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Chapel Hill Astronomical and Observational Society
.

Jordan Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jordan Lake clear sky chart from 2005-11-05 to 2018-03-11.

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
46%
16647
22%
49230
14%
16611
54%
16611
28%
31159

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
30%
776
22%
2522
11%
776
45%
776
23%
1541
January16th to endNight
30%
832
25%
2704
14%
832
45%
832
26%
1627
February1st to 15thNight
27%
761
21%
2482
10%
761
42%
761
24%
1515
February16th to endNight
27%
683
27%
2000
12%
683
44%
683
28%
1213
March1st to 15thNight
32%
764
25%
2101
13%
764
43%
764
29%
1311
March16th to endNight
33%
768
21%
2112
11%
768
42%
768
26%
1390
April1st to 15thNight
35%
718
23%
1847
9%
718
43%
718
32%
1187
April16th to endNight
40%
720
21%
1800
8%
720
46%
720
32%
1184
May1st to 15thNight
44%
559
17%
1639
8%
559
50%
559
27%
1077
May16th to endNight
60%
561
16%
1683
12%
561
62%
561
29%
1074
June1st to 15thNight
59%
540
14%
1620
13%
540
61%
540
25%
1070
June16th to endNight
67%
540
17%
1620
13%
540
69%
540
28%
1048
July1st to 15thNight
69%
510
12%
1530
10%
510
70%
510
26%
1029
July16th to endNight
72%
564
15%
1595
12%
531
74%
531
27%
1037
August1st to 15thNight
67%
539
14%
1609
12%
536
68%
536
24%
1112
August16th to endNight
65%
735
16%
1881
11%
735
66%
735
27%
1308
September1st to 15thNight
64%
720
24%
1830
19%
720
66%
720
34%
1198
September16th to endNight
50%
717
18%
2001
15%
717
53%
717
23%
1409
October1st to 15thNight
49%
720
26%
2160
24%
720
56%
720
30%
1347
October16th to endNight
47%
768
35%
2304
24%
768
63%
768
37%
1278
November1st to 15thNight
49%
760
27%
2281
19%
760
60%
760
33%
1332
November16th to endNight
45%
780
27%
2511
21%
780
56%
780
30%
1463
December1st to 15thNight
38%
780
21%
2694
14%
780
49%
780
22%
1670
December16th to endNight
32%
832
24%
2704
15%
832
44%
832
22%
1739

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%5%2443445%7%43%
49372
Night
13%5%2442445%7%45%
49424
Cloud CoverDay
13%5%2443445%7%43%
49382
Night
13%42442445%7%45%
49428
TransparencyDay
39%7%11%20%17%4
49083
Night
37%6%11%19%19%5%
49231
SeeingDay
16%7%21%25%20%8%
6334
Night
15%6%25%31%18%1
16656

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
18%5%25%423345%44%
1746
16th to endDay
17%5%23223335%49%
1872
February1st to 15thDay
17%6%24423446%45%
1710
16th to endDay
16%5%24423345%46%
1755
March1st to 15thDay
14%413333446%50%
2085
16th to endDay
16%6%245%23345%44%
2112
April1st to 15thDay
10%413223448%52%
2101
16th to endDay
13%4245%25%5%5%6%43%
2160
May1st to 15thDay
13%6%25%5%35%5%5%7%38%
2322
16th to endDay
9%5%145%36%6%6%8%42%
2431
June1st to 15thDay
9%425%5%36%5%7%9%39%
2340
16th to endDay
7%425%5%35%6%7%10%41%
2340
July1st to 15thDay
8%5%26%6%47%7%8%11%32%
2340
16th to endDay
6%426%8%5%7%8%6%10%32%
2465
August1st to 15thDay
10%5%26%6%46%7%7%10%31%
2340
16th to endDay
11%6%36%6%5%6%7%8%9%30%
2309
September1st to 15thDay
10%314435%46%8%45%
2128
16th to endDay
21%6%25%414445%38%
1959
October1st to 15thDay
17%5%13323246%49%
1800
16th to endDay
11%42431233358%
1920
November1st to 15thDay
16%313313346%53%
1907
16th to endDay
12%424423346%51%
1769
December1st to 15thDay
17%6%24322346%45%
1589
16th to endDay
19%7%24424435%42%
1872
January1st to 15thNight
15%5%24424336%46%
2522
16th to endNight
16%5%14413435%47%
2704
February1st to 15thNight
17%5%24323346%47%
2480
16th to endNight
14%41342446%5%48%
2000
March1st to 15thNight
14%324423346%50%
2089
16th to endNight
16%424433446%44%
2112
April1st to 15thNight
13%5%24424346%48%
1847
16th to endNight
13%42442445%8%45%
1800
May1st to 15thNight
12%5%35%6%35%46%8%40%
1639
16th to endNight
11%5%145%346%5%9%43%
1682
June1st to 15thNight
10%436%5%35%6%6%9%38%
1620
16th to endNight
6%425%5%35%6%7%10%42%
1620
July1st to 15thNight
7%415%6%36%6%8%11%37%
1620
16th to endNight
6%4245%46%6%8%13%36%
1703
August1st to 15thNight
11%4246%36%6%7%10%35%
1620
16th to endNight
7%5%35%6%35%6%7%11%35%
1881
September1st to 15thNight
11%5%2342345%7%47%
1830
16th to endNight
17%6%25%5%34436%37%
2001
October1st to 15thNight
17%5%2441223449%
2160
16th to endNight
11%313312245%57%
2304
November1st to 15thNight
13%424323345%53%
2281
16th to endNight
13%32432345%6%50%
2511
December1st to 15thNight
17%6%23423435%45%
2694
16th to endNight
19%7%24423346%41%
2704

