Marshall Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marshall Hill Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
26%
22916
11%
75033
7%
22871
32%
22871
13%
56199

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
8%
758
5%
2649
2%
757
13%
757
3%
2298
January11th to 20thNight
8%
760
3%
2556
2%
760
10%
760
3%
2280
January21st to endNight
10%
820
6%
2664
2%
820
15%
820
4%
2343
February1st to 10thNight
6%
720
5%
2340
0%
720
11%
720
4%
2017
February11th to 20thNight
9%
702
7%
2275
1%
700
16%
700
6%
1897
February21st to endNight
11%
592
6%
1854
2%
592
16%
592
7%
1511
March1st to 10thNight
16%
720
10%
1994
5%
720
22%
720
11%
1581
March11th to 20thNight
19%
720
14%
1980
6%
720
26%
720
14%
1490
March21st to endNight
24%
792
16%
2174
6%
792
31%
792
17%
1610
April1st to 10thNight
20%
720
13%
1800
4%
720
26%
720
16%
1386
April11th to 20thNight
26%
711
15%
1775
5%
711
31%
711
20%
1283
April21st to endNight
28%
562
16%
1642
7%
562
35%
562
19%
1162
May1st to 10thNight
37%
512
14%
1592
9%
512
43%
512
20%
1076
May11th to 20thNight
36%
354
19%
1416
15%
354
47%
354
19%
861
May21st to endNight
45%
388
16%
1551
15%
388
55%
388
23%
889
June1st to 10thNight
38%
360
15%
1440
12%
360
51%
360
18%
922
June11th to 20thNight
44%
360
16%
1440
18%
360
53%
360
22%
847
June21st to endNight
39%
360
14%
1440
14%
360
49%
360
21%
859
July1st to 10thNight
53%
350
17%
1400
21%
350
60%
350
29%
733
July11th to 20thNight
50%
348
17%
1392
21%
348
57%
348
25%
774
July21st to endNight
52%
513
17%
1599
13%
485
57%
485
30%
948
August1st to 10thNight
55%
537
18%
1600
14%
534
59%
534
32%
977
August11th to 20thNight
54%
534
18%
1598
12%
534
57%
534
30%
997
August21st to endNight
48%
724
20%
1912
9%
724
53%
724
32%
1224
September1st to 10thNight
49%
720
17%
1800
8%
720
52%
720
30%
1212
September11th to 20thNight
51%
720
24%
1838
15%
720
55%
720
34%
1199
September21st to endNight
42%
720
18%
2056
14%
720
46%
720
23%
1399
October1st to 10thNight
33%
720
13%
2160
10%
720
38%
720
18%
1546
October11th to 20thNight
22%
720
10%
2160
6%
720
29%
720
14%
1619
October21st to endNight
18%
792
11%
2376
7%
792
25%
792
10%
1887
November1st to 10thNight
26%
740
17%
2439
11%
740
35%
740
15%
1710
November11th to 20thNight
18%
760
12%
2660
8%
760
25%
760
10%
2006
November21st to endNight
14%
760
7%
2660
4%
760
19%
760
7%
2149
December1st to 10thNight
9%
760
3%
2660
2%
760
11%
760
3%
2315
December11th to 20thNight
8%
760
3%
3263
0%
759
12%
759
3%
2810
December21st to endNight
7%
827
2%
4878
0%
817
11%
817
1%
4382

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%11%48%6%45%45%7%31%
74799
Night
17%11%5%8%7%45%5%5%6%28%
75242
TransparencyDay
52%7%13%18%10%1
74511
Night
56%8%11%16%9%1
75064
SeeingDay
23%16%30%20%9%3
13430
Night
24%16%28%21%9%1
22929

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
15%17%9%13%10%5%6%44412%
1517
11th to 20thDay
17%19%9%16%10%4444310%
1619
21st to endDay
13%19%8%17%11%7%6%43310%
1836
