Island Grove Observatory.
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Island Grove Observatory. clear sky chart from 2005-11-06 to 2024-03-23.

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
34%
23022
12%
80621
9%
22934
40%
22934
15%
58520

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
17%
750
6%
4419
3%
737
25%
737
5%
3472
January11th to 20thNight
19%
755
6%
4220
3%
740
23%
740
4%
3396
January21st to endNight
25%
831
6%
4528
6%
814
32%
814
6%
3671
February1st to 10thNight
20%
760
9%
2470
4%
760
26%
760
10%
1966
February11th to 20thNight
22%
740
11%
2405
5%
740
30%
740
12%
1826
February21st to endNight
23%
628
11%
1996
5%
628
31%
628
14%
1494
March1st to 10thNight
25%
760
15%
2090
8%
760
31%
760
19%
1542
March11th to 20thNight
27%
760
17%
2089
9%
760
34%
760
19%
1510
March21st to endNight
34%
801
18%
2201
10%
801
40%
801
22%
1553
April1st to 10thNight
27%
720
14%
1831
6%
720
32%
720
20%
1386
April11th to 20thNight
35%
650
18%
1714
9%
650
40%
650
25%
1209
April21st to endNight
36%
540
16%
1620
9%
540
42%
540
20%
1153
May1st to 10thNight
45%
536
16%
1616
10%
536
51%
536
25%
1062
May11th to 20thNight
46%
354
21%
1416
21%
354
57%
354
21%
826
May21st to endNight
55%
388
17%
1551
19%
388
64%
388
28%
843
June1st to 10thNight
45%
360
13%
1440
12%
360
55%
360
19%
872
June11th to 20thNight
53%
360
15%
1386
18%
360
59%
360
24%
807
June21st to endNight
51%
360
14%
1260
17%
360
55%
360
23%
769
July1st to 10thNight
65%
350
17%
1288
22%
350
69%
350
32%
689
July11th to 20thNight
59%
348
18%
1392
25%
348
64%
348
27%
770
July21st to endNight
62%
536
17%
1621
16%
507
65%
507
32%
963
August1st to 10thNight
64%
537
18%
1600
15%
534
67%
534
35%
947
August11th to 20thNight
67%
534
22%
1598
15%
534
72%
534
36%
922
August21st to endNight
56%
665
19%
1853
11%
665
60%
665
29%
1216
September1st to 10thNight
54%
720
16%
1800
8%
720
56%
720
32%
1183
September11th to 20thNight
55%
720
25%
1898
15%
720
59%
720
35%
1183
September21st to endNight
48%
720
19%
2018
15%
720
51%
720
26%
1351
October1st to 10thNight
40%
720
15%
2160
12%
720
44%
720
20%
1525
October11th to 20thNight
29%
720
11%
2160
7%
720
35%
720
16%
1611
October21st to endNight
23%
792
13%
2376
8%
792
30%
792
12%
1867
November1st to 10thNight
31%
740
15%
2496
12%
740
37%
740
17%
1736
November11th to 20thNight
21%
760
12%
2660
7%
760
30%
760
11%
2012
November21st to endNight
21%
760
8%
2660
5%
760
25%
760
10%
2102
December1st to 10thNight
13%
760
5%
2660
2%
760
17%
760
7%
2215
December11th to 20thNight
15%
760
5%
3262
2%
759
20%
759
6%
2648
December21st to endNight
12%
827
3%
4867
1%
817
18%
817
3%
4223

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%10%48%7%45%5%6%7%31%
78988
Night
15%11%5%8%7%45%5%5%6%30%
80880
TransparencyDay
52%7%12%18%10%1
78702
Night
53%7%12%18%9%1
80703
SeeingDay
17%8%27%26%16%5%
13833
Night
19%8%29%30%12%1
23055

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
16%13%6%9%8%46%5%45%25%
2517
11th to 20thDay
