Tilbury Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tilbury Hill Observatory clear sky chart from 2005-11-05 to 2019-06-19.

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
29%
17173
13%
53976
8%
17141
35%
17141
16%
38757

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
16%
840
9%
2926
5%
836
22%
836
9%
2273
January16th to endNight
13%
896
9%
2971
3%
896
21%
896
9%
2295
February1st to 15thNight
11%
824
8%
2677
2%
821
19%
821
8%
2130
February16th to endNight
15%
735
11%
2237
5%
735
23%
735
11%
1736
March1st to 15thNight
17%
840
13%
2416
6%
840
25%
840
13%
1851
March16th to endNight
21%
896
16%
2391
6%
896
29%
896
18%
1741
April1st to 15thNight
20%
838
13%
2096
4%
838
25%
838
18%
1582
April16th to endNight
26%
831
15%
2077
5%
831
32%
831
24%
1455
May1st to 15thNight
33%
482
16%
1742
10%
482
41%
482
15%
1224
May16th to endNight
39%
438
15%
1753
15%
438
46%
438
18%
1129
June1st to 15thNight
39%
420
15%
1680
14%
420
49%
420
17%
1080
June16th to endNight
51%
398
12%
1592
15%
398
58%
398
23%
941
July1st to 15thNight
59%
370
14%
1480
18%
370
63%
370
24%
865
July16th to endNight
58%
417
15%
1555
18%
395
63%
395
24%
908
August1st to 15thNight
56%
575
14%
1715
11%
572
58%
572
26%
1115
August16th to endNight
59%
808
18%
2050
10%
808
61%
808
33%
1350
September1st to 15thNight
47%
780
17%
1950
9%
780
50%
780
28%
1342
September16th to endNight
49%
777
22%
2120
17%
777
53%
777
31%
1374
October1st to 15thNight
36%
780
17%
2340
13%
780
42%
780
20%
1633
October16th to endNight
22%
832
13%
2539
6%
832
30%
832
15%
1865
November1st to 15thNight
26%
820
15%
2667
9%
820
35%
820
17%
1878
November16th to endNight
23%
840
12%
2926
7%
840
31%
840
13%
2125
December1st to 15thNight
18%
840
7%
2940
5%
840
22%
840
8%
2298
December16th to endNight
13%
896
5%
3136
2%
896
18%
896
7%
2567

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%9%47%6%35%45%6%30%
54748
Night
15%9%47%6%3445%6%32%
54147
Cloud CoverDay
16%9%47%6%35%45%6%30%
54758
Night
15%9%47%6%3445%6%32%
54152
TransparencyDay
53%6%11%17%9%
54437
Night
51%6%13%18%9%
53976
SeeingDay
23%15%27%18%10%4
7773
Night
21%14%28%24%9%
17174

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
13%11%6%12%9%35%45%7%20%
1672
16th to endDay
11%12%7%12%9%45%445%20%
1955
February1st to 15thDay
16%12%6%12%9%45%44419%
1845
16th to endDay
17%11%5%10%9%34445%22%
1807
March1st to 15thDay
17%13%6%8%7%34445%24%
2190
16th to endDay
17%12%46%6%35%35%5%29%
2537
April1st to 15thDay
18%11%46%5%34446%30%
2512
16th to endDay
16%10%35%423436%36%
2488
May1st to 15thDay
21%8%25%5%35%45%6%31%
2881
16th to endDay
17%7%35%5%45%45%7%34%
3066
June1st to 15thDay
19%6%26%6%35%5%5%8%30%
2940
16th to endDay
14%7%26%6%345%5%8%33%
2776
July1st to 15thDay
10%5%25%6%36%6%7%9%35%
2730
16th to endDay
12%5%35%6%46%6%6%8%35%
2867
August1st to 15thDay
15%6%35%6%46%6%6%8%31%
2487
16th to endDay
14%5%26%5%45%5%6%8%34%
2490
September1st to 15thDay
18%6%26%5%35%45%5%35%
2340
16th to endDay
15%6%25%434446%42%
2166
October1st to 15thDay
18%9%35%5%34448%32%
1950
16th to endDay
20%12%36%6%35%45%6%26%
2035
November1st to 15thDay
17%11%36%5%34446%32%
1851
16th to endDay
19%14%37%5%24346%26%
1691
December1st to 15thDay
15%16%7%10%6%25%445%20%
1680
16th to endDay
18%17%7%11%6%45%44415%
1792
January1st to 15thNight
16%11%5%9%8%35%5%6%6%20%
2926
16th to endNight
14%12%5%9%8%45%5%5%5%22%
2971
February1st to 15thNight
14%12%6%10%8%46%5%45%21%
2675
16th to endNight
16%11%410%7%35%446%23%
2237
March1st to 15thNight
17%11%5%8%7%3444427%
2404
16th to endNight
16%10%47%42435%6%33%
2391
April1st to 15thNight
20%9%46%5%24437%31%
2096
16th to endNight
18%8%24424345%41%
2077
May1st to 15thNight
18%7%26%6%3446%6%33%
1742
16th to endNight
12%7%36%435%45%7%39%
1751
June1st to 15thNight
19%7%345%2335%7%38%
1680
16th to endNight
10%3245%45%5%5%9%43%
1592
July1st to 15thNight
7%425%6%36%5%6%8%45%
1560
16th to endNight
11%3234246%6%8%43%
1651
August1st to 15thNight
9%5%26%5%345%6%8%42%
1726
16th to endNight
11%4245%3445%8%45%
2050
September1st to 15thNight
14%5%35%42445%6%43%
1950
16th to endNight
14%41443445%7%46%
2120
October1st to 15thNight
17%8%26%5%23447%37%
2340
16th to endNight
21%10%37%5%33345%30%
2539
November1st to 15thNight
16%10%36%5%345%5%5%34%
2667
16th to endNight
17%12%45%5%34446%30%
2926
December1st to 15thNight
19%11%6%8%6%35%5%5%6%21%
2940
16th to endNight
18%14%7%10%7%345%46%17%
3136

