Chatham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chatham 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
37%
15174
15%
46384
10%
15136
42%
15136
20%
32442

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
16%
860
10%
2470
3%
859
22%
859
11%
1975
January16th to endNight
19%
768
9%
2496
4%
768
27%
768
9%
1995
February1st to 15thNight
19%
702
9%
2287
2%
701
26%
701
11%
1765
February16th to endNight
24%
631
12%
1910
5%
631
32%
631
14%
1444
March1st to 15thNight
26%
720
14%
2037
8%
720
31%
720
16%
1497
March16th to endNight
33%
768
18%
2112
11%
768
39%
768
23%
1483
April1st to 15thNight
30%
647
14%
1831
7%
647
36%
647
21%
1298
April16th to endNight
38%
540
20%
1620
9%
540
46%
540
25%
1068
May1st to 15thNight
44%
540
13%
1620
8%
540
49%
540
20%
1152
May16th to endNight
49%
519
16%
1642
12%
519
54%
519
27%
1028
June1st to 15thNight
55%
360
16%
1356
17%
360
61%
360
26%
796
June16th to endNight
57%
360
19%
1260
19%
360
63%
360
30%
715
July1st to 15thNight
62%
486
17%
1336
15%
486
65%
486
39%
792
July16th to endNight
67%
564
17%
1587
14%
531
70%
531
37%
898
August1st to 15thNight
71%
527
19%
1575
16%
524
73%
524
36%
927
August16th to endNight
67%
525
23%
1575
17%
525
71%
525
36%
915
September1st to 15thNight
60%
647
23%
1764
16%
647
64%
647
33%
1103
September16th to endNight
57%
657
23%
1815
17%
657
61%
657
32%
1141
October1st to 15thNight
48%
660
18%
1930
16%
660
52%
660
26%
1252
October16th to endNight
26%
704
13%
2154
7%
704
34%
704
15%
1573
November1st to 15thNight
35%
700
19%
2276
16%
700
41%
700
21%
1509
November16th to endNight
23%
720
12%
2481
9%
720
28%
720
12%
1826
December1st to 15thNight
18%
720
6%
2520
3%
720
22%
720
8%
2011
December16th to endNight
15%
849
7%
2730
2%
849
20%
849
7%
2279

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
14%8%36%5%35%5%5%7%33%
47559
Night
15%8%46%5%3445%6%34%
46566
Cloud CoverDay
14%8%36%5%35%5%5%7%33%
47569
Night
15%8%36%5%3445%6%34%
46570
TransparencyDay
47%7%13%19%10%1
47259
Night
48%6%12%20%11%1
46384
SeeingDay
16%7%26%26%16%7%
5556
Night
19%8%27%29%14%1
15182

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
18%14%7%10%7%35%345%18%
1463
16th to endDay
17%14%7%10%8%35%5%5%6%16%
1728
February1st to 15thDay
16%14%6%8%6%45%446%21%
1575
16th to endDay
13%12%6%9%7%46%446%24%
1563
March1st to 15thDay
19%10%5%7%5%3445%5%26%
1911
16th to endDay
17%9%5%7%7%345%5%7%27%
2112
April1st to 15thDay
19%9%36%4245%6%7%30%
2125
16th to endDay
17%7%26%4345%46%37%
2340
May1st to 15thDay
16%7%37%5%3446%6%34%
2331
16th to endDay
12%426%5%345%6%9%40%
2470
June1st to 15thDay
10%5%25%5%35%6%6%9%39%
2601
16th to endDay
9%425%5%35%6%8%10%37%
2700
July1st to 15thDay
8%324425%6%6%9%45%
2541
16th to endDay
7%5%245%35%5%7%10%41%
2471
August1st to 15thDay
6%425%6%35%5%8%9%40%
2288
16th to endDay
9%424435%5%6%10%43%
2275
September1st to 15thDay
12%6%25%4346%5%7%41%
1851
16th to endDay
14%424435%6%49%40%
1815
October1st to 15thDay
13%5%36%5%35%5%5%6%38%
1700
16th to endDay
22%7%35%5%34446%32%
1716
November1st to 15thDay
15%8%36%5%245%46%37%
1582
16th to endDay
18%11%5%9%6%3445%7%23%
1476
December1st to 15thDay
19%14%5%9%7%35%446%19%
1440
16th to endDay
21%14%6%10%6%3446%5%14%
1485
January1st to 15thNight
21%13%5%8%6%3435%5%21%
2470
16th to endNight
15%15%7%9%7%4444421%
2496
February1st to 15thNight
15%14%6%9%6%34345%26%
2285
16th to endNight
17%11%48%7%345%45%26%
1910
March1st to 15thNight
18%10%46%6%34446%29%
2025
16th to endNight
16%8%36%434447%36%
2112
April1st to 15thNight
17%8%36%5%34446%34%
1831
16th to endNight
14%6%25%5%25%45%7%39%
1620
May1st to 15thNight
16%5%26%7%45%6%6%6%33%
1620
16th to endNight
11%435%6%33448%44%
1640
June1st to 15thNight
9%4245%4446%8%44%
1356
16th to endNight
9%4234245%6%9%45%
1260
July1st to 15thNight
7%335%42335%8%53%
1416
16th to endNight
5%314425%5%6%10%49%
1694
August1st to 15thNight
5%3145%25%5%6%8%49%
1586
16th to endNight
9%41442335%8%52%
1575
September1st to 15thNight
10%5%2443335%7%48%
1764
16th to endNight
11%5%2442445%6%47%
1815
October1st to 15thNight
15%6%25%423346%43%
1930
16th to endNight
21%6%36%5%3445%6%30%
2154
November1st to 15thNight
13%8%46%434446%38%
2276
16th to endNight
22%11%46%5%2345%6%26%
2481
December1st to 15thNight
21%13%6%8%6%34445%20%
2520
16th to endNight
23%14%6%8%6%45%45%5%16%
2730

