University of Maryland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Maryland Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
37%
15089
22%
46528
11%
15050
49%
15050
26%
28630

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%
2428
10%
716
40%
716
20%
1490
January16th to endNight
23%
768
24%
2496
8%
768
41%
768
22%
1509
February1st to 15thNight
25%
703
20%
2287
7%
701
42%
701
21%
1383
February16th to endNight
19%
631
25%
1910
6%
631
42%
631
22%
1185
March1st to 15thNight
25%
720
22%
2013
9%
720
39%
720
25%
1268
March16th to endNight
22%
768
22%
2082
8%
768
34%
768
24%
1380
April1st to 15thNight
25%
718
20%
1796
5%
718
35%
718
27%
1167
April16th to endNight
37%
720
20%
1800
7%
720
43%
720
30%
1187
May1st to 15thNight
34%
559
18%
1639
8%
559
41%
559
23%
1085
May16th to endNight
51%
374
17%
1497
15%
374
58%
374
25%
845
June1st to 15thNight
51%
360
20%
1440
18%
360
60%
360
26%
798
June16th to endNight
52%
360
17%
1440
16%
360
61%
360
26%
807
July1st to 15thNight
56%
340
14%
1360
16%
340
59%
340
20%
836
July16th to endNight
68%
535
17%
1566
14%
502
73%
502
37%
879
August1st to 15thNight
65%
527
19%
1575
15%
524
66%
524
33%
938
August16th to endNight
59%
700
19%
1750
10%
700
60%
700
34%
1156
September1st to 15thNight
56%
660
27%
1650
15%
660
61%
660
35%
1078
September16th to endNight
52%
657
22%
1806
18%
657
55%
657
29%
1197
October1st to 15thNight
45%
660
25%
1980
21%
660
53%
660
27%
1259
October16th to endNight
35%
704
30%
2112
16%
704
52%
704
32%
1213
November1st to 15thNight
37%
700
26%
2233
15%
700
53%
700
28%
1310
November16th to endNight
32%
720
23%
2460
12%
720
48%
720
24%
1425
December1st to 15thNight
27%
720
19%
2520
8%
720
42%
720
19%
1558
December16th to endNight
23%
768
22%
2688
8%
768
41%
768
18%
1677

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%25%5%345%5%7%41%
47437
Night
14%5%2442445%7%43%
46710
Cloud CoverDay
14%5%245%3445%7%41%
47447
Night
14%5%2442445%7%43%
46714
TransparencyDay
40%411%24%16%2
47137
Night
39%411%24%17%3
46528
SeeingDay
18%9%25%25%14%6%
5543
Night
17%11%27%29%13%1
15094

