Hampden-Sydney College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hampden-Sydney College Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
41%
15573
22%
47052
13%
15536
51%
15536
26%
29419

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
23%
717
23%
2327
9%
716
41%
716
20%
1451
January16th to endNight
20%
768
24%
2496
8%
768
40%
768
23%
1496
February1st to 15thNight
21%
701
22%
2287
7%
701
40%
701
24%
1353
February16th to endNight
21%
631
28%
1892
10%
631
41%
631
26%
1133
March1st to 15thNight
25%
720
23%
1980
9%
720
39%
720
25%
1267
March16th to endNight
27%
768
20%
2112
10%
768
36%
768
24%
1428
April1st to 15thNight
29%
718
22%
1820
6%
718
40%
718
32%
1150
April16th to endNight
37%
720
20%
1800
8%
720
43%
720
31%
1202
May1st to 15thNight
38%
554
17%
1634
8%
554
46%
554
25%
1075
May16th to endNight
56%
516
17%
1638
12%
516
60%
516
30%
1002
June1st to 15thNight
66%
360
19%
1440
23%
360
69%
360
27%
819
June16th to endNight
63%
367
18%
1447
19%
367
67%
367
27%
812
July1st to 15thNight
63%
510
13%
1530
11%
510
65%
510
28%
996
July16th to endNight
69%
564
16%
1595
12%
531
72%
531
33%
976
August1st to 15thNight
65%
539
18%
1609
14%
536
66%
536
27%
1056
August16th to endNight
63%
735
18%
1881
12%
735
65%
735
29%
1277
September1st to 15thNight
61%
720
24%
1800
17%
720
63%
720
32%
1204
September16th to endNight
51%
693
20%
1940
17%
693
54%
693
24%
1336
October1st to 15thNight
46%
660
26%
1980
23%
660
55%
660
27%
1256
October16th to endNight
42%
704
32%
2112
20%
704
57%
704
34%
1216
November1st to 15thNight
41%
700
26%
2101
17%
700
53%
700
30%
1244
November16th to endNight
38%
720
24%
2480
16%
720
50%
720
25%
1428
December1st to 15thNight
31%
720
21%
2520
12%
720
44%
720
20%
1572
December16th to endNight
28%
768
23%
2631
11%
768
43%
768
19%
1670

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%42%
47853
Night
13%5%2442445%7%44%
47246
Cloud CoverDay
13%5%2442445%7%42%
47863
Night
13%5%2442445%7%44%
47250
TransparencyDay
39%5%11%22%16%3
47564
Night
38%5%10%22%18%4
47053
SeeingDay
18%7%24%25%16%6%
5708
Night
16%9%26%30%15%1
15581

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
16%6%2432345%5%43%
1611
16th to endDay
16%6%2331335%6%45%
1728
February1st to 15thDay
13%7%24424348%44%
1575
16th to endDay
14%5%345%34447%43%
1580
March1st to 15thDay
14%6%14335%445%45%
1969
16th to endDay
18%6%25%4245%45%40%
2112
April1st to 15thDay
10%5%23424348%50%
2128
16th to endDay
16%5%24424446%44%
2160
May1st to 15thDay
18%7%25%5%25%45%7%36%
2327
16th to endDay
11%5%24435%6%6%8%40%
2472
June1st to 15thDay
10%324435%5%6%9%44%
2520
16th to endDay
8%324446%5%7%9%43%
2508
July1st to 15thDay
10%426%7%46%5%7%9%35%
2340
16th to endDay
6%425%6%47%6%7%12%36%
2465
August1st to 15thDay
11%5%26%7%36%6%6%10%33%
2340
16th to endDay
11%5%25%6%36%5%7%9%35%
2309
September1st to 15thDay
10%5%25%5%3445%5%46%
2160
16th to endDay
20%5%35%424446%36%
1876
October1st to 15thDay
19%412223236%49%
1650
16th to endDay
13%414423335%53%
1760
November1st to 15thDay
15%424323435%50%
1757
16th to endDay
14%423413347%49%
1476
December1st to 15thDay
15%5%2342445%5%45%
1440
16th to endDay
19%7%35%324335%41%
1590
January1st to 15thNight
15%5%35%333345%45%
2327
16th to endNight
16%5%24323446%46%
2496
February1st to 15thNight
15%6%13323446%47%
2285
16th to endNight
12%5%24423446%47%
1892
March1st to 15thNight
14%424434446%46%
1968
16th to endNight
17%6%26%42335%6%41%
2112
April1st to 15thNight
12%5%24434448%47%
1820
16th to endNight
15%6%14424447%44%
1800
May1st to 15thNight
16%6%2343445%7%39%
1634
16th to endNight
11%42443445%10%44%
1637
June1st to 15thNight
8%424425%5%7%10%44%
1440
16th to endNight
7%4345%25%46%10%45%
1447
July1st to 15thNight
9%5%145%345%7%9%43%
1620
16th to endNight
5%3245%46%6%8%9%42%
1703
August1st to 15thNight
10%425%6%345%7%9%39%
1620
16th to endNight
10%5%145%36%5%7%10%37%
1881
September1st to 15thNight
11%5%25%5%2446%6%45%
1800
16th to endNight
18%6%25%5%2345%6%38%
1940
October1st to 15thNight
17%5%25%423435%46%
1980
16th to endNight
12%423323435%54%
2112
November1st to 15thNight
13%5%24322335%52%
2101
16th to endNight
14%413324446%49%
2480
December1st to 15thNight
16%6%35%43443443%
2520
16th to endNight
18%6%24423446%40%
2631

