Table Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Table Mountain Observatory clear sky chart from 2005-11-05 to 2017-09-17.

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
65%
15853
35%
46751
26%
15807
73%
15807
52%
22638

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
47%
717
28%
2488
20%
716
59%
716
35%
1271
January16th to endNight
45%
768
33%
2304
20%
768
60%
768
35%
1284
February1st to 15thNight
53%
700
33%
2106
25%
697
65%
697
40%
1130
February16th to endNight
47%
632
28%
1895
23%
632
58%
632
33%
1113
March1st to 15thNight
54%
720
28%
2160
22%
720
66%
720
42%
1130
March16th to endNight
56%
768
31%
2016
20%
768
65%
768
46%
1107
April1st to 15thNight
52%
719
31%
1799
14%
719
61%
719
50%
987
April16th to endNight
57%
720
35%
1800
16%
720
64%
720
60%
914
May1st to 15thNight
66%
590
34%
1670
21%
590
71%
590
62%
755
May16th to endNight
61%
561
35%
1685
23%
561
69%
561
62%
725
June1st to 15thNight
76%
501
35%
1581
29%
501
81%
501
72%
615
June16th to endNight
92%
360
42%
1440
49%
360
96%
360
81%
420
July1st to 15thNight
92%
340
36%
1360
43%
340
92%
340
64%
467
July16th to endNight
88%
620
33%
1644
26%
582
89%
582
61%
772
August1st to 15thNight
92%
717
38%
1790
25%
713
92%
713
79%
818
August16th to endNight
92%
764
44%
1910
30%
764
92%
764
78%
879
September1st to 15thNight
91%
720
41%
1842
32%
720
92%
720
76%
839
September16th to endNight
84%
664
46%
1837
40%
664
89%
664
76%
774
October1st to 15thNight
69%
660
38%
1815
32%
660
76%
660
57%
895
October16th to endNight
71%
704
41%
2151
36%
704
80%
704
56%
970
November1st to 15thNight
66%
700
36%
2275
31%
700
78%
700
54%
1001
November16th to endNight
60%
720
36%
2340
32%
720
70%
720
44%
1139
December1st to 15thNight
53%
720
31%
2340
24%
720
65%
720
36%
1251
December16th to endNight
42%
768
32%
2503
21%
768
58%
768
31%
1382

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
1213213346%68%
48000
Night
1213313346%69%
46947
TransparencyDay
17%45%14%27%30%
47690
Night
17%45%14%27%29%
46758
SeeingDay
313%33%34%10%3
5895
Night
26%21%33%30%6%
15863

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
232332345%8%59%
1439
16th to endDay
37%35%423345%55%
1920
February1st to 15thDay
142332345%9%58%
1751
16th to endDay
25%37%6%2335%7%52%
1579
March1st to 15thDay
131442346%9%58%
1800
16th to endDay
1225%4245%5%9%56%
2205
April1st to 15thDay
2333422337%64%
2149
16th to endDay
1111213347%71%
2160
May1st to 15thDay
111221335%8%70%
2280
16th to endDay
1111212236%73%
2429
June1st to 15thDay
12112236%77%
2375
16th to endDay
111111387%
2520
July1st to 15thDay
112113235%74%
2520
16th to endDay
2111213347%69%
2405
August1st to 15thDay
1121123480%
2160
16th to endDay
11121233477%
2292
September1st to 15thDay
11212235%77%
2117
16th to endDay
21222481%
1828
October1st to 15thDay
1213313246%69%
1815
16th to endDay
112122235%75%
1719
November1st to 15thDay
1212323336%68%
1584
16th to endDay
231332335%7%63%
1620
December1st to 15thDay
222442347%10%54%
1620
16th to endDay
45%25%4235%47%53%
1713
January1st to 15thNight
25%2432445%8%55%
2488
16th to endNight
4425%42435%7%55%
2304
February1st to 15thNight
341432335%8%58%
2101
16th to endNight
2335%6%345%5%6%53%
1895
March1st to 15thNight
122342347%8%59%
2160
16th to endNight
224415%45%7%60%
2016
April1st to 15thNight
1324413236%66%
1799
16th to endNight
11122125%7%73%
1800
May1st to 15thNight
111235%7%74%
1670
16th to endNight
222226%79%
1684
June1st to 15thNight
111135%82%
1581
16th to endNight
111389%
1440
July1st to 15thNight
1121223380%
1440
16th to endNight
211223345%72%
1756
August1st to 15thNight
1111122484%
1800
16th to endNight
1121112383%
1910
September1st to 15thNight
111212482%
1842
16th to endNight
111122385%
1837
October1st to 15thNight
1113312335%72%
1815
16th to endNight
12212235%75%
2151
November1st to 15thNight
1213313345%71%
2275
16th to endNight
121331335%8%63%
2340
December1st to 15thNight
341442346%9%54%
2340
16th to endNight
4325%42446%7%54%
2503

