Hallam Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hallam Observatory clear sky chart from 2005-11-05 to 2018-02-14.

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
37%
16060
16%
48752
10%
16022
43%
16022
21%
33118

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 15thNight
17%
964
11%
2708
4%
962
23%
962
14%
2071
January16th to endNight
17%
832
10%
2704
4%
832
25%
832
11%
2071
February1st to 15thNight
19%
757
10%
2468
3%
757
26%
757
12%
1865
February16th to endNight
22%
631
15%
1893
7%
631
30%
631
14%
1397
March1st to 15thNight
26%
720
14%
2049
8%
720
32%
720
17%
1490
March16th to endNight
34%
768
19%
2112
10%
768
40%
768
24%
1448
April1st to 15thNight
29%
632
16%
1830
7%
632
38%
632
21%
1270
April16th to endNight
36%
540
22%
1620
9%
540
47%
540
27%
1027
May1st to 15thNight
39%
540
14%
1620
7%
540
46%
540
20%
1132
May16th to endNight
50%
561
16%
1684
12%
561
55%
561
31%
1039
June1st to 15thNight
54%
369
17%
1347
18%
369
60%
369
27%
779
June16th to endNight
56%
390
19%
1290
18%
390
63%
390
32%
739
July1st to 15thNight
63%
510
17%
1360
16%
510
65%
510
39%
810
July16th to endNight
69%
564
19%
1569
16%
531
72%
531
42%
856
August1st to 15thNight
69%
539
20%
1609
18%
536
70%
536
37%
938
August16th to endNight
69%
573
25%
1719
19%
573
73%
573
39%
962
September1st to 15thNight
59%
696
23%
1935
17%
696
62%
696
35%
1169
September16th to endNight
58%
717
26%
1980
19%
717
62%
717
35%
1201
October1st to 15thNight
46%
720
19%
2094
16%
720
51%
720
28%
1343
October16th to endNight
28%
768
18%
2362
10%
768
39%
768
19%
1617
November1st to 15thNight
36%
760
21%
2471
17%
760
43%
760
22%
1586
November16th to endNight
24%
780
15%
2650
8%
780
33%
780
15%
1865
December1st to 15thNight
18%
780
9%
2730
4%
780
25%
780
11%
2046
December16th to endNight
15%
949
8%
2948
2%
949
21%
949
9%
2397

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
13%8%36%5%35%5%5%8%34%
49300
Night
15%8%36%5%3445%6%35%
48934
Cloud CoverDay
14%8%36%5%35%5%5%7%34%
49310
Night
15%8%36%5%3445%6%35%
48938
TransparencyDay
47%412%22%11%1
49000
Night
48%311%22%12%1
48752
SeeingDay
16%7%26%26%16%7%
6183
Night
19%8%26%29%14%2
16068

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 15thDay
18%14%6%8%7%45%5%46%18%
1556
16th to endDay
16%15%7%10%7%4446%5%17%
1872
February1st to 15thDay
17%14%48%7%4445%5%24%
1695
16th to endDay
14%11%410%7%35%5%5%6%23%
1580
March1st to 15thDay
18%10%48%6%35%45%5%27%
1900
16th to endDay
17%10%5%7%5%345%36%30%
2112
April1st to 15thDay
18%9%36%5%245%5%7%31%
2127
16th to endDay
17%7%25%5%33447%38%
2340
May1st to 15thDay
16%7%46%6%45%5%47%32%
2331
16th to endDay
12%6%25%5%25%45%8%41%
2431
June1st to 15thDay
10%5%26%5%35%5%7%8%40%
2607
16th to endDay
9%416%5%46%6%7%11%37%
2665
July1st to 15thDay
8%4244345%6%8%46%
2520
16th to endDay
7%5%24436%5%6%10%42%
2489
August1st to 15thDay
7%5%245%35%5%6%11%42%
2340
16th to endDay
9%424435%6%6%9%44%
2483
September1st to 15thDay
12%6%25%5%3345%9%41%
2009
16th to endDay
12%5%244345%5%9%42%
1980
October1st to 15thDay
13%5%26%5%35%5%5%7%39%
1866
16th to endDay
20%8%25%5%245%5%5%34%
1860
November1st to 15thDay
13%8%45%5%345%46%37%
1717
16th to endDay
16%11%46%6%35%46%7%26%
1638
December1st to 15thDay
17%12%7%10%6%35%446%20%
1560
16th to endDay
20%13%6%11%6%3445%5%16%
1622
January1st to 15thNight
21%12%47%6%34346%24%
2708
16th to endNight
15%14%6%10%7%3445%5%22%
2704
February1st to 15thNight
16%14%5%9%6%35%445%25%
2466
16th to endNight
16%12%49%7%25%445%27%
1893
March1st to 15thNight
18%10%36%6%45%446%30%
2037
16th to endNight
14%7%36%5%35%5%47%36%
2112
April1st to 15thNight
18%8%35%5%24447%34%
1830
16th to endNight
13%6%26%5%245%6%7%39%
1620
May1st to 15thNight
17%6%26%7%35%6%5%7%32%
1620
16th to endNight
11%435%435%36%7%44%
1682
June1st to 15thNight
10%325%5%35%5%6%7%44%
1347
16th to endNight
8%424425%5%6%9%44%
1290
July1st to 15thNight
6%425%42445%7%52%
1440
16th to endNight
5%3134245%6%10%50%
1676
August1st to 15thNight
6%325%4345%6%8%48%
1620
16th to endNight
9%3233245%5%8%51%
1719
September1st to 15thNight
10%4245%2445%6%48%
1935
16th to endNight
11%5%25%42345%7%46%
1980
October1st to 15thNight
15%5%16%5%2445%6%42%
2094
16th to endNight
19%6%36%5%35%445%34%
2362
November1st to 15thNight
15%7%25%5%3445%6%38%
2471
16th to endNight
21%9%36%5%2445%7%27%
2650
December1st to 15thNight
20%11%47%5%35%45%6%23%
2730
16th to endNight
21%14%6%10%6%345%45%17%
2948

