Avon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Avon clear sky chart from 2005-11-06 to 2024-03-18.

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
47%
24305
13%
80200
9%
24220
52%
24220
13%
58832

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
27%
749
21%
4113
6%
737
36%
737
7%
2531
January11th to 20thNight
33%
753
20%
3979
7%
740
42%
740
7%
2653
January21st to endNight
27%
824
11%
4443
8%
814
37%
814
8%
2954
February1st to 10thNight
33%
760
14%
2392
7%
760
42%
760
13%
1814
February11th to 20thNight
26%
743
12%
2219
6%
740
35%
740
13%
1732
February21st to endNight
33%
628
15%
1884
7%
628
43%
628
14%
1407
March1st to 10thNight
35%
760
15%
2214
10%
760
42%
760
16%
1626
March11th to 20thNight
37%
752
12%
2068
8%
752
43%
752
14%
1631
March21st to endNight
39%
792
13%
2079
6%
792
45%
792
15%
1630
April1st to 10thNight
40%
720
14%
1800
5%
720
45%
720
20%
1356
April11th to 20thNight
50%
711
18%
1775
8%
711
55%
711
21%
1327
April21st to endNight
51%
720
17%
1800
8%
720
53%
720
20%
1363
May1st to 10thNight
52%
720
11%
1800
6%
720
53%
720
18%
1385
May11th to 20thNight
53%
629
10%
1690
6%
629
55%
629
15%
1329
May21st to endNight
66%
389
11%
1551
13%
389
67%
389
12%
1122
June1st to 10thNight
62%
360
12%
1440
15%
360
64%
360
13%
1034
June11th to 20thNight
74%
360
12%
1440
15%
360
74%
360
14%
1018
June21st to endNight
68%
360
10%
1440
12%
360
69%
360
11%
1060
July1st to 10thNight
74%
350
11%
1400
14%
350
75%
350
12%
1015
July11th to 20thNight
70%
489
9%
1533
9%
489
70%
489
15%
1160
July21st to endNight
66%
591
5%
1672
4%
558
67%
558
14%
1306
August1st to 10thNight
64%
645
6%
1708
4%
642
64%
642
18%
1316
August11th to 20thNight
69%
710
11%
1774
7%
710
70%
710
16%
1414
August21st to endNight
60%
792
7%
1980
5%
792
60%
792
15%
1600
September1st to 10thNight
61%
720
15%
1800
10%
720
62%
720
17%
1437
September11th to 20thNight
58%
720
14%
1823
11%
720
59%
720
16%
1454
September21st to endNight
52%
720
11%
1980
11%
720
53%
720
12%
1586
October1st to 10thNight
58%
720
17%
2142
15%
720
62%
720
18%
1586
October11th to 20thNight
52%
720
18%
2160
14%
720
57%
720
15%
1622
October21st to endNight
48%
792
17%
2376
12%
792
56%
792
15%
1755
November1st to 10thNight
49%
740
14%
2317
12%
740
53%
740
14%
1762
November11th to 20thNight
43%
760
13%
2470
11%
760
47%
760
12%
1899
November21st to endNight
44%
760
13%
2470
9%
760
49%
760
13%
1845
December1st to 10thNight
32%
760
13%
2555
6%
760
41%
760
11%
1957
December11th to 20thNight
36%
760
15%
3171
9%
758
44%
758
9%
2315
December21st to endNight
35%
826
11%
4742
6%
817
46%
817
8%
2831

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
12%6%35%5%35%5%6%8%42%
78758
Night
13%6%35%5%35%5%6%8%41%
81011
TransparencyDay
27%35%10%16%10%2
78472
Night
29%33%9%16%12%2
80816
SeeingDay
15%6%19%27%25%8%
14618
Night
16%7%21%28%22%6%
24346

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
11%8%36%43445%8%43%
2691
11th to 20thDay
11%6%26%5%245%6%7%44%
2840
