Preston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Preston clear sky chart from 2005-11-05 to 2018-02-14.

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
29%
15568
18%
48770
8%
15537
40%
15537
21%
32029

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
17%
780
20%
2716
7%
776
35%
776
17%
1700
January16th to endNight
15%
832
19%
2912
5%
832
34%
832
15%
1856
February1st to 15thNight
15%
760
18%
2326
4%
757
31%
757
17%
1556
February16th to endNight
16%
631
24%
1893
7%
631
37%
631
19%
1239
March1st to 15thNight
20%
720
17%
2160
6%
720
33%
720
18%
1460
March16th to endNight
22%
768
22%
2017
7%
768
34%
768
23%
1350
April1st to 15thNight
22%
718
17%
1796
5%
718
30%
718
23%
1278
April16th to endNight
24%
623
16%
1703
5%
623
31%
623
22%
1195
May1st to 15thNight
26%
540
15%
1620
6%
540
35%
540
18%
1145
May16th to endNight
33%
413
10%
1536
7%
413
38%
413
15%
1053
June1st to 15thNight
36%
360
12%
1440
11%
360
44%
360
14%
962
June16th to endNight
49%
360
12%
1440
13%
360
56%
360
23%
850
July1st to 15thNight
45%
340
11%
1360
13%
340
49%
340
19%
868
July16th to endNight
50%
376
13%
1418
15%
354
53%
354
19%
898
August1st to 15thNight
51%
506
16%
1577
12%
504
54%
504
26%
1000
August16th to endNight
57%
764
20%
1910
12%
764
58%
764
33%
1264
September1st to 15thNight
41%
720
19%
1848
10%
720
45%
720
29%
1255
September16th to endNight
47%
717
22%
1980
15%
717
52%
717
30%
1288
October1st to 15thNight
35%
720
22%
2019
12%
720
45%
720
26%
1323
October16th to endNight
25%
768
18%
2475
8%
768
38%
768
21%
1604
November1st to 15thNight
29%
760
20%
2472
10%
760
42%
760
21%
1589
November16th to endNight
25%
780
22%
2535
9%
780
41%
780
21%
1588
December1st to 15thNight
22%
780
17%
2705
5%
780
36%
780
16%
1790
December16th to endNight
15%
832
19%
2912
4%
832
33%
832
15%
1918

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
17%6%24424447%40%
49310
Night
17%6%24424346%41%
48940
Cloud CoverDay
17%5%2442445%6%41%
49320
Night
17%6%24424446%41%
48945
TransparencyDay
42%6%12%20%15%2
48998
Night
43%5%11%20%15%2
48770
SeeingDay
20%13%31%22%9%2
6331
Night
21%12%32%24%8%
15569

