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 2018-04-23.

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%
15721
13%
49519
7%
15690
35%
15690
15%
35565

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%
780
11%
2716
6%
776
23%
776
8%
2071
January16th to endNight
14%
832
10%
2895
3%
832
24%
832
8%
2220
February1st to 15thNight
11%
764
9%
2414
1%
761
19%
761
7%
1940
February16th to endNight
17%
683
13%
2049
5%
683
27%
683
10%
1567
March1st to 15thNight
16%
780
12%
2340
5%
780
25%
780
10%
1802
March16th to endNight
24%
832
20%
2168
7%
832
34%
832
16%
1574
April1st to 15thNight
20%
778
12%
1946
2%
778
26%
778
16%
1486
April16th to endNight
25%
668
15%
1782
4%
668
32%
668
19%
1269
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
48%
717
24%
1980
17%
717
53%
717
32%
1231
October1st to 15thNight
35%
720
17%
2079
11%
720
42%
720
20%
1452
October16th to endNight
23%
768
12%
2448
7%
768
32%
768
13%
1810
November1st to 15thNight
28%
760
16%
2472
10%
760
38%
760
15%
1711
November16th to endNight
23%
780
15%
2607
9%
780
32%
780
12%
1854
December1st to 15thNight
16%
780
8%
2730
3%
780
23%
780
7%
2141
December16th to endNight
15%
832
9%
2912
3%
832
23%
832
6%
2307

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%
50004
Night
15%9%47%6%35%446%32%
49689
Cloud CoverDay
16%9%47%6%35%45%6%31%
50014
Night
15%8%47%6%3445%6%33%
49694
TransparencyDay
52%6%11%17%10%1
49692
Night
51%6%11%18%10%1
49519
SeeingDay
23%17%25%18%10%3
6429
Night
23%18%24%23%8%1
15722

