Durham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Durham 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
28%
23234
11%
74750
8%
23184
33%
23184
14%
56570

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
10%
948
5%
2819
1%
946
13%
946
5%
2487
January11th to 20thNight
11%
791
3%
2464
2%
791
12%
791
4%
2189
January21st to endNight
16%
792
5%
2574
3%
792
18%
792
6%
2215
February1st to 10thNight
11%
720
4%
2340
1%
720
15%
720
5%
2057
February11th to 20thNight
16%
700
7%
2275
2%
700
21%
700
9%
1864
February21st to endNight
13%
592
10%
1821
2%
592
22%
592
9%
1481
March1st to 10thNight
18%
720
13%
2025
6%
720
24%
720
14%
1554
March11th to 20thNight
21%
720
16%
1980
7%
720
29%
720
15%
1509
March21st to endNight
29%
792
15%
2178
7%
792
35%
792
19%
1610
April1st to 10thNight
23%
720
12%
1886
4%
720
28%
720
16%
1456
April11th to 20thNight
33%
569
18%
1633
10%
569
37%
569
21%
1142
April21st to endNight
30%
540
14%
1620
8%
540
34%
540
17%
1183
May1st to 10thNight
38%
540
15%
1620
10%
540
43%
540
22%
1110
May11th to 20thNight
41%
438
19%
1500
15%
438
49%
438
22%
947
May21st to endNight
47%
388
17%
1551
15%
388
59%
388
25%
885
June1st to 10thNight
39%
360
14%
1382
13%
360
48%
360
22%
839
June11th to 20thNight
47%
360
17%
1260
16%
360
55%
360
24%
757
June21st to endNight
46%
360
17%
1260
17%
360
53%
360
25%
774
July1st to 10thNight
63%
350
19%
1225
23%
350
68%
350
36%
639
July11th to 20thNight
53%
396
19%
1337
21%
396
58%
396
30%
779
July21st to endNight
56%
591
15%
1672
12%
558
57%
558
29%
1053
August1st to 10thNight
59%
537
18%
1600
14%
534
61%
534
31%
975
August11th to 20thNight
61%
534
22%
1598
16%
534
65%
534
32%
958
August21st to endNight
50%
608
19%
1796
13%
608
53%
608
26%
1186
September1st to 10thNight
50%
720
16%
1804
9%
720
52%
720
31%
1214
September11th to 20thNight
50%
720
23%
1980
15%
720
54%
720
32%
1240
September21st to endNight
43%
720
17%
1980
12%
720
47%
720
23%
1378
October1st to 10thNight
36%
720
15%
2145
12%
720
41%
720
20%
1516
October11th to 20thNight
26%
720
11%
2160
6%
720
32%
720
13%
1672
October21st to endNight
19%
792
10%
2413
6%
792
25%
792
10%
1941
November1st to 10thNight
24%
740
13%
2447
10%
740
30%
740
13%
1852
November11th to 20thNight
18%
760
10%
2660
7%
760
24%
760
9%
2100
November21st to endNight
15%
760
7%
2660
6%
760
19%
760
7%
2188
December1st to 10thNight
9%
760
3%
2660
2%
760
11%
760
3%
2376
December11th to 20thNight
8%
760
3%
3271
1%
759
12%
759
4%
2747
December21st to endNight
6%
996
2%
5154
0%
985
9%
985
2%
4697

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%12%5%8%6%35%45%7%29%
74841
Night
16%13%5%9%7%35%45%6%27%
74954
TransparencyDay
56%6%10%17%10%1
74548
Night
58%6%10%16%9%1
74782
SeeingDay
21%13%29%22%11%4
13923
Night
22%11%30%26%10%1
23248

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
15%17%11%16%10%5%5%33410%
1346
11th to 20thDay
15%21%10%16%10%4433311%
1618
21st to endDay
12%19%10%15%10%5%5%45%411%
