Tellus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tellus Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
46%
15467
23%
47024
16%
15424
55%
15424
28%
28938

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
24%
720
23%
2327
9%
716
40%
716
19%
1473
January16th to endNight
25%
768
25%
2430
11%
768
41%
768
21%
1557
February1st to 15thNight
27%
704
25%
2112
9%
701
42%
701
25%
1301
February16th to endNight
22%
631
24%
1893
8%
631
36%
631
22%
1214
March1st to 15thNight
29%
720
22%
2085
11%
720
40%
720
25%
1333
March16th to endNight
36%
768
21%
2112
13%
768
43%
768
29%
1325
April1st to 15thNight
40%
600
25%
1858
16%
600
52%
600
32%
1057
April16th to endNight
48%
540
27%
1721
19%
540
57%
540
32%
982
May1st to 15thNight
49%
540
20%
1620
11%
540
58%
540
30%
993
May16th to endNight
63%
561
21%
1682
15%
561
68%
561
34%
991
June1st to 15thNight
73%
540
17%
1620
14%
540
75%
540
33%
1006
June16th to endNight
76%
540
18%
1620
15%
540
77%
540
36%
941
July1st to 15thNight
72%
510
14%
1530
13%
510
73%
510
29%
977
July16th to endNight
71%
564
16%
1595
15%
531
72%
531
27%
1047
August1st to 15thNight
74%
539
20%
1667
22%
536
74%
536
30%
1036
August16th to endNight
64%
573
20%
1910
22%
573
66%
573
24%
1247
September1st to 15thNight
62%
684
18%
1944
17%
684
64%
684
27%
1296
September16th to endNight
59%
693
27%
1914
20%
693
65%
693
33%
1173
October1st to 15thNight
54%
660
34%
1815
25%
660
62%
660
38%
1055
October16th to endNight
49%
704
32%
2117
25%
704
60%
704
35%
1193
November1st to 15thNight
47%
700
30%
2276
24%
700
58%
700
35%
1213
November16th to endNight
36%
720
28%
2340
18%
720
51%
720
27%
1362
December1st to 15thNight
29%
720
23%
2340
13%
720
43%
720
23%
1468
December16th to endNight
28%
768
20%
2496
13%
768
37%
768
18%
1698

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
13%5%25%5%35%5%5%7%40%
47901
Night
12%5%2442445%7%45%
47223
TransparencyDay
40%5%11%19%17%4
47602
Night
36%5%11%21%19%5%
47026
SeeingDay
16%6%18%23%21%13%
5351
Night
15%9%22%27%21%4
15469

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
17%8%25%423346%41%
1611
16th to endDay
17%6%24434436%42%
1794
February1st to 15thDay
18%6%2442445%7%41%
1750
16th to endDay
19%5%25%433335%41%
1580
March1st to 15thDay
17%6%25%5%14448%39%
1875
16th to endDay
15%6%35%43445%7%40%
2111
April1st to 15thDay
12%5%25%5%2445%7%44%
2100
16th to endDay
11%5%15%5%2445%7%45%
2238
May1st to 15thDay
11%5%25%6%35%447%42%
2331
16th to endDay
9%5%25%6%35%6%6%7%41%
2431
June1st to 15thDay
9%426%6%37%6%8%10%34%
2340
16th to endDay
6%3245%37%5%8%10%42%
2340
July1st to 15thDay
10%436%7%47%7%7%10%30%
2340
16th to endDay
8%5%25%7%47%8%9%10%30%
2465
August1st to 15thDay
8%425%6%47%7%9%10%33%
2279
16th to endDay
13%5%25%7%47%8%8%8%27%
2292
September1st to 15thDay
11%5%25%6%45%6%6%9%36%
2004
16th to endDay
14%6%244346%5%6%43%
1905
October1st to 15thDay
12%414423345%53%
1815
16th to endDay
13%5%1242223357%
1750
November1st to 15thDay
13%424212345%54%
1582
16th to endDay
15%5%25%313346%48%
1620
December1st to 15thDay
18%5%26%42445%6%39%
1620
16th to endDay
24%9%34423435%34%
1728
January1st to 15thNight
18%6%2432335%6%41%
2327
16th to endNight
18%7%3432335%5%42%
2430
February1st to 15thNight
15%5%24423336%47%
2103
16th to endNight
18%5%24424436%43%
1893
March1st to 15thNight
14%5%24324447%44%
2085
16th to endNight
12%5%25%423346%47%
2112
April1st to 15thNight
11%41332445%8%50%
1858
16th to endNight
9%42342445%7%49%
1721
May1st to 15thNight
9%4146%35%46%7%45%
1619
16th to endNight
6%324435%5%6%10%47%
1682
June1st to 15thNight
43245%35%6%8%12%42%
1620
16th to endNight
43145%25%7%7%11%47%
1620
July1st to 15thNight
6%5%15%5%37%6%8%10%39%
1620
16th to endNight
5%415%5%46%7%8%10%38%
1703
August1st to 15thNight
5%325%6%446%7%11%42%
1678
16th to endNight
10%5%25%6%25%5%7%10%37%
1910
September1st to 15thNight
10%425%5%35%46%8%41%
1944
16th to endNight
11%3145%2345%8%47%
1914
October1st to 15thNight
11%323323346%56%
1815
16th to endNight
9%5%25%32223358%
2117
November1st to 15thNight
10%5%12323345%56%
2276
16th to endNight
14%5%14323346%49%
2340
December1st to 15thNight
15%6%36%424346%41%
2340
16th to endNight
21%8%25%423446%35%
2496

