Mount Airy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Airy Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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%
23398
20%
74674
11%
23347
47%
23347
24%
46976

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 10thNight
18%
757
23%
2621
7%
757
38%
757
20%
1603
January11th to 20thNight
23%
732
21%
2378
9%
732
38%
732
18%
1552
January21st to endNight
20%
792
23%
2574
8%
792
39%
792
22%
1582
February1st to 10thNight
23%
720
18%
2340
7%
720
36%
720
19%
1505
February11th to 20thNight
14%
702
19%
2230
3%
700
34%
700
18%
1426
February21st to endNight
14%
592
22%
1776
6%
592
34%
592
18%
1199
March1st to 10thNight
22%
720
24%
2019
8%
720
38%
720
26%
1264
March11th to 20thNight
24%
720
20%
1980
9%
720
35%
720
22%
1306
March21st to endNight
20%
792
20%
2142
6%
792
31%
792
24%
1432
April1st to 10thNight
23%
720
18%
1800
3%
720
32%
720
23%
1246
April11th to 20thNight
34%
711
19%
1775
6%
711
40%
711
30%
1181
April21st to endNight
31%
720
19%
1800
5%
720
38%
720
29%
1219
May1st to 10thNight
37%
585
17%
1665
9%
585
43%
585
24%
1121
May11th to 20thNight
38%
402
17%
1463
11%
402
46%
402
20%
948
May21st to endNight
53%
388
15%
1551
14%
388
59%
388
22%
940
June1st to 10thNight
52%
360
20%
1440
20%
360
60%
360
26%
814
June11th to 20thNight
52%
360
16%
1440
18%
360
57%
360
20%
891
June21st to endNight
60%
360
17%
1440
17%
360
67%
360
25%
807
July1st to 10thNight
60%
350
14%
1400
17%
350
63%
350
19%
888
July11th to 20thNight
69%
393
17%
1437
17%
393
72%
393
27%
828
July21st to endNight
69%
591
16%
1672
13%
558
72%
558
32%
1030
August1st to 10thNight
65%
537
16%
1600
13%
534
67%
534
27%
1055
August11th to 20thNight
62%
601
17%
1665
11%
601
63%
601
27%
1117
August21st to endNight
63%
792
19%
1980
12%
792
64%
792
34%
1309
September1st to 10thNight
56%
720
22%
1800
11%
720
59%
720
32%
1206
September11th to 20thNight
60%
720
30%
1800
21%
720
63%
720
39%
1122
September21st to endNight
46%
720
21%
2062
14%
720
53%
720
26%
1390
October1st to 10thNight
46%
720
18%
2160
16%
720
53%
720
27%
1395
October11th to 20thNight
43%
720
25%
2160
18%
720
55%
720
30%
1299
October21st to endNight
36%
792
26%
2376
16%
792
50%
792
28%
1455
November1st to 10thNight
40%
740
27%
2330
17%
740
55%
740
32%
1328
November11th to 20thNight
31%
760
25%
2502
13%
760
47%
760
25%
1485
November21st to endNight
31%
760
22%
2660
12%
760
46%
760
23%
1551
December1st to 10thNight
22%
760
19%
2660
6%
760
39%
760
18%
1696
December11th to 20thNight
23%
760
20%
3176
8%
759
38%
759
18%
1926
December21st to endNight
25%
829
14%
4800
9%
817
41%
817
12%
2860

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%6%35%5%35%5%6%8%41%
74678
Night
14%6%25%5%345%5%8%43%
75062
TransparencyDay
39%7%13%22%15%3
74388
Night
39%7%12%23%16%3
74871
SeeingDay
18%12%27%24%14%5%
13807
Night
17%10%30%29%12%1
23410

By Month

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 10thDay
14%8%36%6%25%5%5%6%42%
1540
11th to 20thDay
