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 2017-12-20.

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
37%
15677
15%
47939
10%
15639
43%
15639
20%
33448

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
15%
860
10%
2470
4%
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
27%
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
34%
768
18%
2112
11%
768
40%
768
23%
1483
April1st to 15thNight
30%
647
15%
1831
7%
647
37%
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
55%
519
27%
1028
June1st to 15thNight
55%
360
16%
1356
17%
360
61%
360
26%
796
June16th to endNight
58%
360
19%
1260
19%
360
64%
360
30%
715
July1st to 15thNight
63%
486
17%
1336
15%
486
66%
486
39%
792
July16th to endNight
67%
564
17%
1587
14%
531
70%
531
37%
898
August1st to 15thNight
71%
539
19%
1609
17%
536
73%
536
37%
943
August16th to endNight
67%
573
25%
1719
19%
573
71%
573
36%
999
September1st to 15thNight
59%
706
22%
1925
15%
706
63%
706
33%
1203
September16th to endNight
58%
717
24%
1980
19%
717
61%
717
33%
1240
October1st to 15thNight
48%
720
18%
2106
16%
720
51%
720
26%
1380
October16th to endNight
27%
768
15%
2350
9%
768
36%
768
16%
1696
November1st to 15thNight
35%
760
18%
2471
15%
760
41%
760
20%
1652
November16th to endNight
23%
780
13%
2688
8%
780
29%
780
13%
1949
December1st to 15thNight
18%
780
7%
2730
3%
780
22%
780
9%
2148
December16th to endNight
15%
869
7%
2797
2%
869
20%
869
7%
2332

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%
48872
Night
15%8%46%5%3445%6%35%
48121
Cloud CoverDay
14%8%36%5%35%5%5%7%33%
48882
Night
15%8%36%5%3445%6%35%
48125
TransparencyDay
47%7%13%19%10%1
48572
Night
48%6%12%20%11%1
47939
SeeingDay
16%7%26%25%16%7%
5992
Night
19%8%26%28%14%2
15685

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%45%6%8%9%40%
2340
16th to endDay
9%424435%5%6%9%43%
2483
September1st to 15thDay
11%6%25%4346%5%8%41%
2019
16th to endDay
13%424435%6%49%42%
1980
October1st to 15thDay
13%6%36%5%35%5%5%6%38%
1854
16th to endDay
20%7%35%5%34446%33%
1872
November1st to 15thDay
15%8%36%5%245%46%36%
1717
16th to endDay
17%11%5%8%6%3445%7%24%
1599
December1st to 15thDay
18%14%5%9%7%35%446%20%
1560
16th to endDay
21%14%6%10%6%3446%5%14%
1517
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%50%
1620
16th to endNight
8%41442345%7%52%
1719
September1st to 15thNight
10%5%2443335%7%48%
1925
16th to endNight
10%5%245%3445%7%47%
1980
October1st to 15thNight
15%6%25%423346%43%
2106
16th to endNight
20%6%36%5%345%5%7%31%
2350
November1st to 15thNight
14%8%46%434446%37%
2471
16th to endNight
22%11%46%5%2345%7%27%
2688
December1st to 15thNight
20%13%5%8%6%34445%21%
2730
16th to endNight
22%14%6%8%6%45%45%5%17%
2797

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%
2340
16th to endDay
9%5%24435%46%9%43%
2483
September1st to 15thDay
11%6%25%5%345%5%8%40%
2019
16th to endDay
14%5%14425%5%48%42%
1980
October1st to 15thDay
13%6%36%5%345%5%6%38%
1854
16th to endDay
20%7%35%5%3445%5%33%
1872
November1st to 15thDay
15%7%36%5%246%45%36%
1717
16th to endDay
17%11%5%9%6%25%448%24%
1599
December1st to 15thDay
18%13%5%8%7%35%5%46%20%
1560
16th to endDay
22%14%6%10%7%25%5%5%6%14%
1517
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%
1620
16th to endNight
9%414423448%52%
1719
September1st to 15thNight
10%5%2443445%7%48%
1925
16th to endNight
11%5%245%24446%47%
1980
October1st to 15thNight
15%6%25%433346%43%
2106
16th to endNight
20%6%26%5%3445%6%32%
2350
November1st to 15thNight
14%8%37%42445%6%38%
2471
16th to endNight
22%10%47%42345%6%27%
2688
December1st to 15thNight
20%13%5%8%6%35%346%21%
2730
16th to endNight
22%14%6%9%6%4445%5%17%
2797

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%27%12%
2327
16th to endDay
34%7%15%26%13%2
2483
September1st to 15thDay
38%6%16%26%11%
2019
16th to endDay
38%7%12%27%13%
1980
October1st to 15thDay
44%7%13%20%13%1
1854
16th to endDay
50%7%15%18%7%
1872
November1st to 15thDay
47%7%13%19%10%1
1717
16th to endDay
58%6%14%14%5%
1599
December1st to 15thDay
63%7%12%10%5%
1560
16th to endDay
69%7%10%9%3
1517
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%32%13%2
1609
16th to endNight
30%6%12%29%18%2
1719
September1st to 15thNight
34%6%13%25%17%2
1925
16th to endNight
35%5%10%30%16%1
1980
October1st to 15thNight
41%6%12%19%19%
2106
16th to endNight
50%7%13%19%8%
2350
November1st to 15thNight
46%5%12%21%11%1
2471
16th to endNight
55%5%13%17%6%
2688
December1st to 15thNight
63%6%12%12%5%
2730
16th to endNight
68%7%9%10%3
2797

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
9%116%31%26%15%
313
16th to endDay
9%315%28%23%18%
332
September1st to 15thDay
13%417%23%29%12%
315
16th to endDay
16%215%21%26%17%
314
October1st to 15thDay
19%7%18%27%20%6%
315
16th to endDay
23%11%33%22%7%1
288
November1st to 15thDay
17%11%26%29%12%2
135
16th to endDay
22%22%32%17%6%
99
December1st to 15thDay
18%25%32%14%8%
90
16th to endDay
25%33%32%6%2
78
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%43%30%7%
539
16th to endNight
9%114%32%37%4
573
September1st to 15thNight
13%415%30%30%5%
706
16th to endNight
13%217%29%27%8%
717
October1st to 15thNight
18%6%20%29%21%3
720
16th to endNight
22%13%30%26%5%
768
November1st to 15thNight
19%9%28%27%12%1
760
16th to endNight
28%16%26%19%8%
780
December1st to 15thNight
28%15%32%17%5%
780
16th to endNight
29%12%35%18%3
869
Page generated at 2017-12-21 07:04:45. Version ac04

Copyright 2017 A. Danko


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