21st to endDay
13%11%46%5%3445%8%36%
3124
February1st to 10thDay
15%6%35%5%346%6%9%39%
1786
11th to 20thDay
19%8%35%5%3445%5%40%
1848
21st to endDay
13%6%35%6%346%6%8%40%
1570
March1st to 10thDay
14%8%25%42445%6%46%
1962
11th to 20thDay
14%7%25%6%3446%7%42%
2058
21st to endDay
15%6%25%4345%5%7%44%
2276
April1st to 10thDay
13%5%3342345%6%51%
2160
11th to 20thDay
10%425%5%35%5%5%7%50%
2134
21st to endDay
9%5%2443445%8%53%
2160
May1st to 10thDay
11%434445%6%6%10%44%
2160
11th to 20thDay
15%6%35%4445%5%8%42%
2213
21st to endDay
11%6%245%345%6%9%44%
2725
June1st to 10thDay
9%5%35%6%46%6%7%10%41%
2520
11th to 20thDay
8%4345%35%5%6%11%45%
2520
21st to endDay
6%425%7%46%6%8%13%38%
2520
July1st to 10thDay
9%5%25%5%46%5%8%11%39%
2520
11th to 20thDay
8%5%35%5%445%8%12%40%
2371
21st to endDay
8%5%25%6%47%6%7%10%39%
2547
August1st to 10thDay
10%7%26%6%35%6%7%12%36%
2202
11th to 20thDay
10%425%5%35%6%8%10%41%
2126
21st to endDay
12%6%26%6%46%6%7%8%36%
2376
September1st to 10thDay
11%6%27%6%35%5%7%10%37%
2160
11th to 20thDay
14%5%26%6%46%6%5%7%38%
2135
21st to endDay
18%7%26%5%35%5%5%6%37%
1980
October1st to 10thDay
13%6%35%5%245%47%47%
1814
11th to 20thDay
13%6%36%5%24346%50%
1800
21st to endDay
12%6%25%43445%6%49%
1980
November1st to 10thDay
16%7%25%5%245%5%7%42%
1758
11th to 20thDay
16%7%25%5%34347%45%
1710
21st to endDay
13%5%25%435%5%5%8%46%
1710
December1st to 10thDay
16%7%46%5%35%5%5%7%38%
1620
11th to 20thDay
16%7%26%5%35%45%6%42%
1910
21st to endDay
13%7%35%4235%6%6%46%
2772
January1st to 10thNight
13%7%35%5%3445%7%43%
4237
11th to 20thNight
15%7%25%5%345%5%7%40%
4120
21st to endNight
13%8%26%5%45%5%5%7%39%
4532
February1st to 10thNight
18%8%3443445%8%39%
2392
11th to 20thNight
18%8%37%5%24446%38%
2218
21st to endNight
16%7%36%5%3445%7%40%
1884
March1st to 10thNight
13%6%345%2435%7%47%
2214
11th to 20thNight
15%7%36%6%25%5%6%7%39%
2068
21st to endNight
15%6%35%5%25%5%5%6%43%
2079
April1st to 10thNight
13%6%25%42435%8%47%
1800
11th to 20thNight
8%6%346%35%6%6%8%45%
1775
21st to endNight
11%5%246%2445%8%49%
1800
May1st to 10thNight
13%6%35%5%336%6%8%43%
1800
11th to 20thNight
14%437%43446%9%42%
1690
21st to endNight
10%5%25%5%35%5%6%10%44%
1551
June1st to 10thNight
11%5%25%5%445%7%10%41%
1440
11th to 20thNight
8%3145%35%7%8%13%44%
1440
21st to endNight
8%5%26%6%46%6%8%14%36%
1440
July1st to 10thNight
8%425%5%37%8%9%12%38%
1440
11th to 20thNight
10%425%6%45%6%7%10%41%
1585
21st to endNight
9%6%26%7%36%7%8%10%35%
1771
August1st to 10thNight
10%6%36%5%36%6%8%13%32%
1719
11th to 20thNight
9%5%27%6%46%6%7%10%37%
1774
21st to endNight
13%7%37%7%446%6%9%34%
1980
September1st to 10thNight
13%6%26%6%46%6%8%9%34%
1800
11th to 20thNight
14%7%37%5%36%5%6%8%35%
1823
21st to endNight
18%6%36%6%445%5%7%34%
1980
October1st to 10thNight
12%6%26%5%3445%7%45%
2142
11th to 20thNight
15%6%2442445%6%47%
2160
21st to endNight
12%6%24435%45%7%48%
2376
November1st to 10thNight
15%7%35%5%4445%7%41%
2317
11th to 20thNight
15%7%37%5%345%47%40%