15%8%36%5%345%47%40%
1638
21st to endDay
15%10%36%5%35%447%39%
1782
February1st to 10thDay
15%7%37%6%36%5%7%5%36%
1620
11th to 20thDay
17%7%36%5%4446%6%39%
1616
21st to endDay
19%9%36%5%33445%39%
1480
March1st to 10thDay
12%5%35%5%34447%47%
1938
11th to 20thDay
16%8%35%5%34447%40%
1980
21st to endDay
16%7%37%6%45%45%6%38%
2214
April1st to 10thDay
13%7%35%5%35%47%9%40%
2160
11th to 20thDay
14%5%36%5%3445%6%46%
2134
21st to endDay
18%7%26%43445%7%40%
2160
May1st to 10thDay
15%7%35%5%35%5%5%8%38%
2310
11th to 20thDay
18%7%26%5%35%45%8%36%
2428
21st to endDay
14%5%36%6%346%5%9%41%
2725
June1st to 10thDay
12%5%345%35%5%6%11%42%
2520
11th to 20thDay
12%6%26%6%35%6%6%10%38%
2520
21st to endDay
10%424435%5%7%11%45%
2520
July1st to 10thDay
9%5%35%6%46%5%7%9%42%
2520
11th to 20thDay
6%3246%47%6%7%11%44%
2470
21st to endDay
10%5%25%7%37%7%7%9%37%
2547
August1st to 10thDay
10%5%36%6%46%7%8%9%37%
2327
11th to 20thDay
11%6%35%6%45%6%6%10%39%
2217
21st to endDay
11%5%246%45%5%7%9%42%
2376
September1st to 10thDay
15%5%25%5%3435%8%44%
2160
11th to 20thDay
12%5%25%42446%8%48%
2160
21st to endDay
17%7%345%2445%8%39%
1892
October1st to 10thDay
17%5%345%2345%8%44%
1800
11th to 20thDay
17%6%24434445%47%
1800
21st to endDay
14%6%25%5%2346%5%47%
1980
November1st to 10thDay
12%6%2433445%8%50%
1746
11th to 20thDay
14%5%2443445%6%48%
1676
21st to endDay
15%5%35%4345%5%6%45%
1520
December1st to 10thDay
16%8%25%435%5%6%7%37%
1520
11th to 20thDay
16%6%26%5%3445%8%40%
1910
21st to endDay
18%7%36%5%35%5%6%8%35%
2772
January1st to 10thNight
14%7%36%4345%5%7%42%
2621
11th to 20thNight
18%6%35%5%345%6%6%39%
2378
21st to endNight
14%7%35%5%25%45%6%44%
2574
February1st to 10thNight
16%6%36%5%3446%8%39%
2340
11th to 20thNight
18%7%36%324447%41%
2228
21st to endNight
18%8%26%6%24446%39%
1776
March1st to 10thNight
13%5%35%435%5%5%6%47%
2019
11th to 20thNight
17%6%25%4335%5%7%42%
1980
21st to endNight
14%7%36%434447%42%
2142
April1st to 10thNight
14%7%36%5%45%5%6%7%39%
1800
11th to 20thNight
12%6%26%5%3445%8%45%
1775
21st to endNight
17%6%245%25%5%6%7%42%
1800
May1st to 10thNight
15%7%36%5%345%6%8%38%
1665
11th to 20thNight
18%7%35%4345%7%8%37%
1463
21st to endNight
12%5%36%6%345%7%10%41%
1550
June1st to 10thNight
10%5%24435%6%5%10%45%
1440
11th to 20thNight
11%5%37%5%45%6%5%8%40%
1440
21st to endNight
9%4345%36%6%6%10%44%
1440
July1st to 10thNight
9%5%36%7%47%6%7%10%37%
1440
11th to 20thNight
6%415%6%36%5%8%9%47%
1485
21st to endNight
8%424446%5%8%13%42%
1771
August1st to 10thNight
9%5%26%5%5%6%6%7%10%39%
1611
11th to 20thNight
10%6%26%6%46%5%6%8%41%
1665
21st to endNight
10%5%25%5%36%5%6%9%43%
1980
September1st to 10thNight
14%7%25%4345%5%7%46%
1800
11th to 20thNight
9%5%36%5%23347%53%
1800
21st to endNight
17%6%25%5%35%45%6%42%
2062
October1st to 10thNight
19%5%25%4335%45%45%
2160
11th to 20thNight
15%5%25%333446%51%
2160
21st to endNight
17%5%2433445%7%47%
2376
November1st to 10thNight
12%5%24433347%53%
2330
11th to 20thNight
