Hildene Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hildene Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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
25%
22945
13%
80539
7%
22847
33%
22847
13%
58360

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
10%
758
12%
4366
4%
737
19%
737
5%
3089
January11th to 20thNight
15%
758
7%
4404
4%
740
22%
740
4%
3558
January21st to endNight
14%
831
7%
4864
5%
814
23%
814
4%
3918
February1st to 10thNight
9%
760
10%
2466
3%
760
18%
760
7%
1928
February11th to 20thNight
11%
722
12%
2155
2%
719
23%
719
8%
1641
February21st to endNight
11%
592
14%
1776
3%
592
24%
592
10%
1329
March1st to 10thNight
15%
720
15%
2160
6%
720
26%
720
12%
1616
March11th to 20thNight
16%
720
13%
2107
5%
720
26%
720
12%
1576
March21st to endNight
21%
792
18%
1980
5%
792
31%
792
16%
1468
April1st to 10thNight
20%
720
14%
1800
4%
720
26%
720
16%
1360
April11th to 20thNight
21%
711
13%
1775
4%
711
27%
711
18%
1310
April21st to endNight
21%
594
14%
1674
4%
594
29%
594
14%
1254
May1st to 10thNight
23%
540
12%
1620
6%
540
29%
540
14%
1203
May11th to 20thNight
29%
357
16%
1419
12%
357
39%
357
14%
962
May21st to endNight
39%
388
13%
1551
12%
388
47%
388
16%
1013
June1st to 10thNight
35%
360
12%
1440
11%
360
44%
360
14%
967
June11th to 20thNight
42%
360
11%
1440
13%
360
49%
360
19%
925
June21st to endNight
40%
360
12%
1440
12%
360
48%
360
17%
914
July1st to 10thNight
47%
350
12%
1400
15%
350
53%
350
20%
890
July11th to 20thNight
50%
348
14%
1392
19%
348
53%
348
21%
864
July21st to endNight
46%
394
16%
1486
17%
372
53%
372
23%
879
August1st to 10thNight
53%
366
16%
1430
16%
364
59%
364
24%
831
August11th to 20thNight
47%
649
13%
1713
7%
649
48%
649
25%
1199
August21st to endNight
46%
792
18%
1980
9%
792
49%
792
31%
1325
September1st to 10thNight
43%
720
17%
1800
7%
720
46%
720
30%
1218
September11th to 20thNight
46%
720
23%
1908
14%
720
51%
720
31%
1234
September21st to endNight
38%
720
21%
1980
13%
720
45%
720
25%
1331
October1st to 10thNight
32%
720
14%
2038
9%
720
38%
720
18%
1472
October11th to 20thNight
28%
720
15%
2191
8%
720
37%
720
18%
1526
October21st to endNight
18%
792
11%
2574
5%
792
26%
792
10%
1954
November1st to 10thNight
23%
740
17%
2407
9%
740
34%
740
16%
1649
November11th to 20thNight
20%
760
17%
2470
8%
760
32%
760
13%
1761
November21st to endNight
16%
760
13%
2588
6%
760
25%
760
11%
1852
December1st to 10thNight
11%
760
8%
2660
3%
760
19%
760
7%
2099
December11th to 20thNight
14%
760
9%
3252
4%
759
21%
759
6%
2494
December21st to endNight
11%
831
6%
4833
2%
817
21%
817
4%
3751

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
17%12%5%8%6%35%45%6%30%
78160
Night
17%12%5%9%6%34445%30%
80912
TransparencyDay
53%6%11%19%10%1
77865
Night
54%6%11%18%10%1
80741
SeeingDay
23%20%29%17%7%2
13797
Night
22%21%27%22%8%1
22960

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 10thDay
12%18%9%13%7%3443323%
2517
11th to 20thDay
12%16%8%16%13%3333419%