10%15%7%15%12%5%5%5%5%5%15%
2715
21st to endDay
18%20%7%11%9%35%445%15%
3124
February1st to 10thDay
12%15%5%13%9%48%6%5%5%18%
1710
11th to 20thDay
12%12%6%12%8%45%45%6%24%
1663
21st to endDay
16%13%5%9%8%5%5%5%6%5%23%
1454
March1st to 10thDay
14%9%5%9%8%45%5%5%5%30%
2087
11th to 20thDay
11%12%6%9%7%45%45%5%31%
2090
21st to endDay
17%9%37%6%35%45%6%36%
2200
April1st to 10thDay
17%12%47%7%35%5%5%5%29%
2128
11th to 20thDay
18%10%35%5%3445%7%36%
2212
21st to endDay
19%9%47%6%3445%6%35%
2340
May1st to 10thDay
15%8%48%5%445%47%36%
2344
11th to 20thDay
13%6%36%6%345%6%7%41%
2478
21st to endDay
11%6%25%436%5%7%7%43%
2725
June1st to 10thDay
15%7%35%5%25%5%7%7%39%
2520
11th to 20thDay
12%6%25%5%35%5%7%9%41%
2565
21st to endDay
13%6%35%6%45%5%5%8%40%
2700
July1st to 10thDay
9%4246%45%5%6%9%46%
2628
11th to 20thDay
10%6%26%5%45%6%8%9%40%
2520
21st to endDay
10%6%35%6%46%7%7%9%38%
2599
August1st to 10thDay
10%6%35%6%36%6%7%10%40%
2327
11th to 20thDay
8%6%26%6%46%6%8%10%40%
2303
21st to endDay
11%6%26%6%35%6%7%9%39%
2484
September1st to 10thDay
14%6%35%5%36%5%6%8%40%
2160
11th to 20thDay
13%6%26%5%345%5%8%42%
2059
21st to endDay
13%7%36%7%45%6%5%7%37%
1940
October1st to 10thDay
19%9%36%6%445%5%6%33%
1800
11th to 20thDay
20%9%410%8%45%46%7%25%
1800
21st to endDay
25%14%48%6%34445%21%
1980
November1st to 10thDay
14%13%47%6%35%5%6%8%28%
1578
11th to 20thDay
13%15%6%9%8%36%5%6%6%22%
1520
21st to endDay
19%16%5%10%8%5%6%446%17%
1520
December1st to 10thDay
15%18%8%15%9%46%445%13%
1520
11th to 20thDay
17%13%5%10%8%5%6%6%5%5%19%
1906
21st to endDay
25%17%6%13%8%35%345%12%
2772
January1st to 10thNight
15%16%7%12%9%46%5%5%6%16%
4429
11th to 20thNight
12%11%7%14%13%5%7%5%45%16%
4244
21st to endNight
18%14%7%9%10%46%6%5%5%16%
4532
February1st to 10thNight
12%14%7%10%10%5%6%5%5%7%19%
2470
11th to 20thNight
12%13%6%10%8%46%5%5%6%23%
2403
21st to endNight
14%11%5%11%8%45%5%6%5%26%
1996
March1st to 10thNight
12%11%5%9%7%45%45%6%32%
2080
11th to 20thNight
13%11%6%7%7%345%45%34%
2089
21st to endNight
16%8%47%6%35%5%46%37%
2201
April1st to 10thNight
19%11%46%6%45%5%46%31%
1831
11th to 20thNight
17%7%36%6%3445%8%37%
1714
21st to endNight
16%10%47%6%445%46%35%
1620
May1st to 10thNight
17%6%35%5%35%46%8%39%
1616
11th to 20thNight
13%6%26%43445%8%44%
1415
21st to endNight
9%5%24435%46%7%51%
1550
June1st to 10thNight
12%6%45%5%45%5%6%8%40%
1440
11th to 20thNight
9%426%6%45%5%6%9%43%
1386
21st to endNight
10%6%35%6%3446%7%43%
1260
July1st to 10thNight
5%425%5%345%6%7%54%
1326
11th to 20thNight
10%5%334345%5%8%48%
1440
21st to endNight
8%434436%5%7%8%48%
1716
August1st to 10thNight
9%5%244245%6%10%49%
1611
11th to 20thNight
7%425%5%345%7%10%50%
1598
21st to endNight
10%6%25%5%45%5%6%7%45%
1853
September1st to 10thNight
11%7%246%35%5%5%8%45%
1800
11th to 20thNight
12%6%25%4235%5%7%49%
1898
21st to endNight
15%7%35%6%34445%43%
2018
October1st to 10thNight
17%7%36%7%45%447%35%
2160
11th to 20thNight
18%9%48%7%45%46%6%29%
2160
21st to endNight
22%10%48%6%45%5%5%6%24%
