Johnston's Harbour
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Johnston's Harbour clear sky chart from 2005-11-05 to 2017-09-16.

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
29%
15229
10%
46742
7%
15187
33%
15187
13%
35545

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
8%
899
1%
2616
0%
895
9%
895
2%
2431
January16th to endNight
14%
847
5%
2575
3%
847
17%
847
4%
2306
February1st to 15thNight
14%
702
4%
2287
1%
701
16%
701
6%
1969
February16th to endNight
22%
631
8%
1967
3%
631
27%
631
9%
1601
March1st to 15thNight
24%
720
12%
2034
7%
720
28%
720
14%
1535
March16th to endNight
32%
768
19%
2112
9%
768
40%
768
20%
1466
April1st to 15thNight
28%
644
16%
1791
7%
644
34%
644
20%
1252
April16th to endNight
32%
540
18%
1620
7%
540
41%
540
19%
1097
May1st to 15thNight
43%
537
14%
1617
12%
537
46%
537
18%
1103
May16th to endNight
41%
375
14%
1482
13%
374
50%
374
19%
860
June1st to 15thNight
42%
360
13%
1260
12%
360
50%
360
20%
792
June16th to endNight
51%
360
11%
1260
10%
360
56%
360
22%
772
July1st to 15thNight
60%
340
17%
1190
19%
340
64%
340
31%
640
July16th to endNight
51%
535
15%
1476
13%
502
59%
502
29%
911
August1st to 15thNight
57%
539
17%
1609
14%
536
61%
536
28%
967
August16th to endNight
55%
573
18%
1719
11%
573
60%
573
27%
1047
September1st to 15thNight
50%
702
14%
1869
9%
702
52%
702
27%
1247
September16th to endNight
48%
658
17%
1818
12%
658
51%
658
25%
1218
October1st to 15thNight
32%
660
13%
1956
10%
660
37%
660
15%
1467
October16th to endNight
20%
704
8%
2172
4%
704
25%
704
8%
1790
November1st to 15thNight
23%
700
11%
2357
10%
700
26%
700
10%
1847
November16th to endNight
16%
720
4%
2520
3%
720
18%
720
4%
2184
December1st to 15thNight
9%
755
1%
2555
0%
755
10%
755
2%
2374
December16th to endNight
6%
960
2%
2880
0%
960
8%
960
2%
2669

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
15%10%46%5%3445%6%33%
48001
Night
16%12%5%8%6%34446%27%
46910
Cloud CoverDay
15%9%36%5%3445%7%33%
48011
Night
17%12%5%8%6%34445%27%
46914
TransparencyDay
50%6%12%19%9%
47687
Night
58%5%10%15%8%
46742
SeeingDay
18%12%21%27%15%4
5836
Night
19%21%21%22%11%2
15232

By Month

Percentage Cloud Cover NE (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
19%20%8%15%8%36%4328%
1319
16th to endDay
16%20%8%12%9%46%4429%
1647
February1st to 15thDay
12%19%7%13%9%45%43415%
1575
16th to endDay
13%17%7%9%8%46%336%19%
1503
March1st to 15thDay
13%10%37%6%45%5%46%31%
1915
16th to endDay
13%7%37%6%36%45%7%35%
2112
April1st to 15thDay
19%9%35%5%24446%34%
2167
16th to endDay
17%7%3432335%7%41%
2340
May1st to 15thDay
17%7%2442445%6%39%
2334
16th to endDay
12%42342446%8%46%
2630
June1st to 15thDay
12%5%25%5%25%5%6%8%39%
2700
16th to endDay
10%5%35%5%345%6%9%40%
2700
July1st to 15thDay
8%42442445%8%49%
2700
16th to endDay
8%41342446%10%47%
2587
August1st to 15thDay
10%5%2442435%8%48%
2340
16th to endDay
11%6%25%5%3445%8%42%
2483
September1st to 15thDay
13%6%16%42445%7%42%
2076
16th to endDay
17%5%35%4345%5%7%38%
1815
October1st to 15thDay
21%9%46%5%24445%30%
1673
16th to endDay
29%12%5%8%6%43335%18%
1697
November1st to 15thDay
20%13%38%6%36%5%46%20%
1501
16th to endDay
23%18%7%9%7%35%43411%
1440
December1st to 15thDay
19%25%7%12%9%343326%
1403
16th to endDay
21%24%9%13%8%333336%
1344
January1st to 15thNight
20%20%8%13%10%45%3237%
2616
16th to endNight
15%18%8%13%10%46%33311%
2575
February1st to 15thNight
12%19%8%14%8%3434414%
2285
16th to endNight
15%13%7%10%7%36%44421%
1967
March1st to 15thNight
15%12%47%6%43346%30%
2022
16th to endNight
13%7%37%5%34445%38%
2112
April1st to 15thNight
21%8%35%323446%35%
1791
16th to endNight
19%7%25%42445%6%37%
1620
May1st to 15thNight
16%7%45%44445%7%34%
1617
16th to endNight
10%6%25%5%35%447%43%
1480
June1st to 15thNight
12%8%25%5%245%48%39%
1260
16th to endNight
9%4346%35%6%5%8%42%
1260
July1st to 15thNight
7%315%42446%9%50%
1260
16th to endNight
7%5%14425%47%7%49%
1579
August1st to 15thNight
7%5%25%425%5%5%8%48%
1620
16th to endNight
9%5%25%4245%5%8%46%
1719
September1st to 15thNight
13%5%145%25%5%6%6%42%
1869
16th to endNight
15%7%34434447%40%
1818
October1st to 15thNight
21%9%37%5%35%337%29%
1956
16th to endNight
26%11%39%7%34345%19%
2172
November1st to 15thNight
20%13%49%6%3445%5%22%
2357
16th to endNight
24%17%5%9%7%35%43414%
2520
December1st to 15thNight
22%22%8%12%8%343338%
2555
16th to endNight
23%22%9%13%7%342237%
2880