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
44%37%16%23%4
1612
16th to endDay
43%36%19%21%5%
1794
February1st to 15thDay
42%38%20%21%3
1752
16th to endDay
45%36%19%20%4
1580
March1st to 15thDay
44%39%17%20%4
1875
16th to endDay
42%412%21%15%3
2112
April1st to 15thDay
39%49%21%19%5%
2100
16th to endDay
36%49%22%21%4
2238
May1st to 15thDay
39%5%12%21%17%3
2331
16th to endDay
38%7%15%22%13%2
2435
June1st to 15thDay
40%9%16%22%9%1
2340
16th to endDay
31%9%16%25%14%2
2340
July1st to 15thDay
42%9%18%18%8%1
2201
16th to endDay
41%11%19%17%9%1
2310
August1st to 15thDay
39%10%16%21%9%1
2266
16th to endDay
48%9%14%14%10%1
2292
September1st to 15thDay
43%7%13%17%14%3
2004
16th to endDay
39%5%11%19%19%4
1905
October1st to 15thDay
33%38%18%26%9%
1815
16th to endDay
32%25%18%29%10%
1750
November1st to 15thDay
32%3417%33%8%
1582
16th to endDay
38%37%16%26%7%
1620
December1st to 15thDay
44%39%20%16%6%
1620
16th to endDay
53%27%17%14%4
1728
January1st to 15thNight
42%47%19%21%4
2327
16th to endNight
43%35%20%20%6%
2430
February1st to 15thNight
39%27%22%20%7%
2112
16th to endNight
43%3422%22%3
1893
March1st to 15thNight
40%49%20%18%7%
2085
16th to endNight
38%39%27%17%3
2112
April1st to 15thNight
32%410%23%23%6%
1858
16th to endNight
31%5%9%25%21%7%
1721
May1st to 15thNight
36%5%14%20%21%2
1620
16th to endNight
30%7%16%26%16%3
1684
June1st to 15thNight
31%10%21%25%11%1
1620
16th to endNight
28%8%20%23%16%3
1620
July1st to 15thNight
34%10%19%21%11%1
1530
16th to endNight
35%11%18%20%11%2
1595
August1st to 15thNight
33%10%17%23%13%1
1667
16th to endNight
39%9%16%18%13%1
1910
September1st to 15thNight
39%8%15%18%13%4
1944
16th to endNight
33%6%10%24%18%5%
1914
October1st to 15thNight
29%49%19%27%9%
1815
16th to endNight
31%2317%31%12%
2117
November1st to 15thNight
30%26%17%30%12%
2276
16th to endNight
35%35%18%27%8%
2340
December1st to 15thNight
42%29%19%18%5%
2340
16th to endNight
50%38%16%17%4
2496

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%15%40%16%32
90
16th to endDay
17%29%42%7%3
96
February1st to 15thDay
24%17%44%11%2
90
16th to endDay
29%14%39%16%
81
March1st to 15thDay
27%16%27%23%41
165
16th to endDay
18%11%31%23%13%1
288
April1st to 15thDay
20%11%28%21%16%1
270
16th to endDay
13%5%23%30%22%4
270
May1st to 15thDay
16%6%28%20%14%14%
288
16th to endDay
14%316%31%20%13%
331
June1st to 15thDay
11%111%25%26%24%
315
16th to endDay
11%13%22%25%26%
315
July1st to 15thDay
11%29%20%26%30%
305
16th to endDay
11%1417%36%27%
326
August1st to 15thDay
15%16%25%28%23%
313
16th to endDay
14%16%27%28%21%
332
September1st to 15thDay
13%5%11%19%30%19%
315
16th to endDay
17%313%26%29%9%
287
October1st to 15thDay
19%419%24%24%7%
255
16th to endDay
17%7%18%26%23%6%
253
November1st to 15thDay
23%16%18%26%12%2
90
16th to endDay
16%12%37%18%11%3
90
December1st to 15thDay
24%16%33%17%7%
90
16th to endDay
36%12%30%18%2
96
January1st to 15thNight
24%18%28%21%6%
720
16th to endNight
22%17%31%24%4
768
February1st to 15thNight
20%16%32%26%3
704
16th to endNight
22%19%33%22%2
631
March1st to 15thNight
19%14%35%24%6%
720
16th to endNight
15%11%30%28%13%
768
April1st to 15thNight
15%11%29%27%15%
600
16th to endNight
12%9%25%33%17%1
540
May1st to 15thNight
11%5%25%30%23%3
540
16th to endNight
9%216%33%32%6%
563
June1st to 15thNight
7%110%32%38%10%
540
16th to endNight
417%32%42%12%
540
July1st to 15thNight
8%5%27%43%14%
510
16th to endNight
7%17%24%45%13%
564
August1st to 15thNight
6%16%30%42%12%
539
16th to endNight
11%9%25%42%10%
573
September1st to 15thNight
13%29%26%36%9%
684
16th to endNight
13%216%33%31%2
693
October1st to 15thNight
12%6%22%30%23%5%
660
16th to endNight
11%9%24%28%22%3
704
November1st to 15thNight
13%8%26%27%21%1
700
16th to endNight
18%14%28%26%12%
720
December1st to 15thNight
21%13%31%23%9%
720
16th to endNight
27%13%27%24%7%
768
Page generated at 2017-09-25 09:02:17. Version ac04

Copyright 2017 A. Danko


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