LAS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LAS Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
36%
15572
20%
46420
12%
15527
44%
15527
22%
30219

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
17%
900
15%
2506
6%
895
26%
895
15%
1836
January16th to endNight
17%
868
16%
2550
5%
868
28%
868
15%
1818
February1st to 15thNight
15%
704
15%
2112
4%
701
25%
701
13%
1533
February16th to endNight
16%
631
15%
1893
5%
631
24%
631
14%
1347
March1st to 15thNight
20%
720
16%
2151
7%
720
28%
720
16%
1497
March16th to endNight
26%
708
14%
2052
6%
708
34%
708
17%
1446
April1st to 15thNight
30%
539
22%
1797
12%
539
44%
539
22%
1086
April16th to endNight
32%
540
20%
1728
10%
540
43%
540
20%
1100
May1st to 15thNight
32%
540
15%
1619
8%
539
39%
539
19%
1121
May16th to endNight
45%
561
19%
1683
10%
561
53%
561
21%
1094
June1st to 15thNight
53%
540
21%
1458
13%
540
60%
540
31%
886
June16th to endNight
55%
540
21%
1440
13%
540
61%
540
35%
853
July1st to 15thNight
58%
510
18%
1360
13%
510
61%
510
33%
850
July16th to endNight
62%
564
23%
1500
16%
531
72%
531
36%
877
August1st to 15thNight
68%
527
24%
1623
23%
524
72%
524
36%
886
August16th to endNight
67%
525
24%
1750
23%
525
72%
525
36%
911
September1st to 15thNight
61%
555
27%
1710
25%
555
65%
555
32%
998
September16th to endNight
54%
657
26%
1815
18%
657
59%
657
31%
1112
October1st to 15thNight
50%
660
33%
1815
22%
660
58%
660
34%
1067
October16th to endNight
35%
704
24%
2235
16%
704
46%
704
25%
1373
November1st to 15thNight
36%
700
28%
2276
19%
700
49%
700
26%
1320
November16th to endNight
26%
720
22%
2340
12%
720
39%
720
17%
1509
December1st to 15thNight
24%
720
14%
2340
8%
720
31%
720
13%
1679
December16th to endNight
15%
939
14%
2667
4%
939
25%
939
12%
2020

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
14%6%35%5%3445%7%39%
47556
Night
14%7%35%424447%41%
46604
TransparencyDay
43%5%11%21%14%2
47242
Night
42%5%10%22%15%3
46422
SeeingDay
16%15%22%21%16%8%
5581
Night
16%21%21%24%14%2
15574

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
21%9%39%413345%32%
1432
16th to endDay
13%10%47%5%2435%7%36%
1671
February1st to 15thDay
15%11%5%7%423336%35%
1750
16th to endDay
15%9%47%5%345%5%6%32%
1580
March1st to 15thDay
23%8%25%424245%36%
1809
16th to endDay
17%7%36%43446%7%33%
2180
April1st to 15thDay
15%8%25%433447%40%
2151
16th to endDay
17%6%35%5%34447%38%
2232
May1st to 15thDay
20%6%35%5%34347%34%
2331
16th to endDay
11%6%25%6%37%6%5%8%35%
2431
June1st to 15thDay
9%325%5%35%5%5%9%44%
2499
16th to endDay
7%334436%5%6%9%43%
2520
July1st to 15thDay
12%6%26%5%346%6%9%36%
2520
16th to endDay
7%425%5%46%6%8%9%39%
2562
August1st to 15thDay
8%415%5%36%6%6%10%40%
2238
16th to endDay
7%425%3345%7%9%44%
2100
September1st to 15thDay
11%6%25%5%2446%7%42%
1909
16th to endDay
12%5%245%34445%46%
1815
October1st to 15thDay
13%5%23323346%51%
1815
16th to endDay
17%6%14413245%47%
1635
November1st to 15thDay
13%5%24324446%48%
1582
16th to endDay
20%7%33423346%41%
1620
December1st to 15thDay
23%11%46%42333432%
1620
16th to endDay
19%11%5%7%5%24345%32%
1554
January1st to 15thNight
18%9%48%6%335%4432%
2506
16th to endNight
17%10%5%6%5%2435%6%32%
2550
February1st to 15thNight
19%9%5%7%5%24445%32%
2103
16th to endNight
17%10%46%42445%6%32%
1893
March1st to 15thNight
17%9%45%5%3334437%
2151
16th to endNight
15%8%36%6%3445%7%34%
2052
April1st to 15thNight
13%5%3441445%8%44%
1797
16th to endNight
16%5%345%25%446%42%
1728
May1st to 15thNight
16%7%35%6%25%347%37%
1619
16th to endNight
8%4247%45%6%6%8%40%
1682
June1st to 15thNight
6%315%425%45%8%50%
1458
16th to endNight
5%213435%5%5%10%50%
1440
July1st to 15thNight
9%415%5%2445%9%47%
1440
16th to endNight
6%3133245%7%10%49%
1603
August1st to 15thNight
6%31333446%10%52%
1634
16th to endNight
432433446%10%52%
1750
September1st to 15thNight
10%32443345%10%48%
1710
16th to endNight
11%5%24433346%50%
1815
October1st to 15thNight
9%423423346%54%
1815
16th to endNight
16%6%2442333448%
2235
November1st to 15thNight
12%5%24424436%50%
2276
16th to endNight
18%7%34324336%41%
2340
December1st to 15thNight
21%10%5%7%423335%31%
2340
16th to endNight
23%11%46%423346%29%
2667

