Hopkins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hopkins Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
28%
14854
13%
46826
7%
14823
35%
14823
15%
33640

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
15%
720
11%
2506
5%
716
22%
716
8%
1927
January16th to endNight
14%
768
11%
2673
3%
768
24%
768
7%
2057
February1st to 15thNight
11%
704
8%
2225
1%
701
18%
701
7%
1793
February16th to endNight
16%
631
13%
1893
5%
631
27%
631
10%
1446
March1st to 15thNight
17%
720
13%
2160
6%
720
26%
720
10%
1638
March16th to endNight
23%
768
19%
2001
7%
768
32%
768
15%
1462
April1st to 15thNight
20%
718
12%
1796
2%
718
26%
718
16%
1371
April16th to endNight
25%
645
15%
1725
4%
645
32%
645
19%
1227
May1st to 15thNight
26%
489
15%
1569
8%
489
34%
489
15%
1119
May16th to endNight
39%
374
12%
1497
11%
374
46%
374
15%
1036
June1st to 15thNight
36%
360
11%
1440
9%
360
43%
360
14%
975
June16th to endNight
48%
360
10%
1440
13%
360
54%
360
21%
888
July1st to 15thNight
51%
340
12%
1360
14%
340
57%
340
20%
847
July16th to endNight
55%
376
14%
1418
17%
354
59%
354
23%
828
August1st to 15thNight
53%
464
15%
1535
11%
462
57%
462
26%
937
August16th to endNight
52%
764
18%
1910
10%
764
54%
764
30%
1270
September1st to 15thNight
44%
720
18%
1812
9%
720
47%
720
28%
1230
September16th to endNight
47%
661
22%
1825
16%
661
52%
661
31%
1145
October1st to 15thNight
33%
660
18%
1906
11%
660
41%
660
19%
1344
October16th to endNight
22%
704
10%
2244
5%
704
30%
704
12%
1678
November1st to 15thNight
28%
700
17%
2277
10%
700
38%
700
16%
1568
November16th to endNight
23%
720
14%
2406
8%
720
31%
720
12%
1737
December1st to 15thNight
16%
720
8%
2520
3%
720
23%
720
7%
1973
December16th to endNight
15%
768
9%
2688
4%
768
23%
768
6%
2144

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
16%9%47%6%35%446%31%
47940
Night
15%9%47%6%35%446%32%
46996
Cloud CoverDay
16%9%47%6%35%45%6%31%
47950
Night
15%8%47%6%3445%6%33%
47001
TransparencyDay
52%6%11%17%10%1
47628
Night
51%6%11%17%10%1
46826
SeeingDay
23%16%25%19%11%4
5745
Night
23%18%24%23%8%1
14855

