Perkins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perkins Observatory clear sky chart from 2005-11-05 to 2017-04-28.

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
42%
15044
18%
45364
13%
15004
47%
15004
23%
30767

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
16%
719
13%
2327
6%
716
23%
716
10%
1835
January16th to endNight
23%
768
13%
2496
8%
768
30%
768
13%
1892
February1st to 15thNight
24%
701
12%
2241
7%
701
30%
701
15%
1638
February16th to endNight
23%
631
13%
1893
7%
631
30%
631
14%
1430
March1st to 15thNight
31%
720
16%
2053
10%
720
36%
720
18%
1481
March16th to endNight
33%
768
18%
2112
11%
768
38%
768
22%
1474
April1st to 15thNight
33%
630
15%
1858
8%
630
39%
630
24%
1256
April16th to endNight
40%
534
21%
1602
10%
534
50%
534
26%
1040
May1st to 15thNight
41%
495
15%
1485
9%
495
47%
495
21%
1049
May16th to endNight
58%
514
18%
1540
14%
513
61%
513
32%
954
June1st to 15thNight
63%
495
17%
1485
14%
495
68%
495
34%
870
June16th to endNight
63%
495
16%
1324
14%
495
65%
495
34%
828
July1st to 15thNight
63%
465
17%
1293
15%
465
64%
465
33%
809
July16th to endNight
72%
522
19%
1469
18%
489
73%
489
38%
833
August1st to 15thNight
70%
494
22%
1474
19%
491
71%
491
34%
883
August16th to endNight
73%
525
23%
1601
21%
525
76%
525
38%
894
September1st to 15thNight
68%
639
29%
1794
23%
639
71%
639
40%
1029
September16th to endNight
62%
657
27%
1815
21%
657
65%
657
35%
1102
October1st to 15thNight
53%
660
25%
1897
22%
660
58%
660
32%
1175
October16th to endNight
34%
704
21%
2153
16%
704
42%
704
22%
1448
November1st to 15thNight
44%
700
25%
2276
21%
700
53%
700
28%
1356
November16th to endNight
30%
720
17%
2340
12%
720
38%
720
17%
1651
December1st to 15thNight
22%
720
11%
2340
7%
720
27%
720
12%
1802
December16th to endNight
19%
768
10%
2496
6%
768
26%
768
9%
2038

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
15%9%46%5%3445%7%33%
46319
Night
15%8%36%5%34446%36%
45552
Cloud CoverDay
15%9%46%5%3445%7%33%
46329
Night
15%8%36%5%34446%36%
45556
TransparencyDay
50%311%21%11%1
46025
Night
48%310%22%13%2
45364
SeeingDay
19%5%20%22%19%11%
5026
Night
20%5%23%31%16%1
15049

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
23%15%6%9%6%2434418%
1611
16th to endDay
15%17%7%9%7%45%445%18%
1728
February1st to 15thDay
16%15%5%10%8%24435%23%
1618
16th to endDay
15%14%5%8%7%34446%25%
1580
March1st to 15thDay
21%10%7%7%5%34345%25%
1895
16th to endDay
19%11%5%6%5%33346%30%
2112
April1st to 15thDay
20%9%36%5%35%345%33%
2100
16th to endDay
17%6%36%5%24446%36%
2305
May1st to 15thDay
18%9%47%5%4445%6%28%
2136
16th to endDay
11%5%25%5%35%6%6%9%38%
2223
June1st to 15thDay
12%6%35%6%245%6%9%37%
2145
16th to endDay
11%425%6%35%7%7%9%36%
2302
July1st to 15thDay
10%7%35%435%5%6%8%38%
2250
16th to endDay
9%325%436%7%6%11%38%
2283
August1st to 15thDay
7%416%5%46%5%7%9%41%
2145
16th to endDay
8%4344446%7%10%41%
2248
September1st to 15thDay
12%5%235%245%6%6%45%
1824
16th to endDay
15%6%246%24446%42%
1815
October1st to 15thDay
14%6%26%5%3445%7%40%
1733
16th to endDay
21%8%35%5%2345%5%34%
1716
November1st to 15thDay
12%8%36%424347%42%
1582
16th to endDay
19%11%5%7%5%23346%29%
1620
December1st to 15thDay
23%17%7%7%6%2433419%
1620
16th to endDay
23%17%6%8%6%34435%17%
1728
January1st to 15thNight
22%13%5%10%7%3433422%
2327
16th to endNight
17%16%5%8%6%34445%23%
2496
February1st to 15thNight
19%12%46%6%34445%28%
2239
16th to endNight
17%13%6%9%6%34345%25%
1893
March1st to 15thNight
18%10%45%5%35%445%31%
2041
16th to endNight
16%9%47%5%24447%34%
2112
April1st to 15thNight
16%7%36%5%2445%7%35%
1858
16th to endNight
14%6%25%5%3445%7%40%
1602
May1st to 15thNight
17%8%36%6%26%45%6%32%
1485
16th to endNight
10%5%24436%5%6%9%42%
1538
June1st to 15thNight
6%424435%5%8%8%44%
1485
16th to endNight
10%414435%5%7%9%42%
1324
July1st to 15thNight
9%425%5%3445%9%45%
1378
16th to endNight
5%325%4246%6%9%48%
1577
August1st to 15thNight
5%315%6%46%6%6%8%44%
1485
16th to endNight
6%4144245%5%9%50%
1601
September1st to 15thNight
10%423424347%53%
1794
16th to endNight
13%424424446%48%
1815
October1st to 15thNight
13%415%425%346%47%
1897
16th to endNight
18%6%36%5%23435%38%
2153
November1st to 15thNight
15%6%24324445%46%
2276
16th to endNight
21%9%37%5%3445%5%30%
2340
December1st to 15thNight
23%14%6%8%5%23335%22%
2340
16th to endNight
22%16%7%8%6%3443418%
2496

