This page made possible by the sponsorship of: Steve Altstadt.

Sunset Farm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Farm Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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
33%
25084
14%
74807
7%
25034
39%
25034
21%
51435

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
24%
949
11%
2837
5%
946
31%
946
15%
2083
January11th to 20thNight
20%
925
10%
2680
4%
925
25%
925
12%
2095
January21st to endNight
20%
990
12%
2772
4%
990
27%
990
13%
2189
February1st to 10thNight
26%
900
12%
2453
3%
900
33%
900
17%
1818
February11th to 20thNight
29%
876
14%
2274
6%
874
34%
874
21%
1625
February21st to endNight
29%
615
19%
1799
6%
615
39%
615
21%
1264
March1st to 10thNight
27%
720
11%
2160
5%
720
34%
720
17%
1630
March11th to 20thNight
31%
540
20%
1980
13%
540
43%
540
19%
1268
March21st to endNight
32%
594
12%
2178
11%
594
38%
594
15%
1524
April1st to 10thNight
33%
540
18%
1764
12%
540
45%
540
23%
1112
April11th to 20thNight
33%
533
15%
1599
11%
533
40%
533
20%
1134
April21st to endNight
38%
540
13%
1620
9%
540
42%
540
22%
1100
May1st to 10thNight
37%
540
13%
1620
6%
540
41%
540
27%
1035
May11th to 20thNight
39%
531
16%
1272
6%
531
42%
531
30%
873
May21st to endNight
39%
584
11%
1358
4%
582
43%
582
29%
951
June1st to 10thNight
45%
540
8%
1260
4%
540
48%
540
34%
861
June11th to 20thNight
40%
540
7%
1260
3%
540
45%
540
34%
851
June21st to endNight
52%
540
8%
1260
5%
540
56%
540
36%
831
July1st to 10thNight
59%
525
10%
1225
6%
525
61%
525
49%
735
July11th to 20thNight
54%
522
14%
1218
6%
522
57%
522
44%
740
July21st to endNight
61%
591
17%
1429
8%
558
63%
558
49%
802
August1st to 10thNight
57%
537
18%
1472
9%
534
61%
534
42%
836
August11th to 20thNight
53%
534
19%
1600
12%
534
60%
534
36%
919
August21st to endNight
51%
594
22%
1782
19%
594
59%
594
36%
1017
September1st to 10thNight
48%
540
20%
1757
18%
540
56%
540
35%
967
September11th to 20thNight
45%
540
19%
1980
16%
540
55%
540
28%
1103
September21st to endNight
36%
540
21%
1980
16%
540
46%
540
23%
1202
October1st to 10thNight
29%
618
17%
2058
11%
618
36%
618
19%
1380
October11th to 20thNight
32%
799
19%
2239
9%
799
39%
799
23%
1503
October21st to endNight
26%
990
14%
2574
4%
990
32%
990
20%
1869
November1st to 10thNight
24%
925
15%
2481
3%
925
31%
925
18%
1815
November11th to 20thNight
21%
950
12%
2660
4%
950
28%
950
15%
2037
November21st to endNight
18%
950
11%
2806
3%
950
25%
950
14%
2096
December1st to 10thNight
23%
950
9%
2850
3%
950
29%
950
14%
2153
December11th to 20thNight
16%
949
7%
3488
1%
949
23%
949
9%
2685
December21st to endNight
19%
1033
8%
5062
4%
1026
27%
1026
8%
3332

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
11%9%47%6%45%5%6%8%35%
74651
Night
10%9%48%7%45%5%6%8%36%
75164
TransparencyDay
46%7%15%22%9%1
74338
Night
46%7%14%22%10%1
74999
SeeingDay
14%11%32%28%12%3
15023
Night
13%10%34%30%11%1
25101

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
6%11%5%11%10%5%5%8%7%9%23%
1328
