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 2017-12-02.

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%
16158
28%
47854
16%
16112
55%
16112
39%
26215

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%
539
22%
1882
12%
539
47%
539
29%
1037
April16th to endNight
35%
540
23%
1638
15%
540
48%
540
29%
1000
May1st to 15thNight
39%
539
23%
1620
17%
539
51%
539
34%
933
May16th to endNight
46%
562
19%
1684
14%
561
54%
561
34%
981
June1st to 15thNight
50%
540
20%
1620
12%
540
58%
540
44%
843
June16th to endNight
55%
540
25%
1620
15%
540
64%
540
53%
785
July1st to 15thNight
67%
510
20%
1530
17%
510
71%
510
46%
801
July16th to endNight
72%
564
26%
1594
26%
531
74%
531
46%
828
August1st to 15thNight
69%
539
22%
1610
23%
536
72%
536
40%
877
August16th to endNight
67%
573
26%
1828
26%
573
73%
573
41%
967
September1st to 15thNight
55%
540
23%
1956
22%
540
63%
540
33%
1047
September16th to endNight
53%
537
33%
1980
28%
537
64%
537
38%
972
October1st to 15thNight
47%
582
34%
2022
25%
582
56%
582
35%
1034
October16th to endNight
47%
798
37%
2334
19%
798
60%
798
52%
1143
November1st to 15thNight
44%
950
31%
2470
12%
950
54%
950
50%
1316
November16th to endNight
42%
975
35%
2535
15%
975
55%
975
47%
1382
December1st to 15thNight
38%
910
27%
2366
11%
910
49%
910
40%
1388
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%54%
48595
Night
7%423323347%56%
48050
Cloud CoverDay
8%42432445%7%54%
48593
Night
7%423323447%56%
48050
TransparencyDay
29%412%27%20%5%
48293
Night
27%411%26%22%6%
47855
SeeingDay
9%6%28%33%18%3
6060
Night
9%7%33%36%12%
16160

