Hallam Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hallam Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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%
23854
16%
74949
11%
23801
43%
23801
20%
51307

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%
948
14%
2648
5%
946
25%
946
15%
2013
January11th to 20thNight
16%
843
8%
2553
3%
843
20%
843
10%
2017
January21st to endNight
20%
835
10%
2711
5%
835
26%
835
11%
2090
February1st to 10thNight
17%
720
8%
2340
3%
720
23%
720
11%
1790
February11th to 20thNight
20%
700
14%
2185
6%
699
28%
699
13%
1630
February21st to endNight
17%
592
16%
1776
4%
592
29%
592
15%
1296
March1st to 10thNight
25%
720
17%
2083
9%
720
33%
720
19%
1478
March11th to 20thNight
27%
720
20%
1980
9%
720
36%
720
24%
1313
March21st to endNight
30%
792
19%
2178
10%
792
37%
792
21%
1551
April1st to 10thNight
29%
680
16%
1940
8%
680
37%
680
23%
1325
April11th to 20thNight
30%
533
22%
1597
9%
533
41%
533
24%
1053
April21st to endNight
35%
540
20%
1620
7%
540
45%
540
25%
1064
May1st to 10thNight
36%
540
12%
1620
7%
540
41%
540
19%
1153
May11th to 20thNight
41%
531
18%
1592
12%
531
48%
531
24%
1065
May21st to endNight
56%
582
16%
1745
12%
582
61%
582
34%
1026
June1st to 10thNight
50%
374
18%
1392
17%
374
59%
374
29%
779
June11th to 20thNight
55%
360
20%
1260
21%
360
62%
360
29%
732
June21st to endNight
56%
405
22%
1305
21%
405
62%
405
33%
741
July1st to 10thNight
64%
525
18%
1400
16%
525
67%
525
41%
821
July11th to 20thNight
64%
522
21%
1441
17%
522
68%
522
38%
843
July21st to endNight
67%
591
20%
1672
16%
558
72%
558
41%
933
August1st to 10thNight
68%
537
21%
1600
19%
534
69%
534
39%
909
August11th to 20thNight
66%
534
24%
1598
18%
534
70%
534
34%
955
August21st to endNight
65%
594
25%
1782
19%
594
70%
594
37%
1020
September1st to 10thNight
60%
684
21%
1912
16%
684
63%
684
34%
1167
September11th to 20thNight
60%
720
28%
1980
21%
720
64%
720
38%
1168
September21st to endNight
50%
720
23%
1980
17%
720
55%
720
30%
1263
October1st to 10thNight
47%
720
21%
2060
17%
720
52%
720
30%
1308
October11th to 20thNight
32%
720
18%
2160
9%
720
42%
720
21%
1463
October21st to endNight
27%
792
18%
2463
11%
792
37%
792
17%
1724
November1st to 10thNight
37%
740
23%
2406
17%
740
47%
740
24%
1503
November11th to 20thNight
23%
760
19%
2470
8%
760
35%
760
17%
1661
November21st to endNight
23%
760
13%
2639
8%
760
31%
760
14%
1888
December1st to 10thNight
19%
760
9%
2660
5%
760
24%
760
11%
2007
December11th to 20thNight
18%
759
10%
3248
5%
758
27%
758
9%
2357
December21st to endNight
15%
1001
4%
4953
2%
988
21%
988
5%
4201

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%35%
74915
Night
15%9%47%6%35%5%5%7%34%
75204
TransparencyDay
48%412%22%12%1
74626
Night
50%311%22%12%1
75013
SeeingDay
17%9%28%26%14%6%
14134
Night
19%8%28%29%13%2
23868

