Ewell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ewell Observatory clear sky chart from 2005-11-05 to 2017-12-02.

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
64%
15722
23%
47854
21%
15685
68%
15685
34%
27676

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
61%
717
17%
2366
17%
716
64%
716
27%
1461
January16th to endNight
58%
768
24%
2496
25%
768
61%
768
26%
1574
February1st to 15thNight
54%
700
22%
2288
21%
700
59%
700
24%
1463
February16th to endNight
50%
632
20%
1903
17%
632
56%
632
24%
1258
March1st to 15thNight
53%
720
17%
1995
15%
720
56%
720
26%
1340
March16th to endNight
51%
768
19%
2095
14%
768
55%
768
28%
1395
April1st to 15thNight
49%
719
20%
1799
11%
719
53%
719
30%
1204
April16th to endNight
57%
720
23%
1800
11%
720
61%
720
42%
1065
May1st to 15thNight
68%
608
23%
1688
16%
608
70%
608
43%
935
May16th to endNight
63%
411
19%
1534
17%
411
69%
411
29%
840
June1st to 15thNight
69%
360
20%
1440
20%
360
75%
360
32%
719
June16th to endNight
85%
360
31%
1440
40%
360
87%
360
52%
555
July1st to 15thNight
85%
401
25%
1421
28%
401
87%
401
56%
579
July16th to endNight
78%
564
24%
1593
23%
531
80%
531
44%
811
August1st to 15thNight
79%
550
22%
1623
18%
547
79%
547
44%
827
August16th to endNight
79%
764
26%
1910
16%
764
80%
764
52%
1051
September1st to 15thNight
81%
720
33%
1800
23%
720
83%
720
57%
953
September16th to endNight
77%
716
36%
1969
32%
716
80%
716
55%
984
October1st to 15thNight
73%
720
26%
2160
28%
720
77%
720
43%
1146
October16th to endNight
71%
768
31%
2304
33%
768
74%
768
38%
1291
November1st to 15thNight
67%
760
24%
2355
25%
760
69%
760
35%
1353
November16th to endNight
64%
780
23%
2639
25%
780
66%
780
30%
1521
December1st to 15thNight
54%
728
20%
2548
19%
728
59%
728
22%
1618
December16th to endNight
46%
768
19%
2688
17%
768
52%
768
21%
1733

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
6%424323347%55%
48590
Night
7%42442345%8%52%
48043
TransparencyDay
27%7%11%17%21%13%
48279
Night
29%7%12%17%19%13%
47855
SeeingDay
8%9%23%33%19%5%
6317
Night
9%413%30%33%8%
15731

