St George Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St George Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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%
23431
22%
78921
19%
23324
63%
23324
28%
46889

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
38%
948
22%
4000
9%
926
48%
926
19%
2438
January11th to 20thNight
42%
945
28%
3927
13%
930
49%
930
24%
2138
January21st to endNight
35%
877
17%
4183
12%
860
45%
860
16%
2629
February1st to 10thNight
36%
759
24%
2275
12%
759
45%
759
25%
1497
February11th to 20thNight
41%
703
20%
2099
12%
700
46%
700
27%
1308
February21st to endNight
30%
592
20%
1776
10%
592
37%
592
24%
1161
March1st to 10thNight
40%
572
26%
2012
18%
572
50%
572
26%
1163
March11th to 20thNight
48%
540
20%
1980
15%
540
55%
540
24%
1189
March21st to endNight
51%
594
26%
2113
21%
594
59%
594
32%
1130
April1st to 10thNight
49%
540
25%
1800
20%
540
56%
540
28%
1075
April11th to 20thNight
45%
533
24%
1775
19%
533
53%
533
27%
1080
April21st to endNight
56%
540
28%
1800
26%
540
64%
540
36%
952
May1st to 10thNight
66%
540
25%
1800
26%
540
70%
540
38%
945
May11th to 20thNight
71%
531
19%
1770
23%
531
73%
531
33%
1013
May21st to endNight
76%
582
22%
1938
25%
582
78%
582
37%
1047
June1st to 10thNight
77%
540
16%
1797
20%
540
77%
540
30%
1068
June11th to 20thNight
80%
540
18%
1620
20%
540
80%
540
35%
958
June21st to endNight
72%
540
14%
1620
16%
540
73%
540
30%
1019
July1st to 10thNight
77%
525
11%
1575
12%
525
77%
525
28%
1012
July11th to 20thNight
78%
522
13%
1566
15%
522
78%
522
28%
999
July21st to endNight
78%
591
10%
1845
14%
558
78%
558
27%
1126
August1st to 10thNight
86%
537
14%
1778
17%
534
86%
534
37%
962
August11th to 20thNight
82%
534
13%
1776
15%
534
82%
534
25%
1132
August21st to endNight
75%
594
15%
1980
18%
594
75%
594
27%
1242
September1st to 10thNight
75%
540
18%
1800
21%
540
76%
540
26%
1112
September11th to 20thNight
70%
540
22%
1800
23%
540
72%
540
28%
1104
September21st to endNight
75%
720
28%
1980
24%
720
75%
720
38%
1178
October1st to 10thNight
73%
720
27%
2119
23%
720
74%
720
44%
1123
October11th to 20thNight
69%
720
35%
2160
30%
720
74%
720
45%
1138
October21st to endNight
59%
792
32%
2376
27%
792
64%
792
38%
1320
November1st to 10thNight
65%
740
31%
2220
28%
740
68%
740
39%
1233
November11th to 20thNight
57%
760
33%
2280
27%
760
64%
760
36%
1301
November21st to endNight
50%
760
29%
2280
22%
760
58%
760
32%
1391
December1st to 10thNight
46%
760
27%
2280
20%
760
54%
760
28%
1438
December11th to 20thNight
46%
758
24%
2674
17%
758
52%
758
23%
1652
December21st to endNight
36%
902
13%
4147
12%
888
42%
888
14%
2616

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
11%6%36%6%46%6%6%8%37%
79179
Night
10%5%25%5%345%6%8%47%
79790
TransparencyDay
42%9%12%17%15%5%
78916
Night
34%8%14%20%17%8%
79574
SeeingDay
12%5%19%25%23%16%
13647
Night
12%5%18%31%26%8%
23452

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
16%7%35%6%35%46%9%36%
2847
