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 2005-11-05 to 2017-09-19.

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
50%
15407
26%
46974
18%
15362
57%
15362
33%
27422

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
41%
897
25%
2327
12%
895
48%
895
32%
1472
January16th to endNight
38%
876
30%
2412
14%
876
48%
876
31%
1484
February1st to 15thNight
35%
704
27%
2106
13%
698
47%
698
30%
1239
February16th to endNight
25%
632
28%
1894
9%
632
40%
632
29%
1117
March1st to 15thNight
32%
558
28%
1998
15%
558
48%
558
25%
1174
March16th to endNight
33%
576
22%
2112
15%
576
43%
576
24%
1226
April1st to 15thNight
29%
539
22%
1977
12%
539
44%
539
24%
1143
April16th to endNight
30%
540
24%
1773
13%
540
45%
540
25%
1115
May1st to 15thNight
35%
539
21%
1620
15%
539
44%
539
28%
1024
May16th to endNight
46%
562
24%
1684
19%
561
56%
561
32%
1011
June1st to 15thNight
65%
540
23%
1620
23%
540
71%
540
47%
838
June16th to endNight
65%
540
21%
1620
21%
540
69%
540
36%
953
July1st to 15thNight
75%
510
20%
1530
22%
510
79%
510
42%
847
July16th to endNight
81%
564
23%
1594
27%
531
85%
531
43%
864
August1st to 15thNight
88%
539
28%
1679
32%
536
88%
536
52%
800
August16th to endNight
77%
573
20%
1973
24%
573
77%
573
32%
1117
September1st to 15thNight
74%
540
20%
1980
25%
540
75%
540
32%
1104
September16th to endNight
69%
504
24%
1857
27%
504
71%
504
29%
1057
October1st to 15thNight
68%
498
34%
1818
35%
498
72%
498
39%
910
October16th to endNight
60%
704
35%
2112
24%
704
67%
704
47%
1075
November1st to 15thNight
51%
712
28%
2112
18%
712
57%
712
34%
1245
November16th to endNight
47%
900
30%
2340
15%
900
54%
900
38%
1404
December1st to 15thNight
40%
900
28%
2340
15%
900
45%
900
30%
1540
December16th to endNight
38%
960
24%
2496
11%
960
44%
960
28%
1663

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
13%5%2442445%7%44%
47831
Night
13%423323347%51%
47176
Cloud CoverDay
13%5%2443445%7%44%
47829
Night
13%423323347%51%
47176
TransparencyDay
39%49%19%17%9%
47529
Night
33%411%20%18%11%
46981
SeeingDay
13%417%30%26%6%
5365
Night
15%5%23%35%18%1
15412

By Month

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
17%5%24324335%47%
1611
16th to endDay
15%5%23123335%52%
1821
February1st to 15thDay
15%5%23323345%50%
1750
16th to endDay
19%6%13222335%48%
1580
March1st to 15thDay
22%6%14313235%45%
1962
16th to endDay
18%5%24313335%47%
2112
April1st to 15thDay
15%5%24434445%44%
1971
16th to endDay
16%6%25%433346%42%
2184
May1st to 15thDay
17%7%36%5%245%46%36%
2331
16th to endDay
18%6%36%5%3446%7%34%
2430
June1st to 15thDay
6%425%436%5%7%8%43%
2340
16th to endDay
12%5%36%6%46%5%7%9%32%
2340
July1st to 15thDay
9%425%6%46%6%7%8%38%
2340
16th to endDay
8%325%5%46%6%7%11%39%
2466
August1st to 15thDay
22123345%5%11%55%
2268
16th to endDay
9%5%248%36%5%6%10%37%
2226
September1st to 15thDay
9%5%27%6%45%46%7%38%
1980
16th to endDay
10%5%25%6%245%5%7%43%
1848
October1st to 15thDay
11%41442445%6%50%
1800
16th to endDay
7%412223335%62%
1760
November1st to 15thDay
14%6%25%5%24346%43%
1743
16th to endDay
15%5%1441323453%
1620
December1st to 15thDay
20%6%23323335%43%
1620
16th to endDay
16%7%24424436%42%
1728
January1st to 15thNight
17%413323435%48%
2327
16th to endNight
18%413313345%50%
2412
February1st to 15thNight
17%5%2331223453%
2102
16th to endNight
17%5%12313336%50%
1894
March1st to 15thNight
17%7%23223325%49%
1998
16th to endNight
18%5%23312445%47%
2112
April1st to 15thNight
16%5%23423337%47%
1977
16th to endNight
19%5%23323345%45%
1773
May1st to 15thNight
15%5%2432445%7%44%
1619
16th to endNight
13%43342335%9%45%
1683
June1st to 15thNight
431232335%9%59%
1620
16th to endNight
6%32442445%9%51%
1620
July1st to 15thNight
6%22332446%10%52%
1620
16th to endNight
6%21231446%11%53%
1702
August1st to 15thNight
111212346%11%62%
1689
16th to endNight
8%214435%5%6%11%46%
1973
September1st to 15thNight
7%415%4235%49%49%
1980
16th to endNight
9%5%1443346%7%48%
1857
October1st to 15thNight
11%313323347%57%
1818
16th to endNight
9%312212135%65%
2112
November1st to 15thNight
12%423423345%52%
2112
16th to endNight
16%42221224456%
2340
December1st to 15thNight
18%5%2432333448%
2340
16th to endNight
18%7%2441333446%
2496

