Hilltop Homeschool Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hilltop Homeschool Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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%
24662
18%
74690
11%
24604
44%
24604
24%
49137

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
27%
950
14%
2648
5%
946
35%
946
21%
1844
January11th to 20thNight
23%
918
11%
2515
4%
918
28%
918
15%
1940
January21st to endNight
23%
990
13%
2574
4%
990
30%
990
17%
1936
February1st to 10thNight
21%
900
13%
2340
3%
900
27%
900
19%
1722
February11th to 20thNight
26%
855
18%
2252
7%
853
32%
853
24%
1546
February21st to endNight
21%
592
19%
1776
5%
592
34%
592
20%
1213
March1st to 10thNight
24%
658
17%
2098
8%
658
33%
658
22%
1361
March11th to 20thNight
32%
540
19%
1980
14%
540
41%
540
20%
1253
March21st to endNight
24%
594
10%
2107
6%
594
31%
594
13%
1551
April1st to 10thNight
24%
540
17%
1800
10%
540
35%
540
21%
1176
April11th to 20thNight
27%
533
19%
1775
13%
533
40%
533
20%
1179
April21st to endNight
34%
540
17%
1787
11%
540
45%
540
22%
1173
May1st to 10thNight
30%
540
17%
1535
9%
540
40%
540
25%
1042
May11th to 20thNight
39%
531
17%
1417
11%
531
44%
531
26%
984
May21st to endNight
45%
584
16%
1551
7%
582
51%
582
30%
1038
June1st to 10thNight
55%
540
17%
1440
9%
540
60%
540
36%
890
June11th to 20thNight
53%
540
18%
1440
10%
540
56%
540
36%
877
June21st to endNight
50%
540
19%
1440
10%
540
54%
540
37%
880
July1st to 10thNight
70%
525
18%
1400
11%
525
71%
525
43%
818
July11th to 20thNight
66%
522
24%
1392
15%
522
68%
522
48%
768
July21st to endNight
66%
591
23%
1542
18%
558
68%
558
41%
888
August1st to 10thNight
68%
537
20%
1601
19%
534
71%
534
38%
905
August11th to 20thNight
66%
534
26%
1636
26%
534
71%
534
37%
929
August21st to endNight
66%
594
25%
1980
28%
594
71%
594
38%
1056
September1st to 10thNight
59%
540
23%
1800
22%
540
65%
540
34%
987
September11th to 20thNight
57%
540
32%
1800
30%
540
63%
540
35%
984
September21st to endNight
50%
667
25%
2033
16%
667
59%
667
37%
1132
October1st to 10thNight
46%
720
23%
2160
15%
720
52%
720
33%
1262
October11th to 20thNight
35%
720
22%
2160
12%
720
43%
720
27%
1357
October21st to endNight
29%
792
20%
2402
12%
792
38%
792
23%
1569
November1st to 10thNight
34%
740
25%
2406
15%
740
47%
740
31%
1378
November11th to 20thNight
25%
867
20%
2577
8%
867
36%
867
23%
1686
November21st to endNight
29%
950
18%
2660
8%
950
38%
950
23%
1817
December1st to 10thNight
23%
950
13%
2660
5%
950
31%
950
19%
1888
December11th to 20thNight
25%
949
15%
3167
5%
948
32%
948
16%
2212
December21st to endNight
19%
1039
9%
4839
4%
1026
29%
1026
7%
3896

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%8%36%6%35%5%5%7%38%
74745
Night
13%8%36%5%3445%6%41%
75034
TransparencyDay
45%6%13%22%13%1
74449
Night
43%5%12%23%14%2
74850
SeeingDay
16%9%29%27%15%4
14679
Night
16%9%31%31%12%1
24671

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
11%10%7%10%8%6%6%5%5%6%25%
1517
