AJP Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AJP Observatory clear sky chart from 2005-11-05 to 2017-09-10.

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
21%
14752
14%
46669
5%
14712
31%
14712
16%
32924

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
12%
750
13%
2685
2%
746
26%
746
11%
1882
January16th to endNight
18%
772
18%
2611
4%
772
35%
772
16%
1711
February1st to 15thNight
16%
704
12%
2286
4%
701
27%
701
15%
1626
February16th to endNight
15%
631
12%
2022
2%
631
27%
631
11%
1533
March1st to 15thNight
16%
720
16%
2016
4%
720
26%
720
15%
1495
March16th to endNight
18%
768
18%
2112
6%
768
29%
768
18%
1527
April1st to 15thNight
18%
572
13%
1764
5%
572
26%
572
13%
1310
April16th to endNight
20%
540
16%
1620
7%
540
28%
540
17%
1151
May1st to 15thNight
24%
540
10%
1530
6%
540
29%
540
15%
1134
May16th to endNight
19%
525
10%
1460
3%
524
26%
524
15%
1088
June1st to 15thNight
28%
360
8%
1260
4%
360
32%
360
14%
907
June16th to endNight
28%
360
5%
1260
2%
360
33%
360
14%
918
July1st to 15thNight
31%
340
10%
1190
5%
340
39%
340
20%
777
July16th to endNight
33%
457
9%
1363
7%
429
37%
429
19%
956
August1st to 15thNight
34%
539
9%
1608
5%
535
38%
535
20%
1142
August16th to endNight
34%
573
15%
1728
6%
573
41%
573
24%
1115
September1st to 15thNight
26%
555
15%
1779
7%
555
35%
555
21%
1131
September16th to endNight
29%
657
22%
1820
10%
657
40%
657
31%
1118
October1st to 15thNight
27%
660
21%
1980
8%
660
38%
660
23%
1296
October16th to endNight
19%
704
15%
2164
5%
704
29%
704
15%
1541
November1st to 15thNight
19%
725
15%
2428
6%
725
30%
725
14%
1724
November16th to endNight
22%
750
13%
2520
6%
750
32%
750
13%
1821
December1st to 15thNight
12%
750
13%
2583
2%
750
24%
750
11%
1862
December16th to endNight
11%
800
12%
2880
3%
800
23%
800
9%
2159

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
21%8%36%5%34446%31%
47965
Night
19%9%36%5%34446%33%
46830
TransparencyDay
53%411%19%9%1
47645
Night
51%411%20%9%
46669
SeeingDay
28%13%30%18%7%1
6966
Night
26%13%33%20%6%
14752

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%14%5%6%6%2435%6%32%
1253
16th to endDay
14%10%5%6%5%33445%37%
1608
February1st to 15thDay
13%8%47%6%45%5%5%6%32%
1575
16th to endDay
16%12%6%9%7%34335%27%
1449
March1st to 15thDay
20%14%46%4245%3430%
1932
16th to endDay
17%12%5%7%424335%33%
2112
April1st to 15thDay
21%10%36%6%34345%29%
2183
16th to endDay
26%9%25%422335%33%
2340
May1st to 15thDay
28%9%35%423335%29%
2421
16th to endDay
26%8%35%424446%27%
2664
June1st to 15thDay
28%8%36%5%25%445%25%
2700
16th to endDay
27%7%25%5%2435%7%27%
2700
July1st to 15thDay
17%5%26%5%36%46%8%33%
2700
16th to endDay
20%6%26%6%46%5%5%8%28%
2694
August1st to 15thDay
19%6%26%6%35%5%6%7%29%
2340
16th to endDay
15%5%25%6%45%5%5%7%35%
2474
September1st to 15thDay
18%8%36%5%2435%5%36%
2066
16th to endDay
19%5%24413235%46%
1810
October1st to 15thDay
19%7%35%423346%39%
1650
16th to endDay
26%7%25%5%23445%32%
1708
November1st to 15thDay
22%11%34434346%31%
1428
16th to endDay
18%10%37%5%24346%31%
1440
December1st to 15thDay
23%9%46%434345%29%
1374
16th to endDay
23%12%5%6%42333427%
1344
January1st to 15thNight
15%10%46%5%345%45%32%
2685
16th to endNight
14%9%36%5%3435%5%37%
2611
February1st to 15thNight
14%12%36%5%345%5%6%31%
2285
16th to endNight
18%14%5%6%5%35%345%26%
2022
March1st to 15thNight
19%10%36%5%35%5%46%29%
2004
16th to endNight
17%10%47%5%2333436%
2112
April1st to 15thNight
23%9%47%5%33445%28%
1764
16th to endNight
24%11%24322436%33%
1620
May1st to 15thNight
24%8%346%24346%30%
1530
16th to endNight
21%7%36%6%35%45%6%28%
1458
June1st to 15thNight
26%8%25%425%35%6%29%
1260
16th to endNight
24%7%25%5%36%45%6%28%
1260
July1st to 15thNight
13%5%26%6%36%6%6%6%36%
1260
16th to endNight
17%6%36%6%36%46%9%29%
1457
August1st to 15thNight
18%5%25%6%45%5%47%33%
1620
16th to endNight
14%5%25%4245%48%41%
1728
September1st to 15thNight
18%5%24422446%42%
1779
16th to endNight
16%413323446%49%
1820
October1st to 15thNight
18%6%24423446%40%
1980
16th to endNight
25%8%25%423335%34%
2164
November1st to 15thNight
22%9%35%42435%6%31%
2428
16th to endNight
21%9%36%444345%31%
2520
December1st to 15thNight
22%11%36%5%2433431%
2583
16th to endNight
24%10%5%6%5%2434426%
2880

