California Institute of Technology
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the California Institute of Technology clear sky chart from 2005-11-05 to 2018-01-12.

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
75%
16320
33%
48251
29%
16273
79%
16273
51%
24040

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
64%
760
27%
2628
25%
760
72%
760
40%
1313
January16th to endNight
61%
768
35%
2340
28%
768
71%
768
42%
1235
February1st to 15thNight
69%
698
35%
2105
34%
697
76%
697
47%
1080
February16th to endNight
60%
632
27%
1895
26%
632
66%
632
36%
1080
March1st to 15thNight
67%
720
29%
2160
26%
720
75%
720
48%
1094
March16th to endNight
71%
768
28%
2004
23%
768
75%
768
48%
1111
April1st to 15thNight
66%
719
29%
1799
18%
719
69%
719
49%
1024
April16th to endNight
67%
720
31%
1800
15%
720
72%
720
54%
975
May1st to 15thNight
68%
617
28%
1697
19%
617
71%
617
46%
937
May16th to endNight
71%
561
30%
1685
24%
561
73%
561
52%
840
June1st to 15thNight
81%
470
28%
1550
28%
470
83%
470
52%
724
June16th to endNight
91%
360
35%
1440
43%
360
91%
360
54%
551
July1st to 15thNight
91%
340
33%
1360
41%
340
91%
340
55%
512
July16th to endNight
90%
630
31%
1650
25%
588
91%
588
56%
830
August1st to 15thNight
89%
717
34%
1790
23%
713
89%
713
65%
895
August16th to endNight
92%
764
41%
1910
29%
764
92%
764
69%
940
September1st to 15thNight
88%
720
38%
1829
31%
720
88%
720
65%
908
September16th to endNight
89%
716
44%
1980
42%
716
90%
716
68%
884
October1st to 15thNight
79%
720
36%
1995
34%
720
82%
720
57%
1001
October16th to endNight
80%
768
41%
2331
42%
768
83%
768
53%
1108
November1st to 15thNight
78%
760
34%
2470
35%
760
83%
760
54%
1112
November16th to endNight
76%
780
38%
2535
39%
780
80%
780
50%
1193
December1st to 15thNight
67%
780
33%
2535
31%
780
75%
780
41%
1304
December16th to endNight
60%
832
34%
2763
30%
832
69%
832
40%
1389

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
5%212212236%68%
49059
Night
5%212212335%68%
48452
TransparencyDay
18%37%16%27%26%
48752
Night
20%37%16%26%26%
48259
SeeingDay
6%8%29%38%12%4
6309
Night
5%212%34%36%8%
16333

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
8%413412235%62%
1550
16th to endDay
12%322312335%57%
1879
February1st to 15thDay
7%323212336%62%
1751
16th to endDay
13%413324236%52%
1579
March1st to 15thDay
5%21332335%7%60%
1800
16th to endDay
5%21332435%9%56%
2217
April1st to 15thDay
6%312224347%60%
2149
16th to endDay
5%212212335%70%
2160
May1st to 15thDay
5%312312345%65%
2250
16th to endDay
5%312323336%65%
2429
June1st to 15thDay
5%312322137%66%
2406
16th to endDay
121112383%
2520
July1st to 15thDay
212312235%72%
2520
16th to endDay
2111212336%73%
2396
August1st to 15thDay
11111212382%
2160
16th to endDay
11223481%
2292
September1st to 15thDay
1111122481%
2130
16th to endDay
21111123380%
1980
October1st to 15thDay
5%11231223471%
1962
16th to endDay
421211233472%
1893
November1st to 15thDay
331231224469%
1719
16th to endDay
5%212313247%64%
1755
December1st to 15thDay
7%212313449%58%
1755
16th to endDay
11%312212337%58%
1807
January1st to 15thNight
7%413222336%61%
2628
16th to endNight
10%312312235%61%
2340
February1st to 15thNight
6%31331223465%
2101
16th to endNight
13%32232434455%
1895
March1st to 15thNight
5%312223348%63%
2160
16th to endNight
421222445%7%60%
2004
April1st to 15thNight
6%412322325%64%
1799
16th to endNight
3211323336%69%
1800
May1st to 15thNight
33125%22337%64%
1697
16th to endNight
3212422235%69%
1684
June1st to 15thNight
121323247%69%
1550
16th to endNight
1121223380%
1440
July1st to 15thNight
22121223475%
1440
16th to endNight
21121235%7%68%
1766
August1st to 15thNight
1211112225%76%
1800
16th to endNight
1111123480%
1910
September1st to 15thNight
12221222378%
1829
16th to endNight
11111122482%
1980
October1st to 15thNight
321222233372%
1995
16th to endNight
5%2121223472%
2331
November1st to 15thNight
3312312235%70%
2470
16th to endNight
5%212212346%66%
2535
December1st to 15thNight
6%213423237%61%
2535
16th to endNight
10%312212335%62%
2763

