James Clerk Maxwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James Clerk Maxwell Observatory clear sky chart from 2005-11-05 to 2017-09-23.

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
66%
15207
28%
46930
25%
15169
70%
15169
39%
25568

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
58%
717
20%
2490
21%
716
62%
716
28%
1457
January16th to endNight
52%
768
25%
2496
23%
768
59%
768
27%
1532
February1st to 15thNight
54%
699
23%
2266
21%
696
60%
696
25%
1434
February16th to endNight
51%
632
24%
1895
21%
632
57%
632
26%
1213
March1st to 15thNight
49%
720
15%
2067
12%
720
53%
720
24%
1423
March16th to endNight
50%
768
20%
2052
14%
768
55%
768
28%
1373
April1st to 15thNight
53%
719
19%
1799
11%
719
55%
719
29%
1241
April16th to endNight
63%
720
24%
1800
12%
720
66%
720
41%
1087
May1st to 15thNight
65%
542
28%
1622
21%
542
69%
542
49%
796
May16th to endNight
70%
375
25%
1496
31%
374
72%
374
37%
695
June1st to 15thNight
75%
360
26%
1440
33%
360
79%
360
42%
620
June16th to endNight
86%
360
35%
1440
48%
360
89%
360
63%
500
July1st to 15thNight
95%
340
34%
1360
46%
340
96%
340
74%
420
July16th to endNight
93%
475
40%
1507
43%
445
93%
445
77%
532
August1st to 15thNight
90%
568
38%
1641
31%
565
90%
565
71%
697
August16th to endNight
94%
764
44%
1910
29%
764
94%
764
82%
851
September1st to 15thNight
91%
720
42%
1800
30%
720
92%
720
79%
815
September16th to endNight
82%
688
44%
1861
37%
688
85%
688
68%
860
October1st to 15thNight
70%
660
31%
1980
32%
660
75%
660
48%
997
October16th to endNight
71%
704
34%
2112
34%
704
76%
704
45%
1097
November1st to 15thNight
66%
700
27%
2261
27%
700
71%
700
38%
1229
November16th to endNight
59%
720
23%
2427
24%
720
63%
720
30%
1385
December1st to 15thNight
52%
720
21%
2520
19%
720
59%
720
24%
1545
December16th to endNight
44%
768
22%
2688
18%
768
51%
768
20%
1769

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%312212235%62%
47972
Night
13%312212335%58%
47120
TransparencyDay
26%27%18%24%20%
47652
Night
29%37%18%22%18%
46940
SeeingDay
14%9%27%32%12%4
5858
Night
14%29%31%33%7%
15216

By Month

Percentage Cloud Cover SW (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
20%5%23322336%46%
1447
16th to endDay
24%6%23213335%43%
1728
February1st to 15thDay
22%5%1331333445%
1590
16th to endDay
24%7%23222436%39%
1579
March1st to 15thDay
24%5%34423336%38%
1890
16th to endDay
22%5%2332335%7%39%
2172
April1st to 15thDay
22%5%2332345%5%41%
2149
16th to endDay
13%413212345%57%
2160
May1st to 15thDay
6%312212346%64%
2322
16th to endDay
8%313313446%58%
2615
June1st to 15thDay
8%313212235%63%
2520
16th to endDay
2111112383%
2520
July1st to 15thDay
11111286%
2520
16th to endDay
11111111286%
2556
August1st to 15thDay
11112285%
2308
16th to endDay
1112287%
2292
September1st to 15thDay
2111122382%
2160
16th to endDay
42111123477%
1933
October1st to 15thDay
8%112222335%65%
1650
16th to endDay
11%313313335%59%
1760
November1st to 15thDay
12%41332436%6%50%
1595
16th to endDay
18%414224345%48%
1530
December1st to 15thDay
22%5%2441335%6%41%
1440
16th to endDay
26%7%24322235%38%
1536
January1st to 15thNight
19%42231335%7%46%
2490
16th to endNight
21%5%23312335%47%
2496
February1st to 15thNight
22%5%1341345%6%41%
2263
16th to endNight
24%5%12213446%42%
1895
March1st to 15thNight
23%5%24414347%37%
2067
16th to endNight
21%5%1332334445%
2052
April1st to 15thNight
18%6%34424346%42%
1799
16th to endNight
13%323212346%57%
1800
May1st to 15thNight
7%211213345%64%
1622
16th to endNight
8%213413445%60%
1496
June1st to 15thNight
7%31221223468%
1440
16th to endNight
11121212381%
1440
July1st to 15thNight
1211286%
1440
16th to endNight
111111287%
1611
August1st to 15thNight
21111212380%
1650
16th to endNight
1123384%
1910
September1st to 15thNight
1111212383%
1800
16th to endNight
21221123377%
1861
October1st to 15thNight
9%21222223466%
1980
16th to endNight
9%312213346%61%
2112
November1st to 15thNight
13%31321235%7%54%
2261
16th to endNight
15%424323345%50%
2427
December1st to 15thNight
25%313223346%43%
2520
16th to endNight
28%5%1431334439%
2688