February1st to 10thDay
17%17%7%13%9%5%6%44413%
1620
11th to 20thDay
13%17%10%15%10%45%43415%
1573
21st to endDay
19%16%5%12%10%5%5%44415%
1401
March1st to 10thDay
15%13%6%13%10%45%44422%
1959
11th to 20thDay
14%15%5%8%6%44445%30%
1980
21st to endDay
19%12%48%6%34445%30%
2182
April1st to 10thDay
18%13%6%6%6%34446%31%
2160
11th to 20thDay
20%8%46%5%34446%37%
2134
21st to endDay
21%9%47%5%3445%5%34%
2333
May1st to 10thDay
16%9%36%6%35%5%6%8%34%
2366
11th to 20thDay
16%7%35%5%445%6%7%38%
2478
21st to endDay
12%6%25%5%35%5%6%7%44%
2725
June1st to 10thDay
15%6%35%6%25%5%5%8%40%
2520
11th to 20thDay
12%5%25%6%36%5%6%9%41%
2520
21st to endDay
14%6%25%435%47%9%41%
2520
July1st to 10thDay
9%5%245%35%46%9%49%
2520
11th to 20thDay
11%5%35%5%36%6%8%9%39%
2520
21st to endDay
10%5%35%7%47%6%7%10%38%
2621
August1st to 10thDay
9%5%25%5%45%6%7%9%42%
2327
11th to 20thDay
10%5%26%6%46%5%7%10%38%
2303
21st to endDay
12%6%35%5%345%6%9%42%
2426
September1st to 10thDay
15%6%36%5%3446%8%41%
2160
11th to 20thDay
14%5%25%4345%6%7%43%
2120
21st to endDay
15%6%36%6%35%47%9%36%
1904
October1st to 10thDay
22%8%35%6%36%45%6%32%
1800
11th to 20thDay
21%10%49%5%345%5%6%27%
1800
21st to endDay
27%13%46%7%34445%21%
1980
November1st to 10thDay
14%14%48%5%36%5%5%7%29%
1637
11th to 20thDay
15%16%6%9%6%35%5%5%7%25%
1520
21st to endDay
19%19%6%9%6%445%5%5%17%
1520
December1st to 10thDay
19%21%8%12%6%35%44413%
1520
11th to 20thDay
19%19%8%15%6%44345%12%
1906
21st to endDay
26%22%9%12%6%3433310%
2772
January1st to 10thNight
16%19%7%11%10%5%5%435%14%
2649
11th to 20thNight
17%19%10%14%10%4444410%
2556
21st to endNight
14%15%8%15%12%5%5%44412%
2664
February1st to 10thNight
17%18%7%13%9%46%435%15%
2340
11th to 20thNight
14%13%7%14%10%46%445%18%
2273
21st to endNight
17%15%6%13%7%345%46%18%
1854
March1st to 10thNight
14%14%5%11%8%445%46%25%
1984
11th to 20thNight
16%12%6%9%7%34445%30%
1979
21st to endNight
16%10%5%9%6%44445%33%
2174
April1st to 10thNight
19%11%6%6%6%3445%5%30%
1800
11th to 20thNight
17%9%35%6%35%5%5%6%35%
1775
21st to endNight
19%8%36%5%35%5%5%8%31%
1642
May1st to 10thNight
18%7%35%5%36%5%5%8%35%
1592
11th to 20thNight
13%5%27%6%35%6%5%7%41%
1415
21st to endNight
11%435%5%346%6%7%46%
1550
June1st to 10thNight
12%6%25%6%5%6%7%7%8%35%
1440
11th to 20thNight
12%425%6%45%5%5%8%43%
1440
21st to endNight
11%435%6%45%5%6%8%41%
1440
July1st to 10thNight
7%6%245%345%5%10%51%
1440
11th to 20thNight
11%4345%445%6%9%45%
1440
21st to endNight
10%5%245%246%6%9%47%
1693
August1st to 10thNight
9%5%245%35%46%10%46%
1611
11th to 20thNight
10%5%25%435%5%6%8%45%
1598
21st to endNight
10%6%25%5%35%5%6%8%46%
1912
September1st to 10thNight
13%6%26%5%2346%8%44%
1800
11th to 20thNight
15%4245%2445%7%47%
1838
21st to endNight
18%5%35%5%345%7%8%37%
2056
October1st to 10thNight
18%7%37%7%345%6%6%34%
2160
11th to 20thNight
22%10%46%5%445%5%7%28%
2160
21st to endNight
29%12%36%6%35%45%5%23%
2376
November1st to 10thNight
16%11%47%5%35%45%6%35%
