West Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Valley Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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%
23167
19%
74643
10%
23121
44%
23121
21%
47898

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
18%
760
21%
2649
7%
757
36%
757
18%
1639
January11th to 20thNight
23%
725
17%
2485
6%
725
38%
725
15%
1666
January21st to endNight
20%
792
21%
2574
7%
792
38%
792
19%
1623
February1st to 10thNight
18%
720
17%
2340
6%
720
32%
720
16%
1540
February11th to 20thNight
12%
703
18%
2174
3%
700
32%
700
15%
1445
February21st to endNight
15%
592
24%
1776
7%
592
35%
592
18%
1176
March1st to 10thNight
23%
720
22%
2133
9%
720
39%
720
21%
1324
March11th to 20thNight
22%
720
22%
1980
8%
720
35%
720
21%
1319
March21st to endNight
22%
792
21%
2070
6%
792
34%
792
23%
1424
April1st to 10thNight
21%
720
17%
1800
3%
720
30%
720
21%
1241
April11th to 20thNight
27%
711
18%
1775
6%
711
33%
711
26%
1218
April21st to endNight
26%
720
18%
1800
5%
720
33%
720
26%
1220
May1st to 10thNight
30%
548
15%
1628
8%
548
36%
548
20%
1126
May11th to 20thNight
33%
354
18%
1416
11%
354
44%
354
17%
913
May21st to endNight
48%
388
14%
1551
14%
388
54%
388
18%
979
June1st to 10thNight
44%
360
18%
1440
18%
360
54%
360
21%
876
June11th to 20thNight
48%
360
14%
1440
14%
360
56%
360
21%
879
June21st to endNight
54%
360
16%
1440
17%
360
61%
360
24%
827
July1st to 10thNight
53%
350
14%
1400
17%
350
57%
350
21%
873
July11th to 20thNight
58%
348
17%
1392
21%
348
62%
348
24%
790
July21st to endNight
57%
442
16%
1532
16%
418
61%
418
25%
909
August1st to 10thNight
62%
537
16%
1600
12%
534
64%
534
27%
1040
August11th to 20thNight
53%
652
16%
1716
10%
652
55%
652
25%
1191
August21st to endNight
54%
792
17%
1980
8%
792
56%
792
30%
1352
September1st to 10thNight
47%
720
21%
1800
9%
720
53%
720
32%
1207
September11th to 20thNight
54%
720
28%
1808
18%
720
58%
720
36%
1142
September21st to endNight
46%
720
24%
2009
17%
720
54%
720
28%
1315
October1st to 10thNight
40%
720
17%
2160
12%
720
47%
720
24%
1404
October11th to 20thNight
40%
720
22%
2160
14%
720
51%
720
28%
1332
October21st to endNight
30%
792
23%
2425
13%
792
45%
792
24%
1531
November1st to 10thNight
35%
740
23%
2407
13%
740
51%
740
27%
1402
November11th to 20thNight
31%
760
24%
2470
13%
760
45%
760
21%
1528
November21st to endNight
29%
760
20%
2643
9%
760
44%
760
21%
1601
December1st to 10thNight
21%
760
16%
2660
6%
760
33%
760
14%
1786
December11th to 20thNight
24%
760
17%
3210
7%
759
38%
759
14%
2014
December21st to endNight
24%
829
11%
4800
7%
817
38%
817
10%
3046

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
16%6%35%5%35%5%6%8%39%
74717
Night
16%6%25%5%3445%7%42%
74979
TransparencyDay
42%8%13%21%14%2
74425
Night
41%7%13%22%15%2
74805
SeeingDay
21%16%28%21%11%3
13777
Night
20%13%29%28%10%1
23175

By Month

Percentage Cloud Cover NE (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%37%5%5%45%6%7%39%
1517
11th to 20thDay
