Foothill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foothill Observatory clear sky chart from 2005-11-06 to 2024-03-18.

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
65%
23981
25%
80577
24%
23896
69%
23896
34%
45110

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 10thNight
51%
748
19%
4164
18%
736
55%
736
15%
2389
January11th to 20thNight
53%
750
13%
4091
17%
740
60%
740
13%
2828
January21st to endNight
54%
830
14%
4447
23%
814
59%
814
12%
3152
February1st to 10thNight
54%
760
25%
2470
23%
760
61%
760
29%
1477
February11th to 20thNight
52%
740
23%
2310
21%
740
58%
740
28%
1400
February21st to endNight
51%
628
23%
1884
19%
628
57%
628
32%
1118
March1st to 10thNight
47%
760
16%
2123
14%
760
51%
760
24%
1447
March11th to 20thNight
52%
752
23%
2067
19%
752
56%
752
28%
1356
March21st to endNight
51%
792
18%
2147
12%
792
57%
792
31%
1362
April1st to 10thNight
51%
720
20%
1800
11%
720
55%
720
33%
1172
April11th to 20thNight
51%
711
25%
1779
12%
711
55%
711
36%
1101
April21st to endNight
58%
720
28%
1800
13%
720
63%
720
49%
994
May1st to 10thNight
61%
662
26%
1742
15%
662
64%
662
48%
964
May11th to 20thNight
69%
493
25%
1556
20%
493
72%
493
45%
771
May21st to endNight
65%
388
22%
1552
20%
388
72%
388
34%
741
June1st to 10thNight
68%
360
25%
1440
25%
360
76%
360
39%
649
June11th to 20thNight
78%
360
35%
1440
38%
360
85%
360
58%
503
June21st to endNight
81%
360
36%
1440
44%
360
85%
360
59%
513
July1st to 10thNight
87%
351
36%
1401
44%
351
90%
351
69%
454
July11th to 20thNight
92%
522
35%
1566
33%
522
92%
522
70%
640
July21st to endNight
88%
591
37%
1674
34%
558
89%
558
67%
697
August1st to 10thNight
82%
537
31%
1602
26%
534
83%
534
54%
733
August11th to 20thNight
86%
655
38%
1723
26%
655
87%
655
71%
785
August21st to endNight
86%
792
38%
1980
24%
792
87%
792
71%
942
September1st to 10thNight
86%
720
41%
1800
29%
720
87%
720
73%
846
September11th to 20thNight
76%
720
39%
1800
29%
720
79%
720
58%
950
September21st to endNight
80%
720
38%
2043
36%
720
85%
720
65%
918
October1st to 10thNight
78%
720
30%
2160
30%
720
83%
720
54%
1025
October11th to 20thNight
71%
720
33%
2160
34%
720
76%
720
45%
1117
October21st to endNight
73%
792
35%
2376
36%
792
77%
792
47%
1201
November1st to 10thNight
66%
740
29%
2242
29%
740
70%
740
41%
1213
November11th to 20thNight
65%
760
27%
2470
26%
760
70%
760
34%
1366
November21st to endNight
65%
760
28%
2603
30%
760
70%
760
38%
1350
December1st to 10thNight
55%
760
23%
2660
20%
760
62%
760
26%
1601
December11th to 20thNight
50%
760
17%
3243
19%
759
55%
759
16%
2264
December21st to endNight
44%
827
12%
4822
16%
817
51%
817
12%
3071

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
6%42442445%7%57%
78833
Night
6%5%25%5%3445%8%53%
80991
TransparencyDay
27%5%11%20%23%15%
78554
Night
31%5%11%20%20%14%
80792
SeeingDay
7%10%25%32%20%6%
14532
Night
8%5%16%34%30%6%
24023

