Nederland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nederland 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%
15840
25%
46503
11%
15796
50%
15796
32%
25556

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
21%
900
27%
2506
6%
895
40%
895
28%
1337
January16th to endNight
16%
798
31%
2526
8%
798
42%
798
28%
1255
February1st to 15thNight
14%
699
22%
2264
3%
698
35%
698
22%
1214
February16th to endNight
14%
632
29%
1894
3%
632
41%
632
27%
1033
March1st to 15thNight
24%
720
25%
2097
6%
720
42%
720
30%
1170
March16th to endNight
27%
690
25%
2034
8%
690
44%
690
29%
1143
April1st to 15thNight
26%
539
23%
1798
9%
539
43%
539
27%
978
April16th to endNight
28%
540
20%
1800
10%
540
43%
540
24%
1047
May1st to 15thNight
36%
540
18%
1593
8%
540
46%
540
24%
1046
May16th to endNight
48%
563
17%
1498
10%
561
55%
561
34%
908
June1st to 15thNight
56%
540
21%
1440
14%
540
62%
540
47%
763
June16th to endNight
70%
540
27%
1440
20%
540
74%
540
56%
730
July1st to 15thNight
71%
510
17%
1360
14%
510
73%
510
36%
836
July16th to endNight
77%
564
22%
1455
21%
531
78%
531
39%
846
August1st to 15thNight
72%
527
19%
1611
18%
524
75%
524
38%
881
August16th to endNight
68%
525
25%
1750
25%
525
72%
525
35%
950
September1st to 15thNight
58%
495
28%
1650
25%
495
67%
495
35%
893
September16th to endNight
62%
500
33%
1769
27%
500
72%
500
42%
842
October1st to 15thNight
40%
660
29%
1980
14%
660
54%
660
37%
1082
October16th to endNight
33%
723
30%
2131
10%
723
48%
723
35%
1141
November1st to 15thNight
27%
875
31%
2275
8%
875
42%
875
40%
1236
November16th to endNight
24%
900
31%
2424
7%
900
43%
900
39%
1259
December1st to 15thNight
16%
900
22%
2520
2%
900
34%
900
26%
1414
December16th to endNight
16%
960
23%
2688
5%
960
33%
960
26%
1552

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
5%5%24435%5%6%8%47%
47480
Night
342432345%8%57%
46679
TransparencyDay
32%8%14%22%16%5%
47158
Night
24%7%15%26%20%5%
46503
SeeingDay
11%15%35%22%11%3
5715
Night
8%16%34%24%13%2
15845

