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 2018-02-14.

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%
16619
25%
48819
11%
16575
50%
16575
33%
26725

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
20%
975
26%
2716
6%
970
39%
970
29%
1443
January16th to endNight
16%
864
31%
2736
8%
864
43%
864
30%
1353
February1st to 15thNight
14%
755
22%
2445
4%
754
35%
754
23%
1300
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%
539
18%
1650
18%
536
75%
536
38%
904
August16th to endNight
70%
573
27%
1910
27%
573
73%
573
36%
1033
September1st to 15thNight
61%
540
27%
1800
23%
540
69%
540
36%
963
September16th to endNight
60%
546
33%
1930
26%
546
71%
546
41%
941
October1st to 15thNight
40%
720
28%
2160
13%
720
54%
720
38%
1168
October16th to endNight
33%
789
32%
2325
11%
789
49%
789
35%
1242
November1st to 15thNight
27%
950
30%
2470
8%
950
42%
950
41%
1332
November16th to endNight
23%
975
32%
2626
7%
975
43%
975
39%
1368
December1st to 15thNight
16%
975
23%
2730
3%
975
34%
975
28%
1512
December16th to endNight
16%
1040
24%
2912
5%
1040
34%
1040
27%
1666

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%48%
49268
Night
342432345%8%57%
48995
TransparencyDay
32%8%14%22%17%5%
48946
Night
23%7%15%26%20%5%
48819
SeeingDay
10%15%36%22%11%3
6313
Night
8%16%34%24%13%2
16624

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%42435%9%57%
1552
16th to endDay
425%41435%9%58%
1850
February1st to 15thDay
426%43435%11%52%
1715
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
8%5%25%445%6%5%10%38%
2301
16th to endDay
6%5%24445%6%7%8%43%
2292
September1st to 15thDay
8%41342446%8%51%
2160
16th to endDay
6%5%13423346%58%
2026
October1st to 15thDay
5%423425%45%6%54%
1800
16th to endDay
4423323436%59%
1887
November1st to 15thDay
131223335%10%61%
1718
16th to endDay
1325%31336%10%59%
1661
December1st to 15thDay
426%5%35%339%54%
1560
16th to endDay
1325%5%2335%9%56%
1664
January1st to 15thNight
21432445%9%62%
2716
16th to endNight
414323338%63%
2736
February1st to 15thNight
424323349%60%
2435
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%
1659
16th to endNight
5%31443446%8%52%
1910
September1st to 15thNight
7%41432335%7%55%
1800
16th to endNight
5%31231335%5%63%
1930
October1st to 15thNight
45%13223347%60%
2160
16th to endNight
45%24313347%59%
2325
November1st to 15thNight
131331345%8%64%
2470
16th to endNight
131421436%7%63%
2626
December1st to 15thNight
1424322349%59%
2730
16th to endNight
1326%42346%9%55%
2912

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%21%25%15%3
1553
16th to endDay
24%5%17%28%18%4
1850
February1st to 15thDay
28%9%22%25%10%3
1716
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%14%14%13%6%
2286
16th to endDay
35%8%12%19%18%4
2292
September1st to 15thDay
29%8%12%20%19%9%
2160
16th to endDay
26%5%9%25%28%3
2026
October1st to 15thDay
29%8%10%20%26%4
1800
16th to endDay
24%6%10%26%25%5%
1887
November1st to 15thDay
20%8%14%24%24%7%
1718
16th to endDay
20%415%28%24%6%
1661
December1st to 15thDay
26%8%17%29%14%2
1560
16th to endDay
25%7%18%27%17%3
1664
January1st to 15thNight
18%8%20%29%18%4
2716
16th to endNight
21%5%17%29%21%5%
2736
February1st to 15thNight
22%9%21%29%14%2
2445
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%8%18%22%17%7%
1650
16th to endNight
26%7%12%22%24%6%
1910
September1st to 15thNight
26%6%13%20%19%12%
1800
16th to endNight
21%5%10%27%29%5%
1930
October1st to 15thNight
24%5%12%22%27%8%
2160
16th to endNight
25%412%25%26%6%
2325
November1st to 15thNight
19%5%14%26%25%9%
2470
16th to endNight
18%5%14%29%25%6%
2626
December1st to 15thNight
22%8%18%28%17%4
2730
16th to endNight
25%7%20%26%16%2
2912

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
8%21%45%12%11%
135
16th to endDay
332%41%18%2
144
February1st to 15thDay
11%41%40%6%
130
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%7%38%30%10%1
315
16th to endDay
9%6%32%36%12%2
332
September1st to 15thDay
10%9%39%27%8%4
315
16th to endDay
10%7%41%25%11%3
193
October1st to 15thDay
6%15%42%20%11%2
135
16th to endDay
10%17%44%19%6%2
144
November1st to 15thDay
412%46%17%15%2
135
16th to endDay
10%32%46%9%
135
December1st to 15thDay
10%36%40%11%
135
16th to endDay
6%37%42%9%4
144
January1st to 15thNight
11%26%38%15%5%1
975
16th to endNight
10%25%43%14%5%
864
February1st to 15thNight
10%28%42%15%3
760
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
539
16th to endNight
6%111%36%33%9%
573
September1st to 15thNight
7%222%33%26%7%
540
16th to endNight
7%422%36%21%6%
546
October1st to 15thNight
8%12%35%26%12%3
720
16th to endNight
8%18%36%22%10%2
789
November1st to 15thNight
6%20%43%20%7%
950
16th to endNight
6%23%44%21%4
975
December1st to 15thNight
11%23%45%16%2
975
16th to endNight
8%24%45%15%5%
1040
Page generated at 2018-02-15 08:58:42. Version ac04

Copyright 2018 A. Danko


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