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 2018-10-27.

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
50%
16822
25%
51339
18%
16777
57%
16777
32%
30140

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%
972
26%
2522
13%
970
49%
970
32%
1589
January16th to endNight
38%
949
29%
2613
13%
949
49%
949
31%
1613
February1st to 15thNight
33%
764
26%
2286
12%
758
45%
758
29%
1372
February16th to endNight
24%
684
26%
2050
8%
684
38%
684
26%
1254
March1st to 15thNight
31%
605
27%
2165
14%
605
46%
605
25%
1280
March16th to endNight
32%
624
22%
2288
15%
624
42%
624
24%
1332
April1st to 15thNight
29%
584
22%
2142
12%
584
44%
584
24%
1232
April16th to endNight
31%
579
24%
1897
14%
579
46%
579
25%
1182
May1st to 15thNight
35%
584
22%
1755
16%
584
44%
584
28%
1108
May16th to endNight
48%
610
24%
1828
19%
609
58%
609
34%
1072
June1st to 15thNight
65%
585
24%
1755
23%
585
72%
585
49%
892
June16th to endNight
65%
585
20%
1755
20%
585
69%
585
36%
1034
July1st to 15thNight
76%
555
19%
1665
20%
555
80%
555
42%
918
July16th to endNight
81%
612
24%
1738
27%
579
86%
579
45%
922
August1st to 15thNight
88%
575
28%
1789
32%
572
88%
572
52%
854
August16th to endNight
79%
621
19%
2138
23%
621
79%
621
33%
1184
September1st to 15thNight
71%
585
19%
2145
23%
585
72%
585
28%
1255
September16th to endNight
66%
582
22%
2145
25%
582
68%
582
26%
1270
October1st to 15thNight
69%
588
32%
2148
33%
588
72%
588
37%
1108
October16th to endNight
58%
816
35%
2448
24%
816
65%
816
44%
1285
November1st to 15thNight
51%
773
26%
2293
17%
773
56%
773
32%
1386
November16th to endNight
48%
975
31%
2535
15%
975
56%
975
39%
1498
December1st to 15thNight
40%
975
28%
2535
15%
975
45%
975
30%
1668
December16th to endNight
36%
1040
23%
2704
11%
1040
43%
1040
27%
1832

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%2443445%7%43%
52204
Night
13%423323347%50%
51541
Cloud CoverDay
13%5%2443445%7%44%
52202
Night
13%423323347%51%
51541
TransparencyDay
39%49%19%17%8%
51902
Night
33%411%20%18%11%
51346
SeeingDay
14%418%30%25%6%
6786
Night
15%5%22%35%18%1
16827

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%
1746
16th to endDay
16%5%23123335%52%
1973
February1st to 15thDay
16%5%2432434447%
1900
16th to endDay
21%6%13222335%45%
1710
March1st to 15thDay
21%6%24313335%44%
2125
16th to endDay
18%5%25%313335%47%
2288
April1st to 15thDay
15%5%24434446%44%
2136
16th to endDay
16%6%25%433446%42%
2335
May1st to 15thDay
16%7%36%5%24446%38%
2526
16th to endDay
17%5%36%5%3446%7%36%
2638
June1st to 15thDay
6%425%437%5%6%9%44%
2535
16th to endDay
12%5%36%6%46%5%7%8%32%
2535
July1st to 15thDay
9%425%6%46%5%7%8%39%
2535
16th to endDay
7%325%5%45%5%6%10%43%
2674
August1st to 15thDay
22133345%5%11%55%
2411
16th to endDay
8%4247%36%5%6%10%38%
2413
September1st to 15thDay
11%6%37%6%45%46%6%35%
2145
16th to endDay
11%6%26%6%3446%7%39%
2145
October1st to 15thDay
10%5%145%25%45%6%48%
2127
16th to endDay
11%412323335%60%
2033
November1st to 15thDay
15%6%26%5%24346%41%
1892
16th to endDay
14%5%1441323454%
1755
December1st to 15thDay
21%6%23323345%42%
1755
16th to endDay
18%7%24424436%41%
1872
January1st to 15thNight
18%413322435%49%
2522
16th to endNight
18%5%13313345%49%
2613
February1st to 15thNight
19%5%2331222451%
2282
16th to endNight
19%5%23313436%47%
2050
March1st to 15thNight
17%7%23323335%48%
2165
16th to endNight
17%5%23313445%47%
2288
April1st to 15thNight
15%5%23423347%47%
2142
16th to endNight
18%5%24323345%46%
1897
May1st to 15thNight
15%5%25%32445%7%44%
1754
16th to endNight
12%42342335%9%47%
1827
June1st to 15thNight
331232335%9%61%
1755
16th to endNight
7%32442445%9%50%
1755
July1st to 15thNight
6%22332446%11%53%
1755
16th to endNight
5%21231446%11%55%
1846
August1st to 15thNight
121212346%11%63%
1799
16th to endNight
8%2133345%6%11%48%
2138
September1st to 15thNight
8%5%25%5%245%5%8%46%
2145
16th to endNight
10%6%15%43346%7%45%
2145
October1st to 15thNight
10%313423347%55%
2148
16th to endNight
12%31221213463%
2448
November1st to 15thNight
12%5%24424445%50%
2293
16th to endNight
15%41221224457%
2535
December1st to 15thNight
19%5%1432333447%
2535
16th to endNight
20%7%2441323444%
2704