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
19%20%9%14%8%35%3328%
1319
16th to endDay
17%20%7%12%10%45%4349%
1647
February1st to 15thDay
13%19%7%13%10%35%34415%
1575
16th to endDay
14%17%7%9%8%36%435%20%
1503
March1st to 15thDay
14%10%46%6%45%45%6%31%
1926
16th to endDay
13%7%37%6%35%5%5%6%35%
2111
April1st to 15thDay
20%9%35%423446%34%
2167
16th to endDay
18%7%3432335%7%41%
2340
May1st to 15thDay
17%6%34425%35%7%39%
2334
16th to endDay
12%41342445%9%46%
2630
June1st to 15thDay
12%6%25%5%35%5%5%9%39%
2700
16th to endDay
11%6%25%5%345%7%9%39%
2700
July1st to 15thDay
8%42442445%8%49%
2700
16th to endDay
8%4134245%6%9%47%
2587
August1st to 15thDay
10%5%2442345%8%48%
2340
16th to endDay
11%6%25%434448%42%
2483
September1st to 15thDay
13%5%25%5%3435%7%42%
2076
16th to endDay
17%5%35%4245%5%8%38%
1815
October1st to 15thDay
22%9%46%4245%45%30%
1673
16th to endDay
29%11%48%6%34335%18%
1697
November1st to 15thDay
21%12%47%6%26%5%46%20%
1501
16th to endDay
24%18%6%9%7%3434412%
1440
December1st to 15thDay
21%24%7%13%8%45%3236%
1403
16th to endDay
21%25%9%13%7%333335%
1344
January1st to 15thNight
20%20%8%14%9%45%3337%
2616
16th to endNight
16%18%8%13%9%45%33411%
2575
February1st to 15thNight
12%19%8%13%9%3434415%
2278
16th to endNight
15%14%7%10%7%45%445%20%
1967
March1st to 15thNight
16%11%47%7%34346%30%
2034
16th to endNight
14%7%46%5%34445%39%
2112
April1st to 15thNight
21%9%25%413446%36%
1791
16th to endNight
19%7%3442445%6%37%
1620
May1st to 15thNight
16%7%44445%45%7%35%
1616
16th to endNight
10%5%35%5%34447%44%
1480
June1st to 15thNight
12%7%35%5%345%5%8%39%
1260
16th to endNight
10%4245%35%6%5%8%41%
1260
July1st to 15thNight
7%325%43446%9%50%
1260
16th to endNight
7%414425%46%8%49%
1579
August1st to 15thNight
7%5%25%435%5%5%8%48%
1620
16th to endNight
9%5%25%4245%5%7%47%
1719
September1st to 15thNight
13%5%244245%6%7%42%
1869
16th to endNight
15%8%34424447%40%
1818
October1st to 15thNight
22%8%37%5%35%346%29%
1956
16th to endNight
27%11%47%7%34345%19%
2172
November1st to 15thNight
20%14%49%5%2445%5%22%
2357
16th to endNight
25%18%5%8%7%35%335%13%
2520
December1st to 15thNight
23%22%8%12%8%233337%
2555
16th to endNight
24%23%9%12%6%233237%
2880

