VLA
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the VLA clear sky chart from 2005-11-05 to 2017-09-18.

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
57%
15481
33%
47155
21%
15435
69%
15435
44%
23441

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 15thNight
57%
719
33%
2327
26%
716
70%
716
46%
1088
January16th to endNight
51%
768
36%
2496
25%
768
67%
768
42%
1191
February1st to 15thNight
49%
701
34%
2255
23%
697
65%
697
49%
1021
February16th to endNight
39%
632
42%
1855
19%
632
63%
632
45%
872
March1st to 15thNight
47%
720
39%
1980
18%
720
63%
720
48%
971
March16th to endNight
48%
768
38%
2112
18%
768
65%
768
53%
1012
April1st to 15thNight
36%
548
36%
1807
14%
548
63%
548
41%
804
April16th to endNight
35%
540
37%
1800
17%
540
63%
540
42%
805
May1st to 15thNight
42%
540
30%
1769
18%
539
59%
539
44%
830
May16th to endNight
52%
563
32%
1684
19%
561
66%
561
53%
769
June1st to 15thNight
55%
540
28%
1620
19%
540
63%
540
48%
791
June16th to endNight
69%
540
23%
1620
20%
540
72%
540
44%
841
July1st to 15thNight
69%
510
17%
1530
15%
510
71%
510
29%
960
July16th to endNight
75%
564
14%
1593
13%
531
76%
531
29%
982
August1st to 15thNight
77%
539
17%
1668
18%
536
77%
536
31%
993
August16th to endNight
74%
573
22%
1910
23%
573
75%
573
29%
1139
September1st to 15thNight
70%
540
25%
1800
26%
540
73%
540
30%
1080
September16th to endNight
74%
613
35%
1842
27%
613
80%
613
51%
900
October1st to 15thNight
62%
660
37%
1980
23%
660
73%
660
57%
904
October16th to endNight
68%
704
41%
2112
27%
704
78%
704
61%
926
November1st to 15thNight
67%
700
43%
2100
27%
700
78%
700
60%
910
November16th to endNight
57%
807
40%
2255
26%
807
67%
807
48%
1106
December1st to 15thNight
52%
900
34%
2520
19%
900
65%
900
46%
1276
December16th to endNight
46%
792
35%
2520
23%
792
60%
792
38%
1270

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
331442445%7%57%
47620
Night
221332445%7%62%
47360
TransparencyDay
25%47%17%27%17%
47316
Night
20%48%20%28%16%
47165
SeeingDay
8%14%24%26%18%6%
5098
Night
410%23%33%24%3
15485

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 15thDay
412312448%64%
1611
16th to endDay
2423322346%64%
1728
February1st to 15thDay
1313223336%68%
1598
16th to endDay
12133235%46%65%
1616
March1st to 15thDay
13233245%6%7%59%
1980
16th to endDay
222232345%7%64%
2112
April1st to 15thDay
221232435%7%65%
2141
16th to endDay
333323335%68%
2160
May1st to 15thDay
1213425%45%7%61%
2178
16th to endDay
2313323345%65%
2429
June1st to 15thDay
2213323448%62%
2340
16th to endDay
22145%35%45%7%57%
2340
July1st to 15thDay
6%426%6%48%6%7%9%37%
2340
16th to endDay
5%5%36%7%5%8%9%9%10%27%
2467
August1st to 15thDay
7%437%8%5%6%6%8%8%34%
2280
16th to endDay
6%415%7%47%7%7%7%39%
2292
September1st to 15thDay
8%5%26%5%45%5%5%6%42%
2160
16th to endDay
4313324446%61%
1840
October1st to 15thDay
3212323435%67%
1650
16th to endDay
113212235%73%
1760
November1st to 15thDay
112112336%72%
1759
16th to endDay
2312313235%69%
1689
December1st to 15thDay
2213212448%65%
1440
16th to endDay
1424424346%60%
1710
January1st to 15thNight
2313222447%64%
2327
16th to endNight
2313324445%64%
2496
February1st to 15thNight
1312212346%68%
2253
16th to endNight
1112313447%67%
1855
March1st to 15thNight
1212313346%68%
1980
16th to endNight
1111213347%71%
2112
April1st to 15thNight
1212323446%67%
1807
16th to endNight
1311323235%70%
1800
May1st to 15thNight
2223223345%66%
1769
16th to endNight
2212212246%69%
1684
June1st to 15thNight
3213313346%65%
1620
16th to endNight
11125%35%5%7%8%57%
1620
July1st to 15thNight
3325%6%48%7%8%10%39%
1620
16th to endNight
3325%6%36%8%8%12%37%
1701
August1st to 15thNight
2316%6%36%6%7%12%41%
1677
16th to endNight
3325%8%45%6%7%11%41%
1910
September1st to 15thNight
5%425%6%36%6%7%9%42%
1800
16th to endNight
3313313447%64%
1842
October1st to 15thNight
2212212345%71%
1980
16th to endNight
1212112235%75%
2112
November1st to 15thNight
111212345%74%
2100
16th to endNight
2312313225%70%
2255
December1st to 15thNight
231222345%6%65%
2520
16th to endNight
132442445%7%58%
2520