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
23%15%6%9%6%3343418%
1611
16th to endDay
15%17%7%9%7%4434418%
1728
February1st to 15thDay
16%15%5%9%7%34345%24%
1618
16th to endDay
15%14%6%8%7%34445%25%
1580
March1st to 15thDay
22%11%6%6%6%34345%25%
1906
16th to endDay
20%10%6%5%5%33346%30%
2111
April1st to 15thDay
20%9%46%5%34346%33%
2100
16th to endDay
18%6%36%6%24446%36%
2305
May1st to 15thDay
19%9%37%6%345%5%6%29%
2136
16th to endDay
12%5%25%5%35%6%7%9%38%
2223
June1st to 15thDay
12%6%35%6%245%6%8%37%
2145
16th to endDay
11%425%5%35%6%7%9%35%
2302
July1st to 15thDay
11%7%35%5%35%5%6%8%37%
2250
16th to endDay
9%415%5%36%7%7%11%38%
2283
August1st to 15thDay
7%326%6%45%5%6%10%40%
2145
16th to endDay
8%415%435%6%7%9%42%
2248
September1st to 15thDay
11%6%245%2446%6%45%
1824
16th to endDay
15%6%25%5%24446%41%
1815
October1st to 15thDay
14%5%35%5%2445%7%41%
1733
16th to endDay
21%8%25%5%2335%5%35%
1716
November1st to 15thDay
12%8%26%434346%43%
1582
16th to endDay
20%11%5%7%5%23345%29%
1620
December1st to 15thDay
24%16%7%8%5%3333419%
1620
16th to endDay
24%16%6%8%6%3444417%
1728
January1st to 15thNight
22%13%6%10%6%4433421%
2327
16th to endNight
17%15%5%8%6%3444424%
2496
February1st to 15thNight
19%12%46%6%34445%28%
2232
16th to endNight
17%12%6%9%6%44345%25%
1893
March1st to 15thNight
18%9%36%5%35%346%31%
2053
16th to endNight
16%8%47%5%2445%6%34%
2112
April1st to 15thNight
16%7%36%5%2445%7%35%
1858
16th to endNight
14%6%25%5%33448%40%
1602
May1st to 15thNight
18%8%35%6%35%45%6%32%
1484
16th to endNight
10%5%24435%5%6%9%41%
1538
June1st to 15thNight
6%334425%5%7%9%44%
1485
16th to endNight
9%425%435%5%7%9%42%
1324
July1st to 15thNight
9%425%5%3346%8%46%
1378
16th to endNight
6%3245%246%5%10%48%
1577
August1st to 15thNight
5%325%5%45%5%7%7%45%
1485
16th to endNight
6%414325%5%5%8%50%
1601
September1st to 15thNight
10%423333247%53%
1794
16th to endNight
13%424424446%48%
1815
October1st to 15thNight
12%5%25%5%24436%47%
1897
16th to endNight
18%7%36%5%24335%38%
2153
November1st to 15thNight
15%6%23423346%46%
2276
16th to endNight
21%9%36%5%3445%5%29%
2340
December1st to 15thNight
24%14%5%8%5%23335%23%
2340
16th to endNight
23%16%6%8%6%3443418%
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
69%37%13%5%
1612
16th to endDay
68%212%12%3
1728
February1st to 15thDay
63%310%14%7%
1620
16th to endDay
60%211%19%5%
1580
March1st to 15thDay
61%212%17%5%
1906
16th to endDay
55%111%24%5%
2112
April1st to 15thDay
52%212%19%10%1
2100
16th to endDay
48%39%21%13%4
2305
May1st to 15thDay
54%313%18%7%1
2136
16th to endDay
40%5%14%25%12%1
2227
June1st to 15thDay
43%311%27%13%1
2145
16th to endDay
40%416%19%16%2
2302
July1st to 15thDay
43%49%22%18%2
2106
16th to endDay
35%5%15%32%9%1
2128
August1st to 15thDay
37%311%27%17%1
2132
16th to endDay
34%412%27%15%4
2248
September1st to 15thDay
37%39%25%20%3
1824
16th to endDay
43%28%20%23%1
1815
October1st to 15thDay
42%39%23%19%1
1733
16th to endDay
50%211%24%10%
1716
November1st to 15thDay
42%29%24%16%4
1582
16th to endDay
