Gettysburg College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gettysburg College Observatory clear sky chart from 2005-11-05 to 2018-02-22.

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
34%
15782
20%
48935
10%
15742
44%
15742
22%
30988

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%
777
19%
2648
9%
776
33%
776
16%
1685
January16th to endNight
19%
832
22%
2704
7%
832
38%
832
19%
1719
February1st to 15thNight
19%
764
17%
2482
6%
761
33%
761
17%
1587
February16th to endNight
15%
659
21%
2017
5%
659
35%
659
17%
1327
March1st to 15thNight
21%
720
20%
2007
8%
720
34%
720
20%
1358
March16th to endNight
22%
768
20%
2088
8%
768
33%
768
22%
1394
April1st to 15thNight
23%
718
17%
1796
5%
718
30%
718
24%
1238
April16th to endNight
33%
720
17%
1800
5%
720
39%
720
25%
1207
May1st to 15thNight
31%
514
16%
1594
8%
514
39%
514
17%
1084
May16th to endNight
40%
374
17%
1497
13%
374
49%
374
17%
902
June1st to 15thNight
45%
360
18%
1440
16%
360
55%
360
21%
819
June16th to endNight
52%
360
18%
1440
16%
360
62%
360
23%
804
July1st to 15thNight
52%
340
13%
1360
15%
340
55%
340
19%
843
July16th to endNight
61%
519
16%
1550
13%
486
68%
486
32%
885
August1st to 15thNight
59%
539
19%
1609
15%
536
62%
536
29%
975
August16th to endNight
61%
741
21%
1887
13%
741
63%
741
33%
1215
September1st to 15thNight
52%
720
24%
1800
14%
720
57%
720
35%
1157
September16th to endNight
52%
717
26%
1977
19%
717
58%
717
28%
1273
October1st to 15thNight
42%
720
21%
2160
17%
720
50%
720
25%
1378
October16th to endNight
34%
768
28%
2304
15%
768
51%
768
30%
1360
November1st to 15thNight
34%
760
23%
2430
14%
760
46%
760
25%
1464
November16th to endNight
27%
780
21%
2703
11%
780
42%
780
20%
1631
December1st to 15thNight
21%
780
15%
2730
5%
780
34%
780
13%
1815
December16th to endNight
18%
832
18%
2912
5%
832
35%
832
16%
1868

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
14%5%245%35%5%5%7%40%
49311
Night
14%5%2442445%7%43%
49117
Cloud CoverDay
14%5%25%5%35%45%7%40%
49321
Night
14%5%2442445%7%43%
49121
TransparencyDay
41%5%12%22%14%2
49011
Night
40%5%12%23%15%2
48935
SeeingDay
17%15%21%22%16%6%
6103
Night
15%20%24%25%11%1
15786

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
13%7%25%6%35%6%5%6%37%
1617
16th to endDay
13%8%27%5%25%35%7%37%
1872
February1st to 15thDay
15%7%35%5%35%6%46%36%
1710
16th to endDay
13%6%36%6%3445%6%39%
1603
March1st to 15thDay
14%6%24435%347%41%
1939
16th to endDay
17%6%35%5%34346%38%
2136
April1st to 15thDay
15%5%25%5%3445%6%40%
2152
16th to endDay
18%5%24425%346%42%
2160
May1st to 15thDay
19%5%25%5%25%46%7%35%
2364
16th to endDay
15%5%35%5%35%447%38%
2618
June1st to 15thDay
12%2245%35%5%7%10%40%
2520
16th to endDay
10%4246%345%7%9%42%
2520
July1st to 15thDay
9%425%5%35%6%7%10%38%
2520
16th to endDay
7%5%246%46%7%6%9%38%
2509
August1st to 15thDay
11%425%5%36%6%6%8%37%
2340
16th to endDay
12%4244345%6%7%43%
2302
September1st to 15thDay
16%423424447%45%
2160
16th to endDay
15%5%24434447%43%
1981
October1st to 15thDay
20%413324446%43%
1800
16th to endDay
14%42442445%6%45%
1920
November1st to 15thDay
14%6%234235%47%46%
1758
16th to endDay
17%5%3432335%6%43%
1586
December1st to 15thDay
16%5%25%5%45%5%6%6%36%
1560
16th to endDay
14%5%35%6%45%5%6%8%35%
1664
January1st to 15thNight
14%6%25%5%2445%7%39%
2648
16th to endNight
14%6%35%423347%42%
2704
February1st to 15thNight
14%7%3443345%7%40%
2480
16th to endNight
13%6%35%6%25%5%5%7%39%
2017
March1st to 15thNight
15%5%34434347%42%
1995
16th to endNight
16%5%25%3345%47%42%
2088
April1st to 15thNight
14%5%444335%6%7%39%
1796
16th to endNight
16%5%2441345%8%43%
1800
May1st to 15thNight
20%5%3442435%6%39%
1594
16th to endNight
14%5%25%42445%7%42%
1495
June1st to 15thNight
11%414325%5%6%9%46%
1440
16th to endNight
10%324335%46%9%46%
1440
July1st to 15thNight
10%425%5%35%5%6%10%39%
1440
16th to endNight
7%3233245%6%11%48%
1657
August1st to 15thNight
8%425%4345%5%9%45%
1620
16th to endNight
11%42342445%8%48%
1887
September1st to 15thNight
12%42442345%6%49%
1800
16th to endNight
15%5%13434445%48%
1977
October1st to 15thNight
19%424323346%44%
2160
16th to endNight
14%41332435%6%49%
2304
November1st to 15thNight
11%7%15%42435%5%48%
2430
16th to endNight
16%414425%35%7%43%
2703
December1st to 15thNight
15%6%25%5%36%5%6%6%35%
2730
16th to endNight
15%5%2443446%8%39%
2912

