El Cajon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the El Cajon clear sky chart from 2005-11-05 to 2017-09-18.

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%
16260
32%
46993
29%
16216
77%
16216
45%
25440

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
71%
717
30%
2315
30%
716
75%
716
42%
1188
January16th to endNight
67%
768
35%
2304
31%
768
73%
768
40%
1285
February1st to 15thNight
73%
698
32%
2109
33%
698
76%
698
43%
1115
February16th to endNight
62%
632
28%
1895
32%
632
64%
632
31%
1160
March1st to 15thNight
70%
720
29%
2160
33%
720
72%
720
41%
1187
March16th to endNight
71%
768
27%
2040
26%
768
73%
768
42%
1197
April1st to 15thNight
66%
719
26%
1799
19%
719
67%
719
42%
1102
April16th to endNight
70%
720
29%
1800
19%
720
71%
720
46%
1070
May1st to 15thNight
68%
587
27%
1667
19%
587
69%
587
38%
1002
May16th to endNight
69%
561
25%
1685
20%
561
69%
561
35%
1025
June1st to 15thNight
73%
540
24%
1620
21%
540
73%
540
38%
954
June16th to endNight
89%
540
33%
1620
30%
540
89%
540
61%
743
July1st to 15thNight
86%
510
25%
1530
21%
510
86%
510
49%
791
July16th to endNight
82%
640
29%
1663
22%
601
82%
601
47%
907
August1st to 15thNight
85%
717
32%
1790
21%
713
85%
713
57%
955
August16th to endNight
87%
764
37%
1910
26%
764
87%
764
59%
1009
September1st to 15thNight
85%
720
34%
1875
29%
720
86%
720
56%
995
September16th to endNight
85%
667
44%
1847
42%
667
86%
667
64%
854
October1st to 15thNight
79%
660
35%
1815
34%
660
81%
660
51%
944
October16th to endNight
76%
704
37%
2098
38%
704
78%
704
47%
1074
November1st to 15thNight
80%
700
34%
2275
39%
700
81%
700
49%
1067
November16th to endNight
77%
720
35%
2340
39%
720
79%
720
46%
1151
December1st to 15thNight
67%
720
32%
2340
34%
720
70%
720
36%
1274
December16th to endNight
66%
768
32%
2496
33%
768
70%
768
36%
1391

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%313313346%63%
47791
Night
6%5%23313347%58%
47198
TransparencyDay
22%2416%26%26%
47493
Night
26%25%15%22%27%
46997
SeeingDay
7%6%27%38%14%5%
5932
Night
8%25%32%38%12%
16272

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
7%414422345%58%
1620
16th to endDay
11%413323346%55%
1920
February1st to 15thDay
7%323323446%58%
1751
16th to endDay
12%5%25%424447%45%
1579
March1st to 15thDay
7%42331335%10%55%
1800
16th to endDay
6%22332435%7%57%
2181
April1st to 15thDay
7%42442345%6%54%
2149
16th to endDay
6%312112236%67%
2160
May1st to 15thDay
5%423223347%61%
2281
16th to endDay
45%23212237%63%
2429
June1st to 15thDay
45%13312337%64%
2340
16th to endDay
12111235%77%
2340
July1st to 15thDay
1412223247%64%
2340
16th to endDay
221221345%9%63%
2388
August1st to 15thDay
1212212235%75%
2160
16th to endDay
1112212337%72%
2292
September1st to 15thDay
1212312245%72%
2085
16th to endDay
21121233476%
1838
October1st to 15thDay
5%313323345%62%
1815
16th to endDay
6%41221222469%
1771
November1st to 15thDay
5%213313336%65%
1584
16th to endDay
5%412413346%61%
1620
December1st to 15thDay
8%313323449%54%
1620
16th to endDay
12%413412446%53%
1728
January1st to 15thNight
6%413323446%59%
2315
16th to endNight
11%413213346%56%
2304
February1st to 15thNight
7%31431335%6%58%
2101
16th to endNight
13%5%3442345%5%46%
1895
March1st to 15thNight
8%5%14313448%54%
2160
16th to endNight
7%5%2332345%9%50%
2040
April1st to 15thNight
9%7%34312338%52%
1799
16th to endNight
7%6%24312346%57%
1800
May1st to 15thNight
6%7%35%323349%49%
1667
16th to endNight
7%10%44213347%49%
1684
June1st to 15thNight
48%3442335%9%49%
1620
16th to endNight
121312349%67%
1620
July1st to 15thNight
15%2432336%9%57%
1620
16th to endNight
232442346%8%56%
1777
August1st to 15thNight
141321235%10%62%
1800
16th to endNight
1313213247%68%
1910
September1st to 15thNight
1324313245%66%
1875
16th to endNight
3212111136%74%
1847
October1st to 15thNight
5%313323347%62%
1815
16th to endNight
6%5%13213247%61%
2098
November1st to 15thNight
5%312313335%66%
2275
16th to endNight
5%313322348%60%
2340
December1st to 15thNight
9%5%24323346%54%
2340
16th to endNight
12%424323346%54%
2496

