This page made possible by the sponsorship of: Delaware Valley Amateur Astronomers.

Colonial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colonial Observatory 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
35%
15797
20%
49115
11%
15763
46%
15763
23%
31223

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%
780
20%
2716
9%
776
39%
776
18%
1708
January16th to endNight
22%
832
23%
2736
8%
832
42%
832
20%
1725
February1st to 15thNight
18%
764
17%
2453
5%
761
33%
761
18%
1610
February16th to endNight
18%
683
25%
2049
7%
683
39%
683
20%
1315
March1st to 15thNight
24%
764
21%
2227
8%
764
38%
764
22%
1429
March16th to endNight
24%
768
22%
2029
9%
768
36%
768
26%
1354
April1st to 15thNight
22%
718
18%
1796
5%
718
30%
718
27%
1209
April16th to endNight
33%
720
18%
1800
6%
720
38%
720
29%
1213
May1st to 15thNight
33%
509
17%
1589
8%
509
42%
509
20%
1069
May16th to endNight
47%
374
15%
1497
15%
374
52%
374
20%
913
June1st to 15thNight
43%
360
15%
1440
14%
360
53%
360
20%
891
June16th to endNight
56%
360
16%
1440
18%
360
63%
360
25%
830
July1st to 15thNight
50%
340
13%
1360
14%
340
54%
340
19%
852
July16th to endNight
61%
403
16%
1443
16%
379
68%
379
27%
812
August1st to 15thNight
64%
581
18%
1651
13%
578
66%
578
32%
1029
August16th to endNight
59%
764
18%
1910
10%
764
61%
764
31%
1280
September1st to 15thNight
49%
720
25%
1800
13%
720
54%
720
34%
1172
September16th to endNight
54%
717
27%
1938
21%
717
59%
717
33%
1235
October1st to 15thNight
44%
720
20%
2160
16%
720
51%
720
27%
1371
October16th to endNight
35%
768
28%
2344
16%
768
51%
768
29%
1395
November1st to 15thNight
35%
760
21%
2472
12%
760
48%
760
26%
1507
November16th to endNight
29%
780
22%
2623
11%
780
45%
780
20%
1641
December1st to 15thNight
25%
780
16%
2730
7%
780
38%
780
15%
1819
December16th to endNight
19%
832
20%
2912
6%
832
36%
832
17%
1844

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%5%244245%5%7%40%
49514
Night
15%6%2442445%7%43%
49288
Cloud CoverDay
15%5%244245%5%7%40%
49523
Night
15%5%2442445%7%43%
49294
TransparencyDay
41%6%11%22%15%2
49205
Night
40%5%11%22%17%2
49115
SeeingDay
21%10%26%23%13%4
6278
Night
19%9%30%28%10%1
15798