By Month

Percentage Cloud Cover SW (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
13%5%25%42345%7%45%
1569
16th to endDay
13%6%245%345%46%43%
1728
February1st to 15thDay
13%8%35%42445%5%41%
1576
16th to endDay
14%6%36%5%3335%7%38%
1571
March1st to 15thDay
10%5%37%5%25%46%10%37%
1962
16th to endDay
9%5%16%7%345%5%8%42%
2127
April1st to 15thDay
11%5%25%42446%7%44%
2149
16th to endDay
6%423313348%58%
2160
May1st to 15thDay
331431325%9%61%
2260
16th to endDay
5%7%34322348%54%
2575
June1st to 15thDay
442331445%8%58%
2520
16th to endDay
1212112237%72%
2520
July1st to 15thDay
212223338%67%
2451
16th to endDay
212322248%68%
2468
August1st to 15thDay
1313323238%64%
2328
16th to endDay
324222248%67%
2292
September1st to 15thDay
1313322237%68%
2160
16th to endDay
3222312238%68%
1989
October1st to 15thDay
5%323323336%63%
1800
16th to endDay
6%413433345%57%
1920
November1st to 15thDay
7%5%333345%5%7%50%
1833
16th to endDay
10%6%35%5%33346%47%
1648
December1st to 15thDay
12%6%46%6%3345%6%39%
1448
16th to endDay
15%6%45%5%2335%7%39%
1536
January1st to 15thNight
12%6%25%42446%8%42%
2366
16th to endNight
13%6%35%42335%6%45%
2496
February1st to 15thNight
10%6%35%5%3445%6%43%
2276
16th to endNight
13%7%25%42445%9%41%
1903
March1st to 15thNight
13%5%36%434448%42%
1995
16th to endNight
10%6%34434446%46%
2095
April1st to 15thNight
9%4245%3446%9%45%
1799
16th to endNight
5%313323347%59%
1800
May1st to 15thNight
231431335%10%59%
1688
16th to endNight
4415%42345%9%54%
1533
June1st to 15thNight
322341336%11%57%
1440
16th to endNight
111212247%73%
1440
July1st to 15thNight
2123123413%62%
1506
16th to endNight
2244245%7%14%48%
1701
August1st to 15thNight
131344447%15%47%
1632
16th to endNight
1135%2337%15%53%
1910
September1st to 15thNight
111342335%10%63%
1800
16th to endNight
1212322247%68%
1969
October1st to 15thNight
4323323336%63%
2160
16th to endNight
5%41442345%6%56%
2304
November1st to 15thNight
7%423423348%54%
2355
16th to endNight
8%5%24424446%50%
2639
December1st to 15thNight
12%6%26%5%24446%43%
2548
16th to endNight
16%7%34423445%43%
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
38%6%11%18%15%9%
1569
16th to endDay
41%7%8%12%18%12%
1728
February1st to 15thDay
43%6%8%17%20%4
1576
16th to endDay
46%6%7%22%12%3
1571
March1st to 15thDay
42%8%11%18%13%5%
1962
16th to endDay
41%8%14%18%14%2
2127
April1st to 15thDay
39%8%13%19%16%2
2149
16th to endDay
26%7%15%23%20%6%
2160
May1st to 15thDay
20%10%11%23%26%6%
2260
16th to endDay
29%10%13%21%19%4
2579
June1st to 15thDay
24%10%14%18%20%12%
2520
16th to endDay
12%6%10%16%31%23%
2520
July1st to 15thDay
12%7%13%9%18%37%
2307
16th to endDay
12%6%12%11%23%33%
2313
August1st to 15thDay
16%8%12%10%27%25%
2312
16th to endDay
14%6%11%14%32%21%
2292
September1st to 15thDay
15%6%8%13%27%28%
2160
16th to endDay
17%47%23%29%17%
1989
October1st to 15thDay
22%5%9%18%24%20%
1800
16th to endDay
27%5%12%20%20%13%
1920
November1st to 15thDay
32%7%12%25%15%6%
1833
16th to endDay
37%6%10%16%24%4
1648
December1st to 15thDay
44%6%11%15%17%4
1448
16th to endDay
45%412%16%16%4
1536
January1st to 15thNight
38%6%13%18%15%9%
2366
16th to endNight
40%6%9%11%16%15%
2496
February1st to 15thNight
40%7%9%17%17%7%
2288
16th to endNight
40%8%12%23%12%2
1903
March1st to 15thNight
41%6%12%18%14%5%
1995
16th to endNight
39%7%12%18%17%5%
2095
April1st to 15thNight
34%10%15%20%16%2
1799
16th to endNight
24%7%14%25%19%8%
1800
May1st to 15thNight
20%10%12%20%25%10%
1689
16th to endNight
26%11%16%20%18%5%
1534
June1st to 15thNight
20%13%15%17%20%12%
1440
16th to endNight
11%6%10%14%31%25%
1440
July1st to 15thNight
10%7%17%10%19%34%
1421
16th to endNight
17%10%20%10%13%26%
1593
August1st to 15thNight
18%9%20%9%20%21%
1623
16th to endNight
15%10%18%10%22%22%
1910
September1st to 15thNight
14%8%11%11%23%30%
1800
16th to endNight
16%5%9%21%26%20%
1969
October1st to 15thNight
23%6%8%18%21%22%
2160
16th to endNight
25%8%9%19%21%16%
2304
November1st to 15thNight
29%7%14%25%16%6%
2355
16th to endNight
33%6%10%22%19%8%
2639
December1st to 15thNight
40%8%9%15%19%6%
2548
16th to endNight
43%6%11%15%16%6%
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
21%7%21%30%14%6%
198
16th to endDay
14%10%22%28%18%5%
256
February1st to 15thDay
17%13%23%22%18%3
236
16th to endDay
18%13%23%28%14%1
237
March1st to 15thDay
15%16%26%22%15%2
270
16th to endDay
11%16%35%27%7%1
288
April1st to 15thDay
14%18%36%20%8%1
269
16th to endDay
7%22%32%25%8%2
270
May1st to 15thDay
6%13%36%30%10%1
288
16th to endDay
10%15%25%29%13%5%
330
June1st to 15thDay
6%13%30%36%12%1
315
16th to endDay
1321%43%24%6%
315
July1st to 15thDay
1217%51%22%4
305
16th to endDay
216%45%26%8%
327
August1st to 15thDay
1218%43%26%6%
315
16th to endDay
1115%46%27%8%
332
September1st to 15thDay
1415%43%25%9%
315
16th to endDay
47%19%33%25%9%
315
October1st to 15thDay
5%7%23%37%19%7%
315
16th to endDay
8%7%22%27%23%9%
336
November1st to 15thDay
10%6%19%27%26%10%
249
16th to endDay
8%9%25%28%28%
110
December1st to 15thDay
14%11%30%25%16%1
62
16th to endDay
10%15%14%23%28%7%
64
January1st to 15thNight
15%5%11%30%30%7%
720
16th to endNight
16%5%12%29%27%8%
768
February1st to 15thNight
12%8%16%30%27%5%
704
16th to endNight
16%6%19%30%21%5%
632
March1st to 15thNight
14%7%15%27%29%5%
720
16th to endNight
13%8%20%32%22%3
768
April1st to 15thNight
11%9%22%32%20%4
719
16th to endNight
7%8%21%29%25%7%
720
May1st to 15thNight
4317%38%29%7%
609
16th to endNight
6%5%17%33%28%8%
412
June1st to 15thNight
3219%38%32%3
360
16th to endNight
118%38%46%3
360
July1st to 15thNight
16%24%53%13%
401
16th to endNight
117%19%51%17%
564
August1st to 15thNight
426%23%51%12%
550
16th to endNight
117%29%48%11%
764
September1st to 15thNight
37%24%48%15%
720
16th to endNight
3210%27%42%13%
716
October1st to 15thNight
6%211%31%38%10%
720
16th to endNight
8%39%27%34%16%
768
November1st to 15thNight
8%213%32%34%9%
760
16th to endNight
11%312%34%29%8%
780
December1st to 15thNight
15%6%14%31%24%7%
728
16th to endNight
18%7%19%29%20%3
768
Page generated at 2017-12-03 08:16:37. Version ac04

Copyright 2017 A. Danko


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