Purdue University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Purdue University clear sky chart from 2005-11-05 to

2017-09-12
.

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
39%
15848
18%
46763
12%
15807
45%
15807
24%
31000

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
19%
900
14%
2506
5%
895
26%
895
16%
1893
January16th to endNight
22%
960
12%
2670
5%
960
28%
960
16%
2018
February1st to 15thNight
21%
712
12%
2123
4%
712
28%
712
17%
1519
February16th to endNight
24%
631
16%
1893
7%
631
32%
631
18%
1347
March1st to 15thNight
26%
720
14%
2160
9%
720
31%
720
18%
1518
March16th to endNight
29%
678
15%
2031
7%
678
37%
678
21%
1415
April1st to 15thNight
29%
539
17%
1797
9%
539
40%
539
21%
1177
April16th to endNight
34%
540
19%
1716
12%
540
44%
540
25%
1103
May1st to 15thNight
36%
540
15%
1620
8%
540
42%
540
22%
1117
May16th to endNight
52%
561
18%
1576
13%
561
56%
561
30%
1016
June1st to 15thNight
57%
540
18%
1440
12%
540
61%
540
40%
859
June16th to endNight
55%
540
19%
1440
13%
540
59%
540
33%
925
July1st to 15thNight
63%
510
18%
1360
13%
510
65%
510
39%
829
July16th to endNight
73%
564
25%
1417
19%
531
76%
531
48%
773
August1st to 15thNight
74%
539
22%
1631
21%
536
75%
536
39%
911
August16th to endNight
68%
573
23%
1910
22%
573
73%
573
37%
1033
September1st to 15thNight
63%
569
27%
1806
25%
569
67%
569
36%
1011
September16th to endNight
61%
657
27%
1815
21%
657
64%
657
36%
1090
October1st to 15thNight
53%
660
30%
1837
22%
660
59%
660
37%
1088
October16th to endNight
29%
704
19%
2260
12%
704
38%
704
22%
1437
November1st to 15thNight
36%
700
27%
2276
19%
700
49%
700
29%
1325
November16th to endNight
28%
720
18%
2340
11%
720
37%
720
18%
1592
December1st to 15thNight
22%
831
11%
2451
5%
831
28%
831
14%
1870
December16th to endNight
18%
960
11%
2688
3%
960
25%
960
13%
2134

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
13%8%36%5%3445%7%36%
47901
Night
14%8%35%5%2445%6%39%
46932
Cloud CoverDay
14%7%36%5%3445%7%36%
47910
Night
15%7%35%5%2445%6%39%
46937
TransparencyDay
46%412%21%12%2
47588
Night
45%311%22%14%2
46763
SeeingDay
16%6%23%25%19%8%
5865
Night
18%8%27%30%14%1
15853

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
19%13%49%6%4435%5%23%
1432
16th to endDay
15%14%7%8%6%23345%26%
1553
February1st to 15thDay
15%13%5%7%6%46%435%26%
1738
16th to endDay
14%13%5%8%5%35%446%28%
1580
March1st to 15thDay
18%12%5%8%5%23446%26%
1790
16th to endDay
16%11%46%5%2445%7%29%
2202
April1st to 15thDay
18%8%34424446%36%
2151
16th to endDay
18%8%35%5%2345%6%35%
2244
May1st to 15thDay
20%7%37%6%35%446%30%
2331
16th to endDay
12%6%36%6%35%5%5%7%37%
2536
June1st to 15thDay
9%4245%35%5%5%8%44%
2520
16th to endDay
10%426%6%35%5%7%10%38%
2520
July1st to 15thDay
10%5%25%5%35%5%5%9%40%
2520
16th to endDay
5%325%435%7%7%10%42%
2655
August1st to 15thDay
6%4245%445%6%10%45%
2315
16th to endDay
7%5%25%5%35%5%7%8%43%
2292
September1st to 15thDay
10%5%135%35%5%5%7%43%
2062
16th to endDay
10%6%2443445%6%47%
1815
October1st to 15thDay
14%6%246%345%46%42%
1790
16th to endDay
18%7%35%335%5%45%37%
1611
November1st to 15thDay
15%6%23422345%47%
1582
16th to endDay
18%8%45%434445%35%
1620
December1st to 15thDay
19%14%5%8%6%24336%25%
1506
16th to endDay
21%15%5%8%5%45%345%21%
1536
January1st to 15thNight
18%12%48%5%3433429%
2506
16th to endNight
15%13%6%9%6%25%345%27%
2670
February1st to 15thNight
18%12%5%6%5%23246%31%
2121
16th to endNight
15%13%47%5%24445%33%
1893
March1st to 15thNight
18%9%45%5%3445%5%33%
2147
16th to endNight
15%9%36%5%25%447%34%
2031
April1st to 15thNight
16%7%35%5%2445%6%37%
1797
16th to endNight
17%7%25%42345%6%39%
1716
May1st to 15thNight
18%7%35%5%3445%7%34%
1620
16th to endNight
11%425%5%35%5%6%8%41%
1574
June1st to 15thNight
9%42432345%6%51%
1440
16th to endNight
8%425%5%35%5%6%9%43%
1440
July1st to 15thNight
6%42443446%10%47%
1440
16th to endNight
5%3133243410%56%
1513
August1st to 15thNight
5%413425%6%7%11%47%
1641
16th to endNight
6%32442446%8%51%
1910
September1st to 15thNight
10%42342345%7%51%
1806
16th to endNight
9%4145%34447%50%
1815
October1st to 15thNight
11%414324446%52%
1837
16th to endNight
19%6%25%4235%45%41%
2260
November1st to 15thNight
13%5%2442345%6%47%
2276
16th to endNight
19%9%46%5%23345%35%
2340
December1st to 15thNight
19%12%5%8%6%2444327%
2451
16th to endNight
23%12%5%7%5%24445%22%
2688