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 15thDay
18%14%5%7%7%46%446%18%
1556
16th to endDay
16%15%6%10%8%3436%5%18%
1872
February1st to 15thDay
17%14%48%6%34446%23%
1695
16th to endDay
15%12%411%6%36%5%5%6%23%
1580
March1st to 15thDay
19%10%5%8%6%35%445%27%
1911
16th to endDay
17%10%5%7%6%345%46%29%
2111
April1st to 15thDay
18%9%36%43445%7%31%
2127
16th to endDay
17%7%25%433448%38%
2340
May1st to 15thDay
16%7%46%6%445%5%7%32%
2331
16th to endDay
11%6%25%5%245%5%9%41%
2431
June1st to 15thDay
10%425%5%35%5%7%7%41%
2607
16th to endDay
9%5%245%36%5%7%11%37%
2665
July1st to 15thDay
8%4244245%6%9%46%
2520
16th to endDay
7%5%245%36%5%7%10%42%
2489
August1st to 15thDay
7%4246%35%5%6%11%42%
2340
16th to endDay
9%4244346%6%9%44%
2483
September1st to 15thDay
12%6%245%2445%8%41%
2009
16th to endDay
12%5%2443445%9%42%
1980
October1st to 15thDay
13%6%26%5%345%5%6%40%
1866
16th to endDay
20%8%25%5%2445%5%33%
1860
November1st to 15thDay
14%8%35%5%25%5%46%37%
1717
16th to endDay
16%11%46%6%45%5%6%7%26%
1638
December1st to 15thDay
17%12%6%10%6%35%5%46%20%
1560
16th to endDay
21%13%6%11%6%35%45%6%15%
1622
January1st to 15thNight
22%11%47%6%34446%24%
2708
16th to endNight
16%13%7%9%6%3445%422%
2704
February1st to 15thNight
16%14%5%8%6%25%445%26%
2459
16th to endNight
16%12%5%9%6%25%435%28%
1893
March1st to 15thNight
18%10%36%6%35%345%30%
2049
16th to endNight
14%7%35%5%35%45%7%36%
2112
April1st to 15thNight
19%7%45%5%34447%34%
1830
16th to endNight
12%6%25%6%245%5%8%39%
1620
May1st to 15thNight
17%5%36%6%35%5%5%6%32%
1619
16th to endNight
11%435%436%35%7%44%
1682
June1st to 15thNight
10%415%5%35%5%5%8%44%
1347
16th to endNight
8%4245%35%5%6%9%44%
1290
July1st to 15thNight
7%42442445%7%52%
1440
16th to endNight
6%31343447%10%51%
1676
August1st to 15thNight
6%414435%5%6%8%48%
1620
16th to endNight
9%42332445%8%51%
1719
September1st to 15thNight
11%42342445%7%48%
1935
16th to endNight
11%425%42336%7%46%
1980
October1st to 15thNight
15%5%16%5%24447%42%
2094
16th to endNight
19%7%26%5%35%45%5%34%
2362
November1st to 15thNight
15%7%35%5%2445%6%38%
2471
16th to endNight
21%9%36%5%25%45%7%27%
2650
December1st to 15thNight
21%11%5%7%5%4445%6%23%
2730
16th to endNight
21%14%6%10%6%44445%17%
2948

