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Salt Lake Astronomical Society
.

Stansbury Park Observatory Complex (SPOC)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stansbury Park Observatory Complex (SPOC) clear sky chart from 2005-11-06 to 2024-01-16.

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
57%
24013
23%
74589
19%
23971
62%
23971
31%
44373

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
47%
756
15%
2457
14%
756
51%
756
20%
1676
January11th to 20thNight
48%
740
16%
2405
15%
740
51%
740
20%
1641
January21st to endNight
49%
792
16%
2574
15%
792
52%
792
19%
1792
February1st to 10thNight
49%
720
15%
2340
15%
720
53%
720
22%
1577
February11th to 20thNight
48%
700
19%
2181
17%
700
53%
700
26%
1403
February21st to endNight
45%
592
23%
1776
19%
592
51%
592
23%
1184
March1st to 10thNight
47%
720
18%
2057
15%
720
52%
720
25%
1401
March11th to 20thNight
55%
720
22%
1980
17%
720
59%
720
33%
1246
March21st to endNight
45%
792
18%
2178
12%
792
50%
792
26%
1480
April1st to 10thNight
48%
720
19%
1980
12%
720
52%
720
31%
1260
April11th to 20thNight
45%
538
26%
1609
16%
538
53%
538
30%
987
April21st to endNight
54%
540
25%
1620
16%
540
59%
540
31%
967
May1st to 10thNight
54%
540
22%
1620
16%
540
58%
540
30%
1004
May11th to 20thNight
64%
531
24%
1595
19%
531
67%
531
38%
892
May21st to endNight
60%
582
18%
1746
16%
582
63%
582
32%
1043
June1st to 10thNight
66%
540
22%
1620
20%
540
71%
540
40%
867
June11th to 20thNight
63%
540
25%
1620
22%
540
69%
540
46%
801
June21st to endNight
70%
381
30%
1461
33%
381
80%
381
48%
609
July1st to 10thNight
72%
457
23%
1507
22%
457
77%
457
44%
735
July11th to 20thNight
71%
522
23%
1566
22%
522
73%
522
41%
850
July21st to endNight
75%
591
25%
1674
23%
558
76%
558
42%
911
August1st to 10thNight
70%
537
29%
1602
21%
534
75%
534
45%
830
August11th to 20thNight
76%
534
33%
1602
26%
534
79%
534
47%
820
August21st to endNight
74%
594
35%
1782
30%
594
76%
594
46%
917
September1st to 10thNight
74%
699
33%
1954
26%
699
79%
699
54%
974
September11th to 20thNight
73%
720
35%
1980
28%
720
77%
720
53%
1013
September21st to endNight
70%
720
36%
1980
29%
720
74%
720
52%
1019
October1st to 10thNight
63%
720
29%
2046
21%
720
68%
720
43%
1140
October11th to 20thNight
66%
720
33%
2160
27%
720
72%
720
43%
1145
October21st to endNight
59%
792
32%
2407
26%
792
66%
792
40%
1306
November1st to 10thNight
58%
740
26%
2405
24%
740
62%
740
31%
1384
November11th to 20thNight
55%
760
26%
2470
23%
760
61%
760
32%
1428
November21st to endNight
56%
788
24%
2498
22%
788
59%
788
29%
1518
December1st to 10thNight
52%
950
16%
2660
13%
950
54%
950
28%
1752
December11th to 20thNight
46%
903
18%
3064
13%
902
48%
902
19%
2026
December21st to endNight
36%
822
9%
4413
11%
817
39%
817
10%
2775

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%7%35%5%35%5%5%7%49%
74757
Night
7%8%35%5%345%5%7%48%
75038
TransparencyDay
33%6%10%20%20%11%
74483
Night
34%6%10%21%19%10%
74833
SeeingDay
8%9%25%34%20%4
13667
Night
11%5%19%36%24%5%
24038