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
16%6%2432445%6%43%
1611
16th to endDay
17%6%14413445%46%
1728
February1st to 15thDay
13%7%2442435%7%44%
1575
16th to endDay
14%5%25%5%34446%43%
1580
March1st to 15thDay
15%6%14425%446%45%
1980
16th to endDay
18%6%24425%436%40%
2111
April1st to 15thDay
10%5%24323347%51%
2128
16th to endDay
17%4244245%46%43%
2160
May1st to 15thDay
19%6%25%5%25%45%7%36%
2327
16th to endDay
11%424436%6%6%8%39%
2472
June1st to 15thDay
10%3135%35%47%9%44%
2520
16th to endDay
8%324435%5%6%10%43%
2508
July1st to 15thDay
10%426%6%36%5%7%10%34%
2340
16th to endDay
6%4346%47%6%7%11%36%
2465
August1st to 15thDay
11%427%7%45%5%7%9%34%
2340
16th to endDay
11%5%246%36%6%7%9%36%
2309
September1st to 15thDay
10%5%245%34446%46%
2160
16th to endDay
21%6%25%42445%6%36%
1876
October1st to 15thDay
20%313223236%50%
1650
16th to endDay
13%424414235%53%
1760
November1st to 15thDay
15%423324335%50%
1757
16th to endDay
15%423413347%49%
1476
December1st to 15thDay
14%5%1432445%5%46%
1440
16th to endDay
19%7%35%32443441%
1590
January1st to 15thNight
15%5%25%424335%45%
2327
16th to endNight
17%423423446%46%
2496
February1st to 15thNight
15%6%13324446%47%
2278
16th to endNight
12%5%2443335%6%48%
1892
March1st to 15thNight
14%43442435%6%45%
1980
16th to endNight
17%5%35%424346%40%
2112
April1st to 15thNight
12%5%14433449%47%
1820
16th to endNight
15%5%2443435%7%44%
1800
May1st to 15thNight
17%6%234345%5%7%39%
1633
16th to endNight
11%5%1443445%10%44%
1637
June1st to 15thNight
9%314425%46%11%43%
1440
16th to endNight
7%433435%46%10%45%
1447
July1st to 15thNight
9%5%15%5%25%5%7%9%43%
1620
16th to endNight
5%3245%47%5%7%10%42%
1703
August1st to 15thNight
10%426%6%4447%9%39%
1620
16th to endNight
10%425%5%36%5%7%11%36%
1881
September1st to 15thNight
12%5%16%4335%5%7%44%
1800
16th to endNight
18%6%26%5%2335%6%38%
1940
October1st to 15thNight
17%5%25%333336%46%
1980
16th to endNight
13%423323335%54%
2112
November1st to 15thNight
13%5%24313235%52%
2101
16th to endNight
14%313324446%49%
2480
December1st to 15thNight
16%5%35%5%2434443%
2520
16th to endNight
18%7%24423446%40%
2631

