Tye Preston Memorial Library
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tye Preston Memorial Library clear sky chart from 2005-11-06 to 2024-01-27.

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
46%
23790
24%
74483
17%
23732
54%
23732
30%
44100

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
38%
947
29%
2458
13%
946
48%
946
35%
1519
January11th to 20thNight
34%
950
26%
2470
10%
950
44%
950
31%
1560
January21st to endNight
33%
896
29%
2536
11%
896
45%
896
27%
1601
February1st to 10thNight
29%
720
26%
2160
10%
720
41%
720
25%
1338
February11th to 20thNight
27%
704
22%
2098
9%
700
38%
700
25%
1308
February21st to endNight
19%
592
25%
1776
6%
592
35%
592
23%
1131
March1st to 10thNight
25%
567
24%
2007
10%
567
40%
567
21%
1243
March11th to 20thNight
29%
540
21%
1980
13%
540
39%
540
20%
1226
March21st to endNight
29%
594
24%
2178
13%
594
43%
594
24%
1261
April1st to 10thNight
27%
540
23%
1980
12%
540
44%
540
23%
1161
April11th to 20thNight
27%
533
21%
1953
12%
533
42%
533
21%
1190
April21st to endNight
27%
540
27%
1670
14%
540
45%
540
26%
1066
May1st to 10thNight
31%
540
24%
1620
16%
540
41%
540
27%
1047
May11th to 20thNight
40%
531
21%
1594
18%
531
48%
531
28%
999
May21st to endNight
43%
583
21%
1745
14%
582
55%
582
29%
1086
June1st to 10thNight
62%
540
23%
1620
22%
540
69%
540
43%
865
June11th to 20thNight
54%
540
23%
1620
20%
540
64%
540
44%
866
June21st to endNight
62%
540
22%
1620
22%
540
67%
540
35%
957
July1st to 10thNight
67%
525
16%
1575
17%
525
70%
525
32%
966
July11th to 20thNight
77%
522
26%
1566
26%
522
82%
522
47%
812
July21st to endNight
78%
591
24%
1673
25%
558
83%
558
47%
868
August1st to 10thNight
83%
537
28%
1613
29%
534
86%
534
59%
743
August11th to 20thNight
77%
534
24%
1778
26%
534
79%
534
35%
981
August21st to endNight
78%
594
19%
2074
22%
594
79%
594
33%
1144
September1st to 10thNight
70%
540
20%
1980
23%
540
72%
540
30%
1121
September11th to 20thNight
68%
540
21%
1980
23%
540
70%
540
26%
1189
September21st to endNight
66%
540
24%
1980
26%
540
69%
540
32%
1077
October1st to 10thNight
72%
540
32%
1980
34%
540
74%
540
39%
987
October11th to 20thNight
54%
634
31%
2074
23%
634
61%
634
33%
1195
October21st to endNight
50%
792
33%
2376
20%
792
58%
792
38%
1313
November1st to 10thNight
54%
740
28%
2220
19%
740
58%
740
33%
1341
November11th to 20thNight
47%
874
26%
2394
15%
874
54%
874
35%
1470
November21st to endNight
38%
950
26%
2470
12%
950
45%
950
29%
1607
December1st to 10thNight
40%
950
30%
2470
15%
950
47%
950
33%
1574
December11th to 20thNight
34%
950
25%
2887
12%
948
41%
948
23%
1900
December21st to endNight
36%
1040
17%
4308
12%
1026
44%
1026
22%
2388

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%5%25%5%3445%7%46%
75011
Night
15%5%24423447%50%
75063
TransparencyDay
39%5%9%20%18%9%
74736
Night
35%5%11%20%18%11%
74858
SeeingDay
15%7%22%30%21%5%
13722
Night
17%7%25%34%16%1
23799

