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 2017-10-06.

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
42%
15555
18%
46843
13%
15515
48%
15515
23%
31631

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
39%
540
21%
1620
10%
540
49%
540
26%
1056
May1st to 15thNight
40%
540
15%
1620
8%
540
46%
540
21%
1141
May16th to endNight
56%
562
18%
1684
14%
561
60%
561
32%
1047
June1st to 15thNight
63%
540
16%
1620
13%
540
68%
540
35%
935
June16th to endNight
61%
540
18%
1444
14%
540
65%
540
35%
897
July1st to 15thNight
62%
510
16%
1418
14%
510
64%
510
33%
893
July16th to endNight
73%
564
21%
1595
20%
531
75%
531
40%
889
August1st to 15thNight
70%
539
21%
1609
17%
536
71%
536
34%
956
August16th to endNight
74%
573
24%
1748
22%
573
76%
573
38%
978
September1st to 15thNight
65%
697
27%
1957
22%
697
68%
697
39%
1137
September16th to endNight
62%
717
29%
1980
23%
717
66%
717
36%
1188
October1st to 15thNight
53%
684
25%
1963
22%
684
58%
684
32%
1213
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%8%36%5%3445%7%33%
48340
Night
15%8%36%5%34446%36%
47031
Cloud CoverDay
15%8%36%5%3445%7%33%
48350
Night
15%8%36%5%3445%6%36%
47035
TransparencyDay
49%311%21%12%1
48046
Night
47%310%22%13%2
46843
SeeingDay
18%5%21%23%19%11%
5687
Night
19%5%23%32%16%2
15560

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
18%6%36%5%24446%36%
2340
May1st to 15thDay
19%9%47%5%4445%6%29%
2331
16th to endDay
11%5%25%5%35%6%7%8%37%
2431
June1st to 15thDay
11%6%25%6%25%5%6%9%37%
2340
16th to endDay
10%425%6%35%6%7%9%37%
2511
July1st to 15thDay
10%7%35%5%35%5%6%8%38%
2454
16th to endDay
9%325%435%7%7%11%39%
2465
August1st to 15thDay
6%416%5%46%5%7%9%41%
2340
16th to endDay
8%4244446%7%10%41%
2453
September1st to 15thDay
13%5%245%345%6%6%44%
1990
16th to endDay
14%6%246%24446%43%
1980
October1st to 15thDay
14%6%26%5%2345%7%41%
1795
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%
1620
May1st to 15thNight
18%8%35%6%25%45%6%33%
1620
16th to endNight
10%5%24435%5%7%9%41%
1682
June1st to 15thNight
6%324436%5%8%8%46%
1620
16th to endNight
10%314435%5%7%9%43%
1444
July1st to 15thNight
9%425%5%3445%9%45%
1503
16th to endNight
5%325%3245%6%9%50%
1703
August1st to 15thNight
4325%6%46%6%7%8%44%
1620
16th to endNight
5%4144346%5%9%50%
1748
September1st to 15thNight
11%423323347%52%
1957
16th to endNight
12%424424446%50%
1980
October1st to 15thNight
13%415%424346%47%
1963
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%35%
2340
May1st to 15thDay
20%9%36%6%24446%29%
2331
16th to endDay
12%5%25%5%35%6%7%8%37%
2431
June1st to 15thDay
11%6%35%6%245%6%9%37%
2340
16th to endDay
11%425%5%35%6%7%9%36%
2511
July1st to 15thDay
10%7%35%5%35%5%7%7%37%
2454
16th to endDay
9%415%5%36%6%7%11%39%
2465
August1st to 15thDay
6%326%6%46%5%6%10%40%
2340
16th to endDay
8%425%435%6%7%9%42%
2453
September1st to 15thDay
12%5%245%3445%6%44%
1990
16th to endDay
14%6%25%5%24436%43%
1980
October1st to 15thDay
14%5%35%5%2445%7%41%
1795
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%39%
1620
May1st to 15thNight
18%7%36%6%35%45%6%33%
1619
16th to endNight
10%5%24436%5%6%9%41%
1682
June1st to 15thNight
6%334425%5%7%9%46%
1620
16th to endNight
9%415%435%5%7%9%42%
1444
July1st to 15thNight
9%425%5%3346%7%45%
1503
16th to endNight
5%3244246%6%10%49%
1703
August1st to 15thNight
5%325%5%45%6%7%8%45%
1620
16th to endNight
5%414425%5%5%8%50%
1748
September1st to 15thNight
11%423323247%52%
1957
16th to endNight
12%424424446%50%
1980
October1st to 15thNight
12%5%25%5%24446%47%
1963
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%12%4
2340
May1st to 15thDay
54%313%18%8%1
2331
16th to endDay
41%5%13%25%12%1
2435
June1st to 15thDay
42%311%27%14%1
2340
16th to endDay
39%5%15%20%16%2
2511
July1st to 15thDay
43%410%21%18%1
2310
16th to endDay
34%415%31%10%2
2310
August1st to 15thDay
37%411%27%16%1
2327
16th to endDay
35%412%27%15%3
2453
September1st to 15thDay
38%39%24%19%3
1990
16th to endDay
42%28%21%24%1
1980
October1st to 15thDay
42%39%23%19%1
1795
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
1620
May1st to 15thNight
51%310%22%10%1
1620
16th to endNight
38%412%27%15%3
1684
June1st to 15thNight
31%5%15%24%20%1
1620
16th to endNight
34%5%15%23%16%3
1444
July1st to 15thNight
37%411%21%21%4
1418
16th to endNight
27%5%13%33%15%3
1595
August1st to 15thNight
33%413%25%21%1
1609
16th to endNight
29%413%27%21%3
1748
September1st to 15thNight
32%210%26%22%5%
1957
16th to endNight
34%27%25%27%1
1980
October1st to 15thNight
37%38%26%20%2
1963
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%7%27%22%15%8%
270
May1st to 15thDay
27%423%21%17%6%
288
16th to endDay
19%419%27%16%13%
331
June1st to 15thDay
11%215%33%23%12%
315
16th to endDay
16%424%25%20%8%
486
July1st to 15thDay
17%214%29%22%13%
414
16th to endDay
10%19%21%28%27%
326
August1st to 15thDay
11%29%20%29%26%
313
16th to endDay
11%210%18%31%25%
332
September1st to 15thDay
15%111%20%32%18%
315
16th to endDay
15%312%19%29%20%
314
October1st to 15thDay
12%5%18%28%23%12%
278
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
540
May1st to 15thNight
22%423%34%14%
540
16th to endNight
12%319%34%26%3
563
June1st to 15thNight
6%120%47%22%1
540
16th to endNight
10%214%44%26%2
540
July1st to 15thNight
11%311%42%27%3
510
16th to endNight
7%17%39%40%3
564
August1st to 15thNight
7%19%40%34%5%
539
16th to endNight
7%7%38%38%6%
573
September1st to 15thNight
13%49%34%31%6%
697
16th to endNight
13%412%30%34%4
717
October1st to 15thNight
15%320%32%23%3
684
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 2017-10-07 14:07:44. Version ac04

Copyright 2017 A. Danko


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