John J. McCarthy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John J. McCarthy Observatory clear sky chart from 2005-11-05 to 2018-02-14.

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
28%
15565
19%
48800
8%
15533
40%
15533
20%
31443

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
18%
780
21%
2716
7%
776
35%
776
18%
1668
January16th to endNight
16%
832
21%
2857
5%
832
36%
832
16%
1784
February1st to 15thNight
15%
760
17%
2388
3%
757
30%
757
15%
1592
February16th to endNight
16%
631
24%
1893
7%
631
35%
631
18%
1212
March1st to 15thNight
18%
720
18%
2160
6%
720
32%
720
16%
1443
March16th to endNight
19%
768
24%
1997
7%
768
33%
768
20%
1314
April1st to 15thNight
19%
718
16%
1796
3%
718
26%
718
21%
1263
April16th to endNight
22%
672
17%
1752
3%
672
32%
672
22%
1208
May1st to 15thNight
23%
513
16%
1593
6%
513
34%
513
16%
1094
May16th to endNight
34%
375
14%
1497
11%
374
43%
374
17%
944
June1st to 15thNight
34%
360
14%
1440
9%
360
47%
360
17%
939
June16th to endNight
47%
360
14%
1440
13%
360
57%
360
25%
815
July1st to 15thNight
47%
340
11%
1360
13%
340
51%
340
21%
839
July16th to endNight
51%
376
15%
1418
17%
354
56%
354
22%
858
August1st to 15thNight
48%
519
17%
1590
10%
517
54%
517
29%
967
August16th to endNight
55%
764
18%
1910
10%
764
57%
764
31%
1285
September1st to 15thNight
41%
720
22%
1808
11%
720
47%
720
32%
1178
September16th to endNight
47%
717
26%
1980
17%
717
53%
717
31%
1238
October1st to 15thNight
35%
720
21%
2082
12%
720
44%
720
24%
1350
October16th to endNight
26%
768
20%
2429
9%
768
40%
768
20%
1562
November1st to 15thNight
29%
760
21%
2472
10%
760
44%
760
21%
1561
November16th to endNight
26%
780
22%
2580
11%
780
40%
780
20%
1621
December1st to 15thNight
23%
780
15%
2730
5%
780
35%
780
14%
1805
December16th to endNight
15%
832
19%
2912
4%
832
33%
832
14%
1903

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%6%25%5%3445%7%39%
49290
Night
16%6%2442445%6%41%
48970
Cloud CoverDay
16%6%25%43445%7%39%
49300
Night
16%6%2442445%6%41%
48975
TransparencyDay
43%5%11%21%15%1
48978
Night
42%5%12%22%15%2
48800
SeeingDay
22%13%29%21%10%3
6224
Night
22%14%30%23%8%
15565

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
15%6%244245%5%6%41%
1552
16th to endDay
14%7%35%42445%7%39%
1718
February1st to 15thDay
16%7%36%6%35%447%35%
1774
16th to endDay
16%6%26%5%25%446%38%
1580
March1st to 15thDay
17%6%35%6%44346%36%
1790
16th to endDay
18%7%25%423346%40%
2226
April1st to 15thDay
19%7%245%24445%38%
2152
16th to endDay
16%5%24323437%43%
2208
May1st to 15thDay
22%7%245%25%346%34%
2368
16th to endDay
16%6%25%425%45%7%38%
2618
June1st to 15thDay
22%5%24425%5%6%7%32%
2520
16th to endDay
13%5%246%35%5%5%10%35%
2520
July1st to 15thDay
13%435%6%35%5%5%8%37%
2520
16th to endDay
12%435%5%36%6%6%8%37%
2654
August1st to 15thDay
13%5%245%25%6%7%8%37%
2346
16th to endDay
11%5%25%5%345%5%8%42%
2292
September1st to 15thDay
13%5%25%5%345%46%43%
2151
16th to endDay
12%5%2442345%6%47%
1980
October1st to 15thDay
19%5%25%42335%5%42%
1878
16th to endDay
18%5%14434446%41%
1794
November1st to 15thDay
17%6%23323347%43%
1716
16th to endDay
17%6%2331435%6%44%
1709
December1st to 15thDay
17%6%45%4345%46%38%
1560
16th to endDay
19%8%36%43445%7%33%
1664
January1st to 15thNight
17%6%2442445%6%42%
2716
16th to endNight
15%8%345%23447%41%
2857
February1st to 15thNight
15%7%35%5%35%45%6%37%
2387
16th to endNight
15%7%25%42335%6%42%
1893
March1st to 15thNight
17%6%25%4345%46%39%
2147
16th to endNight
16%6%25%423346%44%
1997
April1st to 15thNight
17%7%25%5%35%45%6%37%
1796
16th to endNight
18%5%25%423345%41%
1752
May1st to 15thNight
19%7%25%5%33445%36%
1593
16th to endNight
17%6%24434448%38%
1495
June1st to 15thNight
19%6%25%4235%46%39%
1440
16th to endNight
13%4244235%7%8%45%
1440
July1st to 15thNight
12%6%15%43446%9%39%
1440
16th to endNight
9%5%35%5%445%6%8%40%
1514
August1st to 15thNight
11%3145%245%6%8%45%
1600
16th to endNight
13%5%14435%46%8%43%
1910
September1st to 15thNight
15%423423447%48%
1808
16th to endNight
13%5%23424335%51%
1980
October1st to 15thNight
17%5%24433245%45%
2082
16th to endNight
19%424433345%41%
2429
November1st to 15thNight
15%7%24423445%43%
2472
16th to endNight
17%6%25%32435%7%40%
2580
December1st to 15thNight
20%7%2442446%6%36%
2730
16th to endNight
20%6%25%43445%6%36%
2912

