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Putney Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Putney Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
29%
22859
16%
74711
8%
22813
39%
22813
18%
49976

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
18%
760
18%
2649
6%
757
34%
757
14%
1747
January11th to 20thNight
19%
741
13%
2591
4%
741
30%
741
11%
1856
January21st to endNight
18%
792
17%
2772
7%
792
32%
792
15%
1812
February1st to 10thNight
15%
720
13%
2321
4%
720
28%
720
13%
1643
February11th to 20thNight
15%
703
17%
2100
4%
700
32%
700
16%
1430
February21st to endNight
16%
592
19%
1776
6%
592
32%
592
15%
1222
March1st to 10thNight
19%
720
20%
2160
8%
720
34%
720
17%
1454
March11th to 20thNight
20%
720
16%
2116
6%
720
30%
720
16%
1471
March21st to endNight
23%
792
21%
1980
5%
792
34%
792
20%
1394
April1st to 10thNight
23%
720
17%
1800
4%
720
31%
720
20%
1285
April11th to 20thNight
26%
711
15%
1775
5%
711
31%
711
21%
1251
April21st to endNight
23%
572
17%
1652
5%
572
31%
572
17%
1159
May1st to 10thNight
23%
540
13%
1620
5%
540
29%
540
15%
1166
May11th to 20thNight
29%
400
16%
1462
11%
400
39%
400
17%
955
May21st to endNight
41%
389
13%
1551
14%
388
47%
388
17%
986
June1st to 10thNight
36%
360
13%
1440
12%
360
46%
360
17%
932
June11th to 20thNight
35%
360
11%
1440
10%
360
44%
360
17%
924
June21st to endNight
45%
360
12%
1440
14%
360
52%
360
19%
898
July1st to 10thNight
49%
350
13%
1400
16%
350
57%
350
21%
854
July11th to 20thNight
51%
348
14%
1392
17%
348
55%
348
21%
848
July21st to endNight
54%
394
17%
1486
20%
372
60%
372
25%
852
August1st to 10thNight
52%
399
17%
1463
13%
397
58%
397
26%
853
August11th to 20thNight
46%
621
14%
1685
9%
621
48%
621
24%
1149
August21st to endNight
51%
792
20%
1980
10%
792
53%
792
34%
1281
September1st to 10thNight
45%
720
20%
1800
8%
720
51%
720
32%
1179
September11th to 20thNight
47%
720
24%
1930
14%
720
53%
720
34%
1191
September21st to endNight
43%
720
23%
1980
15%
720
50%
720
30%
1269
October1st to 10thNight
37%
720
15%
2002
9%
720
43%
720
20%
1399
October11th to 20thNight
34%
720
18%
2212
10%
720
45%
720
22%
1441
October21st to endNight
23%
792
13%
2574
6%
792
33%
792
15%
1829
November1st to 10thNight
29%
740
20%
2407
11%
740
42%
740
21%
1510
November11th to 20thNight
26%
760
20%
2470
9%
760
39%
760
18%
1604
November21st to endNight
24%
760
18%
2549
7%
760
38%
760
17%
1652
December1st to 10thNight
18%
760
15%
2660
6%
760
31%
760
13%
1850
December11th to 20thNight
18%
760
13%
3231
4%
759
31%
759
11%
2183
December21st to endNight
16%
831
11%
4845
3%
817
30%
817
7%
3447

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
16%7%36%6%35%5%5%7%37%
74808
Night
16%7%35%5%35%5%5%7%39%
74978
TransparencyDay
45%8%13%20%13%2
74513
Night
44%8%13%21%13%2
74807
SeeingDay
24%18%29%19%8%2
13772
Night
21%15%30%25%8%1
22864

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
12%8%47%6%45%5%7%6%36%
1517
11th to 20thDay
16%7%38%5%45%5%7%7%34%
1480
21st to endDay