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
39%38%20%19%8%
1447
16th to endDay
44%26%14%18%13%
1728
February1st to 15thDay
42%29%18%20%5%
1590
16th to endDay
46%39%25%12%2
1579
March1st to 15thDay
47%311%15%15%6%
1890
16th to endDay
44%413%17%15%3
2172
April1st to 15thDay
42%412%22%14%3
2149
16th to endDay
28%413%24%23%6%
2160
May1st to 15thDay
20%28%31%27%8%
2322
16th to endDay
26%312%28%21%7%
2619
June1st to 15thDay
23%210%21%21%20%
2520
16th to endDay
8%1315%38%32%
2520
July1st to 15thDay
6%128%27%52%
2371
16th to endDay
8%1210%23%54%
2397
August1st to 15thDay
7%1210%31%46%
2292
16th to endDay
5%1112%37%42%
2292
September1st to 15thDay
8%1311%33%41%
2160
16th to endDay
13%1422%32%24%
1933
October1st to 15thDay
21%36%25%27%16%
1650
16th to endDay
28%36%20%25%15%
1760
November1st to 15thDay
33%311%20%21%9%
1595
16th to endDay
38%38%20%21%7%
1530
December1st to 15thDay
43%37%18%21%4
1440
16th to endDay
48%29%13%17%7%
1536
January1st to 15thNight
37%39%19%17%10%
2490
16th to endNight
41%27%13%18%16%
2496
February1st to 15thNight
42%47%17%19%9%
2275
16th to endNight
41%48%24%16%4
1895
March1st to 15thNight
47%410%15%14%6%
2067
16th to endNight
41%5%9%20%16%6%
2052
April1st to 15thNight
43%413%19%14%5%
1799
16th to endNight
28%5%11%24%23%6%
1800
May1st to 15thNight
20%310%24%27%12%
1623
16th to endNight
24%413%30%18%8%
1496
June1st to 15thNight
20%410%21%23%19%
1440
16th to endNight
10%1414%32%35%
1440
July1st to 15thNight
6%128%27%53%
1360
16th to endNight
7%110%21%58%
1507
August1st to 15thNight
11%114%26%45%
1641
16th to endNight
6%1212%31%45%
1910
September1st to 15thNight
8%1410%35%40%
1800
16th to endNight
12%15%19%36%25%
1861
October1st to 15thNight
22%26%23%25%19%
1980
16th to endNight
24%35%21%28%16%
2112
November1st to 15thNight
29%412%22%19%11%
2261
16th to endNight
34%47%20%21%10%
2427
December1st to 15thNight
41%48%17%20%7%
2520
16th to endNight
48%36%13%19%8%
2688

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
48%5%20%16%8%2
75
16th to endDay
32%5%19%27%11%3
256
February1st to 15thDay
28%8%25%26%9%1
239
16th to endDay
32%14%22%21%8%
238
March1st to 15thDay
33%14%21%19%8%3
270
16th to endDay
25%18%28%20%6%1
288
April1st to 15thDay
29%18%26%16%6%1
269
16th to endDay
19%18%30%22%6%2
270
May1st to 15thDay
12%16%38%21%8%3
288
16th to endDay
11%13%22%32%13%6%
330
June1st to 15thDay
10%13%30%33%7%4
315
16th to endDay
35%33%39%11%7%
315
July1st to 15thDay
1126%51%10%8%
305
16th to endDay
226%49%13%7%
327
August1st to 15thDay
2326%43%15%7%
315
16th to endDay
223%47%18%8%
332
September1st to 15thDay
3429%38%19%4
315
16th to endDay
7%7%29%34%17%3
284
October1st to 15thDay
7%12%31%29%13%5%
255
16th to endDay
16%7%25%34%16%
272
November1st to 15thDay
8%6%16%28%31%6%
101
16th to endDay
21%9%20%30%16%2
75
December1st to 15thDay
38%5%33%13%6%3
60
16th to endDay
28%318%21%25%3
64
January1st to 15thNight
21%210%35%24%5%
720
16th to endNight
24%212%31%24%4
768
February1st to 15thNight
24%111%36%23%2
704
16th to endNight
25%5%11%31%21%4
632
March1st to 15thNight
24%411%29%25%3
720
16th to endNight
23%413%30%24%3
768
April1st to 15thNight
19%5%13%36%21%3
719
16th to endNight
14%412%33%26%8%
720
May1st to 15thNight
8%317%34%30%5%
543
16th to endNight
9%28%30%41%8%
375
June1st to 15thNight
7%310%30%43%5%
360
16th to endNight
26%34%47%8%
360
July1st to 15thNight
123%64%10%
340
16th to endNight
1122%60%14%
475
August1st to 15thNight
2228%53%12%
568
16th to endNight
229%56%10%
764
September1st to 15thNight
2328%53%11%
720
16th to endNight
419%30%39%14%
688
October1st to 15thNight
9%210%31%35%9%
660
16th to endNight
12%210%28%35%10%
704
November1st to 15thNight
14%29%32%31%8%
700
16th to endNight
16%412%36%23%6%
720
December1st to 15thNight
26%410%32%20%5%
720
16th to endNight
30%313%30%18%2
768
Page generated at 2017-09-24 06:15:47. Version ac04

Copyright 2017 A. Danko


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