1782
February1st to 10thDay
14%20%8%17%12%4433310%
1620
11th to 20thDay
14%15%9%16%11%4445%5%13%
1573
21st to endDay
16%20%8%11%9%45%43415%
1432
March1st to 10thDay
15%14%412%10%45%345%25%
1931
11th to 20thDay
13%12%7%10%8%45%5%45%27%
1980
21st to endDay
19%12%5%7%7%345%5%5%29%
2178
April1st to 10thDay
18%15%6%9%5%25%45%6%25%
2074
11th to 20thDay
20%11%45%43345%6%35%
2292
21st to endDay
21%11%46%5%33447%32%
2340
May1st to 10thDay
19%8%46%5%245%5%7%35%
2340
11th to 20thDay
17%9%35%6%35%45%6%38%
2392
21st to endDay
14%6%35%5%35%5%6%8%41%
2725
June1st to 10thDay
15%8%35%5%35%46%8%38%
2574
11th to 20thDay
14%5%345%35%5%6%9%42%
2700
21st to endDay
17%6%25%5%245%6%8%39%
2700
July1st to 10thDay
11%4345%35%5%6%10%44%
2700
11th to 20thDay
12%7%36%6%36%6%6%8%36%
2576
21st to endDay
12%6%35%6%36%6%7%10%36%
2547
August1st to 10thDay
12%6%27%6%35%6%7%10%36%
2327
11th to 20thDay
12%5%36%7%46%6%7%9%35%
2303
21st to endDay
13%6%37%6%45%5%6%9%37%
2540
September1st to 10thDay
15%8%27%6%345%6%7%37%
2152
11th to 20thDay
15%5%36%5%3446%8%42%
1980
21st to endDay
18%7%46%5%35%5%5%8%33%
1980
October1st to 10thDay
21%10%47%5%34446%32%
1814
11th to 20thDay
26%14%5%7%6%45%446%20%
1800
21st to endDay
28%17%48%5%45%445%17%
1940
November1st to 10thDay
18%18%48%6%44345%26%
1630
11th to 20thDay
18%17%7%10%8%345%5%6%18%
1520
21st to endDay
20%22%9%9%6%34445%13%
1520
December1st to 10thDay
16%25%13%15%7%35%3347%
1520
11th to 20thDay
19%20%9%13%7%4444411%
1906
21st to endDay
25%22%9%14%6%333348%
2489
January1st to 10thNight
16%19%9%16%10%45%34411%
2819
11th to 20thNight
19%19%8%14%10%35%33410%
2464
21st to endNight
12%16%8%14%12%6%6%5%4412%
2574
February1st to 10thNight
14%19%9%13%12%5%5%44311%
2340
11th to 20thNight
14%15%8%14%10%45%445%17%
2273
21st to endNight
16%14%8%11%8%35%5%45%20%
1821
March1st to 10thNight
13%12%6%12%8%35%44428%
2015
11th to 20thNight
13%14%6%8%7%44445%30%
1979
21st to endNight
17%11%5%8%5%5%5%345%32%
2178
April1st to 10thNight
19%12%6%8%5%34446%28%
1886
11th to 20thNight
18%11%45%435%45%6%36%
1633
21st to endNight
17%12%47%5%4445%5%31%
1620
May1st to 10thNight
16%9%35%5%35%5%5%6%37%
1620
11th to 20thNight
15%7%35%5%3446%6%41%
1499
21st to endNight
10%426%435%5%5%8%47%
1550
June1st to 10thNight
14%6%35%6%25%46%8%40%
1382
11th to 20thNight
13%425%5%346%7%9%42%
1260
21st to endNight
14%7%35%43446%7%44%
1260
July1st to 10thNight
7%5%2442346%8%54%
1260
11th to 20thNight
11%6%25%43446%9%45%
1379
21st to endNight
10%5%35%5%35%5%6%8%45%
1771
August1st to 10thNight
11%5%25%435%45%8%48%
1611
11th to 20thNight
10%425%5%345%6%9%47%
1598
21st to endNight
13%6%26%6%45%5%47%42%
1796
September1st to 10thNight
12%6%25%5%35%46%8%44%
1804
11th to 20thNight
14%5%25%43345%8%47%
1980
21st to endNight
16%7%37%6%44446%38%
1980
October1st to 10thNight
19%8%37%5%3445%7%35%
2145
11th to 20thNight
25%12%37%6%345%5%5%24%
2160
21st to endNight
26%14%47%7%445%5%5%20%
2413
November1st to 10thNight