11th to 20thDay
12%5%24333446%53%
2850
21st to endDay
21%9%37%5%3445%6%32%
3386
February1st to 10thDay
14%7%46%6%25%5%7%6%37%
1890
11th to 20thDay
15%7%35%5%3446%7%42%
1748
21st to endDay
16%9%36%5%35%5%5%7%36%
1480
March1st to 10thDay
12%7%46%6%445%5%6%40%
1962
11th to 20thDay
12%7%46%6%45%5%6%8%37%
1980
21st to endDay
10%5%25%436%5%6%8%46%
2240
April1st to 10thDay
10%5%346%35%7%7%9%41%
2160
11th to 20thDay
14%6%36%5%35%5%6%11%38%
2134
21st to endDay
8%5%25%6%45%6%6%9%44%
2160
May1st to 10thDay
8%435%5%35%5%7%10%44%
2160
11th to 20thDay
8%5%35%6%46%6%6%8%43%
2124
21st to endDay
6%5%46%7%5%7%7%7%10%35%
2337
June1st to 10thDay
9%6%48%9%5%8%8%8%9%27%
2160
11th to 20thDay
6%5%37%8%6%10%8%8%11%29%
2336
21st to endDay
7%7%49%10%7%9%9%8%9%21%
2340
July1st to 10thDay
11%7%48%9%6%10%9%8%10%18%
2340
11th to 20thDay
10%7%49%11%6%9%9%8%9%19%
2340
21st to endDay
9%6%5%8%10%6%10%8%7%9%23%
2359
August1st to 10thDay
7%6%38%9%6%8%8%10%12%23%
2148
11th to 20thDay
10%5%49%9%7%9%9%8%9%21%
2125
21st to endDay
9%6%49%9%6%7%7%7%8%28%
2376
September1st to 10thDay
10%7%38%8%5%8%7%7%9%28%
2160
11th to 20thDay
11%6%37%7%6%7%6%7%7%32%
2139
21st to endDay
9%5%35%7%45%6%6%7%42%
1980
October1st to 10thDay
6%425%5%46%45%6%53%
1836
11th to 20thDay
7%6%25%5%4445%7%51%
1800
21st to endDay
10%5%245%3445%7%51%
1980
November1st to 10thDay
8%5%25%5%35%35%8%51%
1858
11th to 20thDay
10%7%25%43445%6%51%
1900
21st to endDay
13%6%25%5%35%6%5%7%42%
1900
December1st to 10thDay
14%6%25%5%345%6%7%43%
1900
11th to 20thDay
16%7%45%5%345%6%7%38%
2396
21st to endDay
17%8%36%6%445%5%6%37%
3348
January1st to 10thNight
16%7%35%5%3445%6%43%
4108
11th to 20thNight
14%5%2342335%7%51%
4120
21st to endNight
24%7%24423347%38%
4265
February1st to 10thNight
14%7%46%5%34447%42%
2275
11th to 20thNight
16%6%35%3245%5%6%46%
2098
21st to endNight
17%7%35%5%3435%8%41%
1776
March1st to 10thNight
12%6%25%435%45%8%47%
2012
11th to 20thNight
12%5%36%435%6%6%8%42%
1980
21st to endNight
9%425%424447%53%
2113
April1st to 10thNight
11%5%34435%5%5%8%47%
1800
11th to 20thNight
11%6%36%5%345%5%7%46%
1775
21st to endNight
6%425%5%345%6%8%54%
1800
May1st to 10thNight
8%32432446%11%53%
1800
11th to 20thNight
6%425%5%35%6%7%9%49%
1769
21st to endNight
6%224435%5%7%11%52%
1938
June1st to 10thNight
6%5%25%435%6%7%11%46%
1797
11th to 20thNight
34245%45%7%9%11%47%
1620
21st to endNight
6%5%345%46%6%8%11%43%
1620
July1st to 10thNight
45%35%5%45%6%9%13%41%
1620
11th to 20thNight
5%3246%46%7%9%12%43%
1620
21st to endNight
4436%5%36%7%8%13%41%
1954
August1st to 10thNight
23244245%7%13%53%
1790
11th to 20thNight
44246%5%6%9%11%13%38%
1776
21st to endNight
6%425%6%45%7%7%14%40%
1980
September1st to 10thNight
6%5%25%5%45%7%9%10%42%
1800
11th to 20thNight
9%425%5%35%6%8%9%44%
1800
21st to endNight
7%4245%35%5%6%9%50%
1980
October1st to 10thNight
6%3145%2435%7%59%
2119
11th to 20thNight
7%223423347%62%
2160
21st to endNight
11%5%23423437%57%
2376
November1st to 10thNight
9%5%234345%47%55%
2220