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
16%6%24325%445%43%
1552
16th to endDay
16%6%24424346%43%
1664
February1st to 15thDay
17%6%34435%5%5%7%36%
1831
16th to endDay
17%6%15%5%345%45%40%
1580
March1st to 15thDay
20%6%2442435%6%39%
1790
16th to endDay
17%7%34423445%42%
2202
April1st to 15thDay
20%5%35%324346%39%
2152
16th to endDay
19%314424437%43%
2256
May1st to 15thDay
25%6%25%423346%35%
2332
16th to endDay
17%5%144245%48%40%
2589
June1st to 15thDay
22%6%35%43445%8%33%
2520
16th to endDay
13%6%245%345%6%9%37%
2520
July1st to 15thDay
14%425%6%35%6%5%8%37%
2520
16th to endDay
14%5%245%25%5%6%9%36%
2654
August1st to 15thDay
14%6%25%5%35%45%8%37%
2359
16th to endDay
10%5%36%5%3445%8%41%
2292
September1st to 15thDay
15%5%24434446%45%
2112
16th to endDay
15%5%14423345%48%
1980
October1st to 15thDay
18%6%25%323335%43%
1938
16th to endDay
15%5%25%5%24345%46%
1749
November1st to 15thDay
19%7%24323346%42%
1716
16th to endDay
16%314434445%46%
1755
December1st to 15thDay
20%7%1432335%6%41%
1583
16th to endDay
21%7%26%323245%38%
1664
January1st to 15thNight
18%6%24423346%43%
2716
16th to endNight
16%9%24423445%41%
2912
February1st to 15thNight
16%8%25%5%33345%40%
2325
16th to endNight
17%7%24423445%43%
1893
March1st to 15thNight
19%6%24423346%39%
2147
16th to endNight
17%7%24424345%44%
2017
April1st to 15thNight
18%7%34424446%38%
1796
16th to endNight
22%5%245%2434440%
1703
May1st to 15thNight
20%8%25%434236%37%
1620
16th to endNight
18%6%245%345%5%8%35%
1534
June1st to 15thNight
22%5%25%41445%7%37%
1440
16th to endNight
15%6%2432445%8%42%
1440
July1st to 15thNight
13%6%25%6%26%46%8%38%
1440
16th to endNight
12%6%35%6%35%5%6%9%35%
1514
August1st to 15thNight
9%5%25%5%345%5%8%42%
1587
16th to endNight
11%4245%345%5%8%44%
1910
September1st to 15thNight
15%43443445%6%43%
1848
16th to endNight
14%6%25%323435%47%
1980
October1st to 15thNight
15%6%25%434345%44%
2019
16th to endNight
18%5%25%423446%41%
2475
November1st to 15thNight
18%6%24423335%44%
2472
16th to endNight
17%7%24423346%43%
2535
December1st to 15thNight
21%7%24334346%38%
2705
16th to endNight
20%7%25%42435%5%38%
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
17%6%23424445%43%
1552
16th to endDay
17%6%34423346%43%
1664
February1st to 15thDay
17%7%23435%46%6%36%
1831
16th to endDay
17%6%25%5%34346%39%
1580
March1st to 15thDay
21%6%2442335%6%39%
1800
16th to endDay
17%8%3441434443%
2202
April1st to 15thDay
20%6%34324446%40%
2152
16th to endDay
19%424424347%43%
2256
May1st to 15thDay
26%5%25%423336%35%
2332
16th to endDay
18%5%2443435%7%40%
2589
June1st to 15thDay
23%5%245%2345%7%34%
2520
16th to endDay
14%6%25%42446%9%37%
2520
July1st to 15thDay
14%5%245%35%5%5%9%38%
2520
16th to endDay
14%5%25%5%25%5%6%8%37%
2654
August1st to 15thDay
14%5%25%5%35%5%5%8%38%
2359
16th to endDay
11%5%25%5%25%45%7%43%
2292
September1st to 15thDay
16%5%23425%345%46%
2112
16th to endDay
15%5%24323345%48%
1980
October1st to 15thDay
19%6%25%423345%43%
1938
16th to endDay
16%5%15%5%23345%46%
1749
November1st to 15thDay
20%6%24323345%43%
1716
16th to endDay
17%3235%24345%46%
1755
December1st to 15thDay
21%6%2432335%5%41%
1583
16th to endDay
22%7%34413336%38%
1664
January1st to 15thNight
18%6%23323336%43%
2716
16th to endNight
17%8%34423335%41%
2912
February1st to 15thNight
16%9%35%5%23345%41%
2318
16th to endNight
17%6%34324335%44%
1893
March1st to 15thNight
19%6%2442445%5%40%
2160
16th to endNight
17%6%33424345%44%
2016
April1st to 15thNight
18%8%345%2445%5%38%
1796
16th to endNight
22%5%245%3334340%
1703
May1st to 15thNight
20%8%34434336%36%
1620
16th to endNight
18%6%244445%5%7%35%
1534
June1st to 15thNight
23%5%24334347%36%
1440
16th to endNight
16%5%243245%5%8%42%
1440
July1st to 15thNight
14%5%25%5%25%45%9%37%
1440
16th to endNight
12%6%35%5%36%6%6%8%35%
1514
August1st to 15thNight
10%5%25%435%6%6%8%41%
1587
16th to endNight
11%425%425%45%7%45%
1910
September1st to 15thNight
15%5%245%34446%43%
1848
16th to endNight
15%5%2441434447%
1980
October1st to 15thNight
16%6%24423335%45%
2019
16th to endNight
19%5%24423446%42%
2475
November1st to 15thNight