11th to 20thDay
13%14%5%11%7%45%447%26%
1514
21st to endDay
14%13%6%10%7%45%45%6%25%
1782
February1st to 10thDay
13%11%5%8%8%446%5%6%29%
1620
11th to 20thDay
15%11%36%6%46%5%48%33%
1596
21st to endDay
15%11%410%6%3346%7%32%
1480
March1st to 10thDay
16%9%5%8%5%24447%35%
1877
11th to 20thDay
13%10%47%6%35%5%5%7%34%
1980
21st to endDay
22%11%47%6%35%45%7%26%
2247
April1st to 10thDay
18%11%47%6%35%45%6%32%
2160
11th to 20thDay
17%10%46%6%445%6%6%32%
2134
21st to endDay
23%7%45%5%34447%35%
2168
May1st to 10thDay
18%8%36%7%36%5%6%7%30%
2425
11th to 20thDay
18%8%36%6%35%45%6%36%
2479
21st to endDay
15%6%37%6%46%5%6%7%34%
2725
June1st to 10thDay
10%5%25%5%45%6%7%9%42%
2520
11th to 20thDay
11%5%25%5%345%6%7%47%
2520
21st to endDay
11%5%36%6%35%5%6%9%39%
2520
July1st to 10thDay
8%425%435%6%7%9%47%
2520
11th to 20thDay
8%425%5%35%5%6%9%47%
2520
21st to endDay
6%5%25%5%35%6%7%10%46%
2681
August1st to 10thDay
7%5%25%7%36%6%7%10%42%
2328
11th to 20thDay
9%6%35%5%35%5%6%9%45%
2265
21st to endDay
9%5%25%5%35%5%5%7%51%
2376
September1st to 10thDay
14%5%25%42435%8%48%
2160
11th to 20thDay
10%4245%345%6%7%51%
2160
21st to endDay
10%5%26%5%35%46%8%45%
1905
October1st to 10thDay
16%6%26%43435%6%45%
1800
11th to 20thDay
19%7%35%42435%6%40%
1800
21st to endDay
17%9%35%5%3335%6%40%
1952
November1st to 10thDay
11%7%35%6%35%5%5%7%44%
1672
11th to 20thDay
14%11%5%7%5%345%5%7%35%
1601
21st to endDay
17%8%47%434347%39%
1520
December1st to 10thDay
14%12%39%6%34448%32%
1520
11th to 20thDay
14%11%5%8%6%45%5%46%32%
1918
21st to endDay
23%19%9%10%6%23435%17%
2783
January1st to 10thNight
11%9%5%9%5%46%5%45%37%
2648
11th to 20thNight
12%13%7%11%8%44446%27%
2515
21st to endNight
13%12%5%10%7%45%446%30%
2574
February1st to 10thNight
13%13%5%7%7%3445%6%33%
2340
11th to 20thNight
12%9%37%7%4445%6%40%
2251
21st to endNight
15%10%49%6%43445%37%
1776
March1st to 10thNight
16%9%36%5%34446%41%
2098
11th to 20thNight
12%8%38%6%35%446%41%
1980
21st to endNight
19%11%48%6%5%5%45%5%28%
2107
April1st to 10thNight
15%9%46%5%3345%5%41%
1800
11th to 20thNight
18%9%36%5%3335%6%39%
1775
21st to endNight
19%7%25%5%245%6%7%39%
1787
May1st to 10thNight
15%6%346%36%5%5%6%41%
1535
11th to 20thNight
16%6%36%5%36%46%6%39%
1416
21st to endNight
12%5%35%6%36%46%8%43%
1551
June1st to 10thNight
8%5%25%4335%7%9%48%
1440
11th to 20thNight
9%425%425%5%8%9%46%
1440
21st to endNight
11%325%3345%6%8%49%
1440
July1st to 10thNight
7%325%4235%8%9%53%
1440
11th to 20thNight
5%42332445%10%59%
1440
21st to endNight
6%4245%35%5%7%8%52%
1633
August1st to 10thNight
5%3245%45%5%6%9%53%
1611
11th to 20thNight
7%4245%3445%8%54%
1636
21st to endNight
9%42342445%7%57%
1980
September1st to 10thNight
11%415%4335%46%54%
1800
11th to 20thNight
11%425%5%24347%55%
1800
21st to endNight
8%425%424447%55%
2033
October1st to 10thNight
13%5%245%24346%53%
2160
11th to 20thNight
14%7%35%434445%47%
2160
21st to endNight
17%8%35%5%2435%6%42%
2402
November1st to 10thNight
