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 2024-02-25.

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
36%
23732
14%
80402
10%
23631
42%
23631
17%
56939

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
17%
943
10%
4397
4%
926
25%
926
8%
3307
January11th to 20thNight
15%
798
4%
4173
3%
778
20%
778
4%
3453
January21st to endNight
21%
829
6%
4503
5%
814
27%
814
6%
3705
February1st to 10thNight
19%
760
8%
2470
3%
760
25%
760
10%
1928
February11th to 20thNight
21%
741
12%
2337
4%
740
29%
740
13%
1721
February21st to endNight
19%
611
12%
1831
4%
611
28%
611
14%
1379
March1st to 10thNight
25%
720
16%
2066
8%
720
31%
720
18%
1477
March11th to 20thNight
27%
720
20%
1980
10%
720
36%
720
23%
1351
March21st to endNight
30%
792
17%
2178
10%
792
36%
792
20%
1575
April1st to 10thNight
29%
702
15%
1942
7%
702
36%
702
21%
1365
April11th to 20thNight
31%
533
19%
1597
10%
533
38%
533
23%
1079
April21st to endNight
36%
540
18%
1620
7%
540
44%
540
23%
1101
May1st to 10thNight
39%
540
12%
1620
8%
540
42%
540
19%
1155
May11th to 20thNight
43%
531
17%
1592
11%
531
49%
531
23%
1092
May21st to endNight
55%
520
16%
1683
14%
520
60%
520
30%
994
June1st to 10thNight
50%
360
16%
1404
17%
360
58%
360
25%
804
June11th to 20thNight
55%
360
20%
1260
21%
360
61%
360
28%
735
June21st to endNight
54%
360
21%
1260
21%
360
60%
360
30%
712
July1st to 10thNight
66%
488
17%
1363
17%
488
68%
488
38%
801
July11th to 20thNight
61%
522
18%
1468
15%
522
65%
522
38%
868
July21st to endNight
66%
591
18%
1672
14%
558
70%
558
36%
969
August1st to 10thNight
70%
537
20%
1600
17%
534
72%
534
37%
920
August11th to 20thNight
68%
534
23%
1598
18%
534
71%
534
34%
968
August21st to endNight
62%
594
24%
1782
18%
594
66%
594
34%
1055
September1st to 10thNight
58%
699
19%
1897
13%
699
61%
699
31%
1208
September11th to 20thNight
61%
720
27%
1980
20%
720
65%
720
38%
1179
September21st to endNight
49%
720
21%
1980
16%
720
53%
720
28%
1315
October1st to 10thNight
47%
720
19%
2078
16%
720
51%
720
28%
1329
October11th to 20thNight
33%
720
16%
2160
9%
720
42%
720
19%
1500
October21st to endNight
26%
792
15%
2444
9%
792
34%
792
15%
1796
November1st to 10thNight
36%
740
20%
2406
15%
740
43%
740
22%
1550
November11th to 20thNight
22%
760
14%
2505
7%
760
30%
760
14%
1778
November21st to endNight
22%
760
12%
2660
9%
760
28%
760
12%
1948
December1st to 10thNight
17%
760
7%
2660
4%
760
22%
760
8%
2120
December11th to 20thNight
17%
759
9%
3257
3%
758
26%
758
8%
2419
December21st to endNight
16%
956
4%
4979
2%
945
21%
945
4%
4283

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%9%47%6%35%5%6%8%32%
78556
Night
16%10%47%6%3445%7%33%
80707
TransparencyDay
48%8%13%19%11%1
78267
Night
50%7%12%19%11%1
80516
SeeingDay
17%10%29%25%14%5%
14137
Night
19%10%28%28%13%2
23759

