
720
22%
2160
15%
720
52%
720
30%
1324
February11th to 20thNight
47%
700
23%
2099
14%
700
56%
700
34%
1216
February21st to endNight
45%
592
29%
1653
18%
592
55%
592
38%
969
March1st to 10thNight
48%
720
30%
1980
18%
720
59%
720
45%
1102
March11th to 20thNight
54%
720
27%
1980
19%
720
61%
720
40%
1145
March21st to endNight
53%
792
28%
2178
19%
792
60%
792
40%
1250
April1st to 10thNight
52%
720
24%
1980
17%
720
57%
720
41%
1135
April11th to 20thNight
51%
711
21%
1903
15%
711
55%
711
34%
1191
April21st to endNight
58%
720
27%
1800
17%
720
62%
720
45%
1059
May1st to 10thNight
66%
637
28%
1717
19%
637
70%
637
45%
960
May11th to 20thNight
67%
531
16%
1592
12%
531
69%
531
31%
991
May21st to endNight
71%
582
20%
1745
17%
582
71%
582
29%
1123
June1st to 10thNight
65%
540
13%
1620
10%
540
66%
540
20%
1166
June11th to 20thNight
72%
540
13%
1620
10%
540
72%
540
23%
1130
June21st to endNight
69%
540
9%
1620
8%
540
69%
540
22%
1149
July1st to 10thNight
70%
525
5%
1575
4%
525
70%
525
17%
1202
July11th to 20thNight
72%
522
6%
1566
6%
522
72%
522
19%
1165
July21st to endNight
73%
591
8%
1672
8%
558
74%
558
19%
1236
August1st to 10thNight
77%
537
7%
1600
6%
534
77%
534
21%
1141
August11th to 20thNight
75%
587
7%
1669
6%
587
75%
587
23%
1193
August21st to endNight
62%
792
8%
2178
7%
792
62%
792
17%
1658
September1st to 10thNight
64%
720
8%
1980
8%
720
64%
720
16%
1499
September11th to 20thNight
58%
720
7%
1980
7%
720
58%
720
17%
1511
September21st to endNight
61%
720
16%
1980
16%
720
62%
720
21%
1418
October1st to 10thNight
61%
720
20%
2016
19%
720
64%
720
27%
1351
October11th to 20thNight
69%
720
27%
2160
27%
720
72%
720
35%
1268
October21st to endNight
61%
792
27%
2376
25%
792
66%
792
33%
1404
November1st to 10thNight
56%
740
25%
2221
22%
740
63%
740
33%
1324
November11th to 20thNight
54%
760
26%
2280
22%
760
61%
760
29%
1424
November21st to endNight
51%
760
25%
2346
20%
760
60%
760
29%
1437
December1st to 10thNight
57%
760
27%
2470
22%
760
66%
760
34%
1385
December11th to 20thNight
50%
760
24%
2946
21%
758
56%
758
24%
1745
December21st to endNight
51%
831
14%
4470
21%
817
57%
817
16%
2817

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%37%7%46%6%7%8%37%
74912
Night
8%5%36%6%35%5%6%9%44%
74905
TransparencyDay
41%10%14%16%13%5%
74635
Night
36%10%16%20%14%5%
74701
SeeingDay
8%5%18%25%24%20%
14492
Night
9%419%34%27%8%
24628

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
11%6%34435%5%5%8%46%
1706
11th to 20thDay
7%6%35%5%35%5%6%8%48%
1671
21st to endDay
13%6%36%5%4346%7%44%
1821
February1st to 10thDay
12%5%36%6%345%6%8%42%
1800
11th to 20thDay
6%425%5%45%5%5%6%53%
1748
21st to endDay
9%7%25%5%36%5%6%7%45%
1601
March1st to 10thDay
4435%5%25%5%5%8%55%
1980
11th to 20thDay
8%4245%36%5%6%8%49%
1980
21st to endDay
7%435%6%35%5%6%7%51%
2178
April1st to 10thDay
6%425%7%46%5%7%8%48%
1980