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%10%5%9%8%48%5%5%5%22%
1552
16th to endDay
12%11%7%10%8%45%446%23%
1680
February1st to 15thDay
12%14%7%11%9%45%45%5%19%
1777
16th to endDay
14%12%6%10%7%45%435%25%
1710
March1st to 15thDay
17%13%6%8%7%24436%24%
1940
16th to endDay
16%10%5%8%423345%34%
2405
April1st to 15thDay
17%12%47%6%34345%29%
2332
16th to endDay
15%8%36%5%3445%6%36%
2293
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
14%5%24423336%48%
1980
October1st to 15thDay
19%9%36%43434436%
1878
16th to endDay
21%13%46%434435%29%
1774
November1st to 15thDay
15%10%45%424446%37%
1716
16th to endDay
16%11%36%5%34445%33%
1680
December1st to 15thDay
17%14%6%10%6%34335%25%
1560
16th to endDay
15%17%7%10%7%4433420%
1664
January1st to 15thNight
13%12%6%10%8%36%436%24%
2716
16th to endNight
12%13%5%9%8%47%45%5%24%
2895
February1st to 15thNight
13%10%6%11%9%46%5%4421%
2413
16th to endNight
13%13%5%8%7%36%5%45%26%
2049
March1st to 15thNight
16%12%5%9%6%35%34427%
2327
16th to endNight
15%10%5%7%433345%35%
2168
April1st to 15thNight
17%10%46%6%35%445%30%
1946
16th to endNight
17%8%36%5%23445%38%
1782
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
13%424423346%50%
1980
October1st to 15thNight
17%8%27%5%34345%38%
2079
16th to endNight
20%11%36%5%44445%29%
2448
November1st to 15thNight
14%9%36%5%25%446%36%
2472
16th to endNight
14%9%46%7%35%5%5%5%31%
2607
December1st to 15thNight
18%13%5%9%7%3444422%
2730
16th to endNight
17%14%6%9%7%35%445%21%
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
13%9%5%6%8%48%6%6%6%23%
1552
16th to endDay
12%11%5%9%9%45%5%5%6%23%
1680
February1st to 15thDay
12%11%6%11%10%46%45%5%21%
1777
16th to endDay
15%10%6%10%7%35%34427%
1710
March1st to 15thDay
17%12%5%9%6%3345%5%25%
1950
16th to endDay
15%9%5%7%5%24436%34%
2405
April1st to 15thDay
17%11%48%6%24345%30%
2332
16th to endDay
14%8%37%6%345%45%36%
2293
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%323445%47%
1980
October1st to 15thDay
19%9%36%5%23335%36%
1878
16th to endDay
19%12%37%5%2444429%
1774
November1st to 15thDay
14%10%46%423347%36%
1716
16th to endDay
17%8%46%5%25%5%5%5%33%
1680
December1st to 15thDay
17%12%6%9%7%3434426%
1560
16th to endDay
17%14%5%10%10%34345%21%
1664
January1st to 15thNight
14%10%48%8%45%45%6%25%
2716
16th to endNight
12%10%5%9%8%5%6%5%5%5%25%
2895
February1st to 15thNight
12%9%6%9%8%6%7%5%5%5%22%
2406
16th to endNight
13%11%5%9%6%35%46%5%28%
2049
March1st to 15thNight
16%11%49%6%35%5%4428%
2340
16th to endNight
14%9%47%6%23445%36%
2167
April1st to 15thNight
16%10%47%5%345%4431%
1946
16th to endNight
17%8%36%5%23445%38%
1782
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
12%424414346%49%
1980
October1st to 15thNight
17%7%27%5%23445%39%
2079
16th to endNight
18%10%46%6%24446%30%
2448
November1st to 15thNight
14%8%37%5%25%35%6%37%
2472
16th to endNight
14%9%47%6%35%5%5%5%32%
2607
December1st to 15thNight
18%11%5%9%8%34445%24%
2730
16th to endNight
17%13%48%8%35%5%5%5%22%
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
60%6%9%15%7%1
1553
16th to endDay
60%7%12%13%5%
1680
February1st to 15thDay
65%7%9%11%5%
1780
16th to endDay
60%6%8%14%9%
1710
March1st to 15thDay
60%6%10%13%8%
1950
16th to endDay
52%49%14%15%3
2405
April1st to 15thDay
57%48%18%9%1
2332
16th to endDay
47%6%8%19%14%3
2293
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
37%410%26%19%1
1980
October1st to 15thDay
51%5%9%20%10%2
1878
16th to endDay
57%5%12%15%7%1
1774
November1st to 15thDay
47%7%13%18%11%1
1716
16th to endDay
52%5%13%16%11%1
1680
December1st to 15thDay
62%5%11%12%7%
1560
16th to endDay
67%6%9%11%5%
1664
January1st to 15thNight
59%6%10%12%9%1
2716
16th to endNight
60%6%10%13%8%1
2895
February1st to 15thNight
63%8%10%10%7%
2414
16th to endNight
57%7%10%15%8%1
2049
March1st to 15thNight
60%5%9%16%7%1
2340
16th to endNight
50%5%8%16%14%4
2168
April1st to 15thNight
54%5%11%17%10%
1946
16th to endNight
47%6%9%17%15%3
1782
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
35%5%10%27%20%1
1980
October1st to 15thNight
48%5%11%19%12%2
2079
16th to endNight
54%7%12%17%8%
2448
November1st to 15thNight
48%6%11%17%13%2
2472
16th to endNight
51%7%13%17%9%1
2607
December1st to 15thNight
62%7%12%12%5%
2730
16th to endNight
64%6%10%11%7%
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
27%30%25%10%6%
90
16th to endDay
24%36%31%8%1
100
February1st to 15thDay
30%33%23%8%3
203
16th to endDay
27%26%28%15%2
291
March1st to 15thDay
33%22%24%16%2
330
16th to endDay
28%22%24%15%6%2
352
April1st to 15thDay
32%22%24%12%7%1
330
16th to endDay
19%21%26%19%8%4
289
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
17%8%21%18%17%15%
314
October1st to 15thDay
21%12%21%21%18%3
315
16th to endDay
29%17%27%20%6%
299
November1st to 15thDay
26%18%20%22%11%1
280
16th to endDay
27%25%20%18%6%1
210
December1st to 15thDay
26%27%28%11%41
90
16th to endDay
38%33%22%41
96
January1st to 15thNight
24%26%29%16%2
780
16th to endNight
24%26%27%19%1
832
February1st to 15thNight
24%27%31%15%1
764
16th to endNight
26%25%26%18%2
683
March1st to 15thNight
30%24%24%18%2
780
16th to endNight
26%22%21%20%7%
832
April1st to 15thNight
25%21%28%20%3
778
16th to endNight
24%20%26%20%8%
668
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
15%10%20%27%20%6%
717
October1st to 15thNight
23%12%22%27%13%1
720
16th to endNight
28%19%23%22%6%
768
November1st to 15thNight
21%20%23%23%9%1
760
16th to endNight
24%21%25%19%7%1
780
December1st to 15thNight
31%25%22%15%5%
780
16th to endNight
30%29%22%15%2
832
Page generated at 2018-04-24 10:03:00. Version ac04

Copyright 2018 A. Danko


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