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
17%4343245%46%41%
1507
16th to endDay
13%7%345%34446%42%
1728
February1st to 15thDay
14%6%25%5%245%5%7%39%
1575
16th to endDay
13%7%26%5%24435%41%
1563
March1st to 15thDay
15%435%5%24345%45%
1933
16th to endDay
16%5%35%5%24347%40%
2142
April1st to 15thDay
13%5%25%335%447%45%
2152
16th to endDay
17%5%15%423346%44%
2160
May1st to 15thDay
16%6%25%425%46%8%36%
2322
16th to endDay
14%5%25%6%35%5%6%8%38%
2618
June1st to 15thDay
11%325%436%5%7%9%41%
2520
16th to endDay
10%425%435%5%7%10%41%
2520
July1st to 15thDay
9%425%6%45%6%7%10%38%
2520
16th to endDay
7%325%5%37%7%8%11%36%
2492
August1st to 15thDay
10%5%25%7%36%6%6%8%35%
2277
16th to endDay
12%415%6%36%5%6%7%40%
2100
September1st to 15thDay
12%5%2442446%8%44%
1980
16th to endDay
15%7%2442446%7%39%
1821
October1st to 15thDay
18%5%2433235%6%45%
1650
16th to endDay
15%316%422445%48%
1760
November1st to 15thDay
15%5%2232445%7%47%
1624
16th to endDay
16%5%24333446%45%
1497
December1st to 15thDay
16%424325%45%6%43%
1440
16th to endDay
18%6%2442435%5%40%
1536
January1st to 15thNight
17%5%3442335%7%42%
2428
16th to endNight
16%6%2332445%5%44%
2496
February1st to 15thNight
14%6%2443445%5%45%
2285
16th to endNight
14%6%24424346%45%
1910
March1st to 15thNight
16%414324447%45%
2001
16th to endNight
15%6%35%4235%46%41%
2082
April1st to 15thNight
13%6%243245%6%6%43%
1796
16th to endNight
16%5%2332445%7%43%
1800
May1st to 15thNight
18%7%25%423448%38%
1639
16th to endNight
13%5%133345%7%9%41%
1495
June1st to 15thNight
10%423425%6%6%10%44%
1440
16th to endNight
8%4246%45%46%9%42%
1440
July1st to 15thNight
8%437%7%5%7%5%6%9%36%
1440
16th to endNight
6%214325%7%9%11%44%
1673
August1st to 15thNight
9%325%5%3446%8%45%
1586
16th to endNight
10%5%25%5%245%5%8%43%
1750
September1st to 15thNight
11%5%245%2445%8%44%
1650
16th to endNight
13%6%245%345%47%41%
1806
October1st to 15thNight
18%5%24423345%45%
1980
16th to endNight
13%412323347%51%
2112
November1st to 15thNight
13%5%24423335%51%
2233
16th to endNight
15%4234235%5%7%44%
2460
December1st to 15thNight
16%6%25%5%3445%6%40%
2520
16th to endNight
16%6%245%24446%41%
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
17%5%344145%46%42%
1507
16th to endDay
14%7%34434447%42%
1728
February1st to 15thDay
14%7%245%2445%7%40%
1575
16th to endDay
13%7%26%5%24436%42%
1563
March1st to 15thDay
15%5%245%34345%45%
1944
16th to endDay
16%6%35%434436%41%
2141
April1st to 15thDay
13%5%25%434347%45%
2152
16th to endDay
17%41442435%5%44%
2160
May1st to 15thDay
16%6%25%435%47%7%36%
2322
16th to endDay
14%4245%35%5%5%8%38%
2618
June1st to 15thDay
12%324425%5%7%9%41%
2520
16th to endDay
10%3245%35%5%7%10%41%
2520
July1st to 15thDay
9%5%246%36%5%7%10%37%
2520
16th to endDay
7%3246%37%7%8%11%36%
2492
August1st to 15thDay
10%6%26%7%36%6%6%8%35%
2277
16th to endDay
12%4146%36%5%6%8%39%
2100
September1st to 15thDay
12%5%2342446%7%45%
1980
16th to endDay
14%6%345%2445%7%39%
1821
October1st to 15thDay
18%5%24223346%45%
1650
16th to endDay
15%325%423446%48%
1760
November1st to 15thDay
14%5%1322345%6%48%
1624
16th to endDay
16%423423447%45%
1497
December1st to 15thDay
16%42432446%6%44%
1440
16th to endDay
18%7%24424345%41%
1536
January1st to 15thNight
17%5%34424347%42%
2428
16th to endNight
16%6%2432445%6%44%
2496
February1st to 15thNight
14%6%1443445%6%45%
2278
16th to endNight
14%6%24424346%45%
1910
March1st to 15thNight
16%42441435%7%45%
2013
16th to endNight
16%5%25%5%1445%5%41%
2082
April1st to 15thNight
14%5%244246%5%7%43%
1796
16th to endNight
17%42342445%8%43%
1800
May1st to 15thNight
19%6%34424447%38%
1638
16th to endNight
13%5%133245%7%10%41%
1495
June1st to 15thNight
10%423335%5%7%10%44%
1440
16th to endNight
8%4246%36%46%10%43%
1440
July1st to 15thNight
7%5%36%6%5%7%5%6%9%37%
1440
16th to endNight
6%223335%7%8%12%44%
1673
August1st to 15thNight
9%416%4345%6%9%45%
1586
16th to endNight
10%5%25%5%35%45%9%42%
1750
September1st to 15thNight
12%424435%36%7%45%
1650
16th to endNight
13%6%25%5%3445%6%41%
1806
October1st to 15thNight
18%5%24423345%45%
1980
16th to endNight
14%323323346%51%
2112
