Landolt Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Landolt Astronomical Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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%
15269
23%
46789
19%
15223
62%
15223
31%
28163

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
38%
900
24%
2327
10%
895
47%
895
30%
1501
January16th to endNight
37%
933
25%
2469
12%
933
45%
933
30%
1605
February1st to 15thNight
40%
704
27%
2111
14%
701
49%
701
29%
1302
February16th to endNight
36%
631
23%
1893
12%
631
44%
631
26%
1204
March1st to 15thNight
43%
543
25%
1983
19%
543
52%
543
24%
1190
March16th to endNight
49%
576
24%
2082
20%
576
57%
576
31%
1120
April1st to 15thNight
48%
539
25%
1797
19%
539
59%
539
31%
1027
April16th to endNight
50%
540
26%
1800
22%
540
59%
540
33%
1007
May1st to 15thNight
57%
539
21%
1799
21%
539
63%
539
33%
1013
May16th to endNight
71%
563
23%
1870
25%
561
75%
561
36%
1029
June1st to 15thNight
80%
540
16%
1655
17%
540
81%
540
33%
983
June16th to endNight
75%
540
15%
1620
16%
540
76%
540
29%
1034
July1st to 15thNight
82%
510
14%
1530
17%
510
82%
510
33%
919
July16th to endNight
79%
564
10%
1647
13%
531
80%
531
25%
1076
August1st to 15thNight
82%
539
13%
1788
17%
536
83%
536
32%
1058
August16th to endNight
73%
573
13%
1910
15%
573
73%
573
22%
1269
September1st to 15thNight
74%
540
21%
1800
23%
540
75%
540
28%
1100
September16th to endNight
71%
588
27%
1757
25%
588
72%
588
32%
1075
October1st to 15thNight
74%
660
32%
1980
28%
660
76%
660
48%
1000
October16th to endNight
62%
704
34%
2112
30%
704
66%
704
40%
1147
November1st to 15thNight
57%
700
32%
2100
27%
700
62%
700
36%
1197
November16th to endNight
52%
720
34%
2160
25%
720
62%
720
37%
1218
December1st to 15thNight
45%
720
27%
2160
19%
720
52%
720
27%
1372
December16th to endNight
36%
903
19%
2439
11%
903
41%
903
23%
1717

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%5%26%6%46%5%6%7%37%
47934
Night
10%42442445%8%47%
46994
TransparencyDay
43%7%10%16%15%5%
47642
Night
34%6%12%19%17%8%
46792
SeeingDay
12%211%21%28%23%
5366
Night
12%418%31%26%6%
15270

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
16%6%24435%435%43%
1611
16th to endDay
20%6%25%42432342%
1752
February1st to 15thDay
14%6%25%424347%42%
1750
16th to endDay
18%6%25%5%2435%5%40%
1586
March1st to 15thDay
16%6%36%5%23446%39%
1980
16th to endDay
11%5%36%5%35%448%41%
2140
April1st to 15thDay
12%5%25%5%345%6%8%39%
2151
16th to endDay
11%5%25%5%36%6%6%9%37%
2160
May1st to 15thDay
11%5%25%5%36%7%5%8%37%
2151
16th to endDay
7%6%25%7%47%6%6%8%37%
2244
June1st to 15thDay
5%5%28%8%5%9%7%8%8%28%
2301
16th to endDay
5%5%37%9%6%10%10%8%9%23%
2340
July1st to 15thDay
8%5%38%8%5%9%7%8%10%25%
2340
16th to endDay
9%5%48%9%6%9%7%7%8%23%
2406
August1st to 15thDay
7%5%37%8%5%8%7%8%11%25%
2160
16th to endDay
12%6%27%7%5%7%7%7%8%25%
2292
September1st to 15thDay
11%5%25%7%46%6%6%9%34%
2160
16th to endDay
9%6%36%5%45%5%5%7%39%
1860
October1st to 15thDay
7%42442445%6%53%
1650
16th to endDay
9%31332335%7%56%
1760
November1st to 15thDay
8%425%4235%5%6%51%
1758
16th to endDay
10%5%24423435%52%
1800
December1st to 15thDay
17%7%36%424446%38%
1800
16th to endDay
19%9%36%5%44347%31%
1782
January1st to 15thNight
15%424424347%45%
2327
16th to endNight
18%6%2332245%5%44%
2469
February1st to 15thNight
14%5%24324446%46%
2103
16th to endNight
18%5%24323346%44%
1893
March1st to 15thNight
14%7%25%334346%44%
1983
16th to endNight
10%415%223447%51%
2082
April1st to 15thNight
9%5%2442345%7%49%
1797
16th to endNight
7%425%42445%8%51%
1800
May1st to 15thNight
8%3144345%6%9%47%
1799
16th to endNight
5%41442445%9%51%
1869
June1st to 15thNight
23245%36%7%8%11%44%
1655
16th to endNight
5%3246%36%8%8%11%39%
1620
July1st to 15thNight
3323326%6%7%13%44%
1620
16th to endNight
5%325%5%37%6%11%14%34%
1759
August1st to 15thNight
4413435%6%8%13%44%
1800
16th to endNight
7%5%26%6%46%6%7%11%35%
1910
September1st to 15thNight
7%425%5%35%5%6%9%43%
1800
16th to endNight
7%4245%35%6%6%8%45%
1757
October1st to 15thNight
5%313223347%62%
1980
16th to endNight
10%214323335%59%
2112
November1st to 15thNight
9%425%32334456%
2100
16th to endNight
10%5%23322325%57%
2160
December1st to 15thNight
14%6%35%423346%45%
2160
16th to endNight
19%8%36%424346%35%
2439