2439
11th to 20thNight
15%12%5%8%7%45%5%6%7%25%
2660
21st to endNight
18%17%6%8%7%36%45%6%19%
2660
December1st to 10thNight
21%19%8%11%7%35%445%13%
2660
11th to 20thNight
17%20%8%12%7%45%5%5%5%14%
3288
21st to endNight
28%18%7%10%7%444448%
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
75%8%8%8%2
1518
11th to 20thDay
78%7%7%6%1
1619
21st to endDay
79%5%8%6%1
1836
February1st to 10thDay
74%6%10%9%1
1620
11th to 20thDay
74%6%7%10%3
1582
21st to endDay
72%6%10%8%4
1401
March1st to 10thDay
64%6%11%14%5%1
1962
11th to 20thDay
56%6%10%16%11%1
1980
21st to endDay
55%5%9%20%10%2
2182
April1st to 10thDay
54%5%11%18%10%1
2160
11th to 20thDay
48%5%10%21%14%2
2134
21st to endDay
52%5%10%18%13%2
2333
May1st to 10thDay
46%6%12%22%12%2
2366
11th to 20thDay
44%7%11%23%13%2
2482
21st to endDay
38%6%17%23%14%1
2725
June1st to 10thDay
42%6%13%22%16%1
2520
11th to 20thDay
38%7%14%25%14%1
2520
21st to endDay
37%7%16%24%16%1
2520
July1st to 10thDay
31%7%14%28%19%2
2450
11th to 20thDay
36%8%18%27%11%
2445
21st to endDay
38%9%15%23%14%1
2474
August1st to 10thDay
35%8%18%25%13%1
2314
11th to 20thDay
37%9%18%24%11%
2303
21st to endDay
37%6%16%29%11%1
2426
September1st to 10thDay
40%7%15%27%11%1
2160
11th to 20thDay
38%7%14%26%14%1
2120
21st to endDay
44%7%15%23%10%1
1904
October1st to 10thDay
51%7%13%17%11%1
1800
11th to 20thDay
56%7%14%16%6%1
1800
21st to endDay
65%5%13%11%6%1
1980
November1st to 10thDay
54%7%12%17%9%1
1637
11th to 20thDay
57%8%16%12%6%1
1520
21st to endDay
67%8%11%11%3
1520
December1st to 10thDay
74%9%9%7%1
1520
11th to 20thDay
76%8%10%5%2
1906
21st to endDay
80%7%7%6%
2772
January1st to 10thNight
74%8%7%8%2
2649
11th to 20thNight
78%6%9%5%1
2556
21st to endNight
75%8%7%8%3
2664
February1st to 10thNight
72%8%8%10%2
2340
11th to 20thNight
69%6%9%11%4
2275
21st to endNight
65%9%10%12%5%
1854
March1st to 10thNight
60%7%11%16%5%
1994
11th to 20thNight
56%5%12%17%9%1
1980
21st to endNight
54%6%8%19%12%2
2174
April1st to 10thNight
56%6%8%19%10%1
1800
11th to 20thNight
48%6%11%20%13%2
1775
21st to endNight
49%7%11%19%13%2
1642
May1st to 10thNight
46%6%11%20%15%2
1592
11th to 20thNight
41%7%12%24%13%3
1416
21st to endNight
35%8%14%22%18%3
1551
June1st to 10thNight
42%9%11%21%16%2
1440
11th to 20thNight
38%7%12%23%19%2
1440
21st to endNight
39%8%13%22%17%1
1440
July1st to 10thNight
30%6%13%26%22%3
1400
11th to 20thNight
34%7%14%27%16%2
1392
21st to endNight
33%8%15%27%15%2
1599
August1st to 10thNight
33%8%17%28%14%1
1600
11th to 20thNight
34%9%17%27%13%
1598
21st to endNight
34%9%14%28%13%2
1912
September1st to 10thNight
38%8%14%26%13%1
1800
11th to 20thNight
36%6%12%28%17%1
1838
21st to endNight
43%7%13%23%13%
2056
October1st to 10thNight
48%9%12%18%12%1
2160
11th to 20thNight
55%7%13%17%7%1
2160
21st to endNight
63%7%11%12%7%1
2376
November1st to 10thNight
49%8%11%19%11%2
2439
11th to 20thNight
57%9%12%14%8%1
2660
21st to endNight
64%8%13%12%2
2660
December1st to 10thNight
