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 2017-04-01.

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
60%
14640
23%
45138
20%
14595
65%
14595
32%
26800

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
53%
497
27%
1657
23%
497
61%
497
31%
954
April16th to endNight
55%
495
26%
1650
24%
495
61%
495
34%
912
May1st to 15thNight
67%
494
21%
1649
22%
494
71%
494
33%
914
May16th to endNight
78%
513
23%
1710
25%
513
80%
513
39%
906
June1st to 15thNight
82%
495
16%
1592
20%
495
82%
495
34%
926
June16th to endNight
78%
495
14%
1485
16%
495
78%
495
32%
910
July1st to 15thNight
81%
465
14%
1395
15%
465
81%
465
31%
860
July16th to endNight
75%
522
8%
1571
11%
489
75%
489
24%
1033
August1st to 15thNight
87%
494
16%
1638
19%
491
87%
491
36%
903
August16th to endNight
79%
525
13%
1750
15%
525
79%
525
27%
1101
September1st to 15thNight
74%
495
19%
1650
21%
495
74%
495
27%
1022
September16th to endNight
74%
603
27%
1761
25%
603
75%
603
34%
1076
October1st to 15thNight
76%
660
31%
1955
27%
660
78%
660
49%
990
October16th to endNight
64%
704
33%
2112
29%
704
69%
704
42%
1130
November1st to 15thNight
64%
700
32%
2100
29%
700
68%
700
36%
1205
November16th to endNight
55%
720
34%
2160
26%
720
63%
720
38%
1213
December1st to 15thNight
51%
720
27%
2160
20%
720
57%
720
29%
1336
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%
45931
Night
8%4244245%5%9%47%
45349
TransparencyDay
44%8%11%16%14%5%
45644
Night
32%7%14%20%16%8%
45141
SeeingDay
11%212%21%27%23%
4562
Night
11%416%30%27%8%
14643

