John's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John's Observatory clear sky chart from 2005-11-05 to 2017-10-06.

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
64%
15965
20%
47179
17%
15927
67%
15927
31%
29333

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
52%
716
19%
2327
15%
716
57%
716
25%
1456
January16th to endNight
50%
768
24%
2382
19%
768
57%
768
30%
1430
February1st to 15thNight
51%
701
24%
2105
19%
699
57%
699
33%
1263
February16th to endNight
55%
631
25%
1857
17%
631
62%
631
36%
1065
March1st to 15thNight
53%
720
30%
1980
17%
720
63%
720
44%
1099
March16th to endNight
58%
768
27%
2112
21%
768
62%
768
40%
1216
April1st to 15thNight
64%
718
22%
1976
18%
718
67%
718
45%
1099
April16th to endNight
61%
718
27%
1888
19%
718
66%
718
43%
1106
May1st to 15thNight
78%
539
28%
1619
25%
539
81%
539
46%
864
May16th to endNight
77%
561
24%
1683
20%
561
78%
561
38%
980
June1st to 15thNight
73%
540
11%
1620
10%
540
73%
540
25%
1103
June16th to endNight
79%
540
9%
1620
8%
540
79%
540
26%
1101
July1st to 15thNight
76%
510
6%
1530
6%
510
76%
510
20%
1132
July16th to endNight
78%
564
7%
1595
7%
531
79%
531
23%
1125
August1st to 15thNight
79%
539
4%
1690
5%
536
79%
536
25%
1141
August16th to endNight
68%
699
8%
2036
8%
699
68%
699
20%
1492
September1st to 15thNight
68%
720
7%
1980
8%
720
68%
720
18%
1460
September16th to endNight
65%
717
13%
1980
12%
717
66%
717
21%
1428
October1st to 15thNight
72%
684
24%
1906
24%
684
74%
684
35%
1160
October16th to endNight
59%
704
25%
2112
24%
704
64%
704
32%
1276
November1st to 15thNight
65%
700
30%
2101
28%
700
70%
700
38%
1195
November16th to endNight
55%
720
28%
2244
23%
720
63%
720
35%
1300
December1st to 15thNight
57%
720
24%
2340
23%
720
61%
720
29%
1408
December16th to endNight
58%
768
24%
2496
25%
768
61%
768
32%
1434

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
8%5%25%6%46%6%7%9%38%
47996
Night
6%425%5%35%5%6%9%44%
47385
TransparencyDay
41%8%14%16%13%5%
47697
Night
34%8%17%20%14%5%
47188
SeeingDay
9%212%21%26%28%
5826
Night
8%215%33%29%10%
15975