11%5%25%424446%53%
2406
11th to 20thNight
13%9%46%5%3445%6%41%
2577
21st to endNight
16%8%36%5%34446%41%
2660
December1st to 10thNight
13%10%48%7%35%5%46%34%
2660
11th to 20thNight
15%10%5%8%5%35%446%34%
3279
21st to endNight
21%15%7%10%7%345%45%19%
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
58%6%11%15%10%1
1518
11th to 20thDay
60%413%18%6%
1514
21st to endDay
60%5%14%13%8%1
1782
February1st to 10thDay
55%415%18%7%1
1620
11th to 20thDay
50%6%13%20%9%1
1605
21st to endDay
51%6%14%19%8%2
1480
March1st to 10thDay
49%5%15%22%9%1
1877
11th to 20thDay
49%5%13%22%10%1
1980
21st to endDay
57%5%12%19%6%1
2247
April1st to 10thDay
53%5%11%21%9%
2160
11th to 20thDay
50%5%10%23%10%1
2134
21st to endDay
49%5%11%21%12%2
2168
May1st to 10thDay
52%7%13%20%8%1
2425
11th to 20thDay
49%5%12%20%12%1
2483
21st to endDay
46%8%12%18%13%2
2725
June1st to 10thDay
36%7%16%25%16%1
2520
11th to 20thDay
35%7%11%22%22%3
2520
21st to endDay
40%8%13%23%15%1
2520
July1st to 10thDay
32%7%14%30%14%2
2451
11th to 20thDay
32%9%15%26%14%4
2445
21st to endDay
30%8%19%25%16%2
2532
August1st to 10thDay
35%8%17%28%11%1
2314
11th to 20thDay
35%7%14%26%16%1
2265
21st to endDay
32%6%11%27%20%3
2376
September1st to 10thDay
35%5%11%25%21%3
2160
11th to 20thDay
31%6%12%31%18%2
2160
21st to endDay
35%5%11%30%17%1
1905
October1st to 10thDay
41%5%9%25%19%1
1800
11th to 20thDay
44%5%11%24%15%2
1800
21st to endDay
45%5%12%22%14%2
1952
November1st to 10thDay
39%5%13%26%14%3
1672
11th to 20thDay
48%414%20%13%1
1601
21st to endDay
46%5%16%24%8%
1520
December1st to 10thDay
53%5%14%20%8%1
1520
11th to 20thDay
52%5%16%17%9%1
1915
21st to endDay
72%410%10%4
2783
January1st to 10thNight
49%414%19%14%1
2648
11th to 20thNight
59%5%15%14%8%
2515
21st to endNight
54%6%13%16%9%1
2574
February1st to 10thNight
53%411%20%11%1
2340
11th to 20thNight
44%5%12%24%12%2
2252
21st to endNight
49%312%20%13%2
1776
March1st to 10thNight
45%414%26%11%1
2098
11th to 20thNight
45%5%13%22%14%2
1980
21st to endNight
58%5%11%16%9%1
2107
April1st to 10thNight
45%5%10%25%14%2
1800
11th to 20thNight
46%5%8%23%15%3
1775
21st to endNight
45%5%12%23%14%2
1787
May1st to 10thNight
43%7%12%23%14%1
1535
11th to 20thNight
43%7%13%23%12%2
1417
21st to endNight
38%8%13%22%16%3
1552
June1st to 10thNight
31%8%14%26%17%3
1440
11th to 20thNight
32%7%14%23%20%5%
1440
21st to endNight
31%7%17%24%16%4
1440
July1st to 10thNight
25%8%16%31%16%4
1400
11th to 20thNight
22%7%17%33%18%3
1392
21st to endNight
26%8%16%29%18%3
1542
August1st to 10thNight
27%8%18%31%13%4
1601
11th to 20thNight
29%7%13%31%20%2
1636
21st to endNight
28%5%11%27%23%6%
1980
September1st to 10thNight
31%5%12%25%25%3
1800
11th to 20thNight
32%410%33%20%3
1800
21st to endNight
30%410%30%23%2
2033
October1st to 10thNight
33%410%26%23%3
2160
11th to 20thNight
39%5%11%23%19%3
2160
21st to endNight
45%411%23%15%2
2402
November1st to 10thNight
33%5%14%26%18%4
2406
11th to 20thNight
44%413%24%13%2
2577
21st to endNight