2376
November1st to 10thNight
17%10%47%6%445%6%7%32%
2496
11th to 20thNight
15%15%5%8%7%45%46%7%24%
2660
21st to endNight
19%14%5%10%6%46%5%5%6%20%
2660
December1st to 10thNight
18%16%8%13%7%45%5%5%5%15%
2660
11th to 20thNight
15%15%6%12%8%47%6%5%5%17%
3288
21st to endNight
26%17%6%14%7%3443411%
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
59%7%16%12%6%
2518
11th to 20thDay
70%6%11%9%4
2715
21st to endDay
71%6%10%8%41
3124
February1st to 10thDay
67%7%8%13%41
1710
11th to 20thDay
60%6%11%15%7%1
1672
21st to endDay
62%5%11%15%7%
1454
March1st to 10thDay
56%5%14%15%10%1
2090
11th to 20thDay
55%6%10%16%12%2
2090
21st to endDay
52%5%9%17%15%2
2200
April1st to 10thDay
56%5%10%17%10%1
2128
11th to 20thDay
48%6%11%20%15%1
2212
21st to endDay
51%6%11%17%12%3
2340
May1st to 10thDay
50%5%10%19%14%2
2344
11th to 20thDay
42%8%10%24%15%1
2482
21st to endDay
39%8%14%22%14%2
2725
June1st to 10thDay
43%8%12%21%15%1
2520
11th to 20thDay
39%8%13%23%16%1
2565
21st to endDay
42%7%15%23%13%1
2700
July1st to 10thDay
34%9%13%25%18%2
2558
11th to 20thDay
38%9%15%26%10%1
2445
21st to endDay
41%10%13%22%12%1
2454
August1st to 10thDay
37%10%16%23%11%2
2314
11th to 20thDay
37%10%17%24%11%
2303
21st to endDay
40%8%15%25%11%1
2484
September1st to 10thDay
41%8%13%26%10%1
2160
11th to 20thDay
39%7%14%27%12%1
2059
21st to endDay
45%8%14%26%8%
1940
October1st to 10thDay
51%6%12%18%11%1
1800
11th to 20thDay
57%7%12%18%5%1
1800
21st to endDay
64%7%12%12%5%
1980
November1st to 10thDay
53%8%14%17%8%1
1578
11th to 20thDay
60%9%11%12%8%1
1520
21st to endDay
67%7%11%12%2
1520
December1st to 10thDay
72%7%10%9%2
1520
11th to 20thDay
66%9%11%10%5%
1906
21st to endDay
77%5%8%8%1
2772
January1st to 10thNight
65%8%13%10%3
4429
11th to 20thNight
68%8%12%9%3
4244
21st to endNight
67%9%10%10%41
4532
February1st to 10thNight
63%7%10%15%41
2470
11th to 20thNight
59%6%12%16%7%
2405
21st to endNight
57%6%12%17%8%1
1996
March1st to 10thNight
54%5%13%18%9%1
2090
11th to 20thNight
51%5%11%20%11%2
2090
21st to endNight
48%6%9%20%15%2
2201
April1st to 10thNight
55%5%10%18%12%1
1831
11th to 20thNight
46%6%9%21%16%2
1714
21st to endNight
50%6%10%17%15%2
1620
May1st to 10thNight
43%6%11%20%17%3
1616
11th to 20thNight
39%6%9%24%18%3
1416
21st to endNight
32%7%16%25%17%4
1551
June1st to 10thNight
41%8%10%24%16%2
1440
11th to 20thNight
37%7%11%22%21%2
1386
21st to endNight
39%7%14%24%15%1
1260
July1st to 10thNight
28%7%11%29%22%2
1288
11th to 20thNight
33%7%12%31%16%1
1392
21st to endNight
31%9%14%29%15%2
1621
August1st to 10thNight
31%7%18%30%13%1
1600
11th to 20thNight
28%9%18%29%15%1
1598
21st to endNight
37%9%13%27%14%1
1853
September1st to 10thNight
37%7%15%26%13%1
1800
11th to 20thNight
34%6%13%31%16%1
1898
21st to endNight
42%7%13%27%11%
2018
October1st to 10thNight
47%9%11%18%14%1
2160
11th to 20thNight
54%7%13%18%7%
2160
21st to endNight
59%9%11%13%8%
2376
November1st to 10thNight
50%6%13%21%9%1
2496
11th to 20thNight
58%8%12%14%7%1
2660
