George Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Observatory clear sky chart from 2005-11-05 to 2018-03-21.

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
57%
16104
23%
49155
19%
16059
61%
16059
31%
29509

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
39%
974
23%
2522
11%
970
45%
970
29%
1691
January16th to endNight
40%
1040
22%
2704
11%
1040
45%
1040
29%
1805
February1st to 15thNight
39%
819
22%
2345
12%
815
45%
815
25%
1559
February16th to endNight
35%
566
24%
1932
13%
566
45%
566
22%
1208
March1st to 15thNight
37%
585
24%
2145
17%
585
47%
585
21%
1334
March16th to endNight
48%
594
23%
2178
19%
594
56%
594
28%
1218
April1st to 15thNight
46%
539
21%
1956
17%
539
55%
539
25%
1171
April16th to endNight
43%
540
22%
1800
18%
540
51%
540
23%
1141
May1st to 15thNight
50%
539
21%
1713
19%
539
55%
539
28%
1032
May16th to endNight
64%
562
24%
1685
25%
561
66%
561
34%
994
June1st to 15thNight
81%
540
20%
1620
23%
540
82%
540
46%
840
June16th to endNight
78%
540
15%
1620
19%
540
79%
540
35%
962
July1st to 15thNight
81%
510
18%
1530
23%
510
81%
510
38%
869
July16th to endNight
84%
564
18%
1594
23%
531
85%
531
39%
891
August1st to 15thNight
91%
539
27%
1610
30%
536
91%
536
55%
771
August16th to endNight
79%
573
17%
1958
21%
573
79%
573
28%
1163
September1st to 15thNight
75%
540
15%
1980
17%
540
76%
540
28%
1148
September16th to endNight
74%
537
27%
1980
31%
537
74%
537
29%
1123
October1st to 15thNight
77%
651
32%
2091
30%
651
78%
651
46%
1028
October16th to endNight
67%
768
36%
2304
28%
768
71%
768
45%
1193
November1st to 15thNight
64%
760
26%
2280
22%
760
66%
760
35%
1331
November16th to endNight
57%
809
33%
2369
24%
809
64%
809
37%
1354
December1st to 15thNight
43%
975
23%
2535
14%
975
47%
975
26%
1762
December16th to endNight
37%
1040
19%
2704
10%
1040
42%
1040
23%
1921

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%36%5%6%8%36%
49686
Night
10%5%2442445%7%47%
49352
TransparencyDay
43%7%11%17%13%6%
49386
Night
34%6%13%21%15%8%
49157
SeeingDay
12%315%27%25%15%
5901
Night
13%418%32%27%4
16107

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
19%6%34434445%39%
1746
16th to endDay
19%8%25%312245%43%
1872
February1st to 15thDay
21%9%26%32333439%
1855
16th to endDay
19%8%445%34335%37%
1827
March1st to 15thDay
19%8%26%5%24435%36%
2145
16th to endDay
14%6%36%5%25%35%6%41%
2171
April1st to 15thDay
13%6%45%5%35%45%6%39%
1992
16th to endDay
13%7%46%5%25%446%38%
2160
May1st to 15thDay
10%7%36%6%36%5%6%8%33%
2235
16th to endDay
8%6%37%6%36%6%8%10%32%
2430
June1st to 15thDay
4326%7%49%7%8%10%34%
2340
16th to endDay
7%437%8%5%10%9%8%10%25%
2340
July1st to 15thDay
9%5%36%7%48%7%8%11%26%
2340
16th to endDay
5%437%9%6%8%8%8%10%25%
2466
August1st to 15thDay
3225%6%6%8%8%9%12%36%
2340
16th to endDay
9%436%8%5%6%7%8%9%28%
2238
September1st to 15thDay
8%5%38%7%5%7%6%7%7%31%
1980
16th to endDay
11%6%36%6%46%5%5%8%33%
1980
October1st to 15thDay
7%5%244445%6%6%47%
1857
16th to endDay
8%323434345%55%
1920
November1st to 15thDay
9%6%35%6%35%47%7%39%
1908
16th to endDay
12%5%35%5%24336%48%
1917
December1st to 15thDay
17%7%36%5%24437%34%
1755
16th to endDay
17%8%36%425%446%34%
1872
January1st to 15thNight
17%6%25%423345%43%
2522
16th to endNight
18%5%25%423345%44%
2704
February1st to 15thNight
18%8%25%5%23336%41%
2337
16th to endNight
16%7%35%433246%43%
1932
March1st to 15thNight
18%7%24423446%41%
2145
16th to endNight
9%6%24424446%50%
2178
April1st to 15thNight
12%5%25%42445%6%45%
1956
16th to endNight
14%5%25%425%45%6%41%
1800
May1st to 15thNight
8%5%245%25%5%6%10%42%
1712
16th to endNight
45%234346%8%10%45%
1683
June1st to 15thNight
321332337%12%55%
1620
16th to endNight
4224425%6%8%10%46%
1620
July1st to 15thNight
4423425%46%11%51%
1620
16th to endNight
3223326%5%6%11%51%
1702
August1st to 15thNight
1112235%5%12%63%
1620
16th to endNight
4425%5%25%5%6%11%45%
1958
September1st to 15thNight
6%425%435%5%6%10%46%
1980
16th to endNight
7%4245%345%6%9%47%
1980
October1st to 15thNight
6%2221345%7%62%
2091
16th to endNight
7%31331223464%
2304
November1st to 15thNight
10%4243245%5%7%49%
2280
16th to endNight
11%423323345%55%
2369
December1st to 15thNight
17%6%25%5%24446%39%
2535
16th to endNight
18%8%36%423445%37%
2704

