Chatham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chatham clear sky chart from 2005-11-06 to 2024-01-13.

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
38%
24467
20%
74664
12%
24406
46%
24406
24%
48665

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 10thNight
20%
949
16%
2648
6%
946
30%
946
19%
1878
January11th to 20thNight
21%
911
15%
2522
5%
911
28%
911
17%
1872
January21st to endNight
21%
990
15%
2574
5%
990
28%
990
18%
1898
February1st to 10thNight
21%
897
13%
2337
4%
897
28%
897
17%
1759
February11th to 20thNight
23%
702
18%
2099
7%
700
32%
700
20%
1426
February21st to endNight
21%
592
18%
1776
6%
592
30%
592
18%
1211
March1st to 10thNight
24%
720
18%
2160
7%
720
33%
720
19%
1458
March11th to 20thNight
31%
590
17%
2030
9%
590
40%
590
19%
1327
March21st to endNight
26%
594
13%
2028
7%
594
34%
594
15%
1462
April1st to 10thNight
29%
540
20%
1800
12%
540
42%
540
23%
1108
April11th to 20thNight
32%
533
22%
1775
14%
533
45%
533
27%
1075
April21st to endNight
28%
540
18%
1800
11%
540
39%
540
20%
1198
May1st to 10thNight
33%
540
16%
1666
9%
540
42%
540
24%
1090
May11th to 20thNight
45%
531
19%
1453
13%
531
50%
531
27%
988
May21st to endNight
50%
584
17%
1551
11%
582
54%
582
29%
1053
June1st to 10thNight
63%
540
19%
1440
14%
540
67%
540
40%
859
June11th to 20thNight
58%
540
23%
1440
17%
540
62%
540
38%
861
June21st to endNight
56%
540
21%
1440
13%
540
61%
540
38%
879
July1st to 10thNight
69%
525
18%
1400
15%
525
70%
525
36%
865
July11th to 20thNight
68%
522
23%
1392
18%
522
71%
522
45%
787
July21st to endNight
69%
591
26%
1502
21%
558
72%
558
41%
887
August1st to 10thNight
72%
537
22%
1620
22%
534
75%
534
36%
911
August11th to 20thNight
70%
534
23%
1776
27%
534
72%
534
33%
1006
August21st to endNight
73%
594
23%
1980
25%
594
76%
594
39%
1053
September1st to 10thNight
62%
540
26%
1800
26%
540
66%
540
34%
995
September11th to 20thNight
64%
576
34%
1836
33%
576
68%
576
38%
994
September21st to endNight
55%
720
29%
1980
19%
720
62%
720
38%
1160
October1st to 10thNight
51%
720
26%
2112
18%
720
57%
720
35%
1231
October11th to 20thNight
36%
720
28%
2160
15%
720
48%
720
30%
1284
October21st to endNight
29%
792
21%
2461
12%
792
39%
792
22%
1598
November1st to 10thNight
33%
740
27%
2406
17%
740
47%
740
30%
1352
November11th to 20thNight
29%
760
26%
2470
13%
760
45%
760
27%
1448
November21st to endNight
27%
821
21%
2531
11%
821
39%
821
22%
1660
December1st to 10thNight
23%
950
14%
2660
7%
950
30%
950
20%
1898
December11th to 20thNight
28%
950
18%
3169
8%
948
36%
948
19%
2108
December21st to endNight
18%
1042
8%
4870
5%
1026
26%
1026
7%
4026

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
13%7%36%5%345%5%8%40%
74767
Night
14%8%36%5%3445%7%42%
74972
TransparencyDay
42%7%13%23%14%2
74470
Night
42%6%11%23%16%3
74791
SeeingDay
15%10%27%26%16%5%
14518
Night
17%9%30%30%13%1
24472

