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 2005-11-05 to 2017-12-02.

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
61%
15442
23%
47661
20%
15397
65%
15397
32%
28277

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 15thNight
43%
900
22%
2327
10%
895
50%
895
32%
1527
January16th to endNight
41%
858
25%
2394
13%
858
48%
858
29%
1540
February1st to 15thNight
44%
704
26%
2111
15%
700
52%
700
30%
1284
February16th to endNight
38%
631
23%
1893
13%
631
44%
631
28%
1178
March1st to 15thNight
44%
561
25%
2001
19%
561
53%
561
26%
1167
March16th to endNight
55%
576
23%
2068
20%
576
61%
576
30%
1136
April1st to 15thNight
54%
539
26%
1797
21%
539
62%
539
32%
1024
April16th to endNight
55%
540
26%
1800
25%
540
62%
540
34%
1003
May1st to 15thNight
67%
539
21%
1799
22%
539
72%
539
35%
981
May16th to endNight
76%
561
22%
1870
25%
561
78%
561
36%
1017
June1st to 15thNight
79%
540
15%
1737
19%
540
79%
540
32%
1034
June16th to endNight
76%
540
14%
1620
16%
540
76%
540
30%
1017
July1st to 15thNight
82%
510
13%
1530
15%
510
82%
510
33%
932
July16th to endNight
76%
564
9%
1705
13%
531
77%
531
24%
1106
August1st to 15thNight
85%
539
14%
1788
17%
536
86%
536
34%
1016
August16th to endNight
77%
573
13%
1910
16%
573
77%
573
25%
1218
September1st to 15thNight
74%
540
19%
1800
21%
540
75%
540
28%
1098
September16th to endNight
76%
658
28%
1921
27%
658
77%
658
35%
1152
October1st to 15thNight
75%
720
29%
2133
26%
720
76%
720
47%
1101
October16th to endNight
64%
768
35%
2304
29%
768
69%
768
43%
1223
November1st to 15thNight
65%
760
31%
2280
28%
760
69%
760
37%
1292
November16th to endNight
56%
780
35%
2340
27%
780
64%
780
38%
1306
December1st to 15thNight
51%
728
26%
2184
20%
728
57%
728
29%
1348
December16th to endNight
40%
813
20%
2349
14%
813
45%
813
23%
1577

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%5%36%6%46%6%6%7%35%
48752
Night
8%4244245%5%9%47%
47872
TransparencyDay
43%8%11%16%14%5%
48465
Night
32%7%14%20%16%8%
47664
SeeingDay
11%212%22%26%22%
5513
Night
11%416%30%28%9%
15445

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 15thDay
14%6%35%5%35%35%7%40%
1611
16th to endDay
16%6%36%423345%42%
1826
February1st to 15thDay
12%6%35%5%2446%7%42%
1750
16th to endDay
15%7%35%5%3435%6%38%
1580
March1st to 15thDay
14%6%36%5%345%5%5%39%
1968
16th to endDay
9%5%35%426%6%6%8%42%
2152
April1st to 15thDay
11%435%5%346%6%9%39%
2151
16th to endDay
10%425%6%45%6%6%9%38%
2160
May1st to 15thDay
8%436%6%36%6%6%9%38%
2151
16th to endDay
7%5%45%6%47%6%7%9%36%
2244
June1st to 15thDay
7%6%49%9%6%9%7%7%9%22%
2217
16th to endDay
6%5%48%9%6%10%9%8%9%20%
2340
July1st to 15thDay
7%5%48%10%6%10%8%7%9%19%
2340
16th to endDay
10%6%5%9%10%5%10%8%6%7%18%
2341
August1st to 15thDay
9%6%39%8%6%8%8%8%10%19%
2160
16th to endDay
10%6%49%10%6%8%7%6%6%22%
2292
September1st to 15thDay
10%6%27%7%5%8%6%7%8%28%
2160
16th to endDay
9%6%36%7%46%6%6%7%35%
2025
October1st to 15thDay
6%5%15%5%35%45%6%50%
1824
16th to endDay
6%414434446%53%
1920
November1st to 15thDay
7%5%245%3445%7%48%
1908
16th to endDay
8%414434446%52%
1950
December1st to 15thDay
15%6%35%5%245%5%5%41%
1813
16th to endDay
18%7%35%5%3436%6%33%
1869
January1st to 15thNight
14%5%3443345%7%44%
2327
16th to endNight
15%6%24433347%44%
2394
February1st to 15thNight
11%5%24324447%48%
2103
16th to endNight
15%5%24423346%46%
1893
March1st to 15thNight
12%6%15%424447%45%
2001
16th to endNight
8%425%324447%50%
2068
April1st to 15thNight
9%43433445%7%51%
1797
16th to endNight
6%425%4235%6%7%51%
1800
May1st to 15thNight
5%3244245%8%11%48%
1799
16th to endNight
5%21333446%10%53%
1869
June1st to 15thNight
44144346%8%12%45%
1737
16th to endNight
45%25%5%35%6%8%11%42%
1620
July1st to 15thNight
3323435%6%8%14%43%
1620
16th to endNight
4425%5%36%7%9%13%37%
1823
August1st to 15thNight
22135%346%8%14%46%
1800
16th to endNight
6%4245%35%7%8%13%38%
1910
September1st to 15thNight
6%415%435%6%7%10%42%
1800
16th to endNight
6%3245%345%7%10%46%
1921
October1st to 15thNight
5%21342325%8%60%
2133
16th to endNight
9%312313336%59%
2304
November1st to 15thNight
9%413424437%54%
2280
16th to endNight
7%423424435%57%
2340
December1st to 15thNight
11%5%35%424446%47%
2184
16th to endNight
14%7%36%5%24447%37%
2349

