Hamilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hamilton clear sky chart from 2005-11-06 to 2024-01-27.

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
41%
24112
17%
74570
12%
24058
46%
24058
22%
50567

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
22%
950
12%
2648
5%
946
28%
946
13%
2088
January11th to 20thNight
17%
843
11%
2553
5%
843
23%
843
12%
2004
January21st to endNight
22%
820
13%
2664
8%
820
28%
820
13%
1995
February1st to 10thNight
23%
720
12%
2241
7%
720
28%
720
13%
1710
February11th to 20thNight
21%
701
13%
2099
7%
700
27%
700
16%
1515
February21st to endNight
20%
592
14%
1776
5%
592
29%
592
15%
1322
March1st to 10thNight
26%
720
16%
2155
9%
720
32%
720
20%
1479
March11th to 20thNight
28%
720
17%
1980
8%
720
36%
720
22%
1378
March21st to endNight
31%
792
15%
2178
8%
792
37%
792
19%
1581
April1st to 10thNight
33%
581
17%
1841
11%
581
42%
581
23%
1212
April11th to 20thNight
38%
533
21%
1729
13%
533
47%
533
24%
1105
April21st to endNight
33%
540
18%
1620
8%
540
42%
540
22%
1104
May1st to 10thNight
33%
540
13%
1620
7%
540
39%
540
19%
1161
May11th to 20thNight
46%
531
15%
1593
10%
531
51%
531
23%
1080
May21st to endNight
59%
583
16%
1745
12%
582
62%
582
30%
1076
June1st to 10thNight
61%
540
17%
1586
15%
540
63%
540
34%
956
June11th to 20thNight
62%
540
21%
1440
15%
540
66%
540
36%
897
June21st to endNight
63%
540
19%
1440
13%
540
68%
540
35%
896
July1st to 10thNight
64%
525
17%
1400
15%
525
65%
525
33%
906
July11th to 20thNight
69%
522
19%
1446
16%
522
71%
522
38%
883
July21st to endNight
67%
591
21%
1672
19%
558
69%
558
34%
999
August1st to 10thNight
70%
537
22%
1600
20%
534
71%
534
34%
962
August11th to 20thNight
67%
534
22%
1598
20%
534
70%
534
34%
960
August21st to endNight
72%
594
23%
1962
22%
594
75%
594
37%
1057
September1st to 10thNight
60%
625
22%
1885
18%
625
64%
625
33%
1113
September11th to 20thNight
68%
720
30%
1980
24%
720
72%
720
40%
1149
September21st to endNight
52%
720
27%
1980
20%
720
57%
720
34%
1230
October1st to 10thNight
56%
720
27%
1980
21%
720
62%
720
38%
1175
October11th to 20thNight
41%
720
22%
2127
17%
720
49%
720
27%
1367
October21st to endNight
35%
792
21%
2520
16%
792
43%
792
22%
1690
November1st to 10thNight
42%
740
24%
2406
18%
740
52%
740
28%
1429
November11th to 20thNight
32%
760
23%
2470
13%
760
45%
760
22%
1600
November21st to endNight
33%
760
17%
2470
13%
760
40%
760
17%
1729
December1st to 10thNight
24%
760
13%
2470
7%
760
32%
760
14%
1813
December11th to 20thNight
26%
759
14%
2992
9%
759
32%
759
13%
2124
December21st to endNight
19%
947
7%
4704
6%
935
25%
935
7%
3822

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
14%8%36%6%35%5%5%7%37%
75198
Night
15%8%36%5%3445%7%38%
74839
TransparencyDay
46%7%12%20%13%2
74911
Night
45%6%11%21%14%2
74646
SeeingDay
17%11%26%23%15%7%
14263
Night
18%8%26%30%15%2
24123

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
17%13%5%8%6%446%36%28%
1517
11th to 20thDay