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%14%6%10%7%46%5%5%7%21%
1517
11th to 20thDay
16%16%7%12%9%45%45%6%16%
1621
21st to endDay
18%17%7%10%7%3436%6%19%
1872
February1st to 10thDay
16%16%49%8%45%45%6%24%
1620
11th to 20thDay
15%12%5%10%8%46%5%5%6%25%
1662
21st to endDay
17%13%410%6%35%5%5%6%26%
1480
March1st to 10thDay
17%11%48%6%35%5%5%5%30%
1876
11th to 20thDay
14%10%5%7%6%345%5%7%34%
1980
21st to endDay
19%12%5%9%7%44446%28%
2178
April1st to 10thDay
14%11%46%6%345%6%7%33%
2034
11th to 20thDay
17%9%36%445%5%6%8%33%
2312
21st to endDay
19%9%36%6%44447%35%
2340
May1st to 10thDay
16%8%47%6%45%5%5%7%35%
2340
11th to 20thDay
17%8%46%5%3446%8%34%
2301
21st to endDay
10%6%25%5%35%6%6%9%43%
2531
June1st to 10thDay
10%5%35%5%35%6%8%9%41%
2560
11th to 20thDay
10%5%25%6%46%5%6%9%42%
2700
21st to endDay
9%5%25%5%46%6%7%11%42%
2648
July1st to 10thDay
9%5%244345%7%10%48%
2520
11th to 20thDay
10%4245%35%6%6%10%45%
2466
21st to endDay
8%5%35%5%36%5%7%10%43%
2547
August1st to 10thDay
8%5%245%35%5%7%10%46%
2327
11th to 20thDay
9%425%6%5%5%7%7%11%39%
2303
21st to endDay
9%5%25%435%6%6%9%45%
2574
September1st to 10thDay
10%6%35%6%346%6%8%42%
2025
11th to 20thDay
9%5%244345%7%11%44%
1980
21st to endDay
16%5%35%6%345%5%8%40%
1980
October1st to 10thDay
12%5%36%5%45%6%5%7%42%
1899
11th to 20thDay
17%7%36%6%35%5%6%8%35%
1800
21st to endDay
20%9%36%7%35%46%7%30%
1890
November1st to 10thDay
11%8%36%7%36%6%5%7%38%
1672
11th to 20thDay
15%10%47%5%35%5%6%7%34%
1710
21st to endDay
18%13%5%8%6%45%5%6%6%26%
1539
December1st to 10thDay
16%13%6%12%7%46%5%5%6%20%
1520
11th to 20thDay
16%15%5%8%6%46%5%6%8%21%
1910
21st to endDay
26%17%8%12%6%3445%5%11%
2681
January1st to 10thNight
18%12%5%9%7%45%447%26%
2648
11th to 20thNight
19%16%6%10%6%35%45%6%20%
2553
21st to endNight
16%14%7%10%7%445%5%423%
2711
February1st to 10thNight
18%15%5%9%7%3445%5%24%
2340
11th to 20thNight
16%13%6%9%7%36%5%46%26%
2184
21st to endNight
19%12%48%6%35%5%45%30%
1776
March1st to 10thNight
16%10%47%7%45%5%5%5%32%
2083
11th to 20thNight
13%8%45%5%35%5%6%8%38%
1980
21st to endNight
19%8%47%5%35%45%6%34%
2178
April1st to 10thNight
16%9%46%6%3445%7%37%
1940
11th to 20thNight
17%7%35%5%35%5%6%9%35%
1597
21st to endNight
16%8%35%5%245%5%6%39%
1620
May1st to 10thNight
16%7%47%7%46%5%6%6%31%
1620
11th to 20thNight
17%6%35%5%45%5%5%6%38%
1592
21st to endNight
9%435%435%5%6%9%47%
1744
June1st to 10thNight
11%5%26%436%46%9%45%
1392
11th to 20thNight
11%415%6%35%6%5%9%44%
1260
21st to endNight
9%5%245%345%6%8%48%
1305
July1st to 10thNight
7%5%25%4245%6%9%51%
1440
11th to 20thNight
8%4245%35%6%5%8%50%
1489
21st to endNight
6%4234446%7%10%49%
1771
August1st to 10thNight
8%4245%345%6%9%50%
1611
11th to 20thNight
8%425%5%35%6%5%9%47%
1598
21st to endNight
8%424435%5%6%8%50%
1782
September1st to 10thNight
10%5%35%5%25%5%6%8%47%
1912
11th to 20thNight
10%425%5%2436%8%51%
1980
21st to endNight
13%6%36%5%4445%7%43%
1980
October1st to 10thNight
13%5%26%5%3445%7%45%
2060
11th to 20thNight
18%8%26%6%35%5%47%36%
2160
21st to endNight
21%9%46%6%35%45%6%32%
2463
November1st to 10thNight
13%8%35%6%3445%6%43%
2406
11th to 20thNight
17%9%36%6%36%46%8%32%
2470
21st to endNight
20%12%5%7%6%3445%6%28%
2639
December1st to 10thNight
19%12%6%8%7%45%5%5%6%21%
2660
11th to 20thNight
15%14%5%9%7%45%5%5%6%25%
3285
21st to endNight
27%20%7%10%6%3343413%
4975