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
17%5%24323335%47%
1611
16th to endDay
15%5%22113335%52%
1821
February1st to 15thDay
15%6%13423345%50%
1750
16th to endDay
19%5%13232335%48%
1580
March1st to 15thDay
21%6%24313335%45%
1962
16th to endDay
18%5%2431334447%
2110
April1st to 15thDay
15%6%2443445%5%44%
1971
16th to endDay
16%6%25%434346%42%
2184
May1st to 15thDay
16%7%36%5%2445%6%36%
2331
16th to endDay
18%6%25%43446%7%34%
2430
June1st to 15thDay
6%435%5%36%6%6%9%43%
2340
16th to endDay
12%6%36%6%36%6%5%10%32%
2340
July1st to 15thDay
9%425%6%46%5%6%9%38%
2340
16th to endDay
8%3246%46%5%6%11%39%
2466
August1st to 15thDay
22123345%5%11%56%
2268
16th to endDay
9%6%247%46%5%6%10%37%
2226
September1st to 15thDay
9%5%36%7%45%46%7%38%
1980
16th to endDay
10%5%25%6%35%45%7%43%
1848
October1st to 15thDay
10%5%24424346%50%
1800
16th to endDay
8%312312335%62%
1760
November1st to 15thDay
13%6%25%5%2435%6%43%
1743
16th to endDay
15%6%2331323353%
1620
December1st to 15thDay
21%6%23323336%43%
1620
16th to endDay
16%7%24423446%43%
1728
January1st to 15thNight
17%5%13313435%48%
2327
16th to endNight
18%413323345%50%
2412
February1st to 15thNight
17%5%2321223453%
2102
16th to endNight
16%5%13313345%51%
1894
March1st to 15thNight
17%6%23223325%49%
1998
16th to endNight
17%5%2331245%5%47%
2112
April1st to 15thNight
16%5%23423347%47%
1977
16th to endNight
19%5%24423345%45%
1773
May1st to 15thNight
15%5%2431445%7%44%
1619
16th to endNight
13%42442336%9%45%
1683
June1st to 15thNight
431222335%9%60%
1620
16th to endNight
6%2244245%5%10%50%
1620
July1st to 15thNight
6%3133245%5%10%53%
1620
16th to endNight
6%212315%46%12%53%
1702
August1st to 15thNight
12222346%11%62%
1689
16th to endNight
8%313435%5%6%11%46%
1973
September1st to 15thNight
7%415%4235%5%9%49%
1980
16th to endNight
9%5%1442436%8%48%
1857
October1st to 15thNight
11%312323346%57%
1818
16th to endNight
9%312221235%65%
2112
November1st to 15thNight
12%5%23413345%52%
2112
16th to endNight
15%422212235%56%
2340
December1st to 15thNight
19%5%1432334447%
2340
16th to endNight
18%7%2432333446%
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
40%2317%20%13%
1612
16th to endDay
36%35%14%24%14%
1821
February1st to 15thDay
38%26%14%21%16%
1751
16th to endDay
39%25%17%20%13%
1580
March1st to 15thDay
43%37%12%19%13%
1962
16th to endDay
40%27%17%20%11%
2112
April1st to 15thDay
41%38%15%18%12%
1971
16th to endDay
44%35%19%15%10%
2184
May1st to 15thDay
48%48%14%17%6%
2331
16th to endDay
48%5%8%16%16%4
2434
June1st to 15thDay
35%7%11%21%20%4
2340
16th to endDay
46%7%10%20%12%2
2340
July1st to 15thDay
39%7%14%25%11%2
2201
16th to endDay
34%7%19%27%9%
2311
August1st to 15thDay
22%6%16%36%16%1
2254
16th to endDay
41%6%15%22%10%2
2226
September1st to 15thDay
42%6%10%18%16%4
1980
16th to endDay
37%5%10%19%17%9%
1848
October1st to 15thDay
33%410%18%19%13%
1800
16th to endDay
25%35%17%27%20%
1760
November1st to 15thDay
41%37%20%16%10%
1743
16th to endDay
36%25%12%21%20%
1620
December1st to 15thDay