2470
21st to endNight
13%6%35%42445%8%46%
2470
December1st to 10thNight
16%8%27%6%3446%6%37%
2555
11th to 20thNight
14%7%35%5%3445%7%43%
3285
21st to endNight
13%8%244345%5%7%45%
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
30%23%5%28%13%1
2692
11th to 20thDay
29%23%5%21%16%5%
2840
21st to endDay
39%23%8%14%12%3
3124
February1st to 10thDay
28%34%8%19%10%2
1786
11th to 20thDay
33%34%6%12%13%3
1857
21st to endDay
29%34%9%16%10%2
1570
March1st to 10thDay
25%34%9%17%13%3
1962
11th to 20thDay
28%35%10%15%9%4
2058
21st to endDay
29%36%10%14%9%1
2276
April1st to 10thDay
22%35%9%15%14%3
2160
11th to 20thDay
22%34%9%19%13%3
2134
21st to endDay
21%36%10%19%11%3
2160
May1st to 10thDay
23%38%14%17%8%
2160
11th to 20thDay
30%38%11%14%6%1
2217
21st to endDay
26%37%13%18%5%
2725
June1st to 10thDay
25%38%13%16%8%
2520
11th to 20thDay
25%38%12%19%6%
2520
21st to endDay
25%39%14%17%6%
2520
July1st to 10thDay
25%38%13%17%6%
2450
11th to 20thDay
22%41%18%12%7%
2300
21st to endDay
25%40%18%15%3
2405
August1st to 10thDay
24%37%20%15%3
2189
11th to 20thDay
26%37%14%15%7%
2126
21st to endDay
31%38%13%11%6%
2376
September1st to 10thDay
28%37%11%13%10%1
2160
11th to 20thDay
28%36%8%14%12%1
2135
21st to endDay
32%36%6%13%12%1
1980
October1st to 10thDay
24%35%6%15%19%1
1814
11th to 20thDay
28%35%416%13%4
1800
21st to endDay
27%36%5%16%14%2
1980
November1st to 10thDay
25%40%6%13%14%2
1758
11th to 20thDay
26%38%8%15%10%3
1710
21st to endDay
26%39%8%14%11%2
1710
December1st to 10thDay
28%38%10%14%8%1
1620
11th to 20thDay
31%31%6%13%15%4
1906
21st to endDay
29%23%9%22%16%1
2772
January1st to 10thNight
30%24%5%21%18%2
4237
11th to 20thNight
35%24%6%18%14%4
4120
21st to endNight
36%23%8%15%15%4
4532
February1st to 10thNight
32%33%6%15%11%3
2392
11th to 20thNight
33%34%7%12%11%4
2220
21st to endNight
30%33%8%15%13%2
1884
March1st to 10thNight
26%34%8%16%13%3
2214
11th to 20thNight
30%37%8%12%9%4
2068
21st to endNight
29%36%8%15%10%2
2079
April1st to 10thNight
24%35%10%14%14%3
1800
11th to 20thNight
23%34%8%17%15%3
1775
21st to endNight
24%36%8%19%11%3
1800
May1st to 10thNight
26%36%11%18%8%1
1800
11th to 20thNight
29%37%12%12%9%1
1691
21st to endNight
24%38%13%17%7%1
1552
June1st to 10thNight
26%36%13%17%8%
1440
11th to 20thNight
23%38%14%18%7%
1440
21st to endNight
26%39%14%16%6%
1440
July1st to 10thNight
23%40%13%16%8%
1400
11th to 20thNight
24%40%16%14%5%
1533
21st to endNight
27%40%17%14%1
1672
August1st to 10thNight
25%38%19%15%2
1708
11th to 20thNight
29%38%13%13%7%1
1774
21st to endNight
32%37%13%12%5%1
1980
September1st to 10thNight
29%37%8%16%10%
1800
11th to 20thNight
30%36%8%16%9%1
1823
21st to endNight
34%35%6%14%10%1
1980
October1st to 10thNight
25%35%416%18%1
2142
11th to 20thNight
28%35%5%16%13%3
2160
21st to endNight
26%35%416%16%2
2376
November1st to 10thNight
25%39%5%15%13%3
2317
11th to 20thNight
28%38%7%12%13%1
2470
21st to endNight
26%38%7%13%13%3
2470
December1st to 10thNight
28%38%8%13%10%2