16%7%37%7%45%5%6%7%32%
1493
21st to endDay
13%8%36%6%36%5%6%7%36%
1782
February1st to 10thDay
17%7%36%6%36%5%6%7%35%
1620
11th to 20thDay
15%7%36%445%5%6%8%37%
1673
21st to endDay
20%9%35%5%35%5%5%6%35%
1480
March1st to 10thDay
13%7%26%5%35%5%5%7%42%
1822
11th to 20thDay
16%7%25%7%45%5%6%8%36%
1980
21st to endDay
20%7%35%435%45%6%38%
2286
April1st to 10thDay
15%7%45%6%35%5%6%8%37%
2160
11th to 20thDay
17%6%26%5%445%5%7%41%
2134
21st to endDay
18%5%346%345%6%8%39%
2160
May1st to 10thDay
20%6%35%5%45%6%6%7%35%
2345
11th to 20thDay
21%6%25%5%35%5%6%8%34%
2478
21st to endDay
16%6%245%35%6%6%9%39%
2725
June1st to 10thDay
13%6%25%6%36%5%7%10%37%
2520
11th to 20thDay
15%6%25%5%46%5%7%9%36%
2520
21st to endDay
12%5%245%46%6%7%9%40%
2520
July1st to 10thDay
10%5%36%6%46%7%7%10%37%
2520
11th to 20thDay
8%5%25%5%45%6%7%10%43%
2520
21st to endDay
10%7%37%6%46%6%6%9%37%
2691
August1st to 10thDay
10%5%37%5%46%6%8%9%37%
2327
11th to 20thDay
12%5%26%6%5%5%7%6%8%39%
2165
21st to endDay
15%6%245%35%6%7%9%39%
2376
September1st to 10thDay
17%5%35%435%45%7%42%
2160
11th to 20thDay
14%42343445%7%49%
2152
21st to endDay
19%6%35%4345%5%6%41%
1947
October1st to 10thDay
18%6%23435%5%5%8%41%
1800
11th to 20thDay
18%424445%45%8%43%
1800
21st to endDay
20%5%35%4335%6%7%39%
1930
November1st to 10thDay
14%6%34322347%51%
1671
11th to 20thDay
17%6%244335%5%6%45%
1710
21st to endDay
18%4344345%6%7%41%
1535
December1st to 10thDay
18%9%36%6%345%5%7%34%
1520
11th to 20thDay
21%6%24435%5%6%6%39%
1906
21st to endDay
19%8%47%6%45%5%7%6%29%
2772
January1st to 10thNight
15%7%25%5%3445%7%42%
2649
11th to 20thNight
17%9%35%5%25%5%5%7%37%
2485
21st to endNight
13%8%35%5%35%5%5%6%43%
2574
February1st to 10thNight
17%9%35%5%245%47%39%
2340
11th to 20thNight
17%6%36%6%35%5%5%6%39%
2172
21st to endNight
19%7%25%5%34446%41%
1776
March1st to 10thNight
15%5%35%42445%7%45%
2123
11th to 20thNight
18%7%35%5%34445%44%
1979
21st to endNight
17%8%35%5%34447%41%
2070
April1st to 10thNight
16%7%45%5%35%45%7%39%
1800
11th to 20thNight
15%5%26%5%345%5%7%42%
1775
21st to endNight
21%42443445%5%44%
1800
May1st to 10thNight
19%8%25%6%3445%6%37%
1628
11th to 20thNight
19%6%35%6%45%45%6%39%
1415
21st to endNight
15%6%25%5%35%46%9%40%
1550
June1st to 10thNight
16%6%35%5%3335%8%42%
1440
11th to 20thNight
13%7%24435%6%7%8%41%
1440
21st to endNight
11%415%5%45%47%8%45%
1440
July1st to 10thNight
11%5%26%6%36%5%6%9%41%
1440
11th to 20thNight
10%425%5%35%5%7%9%45%
1440
21st to endNight
11%5%245%346%8%10%42%
1622
August1st to 10thNight
10%5%26%7%35%7%6%9%42%
1611
11th to 20thNight
13%6%36%7%446%7%7%38%
1716
21st to endNight
14%6%25%435%6%6%8%42%
1980
September1st to 10thNight
15%6%26%5%34347%46%
1800
11th to 20thNight
13%5%245%33447%51%
1808
21st to endNight
17%6%25%5%24447%44%
2009
October1st to 10thNight
21%5%2443345%6%43%
2160
11th to 20thNight
18%42443345%6%47%
2160
21st to endNight
21%6%14423446%45%
2425
November1st to 10thNight
12%5%2443445%5%51%
2407
11th to 20thNight