11th to 20thNight
11%5%23424446%55%
2280
21st to endNight
9%6%35%5%34447%49%
2280
December1st to 10thNight
13%7%36%42445%5%46%
2280
11th to 20thNight
11%8%26%5%2445%8%44%
2799
21st to endNight
16%9%46%5%24446%40%
4290

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
43%6%8%15%19%9%
2848
11th to 20thDay
32%5%6%20%30%7%
2850
21st to endDay
53%5%5%13%18%6%
3386
February1st to 10thDay
45%6%9%14%17%9%
1890
11th to 20thDay
41%7%9%17%18%9%
1758
21st to endDay
45%7%10%16%17%5%
1480
March1st to 10thDay
42%7%8%19%18%5%
1962
11th to 20thDay
43%6%13%14%18%6%
1980
21st to endDay
33%7%13%23%18%6%
2240
April1st to 10thDay
36%8%13%18%17%9%
2160
11th to 20thDay
41%5%12%20%14%8%
2134
21st to endDay
34%7%11%21%21%6%
2160
May1st to 10thDay
33%6%13%24%17%7%
2160
11th to 20thDay
35%10%13%21%19%2
2128
21st to endDay
40%10%15%21%14%1
2337
June1st to 10thDay
49%11%12%19%8%1
2160
11th to 20thDay
43%12%14%19%10%1
2336
21st to endDay
52%14%15%14%6%
2340
July1st to 10thDay
54%15%16%14%1
2276
11th to 20thDay
56%14%17%10%2
2271
21st to endDay
51%13%17%15%4
2228
August1st to 10thDay
46%17%20%15%3
2136
11th to 20thDay
53%14%18%12%21
2125
21st to endDay
51%12%16%13%7%1
2376
September1st to 10thDay
49%11%15%13%10%3
2160
11th to 20thDay
46%12%14%13%12%4
2139
21st to endDay
39%8%11%18%15%8%
1980
October1st to 10thDay
31%7%10%20%21%11%
1836
11th to 20thDay
31%6%10%17%22%14%
1800
21st to endDay
32%5%8%15%26%13%
1980
November1st to 10thDay
31%5%9%21%25%9%
1858
11th to 20thDay
32%6%7%18%24%13%
1900
21st to endDay
39%6%8%17%19%10%
1900
December1st to 10thDay
38%7%10%21%16%7%
1900
11th to 20thDay
43%7%8%15%19%9%
2394
21st to endDay
47%7%9%13%21%3
3348
January1st to 10thNight
42%5%8%14%23%9%
4108
11th to 20thNight
34%46%19%26%11%
4120
21st to endNight
47%46%15%19%9%
4265
February1st to 10thNight
42%6%7%16%18%10%
2275
11th to 20thNight
38%6%9%18%18%12%
2099
21st to endNight
42%6%12%13%15%11%
1776
March1st to 10thNight
35%6%12%18%20%8%
2012
11th to 20thNight
37%8%15%16%16%8%
1980
21st to endNight
29%6%14%21%23%7%
2113
April1st to 10thNight
35%7%13%17%17%10%
1800
11th to 20thNight
37%6%11%20%16%9%
1775
21st to endNight
27%7%12%23%24%7%
1800
May1st to 10thNight
26%6%14%28%18%7%
1800
11th to 20thNight
29%9%15%23%21%3
1770
21st to endNight
24%9%17%29%18%2
1939
June1st to 10thNight
32%10%18%28%13%
1797
11th to 20thNight
27%11%21%28%12%
1620
21st to endNight
33%11%24%25%7%
1620
July1st to 10thNight
33%13%31%21%3
1575
11th to 20thNight
28%17%32%18%5%
1566
21st to endNight
30%14%30%22%4
1845
August1st to 10thNight
20%14%34%28%4
1778
11th to 20thNight
30%18%30%18%3
1776
21st to endNight
32%14%24%21%7%2
1980
September1st to 10thNight
32%13%22%17%13%3
1800
11th to 20thNight
32%12%19%19%14%5%
1800
21st to endNight
28%8%16%25%16%7%
1980
October1st to 10thNight
24%5%14%23%19%14%
2119
11th to 20thNight
23%6%10%22%24%15%
2160
21st to endNight
29%47%16%27%17%
2376
November1st to 10thNight
30%49%22%25%11%
2220
11th to 20thNight
30%5%8%17%27%13%
