Van Horne
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Van Horne clear sky chart from 2005-11-05 to 2017-09-19.

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
38%
16012
18%
46897
11%
15967
45%
15967
26%
30522

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
29%
900
13%
2506
5%
895
36%
895
21%
1762
January16th to endNight
23%
960
16%
2505
5%
960
31%
960
19%
1861
February1st to 15thNight
23%
878
14%
2288
4%
876
30%
876
20%
1654
February16th to endNight
27%
687
19%
1949
7%
687
37%
687
25%
1281
March1st to 15thNight
32%
603
19%
2043
12%
603
40%
603
23%
1261
March16th to endNight
24%
576
11%
2076
7%
576
31%
576
14%
1476
April1st to 15thNight
26%
539
16%
1797
11%
539
35%
539
22%
1177
April16th to endNight
34%
540
18%
1773
13%
540
45%
540
21%
1205
May1st to 15thNight
34%
540
17%
1518
8%
540
42%
540
25%
1038
May16th to endNight
43%
563
18%
1497
7%
561
50%
561
29%
995
June1st to 15thNight
53%
540
15%
1440
7%
540
56%
540
33%
907
June16th to endNight
53%
540
17%
1440
9%
540
56%
540
40%
857
July1st to 15thNight
68%
510
18%
1360
11%
510
70%
510
44%
777
July16th to endNight
71%
564
23%
1469
18%
531
72%
531
47%
809
August1st to 15thNight
69%
539
21%
1610
21%
536
71%
536
40%
895
August16th to endNight
63%
573
22%
1863
24%
573
67%
573
33%
1064
September1st to 15thNight
62%
540
27%
1800
27%
540
67%
540
36%
952
September16th to endNight
53%
573
25%
1839
17%
573
61%
573
36%
1002
October1st to 15thNight
43%
660
26%
1980
18%
660
49%
660
31%
1194
October16th to endNight
36%
704
19%
2117
11%
704
44%
704
27%
1336
November1st to 15thNight
34%
723
24%
2299
14%
723
45%
723
29%
1354
November16th to endNight
28%
900
16%
2520
8%
900
35%
900
24%
1712
December1st to 15thNight
21%
900
12%
2520
4%
900
29%
900
16%
1884
December16th to endNight
23%
960
14%
2688
4%
960
31%
960
15%
2069

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
14%7%35%5%3445%7%37%
47921
Night
13%7%35%424446%43%
47071
Cloud CoverDay
14%7%35%5%3445%7%37%
47930
Night
13%7%35%424446%43%
47075
TransparencyDay
45%6%13%22%11%1
47613
Night
41%5%13%23%14%1
46898
SeeingDay
15%6%27%28%16%4
6032
Night
16%7%30%30%12%1
16014

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 15thDay
10%9%5%8%7%5%5%5%46%30%
1432
16th to endDay
14%12%6%8%6%35%45%7%25%
1716
February1st to 15thDay
12%11%48%6%3445%6%31%
1575
16th to endDay
13%9%46%5%35%46%7%35%
1522
March1st to 15thDay
15%8%45%6%2445%7%34%
1915
16th to endDay
19%10%5%6%5%34448%29%
2148
April1st to 15thDay
18%9%46%5%35%446%32%
2151
16th to endDay
24%9%25%5%23346%30%
2184
May1st to 15thDay
19%8%36%5%35%45%6%30%
2433
16th to endDay
15%5%36%6%35%5%47%36%
2617
June1st to 15thDay
14%5%24425%5%6%8%39%
2520
16th to endDay
10%425%435%5%6%10%41%
2520
July1st to 15thDay
7%4245%35%5%6%8%45%
2520
16th to endDay
8%4245%35%6%7%9%43%
2596
August1st to 15thDay
7%425%5%36%6%6%9%42%
2340
16th to endDay
11%5%25%5%35%45%7%43%
2339
September1st to 15thDay
11%314423347%52%
2160
16th to endDay
12%3245%2445%7%47%
1854
October1st to 15thDay
14%435%42334447%
1650
16th to endDay
17%7%25%5%23445%41%
1755
November1st to 15thDay
12%6%34334446%45%
1558
16th to endDay
17%8%35%424347%38%
1440
December1st to 15thDay
17%13%5%8%5%24435%27%
1440
16th to endDay
18%16%7%9%6%34235%23%
1536
January1st to 15thNight
11%9%47%5%35%45%6%36%
2506
16th to endNight
11%11%5%8%7%34445%33%
2505
February1st to 15thNight
11%12%5%7%5%23445%35%
2285
16th to endNight
14%7%35%424446%43%
1949
March1st to 15thNight
15%7%24423336%45%
2031
16th to endNight
17%9%37%5%35%5%45%32%
2076
April1st to 15thNight
14%8%45%42445%5%39%
1797
16th to endNight
21%6%25%434445%38%
1773
May1st to 15thNight
16%5%25%4245%6%7%39%
1518
16th to endNight
11%5%25%5%35%45%5%44%
1496
June1st to 15thNight
10%5%25%42445%8%46%
1440
16th to endNight
8%3243245%6%10%49%
1440
July1st to 15thNight
5%2133145%6%9%55%
1440
16th to endNight
4212425%37%9%54%
1569
August1st to 15thNight
5%41442445%8%54%
1620
16th to endNight
9%414414437%52%
1863
September1st to 15thNight
10%413323346%56%
1800
16th to endNight
8%424424337%54%
1839
October1st to 15thNight
12%5%15%423245%50%
1980
16th to endNight
14%6%34423346%45%
2117
November1st to 15thNight
12%6%34423245%50%
2299
16th to endNight
17%9%24424435%41%
2520
December1st to 15thNight
17%11%47%5%2444432%
2520
16th to endNight
21%11%46%5%34445%28%
2688