11%8%47%6%446%6%7%38%
1584
February1st to 10thDay
15%11%47%6%46%6%6%8%29%
1634
11th to 20thDay
11%8%48%7%45%5%6%8%34%
1748
21st to endDay
18%9%49%6%4445%5%33%
1480
March1st to 10thDay
13%7%48%6%45%45%8%36%
1797
11th to 20thDay
16%9%46%5%4445%6%36%
1844
21st to endDay
17%7%37%5%34445%39%
2373
April1st to 10thDay
17%9%37%6%3445%6%37%
2160
11th to 20thDay
16%6%35%5%35%6%6%7%37%
2134
21st to endDay
19%7%37%6%45%5%5%6%33%
2306
May1st to 10thDay
22%8%36%5%35%45%6%33%
2340
11th to 20thDay
18%6%25%5%345%5%7%40%
2447
21st to endDay
15%5%36%5%3445%8%42%
2725
June1st to 10thDay
18%5%36%7%45%6%6%8%33%
2520
11th to 20thDay
17%5%35%6%45%46%8%38%
2520
21st to endDay
16%6%36%6%46%6%7%8%33%
2520
July1st to 10thDay
13%435%5%36%6%7%8%40%
2520
11th to 20thDay
12%5%35%6%45%6%6%10%38%
2520
21st to endDay
13%435%5%46%6%7%9%38%
2744
August1st to 10thDay
10%5%36%6%46%5%8%9%39%
2446
11th to 20thDay
12%5%26%6%45%6%7%9%38%
2212
21st to endDay
12%425%5%35%6%6%9%43%
2376
September1st to 10thDay
12%6%35%435%5%6%8%43%
2160
11th to 20thDay
13%5%345%34347%50%
2030
21st to endDay
16%5%25%5%35%448%44%
1980
October1st to 10thDay
22%6%26%5%35%445%37%
1954
11th to 20thDay
14%5%26%6%35%5%5%8%41%
1746
21st to endDay
22%8%36%6%35%5%46%32%
1782
November1st to 10thDay
12%7%35%5%3445%6%46%
1671
11th to 20thDay
17%6%35%6%3445%6%40%
1710
21st to endDay
18%8%35%5%2446%7%38%
1630
December1st to 10thDay
17%10%47%8%36%5%5%6%31%
1520
11th to 20thDay
21%9%26%5%4444437%
1906
21st to endDay
20%12%38%6%45%5%45%29%
2772
January1st to 10thNight
13%9%36%5%35%5%6%7%39%
2649
11th to 20thNight
15%10%47%5%36%6%5%7%32%
2591
21st to endNight
10%8%45%7%45%5%6%8%39%
2772
February1st to 10thNight
13%9%37%7%45%6%5%7%33%
2321
11th to 20thNight
10%8%35%6%5%6%6%5%7%37%
2098
21st to endNight
15%8%27%6%35%46%7%36%
1776
March1st to 10thNight
14%9%36%6%35%5%5%6%40%
2150
11th to 20thNight
14%8%36%5%45%5%5%6%37%
2115
21st to endNight
15%8%35%5%3445%7%41%
1980
April1st to 10thNight
17%7%35%5%35%5%5%7%39%
1800
11th to 20thNight
17%7%35%5%2446%7%40%
1775
21st to endNight
20%7%35%5%3445%7%37%
1652
May1st to 10thNight
22%8%35%6%345%46%34%
1620
11th to 20thNight
17%5%26%6%45%5%6%7%37%
1461
21st to endNight
15%5%247%345%5%9%40%
1550
June1st to 10thNight
18%6%35%5%26%45%7%39%
1440
11th to 20thNight
19%5%25%5%35%45%8%40%
1440
21st to endNight
15%5%25%6%36%5%7%6%41%
1440
July1st to 10thNight
12%425%5%35%5%6%9%43%
1440
11th to 20thNight
12%5%245%46%6%7%10%41%
1440
21st to endNight
11%5%14436%5%5%10%47%
1574
August1st to 10thNight
9%5%25%5%35%45%8%48%
1473
11th to 20thNight
12%5%25%6%45%5%6%9%40%
1685
21st to endNight
14%6%2442345%6%50%
1980
September1st to 10thNight
12%5%244345%5%8%49%
1800
11th to 20thNight
14%5%14324446%53%
1930
21st to endNight
14%4244245%5%6%48%
1980
October1st to 10thNight
21%6%244345%47%39%
2002
11th to 20thNight
15%6%25%5%45%6%6%7%41%
2212
21st to endNight
22%7%35%5%245%5%5%35%
2574
November1st to 10thNight
14%6%35%5%3446%6%45%
2407
11th to 20thNight
16%7%246%3445%7%42%
2470