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
14%7%35%5%35%5%46%38%
1617
16th to endDay
13%9%36%5%35%35%6%38%
1872
February1st to 15thDay
15%8%25%5%35%6%45%36%
1710
16th to endDay
14%6%36%6%34446%39%
1603
March1st to 15thDay
14%6%25%435%447%41%
1950
16th to endDay
16%6%35%5%34446%38%
2135
April1st to 15thDay
15%6%25%5%25%45%6%40%
2152
16th to endDay
17%5%24434446%42%
2160
May1st to 15thDay
18%5%25%5%2446%8%35%
2364
16th to endDay
15%5%25%5%345%5%7%37%
2618
June1st to 15thDay
12%3245%35%5%7%10%39%
2520
16th to endDay
10%325%5%35%5%6%9%42%
2520
July1st to 15thDay
9%325%5%36%5%7%9%39%
2520
16th to endDay
7%425%6%46%6%7%10%37%
2509
August1st to 15thDay
11%5%25%5%47%47%8%37%
2340
16th to endDay
11%5%145%345%5%9%42%
2302
September1st to 15thDay
16%42433435%7%45%
2160
16th to endDay
14%6%245%34447%43%
1981
October1st to 15thDay
19%41332445%5%44%
1800
16th to endDay
14%5%25%42345%6%45%
1920
November1st to 15thDay
14%7%1342345%7%46%
1758
16th to endDay
16%6%34333346%42%
1586
December1st to 15thDay
16%5%25%5%36%5%5%5%37%
1560
16th to endDay
15%6%35%6%35%46%7%35%
1664
January1st to 15thNight
15%6%35%4245%5%7%39%
2648
16th to endNight
15%7%35%423346%42%
2704
February1st to 15thNight
14%7%245%2345%6%41%
2473
16th to endNight
13%6%36%5%245%6%7%39%
2017
March1st to 15thNight
15%5%24434447%42%
2007
16th to endNight
15%6%25%325%446%42%
2088
April1st to 15thNight
13%6%345%335%6%8%38%
1796
16th to endNight
16%6%24423448%43%
1800
May1st to 15thNight
18%7%3442435%6%38%
1593
16th to endNight
12%6%25%4245%5%7%42%
1495
June1st to 15thNight
11%5%134245%5%9%46%
1440
16th to endNight
9%324436%46%8%46%
1440
July1st to 15thNight
9%4246%35%6%5%10%39%
1440
16th to endNight
6%413335%5%6%10%48%
1657
August1st to 15thNight
8%425%5%245%6%10%44%
1620
16th to endNight
11%32442445%7%48%
1887
September1st to 15thNight
12%42442445%7%47%
1800
16th to endNight
14%5%23424346%48%
1977
October1st to 15thNight
19%423423445%44%
2160
16th to endNight
15%32332345%5%49%
2304
November1st to 15thNight
12%6%25%423345%48%
2430
16th to endNight
15%424435%45%6%43%
2703
December1st to 15thNight
15%6%25%5%35%6%5%6%36%
2730
16th to endNight
15%6%244245%6%8%38%
2912

