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Orchard Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Orchard Hill Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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
27%
22626
11%
74770
7%
22584
32%
22584
13%
56408

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
12%
760
5%
2649
3%
757
15%
757
5%
2275
January11th to 20thNight
14%
741
6%
2591
3%
741
18%
741
6%
2137
January21st to endNight
19%
792
9%
2772
5%
792
24%
792
9%
2187
February1st to 10thNight
13%
720
6%
2380
2%
720
19%
720
7%
2000
February11th to 20thNight
17%
703
8%
2149
3%
700
23%
700
8%
1759
February21st to endNight
20%
592
12%
1776
5%
592
29%
592
11%
1407
March1st to 10thNight
22%
720
12%
2160
8%
720
28%
720
12%
1706
March11th to 20thNight
21%
720
13%
2103
6%
720
28%
720
12%
1611
March21st to endNight
24%
792
17%
1980
7%
792
31%
792
17%
1510
April1st to 10thNight
23%
720
13%
1800
3%
720
27%
720
16%
1416
April11th to 20thNight
26%
711
11%
1775
5%
711
29%
711
17%
1364
April21st to endNight
23%
568
14%
1648
6%
568
30%
568
15%
1237
May1st to 10thNight
29%
462
15%
1542
11%
462
35%
462
15%
1091
May11th to 20thNight
31%
354
17%
1416
16%
354
42%
354
16%
928
May21st to endNight
38%
388
11%
1551
12%
388
46%
388
17%
993
June1st to 10thNight
31%
360
10%
1440
12%
360
38%
360
13%
1001
June11th to 20thNight
38%
360
11%
1440
14%
360
44%
360
18%
923
June21st to endNight
40%
360
8%
1440
11%
360
44%
360
15%
983
July1st to 10thNight
46%
350
12%
1400
16%
350
53%
350
21%
872
July11th to 20thNight
55%
348
13%
1392
19%
348
59%
348
22%
845
July21st to endNight
50%
394
12%
1486
16%
372
53%
372
19%
947
August1st to 10thNight
56%
358
16%
1422
18%
356
58%
356
23%
860
August11th to 20thNight
52%
561
15%
1625
10%
561
54%
561
25%
1108
August21st to endNight
45%
792
15%
1980
8%
792
47%
792
28%
1386
September1st to 10thNight
44%
720
12%
1800
6%
720
46%
720
28%
1245
September11th to 20thNight
48%
720
18%
1894
12%
720
50%
720
31%
1226
September21st to endNight
40%
720
20%
1980
15%
720
43%
720
25%
1355
October1st to 10thNight
31%
720
14%
2068
9%
720
36%
720
18%
1503
October11th to 20thNight
27%
720
13%
2207
8%
720
33%
720
15%
1661
October21st to endNight
20%
792
9%
2574
5%
792
27%
792
11%
2018
November1st to 10thNight
21%
740
13%
2407
7%
740
28%
740
13%
1769
November11th to 20thNight
21%
760
15%
2490
11%
760
27%
760
11%
1893
November21st to endNight
16%
760
8%
2660
5%
760
21%
760
8%
2146
December1st to 10thNight
12%
760
3%
2660
1%
760
14%
760
4%
2296
December11th to 20thNight
14%
760
6%
3259
2%
759
18%
759
5%
2713
December21st to endNight
11%
828
3%
4854
1%
817
14%
817
3%
4037

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
18%9%47%6%35%5%5%7%31%
74792
Night
19%11%47%6%35%45%6%30%
75000
TransparencyDay
54%7%12%18%8%1
74497
Night
58%7%11%16%8%1
74829
SeeingDay
25%17%29%20%8%2
13549
Night
26%14%27%24%9%1
22635

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
19%19%6%10%7%45%5%4418%