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 10thDay
14%12%5%8%8%5%5%446%28%
1517
11th to 20thDay
16%12%6%10%8%445%45%25%
1478
21st to endDay
12%13%7%10%6%34345%31%
1782
February1st to 10thDay
16%11%6%10%5%345%5%6%29%
1620
11th to 20thDay
14%10%46%8%45%5%5%7%31%
1748
21st to endDay
17%12%5%8%6%34445%31%
1480
March1st to 10thDay
16%9%5%6%6%44446%36%
1800
11th to 20thDay
15%11%47%6%25%45%8%34%
1944
21st to endDay
20%12%47%7%35%446%27%
2326
April1st to 10thDay
13%7%46%6%35%5%6%8%38%
2160
11th to 20thDay
16%9%36%5%3445%6%39%
2134
21st to endDay
20%6%25%5%45%5%5%6%36%
2160
May1st to 10thDay
15%7%35%5%46%5%6%9%34%
2290
11th to 20thDay
17%7%36%5%35%45%7%38%
2438
21st to endDay
11%6%35%7%45%6%7%9%37%
2725
June1st to 10thDay
7%435%4446%6%9%47%
2520
11th to 20thDay
8%5%25%5%35%5%5%8%49%
2520
21st to endDay
6%5%36%6%47%6%6%9%43%
2520
July1st to 10thDay
8%4245%46%6%7%11%45%
2520
11th to 20thDay
6%425%435%6%8%11%47%
2520
21st to endDay
9%4145%35%5%7%10%47%
2725
August1st to 10thDay
7%5%25%6%446%7%11%43%
2306
11th to 20thDay
7%436%5%45%6%6%10%46%
2125
21st to endDay
7%3245%3447%10%49%
2376
September1st to 10thDay
13%5%2443446%6%48%
2160
11th to 20thDay
9%5%35%43435%7%51%
2106
21st to endDay
7%435%5%245%5%8%52%
1980
October1st to 10thDay
12%42443345%7%52%
1845
11th to 20thDay
16%7%345%3445%6%45%
1800
21st to endDay
18%8%35%5%24445%41%
1895
November1st to 10thDay
11%5%26%5%245%46%51%
1672
11th to 20thDay
15%9%37%42245%7%43%
1710
21st to endDay
17%9%5%6%5%34435%39%
1644
December1st to 10thDay
13%11%5%7%7%35%5%5%6%33%
1520
11th to 20thDay
14%10%48%6%35%6%5%8%31%
1918
21st to endDay
21%15%7%13%5%24445%19%
2783
January1st to 10thNight
12%10%5%8%7%35%445%36%
2648
11th to 20thNight
16%10%49%7%44345%32%
2522
21st to endNight
14%11%6%9%5%35%45%6%31%
2574
February1st to 10thNight
18%10%6%7%6%35%446%31%
2337
11th to 20thNight
14%10%47%6%35%35%6%38%
2098
21st to endNight
18%9%45%6%34445%38%
1776
March1st to 10thNight
16%10%36%6%34445%40%
2160
11th to 20thNight
17%8%46%5%3445%7%39%
2030
21st to endNight
22%10%46%5%3445%6%31%
2028
April1st to 10thNight
13%7%35%5%3445%7%44%
1800
11th to 20thNight
15%7%25%434448%45%
1775
21st to endNight
20%7%36%5%24446%39%
1800
May1st to 10thNight
15%6%36%5%35%46%8%40%
1666
11th to 20thNight
16%6%35%435%5%5%8%42%
1452
21st to endNight
10%6%35%6%35%6%6%10%39%
1551
June1st to 10thNight
7%6%243235%5%10%52%
1440
11th to 20thNight
7%425%5%35%45%11%50%
1440
21st to endNight
6%5%25%5%245%8%10%48%
1440
July1st to 10thNight
9%424435%5%6%9%49%
1440
11th to 20thNight
5%3234346%6%9%56%
1440
21st to endNight
5%4244345%7%10%52%
1590
August1st to 10thNight
7%423335%6%7%9%52%
1630
11th to 20thNight
7%4245%35%5%5%8%52%
1776
21st to endNight
6%3134345%7%8%56%
1980
September1st to 10thNight
11%5%25%423448%53%
1800
11th to 20thNight
7%5%3443446%8%54%
1836
21st to endNight
8%5%2442445%7%54%
1980
October1st to 10thNight
10%4245%34446%55%
2112
11th to 20thNight
16%6%24332447%50%
2160
21st to endNight
18%8%35%424345%43%
2461
November1st to 10thNight
10%6%25%424446%54%
2406
11th to 20thNight
13%7%35%423346%49%
2470
21st to endNight
17%8%35%424445%42%
2531
December1st to 10thNight
17%10%47%6%35%45%7%33%
2660
11th to 20thNight
16%8%48%5%24345%40%
3279
21st to endNight
22%18%5%10%6%3444419%
4884