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
18%14%6%11%7%35%445%18%
1463
16th to endDay
16%14%7%10%7%46%445%16%
1728
February1st to 15thDay
16%15%5%9%6%46%446%21%
1575
16th to endDay
13%11%6%9%8%36%5%5%5%24%
1563
March1st to 15thDay
19%11%5%8%6%35%35%5%26%
1922
16th to endDay
17%9%48%6%345%5%6%26%
2111
April1st to 15thDay
19%8%36%5%245%6%7%30%
2125
16th to endDay
18%7%26%433446%37%
2340
May1st to 15thDay
17%7%36%6%3446%6%33%
2331
16th to endDay
12%5%26%5%3446%8%41%
2470
June1st to 15thDay
10%5%25%5%35%6%6%8%39%
2601
16th to endDay
9%5%25%5%35%5%8%10%37%
2700
July1st to 15thDay
8%4244246%6%9%44%
2541
16th to endDay
7%425%5%36%5%8%9%41%
2471
August1st to 15thDay
6%425%6%45%6%8%10%39%
2288
16th to endDay
9%5%24435%46%9%43%
2275
September1st to 15thDay
12%6%25%5%345%5%7%40%
1851
16th to endDay
15%5%14435%5%48%40%
1815
October1st to 15thDay
13%6%36%5%35%5%5%6%38%
1700
16th to endDay
21%8%35%5%34445%32%
1716
November1st to 15thDay
15%7%35%5%246%45%37%
1582
16th to endDay
17%10%5%9%7%25%448%23%
1476
December1st to 15thDay
19%14%5%8%6%35%6%46%19%
1440
16th to endDay
21%14%6%11%7%25%5%5%6%14%
1485
January1st to 15thNight
21%14%5%8%6%3444421%
2470
16th to endNight
15%15%7%10%8%4444421%
2496
February1st to 15thNight
15%14%6%9%7%34345%25%
2278
16th to endNight
17%11%5%8%7%35%446%26%
1910
March1st to 15thNight
18%10%46%6%34446%29%
2037
16th to endNight
16%7%36%5%34446%36%
2112
April1st to 15thNight
17%8%46%6%34446%34%
1831
16th to endNight
14%6%25%5%25%448%39%
1620
May1st to 15thNight
16%5%26%6%45%5%6%6%33%
1619
16th to endNight
11%5%26%6%2345%8%44%
1640
June1st to 15thNight
9%3246%35%46%9%44%
1356
16th to endNight
8%424425%46%10%45%
1260
July1st to 15thNight
6%425%41335%8%53%
1416
16th to endNight
6%224425%5%7%9%50%
1694
August1st to 15thNight
6%3145%35%5%6%8%50%
1586
16th to endNight
9%414423448%51%
1575
September1st to 15thNight
10%5%2442345%7%48%
1764
16th to endNight
11%5%245%24446%47%
1815
October1st to 15thNight
15%6%25%433346%44%
1930
16th to endNight
21%6%36%5%3445%6%31%
2154
November1st to 15thNight
13%8%37%42445%6%38%
2276
16th to endNight
22%10%47%42345%6%26%
2481
December1st to 15thNight
21%13%5%8%6%35%346%20%
2520
16th to endNight
22%14%6%9%6%4445%5%16%
2730

