Pratt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pratt clear sky chart from 2005-11-05 to 2017-04-01.

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
45%
15342
28%
45366
16%
15296
55%
15296
39%
24881

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
37%
900
31%
2327
13%
895
48%
895
41%
1349
January16th to endNight
38%
960
34%
2496
14%
960
51%
960
45%
1387
February1st to 15thNight
33%
770
23%
2178
10%
766
42%
766
32%
1320
February16th to endNight
33%
632
32%
1894
13%
632
49%
632
37%
1039
March1st to 15thNight
38%
622
27%
2062
14%
622
51%
622
34%
1106
March16th to endNight
31%
576
21%
2112
13%
576
45%
576
28%
1170
April1st to 15thNight
31%
497
24%
1736
13%
497
48%
497
30%
943
April16th to endNight
36%
495
23%
1502
15%
495
49%
495
29%
906
May1st to 15thNight
38%
494
24%
1485
17%
494
51%
494
33%
862
May16th to endNight
47%
514
18%
1540
14%
513
54%
513
34%
897
June1st to 15thNight
49%
495
21%
1485
12%
495
58%
495
44%
776
June16th to endNight
56%
495
24%
1485
14%
495
64%
495
53%
716
July1st to 15thNight
67%
465
21%
1395
17%
465
70%
465
45%
736
July16th to endNight
72%
522
25%
1468
24%
489
74%
489
46%
761
August1st to 15thNight
69%
494
23%
1475
24%
491
73%
491
41%
800
August16th to endNight
66%
525
24%
1675
24%
525
72%
525
40%
896
September1st to 15thNight
54%
495
24%
1793
23%
495
62%
495
31%
986
September16th to endNight
55%
492
32%
1815
28%
492
65%
492
39%
880
October1st to 15thNight
48%
533
35%
1853
27%
533
57%
533
36%
936
October16th to endNight
48%
731
35%
2139
18%
731
59%
731
52%
1055
November1st to 15thNight
45%
875
33%
2275
13%
875
56%
875
51%
1199
November16th to endNight
42%
900
34%
2340
14%
900
54%
900
47%
1276
December1st to 15thNight
38%
900
27%
2340
11%
900
49%
900
39%
1375
December16th to endNight
38%
960
32%
2496
15%
960
49%
960
37%
1510

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
8%42432445%7%53%
45738
Night
7%423323347%56%
45562
Cloud CoverDay
8%42432445%7%54%
45736
Night
7%423323447%56%
45562
TransparencyDay
29%412%27%20%5%
45436
Night
27%311%26%22%6%
45367
SeeingDay
9%6%28%34%17%3
5059
Night
9%7%33%36%12%
15344