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
53%5%13%16%10%1
1254
16th to endDay
49%310%19%16%
1608
February1st to 15thDay
51%412%19%8%2
1578
16th to endDay
59%47%18%8%1
1449
March1st to 15thDay
57%38%19%9%1
1943
16th to endDay
55%37%19%10%3
2112
April1st to 15thDay
57%37%15%12%2
2183
16th to endDay
54%38%16%13%2
2340
May1st to 15thDay
58%37%19%10%
2421
16th to endDay
57%5%10%18%6%
2668
June1st to 15thDay
60%410%14%8%1
2700
16th to endDay
57%413%15%8%
2700
July1st to 15thDay
46%6%16%20%9%
2541
16th to endDay
52%6%15%18%6%
2527
August1st to 15thDay
51%6%13%22%6%
2327
16th to endDay
45%6%17%22%7%
2474
September1st to 15thDay
47%5%13%21%11%
2066
16th to endDay
41%311%28%14%1
1810
October1st to 15thDay
46%412%26%9%
1650
16th to endDay
54%411%19%9%
1708
November1st to 15thDay
54%413%14%11%
1428
16th to endDay
53%415%16%9%
1440
December1st to 15thDay
58%314%16%6%
1374
16th to endDay
60%49%15%9%
1344
January1st to 15thNight
52%5%13%17%9%
2685
16th to endNight
47%312%23%12%
2611
February1st to 15thNight
53%412%18%10%
2286
16th to endNight
60%37%15%10%1
2022
March1st to 15thNight
55%38%18%11%1
2016
16th to endNight
52%37%20%14%2
2112
April1st to 15thNight
58%38%14%11%3
1764
16th to endNight
52%38%16%14%3
1620
May1st to 15thNight
55%5%10%18%8%1
1530
16th to endNight
54%6%13%17%8%
1460
June1st to 15thNight
55%411%19%7%1
1260
16th to endNight
55%6%14%16%7%
1260
July1st to 15thNight
42%8%15%25%7%
1190
16th to endNight
49%6%18%16%8%
1363
August1st to 15thNight
49%7%14%22%6%
1608
16th to endNight
40%6%19%25%7%
1728
September1st to 15thNight
43%413%25%12%
1779
16th to endNight
35%413%30%14%1
1820
October1st to 15thNight
43%411%28%11%
1980
16th to endNight
53%310%23%9%
2164
November1st to 15thNight
53%413%19%8%
2428
16th to endNight
54%5%12%18%7%
2520
December1st to 15thNight
57%211%21%6%
2583
16th to endNight
60%48%17%9%
2880

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
33%11%48%6%
60
16th to endDay
20%17%33%20%7%
163
February1st to 15thDay
19%16%33%22%7%
236
16th to endDay
27%14%31%18%6%1
217
March1st to 15thDay
32%15%31%14%41
270
16th to endDay
28%17%28%16%7%1
288
April1st to 15thDay
31%17%29%15%5%1
270
16th to endDay
31%18%26%15%6%1
270
May1st to 15thDay
39%14%24%14%5%2
377
16th to endDay
33%14%28%17%5%
519
June1st to 15thDay
35%11%27%17%6%1
495
16th to endDay
30%11%32%18%6%
495
July1st to 15thDay
21%9%36%20%10%1
475
16th to endDay
21%7%32%29%8%
461
August1st to 15thDay
18%10%33%22%12%2
313
16th to endDay
20%9%29%26%11%1
332
September1st to 15thDay
21%16%32%19%9%
294
16th to endDay
22%11%35%21%5%3
253
October1st to 15thDay
24%10%28%18%12%5%
225
16th to endDay
35%17%26%11%6%2
240
November1st to 15thDay
31%13%28%16%8%2
235
16th to endDay
27%15%31%16%7%1
240
December1st to 15thDay
32%16%31%16%3
174
16th to endDay
37%20%37%4
64
January1st to 15thNight
25%18%34%18%3
750
16th to endNight
22%16%36%19%5%
772
February1st to 15thNight
23%15%38%17%5%
704
16th to endNight
30%13%32%18%5%
631
March1st to 15thNight
27%15%36%16%3
720
16th to endNight
23%15%35%18%5%
768
April1st to 15thNight
29%15%30%19%6%
572
16th to endNight
32%12%30%18%4
540
May1st to 15thNight
30%13%27%21%6%
540
16th to endNight
25%16%34%16%6%
525
June1st to 15thNight
28%7%28%29%6%
360
16th to endNight
28%9%28%25%8%
360
July1st to 15thNight
14%9%39%26%9%
340
16th to endNight
20%5%35%28%9%
457
August1st to 15thNight
20%434%28%11%
539
16th to endNight
17%10%31%27%10%1
573
September1st to 15thNight
23%10%34%23%7%
555
16th to endNight
18%12%37%23%7%
657
October1st to 15thNight
21%12%35%21%8%
660
16th to endNight
31%15%29%18%5%
704
November1st to 15thNight
28%12%33%17%6%1
725
16th to endNight
27%16%26%20%7%1
750
December1st to 15thNight
30%17%34%13%4
750
16th to endNight
35%15%28%15%31
800
Page generated at 2017-09-11 20:35:23. Version ac04

Copyright 2017 A. Danko


AJP Observatory Clear Sky Chart | Charts Home