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
16%16%7%13%8%45%5%5%5%17%
2527
11th to 20thDay
15%19%8%12%9%6%7%5%46%11%
2759
21st to endDay
26%18%9%10%6%3434413%
3124
February1st to 10thDay
15%16%5%10%7%46%447%23%
1710
11th to 20thDay
14%11%6%10%8%47%5%5%6%25%
1729
21st to endDay
17%12%5%9%8%45%5%5%6%25%
1520
March1st to 10thDay
17%10%48%6%36%5%6%6%29%
1893
11th to 20thDay
13%11%5%8%7%345%6%6%32%
1980
21st to endDay
20%11%5%9%6%345%5%6%27%
2178
April1st to 10thDay
14%10%47%6%35%5%7%8%32%
2030
11th to 20thDay
18%10%35%5%346%5%7%34%
2312
21st to endDay
19%8%37%6%4345%7%33%
2340
May1st to 10thDay
17%8%36%5%445%6%7%34%
2340
11th to 20thDay
16%7%36%6%35%46%9%35%
2301
21st to endDay
9%5%26%5%446%7%9%42%
2589
June1st to 10thDay
10%6%36%5%46%6%6%9%39%
2551
11th to 20thDay
9%6%36%5%35%6%6%9%42%
2700
21st to endDay
10%5%25%5%45%6%8%10%41%
2700
July1st to 10thDay
8%5%25%5%35%6%6%9%46%
2552
11th to 20thDay
9%4245%35%6%8%10%43%
2440
21st to endDay
7%5%25%7%37%6%8%9%42%
2547
August1st to 10thDay
7%425%6%46%6%8%10%43%
2327
11th to 20thDay
8%5%25%5%46%6%8%11%38%
2303
21st to endDay
9%5%25%6%36%5%6%9%44%
2574
September1st to 10thDay
10%6%36%5%45%6%5%8%42%
2040
11th to 20thDay
9%5%245%36%5%6%10%44%
1980
21st to endDay
17%5%26%5%35%6%5%7%39%
1980
October1st to 10thDay
12%6%36%6%35%6%6%7%41%
1881
11th to 20thDay
17%8%36%6%45%5%6%7%33%
1800
21st to endDay
20%9%46%5%45%5%6%7%30%
1908
November1st to 10thDay
12%8%47%6%45%6%5%8%36%
1672
11th to 20thDay
15%11%48%5%36%5%47%32%
1672
21st to endDay
18%12%5%9%8%35%5%5%6%23%
1520
December1st to 10thDay
16%14%6%10%8%46%6%46%19%
1520
11th to 20thDay
18%13%6%9%7%45%6%5%7%20%
1910
21st to endDay
26%19%7%10%6%34445%10%
2647
January1st to 10thNight
16%15%6%9%7%45%45%5%23%
4428
11th to 20thNight
15%18%8%12%8%45%5%5%7%14%
4184
21st to endNight
25%17%8%10%7%3333418%
4532
February1st to 10thNight
17%15%5%10%7%45%446%24%
2470
11th to 20thNight
16%13%6%8%7%45%45%7%26%
2335
21st to endNight
19%11%49%7%35%5%45%27%
1831
March1st to 10thNight
16%10%47%7%45%5%5%7%31%
2066
11th to 20thNight
14%8%36%7%246%5%8%38%
1980
21st to endNight
19%9%5%7%6%3445%6%33%
2178
April1st to 10thNight
15%9%5%6%5%35%5%5%7%35%
1942
11th to 20thNight
18%6%37%7%34449%36%
1597
21st to endNight
17%8%36%5%35%45%7%37%
1620
May1st to 10thNight
17%7%27%7%45%5%6%7%32%
1620
11th to 20thNight
15%6%46%6%45%5%5%7%38%
1592
21st to endNight
10%415%6%335%6%9%48%
1682
June1st to 10thNight
11%435%5%45%46%10%45%
1404
11th to 20thNight
9%5%246%35%5%6%10%45%
1260
21st to endNight
10%5%35%5%3446%9%48%
1260
July1st to 10thNight
7%5%25%4234410%53%
1403
11th to 20thNight
8%425%5%25%46%10%49%
1516
21st to endNight
6%324445%6%8%9%49%
1771
August1st to 10thNight
7%4245%345%6%8%52%
1611
11th to 20thNight
7%5%25%5%35%6%6%9%48%
1598
21st to endNight
8%425%5%345%6%8%49%
1782
September1st to 10thNight
10%6%25%6%345%6%8%45%
1897
11th to 20thNight
8%5%3443445%7%54%
1980
21st to endNight
13%7%36%6%3445%7%43%
1980
October1st to 10thNight
13%6%26%5%33447%46%
2078
11th to 20thNight
17%8%36%6%445%5%7%35%
2160
21st to endNight
22%8%47%6%45%45%6%30%
2444
November1st to 10thNight
13%9%36%5%345%5%6%41%
2406
11th to 20thNight
17%10%38%6%45%5%6%8%29%
2505
21st to endNight
21%13%5%8%5%2345%6%28%
2660
December1st to 10thNight
19%14%6%9%7%45%45%6%20%
2660
11th to 20thNight
16%15%5%8%7%35%5%5%6%25%
3285
21st to endNight
26%19%7%10%6%4444413%
4990