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 15thDay
18%37%21%35%13%
1612
16th to endDay
21%25%18%35%15%
1728
February1st to 15thDay
18%16%23%29%20%
1599
16th to endDay
19%2319%37%17%
1616
March1st to 15thDay
22%4315%35%17%
1980
16th to endDay
20%2316%37%19%
2112
April1st to 15thDay
18%3318%35%20%
2141
16th to endDay
20%2314%35%23%
2160
May1st to 15thDay
22%35%15%30%23%
2178
16th to endDay
22%37%11%28%26%
2432
June1st to 15thDay
20%3414%22%35%
2340
16th to endDay
25%46%12%22%28%
2340
July1st to 15thDay
39%7%13%16%12%10%
2201
16th to endDay
44%10%20%16%6%
2312
August1st to 15thDay
42%9%16%14%13%3
2265
16th to endDay
37%9%13%17%14%6%
2292
September1st to 15thDay
39%5%10%17%17%8%
2160
16th to endDay
23%4420%26%21%
1840
October1st to 15thDay
19%46%16%30%22%
1650
16th to endDay
14%25%15%38%23%
1760
November1st to 15thDay
13%2421%37%20%
1759
16th to endDay
18%1216%36%23%
1689
December1st to 15thDay
17%45%30%28%12%
1440
16th to endDay
25%39%24%29%8%
1710
January1st to 15thNight
20%27%21%36%11%
2327
16th to endNight
21%26%19%35%13%
2496
February1st to 15thNight
17%37%26%25%20%
2263
16th to endNight
16%25%23%36%15%
1855
March1st to 15thNight
17%25%20%37%15%
1980
16th to endNight
14%25%19%39%18%
2112
April1st to 15thNight
17%2422%34%17%
1807
16th to endNight
17%3316%36%22%
1800
May1st to 15thNight
20%36%14%35%19%
1770
16th to endNight
15%37%17%32%22%
1685
June1st to 15thNight
20%3411%23%36%
1620
16th to endNight
23%49%11%21%29%
1620
July1st to 15thNight
35%8%14%17%13%10%
1530
16th to endNight
33%10%26%22%5%1
1593
August1st to 15thNight
31%9%23%18%12%4
1668
16th to endNight
33%8%17%20%13%5%
1910
September1st to 15thNight
34%7%15%18%15%8%
1800
16th to endNight
19%36%20%30%19%
1842
October1st to 15thNight
14%27%22%30%21%
1980
16th to endNight
12%2420%36%23%
2112
November1st to 15thNight
12%2326%34%20%
2100
16th to endNight
18%2318%33%23%
2255
December1st to 15thNight
18%47%25%31%12%
2520
16th to endNight
23%310%28%27%6%
2520

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 15thDay
5%16%38%18%16%3
90
16th to endDay
320%33%18%18%5%
96
February1st to 15thDay
111%30%28%22%6%
90
16th to endDay
6%28%28%18%12%5%
117
March1st to 15thDay
10%22%32%19%11%3
270
16th to endDay
9%32%29%18%6%3
288
April1st to 15thDay
7%31%29%18%10%2
269
16th to endDay
11%32%35%14%5%1
270
May1st to 15thDay
5%26%37%21%6%2
288
16th to endDay
6%17%38%25%10%1
330
June1st to 15thDay
411%29%29%19%6%
315
16th to endDay
4318%35%30%7%
315
July1st to 15thDay
9%212%38%28%8%
305
16th to endDay
12%18%25%26%24%
327
August1st to 15thDay
11%411%29%29%13%
315
16th to endDay
11%411%30%31%11%
332
September1st to 15thDay
13%317%31%26%6%
315
16th to endDay
8%7%22%38%20%2
214
October1st to 15thDay
6%10%25%18%24%14%
90
16th to endDay
3430%32%21%8%
96
November1st to 15thDay
8%10%28%26%23%2
90
16th to endDay
434%24%23%13%
90
December1st to 15thDay
317%33%22%17%5%
90
16th to endDay
19%18%21%32%6%1
96
January1st to 15thNight
48%24%36%21%4
720
16th to endNight
412%27%32%21%2
768
February1st to 15thNight
311%31%33%17%2
704
16th to endNight
222%33%31%9%
632
March1st to 15thNight
416%29%31%17%
720
16th to endNight
314%30%35%14%
768
April1st to 15thNight
221%36%29%9%
548
16th to endNight
318%39%27%10%
540
May1st to 15thNight
39%39%36%10%1
540
16th to endNight
37%30%39%16%1
563
June1st to 15thNight
5%428%42%17%
540
16th to endNight
2315%42%32%2
540
July1st to 15thNight
427%36%41%7%
510
16th to endNight
41322%54%12%
564
August1st to 15thNight
32319%56%14%
539
16th to endNight
415%24%54%8%
573
September1st to 15thNight
7%10%33%44%4
540
16th to endNight
5%212%39%36%4
613
October1st to 15thNight
35%24%36%25%3
660
16th to endNight
25%20%37%27%7%
704
November1st to 15thNight
18%19%37%27%4
700
16th to endNight
415%18%29%25%5%
807
December1st to 15thNight
5%15%24%35%17%1
900
16th to endNight
415%28%35%13%2
792
Page generated at 2017-09-19 05:32:22. Version ac04

Copyright 2017 A. Danko


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