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
23%5%6%17%27%20%
1439
16th to endDay
31%45%15%22%20%
1920
February1st to 15thDay
24%5%5%18%24%21%
1751
16th to endDay
32%7%5%15%28%10%
1579
March1st to 15thDay
22%6%8%22%21%19%
1800
16th to endDay
23%7%9%22%22%14%
2205
April1st to 15thDay
23%46%23%29%12%
2149
16th to endDay
14%5%8%20%31%18%
2160
May1st to 15thDay
13%5%8%18%31%22%
2280
16th to endDay
13%5%6%19%40%16%
2432
June1st to 15thDay
11%4310%31%39%
2375
16th to endDay
6%13422%61%
2520
July1st to 15thDay
12%336%19%54%
2371
16th to endDay
14%35%16%27%33%
2255
August1st to 15thDay
8%118%25%53%
2146
16th to endDay
11%2210%31%42%
2292
September1st to 15thDay
9%25%9%29%43%
2117
16th to endDay
9%228%33%43%
1828
October1st to 15thDay
18%3414%29%29%
1815
16th to endDay
13%5%412%27%36%
1719
November1st to 15thDay
18%35%15%32%24%
1584
16th to endDay
22%5%5%15%26%25%
1620
December1st to 15thDay
24%7%8%18%23%16%
1620
16th to endDay
30%6%7%20%18%15%
1713
January1st to 15thNight
25%5%5%15%25%22%
2488
16th to endNight
29%47%13%22%22%
2304
February1st to 15thNight
25%5%5%16%25%22%
2112
16th to endNight
32%6%5%15%26%14%
1895
March1st to 15thNight
22%6%8%18%22%21%
2160
16th to endNight
22%5%8%21%27%14%
2016
April1st to 15thNight
20%47%25%24%16%
1799
16th to endNight
13%48%21%33%18%
1800
May1st to 15thNight
10%48%21%33%22%
1671
16th to endNight
10%46%21%37%19%
1685
June1st to 15thNight
7%3414%30%39%
1581
16th to endNight
435%21%63%
1440
July1st to 15thNight
9%238%16%59%
1360
16th to endNight
14%35%12%25%38%
1644
August1st to 15thNight
7%127%25%55%
1790
16th to endNight
9%29%32%46%
1910
September1st to 15thNight
8%148%27%48%
1842
16th to endNight
7%219%33%45%
1837
October1st to 15thNight
16%3412%31%31%
1815
16th to endNight
13%448%30%38%
2151
November1st to 15thNight
16%3415%33%26%
2275
16th to endNight
19%6%5%17%25%26%
2340
December1st to 15thNight
25%6%8%14%24%20%
2340
16th to endNight
29%5%7%16%24%16%
2503

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
6%32%29%29%1
61
16th to endDay
9%27%29%26%5%1
288
February1st to 15thDay
6%22%33%27%8%1
266
16th to endDay
9%25%34%22%7%
238
March1st to 15thDay
423%34%23%13%
270
16th to endDay
420%40%25%7%1
288
April1st to 15thDay
6%23%41%18%8%1
269
16th to endDay
228%38%21%5%1
270
May1st to 15thDay
416%42%28%5%3
288
16th to endDay
312%42%31%7%3
330
June1st to 15thDay
211%38%34%9%3
315
16th to endDay
25%34%41%11%4
315
July1st to 15thDay
1228%46%14%6%
305
16th to endDay
2120%53%12%9%
327
August1st to 15thDay
2233%45%9%7%
315
16th to endDay
1125%48%15%8%
332
September1st to 15thDay
2133%45%14%2
315
16th to endDay
27%29%42%16%1
260
October1st to 15thDay
316%38%32%8%1
255
16th to endDay
310%34%33%18%
225
November1st to 15thDay
410%17%33%28%5%
90
16th to endDay
114%45%24%14%
90
December1st to 15thDay
118%20%42%17%
90
16th to endDay
10%20%34%24%9%
93
January1st to 15thNight
5%9%33%31%16%3
720
16th to endNight
6%11%29%29%19%2
768
February1st to 15thNight
48%27%35%19%3
704
16th to endNight
411%33%27%18%3
632
March1st to 15thNight
29%30%35%19%3
720
16th to endNight
19%27%33%23%3
768
April1st to 15thNight
211%29%33%20%3
719
16th to endNight
18%30%32%21%5%
720
May1st to 15thNight
5%25%38%25%4
591
16th to endNight
1430%30%29%3
563
June1st to 15thNight
119%44%30%2
501
16th to endNight
5%43%46%4
360
July1st to 15thNight
1122%65%9%
340
16th to endNight
2330%51%10%
620
August1st to 15thNight
1330%52%11%
717
16th to endNight
1225%55%15%
764
September1st to 15thNight
1431%47%15%
720
16th to endNight
12%28%44%13%
664
October1st to 15thNight
2420%36%30%5%
660
16th to endNight
1418%33%29%12%
704
November1st to 15thNight
2423%37%25%7%
700
16th to endNight
25%26%34%23%8%
720
December1st to 15thNight
6%8%26%35%20%1
720
16th to endNight
5%14%31%33%12%1
768
Page generated at 2017-09-18 05:38:21. Version ac04

Copyright 2017 A. Danko


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