Hilltop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hilltop Observatory clear sky chart from 2005-11-05 to 2019-06-02.

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
33%
16843
9%
53711
8%
16814
35%
16814
13%
41603

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 15thNight
22%
900
8%
3136
5%
896
26%
896
7%
2633
January16th to endNight
27%
898
8%
3030
6%
898
31%
898
9%
2456
February1st to 15thNight
21%
817
6%
2678
4%
817
25%
817
8%
2250
February16th to endNight
24%
736
9%
2387
5%
736
29%
736
9%
1942
March1st to 15thNight
23%
840
7%
2351
5%
840
25%
840
10%
1983
March16th to endNight
23%
896
6%
2454
4%
896
24%
896
8%
2118
April1st to 15thNight
22%
836
3%
2096
1%
836
23%
836
9%
1836
April16th to endNight
34%
624
8%
1872
6%
624
36%
624
12%
1473
May1st to 15thNight
40%
457
12%
1717
14%
457
43%
457
14%
1217
May16th to endNight
35%
439
4%
1752
6%
438
37%
438
10%
1328
June1st to 15thNight
35%
392
6%
1409
6%
392
37%
392
12%
1063
June16th to endNight
45%
390
8%
1365
8%
390
48%
390
17%
950
July1st to 15thNight
58%
370
12%
1295
14%
370
60%
370
28%
765
July16th to endNight
49%
408
13%
1500
16%
386
55%
386
23%
894
August1st to 15thNight
50%
556
13%
1702
13%
554
51%
554
23%
1103
August16th to endNight
52%
661
17%
1903
13%
661
54%
661
27%
1212
September1st to 15thNight
62%
780
19%
1950
12%
780
63%
780
37%
1237
September16th to endNight
46%
776
16%
2155
13%
776
47%
776
21%
1549
October1st to 15thNight
46%
780
14%
2340
15%
780
49%
780
22%
1620
October16th to endNight
38%
832
10%
2526
10%
832
39%
832
15%
1940
November1st to 15thNight
22%
820
6%
2870
5%
820
25%
820
7%
2437
November16th to endNight
24%
840
5%
2940
4%
840
26%
840
8%
2412
December1st to 15thNight
29%
840
10%
2940
8%
840
33%
840
11%
2279
December16th to endNight
15%
955
5%
3343
3%
955
18%
955
5%
2906

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
28%10%36%5%23345%24%
54642
Night
29%10%36%5%23335%25%
53868
Cloud CoverDay
28%9%36%423345%25%
54642
Night
29%10%36%423335%26%
53871
TransparencyDay
61%6%11%13%6%1
54302
Night
61%5%11%14%6%1
53712
SeeingDay
30%9%23%23%10%2
8248
Night
33%7%17%23%15%2
16851

By Month

Percentage Cloud Cover NW (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 15thDay
45%9%35%32323416%
1463
16th to endDay
38%10%46%5%3223318%
1893
February1st to 15thDay
40%12%37%43422414%
1846
16th to endDay
35%13%5%6%5%23235%16%
1658
March1st to 15thDay
41%10%47%42423314%
2268
16th to endDay
37%13%47%5%3433312%
2473
April1st to 15thDay
38%11%47%5%35%435%10%
2509
16th to endDay
28%10%47%6%34445%19%
2692
May1st to 15thDay
22%9%47%5%34445%28%
2892
16th to endDay
27%12%47%6%2445%5%19%
3063
June1st to 15thDay
28%11%46%5%34346%21%
2902
16th to endDay
24%9%46%5%24446%25%
2925
July1st to 15thDay
16%8%34323337%42%
2925
16th to endDay
14%10%34323448%40%
2923
August1st to 15thDay
14%10%25%424349%37%
2500
16th to endDay
16%7%37%423346%39%
2650
September1st to 15thDay
15%7%34422336%44%
2340
16th to endDay
20%8%36%5%3435%6%33%
2132
October1st to 15thDay
26%7%35%42345%6%29%
1950
16th to endDay
31%9%36%5%33445%21%
2048
November1st to 15thDay
40%10%37%5%2343412%
1648
16th to endDay
40%10%35%5%3233417%
1680
December1st to 15thDay
33%9%36%4245%35%21%
1680
16th to endDay
41%10%35%42333415%
1582
January1st to 15thNight
43%9%35%42323317%
3136
16th to endNight
37%9%35%433445%19%
3030
February1st to 15thNight
36%11%47%5%23335%16%
2666
16th to endNight
31%14%46%5%2322420%
2387
March1st to 15thNight
38%11%36%42332417%
2351
16th to endNight
36%12%37%5%3334415%
2450
April1st to 15thNight
31%13%6%9%6%2432414%
2096
16th to endNight
25%8%47%7%34337%22%
1872
May1st to 15thNight
23%10%45%423336%31%
1717
16th to endNight
25%12%46%5%24435%23%
1752
June1st to 15thNight
25%12%46%422326%27%
1409
16th to endNight
19%11%36%324347%32%
1365
July1st to 15thNight
12%7%34413346%46%
1365
16th to endNight
11%11%36%423237%43%
1595
August1st to 15thNight
12%13%36%423347%38%
1710
16th to endNight
13%10%34322447%42%
1903
September1st to 15thNight
8%7%24423346%51%
1950
16th to endNight
22%8%36%423346%35%
2155
October1st to 15thNight
20%8%36%5%23435%37%
2340
16th to endNight
32%8%25%333345%27%
2526
November1st to 15thNight
39%9%36%5%2433316%
2870
16th to endNight
34%10%36%6%2343418%
2940
December1st to 15thNight
31%9%46%5%2433424%
2940
16th to endNight
43%11%46%42332314%
3343