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
13%11%6%13%8%45%5%5%6%20%
1672
16th to endDay
11%11%8%12%9%46%445%20%
1955
February1st to 15thDay
16%12%6%12%9%45%44419%
1845
16th to endDay
15%13%410%9%3444423%
1807
March1st to 15thDay
17%13%7%9%6%3444424%
2201
16th to endDay
17%12%47%6%2445%5%29%
2536
April1st to 15thDay
18%11%5%7%5%34436%30%
2512
16th to endDay
16%10%35%424436%36%
2488
May1st to 15thDay
22%6%25%5%25%45%6%31%
2881
16th to endDay
17%7%36%5%4445%7%34%
3066
June1st to 15thDay
19%7%25%6%35%5%6%8%30%
2940
16th to endDay
15%7%25%6%345%6%8%33%
2776
July1st to 15thDay
9%5%26%6%46%6%7%9%34%
2730
16th to endDay
12%5%36%6%36%5%6%9%34%
2867
August1st to 15thDay
15%6%26%6%36%6%6%8%31%
2487
16th to endDay
14%6%25%6%45%5%6%7%34%
2490
September1st to 15thDay
18%7%35%435%446%34%
2340
16th to endDay
15%6%245%25%446%41%
2166
October1st to 15thDay
17%9%37%5%34447%32%
1950
16th to endDay
20%11%36%7%3445%6%26%
2035
November1st to 15thDay
16%11%36%5%33447%32%
1851
16th to endDay
18%14%37%6%24346%27%
1691
December1st to 15thDay
15%16%7%9%6%35%445%20%
1680
16th to endDay
17%16%8%11%7%35%44416%
1792
January1st to 15thNight
15%11%5%11%7%46%5%6%6%20%
2926
16th to endNight
13%12%49%10%46%5%5%6%22%
2971
February1st to 15thNight
14%11%6%10%9%36%5%4421%
2668
16th to endNight
16%11%5%10%7%25%446%23%
2237
March1st to 15thNight
17%12%5%9%6%4434427%
2416
16th to endNight
15%10%47%42445%6%33%
2391
April1st to 15thNight
19%9%46%5%24447%31%
2096
16th to endNight
19%7%25%423345%40%
2077
May1st to 15thNight
18%7%26%5%2445%6%34%
1742
16th to endNight
12%6%36%5%345%5%7%39%
1751
June1st to 15thNight
19%6%35%423447%38%
1680
16th to endNight
10%4246%35%5%5%9%43%
1592
July1st to 15thNight
7%335%5%36%5%6%8%45%
1560
16th to endNight
12%3235%246%6%9%42%
1651
August1st to 15thNight
9%5%25%5%35%5%6%8%42%
1726
16th to endNight
10%4344245%5%7%45%
2050
September1st to 15thNight
13%6%25%42445%6%42%
1950
16th to endNight
13%41442445%7%45%
2120
October1st to 15thNight
18%7%36%43435%6%37%
2340
16th to endNight
21%10%37%6%33346%30%
2539
November1st to 15thNight
15%11%36%5%25%45%5%33%
2667
16th to endNight
16%12%36%5%345%46%30%
2926
December1st to 15thNight
18%12%6%8%7%345%5%6%21%
2940
16th to endNight
17%14%7%11%7%35%5%45%17%
3136

