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 2018-10-27.

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%
17775
32%
51370
29%
17731
77%
17731
45%
27847

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
70%
777
30%
2509
30%
776
74%
776
40%
1315
January16th to endNight
68%
832
35%
2496
33%
832
74%
832
41%
1371
February1st to 15thNight
74%
758
32%
2289
33%
758
76%
758
43%
1207
February16th to endNight
63%
684
29%
2051
33%
684
65%
684
33%
1240
March1st to 15thNight
69%
780
28%
2340
31%
780
70%
780
38%
1316
March16th to endNight
71%
832
27%
2210
25%
832
72%
832
42%
1300
April1st to 15thNight
66%
779
27%
1949
20%
779
67%
779
41%
1203
April16th to endNight
70%
772
29%
1930
19%
772
71%
772
44%
1159
May1st to 15thNight
68%
636
27%
1806
19%
636
69%
636
37%
1098
May16th to endNight
67%
609
24%
1829
19%
609
67%
609
33%
1138
June1st to 15thNight
74%
585
26%
1755
22%
585
74%
585
40%
1020
June16th to endNight
87%
585
31%
1755
28%
585
87%
585
57%
843
July1st to 15thNight
87%
555
27%
1665
23%
555
87%
555
51%
848
July16th to endNight
83%
694
28%
1813
21%
655
83%
655
48%
982
August1st to 15thNight
86%
765
33%
1910
22%
761
86%
761
58%
1008
August16th to endNight
86%
828
36%
2070
25%
828
86%
828
58%
1098
September1st to 15thNight
85%
780
36%
2031
31%
780
85%
780
55%
1084
September16th to endNight
86%
776
44%
2145
42%
776
87%
776
65%
988
October1st to 15thNight
79%
780
35%
2145
34%
780
80%
780
50%
1126
October16th to endNight
77%
816
38%
2428
40%
816
79%
816
48%
1228
November1st to 15thNight
79%
760
32%
2470
37%
760
81%
760
47%
1189
November16th to endNight
77%
780
36%
2535
40%
780
78%
780
45%
1251
December1st to 15thNight
68%
780
32%
2535
34%
780
71%
780
37%
1358
December16th to endNight
68%
832
33%
2704
35%
832
71%
832
37%
1477

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%
52172
Night
6%5%24313347%57%
51575
TransparencyDay
22%2416%26%26%
51874
Night
26%2415%23%26%
51374
SeeingDay
7%6%25%37%16%6%
7440
Night
8%25%32%39%13%
17787

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%424412345%57%
1756
16th to endDay
10%313323346%56%
2080
February1st to 15thDay
7%323323446%59%
1901
16th to endDay
12%5%25%424447%45%
1709
March1st to 15thDay
7%42331335%10%54%
1950
16th to endDay
6%22342435%7%57%
2363
April1st to 15thDay
7%42432345%6%55%
2329
16th to endDay
6%312212246%66%
2305
May1st to 15thDay
6%423223346%60%
2472
16th to endDay
5%5%23212237%62%
2637
June1st to 15thDay
4413312336%65%
2535
16th to endDay
112212235%77%
2535
July1st to 15thDay
241222325%7%64%
2535
16th to endDay
221221335%9%64%
2589
August1st to 15thDay
1212212235%75%
2293
16th to endDay
1112212337%71%
2484
September1st to 15thDay
1212212235%73%
2259
16th to endDay
2111223476%
2145
October1st to 15thDay
5%413323345%62%
2145
16th to endDay
5%41221122470%
2049
November1st to 15thDay
5%313323346%62%
1719
16th to endDay
5%412413346%61%
1755
December1st to 15thDay
8%312323448%56%
1755
16th to endDay
11%41331345%6%53%
1872
January1st to 15thNight
6%413423446%56%
2509
16th to endNight
10%413213336%57%
2496
February1st to 15thNight
7%313323346%58%
2281
16th to endNight
12%5%3442345%5%48%
2051
March1st to 15thNight
9%6%14313448%52%
2340
16th to endNight
7%5%2332345%10%50%
2210
April1st to 15thNight
9%7%34323338%51%
1949
16th to endNight
7%6%24313346%56%
1930
May1st to 15thNight
6%7%37%42335%9%48%
1806
16th to endNight
7%10%45%314347%46%
1828
June1st to 15thNight
47%3442345%9%50%
1755
16th to endNight
141321335%9%64%
1755
July1st to 15thNight
15%2332336%10%57%
1755
16th to endNight
232432346%8%57%
1927
August1st to 15thNight
131321235%10%63%
1920
16th to endNight
1314213247%67%
2070
September1st to 15thNight
2313313346%65%
2031
16th to endNight
3212111235%74%
2145
October1st to 15thNight
5%313323347%61%
2145
16th to endNight
5%5%13213236%63%
2428
November1st to 15thNight
4413313336%63%
2470
16th to endNight
5%314322347%60%
2535
December1st to 15thNight
8%5%24323346%55%
2535
16th to endNight
11%314323346%55%
2704

