Stull Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stull Observatory clear sky chart from 2005-11-05 to 2018-04-23.

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
26%
15788
12%
49563
7%
15750
32%
15750
14%
36523

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
9%
778
7%
2705
4%
776
14%
776
5%
2239
January16th to endNight
10%
832
7%
2704
3%
832
16%
832
5%
2234
February1st to 15thNight
7%
764
6%
2482
1%
761
13%
761
5%
2059
February16th to endNight
11%
683
9%
2139
2%
683
19%
683
8%
1692
March1st to 15thNight
13%
780
12%
2172
5%
780
20%
780
10%
1740
March16th to endNight
19%
832
15%
2269
6%
832
26%
832
15%
1702
April1st to 15thNight
18%
778
13%
1946
3%
778
24%
778
16%
1493
April16th to endNight
31%
682
17%
1796
7%
682
36%
682
24%
1251
May1st to 15thNight
37%
459
15%
1539
9%
459
44%
459
16%
1092
May16th to endNight
39%
375
15%
1497
16%
374
46%
374
18%
959
June1st to 15thNight
39%
360
13%
1440
11%
360
48%
360
17%
935
June16th to endNight
44%
360
13%
1440
14%
360
51%
360
22%
850
July1st to 15thNight
51%
340
14%
1360
19%
340
55%
340
23%
803
July16th to endNight
53%
459
16%
1494
14%
430
58%
430
28%
891
August1st to 15thNight
54%
539
14%
1609
9%
536
58%
536
30%
992
August16th to endNight
55%
690
20%
1836
12%
690
58%
690
33%
1172
September1st to 15thNight
44%
720
19%
1800
10%
720
48%
720
29%
1213
September16th to endNight
49%
717
21%
1986
15%
717
52%
717
27%
1308
October1st to 15thNight
34%
720
15%
2160
11%
720
41%
720
21%
1484
October16th to endNight
17%
768
12%
2304
5%
768
26%
768
13%
1743
November1st to 15thNight
23%
760
15%
2513
11%
760
31%
760
16%
1771
November16th to endNight
17%
780
11%
2730
7%
780
24%
780
9%
2081
December1st to 15thNight
12%
780
5%
2730
3%
780
14%
780
5%
2272
December16th to endNight
8%
832
4%
2912
1%
832
11%
832
4%
2547

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%11%5%8%7%35%45%6%27%
49950
Night
13%11%5%8%7%35%446%28%
49741
Cloud CoverDay
15%11%48%7%35%45%6%27%
49960
Night
13%10%5%8%7%35%45%6%29%
49746
TransparencyDay
56%6%11%16%9%
49646
Night
54%6%12%17%8%
49563
SeeingDay
22%17%27%18%9%4
6176
Night
21%18%28%20%9%1
15790

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 15thDay
12%15%7%14%10%46%34415%
1562
16th to endDay
7%15%9%15%11%5%6%445%14%
1872
February1st to 15thDay
12%17%9%12%10%46%43315%
1710
16th to endDay
14%14%7%12%9%34345%21%
1620
March1st to 15thDay
15%16%8%10%7%23345%22%
2105
16th to endDay
15%13%6%8%7%34445%26%
2307
April1st to 15thDay
16%15%5%8%6%24345%26%
2332
16th to endDay
17%10%35%43435%5%36%
2290
May1st to 15thDay
17%10%47%6%25%446%29%
2409
16th to endDay
14%8%36%6%345%5%6%34%
2618
June1st to 15thDay
18%8%36%5%35%45%7%31%
2520
16th to endDay
14%6%37%6%36%45%9%32%
2520
July1st to 15thDay
10%6%36%7%5%7%5%6%8%32%
2520
16th to endDay
12%6%37%7%46%6%6%8%30%
2568
August1st to 15thDay
13%6%37%7%46%5%5%8%30%
2340
16th to endDay
13%7%26%8%46%6%6%7%31%
2352
September1st to 15thDay
19%8%26%4345%5%7%34%
2160
16th to endDay
16%6%26%5%34347%38%
1970
October1st to 15thDay
19%9%45%5%33347%32%
1800
16th to endDay
20%16%5%7%6%3334424%
1920
November1st to 15thDay
12%15%5%6%5%3345%7%28%
1671
16th to endDay
16%17%7%8%6%24445%22%
1560
December1st to 15thDay
12%19%9%14%8%3443414%
1560
16th to endDay
13%16%12%15%10%3444411%
1664
January1st to 15thNight
12%14%6%14%11%5%6%446%14%
2705
16th to endNight
9%13%8%13%12%5%6%44415%
2704
February1st to 15thNight
11%13%10%12%12%5%6%435%15%
2480
16th to endNight
10%13%7%14%9%4445%420%
2139
March1st to 15thNight
13%15%6%11%7%44345%23%
2160
16th to endNight
12%11%6%9%6%34346%29%
2269
April1st to 15thNight
16%13%5%7%435%446%28%
1946
16th to endNight
14%10%35%433446%39%
1796
May1st to 15thNight
16%8%246%36%5%6%6%32%
1539
16th to endNight
12%7%35%4345%5%7%40%
1495
June1st to 15thNight
16%5%25%5%3446%8%36%
1440
16th to endNight
10%5%245%245%6%7%43%
1440
July1st to 15thNight
6%435%6%46%45%9%43%
1440
16th to endNight
9%323426%5%8%8%44%
1597
August1st to 15thNight
9%5%24435%6%6%9%43%
1620
16th to endNight
10%5%25%4245%5%7%45%
1836
September1st to 15thNight
16%5%25%423347%43%
1800
16th to endNight
14%5%25%42445%7%41%
1986
October1st to 15thNight
17%9%35%433338%36%
2160
16th to endNight
23%12%36%5%34345%26%
2304
November1st to 15thNight
14%12%37%6%24445%33%
2513
16th to endNight
13%17%6%8%7%2445%5%23%
2730
December1st to 15thNight
16%16%9%11%8%35%445%13%
2730
16th to endNight
14%16%9%15%9%46%45%5%9%
2912