18%15%48%6%45%445%26%
2447
11th to 20thNight
18%16%6%9%6%45%5%5%5%20%
2660
21st to endNight
21%19%7%9%5%4445%5%16%
2660
December1st to 10thNight
20%24%10%12%7%25%3349%
2660
11th to 20thNight
15%21%10%13%7%3444413%
3288
21st to endNight
23%22%9%15%8%343337%
5169

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
79%48%7%2
1347
11th to 20thDay
80%45%6%41
1618
21st to endDay
76%47%8%5%1
1782
February1st to 10thDay
81%448%21
1620
11th to 20thDay
74%46%9%5%1
1582
21st to endDay
73%5%6%11%5%
1432
March1st to 10thDay
62%411%13%8%1
1934
11th to 20thDay
59%6%9%15%9%2
1980
21st to endDay
56%6%9%16%11%2
2178
April1st to 10thDay
61%5%8%16%9%
2074
11th to 20thDay
51%5%9%20%14%1
2292
21st to endDay
54%5%10%17%11%2
2340
May1st to 10thDay
49%6%9%21%14%1
2340
11th to 20thDay
48%5%10%22%13%2
2396
21st to endDay
41%6%15%22%14%2
2725
June1st to 10thDay
44%7%11%22%15%1
2574
11th to 20thDay
40%7%11%25%16%1
2700
21st to endDay
42%8%13%23%13%1
2700
July1st to 10thDay
35%8%12%23%19%2
2626
11th to 20thDay
41%9%12%26%12%1
2495
21st to endDay
42%8%12%23%13%2
2405
August1st to 10thDay
41%9%15%23%10%2
2314
11th to 20thDay
44%8%14%22%11%
2303
21st to endDay
43%9%13%22%11%1
2540
September1st to 10thDay
45%8%11%22%11%3
2152
11th to 20thDay
40%7%13%30%11%
1980
21st to endDay
49%7%12%24%7%1
1980
October1st to 10thDay
54%6%10%16%12%2
1814
11th to 20thDay
65%6%9%13%5%1
1800
21st to endDay
70%6%8%11%5%
1940
November1st to 10thDay
61%6%10%16%7%
1630
11th to 20thDay
67%7%10%9%6%1
1520
21st to endDay
73%6%9%10%2
1520
December1st to 10thDay
83%5%5%6%1
1520
11th to 20thDay
76%5%9%7%3
1906
21st to endDay
82%6%6%5%1
2489
January1st to 10thNight
79%47%7%3
2819
11th to 20thNight
79%47%44
2464
21st to endNight
75%6%8%6%5%1
2574
February1st to 10thNight
78%5%7%9%1
2340
11th to 20thNight
70%5%9%12%5%
2275
21st to endNight
67%5%7%13%7%
1821
March1st to 10thNight
59%5%10%16%9%1
2025
11th to 20thNight
56%7%7%17%11%2
1980
21st to endNight
55%49%17%14%2
2178
April1st to 10thNight
58%5%7%17%12%1
1886
11th to 20thNight
49%5%8%20%17%1
1633
21st to endNight
54%6%9%15%14%2
1620
May1st to 10thNight
47%5%8%22%15%3
1620
11th to 20thNight
43%6%8%24%16%3
1500
21st to endNight
34%8%14%26%15%4
1551
June1st to 10thNight
41%6%11%25%16%2
1382
11th to 20thNight
37%7%13%25%16%1
1260
21st to endNight
39%7%11%27%15%2
1260
July1st to 10thNight
26%7%13%28%23%3
1225
11th to 20thNight
35%5%12%30%16%1
1337
21st to endNight
37%7%13%25%16%2
1672
August1st to 10thNight
35%7%15%29%14%1
1600
11th to 20thNight
32%8%14%29%15%2
1598
21st to endNight
41%7%10%27%13%1
1796
September1st to 10thNight
38%8%12%28%13%2
1804
11th to 20thNight
36%6%13%29%15%1
1980
21st to endNight
46%7%12%24%10%1
1980
October1st to 10thNight
49%7%11%19%14%1
2145
11th to 20thNight
60%6%11%17%6%
2160
21st to endNight
65%5%10%12%7%
2413
November1st to 10thNight
58%7%10%18%7%1
2447
11th to 20thNight
64%6%11%12%5%2
2660
21st to endNight
70%6%12%10%2
2660