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
25%45%14%22%26%
1550
16th to endDay
28%28%14%18%27%
1879
February1st to 15thDay
23%26%17%24%25%
1751
16th to endDay
30%29%18%28%10%
1579
March1st to 15thDay
21%410%20%21%21%
1800
16th to endDay
22%5%13%23%20%14%
2217
April1st to 15thDay
22%311%25%24%11%
2149
16th to endDay
17%210%22%28%17%
2160
May1st to 15thDay
22%39%19%28%16%
2250
16th to endDay
22%37%26%30%10%
2432
June1st to 15thDay
22%38%13%31%20%
2406
16th to endDay
11%237%34%41%
2520
July1st to 15thDay
14%35%9%26%39%
2371
16th to endDay
12%5%7%20%32%21%
2249
August1st to 15thDay
9%2213%36%35%
2146
16th to endDay
8%2411%40%32%
2292
September1st to 15thDay
9%2413%34%36%
2130
16th to endDay
9%2413%35%34%
1980
October1st to 15thDay
17%26%18%26%28%
1962
16th to endDay
15%35%13%21%40%
1893
November1st to 15thDay
18%37%16%23%31%
1719
16th to endDay
20%36%13%23%32%
1755
December1st to 15thDay
23%49%17%18%27%
1755
16th to endDay
26%39%16%21%22%
1807
January1st to 15thNight
25%36%14%21%27%
2628
16th to endNight
26%37%15%16%31%
2340
February1st to 15thNight
23%36%16%22%27%
2112
16th to endNight
31%48%15%24%15%
1895
March1st to 15thNight
21%39%20%23%20%
2160
16th to endNight
21%6%12%21%22%15%
2004
April1st to 15thNight
25%29%26%22%13%
1799
16th to endNight
19%410%23%26%16%
1800
May1st to 15thNight
24%5%9%20%26%14%
1698
16th to endNight
19%39%27%28%11%
1685
June1st to 15thNight
19%311%15%33%16%
1550
16th to endNight
14%25%9%32%35%
1440
July1st to 15thNight
13%35%10%27%39%
1360
16th to endNight
16%5%7%20%28%20%
1650
August1st to 15thNight
13%3310%38%30%
1790
16th to endNight
11%3310%39%31%
1910
September1st to 15thNight
13%349%33%34%
1829
16th to endNight
10%3312%34%35%
1980
October1st to 15thNight
17%3417%30%27%
1995
16th to endNight
17%3411%22%39%
2331
November1st to 15thNight
16%37%15%23%33%
2470
16th to endNight
20%26%16%20%32%
2535
December1st to 15thNight
24%5%7%14%18%28%
2535
16th to endNight
25%38%14%24%22%
2763

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
18%7%33%28%10%
112
16th to endDay
12%13%35%29%8%
281
February1st to 15thDay
11%10%29%33%12%2
266
16th to endDay
15%21%30%25%6%
238
March1st to 15thDay
7%15%33%27%14%1
270
16th to endDay
7%15%36%29%8%1
288
April1st to 15thDay
10%17%42%20%5%3
269
16th to endDay
421%42%23%42
270
May1st to 15thDay
10%15%35%29%6%2
288
16th to endDay
8%10%40%29%10%1
330
June1st to 15thDay
8%8%29%42%7%3
315
16th to endDay
1128%46%13%9%
315
July1st to 15thDay
1118%54%18%6%
305
16th to endDay
113%55%20%8%
327
August1st to 15thDay
3121%56%10%5%
315
16th to endDay
17%59%15%6%
332
September1st to 15thDay
2229%52%9%4
315
16th to endDay
2434%40%11%5%
315
October1st to 15thDay
410%34%37%10%2
315
16th to endDay
6%6%27%37%16%6%
309
November1st to 15thDay
5%5%18%32%28%8%
135
16th to endDay
5%5%25%37%17%8%
135
December1st to 15thDay
15%20%42%24%5%
135
16th to endDay
10%6%25%29%24%3
129
January1st to 15thNight
8%118%43%23%4
764
16th to endNight
12%217%36%26%4
768
February1st to 15thNight
7%214%41%27%5%
704
16th to endNight
13%218%36%24%4
632
March1st to 15thNight
5%220%40%25%5%
720
16th to endNight
5%315%41%29%4
768
April1st to 15thNight
7%316%42%25%3
719
16th to endNight
3420%36%29%5%
720
May1st to 15thNight
5%318%35%32%5%
618
16th to endNight
4219%38%31%4
563
June1st to 15thNight
1110%39%43%2
470
16th to endNight
1430%57%6%
360
July1st to 15thNight
2210%70%15%
340
16th to endNight
21120%57%17%
630
August1st to 15thNight
11421%55%16%
717
16th to endNight
1216%54%23%
764
September1st to 15thNight
15%24%49%17%
720
16th to endNight
25%29%48%13%
716
October1st to 15thNight
311%36%40%7%
720
16th to endNight
6%19%31%37%13%
768
November1st to 15thNight
3111%40%33%10%
760
16th to endNight
5%112%39%33%8%
780
December1st to 15thNight
7%215%41%27%5%
780
16th to endNight
11%5%16%40%21%4
832
Page generated at 2018-01-12 19:01:37. Version ac04

Copyright 2018 A. Danko


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