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
44%5%14%20%11%2
1618
16th to endDay
46%313%22%10%2
1872
February1st to 15thDay
48%5%11%21%10%2
1712
16th to endDay
46%412%24%10%2
1603
March1st to 15thDay
44%413%22%14%1
1950
16th to endDay
47%5%10%18%15%2
2136
April1st to 15thDay
44%412%20%16%2
2152
16th to endDay
42%48%20%19%4
2160
May1st to 15thDay
45%5%11%21%14%1
2364
16th to endDay
43%6%14%20%13%1
2622
June1st to 15thDay
37%9%13%24%13%
2520
16th to endDay
36%7%15%24%14%1
2520
July1st to 15thDay
38%8%13%21%14%2
2371
16th to endDay
36%9%14%23%15%1
2353
August1st to 15thDay
41%7%13%21%14%
2327
16th to endDay
36%7%12%25%14%2
2302
September1st to 15thDay
38%412%20%20%2
2160
16th to endDay
40%49%21%20%3
1981
October1st to 15thDay
41%411%24%16%2
1800
16th to endDay
37%411%27%17%
1920
November1st to 15thDay
37%414%21%16%5%
1758
16th to endDay
42%310%28%13%1
1586
December1st to 15thDay
46%6%15%20%10%1
1560
16th to endDay
46%5%14%21%10%
1664
January1st to 15thNight
43%5%12%20%13%3
2648
16th to endNight
42%411%27%11%1
2704
February1st to 15thNight
43%414%22%12%2
2482
16th to endNight
43%410%23%15%1
2017
March1st to 15thNight
43%5%10%26%12%1
2007
16th to endNight
42%5%12%21%16%2
2088
April1st to 15thNight
42%411%21%15%3
1796
16th to endNight
38%6%11%20%17%6%
1800
May1st to 15thNight
45%6%9%20%15%2
1594
16th to endNight
39%7%15%20%14%2
1497
June1st to 15thNight
35%7%11%28%16%
1440
16th to endNight
33%6%13%27%17%1
1440
July1st to 15thNight
39%9%13%21%12%3
1360
16th to endNight
29%7%19%23%17%2
1550
August1st to 15thNight
33%6%15%25%17%1
1609
16th to endNight
34%7%11%24%19%3
1887
September1st to 15thNight
34%5%12%21%22%3
1800
16th to endNight
38%5%7%23%21%3
1977
October1st to 15thNight
41%410%24%17%1
2160
16th to endNight
34%310%31%18%1
2304
November1st to 15thNight
38%49%25%16%6%
2430
16th to endNight
40%310%25%17%2
2703
December1st to 15thNight
47%5%13%21%10%1
2730
16th to endNight
43%6%15%22%10%
2912

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
21%29%32%11%3
105
16th to endDay
22%22%37%15%2
144
February1st to 15thDay
23%31%34%9%
135
16th to endDay
22%25%33%16%2
224
March1st to 15thDay
17%27%22%25%5%
270
16th to endDay
20%25%26%20%5%2
288
April1st to 15thDay
20%21%27%17%11%1
270
16th to endDay
21%15%22%17%16%5%
270
May1st to 15thDay
27%23%21%13%10%4
288
16th to endDay
19%12%13%26%21%6%
331
June1st to 15thDay
15%14%21%24%17%6%
315
16th to endDay
12%14%21%28%16%4
315
July1st to 15thDay
14%9%17%25%23%9%
305
16th to endDay
8%10%12%28%27%12%
326
August1st to 15thDay
13%6%12%26%25%15%
313
16th to endDay
10%6%9%25%28%20%
332
September1st to 15thDay
15%10%14%19%22%16%
315
16th to endDay
19%7%13%23%20%15%
314
October1st to 15thDay
24%8%13%24%20%7%
315
16th to endDay
14%14%31%22%14%2
336
November1st to 15thDay
19%16%20%24%16%2
290
16th to endDay
18%19%29%19%11%1
116
December1st to 15thDay
18%22%27%17%10%3
90
16th to endDay
22%16%39%15%5%
96
January1st to 15thNight
15%29%31%20%3
780
16th to endNight
17%25%34%21%1
832
February1st to 15thNight
17%26%33%21%1
764
16th to endNight
16%29%35%16%1
659
March1st to 15thNight
17%27%30%20%3
720
16th to endNight
15%24%33%20%5%
768
April1st to 15thNight
15%27%31%20%4
718
16th to endNight
16%21%24%25%10%1
720
May1st to 15thNight
19%21%23%25%9%
514
16th to endNight
14%20%15%30%17%1
375
June1st to 15thNight
10%17%21%29%18%1
360
16th to endNight
8%20%12%42%16%
360
July1st to 15thNight
12%12%12%36%24%1
340
16th to endNight
6%17%8%38%25%3
519
August1st to 15thNight
11%10%7%41%25%2
539
16th to endNight
11%10%8%36%28%3
741
September1st to 15thNight
10%10%16%32%27%2
720
16th to endNight
16%9%14%27%28%4
717
October1st to 15thNight
20%12%19%25%18%3
720
16th to endNight
15%16%30%25%11%
768
November1st to 15thNight
13%19%27%28%10%1
760
16th to endNight
14%28%26%23%7%
780
December1st to 15thNight
17%24%31%19%7%
780
16th to endNight
19%25%33%20%2
832
Page generated at 2018-02-23 11:21:40. Version ac04

Copyright 2018 A. Danko


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