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 2018-04-23.

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
29%
16128
10%
49483
7%
16086
33%
16086
12%
37753

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
9%
974
2%
2835
0%
970
11%
970
2%
2635
January16th to endNight
15%
917
4%
2789
2%
917
17%
917
4%
2491
February1st to 15thNight
14%
762
6%
2482
2%
761
18%
761
7%
2110
February16th to endNight
22%
683
8%
2129
3%
683
27%
683
9%
1727
March1st to 15thNight
24%
780
12%
2204
6%
780
29%
780
15%
1660
March16th to endNight
32%
832
19%
2288
9%
832
41%
832
20%
1572
April1st to 15thNight
28%
698
15%
1941
7%
698
34%
698
19%
1373
April16th to endNight
32%
558
18%
1672
8%
558
42%
558
19%
1127
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
50%
717
19%
1980
14%
717
52%
717
26%
1320
October1st to 15thNight
33%
720
12%
2134
10%
720
37%
720
16%
1596
October16th to endNight
20%
768
8%
2370
4%
768
26%
768
8%
1935
November1st to 15thNight
23%
760
10%
2559
9%
760
25%
760
9%
2035
November16th to endNight
16%
780
4%
2730
3%
780
18%
780
4%
2366
December1st to 15thNight
9%
818
1%
2768
0%
818
10%
818
2%
2575
December16th to endNight
6%
1040
2%
3120
0%
1040
8%
1040
2%
2892

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%32%
50035
Night
16%12%5%8%6%34445%27%
49651
Cloud CoverDay
15%10%36%5%3445%6%33%
50045
Night
17%12%5%8%6%34445%27%
49655
TransparencyDay
51%6%12%19%9%
49721
Night
58%5%10%15%8%
49483
SeeingDay
19%11%22%26%14%4
6510
Night
19%21%21%22%11%2
16131

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%7%36%5%228%
1430
16th to endDay
16%20%8%12%9%46%4438%
1784
February1st to 15thDay
12%19%7%12%9%35%43416%
1710
16th to endDay
14%17%6%8%8%46%336%20%
1627
March1st to 15thDay
13%10%37%7%45%5%45%31%
2075
16th to endDay
13%7%36%5%36%45%6%36%
2288
April1st to 15thDay
20%10%35%5%24446%33%
2348
16th to endDay
17%7%3431335%7%41%
2402
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
16%5%344345%5%7%39%
1980
October1st to 15thDay
21%9%46%5%24446%31%
1825
16th to endDay
28%12%48%6%33335%19%
1851
November1st to 15thDay
21%13%38%6%36%5%46%19%
1629
16th to endDay
22%18%7%9%7%35%43411%
1560
December1st to 15thDay
19%25%7%12%9%343326%
1520
16th to endDay
21%23%9%13%8%333336%
1456
January1st to 15thNight
19%20%7%12%10%46%4337%
2835
16th to endNight
15%18%8%13%10%5%6%33411%
2789
February1st to 15thNight
11%19%8%14%8%3434416%
2480
16th to endNight
15%13%7%10%7%36%45%421%
2129
March1st to 15thNight
14%12%47%6%43346%31%
2192
16th to endNight
13%7%37%6%3445%5%39%
2288
April1st to 15thNight
21%8%35%424446%34%
1941
16th to endNight
19%7%25%42445%6%38%
1672
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
14%7%34434447%41%
1980
October1st to 15thNight
20%9%37%5%35%336%29%
2134
16th to endNight
26%11%49%7%34345%20%
2370
November1st to 15thNight
20%14%49%6%3445%5%21%
2559
16th to endNight
24%17%5%9%7%3443414%
2730
December1st to 15thNight
21%22%9%12%8%343338%
2768
16th to endNight
23%21%9%13%7%342237%
3120