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
68%6%9%11%4
1464
16th to endDay
67%7%10%9%5%
1728
February1st to 15thDay
64%6%10%13%5%
1577
16th to endDay
59%6%10%16%5%1
1563
March1st to 15thDay
59%6%11%14%7%
1922
16th to endDay
54%5%11%18%8%1
2112
April1st to 15thDay
50%7%11%18%11%
2125
16th to endDay
46%5%13%18%13%2
2340
May1st to 15thDay
47%6%11%21%10%2
2331
16th to endDay
38%7%13%24%14%1
2474
June1st to 15thDay
38%8%13%24%12%1
2601
16th to endDay
37%8%16%20%15%1
2700
July1st to 15thDay
33%7%11%26%19%2
2392
16th to endDay
34%8%17%22%15%2
2315
August1st to 15thDay
34%8%16%26%12%
2275
16th to endDay
34%7%16%26%13%2
2275
September1st to 15thDay
39%6%16%25%11%
1851
16th to endDay
39%7%12%27%12%
1815
October1st to 15thDay
43%7%13%21%12%1
1700
16th to endDay
51%6%16%17%7%
1716
November1st to 15thDay
46%7%13%20%11%1
1582
16th to endDay
59%6%14%14%4
1476
December1st to 15thDay
64%8%13%9%3
1440
16th to endDay
69%7%10%9%3
1485
January1st to 15thNight
64%5%10%13%6%
2470
16th to endNight
64%6%10%13%4
2496
February1st to 15thNight
61%5%10%15%5%
2287
16th to endNight
57%5%11%17%6%
1910
March1st to 15thNight
55%411%18%8%1
2037
16th to endNight
46%5%11%22%11%1
2112
April1st to 15thNight
50%410%20%12%1
1831
16th to endNight
41%6%10%19%17%4
1620
May1st to 15thNight
49%6%11%17%12%3
1620
16th to endNight
38%7%12%24%16%2
1642
June1st to 15thNight
36%6%12%27%15%1
1356
16th to endNight
33%6%15%23%20%1
1260
July1st to 15thNight
30%5%9%29%22%2
1336
16th to endNight
28%6%16%25%19%3
1587
August1st to 15thNight
28%8%13%31%13%3
1575
16th to endNight
30%6%13%29%18%2
1575
September1st to 15thNight
34%6%13%24%18%2
1764
16th to endNight
36%5%11%30%15%1
1815
October1st to 15thNight
40%6%13%18%20%
1930
16th to endNight
52%7%13%19%6%
2154
November1st to 15thNight
45%5%12%21%12%1
2276
16th to endNight
57%5%13%17%6%
2481
December1st to 15thNight
64%6%11%12%4
2520
16th to endNight
68%7%9%10%3
2730

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
20%23%34%15%42
90
16th to endDay
32%8%38%17%3
96
February1st to 15thDay
23%18%40%16%1
90
16th to endDay
19%13%45%18%2
81
March1st to 15thDay
28%13%29%20%7%
212
16th to endDay
23%6%31%27%7%3
288
April1st to 15thDay
22%8%31%25%9%2
270
16th to endDay
22%8%30%18%12%7%
270
May1st to 15thDay
20%6%32%19%17%3
288
16th to endDay
15%6%27%26%14%8%
331
June1st to 15thDay
11%426%36%17%3
396
16th to endDay
13%431%29%16%4
495
July1st to 15thDay
10%324%30%21%8%
475
16th to endDay
11%218%25%28%13%
331
August1st to 15thDay
10%115%30%26%15%
297
16th to endDay
10%313%27%25%20%
268
September1st to 15thDay
15%5%12%22%30%13%
255
16th to endDay
17%215%25%25%12%
254
October1st to 15thDay
15%7%20%28%21%6%
255
16th to endDay
25%10%33%22%6%1
228
November1st to 15thDay
12%11%24%36%12%3
90
16th to endDay
18%27%28%21%4
66
December1st to 15thDay
21%26%31%15%5%
60
16th to endDay
21%32%35%7%2
70
January1st to 15thNight
28%13%33%18%4
864
16th to endNight
22%12%33%24%6%
768
February1st to 15thNight
24%12%33%24%4
704
16th to endNight
22%11%30%29%5%
631
March1st to 15thNight
22%10%32%26%7%
720
16th to endNight
20%6%34%26%10%
768
April1st to 15thNight
21%10%30%30%6%
647
16th to endNight
15%7%33%31%11%
540
May1st to 15thNight
18%424%35%15%1
540
16th to endNight
12%326%34%19%3
521
June1st to 15thNight
12%123%45%15%
360
16th to endNight
10%23%47%16%1
360
July1st to 15thNight
9%18%39%28%3
486
16th to endNight
7%115%42%28%3
564
August1st to 15thNight
6%11%42%30%7%
527
16th to endNight
10%114%30%37%5%
525
September1st to 15thNight
12%414%31%29%6%
647
16th to endNight
14%218%30%25%7%
657
October1st to 15thNight
17%6%20%30%21%3
660
16th to endNight
23%13%31%26%5%
704
November1st to 15thNight
18%9%30%26%13%1
700
16th to endNight
29%15%25%19%9%
720
December1st to 15thNight
28%14%33%17%5%
720
16th to endNight
30%12%34%19%3
849
Page generated at 2017-08-13 14:16:18. Version ac04

Copyright 2017 A. Danko


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