1517
11th to 20thDay
19%16%6%11%8%44445%19%
1480
21st to endDay
11%16%5%10%7%45%5%5%6%24%
1584
February1st to 10thDay
17%14%5%13%9%45%5%4419%
1580
11th to 20thDay
14%16%6%11%7%445%45%24%
1699
21st to endDay
17%15%5%8%8%5%445%6%22%
1480
March1st to 10thDay
16%14%5%9%7%35%5%46%26%
1797
11th to 20thDay
18%12%5%7%7%35%446%29%
1853
21st to endDay
20%11%48%5%34345%33%
2376
April1st to 10thDay
22%12%37%6%44445%30%
2160
11th to 20thDay
21%8%36%5%3445%5%36%
2134
21st to endDay
23%8%46%6%35%446%31%
2311
May1st to 10thDay
20%7%46%5%345%5%6%35%
2434
11th to 20thDay
18%7%35%6%35%45%6%37%
2478
21st to endDay
16%6%35%7%35%6%7%8%34%
2725
June1st to 10thDay
23%7%36%5%36%5%5%8%30%
2520
11th to 20thDay
16%7%35%5%45%5%6%8%37%
2520
21st to endDay
20%7%36%7%45%5%6%7%31%
2520
July1st to 10thDay
13%5%37%5%35%5%6%8%40%
2520
11th to 20thDay
13%6%36%6%35%6%7%9%35%
2520
21st to endDay
14%6%25%5%45%6%6%10%36%
2744
August1st to 10thDay
13%5%37%6%46%7%7%7%36%
2506
11th to 20thDay
12%5%26%5%35%6%6%10%39%
2253
21st to endDay
14%5%36%5%35%46%8%41%
2376
September1st to 10thDay
14%5%35%435%5%6%7%43%
2160
11th to 20thDay
17%414434347%47%
2066
21st to endDay
14%5%35%5%35%5%5%9%40%
1980
October1st to 10thDay
22%7%36%7%35%5%5%7%30%
1890
11th to 20thDay
18%9%46%6%36%6%5%6%30%
1751
21st to endDay
25%12%47%7%44445%26%
1782
November1st to 10thDay
19%10%37%6%345%5%6%31%
1671
11th to 20thDay
19%11%49%6%45%45%6%27%
1687
21st to endDay
21%13%5%8%6%45%6%5%6%21%
1520
December1st to 10thDay
23%16%6%10%6%45%345%18%
1520
11th to 20thDay
22%14%5%8%6%35%5%46%22%
1906
21st to endDay
26%15%6%11%8%3444416%
2772
January1st to 10thNight
18%17%7%11%7%44445%17%
2649
11th to 20thNight
18%17%7%10%8%36%44420%
2591
21st to endNight
13%14%6%10%8%45%445%27%
2772
February1st to 10thNight
17%15%7%12%9%5%5%44418%
2380
11th to 20thNight
15%12%6%9%9%5%5%5%46%25%
2147
21st to endNight
17%14%5%9%6%4444429%
1776
March1st to 10thNight
18%14%6%8%6%3444427%
2150
11th to 20thNight
21%13%5%6%6%3443430%
2102
21st to endNight
21%10%36%6%34445%34%
1980
April1st to 10thNight
23%10%37%6%35%35%6%28%
1800
11th to 20thNight
24%10%36%6%34435%33%
1775
21st to endNight
21%9%47%5%45%445%33%
1648
May1st to 10thNight
22%9%37%5%24447%33%
1542
11th to 20thNight
18%7%37%443447%38%
1415
21st to endNight
15%6%35%445%5%5%9%39%
1550
June1st to 10thNight
24%7%35%5%345%6%7%30%
1440
11th to 20thNight
18%6%25%6%25%5%6%8%37%
1440
21st to endNight
18%6%36%5%45%5%6%8%34%
1440
July1st to 10thNight
11%6%346%45%5%6%9%41%
1440
11th to 20thNight
10%5%346%36%6%7%10%40%
1440
21st to endNight
14%435%6%45%6%7%9%38%
1574
August1st to 10thNight
12%5%245%346%6%10%42%
1432
11th to 20thNight
13%6%45%6%45%5%5%7%40%
1625
21st to endNight
13%6%36%5%35%45%8%42%
1980
September1st to 10thNight
14%436%6%345%5%8%43%
1800
11th to 20thNight
15%6%245%2345%6%48%
1894
21st to endNight
18%5%25%5%345%46%43%
1980
October1st to 10thNight
19%7%37%6%35%45%6%35%
2068
11th to 20thNight
21%9%36%6%45%5%5%6%31%
2207