Percentage Cloud Cover SW (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 15thDay
47%7%36%31333316%
1463
16th to endDay
40%9%46%5%2322416%
1893
February1st to 15thDay
41%11%36%43333314%
1846
16th to endDay
36%12%37%5%3333416%
1658
March1st to 15thDay
42%11%46%42432414%
2268
16th to endDay
39%13%47%5%3443311%
2473
April1st to 15thDay
38%11%47%5%34445%11%
2509
16th to endDay
29%10%5%7%7%34345%19%
2692
May1st to 15thDay
23%8%46%5%3445%5%28%
2892
16th to endDay
27%12%47%6%2345%6%20%
3063
June1st to 15thDay
27%11%46%5%24446%22%
2902
16th to endDay
24%9%46%434446%27%
2925
July1st to 15thDay
16%8%24323346%45%
2925
16th to endDay
13%10%25%413347%44%
2923
August1st to 15thDay
14%9%26%323348%40%
2500
16th to endDay
16%7%35%423336%42%
2650
September1st to 15thDay
15%7%25%322246%45%
2340
16th to endDay
21%8%26%424446%32%
2132
October1st to 15thDay
26%8%35%5%13447%29%
1950
16th to endDay
33%9%36%43335%6%20%
2048
November1st to 15thDay
39%11%46%434335%11%
1648
16th to endDay
41%9%46%42323416%
1680
December1st to 15thDay
34%9%35%5%24445%20%
1680
16th to endDay
41%10%36%42423415%
1582
January1st to 15thNight
44%8%35%42332317%
3135
16th to endNight
37%9%36%423345%18%
3030
February1st to 15thNight
35%12%46%5%23335%16%
2666
16th to endNight
31%14%46%43332421%
2387
March1st to 15thNight
40%9%35%42333418%
2351
16th to endNight
36%11%46%5%3334416%
2454
April1st to 15thNight
31%13%48%7%3433414%
2096
16th to endNight
25%8%37%6%45%346%24%
1872
May1st to 15thNight
23%10%34423336%32%
1717
16th to endNight
24%11%46%5%34446%24%
1752
June1st to 15thNight
24%10%46%423445%29%
1409
16th to endNight
17%11%36%423346%35%
1365
July1st to 15thNight
11%6%25%413337%49%
1365
16th to endNight
10%10%25%414337%46%
1595
August1st to 15thNight
11%10%36%423348%41%
1710
16th to endNight
13%8%24314347%45%
1903
September1st to 15thNight
8%7%14413337%53%
1950
16th to endNight
22%8%36%413345%36%
2155
October1st to 15thNight
21%8%34433335%38%
2340
16th to endNight
32%8%25%323345%27%
2526
November1st to 15thNight
40%8%26%5%3433316%
2870
16th to endNight
34%10%46%5%24335%18%
2940
December1st to 15thNight
30%8%47%42433425%
2940
16th to endNight
43%11%37%42322314%
3343