57%210%20%9%
1620
December1st to 15thDay
70%19%13%4
1620
16th to endDay
69%39%13%3
1728
January1st to 15thNight
67%27%15%7%
2327
16th to endNight
63%29%18%6%
2496
February1st to 15thNight
57%312%18%7%
2241
16th to endNight
61%28%20%6%
1893
March1st to 15thNight
55%210%22%7%2
2053
16th to endNight
50%310%24%9%1
2112
April1st to 15thNight
47%312%19%16%1
1858
16th to endNight
42%410%19%18%4
1602
May1st to 15thNight
51%311%22%9%1
1485
16th to endNight
37%411%27%15%3
1540
June1st to 15thNight
33%5%15%24%19%1
1485
16th to endNight
35%5%16%22%15%3
1324
July1st to 15thNight
37%310%21%22%4
1293
16th to endNight
29%5%14%33%14%3
1469
August1st to 15thNight
34%412%25%21%1
1474
16th to endNight
29%412%27%21%3
1601
September1st to 15thNight
31%210%25%23%6%
1794
16th to endNight
36%27%24%27%1
1815
October1st to 15thNight
38%39%27%19%2
1897
16th to endNight
48%28%26%11%1
2153
November1st to 15thNight
39%210%24%19%4
2276
16th to endNight
54%29%22%9%
2340
December1st to 15thNight
64%210%16%6%
2340
16th to endNight
69%38%13%3
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
32%13%33%11%6%3
90
16th to endDay
29%15%33%18%3
96
February1st to 15thDay
27%16%40%15%
90
16th to endDay
20%8%40%24%4
81
March1st to 15thDay
28%12%26%25%7%
196
16th to endDay
26%8%33%20%9%1
288
April1st to 15thDay
26%7%26%24%11%2
270
16th to endDay
18%6%27%23%15%8%
259
May1st to 15thDay
25%324%22%17%7%
228
16th to endDay
19%316%27%17%16%
267
June1st to 15thDay
13%316%31%22%12%
255
16th to endDay
17%320%26%22%9%
412
July1st to 15thDay
18%311%29%22%14%
345
16th to endDay
11%18%20%28%29%
270
August1st to 15thDay
12%28%19%29%27%
253
16th to endDay
12%18%15%31%30%
268
September1st to 15thDay
14%17%20%36%19%
255
16th to endDay
18%211%20%29%16%
254
October1st to 15thDay
10%5%20%29%22%11%
255
16th to endDay
23%6%32%23%13%1
228
November1st to 15thDay
14%7%21%20%24%12%
90
16th to endDay
27%12%38%14%6%
90
December1st to 15thDay
28%17%30%15%43
90
16th to endDay
35%13%36%14%
96
January1st to 15thNight
29%9%35%21%3
720
16th to endNight
26%9%31%28%3
768
February1st to 15thNight
26%7%32%28%4
704
16th to endNight
25%9%32%29%3
631
March1st to 15thNight
25%8%27%29%8%
720
16th to endNight
21%6%32%27%11%
768
April1st to 15thNight
20%9%31%30%7%
630
16th to endNight
17%6%29%30%13%1
534
May1st to 15thNight
22%422%34%15%
495
16th to endNight
12%318%33%28%3
515
June1st to 15thNight
7%120%47%21%1
495
16th to endNight
11%211%44%28%2
495
July1st to 15thNight
12%311%41%27%4
465
16th to endNight
7%17%39%39%4
522
August1st to 15thNight
7%19%41%34%5%
494
16th to endNight
8%7%37%38%6%
525
September1st to 15thNight
12%38%35%32%6%
639
16th to endNight
14%412%32%33%3
657
October1st to 15thNight
15%420%32%23%3
660
16th to endNight
24%5%28%29%10%
704
November1st to 15thNight
18%6%23%32%17%1
700
16th to endNight
25%8%28%27%9%
720
December1st to 15thNight
30%7%28%25%6%1
720
16th to endNight
32%8%29%25%5%
768
Page generated at 2017-04-29 09:44:55. Version ac04

Copyright 2017 A. Danko


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