Oxford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oxford Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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%
14680
14%
46837
8%
14650
35%
14650
16%
33345

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
13%
720
6%
2506
2%
716
19%
716
7%
2038
January16th to endNight
19%
768
10%
2634
6%
768
24%
768
10%
2027
February1st to 15thNight
14%
702
9%
2287
3%
701
22%
701
9%
1785
February16th to endNight
17%
631
12%
1940
5%
631
27%
631
12%
1488
March1st to 15thNight
19%
720
14%
2101
6%
720
27%
720
15%
1545
March16th to endNight
28%
768
20%
2032
8%
768
37%
768
21%
1416
April1st to 15thNight
23%
718
17%
1796
6%
718
29%
718
19%
1331
April16th to endNight
28%
663
18%
1743
6%
663
35%
663
22%
1220
May1st to 15thNight
34%
360
18%
1440
15%
360
45%
360
18%
932
May16th to endNight
31%
375
14%
1497
9%
374
44%
374
19%
935
June1st to 15thNight
33%
360
10%
1440
10%
360
41%
360
15%
966
June16th to endNight
39%
360
10%
1440
13%
360
45%
360
19%
894
July1st to 15thNight
49%
340
15%
1360
21%
340
55%
340
27%
750
July16th to endNight
53%
376
14%
1418
18%
354
58%
354
26%
812
August1st to 15thNight
47%
449
14%
1520
11%
447
52%
447
22%
969
August16th to endNight
49%
720
22%
1866
11%
720
55%
720
38%
1146
September1st to 15thNight
44%
720
17%
1800
7%
720
48%
720
32%
1209
September16th to endNight
44%
658
23%
1796
14%
658
49%
658
31%
1137
October1st to 15thNight
33%
660
18%
1980
12%
660
42%
660
20%
1366
October16th to endNight
23%
704
11%
2197
6%
704
30%
704
13%
1658
November1st to 15thNight
24%
700
17%
2316
12%
700
33%
700
16%
1615
November16th to endNight
20%
720
12%
2520
7%
720
27%
720
10%
1924
December1st to 15thNight
15%
720
7%
2520
3%
720
20%
720
8%
2002
December16th to endNight
13%
768
8%
2688
4%
768
18%
768
7%
2180

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
18%9%36%5%34446%32%
47913
Night
18%10%46%5%24446%32%
47006
Cloud CoverDay
18%9%36%5%34446%32%
47922
Night
18%10%37%5%24446%32%
47012
TransparencyDay
52%411%20%10%1
47600
Night
53%411%19%10%1
46837
SeeingDay
24%13%28%20%9%3
5495
Night
24%12%28%24%9%
14682

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
21%16%49%6%3434419%
1432
16th to endDay
15%14%6%9%9%35%345%22%
1590
February1st to 15thDay
15%14%6%11%8%35%43422%
1575
16th to endDay
19%14%6%7%6%4433424%
1532
March1st to 15thDay
18%12%5%8%7%3433426%
1847
16th to endDay
17%9%36%5%23235%40%
2191
April1st to 15thDay
22%11%35%423435%33%
2152
16th to endDay
19%8%36%5%23345%36%
2226
May1st to 15thDay
20%8%36%424447%34%
2511
16th to endDay
17%6%36%5%345%5%6%36%
2618
June1st to 15thDay
22%6%35%5%2445%7%30%
2520
16th to endDay
18%6%26%5%35%5%5%7%32%
2520
July1st to 15thDay
12%5%345%35%5%5%8%39%
2520
16th to endDay
11%5%25%5%46%6%7%8%36%
2654
August1st to 15thDay
12%6%37%6%45%5%5%7%35%
2427
16th to endDay
11%42443446%8%42%
2324
September1st to 15thDay
17%6%15%334447%41%
2160
16th to endDay
17%6%14423346%44%
1834
October1st to 15thDay
20%8%37%5%33445%34%
1650
16th to endDay
27%11%47%43343425%
1675
November1st to 15thDay
15%7%36%7%46%436%33%
1539
16th to endDay
22%13%5%7%5%34345%25%
1440
December1st to 15thDay
25%14%5%8%7%2433419%
1440
16th to endDay
26%16%6%7%5%2343319%
1536
January1st to 15thNight
20%15%6%10%7%2444419%
2506
16th to endNight
14%15%5%9%7%35%35%5%23%
2634
February1st to 15thNight
15%14%6%11%8%34345%22%
2285
16th to endNight
17%12%5%9%6%3434427%
1940
March1st to 15thNight
20%12%47%5%24335%30%
2088
16th to endNight
17%9%35%5%2333440%
2032
April1st to 15thNight
23%9%35%5%24335%34%
1796
16th to endNight
19%8%25%423446%37%
1743
May1st to 15thNight
20%7%35%424446%36%
1440
16th to endNight
16%6%25%5%2445%7%38%
1495
June1st to 15thNight
20%6%35%6%34447%32%
1440
16th to endNight
16%6%245%346%5%8%36%
1440
July1st to 15thNight
10%3245%35%45%9%45%
1440
16th to endNight
8%3145%25%6%7%10%44%
1514
August1st to 15thNight
11%5%25%5%345%6%8%39%
1530
16th to endNight
10%32442445%7%49%
1866
September1st to 15thNight
15%5%25%5%2335%5%45%
1800
16th to endNight
16%425%323345%47%
1796
October1st to 15thNight
20%6%35%5%2445%7%34%
1980
16th to endNight
26%10%36%5%23445%26%
2197
November1st to 15thNight
19%9%46%434345%33%
2316
16th to endNight
21%12%48%7%35%436%24%
2520
December1st to 15thNight
22%16%5%8%6%2434421%
2520
16th to endNight
25%16%5%8%7%3333419%
2688