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
15%6%2443445%8%40%
1552
16th to endDay
15%7%244245%47%41%
1836
February1st to 15thDay
17%6%25%4245%47%38%
1737
16th to endDay
16%7%25%5%2445%6%38%
1710
March1st to 15thDay
15%5%25%5%35%5%45%39%
1959
16th to endDay
19%5%25%424446%41%
2193
April1st to 15thDay
15%6%34424347%41%
2152
16th to endDay
15%4245%245%46%44%
2160
May1st to 15thDay
18%6%25%5%25%5%5%6%36%
2371
16th to endDay
16%5%25%4345%6%8%37%
2618
June1st to 15thDay
14%5%35%5%25%5%7%9%36%
2520
16th to endDay
11%425%6%35%5%7%10%39%
2520
July1st to 15thDay
9%6%25%5%36%6%6%9%37%
2520
16th to endDay
8%425%5%36%7%7%9%38%
2624
August1st to 15thDay
11%415%5%345%8%7%40%
2286
16th to endDay
11%4245%335%7%8%42%
2292
September1st to 15thDay
14%5%14424447%46%
2160
16th to endDay
14%5%25%434447%42%
2019
October1st to 15thDay
18%5%1232345%6%44%
1800
16th to endDay
17%5%1332345%6%46%
1878
November1st to 15thDay
17%6%24423345%46%
1716
16th to endDay
16%424323446%47%
1667
December1st to 15thDay
16%7%244235%5%8%39%
1560
16th to endDay
18%6%345%24447%37%
1664
January1st to 15thNight
16%6%35%424346%42%
2716
16th to endNight
16%7%24424447%42%
2736
February1st to 15thNight
15%7%35%5%24446%40%
2452
16th to endNight
16%8%25%32344444%
2049
March1st to 15thNight
18%42333435%6%43%
2214
16th to endNight
17%7%24423235%45%
2029
April1st to 15thNight
16%5%2443436%7%43%
1796
16th to endNight
17%5%15%42445%7%42%
1800
May1st to 15thNight
18%6%26%424446%38%
1589
16th to endNight
15%6%15%42445%6%42%
1495
June1st to 15thNight
16%5%25%42345%8%40%
1440
16th to endNight
9%3235%25%6%6%11%43%
1440
July1st to 15thNight
10%326%6%45%5%8%9%37%
1440
16th to endNight
8%4243246%8%11%43%
1541
August1st to 15thNight
9%5%14425%5%6%10%45%
1662
16th to endNight
11%5%25%43445%8%43%
1910
September1st to 15thNight
13%5%24413446%48%
1800
16th to endNight
11%6%25%5%2345%6%47%
1938
October1st to 15thNight
16%6%25%323345%45%
2160
16th to endNight
16%5%23313335%51%
2344
November1st to 15thNight
15%5%24323345%48%
2472
16th to endNight
16%5%2432445%7%42%
2623
December1st to 15thNight
18%6%25%5%35%446%36%
2730
16th to endNight
17%6%35%42345%7%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
15%5%2443445%8%40%
1552
16th to endDay
15%7%2442445%7%41%
1836
February1st to 15thDay
17%6%344245%5%7%39%
1737
16th to endDay
15%7%25%5%25%447%37%
1710
March1st to 15thDay
15%6%25%5%35%445%40%
1969
16th to endDay
19%6%24424346%41%
2192
April1st to 15thDay
16%6%24424348%41%
2152
16th to endDay
15%4245%2445%6%43%
2160
May1st to 15thDay
18%6%25%5%25%5%5%6%35%
2371
16th to endDay
16%6%25%4245%6%9%36%
2618
June1st to 15thDay
14%425%5%35%5%7%9%35%
2520
16th to endDay
11%325%5%35%6%6%9%39%
2520
July1st to 15thDay
9%5%35%5%36%6%7%8%37%
2520
16th to endDay
8%425%6%36%7%7%9%38%
2624
August1st to 15thDay
12%415%5%35%5%8%8%39%
2286
16th to endDay
12%425%5%245%6%8%42%
2292
September1st to 15thDay
14%414423447%45%
2160
16th to endDay
14%5%25%434447%42%
2019
October1st to 15thDay
18%4233235%47%44%
1800
16th to endDay
18%41331435%6%46%
1878
November1st to 15thDay
17%5%24423245%45%
1716
16th to endDay
16%424323446%47%
1667
December1st to 15thDay
16%7%2442345%8%39%
1560
16th to endDay
19%6%24433447%38%
1664
January1st to 15thNight
16%5%25%5%24446%42%
2716
16th to endNight
16%7%24424446%42%
2736
February1st to 15thNight
15%7%25%434446%40%
2445
16th to endNight
16%6%25%42344444%
2049
March1st to 15thNight
17%5%2432345%6%43%
2227
16th to endNight
16%7%35%323236%45%
2029
April1st to 15thNight
16%5%25%33435%6%43%
1796
16th to endNight
16%5%2442445%7%42%
1800
May1st to 15thNight
17%6%35%5%34446%38%
1589
16th to endNight
15%6%25%42435%7%41%
1495
June1st to 15thNight
16%5%3432445%8%40%
1440
16th to endNight
9%324425%5%7%11%42%
1440
July1st to 15thNight
11%326%5%36%6%7%10%37%
1440
16th to endNight
7%4243246%9%10%42%
1541
August1st to 15thNight
9%5%144345%6%9%45%
1662
16th to endNight
11%42443445%8%44%
1910
September1st to 15thNight
14%5%23414446%48%
1800
16th to endNight
12%5%245%2345%6%46%
1938
October1st to 15thNight
17%5%24322345%45%
2160
16th to endNight
16%5%13322346%51%
2344
November1st to 15thNight