11th to 20thDay
9%5%25%7%46%7%7%9%39%
2003
21st to endDay
5%325%6%35%5%7%9%49%
2160
May1st to 10thDay
5%3235%35%46%8%55%
2256
11th to 20thDay
9%437%7%46%6%7%8%38%
2301
21st to endDay
7%6%38%8%5%7%7%8%8%34%
2531
June1st to 10thDay
14%7%49%8%6%8%7%7%8%23%
2340
11th to 20thDay
6%5%49%10%6%8%8%8%9%27%
2340
21st to endDay
9%7%48%9%6%8%8%7%8%27%
2340
July1st to 10thDay
9%6%48%10%6%9%9%7%9%22%
2340
11th to 20thDay
8%6%49%10%6%8%7%8%9%24%
2340
21st to endDay
7%7%49%10%6%9%8%8%10%23%
2547
August1st to 10thDay
5%6%38%10%6%9%9%9%11%23%
2327
11th to 20thDay
6%6%39%9%7%9%8%9%11%23%
2207
21st to endDay
11%7%49%11%47%7%9%9%22%
2178
September1st to 10thDay
8%7%39%10%6%9%8%8%10%23%
1980
11th to 20thDay
9%7%49%9%8%9%7%8%8%22%
1980
21st to endDay
9%7%48%9%5%8%8%6%10%26%
1980
October1st to 10thDay
12%5%38%8%5%6%7%7%8%32%
1944
11th to 20thDay
7%426%6%45%6%6%10%44%
1800
21st to endDay
6%425%7%36%6%6%9%46%
1980
November1st to 10thDay
8%435%6%36%6%7%9%45%
1857
11th to 20thDay
9%6%26%6%45%6%6%8%43%
1900
21st to endDay
9%5%36%7%45%6%6%9%41%
1829
December1st to 10thDay
7%437%6%45%5%5%8%46%
1710
11th to 20thDay
11%7%36%6%35%5%5%8%42%
2153
21st to endDay
12%6%38%7%5%6%6%6%7%35%
3124
January1st to 10thNight
10%5%35%5%35%5%6%7%46%
2459
11th to 20thNight
9%5%35%425%5%6%9%47%
2417
21st to endNight
12%6%25%5%35%5%5%6%45%
2532
February1st to 10thNight
10%7%35%43445%8%46%
2160
11th to 20thNight
9%4245%345%5%7%52%
2098
21st to endNight
8%5%25%5%35%45%8%52%
1653
March1st to 10thNight
5%3234245%5%8%59%
1980
11th to 20thNight
8%325%42446%8%54%
1980
21st to endNight
6%3244345%5%8%55%
2178
April1st to 10thNight
6%424425%45%9%54%
1980
11th to 20thNight
7%5%36%5%35%5%6%8%49%
1903
21st to endNight
5%4245%3446%7%57%
1800
May1st to 10thNight
4224435%36%7%60%
1717
11th to 20thNight
7%436%5%345%6%9%48%
1592
21st to endNight
5%436%5%46%6%7%9%43%
1744
June1st to 10thNight
10%8%37%6%46%7%6%10%32%
1620
11th to 20thNight
6%7%36%6%47%7%7%12%35%
1620
21st to endNight
8%7%37%6%47%6%8%12%32%
1620
July1st to 10thNight
7%7%37%7%5%7%6%9%12%30%
1620
11th to 20thNight
6%5%46%8%5%7%8%9%11%30%
1620
21st to endNight
5%5%38%8%5%7%6%10%12%30%
1771
August1st to 10thNight
45%37%7%48%7%9%13%34%
1611
11th to 20thNight
35%37%7%5%7%8%9%12%32%
1669
21st to endNight
10%7%37%7%47%6%8%10%30%
2178
September1st to 10thNight
7%6%37%8%47%7%10%11%31%
1980
11th to 20thNight
11%7%39%7%47%6%7%10%29%
1980
21st to endNight
10%7%37%7%46%6%7%9%34%
1980
October1st to 10thNight
8%5%36%7%446%7%8%41%
2016
11th to 20thNight
6%425%6%35%6%6%8%50%
2160
21st to endNight
8%6%25%5%3445%8%51%
2376
November1st to 10thNight
9%5%245%35%5%5%8%50%
2221
11th to 20thNight
8%6%346%35%5%6%7%46%
2280
21st to endNight
8%6%35%5%35%46%9%44%
2346
December1st to 10thNight
8%425%5%35%5%6%10%49%
2470
11th to 20thNight
12%7%25%4345%5%6%48%
3042
21st to endNight
14%7%36%6%46%5%6%7%36%