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
21%16%410%7%2444418%
1432
16th to endDay
15%12%6%10%9%45%346%21%
1590
February1st to 15thDay
15%14%6%11%8%35%44422%
1575
16th to endDay
19%14%6%7%6%35%33424%
1532
March1st to 15thDay
18%13%5%8%8%3433326%
1857
16th to endDay
17%10%35%323235%39%
2190
April1st to 15thDay
23%10%45%42334432%
2152
16th to endDay
19%8%25%5%23345%36%
2226
May1st to 15thDay
20%7%35%5%2345%6%34%
2511
16th to endDay
17%6%36%5%34447%36%
2618
June1st to 15thDay
22%6%35%5%245%5%7%30%
2520
16th to endDay
18%6%26%5%35%5%5%7%32%
2520
July1st to 15thDay
12%5%25%5%35%46%8%39%
2520
16th to endDay
11%5%26%5%36%6%8%8%35%
2654
August1st to 15thDay
12%6%27%6%46%5%5%7%35%
2427
16th to endDay
10%5%245%35%46%8%42%
2324
September1st to 15thDay
17%5%24424447%40%
2160
16th to endDay
17%6%24423446%43%
1834
October1st to 15thDay
20%8%37%5%23446%34%
1650
16th to endDay
27%10%48%5%24445%24%
1675
November1st to 15thDay
15%7%46%6%37%5%46%33%
1539
16th to endDay
22%14%47%5%24346%25%
1440
December1st to 15thDay
24%16%5%8%6%3433418%
1440
16th to endDay
27%15%7%8%5%2343319%
1536
January1st to 15thNight
20%16%5%10%6%3443419%
2506
16th to endNight
14%15%6%9%8%34445%23%
2634
February1st to 15thNight
15%13%6%12%7%3444422%
2278
16th to endNight
17%13%6%9%5%34345%26%
1940
March1st to 15thNight
20%12%5%6%5%24235%30%
2101
16th to endNight
18%9%35%5%2324440%
2032
April1st to 15thNight
23%8%36%5%2334434%
1796
16th to endNight
19%8%245%23446%37%
1743
May1st to 15thNight
20%7%25%5%335%5%6%35%
1440
16th to endNight
16%6%25%5%245%5%7%38%
1495
June1st to 15thNight
21%6%36%5%345%5%6%32%
1440
16th to endNight
16%7%25%4346%6%8%36%
1440
July1st to 15thNight
10%3245%35%45%10%45%
1440
16th to endNight
8%415%435%5%6%9%44%
1514
August1st to 15thNight
11%6%26%6%345%6%8%39%
1530
16th to endNight
10%31442446%7%49%
1866
September1st to 15thNight
15%5%25%5%23336%45%
1800
16th to endNight
16%5%2442325%5%47%
1796
October1st to 15thNight
20%7%25%434448%34%
1980
16th to endNight
26%10%46%5%23345%26%
2197
November1st to 15thNight
18%9%46%5%24346%33%
2316
16th to endNight
21%12%47%8%25%435%24%
2520
December1st to 15thNight
22%17%48%5%2434421%
2520
16th to endNight
24%16%6%8%6%2423419%
2688