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
10%14%48%7%47%5%5%8%28%
1708
11th to 20thDay
8%13%5%11%9%447%5%8%26%
1665
21st to endDay
9%15%6%8%8%5%6%6%47%27%
1782
February1st to 10thDay
9%12%5%8%5%445%5%9%34%
1620
11th to 20thDay
9%13%5%8%5%25%5%5%7%35%
1664
21st to endDay
6%12%6%8%7%46%5%5%7%34%
1480
March1st to 10thDay
7%10%47%6%36%5%48%40%
1903
11th to 20thDay
5%9%36%6%45%7%7%9%39%
1980
21st to endDay
10%12%37%6%35%7%5%7%33%
2178
April1st to 10thDay
10%9%35%5%36%5%5%8%41%
1980
11th to 20thDay
8%9%35%6%36%45%6%44%
2296
21st to endDay
7%8%25%5%2446%8%49%
2340
May1st to 10thDay
6%6%26%5%445%7%9%47%
2340
11th to 20thDay
7%5%145%35%6%7%10%48%
2301
21st to endDay
7%5%25%6%46%6%7%8%45%
2530
June1st to 10thDay
3224425%6%5%10%58%
2340
11th to 20thDay
431342445%6%64%
2340
21st to endDay
1212322446%73%
2485
July1st to 10thDay
221333446%7%66%
2394
11th to 20thDay
32233245%5%8%63%
2340
21st to endDay
33134346%8%9%57%
2545
August1st to 10thDay
2213435%5%6%9%59%
2328
11th to 20thDay
22133235%5%7%66%
2299
21st to endDay
321433346%7%64%
2574
September1st to 10thDay
221331245%6%72%
2001
11th to 20thDay
421332445%6%66%
1980
21st to endDay
7%312313336%68%
1980
October1st to 10thDay
7%5%244345%5%7%54%
1913
11th to 20thDay
8%424323447%60%
1800
21st to endDay
6%7%333345%5%6%54%
1944
November1st to 10thDay
8%7%345%35%5%5%7%47%
1674
11th to 20thDay
11%8%45%423448%47%
1710
21st to endDay
7%11%47%7%345%46%41%
1656
December1st to 10thDay
9%11%48%6%45%5%5%7%36%
1520
11th to 20thDay
8%13%5%8%7%47%6%7%10%25%
2021
21st to endDay
8%14%5%8%6%47%6%6%7%29%
3146
January1st to 10thNight
10%12%48%6%45%5%5%8%32%
2457
11th to 20thNight
10%12%47%7%35%5%6%6%34%
2405
21st to endNight
9%15%47%7%36%5%6%6%32%
2574
February1st to 10thNight
8%11%47%6%45%6%6%7%36%
2340
11th to 20thNight
8%12%5%7%5%3445%5%43%
2180
21st to endNight
5%14%5%7%5%35%6%6%7%38%
1776
March1st to 10thNight
8%11%47%6%45%448%37%
2057
11th to 20thNight
5%9%35%5%46%5%7%8%43%
1980
21st to endNight
8%10%47%5%35%7%7%7%37%
2178
April1st to 10thNight
11%9%345%33446%48%
1980
11th to 20thNight
8%9%25%5%245%7%8%46%
1609
21st to endNight
9%6%35%5%345%5%7%49%
1620
May1st to 10thNight
8%6%35%5%35%5%7%7%45%
1620
11th to 20thNight
7%4245%4445%10%51%
1594
21st to endNight
8%5%25%6%35%5%6%11%45%
1746
June1st to 10thNight
5%4244335%6%8%57%
1620
11th to 20thNight
431342346%6%64%
1620
21st to endNight
221242436%8%66%
1461
July1st to 10thNight
3214435%46%8%60%
1552
11th to 20thNight
33246%45%5%6%7%55%
1620
21st to endNight
4324445%5%6%9%54%
1773
August1st to 10thNight
42234245%6%9%58%
1611
11th to 20thNight
2323435%46%8%60%
1602
21st to endNight
32144345%6%7%61%
1782
September1st to 10thNight
4213323346%68%
1954
11th to 20thNight
4313323436%68%
1980
21st to endNight
6%21232334470%
1980
October1st to 10thNight
7%424434445%59%
2046
11th to 20thNight
7%5%24423346%61%
2160
21st to endNight
7%7%2432345%7%57%
2407
November1st to 10thNight
9%9%25%32446%7%49%
2405
11th to 20thNight
8%9%35%5%2445%7%47%
2470
21st to endNight
7%9%5%5%6%34447%45%
2498
December1st to 10thNight
9%10%36%5%345%6%7%40%
2660
11th to 20thNight
10%12%46%6%36%5%5%8%35%
3200
21st to endNight
9%13%5%8%6%35%5%5%6%33%
4521

