T. Spence Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the T. Spence Memorial Observatory clear sky chart from 2005-11-05 to 2017-09-24.

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
52%
15761
22%
46996
15%
15717
59%
15717
28%
29526

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
32%
720
22%
2327
12%
716
47%
716
24%
1417
January16th to endNight
31%
768
23%
2496
13%
768
44%
768
23%
1555
February1st to 15thNight
37%
704
24%
2134
13%
700
50%
700
28%
1312
February16th to endNight
33%
631
25%
1892
12%
631
46%
631
28%
1164
March1st to 15thNight
41%
720
28%
1980
14%
720
53%
720
34%
1201
March16th to endNight
43%
768
21%
2112
12%
768
50%
768
32%
1306
April1st to 15thNight
44%
718
24%
1976
15%
718
52%
718
35%
1188
April16th to endNight
52%
609
27%
1704
16%
609
60%
609
38%
989
May1st to 15thNight
55%
539
22%
1620
13%
539
62%
539
34%
941
May16th to endNight
62%
561
19%
1682
13%
561
66%
561
29%
1072
June1st to 15thNight
66%
540
13%
1620
10%
540
67%
540
26%
1085
June16th to endNight
72%
540
15%
1620
13%
540
73%
540
26%
1077
July1st to 15thNight
69%
510
12%
1530
10%
510
70%
510
23%
1049
July16th to endNight
73%
564
11%
1595
10%
531
74%
531
23%
1107
August1st to 15thNight
72%
539
13%
1609
12%
536
72%
536
22%
1136
August16th to endNight
61%
645
14%
1902
13%
645
61%
645
19%
1371
September1st to 15thNight
66%
720
16%
1980
15%
720
66%
720
26%
1349
September16th to endNight
57%
693
17%
1914
14%
693
60%
693
24%
1335
October1st to 15thNight
58%
660
30%
1884
26%
660
64%
660
35%
1127
October16th to endNight
59%
704
33%
2112
29%
704
67%
704
38%
1173
November1st to 15thNight
57%
700
30%
2131
25%
700
66%
700
36%
1197
November16th to endNight
46%
720
28%
2340
21%
720
57%
720
30%
1337
December1st to 15thNight
44%
720
23%
2340
17%
720
53%
720
26%
1418
December16th to endNight
38%
768
22%
2496
18%
768
46%
768
22%
1620

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
10%5%25%5%35%5%6%8%42%
47924
Night
11%5%244345%5%7%44%
47197
TransparencyDay
38%7%12%17%17%6%
47625
Night
36%7%11%17%18%8%
47000
SeeingDay
12%416%22%25%18%
5617
Night
13%5%22%32%23%3
15764

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 15thDay
13%7%2432335%6%46%
1611
16th to endDay
15%5%2433435%6%45%
1728
February1st to 15thDay
14%5%2432445%6%45%
1723
16th to endDay
14%5%24423346%48%
1580
March1st to 15thDay
12%5%24324346%49%
1980
16th to endDay
12%5%245%235%47%45%
2111
April1st to 15thDay
10%424324346%51%
1972
16th to endDay
8%423424447%52%
2265
May1st to 15thDay
8%424436%5%6%8%45%
2331
16th to endDay
7%5%25%6%36%5%6%8%40%
2431
June1st to 15thDay
8%4246%46%7%8%10%34%
2340
16th to endDay
45%246%47%7%8%11%37%
2340
July1st to 15thDay
7%426%7%5%8%7%9%10%30%
2340
16th to endDay
6%436%7%5%7%8%9%12%28%
2465
August1st to 15thDay
9%5%37%7%5%6%8%8%11%27%
2340
16th to endDay
11%7%37%7%46%7%8%9%28%
2284
September1st to 15thDay
7%437%8%45%5%7%8%37%
1980
16th to endDay
15%6%36%5%45%5%5%5%35%
1905
October1st to 15thDay
14%5%23323336%50%
1746
16th to endDay
8%42332333460%
1760
November1st to 15thDay
11%424323245%54%
1724
16th to endDay
14%5%2431425%6%50%
1620
December1st to 15thDay
14%5%25%5%23337%44%
1620
16th to endDay
18%8%25%423247%39%
1728
January1st to 15thNight
13%6%343235%46%45%
2327
16th to endNight
18%5%2442334445%
2496
February1st to 15thNight
15%424424345%47%
2127
16th to endNight
12%5%245%24345%49%
1892
March1st to 15thNight
8%42442445%6%50%
1980
16th to endNight
10%434324446%50%
2112
April1st to 15thNight
10%323423447%52%
1976
16th to endNight
8%2245%2345%9%51%
1704
May1st to 15thNight
6%424425%5%6%7%49%
1619
16th to endNight
7%316%7%347%7%8%43%
1682
June1st to 15thNight
8%6%245%46%6%7%10%36%
1620
16th to endNight
5%3245%47%7%8%11%36%
1620
July1st to 15thNight
6%6%26%6%45%8%7%12%32%
1620
16th to endNight
5%5%25%7%37%8%8%10%34%
1703
August1st to 15thNight
6%426%7%46%8%10%12%30%
1620
16th to endNight
10%6%35%7%45%6%7%9%30%
1902
September1st to 15thNight
9%5%26%5%46%5%7%10%37%
1980
16th to endNight
12%6%35%5%45%46%8%36%
1914
October1st to 15thNight
14%424212345%53%
1884
16th to endNight
8%414423345%57%
2112
November1st to 15thNight
10%414323346%54%
2131
16th to endNight
13%414323345%53%
2340
December1st to 15thNight
15%5%2442335%6%46%
2340
16th to endNight
17%6%2442345%6%40%
2496