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 15thDay
41%48%16%18%9%
1612
16th to endDay
44%45%17%19%9%
1826
February1st to 15thDay
38%5%8%15%22%8%
1752
16th to endDay
43%5%7%19%18%6%
1580
March1st to 15thDay
43%6%8%18%16%5%
1968
16th to endDay
36%6%10%20%19%5%
2154
April1st to 15thDay
38%7%11%17%17%9%
2151
16th to endDay
39%6%12%21%15%5%
2160
May1st to 15thDay
38%8%12%19%13%7%
2151
16th to endDay
39%8%13%20%15%1
2248
June1st to 15thDay
52%11%10%14%8%
2217
16th to endDay
51%13%14%13%7%
2340
July1st to 15thDay
54%14%15%13%2
2201
16th to endDay
58%12%14%10%2
2196
August1st to 15thDay
53%14%17%11%1
2148
16th to endDay
56%12%14%8%5%1
2292
September1st to 15thDay
49%10%14%13%9%2
2160
16th to endDay
45%8%12%14%14%4
2025
October1st to 15thDay
32%5%11%17%20%12%
1824
16th to endDay
29%46%15%26%17%
1920
November1st to 15thDay
33%5%8%21%22%7%
1908
16th to endDay
30%5%7%15%25%15%
1950
December1st to 15thDay
41%7%9%21%15%5%
1813
16th to endDay
48%6%7%18%14%5%
1869
January1st to 15thNight
38%49%14%21%10%
2327
16th to endNight
40%37%15%19%12%
2394
February1st to 15thNight
35%5%7%19%20%11%
2112
16th to endNight
38%48%15%19%12%
1893
March1st to 15thNight
37%5%11%19%19%7%
2001
16th to endNight
31%5%13%20%20%7%
2068
April1st to 15thNight
31%6%13%20%17%10%
1797
16th to endNight
29%6%12%25%19%7%
1800
May1st to 15thNight
27%7%16%25%15%7%
1800
16th to endNight
25%8%16%26%19%1
1871
June1st to 15thNight
29%10%17%27%14%
1737
16th to endNight
33%10%24%22%8%
1620
July1st to 15thNight
27%13%32%21%4
1530
16th to endNight
32%16%29%18%3
1705
August1st to 15thNight
23%13%32%25%4
1788
16th to endNight
33%14%27%17%6%2
1910
September1st to 15thNight
32%10%20%18%12%4
1800
16th to endNight
29%9%17%22%15%5%
1921
October1st to 15thNight
21%5%14%21%21%15%
2133
16th to endNight
26%36%16%25%21%
2304
November1st to 15thNight
30%410%21%24%8%
2280
16th to endNight
28%47%17%25%16%
2340
December1st to 15thNight
36%5%9%24%16%7%
2184
16th to endNight
44%5%9%18%15%7%
2349

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 15thDay
22%5%33%27%11%
90
16th to endDay
13%14%45%18%6%1
96
February1st to 15thDay
6%16%30%32%12%2
90
16th to endDay
12%7%38%25%16%
81
March1st to 15thDay
27%8%34%20%8%
90
16th to endDay
14%7%22%26%25%2
138
April1st to 15thDay
16%420%35%21%2
270
16th to endDay
13%220%31%26%5%
270
May1st to 15thDay
12%415%31%27%7%
288
16th to endDay
8%212%26%27%22%
331
June1st to 15thDay
11%16%23%27%29%
372
16th to endDay
10%16%24%28%28%
495
July1st to 15thDay
8%317%29%41%
475
16th to endDay
10%212%30%43%
390
August1st to 15thDay
10%27%27%51%
313
16th to endDay
9%7%11%33%37%
332
September1st to 15thDay
11%420%27%34%
315
16th to endDay
9%7%17%43%22%
314
October1st to 15thDay
5%110%33%32%17%
159
16th to endDay
10%420%35%20%9%
144
November1st to 15thDay
16%218%30%27%5%
135
16th to endDay
7%5%30%30%20%6%
135
December1st to 15thDay
20%6%31%23%13%4
94
16th to endDay
30%6%29%21%12%
96
January1st to 15thNight
17%6%27%38%9%1
900
16th to endNight
18%8%28%34%9%
858
February1st to 15thNight
13%9%28%35%11%
704
16th to endNight
18%10%29%34%7%
631
March1st to 15thNight
14%8%27%35%13%
561
16th to endNight
10%5%22%36%24%
576
April1st to 15thNight
12%5%23%39%18%
539
16th to endNight
8%325%37%22%1
540
May1st to 15thNight
7%216%46%25%1
540
16th to endNight
6%9%35%38%8%
563
June1st to 15thNight
5%322%55%12%
540
16th to endNight
6%1417%47%22%
540
July1st to 15thNight
319%47%36%
510
16th to endNight
3128%47%36%
564
August1st to 15thNight
36%45%43%
539
16th to endNight
7%15%10%45%30%
573
September1st to 15thNight
8%415%46%24%
540
16th to endNight
7%424%51%11%
658
October1st to 15thNight
6%9%36%37%8%
720
16th to endNight
11%316%34%28%5%
768
November1st to 15thNight
11%213%41%26%4
760
16th to endNight
10%422%38%22%1
780
December1st to 15thNight
14%7%21%35%19%
728
16th to endNight
18%8%26%31%14%
813
Page generated at 2017-12-03 10:16:48. Version ac04

Copyright 2017 A. Danko


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