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
16%5%25%323346%51%
1707
11th to 20thDay
20%7%3332344448%
1710
21st to endDay
17%5%34323437%50%
1978
February1st to 10thDay
20%7%24423335%45%
1800
11th to 20thDay
20%7%24334446%43%
1749
21st to endDay
21%5%24232335%49%
1480
March1st to 10thDay
20%8%25%424345%42%
1952
11th to 20thDay
17%6%34434445%45%
1980
21st to endDay
18%5%25%5%23346%47%
2178
April1st to 10thDay
15%8%35%5%34446%42%
1980
11th to 20thDay
22%6%2432335%5%43%
1956
21st to endDay
15%6%25%5%34446%45%
2285
May1st to 10thDay
17%7%36%5%35%5%5%6%39%
2340
11th to 20thDay
15%7%36%5%45%46%6%38%
2302
21st to endDay
18%5%25%6%45%5%7%8%33%
2531
June1st to 10thDay
7%424436%7%6%10%47%
2340
11th to 20thDay
9%5%35%5%36%5%6%9%45%
2340
21st to endDay
9%6%36%7%47%7%6%10%35%
2340
July1st to 10thDay
12%5%36%6%5%7%6%6%9%36%
2340
11th to 20thDay
5%3245%46%6%7%10%48%
2340
21st to endDay
6%3245%36%6%6%12%48%
2546
August1st to 10thDay
22234345%5%10%60%
2311
11th to 20thDay
6%4145%45%5%6%10%49%
2123
21st to endDay
6%5%247%5%6%7%7%11%42%
2280
September1st to 10thDay
9%5%37%7%45%46%7%42%
1980
11th to 20thDay
12%7%36%6%45%6%7%7%39%
1980
21st to endDay
9%425%6%36%46%7%48%
1980
October1st to 10thDay
7%5%245%35%45%7%53%
1980
11th to 20thDay
14%5%1443445%5%50%
1867
21st to endDay
13%423423346%57%
1980
November1st to 10thDay
16%7%35%5%24347%44%
1858
11th to 20thDay
12%6%25%5%3435%5%49%
1782
21st to endDay
19%7%36%5%2433445%
1710
December1st to 10thDay
18%5%34433346%46%
1710
11th to 20thDay
18%7%345%34445%44%
2161
21st to endDay
19%5%1332234454%
3135
January1st to 10thNight
18%5%13322446%52%
2458
11th to 20thNight
19%6%24323335%51%
2470
21st to endNight
20%6%23423345%48%
2536
February1st to 10thNight
20%7%24422335%47%
2160
11th to 20thNight
21%7%23323235%49%
2098
21st to endNight
22%7%23414345%45%
1776
March1st to 10thNight
19%7%24433345%44%
2007
11th to 20thNight
19%8%34324445%45%
1980
21st to endNight
18%6%24323445%48%
2178
April1st to 10thNight
16%6%24434448%45%
1980
11th to 20thNight
21%6%24423345%45%
1953
21st to endNight
20%6%24423345%47%
1670
May1st to 10thNight
17%6%245%24446%44%
1620
11th to 20thNight
15%5%35%43445%8%45%
1593
21st to endNight
16%5%25%42446%10%42%
1745
June1st to 10thNight
7%41332435%9%59%
1620
11th to 20thNight
5%32443445%10%57%
1620
21st to endNight
9%4245%345%6%10%49%
1620
July1st to 10thNight
10%5%34335%5%6%10%46%
1620
11th to 20thNight
41134145%6%10%59%
1620
21st to endNight
6%31332446%12%57%
1772
August1st to 10thNight
22222235%11%69%
1623
11th to 20thNight
5%3134346%7%11%50%
1778
21st to endNight
7%423435%6%7%10%49%
2074
September1st to 10thNight
8%5%26%4245%6%8%50%
1980
11th to 20thNight
11%6%25%5%345%5%8%46%
1980
21st to endNight
10%5%2442446%8%51%
1980
October1st to 10thNight
7%41342445%7%58%
1980
11th to 20thNight
15%5%24333346%52%
2074
21st to endNight
16%41332223461%
2376
November1st to 10thNight
15%6%25%423345%53%
2220
11th to 20thNight
13%5%24424445%53%
2394
21st to endNight
21%5%2442335%5%46%
2470
December1st to 10thNight
18%6%24423345%50%
2470
11th to 20thNight
21%7%25%42333446%
3036
21st to endNight
21%5%23212235%53%
4532