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
64%5%11%12%5%
1673
16th to endDay
64%5%13%10%5%
1955
February1st to 15thDay
67%6%8%11%5%
1847
16th to endDay
63%5%10%14%6%
1807
March1st to 15thDay
62%5%11%12%6%
2201
16th to endDay
56%49%17%11%1
2537
April1st to 15thDay
55%5%9%18%9%1
2512
16th to endDay
48%5%9%18%14%2
2488
May1st to 15thDay
52%7%10%19%8%1
2881
16th to endDay
48%8%12%19%10%
3070
June1st to 15thDay
50%7%12%19%9%
2940
16th to endDay
46%7%14%20%10%1
2776
July1st to 15thDay
42%9%13%19%13%1
2581
16th to endDay
43%9%16%18%11%
2700
August1st to 15thDay
48%9%14%17%9%
2474
16th to endDay
44%9%11%22%11%
2490
September1st to 15thDay
49%6%11%19%12%
2340
16th to endDay
42%6%10%22%16%1
2166
October1st to 15thDay
51%6%9%18%12%
1950
16th to endDay
57%6%12%16%6%
2035
November1st to 15thDay
51%5%14%16%8%2
1851
16th to endDay
57%5%13%15%6%
1691
December1st to 15thDay
65%6%13%11%4
1680
16th to endDay
71%5%11%9%2
1792
January1st to 15thNight
61%7%12%11%5%
2926
16th to endNight
61%5%14%13%5%
2971
February1st to 15thNight
64%6%11%12%5%
2677
16th to endNight
60%5%12%13%6%
2237
March1st to 15thNight
58%5%11%17%6%
2416
16th to endNight
51%5%10%19%11%1
2391
April1st to 15thNight
53%5%10%18%10%1
2096
16th to endNight
45%411%19%16%2
2077
May1st to 15thNight
49%9%9%17%12%1
1742
16th to endNight
42%8%12%21%12%1
1753
June1st to 15thNight
45%6%11%23%12%
1680
16th to endNight
36%8%16%24%13%1
1592
July1st to 15thNight
35%8%13%25%15%1
1480
16th to endNight
34%9%16%22%14%2
1555
August1st to 15thNight
37%10%15%25%10%
1715
16th to endNight
36%7%14%26%14%
2050
September1st to 15thNight
40%7%14%21%13%1
1950
16th to endNight
37%6%12%26%17%
2120
October1st to 15thNight
47%7%12%19%13%1
2340
16th to endNight
55%5%14%17%7%
2539
November1st to 15thNight
50%5%13%17%10%2
2667
16th to endNight
53%7%14%17%7%
2926
December1st to 15thNight
61%7%14%12%3
2940
16th to endNight
67%6%12%10%2
3136

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%30%33%6%5%1
120
16th to endDay
24%33%32%7%2
175
February1st to 15thDay
31%30%31%5%1
180
16th to endDay
30%28%30%8%2
310
March1st to 15thDay
28%21%31%14%4
390
16th to endDay
25%24%26%15%42
416
April1st to 15thDay
32%21%27%13%41
390
16th to endDay
25%22%24%13%10%3
379
May1st to 15thDay
32%15%26%12%9%3
408
16th to endDay
20%14%30%20%11%3
459
June1st to 15thDay
20%14%30%22%10%1
435
16th to endDay
21%10%31%21%12%1
388
July1st to 15thDay
12%330%29%16%7%
365
16th to endDay
14%5%27%30%17%4
390
August1st to 15thDay
18%5%26%22%18%8%
358
16th to endDay
13%6%22%26%16%14%
396
September1st to 15thDay
21%8%19%21%20%9%
375
16th to endDay
16%9%24%21%16%12%
374
October1st to 15thDay
24%8%25%21%14%4
375
16th to endDay
27%20%29%15%6%
386
November1st to 15thDay
27%15%24%20%10%
325
16th to endDay
35%17%32%9%3
131
December1st to 15thDay
24%26%26%17%31
120
16th to endDay
36%28%25%8%
128
January1st to 15thNight
23%24%31%17%3
840
16th to endNight
21%21%36%18%1
896
February1st to 15thNight
25%22%35%15%1
824
16th to endNight
26%21%32%16%2
735
March1st to 15thNight
27%22%29%18%2
840
16th to endNight
21%19%32%19%6%
896
April1st to 15thNight
27%18%30%19%2
838
16th to endNight
24%14%31%20%9%
831
May1st to 15thNight
22%9%27%30%8%
482
16th to endNight
14%9%30%32%12%
439
June1st to 15thNight
20%8%27%32%10%
420
16th to endNight
11%328%43%13%
398
July1st to 15thNight
8%320%46%18%1
370
16th to endNight
14%220%42%18%1
417
August1st to 15thNight
12%321%36%24%2
575
16th to endNight
13%218%33%29%2
808
September1st to 15thNight
16%5%23%31%20%2
780
16th to endNight
15%7%22%26%23%5%
777
October1st to 15thNight
22%9%25%26%14%1
780
16th to endNight
28%15%29%22%4
832
November1st to 15thNight
21%15%32%22%7%
820
16th to endNight
26%18%28%20%5%
840
December1st to 15thNight
25%21%28%20%4
840
16th to endNight
27%21%34%14%2
896
Page generated at 2019-06-20 08:02:27. Version ac04

Copyright 2019 A. Danko


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