11th to 20thDay
7%13%7%13%10%5%7%6%8%8%17%
1385
21st to endDay
8%13%7%12%10%5%7%6%9%7%15%
1584
February1st to 10thDay
8%12%5%11%9%46%7%7%8%23%
1506
11th to 20thDay
5%10%5%9%10%6%8%9%9%8%22%
1574
21st to endDay
7%9%6%11%8%5%6%8%6%8%24%
1471
March1st to 10thDay
9%13%7%12%10%5%7%5%6%7%20%
1800
11th to 20thDay
7%10%48%9%5%6%6%6%9%30%
1980
21st to endDay
13%12%5%9%6%45%46%7%28%
2178
April1st to 10thDay
9%11%6%8%6%35%5%47%34%
2194
11th to 20thDay
13%12%48%6%45%5%5%6%31%
2309
21st to endDay
16%10%47%6%35%45%6%32%
2340
May1st to 10thDay
15%9%36%5%345%5%7%37%
2340
11th to 20thDay
17%9%35%4345%5%7%38%
2618
21st to endDay
18%7%35%6%35%5%5%8%36%
2918
June1st to 10thDay
13%6%35%6%46%5%6%9%38%
2700
11th to 20thDay
14%5%36%6%45%5%6%8%36%
2700
21st to endDay
12%6%26%6%45%6%7%10%37%
2700
July1st to 10thDay
7%5%245%45%6%7%9%46%
2700
11th to 20thDay
9%5%25%6%36%5%6%9%44%
2700
21st to endDay
6%3144347%6%9%53%
2803
August1st to 10thDay
7%5%26%6%35%6%5%8%46%
2459
11th to 20thDay
10%4245%35%6%7%7%46%
2301
21st to endDay
11%6%35%5%3345%8%48%
2574
September1st to 10thDay
12%6%2442445%6%50%
2201
11th to 20thDay
14%6%25%5%4445%8%42%
1980
21st to endDay
14%7%35%435%46%9%41%
1980
October1st to 10thDay
16%12%46%4245%46%36%
1883
11th to 20thDay
13%12%47%5%3445%8%34%
1720
21st to endDay
15%12%5%8%5%4446%9%27%
1782
November1st to 10thDay
14%12%6%11%6%345%5%6%29%
1595
11th to 20thDay
9%12%6%14%10%445%5%6%26%
1520
21st to endDay
9%14%5%12%10%45%5%46%27%
1373
December1st to 10thDay
8%11%6%11%10%5%7%7%6%7%21%
1330
11th to 20thDay
10%12%6%13%11%5%7%5%5%8%19%
1683
21st to endDay
8%9%5%9%7%46%5%6%7%32%
2442
January1st to 10thNight
7%11%6%10%8%5%6%6%7%10%25%
2837
11th to 20thNight
9%13%6%11%8%5%7%6%6%8%20%
2680
21st to endNight
7%12%6%12%10%5%7%7%7%8%19%
2772
February1st to 10thNight
7%10%5%9%9%5%7%7%7%7%26%
2453
11th to 20thNight
6%8%48%8%5%8%7%7%9%31%
2273
21st to endNight
7%10%5%9%7%46%5%7%8%32%
1799
March1st to 10thNight
10%11%7%10%8%45%6%5%8%27%
2160
11th to 20thNight
7%8%48%7%46%6%5%7%39%
1980
21st to endNight
11%10%5%10%7%45%5%5%6%33%
2178
April1st to 10thNight
7%7%47%6%35%6%6%7%41%
1764
11th to 20thNight
12%11%5%7%5%35%6%5%7%34%
1599
21st to endNight
14%8%46%5%45%5%5%7%37%
1620
May1st to 10thNight
13%7%45%5%35%45%7%42%
1620
11th to 20thNight
12%10%26%433448%42%
1272
21st to endNight
16%7%4442446%7%42%
1358
June1st to 10thNight
10%425%7%35%46%9%45%
1260
11th to 20thNight
11%5%35%5%3447%9%44%
1260
21st to endNight
10%4245%35%47%10%47%
1260
July1st to 10thNight
6%3244335%5%8%58%
1260
11th to 20thNight
5%3244246%6%8%57%
1260
21st to endNight
43143345%7%8%58%
1513
August1st to 10thNight
6%415%43336%8%56%
1480
11th to 20thNight
7%3235%35%5%5%9%54%
1600
21st to endNight
9%5%1443445%6%54%
1782
September1st to 10thNight
9%5%25%32346%7%54%
1757
11th to 20thNight
12%425%43445%7%51%
1980
21st to endNight
14%7%25%5%44447%44%
1980
October1st to 10thNight
14%9%46%6%345%45%39%
2058
11th to 20thNight
14%10%35%5%3446%8%39%
2239
21st to endNight
11%12%48%6%45%45%8%34%
2574