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
53%310%19%11%
1433
16th to endDay
47%5%11%26%8%1
1671
February1st to 15thDay
51%39%24%9%1
1752
16th to endDay
51%5%13%21%8%
1580
March1st to 15thDay
51%310%19%12%1
1809
16th to endDay
49%7%12%22%7%1
2181
April1st to 15thDay
44%5%9%22%18%
2151
16th to endDay
44%6%9%23%12%3
2232
May1st to 15thDay
50%6%11%19%11%
2331
16th to endDay
44%8%13%17%13%2
2435
June1st to 15thDay
36%6%11%24%17%2
2499
16th to endDay
34%8%13%21%19%3
2520
July1st to 15thDay
42%7%12%19%14%4
2371
16th to endDay
35%10%14%22%14%2
2402
August1st to 15thDay
36%7%12%21%18%2
2225
16th to endDay
32%7%14%21%19%5%
2100
September1st to 15thDay
39%6%9%20%19%5%
1909
16th to endDay
39%5%7%20%23%3
1815
October1st to 15thDay
33%47%26%24%3
1815
16th to endDay
40%39%27%17%2
1635
November1st to 15thDay
36%39%24%18%7%
1582
16th to endDay
45%39%23%16%2
1620
December1st to 15thDay
56%39%19%8%1
1620
16th to endDay
55%410%21%7%1
1554
January1st to 15thNight
53%49%18%12%1
2506
16th to endNight
52%411%21%8%1
2550
February1st to 15thNight
54%410%22%7%1
2112
16th to endNight
51%5%12%20%9%
1893
March1st to 15thNight
50%49%21%11%3
2151
16th to endNight
48%5%14%21%8%1
2052
April1st to 15thNight
37%5%11%22%19%3
1797
16th to endNight
42%49%24%14%5%
1728
May1st to 15thNight
46%6%10%22%11%1
1620
16th to endNight
38%9%12%18%17%4
1684
June1st to 15thNight
30%7%12%24%21%3
1458
16th to endNight
28%6%16%20%21%6%
1440
July1st to 15thNight
33%6%12%23%18%4
1360
16th to endNight
26%7%15%28%18%3
1500
August1st to 15thNight
26%6%16%25%21%3
1623
16th to endNight
25%6%14%28%20%5%
1750
September1st to 15thNight
32%5%9%23%21%7%
1710
16th to endNight
35%47%21%24%6%
1815
October1st to 15thNight
31%48%24%26%5%
1815
16th to endNight
40%37%26%18%4
2235
November1st to 15thNight
36%38%21%23%7%
2276
16th to endNight
44%48%22%17%2
2340
December1st to 15thNight
55%310%19%9%1
2340
16th to endNight
58%410%17%8%1
2667

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
35%17%32%11%3
90
16th to endDay
18%29%40%9%2
96
February1st to 15thDay
22%33%33%10%1
90
16th to endDay
23%18%43%13%1
81
March1st to 15thDay
30%18%35%14%2
99
16th to endDay
21%23%33%13%6%1
288
April1st to 15thDay
18%21%27%22%9%
270
16th to endDay
21%18%27%17%11%4
270
May1st to 15thDay
22%21%26%15%10%3
288
16th to endDay
16%16%24%21%10%9%
331
June1st to 15thDay
10%15%16%30%18%8%
474
16th to endDay
8%17%24%25%16%7%
495
July1st to 15thDay
15%11%17%25%20%9%
475
16th to endDay
6%12%9%28%27%15%
422
August1st to 15thDay
14%6%11%21%24%21%
297
16th to endDay
11%6%11%25%26%19%
268
September1st to 15thDay
11%7%14%19%31%15%
255
16th to endDay
14%9%12%20%30%11%
254
October1st to 15thDay
12%10%23%23%20%9%
255
16th to endDay
16%15%29%29%7%
117
November1st to 15thDay
13%5%33%26%15%5%
90
16th to endDay
38%13%23%20%4
90
December1st to 15thDay
23%18%31%22%13
90
16th to endDay
34%22%38%22
96
January1st to 15thNight
19%28%30%18%2
900
16th to endNight
22%29%27%18%2
868
February1st to 15thNight
23%27%28%19%
704
16th to endNight
21%26%31%19%1
631
March1st to 15thNight
20%26%28%20%3
720
16th to endNight
17%25%27%22%6%
708
April1st to 15thNight
15%23%29%23%8%
539
16th to endNight
18%22%21%28%8%1
540
May1st to 15thNight
17%21%22%26%11%1
540
16th to endNight
10%19%16%30%17%5%
563
June1st to 15thNight
7%17%14%35%21%2
540
16th to endNight
5%17%14%35%25%1
540
July1st to 15thNight
11%12%9%29%34%2
510
16th to endNight
5%17%8%25%34%8%
564
August1st to 15thNight
8%10%7%30%35%7%
527
16th to endNight
410%10%27%40%6%
525
September1st to 15thNight
10%10%12%26%32%7%
555
16th to endNight
12%12%12%27%29%5%
657
October1st to 15thNight
12%13%19%25%23%5%
660
16th to endNight
17%15%27%26%11%
704
November1st to 15thNight
13%22%24%23%15%1
700
16th to endNight
21%27%21%20%8%
720
December1st to 15thNight
23%26%20%19%8%1
720
16th to endNight
26%26%27%17%2
939
Page generated at 2017-08-13 14:16:53. Version ac04

Copyright 2017 A. Danko


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