Norwich
Clear Sky Chart
History

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

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
28%
22897
13%
74715
8%
22855
34%
22855
14%
54495

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
11%
760
9%
2649
4%
757
18%
757
6%
2110
January11th to 20thNight
16%
724
7%
2532
4%
724
21%
724
7%
2070
January21st to endNight
14%
792
10%
2613
4%
792
21%
792
8%
2087
February1st to 10thNight
12%
720
8%
2340
3%
720
19%
720
7%
1912
February11th to 20thNight
11%
703
9%
2204
3%
700
18%
700
7%
1795
February21st to endNight
13%
592
13%
1776
5%
592
22%
592
11%
1386
March1st to 10thNight
17%
720
11%
2157
5%
720
25%
720
12%
1663
March11th to 20thNight
19%
720
14%
1980
7%
720
25%
720
14%
1525
March21st to endNight
23%
792
16%
2057
6%
792
30%
792
15%
1546
April1st to 10thNight
20%
720
12%
1800
3%
720
26%
720
16%
1397
April11th to 20thNight
25%
711
15%
1775
6%
711
28%
711
20%
1302
April21st to endNight
26%
720
14%
1800
5%
720
31%
720
20%
1342
May1st to 10thNight
31%
419
15%
1499
12%
419
35%
419
15%
1060
May11th to 20thNight
33%
354
18%
1416
13%
354
43%
354
18%
885
May21st to endNight
42%
388
15%
1551
15%
388
50%
388
20%
964
June1st to 10thNight
42%
360
15%
1440
18%
360
51%
360
19%
889
June11th to 20thNight
45%
360
14%
1440
17%
360
52%
360
20%
898
June21st to endNight
48%
360
15%
1440
16%
360
57%
360
23%
852
July1st to 10thNight
56%
350
13%
1400
18%
350
60%
350
26%
790
July11th to 20thNight
56%
348
17%
1392
21%
348
60%
348
24%
819
July21st to endNight
52%
394
17%
1486
18%
372
56%
372
22%
897
August1st to 10thNight
53%
485
16%
1549
12%
483
56%
483
28%
951
August11th to 20thNight
54%
613
14%
1677
10%
613
56%
613
26%
1152
August21st to endNight
53%
792
19%
1980
10%
792
55%
792
32%
1308
September1st to 10thNight
46%
720
16%
1800
7%
720
48%
720
30%
1221
September11th to 20thNight
48%
720
25%
1813
15%
720
53%
720
33%
1174
September21st to endNight
42%
720
21%
2006
16%
720
47%
720
26%
1353
October1st to 10thNight
35%
720
15%
2160
11%
720
41%
720
19%
1538
October11th to 20thNight
29%
720
15%
2160
8%
720
38%
720
18%
1539
October21st to endNight
21%
792
12%
2477
7%
792
30%
792
12%
1911
November1st to 10thNight
27%
740
18%
2407
11%
740
37%
740
17%
1653
November11th to 20thNight
20%
760
16%
2526
10%
760
29%
760
12%
1854
November21st to endNight
20%
760
10%
2660
6%
760
27%
760
11%
1969
December1st to 10thNight
14%
760
7%
2660
4%
760
18%
760
7%
2146
December11th to 20thNight
15%
760
7%
3257
3%
759
20%
759
6%
2591
December21st to endNight
12%
828
4%
4836
2%
817
16%
817
3%
3946

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
17%10%48%6%45%5%5%7%29%
74726
Night
17%11%5%8%6%35%45%6%30%
74965
TransparencyDay
55%5%11%18%9%1
74431
Night
55%5%11%18%9%1
74794
SeeingDay
27%15%28%19%9%3
13608
Night
24%15%27%25%9%1
22906

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 10thDay
13%16%7%12%7%45%5%5%421%
1517
11th to 20thDay
16%17%7%14%8%46%44416%
1440
21st to endDay