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
51%9%17%18%5%
1709
11th to 20thDay
51%7%14%19%7%1
1665
21st to endDay
54%9%16%13%6%2
1782
February1st to 10thDay
49%7%13%21%10%2
1620
11th to 20thDay
49%6%12%19%13%1
1673
21st to endDay
51%6%13%21%10%1
1480
March1st to 10thDay
42%8%12%23%13%2
1903
11th to 20thDay
41%6%14%25%13%2
1980
21st to endDay
49%8%13%19%9%2
2178
April1st to 10thDay
43%7%10%19%16%5%
1980
11th to 20thDay
41%6%10%20%19%4
2299
21st to endDay
34%6%10%26%22%2
2340
May1st to 10thDay
33%7%8%25%22%5%
2340
11th to 20thDay
31%7%8%26%23%6%
2304
21st to endDay
35%7%10%24%19%5%
2530
June1st to 10thDay
23%5%8%22%32%10%
2340
11th to 20thDay
21%5%5%18%30%20%
2340
21st to endDay
13%4312%32%35%
2485
July1st to 10thDay
18%45%12%25%36%
2330
11th to 20thDay
19%5%5%13%26%32%
2271
21st to endDay
23%6%7%13%24%27%
2404
August1st to 10thDay
22%6%5%17%22%27%
2313
11th to 20thDay
18%5%413%32%27%
2299
21st to endDay
21%5%415%33%22%
2574
September1st to 10thDay
14%5%417%30%29%
2001
11th to 20thDay
20%5%7%20%26%23%
1980
21st to endDay
20%5%5%18%35%17%
1980
October1st to 10thDay
31%5%7%20%29%8%
1913
11th to 20thDay
26%5%5%31%28%6%
1800
21st to endDay
28%7%13%27%22%2
1944
November1st to 10thDay
36%6%12%22%20%3
1674
11th to 20thDay
36%6%11%31%15%1
1710
21st to endDay
43%413%24%14%1
1656
December1st to 10thDay
44%8%17%21%8%2
1520
11th to 20thDay
49%6%14%22%10%
2020
21st to endDay
47%8%15%16%13%1
3146
January1st to 10thNight
47%7%16%22%7%1
2457
11th to 20thNight
48%6%15%21%9%1
2405
21st to endNight
47%8%16%19%7%3
2574
February1st to 10thNight
45%7%15%24%8%1
2340
11th to 20thNight
42%5%10%23%17%2
2181
21st to endNight
43%7%10%23%15%2
1776
March1st to 10thNight
45%5%11%25%13%2
2057
11th to 20thNight
36%6%13%27%16%3
1980
21st to endNight
41%9%12%24%11%3
2178
April1st to 10thNight
37%6%10%23%17%7%
1980
11th to 20thNight
35%7%9%23%23%4
1609
21st to endNight
34%6%11%24%23%2
1620
May1st to 10thNight
35%7%8%23%23%4
1620
11th to 20thNight
28%6%8%26%24%8%
1595
21st to endNight
32%7%13%26%17%4
1746
June1st to 10thNight
25%6%7%23%27%11%
1620
11th to 20thNight
20%47%19%28%21%
1620
21st to endNight
17%4313%27%36%
1461
July1st to 10thNight
22%5%5%13%18%37%
1507
11th to 20thNight
27%45%11%22%31%
1566
21st to endNight
25%6%8%14%19%28%
1674
August1st to 10thNight
22%6%7%15%22%27%
1602
11th to 20thNight
22%5%412%28%29%
1602
21st to endNight
22%5%5%12%28%27%
1782
September1st to 10thNight
19%3415%25%34%
1954
11th to 20thNight
19%45%16%29%26%
1980
21st to endNight
20%4316%32%25%
1980
October1st to 10thNight
27%48%21%30%10%
2046
11th to 20thNight
25%46%26%30%8%
2160
21st to endNight
27%5%9%27%27%5%
2407
November1st to 10thNight
33%6%9%25%21%6%
2405
11th to 20thNight
34%5%12%29%17%3
2470
21st to endNight
37%5%10%29%16%3
2498
December1st to 10thNight
40%7%16%24%11%3
2660
11th to 20thNight
44%7%14%23%12%
3200
21st to endNight
48%7%15%17%13%1
4521