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
6%424313336%60%
1611
16th to endDay
7%423213348%56%
1728
February1st to 15thDay
6%6%35%434336%51%
1694
16th to endDay
5%6%2221336%5%59%
1580
March1st to 15thDay
8%5%15%42345%7%50%
1897
16th to endDay
13%5%24414346%49%
2112
April1st to 15thDay
10%42442446%8%46%
1894
16th to endDay
12%6%25%423445%48%
2128
May1st to 15thDay
10%424224446%51%
2136
16th to endDay
9%325%435%5%6%7%44%
2222
June1st to 15thDay
6%31332445%9%53%
2145
16th to endDay
5%21232447%7%57%
2145
July1st to 15thDay
6%424435%5%6%7%48%
2145
16th to endDay
6%2134345%6%9%54%
2284
August1st to 15thDay
5%3135%245%6%10%50%
2145
16th to endDay
6%314434449%54%
2173
September1st to 15thDay
9%425%42445%6%50%
1837
16th to endDay
5%313424346%59%
1815
October1st to 15thDay
12%313313235%58%
1766
16th to endDay
5%323422345%62%
1730
November1st to 15thDay
5%313322336%64%
1583
16th to endDay
7%413212235%64%
1620
December1st to 15thDay
9%7%24313337%53%
1620
16th to endDay
9%7%25%323435%52%
1728
January1st to 15thNight
5%423423346%57%
2327
16th to endNight
5%423313235%62%
2496
February1st to 15thNight
9%6%35%423445%50%
2168
16th to endNight
7%5%24323345%57%
1894
March1st to 15thNight
7%424424347%53%
2062
16th to endNight
11%5%25%423345%51%
2112
April1st to 15thNight
9%5%1333345%6%52%
1736
16th to endNight
11%5%24424446%48%
1502
May1st to 15thNight
11%433223347%52%
1484
16th to endNight
8%3144345%7%10%47%
1539
June1st to 15thNight
6%31232345%11%56%
1485
16th to endNight
411122445%10%61%
1485
July1st to 15thNight
5%2134245%6%10%53%
1485
16th to endNight
6%21231447%10%55%
1576
August1st to 15thNight
5%31442447%10%50%
1485
16th to endNight
441432435%8%56%
1675
September1st to 15thNight
8%42442346%8%50%
1793
16th to endNight
5%31433335%6%60%
1815
October1st to 15thNight
11%41222233461%
1853
16th to endNight
5%3231223469%
2139
November1st to 15thNight
5%312213346%64%
2275
16th to endNight
8%312213336%63%
2340
December1st to 15thNight
8%5%23324447%54%
2340
16th to endNight
9%6%34323246%53%
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
6%424313336%60%
1611
16th to endDay
7%5%14223347%56%
1728
February1st to 15thDay
6%6%35%434336%52%
1694
16th to endDay
5%6%1321345%5%59%
1580
March1st to 15thDay
8%5%2442445%7%49%
1897
16th to endDay
13%5%24313346%49%
2110
April1st to 15thDay
9%5%2432445%9%47%
1894
16th to endDay
12%6%2442345%5%48%
2128
May1st to 15thDay
11%42332445%6%51%
2136
16th to endDay
9%325%5%35%5%6%7%44%
2222
June1st to 15thDay
6%41332445%8%54%
2145
16th to endDay
5%21232446%8%57%
2145
July1st to 15thDay
6%423436%5%6%7%48%
2145
16th to endDay
5%2134245%6%9%54%
2284
August1st to 15thDay
5%3135%235%6%9%51%
2145
16th to endDay
6%314434448%54%
2173
September1st to 15thDay
9%4245%245%46%50%
1837
16th to endDay
5%413424345%60%
1815
October1st to 15thDay
12%313322335%58%
1766
16th to endDay
5%413322345%62%
1730
November1st to 15thDay
5%222222346%65%
1583
16th to endDay
7%41321223465%
1620
December1st to 15thDay
9%7%24313336%53%
1620
16th to endDay
10%6%35%313335%52%
1728
January1st to 15thNight
5%423433346%57%
2327
16th to endNight
5%423413235%62%
2496
February1st to 15thNight
8%7%35%423345%50%
2168
16th to endNight
7%5%24323435%57%
1894
March1st to 15thNight
7%424424347%54%
2062
16th to endNight
11%6%14423446%51%
2112
April1st to 15thNight
9%5%2332345%7%52%
1736
16th to endNight
12%5%25%32445%6%48%
1502
May1st to 15thNight
10%5%24313337%53%
1484
16th to endNight
8%3134246%6%10%47%
1539
June1st to 15thNight
7%21222345%10%56%
1485
16th to endNight
41112235%410%61%
1485
July1st to 15thNight
42134245%6%11%52%
1485
16th to endNight
6%21232346%11%55%
1576
August1st to 15thNight
5%314425%47%10%50%
1485
16th to endNight
442443345%8%56%
1675
September1st to 15thNight
8%42432446%7%50%
1793
16th to endNight
5%313322445%61%
1815
October1st to 15thNight
11%41222233461%
1853
16th to endNight
6%21231223469%
2139
November1st to 15thNight
5%212213346%64%
2275
16th to endNight
8%312213335%63%
2340
December1st to 15thNight