19%13%6%10%7%34445%25%
1620
21st to endDay
17%14%6%9%7%4445%6%25%
1836
February1st to 10thDay
20%14%49%6%4445%427%
1717
11th to 20thDay
16%12%5%9%7%34446%30%
1748
21st to endDay
19%12%5%10%5%33345%31%
1480
March1st to 10thDay
18%9%37%6%345%46%34%
1804
11th to 20thDay
19%11%46%6%35%45%6%32%
1980
21st to endDay
23%9%48%6%3445%6%29%
2178
April1st to 10thDay
14%9%27%6%35%5%48%37%
2132
11th to 20thDay
17%8%35%6%3445%7%38%
2178
21st to endDay
19%7%46%5%45%46%6%35%
2340
May1st to 10thDay
20%8%36%6%35%5%5%6%34%
2340
11th to 20thDay
20%8%46%5%3445%6%36%
2301
21st to endDay
8%6%36%7%46%7%8%9%35%
2531
June1st to 10thDay
13%5%25%5%36%5%6%8%41%
2367
11th to 20thDay
9%5%26%6%35%6%6%9%43%
2520
21st to endDay
9%5%36%5%46%5%7%11%39%
2520
July1st to 10thDay
11%6%26%6%46%7%7%9%36%
2520
11th to 20thDay
8%5%35%6%35%7%6%10%41%
2462
21st to endDay
10%5%25%5%47%5%7%10%41%
2547
August1st to 10thDay
8%436%6%36%6%6%10%44%
2327
11th to 20thDay
9%5%36%6%36%6%6%10%40%
2303
21st to endDay
8%5%25%5%35%5%6%9%46%
2390
September1st to 10thDay
13%5%24435%5%6%7%47%
2057
11th to 20thDay
9%5%25%43446%9%49%
1980
21st to endDay
13%6%34424346%50%
1980
October1st to 10thDay
11%436%5%445%5%7%46%
1980
11th to 20thDay
14%6%25%5%3446%6%45%
1832
21st to endDay
21%8%36%5%23446%37%
1832
November1st to 10thDay
8%6%345%25%5%36%53%
1672
11th to 20thDay
14%9%47%6%34447%38%
1710
21st to endDay
18%10%47%5%34346%35%
1710
December1st to 10thDay
21%11%6%9%7%35%445%25%
1710
11th to 20thDay
15%14%5%8%6%35%45%5%29%
2153
21st to endDay
22%21%6%10%6%34445%17%
2924
January1st to 10thNight
18%11%6%10%8%45%5%35%26%
2648
11th to 20thNight
20%15%5%9%8%34346%24%
2553
21st to endNight
18%13%6%9%6%35%5%45%28%
2664
February1st to 10thNight
22%11%48%6%3444428%
2241
11th to 20thNight
17%9%5%8%7%45%5%6%7%30%
2098
21st to endNight
21%12%48%5%34446%29%
1776
March1st to 10thNight
17%7%45%5%4445%5%38%
2155
11th to 20thNight
17%9%36%6%3446%6%37%
1980
21st to endNight
19%8%47%5%35%5%46%34%
2178
April1st to 10thNight
14%7%36%5%445%6%7%39%
1841
11th to 20thNight
14%7%36%4345%5%8%41%
1729
21st to endNight
17%6%35%6%45%5%6%7%37%
1620
May1st to 10thNight
18%9%37%6%35%45%7%33%
1620
11th to 20thNight
16%6%36%6%345%6%7%38%
1592
21st to endNight
8%4246%35%5%8%12%42%
1744
June1st to 10thNight
8%5%25%5%35%449%50%
1586
11th to 20thNight
8%5%245%245%7%10%47%
1440
21st to endNight
6%5%24436%5%9%10%46%
1440
July1st to 10thNight
11%425%4345%5%9%46%
1440
11th to 20thNight
7%4244445%6%10%50%
1494
21st to endNight
9%426%4345%5%8%48%
1771
August1st to 10thNight
6%415%6%35%5%6%10%48%
1611
11th to 20thNight
7%425%5%35%5%8%9%47%
1598
21st to endNight
6%315%4345%6%9%53%
1962
September1st to 10thNight
12%5%2442445%7%50%
1885
11th to 20thNight
9%41443345%9%54%
1980
21st to endNight
12%6%25%5%23346%51%
1980
October1st to 10thNight
9%425%434446%54%
1980
11th to 20thNight
15%7%26%5%3445%7%42%
2127
21st to endNight
18%8%36%5%345%45%40%
2520
November1st to 10thNight
13%7%35%424447%47%
2406
11th to 20thNight