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%8%14%8%36%4328%
1430
16th to endDay
17%19%7%12%9%45%5%449%
1784
February1st to 15thDay
13%19%6%13%10%35%34415%
1710
16th to endDay
14%16%7%9%8%36%435%20%
1627
March1st to 15thDay
13%10%47%6%45%446%31%
2086
16th to endDay
13%6%37%6%36%5%5%6%37%
2287
April1st to 15thDay
21%9%35%5%23446%33%
2348
16th to endDay
18%7%23323347%42%
2402
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
16%5%35%4245%5%8%39%
1980
October1st to 15thDay
22%9%36%4245%46%31%
1825
16th to endDay
29%11%48%6%34335%19%
1851
November1st to 15thDay
23%12%36%7%36%5%5%6%19%
1629
16th to endDay
24%19%6%9%7%3434412%
1560
December1st to 15thDay
20%25%7%12%8%45%3236%
1520
16th to endDay
20%24%8%13%8%243336%
1456
January1st to 15thNight
20%20%8%13%9%45%3337%
2835
16th to endNight
15%18%8%12%10%46%33411%
2789
February1st to 15thNight
11%19%8%13%9%3434416%
2473
16th to endNight
15%13%7%10%7%45%45%5%20%
2129
March1st to 15thNight
15%11%47%7%34345%30%
2204
16th to endNight
13%7%36%6%34445%39%
2288
April1st to 15thNight
21%9%35%4235%46%34%
1941
16th to endNight
20%7%24424446%38%
1672
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
14%7%25%42445%7%41%
1980
October1st to 15thNight
22%8%36%6%35%346%29%
2134
16th to endNight
26%11%47%7%34345%20%
2370
November1st to 15thNight
21%15%39%5%2445%5%21%
2559
16th to endNight
24%18%5%8%7%35%335%13%
2730
December1st to 15thNight
22%22%8%12%8%243237%
2768
16th to endNight
23%22%8%12%7%343237%
3120

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
1431
16th to endDay
80%5%6%5%1
1784
February1st to 15thDay
74%49%9%1
1712
16th to endDay
67%411%10%5%
1627
March1st to 15thDay
53%5%13%18%8%
2086
16th to endDay
45%6%13%20%12%2
2288
April1st to 15thDay
52%49%21%11%1
2348
16th to endDay
42%5%11%22%15%2
2402
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
42%6%15%26%7%
1980
October1st to 15thDay
54%5%11%18%9%
1825
16th to endDay
67%7%10%10%3
1851
November1st to 15thDay
63%8%11%10%5%
1629
16th to endDay
75%5%9%7%1
1560
December1st to 15thDay
86%4421
1520
16th to endDay
85%45%4
1456
January1st to 15thNight
83%6%5%4
2835
16th to endNight
78%5%7%6%1
2789
February1st to 15thNight
72%6%7%10%3
2482
16th to endNight
66%6%10%11%5%
2129
March1st to 15thNight
55%5%11%18%7%1
2204
16th to endNight
46%410%21%14%2
2288
April1st to 15thNight
50%48%22%12%1
1941
16th to endNight
45%48%19%18%2
1672
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
42%5%14%27%9%
1980
October1st to 15thNight
55%5%9%19%9%
2134
16th to endNight
67%5%10%12%3
2370
November1st to 15thNight
63%6%11%13%3
2559
16th to endNight
74%6%10%6%1
2730
December1st to 15thNight
85%45%21
2768
16th to endNight
83%5%6%3
3120

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
30%7%36%21%3
107
16th to endDay
35%6%28%23%6%
144
February1st to 15thDay
26%17%36%17%2
135
16th to endDay
29%14%31%23%1
120
March1st to 15thDay
19%12%30%28%7%
271
16th to endDay
20%14%26%28%7%3
352
April1st to 15thDay
26%12%26%22%9%1
330
16th to endDay
22%13%22%24%11%4
289
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%7%17%23%25%7%
314
October1st to 15thDay
25%11%22%26%11%3
315
16th to endDay
31%15%25%20%5%1
267
November1st to 15thDay
26%6%35%23%6%
115
16th to endDay
25%18%37%16%1
90
December1st to 15thDay
31%21%27%15%4
90
16th to endDay
32%28%32%7%
96
January1st to 15thNight
24%30%24%15%4
975
16th to endNight
19%26%24%22%6%1
917
February1st to 15thNight
18%25%31%19%4
764
16th to endNight
20%20%27%24%7%
683
March1st to 15thNight
18%22%25%23%8%1
780
16th to endNight
15%20%23%27%11%1
832
April1st to 15thNight
21%20%24%25%7%1
698
16th to endNight
20%21%20%21%13%1
558
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
16%12%16%22%24%7%
717
October1st to 15thNight
23%14%21%23%13%3
720
16th to endNight
30%19%24%21%31
768
November1st to 15thNight
23%24%23%18%9%1
760
16th to endNight
28%29%19%15%5%
780
December1st to 15thNight
28%30%25%11%3
818
16th to endNight
28%29%26%12%3
1040
Page generated at 2018-04-24 10:02:59. Version ac04

Copyright 2018 A. Danko


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