Valley View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Valley View Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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
35%
24037
15%
97381
10%
23784
43%
23784
16%
64625

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%
752
16%
4372
7%
737
31%
737
9%
2859
January11th to 20thNight
22%
759
13%
4233
6%
740
31%
740
8%
2961
January21st to endNight
19%
832
10%
4505
6%
814
31%
814
8%
3313
February1st to 10thNight
23%
760
15%
2470
8%
760
34%
760
15%
1713
February11th to 20thNight
15%
743
14%
2358
3%
740
30%
740
14%
1649
February21st to endNight
18%
628
17%
1884
7%
628
31%
628
16%
1338
March1st to 10thNight
23%
758
17%
3551
9%
744
34%
744
11%
2500
March11th to 20thNight
24%
755
19%
3463
8%
740
36%
740
14%
2057
March21st to endNight
21%
831
12%
3562
7%
814
30%
814
14%
2189
April1st to 10thNight
23%
760
15%
1900
4%
760
29%
760
19%
1414
April11th to 20thNight
28%
751
16%
1875
5%
751
34%
751
25%
1308
April21st to endNight
30%
760
16%
1900
4%
760
36%
760
26%
1309
May1st to 10thNight
34%
537
16%
2698
9%
530
40%
530
11%
1925
May11th to 20thNight
34%
369
11%
2483
13%
364
46%
364
9%
1719
May21st to endNight
47%
403
11%
2725
14%
399
54%
399
13%
1519
June1st to 10thNight
49%
380
18%
1520
19%
380
58%
380
22%
922
June11th to 20thNight
49%
380
14%
1520
14%
380
57%
380
22%
915
June21st to endNight
55%
380
15%
1520
17%
380
60%
380
25%
887
July1st to 10thNight
51%
364
15%
2352
15%
360
56%
360
13%
1364
July11th to 20thNight
62%
361
12%
2234
21%
358
65%
358
15%
1329
July21st to endNight
57%
508
11%
2569
14%
476
61%
476
16%
1621
August1st to 10thNight
62%
558
12%
2698
14%
545
65%
545
16%
1869
August11th to 20thNight
57%
615
15%
2660
10%
609
59%
609
17%
1732
August21st to endNight
57%
819
14%
3133
10%
814
59%
814
23%
1902
September1st to 10thNight
51%
760
23%
1900
11%
760
55%
760
32%
1258
September11th to 20thNight
59%
760
28%
1900
20%
760
61%
760
40%
1178
September21st to endNight
46%
760
23%
2158
17%
760
52%
760
27%
1422
October1st to 10thNight
43%
760
12%
3129
15%
740
48%
740
16%
2253
October11th to 20thNight
39%
760
15%
2685
13%
740
47%
740
20%
1778
October21st to endNight
33%
836
16%
3001
14%
814
41%
814
19%
1893
November1st to 10thNight
39%
771
23%
2505
15%
771
51%
771
27%
1493
November11th to 20thNight
30%
760
19%
2530
12%
760
41%
760
19%
1682
November21st to endNight
30%
760
17%
2660
11%
760
40%
760
18%
1758
December1st to 10thNight
22%
760
12%
2660
6%
760
30%
760
12%
1918
December11th to 20thNight
26%
759
13%
3231
6%
759
35%
759
11%
2242
December21st to endNight
23%
828
8%
4837
6%
817
33%
817
8%
3436

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%8%37%5%5%5%46%7%35%
101464
Night
17%9%37%44445%7%36%
97651
TransparencyDay
46%6%12%22%12%2
105052
Night
46%6%12%22%13%2
102151
SeeingDay
22%15%27%21%11%4
14836
Night
20%12%28%29%11%1
24151

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
15%12%48%5%4435%5%34%
2517
11th to 20thDay