November1st to 10thNight
11%11%49%7%35%5%6%8%31%
2481
11th to 20thNight
9%13%6%10%9%45%45%7%28%
2660
21st to endNight
10%12%410%8%45%5%6%7%29%
2806
December1st to 10thNight
7%11%6%11%10%5%7%6%6%7%25%
2850
11th to 20thNight
10%12%7%12%10%5%6%46%7%20%
3514
21st to endNight
10%11%5%9%8%5%5%45%6%32%
5225

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
55%7%14%18%5%1
1329
11th to 20thDay
63%6%10%14%8%
1385
21st to endDay
65%5%9%14%7%
1584
February1st to 10thDay
55%6%12%19%8%1
1506
11th to 20thDay
54%8%12%16%10%1
1583
21st to endDay
55%6%12%17%9%1
1471
March1st to 10thDay
62%8%13%12%6%
1800
11th to 20thDay
49%9%14%20%8%
1980
21st to endDay
54%8%13%18%7%
2178
April1st to 10thDay
49%6%14%23%8%1
2194
11th to 20thDay
52%6%12%19%10%1
2312
21st to endDay
52%6%16%16%8%1
2340
May1st to 10thDay
46%6%12%25%10%1
2340
11th to 20thDay
44%6%13%25%10%2
2622
21st to endDay
46%8%15%22%9%1
2918
June1st to 10thDay
41%9%19%21%9%1
2700
11th to 20thDay
44%9%15%22%9%1
2700
21st to endDay
39%11%20%24%5%1
2700
July1st to 10thDay
31%8%21%31%9%1
2626
11th to 20thDay
34%9%18%27%11%
2619
21st to endDay
26%8%21%32%13%1
2647
August1st to 10thDay
35%10%17%27%10%1
2443
11th to 20thDay
32%9%19%31%8%
2301
21st to endDay
34%7%15%32%13%
2574
September1st to 10thDay
34%6%19%28%12%1
2201
11th to 20thDay
40%8%17%24%10%1
1980
21st to endDay
40%7%15%31%7%
1980
October1st to 10thDay
48%8%14%22%9%
1883
11th to 20thDay
48%5%15%23%9%
1720
21st to endDay
53%7%18%15%5%
1782
November1st to 10thDay
55%8%11%17%8%1
1595
11th to 20thDay
58%7%15%13%6%
1520
21st to endDay
59%8%12%15%7%
1373
December1st to 10thDay
60%8%14%13%5%
1330
11th to 20thDay
62%7%11%13%6%1
1680
21st to endDay
48%7%12%20%13%
2442
January1st to 10thNight
52%8%13%19%8%1
2837
11th to 20thNight
60%7%11%14%8%
2680
21st to endNight
60%7%10%13%9%
2772
February1st to 10thNight
53%7%13%17%10%1
2453
11th to 20thNight
48%7%15%18%12%1
2274
21st to endNight
47%7%13%22%11%1
1799
March1st to 10thNight
54%8%13%17%7%1
2160
11th to 20thNight
44%6%12%24%12%1
1980
21st to endNight
50%8%14%17%9%1
2178
April1st to 10thNight
40%7%13%23%15%2
1764
11th to 20thNight
49%7%11%18%13%2
1599
21st to endNight
45%6%15%21%11%1
1620
May1st to 10thNight
40%6%15%26%13%1
1620
11th to 20thNight
41%7%13%24%14%2
1273
21st to endNight
41%7%13%25%13%1
1358
June1st to 10thNight
34%10%19%28%8%2
1260
11th to 20thNight
36%7%18%25%13%2
1260
21st to endNight
31%10%21%31%6%1
1260
July1st to 10thNight
24%6%19%37%13%1
1225
11th to 20thNight
23%10%19%32%16%1
1218
21st to endNight
21%6%18%37%16%2
1429
August1st to 10thNight
26%6%19%34%13%2
1472
11th to 20thNight
27%9%21%31%12%1
1600
21st to endNight
29%7%15%32%17%1
1782
September1st to 10thNight
28%7%15%32%16%2
1757
11th to 20thNight
32%6%19%28%14%1
1980
21st to endNight
40%6%13%30%10%1
1980
October1st to 10thNight
45%8%14%24%9%
2058
11th to 20thNight
43%6%14%26%10%
2239
21st to endNight
48%7%16%22%6%1
2574
November1st to 10thNight
49%8%14%18%9%2
2481
11th to 20thNight
56%7%14%14%8%1
2660
21st to endNight