November1st to 15thNight
14%423423335%51%
2233
16th to endNight
16%41342445%7%45%
2460
December1st to 15thNight
16%5%25%5%2445%6%40%
2520
16th to endNight
17%5%2442445%6%41%
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
41%413%22%13%3
1508
16th to endDay
42%211%25%14%4
1728
February1st to 15thDay
42%410%26%14%2
1577
16th to endDay
43%310%26%14%1
1563
March1st to 15thDay
41%311%25%15%3
1944
16th to endDay
44%310%20%17%3
2142
April1st to 15thDay
39%39%24%18%4
2152
16th to endDay
40%39%19%21%6%
2160
May1st to 15thDay
44%411%21%15%2
2322
16th to endDay
41%414%24%12%2
2622
June1st to 15thDay
36%6%13%27%14%1
2520
16th to endDay
36%6%14%22%18%2
2520
July1st to 15thDay
39%5%13%22%15%3
2371
16th to endDay
35%7%14%25%15%2
2336
August1st to 15thDay
42%5%11%21%17%1
2264
16th to endDay
39%6%10%24%17%2
2100
September1st to 15thDay
36%413%21%21%3
1980
16th to endDay
42%47%23%20%1
1821
October1st to 15thDay
39%49%25%18%3
1650
16th to endDay
37%27%29%20%2
1760
November1st to 15thDay
35%312%23%19%5%
1624
16th to endDay
39%29%27%17%2
1497
December1st to 15thDay
39%315%25%12%3
1440
16th to endDay
45%311%25%13%1
1536
January1st to 15thNight
41%310%21%18%4
2428
16th to endNight
40%211%27%17%1
2496
February1st to 15thNight
39%211%25%17%3
2287
16th to endNight
39%39%26%19%2
1910
March1st to 15thNight
39%312%27%14%2
2013
16th to endNight
42%310%21%17%3
2082
April1st to 15thNight
38%49%21%22%3
1796
16th to endNight
38%410%21%18%7%
1800
May1st to 15thNight
45%311%21%14%3
1639
16th to endNight
35%6%20%20%14%2
1497
June1st to 15thNight
34%5%13%28%16%1
1440
16th to endNight
35%5%14%24%17%3
1440
July1st to 15thNight
41%6%15%18%12%4
1360
16th to endNight
27%7%17%28%17%2
1566
August1st to 15thNight
34%5%14%24%19%1
1575
16th to endNight
38%410%25%17%3
1750
September1st to 15thNight
37%410%22%20%4
1650
16th to endNight
41%47%22%20%3
1806
October1st to 15thNight
41%36%26%18%3
1980
16th to endNight
33%28%29%22%3
2112
November1st to 15thNight
36%27%24%21%7%
2233
16th to endNight
37%211%24%19%4
2460
December1st to 15thNight
43%312%23%14%2
2520
16th to endNight
43%411%25%13%2
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
24%23%41%9%1
73
16th to endDay
29%23%28%14%4
96
February1st to 15thDay
27%24%37%6%3
90
16th to endDay
19%19%40%18%2
222
March1st to 15thDay
21%15%32%22%6%
270
16th to endDay
22%13%36%18%5%3
288
April1st to 15thDay
19%14%32%21%9%1
270
16th to endDay
21%10%30%22%9%5%
270
May1st to 15thDay
29%17%20%18%9%4
288
16th to endDay
19%422%30%17%4
331
June1st to 15thDay
15%7%22%27%16%8%
315
16th to endDay
13%5%28%32%15%4
315
July1st to 15thDay
13%123%35%20%4
305
16th to endDay
9%116%33%27%10%
326
August1st to 15thDay
13%216%34%22%11%
297
16th to endDay
11%10%37%22%17%
268
September1st to 15thDay
12%415%29%20%17%
255
16th to endDay
23%120%25%13%14%
254
October1st to 15thDay
24%7%18%22%22%4
255
16th to endDay
15%14%29%26%12%1
272
November1st to 15thDay
18%12%25%23%14%5%
242
16th to endDay
21%12%33%20%10%1
117
December1st to 15thDay
20%31%21%15%10%1
60
16th to endDay
37%17%25%17%3
64
January1st to 15thNight
20%16%33%24%4
720
16th to endNight
20%18%34%25%1
768
February1st to 15thNight
20%17%31%27%2
704
16th to endNight
20%21%33%22%1
631
March1st to 15thNight
20%15%35%24%3
720
16th to endNight
19%15%38%19%5%1
768
April1st to 15thNight
17%18%35%23%4
718
16th to endNight
20%10%26%30%11%
720
May1st to 15thNight
21%8%27%30%11%
559
16th to endNight
14%421%38%18%1
375
June1st to 15thNight
11%5%25%37%19%
360
16th to endNight
10%322%46%16%
360
July1st to 15thNight
10%221%41%24%
340
16th to endNight
7%115%41%30%3
535
August1st to 15thNight
11%112%42%30%1
527
16th to endNight
13%215%35%29%4
700
September1st to 15thNight
14%218%31%28%3
660
16th to endNight
16%318%31%26%3
657
October1st to 15thNight
21%7%17%28%20%3
660
16th to endNight
16%13%30%26%12%
704
November1st to 15thNight
15%11%30%27%13%2
700
16th to endNight
19%15%27%26%9%1
720
December1st to 15thNight
20%14%31%23%9%
720
16th to endNight
21%15%32%26%3
768
Page generated at 2017-08-13 14:16:28. Version ac04

Copyright 2017 A. Danko


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