17%7%37%5%24446%41%
2470
21st to endNight
20%10%46%5%3445%7%32%
2470
December1st to 10thNight
21%10%48%6%3445%6%29%
2470
11th to 20thNight
19%11%6%7%6%3445%5%29%
3042
21st to endNight
22%17%5%9%7%4445%5%17%
4728

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
57%6%11%17%10%
1518
11th to 20thDay
62%6%11%14%7%
1620
21st to endDay
60%6%12%16%5%1
1836
February1st to 10thDay
60%5%11%16%8%1
1717
11th to 20thDay
57%5%10%20%8%1
1758
21st to endDay
56%5%10%19%8%2
1480
March1st to 10thDay
51%6%10%23%10%1
1804
11th to 20thDay
52%7%12%18%10%1
1980
21st to endDay
55%7%11%19%7%1
2178
April1st to 10thDay
46%7%12%22%13%1
2132
11th to 20thDay
45%6%11%20%14%4
2178
21st to endDay
49%6%10%21%11%2
2340
May1st to 10thDay
50%8%11%20%10%1
2340
11th to 20thDay
50%7%12%18%12%1
2305
21st to endDay
40%12%15%20%11%1
2531
June1st to 10thDay
39%8%12%23%16%2
2367
11th to 20thDay
36%9%15%22%17%2
2520
21st to endDay
39%9%15%19%14%4
2520
July1st to 10thDay
42%8%13%19%16%2
2451
11th to 20thDay
35%10%14%23%17%2
2387
21st to endDay
37%9%16%22%14%3
2405
August1st to 10thDay
33%10%16%26%14%2
2314
11th to 20thDay
38%7%14%25%14%1
2303
21st to endDay
33%8%16%21%17%6%
2390
September1st to 10thDay
35%7%12%23%17%5%
2057
11th to 20thDay
32%7%12%26%21%2
1980
21st to endDay
36%6%11%24%22%2
1980
October1st to 10thDay
37%7%11%22%21%2
1980
11th to 20thDay
38%6%10%25%19%3
1832
21st to endDay
48%411%21%14%2
1832
November1st to 10thDay
34%5%10%29%20%3
1672
11th to 20thDay
47%410%21%15%3
1710
21st to endDay
50%5%10%22%12%1
1710
December1st to 10thDay
62%5%10%17%5%
1710
11th to 20thDay
57%5%10%18%10%
2150
21st to endDay
70%6%9%10%6%
2924
January1st to 10thNight
61%6%8%16%9%
2648
11th to 20thNight
63%411%15%6%1
2553
21st to endNight
59%5%10%18%8%1
2664
February1st to 10thNight
60%5%10%18%6%1
2241
11th to 20thNight
53%5%12%19%10%1
2099
21st to endNight
56%6%11%16%9%2
1776
March1st to 10thNight
48%6%9%22%14%2
2155
11th to 20thNight
47%6%13%21%11%2
1980
21st to endNight
51%6%10%22%9%2
2178
April1st to 10thNight
42%7%12%26%12%2
1841
11th to 20thNight
40%7%11%17%19%6%
1729
21st to endNight
45%7%12%20%13%4
1620
May1st to 10thNight
51%6%11%20%9%3
1620
11th to 20thNight
45%6%14%19%13%2
1593
21st to endNight
32%10%18%22%14%4
1745
June1st to 10thNight
32%7%11%27%20%3
1586
11th to 20thNight
30%9%14%24%19%5%
1440
21st to endNight
30%10%17%21%18%5%
1440
July1st to 10thNight
34%9%14%20%18%5%
1400
11th to 20thNight
29%9%14%27%18%3
1446
21st to endNight
31%8%14%29%16%2
1672
August1st to 10thNight
30%10%15%28%16%3
1600
11th to 20thNight
31%9%15%28%17%1
1598
21st to endNight
25%8%16%23%22%5%
1962
September1st to 10thNight
33%6%14%23%17%7%
1885
11th to 20thNight
27%8%11%27%25%3
1980
21st to endNight
36%410%21%26%2
1980
October1st to 10thNight
30%6%10%27%23%3
1980
11th to 20thNight
41%5%9%24%20%2
2127
21st to endNight
46%5%10%21%16%3
2520
November1st to 10thNight
37%412%21%22%3
2406
11th to 20thNight
44%5%9%22%17%3
2470
21st to endNight
52%5%11%20%10%1