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
40%49%21%19%3
1612
16th to endDay
39%39%24%17%5%
1728
February1st to 15thDay
40%28%23%20%4
1577
16th to endDay
41%410%22%17%3
1580
March1st to 15thDay
40%310%21%18%5%
1980
16th to endDay
45%5%8%21%14%4
2112
April1st to 15thDay
33%310%27%18%6%
2128
16th to endDay
40%5%10%18%18%6%
2160
May1st to 15thDay
46%6%13%18%11%2
2327
16th to endDay
39%7%15%25%10%1
2476
June1st to 15thDay
34%7%17%26%14%
2520
16th to endDay
34%8%16%20%16%2
2508
July1st to 15thDay
41%8%14%16%14%3
2201
16th to endDay
38%8%15%22%14%
2310
August1st to 15thDay
43%7%13%22%10%1
2327
16th to endDay
41%7%15%22%11%1
2309
September1st to 15thDay
37%7%11%17%22%3
2160
16th to endDay
47%49%17%19%1
1876
October1st to 15thDay
37%37%26%20%4
1650
16th to endDay
35%37%26%21%6%
1760
November1st to 15thDay
37%37%20%23%7%
1757
16th to endDay
36%310%23%23%4
1476
December1st to 15thDay
38%310%23%17%5%
1440
16th to endDay
47%38%20%17%2
1590
January1st to 15thNight
41%48%19%19%6%
2327
16th to endNight
39%39%24%18%4
2496
February1st to 15thNight
37%37%25%19%5%
2287
16th to endNight
36%38%26%20%4
1892
March1st to 15thNight
38%49%25%17%4
1980
16th to endNight
44%48%21%17%3
2112
April1st to 15thNight
35%311%22%21%5%
1820
16th to endNight
39%411%18%18%7%
1800
May1st to 15thNight
42%5%13%21%14%2
1634
16th to endNight
35%6%18%24%12%2
1639
June1st to 15thNight
32%8%16%24%15%1
1440
16th to endNight
33%7%17%19%18%4
1447
July1st to 15thNight
37%9%14%18%15%5%
1530
16th to endNight
32%8%19%24%13%1
1595
August1st to 15thNight
38%8%14%21%15%1
1609
16th to endNight
38%9%14%22%11%2
1881
September1st to 15thNight
37%7%11%19%18%5%
1800
16th to endNight
46%5%8%16%20%3
1940
October1st to 15thNight
40%47%23%19%3
1980
16th to endNight
33%36%25%26%5%
2112
November1st to 15thNight
35%38%17%25%8%
2101
16th to endNight
36%47%22%22%6%
2480
December1st to 15thNight
43%47%22%18%3
2520
16th to endNight
44%310%20%16%3
2631

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
31%15%44%5%3
90
16th to endDay
27%33%31%8%
96
February1st to 15thDay
24%30%38%42
90
16th to endDay
20%17%37%21%3
186
March1st to 15thDay
21%14%35%21%6%
270
16th to endDay
21%13%36%19%6%2
288
April1st to 15thDay
18%13%31%26%7%2
270
16th to endDay
22%6%28%25%12%4
270
May1st to 15thDay
29%10%30%15%10%3
288
16th to endDay
19%324%25%18%9%
331
June1st to 15thDay
10%6%23%28%21%8%
315
16th to endDay
13%323%33%20%6%
315
July1st to 15thDay
13%218%39%19%6%
305
16th to endDay
7%213%33%30%11%
326
August1st to 15thDay
15%216%25%28%12%
313
16th to endDay
12%12%30%26%17%
332
September1st to 15thDay
13%6%18%25%20%16%
315
16th to endDay
31%212%21%20%11%
287
October1st to 15thDay
26%418%24%18%6%
255
16th to endDay
14%9%29%29%16%1
272
November1st to 15thDay
18%9%28%21%19%3
265
16th to endDay
20%11%27%26%9%5%
96
December1st to 15thDay
18%13%35%25%6%1
60
16th to endDay
38%20%17%21%1
73
January1st to 15thNight
20%18%34%22%4
720
16th to endNight
19%17%38%23%1
768
February1st to 15thNight
21%14%38%23%2
704
16th to endNight
19%15%38%23%2
631
March1st to 15thNight
17%15%36%25%4
720
16th to endNight
20%10%35%25%7%
768
April1st to 15thNight
14%17%35%24%7%
718
16th to endNight
16%8%33%31%9%
720
May1st to 15thNight
19%7%29%29%11%1
554
16th to endNight
14%219%38%23%2
518
June1st to 15thNight
9%116%40%29%2
360
16th to endNight
8%214%49%22%1
367
July1st to 15thNight
12%212%37%32%2
510
16th to endNight
6%13%37%37%4
564
August1st to 15thNight
12%17%41%32%4
539
16th to endNight
12%111%35%33%5%
735
September1st to 15thNight
14%212%35%30%5%
720
16th to endNight
21%314%29%27%4
693
October1st to 15thNight
20%5%19%28%21%3
660
16th to endNight
15%9%29%29%15%
704
November1st to 15thNight
16%8%27%31%14%2
700
16th to endNight
16%13%28%28%12%
720
December1st to 15thNight
20%10%30%29%7%
720
16th to endNight
23%13%30%29%3
768
Page generated at 2017-09-25 09:02:18. Version ac04

Copyright 2017 A. Danko


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