11%15%8%12%11%5%5%45%6%20%
1742
February1st to 10thDay
17%15%6%13%11%45%5%45%15%
1620
11th to 20thDay
13%13%8%14%10%45%6%35%19%
1643
21st to endDay
21%14%411%11%34435%20%
1480
March1st to 10thDay
14%12%8%12%7%45%445%25%
1797
11th to 20thDay
16%17%6%9%5%25%345%28%
1980
21st to endDay
21%12%5%7%6%34445%30%
2298
April1st to 10thDay
18%13%5%8%5%35%345%31%
2160
11th to 20thDay
18%10%47%5%45%446%35%
2134
21st to endDay
21%13%36%5%44446%31%
2160
May1st to 10thDay
21%11%47%5%34446%31%
2474
11th to 20thDay
19%7%35%6%345%6%6%35%
2478
21st to endDay
15%7%36%6%345%5%8%37%
2725
June1st to 10thDay
19%7%36%6%45%5%5%9%32%
2520
11th to 20thDay
18%6%36%5%46%5%5%9%33%
2520
21st to endDay
17%6%36%6%46%6%6%9%32%
2520
July1st to 10thDay
13%7%36%6%46%6%7%8%34%
2520
11th to 20thDay
13%5%35%6%36%6%6%10%37%
2520
21st to endDay
14%6%37%6%46%5%6%9%34%
2744
August1st to 10thDay
13%5%27%7%45%6%7%10%34%
2377
11th to 20thDay
15%8%37%7%5%6%6%6%9%30%
2206
21st to endDay
17%6%36%6%46%46%9%34%
2376
September1st to 10thDay
18%7%25%6%345%5%8%37%
2160
11th to 20thDay
16%7%26%43445%7%43%
2147
21st to endDay
19%5%36%6%36%5%6%7%35%
1949
October1st to 10thDay
22%9%38%6%3445%6%31%
1800
11th to 20thDay
20%9%36%6%35%5%5%6%31%
1800
21st to endDay
25%15%46%5%3446%6%24%
1877
November1st to 10thDay
14%12%46%5%445%5%7%35%
1671
11th to 20thDay
18%16%49%6%35%445%26%
1653
21st to endDay
18%16%6%8%7%45%446%22%
1520
December1st to 10thDay
20%17%8%11%7%35%445%16%
1520
11th to 20thDay
21%15%9%11%6%3434322%
1906
21st to endDay
23%20%9%9%6%3435%414%
2772
January1st to 10thNight
15%14%6%12%7%46%6%5%6%18%
2649
11th to 20thNight
16%15%7%13%9%45%35%5%17%
2532
21st to endNight
11%14%7%11%9%6%6%5%6%6%18%
2613
February1st to 10thNight
14%14%7%14%8%46%445%19%
2340
11th to 20thNight
13%12%8%13%10%47%6%5%5%18%
2202
21st to endNight
18%12%5%12%8%3445%6%23%
1776
March1st to 10thNight
14%11%7%12%7%445%5%6%26%
2147
11th to 20thNight
17%13%5%9%7%35%445%28%
1979
21st to endNight
17%12%5%7%6%445%46%31%
2057
April1st to 10thNight
20%13%5%6%6%34346%29%
1800
11th to 20thNight
18%11%36%5%3445%6%35%
1775
21st to endNight
20%10%46%5%35%45%6%33%
1800
May1st to 10thNight
23%10%36%5%3435%6%31%
1499
11th to 20thNight
15%7%36%5%35%47%9%37%
1415
21st to endNight
13%6%36%5%46%5%6%8%39%
1550
June1st to 10thNight
17%7%35%5%35%45%7%41%
1440
11th to 20thNight
17%6%345%345%6%9%39%
1440
21st to endNight
13%5%245%445%6%9%44%
1440
July1st to 10thNight
9%5%35%5%246%7%11%43%
1440
11th to 20thNight
12%5%245%35%6%5%8%45%
1440
21st to endNight
13%5%25%5%45%5%6%7%44%
1574
August1st to 10thNight
12%5%25%5%25%5%6%9%44%
1559
11th to 20thNight
13%6%36%6%45%5%6%7%38%
1677
21st to endNight
16%5%3442445%8%44%
1980
September1st to 10thNight
17%5%35%43445%7%44%
1800
11th to 20thNight
16%5%24423447%48%
1813
21st to endNight
17%7%25%435%5%47%40%
2006
October1st to 10thNight
21%7%26%6%3445%5%35%
2160
11th to 20thNight
20%8%37%5%345%5%6%34%
2160
21st to endNight
27%12%36%5%34447%25%