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 10thDay
12%7%28%6%35%5%6%8%38%
2735
11th to 20thDay
10%9%410%8%46%5%5%6%32%
2860
21st to endDay
13%10%49%7%35%6%6%8%29%
3146
February1st to 10thDay
10%7%45%4445%6%7%45%
1710
11th to 20thDay
10%7%38%5%25%47%7%43%
1759
21st to endDay
10%6%36%6%3345%6%47%
1570
March1st to 10thDay
11%7%47%7%46%6%8%8%34%
2052
11th to 20thDay
9%6%36%7%5%5%6%6%9%38%
2059
21st to endDay
9%5%36%6%35%5%5%7%45%
2208
April1st to 10thDay
10%6%46%5%3446%8%45%
2160
11th to 20thDay
8%5%2442336%7%55%
2129
21st to endDay
5%42442445%9%59%
2160
May1st to 10thDay
342432345%8%62%
2214
11th to 20thDay
441332345%8%64%
2335
21st to endDay
4424424448%60%
2723
June1st to 10thDay
442322345%8%63%
2520
11th to 20thDay
2112112247%77%
2520
21st to endDay
2112212235%78%
2520
July1st to 10thDay
1112212336%79%
2514
11th to 20thDay
1212212348%75%
2340
21st to endDay
111112237%79%
2546
August1st to 10thDay
2212223338%73%
2328
11th to 20thDay
112112347%78%
2178
21st to endDay
111112348%76%
2376
September1st to 10thDay
1111112335%81%
2160
11th to 20thDay
2313423347%68%
2160
21st to endDay
2212212246%76%
1910
October1st to 10thDay
4223423346%68%
1800
11th to 20thDay
431332345%8%64%
1800
21st to endDay
442343345%7%61%
1980
November1st to 10thDay
6%425%435%5%5%7%54%
1836
11th to 20thDay
6%5%35%5%346%5%7%51%
1710
21st to endDay
7%5%245%245%5%8%53%
1573
December1st to 10thDay
7%437%8%45%5%7%8%43%
1520
11th to 20thDay
13%9%48%7%46%5%6%7%31%
1928
21st to endDay
9%9%5%8%7%45%5%6%8%33%
2794
January1st to 10thNight
11%8%37%5%3447%7%42%
4213
11th to 20thNight
11%9%5%9%8%46%5%6%6%31%
4110
21st to endNight
13%12%38%7%5%6%5%46%30%
4521
February1st to 10thNight
9%6%45%5%35%446%49%
2470
11th to 20thNight
8%6%36%5%35%6%6%9%44%
2308
21st to endNight
8%6%26%5%345%48%49%
1884
March1st to 10thNight
11%6%47%7%35%6%6%7%39%
2123
11th to 20thNight
10%7%36%6%245%6%8%43%
2067
21st to endNight
9%6%35%5%346%5%5%51%
2147
April1st to 10thNight
8%5%36%5%35%47%7%46%
1800
11th to 20thNight
6%6%25%5%25%348%55%
1779
21st to endNight
42123335%5%9%62%
1800
May1st to 10thNight
342342446%8%60%
1742
11th to 20thNight
2324325%46%10%59%
1555
21st to endNight
33133246%6%11%58%
1552
June1st to 10thNight
331442345%10%63%
1440
11th to 20thNight
1112222246%78%
1440
21st to endNight
1212212336%77%
1440
July1st to 10thNight
21112327%79%
1441
11th to 20thNight
111122325%7%76%
1620
21st to endNight
1123235%6%12%66%
1773
August1st to 10thNight
231332345%10%64%
1611
11th to 20thNight
11222236%11%70%
1723
21st to endNight
12222335%9%72%
1980
September1st to 10thNight
111121245%7%76%
1800
11th to 20thNight
341332345%7%66%
1800
21st to endNight
2212212247%75%
2043
October1st to 10thNight
3313323347%68%
2160
11th to 20thNight
4415%423447%62%
2160
21st to endNight
432442345%7%63%
2376
November1st to 10thNight
5%43442446%9%56%
2242
11th to 20thNight
6%6%35%4345%6%8%51%
2470
21st to endNight
6%4235%2445%7%57%
2603
December1st to 10thNight
8%5%46%6%35%5%5%7%48%
2660
11th to 20thNight
14%9%47%7%45%5%5%8%31%
3276
21st to endNight
10%7%37%7%45%6%6%8%35%
4862