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
18%5%24423345%45%
1746
16th to endDay
17%5%23223335%49%
1872
February1st to 15thDay
17%5%24323445%45%
1710
16th to endDay
17%5%24323345%46%
1755
March1st to 15thDay
14%413233446%50%
2096
16th to endDay
17%6%24433345%44%
2111
April1st to 15thDay
11%413323348%53%
2101
16th to endDay
13%425%425%46%7%43%
2160
May1st to 15thDay
13%6%25%6%36%5%5%7%38%
2322
16th to endDay
9%4245%35%5%7%8%42%
2431
June1st to 15thDay
9%5%25%5%45%5%7%9%39%
2340
16th to endDay
7%4245%35%6%7%10%41%
2340
July1st to 15thDay
8%5%26%6%47%7%8%10%32%
2340
16th to endDay
6%5%26%8%5%7%7%7%10%33%
2465
August1st to 15thDay
10%5%26%7%46%6%7%10%31%
2340
16th to endDay
11%6%26%6%46%6%8%9%30%
2309
September1st to 15thDay
11%31443446%8%46%
2128
16th to endDay
21%6%26%423345%39%
1959
October1st to 15thDay
17%5%13323346%49%
1800
16th to endDay
11%32431233458%
1920
November1st to 15thDay
15%413313246%53%
1907
16th to endDay
13%424424246%50%
1769
December1st to 15thDay
17%6%24323346%44%
1589
16th to endDay
19%7%24424435%42%
1872
January1st to 15thNight
15%5%24424437%46%
2522
16th to endNight
16%5%24423445%48%
2704
February1st to 15thNight
17%5%24323345%47%
2473
16th to endNight
14%31332446%5%48%
2000
March1st to 15thNight
14%324323346%49%
2101
16th to endNight
15%425%423446%45%
2112
April1st to 15thNight
13%424424337%48%
1847
16th to endNight
13%42443445%8%45%
1800
May1st to 15thNight
12%5%35%6%3445%8%40%
1638
16th to endNight
11%5%145%345%6%9%42%
1682
June1st to 15thNight
10%5%26%5%35%6%6%9%38%
1620
16th to endNight
6%425%5%35%6%7%10%42%
1620
July1st to 15thNight
8%325%6%36%6%8%11%37%
1620
16th to endNight
6%4345%46%6%8%13%37%
1703
August1st to 15thNight
11%5%245%36%6%7%10%35%
1620
16th to endNight
8%5%36%6%35%6%7%11%35%
1881
September1st to 15thNight
11%5%2442345%7%47%
1830
16th to endNight
17%6%25%5%34436%37%
2001
October1st to 15thNight
17%5%3441233450%
2160
16th to endNight
12%313322335%57%
2304
November1st to 15thNight
14%414323345%53%
2281
16th to endNight
13%32432345%6%50%
2511
December1st to 15thNight
17%6%24324435%45%
2694
16th to endNight
19%6%24423346%41%
2704