December1st to 10thNight
80%5%7%6%2
2660
11th to 20thNight
74%5%11%9%2
3288
21st to endNight
83%5%6%5%1
5169

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%19%36%17%41
189
11th to 20thDay
19%25%38%12%5%
257
21st to endDay
25%17%36%18%3
264
February1st to 10thDay
29%24%35%8%3
240
11th to 20thDay
25%24%40%10%1
240
21st to endDay
31%32%29%7%1
198
March1st to 10thDay
25%18%37%13%6%1
374
11th to 20thDay
25%14%38%16%6%1
420
21st to endDay
29%13%32%18%6%1
462
April1st to 10thDay
25%17%33%19%5%1
420
11th to 20thDay
29%17%25%16%9%4
412
21st to endDay
28%12%29%22%7%2
420
May1st to 10thDay
25%11%28%24%9%2
423
11th to 20thDay
20%10%29%22%15%4
447
21st to endDay
19%9%24%28%15%5%
485
June1st to 10thDay
17%12%26%31%13%2
504
11th to 20thDay
16%8%32%29%11%3
630
21st to endDay
18%8%33%27%12%2
630
July1st to 10thDay
11%7%27%29%20%5%
621
11th to 20thDay
14%8%28%28%14%8%
546
21st to endDay
10%5%31%33%15%6%
488
August1st to 10thDay
13%8%22%29%18%10%
446
11th to 20thDay
16%420%26%23%10%
439
21st to endDay
16%6%25%27%18%8%
495
September1st to 10thDay
17%9%25%27%14%9%
450
11th to 20thDay
11%8%22%27%23%9%
450
21st to endDay
17%13%26%24%14%5%
450
October1st to 10thDay
24%10%28%20%15%3
450
11th to 20thDay
30%20%29%18%4
450
21st to endDay
37%16%27%16%4
455
November1st to 10thDay
32%14%31%16%7%
250
11th to 20thDay
26%22%38%10%23
180
21st to endDay
32%19%37%11%1
180
December1st to 10thDay
31%26%28%14%1
180
11th to 20thDay
29%24%33%10%21
180
21st to endDay
35%19%35%10%1
198
January1st to 10thNight
24%17%36%18%5%1
950
11th to 20thNight
29%17%31%19%41
791
21st to endNight
19%12%36%26%6%
792
February1st to 10thNight
24%19%37%18%3
720
11th to 20thNight
22%15%36%22%4
703
21st to endNight
24%20%35%19%3
592
March1st to 10thNight
20%16%38%20%6%1
720
11th to 20thNight
21%16%37%20%6%1
720
21st to endNight
23%11%30%25%8%1
792
April1st to 10thNight
26%13%32%25%4
720
11th to 20thNight
24%14%28%23%10%2
569
21st to endNight
24%9%31%26%9%
540
May1st to 10thNight
21%6%29%32%11%1
540
11th to 20thNight
19%5%28%36%11%1
438
21st to endNight
13%429%36%17%
389
June1st to 10thNight
17%433%35%11%1
360
11th to 20thNight
13%8%28%39%12%
360
21st to endNight
18%6%25%36%14%1
360
July1st to 10thNight
8%418%51%18%1
350
11th to 20thNight
15%323%38%20%1
396
21st to endNight
12%322%43%19%1
591
August1st to 10thNight
12%318%38%29%1
537
11th to 20thNight
12%316%38%28%3
534
21st to endNight
17%323%29%26%2
608
September1st to 10thNight
15%5%24%33%18%5%
720
11th to 20thNight
16%424%34%18%5%
720
21st to endNight
20%7%23%31%15%4
720
October1st to 10thNight
22%8%28%26%14%1
720
11th to 20thNight
30%12%26%24%7%1
720
21st to endNight
32%13%31%20%31
792
November1st to 10thNight
27%12%30%23%7%1
740
11th to 20thNight
27%18%31%18%6%1
760
21st to endNight
29%15%34%19%3
760
December1st to 10thNight
31%18%32%15%4
760
11th to 20thNight
23%20%39%15%3
760
21st to endNight
32%16%34%16%2
1004
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