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
36%6%10%20%19%9%
1707
11th to 20thDay
33%6%8%23%23%8%
1671
21st to endDay
40%410%17%20%10%
1821
February1st to 10thDay
38%7%11%14%20%10%
1800
11th to 20thDay
30%6%11%19%23%11%
1758
21st to endDay
36%6%10%16%23%8%
1601
March1st to 10thDay
27%6%11%22%25%8%
1980
11th to 20thDay
31%7%12%24%16%10%
1980
21st to endDay
31%7%12%22%20%8%
2178
April1st to 10thDay
33%8%13%22%17%8%
1980
11th to 20thDay
38%10%11%19%18%5%
2003
21st to endDay
30%7%11%21%23%9%
2160
May1st to 10thDay
26%6%11%23%24%10%
2256
11th to 20thDay
40%10%15%18%15%2
2305
21st to endDay
44%10%12%17%15%1
2531
June1st to 10thDay
54%12%15%14%4
2340
11th to 20thDay
48%13%20%16%3
2340
21st to endDay
51%13%20%12%31
2340
July1st to 10thDay
53%17%20%8%2
2275
11th to 20thDay
51%16%22%11%
2271
21st to endDay
50%17%20%12%2
2405
August1st to 10thDay
47%19%25%9%1
2314
11th to 20thDay
49%19%23%8%1
2207
21st to endDay
53%16%19%9%3
2178
September1st to 10thDay
51%16%18%10%4
1980
11th to 20thDay
52%15%19%11%3
1980
21st to endDay
48%12%16%12%10%3
1980
October1st to 10thDay
47%10%11%14%11%7%
1944
11th to 20thDay
34%8%12%23%19%4
1800
21st to endDay
32%8%11%17%21%10%
1980
November1st to 10thDay
34%7%13%17%20%9%
1857
11th to 20thDay
38%7%10%19%20%6%
1900
21st to endDay
38%7%10%20%19%6%
1829
December1st to 10thDay
36%7%12%24%16%5%
1710
11th to 20thDay
40%6%11%16%18%10%
2150
21st to endDay
46%8%11%16%13%6%
3124
January1st to 10thNight
36%5%11%20%18%9%
2459
11th to 20thNight
34%6%10%23%19%8%
2417
21st to endNight
39%5%10%19%18%9%
2532
February1st to 10thNight
36%6%13%16%19%11%
2160
11th to 20thNight
31%6%12%20%21%10%
2099
21st to endNight
30%6%14%19%24%7%
1653
March1st to 10thNight
23%7%13%28%23%7%
1980
11th to 20thNight
28%6%15%27%15%9%
1980
21st to endNight
26%7%13%26%20%8%
2178
April1st to 10thNight
26%7%18%25%16%8%
1980
11th to 20thNight
32%8%13%23%19%5%
1903
21st to endNight
24%8%16%23%22%7%
1800
May1st to 10thNight
23%7%12%26%22%9%
1717
11th to 20thNight
32%10%20%23%13%2
1593
21st to endNight
35%10%15%21%18%1
1745
June1st to 10thNight
44%12%18%20%5%
1620
11th to 20thNight
39%14%22%23%2
1620
21st to endNight
42%14%24%15%5%
1620
July1st to 10thNight
44%19%25%10%2
1575
11th to 20thNight
42%18%27%13%1
1566
21st to endNight
39%20%26%13%1
1672
August1st to 10thNight
36%20%32%12%1
1600
11th to 20thNight
37%18%32%11%2
1669
21st to endNight
46%17%22%13%2
2178
September1st to 10thNight
43%20%20%13%4
1980
11th to 20thNight
48%15%19%16%2
1980
21st to endNight
45%11%17%17%9%2
1980
October1st to 10thNight
37%11%14%20%11%6%
2016
11th to 20thNight
30%8%12%26%20%4
2160
21st to endNight
32%7%11%19%22%9%
2376
November1st to 10thNight
32%7%12%20%20%9%
2221
11th to 20thNight
36%8%12%19%18%8%
2280
21st to endNight
35%8%10%21%20%6%
2346
December1st to 10thNight
30%7%16%28%13%6%
2470
11th to 20thNight
37%6%10%20%17%11%
3044