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
43%45%18%22%5%
1747
16th to endDay
37%38%21%20%8%
1872
February1st to 15thDay
41%37%20%21%5%
1712
16th to endDay
39%39%21%19%4
1755
March1st to 15thDay
35%410%21%21%6%
2096
16th to endDay
42%5%9%20%17%4
2112
April1st to 15thDay
29%5%10%27%19%7%
2101
16th to endDay
39%7%10%17%18%7%
2160
May1st to 15thDay
43%8%13%19%13%2
2322
16th to endDay
37%8%16%22%13%1
2435
June1st to 15thDay
37%9%17%20%13%
2340
16th to endDay
35%11%14%20%15%2
2340
July1st to 15thDay
41%12%15%17%9%2
2201
16th to endDay
42%11%18%18%8%1
2310
August1st to 15thDay
44%10%16%17%9%1
2327
16th to endDay
44%11%14%19%9%1
2309
September1st to 15thDay
35%8%14%19%17%5%
2128
16th to endDay
47%6%8%17%17%3
1959
October1st to 15thDay
36%310%24%19%4
1800
16th to endDay
30%37%21%28%8%
1920
November1st to 15thDay
32%49%18%26%8%
1907
16th to endDay
34%48%20%25%6%
1769
December1st to 15thDay
40%310%22%17%5%
1589
16th to endDay
44%49%19%17%3
1872
January1st to 15thNight
39%49%16%23%6%
2522
16th to endNight
39%37%21%22%6%
2704
February1st to 15thNight
39%38%19%23%6%
2482
16th to endNight
35%5%10%22%21%5%
2000
March1st to 15thNight
35%5%8%21%21%7%
2101
16th to endNight
39%6%8%20%20%4
2112
April1st to 15thNight
37%38%20%23%6%
1847
16th to endNight
36%7%10%20%16%9%
1800
May1st to 15thNight
40%6%13%20%16%2
1639
16th to endNight
35%8%16%22%14%1
1684
June1st to 15thNight
40%8%16%18%13%2
1620
16th to endNight
34%12%15%20%14%3
1620
July1st to 15thNight
35%14%16%20%11%2
1530
16th to endNight
34%12%19%20%10%2
1595
August1st to 15thNight
40%11%17%17%11%1
1609
16th to endNight
37%12%18%20%8%1
1881
September1st to 15thNight
35%7%13%20%17%5%
1830
16th to endNight
46%7%11%13%18%3
2001
October1st to 15thNight
39%38%18%24%4
2160
16th to endNight
29%37%21%26%11%
2304
November1st to 15thNight
33%48%17%25%10%
2281
16th to endNight
34%47%18%25%9%
2511
December1st to 15thNight
41%5%8%20%18%5%
2694
16th to endNight
44%39%19%18%4
2704

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
22%17%36%22%1
135
16th to endDay
19%20%34%16%9%
144
February1st to 15thDay
25%16%45%11%1
135
16th to endDay
20%13%37%23%4
229
March1st to 15thDay
17%16%37%21%6%
312
16th to endDay
18%13%31%25%9%1
288
April1st to 15thDay
17%12%28%24%13%4
270
16th to endDay
18%422%32%15%5%
270
May1st to 15thDay
16%13%27%23%12%6%
288
16th to endDay
15%316%24%29%10%
331
June1st to 15thDay
13%314%28%29%10%
315
16th to endDay
10%119%28%29%10%
315
July1st to 15thDay
10%10%31%34%12%
305
16th to endDay
7%28%23%38%18%
326
August1st to 15thDay
15%13%22%29%18%
313
16th to endDay
13%19%25%28%22%
332
September1st to 15thDay
13%213%23%28%19%
315
16th to endDay
27%312%26%21%8%
314
October1st to 15thDay
20%317%25%22%9%
315
16th to endDay
11%5%23%29%20%8%
336
November1st to 15thDay
20%6%24%26%19%3
325
16th to endDay
10%12%21%34%18%3
180
December1st to 15thDay
24%11%38%14%11%
97
16th to endDay
26%14%34%20%4
144
January1st to 15thNight
17%13%32%26%8%
780
16th to endNight
19%9%37%28%4
832
February1st to 15thNight
20%12%35%29%1
764
16th to endNight
15%13%39%26%4
683
March1st to 15thNight
15%13%35%28%6%
764
16th to endNight
17%9%34%28%8%
768
April1st to 15thNight
17%11%33%27%10%
718
16th to endNight
14%7%34%30%12%1
720
May1st to 15thNight
14%5%30%36%12%
559
16th to endNight
14%115%38%27%1
563
June1st to 15thNight
12%115%38%28%3
540
16th to endNight
8%15%38%33%3
540
July1st to 15thNight
8%11%37%38%2
510
16th to endNight
7%9%34%40%6%
564
August1st to 15thNight
13%7%35%37%5%
539
16th to endNight
10%19%33%39%6%
735
September1st to 15thNight
13%111%31%36%4
720
16th to endNight
20%214%31%28%1
717
October1st to 15thNight
20%320%28%23%4
720
16th to endNight
13%7%28%26%22%2
768
November1st to 15thNight
16%5%26%33%16%2
760
16th to endNight
14%9%26%31%17%
780
December1st to 15thNight
20%8%28%33%9%
780
16th to endNight
21%9%29%31%6%
832
Page generated at 2018-03-12 13:28:05. Version ac04

Copyright 2018 A. Danko


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