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 15thDay
65%310%13%5%
1557
16th to endDay
66%413%11%5%
1872
February1st to 15thDay
62%49%16%6%
1697
16th to endDay
60%311%18%5%1
1580
March1st to 15thDay
59%412%16%7%
1911
16th to endDay
55%29%23%7%1
2112
April1st to 15thDay
51%411%20%11%
2127
16th to endDay
45%313%20%15%2
2340
May1st to 15thDay
50%410%21%11%2
2331
16th to endDay
39%413%25%14%2
2435
June1st to 15thDay
39%414%26%13%1
2607
16th to endDay
38%5%14%23%16%1
2665
July1st to 15thDay
33%5%9%28%20%3
2371
16th to endDay
33%6%15%26%15%3
2333
August1st to 15thDay
35%414%30%14%
2327
16th to endDay
33%5%14%27%16%2
2483
September1st to 15thDay
38%412%29%14%
2009
16th to endDay
37%311%31%14%
1980
October1st to 15thDay
41%412%25%13%1
1866
16th to endDay
50%315%20%8%
1860
November1st to 15thDay
46%412%22%11%1
1717
16th to endDay
56%315%19%5%
1638
December1st to 15thDay
64%411%14%5%
1560
16th to endDay
68%310%13%3
1622
January1st to 15thNight
61%210%15%9%
2708
16th to endNight
63%212%15%5%
2704
February1st to 15thNight
60%310%17%6%
2468
16th to endNight
58%29%20%7%
1893
March1st to 15thNight
55%310%20%9%1
2049
16th to endNight
47%311%24%11%1
2112
April1st to 15thNight
49%210%22%13%
1830
16th to endNight
41%311%21%17%4
1620
May1st to 15thNight
50%49%20%13%2
1620
16th to endNight
38%311%27%16%2
1684
June1st to 15thNight
37%410%30%16%
1347
16th to endNight
34%414%25%19%1
1290
July1st to 15thNight
31%48%29%23%2
1360
16th to endNight
27%316%27%20%4
1569
August1st to 15thNight
31%413%30%16%2
1609
16th to endNight
29%412%31%19%2
1719
September1st to 15thNight
34%412%27%18%2
1935
16th to endNight
35%310%31%17%1
1980
October1st to 15thNight
41%311%22%19%1
2094
16th to endNight
50%211%24%9%
2362
November1st to 15thNight
45%311%25%13%1
2471
16th to endNight
55%212%22%6%
2650
December1st to 15thNight
61%211%15%7%
2730
16th to endNight
68%310%12%4
2948

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 15thDay
22%15%38%14%6%1
135
16th to endDay
29%10%38%19%2
144
February1st to 15thDay
25%14%36%21%1
130
16th to endDay
18%16%46%17%1
81
March1st to 15thDay
30%12%27%22%5%
201
16th to endDay
22%9%30%25%9%2
288
April1st to 15thDay
24%10%28%23%11%2
270
16th to endDay
20%7%35%17%11%8%
270
May1st to 15thDay
23%428%23%14%4
288
16th to endDay
15%9%24%28%14%7%
331
June1st to 15thDay
11%6%28%35%14%3
414
16th to endDay
11%6%32%30%14%4
495
July1st to 15thDay
10%225%32%20%8%
475
16th to endDay
9%316%33%23%14%
349
August1st to 15thDay
10%114%27%29%17%
313
16th to endDay
10%314%25%26%19%
332
September1st to 15thDay
11%317%26%28%12%
315
16th to endDay
12%315%23%26%18%
314
October1st to 15thDay
17%6%21%26%20%7%
315
16th to endDay
21%11%34%23%6%1
276
November1st to 15thDay
14%13%26%28%14%2
135
16th to endDay
18%17%44%16%2
108
December1st to 15thDay
21%20%28%21%8%
90
16th to endDay
22%21%40%14%1
114
January1st to 15thNight
26%13%33%20%4
967
16th to endNight
21%14%33%24%6%
832
February1st to 15thNight
23%14%33%24%4
760
16th to endNight
22%13%30%27%5%
631
March1st to 15thNight
24%10%30%28%6%
720
16th to endNight
18%9%30%27%12%
768
April1st to 15thNight
21%12%29%29%5%
632
16th to endNight
15%7%34%30%12%1
540
May1st to 15thNight
19%6%25%35%12%1
540
16th to endNight
14%324%33%22%2
563
June1st to 15thNight
11%222%46%16%
369
16th to endNight
10%221%44%18%1
390
July1st to 15thNight
9%115%42%28%3
510
16th to endNight
6%214%44%27%3
564
August1st to 15thNight
6%112%41%30%7%
539
16th to endNight
10%112%34%36%4
573
September1st to 15thNight
12%415%31%29%6%
696
16th to endNight
13%216%30%28%8%
717
October1st to 15thNight
17%422%28%21%4
720
16th to endNight
22%11%32%26%7%
768
November1st to 15thNight
19%10%26%28%12%2
760
16th to endNight
28%12%29%19%10%
780
December1st to 15thNight
26%13%32%19%6%
780
16th to endNight
26%12%36%20%4
949
Page generated at 2018-02-15 10:58:49. Version ac04

Copyright 2018 A. Danko


Hallam Observatory Clear Sky Chart |Charts Home