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%6%2443445%5%42%
1552
16th to endDay
15%7%35%42445%7%40%
1718
February1st to 15thDay
16%6%26%5%35%5%47%36%
1774
16th to endDay
16%6%35%5%2445%6%39%
1580
March1st to 15thDay
17%7%25%5%35%446%36%
1800
16th to endDay
17%7%24433345%41%
2226
April1st to 15thDay
19%7%245%24446%38%
2152
16th to endDay
17%5%25%423437%44%
2208
May1st to 15thDay
21%7%25%42435%6%34%
2368
16th to endDay
16%6%25%425%45%7%38%
2618
June1st to 15thDay
22%5%24435%45%8%32%
2520
16th to endDay
12%6%245%35%5%7%9%36%
2520
July1st to 15thDay
13%5%25%5%46%5%5%7%38%
2520
16th to endDay
11%5%25%5%36%6%6%8%37%
2654
August1st to 15thDay
14%425%5%25%6%6%9%37%
2346
16th to endDay
10%5%26%5%35%45%8%41%
2292
September1st to 15thDay
12%5%15%5%346%45%43%
2151
16th to endDay
13%4245%245%45%47%
1980
October1st to 15thDay
18%7%24424346%41%
1878
16th to endDay
18%5%145%3445%5%41%
1794
November1st to 15thDay
17%6%25%323348%42%
1716
16th to endDay
17%6%24313347%44%
1709
December1st to 15thDay
17%6%35%424446%39%
1560
16th to endDay
19%8%35%42445%6%34%
1664
January1st to 15thNight
17%6%1431445%6%43%
2716
16th to endNight
15%7%35%423346%42%
2857
February1st to 15thNight
15%8%35%5%34446%38%
2380
16th to endNight
16%6%35%423346%43%
1893
March1st to 15thNight
18%6%25%42445%5%39%
2160
16th to endNight
16%6%34423447%43%
1996
April1st to 15thNight
17%7%34434447%37%
1796
16th to endNight
18%6%25%423435%42%
1752
May1st to 15thNight
19%6%35%5%34446%35%
1593
16th to endNight
17%7%2442345%7%38%
1495
June1st to 15thNight
21%5%245%24446%39%
1440
16th to endNight
12%5%25%41446%8%45%
1440
July1st to 15thNight
12%435%5%245%6%10%38%
1440
16th to endNight
9%5%35%5%35%5%6%8%40%
1514
August1st to 15thNight
11%3246%25%45%8%45%
1600
16th to endNight
13%4235%3446%7%43%
1910
September1st to 15thNight
14%424423347%47%
1808
16th to endNight
13%5%24324335%51%
1980
October1st to 15thNight
16%5%245%33346%44%
2082
16th to endNight
19%5%2442345%5%42%
2429
November1st to 15thNight
16%6%24413345%44%
2472
16th to endNight
17%6%2443335%6%41%
2580
December1st to 15thNight
20%7%2443445%6%36%
2730
16th to endNight
20%6%2442345%6%37%
2912