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
46%36%14%19%8%
1747
16th to endDay
43%35%16%17%12%
1872
February1st to 15thDay
50%45%14%15%10%
1856
16th to endDay
48%48%14%17%7%
1827
March1st to 15thDay
48%5%8%13%17%6%
2145
16th to endDay
43%49%13%22%6%
2173
April1st to 15thDay
42%5%9%17%15%8%
1992
16th to endDay
46%5%8%18%14%7%
2160
May1st to 15thDay
46%7%10%15%12%7%
2235
16th to endDay
41%8%13%19%14%2
2434
June1st to 15thDay
37%10%15%19%15%1
2340
16th to endDay
46%11%16%18%7%
2340
July1st to 15thDay
43%10%19%20%5%
2201
16th to endDay
44%14%19%19%1
2311
August1st to 15thDay
32%12%21%27%5%
2326
16th to endDay
47%13%16%15%6%1
2238
September1st to 15thDay
46%10%15%15%8%3
1980
16th to endDay
45%7%12%14%16%4
1980
October1st to 15thDay
35%5%9%22%14%12%
1857
16th to endDay
30%46%15%26%17%
1920
November1st to 15thDay
40%5%13%17%16%7%
1908
16th to endDay
38%45%15%20%15%
1917
December1st to 15thDay
50%47%15%14%7%
1755
16th to endDay
50%6%7%15%13%6%
1872
January1st to 15thNight
42%36%15%20%11%
2522
16th to endNight
42%36%15%18%13%
2704
February1st to 15thNight
44%48%12%17%13%
2347
16th to endNight
42%46%15%19%10%
1932
March1st to 15thNight
43%49%16%16%9%
2145
16th to endNight
34%413%16%21%9%
2178
April1st to 15thNight
38%5%9%19%16%10%
1956
16th to endNight
42%5%11%17%14%9%
1800
May1st to 15thNight
35%7%13%20%13%9%
1713
16th to endNight
29%8%17%26%14%2
1685
June1st to 15thNight
21%6%20%28%21%1
1620
16th to endNight
29%9%24%27%9%
1620
July1st to 15thNight
26%8%29%29%6%
1530
16th to endNight
23%9%32%30%3
1594
August1st to 15thNight
14%6%28%42%7%
1610
16th to endNight
31%11%25%25%5%1
1958
September1st to 15thNight
31%10%22%21%11%3
1980
16th to endNight
32%7%14%21%17%5%
1980
October1st to 15thNight
20%6%11%30%18%12%
2091
16th to endNight
24%36%17%27%20%
2304
November1st to 15thNight
31%5%14%23%16%7%
2280
16th to endNight
30%48%19%22%15%
2369
December1st to 15thNight
44%6%8%17%14%8%
2535
16th to endNight
48%47%15%13%10%
2704

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
25%6%27%29%7%2
135
16th to endDay
19%9%31%30%8%
144
February1st to 15thDay
25%6%28%29%8%
135
16th to endDay
17%8%41%25%7%
120
March1st to 15thDay
26%8%38%18%6%
135
16th to endDay
14%11%31%27%11%3
113
April1st to 15thDay
11%11%26%29%20%
111
16th to endDay
18%5%26%27%20%2
270
May1st to 15thDay
15%7%28%32%15%1
372
16th to endDay
13%223%36%19%4
517
June1st to 15thDay
6%18%33%38%11%
495
16th to endDay
8%18%28%31%20%
495
July1st to 15thDay
7%429%32%25%
475
16th to endDay
7%1423%33%31%
515
August1st to 15thDay
4219%37%34%
493
16th to endDay
14%139%33%37%
278
September1st to 15thDay
5%7%25%37%24%
135
16th to endDay
6%417%38%33%
135
October1st to 15thDay
3334%36%21%
135
16th to endDay
13%219%27%26%11%
144
November1st to 15thDay
13%223%32%21%5%
135
16th to endDay
14%227%32%14%8%
135
December1st to 15thDay
25%322%26%11%9%
135
16th to endDay
27%6%36%22%41
144
January1st to 15thNight
19%7%26%34%11%
975
16th to endNight
20%7%25%35%10%
1040
February1st to 15thNight
22%7%25%35%8%
819
16th to endNight
18%9%32%33%6%
566
March1st to 15thNight
21%7%28%33%7%
585
16th to endNight
12%6%27%38%14%1
594
April1st to 15thNight
12%7%27%41%10%
539
16th to endNight
17%6%25%35%13%
540
May1st to 15thNight
12%229%44%11%
540
16th to endNight
7%219%45%24%1
563
June1st to 15thNight
3442%45%2
540
16th to endNight
5%5%27%52%9%
540
July1st to 15thNight
5%218%61%11%
510
16th to endNight
3117%58%17%
564
August1st to 15thNight
111%72%13%
539
16th to endNight
7%216%57%16%
573
September1st to 15thNight
7%15%15%55%15%
540
16th to endNight
7%422%51%11%
537
October1st to 15thNight
5%110%30%44%8%
651
16th to endNight
8%315%37%31%3
768
November1st to 15thNight
10%116%39%28%2
760
16th to endNight
13%420%36%22%1
809
December1st to 15thNight
20%5%23%32%17%1
975
16th to endNight
23%6%25%31%11%1
1040
Page generated at 2018-03-22 10:34:21. Version ac04

Copyright 2018 A. Danko


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