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
435%32435%9%57%
1432
16th to endDay
425%41335%9%58%
1707
February1st to 15thDay
426%43435%11%51%
1596
16th to endDay
446%42335%10%53%
1580
March1st to 15thDay
25%35%5%25%5%5%9%50%
1860
16th to endDay
49%343245%7%11%42%
2190
April1st to 15thDay
48%33436%5%6%8%45%
2150
16th to endDay
10%11%35%435%5%6%7%36%
2160
May1st to 15thDay
11%11%346%35%45%8%35%
2355
16th to endDay
10%7%245%36%6%7%9%35%
2616
June1st to 15thDay
6%4145%36%6%7%8%45%
2520
16th to endDay
3313435%6%7%8%51%
2520
July1st to 15thDay
9%5%25%5%36%6%7%8%39%
2520
16th to endDay
7%436%6%36%6%7%9%39%
2611
August1st to 15thDay
9%6%35%445%6%5%10%38%
2254
16th to endDay
7%5%24446%6%7%8%42%
2100
September1st to 15thDay
8%41342446%7%50%
1980
16th to endDay
45%13423336%60%
1857
October1st to 15thDay
5%424425%447%54%
1650
16th to endDay
45%23323446%59%
1730
November1st to 15thDay
132333335%10%59%
1583
16th to endDay
1326%31335%10%59%
1533
December1st to 15thDay
5%26%5%36%349%52%
1440
16th to endDay
2325%5%2336%10%54%
1536
January1st to 15thNight
31432445%9%61%
2506
16th to endNight
424323338%63%
2526
February1st to 15thNight
425%32335%9%59%
2254
16th to endNight
5%25%42335%9%58%
1894
March1st to 15thNight
131442446%9%56%
2097
16th to endNight
36%2431435%8%56%
2034
April1st to 15thNight
36%2432347%7%55%
1798
16th to endNight
8%8%33313348%49%
1800
May1st to 15thNight
11%7%243145%6%8%43%
1592
16th to endNight
7%5%2342446%9%51%
1497
June1st to 15thNight
331222337%9%60%
1440
16th to endNight
121221347%10%63%
1440
July1st to 15thNight
5%214425%7%7%10%48%
1440
16th to endNight
4323325%5%6%9%52%
1554
August1st to 15thNight
431342445%9%54%
1620
16th to endNight
5%31443446%8%51%
1750
September1st to 15thNight
8%41432335%7%54%
1650
16th to endNight
421231345%5%64%
1769
October1st to 15thNight
45%14223347%59%
1980
16th to endNight
45%24313347%59%
2131
November1st to 15thNight
131331345%8%63%
2275
16th to endNight
131421336%7%63%
2424
December1st to 15thNight
42432335%10%58%
2520
16th to endNight
1326%42346%9%54%
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
24%8%22%26%14%3
1433
16th to endDay
24%5%17%29%18%5%
1707
February1st to 15thDay
29%9%20%26%10%3
1597
16th to endDay
27%5%16%28%17%3
1580
March1st to 15thDay
28%8%14%26%17%3
1860
16th to endDay
34%7%15%25%12%3
2190
April1st to 15thDay
34%7%12%25%14%5%
2150
16th to endDay
43%9%12%19%11%2
2160
May1st to 15thDay
46%10%11%20%10%
2355
16th to endDay
43%11%12%19%9%3
2620
June1st to 15thDay
31%11%13%16%18%8%
2520
16th to endDay
26%9%10%19%25%8%
2520
July1st to 15thDay
39%10%12%15%13%9%
2371
16th to endDay
37%11%14%20%10%5%
2447
August1st to 15thDay
39%11%13%14%13%7%
2239
16th to endDay
36%8%12%19%17%4
2100
September1st to 15thDay
30%8%11%19%19%10%
1980
16th to endDay
25%5%9%25%30%3
1857
October1st to 15thDay
29%9%11%20%26%3
1650
16th to endDay
24%6%10%26%25%5%
1730
November1st to 15thDay
21%9%15%23%23%7%
1583
16th to endDay
20%416%29%23%6%
1533
December1st to 15thDay
28%8%18%29%13%1
1440
16th to endDay
27%8%18%26%15%3
1536
January1st to 15thNight
18%8%20%28%19%3
2506
16th to endNight
21%5%18%29%20%5%
2526
February1st to 15thNight
23%8%19%30%14%3
2264
16th to endNight
22%8%13%29%20%5%
1894
March1st to 15thNight
21%9%13%29%21%4
2097
16th to endNight
26%6%14%30%18%2
2034
April1st to 15thNight
26%8%13%27%18%5%
1798
16th to endNight
33%6%14%26%16%2
1800
May1st to 15thNight
37%10%13%22%13%1
1593
16th to endNight
29%8%19%24%14%3
1498
June1st to 15thNight
19%7%18%24%19%10%
1440
16th to endNight
14%7%12%23%28%13%
1440
July1st to 15thNight
29%9%15%19%15%9%
1360
16th to endNight
25%9%16%25%14%7%
1455
August1st to 15thNight
25%7%18%22%18%7%
1611
16th to endNight
27%7%13%23%21%6%
1750
September1st to 15thNight
28%6%12%20%18%13%
1650
16th to endNight
19%6%10%28%29%5%
1769
October1st to 15thNight
24%6%13%22%27%6%
1980
16th to endNight
25%313%25%26%5%
2131
November1st to 15thNight
19%5%15%25%24%10%
2275
16th to endNight
18%5%14%29%25%6%
2424
December1st to 15thNight
22%9%19%28%16%3
2520
16th to endNight
26%7%20%27%15%2
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
12%16%38%14%16%1
90
16th to endDay
5%31%37%22%3
96
February1st to 15thDay
13%41%36%7%1
90
16th to endDay
7%32%35%19%4
81
March1st to 15thDay
12%35%32%16%2
150
16th to endDay
14%33%40%9%1
288
April1st to 15thDay
15%28%37%13%3
269
16th to endDay
24%27%35%9%2
270
May1st to 15thDay
16%15%39%18%7%1
411
16th to endDay
16%14%31%23%11%2
517
June1st to 15thDay
7%8%32%30%15%5%
495
16th to endDay
35%39%26%19%5%
495
July1st to 15thDay
11%425%29%20%8%
475
16th to endDay
6%229%28%23%9%
470
August1st to 15thDay
11%8%39%30%9%1
300
16th to endDay
10%8%35%33%10%1
268
September1st to 15thDay
12%10%45%25%4
255
16th to endDay
45%43%27%14%4
143
October1st to 15thDay
415%40%22%14%3
90
16th to endDay
14%13%37%25%7%2
96
November1st to 15thDay
6%10%37%20%21%4
90
16th to endDay
13%35%36%13%1
90
December1st to 15thDay
13%34%41%11%
90
16th to endDay
432%47%10%5%
96
January1st to 15thNight
12%27%36%16%6%1
900
16th to endNight
10%26%41%14%5%1
798
February1st to 15thNight
10%28%41%15%4
704
16th to endNight
8%25%48%14%3
632
March1st to 15thNight
6%23%42%19%8%
720
16th to endNight
8%22%38%25%4
690
April1st to 15thNight
10%18%41%23%6%
539
16th to endNight
13%17%36%23%8%
540
May1st to 15thNight
15%6%36%26%11%2
540
16th to endNight
10%7%27%33%17%3
563
June1st to 15thNight
6%230%38%19%2
540
16th to endNight
3120%42%29%2
540
July1st to 15thNight
7%12%36%37%5%
510
16th to endNight
5%16%32%45%8%
564
August1st to 15thNight
5%10%42%35%4
527
16th to endNight
7%112%37%31%9%
525
September1st to 15thNight
7%223%35%24%6%
495
16th to endNight
6%5%22%36%23%7%
500
October1st to 15thNight
8%13%35%25%13%3
660
16th to endNight
9%18%34%22%10%3
723
November1st to 15thNight
6%21%41%20%8%1
875
16th to endNight
6%22%43%22%4
900
December1st to 15thNight
11%23%44%16%3
900
16th to endNight
8%24%44%15%5%
960
Page generated at 2017-08-13 12:16:39. Version ac04

Copyright 2017 A. Danko


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