46%412%26%11%1
2660
December1st to 10thNight
50%5%13%23%9%1
2660
11th to 20thNight
51%413%24%6%1
3281
21st to endNight
67%5%9%13%6%
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
22%10%34%23%9%2
270
11th to 20thDay
21%15%38%21%41
249
21st to endDay
22%11%38%23%5%2
264
February1st to 10thDay
22%15%42%17%3
240
11th to 20thDay
23%17%35%18%7%
240
21st to endDay
26%14%44%12%5%
198
March1st to 10thDay
18%21%41%14%5%
240
11th to 20thDay
23%16%30%22%8%2
240
21st to endDay
32%14%27%19%6%1
333
April1st to 10thDay
24%12%39%18%7%
420
11th to 20thDay
23%15%30%21%10%
412
21st to endDay
26%11%33%18%11%1
428
May1st to 10thDay
20%11%36%24%7%2
603
11th to 20thDay
21%13%26%26%12%2
625
21st to endDay
14%10%27%28%15%6%
679
June1st to 10thDay
7%327%35%23%5%
630
11th to 20thDay
13%6%28%33%18%3
630
21st to endDay
11%7%31%33%13%4
630
July1st to 10thDay
7%318%33%26%13%
621
11th to 20thDay
9%324%35%22%7%
618
21st to endDay
8%123%38%24%5%
684
August1st to 10thDay
7%320%38%24%8%
626
11th to 20thDay
11%418%32%25%9%
579
21st to endDay
9%318%38%25%8%
495
September1st to 10thDay
14%8%19%30%22%8%
450
11th to 20thDay
10%422%36%22%7%
450
21st to endDay
13%10%28%23%23%3
341
October1st to 10thDay
20%9%26%27%13%4
270
11th to 20thDay
21%21%34%13%9%
270
21st to endDay
27%12%34%19%6%3
297
November1st to 10thDay
14%9%43%22%10%2
270
11th to 20thDay
19%18%30%25%6%2
270
21st to endDay
22%12%37%22%6%
270
December1st to 10thDay
14%13%43%21%7%2
270
11th to 20thDay
16%13%39%22%9%1
270
21st to endDay
28%14%39%17%2
297
January1st to 10thNight
14%13%38%28%7%
950
11th to 20thNight
19%13%32%29%7%
918
21st to endNight
18%11%36%28%6%
990
February1st to 10thNight
20%12%38%25%5%
900
11th to 20thNight
17%12%39%27%5%
857
21st to endNight
21%11%40%22%5%1
592
March1st to 10thNight
20%13%37%22%7%1
658
11th to 20thNight
17%12%32%30%8%
540
21st to endNight
22%11%36%25%5%1
594
April1st to 10thNight
19%14%39%24%3
540
11th to 20thNight
23%13%31%26%7%
533
21st to endNight
21%8%33%28%9%1
540
May1st to 10thNight
18%6%40%28%6%1
540
11th to 20thNight
18%10%29%32%11%1
531
21st to endNight
14%429%34%17%2
584
June1st to 10thNight
9%426%42%17%1
540
11th to 20thNight
10%427%44%14%1
540
21st to endNight
12%428%40%15%1
540
July1st to 10thNight
8%114%42%33%3
525
11th to 20thNight
5%319%43%26%3
522
21st to endNight
6%319%44%27%2
591
August1st to 10thNight
5%215%47%28%2
537
11th to 20thNight
7%316%42%27%5%
534
21st to endNight
12%214%42%27%2
594
September1st to 10thNight
12%419%39%22%4
540
11th to 20thNight
11%322%35%25%4
540
21st to endNight
9%7%28%34%19%1
667
October1st to 10thNight
14%6%28%32%17%2
720
11th to 20thNight
19%11%32%29%9%
720
21st to endNight
22%11%32%27%8%
792
November1st to 10thNight
13%9%38%29%10%1
740
11th to 20thNight
18%13%39%22%7%
867
21st to endNight
20%11%35%27%7%
950
December1st to 10thNight
18%14%36%26%6%1
950
11th to 20thNight
23%10%34%27%6%1
950
21st to endNight
25%13%34%23%4
1045
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