21st to endNight
14%8%25%5%345%6%6%42%
2549
December1st to 10thNight
17%9%47%6%45%5%5%6%34%
2660
11th to 20thNight
19%8%35%5%35%45%6%37%
3288
21st to endNight
21%11%5%8%5%35%45%5%28%
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
45%7%12%24%11%1
1518
11th to 20thDay
49%10%14%15%10%2
1480
21st to endDay
44%7%13%22%13%3
1584
February1st to 10thDay
52%9%13%16%9%1
1634
11th to 20thDay
49%6%11%19%12%3
1757
21st to endDay
54%5%10%16%13%2
1480
March1st to 10thDay
48%6%13%18%12%4
1800
11th to 20thDay
48%6%12%15%15%4
1844
21st to endDay
48%5%9%17%16%5%
2373
April1st to 10thDay
48%6%10%20%13%3
2160
11th to 20thDay
43%9%11%18%16%2
2134
21st to endDay
51%7%10%15%14%3
2306
May1st to 10thDay
54%6%9%18%12%2
2340
11th to 20thDay
44%7%10%20%16%3
2451
21st to endDay
40%8%16%22%12%2
2725
June1st to 10thDay
47%9%14%19%10%1
2520
11th to 20thDay
45%8%12%19%15%2
2520
21st to endDay
47%10%14%19%10%1
2520
July1st to 10thDay
40%9%13%21%15%1
2450
11th to 20thDay
38%11%17%24%10%1
2445
21st to endDay
40%10%14%22%13%1
2591
August1st to 10thDay
38%11%16%23%11%1
2432
11th to 20thDay
40%9%15%24%10%1
2212
21st to endDay
36%9%15%25%13%2
2376
September1st to 10thDay
38%8%14%24%15%1
2160
11th to 20thDay
36%7%11%23%22%2
2030
21st to endDay
39%7%12%23%16%3
1980
October1st to 10thDay
49%7%9%20%12%3
1954
11th to 20thDay
40%8%14%27%10%1
1746
21st to endDay
52%7%11%17%10%3
1782
November1st to 10thDay
39%7%15%23%15%2
1671
11th to 20thDay
43%8%11%20%16%2
1710
21st to endDay
46%6%17%21%9%2
1630
December1st to 10thDay
53%7%14%14%10%1
1520
11th to 20thDay
50%6%13%17%13%1
1906
21st to endDay
55%9%11%16%9%
2772
January1st to 10thNight
44%7%12%20%13%2
2649
11th to 20thNight
51%9%14%17%7%2
2591
21st to endNight
45%7%12%19%15%2
2772
February1st to 10thNight
50%8%14%17%10%2
2321
11th to 20thNight
44%7%15%19%12%2
2100
21st to endNight
47%6%13%21%10%3
1776
March1st to 10thNight
44%7%12%23%12%2
2160
11th to 20thNight
47%7%13%17%13%3
2116
21st to endNight
45%6%11%16%18%4
1980
April1st to 10thNight
45%7%12%19%14%2
1800
11th to 20thNight
43%9%11%20%16%2
1775
21st to endNight
47%7%11%17%13%4
1652
May1st to 10thNight
53%5%9%17%13%2
1620
11th to 20thNight
45%8%13%17%14%3
1462
21st to endNight
43%9%13%23%12%1
1551
June1st to 10thNight
45%8%14%22%11%1
1440
11th to 20thNight
44%8%16%19%12%1
1440
21st to endNight
39%11%16%20%14%1
1440
July1st to 10thNight
37%11%15%23%13%1
1400
11th to 20thNight
36%11%15%26%11%
1392
21st to endNight
32%10%17%26%12%2
1486
August1st to 10thNight
33%9%17%26%13%1
1463
11th to 20thNight
41%9%16%22%11%
1685
21st to endNight
35%7%14%28%16%1
1980
September1st to 10thNight
35%8%14%25%18%1
1800
11th to 20thNight
32%7%13%25%21%2
1930
21st to endNight
36%7%11%26%17%2
1980
October1st to 10thNight
46%7%13%21%11%2
2002
11th to 20thNight
40%8%15%25%11%1
2212
21st to endNight
50%8%13%18%10%1
2574
November1st to 10thNight
40%7%13%22%16%3
2407
11th to 20thNight
43%7%12%21%14%3
2470
21st to endNight
43%7%17%23%10%1
2549
December1st to 10thNight
50%8%14%20%9%1
2660