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
20%12%5%8%7%34346%23%
1432
16th to endDay
15%15%7%7%7%33345%26%
1553
February1st to 15thDay
15%13%5%7%6%45%345%26%
1738
16th to endDay
14%12%5%8%6%25%445%28%
1580
March1st to 15thDay
20%11%5%8%5%23346%27%
1800
16th to endDay
17%11%36%5%3445%7%29%
2201
April1st to 15thDay
19%8%24434446%36%
2151
16th to endDay
19%8%25%5%2345%6%35%
2244
May1st to 15thDay
21%6%37%5%35%446%30%
2331
16th to endDay
12%5%36%6%35%45%7%37%
2536
June1st to 15thDay
10%415%5%345%5%8%44%
2520
16th to endDay
10%425%5%445%7%10%38%
2520
July1st to 15thDay
10%5%25%5%35%5%6%8%40%
2520
16th to endDay
6%415%5%36%6%8%11%42%
2655
August1st to 15thDay
7%4145%345%6%9%46%
2315
16th to endDay
8%4245%35%5%7%9%43%
2292
September1st to 15thDay
11%5%135%35%5%6%7%43%
2062
16th to endDay
11%6%24334446%47%
1815
October1st to 15thDay
14%5%15%6%3445%6%42%
1790
16th to endDay
19%6%26%3245%45%37%
1611
November1st to 15thDay
15%6%23422245%47%
1582
16th to endDay
19%8%46%433445%35%
1620
December1st to 15thDay
20%13%6%8%5%24335%25%
1506
16th to endDay
22%14%48%5%45%335%21%
1536
January1st to 15thNight
19%12%49%5%3423430%
2506
16th to endNight
15%13%6%8%6%34445%28%
2670
February1st to 15thNight
18%12%5%7%5%23246%31%
2114
16th to endNight
15%12%47%5%23445%33%
1893
March1st to 15thNight
18%9%45%5%3345%5%34%
2160
16th to endNight
16%9%36%5%34447%34%
2031
April1st to 15thNight
16%7%35%5%245%5%5%38%
1797
16th to endNight
18%6%25%32435%7%39%
1716
May1st to 15thNight
19%6%35%5%25%45%7%34%
1619
16th to endNight
12%315%5%35%5%6%7%42%
1574
June1st to 15thNight
10%42443345%6%51%
1440
16th to endNight
8%4245%35%5%5%9%44%
1440
July1st to 15thNight
6%5%244345%6%10%47%
1440
16th to endNight
5%31331345%10%56%
1513
August1st to 15thNight
6%3134345%7%10%48%
1641
16th to endNight
7%3145%2446%8%51%
1910
September1st to 15thNight
11%42332345%6%52%
1806
16th to endNight
10%3135%33446%50%
1815
October1st to 15thNight
12%414323446%52%
1837
16th to endNight
19%6%25%424446%41%
2260
November1st to 15thNight
13%5%1432345%6%48%
2276
16th to endNight
19%9%37%5%23235%35%
2340
December1st to 15thNight
20%12%5%8%6%2334427%
2451
16th to endNight
23%12%5%7%5%3435%423%
2688