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 15thDay
73%36%8%6%1
1463
16th to endDay
72%46%8%5%2
1893
February1st to 15thDay
75%48%7%3
1846
16th to endDay
72%47%10%4
1658
March1st to 15thDay
76%46%10%2
2268
16th to endDay
78%47%7%1
2473
April1st to 15thDay
76%7%8%7%1
2509
16th to endDay
67%5%11%11%3
2692
May1st to 15thDay
56%7%14%14%6%
2892
16th to endDay
65%9%12%10%1
3067
June1st to 15thDay
62%7%14%10%4
2902
16th to endDay
58%8%14%13%4
2925
July1st to 15thDay
41%6%14%22%13%1
2766
16th to endDay
40%8%14%23%11%1
2755
August1st to 15thDay
43%8%15%20%9%2
2483
16th to endDay
44%6%13%24%9%1
2650
September1st to 15thDay
41%6%11%20%15%4
2340
16th to endDay
51%7%15%14%8%2
2132
October1st to 15thDay
55%5%13%16%8%1
1950
16th to endDay
64%6%10%12%6%
2048
November1st to 15thDay
75%5%8%6%21
1648
16th to endDay
71%46%10%6%
1680
December1st to 15thDay
65%6%8%12%6%
1680
16th to endDay
73%5%7%7%5%
1582
January1st to 15thNight
72%46%8%6%1
3136
16th to endNight
66%6%7%9%6%2
3030
February1st to 15thNight
70%5%8%9%4
2678
16th to endNight
68%49%11%5%
2387
March1st to 15thNight
70%47%14%2
2351
16th to endNight
72%6%9%10%1
2454
April1st to 15thNight
74%5%9%8%1
2096
16th to endNight
61%6%12%12%5%
1872
May1st to 15thNight
54%6%13%18%7%
1718
16th to endNight
59%8%17%11%2
1752
June1st to 15thNight
57%7%14%16%3
1409
16th to endNight
50%7%20%17%41
1365
July1st to 15thNight
35%6%16%27%11%2
1295
16th to endNight
37%7%18%21%11%3
1500
August1st to 15thNight
43%6%16%21%8%3
1702
16th to endNight
38%6%17%24%10%1
1903
September1st to 15thNight
31%7%14%24%15%6%
1950
16th to endNight
50%6%11%18%10%2
2155
October1st to 15thNight
48%412%22%9%1
2340
16th to endNight
59%5%10%15%8%
2526
November1st to 15thNight
71%6%8%8%21
2870
16th to endNight
68%5%8%9%6%1
2940
December1st to 15thNight
62%5%10%13%7%1
2940
16th to endNight
76%45%8%4
3343

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 15thDay
38%8%23%24%5%
120
16th to endDay
49%7%16%18%6%1
240
February1st to 15thDay
48%10%19%16%5%
326
16th to endDay
41%13%26%13%5%
290
March1st to 15thDay
47%11%21%13%5%
390
16th to endDay
41%13%19%16%7%1
416
April1st to 15thDay
46%13%20%15%3
389
16th to endDay
33%14%26%18%42
378
May1st to 15thDay
27%11%21%27%9%2
408
16th to endDay
31%9%22%20%10%4
458
June1st to 15thDay
27%12%26%23%8%2
536
16th to endDay
22%12%31%24%8%1
570
July1st to 15thDay
18%6%27%33%10%3
550
16th to endDay
10%5%29%32%16%5%
432
August1st to 15thDay
16%421%33%17%6%
359
16th to endDay
17%7%19%31%16%8%
396
September1st to 15thDay
18%6%20%27%20%6%
375
16th to endDay
22%7%22%27%16%3
375
October1st to 15thDay
31%7%21%25%11%2
375
16th to endDay
36%10%20%16%13%3
377
November1st to 15thDay
46%11%25%13%1
120
16th to endDay
48%8%10%21%10%1
120
December1st to 15thDay
40%10%19%22%7%
120
16th to endDay
38%11%24%21%3
128
January1st to 15thNight
48%8%17%19%7%
900
16th to endNight
41%7%16%24%9%
898
February1st to 15thNight
40%10%18%21%7%
824
16th to endNight
38%8%20%21%8%1
736
March1st to 15thNight
44%8%18%16%10%1
840
16th to endNight
39%9%17%21%8%2
896
April1st to 15thNight
38%9%19%20%10%1
836
16th to endNight
28%8%18%27%13%2
624
May1st to 15thNight
28%6%14%23%22%3
458
16th to endNight
27%7%15%27%19%2
439
June1st to 15thNight
26%8%18%28%18%1
392
16th to endNight
23%6%16%32%20%1
390
July1st to 15thNight
13%313%35%31%2
370
16th to endNight
14%221%25%31%4
408
August1st to 15thNight
19%112%23%32%10%
556
16th to endNight
19%315%24%27%10%
661
September1st to 15thNight
13%313%26%33%10%
780
16th to endNight
26%413%30%22%3
776
October1st to 15thNight
25%314%30%23%3
780
16th to endNight
36%414%25%17%2
832
November1st to 15thNight
43%10%16%22%5%
820
16th to endNight
37%9%21%22%8%
840
December1st to 15thNight
36%7%19%26%8%1
840
16th to endNight
49%11%17%15%5%
955
Page generated at 2019-06-02 16:43:05. Version ac04

Copyright 2019 A. Danko


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