21st to endNight
25%10%36%6%4445%5%26%
2574
November1st to 10thNight
21%9%36%6%45%446%31%
2407
11th to 20thNight
19%10%47%7%34446%31%
2490
21st to endNight
23%13%48%6%35%5%5%6%23%
2660
December1st to 10thNight
21%17%7%10%8%45%5%4416%
2660
11th to 20thNight
24%15%5%9%7%35%44420%
3288
21st to endNight
27%15%5%10%7%3333419%
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
72%6%9%11%2
1518
11th to 20thDay
67%7%9%10%6%1
1480
21st to endDay
64%6%12%12%5%1
1584
February1st to 10thDay
69%6%7%13%41
1580
11th to 20thDay
65%6%10%13%5%1
1708
21st to endDay
66%5%9%11%7%1
1480
March1st to 10thDay
63%5%9%14%6%2
1800
11th to 20thDay
60%5%11%11%10%1
1853
21st to endDay
58%48%18%10%3
2376
April1st to 10thDay
58%5%8%19%8%2
2160
11th to 20thDay
52%6%7%19%14%2
2134
21st to endDay
57%5%10%16%10%3
2311
May1st to 10thDay
52%5%9%19%13%1
2434
11th to 20thDay
47%6%10%20%15%2
2482
21st to endDay
46%9%14%18%11%2
2725
June1st to 10thDay
53%7%13%17%9%1
2520
11th to 20thDay
44%9%11%22%12%2
2520
21st to endDay
52%8%12%20%8%1
2520
July1st to 10thDay
40%9%14%24%12%
2450
11th to 20thDay
42%10%14%26%8%
2445
21st to endDay
44%10%16%22%8%1
2591
August1st to 10thDay
43%10%17%22%7%
2492
11th to 20thDay
42%9%19%22%9%
2253
21st to endDay
41%8%14%24%12%1
2376
September1st to 10thDay
39%8%13%27%12%1
2160
11th to 20thDay
38%6%11%31%13%1
2066
21st to endDay
42%8%16%23%9%2
1980
October1st to 10thDay
56%7%9%19%9%
1890
11th to 20thDay
53%9%15%17%7%
1751
21st to endDay
65%6%9%14%5%1
1782
November1st to 10thDay
57%7%12%17%7%
1671
11th to 20thDay
62%7%11%12%7%1
1687
21st to endDay
65%8%13%11%3
1520
December1st to 10thDay
73%6%10%9%3
1520
11th to 20thDay
65%8%10%12%4
1906
21st to endDay
71%10%10%7%2
2772
January1st to 10thNight
75%5%10%8%2
2649
11th to 20thNight
70%6%10%9%4
2591
21st to endNight
65%6%11%12%7%1
2772
February1st to 10thNight
71%6%8%9%5%
2380
11th to 20thNight
67%7%8%12%6%
2149
21st to endNight
63%5%9%14%8%1
1776
March1st to 10thNight
63%5%9%15%7%1
2160
11th to 20thNight
60%5%8%16%9%2
2103
21st to endNight
56%47%18%11%2
1980
April1st to 10thNight
60%5%9%15%9%1
1800
11th to 20thNight
56%5%9%17%12%1
1775
21st to endNight
55%6%8%15%13%3
1648
May1st to 10thNight
54%5%6%17%15%3
1542
11th to 20thNight
47%7%9%19%16%2
1416
21st to endNight
43%9%15%19%13%2
1551
June1st to 10thNight
52%7%11%20%8%1
1440
11th to 20thNight
45%7%14%21%11%3
1440
21st to endNight
48%9%14%18%10%
1440
July1st to 10thNight
40%9%15%21%14%1
1400
11th to 20thNight
38%9%16%26%10%1
1392
21st to endNight
40%11%16%23%10%
1486
August1st to 10thNight
38%9%19%26%9%
1422
11th to 20thNight
42%8%15%22%12%
1625
21st to endNight
41%8%14%24%12%
1980
September1st to 10thNight
41%8%13%26%11%1
1800
11th to 20thNight
37%6%15%30%11%1
1894
21st to endNight
41%9%12%24%12%2
1980
October1st to 10thNight
52%6%12%21%9%
2068
11th to 20thNight
55%7%12%19%6%
2207
21st to endNight
62%7%11%13%7%1
2574
November1st to 10thNight
55%6%13%17%8%1
2407
11th to 20thNight
58%7%11%15%7%2
2490
21st to endNight