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
37%3418%22%15%
1708
11th to 20thDay
40%3415%21%16%
1710
21st to endDay
38%45%13%25%15%
1978
February1st to 10thDay
44%35%13%21%15%
1800
11th to 20thDay
44%36%17%18%12%
1759
21st to endDay
41%36%16%18%16%
1480
March1st to 10thDay
46%48%13%18%10%
1952
11th to 20thDay
43%47%13%20%15%
1980
21st to endDay
41%38%18%18%12%
2178
April1st to 10thDay
44%48%17%17%11%
1980
11th to 20thDay
44%46%16%19%11%
1956
21st to endDay
42%45%21%19%10%
2285
May1st to 10thDay
47%47%16%19%7%
2340
11th to 20thDay
46%5%8%19%17%4
2306
21st to endDay
47%6%11%19%15%2
2531
June1st to 10thDay
31%7%13%26%19%3
2340
11th to 20thDay
36%6%11%25%17%4
2340
21st to endDay
43%8%10%23%13%3
2340
July1st to 10thDay
43%8%13%25%10%2
2276
11th to 20thDay
28%7%18%27%17%2
2271
21st to endDay
27%7%19%33%13%1
2405
August1st to 10thDay
21%6%16%37%19%1
2297
11th to 20thDay
30%6%17%30%13%4
2123
21st to endDay
35%8%16%25%13%3
2280
September1st to 10thDay
41%6%9%20%16%8%
1980
11th to 20thDay
40%8%12%18%17%5%
1980
21st to endDay
34%5%10%22%17%11%
1980
October1st to 10thDay
30%5%11%20%24%10%
1980
11th to 20thDay
36%5%8%15%19%16%
1867
21st to endDay
32%25%16%25%20%
1980
November1st to 10thDay
41%47%19%18%10%
1858
11th to 20thDay
38%38%17%20%13%
1782
21st to endDay
45%35%14%19%13%
1710
December1st to 10thDay
40%39%19%20%9%
1710
11th to 20thDay
44%46%11%20%15%
2159
21st to endDay
37%25%15%27%14%
3135
January1st to 10thNight
34%36%16%22%18%
2458
11th to 20thNight
38%26%17%22%16%
2470
21st to endNight
41%3414%19%19%
2536
February1st to 10thNight
41%37%13%20%17%
2160
11th to 20thNight
41%35%16%19%16%
2098
21st to endNight
43%36%15%17%16%
1776
March1st to 10thNight
44%38%16%18%11%
2007
11th to 20thNight
44%48%12%17%16%
1980
21st to endNight
39%410%16%18%14%
2178
April1st to 10thNight
39%49%18%17%13%
1980
11th to 20thNight
43%47%14%17%15%
1953
21st to endNight
42%48%19%17%10%
1670
May1st to 10thNight
41%5%9%18%17%9%
1620
11th to 20thNight
39%5%10%25%16%4
1594
21st to endNight
38%6%16%25%14%1
1746
June1st to 10thNight
24%5%16%29%23%3
1620
11th to 20thNight
24%5%16%31%19%5%
1620
21st to endNight
31%7%18%27%14%3
1620
July1st to 10thNight
31%10%20%28%9%1
1575
11th to 20thNight
19%8%23%32%17%1
1566
21st to endNight
19%8%28%35%9%1
1673
August1st to 10thNight
13%6%24%40%17%1
1613
11th to 20thNight
25%10%21%30%11%2
1778
21st to endNight
28%9%22%27%11%3
2074
September1st to 10thNight
32%7%16%22%16%7%
1980
11th to 20thNight
37%7%15%22%14%5%
1980
21st to endNight
31%6%14%21%17%12%
1980
October1st to 10thNight
26%412%25%21%11%
1980
11th to 20thNight
35%48%17%19%17%
2074
21st to endNight
31%26%17%23%22%
2376
November1st to 10thNight
36%48%18%21%13%
2220
11th to 20thNight
34%5%8%18%19%16%
2394
21st to endNight
41%38%16%19%14%
2470
December1st to 10thNight
38%37%20%20%12%
2470
11th to 20thNight
43%3411%19%19%
3037
21st to endNight
37%3414%29%13%
4532

