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 2017-04-30.

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%
15012
23%
45680
21%
14975
67%
14975
34%
26601

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%
716
23%
1790
11%
716
61%
716
42%
1061
May1st to 15thNight
68%
557
24%
1547
16%
557
70%
557
43%
864
May16th to endNight
63%
376
19%
1403
15%
376
70%
376
30%
760
June1st to 15thNight
69%
330
21%
1320
21%
330
76%
330
31%
667
June16th to endNight
84%
330
29%
1320
38%
330
87%
330
51%
515
July1st to 15thNight
84%
365
26%
1295
30%
365
86%
365
54%
537
July16th to endNight
78%
522
22%
1467
21%
489
80%
489
43%
759
August1st to 15thNight
79%
504
23%
1487
19%
501
79%
501
45%
751
August16th to endNight
78%
700
25%
1750
15%
700
79%
700
51%
974
September1st to 15thNight
82%
660
34%
1650
24%
660
83%
660
59%
862
September16th to endNight
76%
656
33%
1804
30%
656
79%
656
54%
917
October1st to 15thNight
71%
660
26%
1980
28%
660
75%
660
40%
1086
October16th to endNight
70%
704
29%
2112
30%
704
73%
704
37%
1202
November1st to 15thNight
67%
700
26%
2169
27%
700
70%
700
36%
1231
November16th to endNight
64%
720
23%
2436
25%
720
67%
720
31%
1385
December1st to 15thNight
53%
720
20%
2520
19%
720
59%
720
22%
1602
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%424423347%54%
46044
Night
7%42442345%8%51%
45869
TransparencyDay
28%7%11%17%21%13%
45733
Night
29%7%12%17%19%12%
45681
SeeingDay
8%10%24%34%17%4
5493
Night
9%414%30%32%8%
15021

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%423323348%58%
2150
May1st to 15thDay
341432325%9%60%
2071
16th to endDay
5%7%34322348%53%
2354
June1st to 15thDay
442331445%8%57%
2310
16th to endDay
2212112237%72%
2310
July1st to 15thDay
222224338%66%
2247
16th to endDay
213323348%66%
2286
August1st to 15thDay
313323248%65%
2134
16th to endDay
324222248%66%
2100
September1st to 15thDay
1313322237%69%
1980
16th to endDay
3322312138%67%
1824
October1st to 15thDay
5%323423336%61%
1650
16th to endDay
7%5%13433346%57%
1760
November1st to 15thDay
7%5%333345%5%7%51%
1689
16th to endDay
10%6%35%5%33345%47%
1521
December1st to 15thDay
12%7%46%6%3345%6%39%
1440
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%
1790
May1st to 15thNight
231431345%11%58%
1547
16th to endNight
441442345%8%55%
1402
June1st to 15thNight
222341347%12%55%
1320
16th to endNight
111312237%72%
1320
July1st to 15thNight
121231335%14%60%
1380
16th to endNight
2245%25%5%8%15%46%
1575
August1st to 15thNight
13134445%8%15%47%
1496
16th to endNight
2135%2348%15%52%
1750
September1st to 15thNight
111342335%10%63%
1650
16th to endNight
2213322247%67%
1804
October1st to 15thNight
5%323423346%60%
1980
16th to endNight
5%41443345%6%55%
2112
November1st to 15thNight
7%423423348%55%
2169
16th to endNight
7%5%24424446%51%
2436
December1st to 15thNight
12%6%26%5%24446%43%
2520
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%24%20%6%
2150
May1st to 15thDay
22%11%11%22%25%7%
2071
16th to endDay
30%10%14%22%18%4
2358
June1st to 15thDay
25%10%14%17%19%13%
2310
16th to endDay
12%6%9%16%33%21%
2310
July1st to 15thDay
13%7%15%10%19%35%
2103
16th to endDay
12%7%12%11%25%30%
2131
August1st to 15thDay
15%8%11%10%27%26%
2118
16th to endDay
14%6%11%14%33%20%
2100
September1st to 15thDay
14%5%9%12%27%29%
1980
16th to endDay
17%47%22%28%19%
1824
October1st to 15thDay
24%5%9%19%25%15%
1650
16th to endDay
27%5%11%21%20%12%
1760
November1st to 15thDay
32%6%12%25%15%7%
1689
16th to endDay
37%6%10%15%25%4
1521
December1st to 15thDay
44%6%11%15%17%4
1440
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%
1790
May1st to 15thNight
21%10%12%18%25%11%
1548
16th to endNight
25%11%17%20%18%5%
1403
June1st to 15thNight
20%13%16%16%18%14%
1320
16th to endNight
11%6%10%14%32%24%
1320
July1st to 15thNight
10%7%16%11%20%32%
1295
16th to endNight
18%11%21%10%14%23%
1467
August1st to 15thNight
18%8%20%10%20%21%
1487
16th to endNight
16%10%17%10%22%22%
1750
September1st to 15thNight
14%6%12%10%24%31%
1650
16th to endNight
17%5%9%20%26%20%
1804
October1st to 15thNight
25%7%9%19%21%17%
1980
16th to endNight
27%8%8%20%21%13%
2112
November1st to 15thNight
28%7%14%25%16%7%
2169
16th to endNight
32%6%10%22%19%8%
2436
December1st to 15thNight
40%8%8%15%20%6%
2520
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%23%32%25%7%2
267
May1st to 15thDay
8%12%38%30%9%1
228
16th to endDay
11%16%27%27%12%3
266
June1st to 15thDay
411%32%40%9%1
255
16th to endDay
1423%44%21%4
255
July1st to 15thDay
2219%53%19%3
245
16th to endDay
215%48%27%6%
271
August1st to 15thDay
121%46%25%3
255
16th to endDay
1116%51%22%5%
268
September1st to 15thDay
412%47%25%9%
255
16th to endDay
37%19%33%26%9%
255
October1st to 15thDay
6%7%20%39%19%6%
255
16th to endDay
10%7%26%29%20%5%
272
November1st to 15thDay
9%6%18%24%29%10%
195
16th to endDay
5%13%27%27%26%
73
December1st to 15thDay
15%11%31%23%16%1
60
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%
716
May1st to 15thNight
4217%38%28%7%
558
16th to endNight
6%5%18%32%29%8%
377
June1st to 15thNight
3318%39%31%3
330
16th to endNight
119%40%43%3
330
July1st to 15thNight
116%26%50%13%
365
16th to endNight
218%18%52%17%
522
August1st to 15thNight
316%25%50%12%
504
16th to endNight
218%28%49%10%
700
September1st to 15thNight
37%22%50%15%
660
16th to endNight
3210%26%43%14%
656
October1st to 15thNight
7%211%28%38%10%
660
16th to endNight
8%39%28%33%15%
704
November1st to 15thNight
8%213%30%34%10%
700
16th to endNight
10%412%34%29%9%
720
December1st to 15thNight
15%6%14%30%24%7%
720
16th to endNight
18%7%19%29%20%3
768
Page generated at 2017-04-30 17:40:00. Version ac04

Copyright 2017 A. Danko


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