14%13%6%14%7%6%4445%22%
2662
21st to endDay
21%14%410%6%44346%23%
3124
February1st to 10thDay
17%10%49%7%7%5%35%6%28%
1710
11th to 20thDay
15%9%39%7%5%6%45%7%31%
1710
21st to endDay
20%10%49%6%44446%31%
1570
March1st to 10thDay
25%9%38%444345%31%
3266
11th to 20thDay
12%10%48%5%4435%7%38%
3480
21st to endDay
19%10%36%434246%39%
3940
April1st to 10thDay
17%10%27%6%45%447%34%
2280
11th to 20thDay
18%9%26%5%4435%7%37%
2254
21st to endDay
17%6%36%5%4446%7%37%
2280
May1st to 10thDay
28%6%26%44435%6%31%
4208
11th to 20thDay
22%9%38%6%5%445%7%26%
4398
21st to endDay
14%5%25%445%46%10%42%
4837
June1st to 10thDay
13%6%37%6%5%5%46%9%35%
2660
11th to 20thDay
14%7%26%5%5%5%5%6%9%38%
2660
21st to endDay
11%6%25%5%5%5%5%7%9%41%
2660
July1st to 10thDay
7%5%38%6%6%6%5%7%10%37%
4440
11th to 20thDay
7%6%27%5%6%6%5%8%10%39%
4440
21st to endDay
11%8%38%6%6%6%5%7%7%34%
4632
August1st to 10thDay
20%6%28%5%5%5%47%8%30%
4104
11th to 20thDay
16%6%26%45%6%5%7%8%34%
3894
21st to endDay
15%5%25%34446%9%43%
4180
September1st to 10thDay
15%6%25%45%445%6%44%
2280
11th to 20thDay
13%5%25%34435%7%50%
2280
21st to endDay
22%7%26%44435%6%37%
2017
October1st to 10thDay
18%6%26%445%347%40%
1900
11th to 20thDay
18%6%36%45%445%7%38%
1900
21st to endDay
20%6%37%45%5%446%36%
2080
November1st to 10thDay
13%8%26%5%4335%7%45%
1734
11th to 20thDay
17%9%36%5%4445%6%37%
1649
21st to endDay
17%9%35%5%4446%6%37%
1520
December1st to 10thDay
20%11%38%6%5%5%45%7%26%
1520
11th to 20thDay
22%10%36%5%4445%7%32%
1906
21st to endDay
20%12%412%7%6%5%35%5%21%
2772
January1st to 10thNight
15%12%49%5%5%5%347%31%
4429
11th to 20thNight
17%10%411%6%45%35%6%28%
4284
21st to endNight
24%12%310%5%5%435%5%26%
4532
February1st to 10thNight
17%11%36%6%5%5%46%6%33%
2470
11th to 20thNight
16%11%38%6%5%5%45%7%31%
2356
21st to endNight
19%10%38%5%5%445%6%32%
1884
March1st to 10thNight
22%12%47%44435%5%31%
3573
11th to 20thNight
12%13%37%444347%39%
3479
21st to endNight
16%9%36%444345%42%
3690
April1st to 10thNight
19%9%38%5%5%445%6%33%
1900
11th to 20thNight
17%7%37%44346%7%38%
1875
21st to endNight
19%5%26%4345%5%6%42%
1900
May1st to 10thNight
29%8%26%43435%6%29%
2762
11th to 20thNight
22%8%37%5%5%445%6%29%
2495
21st to endNight
12%6%27%5%4447%9%40%
2738
June1st to 10thNight
13%7%26%4445%5%8%42%
1520
11th to 20thNight
12%6%36%5%5%447%10%39%
1520
21st to endNight
12%5%25%436%5%7%11%40%
1520
July1st to 10thNight
9%5%27%6%5%5%46%8%42%
2520
11th to 20thNight
8%5%36%5%5%5%5%8%11%38%
2520
21st to endNight
14%6%36%5%45%46%7%41%
2918
August1st to 10thNight
20%7%27%45%5%46%7%34%
2825
11th to 20thNight
15%8%37%44445%6%41%
2903
21st to endNight
14%5%15%445%46%8%45%
3476
September1st to 10thNight
14%5%25%44435%6%45%
1900
11th to 20thNight
11%5%25%33335%8%51%
1900
21st to endNight
19%7%26%43335%7%40%
2158
October1st to 10thNight
21%7%26%33435%6%41%
2280
11th to 20thNight
16%7%26%444346%43%
2280
21st to endNight
20%8%27%43425%6%38%
2517
November1st to 10thNight