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
15%13%28%26%14%4
270
11th to 20thDay
15%11%26%28%17%3
255
21st to endDay
17%10%23%33%13%3
264
February1st to 10thDay
14%14%29%30%10%3
240
11th to 20thDay
12%15%32%26%14%1
329
21st to endDay
8%17%36%29%9%1
346
March1st to 10thDay
11%13%30%34%10%1
420
11th to 20thDay
9%12%35%32%11%2
420
21st to endDay
16%15%34%28%5%
462
April1st to 10thDay
13%13%33%25%15%2
420
11th to 20thDay
15%22%26%26%9%2
412
21st to endDay
12%15%30%23%18%2
420
May1st to 10thDay
6%12%38%29%12%3
423
11th to 20thDay
7%10%28%30%19%6%
448
21st to endDay
6%10%26%31%22%5%
485
June1st to 10thDay
37%29%34%22%5%
450
11th to 20thDay
5%12%30%33%18%4
450
21st to endDay
26%25%42%21%4
450
July1st to 10thDay
2218%49%24%4
446
11th to 20thDay
2221%46%23%5%
444
21st to endDay
3111%49%29%7%
487
August1st to 10thDay
3219%47%25%4
447
11th to 20thDay
3313%46%28%7%
437
21st to endDay
4216%39%32%6%
495
September1st to 10thDay
35%17%36%28%11%
450
11th to 20thDay
5%7%20%34%28%6%
450
21st to endDay
7%6%19%32%31%5%
450
October1st to 10thDay
7%9%26%30%23%5%
383
11th to 20thDay
6%11%20%28%26%9%
270
21st to endDay
11%7%24%33%23%2
297
November1st to 10thDay
16%12%21%26%20%6%
270
11th to 20thDay
11%10%21%34%16%8%
270
21st to endDay
16%8%19%26%27%5%
270
December1st to 10thDay
13%10%24%33%16%4
270
11th to 20thDay
15%7%22%39%14%1
270
21st to endDay
19%11%26%30%14%1
297
January1st to 10thNight
14%9%19%32%20%6%
760
11th to 20thNight
16%7%19%33%21%6%
740
21st to endNight
18%6%17%35%20%5%
792
February1st to 10thNight
15%421%39%16%4
720
11th to 20thNight
14%5%21%37%19%4
704
21st to endNight
12%8%23%39%16%2
592
March1st to 10thNight
13%8%23%38%16%3
720
11th to 20thNight
8%7%21%39%21%3
720
21st to endNight
14%6%28%38%13%2
792
April1st to 10thNight
16%8%23%35%16%2
720
11th to 20thNight
12%8%28%34%16%2
538
21st to endNight
13%7%20%37%20%3
540
May1st to 10thNight
9%6%25%39%19%3
540
11th to 20thNight
8%419%34%30%5%
531
21st to endNight
9%6%17%34%29%5%
584
June1st to 10thNight
6%6%16%38%30%4
540
11th to 20thNight
48%20%40%26%2
540
21st to endNight
2318%38%32%6%
381
July1st to 10thNight
4214%36%39%5%
457
11th to 20thNight
5%214%44%32%3
522
21st to endNight
5%111%30%48%5%
591
August1st to 10thNight
4216%38%37%3
537
11th to 20thNight
1114%43%36%4
534
21st to endNight
3314%37%37%6%
594
September1st to 10thNight
4412%35%36%8%
699
11th to 20thNight
5%314%35%34%9%
720
21st to endNight
7%415%31%31%12%
720
October1st to 10thNight
8%5%18%37%26%6%
720
11th to 20thNight
10%316%35%28%8%
720
21st to endNight
10%5%22%31%26%6%
792
November1st to 10thNight
13%7%16%32%25%6%
740
11th to 20thNight
13%421%38%17%7%
760
21st to endNight
11%5%18%38%21%6%
788
December1st to 10thNight
15%5%19%39%20%2
950
11th to 20thNight
19%5%20%36%17%3
904
21st to endNight
19%9%23%33%15%1
836
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