15%6%25%322345%48%
2472
16th to endNight
16%5%2442435%7%42%
2623
December1st to 15thNight
18%7%25%5%35%446%36%
2730
16th to endNight
17%5%35%42435%6%40%
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
41%5%13%22%12%4
1553
16th to endDay
41%5%12%24%13%2
1836
February1st to 15thDay
43%5%12%22%13%2
1740
16th to endDay
44%5%11%25%12%1
1710
March1st to 15thDay
44%412%22%13%2
1969
16th to endDay
43%49%21%17%2
2193
April1st to 15thDay
42%310%23%14%4
2152
16th to endDay
39%5%10%18%21%4
2160
May1st to 15thDay
47%6%9%19%14%2
2371
16th to endDay
42%7%12%22%13%1
2622
June1st to 15thDay
42%8%12%24%11%1
2520
16th to endDay
39%8%12%24%13%1
2520
July1st to 15thDay
41%7%11%20%15%3
2371
16th to endDay
37%8%13%24%13%1
2459
August1st to 15thDay
39%7%12%22%17%1
2273
16th to endDay
37%7%9%22%20%2
2292
September1st to 15thDay
37%5%10%20%23%2
2160
16th to endDay
40%5%7%20%23%2
2019
October1st to 15thDay
39%5%12%22%18%1
1800
16th to endDay
37%5%11%26%18%1
1878
November1st to 15thDay
41%412%21%17%2
1716
16th to endDay
38%311%28%16%2
1667
December1st to 15thDay
42%6%16%22%11%1
1560
16th to endDay
45%5%15%20%12%
1664
January1st to 15thNight
42%5%10%21%15%3
2716
16th to endNight
42%410%26%15%1
2736
February1st to 15thNight
44%5%12%21%13%2
2453
16th to endNight
42%46%24%18%3
2049
March1st to 15thNight
40%5%11%23%16%2
2227
16th to endNight
42%310%20%18%4
2029
April1st to 15thNight
40%5%11%21%17%3
1796
16th to endNight
40%5%9%18%21%4
1800
May1st to 15thNight
45%6%10%17%18%2
1589
16th to endNight
40%7%15%16%17%3
1497
June1st to 15thNight
42%6%11%21%17%1
1440
16th to endNight
33%8%12%28%15%1
1440
July1st to 15thNight
39%10%14%18%13%3
1360
16th to endNight
32%8%18%24%14%2
1443
August1st to 15thNight
33%7%13%23%19%2
1651
16th to endNight
37%7%10%19%21%2
1910
September1st to 15thNight
35%6%9%21%23%2
1800
16th to endNight
36%5%6%21%26%2
1938
October1st to 15thNight
40%412%23%16%2
2160
16th to endNight
35%49%30%18%2
2344
November1st to 15thNight
38%411%21%17%6%
2472
16th to endNight
41%410%24%16%2
2623
December1st to 15thNight
47%5%12%20%12%1
2730
16th to endNight
43%414%22%12%1
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
26%28%30%6%5%2
90
16th to endDay
27%20%33%14%2
136
February1st to 15thDay
32%26%26%12%1
164
16th to endDay
24%17%39%15%2
291
March1st to 15thDay
25%16%30%22%4
312
16th to endDay
24%15%32%17%6%2
288
April1st to 15thDay
26%15%30%17%7%2
270
16th to endDay
21%11%31%20%10%4
270
May1st to 15thDay
34%12%22%16%10%3
288
16th to endDay
24%322%25%19%4
331
June1st to 15thDay
16%11%23%26%18%3
315
16th to endDay
14%7%27%31%14%4
315
July1st to 15thDay
17%124%30%20%5%
305
16th to endDay
9%322%31%25%7%
326
August1st to 15thDay
16%217%33%21%8%
313
16th to endDay
11%218%32%21%14%
332
September1st to 15thDay
17%9%21%22%19%9%
315
16th to endDay
21%5%15%27%14%14%
314
October1st to 15thDay
23%321%25%18%7%
315
16th to endDay
17%13%29%24%13%2
325
November1st to 15thDay
21%9%27%26%13%1
280
16th to endDay
19%19%33%18%8%1
197
December1st to 15thDay
28%21%27%10%12%
90
16th to endDay
33%22%38%41
96
January1st to 15thNight
21%15%33%24%4
780
16th to endNight
21%16%36%24%1
832
February1st to 15thNight
20%16%39%23%
764
16th to endNight
21%17%39%19%2
683
March1st to 15thNight
23%15%33%23%3
764
16th to endNight
20%10%39%20%8%
768
April1st to 15thNight
18%15%40%21%4
718
16th to endNight
19%8%36%27%9%
720
May1st to 15thNight
22%8%30%31%6%
509
16th to endNight
18%324%39%13%1
375
June1st to 15thNight
19%326%38%12%
360
16th to endNight
9%127%48%12%
360
July1st to 15thNight
14%222%43%17%
340
16th to endNight
8%121%41%24%2
403
August1st to 15thNight
13%114%45%23%1
581
16th to endNight
14%115%37%28%2
764
September1st to 15thNight
16%324%35%18%2
720
16th to endNight
16%219%31%26%4
717
October1st to 15thNight
20%6%22%28%18%3
720
16th to endNight
19%12%29%27%10%
768
November1st to 15thNight
18%9%31%26%12%1
760
16th to endNight
20%13%30%25%9%
780
December1st to 15thNight
24%11%31%23%7%
780
16th to endNight
22%16%36%22%3
832
Page generated at 2018-03-12 13:28:08. Version ac04

Copyright 2018 A. Danko


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