54%8%15%17%7%
2806
December1st to 10thNight
56%7%16%14%6%
2850
11th to 20thNight
61%7%15%12%41
3516
21st to endNight
51%7%10%19%12%1
5225

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
11%12%37%29%10%2
270
11th to 20thDay
10%12%39%27%11%1
254
21st to endDay
15%10%41%23%9%2
264
February1st to 10thDay
13%16%43%23%31
240
11th to 20thDay
8%16%43%25%7%1
240
21st to endDay
7%9%49%27%8%1
198
March1st to 10thDay
12%11%37%34%5%1
240
11th to 20thDay
15%12%30%30%11%2
240
21st to endDay
13%14%33%28%9%2
264
April1st to 10thDay
12%21%34%24%7%1
454
11th to 20thDay
20%12%32%25%10%1
590
21st to endDay
20%13%31%24%10%2
600
May1st to 10thDay
15%13%29%29%12%2
603
11th to 20thDay
19%12%30%25%12%3
625
21st to endDay
19%9%24%32%13%2
679
June1st to 10thDay
12%12%30%27%15%5%
630
11th to 20thDay
14%13%35%24%12%3
630
21st to endDay
12%8%28%33%15%3
630
July1st to 10thDay
8%6%29%32%19%5%
621
11th to 20thDay
11%8%32%26%18%4
618
21st to endDay
45%28%37%20%6%
684
August1st to 10thDay
6%7%29%32%20%6%
626
11th to 20thDay
12%5%25%33%19%7%
615
21st to endDay
11%9%26%31%18%4
693
September1st to 10thDay
12%11%36%26%12%3
491
11th to 20thDay
19%11%30%27%12%1
270
21st to endDay
23%10%31%26%9%1
270
October1st to 10thDay
21%16%26%26%12%
270
11th to 20thDay
16%12%41%22%7%2
270
21st to endDay
21%15%37%18%6%2
297
November1st to 10thDay
23%13%32%23%8%1
270
11th to 20thDay
14%15%42%22%6%
270
21st to endDay
16%17%43%20%4
270
December1st to 10thDay
12%13%39%27%7%2
270
11th to 20thDay
15%15%43%23%31
270
21st to endDay
11%10%39%28%9%3
297
January1st to 10thNight
12%13%37%29%8%1
950
11th to 20thNight
14%12%40%28%6%
925
21st to endNight
13%13%41%25%6%1
990
February1st to 10thNight
11%12%37%31%7%1
900
11th to 20thNight
9%11%39%29%10%2
879
21st to endNight
11%11%39%29%9%1
615
March1st to 10thNight
13%12%33%28%12%3
720
11th to 20thNight
9%10%40%28%11%2
540
21st to endNight
16%10%28%32%11%3
594
April1st to 10thNight
10%11%39%30%9%1
540
11th to 20thNight
17%8%34%31%10%
533
21st to endNight
16%8%29%35%11%1
540
May1st to 10thNight
16%7%33%32%11%1
540
11th to 20thNight
16%8%31%29%15%
531
21st to endNight
20%8%28%34%10%1
584
June1st to 10thNight
12%7%28%38%13%1
540
11th to 20thNight
13%6%34%32%16%
540
21st to endNight
10%6%27%39%17%1
540
July1st to 10thNight
7%426%41%22%1
525
11th to 20thNight
6%6%29%40%18%1
522
21st to endNight
5%425%44%19%2
591
August1st to 10thNight
8%325%43%20%1
537
11th to 20thNight
8%5%28%37%19%4
534
21st to endNight
11%5%29%38%16%1
594
September1st to 10thNight
12%432%34%16%2
540
11th to 20thNight
12%9%29%38%12%1
540
21st to endNight
17%8%33%29%12%1
540
October1st to 10thNight
20%11%32%29%8%
618
11th to 20thNight
19%11%32%29%9%
799
21st to endNight
17%13%37%25%8%1
990
November1st to 10thNight
16%15%36%26%7%1
925
11th to 20thNight
14%14%40%23%7%2
950
21st to endNight
18%15%40%21%5%1
950
December1st to 10thNight
11%13%39%28%7%1
950
11th to 20thNight
14%15%38%25%7%1
950
21st to endNight
19%9%36%27%8%1
1045
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