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
38%5%8%15%24%7%
1612
16th to endDay
39%47%16%23%8%
1728
February1st to 15thDay
37%48%18%22%7%
1725
16th to endDay
37%46%18%24%9%
1580
March1st to 15thDay
36%39%19%19%11%
1980
16th to endDay
37%5%11%16%21%6%
2112
April1st to 15thDay
33%410%20%21%9%
1972
16th to endDay
31%5%10%21%22%8%
2265
May1st to 15thDay
33%8%12%20%21%3
2331
16th to endDay
37%9%14%23%12%2
2435
June1st to 15thDay
39%11%16%20%10%1
2340
16th to endDay
36%11%19%18%12%1
2340
July1st to 15thDay
40%13%20%15%6%3
2201
16th to endDay
41%14%19%17%41
2310
August1st to 15thDay
45%13%15%17%7%
2327
16th to endDay
47%12%16%13%8%
2284
September1st to 15thDay
41%11%15%12%15%4
1980
16th to endDay
47%7%11%14%14%5%
1905
October1st to 15thDay
35%39%19%20%11%
1746
16th to endDay
28%35%16%28%16%
1760
November1st to 15thDay
32%35%18%24%15%
1724
16th to endDay
35%48%17%22%12%
1620
December1st to 15thDay
40%410%16%19%8%
1620
16th to endDay
45%48%18%15%7%
1728
January1st to 15thNight
38%48%16%23%8%
2327
16th to endNight
41%45%12%24%11%
2496
February1st to 15thNight
38%36%17%19%13%
2136
16th to endNight
37%46%15%23%11%
1892
March1st to 15thNight
31%6%9%19%22%10%
1980
16th to endNight
33%6%12%17%19%10%
2112
April1st to 15thNight
32%5%9%19%21%11%
1976
16th to endNight
28%6%13%19%21%10%
1704
May1st to 15thNight
30%6%15%19%21%5%
1620
16th to endNight
34%10%13%22%16%2
1684
June1st to 15thNight
38%10%19%20%10%
1620
16th to endNight
35%12%19%19%11%2
1620
July1st to 15thNight
38%13%20%18%6%2
1530
16th to endNight
37%15%20%18%7%
1595
August1st to 15thNight
39%15%18%18%7%
1609
16th to endNight
45%12%17%17%6%
1902
September1st to 15thNight
39%11%16%14%12%5%
1980
16th to endNight
43%9%14%14%13%4
1914
October1st to 15thNight
33%49%18%22%11%
1884
16th to endNight
29%37%17%24%17%
2112
November1st to 15thNight
30%37%15%25%16%
2131
16th to endNight
34%46%16%22%14%
2340
December1st to 15thNight
38%411%17%20%7%
2340
16th to endNight
44%48%17%17%7%
2496

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
17%7%34%24%10%5%
90
16th to endDay
14%21%39%12%11%
96
February1st to 15thDay
8%17%42%28%2
90
16th to endDay
11%9%40%33%4
81
March1st to 15thDay
14%10%29%30%15%
270
16th to endDay
14%431%30%15%2
288
April1st to 15thDay
14%10%21%22%24%5%
270
16th to endDay
8%320%28%24%14%
270
May1st to 15thDay
9%8%21%24%20%15%
288
16th to endDay
11%311%19%27%26%
331
June1st to 15thDay
12%8%15%31%31%
315
16th to endDay
7%420%33%32%
315
July1st to 15thDay
6%218%31%39%
305
16th to endDay
5%1318%32%38%
326
August1st to 15thDay
14%419%32%29%
313
16th to endDay
12%6%15%31%33%
332
September1st to 15thDay
11%11%19%35%21%
315
16th to endDay
19%110%26%25%15%
287
October1st to 15thDay
21%218%25%21%10%
255
16th to endDay
9%417%26%32%10%
272
November1st to 15thDay
15%6%21%25%26%3
232
16th to endDay
10%6%24%28%25%4
90
December1st to 15thDay
22%5%32%24%13%2
90
16th to endDay
27%14%20%22%11%3
96
January1st to 15thNight
16%10%33%28%10%
720
16th to endNight
19%10%33%28%6%
768
February1st to 15thNight
16%10%30%35%6%
704
16th to endNight
13%10%37%33%4
631
March1st to 15thNight
12%8%35%35%8%
720
16th to endNight
13%8%30%32%14%
768
April1st to 15thNight
12%7%31%32%15%
718
16th to endNight
9%5%26%39%18%
609
May1st to 15thNight
7%327%38%18%2
540
16th to endNight
7%117%34%33%4
563
June1st to 15thNight
12%210%35%32%7%
540
16th to endNight
8%6%33%40%10%
540
July1st to 15thNight
8%7%28%45%9%
510
16th to endNight
8%5%31%47%6%
564
August1st to 15thNight
7%5%26%49%10%
539
16th to endNight
12%17%25%43%10%
645
September1st to 15thNight
12%18%25%43%7%
720
16th to endNight
16%114%33%32%2
693
October1st to 15thNight
17%216%33%25%3
660
16th to endNight
10%321%34%27%2
704
November1st to 15thNight
13%5%20%36%22%2
700
16th to endNight
16%6%27%29%20%
720
December1st to 15thNight
16%6%27%31%15%2
720
16th to endNight
19%9%27%30%12%
768
Page generated at 2017-09-25 09:02:22. Version ac04

Copyright 2017 A. Danko


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