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
65%9%11%9%5%
2528
11th to 20thDay
74%8%9%6%2
2759
21st to endDay
74%6%9%7%4
3124
February1st to 10thDay
63%5%11%15%7%
1710
11th to 20thDay
59%6%11%15%7%1
1738
21st to endDay
57%7%12%16%7%2
1520
March1st to 10thDay
54%7%12%17%8%1
1893
11th to 20thDay
51%6%12%17%13%2
1980
21st to endDay
58%6%10%18%8%1
2178
April1st to 10thDay
48%7%13%19%12%1
2030
11th to 20thDay
48%6%12%18%13%2
2312
21st to endDay
49%6%13%19%10%2
2340
May1st to 10thDay
46%8%11%22%11%3
2340
11th to 20thDay
45%8%13%18%13%2
2305
21st to endDay
36%8%15%23%16%2
2589
June1st to 10thDay
38%8%13%26%13%2
2551
11th to 20thDay
36%9%14%23%16%2
2700
21st to endDay
35%8%15%23%17%2
2700
July1st to 10thDay
32%8%11%26%20%4
2483
11th to 20thDay
32%8%15%27%16%2
2365
21st to endDay
33%9%17%21%17%3
2405
August1st to 10thDay
32%9%17%30%13%1
2314
11th to 20thDay
35%9%17%27%10%2
2303
21st to endDay
35%8%14%24%16%2
2574
September1st to 10thDay
37%8%17%23%13%1
2040
11th to 20thDay
33%8%15%30%14%1
1980
21st to endDay
42%7%14%24%12%1
1980
October1st to 10thDay
41%7%13%24%14%1
1881
11th to 20thDay
48%9%14%19%10%1
1800
21st to endDay
52%8%14%17%8%1
1908
November1st to 10thDay
47%7%12%22%10%2
1672
11th to 20thDay
50%7%16%16%10%1
1672
21st to endDay
59%8%12%16%5%
1520
December1st to 10thDay
64%8%13%11%4
1520
11th to 20thDay
61%8%14%12%4
1906
21st to endDay
75%7%9%6%2
2647
January1st to 10thNight
59%8%13%14%6%
4428
11th to 20thNight
68%10%11%8%2
4184
21st to endNight
71%6%9%12%3
4532
February1st to 10thNight
62%6%11%15%5%1
2470
11th to 20thNight
57%5%11%17%9%
2337
21st to endNight
57%5%11%17%8%1
1831
March1st to 10thNight
52%5%12%21%9%1
2066
11th to 20thNight
44%6%12%21%14%3
1980
21st to endNight
52%5%9%21%12%1
2178
April1st to 10thNight
48%6%11%20%13%2
1942
11th to 20thNight
45%6%11%20%16%2
1597
21st to endNight
44%7%10%19%15%4
1620
May1st to 10thNight
50%7%11%17%11%4
1620
11th to 20thNight
45%7%9%18%17%3
1593
21st to endNight
33%8%13%26%18%2
1683
June1st to 10thNight
36%7%11%26%17%3
1404
11th to 20thNight
34%8%14%25%17%3
1260
21st to endNight
33%5%14%25%22%1
1260
July1st to 10thNight
29%7%10%27%23%4
1363
11th to 20thNight
30%7%14%30%17%3
1468
21st to endNight
27%9%16%23%20%4
1672
August1st to 10thNight
28%8%15%33%14%2
1600
11th to 20thNight
31%9%14%30%13%2
1598
21st to endNight
31%8%14%28%19%2
1782
September1st to 10thNight
35%8%13%26%15%3
1897
11th to 20thNight
30%6%13%30%20%1
1980
21st to endNight
40%7%12%26%14%1
1980
October1st to 10thNight
37%8%13%22%20%1
2078
11th to 20thNight
48%6%13%18%14%
2160
21st to endNight
53%7%11%18%9%1
2444
November1st to 10thNight
43%6%12%24%14%1
2406
11th to 20thNight
52%7%14%18%9%1
2505
21st to endNight
57%6%14%17%6%
2660
December1st to 10thNight
64%7%11%13%4
2660
11th to 20thNight
58%8%14%14%6%
3288
21st to endNight
75%6%9%7%3
4990