2470
December1st to 10thNight
55%6%11%19%8%1
2470
11th to 20thNight
54%6%10%20%9%1
3044
21st to endNight
68%7%9%11%4
4728

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
25%19%35%15%6%
270
11th to 20thDay
28%23%34%12%3
270
21st to endDay
30%22%29%15%3
282
February1st to 10thDay
28%25%32%13%2
240
11th to 20thDay
29%25%33%13%
240
21st to endDay
29%20%41%8%2
198
March1st to 10thDay
22%26%38%12%2
244
11th to 20thDay
26%13%36%18%7%1
420
21st to endDay
30%15%32%15%8%
462
April1st to 10thDay
20%15%32%21%10%2
420
11th to 20thDay
19%20%25%20%12%4
412
21st to endDay
24%15%33%17%9%3
420
May1st to 10thDay
23%15%31%18%9%4
423
11th to 20thDay
23%8%29%23%12%6%
447
21st to endDay
12%6%24%27%18%12%
485
June1st to 10thDay
14%5%22%30%20%9%
477
11th to 20thDay
11%6%25%30%22%6%
630
21st to endDay
10%7%24%33%18%8%
630
July1st to 10thDay
12%316%26%24%18%
621
11th to 20thDay
9%5%21%27%25%13%
560
21st to endDay
11%318%27%27%14%
488
August1st to 10thDay
8%417%27%26%19%
446
11th to 20thDay
13%5%17%28%23%14%
439
21st to endDay
9%315%30%28%14%
495
September1st to 10thDay
12%317%26%31%11%
450
11th to 20thDay
9%416%26%32%14%
450
21st to endDay
14%11%20%24%22%8%
450
October1st to 10thDay
10%7%25%32%20%7%
450
11th to 20thDay
16%13%32%24%13%2
450
21st to endDay
23%15%34%22%5%1
347
November1st to 10thDay
11%13%38%23%12%3
270
11th to 20thDay
22%18%28%22%9%2
270
21st to endDay
24%14%39%18%5%
270
December1st to 10thDay
20%21%36%17%5%1
270
11th to 20thDay
21%20%39%14%7%
270
21st to endDay
30%19%31%16%4
297
January1st to 10thNight
22%16%32%24%6%
950
11th to 20thNight
28%13%34%21%3
843
21st to endNight
23%14%34%26%3
820
February1st to 10thNight
31%10%30%27%3
720
11th to 20thNight
22%16%37%23%3
703
21st to endNight
28%13%33%24%2
592
March1st to 10thNight
21%12%37%25%5%
720
11th to 20thNight
21%14%33%25%6%1
720
21st to endNight
22%12%30%28%7%
792
April1st to 10thNight
17%11%36%27%9%
581
11th to 20thNight
17%10%31%30%11%1
533
21st to endNight
17%8%37%27%9%1
540
May1st to 10thNight
22%9%30%27%12%1
540
11th to 20thNight
18%425%33%18%2
531
21st to endNight
9%222%36%28%3
584
June1st to 10thNight
10%219%43%24%1
540
11th to 20thNight
10%318%42%23%3
540
21st to endNight
6%121%43%26%2
540
July1st to 10thNight
12%212%34%34%7%
525
11th to 20thNight
7%214%38%34%5%
522
21st to endNight
10%313%38%33%3
591
August1st to 10thNight
8%212%38%38%3
537
11th to 20thNight
8%115%41%30%5%
534
21st to endNight
6%113%35%39%6%
594
September1st to 10thNight
14%313%31%34%4
625
11th to 20thNight
9%413%34%35%5%
720
21st to endNight
15%5%20%32%27%2
720
October1st to 10thNight
12%422%34%23%5%
720
11th to 20thNight
19%8%27%31%14%1
720
21st to endNight
23%10%31%26%11%1
792
November1st to 10thNight
17%8%30%29%16%1
740
11th to 20thNight
21%13%30%27%10%
760
21st to endNight
24%11%26%32%7%1
760
December1st to 10thNight
26%12%32%25%6%
760
11th to 20thNight
24%9%34%26%6%1
760
21st to endNight
29%15%29%23%3
954
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