2477
November1st to 10thNight
13%13%47%5%345%47%36%
2407
11th to 20thNight
20%13%47%5%3445%6%29%
2526
21st to endNight
15%14%6%8%6%45%47%7%25%
2660
December1st to 10thNight
18%16%9%11%7%345%45%18%
2660
11th to 20thNight
21%13%7%12%7%45%34421%
3288
21st to endNight
26%15%9%9%6%34445%16%
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
65%410%14%6%
1518
11th to 20thDay
71%5%8%8%6%2
1440
21st to endDay
66%410%12%7%1
1742
February1st to 10thDay
73%5%7%10%4
1620
11th to 20thDay
69%5%6%14%6%
1652
21st to endDay
67%5%8%13%7%
1480
March1st to 10thDay
62%6%10%14%6%1
1800
11th to 20thDay
60%49%15%11%2
1980
21st to endDay
57%47%15%15%2
2298
April1st to 10thDay
57%46%21%9%3
2160
11th to 20thDay
52%5%10%20%13%2
2134
21st to endDay
56%48%19%11%3
2160
May1st to 10thDay
55%5%8%20%11%2
2474
11th to 20thDay
48%6%9%24%11%2
2482
21st to endDay
45%7%15%22%10%2
2725
June1st to 10thDay
50%7%11%20%12%1
2520
11th to 20thDay
49%5%13%22%10%1
2520
21st to endDay
48%7%13%21%10%1
2520
July1st to 10thDay
46%6%13%23%12%1
2450
11th to 20thDay
42%7%16%26%9%
2445
21st to endDay
46%7%13%22%12%1
2591
August1st to 10thDay
44%7%15%22%11%1
2363
11th to 20thDay
50%7%13%22%7%
2206
21st to endDay
47%6%11%24%10%1
2376
September1st to 10thDay
45%6%12%25%11%1
2160
11th to 20thDay
43%411%23%18%1
2147
21st to endDay
47%6%11%22%12%1
1949
October1st to 10thDay
54%5%10%19%11%1
1800
11th to 20thDay
52%6%11%22%8%1
1800
21st to endDay
63%411%13%8%1
1877
November1st to 10thDay
49%5%13%19%12%1
1671
11th to 20thDay
62%410%15%8%2
1653
21st to endDay
64%313%14%5%
1520
December1st to 10thDay
72%410%11%4
1520
11th to 20thDay
70%312%11%4
1906
21st to endDay
75%47%10%4
2772
January1st to 10thNight
67%5%9%11%7%1
2649
11th to 20thNight
70%5%9%11%5%1
2532
21st to endNight
66%5%9%12%6%1
2613
February1st to 10thNight
68%5%9%11%6%
2340
11th to 20thNight
67%6%8%13%6%1
2204
21st to endNight
62%5%10%14%8%1
1776
March1st to 10thNight
61%5%11%16%7%1
2157
11th to 20thNight
59%5%9%18%8%1
1980
21st to endNight
55%48%17%13%2
2057
April1st to 10thNight
58%5%10%17%9%1
1800
11th to 20thNight
50%5%10%19%15%1
1775
21st to endNight
53%49%19%12%3
1800
May1st to 10thNight
54%5%9%16%13%2
1499
11th to 20thNight
43%7%12%23%13%2
1416
21st to endNight
43%6%15%23%11%2
1551
June1st to 10thNight
43%6%12%24%14%1
1440
11th to 20thNight
42%7%15%24%12%1
1440
21st to endNight
37%8%14%28%13%1
1440
July1st to 10thNight
33%8%17%25%15%2
1400
11th to 20thNight
37%7%15%29%11%1
1392
21st to endNight
40%6%14%24%14%1
1486
August1st to 10thNight
36%8%17%28%11%1
1549
11th to 20thNight
43%7%16%25%9%
1677
21st to endNight
38%6%15%26%14%1
1980
September1st to 10thNight
41%6%15%26%12%1
1800
11th to 20thNight
37%5%11%27%19%1
1813
21st to endNight
44%5%12%24%14%2
2006
October1st to 10thNight
50%6%11%20%12%1
2160
11th to 20thNight
52%5%13%20%9%1
2160
21st to endNight
62%5%10%14%9%1
2477
November1st to 10thNight
50%5%10%19%14%2
2407
11th to 20thNight
56%6%10%18%8%2
2526
21st to endNight