43%37%17%18%9%
1620
16th to endDay
43%37%14%19%10%
1728
January1st to 15thNight
37%26%16%19%17%
2327
16th to endNight
37%25%15%21%17%
2412
February1st to 15thNight
36%26%13%21%20%
2112
16th to endNight
36%25%17%21%15%
1894
March1st to 15thNight
40%27%15%19%15%
1998
16th to endNight
38%49%15%20%11%
2112
April1st to 15thNight
38%38%16%17%15%
1977
16th to endNight
41%48%17%14%12%
1773
May1st to 15thNight
38%5%11%17%16%9%
1620
16th to endNight
35%415%25%15%3
1685
June1st to 15thNight
22%414%28%26%3
1620
16th to endNight
30%7%17%27%14%2
1620
July1st to 15thNight
23%8%22%32%12%1
1530
16th to endNight
22%8%28%31%8%
1594
August1st to 15thNight
14%8%22%39%14%
1679
16th to endNight
29%9%22%27%9%2
1973
September1st to 15thNight
31%6%18%22%15%5%
1980
16th to endNight
33%6%14%19%16%9%
1857
October1st to 15thNight
27%311%26%19%13%
1818
16th to endNight
24%16%17%27%21%
2112
November1st to 15thNight
34%39%20%18%13%
2112
16th to endNight
32%27%13%20%22%
2340
December1st to 15thNight
41%35%18%20%10%
2340
16th to endNight
42%3413%22%13%
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
16%7%35%32%7%
90
16th to endDay
19%10%35%19%12%2
96
February1st to 15thDay
14%15%35%26%6%1
90
16th to endDay
22%16%37%22%11
81
March1st to 15thDay
27%12%38%13%7%
90
16th to endDay
14%7%33%34%9%1
96
April1st to 15thDay
14%13%33%34%4
90
16th to endDay
19%8%39%24%8%
294
May1st to 15thDay
17%9%36%29%6%
468
16th to endDay
25%5%20%31%15%1
517
June1st to 15thDay
8%12%44%31%3
495
16th to endDay
11%110%35%30%10%
495
July1st to 15thDay
7%19%36%36%7%
475
16th to endDay
6%427%49%12%
515
August1st to 15thDay
41226%50%14%
421
16th to endDay
13%2435%27%16%
266
September1st to 15thDay
5%7%20%48%17%
135
16th to endDay
8%116%53%21%
99
October1st to 15thDay
4240%44%8%
90
16th to endDay
16%214%35%26%5%
96
November1st to 15thDay
26%5%26%25%10%5%
90
16th to endDay
23%14%25%21%12%3
90
December1st to 15thDay
27%222%26%8%12%
90
16th to endDay
26%10%34%22%5%1
96
January1st to 15thNight
20%8%25%35%9%
900
16th to endNight
20%9%28%32%9%
876
February1st to 15thNight
21%10%29%29%8%
704
16th to endNight
22%10%40%23%3
632
March1st to 15thNight
21%7%33%31%5%
558
16th to endNight
19%8%35%30%5%
576
April1st to 15thNight
17%10%37%29%4
539
16th to endNight
20%7%38%24%7%
540
May1st to 15thNight
16%6%37%32%6%
540
16th to endNight
15%428%35%16%
563
June1st to 15thNight
421%48%22%1
540
16th to endNight
7%115%49%23%2
540
July1st to 15thNight
5%10%48%32%1
510
16th to endNight
6%5%46%35%4
564
August1st to 15thNight
1444%43%5%
539
16th to endNight
9%340%41%4
573
September1st to 15thNight
8%436%42%6%
540
16th to endNight
10%9%30%43%4
504
October1st to 15thNight
12%112%39%28%4
498
16th to endNight
11%320%34%26%2
704
November1st to 15thNight
13%5%23%33%23%1
712
16th to endNight
17%8%22%34%16%1
900
December1st to 15thNight
22%7%26%29%12%1
900
16th to endNight
23%7%25%32%9%
960
Page generated at 2017-09-20 03:49:47. Version ac04

Copyright 2017 A. Danko


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