2280
21st to endNight
36%5%9%19%21%10%
2280
December1st to 10thNight
39%6%9%21%19%7%
2280
11th to 20thNight
39%6%10%15%17%15%
2800
21st to endNight
46%6%9%14%18%7%
4290

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
21%12%34%24%7%2
270
11th to 20thDay
12%17%33%30%8%
270
21st to endDay
20%15%40%19%5%1
297
February1st to 10thDay
18%14%37%24%7%1
267
11th to 20thDay
23%9%34%23%10%1
240
21st to endDay
13%18%44%20%5%
198
March1st to 10thDay
15%12%41%24%8%
240
11th to 20thDay
18%13%37%24%8%1
240
21st to endDay
14%13%33%28%12%1
326
April1st to 10thDay
12%9%33%31%13%2
420
11th to 20thDay
19%10%26%25%16%3
412
21st to endDay
9%5%27%34%20%3
420
May1st to 10thDay
10%5%23%31%25%6%
423
11th to 20thDay
10%319%31%26%11%
447
21st to endDay
6%210%31%32%20%
485
June1st to 10thDay
12%37%23%31%24%
450
11th to 20thDay
8%110%32%28%21%
626
21st to endDay
9%210%24%29%26%
630
July1st to 10thDay
12%17%20%27%32%
621
11th to 20thDay
10%417%34%35%
618
21st to endDay
8%15%15%33%37%
497
August1st to 10thDay
5%25%13%32%43%
446
11th to 20thDay
10%15%8%30%46%
439
21st to endDay
9%39%16%35%29%
495
September1st to 10thDay
10%15%19%34%31%
450
11th to 20thDay
11%18%22%38%20%
450
21st to endDay
10%29%24%38%17%
450
October1st to 10thDay
6%114%40%27%11%
306
11th to 20thDay
8%316%35%30%9%
270
21st to endDay
19%9%18%32%17%6%
297
November1st to 10thDay
12%419%34%27%4
270
11th to 20thDay
14%8%29%31%14%4
270
21st to endDay
17%9%31%27%12%3
270
December1st to 10thDay
15%6%33%24%14%7%
270
11th to 20thDay
17%12%37%27%6%
270
21st to endDay
24%11%29%23%13%1
297
January1st to 10thNight
18%8%30%35%8%1
950
11th to 20thNight
16%8%27%38%10%
950
21st to endNight
20%10%32%32%7%
882
February1st to 10thNight
16%10%31%35%7%
759
11th to 20thNight
19%11%26%34%9%1
703
21st to endNight
18%13%35%30%4
592
March1st to 10thNight
14%10%31%34%11%
572
11th to 20thNight
14%6%26%37%16%1
540
21st to endNight
11%8%26%37%18%
594
April1st to 10thNight
14%8%26%38%13%1
540
11th to 20thNight
14%7%28%32%17%2
533
21st to endNight
6%6%26%41%21%1
540
May1st to 10thNight
9%318%45%23%2
540
11th to 20thNight
7%311%47%30%3
531
21st to endNight
6%19%34%38%11%
584
June1st to 10thNight
7%25%29%47%10%
540
11th to 20thNight
31423%54%14%
540
21st to endNight
10%15%19%46%20%
540
July1st to 10thNight
5%412%47%31%
525
11th to 20thNight
5%135%53%34%
522
21st to endNight
41313%50%29%
591
August1st to 10thNight
318%45%43%
537
11th to 20thNight
5%137%48%37%
534
21st to endNight
8%25%13%45%27%
594
September1st to 10thNight
7%415%50%25%
540
11th to 20thNight
9%17%18%51%14%
540
21st to endNight
8%16%29%46%9%
720
October1st to 10thNight
8%19%39%37%6%
720
11th to 20thNight
8%314%37%32%6%
720
21st to endNight
14%418%36%25%3
792
November1st to 10thNight
12%314%42%25%4
740
11th to 20thNight
13%5%19%40%21%2
760
21st to endNight
12%6%25%39%18%1
760
December1st to 10thNight
16%8%24%32%19%1
760
11th to 20thNight
15%6%26%40%12%1
760
21st to endNight
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