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 15thDay
11%14%7%14%10%5%5%44415%
1562
16th to endDay
8%15%9%14%10%6%6%45%5%14%
1872
February1st to 15thDay
13%17%9%11%11%45%43316%
1710
16th to endDay
14%13%7%11%9%33445%21%
1620
March1st to 15thDay
15%15%8%10%7%2344423%
2116
16th to endDay
15%13%6%8%7%34345%27%
2306
April1st to 15thDay
16%15%5%8%5%34345%26%
2332
16th to endDay
17%10%25%5%2445%5%36%
2290
May1st to 15thDay
18%11%37%5%3445%6%29%
2409
16th to endDay
15%7%36%6%4445%6%34%
2618
June1st to 15thDay
18%8%37%5%3445%6%32%
2520
16th to endDay
14%6%27%6%45%5%5%9%32%
2520
July1st to 15thDay
10%6%36%7%47%5%5%8%32%
2520
16th to endDay
13%6%36%6%46%6%6%8%30%
2568
August1st to 15thDay
12%7%28%7%46%6%5%8%30%
2340
16th to endDay
13%6%26%7%5%6%5%5%8%30%
2352
September1st to 15thDay
19%7%25%5%335%5%6%34%
2160
16th to endDay
16%7%25%434347%38%
1970
October1st to 15thDay
19%9%27%5%33446%33%
1800
16th to endDay
20%15%48%6%3344424%
1920
November1st to 15thDay
12%15%46%5%2445%7%30%
1671
16th to endDay
16%17%6%8%6%24346%22%
1560
December1st to 15thDay
13%19%9%13%8%34435%15%
1560
16th to endDay
13%15%11%15%10%34445%11%
1664
January1st to 15thNight
12%13%6%14%10%46%445%15%
2705
16th to endNight
10%12%8%13%12%5%6%445%16%
2704
February1st to 15thNight
11%12%9%11%11%5%6%445%16%
2473
16th to endNight
10%12%8%13%9%445%4421%
2139
March1st to 15thNight
13%14%6%11%7%34445%23%
2172
16th to endNight
12%11%5%9%6%3445%5%30%
2269
April1st to 15thNight
16%12%5%7%5%35%45%5%28%
1946
16th to endNight
14%10%36%424347%39%
1796
May1st to 15thNight
17%8%245%36%5%5%6%33%
1539
16th to endNight
12%7%346%245%5%7%40%
1495
June1st to 15thNight
17%6%25%6%2436%8%36%
1440
16th to endNight
10%425%43446%8%43%
1440
July1st to 15thNight
6%5%25%6%36%5%6%9%42%
1440
16th to endNight
10%324426%5%6%9%44%
1597
August1st to 15thNight
8%5%25%3346%6%9%44%
1620
16th to endNight
10%5%25%4245%6%7%45%
1836
September1st to 15thNight
16%6%24433347%44%
1800
16th to endNight
15%5%245%3445%8%41%
1986
October1st to 15thNight
17%8%26%5%23348%36%
2160
16th to endNight
22%12%35%434345%27%
2304
November1st to 15thNight
14%12%37%6%34445%34%
2513
16th to endNight
14%15%6%9%6%4445%6%23%
2730
December1st to 15thNight
16%15%9%11%8%36%5%45%14%
2730
16th to endNight
14%15%10%13%10%46%45%410%
2912

