NFCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NFCC Observatory clear sky chart from 2005-11-05 to 2017-12-20.

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
53%
15982
22%
48094
16%
15942
60%
15942
29%
29531

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
32%
718
21%
2327
11%
716
45%
716
23%
1425
January16th to endNight
33%
768
24%
2379
13%
768
45%
768
24%
1496
February1st to 15thNight
35%
702
25%
2109
12%
700
49%
700
27%
1292
February16th to endNight
33%
631
26%
1893
12%
631
46%
631
28%
1152
March1st to 15thNight
41%
720
27%
2034
15%
720
51%
720
34%
1199
March16th to endNight
44%
768
26%
2112
14%
768
52%
768
35%
1245
April1st to 15thNight
47%
709
24%
1967
17%
709
53%
709
34%
1191
April16th to endNight
55%
540
29%
1737
21%
540
65%
540
39%
924
May1st to 15thNight
59%
539
27%
1619
20%
539
65%
539
39%
897
May16th to endNight
69%
561
23%
1682
19%
561
72%
561
36%
987
June1st to 15thNight
71%
540
14%
1620
13%
540
71%
540
28%
1030
June16th to endNight
74%
540
14%
1620
12%
540
75%
540
27%
1073
July1st to 15thNight
79%
510
10%
1530
9%
510
80%
510
28%
991
July16th to endNight
78%
564
9%
1595
10%
531
79%
531
25%
1077
August1st to 15thNight
76%
539
9%
1714
9%
536
77%
536
22%
1172
August16th to endNight
70%
600
13%
1937
14%
600
71%
600
20%
1370
September1st to 15thNight
63%
720
12%
1980
11%
720
63%
720
20%
1439
September16th to endNight
59%
717
20%
1980
16%
717
62%
717
25%
1363
October1st to 15thNight
58%
720
29%
1980
23%
720
64%
720
39%
1153
October16th to endNight
55%
768
32%
2292
27%
768
64%
768
37%
1285
November1st to 15thNight
51%
760
29%
2358
22%
760
63%
760
36%
1276
November16th to endNight
45%
780
29%
2535
21%
780
58%
780
33%
1374
December1st to 15thNight
42%
780
25%
2535
18%
780
52%
780
28%
1499
December16th to endNight
40%
788
23%
2559
17%
788
49%
788
23%
1621

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
10%5%25%6%35%5%6%8%39%
48732
Night
9%5%244245%6%8%45%
48297
TransparencyDay
40%8%12%16%15%7%
48433
Night
34%7%13%18%17%9%
48100
SeeingDay
11%5%18%24%24%14%
5830
Night
11%6%21%30%24%5%
15989

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
15%5%25%435%36%7%41%
1611
16th to endDay
14%435%423346%46%
1842
February1st to 15thDay
15%6%24323436%46%
1750
16th to endDay
13%6%24323336%48%
1580
March1st to 15thDay
9%5%245%2435%6%49%
1926
16th to endDay
10%425%434346%50%
2111
April1st to 15thDay
10%425%5%34447%47%
1990
16th to endDay
7%324434448%51%
2220
May1st to 15thDay
7%325%5%45%5%5%8%47%
2331
16th to endDay
6%426%7%5%7%6%6%7%39%
2431
June1st to 15thDay
8%5%27%9%5%7%7%8%10%27%
2340
16th to endDay
7%5%26%8%5%9%8%9%10%27%
2340
July1st to 15thDay
7%5%37%8%5%9%8%7%10%24%
2340
16th to endDay
7%5%37%10%6%9%9%8%9%23%
2465
August1st to 15thDay
8%7%49%10%5%7%8%8%10%18%
2231
16th to endDay
12%7%38%9%5%7%6%6%7%24%
2253
September1st to 15thDay
11%6%48%8%47%6%7%10%24%
1980
16th to endDay
12%7%25%5%45%7%6%7%36%
1980
October1st to 15thDay
9%5%25%435%446%49%
1980
16th to endDay
8%4245%2334456%
1929
November1st to 15thDay
7%213322446%59%
1828
16th to endDay
10%324423346%53%
1755
December1st to 15thDay
13%6%244335%37%45%
1755
16th to endDay
17%7%35%42445%6%37%
1764
January1st to 15thNight
13%7%24424447%44%
2327
16th to endNight
14%6%35%5%23446%44%
2379
February1st to 15thNight
13%6%24323445%50%
2103
16th to endNight
13%6%24323245%49%
1893
March1st to 15thNight
9%5%2432345%7%51%
2034
16th to endNight
9%5%14423446%54%
2112
April1st to 15thNight
10%423424447%51%
1967
16th to endNight
5%31332445%8%56%
1737
May1st to 15thNight
6%22332446%11%52%
1619
16th to endNight
5%313425%5%7%11%48%
1682
June1st to 15thNight
7%3345%35%7%8%12%38%
1620
16th to endNight
6%325%6%27%7%7%13%36%
1620
July1st to 15thNight
4324437%8%10%13%36%
1620
16th to endNight
43146%37%9%10%15%32%
1703
August1st to 15thNight
44346%46%9%11%15%29%
1725
16th to endNight
8%425%6%46%7%8%12%32%
1937
September1st to 15thNight
8%6%36%7%45%7%9%10%29%
1980
16th to endNight
8%6%26%6%45%6%7%9%36%
1980
October1st to 15thNight
5%425%433348%54%
1980
16th to endNight
6%5%235%23445%56%
2292
November1st to 15thNight
7%413434347%55%
2358
16th to endNight
9%424323345%55%
2535
December1st to 15thNight
11%5%2442445%7%46%
2535
16th to endNight
14%7%35%5%24347%41%
2559