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
63%210%15%8%
1433
16th to endDay
59%214%16%5%
1553
February1st to 15thDay
59%311%15%8%
1740
16th to endDay
56%313%20%6%
1580
March1st to 15thDay
57%412%18%6%
1800
16th to endDay
52%314%21%6%1
2202
April1st to 15thDay
48%312%21%13%
2151
16th to endDay
48%313%19%10%4
2244
May1st to 15thDay
54%411%20%8%
2331
16th to endDay
45%5%11%19%15%3
2540
June1st to 15thDay
36%412%24%18%2
2520
16th to endDay
39%5%14%21%16%3
2520
July1st to 15thDay
40%411%21%18%2
2371
16th to endDay
33%6%17%27%13%2
2488
August1st to 15thDay
32%6%13%26%19%1
2301
16th to endDay
33%6%13%25%15%5%
2292
September1st to 15thDay
37%5%11%23%17%5%
2062
16th to endDay
36%38%26%21%2
1815
October1st to 15thDay
41%49%25%17%2
1790
16th to endDay
47%311%24%12%1
1611
November1st to 15thDay
39%211%23%17%4
1582
16th to endDay
50%212%24%9%
1620
December1st to 15thDay
61%110%19%5%
1506
16th to endDay
65%310%13%4
1536
January1st to 15thNight
58%38%18%11%
2506
16th to endNight
58%313%17%7%
2670
February1st to 15thNight
55%213%18%8%1
2123
16th to endNight
53%312%21%8%
1893
March1st to 15thNight
51%411%21%9%2
2160
16th to endNight
49%414%21%9%1
2031
April1st to 15thNight
45%49%23%14%1
1797
16th to endNight
43%411%20%13%6%
1716
May1st to 15thNight
48%5%12%21%11%
1620
16th to endNight
38%6%13%21%15%4
1576
June1st to 15thNight
31%413%27%21%2
1440
16th to endNight
35%6%12%22%17%4
1440
July1st to 15thNight
31%5%14%24%20%3
1360
16th to endNight
23%317%32%17%4
1417
August1st to 15thNight
28%5%16%25%22%2
1631
16th to endNight
28%412%26%20%6%
1910
September1st to 15thNight
31%310%26%19%7%
1806
16th to endNight
33%48%26%24%3
1815
October1st to 15thNight
32%310%27%23%2
1837
16th to endNight
43%310%25%13%1
2260
November1st to 15thNight
36%38%25%21%4
2276
16th to endNight
51%211%22%11%
2340
December1st to 15thNight
60%39%16%8%
2451
16th to endNight
64%310%15%6%
2688

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
28%15%32%16%42
90
16th to endDay
25%15%40%17%1
96
February1st to 15thDay
24%13%44%14%3
90
16th to endDay
23%9%35%17%12%1
81
March1st to 15thDay
32%16%36%11%3
90
16th to endDay
30%8%29%20%10%1
279
April1st to 15thDay
20%13%31%21%11%2
270
16th to endDay
22%8%25%24%12%5%
270
May1st to 15thDay
22%11%26%22%13%3
288
16th to endDay
17%428%25%15%8%
436
June1st to 15thDay
12%423%34%18%6%
495
16th to endDay
12%6%30%29%15%5%
495
July1st to 15thDay
14%224%25%22%10%
475
16th to endDay
7%113%31%27%17%
515
August1st to 15thDay
9%19%23%33%22%
318
16th to endDay
10%212%24%34%15%
332
September1st to 15thDay
14%49%30%30%11%
299
16th to endDay
14%6%10%20%36%11%
254
October1st to 15thDay
13%420%28%27%5%
230
16th to endDay
18%9%44%25%2
96
November1st to 15thDay
11%8%20%23%26%10%
90
16th to endDay
33%13%32%14%6%
90
December1st to 15thDay
21%16%37%17%5%1
90
16th to endDay
28%18%42%8%2
96
January1st to 15thNight
23%15%35%22%3
900
16th to endNight
21%12%32%27%4
960
February1st to 15thNight
25%12%34%24%3
715
16th to endNight
22%11%34%26%4
631
March1st to 15thNight
22%12%31%26%5%
720
16th to endNight
19%9%36%25%7%1
678
April1st to 15thNight
20%11%33%24%9%
539
16th to endNight
20%6%34%26%11%
540
May1st to 15thNight
21%6%29%30%11%
540
16th to endNight
12%322%39%18%2
563
June1st to 15thNight
11%221%44%20%
540
16th to endNight
10%222%42%22%
540
July1st to 15thNight
7%218%42%26%3
510
16th to endNight
6%11%40%36%4
564
August1st to 15thNight
7%19%44%30%6%
539
16th to endNight
7%211%32%40%5%
573
September1st to 15thNight
13%215%36%29%3
569
16th to endNight
12%314%32%33%3
657
October1st to 15thNight
13%5%23%34%19%3
660
16th to endNight
22%12%31%23%9%
704
November1st to 15thNight
16%9%33%27%11%1
700
16th to endNight
23%13%28%24%9%
720
December1st to 15thNight
26%10%31%22%7%
831
16th to endNight
29%10%31%23%4
960
Page generated at 2017-09-13 07:39:09. Version ac04

Copyright 2017 A. Danko


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