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
21%16%36%22%5%
270
11th to 20thDay
29%14%29%21%7%1
270
21st to endDay
21%16%38%20%6%
282
February1st to 10thDay
34%11%33%17%5%
240
11th to 20thDay
31%16%32%18%31
240
21st to endDay
29%24%34%13%1
198
March1st to 10thDay
27%16%40%15%2
240
11th to 20thDay
20%19%41%13%6%
240
21st to endDay
22%15%36%22%4
264
April1st to 10thDay
21%17%39%20%3
240
11th to 20thDay
25%17%32%23%4
234
21st to endDay
19%13%37%23%8%
545
May1st to 10thDay
19%11%36%28%6%
603
11th to 20thDay
20%9%26%33%11%1
625
21st to endDay
19%5%23%37%14%1
679
June1st to 10thDay
9%216%40%29%4
630
11th to 20thDay
9%216%46%25%2
630
21st to endDay
10%214%38%27%9%
630
July1st to 10thDay
10%39%35%34%9%
621
11th to 20thDay
47%34%46%8%
618
21st to endDay
5%6%36%44%8%
684
August1st to 10thDay
316%35%44%11%
609
11th to 20thDay
6%1338%42%10%
437
21st to endDay
6%16%27%41%20%
399
September1st to 10thDay
11%17%26%41%14%
270
11th to 20thDay
8%8%29%43%13%
270
21st to endDay
10%110%28%38%12%
270
October1st to 10thDay
6%415%38%30%7%
270
11th to 20thDay
13%6%20%37%18%5%
270
21st to endDay
22%10%27%24%13%3
297
November1st to 10thDay
20%6%29%29%13%3
270
11th to 20thDay
21%11%27%28%10%2
270
21st to endDay
26%18%30%16%9%1
270
December1st to 10thDay
20%8%33%27%6%5%
270
11th to 20thDay
23%16%31%23%7%
270
21st to endDay
24%8%39%22%6%1
297
January1st to 10thNight
21%9%28%33%8%1
950
11th to 20thNight
22%11%29%30%8%
950
21st to endNight
23%10%28%30%8%
896
February1st to 10thNight
27%12%29%26%7%
720
11th to 20thNight
28%12%32%24%4
704
21st to endNight
28%13%38%19%2
592
March1st to 10thNight
24%12%35%26%4
567
11th to 20thNight
24%10%32%29%5%
540
21st to endNight
21%9%39%27%41
594
April1st to 10thNight
18%11%39%27%5%
540
11th to 20thNight
23%10%36%26%4
533
21st to endNight
21%10%38%23%7%
540
May1st to 10thNight
18%9%39%28%6%
540
11th to 20thNight
17%7%32%34%10%1
531
21st to endNight
17%5%29%36%13%
584
June1st to 10thNight
7%222%45%22%1
540
11th to 20thNight
6%229%48%15%1
540
21st to endNight
9%218%42%26%3
540
July1st to 10thNight
10%114%43%30%3
525
11th to 20thNight
3112%50%31%2
522
21st to endNight
6%19%50%31%4
591
August1st to 10thNight
219%49%35%4
537
11th to 20thNight
5%19%48%34%3
534
21st to endNight
7%15%40%42%5%
594
September1st to 10thNight
8%110%37%39%6%
540
11th to 20thNight
12%18%37%39%3
540
21st to endNight
11%113%33%39%4
540
October1st to 10thNight
8%212%44%29%4
540
11th to 20thNight
17%319%38%21%3
634
21st to endNight
18%7%22%32%20%2
792
November1st to 10thNight
17%5%18%38%21%1
740
11th to 20thNight
15%7%25%35%15%2
874
21st to endNight
22%9%26%30%12%1
950
December1st to 10thNight
21%7%28%30%12%2
950
11th to 20thNight
27%8%26%31%8%
950
21st to endNight
23%7%29%33%8%
1045
Page generated at 2024-01-24 08:37:28. Version ac07

Copyright 2024 Attilla Danko


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