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
68%39%11%5%1
1433
16th to endDay
63%410%12%7%
1590
February1st to 15thDay
65%38%18%3
1577
16th to endDay
64%39%11%9%
1532
March1st to 15thDay
63%310%13%7%
1857
16th to endDay
48%38%18%15%5%
2191
April1st to 15thDay
54%36%18%13%2
2152
16th to endDay
49%38%19%16%3
2226
May1st to 15thDay
50%49%20%13%1
2511
16th to endDay
46%5%12%21%11%1
2622
June1st to 15thDay
51%5%11%21%9%
2520
16th to endDay
49%5%12%20%11%
2520
July1st to 15thDay
39%5%12%29%12%
2371
16th to endDay
43%7%15%23%9%
2487
August1st to 15thDay
46%5%14%22%9%
2413
16th to endDay
37%415%29%12%
2324
September1st to 15thDay
42%5%10%27%12%
2160
16th to endDay
40%410%32%11%
1834
October1st to 15thDay
51%312%21%10%
1650
16th to endDay
62%5%10%14%6%
1675
November1st to 15thDay
50%6%14%19%8%
1539
16th to endDay
61%411%13%7%
1440
December1st to 15thDay
69%411%12%2
1440
16th to endDay
69%5%9%10%5%
1536
January1st to 15thNight
68%5%10%10%5%
2506
16th to endNight
62%5%8%15%6%
2634
February1st to 15thNight
65%311%14%4
2287
16th to endNight
61%38%17%8%
1940
March1st to 15thNight
56%311%19%8%
2101
16th to endNight
48%29%19%15%3
2032
April1st to 15thNight
53%26%20%14%1
1796
16th to endNight
47%38%19%17%3
1743
May1st to 15thNight
48%48%19%14%4
1440
16th to endNight
43%5%12%21%13%2
1497
June1st to 15thNight
49%5%13%19%10%
1440
16th to endNight
43%6%13%21%14%
1440
July1st to 15thNight
33%5%13%30%15%
1360
16th to endNight
34%7%16%26%15%
1418
August1st to 15thNight
40%7%15%22%13%
1520
16th to endNight
32%415%32%14%
1866
September1st to 15thNight
40%411%31%11%1
1800
16th to endNight
38%311%33%12%
1796
October1st to 15thNight
48%410%23%12%
1980
16th to endNight
59%413%16%6%
2197
November1st to 15thNight
52%313%17%10%1
2316
16th to endNight
62%410%16%5%
2520
December1st to 15thNight
66%411%13%3
2520
16th to endNight
70%310%11%4
2688

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
28%26%23%13%8%
60
16th to endDay
36%19%28%9%5%
73
February1st to 15thDay
32%26%32%8%
90
16th to endDay
32%19%28%17%2
191
March1st to 15thDay
26%17%29%19%6%
270
16th to endDay
25%18%31%13%8%3
288
April1st to 15thDay
30%19%28%12%8%1
270
16th to endDay
26%17%27%16%7%4
270
May1st to 15thDay
30%13%25%18%9%2
288
16th to endDay
25%16%30%15%10%1
331
June1st to 15thDay
26%12%29%21%9%1
315
16th to endDay
20%9%35%20%11%2
315
July1st to 15thDay
19%7%27%27%14%4
305
16th to endDay
12%7%32%30%12%4
326
August1st to 15thDay
16%6%28%30%13%5%
313
16th to endDay
11%8%27%24%15%11%
332
September1st to 15thDay
21%5%24%22%13%13%
315
16th to endDay
20%8%31%18%13%7%
254
October1st to 15thDay
23%15%24%20%12%4
255
16th to endDay
36%12%27%18%4
257
November1st to 15thDay
19%15%31%22%8%2
193
16th to endDay
35%11%28%25%
60
December1st to 15thDay
28%20%36%8%6%
60
16th to endDay
48%25%23%11
64
January1st to 15thNight
28%19%28%18%4
720
16th to endNight
25%15%31%22%4
768
February1st to 15thNight
22%18%36%18%3
704
16th to endNight
25%16%34%19%4
631
March1st to 15thNight
28%15%31%21%2
720
16th to endNight
22%17%27%23%8%
768
April1st to 15thNight
29%14%28%22%4
718
16th to endNight
26%13%28%22%7%
663
May1st to 15thNight
25%6%28%28%8%1
360
16th to endNight
18%8%36%24%10%
375
June1st to 15thNight
21%9%25%34%9%
360
16th to endNight
18%5%30%32%12%
360
July1st to 15thNight
12%6%25%41%13%
340
16th to endNight
9%227%43%15%1
376
August1st to 15thNight
15%324%36%18%1
449
16th to endNight
12%426%35%19%1
720
September1st to 15thNight
19%426%28%17%2
720
16th to endNight
18%8%25%25%21%1
658
October1st to 15thNight
24%10%27%23%13%1
660
16th to endNight
32%12%25%22%6%
704
November1st to 15thNight
24%13%29%20%10%
700
16th to endNight
31%15%25%19%8%
720
December1st to 15thNight
33%19%27%15%3
720
16th to endNight
36%13%28%17%2
768
Page generated at 2017-09-16 17:06:59. Version ac04

Copyright 2017 A. Danko


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