Percentage Cloud Cover SE (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
11%9%5%8%7%5%6%446%29%
1432
16th to endDay
14%12%6%8%6%36%447%25%
1716
February1st to 15thDay
12%10%5%8%6%3445%6%31%
1575
16th to endDay
12%10%36%5%45%46%7%34%
1522
March1st to 15thDay
15%8%46%6%2445%7%34%
1926
16th to endDay
19%10%46%5%2435%7%29%
2146
April1st to 15thDay
18%9%46%5%34446%32%
2151
16th to endDay
23%9%26%5%24346%31%
2184
May1st to 15thDay
18%9%26%5%35%446%30%
2433
16th to endDay
15%5%26%6%35%5%47%36%
2617
June1st to 15thDay
14%5%245%25%5%5%8%39%
2520
16th to endDay
9%425%435%47%9%42%
2520
July1st to 15thDay
7%4245%35%6%6%8%45%
2520
16th to endDay
8%4245%36%6%7%9%43%
2596
August1st to 15thDay
7%425%5%45%6%6%8%42%
2340
16th to endDay
10%5%345%35%45%7%43%
2339
September1st to 15thDay
11%413423347%51%
2160
16th to endDay
11%4245%245%5%6%47%
1854
October1st to 15thDay
14%5%35%433445%47%
1650
16th to endDay
17%6%35%5%23345%41%
1755
November1st to 15thDay
13%6%35%335%346%45%
1558
16th to endDay
17%9%35%324347%38%
1440
December1st to 15thDay
17%14%5%8%5%335%36%27%
1440
16th to endDay
18%16%6%9%6%34235%23%
1536
January1st to 15thNight
11%9%47%5%36%446%36%
2506
16th to endNight
11%11%5%8%7%34445%33%
2505
February1st to 15thNight
11%12%5%7%5%23445%36%
2278
16th to endNight
14%7%35%5%24436%42%
1949
March1st to 15thNight
15%7%24423336%45%
2043
16th to endNight
17%9%46%6%35%5%45%32%
2076
April1st to 15thNight
15%8%36%5%2445%5%39%
1797
16th to endNight
21%6%25%5%24445%38%
1773
May1st to 15thNight
16%6%245%2446%7%39%
1517
16th to endNight
10%6%25%5%35%5%5%6%44%
1496
June1st to 15thNight
10%5%25%3245%5%8%46%
1440
16th to endNight
8%3244235%7%9%49%
1440
July1st to 15thNight
5%3133245%6%9%55%
1440
16th to endNight
4212425%47%11%53%
1569
August1st to 15thNight
5%41442345%9%54%
1620
16th to endNight
9%424414447%52%
1863
September1st to 15thNight
10%413323446%56%
1800
16th to endNight
7%414423338%54%
1839
October1st to 15thNight
12%5%15%423245%50%
1980
16th to endNight
14%7%34423346%45%
2117
November1st to 15thNight
12%6%25%423345%50%
2299
16th to endNight
17%8%24424435%41%
2520
December1st to 15thNight
17%11%47%5%2444431%
2520
16th to endNight
21%12%46%424445%28%
2688