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
28%2415%23%25%
1757
16th to endDay
29%26%15%16%29%
2080
February1st to 15thDay
26%2415%22%28%
1901
16th to endDay
38%2415%23%15%
1709
March1st to 15thDay
26%26%20%21%22%
1950
16th to endDay
27%36%24%22%15%
2363
April1st to 15thDay
29%25%23%26%12%
2329
16th to endDay
20%25%23%28%19%
2305
May1st to 15thDay
24%3420%28%19%
2472
16th to endDay
23%2322%34%12%
2640
June1st to 15thDay
21%2315%29%27%
2535
16th to endDay
11%226%23%53%
2535
July1st to 15thDay
18%3312%24%35%
2396
16th to endDay
17%412%23%29%12%
2440
August1st to 15thDay
13%1314%36%30%
2279
16th to endDay
14%3215%35%28%
2484
September1st to 15thDay
14%2210%34%35%
2259
16th to endDay
11%2110%35%38%
2145
October1st to 15thDay
23%3315%27%26%
2145
16th to endDay
19%1211%21%42%
2049
November1st to 15thDay
24%25%14%19%33%
1719
16th to endDay
24%2413%21%34%
1755
December1st to 15thDay
27%27%14%18%29%
1755
16th to endDay
31%26%15%22%20%
1872
January1st to 15thNight
27%35%13%21%28%
2509
16th to endNight
29%25%14%15%32%
2496
February1st to 15thNight
27%16%13%21%29%
2292
16th to endNight
37%2314%24%16%
2051
March1st to 15thNight
31%25%19%17%22%
2340
16th to endNight
29%48%19%22%15%
2210
April1st to 15thNight
33%26%22%22%12%
1949
16th to endNight
29%35%17%22%21%
1930
May1st to 15thNight
34%3416%22%17%
1807
16th to endNight
36%4420%21%13%
1829
June1st to 15thNight
30%3312%27%20%
1755
16th to endNight
17%317%20%49%
1755
July1st to 15thNight
21%3313%24%33%
1665
16th to endNight
23%49%24%27%11%
1813
August1st to 15thNight
19%2417%31%25%
1910
16th to endNight
18%3312%31%30%
2070
September1st to 15thNight
19%429%30%34%
2031
16th to endNight
14%119%35%37%
2145
October1st to 15thNight
23%2418%27%24%
2145
16th to endNight
23%2111%20%40%
2428
November1st to 15thNight
22%25%17%16%35%
2470
16th to endNight
24%2416%19%33%
2535
December1st to 15thNight
31%26%14%17%28%
2535
16th to endNight
31%27%14%20%23%
2704

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
24%15%33%20%4
145
16th to endDay
12%8%32%31%11%3
352
February1st to 15thDay
6%10%30%30%14%6%
326
16th to endDay
15%12%34%25%10%2
290
March1st to 15thDay
8%12%34%28%12%3
330
16th to endDay
6%12%34%31%10%3
352
April1st to 15thDay
9%13%37%25%10%4
329
16th to endDay
8%13%39%28%6%4
318
May1st to 15thDay
13%10%32%30%9%4
348
16th to endDay
9%9%32%35%10%1
394
June1st to 15thDay
3327%41%16%7%
375
16th to endDay
1114%47%24%10%
375
July1st to 15thDay
4110%45%27%10%
365
16th to endDay
5%5%51%25%12%
391
August1st to 15thDay
113%49%24%10%
359
16th to endDay
219%54%21%10%
396
September1st to 15thDay
423%48%15%7%
375
16th to endDay
5%325%43%14%7%
375
October1st to 15thDay
6%9%36%31%13%2
375
16th to endDay
9%426%29%22%8%
321
November1st to 15thDay
8%5%17%33%28%6%
135
16th to endDay
12%5%14%31%28%8%
135
December1st to 15thDay
3416%44%26%4
135
16th to endDay
11%318%34%25%7%
144
January1st to 15thNight
8%29%38%28%12%
780
16th to endNight
14%18%36%30%9%
832
February1st to 15thNight
8%26%39%31%11%
764
16th to endNight
12%39%43%25%5%
684
March1st to 15thNight
12%25%41%29%7%
780
16th to endNight
9%25%38%35%8%
832
April1st to 15thNight
10%37%42%29%6%
779
16th to endNight
9%47%40%28%9%
772
May1st to 15thNight
8%28%34%36%9%
637
16th to endNight
10%47%34%37%5%
611
June1st to 15thNight
5%37%36%39%7%
585
16th to endNight
21230%52%10%
585
July1st to 15thNight
21118%61%15%
555
16th to endNight
3211%60%21%
694
August1st to 15thNight
2215%55%23%
765
16th to endNight
219%57%29%
828
September1st to 15thNight
3118%56%18%
780
16th to endNight
4229%45%18%
776
October1st to 15thNight
7%337%40%8%
780
16th to endNight
9%2330%35%19%
816
November1st to 15thNight
7%336%36%14%
760
16th to endNight
7%5%34%34%17%
780
December1st to 15thNight
12%17%38%27%11%
780
16th to endNight
12%18%36%29%11%
832
Page generated at 2018-10-28 05:41:05. Version ac04

Copyright 2018 A. Danko


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