MAC-Johnson Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the MAC-Johnson Site clear sky chart from 2005-11-05 to 2017-09-21.

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
50%
15825
22%
46943
15%
15784
57%
15784
29%
29200

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
34%
718
22%
2327
11%
716
48%
716
25%
1402
January16th to endNight
31%
768
24%
2496
14%
768
45%
768
24%
1532
February1st to 15thNight
35%
704
23%
2204
12%
701
49%
701
26%
1343
February16th to endNight
31%
631
27%
1857
12%
631
46%
631
30%
1117
March1st to 15thNight
36%
720
27%
1980
14%
720
47%
720
33%
1213
March16th to endNight
41%
768
21%
2112
13%
768
48%
768
30%
1323
April1st to 15thNight
44%
718
24%
1940
14%
718
51%
718
36%
1167
April16th to endNight
47%
657
25%
1737
13%
657
53%
657
34%
1064
May1st to 15thNight
49%
539
19%
1620
10%
539
54%
539
30%
998
May16th to endNight
64%
561
19%
1682
13%
561
67%
561
30%
1037
June1st to 15thNight
64%
540
15%
1620
13%
540
65%
540
26%
1048
June16th to endNight
71%
540
15%
1620
14%
540
71%
540
28%
1042
July1st to 15thNight
66%
510
12%
1530
10%
510
67%
510
23%
1051
July16th to endNight
70%
564
11%
1595
10%
531
72%
531
26%
1065
August1st to 15thNight
70%
539
15%
1609
14%
536
70%
536
23%
1106
August16th to endNight
64%
675
15%
1842
12%
675
65%
675
24%
1290
September1st to 15thNight
68%
720
19%
1980
17%
720
69%
720
29%
1301
September16th to endNight
55%
681
17%
1881
14%
681
58%
681
24%
1317
October1st to 15thNight
56%
660
30%
1922
28%
660
61%
660
35%
1136
October16th to endNight
57%
704
33%
2112
28%
704
67%
704
40%
1157
November1st to 15thNight
56%
700
31%
2101
25%
700
64%
700
39%
1154
November16th to endNight
44%
720
27%
2340
20%
720
56%
720
30%
1352
December1st to 15thNight
45%
720
24%
2340
18%
720
56%
720
27%
1403
December16th to endNight
38%
768
23%
2496
17%
768
48%
768
23%
1582

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
12%5%245%345%5%7%42%
47906
Night
11%5%2442445%7%45%
47143
TransparencyDay
38%6%11%20%17%5%
47607
Night
37%6%11%19%18%7%
46946
SeeingDay
13%416%24%24%15%
5669
Night
14%5%21%33%22%3
15830

By Month

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
15%5%2442425%6%45%
1611
16th to endDay
16%6%24323346%46%
1728
February1st to 15thDay
15%6%35%323446%46%
1654
16th to endDay
16%5%2442333449%
1614
March1st to 15thDay
13%423424445%49%
1980
16th to endDay
14%5%35%5%24445%45%
2111
April1st to 15thDay
10%41442445%7%50%
2005
16th to endDay
9%4145%2446%7%49%
2232
May1st to 15thDay
10%425%5%345%5%7%46%
2331
16th to endDay
9%5%35%6%45%6%6%8%39%
2431
June1st to 15thDay
10%425%5%35%6%7%10%38%
2340
16th to endDay
5%3245%46%6%8%10%41%
2340
July1st to 15thDay
8%437%6%5%7%6%7%11%32%
2340
16th to endDay
7%5%347%48%7%8%11%29%
2465
August1st to 15thDay
9%6%25%7%47%8%8%9%30%
2340
16th to endDay
10%5%36%7%46%8%7%10%28%
2344
September1st to 15thDay
9%5%25%5%35%5%6%8%41%
1980
16th to endDay
17%6%25%4235%6%7%38%
1872
October1st to 15thDay
14%423323345%52%
1706
16th to endDay
10%5%23322225%58%
1760
November1st to 15thDay
13%5%13222335%55%
1754
16th to endDay
13%5%24324345%50%
1620
December1st to 15thDay
17%6%25%413347%42%
1620
16th to endDay
20%6%1442445%6%40%
1728
January1st to 15thNight
13%6%24423435%47%
2327
16th to endNight
16%6%3432334446%
2496
February1st to 15thNight
16%414424436%46%
2196
16th to endNight
14%415%313446%48%
1857
March1st to 15thNight
11%5%144245%5%6%48%
1980
16th to endNight
12%423424346%48%
2112
April1st to 15thNight
11%313323347%52%
1940
16th to endNight
9%325%5%2445%7%49%
1737
May1st to 15thNight
7%5%25%435%5%5%7%45%
1619
16th to endNight
7%425%436%5%6%9%44%
1682
June1st to 15thNight
11%5%25%335%5%8%11%36%
1620
16th to endNight
4436%6%35%7%8%10%39%
1620
July1st to 15thNight
7%6%36%6%37%7%7%9%34%
1620
16th to endNight
5%336%6%37%7%9%10%35%
1703
August1st to 15thNight
7%5%26%6%5%6%6%9%11%33%
1620
16th to endNight
10%5%26%6%36%6%7%11%32%
1842
September1st to 15thNight
8%4345%45%6%6%9%40%
1980
16th to endNight
14%7%25%7%45%446%38%
1881
October1st to 15thNight
15%414312346%52%
1922
16th to endNight
9%323412345%57%
2112
November1st to 15thNight
10%412313435%58%
2101
16th to endNight
13%42441335%5%50%
2340
December1st to 15thNight
14%5%13424446%46%
2340
16th to endNight
18%6%24423446%41%
2496