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
42%45%15%22%9%
1612
16th to endDay
47%2413%21%10%
1752
February1st to 15thDay
42%37%14%21%9%
1752
16th to endDay
45%45%16%19%7%
1586
March1st to 15thDay
45%5%7%18%17%5%
1980
16th to endDay
42%48%18%21%5%
2142
April1st to 15thDay
40%6%9%16%18%8%
2151
16th to endDay
40%6%10%20%17%5%
2160
May1st to 15thDay
41%6%12%17%14%6%
2151
16th to endDay
41%7%12%21%15%1
2248
June1st to 15thDay
46%11%10%18%11%
2301
16th to endDay
50%12%14%15%5%
2340
July1st to 15thDay
48%12%17%15%6%
2201
16th to endDay
53%11%16%14%3
2256
August1st to 15thDay
47%12%19%14%6%
2148
16th to endDay
51%11%16%11%6%1
2292
September1st to 15thDay
44%9%15%16%11%3
2160
16th to endDay
41%7%10%16%18%6%
1860
October1st to 15thDay
31%5%7%17%22%14%
1650
16th to endDay
27%3414%30%17%
1760
November1st to 15thDay
32%47%20%26%8%
1758
16th to endDay
34%35%14%26%15%
1800
December1st to 15thDay
46%49%17%15%6%
1800
16th to endDay
53%35%16%15%4
1782
January1st to 15thNight
39%37%15%22%11%
2327
16th to endNight
40%36%15%21%11%
2469
February1st to 15thNight
38%36%18%20%12%
2112
16th to endNight
41%45%17%19%11%
1893
March1st to 15thNight
40%49%19%19%7%
1983
16th to endNight
31%412%21%22%8%
2082
April1st to 15thNight
33%5%11%20%18%10%
1797
16th to endNight
31%5%11%21%20%9%
1800
May1st to 15thNight
30%6%13%23%17%8%
1800
16th to endNight
28%6%15%29%18%1
1871
June1st to 15thNight
29%10%17%28%13%1
1655
16th to endNight
33%11%22%21%10%
1620
July1st to 15thNight
27%12%30%21%7%
1530
16th to endNight
33%15%28%18%3
1647
August1st to 15thNight
26%13%30%21%6%
1788
16th to endNight
38%12%24%13%7%3
1910
September1st to 15thNight
34%8%17%21%11%5%
1800
16th to endNight
34%6%13%19%18%7%
1757
October1st to 15thNight
21%410%20%26%16%
1980
16th to endNight
28%3415%25%22%
2112
November1st to 15thNight
31%47%18%26%11%
2100
16th to endNight
30%35%16%23%19%
2160
December1st to 15thNight
40%48%22%18%6%
2160
16th to endNight
49%37%16%15%7%
2439

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
30%6%30%21%11%1
90
16th to endDay
17%11%42%18%9%
96
February1st to 15thDay
12%13%24%37%8%3
90
16th to endDay
14%6%34%33%11%
81
March1st to 15thDay
33%6%37%16%5%
90
16th to endDay
17%7%19%26%23%4
126
April1st to 15thDay
16%8%23%28%21%1
270
16th to endDay
15%417%34%24%3
270
May1st to 15thDay
12%315%32%27%7%
288
16th to endDay
11%110%24%32%19%
331
June1st to 15thDay
10%15%23%34%23%
456
16th to endDay
9%1420%33%29%
495
July1st to 15thDay
6%317%34%36%
475
16th to endDay
7%1211%34%42%
455
August1st to 15thDay
9%36%30%49%
313
16th to endDay
14%29%31%40%
332
September1st to 15thDay
9%1415%36%31%
315
16th to endDay
7%8%21%39%22%
241
October1st to 15thDay
2115%28%31%21%
90
16th to endDay
14%315%25%20%20%
96
November1st to 15thDay
17%18%37%17%7%
90
16th to endDay
14%6%30%25%16%6%
90
December1st to 15thDay
33%21%28%12%4
90
16th to endDay
27%6%29%27%10%
96
January1st to 15thNight
17%7%30%33%9%
900
16th to endNight
20%9%28%33%8%
933
February1st to 15thNight
17%8%28%37%7%
704
16th to endNight
19%10%29%34%5%
631
March1st to 15thNight
16%7%28%35%11%
543
16th to endNight
11%6%27%37%17%
576
April1st to 15thNight
12%6%28%36%15%
539
16th to endNight
10%329%39%15%1
540
May1st to 15thNight
10%320%44%20%
540
16th to endNight
7%111%39%35%4
563
June1st to 15thNight
3333%50%8%
540
16th to endNight
5%1426%45%17%
540
July1st to 15thNight
4116%50%26%
510
16th to endNight
6%111%53%25%
564
August1st to 15thNight
5%112%51%28%
539
16th to endNight
7%1314%47%24%
573
September1st to 15thNight
9%1224%43%18%
540
16th to endNight
10%6%27%45%9%
588
October1st to 15thNight
6%112%37%38%5%
660
16th to endNight
11%5%17%32%30%2
704
November1st to 15thNight
12%221%39%21%2
700
16th to endNight
14%5%23%37%17%1
720
December1st to 15thNight
17%7%24%35%14%
720
16th to endNight
25%8%24%30%11%
903
Page generated at 2017-09-16 16:07:02. Version ac04

Copyright 2017 A. Danko


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