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
12%435%5%346%6%9%38%
1976
16th to endDay
10%426%5%45%6%6%9%37%
1980
May1st to 15thDay
8%436%6%36%6%6%9%36%
1971
16th to endDay
7%5%45%6%47%6%7%9%36%
2052
June1st to 15thDay
6%5%49%9%6%9%8%7%9%22%
2032
16th to endDay
45%48%9%6%10%10%8%9%21%
2145
July1st to 15thDay
8%6%48%10%6%9%8%7%9%18%
2145
16th to endDay
10%6%5%10%11%5%10%8%6%7%17%
2168
August1st to 15thDay
8%6%38%8%6%8%8%9%10%20%
1980
16th to endDay
10%6%49%9%5%8%8%6%7%22%
2100
September1st to 15thDay
11%6%28%7%5%8%6%6%8%26%
1980
16th to endDay
9%7%36%7%46%6%6%6%34%
1856
October1st to 15thDay
6%5%15%5%3445%6%51%
1672
16th to endDay
7%41443445%6%52%
1760
November1st to 15thDay
8%6%25%5%34347%47%
1758
16th to endDay
9%415%424446%51%
1800
December1st to 15thDay
15%6%35%5%245%5%5%40%
1800
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%4343345%5%7%49%
1657
16th to endNight
6%426%4235%5%7%51%
1650
May1st to 15thNight
5%3244245%8%11%46%
1649
16th to endNight
421333446%10%55%
1709
June1st to 15thNight
2313435%6%8%12%46%
1592
16th to endNight
3424435%6%8%11%43%
1485
July1st to 15thNight
3323435%7%9%15%41%
1485
16th to endNight
4425%5%36%7%9%12%36%
1689
August1st to 15thNight
22134346%8%14%48%
1650
16th to endNight
5%3235%35%7%8%14%39%
1750
September1st to 15thNight
7%325%5%45%6%8%11%40%
1650
16th to endNight
7%3245%346%7%10%44%
1761
October1st to 15thNight
4213313249%62%
1955
16th to endNight
9%312313337%59%
2112
November1st to 15thNight
9%413424446%52%
2100
16th to endNight
7%423423435%57%
2160
December1st to 15thNight
12%5%35%42445%6%47%
2160
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
39%7%11%17%15%8%
1976
16th to endDay
39%6%11%21%14%5%
1980
May1st to 15thDay
39%9%13%19%10%6%
1971
16th to endDay
39%8%13%20%16%1
2056
June1st to 15thDay
52%12%11%15%8%
2032
16th to endDay
50%13%14%13%7%
2145
July1st to 15thDay
54%15%14%12%2
2006
16th to endDay
60%13%14%9%2
2023
August1st to 15thDay
52%14%17%11%1
1968
16th to endDay
56%13%15%8%5%1
2100
September1st to 15thDay
51%10%15%12%8%1
1980
16th to endDay
46%8%11%12%15%5%
1856
October1st to 15thDay
31%5%10%17%21%13%
1672
16th to endDay
30%46%15%26%16%
1760
November1st to 15thDay
33%5%7%20%23%8%
1758
16th to endDay
31%5%6%15%24%15%
1800
December1st to 15thDay
41%7%9%21%15%5%
1800
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%14%20%16%10%
1657
16th to endNight
29%5%11%26%18%7%
1650
May1st to 15thNight
28%7%17%26%12%7%
1650
16th to endNight
23%8%17%28%20%1
1711
June1st to 15thNight
27%10%18%28%15%
1592
16th to endNight
31%10%26%23%9%
1485
July1st to 15thNight
29%12%32%21%4
1395
16th to endNight
34%15%29%17%3
1571
August1st to 15thNight
21%13%32%27%4
1638
16th to endNight
31%14%28%16%6%2
1750
September1st to 15thNight
33%11%22%18%11%3
1650
16th to endNight
30%9%16%20%16%6%
1761
October1st to 15thNight
20%5%13%21%22%16%
1955
16th to endNight
26%36%17%25%20%
2112
November1st to 15thNight
31%48%21%25%8%
2100
16th to endNight
29%47%16%24%17%
2160
December1st to 15thNight
37%5%9%23%16%7%
2160
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
20%219%32%22%2
212
16th to endDay
15%20%30%26%5%
210
May1st to 15thDay
13%315%32%29%5%
228
16th to endDay
8%18%23%31%26%
267
June1st to 15thDay
8%6%23%27%33%
307
16th to endDay
6%1422%32%32%
420
July1st to 15thDay
8%419%31%36%
400
16th to endDay
8%213%28%45%
329
August1st to 15thDay
9%26%28%52%
253
16th to endDay
8%411%35%39%
268
September1st to 15thDay
13%313%29%38%
255
16th to endDay
11%17%17%40%20%
254
October1st to 15thDay
213%31%38%13%
112
16th to endDay
12%416%33%22%10%
96
November1st to 15thDay
23%220%32%20%2
90
16th to endDay
11%6%31%27%16%6%
90
December1st to 15thDay
21%6%33%24%10%4
90
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%24%39%17%1
497
16th to endNight
8%325%39%21%1
495
May1st to 15thNight
7%315%46%25%1
495
16th to endNight
5%9%34%40%9%
515
June1st to 15thNight
4322%56%12%
495
16th to endNight
5%1316%48%24%
495
July1st to 15thNight
4210%50%31%
465
16th to endNight
4129%47%35%
522
August1st to 15thNight
37%45%42%
494
16th to endNight
7%49%46%30%
525
September1st to 15thNight
9%413%46%26%
495
16th to endNight
8%5%25%49%10%
603
October1st to 15thNight
6%19%37%38%6%
660
16th to endNight
11%316%34%28%5%
704
November1st to 15thNight
12%214%40%25%4
700
16th to endNight
10%5%23%38%20%1
720
December1st to 15thNight
15%7%21%35%19%
720
16th to endNight
18%8%26%31%14%
813
Page generated at 2017-04-02 06:39:07. Version ac04

Copyright 2017 A. Danko


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