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
27%2414%23%27%
1621
16th to endDay
30%26%14%16%29%
1920
February1st to 15thDay
27%2415%21%28%
1751
16th to endDay
38%25%16%23%13%
1579
March1st to 15thDay
25%25%20%23%23%
1800
16th to endDay
27%36%24%22%16%
2181
April1st to 15thDay
30%25%24%27%10%
2149
16th to endDay
19%26%23%27%20%
2160
May1st to 15thDay
23%3421%27%19%
2281
16th to endDay
23%2422%34%13%
2432
June1st to 15thDay
22%2315%29%26%
2340
16th to endDay
10%226%22%55%
2340
July1st to 15thDay
19%3311%25%36%
2201
16th to endDay
17%410%23%29%13%
2239
August1st to 15thDay
14%1315%36%29%
2146
16th to endDay
13%3213%35%31%
2292
September1st to 15thDay
14%2211%34%34%
2085
16th to endDay
12%2110%34%38%
1838
October1st to 15thDay
23%3316%26%26%
1815
16th to endDay
20%2212%21%40%
1771
November1st to 15thDay
21%25%14%19%36%
1584
16th to endDay
23%2414%21%34%
1620
December1st to 15thDay
28%28%15%18%26%
1620
16th to endDay
32%27%15%22%18%
1728
January1st to 15thNight
26%35%13%21%30%
2315
16th to endNight
30%26%12%16%32%
2304
February1st to 15thNight
27%16%13%21%28%
2112
16th to endNight
39%2415%22%16%
1895
March1st to 15thNight
29%26%19%18%23%
2160
16th to endNight
29%48%19%22%16%
2040
April1st to 15thNight
32%26%23%23%11%
1799
16th to endNight
29%26%18%21%21%
1800
May1st to 15thNight
33%35%17%21%18%
1668
16th to endNight
34%4420%21%14%
1685
June1st to 15thNight
32%3413%26%19%
1620
16th to endNight
14%217%20%51%
1620
July1st to 15thNight
22%3310%24%34%
1530
16th to endNight
24%47%23%27%12%
1663
August1st to 15thNight
19%2317%32%23%
1790
16th to endNight
17%3210%32%33%
1910
September1st to 15thNight
19%3210%31%32%
1875
16th to endNight
14%119%34%37%
1847
October1st to 15thNight
22%2418%27%24%
1815
16th to endNight
24%2212%20%38%
2098
November1st to 15thNight
21%2416%17%38%
2275
16th to endNight
24%2416%18%33%
2340
December1st to 15thNight
32%26%14%18%25%
2340
16th to endNight
32%28%15%18%22%
2496

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
27%120%33%16%2
99
16th to endDay
15%10%35%30%7%1
288
February1st to 15thDay
7%12%33%31%10%4
266
16th to endDay
15%14%36%24%7%2
238
March1st to 15thDay
8%12%34%28%12%4
270
16th to endDay
6%14%34%31%9%3
288
April1st to 15thDay
10%16%40%21%8%3
269
16th to endDay
8%14%40%27%5%3
270
May1st to 15thDay
13%11%33%29%8%3
288
16th to endDay
9%9%33%36%10%1
330
June1st to 15thDay
4430%40%15%4
315
16th to endDay
1116%51%20%8%
315
July1st to 15thDay
5%112%49%24%7%
305
16th to endDay
6%6%54%22%10%
327
August1st to 15thDay
214%53%20%7%
315
16th to endDay
2110%56%19%9%
332
September1st to 15thDay
425%51%12%5%
315
16th to endDay
4331%45%9%6%
264
October1st to 15thDay
5%9%45%27%10%1
255
16th to endDay
11%430%30%18%4
217
November1st to 15thDay
5%6%17%24%35%10%
90
16th to endDay
13%6%17%32%26%3
90
December1st to 15thDay
5%316%40%28%5%
90
16th to endDay
15%5%18%40%17%2
96
January1st to 15thNight
7%29%39%27%12%
720
16th to endNight
14%18%36%29%9%
768
February1st to 15thNight
8%26%39%32%9%
704
16th to endNight
13%39%42%25%6%
632
March1st to 15thNight
11%36%41%30%7%
720
16th to endNight
10%26%37%35%8%
768
April1st to 15thNight
11%37%42%28%5%
719
16th to endNight
9%47%40%28%10%
720
May1st to 15thNight
8%38%35%36%9%
588
16th to endNight
10%47%33%39%5%
563
June1st to 15thNight
6%37%37%38%6%
540
16th to endNight
2231%53%9%
540
July1st to 15thNight
22119%61%13%
510
16th to endNight
3211%60%20%
640
August1st to 15thNight
2216%56%21%
717
16th to endNight
29%56%29%
764
September1st to 15thNight
318%57%18%
720
16th to endNight
4228%44%19%
667
October1st to 15thNight
7%436%40%9%
660
16th to endNight
10%2330%35%18%
704
November1st to 15thNight
7%435%36%15%
700
16th to endNight
7%6%34%34%16%
720
December1st to 15thNight
13%27%37%27%11%
720
16th to endNight
14%18%36%28%10%
768
Page generated at 2017-09-19 04:32:31. Version ac04

Copyright 2017 A. Danko


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