12%7%27%33435%5%49%
2505
11th to 20thNight
17%10%36%5%4435%6%38%
2530
21st to endNight
15%10%37%5%5%437%8%34%
2660
December1st to 10thNight
20%11%48%44446%7%28%
2660
11th to 20thNight
19%10%37%5%5%446%6%31%
3288
21st to endNight
21%13%49%5%4435%6%25%
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
50%6%15%20%7%2
2518
11th to 20thDay
63%5%10%14%6%1
2662
21st to endDay
62%6%10%16%6%1
3124
February1st to 10thDay
56%5%12%17%9%1
1710
11th to 20thDay
52%6%9%24%8%1
1719
21st to endDay
55%410%22%7%2
1570
March1st to 10thDay
55%5%10%23%6%1
3269
11th to 20thDay
43%6%14%23%11%3
3480
21st to endDay
46%5%10%19%17%2
3940
April1st to 10thDay
51%410%20%12%3
2280
11th to 20thDay
46%5%11%21%14%3
2254
21st to endDay
44%6%10%21%15%4
2280
May1st to 10thDay
53%6%9%20%10%2
4220
11th to 20thDay
55%7%11%17%7%1
4402
21st to endDay
35%7%13%25%16%2
4837
June1st to 10thDay
44%7%12%22%13%2
2660
11th to 20thDay
42%7%13%23%13%2
2660
21st to endDay
38%8%13%27%13%
2660
July1st to 10thDay
41%7%14%25%11%1
4370
11th to 20thDay
37%7%17%25%14%1
4365
21st to endDay
47%7%11%18%16%1
4484
August1st to 10thDay
51%7%13%19%9%
4091
11th to 20thDay
44%7%14%26%8%1
3933
21st to endDay
37%5%12%33%12%1
4180
September1st to 10thDay
40%6%11%23%17%2
2280
11th to 20thDay
33%5%12%25%23%2
2280
21st to endDay
48%5%11%18%17%2
2017
October1st to 10thDay
40%5%9%24%18%4
3160
11th to 20thDay
36%49%28%20%2
3140
21st to endDay
32%415%31%17%1
3406
November1st to 10thDay
40%414%24%16%2
1734
11th to 20thDay
48%411%22%13%2
1649
21st to endDay
46%6%13%26%8%1
1520
December1st to 10thDay
59%6%13%16%7%1
1520
11th to 20thDay
52%418%20%6%
1906
21st to endDay
63%8%11%14%5%
2772
January1st to 10thNight
52%5%13%21%7%1
4429
11th to 20thNight
55%7%13%16%7%1
4284
21st to endNight
60%6%10%17%7%1
4532
February1st to 10thNight
51%5%13%22%8%1
2470
11th to 20thNight
52%5%12%19%11%2
2358
21st to endNight
52%5%11%19%12%2
1884
March1st to 10thNight
54%5%8%22%9%1
3583
11th to 20thNight
43%6%16%24%11%1
3480
21st to endNight
45%48%19%19%4
3690
April1st to 10thNight
52%5%9%19%12%2
1900
11th to 20thNight
44%6%10%21%17%2
1875
21st to endNight
42%7%11%21%17%3
1900
May1st to 10thNight
56%5%8%21%8%2
2769
11th to 20thNight
52%10%12%15%10%1
2496
21st to endNight
39%8%15%19%14%4
2739
June1st to 10thNight
39%8%11%25%17%1
1520
11th to 20thNight
38%8%14%24%13%2
1520
21st to endNight
36%8%14%29%13%1
1520
July1st to 10thNight
41%7%15%18%17%3
2480
11th to 20thNight
37%8%15%24%15%1
2472
21st to endNight
40%7%14%17%20%1
2825
August1st to 10thNight
47%9%15%21%9%1
2814
11th to 20thNight
43%6%13%23%14%
2930
21st to endNight
35%7%11%32%14%1
3476
September1st to 10thNight
38%6%11%26%17%3
1900
11th to 20thNight
31%5%11%29%21%2
1900
21st to endNight
42%5%9%23%20%1
2158
October1st to 10thNight
44%6%10%19%20%1
3800
11th to 20thNight
32%411%28%22%3
3779
21st to endNight
32%415%29%19%1
4141
November1st to 10thNight
37%5%13%24%18%3
2505
11th to 20thNight
46%5%10%23%13%3
2530
21st to endNight
46%6%14%23%10%1