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
39%49%16%23%5%
1612
16th to endDay
39%26%19%23%7%
1728
February1st to 15thDay
39%26%20%23%7%
1656
16th to endDay
39%3421%23%6%
1614
March1st to 15thDay
36%38%21%19%9%
1980
16th to endDay
40%410%19%19%5%
2112
April1st to 15thDay
33%49%20%24%7%
2005
16th to endDay
32%6%10%21%21%6%
2232
May1st to 15thDay
36%6%13%20%19%2
2331
16th to endDay
40%7%13%23%13%1
2435
June1st to 15thDay
37%8%18%24%10%
2340
16th to endDay
34%10%18%23%12%1
2340
July1st to 15thDay
41%9%17%20%7%3
2201
16th to endDay
42%12%19%17%7%1
2310
August1st to 15thDay
44%10%15%20%8%1
2327
16th to endDay
46%11%15%16%7%
2344
September1st to 15thDay
38%8%14%15%17%4
1980
16th to endDay
44%6%10%16%16%5%
1872
October1st to 15thDay
34%38%21%22%8%
1706
16th to endDay
30%3418%29%13%
1760
November1st to 15thDay
32%35%19%26%13%
1754
16th to endDay
36%38%18%23%10%
1620
December1st to 15thDay
41%49%19%17%7%
1620
16th to endDay
43%39%18%17%7%
1728
January1st to 15thNight
39%38%15%25%7%
2327
16th to endNight
40%3417%21%11%
2496
February1st to 15thNight
38%46%18%20%11%
2205
16th to endNight
36%38%19%22%9%
1857
March1st to 15thNight
34%410%18%22%9%
1980
16th to endNight
37%410%19%19%7%
2112
April1st to 15thNight
32%48%22%23%8%
1940
16th to endNight
34%49%20%22%8%
1737
May1st to 15thNight
36%6%14%20%18%3
1620
16th to endNight
33%8%15%24%14%1
1684
June1st to 15thNight
37%9%16%25%9%
1620
16th to endNight
34%11%18%22%10%2
1620
July1st to 15thNight
40%12%18%17%8%3
1530
16th to endNight
36%13%22%16%9%1
1595
August1st to 15thNight
40%11%18%18%9%
1609
16th to endNight
43%9%19%18%8%
1842
September1st to 15thNight
36%10%14%19%13%6%
1980
16th to endNight
45%6%10%17%13%4
1881
October1st to 15thNight
34%29%20%23%9%
1922
16th to endNight
29%27%20%25%14%
2112
November1st to 15thNight
29%27%15%28%15%
2101
16th to endNight
35%36%16%23%14%
2340
December1st to 15thNight
38%410%18%19%8%
2340
16th to endNight
42%49%18%17%8%
2496

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
21%11%33%24%8%1
90
16th to endDay
18%16%37%21%41
96
February1st to 15thDay
15%17%45%17%3
90
16th to endDay
13%8%39%31%6%
115
March1st to 15thDay
10%11%32%30%14%1
270
16th to endDay
14%7%32%27%15%1
288
April1st to 15thDay
14%11%20%24%21%7%
270
16th to endDay
8%421%28%23%13%
270
May1st to 15thDay
10%6%22%28%23%8%
288
16th to endDay
12%214%19%22%28%
331
June1st to 15thDay
12%8%20%31%26%
315
16th to endDay
9%19%25%27%26%
315
July1st to 15thDay
8%424%31%29%
305
16th to endDay
8%16%18%38%26%
326
August1st to 15thDay
16%1418%35%23%
313
16th to endDay
13%6%16%30%32%
332
September1st to 15thDay
13%28%22%31%21%
315
16th to endDay
24%25%28%24%16%
275
October1st to 15thDay
19%215%32%21%8%
255
16th to endDay
12%318%22%31%11%
272
November1st to 15thDay
19%416%30%23%5%
262
16th to endDay
6%7%24%33%22%5%
90
December1st to 15thDay
22%334%25%11%3
90
16th to endDay
29%8%29%21%9%2
96
January1st to 15thNight
17%9%31%30%9%
720
16th to endNight
18%11%34%30%4
768
February1st to 15thNight
18%11%29%35%4
704
16th to endNight
16%10%35%32%4
631
March1st to 15thNight
14%9%36%33%6%
720
16th to endNight
13%7%31%32%14%
768
April1st to 15thNight
14%9%28%32%15%
718
16th to endNight
12%7%25%38%15%
657
May1st to 15thNight
11%228%37%16%2
540
16th to endNight
9%214%36%32%4
563
June1st to 15thNight
14%110%36%28%7%
540
16th to endNight
5%18%37%37%9%
540
July1st to 15thNight
9%15%36%38%7%
510
16th to endNight
6%1434%45%7%
564
August1st to 15thNight
10%5%26%48%8%
539
16th to endNight
14%8%26%40%9%
675
September1st to 15thNight
9%27%30%44%6%
720
16th to endNight
17%112%32%31%4
681
October1st to 15thNight
18%317%31%26%4
660
16th to endNight
11%422%33%25%3
704
November1st to 15thNight
12%5%22%37%19%3
700
16th to endNight
15%7%27%28%18%1
720
December1st to 15thNight
17%6%24%33%16%1
720
16th to endNight
21%9%26%32%9%1
768
Page generated at 2017-09-22 07:35:38. Version ac04

Copyright 2017 A. Danko


MAC-Johnson Site Clear Sky Chart | Charts Home