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
70%5%7%9%6%
1563
16th to endDay
70%5%11%8%3
1872
February1st to 15thDay
72%5%8%9%4
1712
16th to endDay
63%49%15%5%
1620
March1st to 15thDay
63%410%13%6%
2116
16th to endDay
58%48%17%9%1
2307
April1st to 15thDay
59%49%15%10%
2332
16th to endDay
48%49%18%17%2
2290
May1st to 15thDay
53%5%10%18%9%1
2409
16th to endDay
48%7%12%17%12%1
2622
June1st to 15thDay
51%6%11%18%10%1
2520
16th to endDay
48%7%13%20%10%
2520
July1st to 15thDay
47%8%9%21%11%
2371
16th to endDay
47%8%15%17%10%
2408
August1st to 15thDay
49%8%13%18%10%
2327
16th to endDay
48%8%11%20%10%1
2352
September1st to 15thDay
47%7%10%21%11%1
2160
16th to endDay
45%5%9%24%14%
1970
October1st to 15thDay
50%6%11%16%13%1
1800
16th to endDay
62%5%12%15%4
1920
November1st to 15thDay
53%5%12%16%8%2
1671
16th to endDay
62%6%10%15%5%
1560
December1st to 15thDay
72%5%10%9%2
1560
16th to endDay
73%6%11%7%1
1664
January1st to 15thNight
70%5%9%8%5%
2705
16th to endNight
69%6%11%9%3
2704
February1st to 15thNight
69%6%10%9%3
2482
16th to endNight
64%5%10%13%6%
2139
March1st to 15thNight
62%5%9%15%5%
2172
16th to endNight
55%5%11%15%11%1
2269
April1st to 15thNight
55%5%10%16%10%
1946
16th to endNight
45%410%20%15%2
1796
May1st to 15thNight
48%9%9%21%10%
1539
16th to endNight
41%9%11%21%14%1
1497
June1st to 15thNight
44%7%13%22%11%
1440
16th to endNight
37%7%15%24%13%1
1440
July1st to 15thNight
36%8%12%24%16%1
1360
16th to endNight
33%8%16%25%13%2
1494
August1st to 15thNight
33%9%19%26%10%
1609
16th to endNight
34%8%14%27%14%1
1836
September1st to 15thNight
40%6%13%24%13%2
1800
16th to endNight
40%6%12%24%15%1
1986
October1st to 15thNight
47%5%12%19%14%
2160
16th to endNight
57%5%15%15%5%
2304
November1st to 15thNight
51%5%13%17%9%3
2513
16th to endNight
60%6%11%15%5%
2730
December1st to 15thNight
69%7%11%8%2
2730
16th to endNight
74%7%8%7%1
2912

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
32%29%27%5%5%
92
16th to endDay
22%36%30%8%1
144
February1st to 15thDay
34%40%22%2
135
16th to endDay
31%30%31%6%
201
March1st to 15thDay
30%26%25%14%3
330
16th to endDay
25%26%26%13%6%1
352
April1st to 15thDay
33%20%27%12%41
330
16th to endDay
22%18%28%16%10%4
289
May1st to 15thDay
30%19%21%15%9%3
288
16th to endDay
18%16%33%17%10%4
331
June1st to 15thDay
22%13%29%23%9%2
315
16th to endDay
15%14%36%23%7%2
315
July1st to 15thDay
12%8%33%30%9%6%
305
16th to endDay
12%433%29%13%5%
326
August1st to 15thDay
15%7%28%23%17%8%
313
16th to endDay
14%8%23%23%17%12%
332
September1st to 15thDay
18%8%23%19%21%8%
315
16th to endDay
16%10%20%18%21%13%
314
October1st to 15thDay
20%12%26%21%15%3
315
16th to endDay
29%23%24%18%4
336
November1st to 15thDay
24%12%32%19%9%1
222
16th to endDay
24%28%32%13%1
90
December1st to 15thDay
33%28%24%6%5%1
90
16th to endDay
42%34%18%4
96
January1st to 15thNight
26%27%32%10%2
780
16th to endNight
23%31%30%14%
832
February1st to 15thNight
25%31%31%11%
764
16th to endNight
25%29%30%12%1
683
March1st to 15thNight
27%27%28%14%2
780
16th to endNight
20%24%31%15%5%1
832
April1st to 15thNight
27%20%31%18%2
778
16th to endNight
21%11%32%25%7%
682
May1st to 15thNight
21%9%24%32%11%
459
16th to endNight
13%13%26%30%14%1
375
June1st to 15thNight
18%10%28%30%12%
360
16th to endNight
10%7%34%36%11%
360
July1st to 15thNight
9%5%27%39%17%1
340
16th to endNight
10%5%24%33%22%2
459
August1st to 15thNight
10%426%35%21%2
539
16th to endNight
11%422%30%28%2
690
September1st to 15thNight
18%7%24%26%21%2
720
16th to endNight
16%9%20%21%25%6%
717
October1st to 15thNight
22%13%25%22%14%1
720
16th to endNight
30%18%29%17%3
768
November1st to 15thNight
21%18%32%18%7%
760
16th to endNight
26%25%26%15%5%1
780
December1st to 15thNight
30%27%26%12%3
780
16th to endNight
26%31%29%10%2
832
Page generated at 2018-04-24 10:03:04. Version ac04

Copyright 2018 A. Danko


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