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
14%10%6%9%8%48%5%5%422%
1432
16th to endDay
12%11%7%10%9%36%445%22%
1551
February1st to 15thDay
11%14%7%11%9%45%45%5%20%
1636
16th to endDay
13%12%6%11%7%45%43425%
1580
March1st to 15thDay
16%12%5%8%7%345%36%26%
1790
16th to endDay
16%10%5%8%423345%34%
2220
April1st to 15thDay
18%12%47%6%3333430%
2152
16th to endDay
15%8%36%5%3445%6%35%
2235
May1st to 15thDay
22%9%36%5%34345%30%
2391
16th to endDay
18%7%26%5%345%5%6%34%
2618
June1st to 15thDay
23%7%36%5%35%46%8%26%
2520
16th to endDay
19%6%36%5%36%5%5%7%28%
2520
July1st to 15thDay
13%6%36%5%35%6%7%8%32%
2520
16th to endDay
14%5%26%5%35%6%6%8%33%
2654
August1st to 15thDay
14%7%36%6%45%5%6%8%32%
2400
16th to endDay
12%7%26%6%4446%9%36%
2292
September1st to 15thDay
16%6%25%5%35%446%38%
2148
16th to endDay
15%5%24423336%47%
1825
October1st to 15thDay
20%9%36%5%3334434%
1722
16th to endDay
21%13%46%434445%27%
1626
November1st to 15thDay
15%11%45%424446%37%
1581
16th to endDay
16%11%47%5%34445%32%
1551
December1st to 15thDay
18%13%6%9%6%34335%25%
1440
16th to endDay
16%17%7%10%8%4433419%
1536
January1st to 15thNight
13%12%7%10%8%35%435%23%
2506
16th to endNight
12%13%6%9%7%46%45%5%23%
2673
February1st to 15thNight
12%10%6%12%9%46%5%4420%
2224
16th to endNight
12%13%5%8%6%46%5%4426%
1893
March1st to 15thNight
16%11%5%9%6%36%34429%
2147
16th to endNight
15%10%5%6%5%33345%35%
2001
April1st to 15thNight
17%10%46%6%35%445%31%
1796
16th to endNight
17%8%35%5%23445%38%
1725
May1st to 15thNight
19%9%36%5%25%346%33%
1569
16th to endNight
17%7%36%6%345%5%7%33%
1495
June1st to 15thNight
22%6%345%34348%33%
1440
16th to endNight
14%4235%25%5%48%41%
1440
July1st to 15thNight
12%5%35%5%36%46%8%38%
1440
16th to endNight
13%42443446%9%41%
1514
August1st to 15thNight
11%414435%47%8%43%
1545
16th to endNight
13%5%145%35%35%8%43%
1910
September1st to 15thNight
16%6%2432345%5%44%
1812
16th to endNight
14%423423346%49%
1825
October1st to 15thNight
18%8%27%5%34345%37%
1906
16th to endNight
20%11%36%5%43445%29%
2244
November1st to 15thNight
14%10%36%5%25%446%36%
2277
16th to endNight
15%9%46%8%35%5%5%5%29%
2406
December1st to 15thNight
18%13%5%9%7%25%44422%
2520
16th to endNight
18%14%6%9%7%35%445%20%
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
14%10%5%7%8%48%6%6%5%23%
1432
16th to endDay
12%11%5%9%10%45%5%5%6%23%
1551
February1st to 15thDay
12%11%6%11%10%46%45%5%21%
1636
16th to endDay
14%10%6%11%7%35%34427%
1580
March1st to 15thDay
16%11%5%9%6%3445%6%27%
1800
16th to endDay
15%9%5%7%5%24436%35%
2220
April1st to 15thDay
17%11%5%8%7%2433431%
2152
16th to endDay
14%8%38%6%345%45%36%
2235
May1st to 15thDay
22%10%36%5%2444429%
2391
16th to endDay
18%6%345%345%5%7%34%
2618
June1st to 15thDay
22%7%36%5%35%5%5%8%27%
2520
16th to endDay
18%7%36%5%36%5%5%8%28%
2520
July1st to 15thDay
12%7%36%6%35%7%6%7%32%
2520
16th to endDay
14%6%35%5%26%5%6%8%33%
2654
August1st to 15thDay
13%8%36%6%36%5%6%8%31%
2400
16th to endDay
12%6%35%6%345%5%9%36%
2292
September1st to 15thDay
14%7%345%35%45%6%38%
2148
16th to endDay
14%5%25%323345%46%
1825
October1st to 15thDay
20%9%36%5%2333434%
1722
16th to endDay
19%13%37%5%24445%28%
1626
November1st to 15thDay
14%10%46%423347%37%
1581
16th to endDay
17%8%46%5%35%45%5%32%
1551
December1st to 15thDay
18%12%6%8%7%3434426%
1440
16th to endDay
17%14%5%10%10%24445%20%
1536
January1st to 15thNight
14%10%5%9%8%45%446%25%
2506
16th to endNight
12%10%5%8%8%5%6%5%6%5%25%
2673
February1st to 15thNight
12%9%6%9%9%6%7%5%5%5%22%
2217
16th to endNight
13%11%5%8%7%35%5%6%5%27%
1893
March1st to 15thNight
16%10%49%6%35%5%4429%
2160
16th to endNight
14%9%5%7%6%23445%36%
2000
April1st to 15thNight
17%10%47%5%345%5%431%
1796
16th to endNight
17%8%36%5%23445%37%
1725
May1st to 15thNight
19%8%47%5%2335%5%33%
1569
16th to endNight
17%8%35%6%25%45%7%33%
1495
June1st to 15thNight
21%7%35%424448%34%
1440
16th to endNight
14%4344345%5%9%41%
1440
July1st to 15thNight
11%6%35%5%345%6%9%39%
1440
16th to endNight
12%4245%3446%9%42%
1514
August1st to 15thNight
10%425%43447%8%44%
1545
16th to endNight
12%5%25%43445%8%43%
1910
September1st to 15thNight
15%6%24414446%44%
1812
16th to endNight
13%5%24424346%48%
1825