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 10thDay
43%8%12%26%8%3
2735
11th to 20thDay
51%7%11%13%10%8%
2860
21st to endDay
52%7%9%15%13%5%
3146
February1st to 10thDay
39%5%9%20%17%10%
1710
11th to 20thDay
39%5%10%18%21%7%
1768
21st to endDay
38%6%8%22%19%6%
1570
March1st to 10thDay
45%6%11%19%14%4
2052
11th to 20thDay
43%6%11%18%17%4
2059
21st to endDay
39%7%13%20%18%3
2208
April1st to 10thDay
38%5%14%19%19%5%
2160
11th to 20thDay
30%5%16%23%21%5%
2132
21st to endDay
24%5%15%28%23%5%
2160
May1st to 10thDay
22%6%12%25%29%6%
2214
11th to 20thDay
20%416%26%26%8%
2338
21st to endDay
24%6%20%24%21%4
2724
June1st to 10thDay
21%6%15%25%24%9%
2520
11th to 20thDay
10%412%25%33%17%
2520
21st to endDay
11%27%18%32%30%
2520
July1st to 10thDay
8%26%18%26%40%
2445
11th to 20thDay
10%28%17%25%37%
2271
21st to endDay
7%25%16%32%38%
2405
August1st to 10thDay
14%28%19%31%27%
2312
11th to 20thDay
8%26%20%33%32%
2178
21st to endDay
9%17%21%33%28%
2376
September1st to 10thDay
9%25%13%30%41%
2160
11th to 20thDay
17%310%25%28%18%
2160
21st to endDay
12%28%22%34%22%
1910
October1st to 10thDay
19%39%18%29%24%
1800
11th to 20thDay
20%5%10%20%22%23%
1800
21st to endDay
23%49%20%23%20%
1980
November1st to 10thDay
30%6%9%22%21%12%
1836
11th to 20thDay
32%6%11%21%22%8%
1710
21st to endDay
31%410%17%30%9%
1573
December1st to 10thDay
37%7%11%18%16%10%
1520
11th to 20thDay
54%6%10%13%13%5%
1928
21st to endDay
48%6%13%19%11%3
2794
January1st to 10thNight
41%7%17%22%10%3
4213
11th to 20thNight
53%6%14%11%9%7%
4110
21st to endNight
54%6%10%11%12%7%
4521
February1st to 10thNight
37%49%19%19%12%
2470
11th to 20thNight
37%5%11%19%22%7%
2310
21st to endNight
34%5%12%29%16%4
1884
March1st to 10thNight
44%6%15%16%14%5%
2123
11th to 20thNight
39%6%9%21%18%6%
2067
21st to endNight
36%6%15%20%17%6%
2147
April1st to 10thNight
36%6%17%21%16%5%
1800
11th to 20thNight
30%6%13%24%19%8%
1779
21st to endNight
20%6%15%31%21%7%
1800
May1st to 10thNight
23%5%13%23%24%11%
1742
11th to 20thNight
21%6%17%24%24%8%
1556
21st to endNight
23%8%21%23%21%4
1552
June1st to 10thNight
19%7%13%26%26%9%
1440
11th to 20thNight
10%411%23%31%21%
1440
21st to endNight
13%16%19%31%30%
1440
July1st to 10thNight
8%15%18%30%37%
1401
11th to 20thNight
10%29%19%24%36%
1566
21st to endNight
12%27%21%28%30%
1674
August1st to 10thNight
18%39%21%26%24%
1602
11th to 20thNight
11%26%20%30%31%
1723
21st to endNight
11%28%21%29%29%
1980
September1st to 10thNight
9%37%15%24%42%
1800
11th to 20thNight
18%48%25%28%18%
1800
21st to endNight
12%38%21%30%26%
2043
October1st to 10thNight
18%49%19%25%26%
2160
11th to 20thNight
23%48%20%21%24%
2160
21st to endNight
22%49%20%25%21%
2376
November1st to 10thNight
26%5%12%24%19%14%
2242
11th to 20thNight
30%6%11%24%21%9%
2470
21st to endNight
27%411%21%23%13%
2603
December1st to 10thNight
37%7%9%18%17%12%
2660
11th to 20thNight
53%5%10%14%15%4
3276
21st to endNight
48%7%15%16%11%4
4862