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
40%5%9%15%17%11%
1612
16th to endDay
39%37%15%20%11%
1842
February1st to 15thDay
39%37%15%20%14%
1752
16th to endDay
37%47%14%23%12%
1580
March1st to 15thDay
32%5%9%18%24%9%
1926
16th to endDay
35%410%18%19%11%
2112
April1st to 15thDay
36%5%9%17%20%11%
1990
16th to endDay
30%5%10%22%20%10%
2220
May1st to 15thDay
32%6%11%18%20%10%
2331
16th to endDay
38%8%14%18%15%4
2435
June1st to 15thDay
47%11%16%16%6%1
2340
16th to endDay
45%13%18%14%6%1
2340
July1st to 15thDay
48%14%21%12%2
2201
16th to endDay
49%15%20%11%3
2310
August1st to 15thDay
54%15%18%8%3
2218
16th to endDay
54%11%15%13%4
2253
September1st to 15thDay
49%13%15%12%6%1
1980
16th to endDay
43%9%13%15%13%4
1980
October1st to 15thDay
34%5%10%15%21%11%
1980
16th to endDay
30%36%14%27%18%
1929
November1st to 15thDay
24%48%21%26%14%
1828
16th to endDay
32%46%17%23%16%
1755
December1st to 15thDay
38%5%10%18%17%9%
1755
16th to endDay
45%410%19%12%8%
1764
January1st to 15thNight
39%410%14%20%11%
2327
16th to endNight
40%47%13%22%12%
2379
February1st to 15thNight
36%46%14%20%16%
2112
16th to endNight
37%37%16%18%16%
1893
March1st to 15thNight
31%410%21%21%9%
2034
16th to endNight
31%49%18%24%11%
2112
April1st to 15thNight
33%5%8%17%23%11%
1967
16th to endNight
25%6%11%23%22%10%
1737
May1st to 15thNight
24%7%13%21%21%11%
1620
16th to endNight
26%10%16%24%17%4
1684
June1st to 15thNight
33%12%23%21%8%
1620
16th to endNight
34%13%22%20%8%1
1620
July1st to 15thNight
31%15%30%19%3
1530
16th to endNight
32%17%29%15%4
1595
August1st to 15thNight
34%18%28%14%4
1714
16th to endNight
39%17%21%15%41
1937
September1st to 15thNight
44%12%18%13%9%1
1980
16th to endNight
40%11%14%16%12%5%
1980
October1st to 15thNight
28%6%12%18%21%12%
1980
16th to endNight
29%45%15%24%20%
2292
November1st to 15thNight
29%38%20%24%14%
2358
16th to endNight
30%39%17%22%17%
2535
December1st to 15thNight
36%411%20%19%8%
2535
16th to endNight
42%48%21%14%8%
2559

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
17%17%37%8%11%6%
90
16th to endDay
23%19%35%15%5%
96
February1st to 15thDay
16%24%33%17%7%
90
16th to endDay
9%25%40%20%2
81
March1st to 15thDay
11%15%32%27%11%1
216
16th to endDay
13%8%29%27%18%2
288
April1st to 15thDay
12%15%21%30%19%1
270
16th to endDay
7%5%29%28%22%7%
270
May1st to 15thDay
11%7%20%30%19%10%
288
16th to endDay
8%313%29%28%17%
331
June1st to 15thDay
12%11%21%32%21%
315
16th to endDay
11%7%20%30%30%
315
July1st to 15thDay
5%5%20%29%38%
305
16th to endDay
7%417%38%30%
326
August1st to 15thDay
11%16%20%35%24%
313
16th to endDay
9%9%18%32%27%
332
September1st to 15thDay
17%111%19%26%22%
315
16th to endDay
14%111%20%37%14%
314
October1st to 15thDay
12%5%20%28%24%9%
315
16th to endDay
9%6%28%28%22%4
336
November1st to 15thDay
10%229%32%19%5%
246
16th to endDay
5%11%37%28%12%4
135
December1st to 15thDay
14%14%34%25%8%1
135
16th to endDay
28%6%25%25%12%
108
January1st to 15thNight
17%13%30%25%10%1
720
16th to endNight
17%13%31%28%8%
768
February1st to 15thNight
16%13%30%30%8%
704
16th to endNight
18%11%32%33%5%
631
March1st to 15thNight
12%11%31%34%9%
720
16th to endNight
13%8%31%31%15%
768
April1st to 15thNight
12%7%28%35%15%
709
16th to endNight
8%427%34%22%2
540
May1st to 15thNight
7%325%38%23%2
540
16th to endNight
6%214%35%36%4
563
June1st to 15thNight
9%7%35%36%10%
540
16th to endNight
7%5%27%39%17%
540
July1st to 15thNight
5%422%43%23%
510
16th to endNight
41321%50%18%
564
August1st to 15thNight
6%1219%54%15%
539
16th to endNight
10%15%24%38%20%
600
September1st to 15thNight
10%110%26%39%11%
720
16th to endNight
12%115%33%33%3
717
October1st to 15thNight
8%322%32%29%2
720
16th to endNight
8%6%24%32%25%2
768
November1st to 15thNight
9%7%27%32%21%1
760
16th to endNight
11%10%28%31%17%
780
December1st to 15thNight
15%8%26%30%17%
780
16th to endNight
17%11%26%26%16%1
788
Page generated at 2017-12-21 07:04:44. Version ac04

Copyright 2017 A. Danko


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