October1st to 15thNight
18%7%27%5%2344437%
1906
16th to endNight
18%11%46%6%2445%6%30%
2244
November1st to 15thNight
14%8%37%5%25%45%6%37%
2277
16th to endNight
14%8%47%6%35%5%5%5%31%
2406
December1st to 15thNight
18%11%5%9%7%34445%24%
2520
16th to endNight
17%13%47%8%35%5%5%5%21%
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
61%6%9%15%6%1
1433
16th to endDay
60%6%11%13%5%
1551
February1st to 15thDay
64%7%10%11%6%
1639
16th to endDay
60%5%9%13%9%
1580
March1st to 15thDay
58%6%11%14%9%
1800
16th to endDay
51%49%15%14%3
2220
April1st to 15thDay
57%47%18%10%1
2152
16th to endDay
48%6%8%20%13%2
2235
May1st to 15thDay
56%5%8%18%10%1
2391
16th to endDay
47%8%12%19%10%1
2622
June1st to 15thDay
54%7%13%16%7%
2520
16th to endDay
52%8%12%16%10%
2520
July1st to 15thDay
46%8%12%21%9%
2371
16th to endDay
44%8%15%20%10%
2487
August1st to 15thDay
48%8%13%20%9%
2386
16th to endDay
42%6%13%24%11%
2292
September1st to 15thDay
45%6%11%21%14%
2148
16th to endDay
38%49%26%18%1
1825
October1st to 15thDay
52%5%9%19%10%2
1722
16th to endDay
58%6%12%13%7%1
1626
November1st to 15thDay
46%7%14%18%11%1
1581
16th to endDay
52%5%12%15%10%1
1551
December1st to 15thDay
62%5%11%13%6%
1440
16th to endDay
67%6%9%11%4
1536
January1st to 15thNight
60%6%10%12%8%1
2506
16th to endNight
60%6%9%13%8%1
2673
February1st to 15thNight
63%8%10%10%6%
2225
16th to endNight
56%7%10%15%8%1
1893
March1st to 15thNight
58%5%9%16%8%1
2160
16th to endNight
50%5%8%16%13%4
2001
April1st to 15thNight
54%5%11%16%10%
1796
16th to endNight
47%6%10%18%15%2
1725
May1st to 15thNight
51%5%9%17%13%1
1569
16th to endNight
48%9%12%18%10%1
1497
June1st to 15thNight
49%7%13%20%8%
1440
16th to endNight
39%8%17%23%11%
1440
July1st to 15thNight
39%9%13%26%11%
1360
16th to endNight
36%8%17%26%10%1
1418
August1st to 15thNight
36%7%17%26%11%
1535
16th to endNight
38%7%12%28%12%
1910
September1st to 15thNight
40%6%12%24%15%1
1812
16th to endNight
36%410%27%19%1
1825
October1st to 15thNight
49%5%11%18%13%2
1906
16th to endNight
55%7%12%16%7%
2244
November1st to 15thNight
48%6%11%17%13%2
2277
16th to endNight
53%7%13%15%8%1
2406
December1st to 15thNight
61%7%12%12%5%
2520
16th to endNight
64%6%10%11%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%31%23%8%8%
60
16th to endDay
29%35%25%7%1
67
February1st to 15thDay
26%34%25%9%4
152
16th to endDay
24%26%29%17%2
239
March1st to 15thDay
31%22%23%19%2
270
16th to endDay
28%24%21%15%7%2
288
April1st to 15thDay
30%21%22%14%9%1
270
16th to endDay
19%20%27%19%8%4
270
May1st to 15thDay
37%14%22%12%8%4
288
16th to endDay
21%15%32%20%8%1
331
June1st to 15thDay
26%11%30%20%11%
315
16th to endDay
20%14%27%21%13%1
315
July1st to 15thDay
14%9%35%24%12%2
305
16th to endDay
14%8%32%24%15%3
326
August1st to 15thDay
15%12%24%25%14%7%
313
16th to endDay
15%6%21%23%18%14%
332
September1st to 15thDay
17%11%21%20%19%9%
315
16th to endDay
19%8%22%17%16%15%
257
October1st to 15thDay
24%12%18%21%18%4
255
16th to endDay
27%19%27%19%6%
247
November1st to 15thDay
26%15%21%22%12%1
235
16th to endDay
26%25%18%18%8%2
171
December1st to 15thDay
26%31%23%13%31
60
16th to endDay
37%26%28%6%1
64
January1st to 15thNight
24%26%29%15%2
720
16th to endNight
25%26%27%19%1
768
February1st to 15thNight
23%27%31%15%1
704
16th to endNight
25%27%26%17%2
631
March1st to 15thNight
29%24%24%19%2
720
16th to endNight
27%23%21%19%7%
768
April1st to 15thNight
26%21%28%20%3
718
16th to endNight
24%20%26%19%8%
645
May1st to 15thNight
24%15%27%24%6%
489
16th to endNight
18%12%24%31%12%1
375
June1st to 15thNight
25%10%21%30%11%
360
16th to endNight
16%323%42%12%
360
July1st to 15thNight
12%6%22%42%15%
340
16th to endNight
13%5%20%39%19%1
376
August1st to 15thNight
14%421%37%19%2
464
16th to endNight
15%419%32%23%4
764
September1st to 15thNight
20%8%21%29%18%1
720
16th to endNight
16%10%21%27%19%5%
661
October1st to 15thNight
24%13%22%26%11%1
660
16th to endNight
28%19%23%22%5%
704
November1st to 15thNight
21%20%24%23%8%1
700
16th to endNight
25%20%25%19%7%1
720
December1st to 15thNight
31%25%22%14%5%
720
16th to endNight
30%28%22%15%2
768
Page generated at 2017-09-17 12:05:47. Version ac04

Copyright 2017 A. Danko


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