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
42%411%23%13%4
1553
16th to endDay
42%413%23%13%1
1718
February1st to 15thDay
47%5%12%19%13%1
1777
16th to endDay
44%311%24%13%1
1580
March1st to 15thDay
47%413%21%10%2
1800
16th to endDay
45%310%18%18%3
2226
April1st to 15thDay
46%410%20%15%3
2152
16th to endDay
39%5%9%22%19%3
2208
May1st to 15thDay
49%5%8%19%14%1
2368
16th to endDay
44%7%13%19%13%1
2622
June1st to 15thDay
49%6%12%20%9%1
2520
16th to endDay
41%7%14%23%11%
2520
July1st to 15thDay
44%7%10%21%15%1
2371
16th to endDay
41%7%14%21%13%2
2487
August1st to 15thDay
40%8%13%21%14%
2332
16th to endDay
39%6%9%25%18%1
2292
September1st to 15thDay
39%7%10%21%19%1
2151
16th to endDay
38%6%8%21%23%2
1980
October1st to 15thDay
43%49%23%16%2
1878
16th to endDay
42%410%24%14%2
1794
November1st to 15thDay
41%412%21%16%2
1716
16th to endDay
39%211%27%15%2
1709
December1st to 15thDay
46%415%18%13%1
1560
16th to endDay
49%414%18%11%1
1664
January1st to 15thNight
41%312%23%14%3
2716
16th to endNight
42%311%22%15%2
2857
February1st to 15thNight
46%412%19%12%2
2388
16th to endNight
42%310%22%17%3
1893
March1st to 15thNight
44%5%12%21%13%3
2160
16th to endNight
41%39%21%19%4
1997
April1st to 15thNight
45%5%11%20%15%1
1796
16th to endNight
44%48%20%18%3
1752
May1st to 15thNight
49%410%18%14%2
1593
16th to endNight
44%6%15%19%12%1
1497
June1st to 15thNight
47%5%12%20%12%1
1440
16th to endNight
35%6%15%26%14%1
1440
July1st to 15thNight
40%8%13%23%13%1
1360
16th to endNight
38%8%14%22%12%2
1418
August1st to 15thNight
35%6%13%26%15%1
1590
16th to endNight
36%9%11%23%18%
1910
September1st to 15thNight
36%6%10%25%19%1
1808
16th to endNight
36%5%6%23%26%1
1980
October1st to 15thNight
42%410%23%17%2
2082
16th to endNight
42%5%12%23%14%
2429
November1st to 15thNight
41%411%21%17%3
2472
16th to endNight
43%312%25%12%2
2580
December1st to 15thNight
47%5%14%19%13%
2730
16th to endNight
46%412%21%13%1
2912

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
26%30%27%11%31
90
16th to endDay
27%18%40%12%
109
February1st to 15thDay
30%26%29%13%
210
16th to endDay
22%23%35%14%4
239
March1st to 15thDay
28%18%32%15%5%
270
16th to endDay
25%23%32%13%41
288
April1st to 15thDay
28%18%27%15%8%1
270
16th to endDay
24%12%35%15%10%2
270
May1st to 15thDay
37%12%23%14%8%2
288
16th to endDay
20%10%30%25%10%2
331
June1st to 15thDay
24%11%26%22%13%1
315
16th to endDay
16%10%32%26%11%2
315
July1st to 15thDay
17%7%31%26%14%2
305
16th to endDay
12%7%29%30%18%2
326
August1st to 15thDay
15%428%29%13%7%
313
16th to endDay
11%425%29%17%11%
332
September1st to 15thDay
14%10%29%22%14%8%
315
16th to endDay
15%9%27%20%13%13%
314
October1st to 15thDay
24%7%26%21%14%4
315
16th to endDay
23%16%26%23%10%
304
November1st to 15thDay
22%15%26%23%9%2
280
16th to endDay
22%23%23%20%7%1
239
December1st to 15thDay
30%23%28%12%41
90
16th to endDay
33%26%31%8%1
96
January1st to 15thNight
24%19%33%19%3
780
16th to endNight
24%21%33%18%2
832
February1st to 15thNight
24%18%37%19%1
760
16th to endNight
25%20%32%17%2
631
March1st to 15thNight
26%20%32%17%2
720
16th to endNight
22%22%30%18%5%
768
April1st to 15thNight
24%19%34%17%3
718
16th to endNight
22%18%33%19%5%
672
May1st to 15thNight
24%16%33%20%5%
513
16th to endNight
22%7%30%31%6%1
375
June1st to 15thNight
22%6%29%29%11%
360
16th to endNight
12%429%42%10%
360
July1st to 15thNight
15%228%36%17%
340
16th to endNight
11%325%38%20%1
376
August1st to 15thNight
14%326%37%15%1
519
16th to endNight
15%317%36%23%3
764
September1st to 15thNight
16%6%29%30%15%1
720
16th to endNight
17%7%22%26%21%4
717
October1st to 15thNight
20%10%28%26%12%1
720
16th to endNight
24%15%31%20%7%
768
November1st to 15thNight
21%14%29%24%8%1
760
16th to endNight
23%15%30%21%8%
780
December1st to 15thNight
27%17%28%20%4
780
16th to endNight
27%20%33%16%2
832
Page generated at 2018-02-15 10:58:40. Version ac04

Copyright 2018 A. Danko


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