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
64%410%15%7%1
1518
11th to 20thDay
70%412%10%4
1621
21st to endDay
66%210%14%7%
1872
February1st to 10thDay
62%310%16%8%
1620
11th to 20thDay
61%310%18%7%1
1671
21st to endDay
58%411%18%7%2
1480
March1st to 10thDay
55%313%19%8%1
1876
11th to 20thDay
51%310%21%13%2
1980
21st to endDay
59%28%22%8%1
2178
April1st to 10thDay
50%412%20%13%1
2034
11th to 20thDay
49%311%21%14%2
2312
21st to endDay
51%213%20%12%2
2340
May1st to 10thDay
49%49%24%12%3
2340
11th to 20thDay
49%311%19%15%3
2305
21st to endDay
38%415%25%16%2
2531
June1st to 10thDay
38%413%29%14%2
2560
11th to 20thDay
39%415%23%18%2
2700
21st to endDay
35%5%14%27%17%2
2648
July1st to 10thDay
33%49%29%21%4
2451
11th to 20thDay
33%413%29%17%3
2391
21st to endDay
34%5%14%26%18%3
2405
August1st to 10thDay
33%316%34%14%1
2314
11th to 20thDay
38%5%15%29%12%1
2303
21st to endDay
34%414%27%19%2
2574
September1st to 10thDay
39%416%25%14%1
2025
11th to 20thDay
34%413%33%16%
1980
21st to endDay
42%312%28%14%1
1980
October1st to 10thDay
41%412%26%14%2
1899
11th to 20thDay
48%413%24%12%1
1800
21st to endDay
54%413%20%9%1
1890
November1st to 10thDay
45%5%12%24%12%2
1672
11th to 20thDay
50%215%21%10%1
1710
21st to endDay
59%313%19%6%1
1539
December1st to 10thDay
66%411%14%5%
1520
11th to 20thDay
63%313%16%5%
1906
21st to endDay
76%210%9%3
2681
January1st to 10thNight
60%29%19%9%
2648
11th to 20thNight
67%212%14%5%
2553
21st to endNight
65%210%16%6%1
2711
February1st to 10thNight
63%311%17%6%1
2340
11th to 20thNight
60%38%18%10%1
2186
21st to endNight
58%210%20%9%1
1776
March1st to 10thNight
54%210%24%10%1
2083
11th to 20thNight
44%412%23%15%2
1980
21st to endNight
53%28%23%13%1
2178
April1st to 10thNight
48%310%23%14%2
1940
11th to 20thNight
46%311%21%18%2
1597
21st to endNight
45%310%21%17%4
1620
May1st to 10thNight
53%49%19%12%3
1620
11th to 20thNight
47%38%20%19%3
1593
21st to endNight
35%414%28%19%2
1745
June1st to 10thNight
37%311%30%17%3
1392
11th to 20thNight
37%414%26%17%3
1260
21st to endNight
34%312%28%22%1
1305
July1st to 10thNight
31%310%28%24%4
1400
11th to 20thNight
32%411%33%17%3
1441
21st to endNight
29%317%25%21%4
1672
August1st to 10thNight
31%315%33%16%2
1600
11th to 20thNight
35%414%31%14%3
1598
21st to endNight
31%412%31%19%2
1782
September1st to 10thNight
35%5%13%28%17%3
1912
11th to 20thNight
32%310%32%20%2
1980
21st to endNight
41%313%27%16%1
1980
October1st to 10thNight
40%411%24%21%1
2060
11th to 20thNight
49%210%22%15%1
2160
21st to endNight
54%210%23%11%1
2463
November1st to 10thNight
43%311%27%16%1
2406
11th to 20thNight
51%211%23%11%1
2470
21st to endNight
58%214%20%6%
2639
December1st to 10thNight
63%311%17%6%
2660
11th to 20thNight
61%213%16%7%
3288
21st to endNight
77%29%9%3
4975