2520
21st to endDay
16%19%10%12%7%35%44419%
2773
February1st to 10thDay
14%15%6%12%10%35%45%5%21%
1713
11th to 20thDay
10%15%8%16%8%45%33424%
1788
21st to endDay
17%15%7%10%7%34335%26%
1480
March1st to 10thDay
14%12%8%10%8%34445%27%
1797
11th to 20thDay
15%16%6%8%5%33445%30%
1850
21st to endDay
18%14%5%6%6%24335%34%
2376
April1st to 10thDay
17%15%5%8%433345%31%
2160
11th to 20thDay
15%12%5%7%6%35%5%45%34%
2134
21st to endDay
21%14%47%5%35%45%5%28%
2286
May1st to 10thDay
23%12%46%5%34445%30%
2351
11th to 20thDay
19%10%45%5%35%446%36%
2478
21st to endDay
17%7%36%6%35%5%5%8%36%
2725
June1st to 10thDay
20%8%47%6%36%6%6%7%27%
2520
11th to 20thDay
20%7%45%5%45%5%6%9%30%
2520
21st to endDay
21%8%45%6%46%6%6%8%28%
2520
July1st to 10thDay
15%7%36%6%45%5%5%8%37%
2520
11th to 20thDay
14%6%26%6%46%6%7%9%34%
2520
21st to endDay
16%6%46%5%46%6%6%8%33%
2744
August1st to 10thDay
14%6%36%7%46%6%7%9%33%
2480
11th to 20thDay
16%7%36%6%47%5%6%8%32%
2183
21st to endDay
15%7%36%5%35%6%6%8%37%
2376
September1st to 10thDay
16%6%345%25%6%6%7%39%
2160
11th to 20thDay
17%7%2443436%7%44%
2051
21st to endDay
16%8%245%35%5%5%7%39%
1980
October1st to 10thDay
23%11%46%5%3434432%
1921
11th to 20thDay
16%11%47%5%35%45%6%34%
1769
21st to endDay
24%15%46%6%3345%5%25%
1782
November1st to 10thDay
14%13%36%5%2345%7%38%
1671
11th to 20thDay
17%14%5%7%6%34445%32%
1710
21st to endDay
19%12%5%9%7%35%445%27%
1587
December1st to 10thDay
18%19%8%11%6%35%33420%
1520
11th to 20thDay
18%17%9%8%6%3332428%
1906
21st to endDay
17%20%6%11%7%3433421%
2772
January1st to 10thNight
14%13%8%15%6%34445%24%
4429
11th to 20thNight
11%16%10%14%13%5%443416%
4440
21st to endNight
19%17%7%13%8%5%5%42318%
4880
February1st to 10thNight
12%15%7%12%9%5%5%5%45%21%
2466
11th to 20thNight
11%12%7%10%10%5%7%5%5%5%24%
2153
21st to endNight
14%14%7%11%7%3445%5%26%
1776
March1st to 10thNight
15%12%5%9%9%45%445%27%
2150
11th to 20thNight
15%15%5%10%8%4444429%
2106
21st to endNight
18%13%47%5%34345%34%
1980
April1st to 10thNight
18%12%6%8%5%3445%7%29%
1800
11th to 20thNight
18%12%47%4245%5%6%32%
1775
21st to endNight
20%13%5%7%5%34436%31%
1674
May1st to 10thNight
22%14%46%6%32445%30%
1620
11th to 20thNight
17%10%47%6%4445%6%34%
1418
21st to endNight
17%6%36%5%36%46%8%36%
1550
June1st to 10thNight
21%8%35%5%45%46%6%33%
1440
11th to 20thNight
21%6%35%42445%8%38%
1440
21st to endNight
16%6%36%5%45%5%6%7%38%
1440
July1st to 10thNight
16%6%35%6%345%6%7%39%
1440
11th to 20thNight
15%5%35%5%45%5%6%8%38%
1440
21st to endNight
13%6%25%6%45%5%6%8%43%
1574
August1st to 10thNight
13%6%2344447%9%43%
1440
11th to 20thNight
14%8%35%5%46%6%6%8%36%
1713
21st to endNight
17%6%25%43445%8%43%
1980
September1st to 10thNight
16%7%35%4245%46%44%
1800
11th to 20thNight
18%5%2442345%5%48%
1908
21st to endNight
17%7%26%5%34445%43%
1980
October1st to 10thNight
23%9%36%5%34445%34%
2038
11th to 20thNight
16%11%37%5%35%6%5%6%34%
2191
21st to endNight
23%14%47%7%345%46%25%
2574