8%5%23324446%54%
2340
16th to endNight
9%6%25%322346%53%
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
25%312%30%24%3
1612
16th to endDay
27%213%33%18%5%
1728
February1st to 15thDay
34%414%25%15%5%
1695
16th to endDay
24%314%29%20%7%
1580
March1st to 15thDay
32%410%23%21%6%
1897
16th to endDay
35%5%11%24%18%3
2112
April1st to 15thDay
33%5%11%26%16%5%
1894
16th to endDay
36%410%28%17%2
2128
May1st to 15thDay
32%5%14%23%21%2
2136
16th to endDay
36%6%14%25%13%3
2226
June1st to 15thDay
26%6%16%25%17%6%
2145
16th to endDay
23%5%13%26%23%8%
2145
July1st to 15thDay
30%5%10%28%21%2
2006
16th to endDay
24%413%30%21%5%
2129
August1st to 15thDay
26%6%17%26%15%6%
2131
16th to endDay
28%5%11%28%18%7%
2173
September1st to 15thDay
32%5%12%25%15%9%
1837
16th to endDay
25%310%30%24%5%
1815
October1st to 15thDay
29%211%24%25%6%
1766
16th to endDay
24%29%28%26%8%
1730
November1st to 15thDay
21%210%25%28%11%
1583
16th to endDay
24%18%28%27%8%
1620
December1st to 15thDay
32%213%31%17%3
1620
16th to endDay
34%310%26%20%3
1728
January1st to 15thNight
27%39%29%25%4
2327
16th to endNight
25%29%28%27%5%
2496
February1st to 15thNight
35%313%23%17%5%
2178
16th to endNight
29%29%28%20%9%
1894
March1st to 15thNight
29%310%24%22%8%
2062
16th to endNight
34%313%26%18%4
2112
April1st to 15thNight
30%413%23%21%6%
1736
16th to endNight
36%412%22%20%3
1502
May1st to 15thNight
32%412%25%19%4
1485
16th to endNight
30%6%20%25%12%3
1541
June1st to 15thNight
22%5%18%28%20%4
1485
16th to endNight
18%314%30%23%8%
1485
July1st to 15thNight
23%5%13%29%23%4
1395
16th to endNight
22%414%29%24%4
1468
August1st to 15thNight
26%5%16%27%17%6%
1475
16th to endNight
25%6%11%27%19%8%
1675
September1st to 15thNight
30%6%12%24%15%10%
1793
16th to endNight
24%49%26%27%7%
1815
October1st to 15thNight
27%39%24%27%8%
1853
16th to endNight
19%25%27%33%10%
2139
November1st to 15thNight
20%310%22%30%13%
2275
16th to endNight
24%27%28%27%10%
2340
December1st to 15thNight
29%312%28%22%3
2340
16th to endNight
32%211%26%18%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
10%11%35%33%7%2
90
16th to endDay
14%13%40%25%6%
96
February1st to 15thDay
10%8%46%27%5%1
90
16th to endDay
111%55%19%11%1
81
March1st to 15thDay
8%12%57%15%41
90
16th to endDay
13%8%44%26%7%
96
April1st to 15thDay
13%16%36%28%5%
130
16th to endDay
16%14%34%27%5%1
358
May1st to 15thDay
11%11%36%31%9%1
393
16th to endDay
13%6%34%30%11%3
437
June1st to 15thDay
7%5%30%37%16%2
420
16th to endDay
4325%37%25%4
420
July1st to 15thDay
6%315%44%26%4
400
16th to endDay
8%11%40%31%6%
445
August1st to 15thDay
6%211%43%33%2
418
16th to endDay
8%217%43%24%3
341
September1st to 15thDay
4323%51%14%2
112
16th to endDay
10%420%28%26%10%
90
October1st to 15thDay
8%431%34%16%4
90
16th to endDay
4341%26%15%9%
96
November1st to 15thDay
11%431%33%11%8%
90
16th to endDay
17%14%40%16%10%1
90
December1st to 15thDay
11%11%47%17%12%
90
16th to endDay
18%10%50%17%3
96
January1st to 15thNight
7%11%38%34%8%
900
16th to endNight
7%9%39%34%7%
960
February1st to 15thNight
12%10%37%33%5%
770
16th to endNight
9%10%43%30%6%
632
March1st to 15thNight
7%11%38%33%8%
622
16th to endNight
13%10%42%28%6%
576
April1st to 15thNight
13%12%40%27%6%
497
16th to endNight
14%10%36%29%7%
495
May1st to 15thNight
12%8%34%32%10%
495
16th to endNight
9%6%30%35%16%1
515
June1st to 15thNight
7%5%35%40%10%
495
16th to endNight
4431%43%16%
495
July1st to 15thNight
5%118%54%18%1
465
16th to endNight
6%15%49%27%
522
August1st to 15thNight
6%114%51%24%1
494
16th to endNight
7%217%43%26%2
525
September1st to 15thNight
10%326%39%19%1
495
16th to endNight
5%329%38%20%2
492
October1st to 15thNight
13%7%28%33%16%
533
16th to endNight
8%5%36%35%13%1
731
November1st to 15thNight
7%7%36%34%13%
875
16th to endNight
10%9%35%34%10%
900
December1st to 15thNight
11%9%37%35%5%
900
16th to endNight
13%9%35%35%6%
960
Page generated at 2017-04-02 06:39:08. Version ac04

Copyright 2017 A. Danko


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