17%6%25%5%23347%46%
2470
21st to endNight
15%5%25%5%35%45%8%43%
2643
December1st to 10thNight
19%7%36%5%345%5%6%36%
2660
11th to 20thNight
19%6%25%5%35%5%5%7%38%
3288
21st to endNight
20%8%36%5%3445%6%36%
4884

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%6%14%23%11%3
1518
11th to 20thDay
50%7%13%17%10%2
1493
21st to endDay
46%7%12%21%12%2
1782
February1st to 10thDay
47%8%14%19%11%1
1620
11th to 20thDay
44%7%12%23%12%2
1682
21st to endDay
49%6%11%20%11%3
1480
March1st to 10thDay
40%7%13%26%12%3
1825
11th to 20thDay
45%7%12%19%14%3
1980
21st to endDay
47%6%11%17%16%3
2286
April1st to 10thDay
44%7%10%21%13%5%
2160
11th to 20thDay
41%7%11%22%15%4
2134
21st to endDay
42%7%12%20%15%5%
2160
May1st to 10thDay
47%7%11%18%14%3
2345
11th to 20thDay
48%7%11%19%13%2
2482
21st to endDay
41%9%13%21%13%2
2725
June1st to 10thDay
40%9%14%22%13%2
2520
11th to 20thDay
42%10%15%21%10%2
2520
21st to endDay
37%10%15%24%13%1
2520
July1st to 10thDay
40%10%14%21%14%1
2450
11th to 20thDay
33%10%15%25%15%2
2445
21st to endDay
41%10%12%21%15%3
2540
August1st to 10thDay
39%10%15%20%14%1
2314
11th to 20thDay
39%9%16%21%13%1
2165
21st to endDay
38%9%13%21%17%3
2376
September1st to 10thDay
41%7%11%20%17%3
2160
11th to 20thDay
33%6%11%23%23%3
2152
21st to endDay
45%6%10%17%21%2
1947
October1st to 10thDay
41%6%14%22%16%2
1800
11th to 20thDay
40%6%12%24%17%1
1800
21st to endDay
43%7%11%22%15%2
1930
November1st to 10thDay
35%415%25%19%3
1671
11th to 20thDay
39%6%11%24%18%2
1710
21st to endDay
39%8%13%29%9%2
1535
December1st to 10thDay
49%6%16%18%10%1
1520
11th to 20thDay
45%6%17%23%8%1
1906
21st to endDay
50%9%14%19%7%
2772
January1st to 10thNight
41%5%13%23%15%3
2649
11th to 20thNight
47%7%15%21%9%2
2485
21st to endNight
40%6%12%24%15%2
2574
February1st to 10thNight
45%5%14%24%10%2
2340
11th to 20thNight
45%5%12%19%15%3
2174
21st to endNight
44%5%9%22%16%3
1776
March1st to 10thNight
38%6%13%26%15%3
2133
11th to 20thNight
43%5%10%23%17%3
1980
21st to endNight
44%6%11%18%17%4
2070
April1st to 10thNight
43%6%12%22%14%3
1800
11th to 20thNight
40%7%11%20%20%2
1775
21st to endNight
40%7%12%20%17%5%
1800
May1st to 10thNight
47%6%12%18%14%3
1628
11th to 20thNight
45%8%12%17%17%3
1416
21st to endNight
40%11%15%18%14%3
1551
June1st to 10thNight
40%7%11%22%18%1
1440
11th to 20thNight
38%10%15%22%14%1
1440
21st to endNight
34%9%15%28%13%1
1440
July1st to 10thNight
39%10%14%20%14%2
1400
11th to 20thNight
34%9%16%24%14%2
1392
21st to endNight
32%11%18%22%14%2
1532
August1st to 10thNight
36%11%15%21%16%1
1600
11th to 20thNight
42%10%15%17%15%2
1716
21st to endNight
38%10%13%19%19%2
1980
September1st to 10thNight
38%7%11%22%19%3
1800
11th to 20thNight
34%6%10%22%25%3
1808
21st to endNight
40%6%9%22%22%2
2009
October1st to 10thNight
41%6%13%23%15%2
2160
11th to 20thNight
37%6%13%28%14%2
2160
21st to endNight
40%5%11%26%16%2
2425
November1st to 10thNight
34%5%14%24%19%4
2407
11th to 20thNight
40%410%24%17%4
2470
21st to endNight