58%5%15%16%5%
2660
December1st to 10thNight
69%411%13%3
2660
11th to 20thNight
67%6%10%13%4
3288
21st to endNight
71%6%9%11%3
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
33%19%29%14%41
180
11th to 20thDay
43%24%21%9%21
160
21st to endDay
29%27%30%11%3
246
February1st to 10thDay
38%31%28%4
240
11th to 20thDay
37%27%28%7%
310
21st to endDay
40%18%27%12%3
346
March1st to 10thDay
32%22%30%14%3
420
11th to 20thDay
35%21%25%15%4
420
21st to endDay
39%20%23%12%43
462
April1st to 10thDay
37%17%26%15%5%1
420
11th to 20thDay
32%19%26%12%8%4
412
21st to endDay
35%16%27%14%6%2
420
May1st to 10thDay
33%15%28%13%8%4
423
11th to 20thDay
23%13%34%19%7%4
447
21st to endDay
19%10%32%25%9%4
485
June1st to 10thDay
26%15%25%23%9%2
450
11th to 20thDay
20%12%30%25%11%2
450
21st to endDay
21%11%34%24%8%2
450
July1st to 10thDay
15%8%27%28%17%6%
445
11th to 20thDay
16%5%27%34%14%5%
444
21st to endDay
18%6%34%28%11%3
488
August1st to 10thDay
13%8%28%27%18%6%
446
11th to 20thDay
17%7%27%27%14%8%
439
21st to endDay
16%11%27%24%14%8%
495
September1st to 10thDay
20%9%24%23%16%9%
450
11th to 20thDay
16%8%28%21%18%9%
450
21st to endDay
22%12%26%22%14%5%
450
October1st to 10thDay
27%12%24%18%12%7%
450
11th to 20thDay
29%16%28%17%10%1
450
21st to endDay
39%13%25%19%4
444
November1st to 10thDay
27%20%23%19%9%2
365
11th to 20thDay
34%23%23%14%5%1
313
21st to endDay
36%18%32%13%1
180
December1st to 10thDay
37%22%26%12%31
180
11th to 20thDay
37%20%32%10%1
180
21st to endDay
38%21%32%7%11
198
January1st to 10thNight
27%24%32%14%2
760
11th to 20thNight
27%22%27%19%3
724
21st to endNight
22%23%33%18%3
792
February1st to 10thNight
29%22%30%18%1
720
11th to 20thNight
27%24%32%15%2
703
21st to endNight
30%22%30%16%2
592
March1st to 10thNight
26%22%30%19%3
720
11th to 20thNight
29%19%28%19%3
720
21st to endNight
26%21%24%20%7%1
792
April1st to 10thNight
30%15%30%21%3
720
11th to 20thNight
25%18%27%21%8%1
711
21st to endNight
29%13%27%23%7%
720
May1st to 10thNight
31%10%22%29%9%
419
11th to 20thNight
20%10%33%30%7%1
354
21st to endNight
14%6%30%37%12%1
389
June1st to 10thNight
18%8%25%36%12%1
360
11th to 20thNight
19%8%23%34%16%
360
21st to endNight
13%328%41%14%1
360
July1st to 10thNight
10%322%45%19%2
350
11th to 20thNight
12%221%43%21%2
348
21st to endNight
16%222%44%14%2
394
August1st to 10thNight
14%223%37%22%2
485
11th to 20thNight
16%413%38%25%4
613
21st to endNight
18%5%18%35%22%2
792
September1st to 10thNight
19%6%22%34%17%2
720
11th to 20thNight
19%6%21%28%22%4
720
21st to endNight
20%9%24%28%17%2
720
October1st to 10thNight
24%11%24%25%15%2
720
11th to 20thNight
26%12%28%27%7%1
720
21st to endNight
33%12%28%22%4
792
November1st to 10thNight
21%16%30%24%9%
740
11th to 20thNight
31%17%27%17%6%1
760
21st to endNight
25%19%31%20%5%
760
December1st to 10thNight
31%23%25%17%3
760
11th to 20thNight
30%21%28%18%3
760
21st to endNight
30%23%30%16%2
836
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