November1st to 10thNight
14%12%47%6%34445%36%
2407
11th to 20thNight
16%13%6%6%6%34446%32%
2470
21st to endNight
14%14%6%9%6%3445%6%28%
2588
December1st to 10thNight
18%18%9%11%6%34445%20%
2660
11th to 20thNight
19%17%6%11%6%34345%23%
3288
21st to endNight
20%17%8%13%6%3334420%
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
61%6%10%18%5%
2518
11th to 20thDay
68%5%6%15%5%1
2520
21st to endDay
68%48%14%5%1
2773
February1st to 10thDay
61%6%10%14%8%1
1713
11th to 20thDay
61%7%7%15%9%1
1797
21st to endDay
61%6%8%14%11%1
1480
March1st to 10thDay
57%5%11%15%10%3
1800
11th to 20thDay
56%411%12%14%3
1850
21st to endDay
54%47%16%15%4
2376
April1st to 10thDay
56%6%8%19%10%2
2160
11th to 20thDay
51%5%10%19%13%1
2134
21st to endDay
57%6%8%16%10%3
2286
May1st to 10thDay
56%5%6%19%12%1
2351
11th to 20thDay
49%6%9%21%13%2
2482
21st to endDay
46%7%15%21%9%1
2725
June1st to 10thDay
52%8%15%16%9%
2520
11th to 20thDay
49%6%12%19%13%1
2520
21st to endDay
53%7%13%19%8%
2520
July1st to 10thDay
45%7%12%22%13%1
2450
11th to 20thDay
43%7%16%26%8%
2445
21st to endDay
46%7%13%22%10%1
2591
August1st to 10thDay
44%10%14%22%9%
2466
11th to 20thDay
46%9%15%21%8%1
2183
21st to endDay
43%7%14%25%11%1
2376
September1st to 10thDay
41%8%12%28%11%1
2160
11th to 20thDay
40%6%10%25%18%1
2051
21st to endDay
42%7%11%24%14%2
1980
October1st to 10thDay
55%5%8%17%13%2
1921
11th to 20thDay
49%6%14%23%8%1
1769
21st to endDay
62%5%10%13%8%2
1782
November1st to 10thDay
45%7%13%22%13%1
1671
11th to 20thDay
54%6%9%16%14%2
1710
21st to endDay
57%5%14%16%6%1
1587
December1st to 10thDay
67%5%9%12%7%1
1520
11th to 20thDay
62%410%16%8%1
1906
21st to endDay
65%5%8%11%10%
2772
January1st to 10thNight
59%5%10%17%8%1
4429
11th to 20thNight
67%8%7%11%6%1
4440
21st to endNight
69%6%6%11%7%1
4880
February1st to 10thNight
62%6%10%13%6%2
2466
11th to 20thNight
58%7%10%15%10%1
2155
21st to endNight
57%6%10%17%8%2
1776
March1st to 10thNight
57%5%9%19%9%2
2160
11th to 20thNight
56%6%8%17%10%3
2107
21st to endNight
52%5%9%15%16%4
1980
April1st to 10thNight
53%6%10%19%11%1
1800
11th to 20thNight
51%6%10%20%12%1
1775
21st to endNight
55%7%8%16%11%3
1674
May1st to 10thNight
58%48%15%13%2
1620
11th to 20thNight
51%5%11%19%12%2
1419
21st to endNight
45%8%13%22%11%2
1551
June1st to 10thNight
50%6%14%20%10%
1440
11th to 20thNight
45%7%16%20%12%
1440
21st to endNight
44%8%15%23%10%1
1440
July1st to 10thNight
41%9%14%23%12%1
1400
11th to 20thNight
40%9%16%26%9%
1392
21st to endNight
36%9%16%26%11%2
1486
August1st to 10thNight
35%8%18%27%11%1
1430
11th to 20thNight
43%9%17%22%9%
1713
21st to endNight
39%7%13%28%12%1
1980
September1st to 10thNight
40%6%13%27%13%1
1800
11th to 20thNight
38%6%10%27%19%1
1908
21st to endNight
43%5%10%25%14%2
1980
October1st to 10thNight
51%6%10%21%11%2
2038
11th to 20thNight
47%7%15%21%9%1
2191
21st to endNight
59%6%10%16%7%2
2574
November1st to 10thNight
48%6%11%19%13%3
2407
11th to 20thNight
51%7%10%17%12%3
2470
21st to endNight