14%6%35%43445%5%49%
2502
21st to endNight
14%435%5%345%5%7%46%
2660
December1st to 10thNight
16%8%36%5%3445%6%39%
2660
11th to 20thNight
17%6%26%5%345%5%6%42%
3285
21st to endNight
16%7%25%5%35%5%7%9%35%
4884

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 10thDay
42%6%12%23%15%3
1541
11th to 20thDay
43%6%14%23%12%2
1638
21st to endDay
46%411%21%14%4
1782
February1st to 10thDay
46%7%13%23%10%2
1620
11th to 20thDay
44%5%10%25%13%2
1625
21st to endDay
47%5%11%20%13%4
1480
March1st to 10thDay
37%6%12%24%19%2
1938
11th to 20thDay
43%6%12%17%17%5%
1980
21st to endDay
46%6%10%18%16%3
2214
April1st to 10thDay
39%6%13%20%17%4
2160
11th to 20thDay
38%6%11%21%18%6%
2134
21st to endDay
42%6%11%20%17%4
2160
May1st to 10thDay
42%8%13%20%15%2
2310
11th to 20thDay
45%8%12%18%13%3
2432
21st to endDay
40%8%15%24%12%2
2725
June1st to 10thDay
35%8%15%25%14%2
2520
11th to 20thDay
39%11%14%21%13%2
2520
21st to endDay
31%10%16%25%17%1
2520
July1st to 10thDay
36%11%14%20%16%3
2450
11th to 20thDay
30%11%16%25%16%3
2395
21st to endDay
38%11%14%20%15%2
2405
August1st to 10thDay
39%10%16%23%11%1
2314
11th to 20thDay
40%8%15%21%14%2
2217
21st to endDay
36%8%15%24%14%3
2376
September1st to 10thDay
39%7%13%20%18%4
2160
11th to 20thDay
34%6%10%26%21%3
2160
21st to endDay
43%7%10%19%19%2
1892
October1st to 10thDay
39%6%11%24%18%3
1800
11th to 20thDay
38%6%12%23%20%2
1800
21st to endDay
36%6%10%26%19%2
1980
November1st to 10thDay
33%414%26%19%4
1746
11th to 20thDay
35%5%11%24%21%4
1676
21st to endDay
39%6%11%28%15%2
1520
December1st to 10thDay
44%6%14%23%11%2
1520
11th to 20thDay
40%5%13%30%10%2
1906
21st to endDay
43%9%16%22%10%
2772
January1st to 10thNight
40%6%10%24%16%4
2621
11th to 20thNight
44%6%14%22%13%1
2378
21st to endNight
40%411%27%16%2
2574
February1st to 10thNight
43%6%14%23%13%1
2340
11th to 20thNight
43%410%22%16%4
2230
21st to endNight
45%6%10%21%14%5%
1776
March1st to 10thNight
36%5%11%27%18%3
2019
11th to 20thNight
41%6%11%20%18%3
1980
21st to endNight
42%6%12%20%15%5%
2142
April1st to 10thNight
43%6%11%18%18%4
1800
11th to 20thNight
37%6%13%20%18%6%
1775
21st to endNight
39%7%12%23%14%5%
1800
May1st to 10thNight
42%7%13%21%13%4
1665
11th to 20thNight
44%7%12%18%17%2
1464
21st to endNight
36%10%19%21%12%2
1551
June1st to 10thNight
34%8%13%26%17%2
1440
11th to 20thNight
40%9%13%22%14%2
1440
21st to endNight
33%8%15%23%18%2
1440
July1st to 10thNight
41%10%16%17%12%4
1400
11th to 20thNight
30%12%15%25%16%3
1437
21st to endNight
30%11%18%22%17%2
1672
August1st to 10thNight
37%10%16%22%13%1
1600
11th to 20thNight
40%8%15%21%15%2
1665
21st to endNight
35%9%15%24%14%4
1980
September1st to 10thNight
38%8%11%21%18%5%
1800
11th to 20thNight
32%5%12%25%23%3
1800
21st to endNight
42%7%10%19%18%3
2062
October1st to 10thNight
41%5%12%22%17%3
2160
11th to 20thNight
35%6%9%27%21%2
2160
21st to endNight
37%5%10%26%20%3
2376
November1st to 10thNight
32%5%10%27%22%5%
2330
11th to 20thNight
37%5%9%23%21%5%
2502
21st to endNight