11th to 20thNight
47%9%15%14%14%2
3288
21st to endNight
54%10%10%19%6%
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
28%23%27%17%4
180
11th to 20thDay
32%22%34%10%21
170
21st to endDay
23%27%35%10%5%
176
February1st to 10thDay
35%29%26%10%1
266
11th to 20thDay
30%25%31%12%1
416
21st to endDay
33%22%30%13%2
346
March1st to 10thDay
28%21%34%14%3
420
11th to 20thDay
30%26%26%13%41
420
21st to endDay
30%24%27%11%5%3
462
April1st to 10thDay
32%24%24%12%7%1
420
11th to 20thDay
28%25%25%14%6%1
412
21st to endDay
32%18%31%12%6%1
420
May1st to 10thDay
32%22%25%12%8%2
423
11th to 20thDay
22%19%35%17%5%1
447
21st to endDay
19%15%31%24%8%2
485
June1st to 10thDay
18%18%34%19%10%1
450
11th to 20thDay
19%15%34%22%8%2
450
21st to endDay
17%17%33%20%9%3
450
July1st to 10thDay
16%11%30%27%13%3
445
11th to 20thDay
16%7%33%30%11%2
444
21st to endDay
14%11%36%27%10%3
488
August1st to 10thDay
11%11%28%30%15%4
446
11th to 20thDay
14%10%28%28%16%5%
439
21st to endDay
13%12%28%27%13%6%
495
September1st to 10thDay
19%10%24%24%17%6%
450
11th to 20thDay
18%14%27%19%12%9%
450
21st to endDay
17%15%30%21%11%6%
450
October1st to 10thDay
27%16%23%18%12%4
450
11th to 20thDay
19%18%27%24%10%2
423
21st to endDay
33%13%27%19%7%1
396
November1st to 10thDay
24%16%25%23%10%1
365
11th to 20thDay
29%19%28%19%41
370
21st to endDay
31%18%27%20%41
290
December1st to 10thDay
24%27%31%13%41
180
11th to 20thDay
38%19%31%9%2
180
21st to endDay
31%20%33%13%4
198
January1st to 10thNight
20%23%34%20%3
760
11th to 20thNight
26%19%32%20%2
741
21st to endNight
19%19%37%22%3
792
February1st to 10thNight
22%19%36%20%2
720
11th to 20thNight
20%23%39%17%2
703
21st to endNight
25%24%31%18%2
592
March1st to 10thNight
23%21%33%20%2
720
11th to 20thNight
23%23%31%19%4
720
21st to endNight
25%18%31%20%6%
792
April1st to 10thNight
26%18%31%22%4
720
11th to 20thNight
24%20%28%22%6%
711
21st to endNight
27%18%28%22%4
572
May1st to 10thNight
27%15%30%23%6%1
540
11th to 20thNight
19%16%32%28%6%
400
21st to endNight
17%10%25%36%10%1
389
June1st to 10thNight
20%8%29%31%11%
360
11th to 20thNight
18%9%33%31%9%
360
21st to endNight
16%8%26%41%9%1
360
July1st to 10thNight
13%6%26%43%11%1
350
11th to 20thNight
14%326%39%16%2
348
21st to endNight
11%328%42%15%1
394
August1st to 10thNight
13%5%22%42%19%1
399
11th to 20thNight
14%6%26%35%18%2
621
21st to endNight
15%7%22%35%19%2
792
September1st to 10thNight
15%6%29%32%17%1
720
11th to 20thNight
16%8%23%33%17%3
720
21st to endNight
18%8%26%31%15%2
720
October1st to 10thNight
26%12%20%28%12%2
720
11th to 20thNight
19%12%31%27%10%1
720
21st to endNight
29%16%27%23%5%
792
November1st to 10thNight
20%16%29%26%9%
740
11th to 20thNight
22%18%30%22%7%1
760
21st to endNight
22%18%32%22%6%
760
December1st to 10thNight
24%19%34%19%4
760
11th to 20thNight
25%18%34%19%3
760
21st to endNight
26%18%35%18%2
836
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Copyright 2024 Attilla Danko


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