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
11%6%25%5%245%6%10%38%
1611
16th to endDay
10%5%245%245%5%7%47%
1839
February1st to 15thDay
11%6%244245%5%6%47%
1750
16th to endDay
8%4145%2435%9%51%
1613
March1st to 15thDay
43143245%5%7%56%
1980
16th to endDay
8%41343445%8%50%
2111
April1st to 15thDay
6%425%5%35%5%6%9%47%
1972
16th to endDay
6%314435%5%5%7%50%
2076
May1st to 15thDay
43133245%7%8%54%
2331
16th to endDay
7%4246%47%7%7%9%38%
2431
June1st to 15thDay
10%6%37%7%47%8%7%10%26%
2340
16th to endDay
7%5%46%7%6%9%8%9%10%23%
2340
July1st to 15thDay
8%5%37%9%5%12%9%8%9%20%
2340
16th to endDay
8%6%37%9%5%9%9%10%10%20%
2465
August1st to 15thDay
5%6%27%8%6%10%9%10%10%20%
2255
16th to endDay
10%6%37%8%6%8%9%8%9%22%
2154
September1st to 15thDay
10%5%38%8%6%8%8%8%9%22%
1980
16th to endDay
8%7%39%8%5%7%7%7%10%23%
1980
October1st to 15thDay
6%336%7%446%6%8%40%
1850
16th to endDay
9%6%245%2445%7%46%
1760
November1st to 15thDay
5%4245%35%6%5%7%49%
1757
16th to endDay
8%4235%3445%7%48%
1713
December1st to 15thDay
8%6%36%5%35%5%6%9%40%
1620
16th to endDay
7%5%25%5%25%45%9%47%
1728
January1st to 15thNight
9%5%26%5%36%5%6%9%39%
2327
16th to endNight
11%5%245%3446%8%43%
2382
February1st to 15thNight
11%5%24424347%47%
2103
16th to endNight
6%4244345%7%8%48%
1857
March1st to 15thNight
42134245%7%8%54%
1980
16th to endNight
6%32442445%8%54%
2112
April1st to 15thNight
43134245%6%8%55%
1976
16th to endNight
4413435%45%9%52%
1888
May1st to 15thNight
331332445%9%59%
1619
16th to endNight
43244245%6%8%52%
1682
June1st to 15thNight
9%5%345%46%6%7%11%36%
1620
16th to endNight
45%246%46%7%9%13%36%
1620
July1st to 15thNight
5%5%26%5%47%7%9%12%32%
1620
16th to endNight
3425%7%49%7%10%15%29%
1703
August1st to 15thNight
5%4146%36%7%9%14%36%
1701
16th to endNight
8%425%7%47%5%8%12%30%
2036
September1st to 15thNight
6%5%27%8%6%8%7%7%11%28%
1980
16th to endNight
8%7%36%6%5%6%6%7%10%30%
1980
October1st to 15thNight
5%3245%345%6%11%46%
1906
16th to endNight
7%5%25%43445%8%47%
2112
November1st to 15thNight
5%325%4245%5%8%51%
2101
16th to endNight
6%425%5%2436%8%48%
2244
December1st to 15thNight
9%5%35%5%25%46%8%44%
2340
16th to endNight
6%425%435%5%5%8%46%
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
40%5%10%21%17%4
1612
16th to endDay
34%48%20%23%7%
1839
February1st to 15thDay
37%48%16%19%13%
1752
16th to endDay
31%412%17%23%10%
1613
March1st to 15thDay
26%411%26%22%8%
1980
16th to endDay
32%411%21%21%8%
2112
April1st to 15thDay
32%6%13%19%19%6%
1972
16th to endDay
31%6%12%18%24%6%
2076
May1st to 15thDay
26%7%13%24%20%7%
2331
16th to endDay
40%8%14%19%15%1
2435
June1st to 15thDay
51%9%18%16%3
2340
16th to endDay
50%14%20%12%1
2340
July1st to 15thDay
56%15%19%5%1
2201
16th to endDay
51%14%22%10%
2310
August1st to 15thDay
51%15%25%7%
2242
16th to endDay
54%14%20%9%
2154
September1st to 15thDay
54%14%19%10%2
1980
16th to endDay
54%10%14%13%41
1980
October1st to 15thDay
39%8%10%15%16%8%
1850
16th to endDay
37%5%8%16%20%12%
1760
November1st to 15thDay
32%48%22%22%8%
1757
16th to endDay
34%5%9%19%18%13%
1713
December1st to 15thDay
39%5%12%19%17%5%
1620
16th to endDay
34%5%12%21%19%6%
1728
January1st to 15thNight
40%5%11%23%14%4
2327
16th to endNight
36%410%20%20%7%
2382
February1st to 15thNight
35%37%18%22%12%
2112
16th to endNight
30%5%14%20%22%7%
1857
March1st to 15thNight
24%5%12%29%20%7%
1980
16th to endNight
28%5%11%25%21%8%
2112
April1st to 15thNight
24%5%18%25%20%6%
1976
16th to endNight
27%5%13%23%24%5%
1888
May1st to 15thNight
21%7%13%30%20%7%
1620
16th to endNight
27%7%18%26%17%1
1684
June1st to 15thNight
40%10%24%18%5%
1620
16th to endNight
34%15%28%16%5%
1620
July1st to 15thNight
41%17%30%8%1
1530
16th to endNight
36%17%31%13%
1595
August1st to 15thNight
32%17%38%10%
1690
16th to endNight
43%15%27%12%1
2036
September1st to 15thNight
45%14%24%14%2
1980
16th to endNight
44%12%20%16%41
1980
October1st to 15thNight
31%7%14%19%19%7%
1906
16th to endNight
34%6%10%15%23%9%
2112
November1st to 15thNight
29%5%9%22%22%10%
2101
16th to endNight
33%411%22%16%12%
2244
December1st to 15thNight
36%6%12%23%16%4
2340
16th to endNight
33%5%13%21%18%7%
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
23%6%25%24%15%4
90
16th to endDay
10%14%36%26%10%2
96
February1st to 15thDay
11%11%26%28%17%4
90
16th to endDay
11%234%35%16%
114
March1st to 15thDay
6%331%35%20%2
270
16th to endDay
10%5%21%32%25%5%
288
April1st to 15thDay
7%321%32%28%6%
270
16th to endDay
10%112%32%30%12%
270
May1st to 15thDay
6%211%23%34%21%
288
16th to endDay
7%15%18%32%35%
331
June1st to 15thDay
18%16%12%21%39%
315
16th to endDay
6%146%23%57%
315
July1st to 15thDay
7%418%68%
305
16th to endDay
12%37%19%56%
326
August1st to 15thDay
8%46%20%59%
313
16th to endDay
9%15%7%25%50%
332
September1st to 15thDay
10%15%13%24%44%
315
16th to endDay
8%214%38%35%
314
October1st to 15thDay
6%110%28%38%13%
278
16th to endDay
11%215%29%29%11%
272
November1st to 15thDay
7%112%35%33%10%
265
16th to endDay
8%24%36%25%4
183
December1st to 15thDay
11%333%36%14%1
90
16th to endDay
8%5%10%35%33%7%
96
January1st to 15thNight
12%423%40%18%1
720
16th to endNight
12%5%25%39%15%
768
February1st to 15thNight
13%425%42%13%
704
16th to endNight
7%427%45%15%
631
March1st to 15thNight
6%431%42%13%1
720
16th to endNight
7%224%44%19%1
768
April1st to 15thNight
5%221%46%23%
718
16th to endNight
7%222%44%22%
718
May1st to 15thNight
4110%48%32%3
540
16th to endNight
5%17%35%41%8%
563
June1st to 15thNight
9%1320%48%17%
540
16th to endNight
5%1210%44%35%
540
July1st to 15thNight
6%26%42%41%
510
16th to endNight
427%44%41%
564
August1st to 15thNight
6%5%44%41%
539
16th to endNight
10%15%12%40%30%
699
September1st to 15thNight
8%1418%43%23%
720
16th to endNight
12%5%22%42%15%
717
October1st to 15thNight
9%8%39%36%5%
684
16th to endNight
11%118%33%30%3
704
November1st to 15thNight
6%119%45%24%2
700
16th to endNight
6%326%42%19%2
720
December1st to 15thNight
10%320%41%23%
720
16th to endNight
7%322%42%22%1
768
Page generated at 2017-10-07 14:07:45. Version ac04

Copyright 2017 A. Danko


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