18%6%24423335%44%
2472
16th to endNight
18%6%24423346%43%
2535
December1st to 15thNight
21%7%24324345%38%
2705
16th to endNight
20%7%24423446%38%
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
42%6%11%22%13%4
1553
16th to endDay
41%411%23%15%1
1664
February1st to 15thDay
46%6%13%20%12%1
1834
16th to endDay
45%411%22%12%3
1580
March1st to 15thDay
46%414%22%10%2
1800
16th to endDay
43%411%19%16%4
2202
April1st to 15thDay
44%5%11%19%16%3
2152
16th to endDay
40%6%10%20%18%3
2256
May1st to 15thDay
50%5%9%18%13%2
2332
16th to endDay
42%7%14%20%12%2
2593
June1st to 15thDay
49%7%12%19%10%
2520
16th to endDay
40%8%14%22%13%1
2520
July1st to 15thDay
41%8%12%22%13%1
2371
16th to endDay
40%7%16%21%11%2
2487
August1st to 15thDay
42%7%15%19%14%
2345
16th to endDay
37%7%11%23%18%2
2292
September1st to 15thDay
39%6%12%19%20%2
2112
16th to endDay
37%5%7%22%24%2
1980
October1st to 15thDay
42%411%21%16%2
1938
16th to endDay
39%5%12%22%17%2
1749
November1st to 15thDay
43%5%14%16%17%2
1716
16th to endDay
39%410%25%16%2
1755
December1st to 15thDay
43%5%14%18%16%1
1583
16th to endDay
48%5%13%18%13%
1664
January1st to 15thNight
42%412%22%14%2
2716
16th to endNight
45%411%20%15%2
2912
February1st to 15thNight
46%413%21%12%2
2326
16th to endNight
43%49%21%17%2
1893
March1st to 15thNight
45%5%11%20%13%2
2160
16th to endNight
42%49%18%19%5%
2017
April1st to 15thNight
46%5%10%20%14%2
1796
16th to endNight
47%5%8%18%16%4
1703
May1st to 15thNight
49%5%10%16%13%3
1620
16th to endNight
47%9%17%14%9%1
1536
June1st to 15thNight
48%7%12%16%14%
1440
16th to endNight
38%6%15%25%12%1
1440
July1st to 15thNight
44%7%12%20%13%
1360
16th to endNight
41%8%15%21%10%2
1418
August1st to 15thNight
37%8%13%23%14%2
1577
16th to endNight
36%7%11%25%17%
1910
September1st to 15thNight
41%6%11%22%17%
1848
16th to endNight
40%49%18%25%2
1980
October1st to 15thNight
42%410%22%16%3
2019
16th to endNight
42%6%13%20%15%1
2475
November1st to 15thNight
43%411%20%17%3
2472
16th to endNight
42%412%24%13%2
2535
December1st to 15thNight
47%413%20%13%
2705
16th to endNight
47%411%21%13%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
17%35%26%17%2
90
16th to endDay
22%22%43%10%
96
February1st to 15thDay
25%26%34%11%2
272
16th to endDay
15%23%40%15%4
239
March1st to 15thDay
25%20%32%17%3
270
16th to endDay
23%16%38%14%41
288
April1st to 15thDay
23%17%34%16%8%
270
16th to endDay
22%13%36%17%6%3
270
May1st to 15thDay
32%16%25%17%5%1
288
16th to endDay
19%9%34%25%9%1
331
June1st to 15thDay
23%12%26%25%10%2
315
16th to endDay
18%11%30%29%7%2
315
July1st to 15thDay
20%428%28%15%1
305
16th to endDay
13%6%30%32%12%3
326
August1st to 15thDay
15%427%32%14%5%
313
16th to endDay
11%5%26%29%18%7%
332
September1st to 15thDay
16%8%29%25%14%5%
315
16th to endDay
17%5%31%22%13%9%
314
October1st to 15thDay
21%8%25%24%14%4
315
16th to endDay
18%16%31%22%9%1
293
November1st to 15thDay
22%13%29%22%9%1
280
16th to endDay
20%18%29%21%10%
285
December1st to 15thDay
26%20%32%12%7%
113
16th to endDay
28%31%31%9%
96
January1st to 15thNight
22%21%35%18%2
780
16th to endNight
22%21%36%18%1
832
February1st to 15thNight
20%18%40%18%1
760
16th to endNight
21%20%38%17%2
631
March1st to 15thNight
23%17%36%19%3
720
16th to endNight
22%14%37%20%4
768
April1st to 15thNight
24%12%36%22%4
718
16th to endNight
23%9%37%22%5%
623
May1st to 15thNight
23%9%35%23%7%
540
16th to endNight
22%6%31%31%7%
414
June1st to 15thNight
25%6%27%30%10%
360
16th to endNight
19%324%40%12%
360
July1st to 15thNight
19%225%35%17%
340
16th to endNight
14%125%43%15%
376
August1st to 15thNight
12%323%45%13%
506
16th to endNight
14%117%40%22%3
764
September1st to 15thNight
19%328%31%15%1
720
16th to endNight
19%424%29%18%4
717
October1st to 15thNight
20%7%32%24%13%1
720
16th to endNight
23%13%35%19%9%
768
November1st to 15thNight
20%13%31%22%10%1
760
16th to endNight
21%16%32%21%7%
780
December1st to 15thNight
25%16%31%20%5%
780
16th to endNight
24%19%37%16%2
832
Page generated at 2018-02-15 10:58:49. Version ac04

Copyright 2018 A. Danko


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