21st to endNight
62%8%15%12%2
2660
December1st to 10thNight
70%6%11%10%2
2660
11th to 20thNight
67%8%11%11%3
3288
21st to endNight
77%6%8%7%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
16%12%39%23%11%1
180
11th to 20thDay
19%10%42%21%6%2
236
21st to endDay
21%9%35%27%6%1
297
February1st to 10thDay
18%13%45%17%6%1
270
11th to 20thDay
16%18%40%24%2
270
21st to endDay
24%19%32%23%2
238
March1st to 10thDay
19%10%38%25%8%
460
11th to 20thDay
17%14%36%25%8%1
460
21st to endDay
26%8%31%24%10%1
470
April1st to 10thDay
20%11%32%24%10%2
420
11th to 20thDay
26%9%28%20%13%5%
412
21st to endDay
22%9%26%27%12%4
420
May1st to 10thDay
18%6%26%31%14%5%
423
11th to 20thDay
18%5%22%31%18%6%
447
21st to endDay
14%421%30%22%8%
485
June1st to 10thDay
16%5%25%30%21%3
450
11th to 20thDay
14%6%21%30%22%6%
495
21st to endDay
15%6%24%33%17%5%
630
July1st to 10thDay
10%320%31%29%8%
553
11th to 20thDay
12%220%28%27%10%
444
21st to endDay
10%317%39%22%9%
488
August1st to 10thDay
10%316%28%29%13%
446
11th to 20thDay
9%315%30%28%14%
439
21st to endDay
13%422%28%23%11%
495
September1st to 10thDay
12%6%20%26%24%12%
450
11th to 20thDay
10%5%19%26%25%15%
450
21st to endDay
14%7%26%25%20%9%
450
October1st to 10thDay
21%6%26%23%16%8%
450
11th to 20thDay
23%10%34%24%7%2
450
21st to endDay
30%10%31%22%7%1
495
November1st to 10thDay
20%17%30%20%12%2
242
11th to 20thDay
21%19%31%22%6%2
180
21st to endDay
26%14%38%19%4
180
December1st to 10thDay
19%16%41%20%41
180
11th to 20thDay
19%21%36%17%6%1
180
21st to endDay
27%9%41%19%3
198
January1st to 10thNight
21%12%39%20%7%1
760
11th to 20thNight
23%10%34%26%7%1
760
21st to endNight
19%7%33%32%8%1
836
February1st to 10thNight
19%9%41%26%4
760
11th to 20thNight
20%10%37%28%4
743
21st to endNight
19%10%37%28%5%
628
March1st to 10thNight
18%9%36%30%6%
760
11th to 20thNight
18%11%36%27%7%1
760
21st to endNight
20%8%30%30%11%
801
April1st to 10thNight
22%9%33%29%6%
720
11th to 20thNight
19%8%32%29%11%2
650
21st to endNight
21%7%28%31%13%
540
May1st to 10thNight
18%324%36%16%2
536
11th to 20thNight
16%5%26%39%14%1
354
21st to endNight
11%423%40%21%1
389
June1st to 10thNight
13%6%28%38%14%1
360
11th to 20thNight
9%7%22%44%17%1
360
21st to endNight
13%321%42%20%1
360
July1st to 10thNight
8%315%47%25%1
350
11th to 20thNight
12%320%41%22%2
348
21st to endNight
10%318%45%24%1
536
August1st to 10thNight
11%213%42%30%2
537
11th to 20thNight
6%215%38%34%4
534
21st to endNight
12%420%35%26%3
665
September1st to 10thNight
12%5%21%35%22%6%
720
11th to 20thNight
15%420%31%23%7%
720
21st to endNight
18%6%20%31%19%5%
720
October1st to 10thNight
19%5%28%28%18%2
720
11th to 20thNight
23%10%30%28%8%1
720
21st to endNight
27%11%31%25%6%
792
November1st to 10thNight
24%10%28%26%12%1
740
11th to 20thNight
23%14%33%21%9%1
760
21st to endNight
27%12%33%24%4
760
December1st to 10thNight
27%13%37%18%41
760
11th to 20thNight
22%13%38%21%5%1
760
21st to endNight
28%11%35%22%3
836
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