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 10thDay
21%15%25%23%14%2
298
11th to 20thDay
14%11%27%30%14%4
370
21st to endDay
14%11%28%27%17%4
407
February1st to 10thDay
13%19%25%24%14%5%
370
11th to 20thDay
14%20%30%24%11%2
410
21st to endDay
12%16%31%28%12%2
382
March1st to 10thDay
16%18%33%24%9%1
460
11th to 20thDay
15%20%27%21%13%4
449
21st to endDay
11%19%35%24%8%2
462
April1st to 10thDay
12%15%35%24%11%2
420
11th to 20thDay
12%18%30%25%11%3
412
21st to endDay
421%30%28%11%6%
420
May1st to 10thDay
6%13%33%30%13%4
423
11th to 20thDay
7%13%34%30%11%4
448
21st to endDay
6%13%29%31%16%5%
485
June1st to 10thDay
411%30%31%18%6%
450
11th to 20thDay
210%30%35%18%4
450
21st to endDay
1425%40%24%6%
450
July1st to 10thDay
2318%44%29%4
446
11th to 20thDay
115%43%31%9%
444
21st to endDay
114%44%31%10%
487
August1st to 10thDay
2222%41%25%8%
447
11th to 20thDay
1119%41%26%12%
437
21st to endDay
117%43%29%9%
495
September1st to 10thDay
1414%40%31%10%
450
11th to 20thDay
35%24%38%21%8%
450
21st to endDay
38%19%35%27%8%
450
October1st to 10thDay
39%22%34%22%10%
450
11th to 20thDay
5%9%20%30%26%10%
450
21st to endDay
7%6%24%31%24%7%
495
November1st to 10thDay
7%11%24%25%25%9%
432
11th to 20thDay
49%19%31%26%11%
270
21st to endDay
6%8%24%31%25%6%
205
December1st to 10thDay
7%13%23%26%23%8%
180
11th to 20thDay
11%17%22%21%21%9%
180
21st to endDay
16%18%22%25%15%4
198
January1st to 10thNight
17%7%18%31%22%5%
760
11th to 20thNight
14%8%15%34%23%7%
760
21st to endNight
16%7%15%34%23%6%
836
February1st to 10thNight
13%8%17%35%22%5%
760
11th to 20thNight
13%9%19%33%21%5%
744
21st to endNight
12%10%22%35%19%3
628
March1st to 10thNight
13%12%21%31%20%3
760
11th to 20thNight
14%9%17%33%23%4
752
21st to endNight
11%9%23%37%18%2
792
April1st to 10thNight
10%8%23%36%20%3
720
11th to 20thNight
9%13%21%33%19%5%
711
21st to endNight
5%7%26%32%22%7%
720
May1st to 10thNight
5%425%36%26%5%
662
11th to 20thNight
4318%37%32%6%
493
21st to endNight
45%22%36%29%4
389
June1st to 10thNight
4419%42%29%3
360
11th to 20thNight
1315%48%32%1
360
21st to endNight
1114%39%43%3
360
July1st to 10thNight
1110%34%51%3
351
11th to 20thNight
1235%49%12%
522
21st to endNight
1331%55%10%
591
August1st to 10thNight
417%33%47%7%
537
11th to 20thNight
118%39%45%7%
655
21st to endNight
28%34%46%10%
792
September1st to 10thNight
118%37%43%11%
720
11th to 20thNight
4212%37%38%8%
720
21st to endNight
3211%32%41%11%
720
October1st to 10thNight
4212%37%35%10%
720
11th to 20thNight
6%411%31%40%9%
720
21st to endNight
6%5%10%34%32%14%
792
November1st to 10thNight
7%416%31%34%9%
740
11th to 20thNight
9%314%35%31%7%
760
21st to endNight
9%415%39%26%7%
760
December1st to 10thNight
11%6%19%34%25%6%
760
11th to 20thNight
18%5%20%32%21%4
760
21st to endNight
15%11%22%31%19%3
836
Page generated at 2024-03-15 11:01:41. Version ac07

Copyright © 2024 Attilla Danko.


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