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
24%17%32%19%7%1
270
11th to 20thDay
23%16%41%13%5%1
270
21st to endDay
30%10%31%24%6%
294
February1st to 10thDay
26%19%33%20%1
240
11th to 20thDay
22%17%42%16%3
240
21st to endDay
22%21%41%14%2
198
March1st to 10thDay
25%16%36%19%4
316
11th to 20thDay
21%14%33%23%7%1
420
21st to endDay
26%13%31%24%5%1
462
April1st to 10thDay
19%11%36%24%8%2
420
11th to 20thDay
25%11%31%19%9%5%
412
21st to endDay
24%10%34%18%10%3
420
May1st to 10thDay
25%9%29%22%10%4
423
11th to 20thDay
20%7%29%25%13%6%
447
21st to endDay
14%6%23%32%18%7%
485
June1st to 10thDay
12%6%30%35%13%4
508
11th to 20thDay
12%7%30%33%16%2
630
21st to endDay
12%5%31%33%15%5%
630
July1st to 10thDay
10%222%31%24%11%
621
11th to 20thDay
12%424%32%18%10%
564
21st to endDay
10%319%33%26%9%
488
August1st to 10thDay
10%217%27%27%17%
446
11th to 20thDay
12%315%32%25%13%
439
21st to endDay
11%118%28%27%15%
495
September1st to 10thDay
11%322%30%22%12%
450
11th to 20thDay
10%217%28%31%12%
450
21st to endDay
18%5%22%28%17%10%
450
October1st to 10thDay
15%6%26%26%20%7%
450
11th to 20thDay
18%13%33%25%9%2
450
21st to endDay
27%13%35%18%6%1
405
November1st to 10thDay
13%17%33%24%10%3
270
11th to 20thDay
17%20%31%24%6%2
270
21st to endDay
26%10%43%19%21
189
December1st to 10thDay
23%14%37%19%6%1
180
11th to 20thDay
26%13%37%18%6%
180
21st to endDay
27%17%35%18%4
252
January1st to 10thNight
23%13%36%20%6%1
950
11th to 20thNight
28%14%32%22%4
843
21st to endNight
23%12%32%26%6%
835
February1st to 10thNight
26%13%35%24%3
720
11th to 20thNight
24%13%31%25%7%
703
21st to endNight
26%16%36%20%2
592
March1st to 10thNight
22%10%36%28%5%
720
11th to 20thNight
18%12%37%26%6%1
720
21st to endNight
23%9%31%26%11%
792
April1st to 10thNight
19%13%33%30%5%
680
11th to 20thNight
20%9%34%28%8%2
533
21st to endNight
19%6%33%30%11%
540
May1st to 10thNight
20%6%29%34%9%2
540
11th to 20thNight
21%425%34%15%1
531
21st to endNight
12%323%37%24%2
584
June1st to 10thNight
12%326%43%15%1
374
11th to 20thNight
13%122%44%19%
360
21st to endNight
12%421%46%16%1
405
July1st to 10thNight
9%213%41%31%5%
525
11th to 20thNight
11%215%39%29%4
522
21st to endNight
6%216%46%27%3
591
August1st to 10thNight
9%112%42%29%7%
537
11th to 20thNight
9%13%38%35%5%
534
21st to endNight
10%214%37%31%5%
594
September1st to 10thNight
12%415%34%30%6%
684
11th to 20thNight
12%317%33%29%7%
720
21st to endNight
16%5%20%32%22%5%
720
October1st to 10thNight
16%424%31%21%4
720
11th to 20thNight
21%10%30%26%12%2
720
21st to endNight
25%11%31%27%6%
792
November1st to 10thNight
17%11%29%29%13%2
740
11th to 20thNight
24%11%33%23%8%1
760
21st to endNight
28%11%31%21%8%
760
December1st to 10thNight
25%14%35%19%7%1
760
11th to 20thNight
25%12%36%22%4
760
21st to endNight
29%12%33%22%4
1007
Page generated at 2024-01-28 05:26:32. Version ac07

Copyright 2024 Attilla Danko


Hallam Observatory Clear Sky Chart |Charts Home