73%9%10%6%2
2660
11th to 20thNight
72%10%9%8%2
3288
21st to endNight
77%10%8%41
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
32%27%26%12%21
180
11th to 20thDay
35%30%25%8%11
227
21st to endDay
30%23%33%10%4
282
February1st to 10thDay
36%30%29%5%
240
11th to 20thDay
30%35%28%7%
240
21st to endDay
39%26%26%7%1
267
March1st to 10thDay
27%26%29%15%3
420
11th to 20thDay
28%22%32%14%31
420
21st to endDay
30%20%31%14%5%
462
April1st to 10thDay
28%21%31%16%5%
420
11th to 20thDay
30%20%25%16%7%1
412
21st to endDay
27%16%38%13%5%1
420
May1st to 10thDay
24%14%33%18%7%3
423
11th to 20thDay
19%12%34%24%8%3
447
21st to endDay
17%11%30%26%13%3
485
June1st to 10thDay
18%12%34%27%8%1
450
11th to 20thDay
16%14%28%29%12%2
450
21st to endDay
16%10%35%26%10%2
450
July1st to 10thDay
10%7%26%33%19%4
445
11th to 20thDay
15%6%26%33%16%4
444
21st to endDay
11%8%35%31%11%3
488
August1st to 10thDay
9%7%29%33%17%6%
446
11th to 20thDay
12%6%27%33%15%6%
439
21st to endDay
12%8%32%29%14%5%
495
September1st to 10thDay
14%12%25%28%15%6%
450
11th to 20thDay
14%8%26%25%18%10%
450
21st to endDay
15%14%30%22%13%6%
450
October1st to 10thDay
26%14%24%18%13%5%
450
11th to 20thDay
27%19%30%17%5%1
450
21st to endDay
36%18%27%13%6%
495
November1st to 10thDay
30%18%31%13%8%1
315
11th to 20thDay
33%22%27%13%32
180
21st to endDay
34%23%28%13%2
180
December1st to 10thDay
39%21%28%9%31
180
11th to 20thDay
39%21%31%8%1
180
21st to endDay
39%27%25%8%1
198
January1st to 10thNight
32%27%26%12%3
760
11th to 20thNight
34%23%28%13%2
760
21st to endNight
26%26%30%15%3
820
February1st to 10thNight
32%28%28%11%1
720
11th to 20thNight
28%26%31%12%2
703
21st to endNight
30%23%32%13%2
592
March1st to 10thNight
25%24%30%17%3
720
11th to 20thNight
28%18%30%18%41
720
21st to endNight
22%15%34%20%8%1
792
April1st to 10thNight
28%17%30%22%3
720
11th to 20thNight
23%17%30%22%7%1
711
21st to endNight
25%10%31%25%8%
562
May1st to 10thNight
20%12%25%26%15%2
512
11th to 20thNight
15%8%34%33%10%1
354
21st to endNight
11%9%30%32%16%2
389
June1st to 10thNight
16%7%32%31%13%1
360
11th to 20thNight
13%9%28%34%15%1
360
21st to endNight
12%6%34%36%12%
360
July1st to 10thNight
9%5%26%37%22%2
350
11th to 20thNight
12%428%35%19%3
348
21st to endNight
11%428%39%17%2
513
August1st to 10thNight
11%424%35%23%4
537
11th to 20thNight
11%324%35%22%5%
534
21st to endNight
12%5%27%31%22%2
724
September1st to 10thNight
14%9%22%31%20%3
720
11th to 20thNight
17%6%21%28%23%6%
720
21st to endNight
20%12%21%26%16%5%
720
October1st to 10thNight
22%12%26%21%16%2
720
11th to 20thNight
29%17%29%18%7%1
720
21st to endNight
38%14%25%19%4
792
November1st to 10thNight
28%14%29%20%7%1
740
11th to 20thNight
25%22%30%14%7%2
760
21st to endNight
33%22%25%14%5%
760
December1st to 10thNight
37%24%24%11%3
760
11th to 20thNight
32%25%28%12%2
760
21st to endNight
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836
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