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 10thDay
56%5%10%17%12%1
1518
11th to 20thDay
61%6%10%17%6%
1478
21st to endDay
56%5%12%19%7%1
1782
February1st to 10thDay
56%5%13%18%7%1
1620
11th to 20thDay
52%6%11%20%10%1
1758
21st to endDay
55%5%10%19%9%2
1480
March1st to 10thDay
49%6%12%22%11%1
1800
11th to 20thDay
50%6%13%20%9%1
1944
21st to endDay
57%6%12%18%7%1
2326
April1st to 10thDay
43%7%13%23%13%1
2160
11th to 20thDay
45%6%11%23%13%2
2134
21st to endDay
47%8%12%20%12%2
2160
May1st to 10thDay
45%9%16%21%8%1
2290
11th to 20thDay
44%8%13%21%12%1
2442
21st to endDay
41%10%14%19%14%2
2725
June1st to 10thDay
31%8%16%26%17%3
2520
11th to 20thDay
32%8%12%23%22%3
2520
21st to endDay
34%11%14%21%16%3
2520
July1st to 10thDay
32%10%14%27%14%4
2451
11th to 20thDay
29%10%16%27%15%3
2445
21st to endDay
30%10%14%29%16%2
2574
August1st to 10thDay
32%10%17%28%12%2
2292
11th to 20thDay
32%9%16%27%15%1
2125
21st to endDay
28%9%14%25%19%6%
2376
September1st to 10thDay
35%6%11%26%17%5%
2160
11th to 20thDay
33%5%9%26%22%4
2106
21st to endDay
30%6%10%29%22%3
1980
October1st to 10thDay
32%6%12%25%21%3
1845
11th to 20thDay
40%5%10%23%21%3
1800
21st to endDay
44%5%10%24%15%2
1895
November1st to 10thDay
34%5%13%25%19%4
1672
11th to 20thDay
42%311%25%15%3
1710
21st to endDay
48%6%10%24%11%
1644
December1st to 10thDay
51%7%13%18%10%
1520
11th to 20thDay
49%6%12%25%6%2
1915
21st to endDay
65%7%10%12%6%
2783
January1st to 10thNight
50%5%10%21%14%1
2648
11th to 20thNight
54%5%11%21%8%
2522
21st to endNight
52%5%12%19%10%1
2574
February1st to 10thNight
56%411%17%10%2
2337
11th to 20thNight
48%411%21%14%2
2099
21st to endNight
48%5%12%21%11%3
1776
March1st to 10thNight
47%5%9%23%14%1
2160
11th to 20thNight
46%5%13%20%13%2
2030
21st to endNight
53%7%11%19%8%1
2028
April1st to 10thNight
40%5%12%25%15%3
1800
11th to 20thNight
38%5%12%25%17%3
1775
21st to endNight
47%5%10%20%15%3
1800
May1st to 10thNight
41%7%13%23%13%1
1666
11th to 20thNight
41%8%11%23%16%1
1453
21st to endNight
39%8%15%17%16%4
1552
June1st to 10thNight
28%7%14%28%19%3
1440
11th to 20thNight
29%7%11%25%23%5%
1440
21st to endNight
29%8%15%24%18%5%
1440
July1st to 10thNight
31%8%13%28%16%4
1400
11th to 20thNight
24%7%15%31%19%3
1392
21st to endNight
26%8%15%28%19%3
1502
August1st to 10thNight
25%10%16%30%17%2
1620
11th to 20thNight
29%8%11%29%20%2
1776
21st to endNight
23%9%16%25%20%7%
1980
September1st to 10thNight
31%5%11%26%20%7%
1800
11th to 20thNight
27%6%10%26%27%5%
1836
21st to endNight
28%7%10%24%26%6%
1980
October1st to 10thNight
31%6%9%24%25%5%
2112
11th to 20thNight
35%49%27%21%4
2160
21st to endNight
44%5%9%21%19%3
2461
November1st to 10thNight
31%6%11%25%20%6%
2406
11th to 20thNight
37%410%28%19%2
2470
21st to endNight
44%5%10%24%16%1
2531
December1st to 10thNight
50%5%14%20%10%1
2660
11th to 20thNight
46%5%12%25%12%1
3281
21st to endNight
68%6%8%11%6%1
4884