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
54%6%12%16%10%
1433
16th to endDay
58%5%14%13%7%
1716
February1st to 15thDay
52%415%20%6%
1576
16th to endDay
47%6%16%20%8%1
1522
March1st to 15thDay
48%6%15%20%8%
1926
16th to endDay
53%6%13%19%6%
2148
April1st to 15thDay
51%5%10%23%8%
2151
16th to endDay
53%5%10%18%10%
2184
May1st to 15thDay
53%6%12%20%7%
2433
16th to endDay
46%7%12%20%12%1
2621
June1st to 15thDay
40%7%13%22%13%1
2520
16th to endDay
36%8%13%23%16%1
2520
July1st to 15thDay
35%9%13%26%13%2
2371
16th to endDay
34%8%17%22%15%1
2434
August1st to 15thDay
35%8%14%26%13%1
2326
16th to endDay
38%6%12%25%14%2
2339
September1st to 15thDay
31%5%11%27%21%1
2160
16th to endDay
36%5%13%29%13%1
1854
October1st to 15thDay
39%410%24%19%1
1650
16th to endDay
44%413%24%12%1
1755
November1st to 15thDay
39%412%27%14%1
1558
16th to endDay
45%5%19%22%6%
1440
December1st to 15thDay
59%5%12%16%6%
1440
16th to endDay
64%412%11%7%
1536
January1st to 15thNight
48%5%15%17%12%
2506
16th to endNight
53%414%17%9%
2505
February1st to 15thNight
50%5%12%22%9%
2288
16th to endNight
43%414%24%11%1
1949
March1st to 15thNight
41%413%26%13%
2043
16th to endNight
53%6%13%17%8%
2076
April1st to 15thNight
45%311%24%11%2
1797
16th to endNight
47%5%9%21%12%2
1773
May1st to 15thNight
41%8%13%22%13%
1518
16th to endNight
40%6%12%21%17%2
1498
June1st to 15thNight
34%6%15%25%14%3
1440
16th to endNight
29%6%15%24%19%4
1440
July1st to 15thNight
24%6%17%30%16%3
1360
16th to endNight
22%6%18%31%19%1
1469
August1st to 15thNight
26%6%15%31%16%3
1610
16th to endNight
31%6%10%26%18%5%
1863
September1st to 15thNight
28%5%10%27%26%2
1800
16th to endNight
29%412%31%19%1
1839
October1st to 15thNight
36%38%24%22%3
1980
16th to endNight
40%414%23%14%2
2117
November1st to 15thNight
36%312%23%19%3
2299
16th to endNight
45%413%26%9%
2520
December1st to 15thNight
55%5%11%19%7%
2520
16th to endNight
58%5%11%15%7%
2688

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
28%6%38%21%22
90
16th to endDay
23%6%32%27%7%3
96
February1st to 15thDay
8%23%38%23%5%
90
16th to endDay
19%7%45%13%13%
81
March1st to 15thDay
22%18%38%15%4
90
16th to endDay
37%9%25%18%5%2
132
April1st to 15thDay
20%12%32%23%10%
270
16th to endDay
30%8%29%17%12%
294
May1st to 15thDay
19%13%31%26%7%1
468
16th to endDay
16%10%29%26%13%2
517
June1st to 15thDay
14%327%31%18%4
495
16th to endDay
9%5%33%29%17%4
495
July1st to 15thDay
7%125%34%21%9%
475
16th to endDay
10%120%34%27%5%
515
August1st to 15thDay
11%219%35%25%6%
493
16th to endDay
12%217%33%25%8%
379
September1st to 15thDay
10%320%32%24%9%
315
16th to endDay
10%6%28%27%24%3
185
October1st to 15thDay
11%8%30%24%17%7%
90
16th to endDay
19%8%38%23%9%
96
November1st to 15thDay
46%32%32%17%6%
90
16th to endDay
40%13%32%11%3
90
December1st to 15thDay
22%15%37%16%6%1
90
16th to endDay
23%22%40%9%3
96
January1st to 15thNight
15%13%36%26%8%
900
16th to endNight
17%11%36%28%6%
960
February1st to 15thNight
18%11%36%26%6%
880
16th to endNight
17%10%38%24%6%1
687
March1st to 15thNight
18%10%34%25%9%1
603
16th to endNight
21%8%37%24%7%
576
April1st to 15thNight
18%10%38%27%4
539
16th to endNight
22%7%31%29%8%1
540
May1st to 15thNight
17%8%35%29%7%
540
16th to endNight
13%5%31%34%13%
563
June1st to 15thNight
12%325%42%15%
540
16th to endNight
9%128%43%15%1
540
July1st to 15thNight
7%16%45%28%2
510
16th to endNight
6%116%42%30%2
564
August1st to 15thNight
7%114%42%31%2
539
16th to endNight
12%16%38%28%3
573
September1st to 15thNight
11%413%38%25%5%
540
16th to endNight
8%427%33%24%1
573
October1st to 15thNight
15%5%30%30%16%1
660
16th to endNight
17%8%31%31%10%
704
November1st to 15thNight
14%9%36%28%10%
723
16th to endNight
21%11%35%23%8%
900
December1st to 15thNight
23%12%34%23%5%
900
16th to endNight
26%9%31%26%5%
960
Page generated at 2017-09-20 03:49:53. Version ac04

Copyright 2017 A. Danko


Van Horne Clear Sky Chart | Charts Home