52%8%14%18%7%1
2588
December1st to 10thNight
65%6%10%13%6%
2660
11th to 20thNight
63%6%11%11%9%2
3288
21st to endNight
67%5%9%10%9%
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
29%26%29%12%4
180
11th to 20thDay
32%28%28%9%21
180
21st to endDay
21%34%32%10%41
199
February1st to 10thDay
34%39%21%5%1
273
11th to 20thDay
26%35%28%10%2
432
21st to endDay
31%28%27%12%2
346
March1st to 10thDay
28%27%30%14%2
420
11th to 20thDay
30%29%27%10%31
420
21st to endDay
32%24%24%11%6%2
462
April1st to 10thDay
33%23%25%12%5%1
420
11th to 20thDay
29%25%23%15%6%1
412
21st to endDay
30%20%30%13%42
420
May1st to 10thDay
33%21%27%11%7%2
423
11th to 20thDay
21%21%34%18%42
447
21st to endDay
20%17%29%22%9%2
485
June1st to 10thDay
20%19%33%18%9%1
450
11th to 20thDay
21%13%32%24%8%2
450
21st to endDay
20%17%34%19%8%2
450
July1st to 10thDay
16%12%31%25%14%2
445
11th to 20thDay
15%5%39%28%11%2
444
21st to endDay
13%14%39%24%9%1
488
August1st to 10thDay
11%12%31%28%13%5%
446
11th to 20thDay
15%12%31%22%14%6%
439
21st to endDay
15%14%26%27%11%6%
495
September1st to 10thDay
17%13%26%24%13%7%
450
11th to 20thDay
16%17%26%18%14%9%
450
21st to endDay
16%16%31%19%11%7%
450
October1st to 10thDay
28%16%24%16%12%4
450
11th to 20thDay
20%22%29%19%9%1
435
21st to endDay
32%18%28%18%41
396
November1st to 10thDay
25%22%25%19%8%1
365
11th to 20thDay
25%28%27%13%5%1
370
21st to endDay
28%26%33%10%3
247
December1st to 10thDay
32%30%21%13%4
180
11th to 20thDay
36%31%25%7%2
180
21st to endDay
35%24%28%11%2
198
January1st to 10thNight
21%30%32%15%2
760
11th to 20thNight
25%28%27%18%21
760
21st to endNight
17%30%31%18%3
836
February1st to 10thNight
21%31%33%15%1
760
11th to 20thNight
20%33%31%14%2
722
21st to endNight
25%30%30%13%2
592
March1st to 10thNight
23%24%32%18%2
720
11th to 20thNight
26%26%29%16%4
720
21st to endNight
25%25%24%19%7%1
792
April1st to 10thNight
24%24%28%20%4
720
11th to 20thNight
23%22%28%19%8%
711
21st to endNight
29%20%26%20%6%
594
May1st to 10thNight
27%17%26%22%8%
540
11th to 20thNight
20%18%27%28%7%
357
21st to endNight
17%11%27%31%13%1
389
June1st to 10thNight
18%13%28%28%12%1
360
11th to 20thNight
21%11%21%35%12%
360
21st to endNight
16%9%28%36%11%1
360
July1st to 10thNight
13%6%26%37%16%1
350
11th to 20thNight
15%5%23%39%16%2
348
21st to endNight
12%6%30%35%15%2
394
August1st to 10thNight
13%6%20%40%20%1
366
11th to 20thNight
15%8%22%32%19%4
649
21st to endNight
17%7%23%30%20%2
792
September1st to 10thNight
15%10%26%31%15%2
720
11th to 20thNight
18%10%22%28%17%6%
720
21st to endNight
20%13%24%26%14%3
720
October1st to 10thNight
26%14%20%24%13%3
720
11th to 20thNight
20%17%28%24%10%1
720
21st to endNight
28%22%26%19%5%1
792
November1st to 10thNight
21%22%29%21%7%1
740
11th to 20thNight
23%26%27%16%7%2
760
21st to endNight
22%26%30%14%6%1
760
December1st to 10thNight
28%30%26%14%3
760
11th to 20thNight
28%29%24%16%4
760
21st to endNight
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