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%20%34%15%7%1
270
11th to 20thDay
25%15%42%11%5%1
270
21st to endDay
28%14%31%23%41
297
February1st to 10thDay
21%19%37%17%5%
270
11th to 20thDay
21%18%37%20%3
270
21st to endDay
23%22%39%14%2
210
March1st to 10thDay
22%17%35%20%5%
333
11th to 20thDay
20%13%34%26%6%2
420
21st to endDay
25%13%33%23%5%1
462
April1st to 10thDay
15%13%38%25%8%1
420
11th to 20thDay
25%14%27%18%11%4
412
21st to endDay
24%10%33%20%11%2
420
May1st to 10thDay
20%10%34%22%11%3
423
11th to 20thDay
17%9%31%24%15%4
447
21st to endDay
13%5%25%29%19%9%
485
June1st to 10thDay
12%431%35%16%4
481
11th to 20thDay
12%8%28%33%17%3
630
21st to endDay
12%6%32%31%15%4
630
July1st to 10thDay
10%321%31%24%10%
621
11th to 20thDay
12%425%29%22%9%
538
21st to endDay
9%224%30%27%9%
488
August1st to 10thDay
9%318%31%25%14%
446
11th to 20thDay
10%5%18%33%23%12%
439
21st to endDay
9%319%32%26%11%
495
September1st to 10thDay
14%422%30%20%11%
450
11th to 20thDay
11%319%27%28%11%
450
21st to endDay
16%9%24%26%16%9%
450
October1st to 10thDay
16%8%24%24%21%6%
450
11th to 20thDay
18%16%31%25%7%2
450
21st to endDay
24%16%35%18%6%1
423
November1st to 10thDay
13%18%32%26%9%2
270
11th to 20thDay
23%21%28%20%7%1
252
21st to endDay
23%17%36%20%31
180
December1st to 10thDay
23%19%33%16%9%1
180
11th to 20thDay
27%22%31%16%4
180
21st to endDay
28%20%32%16%4
225
January1st to 10thNight
25%15%35%18%6%1
950
11th to 20thNight
28%16%32%20%4
798
21st to endNight
23%12%32%26%7%
836
February1st to 10thNight
26%12%34%24%4
760
11th to 20thNight
23%14%31%25%6%
743
21st to endNight
24%14%37%23%2
611
March1st to 10thNight
20%12%36%26%7%
720
11th to 20thNight
18%11%39%24%7%1
720
21st to endNight
23%9%34%24%10%
792
April1st to 10thNight
20%12%32%30%6%1
702
11th to 20thNight
19%9%35%27%9%1
533
21st to endNight
20%7%32%32%9%
540
May1st to 10thNight
19%7%27%33%13%1
540
11th to 20thNight
17%427%35%16%2
531
21st to endNight
11%323%37%23%2
522
June1st to 10thNight
13%326%42%16%1
360
11th to 20thNight
12%323%46%15%1
360
21st to endNight
11%224%46%16%1
360
July1st to 10thNight
8%216%39%32%4
488
11th to 20thNight
11%318%37%27%4
522
21st to endNight
7%217%43%28%2
591
August1st to 10thNight
9%111%42%31%7%
537
11th to 20thNight
7%213%38%34%6%
534
21st to endNight
10%218%34%32%5%
594
September1st to 10thNight
13%416%32%29%5%
699
11th to 20thNight
10%5%17%32%30%6%
720
21st to endNight
17%5%22%30%20%6%
720
October1st to 10thNight
15%7%24%31%19%4
720
11th to 20thNight
20%15%27%26%11%1
720
21st to endNight
26%14%29%26%5%1
792
November1st to 10thNight
18%11%30%26%14%1
740
11th to 20thNight
23%16%32%20%8%1
760
21st to endNight
28%15%30%20%7%
760
December1st to 10thNight
27%17%33%17%6%1
760
11th to 20thNight
24%17%35%20%4
760
21st to endNight
31%13%31%23%2
964
Page generated at 2024-02-22 06:05:47. Version ac07

Copyright 2024 Attilla Danko


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