21st to endNight
44%8%13%18%13%4
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
15%9%38%27%9%2
270
11th to 20thDay
5%11%37%28%14%5%
257
21st to endDay
14%14%41%23%9%
264
February1st to 10thDay
12%11%33%22%20%3
240
11th to 20thDay
8%5%35%29%16%6%
240
21st to endDay
11%7%29%29%24%1
319
March1st to 10thDay
410%33%35%17%1
420
11th to 20thDay
9%7%27%30%24%4
420
21st to endDay
7%7%27%31%24%4
462
April1st to 10thDay
7%10%26%33%19%5%
420
11th to 20thDay
10%8%25%31%19%8%
412
21st to endDay
7%321%35%22%11%
420
May1st to 10thDay
5%420%29%30%11%
423
11th to 20thDay
10%114%28%28%17%
447
21st to endDay
7%38%24%30%28%
485
June1st to 10thDay
17%311%18%27%24%
450
11th to 20thDay
7%18%24%25%35%
450
21st to endDay
6%25%15%28%45%
450
July1st to 10thDay
6%2411%26%51%
445
11th to 20thDay
7%149%30%49%
444
21st to endDay
5%25%17%24%47%
488
August1st to 10thDay
5%25%15%32%42%
446
11th to 20thDay
5%1313%26%53%
439
21st to endDay
11%27%16%25%39%
495
September1st to 10thDay
8%27%13%27%44%
450
11th to 20thDay
10%28%21%27%32%
450
21st to endDay
10%26%20%33%28%
450
October1st to 10thDay
11%317%32%25%11%
450
11th to 20thDay
7%5%17%28%33%10%
450
21st to endDay
6%320%35%29%6%
495
November1st to 10thDay
9%6%20%31%29%5%
455
11th to 20thDay
12%8%22%33%20%4
460
21st to endDay
10%6%23%39%19%3
389
December1st to 10thDay
49%34%30%17%5%
270
11th to 20thDay
13%5%30%31%19%2
270
21st to endDay
7%8%29%34%18%3
297
January1st to 10thNight
12%9%31%34%13%1
760
11th to 20thNight
10%7%26%42%13%1
744
21st to endNight
15%8%32%35%10%
792
February1st to 10thNight
13%7%29%37%13%
720
11th to 20thNight
10%6%32%39%12%1
703
21st to endNight
10%7%33%40%11%
592
March1st to 10thNight
5%8%35%39%13%1
720
11th to 20thNight
8%6%27%41%16%1
720
21st to endNight
7%5%30%41%16%
792
April1st to 10thNight
6%7%30%42%14%1
720
11th to 20thNight
8%6%28%43%16%1
711
21st to endNight
6%5%27%42%20%1
720
May1st to 10thNight
5%219%47%24%2
637
11th to 20thNight
8%213%42%31%5%
531
21st to endNight
7%29%36%40%7%
584
June1st to 10thNight
11%37%30%41%8%
540
11th to 20thNight
6%16%23%42%23%
540
21st to endNight
8%25%19%46%21%
540
July1st to 10thNight
8%1313%47%27%
525
11th to 20thNight
6%1413%45%32%
522
21st to endNight
5%1314%50%27%
591
August1st to 10thNight
41315%52%27%
537
11th to 20thNight
31413%47%32%
587
21st to endNight
13%26%19%38%22%
792
September1st to 10thNight
8%25%20%42%22%
720
11th to 20thNight
11%310%23%41%12%
720
21st to endNight
11%27%30%41%9%
720
October1st to 10thNight
10%117%37%29%5%
720
11th to 20thNight
7%313%38%34%5%
720
21st to endNight
10%218%38%30%2
792
November1st to 10thNight
11%421%35%26%3
740
11th to 20thNight
8%6%22%41%19%2
760
21st to endNight
10%426%43%15%1
760
December1st to 10thNight
7%424%40%23%2
760
11th to 20thNight
12%5%26%37%19%1
760
21st to endNight
11%6%25%39%18%1
836
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