36%5%13%25%18%3
2660
December1st to 10thNight
44%5%12%25%12%1
2660
11th to 20thNight
41%6%11%22%17%3
3288
21st to endNight
41%9%17%22%10%1
4884

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 10thDay
27%25%30%15%31
191
11th to 20thDay
23%26%35%13%3
246
21st to endDay
22%30%31%13%31
264
February1st to 10thDay
25%23%39%10%3
240
11th to 20thDay
29%23%41%6%
283
21st to endDay
30%18%34%16%1
346
March1st to 10thDay
19%25%30%22%4
420
11th to 20thDay
26%20%30%18%42
420
21st to endDay
22%20%31%19%6%1
462
April1st to 10thDay
20%18%35%19%7%2
420
11th to 20thDay
17%17%32%16%14%5%
412
21st to endDay
26%13%32%19%8%2
420
May1st to 10thDay
23%16%29%18%10%4
423
11th to 20thDay
20%11%28%25%11%4
447
21st to endDay
17%5%22%26%22%7%
485
June1st to 10thDay
14%6%28%26%19%7%
450
11th to 20thDay
16%7%26%27%18%6%
450
21st to endDay
10%426%35%20%6%
450
July1st to 10thDay
10%418%31%23%13%
445
11th to 20thDay
5%218%32%33%11%
444
21st to endDay
13%418%33%25%7%
488
August1st to 10thDay
13%316%28%28%12%
446
11th to 20thDay
11%316%31%26%12%
439
21st to endDay
8%320%30%24%15%
495
September1st to 10thDay
18%420%22%21%14%
450
11th to 20thDay
13%321%29%22%12%
450
21st to endDay
21%9%23%28%15%5%
450
October1st to 10thDay
20%7%20%25%21%7%
450
11th to 20thDay
18%12%25%28%14%2
450
21st to endDay
15%11%37%26%9%2
495
November1st to 10thDay
14%12%30%28%13%3
402
11th to 20thDay
19%13%31%24%11%1
336
21st to endDay
19%20%36%15%9%
180
December1st to 10thDay
23%19%33%19%32
180
11th to 20thDay
24%23%25%21%7%
180
21st to endDay
24%16%35%20%6%
198
January1st to 10thNight
19%17%43%19%3
760
11th to 20thNight
21%16%33%27%3
732
21st to endNight
19%16%42%22%2
792
February1st to 10thNight
20%16%37%26%2
720
11th to 20thNight
24%18%39%17%1
703
21st to endNight
25%18%39%16%2
592
March1st to 10thNight
20%15%41%23%2
720
11th to 20thNight
22%15%35%22%5%1
720
21st to endNight
19%14%42%21%41
792
April1st to 10thNight
19%17%38%22%4
720
11th to 20thNight
15%12%35%30%8%1
711
21st to endNight
20%9%36%27%7%1
720
May1st to 10thNight
19%8%31%30%12%
585
11th to 20thNight
20%8%27%35%10%
402
21st to endNight
14%5%20%41%19%1
389
June1st to 10thNight
13%227%34%24%1
360
11th to 20thNight
12%323%43%18%1
360
21st to endNight
9%122%48%20%
360
July1st to 10thNight
10%315%42%27%3
350
11th to 20thNight
7%212%41%35%2
393
21st to endNight
9%114%45%28%3
591
August1st to 10thNight
10%212%43%30%3
537
11th to 20thNight
11%413%41%27%3
601
21st to endNight
12%314%39%28%4
792
September1st to 10thNight
16%318%32%27%4
720
11th to 20thNight
11%317%37%28%4
720
21st to endNight
21%422%33%19%1
720
October1st to 10thNight
21%423%30%20%3
720
11th to 20thNight
17%8%28%32%13%1
720
21st to endNight
19%9%33%28%11%
792
November1st to 10thNight
14%8%32%36%9%1
740
11th to 20thNight
18%14%33%28%6%1
760
21st to endNight
16%14%34%28%7%1
760
December1st to 10thNight
23%12%37%21%7%
760
11th to 20thNight
19%8%45%23%5%
760
21st to endNight
18%13%39%28%2
836
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