2660
December1st to 10thNight
53%8%13%18%8%
2660
11th to 20thNight
52%6%13%18%9%1
3288
21st to endNight
58%5%15%17%4
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
28%26%29%13%32
180
11th to 20thDay
29%27%32%11%1
222
21st to endDay
24%27%30%14%41
297
February1st to 10thDay
25%28%31%14%2
270
11th to 20thDay
27%23%39%9%1
317
21st to endDay
31%20%30%16%2
382
March1st to 10thDay
29%20%29%17%31
460
11th to 20thDay
27%25%27%17%32
460
21st to endDay
29%23%24%16%5%3
506
April1st to 10thDay
27%23%26%16%6%2
460
11th to 20thDay
24%26%23%15%9%3
452
21st to endDay
25%17%32%16%8%2
460
May1st to 10thDay
31%15%28%15%8%3
463
11th to 20thDay
20%17%29%21%9%3
487
21st to endDay
21%8%26%22%16%6%
529
June1st to 10thDay
16%15%28%26%12%3
490
11th to 20thDay
18%13%26%26%14%3
490
21st to endDay
15%11%33%26%13%2
490
July1st to 10thDay
12%7%25%31%18%6%
485
11th to 20thDay
8%6%26%33%23%5%
484
21st to endDay
13%9%26%30%18%5%
532
August1st to 10thDay
13%8%23%25%22%8%
486
11th to 20thDay
15%6%22%29%19%9%
479
21st to endDay
14%7%24%27%16%12%
539
September1st to 10thDay
19%10%20%20%19%11%
490
11th to 20thDay
14%5%21%26%21%12%
490
21st to endDay
25%11%21%23%14%5%
490
October1st to 10thDay
23%9%23%21%15%8%
490
11th to 20thDay
21%15%26%24%12%2
490
21st to endDay
24%16%28%21%10%1
533
November1st to 10thDay
22%14%25%24%12%3
386
11th to 20thDay
27%16%28%19%8%1
309
21st to endDay
25%23%31%17%4
180
December1st to 10thDay
29%28%27%11%6%1
180
11th to 20thDay
32%23%26%15%4
180
21st to endDay
30%21%29%15%5%
198
January1st to 10thNight
19%20%36%22%3
760
11th to 20thNight
24%17%31%25%3
760
21st to endNight
18%18%38%23%2
836
February1st to 10thNight
23%15%33%28%2
760
11th to 20thNight
22%22%37%18%1
743
21st to endNight
24%18%36%19%3
628
March1st to 10thNight
24%16%32%24%4
760
11th to 20thNight
25%17%31%22%5%
760
21st to endNight
22%15%36%20%5%1
836
April1st to 10thNight
24%18%32%21%5%
760
11th to 20thNight
19%16%33%24%7%
751
21st to endNight
21%15%30%28%6%
760
May1st to 10thNight
26%12%25%27%11%
538
11th to 20thNight
17%7%33%34%8%1
374
21st to endNight
16%5%23%39%17%
411
June1st to 10thNight
14%7%21%41%17%
380
11th to 20thNight
15%5%25%42%13%
380
21st to endNight
11%325%49%12%
380
July1st to 10thNight
13%421%40%20%3
370
11th to 20thNight
9%218%46%23%2
368
21st to endNight
13%318%40%24%1
520
August1st to 10thNight
14%211%43%27%2
567
11th to 20thNight
15%313%38%29%3
630
21st to endNight
14%416%35%29%2
836
September1st to 10thNight
18%321%34%22%2
760
11th to 20thNight
14%417%33%28%4
760
21st to endNight
21%5%21%34%17%2
760
October1st to 10thNight
22%8%20%27%19%3
760
11th to 20thNight
18%12%26%32%12%1
760
21st to endNight
24%10%28%31%7%
836
November1st to 10thNight
15%11%30%33%11%1
771
11th to 20thNight
22%15%28%27%8%1
760
21st to endNight
19%16%30%29%6%1
760
December1st to 10thNight
25%17%31%22%6%
760
11th to 20thNight
22%14%33%26%5%
760
21st to endNight
24%14%32%27%3
836
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