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%42442445%8%47%
2063
16th to endDay
13%6%25%423445%47%
2322
May1st to 15thDay
10%424324446%52%
2331
16th to endDay
9%325%435%5%6%7%45%
2430
June1st to 15thDay
6%31332445%8%54%
2340
16th to endDay
421232446%7%58%
2340
July1st to 15thDay
5%424435%5%6%7%49%
2340
16th to endDay
6%2144345%5%9%53%
2466
August1st to 15thDay
6%3135%245%6%10%49%
2340
16th to endDay
6%314434448%55%
2372
September1st to 15thDay
8%32442445%5%51%
2004
16th to endDay
6%313424346%58%
1980
October1st to 15thDay
12%313313235%57%
1926
16th to endDay
5%313412345%63%
1887
November1st to 15thDay
5%413323346%62%
1718
16th to endDay
7%313212235%65%
1755
December1st to 15thDay
9%7%24313337%53%
1631
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
10%5%2332345%7%51%
1882
16th to endNight
12%6%24423446%47%
1638
May1st to 15thNight
11%433223347%53%
1619
16th to endNight
8%3144345%6%9%47%
1683
June1st to 15thNight
6%31232345%11%56%
1620
16th to endNight
411221445%11%60%
1620
July1st to 15thNight
5%2134245%7%10%54%
1620
16th to endNight
5%21231446%10%55%
1702
August1st to 15thNight
5%31442447%10%50%
1620
16th to endNight
431433435%7%57%
1828
September1st to 15thNight
7%41342446%7%53%
1956
16th to endNight
5%31433345%6%58%
1980
October1st to 15thNight
11%41232233460%
2022
16th to endNight
5%2221223469%
2334
November1st to 15thNight
5%312213346%63%
2470
16th to endNight
8%312212336%63%
2535
December1st to 15thNight
8%5%23324447%54%
2366
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
10%5%2432445%9%47%
2063
16th to endDay
14%6%2442335%5%47%
2322
May1st to 15thDay
10%42332445%6%52%
2331
16th to endDay
9%325%435%5%6%7%45%
2430
June1st to 15thDay
6%31332445%8%54%
2340
16th to endDay
422232446%8%58%
2340
July1st to 15thDay
5%423436%5%6%7%49%
2340
16th to endDay
5%213425%5%5%8%53%
2466
August1st to 15thDay
6%3145%235%6%9%50%
2340
16th to endDay
6%314434448%55%
2372
September1st to 15thDay
8%4245%245%46%51%
2004
16th to endDay
6%413423245%59%
1980
October1st to 15thDay
12%313322235%58%
1926
16th to endDay
5%313322345%63%
1887
November1st to 15thDay
6%323223345%62%
1718
16th to endDay
7%413212235%65%
1755
December1st to 15thDay
9%7%24313337%54%
1631
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
10%42332345%7%51%
1882
16th to endNight
13%5%25%323446%47%
1638
May1st to 15thNight
10%5%24313347%53%
1619
16th to endNight
8%2134245%6%9%48%
1683
June1st to 15thNight
6%21222345%10%56%
1620
16th to endNight
41122135%5%11%60%
1620
July1st to 15thNight
42134245%6%11%53%
1620
16th to endNight
5%21232346%11%55%
1702
August1st to 15thNight
5%314425%47%10%50%
1620
16th to endNight
431443345%7%57%
1828
September1st to 15thNight
7%41332446%7%52%
1956
16th to endNight
5%31442345%6%59%
1980
October1st to 15thNight
11%41232233460%
2022
16th to endNight
5%21231223470%
2334
November1st to 15thNight
5%312223346%63%
2470
16th to endNight
8%213213336%63%
2535
December1st to 15thNight
8%5%2332445%6%54%
2366
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%27%16%5%
2063
16th to endDay
37%410%28%16%2
2322
May1st to 15thDay
32%413%24%22%2
2331
16th to endDay
35%6%14%27%13%2
2434
June1st to 15thDay
26%6%16%26%17%6%
2340
16th to endDay
22%5%13%26%23%8%
2340
July1st to 15thDay
29%5%10%27%23%2
2201
16th to endDay
25%413%30%20%4
2311
August1st to 15thDay
27%6%18%25%15%6%
2326
16th to endDay
27%5%11%28%20%7%
2372
September1st to 15thDay
31%412%26%16%8%
2004
16th to endDay
26%310%30%23%4
1980
October1st to 15thDay
29%211%22%25%7%
1926
16th to endDay
23%39%27%28%8%
1887
November1st to 15thDay
23%29%24%27%11%
1718
16th to endDay
23%17%29%28%8%
1755
December1st to 15thDay
32%213%31%17%3
1631
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
31%412%24%20%5%
1882
16th to endNight
37%412%22%19%3
1638
May1st to 15thNight
31%412%26%20%4
1620
16th to endNight
30%7%19%26%13%3
1685
June1st to 15thNight
22%5%18%28%20%4
1620
16th to endNight
18%314%31%23%8%
1620
July1st to 15thNight
23%5%13%29%23%4
1530
16th to endNight
22%5%13%29%24%4
1594
August1st to 15thNight
26%5%17%27%16%5%
1610
16th to endNight
25%5%11%27%21%8%
1828
September1st to 15thNight
29%6%12%26%16%9%
1956
16th to endNight
24%410%26%27%7%
1980
October1st to 15thNight
28%38%23%27%8%
2022
16th to endNight
19%25%27%33%11%
2334
November1st to 15thNight
21%310%23%29%12%
2470
16th to endNight
23%26%29%28%9%
2535
December1st to 15thNight
29%312%29%21%3
2366
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
14%13%37%27%6%
182
16th to endDay
18%13%36%24%5%1
432
May1st to 15thDay
10%10%36%30%10%1
468
16th to endDay
13%6%34%31%11%2
517
June1st to 15thDay
7%5%28%37%18%3
495
16th to endDay
3325%40%23%3
495
July1st to 15thDay
5%213%42%28%6%
475
16th to endDay
8%10%39%32%8%
515
August1st to 15thDay
7%212%42%32%2
493
16th to endDay
8%216%41%27%4
412
September1st to 15thDay
3222%42%27%3
159
16th to endDay
12%5%22%30%20%8%
135
October1st to 15thDay
12%5%38%28%11%2
135
16th to endDay
48%37%25%18%6%
144
November1st to 15thDay
14%5%31%31%9%6%
135
16th to endDay
11%14%38%17%15%2
135
December1st to 15thDay
10%11%47%17%12%
94
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%11%40%27%6%
539
16th to endNight
15%10%37%29%7%
540
May1st to 15thNight
12%8%34%32%10%1
540
16th to endNight
9%6%31%34%15%1
563
June1st to 15thNight
7%434%39%11%1
540
16th to endNight
3432%43%15%
540
July1st to 15thNight
5%118%53%19%1
510
16th to endNight
6%15%48%28%
564
August1st to 15thNight
5%115%51%24%
539
16th to endNight
6%217%43%27%2
573
September1st to 15thNight
9%325%40%19%1
540
16th to endNight
6%330%37%19%1
537
October1st to 15thNight
13%7%29%33%15%
582
16th to endNight
7%5%37%35%12%1
798
November1st to 15thNight
6%7%37%33%13%
950
16th to endNight
9%9%35%33%10%
975
December1st to 15thNight
11%9%37%35%6%
910
16th to endNight
13%9%35%35%6%
960
Page generated at 2017-12-03 10:16:42. Version ac04

Copyright 2017 A. Danko


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