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%
1746
16th to endDay
15%5%23213335%52%
1973
February1st to 15thDay
16%6%24423335%47%
1900
16th to endDay
21%6%24232335%45%
1710
March1st to 15thDay
20%6%24313335%44%
2125
16th to endDay
17%5%2432334447%
2286
April1st to 15thDay
15%6%24434445%44%
2136
16th to endDay
15%6%25%434346%42%
2335
May1st to 15thDay
16%7%36%5%34446%38%
2526
16th to endDay
17%6%25%5%3446%7%36%
2638
June1st to 15thDay
5%4245%36%6%6%9%44%
2535
16th to endDay
12%5%36%6%36%6%6%9%32%
2535
July1st to 15thDay
8%425%6%46%5%6%9%39%
2535
16th to endDay
7%3245%46%5%6%11%42%
2674
August1st to 15thDay
22133345%5%10%55%
2411
16th to endDay
8%5%147%45%5%6%9%39%
2413
September1st to 15thDay
11%6%36%7%45%46%6%36%
2145
16th to endDay
11%6%26%6%35%46%7%39%
2145
October1st to 15thDay
10%5%24435%347%48%
2127
16th to endDay
11%31231233460%
2033
November1st to 15thDay
14%6%25%5%2435%6%41%
1892
16th to endDay
13%5%2331323454%
1755
December1st to 15thDay
21%6%23323335%42%
1755
16th to endDay
18%7%25%423446%41%
1872
January1st to 15thNight
17%5%13313435%49%
2522
16th to endNight
18%5%23323345%49%
2613
February1st to 15thNight
18%5%2331223451%
2282
16th to endNight
18%5%13313345%47%
2050
March1st to 15thNight
17%6%23323335%48%
2165
16th to endNight
17%5%23312445%47%
2288
April1st to 15thNight
15%5%23423347%47%
2142
16th to endNight
18%5%24423345%46%
1897
May1st to 15thNight
15%5%2431435%7%44%
1754
16th to endNight
12%42442336%9%47%
1827
June1st to 15thNight
431222335%9%61%
1755
16th to endNight
7%3244335%5%9%50%
1755
July1st to 15thNight
6%3132245%5%10%54%
1755
16th to endNight
6%21231445%11%55%
1846
August1st to 15thNight
12222346%11%63%
1799
16th to endNight
7%213435%5%6%11%48%
2138
September1st to 15thNight
8%5%25%4245%5%8%46%
2145
16th to endNight
10%6%245%2436%8%45%
2145
October1st to 15thNight
10%313424347%55%
2148
16th to endNight
11%41221213463%
2448
November1st to 15thNight
12%5%24414445%50%
2293
16th to endNight
14%42221223457%
2535
December1st to 15thNight
19%5%1432334447%
2535
16th to endNight
20%7%2442223444%
2704