65%7%11%13%3
2660
December1st to 10thNight
75%6%9%8%1
2660
11th to 20thNight
71%7%9%10%3
3288
21st to endNight
73%6%8%10%2
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%22%26%17%6%1
180
11th to 20thDay
32%25%29%10%31
170
21st to endDay
26%18%35%18%4
176
February1st to 10thDay
34%32%27%6%1
222
11th to 20thDay
28%28%33%10%2
367
21st to endDay
28%22%32%16%2
346
March1st to 10thDay
32%18%29%19%2
420
11th to 20thDay
25%26%27%16%41
420
21st to endDay
34%20%23%15%6%1
462
April1st to 10thDay
31%19%25%19%41
420
11th to 20thDay
29%21%23%16%8%3
412
21st to endDay
32%18%29%15%5%2
420
May1st to 10thDay
29%18%27%17%7%2
423
11th to 20thDay
23%16%33%19%8%1
447
21st to endDay
20%16%32%22%9%1
485
June1st to 10thDay
28%12%34%18%6%1
450
11th to 20thDay
20%16%28%23%10%2
450
21st to endDay
24%12%34%20%9%1
450
July1st to 10thDay
17%8%31%30%12%2
445
11th to 20thDay
15%9%35%26%12%2
444
21st to endDay
18%6%34%31%9%1
488
August1st to 10thDay
14%10%31%30%13%2
446
11th to 20thDay
15%9%27%30%15%4
439
21st to endDay
15%12%31%24%11%6%
495
September1st to 10thDay
17%13%30%21%13%7%
450
11th to 20thDay
18%14%21%23%17%7%
450
21st to endDay
14%14%31%23%13%5%
450
October1st to 10thDay
26%15%24%15%15%3
450
11th to 20thDay
28%19%24%20%7%2
426
21st to endDay
38%15%26%17%3
396
November1st to 10thDay
31%19%28%15%6%1
365
11th to 20thDay
29%29%23%13%6%1
347
21st to endDay
35%22%26%16%2
180
December1st to 10thDay
28%24%29%16%2
180
11th to 20thDay
27%28%34%10%1
180
21st to endDay
30%28%28%12%21
198
January1st to 10thNight
29%22%30%16%3
760
11th to 20thNight
29%21%27%18%4
741
21st to endNight
22%16%33%24%41
792
February1st to 10thNight
25%17%33%22%2
720
11th to 20thNight
24%19%33%20%3
703
21st to endNight
24%20%30%22%4
592
March1st to 10thNight
28%17%28%25%21
720
11th to 20thNight
30%19%25%21%6%
720
21st to endNight
28%17%24%21%8%1
792
April1st to 10thNight
31%13%27%24%5%
720
11th to 20thNight
30%16%23%23%8%
711
21st to endNight
30%14%28%21%6%
568
May1st to 10thNight
29%10%27%25%8%
462
11th to 20thNight
23%10%31%27%8%1
354
21st to endNight
19%8%29%34%10%1
389
June1st to 10thNight
28%7%25%30%9%1
360
11th to 20thNight
18%8%29%30%15%1
360
21st to endNight
20%7%26%37%10%1
360
July1st to 10thNight
14%5%29%37%15%
350
11th to 20thNight
10%324%44%17%1
348
21st to endNight
16%321%44%15%
394
August1st to 10thNight
15%220%44%19%1
358
11th to 20thNight
14%5%21%36%21%3
561
21st to endNight
16%7%23%33%20%1
792
September1st to 10thNight
17%7%26%32%17%2
720
11th to 20thNight
19%8%20%30%20%3
720
21st to endNight
20%10%26%23%18%2
720
October1st to 10thNight
28%12%24%22%14%1
720
11th to 20thNight
28%12%27%24%8%1
720
21st to endNight
32%14%28%22%4
792
November1st to 10thNight
29%16%29%19%6%1
740
11th to 20thNight
29%18%27%18%7%1
760
21st to endNight
33%20%25%16%5%1
760
December1st to 10thNight
34%20%27%15%4
760
11th to 20thNight
29%21%28%17%5%1
760
21st to endNight
32%21%28%17%3
836
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