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
84%5%5%4
1320
16th to endDay
81%5%5%5%1
1647
February1st to 15thDay
74%49%9%1
1577
16th to endDay
67%5%11%10%5%
1503
March1st to 15thDay
52%5%14%19%7%
1926
16th to endDay
46%6%13%18%12%2
2112
April1st to 15thDay
50%410%21%11%1
2167
16th to endDay
42%5%11%22%14%2
2340
May1st to 15thDay
44%5%9%23%14%2
2334
16th to endDay
35%5%15%24%16%1
2634
June1st to 15thDay
39%7%12%27%12%
2700
16th to endDay
39%7%14%25%12%
2700
July1st to 15thDay
29%7%14%29%16%1
2541
16th to endDay
31%9%15%28%13%1
2427
August1st to 15thDay
32%6%18%27%13%
2327
16th to endDay
38%7%15%26%10%
2483
September1st to 15thDay
40%6%14%28%9%
2076
16th to endDay
43%7%16%24%8%
1815
October1st to 15thDay
55%5%11%17%9%
1673
16th to endDay
69%7%10%9%3
1697
November1st to 15thDay
63%7%11%11%5%
1501
16th to endDay
76%5%8%7%1
1440
December1st to 15thDay
86%45%21
1403
16th to endDay
85%46%3
1344
January1st to 15thNight
83%5%5%4
2616
16th to endNight
79%5%6%6%1
2575
February1st to 15thNight
73%6%8%9%2
2287
16th to endNight
66%6%10%11%5%
1967
March1st to 15thNight
55%5%12%18%6%
2034
16th to endNight
47%410%20%15%2
2112
April1st to 15thNight
49%49%23%12%1
1791
16th to endNight
46%48%19%18%2
1620
May1st to 15thNight
46%5%8%20%14%2
1617
16th to endNight
38%5%13%22%16%2
1482
June1st to 15thNight
41%6%9%24%16%1
1260
16th to endNight
38%5%14%25%14%
1260
July1st to 15thNight
28%6%13%28%19%2
1190
16th to endNight
33%8%14%25%17%
1476
August1st to 15thNight
32%7%16%26%15%1
1609
16th to endNight
35%6%17%26%12%1
1719
September1st to 15thNight
39%7%15%25%11%
1869
16th to endNight
44%415%25%9%
1818
October1st to 15thNight
56%5%10%18%9%
1956
16th to endNight
68%5%10%11%3
2172
November1st to 15thNight
62%6%11%14%4
2357
16th to endNight
75%6%10%6%1
2520
December1st to 15thNight
85%45%21
2555
16th to endNight
84%46%3
2880

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
25%11%35%23%4
71
16th to endDay
36%428%22%8%
96
February1st to 15thDay
27%15%34%21%1
90
16th to endDay
29%13%25%28%2
81
March1st to 15thDay
17%16%31%29%4
216
16th to endDay
20%15%23%27%7%3
288
April1st to 15thDay
24%14%25%23%10%1
270
16th to endDay
22%14%24%22%10%4
270
May1st to 15thDay
21%11%21%26%15%3
288
16th to endDay
13%12%23%29%17%4
343
June1st to 15thDay
13%14%21%34%13%3
495
16th to endDay
13%13%19%31%17%3
495
July1st to 15thDay
10%9%17%35%20%6%
475
16th to endDay
11%11%19%28%23%5%
419
August1st to 15thDay
12%7%14%28%28%8%
313
16th to endDay
21%7%12%25%23%9%
332
September1st to 15thDay
13%7%17%24%23%13%
315
16th to endDay
17%8%20%25%19%8%
254
October1st to 15thDay
25%11%21%27%10%2
255
16th to endDay
32%15%25%19%6%1
209
November1st to 15thDay
16%6%35%32%7%1
77
16th to endDay
23%15%43%16%1
60
December1st to 15thDay
28%25%30%11%5%
60
16th to endDay
34%35%25%4
64
January1st to 15thNight
24%31%24%14%4
900
16th to endNight
19%27%24%21%6%1
847
February1st to 15thNight
19%26%29%18%5%
704
16th to endNight
19%21%26%24%7%
631
March1st to 15thNight
18%24%25%23%7%1
720
16th to endNight
16%21%22%26%11%1
768
April1st to 15thNight
21%21%24%24%6%1
644
16th to endNight
20%22%21%21%13%1
540
May1st to 15thNight
18%18%13%29%18%2
537
16th to endNight
11%19%18%29%18%1
375
June1st to 15thNight
15%19%16%31%15%2
360
16th to endNight
10%18%15%28%25%2
360
July1st to 15thNight
8%14%11%39%22%4
340
16th to endNight
9%17%14%31%23%2
535
August1st to 15thNight
10%11%15%31%25%5%
539
16th to endNight
11%11%14%27%28%5%
573
September1st to 15thNight
14%13%17%26%18%10%
702
16th to endNight
17%13%16%23%22%7%
658
October1st to 15thNight
23%15%21%22%13%3
660
16th to endNight
30%19%24%20%31
704
November1st to 15thNight
21%25%24%18%9%1
700
16th to endNight
28%29%19%16%5%
720
December1st to 15thNight
29%31%25%10%3
755
16th to endNight
29%30%24%12%3
960
Page generated at 2017-09-16 17:07:02. Version ac04

Copyright 2017 A. Danko


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