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%21%13%
1747
16th to endDay
36%35%14%26%14%
1973
February1st to 15thDay
40%26%14%20%15%
1901
16th to endDay
43%25%16%19%12%
1710
March1st to 15thDay
44%37%12%19%12%
2125
16th to endDay
40%28%17%19%11%
2288
April1st to 15thDay
41%38%16%17%12%
2136
16th to endDay
43%36%20%16%10%
2335
May1st to 15thDay
47%48%15%18%6%
2526
16th to endDay
46%5%8%18%16%3
2642
June1st to 15thDay
33%7%12%22%19%4
2535
16th to endDay
46%7%10%20%13%2
2535
July1st to 15thDay
38%7%15%24%12%2
2396
16th to endDay
32%7%18%29%11%
2519
August1st to 15thDay
23%6%16%35%16%1
2397
16th to endDay
39%6%16%23%11%2
2413
September1st to 15thDay
45%6%10%17%15%4
2145
16th to endDay
41%6%10%17%15%8%
2145
October1st to 15thDay
35%412%18%17%11%
2127
16th to endDay
28%25%15%26%20%
2033
November1st to 15thDay
42%38%19%15%10%
1892
16th to endDay
35%25%14%21%19%
1755
December1st to 15thDay
44%37%16%18%9%
1755
16th to endDay
45%38%14%18%9%
1872
January1st to 15thNight
37%26%16%20%17%
2522
16th to endNight
37%25%15%21%17%
2613
February1st to 15thNight
37%26%14%20%19%
2292
16th to endNight
39%35%16%20%14%
2050
March1st to 15thNight
40%38%15%18%14%
2165
16th to endNight
38%49%15%20%11%
2288
April1st to 15thNight
38%39%16%17%15%
2142
16th to endNight
41%48%18%15%12%
1897
May1st to 15thNight
38%5%10%18%17%8%
1755
16th to endNight
33%414%28%15%3
1829
June1st to 15thNight
20%5%15%29%25%3
1755
16th to endNight
30%6%17%28%13%2
1755
July1st to 15thNight
23%8%24%30%12%1
1665
16th to endNight
21%7%26%34%10%
1738
August1st to 15thNight
14%7%23%38%14%1
1789
16th to endNight
27%9%23%27%9%2
2138
September1st to 15thNight
35%6%17%21%14%5%
2145
16th to endNight
36%7%14%19%14%8%
2145
October1st to 15thNight
28%313%25%18%11%
2148
16th to endNight
26%15%17%26%21%
2448
November1st to 15thNight
35%410%19%16%12%
2293
16th to endNight
31%27%15%22%20%
2535
December1st to 15thNight
41%35%18%20%11%
2535
16th to endNight
44%3413%21%12%
2704

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
19%10%34%25%9%
135
16th to endDay
20%9%34%21%11%1
144
February1st to 15thDay
24%14%30%25%5%
135
16th to endDay
32%16%30%18%
120
March1st to 15thDay
25%11%40%16%5%
135
16th to endDay
15%5%39%30%7%
144
April1st to 15thDay
13%15%36%29%5%
135
16th to endDay
18%8%36%28%8%
347
May1st to 15thDay
17%8%35%30%7%
543
16th to endDay
22%5%19%34%16%1
597
June1st to 15thDay
7%13%45%29%3
570
16th to endDay
12%110%37%29%9%
570
July1st to 15thDay
7%18%33%39%9%
550
16th to endDay
5%428%50%11%
595
August1st to 15thDay
41429%47%13%
474
16th to endDay
11%1330%36%17%
325
September1st to 15thDay
17%7%16%41%17%
180
16th to endDay
13%7%22%41%14%
180
October1st to 15thDay
8%10%31%33%15%
180
16th to endDay
25%417%26%20%5%
178
November1st to 15thDay
30%320%28%13%3
135
16th to endDay
15%11%27%25%15%4
135
December1st to 15thDay
28%225%23%10%8%
135
16th to endDay
31%9%31%19%6%1
144
January1st to 15thNight
21%9%24%34%10%
975
16th to endNight
20%8%27%32%9%
949
February1st to 15thNight
23%9%30%28%8%
764
16th to endNight
24%10%39%22%3
684
March1st to 15thNight
20%7%35%30%5%
605
16th to endNight
19%7%36%30%4
624
April1st to 15thNight
17%10%38%29%4
584
16th to endNight
19%7%38%25%7%
579
May1st to 15thNight
16%6%37%31%7%
585
16th to endNight
13%428%36%16%
611
June1st to 15thNight
423%48%21%1
585
16th to endNight
7%115%49%22%2
585
July1st to 15thNight
5%10%47%33%2
555
16th to endNight
5%5%48%35%4
612
August1st to 15thNight
1446%41%5%
575
16th to endNight
8%342%39%4
621
September1st to 15thNight
10%435%42%6%
585
16th to endNight
12%9%30%42%4
582
October1st to 15thNight
11%111%38%31%5%
588
16th to endNight
14%319%34%25%2
816
November1st to 15thNight
13%5%22%33%22%1
773
16th to endNight
16%8%22%33%17%1
975
December1st to 15thNight
23%7%25%29%13%1
975
16th to endNight
25%6%25%32%9%
1040
Page generated at 2018-10-28 07:41:03. Version ac04

Copyright 2018 A. Danko


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