39%6%13%28%13%2
2643
December1st to 10thNight
47%7%13%20%12%1
2660
11th to 20thNight
44%6%15%21%13%2
3288
21st to endNight
47%7%15%22%9%
4884

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
26%31%24%15%31
180
11th to 20thDay
34%25%28%10%2
182
21st to endDay
22%31%32%11%31
264
February1st to 10thDay
27%33%30%8%2
240
11th to 20thDay
28%26%36%9%1
341
21st to endDay
33%21%29%16%1
346
March1st to 10thDay
25%23%31%19%2
420
11th to 20thDay
24%28%31%12%32
420
21st to endDay
30%23%28%13%6%1
462
April1st to 10thDay
25%26%28%15%5%1
420
11th to 20thDay
21%26%27%17%8%3
412
21st to endDay
28%20%31%14%43
420
May1st to 10thDay
29%20%26%15%6%3
423
11th to 20thDay
23%17%30%21%7%1
447
21st to endDay
21%9%26%24%16%4
485
June1st to 10thDay
16%16%29%23%14%3
450
11th to 20thDay
20%14%29%23%12%3
450
21st to endDay
14%12%33%24%14%4
450
July1st to 10thDay
12%6%25%28%21%7%
445
11th to 20thDay
8%6%25%32%24%4
444
21st to endDay
12%7%31%32%13%5%
488
August1st to 10thDay
11%6%23%32%20%8%
446
11th to 20thDay
14%6%21%33%19%7%
439
21st to endDay
15%6%27%26%17%9%
495
September1st to 10thDay
20%9%20%24%19%7%
450
11th to 20thDay
14%5%27%26%17%10%
450
21st to endDay
23%15%22%21%14%5%
450
October1st to 10thDay
25%8%25%21%14%6%
450
11th to 20thDay
20%16%27%24%11%2
450
21st to endDay
23%17%32%18%9%1
470
November1st to 10thDay
18%16%27%24%12%3
365
11th to 20thDay
22%21%29%21%5%1
370
21st to endDay
24%21%35%13%6%1
195
December1st to 10thDay
29%26%28%11%6%
180
11th to 20thDay
33%27%25%12%4
180
21st to endDay
27%23%30%14%42
198
January1st to 10thNight
20%20%37%20%3
760
11th to 20thNight
24%16%32%24%3
725
21st to endNight
18%19%37%24%2
792
February1st to 10thNight
23%18%36%22%2
720
11th to 20thNight
24%21%37%16%2
703
21st to endNight
25%20%37%17%2
592
March1st to 10thNight
21%20%32%23%3
720
11th to 20thNight
24%18%33%19%5%1
720
21st to endNight
21%15%38%20%5%1
792
April1st to 10thNight
21%21%33%20%5%
720
11th to 20thNight
18%15%36%24%7%
711
21st to endNight
23%13%33%23%7%
720
May1st to 10thNight
24%11%31%25%8%1
548
11th to 20thNight
19%8%34%29%9%
354
21st to endNight
18%6%20%42%13%1
389
June1st to 10thNight
18%5%26%35%17%
360
11th to 20thNight
15%8%25%40%11%1
360
21st to endNight
10%427%46%13%
360
July1st to 10thNight
11%320%40%23%2
350
11th to 20thNight
9%422%39%24%1
348
21st to endNight
12%5%19%41%21%2
442
August1st to 10thNight
12%315%44%24%2
537
11th to 20thNight
15%5%15%36%27%2
652
21st to endNight
16%419%35%25%2
792
September1st to 10thNight
17%424%33%19%2
720
11th to 20thNight
16%5%19%34%22%5%
720
21st to endNight
20%6%22%31%20%1
720
October1st to 10thNight
22%8%24%26%17%3
720
11th to 20thNight
19%11%27%32%11%1
720
21st to endNight
25%12%29%26%8%
792
November1st to 10thNight
15%11%35%27%12%1
740
11th to 20thNight
20%17%27%28%8%1
760
21st to endNight
18%17%33%26%7%
760
December1st to 10thNight
24%17%32%22%5%1
760
11th to 20thNight
21%16%33%25%5%
760
21st to endNight
22%16%33%27%2
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