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 10thDay
23%19%36%16%6%
270
11th to 20thDay
28%17%36%17%1
246
21st to endDay
18%21%39%17%5%
264
February1st to 10thDay
28%20%33%16%3
240
11th to 20thDay
27%22%30%18%3
240
21st to endDay
30%15%42%12%1
198
March1st to 10thDay
23%18%41%17%3
240
11th to 20thDay
30%17%33%16%4
240
21st to endDay
32%16%28%20%5%
412
April1st to 10thDay
18%15%34%26%6%
420
11th to 20thDay
19%17%30%22%10%2
412
21st to endDay
24%13%35%18%8%1
420
May1st to 10thDay
20%13%32%25%9%2
423
11th to 20thDay
18%10%28%28%14%3
584
21st to endDay
12%7%30%29%15%8%
679
June1st to 10thDay
8%5%23%31%25%7%
630
11th to 20thDay
9%5%25%33%23%6%
630
21st to endDay
8%7%27%32%21%5%
630
July1st to 10thDay
9%315%29%29%16%
621
11th to 20thDay
7%220%34%25%11%
618
21st to endDay
10%218%32%29%10%
684
August1st to 10thDay
9%116%35%28%9%
604
11th to 20thDay
8%5%17%28%28%15%
439
21st to endDay
7%217%30%34%11%
495
September1st to 10thDay
13%5%18%29%25%10%
450
11th to 20thDay
10%312%31%30%13%
450
21st to endDay
8%9%24%24%25%9%
450
October1st to 10thDay
12%10%25%31%19%3
315
11th to 20thDay
20%20%30%22%7%1
270
21st to endDay
32%13%33%17%6%
297
November1st to 10thDay
9%16%37%21%13%4
270
11th to 20thDay
21%19%32%19%8%2
270
21st to endDay
23%13%37%21%6%
270
December1st to 10thDay
14%23%37%17%6%3
270
11th to 20thDay
19%17%39%22%3
270
21st to endDay
27%21%36%13%3
297
January1st to 10thNight
18%16%38%22%5%
950
11th to 20thNight
21%14%37%22%5%
911
21st to endNight
18%17%35%26%3
990
February1st to 10thNight
24%13%35%25%3
897
11th to 20thNight
21%15%36%26%2
703
21st to endNight
25%14%35%24%2
592
March1st to 10thNight
22%13%36%24%4
720
11th to 20thNight
23%10%33%27%7%
590
21st to endNight
28%9%32%24%7%
594
April1st to 10thNight
18%15%34%27%6%
540
11th to 20thNight
17%11%36%26%10%1
533
21st to endNight
24%8%34%26%7%1
540
May1st to 10thNight
17%6%37%30%9%1
540
11th to 20thNight
18%6%26%36%13%2
531
21st to endNight
11%327%35%20%2
584
June1st to 10thNight
9%320%47%21%
540
11th to 20thNight
9%223%44%20%1
540
21st to endNight
7%425%41%22%1
540
July1st to 10thNight
11%38%34%38%6%
525
11th to 20thNight
5%217%42%31%3
522
21st to endNight
6%115%45%30%3
591
August1st to 10thNight
9%213%40%33%3
537
11th to 20thNight
8%112%39%34%5%
534
21st to endNight
6%115%37%38%4
594
September1st to 10thNight
11%417%37%28%4
540
11th to 20thNight
8%317%36%32%5%
576
21st to endNight
9%7%22%31%27%4
720
October1st to 10thNight
12%428%31%22%3
720
11th to 20thNight
18%12%31%29%11%
720
21st to endNight
23%12%34%25%7%
792
November1st to 10thNight
12%10%40%28%9%
740
11th to 20thNight
17%15%36%25%7%1
760
21st to endNight
22%13%34%25%6%
821
December1st to 10thNight
22%12%37%22%6%1
950
11th to 20thNight
23%9%33%27%7%
950
21st to endNight
27%16%32%22%3
1045
Page generated at 2024-01-10 00:24:59. Version ac07

Copyright 2024 Attilla Danko


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