2555
11th to 20thNight
31%32%6%13%15%3
3288
21st to endNight
29%23%10%21%16%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
17%12%36%26%8%1
231
11th to 20thDay
18%10%30%33%8%1
270
21st to endDay
18%11%35%24%11%1
297
February1st to 10thDay
17%9%33%32%8%1
346
11th to 20thDay
26%12%32%25%5%
456
21st to endDay
18%10%29%31%11%1
382
March1st to 10thDay
18%12%31%26%12%2
460
11th to 20thDay
18%10%31%27%12%1
449
21st to endDay
15%9%31%29%13%2
462
April1st to 10thDay
18%11%28%26%15%2
420
11th to 20thDay
10%6%24%36%21%2
412
21st to endDay
9%5%23%39%19%5%
420
May1st to 10thDay
14%424%33%19%5%
423
11th to 20thDay
15%217%30%28%8%
447
21st to endDay
15%28%29%27%18%
485
June1st to 10thDay
12%313%30%32%10%
450
11th to 20thDay
11%210%29%34%14%
450
21st to endDay
8%210%22%48%10%
450
July1st to 10thDay
11%26%20%44%15%
445
11th to 20thDay
9%2421%45%18%
444
21st to endDay
9%18%26%46%10%
488
August1st to 10thDay
10%28%25%39%16%
446
11th to 20thDay
11%15%20%38%26%
439
21st to endDay
15%15%22%41%15%
495
September1st to 10thDay
12%19%24%32%22%
450
11th to 20thDay
16%310%27%33%11%
450
21st to endDay
20%412%26%28%10%
450
October1st to 10thDay
17%414%22%32%11%
450
11th to 20thDay
16%420%29%24%8%
450
21st to endDay
13%6%20%30%24%6%
495
November1st to 10thDay
20%8%16%29%21%6%
408
11th to 20thDay
20%7%26%30%15%2
370
21st to endDay
14%8%25%31%21%1
370
December1st to 10thDay
18%10%34%28%10%1
280
11th to 20thDay
18%12%29%26%11%3
180
21st to endDay
18%8%30%29%15%1
198
January1st to 10thNight
17%17%33%23%10%1
760
11th to 20thNight
18%14%28%29%10%1
760
21st to endNight
20%14%32%26%7%
836
February1st to 10thNight
21%10%32%28%9%1
760
11th to 20thNight
25%13%31%25%6%
743
21st to endNight
19%13%29%29%10%1
628
March1st to 10thNight
15%12%33%28%11%1
760
11th to 20thNight
17%10%28%31%10%4
752
21st to endNight
18%10%27%30%13%2
792
April1st to 10thNight
15%8%32%29%14%2
720
11th to 20thNight
10%7%26%33%19%4
711
21st to endNight
13%6%24%34%21%2
720
May1st to 10thNight
16%6%19%38%18%3
720
11th to 20thNight
14%319%36%22%6%
629
21st to endNight
14%27%27%33%16%
390
June1st to 10thNight
13%312%32%33%8%
360
11th to 20thNight
10%18%33%38%11%
360
21st to endNight
10%47%34%36%9%
360
July1st to 10thNight
9%16%30%43%11%
350
11th to 20thNight
11%2324%44%16%
489
21st to endNight
11%26%30%41%10%
591
August1st to 10thNight
13%18%27%42%9%
645
11th to 20thNight
11%25%24%39%20%
710
21st to endNight
15%25%20%41%16%
792
September1st to 10thNight
14%28%23%35%18%
720
11th to 20thNight
18%310%23%37%9%
720
21st to endNight
22%311%22%33%9%
720
October1st to 10thNight
14%414%26%31%11%
720
11th to 20thNight
18%6%18%29%25%5%
720
21st to endNight
16%6%23%25%24%6%
792
November1st to 10thNight
19%7%18%30%21%5%
740
11th to 20thNight
19%10%20%32%17%3
760
21st to endNight
16%11%23%29%19%2
760
December1st to 10thNight
19%10%31%25%13%1
760
11th to 20thNight
17%11%30%28%12%2
760
21st to endNight
16%12%28%31%12%1
836
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